Dispersion of InSb Nanoinclusions in Cu3SbS4 for Improved Stability and Thermoelectric Efficiency

Thermoelectric‐based waste heat recovery requires efficient materials to replace conventional non‐eco‐friendly Te‐ and Pb‐based commercial devices. Ternary copper chalcogenide‐based famatinite (Cu3SbS4) compound is one of the practical substitutes for traditional thermoelectric materials. However, the pristine Cu3SbS4 inherits poor structural complexion, large thermal conductivity, and low power conversion efficiency. To develop high‐efficiency Cu3SbS4, InSb nanoinclusions are incorporated via high‐energy ball milling followed by the hot‐press densification method. Incorporating InSb nanoinclusions to lower thermal conductivity via phonon scattering while increasing the thermopower via a carrier energy filtering process. The thermoelectric performance (ZT) of ≈0.4 at 623 K is obtained in Cu3SbS4‐3 mol% InSb nanocomposite, which is ≈140% higher than pure Cu3SbS4. Both mechanical and thermal stability are improved by grain boundary hardening and dispersion strengthening. Thus, a facile nanostructured Cu3SbS4 with added InSb nanoinclusions is delivered as a highly efficient, eco‐friendly, structurally‐, thermally‐, and mechanically‐stable material for next‐generation thermoelectric generators.


Introduction
Over the past few years, extensive research on thermoelectric materials has intensified due to their potent capacity to effectively convert heat into electrical energy.The advantages of solid-state, emission-free, and operation capacity over a wide temperature range make them important technology for waste heat recovery. [1]hermoelectric conversion efficiencies are quantified by the relation, ZT ¼ α 2 σT=κ, where κ and σ represent thermal and electrical conductivities, α represents the thermopower or Seebeck coefficient, and T represents applicable temperature, respectively.The strong negative interrelation between the thermoelectric parameters (α, σ, and κ) complicates the challenge of improving the thermoelectric conversion efficiency in materials. [2]The phonon and electronic components of thermal conductivity are combined to make up the total thermal conductivity.The Wiedemann-Franz law relates the electrical conductivity and electronic components of thermal conductivity.One of the easy steps to improve thermoelectric performance is reducing the phonon component of thermal conductivity without interfering with electrical transport. [3]For this case, various strategies to reduce the phonon component of thermal conductivity are established, such as nanocompositing, [4][5][6] nanostructuring, [7,8] adding rattlers, [9] doping or alloying, [10][11][12] and defect engineering, [13][14][15] etc. Advantageously, the nanostructuring technique does not require extrinsic elemental additions and can be controlled by sintering.
According to statistics, 80% of waste heat from industries is released at temperatures between 373 and 573 K. [16] Though commercially available Te and Pb-based thermoelectric devices suit this intermediate temperature regime, they are toxic and pollute the environment.Generally, high conversion efficiency, good mechanical & thermal stability, low cost, and low toxicity material characteristics are essential for the large-scale production and commercialization of thermoelectric generators. [17][19] Cu 3 SbS 4 is one of the best alternative p-type thermoelectric compounds for intermediatetemperature applications. [20,21]Below their melting point, Cu 3 SbS 4 is highly stable with a tetragonal crystal structure derived from its asymmetric zinc blend structure and does not show any secondary phase formations. [18]However, despite Cu 3 SbS 4 good electrical characteristics, the issue of quite high thermal conductivity due to the uncomplicated lattice structure DOI: 10.1002/aesr.202300125Thermoelectric-based waste heat recovery requires efficient materials to replace conventional non-eco-friendly Te-and Pb-based commercial devices.Ternary copper chalcogenide-based famatinite (Cu 3 SbS 4 ) compound is one of the practical substitutes for traditional thermoelectric materials.However, the pristine Cu 3 SbS 4 inherits poor structural complexion, large thermal conductivity, and low power conversion efficiency.To develop high-efficiency Cu 3 SbS 4 , InSb nanoinclusions are incorporated via high-energy ball milling followed by the hotpress densification method.Incorporating InSb nanoinclusions to lower thermal conductivity via phonon scattering while increasing the thermopower via a carrier energy filtering process.The thermoelectric performance (ZT) of %0.4 at 623 K is obtained in Cu 3 SbS 4 -3 mol% InSb nanocomposite, which is %140% higher than pure Cu 3 SbS 4 .Both mechanical and thermal stability are improved by grain boundary hardening and dispersion strengthening.Thus, a facile nanostructured Cu 3 SbS 4 with added InSb nanoinclusions is delivered as a highly