Recent Progress on Barium Titanate‐Based Ferroelectrics for Sensor Applications

Sensors, functioning as primary conveyors of perceptual data, stand ready to illuminate the landscape of the intelligent era. Barium titanate, an exceedingly pivotal class of ferroelectric materials for sensor applications, has attracted considerable attention from both commercial and industrial sectors in recent years. Against this backdrop, this paper embarks on a comprehensive examination of sensors founded upon barium titanate across a spectrum of applications. Our investigation commences with a historical analysis of ferroelectric materials, with a specific emphasis on the developmental trajectory of barium titanate. Subsequently, an in‐depth exposition elucidates the attributes and manufacturing processes linked to barium titanate materials, providing readers with insight into the structural and manufacturing aspects of these materials. Ultimately, we introduce a diverse array of sensors tailored to distinct functions within a myriad of domains.


Introduction
In recent years, amid the rapid advancement of the information age, global attention has been notably drawn to the realms of the Internet of Things, artificial intelligence, big data analytics, and cloud computing.These transformative forces have heralded a new era of technological possibilities.Notably, at the heart of these technological marvels lie sensors-sophisticated instruments designed to perceive a wide spectrum of environmental stimuli. [1,2]Sensors bridge the divide between the digital and physical worlds, providing the essential conduits through which data flows into the digital realm for analysis, interpretation, and DOI: 10.1002/adsr.202300168decision-making.[5][6] Table 1 provides a comprehensive overview of pressure sensor performance across diverse ferroelectric materials.Notably, the pressure sensor employing barium titanate exhibits a remarkable breadth in its operational range, rendering it wellsuited for an array of application scenarios.Moreover, its rapid response time underscores its capability to meet stringent requirements.
Barium titanate (BTO), a ferroelectric material of paramount importance, has been at the forefront of sensor technology in recent years.Its unique properties, including high permittivity and piezoelectric characteristics, endow sensors with the capability to detect and respond to a wide range of physical phenomena.This versatility has positioned barium titanate as a core material for sensors deployed across various domains.Figure 1 illustrates the breadth of applications where barium titanate-based sensors play a crucial role.In the realm of environmental monitoring, these sensors enable the collection of data pertaining to air and water quality, weather conditions, and pollution levels.Such information underpins informed decisions related to resource management and public health.Barium titanate sensors contribute to these systems, ensuring seamless operation and enhancing the quality of life.

Ferroelectric History
In 1920, ferroelectricity was first observed in Rochelle salt, marking a pivotal moment in the realm of material science.Subsequent to this milestone discovery, a myriad of materials and technologies have emerged, catalyzing profound developments in both industrial and commercial applications.The revelation of ferroelectricity has since proven to be of paramount significance in advancing human technological progress.Actually, the origins of ferroelectricity can be traced back to the mid-17th century when the French chemist Elie Seignette first synthesized Rochelle salt for medicinal purposes.Over two centuries later, investigations into the physical properties of water-soluble crystalline materials, including their pyroelectric and piezoelectric characteristics, commenced.It was in 1912 that the terms "spontaneous polarization" and "ferroelectricity" were formally Pb(Zr,Ti)O 3 Inorganic-based laser 1-65 kPa 65 ms [8]   Hf 0.5 Zr 0.5 O 2 Microwave annealed 2-25 kPa 6 ms [9]   PVDF Electrospinning 1-30 kPa 16 ms [10]   Pb[(Mg 1/3 Nb 2/3 ) 0.7 Ti 0.3 ]O 3 Solid phase reaction 25-250 kPa - [11]   LiNbO 3 Hot pressing 8 Pa-3 kPa 7 ms [12]   PbTiO 3 Sol-gel 2-22 MPa - [13]   P(VDF-TrFE) Spin coating 1.7 Pa-5.5 kPa 100 ms [14]   introduced, marking a critical juncture in the understanding of these phenomena.At the onset of the 20th century, Joseph Valasek made the groundbreaking discovery of ferroelectricity in Rochelle salt, although at that time, this property was still a hypothesis awaiting conclusive proof. [15]Valasek's research encompassed the study of ferroelectric hysteresis loops and the proposal of concepts such as spontaneous polarization and Curie temperature.Building on this foundation, it took just over a decade for researchers to also identify ferroelectricity in Potassium Phosphate Monobasic (KH 2 PO 4 ). [16]The focus then shifted towards comprehensively exploring the various attributes of this material.
In the 1940s, a momentous development in the history of ferroelectricity unfolded as Vul and Vereshtschagin announced the discovery of ferroelectricity in the ABO 3 -type perovskite BTO.This discovery marked a significant milestone, given that BTO was the first known ferroelectric material devoid of hydrogen bonds in its structure.Consequently, it exhibited the ability to manifest multiple ferroelectric phases.In-depth investigations into the properties and structure of BTO revealed its wideranging applicability in industrial and military technology.The subsequent decade, often referred to as the "golden age" of ferro- electric materials, witnessed the discovery of approximately 100 compounds possessing ferroelectric properties.These discoveries laid a robust foundation for the subsequent development of ferroelectric materials.
Among these breakthroughs, the discovery and development of Pb(Zr,Ti)O 3 (PZT) emerged as a hallmark event. [17,18]This advancement marked another pivotal moment in the history of ferroelectricity.Currently, PZT stands as one of the most widely utilized ferroelectric ceramics in both academic research and industrial applications.It has become an indispensable material in various industrial and commercial sectors.Through precise manipulation of the Zr-to-Ti atom ratio and careful control of doping concentrations, an array of electrical properties can be achieved in PZT. [19,20]These versatile electrical characteristics can be harnessed across diverse industries and fields to address specific needs.The extensive research in the field of ferroelectric materials has led to the discovery of more than 1000 different ferroelectric materials.Table 2 presents a selection of common ferroelectrics, along with the years of their discovery.
In the 1950s, intensive research efforts were directed toward enhancing the electrical properties of ferroelectric materials, Table 2.A list of some notable ferroelectric materials and their respective years of discovery.with a particular emphasis on expanding their utility in diverse environments, including high-temperature, high-humidity conditions, and the creation of lead-free compositions. [21,22]Furthermore, as technology advanced, there emerged a growing demand for smaller and more advanced devices, necessitating the reduction of material thickness and area.Although early research interest was evident in ferroelectric thin films, technological and instrumental limitations of the time precluded the production of the high-quality ferroelectric thin films required. [23,24]It wasn't until three decades later, around the 1980s, that the development of the precise processes and instruments became capable of producing the desired high-quality ferroelectric thin films. [25,26]ubsequently, in the 1980s, a new chapter unfolded with the discovery and investigation of relaxation and polymer ferroelectrics, such as polyvinylidene fluoride.These materials found applications in various industrial sectors, particularly in sensors and other technologies.The 1990s marked another milestone, as ongoing advancements in process and instrument precision enabled the production of nano-scale ferroelectric films.This development significantly bolstered industrial and commercial applications, with memory, sensors, and detectors emerging as the most common and widespread uses of these materials. [27]or a more comprehensive exploration of the early history of ferroelectricity, this article maintains a concise focus.30][31] Post-2000, the development of ferroelectricity embarked on a new trajectory, marked by groundbreaking advancements.Reports indicate that ferroelectric ultra-thin films with thicknesses on the order of several nanometers, such as inorganic perovskite ferroelectric thin films, [32] have been achieved.This breakthrough has paved the way for the realization of increasingly miniaturized equipment.In 2008, research efforts delved deeply into negative capacitance field-effect transistors (NCFETs) with the aim of creating more efficient and energy-saving computing systems.Subsequently, in 2009, the tunneling resistance effect of polarization-controlled conductance was experimentally verified.This research unveiled the remarkable memory function of ferroelectric tunnel junctions (FTJs), leading to their application in memory devices.The principle involves applying a voltage to FTJs, modifying polarization, which in turn alters resistance values.Information can be read through the application of a small voltage.Since 2011, ferroelectrics with fluorite structures have garnered attention due to their compatibility with siliconbased technologies and the potential for thickness scalability.In recent years, research on ultra-thin ferroelectrics has extended to two-dimensional materials, [33,34] opening doors for potential applications in nanoscale integrated electronic devices.

Barium Titanate
The chemical composition of barium titanate is denoted as BTO, exemplifying a classic ABO 3 -type perovskite structure-a distinguished ferroelectric material.It merits distinction as the pioneering polycrystalline ceramic material found to exhibit ferroelectric properties in contemporary research.As an important electronic material, barium titanate has many advantages and properties.It is a strong dielectric material and can play an important role in the field of capacitors.Due to the properties of high dielectric constant and high piezoelectric effect, high resistivity and high breakdown field strength, low thermal conductivity and good thermal stability, it can be used to manufacture high-capacity capacitors, high-voltage capacitors and high-temperature capacitors.In addition to the field of capacitors, barium titanate also occupies a certain market in the fields of sensors, non-volatile memory, photovoltaic cells, electro-optical display panels, etc.In addition, it can also be used as a reinforcing agent to prepare composite materials with high mechanical and electrical properties.Therefore, barium titanate is widely used in the electronics industry.It can be used to make consumer electronics such as smartphones, tablets and televisions, as well as electronic devices in vehicles such as cars and airplanes.In recent years, the global demand for barium titanate has gradually increased, indicating that barium titanate can be used in more and more devices, and the future prospect is very broad.

Barium Titanate History
Discovered in 1941 amidst the backdrop of World War II, BTO swiftly emerged as a strategically significant material, driven by the challenges in securing supplies of the widely used dielectric, mica, at the time.This unique historical context fostered independent discoveries of BTO materials in various nations worldwide, including the United States, the United Kingdom, Russia, and Japan. [28]Scientists of the era conjectured that its exceptional dielectric properties might be entwined with its ferroelectric attributes.Post-World War II, a surge of scientific interest was devoted to the exploration of the physical properties and phases of BTO.Notably, pioneering advancements arose from the realm of crystallography.British and Japanese scientists offered the first comprehensive insights into crystal structures and the symmetrical transformations they underwent during phase transitions.In 1947, the inaugural BTO single crystals were successfully synthesized in Switzerland using ternary melt methods. [35]The 1940s witnessed a burgeoning field of ferroelectricity, driven by inquiries into the evolution of ferroelectric switches, domain structures, and BTO's crystal characteristics.The late 1940s and early 1950s ushered in the discovery of a plethora of novel perovskitestructured ferroelectric materials, characterized by continuous enhancements in their dielectric and piezoelectric properties.
Subsequent decades saw a shifting focus within the scientific community, with an intensified exploration of BTO-based materials, particularly in the realm of dielectric properties and their applicability in capacitors.The period spanning the 1970s to the 1990s marked a diversification of the ferroelectric community, with a significant development being the emergence of innovative domains related to photovoltaics and thermistors.The positive resistance temperature coefficient of BTO-based materials significantly contributed to their relevance in thermistor technology.During the 1980s and 1990s, an era of integration and miniaturization unfolded, as ferroelectrics began to find their place in compact devices.BTO single crystals and ceramics emerged as pivotal materials, serving as models for the exploration of strain mechanisms, domain structures, and microstructure engineering. [36,37]BTO films, distinguished by their excellent dielectric response and optical properties, found broad applications in dynamic random access memory and electro-optical devices.Presently, numerous applications are underpinned by the distinctive properties of BTO films. [38,39]2.2.Barium Titanate Structure Figure 3 illustrates the phase evolution of perovskite BTO.[40] In this arrangement, Ti 4+ occupies the body center of the cubic unit cell, while Ba 2+ is situated at the eight vertex corners of the cube.O 2− , on the other hand, resides at the six face centers of the cube, establishing an oxygen octahedron in conjunction with Ti 4+ .Barium titanate is renowned for its exceptional dielectric properties, particularly in proximity to its Curie temperature. At tis critical point, the dielectric constant approaches an impressive value of ≈10,000.Consequently, barium titanate finds widespread application in the manufacturing of ceramic capacitors, piezoelectric sensors, transducers, and various electronic devices.
The crystal structure of Barium titanate is notably sensitive to temperature variations, displaying distinct structural phases within specific temperature ranges, including rhombohedral, orthorhombic, tetragonal, cubic, and hexagonal phases.Generally, BTO ceramics exhibit a dynamic interplay of crystal structures across temperature gradients.Phase transitions within BTO ceramic crystals manifest within defined temperature ranges, characterized as follows: At temperatures below −90 °C, the crystal structure of BTO assumes a trigonal form.Subsequently, as the temperature rises, BTO undergoes a sequence of phase transitions, transitioning from the trigonal phase to the orthorhombic phase, and upon reaching 5 °C, it further evolves from orthorhombic to tetragonal.The tetragonal phase remains stable at room temperature, with the trigonal, orthorhombic, and tetragonal phases all featuring ferroelectric properties.As the temperature gradually escalates to the Curie temperature, T c (120 °C), the structural symmetry of the BTO crystal evolves, transitioning from the tetragonal phase to the cubic phase.Temperatures exceeding 1460 °C lead to the manifestation of the hexagonal phase within BTO.

