Alumina‐Epoxy‐Pore Composites Fabricated by Inkjet Printing for Wireless Communication Technology

Reducing transmission loss via composites with pores has emerged as a breakthrough method for information and communications technology applications. The super/extremely high‐frequency signals used in the technologies result in significant transmission loss. Demand for low‐loss materials with low dielectric constant (Dk) and low dissipation factor (Df) has increased in recent years. Here, it is demonstrated that the pore in composites can be reduced Dk and Df of composites with pores (air). In fabrication of composites, a drop‐on‐demand (DOD) inkjet printing method to fabricate composites with pores (air) for use in wireless communications technologies. The DOD method is a simple but versatile manufacturing process at low cost. Its versatile process makes it possible to control the amount of resin and pore in fabrication of the composites and moreover to distribute all their components isotopically and homogeneously. It is possible to fabricate the composites with a lower Dk and Df value by increasing pore amount in the composites. This strategy enabled us to inexpensively manufacture dielectric substrates with low Dk and Df values, greatly contributing to the communications technology industry.


Introduction
Wireless communication technology refers to a system that transmits and receives data without wires through electromagnetic waves or sound waves.Most wireless systems use RF, which stands for radio, and infrared (IR) frequency.Aside from commonly used information technology (IT) devices, wireless technology is being used in a wide range of fields, one of which is satellites and satellite communication systems.The advantage of this wireless communication system is that there are no space restrictions.In fact, in the case of wireless communication, if there are no problems with the transmitting and receiving devices of the two transmitters and receivers, and there are no variables that affect communication, there are no restrictions on communication.
Modern communication networks such as 5G/6G networks enable various kinds of connections. [1,2]For example, people, things, cities, transportation systems, etc. can be connected in the networks.As proof of this, the scale of mobile and wireless traffic has increased 1000 times compared to 2010.Moreover, the number of wirelessly connected devices has increased to tens of billions, which has had a significant impact on society.Modern networks provide many convenient and useful applications such as smart cities, smart factories, cloud services, remote medical services, and virtual reality, where a huge amount of data can be transported with fast speed and low latency. [3]s various applications are created and used, requirements for services, devices, and networks have also become more diverse.In addition to these requirements, it is expected that the explosively increasing number of traffic will become increasingly difficult to respond to with existing wireless communication technologies.To process large amounts of data quickly, high-frequency signals must be sent.The super (3-30 GHz, SHF)/extremely (> 30 GHz, EHF) high-frequency signals used in the networks result in significant transmission loss and generate more heat.To solve this, the signal transmission loss that occurs at high frequencies must be lowered.
The signal transmission loss of electromagnetic waves is determined by the sum of conductor loss and dielectric loss.Conductor loss is caused by the roughness of the interface between the conductor and dielectric.13] Demand for low-loss materials has been increasing a lot in recent years.Those dielectrics commercially available these days are copper-clad laminate (CCL), low temperature co-fired ceramic (LTCC), poly tetra fluoro ethylene (PTFE), and liquid crystal polymer (LCP).16][17][18][19][20] Also, common manufacturing methods for dielectric include Etching (Photolithography) and screen printing, which suffer from the use of harsh chemicals, high manufacturing costs, and complex manufacturing processes.The development of new dielectric requires efforts to improve the versatility and cost of manufacturing.Inkjet printing is a method of additive electronic manufacturing that is gaining interest for both consumer and industrial applications as a low-cost, highly scalable, and environmentally friendly alternative to typical laminate-based manufacturing methods. [21]xperiments on manufacturing devices, including antennas, through inkjet printing were conducted in various literature.These papers showed that antennas can be fabricated through inkjet printing and can operate at high frequencies.However, in most cases, only electrode printing was done with inkjet printing, and in some cases, dielectric was also printed, but only polymer was used, which has poor usability and applicability.24][25][26][27][28] Our research goal is to manufacture a product that complements the disadvantages while inheriting the advantages of commercially available dielectrics.We want to fabricate hybrid composites with pores (air) as a new type of low-loss material for such a goal.The features of the composites are as follows: Its manufacturing cost is very low compared to commercially available dielectrics, its manufacturing process is simple, and it is easy to produce products that flexibly follow the user's needs, it can be formed on various substrates, it is easy to laminate, it is easy to change D k and D f values, and it has hermeticity.
The composites are composed of matrix, binding, and pores to have physical strength without high-temperature heat treatment.These materials must have low D k and D f values to achieve our research goal.Therefore, matrix material's candidate groups are alumina, PTFE, etc. Binding material's candidate group is resin.This is because binding materials have low D k and D f values while binding matrix materials.In addition, the shape of the pores must be closed to have strong resistance to moisture.
Existing polymer composites have the advantage of being able to easily manufacture various materials in the form of composites using polymer as the main material.However, in general, polymer occupies a majority of the vol%, which has the disadvantage of being unfavorable from the perspective of dielectric properties.
In this paper, we introduce a method to fabricate hybrid composites with pores.Through the inkjet printing process with drop-on-demand (DOD) characteristics, the volume fraction of the materials that make up the composite can be accurately controlled, and through this, the D k and D f values can also be accurately controlled.This means that the method to fabricate hybrid composites with pores makes it possible to fabricate dielectric substrates with various dielectric properties in a radically simple manner.