efficient, eco-friendly, structurally-, thermally-, and mechanically-stable material for next-generation thermoelectric generators.and the absence of lone pairs of electrons impedes its thermoelectric efficiency. [17]The techniques such as nanostructuring by forming nanocomposite structures have been adopted to enhance their conversion efficiency. [22]anostructuring in a thermoelectric material is achieved by the facile incorporation of secondary nanoinclusions in the matrix material.Nanoinclusions contribute to the reduction of thermal conductivity by utilizing interfacial boundary-based phonon scattering. [4,5,20,23]Given the similarity in sizes of the nanoinclusions with the phonon mean free path (MFP of phonons is greater than electrons), these interfacial boundaries effectively scatter phonons more than electrons. [24][27][28][29][30][31][32][33] Semiconducting InSb nanoinclusions are added in both ex situ and in situ manner.Ex situ processing route has advantages in terms of homogeneous distribution, eliminating unwanted secondary phase formations, and controlled particle size in maximizing the interfacial boundary region. [25,28,30]Huang et al. [26] demonstrated that 3 mol% InSb nanoinclusions in the Cu 12 Sb 4 S 13 matrix enhanced the thermopower via carrier energy filtering and degraded thermal conductivity via phonon scattering for maximized thermoelectric performance.Ghosh et al. [27] showed that ex-situ InSb nanoinclusions reduced the CoSb 4 matrix conductivity while improving the thermopower and reducing the thermal conductivity for overall improvement in thermoelectric performance.
On the other hand, solid-state mechanical alloying is advantageous over wet chemical synthesis in producing nanocrystalline powders in terms of product purity, synthesis simplicity, and processing speed. [22,34]In this work, ex situ InSb nanoinclusions were added to improve the thermoelectric performance of mechanically alloyed Cu 3 SbS 4 , employing interfacial boundaryassisted phonon scattering and energy filtering mechanisms.The nanocomposite Cu 3 SbS 4 -3 mol% InSb structure resulted in improved thermopower and reduced thermal conductivity.Additionally, InSb nanocomposite helps to improve the structural, thermal, and mechanical stabilities of the Cu 3 SbS 4 structure.Through the microstructure and processing design, we achieve higher thermoelectric efficiency and improved thermal and mechanical stability using Cu 3 SbS 4 thermoelectric material.

Results and Discussion
Table 1 shows the physical and structural parameters of the Cu 3 SbS 4 samples we produced in this study.The measured densification in all the Cu 3 SbS 4 samples exceeded 90%.The densification was lowered by adding InSb nanoinclusions due to the generation of back stress at the interfaces during sintering in the composites. [35]he complete mechanical alloying route of the nanostructured Cu 3 SbS 4 synthesis process is schematically illustrated in Figure 1a.The Cu 3 SbS 4 crystal structure (tetragonal structure with I42m space group) is shown in Figure 1b.X-ray diffraction (XRD) patterned peaks match those of the famatinite (Cu 3 SbS 4 , JCPDS: 35-0581) and the peak corresponding to the InSb (InSb, JCPDS: 06-0208) phase observed, as shown in Figure 1c.As noted in Table 1, the average size of grains is calculated from the XRD data using the Scherrer equation (Equation ( 1)) and from field emission scanning electron microscope fractured surface morphology (as shown in Figure S1, Supporting Information) using the ImageJ software.As mentioned in Table 1, the average grain size decreases with the increase in the concentration of InSb nanoinclusions.Adding harder nanoinclusions pins the grain boundaries and hinders grain growth during sintering. [36]However, the added InSb amount is too small, and most of the standard XRD Cu 3 SbS 4 and InSb peak positions are matching; we carried out the Raman analysis to confirm the presence of the InSb phase.Raman spectra shown in Figure 1d revealed that the Cu 3 SbS 4 -related peaks displayed vibration modes at Raman shifts of 247, 273, 317, 344, 358, and 634 cm À1 .As shown in Figure 1d, Raman spectra also revealed the InSb peak at 186 cm À1 in composite samples that correspond to the longitudinal-optical (LO) phonon mode, which confirms the formation of the InSb phase. [37]As shown in Figure 1e, the backscattered electron scanning electron microscope (BSE-SEM) image indicates that the InSb nanoinclusions are distributed homogeneously in the Cu 3 SbS 4 nanocomposite.The elemental mapping shown in Figure 1f exposes the elements that are distributed uniformly in the matrix and nanophase.The energy dispersive x-ray spectroscopy (EDX) compositional breakdown in Figure 1g revealed the presence of Cu, Sb, S, and In without oxygen in the Cu 3 SbS 4 composite.From the XRD and EDX analyses, the absence of oxides or secondary phase peaks indicates the high phase purity of the synthesized Cu 3 SbS 4 nanocomposite pellets.