Neural Computing Based on Barium Titanate
In the contemporary landscape, artificial intelligence (AI) systems have attained unprecedented ubiquity, exerting a profound influence across myriad facets of modern civilized life.This pervasive impact is manifest in a diverse array of applications, wherein the evolution of sensors, memory, and processors plays a pivotal role.Within the traditional confines of the von Neumann computing architecture, the discrete operation of processors and memory imparts a notable disparity in speed, with memory lagging significantly behind processing capabilities.This discrepancy underscores the exigent need for a non-volatile memory solution characterized by expansive storage capacity and rapid data retrieval rates.Furthermore, this imperative for advanced memory architecture transcends the established paradigms of computing, necessitating a departure from the conventional structures.This departure heralds a transformative phase wherein the integration of non-volatile memory not only addresses the speed differentials but also catalyzes a paradigm shift, enabling the  [40] Copyright 2019 John Wiley and Sons.
seamless realization of neural network computations.Such innovations collectively propel AI systems beyond the constraints of traditional computing architectures, ushering in a new era of computational efficiency and neural network functionality.
Currently, ferroelectric materials, recognized for their significance in neural state computation, warrant intensified scrutiny.Barium titanate, an exemplar within the ferroelectric category, stands poised for prominent contributions in the future landscape of neural state computing.Notably, the ferroelectric tunnel junction leveraging barium titanate emerges as a frontrunner among prospective candidates for neuromorphic computing devices.Its appeal lies in attributes such as rapid operational speed, minimal energy consumption, high storage density, and the capability for three-dimensional stacking.Recent advancements by Wen's research team exemplify this trend.They successfully engineered a ferroelectric tunnel junction utilizing barium titanate for neural state computation, as illustrated in Figure 4. [41] In their configuration, a doped strontium titanate serves as the bottom electrode, barium titanate operates as the ferroelectric layer, and Pt deposition functions as the top electrode.Additionally, a dielectric layer is strategically incorporated between the two electrodes and the ferroelectric layer.This innovative device exhibits continuous resistance changes spanning two orders of magnitude and displays enduring plasticity across multiple synaptic weights.When applied in artificial synapses employing the backpropagation algorithm, recognition accuracy for handwritten digits surpasses 97%.In addressing challenges related to the low switching ratio of ferroelectric tunnel junctions, Hong Fang et al. devised a novel structural solution. [42]Their approach involved the insertion of an ultra-thin strontium titanate (STO) dielectric layer, augmenting the ferroelectric properties of the device.This enhancement significantly improved the memoryswitch ratio.Leveraging these artificial synapses, they simulated an artificial neural network using a backpropagation algorithm, achieving an impressive classification accuracy of up to 92%.

Ferroelectric Sensor
Ferroelectric materials, by virtue of their favorable attributes, are well-suited for the development of future smart sensors.They possess multi-modal sensitivity rooted in reversible piezoelectric and pyroelectric transduction, boast low energy consumption, potentially exhibit self-powered capabilities, and offer mechanical  [41] Copyright 2020 American Chemical Society.
flexibility and ease of processing.These characteristics render ferroelectric materials ideal for the advancement of smart sensors in the modern era.As modern society evolves, communication and interaction with the environment increasingly permeate various facets of daily life.The future promises not only human-machine communication but also the exchange of information among intelligent machines.This progression not only enhances convenience but also augments comfort in our daily lives.This evolving landscape is driving pivotal societal and industrial trends, including the development of smart cities, active assisted living, smart healthcare, structural health monitoring, collaborative robots, and cyber-physical systems.
In the last decade, there has been a growing interest in the study of nano-ferroelectrics, [43,44] driven by their exceptional properties, including inverse piezoelectricity, thermoelectricity, ferroelectric photovoltaics, and nonlinear optical activity.These diverse characteristics find applications in non-volatile memory, [45] capacitors, photovoltaic cells, and various sensors.Scientists are currently focusing on harnessing different ferroelectric materials to enhance sensor performance.Ferroelectrics are particularly suited for sensor applications as their polarization responds to a range of external factors, including electric fields, mechanical stress, temperature variations, as well as chemical and biological influences.Ferroelectric films hold significant promise for flexible sensor components and are poised to play a pivotal role in the development of future smart sensors, such as electronic skin and various biosensors.This review centers on the utilization of barium titanate as a ferroelectric material in sensor components and presents the latest advancements in this rapidly evolving field.It provides a comprehensive review of different sensor types, including mechanical-thermal signal sensors, piezoelectric sensors, pyroelectric sensors, piezoresistive sensors, and biosensors.

Dielectric Properties
The dielectric property is of paramount importance in ferroelectric materials.Pure BTO ceramic material exhibits a significant dielectric constant and loss, with values around 2000 to 3000 at room temperature.Near its Curie point, the peak dielectric constant can reach approximately 10,000.Ferroelectric materials like barium titanate exhibit a distinctive "dielectric anomaly" near their Curie temperature (T c ). Above the Curie temperature, the relative dielectric constant follows the Curie-Weiss law: [46]  r = C T − T 0 (1)   where C represents the Curie-Weiss constant, T is the test temperature, and T 0 is the Curie-Weiss temperature.This law primarily characterizes the relationship between the relative dielectric constant and temperature for ferroelectrics above their Curie temperature and holds significant importance in the study of dielectric materials.Figure 5 illustrates the temperaturedependent variation of the relative dielectric constant for barium titanate. [47]he primary parameters for assessing dielectric properties include the dielectric constant, dielectric loss, and the tunability of the dielectric constant concerning an applied electric field.Permittivity, also referred to as dielectric constant or dielectric coefficient, quantifies a material's ability to polarize under specific conditions.Generally, the stronger a material's polarization ability, the higher its degree of polarization and dielectric constant.Notably, the magnitude of the dielectric constant depends not only on the material's intrinsic properties but also on external Temperature-dependent dielectric constant of barium titanate. [47]Copyright 2020 Elsevier.
factors such as the frequency of the applied electric field, test conditions (e.g., temperature, humidity), and applied voltage.Table 3 delineates the polarization characteristics of prevalent ferroelectric materials.
Dielectric loss, on the other hand, is the tangent of the phase angle between voltage and current.It represents the energy dissipation in a material subjected to an electric field, typically manifesting as heat energy.Dielectric materials contain a certain number of charge carriers that, when exposed to an electric field, generate local currents, consuming electric energy and converting it into heat energy.At higher frequencies, rapid changes in the electric field lead to hysteresis, causing local charge buildup, current generation, and the dissipation of thermal energy.In practical applications, reducing dielectric loss is a topic that warrants further investigation.Given these properties, BTO's exceptional dielectric characteristics make it suitable for applications such as capacitors, memory devices, and sensors. [60]

Piezoelectric Properties
Piezoelectricity, fundamental phenomenon in materials science, was initially discovered by Pierre and Curie in the 1880s.It is characterized by the linear coupling between electric and mechanical fields.The piezoelectric effect, a cornerstone of this phenomenon, results in the generation of an electric charge on the surface of a material when subjected to mechanical stress.The magnitude of the generated charge is directly proportional to the applied stress, and its polarity depends on whether the stress is compressive or tensile in nature.The direct conversion of mechanical energy into electrical energy, exemplified by the generation of an electric charge under mechanical stress, is referred to as the "direct piezoelectric effect".Conversely, when an electric field is applied to a piezoelectric material, it undergoes changes in size and shape, contracting or expanding based on the direction of the applied electric field.This behavior is termed the "inverse piezoelectric effect".Table 4 summarizes a series of BTO-based piezoelectric materials.Generally, the piezoelectric properties of materials are closely linked to their asymmetric crystal structures.Within these structures, there exist 32 sets of points, of which 21 exhibit asymmetry.Notably, 20 of these sets are characterized by piezoelectricity.The relationship between mechanical stress and electric displacement can be elegantly described by the equation: where D represents the dielectric displacement, d symbolizes the piezoelectric constant, and X corresponds to the applied stress.D signifies the dielectric displacement generated in the piezoelectric material under unit stress, revealing its piezoelectric attributes.The anisotropic nature of the piezoelectric coefficient d is a direct consequence of the crystal structure and the stress applied to the material.Typically denoted as d ij , with the first subscript denoting the direction of electrical displacement (1, 2, and 3 corresponding to the x, y, and z axes, respectively) and the second subscript representing the direction of the applied stress, d 31 and d 33 , for instance, signify transverse and longitudinal piezoelectric coefficients, respectively.Inspired by the remarkable pressure-sensing capabilities of human skin, the concept has emerged to convert external stimuli into electronic signals for perceiving the environment, creating receptors that emulate the characteristics of human skin.Tactile sensors, commonly designed to detect touch and pressure, play a pivotal role in this endeavor.Tactile sensors translate externally applied pressure into electronic signals, permitting the identification of both pressure values and directions through signal processing.Furthermore, these sensors can be seamlessly integrated into machines or devices designed to detect a spectrum of external stimuli, including strain, temperature, and humidity.Tactile sensors based on ferroelectric materials are predominantly piezoelectric sensors, lauded for their high sensitivity, dynamic response, reproducibility, and expansive dynamic range.These sensors represent the most promising candidates for the detection of dynamic pressure representations, encompassing phenomena like vibration and texture.A noteworthy example is the work of Andreas Petritz and colleagues, who reported on the development of a ferroelectric wireless electronic health patch, capable of monitoring pulse rate and measuring blood pressure. [66]

