Investigation on the improvement of microwave dielectric properties of plasma-prepared fluoropolymer-coated CaTiO3-filled PTFE composites

High-performance polymer matrix composites preparation has been urgently hampered by poor interfacial adhesion between two phases. In this article, a thorough analysis of the microstructure and overall properties of composites made of polytetrafluoroethylene filled with fluorocarbon polymer film-coated CaTiO3 was conducted. The similar fluorocarbon molecular structure of CaTiO3 surface and PTFE provides a bridge between the two, significantly improving and promoting the microstructure and interface adhesion of the composite. The comprehensive properties of modified composites are obviously superior to those of untreated composites. Composites filled with 30 wt% CaTiO3 have an excellent dielectric constant at 5.53, acceptable dielectric loss at 0.0047, an outstanding heat resistance index of 281.1°C, and relatively low hygroscopicity of 0.10%. At the same time, the mechanical properties are maintained at a high application level, which makes them promising for microwave devices.


Introduction
Nowadays, in the high-speed development of the information age, electronic devices dominated by 5G communication, satellite navigation, automotive radar, etc., show the development trend of highfrequency and high-speed information processing, high integration, and high function; the demand for microwave dielectric materials is increasingly urgent [1] .Polymer/ceramic microwave dielectric composites show promise because they combine the benefits of ceramic and polymer materials, have great dielectric tunability, are easy to process, and are more suitable for industrial production [2] .Polytetrafluoroethylene (PTFE) has a significant advantage over other polymers in microwave dielectric materials due to its extremely low dielectric loss, stable dielectric constant, strong chemical stability, excellent thermal stability, and mechanical properties [3,4] .However, a major challenge in ceramic-filled composites is the incompatible interface between the organic polymer and the inorganic ceramic filler, where the considerable disparity in surface characteristics between the two causes not only uneven filler dispersion but also the formation of pores at the interface of the distinct phases, deteriorating the properties of the composites.
The current effective method is to functionalize the surface of the ceramic filler, and many scholars have tried numerous surface functionalization reagents for PTFE-based composites.It has been proved that surface treatment with C 14 H 19 F 13 O 3 Si (F8261) can significantly improve the surface properties of ceramics, making them exhibit strong hydrophobicity.The modified ceramic surface, because the molecular structure (carbon-fluorine bonding) is similar to that of PTFE, can be a better bridge to establish with the PTFE and promote the formation of a compact structure [5][6][7] .However, these treatment methods are mostly chemical methods, which have the defects of complicated operation, harsh reaction conditions, low efficiency, and producing a large amount of waste organic solvents.On the contrary, low-temperature plasma surface treatment technology has a short action time, simple and controllable operation, and high efficiency.It only occurs in the surface layer, has almost no effect on the performance of the body, and belongs to the dry treatment, with no pollution.Thus, it shows the advantages and potentials that traditional methods do not have.However, plasma technology is currently largely applied in other fields, such as polymer surface treatment, and the surface modification of micro-nano ceramic particles is less studied [8][9][10] .To this end, in earlier work, we fabricated a low surface energy fluorocarbon polymer-coated core-shell CaTiO 3 using CH 4 /CF 4 plasma.It is expected to be used for PTFE composites to strengthen the interface bonding between the two phases and improve the comprehensive properties of the composites.
This study aims to improve the microstructure and balance the overall performance of composites by resolving the issue of poor interfacial compatibility of polymer-based microwave dielectric materials in an efficient and environmentally friendly method.Fluorocarbon polymer-coated coreshell CaTiO 3 was used as filler to fabricate the PTFE-based microwave dielectric composites through cold pressing and sintering.The microstructure and dielectric characteristics of the composite were examined in relation to filler content and surface modification.Moreover, the mechanical, hygroscopic, and thermal characteristics of the composites were studied from an application standpoint.The composite has the benefits of being easy to prepare and ecologically friendly, exhibiting excellent properties, and having a high application value in the field of microwave media.