where β denotes full width at half maxima, θ is the diffraction angle, λ is the wavelength of the XRD source, and D is the grain size.
From Figure 2a, according to measurements of the Hall effect, as the concentration of InSb nanoinclusions increased, carrier concentration (Â10 20 cm À3 ) increased, and carrier mobility decreased.The temperature-dependent Hall effect measurements of carrier concentration and carrier mobility are shown in Figure S2, Supporting Information.InSb is a nearly metallic phase with a narrow bandgap of 0.17 eV and possesses a very low electrical resistance of %18 μΩm at 300 K. [28] Therefore, the enhanced carrier concentration is significantly caused by the additional InSb semiconducting nanoinclusions.The mobility reduction observed is associated with the domination of carrier scattering in the high-density nanograin/interfacial structures. [34]ccording to Figure 2b and S2a, Supporting Information, it is observed that electrical conductivity increased with increasing temperatures due to the excitation of secondary carriers and the generation of electron-hole pairs with increasing temperatures in all the Cu 3 SbS 4 samples.All Cu 3 SbS 4 samples show increased electrical conductivity with the temperature that displays nondegenerate semiconducting behavior.According to Figure 2b, as the amount of InSb nanophase increased, the electrical conductivity decreased as a result of the predominance of reduced mobility through carrier scattering. [27]The Fermi level is theoretically determined by data from experiments using Equation ( 2) and (3).
where η is the reduced Fermi level, m 0 is the mass of the electron, k B denotes Boltzmann's constant, T is applicable temperature, h is Planck's constant, and Fermi integral functions for η are F 0 ðηÞ, and F 1 ðηÞ.In all Cu 3 SbS 4 samples, the Fermi level (E f ) increases with temperature as well as decreases due to an increase in the amount of InSb nanoinclusions, as shown in Figure 2c.All synthesized Cu 3 SbS 4 samples exhibit positive thermopower values, as illustrated in Figure 2d, which means p-type semiconducting behavior, thus indicating holes as major carriers.In all Cu 3 SbS 4 samples, with increasing temperature, the thermopower values slightly increase.The thermopower also increases with the rising concentration of InSb nanoinclusions.
Based on the bandgap difference and band alignment connecting the matrix and secondary phase, InSb nanoinclusions create interfacial boundaries that filter the carriers based on their relative energies.Gayner et al. [38] explained that the thermopower improved due to filtering energy-dependent charge carriers, which adds up to the enhanced normalized thermopower distribution.In Cu 3 SbS 4 composites, nanocomposite structure results in carrier filtering or low-energy (energy-dependent) charge carrier scattering, as shown in the inset of Figure 1g.Mott's formula (Equation ( 4)) is used to theoretically compute the effective mass from the experimental data using the singleparabolic (SPB) model.