Electro-Optic Properties
The manipulation and transmission of information in our daily lives heavily rely on the use of electrons, photons, and phonons.Among these, electrons and phonons form the fundamental basis for modern information exchange.The electro-optical effect serves as a bridge between these two essential technologies, merging electronics and optics.It provides a means to convert information between the electrical and optical domains, playing a crucial role in optical communication modulation.When a sufficiently strong external electric field is applied to a crystal, it induces changes in the atomic structure of the crystal.Ions within the crystal relocate due to the electric field, leading to the emergence of anisotropy within the crystal.This collective transformation in the optical properties of the substance is referred to as the electro-optical effect.While the change in refractive index is often not substantial, it is adequate to facilitate the conversion of optical and electrical signals.The relationship between the applied electric field and the dielectric constant of the material can be described as follows: [20] n where n represents the refractive index, E is the electric field, r corresponds to the linear electro-optic effect or the Pockels effect, and R also known as the Kerr electro-optic effect.Barium titanate stands as a compelling material with remarkable attributes, rendering it a focal point of research attention.[69] These properties, along with its stability in terms of chemical and physical characteristics, even at ambient temperatures and above, have garnered the keen interest of researchers.Within the domain of optical applications, BTO crystals emerge as powerful assets, facilitating single-channel ultrahigh bandwidth electro-optic modulation.The striking electrooptic coefficient coupled with its unique modulation mechanism positions BTO as a formidable candidate for manipulating light. [68]Practically, the electro-optical effect exhibited by these crystals has found diverse applications in the development of laser devices.These include photodetectors, beam deflectors, and electro-optical deflectors, which serve as indispensable tools for the modulation of light. [70]These devices have permeated various fields, notably making an impact in lidar, laser ranging, and the domain of biomedical microscopic imaging.

Pyroelectric Properties
The pyroelectric effect, a phenomenon intrinsic to ferroelectric materials, hinges on the alteration of the material's internal spontaneous polarization in response to temperature fluctuations.When the temperature of a ferroelectric crystal changes, it induces the generation of surface charge within the crystal, aligning in a specific direction.Due to the inherent spontaneous polarization of ferroelectrics, opposite charges become bound on both sides of the crystal.In a state of thermal equilibrium, the bound charges are electrically neutralized by an equal and opposite amount of free surface charges, rendering them electrically inconspicuous.However, as the temperature increases, the internal spontaneous polarization of the crystal diminishes.Consequently, the free surface charges are unable to entirely counteract the bound charges, allowing the free charges to manifest electrical properties.This gives rise to the generation of an electric field in the vicinity of the crystal, thereby exerting attractive or repulsive forces on charged particles.Figure 6 illustrates the fundamental principles of the pyroelectric effect.When a pyroelectric material is configured as a pyroelectric capacitor, electrodes are affixed to its upper and lower surfaces, establishing electrical connections through external wires.As the distribution of charge between the capacitor's surfaces evolves due to temperature changes, an electric current is generated.The pyroelectric current is quantified by the following equation: where A represents the effective area of the capacitor, p signifies the pyroelectric coefficient, and dT/dt denotes the rate of temperature change over time.It is noteworthy that the direction of the current is also subject to the direction of the temperature change.
When the temperature decreases, the current reverses.Thus, pyroelectricity is characterized as a reversible and dynamic effect.It solely responds to fluctuations in temperature, and once the material reaches a temperature equilibrium, polarization stabilizes, and the generation of additional current ceases.One of the pivotal applications stemming from pyroelectric properties is the pyroelectric detector.These detectors have found utility in various domains, encompassing military surveillance, fire detection, gas analysis, and more.In recent years, with advances in heatdetection performance, the scope of application has extended from military use to civilian applications.These include environmental pollution monitoring, climate change assessment, crop yield prediction, chemical process monitoring, fire hazard detection, medical diagnosis, and beyond.Moreover, pyroelectric detectors excel in distinguishing active biological heat sources from ambient thermal sources, rendering them invaluable for tasks like personnel intrusion detection, position monitoring, infrared imaging, and more aspects. [71]

Acoustoelectric Properties
When surface acoustic waves interact with a carrier and become coupled with the electronic system, two significant effects come into play.First, carriers influence the speed and amplitude of sound waves.Second, sound waves trap charged particles at the crystal's lowest potential, generating an acoustic current aligned with the sound wave's direction.This phenomenon is commonly referred to as the acoustoelectric effect.The formula for the acoustic current field is given by: Here, S represents the energy density of the sound wave, q stands for the charge of the sound wave,  is the relaxation time,  is the wavelength, k denotes the Boltzmann constant, and T is the absolute temperature.This expression can be further simplified to describe acoustic currents in diverse systems: In this formula,  signifies carrier mobility, Γ represents the loss coefficient, F is the surface acoustic wave intensity, and v corresponds to the sound velocity.The Weinreich model of acoustic current, based on this formula, serves as the cornerstone for discussions on acoustic currents.
The acoustoelectric effect has underpinned the development of ferroelectric materials for various applications, including seismic wave detection, depth logging, and biomedical uses.A significant stride has been made in the realm of textile technology, granting clothing newfound functions and roles.Recent innovations have empowered clothing to communicate, generate heat, store energy, display information, cool the wearer, and even store and process digital data.Wei Yan and colleagues have introduced a novel clothing concept based on the ferroelectric material BTO. [72]This fabric can serve as a shirt with dual acoustic fibers, enabling precise acoustic pulse direction measurement.It also supports two-way communication between different pieces of clothing, one serving as the sound transmitter and the other as the receiver.This development allows clothing to communicate acoustically, relying on the acoustoelectric effect to collect acoustic health data from the body.It represents a significant advancement in biomedicine's ability to monitor patient well-being.

Other
The photovoltaic effect was first observed in ferroelectrics in 1956 when researchers documented the presence of subtle photopyroelectric currents in barium titanate and stable photovoltaic currents above the Curie temperature.At the time, this was attributed to ferroelectrics exhibiting the persistence of a charged layer.Subsequently, A. M. Glass and colleagues established that due to the non-centrosymmetric nature of ferroelectric crystal cells, these materials exhibit an extraordinary photovoltaic effect known as the bulk photovoltaic effect.In the earliest photovoltaic devices, the principle of converting light into electricity relied on inherent material asymmetry to separate electron-hole pairs, typically through structures like p-n junctions or heterojunctions. [73]owever, ferroelectric materials can generate photovoltaic voltages significantly higher than their electronic bandgap.Ji's research team conducted experimental investigations into the bulk photovoltaic effect in epitaxial ferroelectric BiFeO 3 thin films at visible wavelengths.They confirmed that the direction of photovoltage and photocurrent changes when controlling the polarization direction of the BiFeO 3 thin film.In 2016, Jonathan E. Spanier and colleagues demonstrated that the solar power conversion efficiency of ferroelectric BTO surpasses the Shockley-Queisser limit.Their work highlighted the potential of nanoscale bulk photovoltaic effects as a promising avenue for efficient photovoltaic solar conversion. [74]Furthermore, the versatility of these materials renders them exceptionally appealing for specific applications.In light of the evolving demands in the era of the Internet of Things, where self-powered sensing equipment is increasingly vital, ferroelectric materials based on the photovoltaic effect could play a significant role in this expanding field.

BTO Bulk Ceramics
BTO bulk ceramics, a class of materials that has attracted considerable attention in the fields of materials science and engineering, are characterized by a notably high dielectric constant, exceptional piezoelectric properties, and consistent ferroelectric characteristics. [75,76]These features are increasingly finding applications in a variety of technological domains.This issue highlights the growing significance of BTO ceramics in the realm of sensing technologies.Recent research has yielded remarkable progress in optimizing these materials for use in pressure sensors, accelerometers, and other state-of-the-art sensing devices.These advancements hold the promise of enhanced sensitivity and reliability.

Preparation Methods
BTO bulk ceramics can be synthesized through a range of methodologies.These principal techniques encompass solidstate reactions, the hydrothermal method, pressing and sintering, as well as the sol-gel method, among others.A comprehensive understanding of the distinctive attributes of each method, along with the specific characteristics and potential applications of the resulting BTO ceramics, serves as a catalyst for advancing the ongoing research and exploration of this remarkable material.
Solid-State Reaction: The solid-state reaction is a classical method of preparation, typically involving starting materials such as BaCO 3 and TiO 2 to synthesize BTO ceramic bulk through a high-temperature solid-state reaction.This method boasts a wellestablished track record, offering precise control over material composition and crystal structure.However, it is worth noting that the solid-state reaction procedure is time-intensive, necessitates high-temperature conditions, and may result in material density and crystal structure variations.Barium titanate bulk ceramics prepared via the solid-state reaction method exhibit commendable crystallinity, density, and chemical stability.They find notable applications in the realm of electronic devices, including their use in ceramic capacitors, piezoelectric ceramic transducers, and acoustic devices.
Pressing and Sintering: Pressing and sintering, a technique involving the compaction of BTO powder followed by hightemperature sintering, is a process renowned for producing ceramic products with exceptional density and uniform microstructure.This method is particularly versatile, allowing for the fabrication of BTO ceramics in various shapes and sizes.Barium titanate bulk ceramics prepared through pressing and sintering showcase superior density and a consistent microstructure. [77,78]heir excellence extends to diverse applications in the realms of electronics, piezoelectrics, and acoustics, making them indispensable components in ceramic capacitors, piezoelectric ceramic transducers, and acoustic filters.
Hydrothermal Method: The hydrothermal method involves a reaction conducted under high-temperature, high-pressure hydrothermal conditions, wherein precursors of barium titanate and an appropriate solvent engage in a hydrothermal process, culminating in the production of BTO bulk ceramics.Characterized by a low preparation temperature, a short preparation duration, and a straightforward process, the hydrothermal method is particularly advantageous for yielding barium titanate ceramics and boasting high density and small crystal size.Barium titanate bulk ceramics synthesized exhibit high density, fine crystal dimensions, and excellent chemical uniformity.Their performance excels in the domains of optical devices, electronic devices, and sensors, proving invaluable in the manufacturing of optical windows, optical waveguide devices, and optical sensors. [79]ol-Gel Method: The sol-gel method involves the creation of a sol by mixing barium titanate precursors with a solvent, followed by a subsequent heating and gelation process resulting in the formation of a gel.This gel is ultimately sintered to produce bulk ceramics.The sol-gel method distinguishes itself with a straightforward preparation process, the ability for precise composition control, and the capacity to fine-tune microstructures, providing a means to exert exacting control over material properties. [80,81]his method unveils a spectrum of promising applications, spanning photoelectric devices, ceramic precursors, catalyst carriers, gas sensors, and other multifaceted fields.
Other Methods: Beyond the traditional preparation methods mentioned above, there exist lesser-used techniques for crafting BTO bulk ceramics, each offering unique potential and attributes.[83] While these alternative methods share some similarities with traditional techniques, they also introduce distinctive characteristics.Notably, they commonly deliver rapid and uniform preparation rates, precise control over material properties, and versatility in producing ceramics of varying shapes and sizes.The wide array of methods available for the preparation of BTO ceramics underscores the importance of selecting the most appropriate one, contingent upon factors such as the desired material properties, efficiency of preparation, and cost considerations.In practical applications, the choice of method should align with specific requirements to yield BTO bulk ceramics possessing exceptional properties.