Sample preparation
PTFE/CaTiO 3 composites were prepared by cold press molding and hot press sintering.CaTiO 3 powder (Shanghai Dianyang Industrial Co., Ltd, China) was pre-treated by a homemade microwave plasma device using CH 4 /CF 4 gas to deposit a fluorocarbon polymer film on its surface to enhance its interaction with PTFE.Firstly, different contents (5 wt.%-50 wt.%) of CaTiO 3 were mixed with PTFE emulsion (60 wt.%, DISP30, Shanghai JinFu Chemical Technology Co., Ltd, China) while adding the appropriate amount of emulsion breaker in a high-speed disperser (JFS-750, Hangzhou Qi Wei Instrument Co., Ltd, China) for 1 h.The resulting mixture was then dried for 12 hours at 120°C in a vacuum oven and further cold-pressed into the required size and thickness.Finally, hot press sintering was carried out at 370°C and 10 MPa for 2 h using hot press equipment (SY-6210B, Dongguan ShiYan Precision Instrument Co., Ltd, China) to obtain a series of samples required for the test.

Characterization
The cross-section morphology of the CaTiO 3 /PTFE composite was observed under a scanning electron microscope (SEM, FEI Quanta 600FEG).The vector network analyzer (VAN, Agilent Vector X) was used to test the composites' microwave dielectric characteristics according to the rectangular waveguide method.TA Instruments Q50 was employed for thermogravimetric analysis (TG).TA Instruments Q100 was used for differential scanning calorimetry (DSC).To erase thermal history, the temperature was first raised quickly to 370°C and isothermalised for 3 minutes.It is followed by cooling to 40°C with a rate of 10°C/min to obtain a crystallization curve.Finally, the melting curve was obtained by heating to 370°C with rate of 10°C/min.The density and porosity of the composites were qualified using Archimedes' principle, and the hygroscopicity of the composites was tested according to IPC-TM-650 2.6.2.Bending tests were carried out using a universal materials machine (CMT4304) at room temperature, according to IPC-TM-650 2.4.4B.  1 (a).Ca elemental scans of CaTiO 3 /PTFE composites before and after modification at 5 wt % filler content were also performed to better analyze the variations in filler distribution.Results show that the modified CaTiO 3 has better connectivity with PTFE and a more uniform distribution of ceramic particles in the matrix.On the contrary, it was found in Figures 1 (a) and 1 (h) that the particles were obviously agglomerated, showing a porous microstructure.This is due to the surface energy difference between PTFE and CaTiO 3 which makes the two poor compatibility.After fluoride modification in the CaTiO 3 surface coating with a similar fluorocarbon structure with PTFE, a polymer layer can be established between the two "bridges", significantly improving and promoting the composite microstructure and interfacial adhesion.As the CaTiO 3 content increases, it can be observed that the ceramic particles start to become dense, and some agglomeration occurs, leading to the creation of voids between the separated particles.At the same time, the low mobility of PTFE also promotes the formation of pores [11] .In short, the modified CaTiO 3 /PTFE composite has a relatively complete and compact internal structure, which will be conducive to improving the comprehensive properties of the composite.The increase in dielectric constant is primarily due to the fact that CaTiO 3 has a higher dielectric constant than PTFE.The dielectric loss depends not just on the dielectric properties of each component of the composites.Furthermore, since the dielectric relaxation at such high frequencies is negligible or nonexistent, the effect of defects in the composites on the dielectric loss is very significant.It is noteworthy that the dielectric constants of modified CaTiO 3 /PTFE composites are significantly higher than those of unmodified samples.In contrast, the dielectric loss exhibits the opposite trend, especially at high filler contents where the difference between the two is more pronounced.The optimization of the dielectric properties of the composite is mainly attributed to the improved interface connection between CaTiO 3 and PTFE after plasma fluorination and the compact microstructure of the composite.When the CaTiO 3 content is 30%, CaTiO 3 /PTFE composites reach a dielectric constant of 5.53, with a dielectric loss of less than 6×10 -3 , which is kept at 0.0047.The results show that the modified CaTiO 3 /PTFE composites have excellent dielectric properties, which can be used for high-frequency microwave substrates and electronic packaging materials.