As noted in Table 1, the density of states' associated effective mass (m Ã d Þincreased with increasing the concentration of InSb nanoinclusions.Carrier mobility has an inverse relationship with , which agrees well with increasing the concentration of InSb nanoinclusions' increased effective mass.The SPB is supported by the Pisarenko relationship between the empirically determined thermopower and carrier concentration, which also explains the carrier energy filtering in nanocomposite structures, as illustrated in Figure 2e. Figure 2f shows that the temperature-dependent power factor increases with temperature in all the Cu 3 SbS 4 samples.According to Figure 2f, the thermopower enhancement by interfacial boundary filtering outweighs the reduction in electrical conductivity by boundary scattering; hence, the power factor increased as the concentration of InSb nanoinclusions increased. According to Figure 3a, the total thermal conductivity is reduced with increasing temperature in all the Cu 3 SbS 4 samples.Fine and high-density interfacial boundaries mainly scatter in the medium-to-long-wavelength phonons. [20]As per Figure 3a, Cu 3 SbS 4 with 3% InSb nanoinclusions shows the smallest possible thermal conductivity of 0.73 W m À1 K À1 ; it is %40% inferior to that of the pristine Cu 3 SbS 4 .The interfacial boundary-assisted phonon scattering generated in the Cu 3 SbS 4 nanocomposite resulted in reduced thermal conductivity.As shown in the inset of Figure 1g and S1a-c, Supporting Information, mechanical alloying resulting in high densities of nanograin boundaries, embedded InSb nanoinclusions, and nanopores together contribute to the nanocomposite structures' reduced thermal conductivity.Kim et al. [39] explained the way to determine the Lorenz number using the equation L ¼ 1.5 þ expðÀjαj=116Þ from the measured thermopower, where α is shown in μV K À1 ; the derived values are shown in Figure 3b.As shown in Figure 3c, The Wiedemann-Franz law, κ electronic ¼ LσT, is used to compute the thermal conductivity from electronic component (κ electronic ), where L denotes the Lorenz number, T denotes the applicable temperature, and σ represents the electrical conductivity.The electronic component of thermal conductivity exhibits the same electrical conductivity trend as minority carriers' excitation and increased electrical conductivity with temperature.According to Figure 3d, the thermal conductivity of phonon component (κ phonon ) is calculated using the equation: κ total ¼ κ electronic þ κ phonon .From Figure 3e, κ phonon contributes higher than κ electronic toward total κ due to the relatively lower electrical conductivity of Cu 3 SbS 4 compared to traditional TE materials.As shown in Figure 3f, we showed κ phonon versus 1000/T to illustrate the fundamental process underlying the phonon thermal conductivity in all the Cu 3 SbS 4 samples.The negative slope of the solid linear connection with the increasing temperature between κ phonon and 1000/T suggests the supremacy of the Umklapp phonon-phonon scattering process at higher temperatures in all Cu 3 SbS 4 samples. [19,40]The thermoelectric performance (ZT) of InSb nanoinclusions added to Cu 3 SbS 4 nanocomposite samples is shown in Figure 4a.We demonstrated a maximum thermoelectric performance (ZT) of %0.4 in the 3 mol% InSb nanoinclusions added Cu 3 SbS 4 , which is %140% higher than pristine Cu 3 SbS 4 .The achieved ZT in the pristine Cu 3 SbS 4 is higher than those reported in previous literature, as shown in Figure 4b.Using the determined ZT, we calculated the thermoelectric efficiency with Equation (5).
As shown in Figure 4c, we achieved maximum thermoelectric efficiency of %5% in the Cu 3 SbS 4 -3 mol% InSb nanocomposite at 623 K.
According to Figure 4d, the differential scanning calorimetry (DSC) analysis revealed strong phase stability until the TE applicable temperature in all the Cu 3 SbS 4 samples.From the DSC curves, it is evident that there are no exothermic peaks or thermal decomposition-related phase changes in all the Cu 3 SbS 4 samples.Vickers microhardness, shown in Figure 4e, indicated that hardness improved sequentially (from 1.7 GPa in pristine Cu 3 SbS 4 to 2.9 GPa in Cu 3 SbS 4 -3 mol% InSb sample) with increasing concentration of InSb nanoinclusions in Cu 3 SbS 4 due to the dispersion strengthening and grain boundary hardening mechanisms. [41]The schematic illustrations of the improved mechanical stability-assisted mechanisms in the composite materials are shown in the Figure S4, Supporting Information.As shown in Figure 4f, thermogravimetric analysis (TGA) plots revealed the thermal stability (change in weight of greater than 98%) of both pristine and nanocomposite Cu 3 SbS 4 .As shown in Figure 4f, increasing the amount of InSb nanoinclusions in Cu 3 SbS 4 composites improved thermal stability.To reassure the thermal stability, thermoelectric parameters have been measured repeatedly in three cycles and corresponding data (with error bars) shown in supporting information as Figure S3, Supporting Information.Therefore, synthesis of Cu 3 SbS 4 by combining facile mechanical alloying with the incorporation of InSb nanoinclusions produces a nanostructured (fine grain boundaries plus interfaces) material with high thermoelectric performance, strong phase stability, and improved mechanical and thermal stability for next-generation intermediate-temperature applications.