Material Selection
To achieve the research goal, we attempted to confirm whether the D k and D f values of the composite were also accurately controlled when the volume fraction of the materials constituting the composite was accurately controlled through the inkjet printing method.For this purpose, composite matrix and binding materials were selected.The materials considered as matrix materials are alumina, which is cheap among ceramics with low D f value and has low D k value, and PTFE, which is one of the materials currently attracting attention due to its very low D k and D f values.
Materials considered as binding materials include resin types such as epoxy, polyimide, and cyanate ester.Among the materials suitable for achieving our research goal, alumina was selected as matrix material and epoxy as the binding material.Alumina has a low D f value, but also has a low D k value among ceramics and is inexpensive.Epoxy has a low D k value, is inexpensive, and exhibits high physical properties when cured.In other words, these two materials have the properties we want and have the advantage of being very cheap.Additionally, as the composite is manufactured through inkjet printing, lower D k and D f values can be achieved due to the pores created.Therefore, it is suitable for producing hybrid composites with pores and checking whether the composite has the characteristics we want.

Morphology and Structure of Alumina Particles
Since we want to fabricate composites for good dielectric performance, we use the particles with high purity and almost the same size, i.e. narrow size distribution.Therefore, the alumina particles synthesized by an aluminum alkoxide hydrolysis process are used. [29]Figure 1a-d shows the morphology of alumina particles before and after jet milling through the field emission scanning electron microscopy (SEM, JEOL, 7500F), respectively.The particle sizes after jet milling became smaller and more uniform.The Particle sinze corresponding to 50 % of the weight percentage ins the particle size distribution curve (D 50 ) of the alumina particles before jet milling was 270 nm, and the D 50 after jet milling was 220 nm.To confirm the change in crystallinity of alumina particles before and after jet milling, X-ray diffraction (XRD, Bruker, D8 FOCUS) patterns were measured (Figure 1e).The XRD patterns are in exact agreement with international center for diffraction data (ICDD) 46-1212.The alumina particles made through ink manufacturing process do not change their phase and have a more uniform and smaller particle size.

Fabrication of Ink
Alumina ink was prepared by mixing a dispersant (DISPERBYK-111) and alumina nano-powder (Sumitomo, AKP-30, D 50 = 270 nm) in the volume ratio of 1:10 and mixing the resultant into Ethylene Glycol (EG, Sigma-Aldrich, anhydrous, 99.8%) by a homogenizer at a speed of 3000 rpm for 1 h.The head of the homogenizer used a square hole high shear screen.It was then milled for 24 h using a jet mill.The diameter of the beads used in the jet mill is 100 μm.For further deflocculation of the residual agglomerates, the mixture solution was filtered by a 6 μm polypropylene filter (Pall, LCF-11100).The ink of epoxy, thermoset resin, (KUKDO, KDL-190DG60) is formulated by a mixing Diethylene Glycol Dimethyl Ether (Sigma-Aldrich, DMDG, anhydrous, 99.5%), maintaining the epoxy concen-tration at 3 vol%.It is also filtered by a 6 μm filter to remove clogging.