Thermal properties
Resonance-type relaxation polarization determines the dielectric characteristics of composites at high frequencies, such as electron cloud distortions, atomic polarisations, small changes in bond lengths and angles, or small vibrations of the lattice, whereas the crystal morphology has an important influence on the resonance-type relaxation [12,13] .As shown in Figure 3, the crystallization behavior of CaTiO 3 /PTFE composites with different CaTiO 3 content was studied via DSC, and the statistical data on melting and crystallization were obtained.Where the degree of crystallinity (χ c ) of the PTFE composite can be calculated by: Where ΔH m Is the experimentally measured enthalpy of melting, ΔH m 0 Is the enthalpy of melting for 100% crystallization of PTFE (69 J/g) [14] , and α is the mass fraction of PTFE.It can be seen that the melting points (T m ) of all the composites do not differ much from each other and are similar to the stable crystalline melting point of neat PTFE, implying that the catio 3 content has little influence on the melting point of catio 3 /PTFE composites.The crystallization peak temperature ( c ) of the composites shifted obviously towards high temperature with the increase of catio 3 content.For instance, the   of neat PTFE is 309.7°C.When the catio 3 content is 50 wt%, the  c of the composite increases by 5.3°C and reaches 315°C.This suggests that the addition of catio 3 promotes the crystallization of PTFE and plays the role of heterogeneous nucleation.However, the crystallinity showed a different trend.The crystallinity of catio 3 /PTFE composites remains around 48% at low filler content and decreases obviously when it exceeds 10 wt.%.This is because catio 3 plays the role of heterogeneous nucleation at low content.At the same time, the excessive addition of catio 3 will reduce the relative content of PTFE components, and it will impede the movement of molecular chains, which will make grain growth difficult [15] .The interaction of these factors makes the crystallinity of catio 3 /PTFE composites exhibits the above trend.It is noteworthy that the dielectric constant of the catio 3 /PTFE composite rises as the catio 3 content does, indicating that the filler content is more important in improving the composite's dielectric properties than crystallinity in the high-filling system.Thermal stability is a key aspect influencing the application of composites.Figure 4 (a) shows the thermal degradation behavior of CaTiO 3 /PTFE composites for different content of CaTiO 3 .It can be seen that the curve trend of all samples is the same, indicating that the addition of CaTiO 3 to the PTFE matrix did not affect the decomposition mechanism of PTFE itself.The residual mass of the sample is basically the same as the content of CaTiO 3 filler, indicating that CaTiO 3 is uniformly dispersed in PTFE.As demonstrated in Figure 4 (b), the thermal stability of the CaTiO 3 /PTFE composite was assessed using the Heat resistance index (THRI) [16] .The steady increase in the thermal degradation temperature of CaTiO 3 /PTFE composites with increasing CaTiO 3 content shows that CaTiO 3 improves composites' thermal stability.This is attributed to the fact that CaTiO 3 has a higher specific heat capacity compared to PTFE, which can absorb heat better, causing the PTFE molecular chains to degrade at higher temperatures.When filled with 30 wt.% CaTiO 3 , the decomposition temperatures of the composites reached 551℃ and 588.7 ℃ at weight loss of 5 wt.% and 30 wt.%, respectively, with a