Conclusion
In summary, InSb nanoinclusions incorporated Cu 3 SbS 4 nanocomposite are synthesized through facile mechanical alloying followed by hot-press densification.By incorporating ex situ InSb nanoinclusions into Cu 3 SbS 4 , nanograin and interfacial barriers are created, which filtered low-energy charge carriers to increase the thermopower and minimize phonon thermal conductivity.As a result, the Cu 3 SbS 4 -3 mol% InSb nanocomposites show a maximum ZT of %0.4, which is %140% higher than pristine Cu 3 SbS 4 .Improved phase, thermal and mechanical stabilities are demonstrated with increasing InSb concentration due to dispersion strengthening and grain boundary hardening.From these observations, we believe that incorporating InSb nanoinclusions effectively generates a nanocomposite structure to improve the thermoelectric performance of Cu 3 SbS 4 .

Experimental Section
Stoichiometric high-purity powders of copper (company: Kurt.J. Lesker, purity: 99%), antimony (company: Kurt.J. Lesker, purity: 99.999%), sulfur (company: Sigma Aldrich, purity: 99.98%), and indium antimonide (company: Sigma Aldrich, purity: 99.99%) were weighed.The weighed powders were mixed and ground homogeneously using a pestle and mortar.In a high-energy planetary ball-milling machine, mechanical alloying was performed using ZrO 2 balls at 300 rpm for 24 h in a toluene medium with a ball-to-powder weight ratio of 20.Then the powders were compacted by hot pressing sintering in a cylindrical high-density graphite mold with a die  [5,18,[20][21][22]42,43] c) obtained maximum efficiency, d) DSC phase stability, e) room-temperature Vickers hardness, and f ) TGA thermal stability of Cu 3 SbS 4 þ x% InSb (x = 0, 1, and 3) nanoinclusion samples. of 15 mm diameter at 350 °C under uniaxial pressure of 120 MPa for 10 min. Hot prssing was done in a hydraulic laboratory press (Model: 944 955, Dake Corp., USA).The processed samples' relative densities were then calculated using the conventional Archimedes method.The powder XRD method was used in a Bruker D2 Phaser with a Lynxeye detector to perform phase analysis using Cu Kα radiation of wavelength 1.54 Å.In order to comprehend the phase formations using vibrational intensities associated with chemical bonding, the WITec RAMAN alpha 300R system was used to record the Raman spectra at an excitation cobalt DPL laser wavelength of 532 nm.The JEOL JSM-IT500 SEM in MSE mode, elemental mapping, and EDX were employed for highmagnification fractography.
The electrical conductivity (σ) and thermopower (α) were measured using a typical four-probe direct current apparatus, that is, the SBA 458 Nemesis (Netzsch), in an argon environment.The Lakeshore Hall measuring system was used to evaluate the charge carrier transport (carrier mobility and concentration) in the magnetic field of À2-þ2 T. Using measured carrier concentration and the thermopower, the density of the states associated effective mass (m Ã d Þand Fermi level (E f Þ were predicted.Using the laser flash method, thermal diffusivity measurements are carried out on disk samples with a diameter of 12.7 mm and thickness of 3 mm in the Netzsch LFA 467 equipment.The standard equation of thermal conductivity, κ ¼ ραC p , was used in Netzsch-assisted software to automatically calculate thermal conductivity, where C p stands for specific heat capacity, α stands for thermal diffusivity, and ρ is the sample's calculated density.The acquired thermopower, thermal, and electrical conductivity data were then used to calculate the power factor (α 2 σ) and the thermoelectric performance (ZT).Micro-Vickers hardness machine was used with a load of 5 kgf and a dwell duration of 10 s on test samples that were meticulously polished.The PerkinElmer STA6000 Simultaneous Thermal Analyzer performed TGA and DSC measurements to understand the phase and thermal stability.