Dispersion and Jetting Stability of Ink
Measurements of alumina ink through a dispersion stability analyzer (Turbiscan, Lab) are shown in Figure 2a,b.The movement of alumina particles in solvent for 24 h is shown through transmitted and backscattered intensities (%) graph (Figure 2a,b).The X-axis in Figure 2a,b is the height of the ink.As a result of measuring the alumina ink for 24 h, it shows that there are slight changes in the transmitted and backscattered intensities appeared at the top of the ink in each graph.Through these changes, we know that the alumina particles are aggregated and sedimented at the  top of the ink over time. [30,31]It means that the alumina ink has high dispersion stability for 24 h.An ink circulation system is used to prevent ink from precipitation when printing for longer than 24 h.

Fabrication of Hybrid Composites with Pores
We want to fabricate dielectrics with a low D k and D f values and resistant to heat.[34][35][36] However, alumina itself is not a suitable candidate for low-loss dielectrics because of its high D k value ( = 10 at 10 GHz). [32]Moreover, alumina particles do not have strong bonding between them and thus it is impossible to fabricate dielectrics made by only alumina particles.We need other component which can make up for alumina's shortcomings if we want to use alumina. If we fabricate alumina-resin hybrid composites, we can overcome the problems alumina has.Although resin has a low D k value (= 3 at 10 GHz), there are still limitations in the D k value alumina-resin hybrid composites have.If we can add air in the composites, the D k value of the composites can be decreased a lot because air has a very low D k value close to 1. [41] Thus, we want to fabricate alumina-resin hybrid composites with pores (air).The fabrication of such composites is possible if we use the DOD feature of an inkjet printing process, which has many advantages which meet our research goals.
We use alumina and resin ink to fabricate alumina-resin hybrid composites with pores.The inks should satisfy a certain condition to avoid for ink droplets failing to jet or scatter.The condition is that the Z number (1/Oh) of them should be within the range of 1 < Z < 10 (Equation 1).This number is a dimensionless constant that describes the tendency of droplets to fail to jet or scatter by comparing viscous forces to inertial and surface tension forces.In Equation 1, Oh means Ohnesorge number, and the main factors determining Z number are viscosity , nozzle diameter , density , and surface tension .Most inks used in inkjet printing can have jetting stability when the Z number is in the range of 1 < Z < 10.If Z exceeds 10, satellite droplets are generated, and if Z is less than 1, jetting does not occur.[44][45][46][47] For the fabrication of alumina-resin hybrid composites, we use alumina and resin ink, whose the Z numbers are 7.1 and 4.  the nozzle, a spherical droplet with a tail was formed, and over time, the tail merged into the spherical droplet to form a complete spherical droplet and was uniformly printed to the substrate.In addition, to obtain a uniform printed shape, the jetting speed of a droplet was maintained at 2.5 m s −1 or more so that the droplets move straight during deposition, and no satellite droplets were generated. [48,49]The volume of each droplet jetted was maintained at 15 pL.The jetting speed and volume of each droplet are measured by UJ 200 software.The uniformly printed droplets and lines formed by them were measured with an optical camera (HiMaxTech, HNM005) and shown in Figures 3b,c.In Figure 3b, the distance between the droplets was set to 100 μm in the X-axis and 100 μm in the Y-axis to see the shape of the printed droplets.The size of the printed droplets is 34-36 μm in diameter.To make fine lines by printed droplets, the optimized Y-axis pitch (Y pitch) was chosen (Figure S2, Supporting Information).As a result of optimization, the line had a uniform shape when the Y pitch was 63.5 μm.In Figure 3c, the distance between the droplets was set to 300 μm in the X-axis and 63.5 μm in the Y-axis to see the shape of the printed lines.The printing error of inkjet printing was less than 2 μm.