Ferroelectric Properties
BTO ceramics is widely regarded as a material of profound significance in the realm of ferroelectrics.The unique ferroelectric properties of BTO bulk ceramics play a pivotal role in shaping their diverse applications, underlining the importance of delving into a comprehensive exploration of these characteristics.The subsequent sections provide an in-depth analysis of the ferroelectric attributes exhibited by BTO ceramics, shedding light on their underlying mechanisms, material structure-property relationships, and their impact on various technological domains.
Ferroelectric Performances: he ferroelectric performances of BTO ceramics encompass several key attributes, including the strength of spontaneous polarization, dielectric constant, and the piezoelectric effect. [84]The strength of spontaneous polarization signifies the maximum achievable value of the material's intrinsic polarization, which can be modulated by an external electric field.The dielectric constant characterizes the material's responsiveness to an electric field, profoundly affecting its capacitive behavior.The piezoelectric effect denotes the generation of charge separation within the material in response to external mechanical forces, leading to the development of a potential difference.These ferroelectric properties collectively bestow BTO bulk ceramics with substantial potential applications in various domains, spanning electronic devices, sensors, capacitors, and a myriad of other technological fields.
Phase Transition: At ambient room temperature, BTO exists in a cubic phase, featuring the presence of spontaneous polarization.However, when the temperature descends below a critical point known as the Curie temperature, barium titanate undergoes a significant ferroelectric phase transition, transitioning from its initial cubic phase to a tetragonal phase. [85]This phase transition is characterized by a distinct alteration in spontaneous polarization, thereby conferring upon the material its remarkable ferroelectric properties.
Domain Structure: The ferroelectric characteristics exhibited by BTO ceramics are intimately intertwined with their internal ferroelectric domain structure.These ferroelectric domains represent microstructures that encompass localized areas characterized by distinct spontaneous polarization orientations.During the ferroelectric phase transition, the arrangement of these domains undergoes reconfiguration, leading to a profound alteration in the overall direction of the material's polarization.It is worth noting that the ferroelectric domain structure plays a pivotal role in shaping the electrical and thermal properties of BTO, thus exerting a significant influence on the material's behavior and performance.
Thermal Depoling: BTO ceramics, under elevated temperature conditions, may experience a noteworthy thermal depoling phenomenon whereby the material's spontaneous polarization gradually diminishes, effectively disappearing above a specific critical temperature.This thermal depoling leads to the loss of the material's ferroelectric characteristics, thereby constraining its utility in high-temperature environments.
To sum, the distinctive ferroelectric attributes of BTO bulk ceramics encompass aspects such as ferroelectric phase transitions, the complex ferroelectric domain structure, ferroelectric performance, and the notable thermal depoling phenomenon.A comprehensive exploration and understanding of these ferroelectric properties are instrumental in unveiling the intricate physical mechanisms governing barium titanate materials.Moreover, such insights provide invaluable guidance for the optimization and enhancement of their applications in electronic devices and sensor technologies.

Applications of BTO Bulk Ceramics
Owing to their remarkable ferroelectric performance, BTO block ceramics have found wide-ranging applications across various fields.The following section provides an overview of some of the key and prevalent applications of these ceramics, highlighting their invaluable contributions to technological advancement and scientific exploration.
Electronic Devices: BTO bulk ceramics have emerged as indispensable materials in the field of electronics, particularly as a preferred choice for the manufacture of high-performance ceramic capacitors and ferroelectric memories. [86]These ceramics are esteemed for their exceptional dielectric properties, boasting a high dielectric constant and commendable dielectric performance.This exceptional combination of properties makes BTO ceramics highly suitable for seamless integration into electronic circuits, playing a pivotal role in a wide array of applications that include energy storage, signal coupling, and signal filtering.
Piezoelectric Devices: BTO ceramics have emerged as a leading force in the realm of piezoelectric materials, underpinning the creation of a diverse array of cutting-edge piezoelectric devices. [87]Their remarkable piezoelectric performance offers a myriad of opportunities for technological innovation and advancement.These ceramics serve as the foundation for piezoelectric ceramic sensors, meticulously engineered to measure and detect an array of physical quantities, encompassing pressure, strain, and acceleration.Furthermore, the versatility of BTO ceramics extends to their utility as piezoelectric ceramic actuators, capable of propelling a wide range of mechanical and acoustic devices with unparalleled efficiency and accuracy.
Ultrasonic Facilities: The exceptional piezoelectric properties and remarkable high-temperature resistance of BTO bulk ceramics have positioned them as cornerstones in the domain of ultrasonic technology, catalyzing innovative applications in a multitude of industries. [88]BTO ceramics play a pivotal role in the creation of ultrasonic equipment, redefining the standards of efficiency and performance.Notably, these ceramics find their place in the development of ultrasonic cleaners, engineered to tackle the meticulous task of removing surface dirt and impurities.Beyond cleaning, BTO ceramics also serve as the bedrock for ultrasonic humidifiers, designed to enhance the humidity of the air.The exceptional high-temperature resistance of these ceramics ensures the reliability and longevity of such humidifiers.
Acoustic Devices: BTO ceramics are instrumental in the development of acoustic filters, meticulously engineered to regulate and fine-tune the transmission and frequency of sound waves. [89]ith their exceptional piezoelectric properties, BTO ceramics enable the creation of highly precise and responsive filters that find applications across diverse domains, from telecommunications to audio engineering.In addition to their role in filtering, BTO takes center stage in the construction of sonar sensors, heralding a new era of sound wave reception and emission technology.These sensors are designed to detect and transmit sound waves, a capability with far-reaching implications in fields such as marine navigation, underwater exploration, and defense.
High-Temperature Applications: The remarkable thermal stability and outstanding high-temperature performance of BTO bulk ceramics have positioned them as indispensable assets in environments characterized by extreme heat, paving the way for a spectrum of high-temperature applications. [75]Firstly, BTO plays a pivotal role in the development of high-temperature sensors, engineered to measure and monitor temperature and pressure in settings where conventional materials would falter.These sensors, underpinned by the exceptional heat-resistant properties of BTO ceramics, offer unprecedented accuracy and reliability, making them invaluable instruments in sectors such as aerospace, industrial manufacturing, and energy production, where precise monitoring under extreme conditions is essential.Furthermore, the versatility of BTO ceramics extends to the fabrication of hightemperature capacitors, which serve as vital components in energy storage systems for high-temperature circuits and electronic devices.
Overall, BTO bulk ceramics have proven to be remarkably versatile materials, finding extensive use in an array of scientific and technological domains.Their impressive ferroelectric, dielectric, and piezoelectric properties make them pivotal candidates for research and application in the realm of functional materials.Beyond their conventional role in electronics, BTO ceramics have made notable contributions to fields such as piezoelectrics, ultrasonics, acoustics, and high-temperature environments.As the boundaries of knowledge continue to expand, BTO bulk ceramics stand poised to evolve, pushing the frontiers of multidisciplinary applications and sparking fresh insights into the possibilities they hold.

Synthesis Techniques
Physical Vapor Deposition: Physical vapor deposition (PVD) has emerged as a pivotal technique for the deposition of BTO onto substrates, offering unprecedented control over film quality and characteristics.Common PVD methods, including evaporation, sputtering, and molecular beam epitaxy, [90][91][92][93][94] have played a transformative role in the production of BTO thin films, with diverse applications in electronic and optical devices, as well as sensors.The evaporation method involves the controlled heating of BTO film, causing it to evaporate and condense onto a substrate, which results in the formation of thin films characterized by their high purity, compactness, and crystal quality.Sputtering, on the other hand, employs ion or electron bombardment to sputter BTO material onto a substrate, yielding films with exceptional properties.Molecular beam epitaxy, an advanced PVD method, transports BTO material to the substrate using a high-energy molecular beam, ensuring precise film deposition and crystalline perfection.The ongoing exploration of PVD techniques promises to advance the production of BTO thin films, opening up new frontiers in electronics, optics, and sensing technologies.
Chemical Vapor Deposition: Chemical vapor deposition (CVD) stands as a powerful technique for the precise deposition of BTO material onto substrates, achieved through controlled chemical reactions.During a typical CVD process, the reaction source undergoes thermal decomposition or chemical reactions, ultimately resulting in the formation of a thin film on the substrate.This method's exceptional advantage lies in its ability to finely tune the composition and structure of the thin film.Utilizing sources such as metal-organic compounds and metal oxides, [95][96][97] CVD has proven to be a versatile method that plays a crucial role in the production of high-purity, uniform, and structurally controlled BTO thin films.This precision is instrumental in tailoring thin films to meet the stringent demands of various applications in electronic devices, optoelectronic devices, and catalysts.

Solution Method:
The solution method presents a straightforward and cost-effective approach to generate BTO thin films on substrates by dissolving BTO precursors in a solvent. [96,98,99]The beauty of this method lies in its adaptability, achieved through the precise adjustment of solution parameters such as composition, pH value, and temperature.These adjustments trigger the dissolution and subsequent reaction of barium titanate precursors, culminating in the formation of a thin film on the substrate.The solution method offers unique advantages, including control over film thickness and properties, all while maintaining a cost-efficient and uncomplicated process.Therefore, the solution method demonstrates remarkable flexibility and plasticity, allowing for the customization of thin films to meet specific application requirements.
[93]100] By directing a laser beam onto the BTO target, local heating is induced, rapidly melting the material, and then allowing it to solidify into a thin film on the substrate.This method is distinguished by its remarkable degree of controllability and selectivity, enabling precise manipulation of the thin film's morphology and crystal structure.Notably, PLD stands as a cornerstone in the production of epitaxial BTO thin films, with applications spanning optical devices, microelectronic devices, and fiber-optic communications.
Other Methods: Besides the well-established synthesis techniques discussed earlier, there exist lesser-utilized methods for the preparation of BTO thin films, including magnetron sputtering, electrochemical deposition, and self-assembly. [76,80,82]These methods share the unique ability to finely control the composition, structure, and morphology of thin films, offering intriguing opportunities for specialized research fields or under unique demands.Magnetron sputtering, for instance, lends itself to the production of magnetic thin films, presenting exciting prospects for magnetic technologies. [101,102]Electrochemical deposition, on the other hand, is well-suited for the creation of nanostructures, providing a pathway to delve into the world of nanoscale materials and devices.Self-assembly methods allow for the orderly arrangement of thin films, unlocking possibilities for precise, well-structured surface coatings.Future research endeavors can further explore the optimization and expansion of these techniques, unveiling novel avenues for innovation and uncovering fresh insights into the versatile capabilities of BTO thin films.