Moisture absorption properties
Moisture absorption properties are critical to achieving stable and reliable applications of composites in a variety of environments.As water has a high loss, the dielectric properties deteriorate when water absorption increases.A composite's water absorption is primarily influenced by the water absorption characteristics of the component parts and the structural porosity.Figure 5 demonstrates the change of CaTiO 3 /PTFE composite density, porosity, and water absorption with ceramic content.The theoretical density of CaTiO 3 /PTFE composites can be computed using the mixing law : Where ρ n , V n , and m n denote the density, volume fraction, and weight of each component in the composite, respectively.The volume percentage of the interior pores determines the porosity of the composite: Where  and ρ theory stand for the experimental and theoretical densities, respectively.As seen in Figure 5, the density of the composites increases essentially linearly as the filler amount increases.This is attributed to the fact that CaTiO 3 ceramics have a higher density than PTFE.In addition, especially at lower ceramic fillings, the experimental data are in good agreement with the theoretical densities.However, the experimental densities of CaTiO 3 /PTFE composites at high filler contents show an increased deviation from the theoretical values.The reason for this is that increasing the filler content causes agglomeration of CaTiO 3 fillers in the matrix and an increase in pores.The existence of pores not only reduces the density of CaTiO 3 /PTFE composites but also has a negative impact on the composites' water absorption.In Figure 5, it is evident that the trends of water absorption and porosity of the composites are similar and that both rise with an increase in filler content.In particular, at high filler contents, the ceramic filler agglomeration is intensified, which further increases the porosity.As a result, when the CaTiO 3 ceramic filler content is low (5 wt%), the porosity and water absorption are low as well, at 0.11% and 0.02%, respectively.As the CaTiO 3 concentration is 30 wt.%, the CaTiO 3 /PTFE composite has porosity and water absorption of 0.86% and 0.10%, respectively, meeting the requirements for engineering applications.The porosity of the composite rapidly increases as the filler content is raised to 50 wt.%.The water absorption of the composite still remained within an acceptable range, reaching 0.18%, due to the enhanced interfacial compatibility of the modified composites.

Bending strength
Mechanical properties are also an important indicator of the practical value of composites.Figure 6 shows the typical force-displacement curves of CaTiO 3 /PTFE composites with CaTiO 3 content.The corresponding bending strength and maximum bending force data are listed in Table 1.It is clear that the bending strength of the composite steadily decreases as the CaTiO 3 content increases.As the filler content increased from 5 wt.% to 50 wt.%, the bending strength decreased by only 22% from 18.98 MPa to 14.80 MPa.The interface between the two phases has a strong influence on the mechanical properties of the composite.In general, the large surface difference between the ceramic filler and the substrate tends to form a large number of interfacial defects and pores in the matrix.Thus, it weakens the interaction between the filler and matrix, resulting in a decrease in the composite's mechanical characteristics.Surface fluorination modification, which effectively promotes the bridging between filler and PTFE, improves the interfacial bonding state to make the composite material structure relatively dense and lowers the composite's mechanical properties loss.

Conclusion
In this paper, microwave dielectric composites were prepared using fluorocarbon polymer-coated CaTiO 3-filled PTFE.The study systematically examined how the filler content affected the composites' microstructure, dielectric, thermal stability, hygroscopic, and mechanical properties.The results show that the introduction of PTFE-like fluorocarbon molecular structure on the surface of the filler improves the compatibility between the filler and PTFE, which makes the composites exhibit excellent dielectric properties, good heat resistance, very low hygroscopicity, and excellent mechanical properties.In contrast to the unmodified composites, the 30 wt% filled modified CaTiO 3 /PTFE composites showed a 10% increase in dielectric constant to 5.53 and a 27% reduction in dielectric loss to 0.0047.In parallel, the composites' heat resistance dramatically increased, with a heat resistance index of 281.1°C, a water absorption rate of only 0.10%, and a bending strength maintained at 15.5 MPa.Hence, the obtained modified CaTiO 3 /PTFE composites have excellent potential for use as highfrequency microwave substrates and electronic packaging materials.
SEM was used to examine the micromorphology of CaTiO 3 /PTFE composites with different modified CaTiO 3 content, as shown in Figure1.As a comparison, SEM of unmodified CaTiO 3 /PTFE composites at 5 wt % content is shown in Figure

Figure 2 .
Figure 2. Variation of the dielectric characteristics of modified and unmodified CaTiO 3 /PTFE composites with CaTiO 3 content.