Equations 2-4 show how transmission loss is determined by conductor loss and dielectric loss.In Equation 3, the conductor loss is proportional to the conductor's skin resistance. [50]It is caused by skin effect.Due to the skin effect, generally the higher the frequency, the closer the current signal flows along the interface between the conductor and dielectric.In the case of a con-ductor with a rougher interface, the conductor loss is greater because the current signal travels a longer distance along the interface.It means that the interface roughness is proportional to the conductor loss.Therefore, it may be a meaningful factor related to the transmission loss. [10,50,51]Thus, it is important to fabricate the composites with a small roughness value.
[57] We measured changes in surface roughness value of the alumina layer according to different X pitch values by using a stylus surface profilometer (alpha-step, d-500).We found that the smoothest surface is obtained when the X pitch of alumina ink is 15 μm. Figure 4a,b show the images of the alumina-resin hybrid composite measured by the optical camera.In Figure 4a,b, lines are printed for 30 and 15 μm X pitch, respectively.When the X pitch is 30 μm, the boundary between the lines appears clearly, but when it is 15 μm, it becomes invisible.Figure 4c,d are a profile graph of surface roughness.When the X pitch is 30 μm, arithmetical mean roughness value (R a in Equation 5) is 0.9 μm, but when the X pitch is 15 μm, the R a is 0.4 μm.If the X pitch is less than 15 μm, another droplet falls nearby before the printed droplet evaporates, and the two droplets are merged into a single droplet by attraction force, which leads to broken of fine shape of each line in the Y-axis and rougher surface in the X-axis direction.Therefore, if the X pitch is less than 15 μm, the lines by droplets in the Y-axis are distributed non-uniformly, resulting in rougher surface.The X pitch is less than 15 μm also results in rougher surface.
Alumina layer is first printed by alumina ink and then resin ink is jetted on the layer.Curing of the alumina-resin hybrid composites is a necessary step to get them to solidify.The composites are cured at 180 °C for 6 h in an infrared curing oven (LAB HOUSE, FTIR-301).The curing temperature was measured by a differential scanning calorimetry (DSC, TA, Q20) (Figure S3, Supporting Information).The fabrication process of the composites is shown in Figure 5.The composite with a thickness of 100 um for use in measurement was manufactured in the form of a multilayer by repeating the process of printing alumina with a thickness of 50 um, infiltrating epoxy, and then curing.When the thickness of alumina is high, the infiltration of epoxy does not reach the bottom of the alumina, and the epoxy is distributed unevenly.To prevent this, the thickness of the alumina was fixed at 50 μm.
Pores are naturally formed after alumina ink is printed on a substrate, the solvent evaporates, and alumina has a packing density of 60 vol.% through random packing.The error in packing density was less than 2 vol.%.t this time, the pore volume % of the composite is adjusted depending on the vol.% of epoxy printed on the alumina layer.In other words, the pore volume %, V p , can be accurately controlled by using the DOD feature of the inkjet printing process.We can change the ratio of components in the composites easily and thus fabricate composites with different D k and D f values by adjusting the ratio of components (See Figures 5 and 6). Figure 6a-e shows the cross sections of the composites according to V p .The cross-section was formed by a focused ion beam (FIB, SII, SMI3050TB) and then measured by a scanning electron microscopy (SEM).As shown in Figure 6a-e, pores exist between the alumina particles, and the density of the pores decreases as the resin volume %, V r , increases.When V p is 40 vol.%,only alumina and pores exist.As V p decreases, V r increases gradually, and when V p is 0 vol.%,only alumina and resin exist with the composites.Through this, we can know that V p and V r in the composites can be controlled.One important point to be noted is alumina, pore, and resin's distribution within the composites.When we fabricate composites, we deposit same amount of alumina and resin in the positions, where every neighboring positions has the same distance, by using the DOD feature of the inkjet printing although alumina and resin ink have different pitch values.Thus, we expect the components of the composites to be distributed homogeneously.The homogeneous distribution is proved by a few theoretical calculations (See Section 4).