Ferroelectric Properties of BTO Thin Films
Dielectric Constant: BTO thin films, with their elevated dielectric constant ranging from tens to thousands, possess a remarkable capacity to respond strongly to electric fields.This characteristic underscores the pivotal role of BTO in various capacitive devices, including capacitors and dielectric layers. [103]Their high dielectric constant is a key feature that enables these films to store and release electric charge efficiently, making them indispensable components in a wide array of electronic applications.As the demand for miniaturization and high-performance electronics continues to grow, Barium Titanate thin films remain at the forefront of innovation, facilitating the development of advanced capacitive devices that drive progress in various technological domains.
Ferroelectric Domain: BTO thin films are notable for their intriguing domain structure variations, which can be attributed to the partial relaxation of epitaxial coherency strains.These domain structures are a result of the unique crystalline arrangement in the film and play a significant role in its properties.Under the influence of this field, the ferroelectric domains within the film undergo a dynamic reconfiguration.They realign themselves in response to the electric field, ultimately forming a polarization that is opposite to the direction of the external field. [104]This behavior is a fundamental characteristic of ferroelectric materials and is a central feature in the application of BTO thin films in various electronic and electro-optic devices.The ability to manipulate domain structures through the application of an electric field opens up exciting possibilities for the design of advanced technologies, including sensors, memory devices, and voltage-tunable optical components.
Saturation Polarization: The concept of saturation polarization (P s ) in barium titanate thin films is a fundamental characteristic that plays a critical role in various electronic and electrooptic applications. [101,105]As the external electric field strength increases, the polarization of the film intensifies until it reaches a point of saturation.This saturation represents the maximum polarization that can be reversed under the influence of an external field.Understanding and carefully managing saturation polarization are essential for optimizing the performance of BTO thin films in different applications, ensuring that they operate within the desired range of polarization and electrical response.Engineers and researchers rely on this understanding to design and fine-tune electronic and electro-optic devices, making the most of BTO thin films' unique properties to achieve specific performance goals.
Piezoelectric Effect: BTO thin films are indeed notable for their significant piezoelectric effect, a phenomenon in which they respond to mechanical stress or pressure by generating charge separation and polarization.This response results in the creation of a potential difference, making BTO thin films indispensable in applications such as piezoelectric sensors and piezoelectric actuators. [106]The piezoelectric effect in these thin films enables them to convert mechanical energy into electrical signals or vice versa.Piezoelectric sensors utilize this property to detect various forms of mechanical input, including pressure, vibration, and deformation, and translate them into electrical signals for monitoring or control purposes.On the other hand, piezoelectric actuators leverage the ability of BTO thin films to generate mechanical motion or force in response to an applied electrical signal.
Domain Wall Motion: The ability of ferroelectric domains in BTO thin films to undergo domain wall motion under the influence of an external electric field holds great promise for memory device technology.This domain wall motion is a dynamic process that can be harnessed for the writing and erasing of information in memory devices, presenting exciting opportunities for the development of novel memory technologies. [104]By manipulating the arrangement of ferroelectric domains within the thin film through the controlled application of electric fields, it becomes possible to write and erase data.Exploring and understanding the behavior of ferroelectric domain walls in BTO thin films is crucial for advancing the field of memory technology.Researchers are continually investigating how to harness these properties to develop memory devices that are more efficient, reliable, and versatile, with the potential to revolutionize data storage and processing capabilities.

Applications of BTO Thin Film
Non-Volatile Memory: The ferroelectric properties of barium titanate thin films have positioned them as a crucial component in non-volatile memory, particularly in the case of ferroelectric random access memory (FeRAM).FeRAM leverages the ferroelectric nature of BTO thin films for information storage and retrieval, offering several distinct advantages, including high-speed read-write operations, low power consumption, and a long operational lifespan. [107]FeRAM's ability to store data in ferroelectric capacitors within BTO films allows for fast data access and retention even without a constant power supply.The incorporation of BTO films in FeRAM technology has significantly contributed to the development of non-volatile memory solutions that find utility in various electronic devices and systems.Their unique properties make them well-suited for high-performance memory applications, ensuring that data can be written and read swiftly while maintaining low power demands and extended operational lifespans.
Sensors and Actuators: The piezoelectric properties of BTO thin films have found extensive applications in the fields of sensing and control.These thin films are integral components in piezoelectric sensors, piezoelectric actuators, and piezoelectric ceramic transducers, where their sensitivity to mechanical stress and pressure enables precise and reliable functionality. [108,109]In piezoelectric sensors, BTO thin films are employed to convert mechanical input, such as pressure, vibration, or strain, into electrical signals.This makes them essential for a wide range of applications, from industrial monitoring to medical devices, where accurate and responsive sensing is critical.Additionally, piezoelectric actuators utilize these films to generate mechanical motion or force in response to electrical signals.This technology is invaluable in applications where precise control and actuation are required, such as in positioning systems and micro-robotics.The versatility of BTO thin films in these piezoelectric applications has greatly advanced the fields of sensing, actuation, and transduction, contributing to the development of more responsive, accurate, and efficient devices across multiple industries.
Optoelectronic Devices: The ferroelectric properties of BTO thin films play a significant role in the realm of optoelectronic devices.These films exhibit the electro-optic and photovoltaic effects, which enables their use in various optoelectronic applications, including mach-zehnder modulators (MZM), optoelectronic memories, and optical switches.In optoelectronic modulators, BTO films can be employed to control the transmission of light by manipulating their ferroelectric properties.This allows for the modulation of optical signals, which is vital in optical communication systems and signal processing.Therefore, the incorporation of barium titanate thin films in these optoelectronic devices underscores their versatility and utility in modern  7. Ferroelectric hysteresis loops for the rare-earth doped BTO ceramics. [114]Copyright 2016 Royal Society of Chemistry.
technology, contributing to the advancement of optical communication, data processing, and optoelectronic systems.
Micro and Nano Fabrication in Integrated Circuits: BTO thin films serve as vital components in the realm of micro-nano fabrication and integrated circuits.They find applications in a range of essential electronic components, including capacitors, inductors, and h-k dielectric layers. [110,111]These components are instrumental in the creation of functional micro-nano structures and integrated circuits, driving progress in the world of electronics and miniaturization.Notably, high-k dielectric layers, often employing BTO thin films, provide insulation and capacitance, enhancing the performance of transistors and other semiconductor devices within integrated circuits.The use of BTO thin films in these essential components is paramount for achieving compact, high-performance electronic systems, contributing to the development of cutting-edge micro-nano structures and integrated circuits in various technological applications.
Indeed, the ferroelectric properties of BTO thin films present a vast array of opportunities for exploration and innovation, extending beyond their existing application fields.These versatile thin films hold great potential in diverse areas and applications, including energy storage, electronic devices, sensor technology, and flexible electronics. [112]Importantly, sensor technology stands to benefit from the unique properties of barium titanate thin films, enabling the creation of more sensitive and accurate sensors for various applications, from environmental monitoring to healthcare.Future research efforts hold the promise of unlocking the full potential of BTO thin films, leading to the optimization and expansion of their ferroelectric properties.This will not only foster their significant roles in the applications mentioned but also open doors to new and exciting possibilities in the world of materials science and technology.

A-Site Doped BTO
Barium titanate adopts the perovskite ABO 3 crystal structure, where the A-site is occupied by Ba 2+ at the corners, the B-site by Ti 4+ at the center, and the O 2− ions at the face center.Notably, the qualifiers "large" and "small" here are in reference to the ionic radii of Ba 2+ (1.61 Å) and Ti 4+ (0.605 Å), respectively.Common elements such as La 3+ , Al 3+ , Bi 3+ , Ce 3+ , Pr 3+ , Nd 3+ , Sm 3+ , among others, find a home at the A-sites, functioning as donor dopants. [113]La, in particular, stands out for its exceptional stability when occupying the A-sites.Such A-site doping represents the strategic introduction of various elements into the A-sites of the barium titanate crystal structure.This deliberate alteration imparts modifications in lattice parameters, crystal structure, and charge distribution within the material, thereby exerting profound influence over the electrical properties of barium titanate.These modifications extend to key attributes such as the dielectric constant, piezoelectric behavior, and thermal stability.
Figure 7 showcases room-temperature ferroelectric hysteresis loops for a range of rare-earth element-doped BTO ceramics. [114]ntriguingly, A-site doped barium titanate derivatives unveil a wealth of physical and electrical properties.The judicious selection of A-site dopants and their concentrations empowers researchers to exercise precise control over the material's performance and characteristics.This doping strategy serves as an effective avenue for tailoring the features of barium titanate materials, broadening their utility in electronics, sensors, and energy-related applications.Copyright 2016 Royal Society of Chemistry.
Furthermore, the selection of specific doping elements and their concentrations necessitates meticulous consideration to strike a harmonious balance between performance and stability requisites.

B-Site Doped BTO
B-site doping involves the introduction of various elements into the B-sites of the barium titanate crystal structure.This perturbation has the capacity to modulate the crystal lattice parameters, alter the crystal structure, and redistribute consequently exerting a profound influence on the electrical properties and ferroelectric characteristics of barium titanate materials.In the context of Ti 4+ substitution, the most frequently employed cations include valence-stable Zr 4+ , Hf 4+ , and Sn 4+ , due to their closely matching ionic radii and equivalent valence states with Ti 4+ .The phase diagrams for (Ba 0.94 Ca 0.06 )(Ti 1-x M x )O 3 (abbreviated as BCTM, M = Sn, Hf, Zr) ceramics are depicted in Figure 8a-c.Notably, as the Sn, Hf, or Zr content increases, both T R-O and T O-T transition to higher temperatures, while T C shifts to a lower temperature, with T R-O displaying a swifter movement compared to T O-T . [115]To elucidate the comparative effects of Sn, Hf, and Zr on these phase transition temperatures, the compositional dependence of T R-O , T O-T , and T C is illustrated in Figure 8d-f.These observations suggest that the incorporation of Sn leads to a more pronounced reduction in T C compared to Hf or Zr-doped BT ceramics, and that the rate of T R-O and T O-T adjustments is relatively slower with Sn addition in contrast to Hf or Zr. [115]vidently, B-site doping has the capacity to modify the polarization behavior and Curie temperature of barium titanate crystals, thereby exerting a discernible impact on the material's ferroelectric and dielectric properties.Moreover, a range of aliovalent cations, including but not limited to Zn 2+ , Cu 2+ , Mg 2+ , Mn 2+/3+ , Fe 2+/3+ , Ni 3+ , Cr 3+ , Co 3+ , Sc 3+ , Ga 3+ , In 3+ , Y 3+ , Ho 3+ , Dy 3+ , Yb 3+ , Nb 5+ , Sb 5+ , and W 6+ , have been identified as capable of incorporating into the Ti 4+ site in barium titanate. [101,113,116,117]his diverse group of dopants further extends the potential for tailoring the properties of BT materials.B-site doping not only impacts electrical characteristics but also demonstrates a noteworthy effect on thermal stability and phase transition behavior, thereby enhancing the overall stability and performance of barium titanate in high-temperature environments. [118]However, it is imperative to acknowledge the challenges and constraints associated with B-site doping.Precise control over the choice of dopant elements, doping concentration, and doping location is paramount to achieving the desired material performance and stability.

Multi-Doped BTO
Multi-doping strategies can be realized through various approaches, including co-doping at both A-site and B-site, sequential doping at these locations, and more.Barium titanate derivatives subjected to multi-doping exhibit a diverse range of electrical properties and hold significant application potential.By introducing multiple elements, it becomes feasible to finely tune essential performance parameters of barium titanate materials, such as lattice parameters, dielectric constant, and piezoelectric coefficient.Kim et al. successfully enhanced ceramic properties through co-doping with BTO, as illustrated in Figure 9. [119] Meanwhile, Jinghui Gao et al. have demonstrated high-performance system. [119]Copyright 2020 Elsevier.
systems utilizing BaTiO 3 , such as Ba(Zr,Ti)O 3 -(Ba,Ca)TiO 3 (BZT-BCT), Ba(Sn,Ti)O 3 -(Ba,Ca)TiO 3 (BST-BCT), and Ba(Hf,Ti)O 3 -(Ba,Ca)TiO 3 (BHT-BCT).Notably, these systems exhibit analogous phase diagrams. [120]These diagrams feature a PZT-like phase boundary characterized by a morphotropic phase boundary (MPB). [20]This boundary originates from a point in the phase diagrams where cubic (C), tetragonal (T), and rhombohedral (R) phases coexist.Notably, at this MPB, the d33 value can reach a range of 500-620 pC/N, comparable to that of soft PZT. [120]Additionally, it is worth noting that the permittivity peaks at the MPB, with a value of  r approximately significantly surpassing the value for pure BTO ceramic, which is around 10,000.
However, multi-doping is not without its challenges and complexities.The selection of dopant elements, their concentrations, and precise incorporation sites must be meticulously controlled to attain the desired material performance and stability. [121]Multidoping can introduce structural complexity and inhomogeneity, impacting the electrical properties and phase transition behavior of the material.Hence, an in-depth exploration of the effects and underlying mechanisms of multi-doping is essential, along with the development of optimized doping techniques to enhance material performance and stability. [122,123]Research on multi-doped barium titanate derivatives opens novel avenues for expanding the functionality of barium titanate materials.Through welldesigned and optimized doping strategies, it becomes possible to finely regulate and enhance the performance of these materials.Future research endeavors will delve deeper into the mechanisms and applications of multi-doping, fostering innovation and development of barium titanate materials in fields such as electronics, sensing, energy, and beyond.

Sensor Applications
In recent years, ferroelectric materials have garnered significant attention as critical components in the field of sensor technology.The dawn of the information age has ushered in a fundamental shift in our daily lives, where the acquisition and exchange of environmental, medical, and communication data are increasingly reliant on sensors.To meet the evolving demands of this digital era, there is a pressing need for the development of sensors that are not only more sensitive but also secure and energyefficient, thereby enhancing the comfort and convenience of our daily lives.The journey towards enhancing sensor performance begins at the core: the basic materials.In this context, barium titanate properties have emerged as a promising avenue for research, with the potential to underpin a diverse array of sensor applications.This encompassing exploration extends to various sensor types, including biomedical, mechanical, optical, and thermal sensors.The versatile properties of barium titanate materials hold the key to advancing the sensor landscape, enabling us to create the devices necessary for modern life.

Biomedical Sensor
In an era marked by remarkable technological advancements in smart textiles, micro-biosensing devices, wireless communication, and microelectronics, the intersection of industry and scientific inquiry is witnessing an unprecedented surge in interest towards biosensor systems tailored for health monitoring.The assessment of daily health hinges on the monitoring of critical physiological parameters such as body temperature, blood pressure, heart rate, and human movement.These variables serve as indispensable indicators of our overall well-being.To address these diverse needs, a plethora of medical sensors has been conceived, each tailored to record and collect crucial human health information.The pressing global impact of Corona Virus Disease 2019 (COVID-19) has heightened individuals' awareness of their own health status, further catalyzing the development and application of these innovative technologies.Wearable sensors, the focal point of this revolution, offer humans the ability to monitor their health data in real-time, effectively issuing warnings when needed.
These versatile devices are pivotal in healthcare's transition towards personalized and proactive management.These sensors can be categorized into two principal types: physical sensors and chemical sensors, each attuned to distinct physiological signals emitted by the human body.Physical sensors encompass temperature sensors, electrical sensors, and motion sensors, capturing critical data related to body temperature, electrical signals generated by physiological activities, and bodily movements.Precise positioning, sizing, and sensor numbers are tailored to optimize the detection of each specific physiological signal, ensuring utmost accuracy.On the other hand, chemical sensors specialize in detecting an array of biochemical indicators present in various bodily fluids.These sensors play a pivotal role in monitoring health conditions and diagnosing diseases.In addition, the development of these sensors must align with rigorous medical standards, necessitating adherence to a set of key principles.They must minimize electronic device radiation to ensure signal stability, maintain data privacy and security, adhere to energy efficiency and environmental protection standards, and be constructed from materials that are biocompatible and harmless to the human body.

Respiration Rate Sensor
The measurement of respiration rate stands as a crucial criterion in assessing an individual's health status, with far-reaching implications for both infants and adults.The ability to gauge respiratory parameters has proven invaluable, particularly in the early The output of device used as a highly sensitive wearable sensor for detecting the breathing motion of a volunteer. [128]Copyright 2022 American Chemical Society.detection of respiratory diseases in infants and the diagnosis of conditions like obstructive sleep apnea in adults.In infants, the measurement of respiration rate is pivotal, as it plays a pivotal role in the early detection of respiratory diseases.Respiratory infections are a major source of morbidity and mortality in the neonatal period.Disruptions in normal breathing can significantly impact the physical and neurological health of newborns, making the timely and accurate measurement of respiration rate a critical tool in pediatric care.For adults, the measurement of respiration rate is equally important, given the prevalence of obstructive sleep apnea-a condition characterized by partial or complete upper respiratory tract obstruction, leading to disturbances in breathing patterns during sleep.This condition not only impairs sleep quality but also increases the risk of long-term neurological dysfunction.Accurate monitoring of respiration rate is integral to early diagnosis and intervention in such cases.
To facilitate the precise measurement of respiration rate, various sensor technologies have emerged.These sensors, including piezoelectric, piezoresistive, gyroscopic, and optical mechanisms, [124] are designed to detect the periodic volume changes that occur in the chest and abdomen during inhalation and exhalation.Recently, strain sensors have gained prominence due to their flexibility, compactness, durability, and wearresistant properties.Chu and colleagues harnessed the power of strain sensors to monitor local tension changes in the chest and abdomen during respiration. [125]By utilizing wireless bluetooth connectivity, these sensors could effectively measure both respiration rate and respiratory volume, aiding in the early prediction of disease in infants and adults.Another innovation features a body motion sensor made from piezoelectric material, as thin as a sheet of paper or bedding.This sensor enables the prediction of various respiratory diseases by monitoring individuals' breathing rates during sleep. [126]Ian K. Small et al. created a BTO-based breath sensor that is highly sensitive to weak breathing and water vapor. [127]Zhou et al. realized self-powered sensing with applications in respiration by dispersing lead-free BaTiO 3 piezoelectric nanoparticles within an elastic polydimethylsiloxane, as depicted in Figure 10. [128]The composite material has a thickness of 1 mm and covers an area of 1 × 1.5 cm 2 .Hung-Yu Yeh et al. have created a novel wearable breath monitoring device created by micromachined thin-film technology. [129]The device has precise respiration rate sensing, breathing air temperature and heating functions, all on a barium titanate film with different layout designs.These promising developments above not only enhance our understanding of respiratory physiology but also empower medical professionals and individuals alike in their quest for better health.As these sensors become more refined and accessible, the early prediction of respiratory diseases in infants and adults takes a significant leap forward, promising improved healthcare outcomes and enhanced quality of life.

Heart Rate Monitoring
Cardiovascular diseases, responsible for a significant annual toll of human lives, underscore the critical importance of heart rate monitoring for timely disease diagnosis.Notably, medical sensors offer precise measurements of cardiovascular parameters, with pulse pressure and heart rate standing as two fundamental indicators.These parameters can be effectively tracked (e) Wearable performance test for the packaged sensors. [130]Copyright 2022 Elsevier.
through the utilization of strain sensors, pressure sensors, and accelerometers, strategically positioned at vital locations such as the neck, chest, and wrist.The authors present a health sensor utilizing a polyvinylidene difluoride (PVDF)/BTO composite fiber within a polydimethylsiloxane (PDMS) microcylinder structure, illustrated in Figure 11. [130]The sensor is fabricated through the flexible polyimide (PI) manufacturing process.Notably, recent advancements in cardiovascular health monitoring have led to the development of an innovative pressure sensor. [131]his flexible, multi-layered elastomer-based self-powered sensor showcases exceptional sensitivity and an extensive pressure range.Rigorously tested over multiple cycles, the sensor has demonstrated outstanding stability and performance, promising a robust tool for cardiovascular parameter detection.Additionally, Andreas Petritz and colleagues introduce ferroelectric wireless electronic health patches, designed to monitor pulse rate and measure blood pressure with remarkable precision. [66]These ultrathin patches, measuring just 1 micron and constructed from parylene, blend seamlessly with human skin, rendering them virtually imperceptible.This technological innovation, holds the potential to redefine early cardiovascular disease monitoring.Therefore, these advancements in cardiovascular health monitoring stand as a testament to the relentless pursuit of precision and convenience in the field, offering hope for early disease detection and improved cardiovascular care.

Temperature Sensor
The monitoring of body temperature is of paramount significance in medicine, as elevated temperature often serves as the earliest indicator of various diseases.Beyond merely detecting fever, continuous temperature monitoring empowers healthcare providers with comprehensive data for more efficient treatment.Traditional approaches employ adhesive temperature sensors or infrared digital cameras, but they fall short in delivering cost-effective and continuous temperature mapping.A recent study has led to the development of a groundbreaking wearable temperature sensor. [132]This device leverages single-walled carbon nanotubes, electrically interconnected via embedded interconnections, boasting a rapid response time of 1.8 seconds.The sensor can be applied to electronic skin, allowing accurate and real-time temperature data collection.The sensing material is composed of polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) and barium titanate nanoparticles. [133]Commonly employed thermistor sensors have limitations related to the distance between the sensor and the human body.To address this challenge, flexible polymer temperature sensors have emerged.These sensors can be fabricated on thin, soft polymer substrates using versatile printing techniques like inkjet, screen printing, and gravure printing. [134]Their biocompatibility and adhesion properties make them ideally suited for direct application to human skin, facilitating continuous and unobtrusive monitoring of body temperature.These innovations represent a significant step forward in the field of medical monitoring, offering the promise of early disease detection and precise temperature tracking, thereby enhancing the quality of patient care.

Mechanical Sensor
Mechanical sensors, designed to convert external mechanical stimuli, including pressure, strain, shear, and vibration, into electrical signals, constitute a pivotal component of sensing technology. [135]Comprising elastic components, mechanical structures, and signal processing, they find extensive utility across manufacturing, commercial, and military sectors.When subjected to pressure from the measured medium, the force is transmitted through a mechanical structure to deform an elastic element slightly, generating an electrical signal.By precisely measuring the extent of deformation of the elastic component, pressure information from the medium can be accurately derived.walking and (d) running, respectively. [136]Copyright 2022 Elsevier.
Over the years, mechanical sensors have evolved from highly sensitive piezoresistive and capacitive sensors to energy-efficient piezoelectric and triboelectric sensors.These developments span various sensing mechanisms, each catering to unique perceptual characteristics such as sensitivity, physical parameters, linearity, and working range.This versatility allows mechanical sensors to serve diverse applications in everyday life.