Analysis of Experiment Data
We fabricated alumina-resin hybrid composites with pores with a small surface roughness value.The D k and D f of composites were measured by vector network analyzer (Rohde & Schwarz, ZNLE14) at 10 GHz and room temperature (RT) with split-post dielectric resonators (SPDR) (QWED, 10 GHz SPDR).Test samples are squared shaped, and their length, width, and thickness are 2.5 cm, 2.5 cm, and 100 μm (Figure S4, Supporting Information).The error of D k was less than 0.1, and the error of D f was less than 0.001. he composites have D k values ranging from 5.3 to 4.0 by increasing V p from 0 to 35 vol.% when alumina has a V a = 60 vol.%,where V a means alumina volume % (Figure 7).The D k values of the composites are determined by the V a , V r , and V p (Figure 7a).60] In Equation 6, where V i and D i mean the volume % and the D k value of a component i, respectively.This formula assumes that the components used have a homogeneous distribution within complexes.Using D 1 = pore (air) permittivity = 1, D 2 = resin permittivity = 3, and D 3 = alumina permittivity = 10, the D k values of our alumina-resin hybrid composites were obtained.The values obtained in this way agree relatively well with the experimental data (Figure 7b).[63] The HS bound expression is as follows in Equations 7 and 8 The D k values obtained in the experiments lie in the middle between the HS upper and lower bounds (Figure 7b).These two results prove that the components in the composites we fabricate have isotropic and homogeneous distribution.The reason why the distribution is possible is because alumina and resin inks are deposited regularly during fabrication by using DOD method of an inkjet printing process.
Figure 8a shows the D f values according to V p of alumina-resin hybrid composites.The D f values tend to decrease linearly as the D k ones do as shown in the plot of D f versus D k (Figure 8b).At high frequency, the D f value of a dielectric can be inferred through a simplified model consisting of a series circuit of a resistor (R s ) and a serial capacitor (C s ) (Equation 9).Here, since C s and tan s are proportional to D k and D f , the value of D f has a linear relationship with that of D k (Figure 8b).
It is very important to investigate the amount of the pores, pore size, and shape within the alumina-resin hybrid composites for calculating closed pore volume %, V cp .Pores can have two forms, open and closed one.Open pores in the composites can deteriorate performance because moisture can be absorbed in the open pores at everyday environment.To estimate closed pore amount in the composites, we carried out N 2 adsorption-desorption experiments (Micromeritics, TristarII 3020 at 77 K with N 2 gas).The experiment data were analyzed by using Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) theory.BET analysis provides specific surface area evaluation of materials by nitrogen multilayer adsorption measured as a function of relative pressure. [64]The analysis encompasses external area and pore area evaluations to determine the total specific surface area in m 2 g −1 .BJH analysis also be employed to determine pore area and specific pore volume using adsorption and desorption techniques.This analysis characterizes pore size distribution independent of external area due to particle size of the sample. [65]igure 9 shows N 2 adsorption-desorption isotherm for V p = 10, 20, and 30 vol.% (Since there are no pores for N 2 to be adsorbed in case V p = 0 vol.%, the measurement is not carried out properly (see Figure S5, Supporting Information)).The three isotherms for V p = 0, 10, and 20 vol.% are nearly flat and the rest one for V p = 30 vol.% has small slope until P/P o = 0.95 in Figure 9 and the BET surface area in Table 1 also has small values for V p = 10, 20, and 30 vol.%, which means little adsorption of N 2 when compared to porous materials.[66][67][68] A sharp hysteresis loop appears for P/P o > 0.95, which is attributed to capillary condensation occurring in the pores.[69][70][71][72] The four isotherms show very similar behavior with nonporous materials ones.[72][73][74] Figure 9b shows BJH adsorption pore size distribution for V p = 10, 20, and 30 vol.%.BJH adsorption average pore diameter is 22.8575, 22.4344, and 24.4433 nm respectively.Open pore volume %, V op , can be calculated by using the total open pore volume ( = V t ) inside the composite and the BJH adsorption total pore volume ( = V o ), which is considered as the total open pore volume (N 2 adsorptiondesorption can measure only open pores in the sample).[53] For example, V t and V o are 0.043 and 0.00202 cm 3 g −1 for V p = 10 vol.% in Table 1.V op and V cp are calculated as follows: V op = (V o /V t ) × V p = (0.00202/0.0429)× 10 vol.% = 0.47 vol.% and V cp = (V t -V o )/V t × V p = (0.0429-0.00202)/0.0429× 10 vol% = 9.53 vol%.The values of V cp and V op for V p = 10, 20, and 30 vol% are shown in Table 1.V op is 0.47, 0.73, 4.77 vol.% for V p = 10, 20, and 30 vol.%, respectively.These small V op values indicate that most pores in our alumina-resin hybrid composites have closed form and thus the composites are strong against moisture in the everyday environment.
[77][78] Papers saying that the D f value increases as the pore ratio increases explain that the causes are local dipole generation, pore surface defects, and adsorption of foreign substances on the pore surface (in particular, most of the adsorbed material is water). [86,87]The reason why the D k and D f values increase when water is adsorbed in the pores is because moisture has very high values of D k ( = 80 at 3 GHz) and D f (= 0.157 at 3 GHz).[90] There were several research to understand the conflicting results that appear as pore amount increases.[85] Our experiment data support the research results because the components in the composites we fabricate are distributed homogeneously and most pores in them have closed form.
So far, we have discussed only the fabrication of low-loss dielectrics, alumina-resin hybrid composites with pores, and their performance of them, but the experiment about transmission loss of the composites is very interesting because high transmission loss in the SHF/EHF occurs.We performed the experiment with our composites and found very low transmission loss (Insertion loss (S 21 ): 0.072@40 GHz), which shows that our  where Y = Absolute value of the length from the centerline to the section curve of the surface log HS upper D max = −2D 3 + where f 1 , f 2, and f 3 mean the volume percent of pore (air), resin, and alumina, respectively.tan s ≈ R s C s (9)   where  means frequency.

Conclusion
We proposed a new fabrication method for low-loss hybrid composites with pores, which can be used widely in wireless communication networks.The most serious problem occurring in the super/extremely high frequency is transmission loss. [1,2][13] We succeeded in fabricating the composites with (D k , D f ) values ranging from (5.3, 0.0099) to (4.0, 0.0032) by increasing pore volume from 0% to 35% when the alumina in the composites has a 60 vol.%.We measured transmission loss after conductor was inkjet printed on the dielectric substrates we fabricated and found very low transmission loss (Insertion loss (S 21 ): 0.072@40 GHz) in comparison with previously known loss values.Fabrication of such low-loss composites was possible because we could control the number of pores in the composites and make the components of the composites distributed homogeneously and the composites have a small surface roughness value by using the DOD method of inkjet printing.We investigated pore amount and shape in the composites via N 2 adsorption-desorption experiments and found that most pores have closed form, which means that the composites are strong against moisture and thus we don't need to be concerned about deterioration of performance by moisture at everyday environment.It is expected that the composites with pores fabricated by inkjet printing will be widely used in wireless communication networks.

Figure 1 .
Figure 1.SEM images of alumina particles: a,b) Before jet milling.c,d) After jet milling.e) X-ray diffraction (XRD) patterns of before and after jet milling.

Figure 2 .
Figure 2. Dispersion stability measurement: a) Transmission change and b) backscatter change over 24 h for both cases.

Figure 4 .
Figure 4. Optical image of an alumina film a,b): the X pitches are 30 a) and 15 μm b), respectively.Alpha-step profile of an alumina layer c,d): the X pitches are 30 c) and 15 μm d), respectively.

Figure 5 .
Figure 5. Schemes of sequential process for the fabrication of alumina-resin hybrid composites.

Figure 7 .
Figure 7. Plot of D k versus V p @10 GHz, RT.The solid line is obtained by the Lichtenecker formula a) and the two solid lines mean HS bounds b).

Figure 8 .
Figure 8. a) Plot of D f versus V p @10 GHz, RT. b) Plot of D k versus D f .The solid line is a regression one.

Table 1 .
BET and BJH experimental data and calculated pore volume ones for V p = 0, 10, 20, and 30 vol%.