Pressure Sensors
Stress, a ubiquitous mechanical stimulus in both human society and nature, necessitates a diverse range of pressure sensors tailored for applications spanning a wide spectrum of pressure values.These sensors are classified based on their ability to detect micro-pressure (1 Pa), low-pressure (1 kPa), medium-pressure (10 kPa), or high-pressure (>100 kPa).The significance of detecting extremely small pressure states, ranging from 1 Pa to 1 kPa, cannot be overstated, as they are integral to discerning weak stress interactions and the minuscule pressures exerted by numerous tiny objects.
In Figure 12, the researchers enhanced the characteristics of the BaTiO 3 composite, resulting in the development of a pressure sensor tailored for human activity detection. [136]The depicted electronic skin exhibits a distinctive fingerprint-like pattern along with intricately interlocked microstructures. [137]This innovative development demonstrates the sensor's capacity to simultaneously detect pressure and temperature fluctuations, employing distinctive signal generation patterns, including sustained pressure (piezoresistive), slight and time-based pressure (piezoelectric), and temperature (thermoelectric).As evidence of its vast po-tential, this electronic skin has exhibited the simultaneous monitoring of pulse pressure, skin temperature, high-frequency dynamic sound waves, and diverse surface textures with varying topological patterns, roughness, and hardness. [138]The burgeoning interest in health and self-monitoring has further propelled the utilization of high-performance pressure sensors in electronic skin and health monitoring and diagnostic systems. [139,140]hese breakthroughs signify a significant step towards versatile and precision-driven applications, promising to transform electronic skin and healthcare monitoring systems.

Strain Sensor
Strain sensors, revered for their ability to measure deformation in various objects, have emerged as indispensable tools in diverse fields.These sensors are instrumental in monitoring alterations in distinct positions during human movement, categorically divided into two key types: Monitoring small deformations in skin and Tracking large skin deformations.The former is associated with observing subtle deformations in regions like the chest, face, and neck, intrinsically linked to activities such as breathing, speech, emotional expressions, and swallowing.In contrast, the latter focuses on the comprehensive analysis of substantial skin deformations, spanning the bending of fingers, wrists, arms, legs, and the spinal column.Sensors aimed at detecting minor deformations, often referred to as pressure sensors, present a unique challenge due to the intricacies of distinguishing electrical signal changes caused by either pressure or strain stimulation.These elastic strain sensors find their place on human skin or smart clothing, boasting a wide array of applications.Various applications to show the highly sensitive, linear and large-range pressure sensing by multilayer e-skins. [149]Copyright 2018 American Chemical Society.
148] Figure 13 introduces an innovative flexible ferroelectric sensor. [149]Crafted from ferroelectric composite materials and featuring a multi-layer interlocking microsphere array, this sensor stands out for its exceptional sensitivity and extensive linear response to pressure.It offers a remarkable pressure detection range spanning from 0.0013 kPa to 353 kPa, characterized by a rapid response time of approximately 20 milliseconds.The sensor exhibits exceptional reliability, enduring over 5000 cycles of detection.Its extensive measurement range enables the monitoring of various strain stimuli, encompassing wrist pulse pressure, respiratory patterns, foot pressure, gas flow, and acoustic sound.One of the compelling advantages of these sensors lies in their large dynamic range linear sensing capabilities, which negate the necessity for complex signal processing.This characteristic meets the growing demand for device miniaturization and reduced power consumption, promising a future characterized by enhanced performance and convenience.

Multimodal Sensors
Multimodal sensors, a focal point of contemporary research, have opened up new horizons in sensing technology.These sensors possess the unique capability to detect and distinguish a multitude of stimuli concurrently, including temperature, humidity, stress, pressure, and more.Unlike their single-detection counterparts, multimodal mechanical sensors are engineered to capture an extensive range of mechanical stimuli, including shear, pressure, vibration, and strain, making them exceptionally effective in navigating complex environments.Figure 14 illustrates the detection of diverse human motion patterns and the measurement of flexural strain using a flexible multifunctional Polyacrylamide (PAM)/BTO composite film-based sensor. [150]his sensor capitalizes on the integration of piezoresistive, piezoelectric, and triboelectric phenomena, making it a versatile platform for biomechanical sensing.Optimized TENG (Tribo-electric nanogenerator) and piezoresistive sensors are harnessed as multimodal sensors, enabling the precise detection of human body movements, pressure variations, and curvature changes with remarkable sensitivity.However, it is worthy of consideration the sensitivity and trade-off in these sensing devices.While the versatility of multimodal sensors is unparalleled, it is crucial to acknowledge that their sensitivity may be relatively reduced compared to single-detection sensors.As an illustrative example, consider tactile sensors within electronic skin systems, which adeptly perceive shear stress and pressure in distinct directions.These sensors empower robots with the capacity for flexible object manipulation and intricate object recognition, enhancing their functionality in diverse applications.
To attain simultaneous detection of various mechanical signals, several approaches have emerged.Constructing sensor arrays with multiple sensor units, implementing integrated layouts, and pioneering novel materials housing diverse sensing mechanisms are all avenues explored by researchers.Such endeavors enable the concurrent capture of an array of mechanical data.Drawing on the previous example of the pressure sensor, [137] it is notable that ferroelectric materials offer an enticing frontier.They encompass diverse sensing mechanisms, including piezoelectric and pyroelectric responses.In this context, the same sensor can effectively detect both pressure and temperature, demonstrating the adaptability and utility of multimodal sensors.

Optical Sensor
Ferroelectric materials with semiconductor characteristics, often referred to as photoferroelectric materials, [151] have captivated researchers for their unique sensitivity to light.In semiconductors, incident photons generate surplus free carriers, inducing changes in the electronic state.However, traditional photoelectric sensors typically depend on external power sources for optical detection, a reliance that often introduces high dark current and minimal switching ratios, curtailing their practical utility.In the contemporary landscape, self-powered photodetectors have  [150] Copyright 2022 American Chemical Society.
emerged as a beacon of innovation.They offer enduring selfpowered detection, minimal energy consumption, and seamless integration into compact functional devices. [152]Particularly, sensors based on ferroelectric materials predominantly operate on two principles: the ferroelectric photovoltaic effect and the pyroelectric effect.The former harnesses high-energy photons to generate photogenerated electron-hole pairs, while the latter is designed for the detection of infrared radiation, often employed in non-contact temperature sensing.
A recent study has introduced an ultra-thin BaTiO 3 nanostructure engineered to detect carbon oxides through light signals. [153]his structure addresses the challenge of achieving lower detection limits.Employing density functional theory, the authors simulated the optical properties and electrical sensitivity of distinct adsorption sites on the surface of ultra-thin BTO nanostructures.Supercell model research revealed a direct correlation between molecular concentration and light signal response, unveiling the potential for manufacturing user-friendly and highly sensitive carbon oxide sensors.This groundbreaking exploration highlights the remarkable capabilities of photoferroelectric materials, ushering in new possibilities in sensor technology with broad-reaching applications, from environmental monitoring to gas sensing.
Among the wide array of optical sensors based on BTO, ultraviolet photodetectors have claimed the spotlight.Nevertheless, the pursuit of effectively enhancing the photocurrent of ferroelectric materials remains a paramount challenge in the development of UV photodetectors.Zhang's research team has proposed a pioneering approach by combining BTO thin films with a GaN layer, [154] forming a heterojunction structure, as depicted in Figure 15a,b.Through meticulous testing, the researchers discovered that cooling and pre-polarization treatments lead to a substantial increase in photocurrent, accompanied by a reduction in the response time to a mere 2.35 seconds.This heterostructure opens new vistas for the realization of ultrafast UV sensors at room temperature, showcasing remarkable  [154] Copyright 2021 American Chemical Society.(c) Schematic diagram of a photodetector with a two-dimensional planar structure when illuminated.(d) Physical drawings of photodetector. [155]Copyright 2018 Elsevier.(e) Schematic diagram of flexible photodetector.(f) A physical picture of the flexible photodetector when it is bent. [156]Copyright 2020 Elsevier.potential.Figure 15c,d introduces a BTO-based photodetector featuring a two-dimensional planar structure with contact electrode configuration. [155]This novel configuration significantly enhances stable photocurrent generation.In a comparative analysis, this structure demonstrated its prowess by achieving larger photocurrents when exposed to 365 nm ultraviolet light.Taking innovation to the next level, a pioneering sensing system that amalgamates the thermoelectric and photovoltaic effects is illustrated in Figure 15e,f.This system records variations in the sensor's electrical signals, enabling the simultaneous detection and identification of changes in light and temperature. [156]hese transformative developments underscore the growing capabilities of perovskite and semiconductor materials in the realm of optical sensors, promising to redefine the landscape of sensing technology.

Thermal Sensor
Thermal sensors, designed to translate changes in temperature into distinct electrical signals, stand as a cornerstone of sensor technology.Among the versatile array of materials, ferroelectric substances have notably harnessed pyroelectric properties to detect temperature-induced shifts in electrical signals.Temperature sensors are broadly categorized into two classes based on their mode of measurement: contact sensors and non-contact sensors.Within the sphere of non-contact sensors, the infrared sensor reigns supreme as the most prevalent and accessible choice.Copyright 2022 Elsevier.

Contact Temperature Sensor
Contact temperature sensors have played a pivotal role in temperature measurement across various sectors, including industry, agriculture, and commerce.They rely on the establishment of direct thermal contact between the sensor and the measured object, facilitating conduction or convection to reach thermal equilibrium.This fundamental principle ensures a higher level of measurement accuracy compared to their non-contact counterparts, making them the preferred choice within a defined temperature range.Moreover, contact temperature sensors have the unique capacity to probe the temperature distribution within an object.The expanding application of cryogenic technology, from national defense and space endeavors to metallurgy, electronics, food, medicine, and petrochemical industries, necessitates temperature sensors with small sensing components, exceptional accuracy, and unwavering reproducibility and stability.Presently, contact temperature sensors encompass a range of technologies, with thermal resistance temperature sensors, thermocouple temperature sensors, and thermistor temperature sensors among the most prominent.
In the medical field, the precise and continuous monitoring of local body temperature holds a paramount role in unraveling the thermal dynamics governing homeostasis.Pioneering research by Suman Mandal et al introduces a substrate based on BTO nanocrystals, [157] designed as a double-layer medium, enabling seamless integration into wearable electronic skin applications.This breakthrough technology facilitates the accurate and continuous measurement of temperatures within the range of 20 to 45 °C.These sensors exhibit low power consumption, rapid response times, and robust functionality in complex environments, including those with varying pH levels or aqueous conditions.Figure 16 exhibits a self-powered temperature sensor grounded in the principle that fluctuations in the polarization of ferroelectric BTO materials induce shifts in surface free electron density, leading to augmented pyroelectric currents. [158]The device features Ag electrodes on both sides, creating a three-layer super-imposed structure: Ag/BTO/Ag, with electrode thickness measuring 0.65 mm.This pioneering sensor demonstrates that the output current is directly proportional to temperature within the Lorentz relation.Simultaneously, it accurately detects temperature changes, elevating energy conversion efficiency.This advancement bodes well for the future development and production of self-powered temperature sensors, poised to revolutionize various fields, including pyrolytic appliances.The convergence of precision temperature sensing in healthcare and self-powered sensor technology underscores the transformative potential of these innovations in healthcare and beyond.

Infrared Sensor
In the realm of infrared detection, scientific advancements over nearly two centuries have seen the development of various heat-sensitive detectors, driven by the foundational theory of infrared radiation.In the current landscape, as safety awareness continues to improve, the utilization of pyroelectric infrared sensors in safety detection applications is on a gradual ascent.The uniqueness of individuals' exercise habits, body types, and other distinguishing characteristics results in distinct infrared signals during physical activity.Leveraging these individualistic signals, researchers are harnessing pyroelectric infrared sensors for the dual purposes of detection and positioning.Sujay Narayana and colleagues have engineered a pyroelectric infrared sensor array tower, which serves as a platform for collecting and analyzing analog signals produced by pyroelectric infrared sensors. [159]heir research revolves around establishing the correlation between these signals and various attributes, such as distance, speed, and size of moving objects.In a similar vein, Jaeseok Yun et al have developed a pyroelectric infrared data acquisition unit featuring two pairs of sensor elements arranged orthogonally. [160]his configuration allows for the detection of human motion in eight evenly distributed directions.Their methodology, bolstered by case-based learning and support vector machine (SVM)  [161] Copyright 2023 John Wiley and Sons.
algorithms, achieves an impressive recognition accuracy of over 98% in motion direction on the original dataset.Even with a reduced feature set, they still achieve recognition accuracy in the range of 89% to 95%.
These pioneering efforts exemplify the growing potential of pyroelectric infrared sensors in safety detection, offering high precision and adaptability in identifying and tracking moving objects based on their unique thermal signatures.The inherent pyroelectric properties of ferroelectric materials have long been harnessed in the creation of thermal infrared detectors, finding diverse applications in safety systems, non-contact temperature measurements, and animal presence sensors.Capitalizing on the small heat capacity of ferroelectric films, these detectors surpass conventional pyroelectric devices.Key to their performance are the fundamental pyroelectric properties, including detection limits, thermal resolution, and responsiveness.Researchers have diligently explored techniques to enhance these properties, such as compositional gradient structures, multilayer configurations, nano-pore integration, and heterogeneous structures.Figure 17 presents an innovative lead-free pyroelectric infrared (PIR) detector, leveraging (Bi 1/2 Na 1/2 TiO 3 )-BaTiO 3 (BNT-BT) ferroelectric ceramics as its sensitive material. [161]This study delves into an exploration of key performance metrics, specifically the noise density, voltage response rate (R V ), and specific detection rate (D*), concerning modulation frequency under the influence of a current mode amplification circuit.The results unveil an intriguing feature of the lead-free PIR detector-its remarkable performance in the low-frequency spectrum.At a modulation frequency of 10 Hz, the R V impressively attains 1.51 × 10 5 V/W, signifying a noteworthy voltage response rate.Simultaneously, the specific detection rate (D*) achieves an impressive 2.02 × 10 8 cmHz 1/2 W −1 at the same frequency.These findings underscore the efficacy of the lead-free PIR detector and its potential for a range of applications requiring exceptional sensitivity to pyro-electric effects in the low-frequency domain.The development of such lead-free materials in pyroelectric infrared detection is promising, not only for its environmental benefits but also for its impressive performance characteristics

Other Sensor
In our daily lives, the detection of various gases holds paramount importance.Notably, in 2004, K. Park and collaborators made significant strides in gas sensing technology with their publication on a gas sensor incorporating BaTiO 3 for the purpose of carbon monoxide (CO) detection. [162]Their findings reveal a compelling relationship between the sensor's resistivity, measured in both high-temperature air and varying CO gas concentrations, and the extent of graphite doping.Moreover, the sensitivity of CO gas detection is significantly enhanced through the augmentation of ferroelectric material porosity.A higher degree of porosity fosters increased oxygen adsorption and, notably, provides an amplified number of reaction sites for the interaction between carbon monoxide and oxygen ions (O-).In 2006, Mandayo et al pioneered the creation of a thin-film sensor designed for the detection of carbon dioxide (CO 2 ). [163]This innovative sensor fulfills a crucial role in monitoring indoor air quality, not only enabling the realtime assessment of target gas response rates but also accurately measuring relative humidity concentrations.
Numerous endeavors have been dedicated to enhancing the reactivity, sensitivity, and specificity of sensors based on BaTiO 3 by leveraging additives and doping techniques.Recently, R.P. Patil and colleagues conducted a comprehensive study on a gas sensor rooted in BTO, [164] demonstrating its proficiency in detecting ammonia (NH 3 ) and nitrogen dioxide (NO 2 ).The researchers employed a low-temperature hydrothermal method to synthesize strontium-doped barium titanate and meticulously examined the  [168] Copyright 2017 Elsevier.(b) 3-D schematic of BaTiO 3 aerosol-based hygroscopic film fabrication. [169]Copyright 2021 Elsevier.
influence of three distinct doping concentrations on gas sensitivity.Their investigation unveiled that 0.2% strontium doping yielded superior sensing performance for both NO 2 and NH 3 gases.Additionally, Anshika Singh's research team introduced an ethanol vapor sensor designed around barium titanate's ethanol gas sensitivity. [165]Ethanol, widely used as a solvent in fragrance, preservative, and perfume manufacturing, necessitates vigilant monitoring due to potential health concerns, particularly central nervous system damage caused by prolonged exposure.Ethanol sensors play a pivotal role in industries such as chemical process monitoring, biomedical research, and respiratory analysis.Remarkably, the ethanol gas tests revealed that after modifying SnO at 250 °C, the sensor exhibited an outstanding 343% sensing response to 500 ppm ethanol vapor, while the unmodified BaTiO 3 sensor yielded a modest 15% response.These findings chart a promising course for the evolution of ethanol gas sensors, offering a remarkable leap in sensitivity and performance. [166,167]n an independent investigation, R.P. Patil et al. delved into the impact of physical attributes such as size, shape, and band gap of BTO films on the gas-sensitive mechanism, with a specific focus on hydrogen sulfide (H 2 S), as illustrated in Figure 18a. [168]Their research revealed that the sensor exhibited selectivity for H 2 S and delivered a robust response when operated above 150 °C, with the highest response recorded at 300 °C.This work expands the application domain of BaTiO 3 -based gas sensors into the realm of H 2 S detection, with potential implications for gas monitoring and safety.Moreover, sensors based on BTO have demonstrated efficacy in humidity sensing, as depicted in Figure 18b. [169]The author proposes an ultra-highly sensitive approach using BaTiO 3 film integrated with IDCs and an SSCs for humidity sensing applications.These studies collectively demonstrate the versatility and potential of BTO-based sensors in addressing a range of gas sensing and environmental monitoring needs, propelling advancements in sensitivity, selectivity, and responsiveness.

Conclusion and Outlook
In the dawn of the Internet of Things (IoT) era, sensors are taking center stage, with their applications expanding and evolving at an unprecedented pace.The prevailing trend in sensor development encompasses miniaturization, heightened resolution, and enhanced accuracy, in addition to their deployment under extreme environmental conditions.In the age of intelligence, sensors are now tasked with functions ranging from information induction, processing, and exchange, to the potential for self-power generation.Notably, barium titanate, characterized by its inherent dielectric, pyroelectric, piezoelectric, and photoelectric properties, stands as a rapidly developing core material for sensors.Its multifaceted attributes are set to be rigorously examined in the context of sensor technology.Barium titanate, serving not only as a standalone sensor material but also in pivotal auxiliary roles across various sensing applications, underscores its versatility and importance in this field.With the rapid development of the era of artificial intelligence, the research of neural computing and artificial synapses will be a hot spot in the future.Barium titanate, as a representative ferroelectric material, will play a great role in these fields.
In the evolution of ferroelectric materials, the exploration of 2D materials emerges as a significant future research focus.With the evolving demands of various industries pushing towards smaller and more energy-efficient devices, the development of 2D ferroelectrics holds immense potential in the realm of high-performance and low-energy consumption micro and nano intelligent components.However, the field of 2D ferroelectrics faces several challenges, such as the existence of multiple stable states, some of which remain unexplored.In the sensor domain, the detection and quantification of the subtle responses from 2D ferroelectrics pose substantial challenges.Presently, the research on 2D ferroelectrics is at a nascent stage.Not only are there numerous potential mechanisms to be unraveled, but the diverse material properties also warrant further exploration and understanding.The journey in understanding and harnessing the full potential of 2D ferroelectrics is ongoing, marked by various potential avenues for exploration and development.

Figure 4 .
Figure 4. (a) Device structure of ferroelectric tunnel junction.(b) Schematic of a two-layer neural network for the recognition of handwritten digit images.(c) The recognition rate as a function of training times within one training epoch.[41]Copyright 2020 American Chemical Society.

Figure
Figure7.Ferroelectric hysteresis loops for the rare-earth doped BTO ceramics.[114]Copyright 2016 Royal Society of Chemistry.

Figure 10 .
Figure 10.(a) The Structure based on PDMS@BTO d+s .(b)The output of device used as a highly sensitive wearable sensor for detecting the breathing motion of a volunteer.[128]Copyright 2022 American Chemical Society.

Figure 11 .
Figure 11.(a) Assembly diagram of flexible capacitive pressure sensor (1.5 cm × 1.5 cm).(b) Physical diagram of device based on BTO.(c) Photographs of the sensor attached to the human throat.(d) Real-time pulse monitoring of capacitive response after normal heartbeat and exercise was performed respectively.(e)Wearable performance test for the packaged sensors.[130]Copyright 2022 Elsevier.

Figure 12 .
Figure 12.(a) Photographs of piezoelectric pressure sensor integrated into the insole of a shoe, and the output voltages generated by (b) jumping, (c)walking and (d) running, respectively.[136]Copyright 2022 Elsevier.

Figure 13 .
Figure13.Various applications to show the highly sensitive, linear and large-range pressure sensing by multilayer e-skins.[149]Copyright 2018 American Chemical Society.

Figure 14 .
Figure 14.A self-powered PAM/BTO-based TENG multiple sensor, (a) mounted on the inner surface of a finger it or pressure applied to a mobile phone, (b) affixed to the knee, and (c) attached to the heel, facilitated the discrimination of distinct weight distributions.The hydrogel-based piezoresistive sensor,assessed by ΔR/R 0 , demonstrated its sensitivity in detecting (d) variations in contact strength and (e) bending of a finger, (f) wrist, and (g) elbow.[150]Copyright 2022 American Chemical Society.

Figure 15 .
Figure 15.(a) Structure diagram of BTO/GaN heterojunction.(b)BTO/GaN physical photos.[154]Copyright 2021 American Chemical Society.(c) Schematic diagram of a photodetector with a two-dimensional planar structure when illuminated.(d) Physical drawings of photodetector.[155]Copyright 2018 Elsevier.(e) Schematic diagram of flexible photodetector.(f) A physical picture of the flexible photodetector when it is bent.[156]Copyright 2020 Elsevier.

Figure 17 .
Figure 17.(a) The photograph of the pyroelectric infrared (PIR) detector, (b) the current mode circuit of the PIR detector,(c) the 3D view of the PIR detector.[161]Copyright 2023 John Wiley and Sons.

Table 1 .
Performances of pressure sensors based on ferroelectric materials.

Table 3 .
Comparison of polarization characteristics of prevalent ferroelectric materials.

Table 4 .
Piezoelectric and dielectric properties of BTO-based materials.