Preparation Of H₃BO₃-based Composites For Dielectric Substrates And Study Of Dielectric Properties

With the rapid development of 5G communication technology,in order to meet the high requirements of high frequency,high efficiency and good thermal stability of microwave communication devices for electronic packaging,it is necessary to continue to explore and develop a series of microwave dielectric materials with inherent sintering temperature,low dielectric constant（ε_r）,high quality factor（Q=1/tanδ）and near zero temperature coefficient of resonant frequency（τ_f）.Currently,the research on dielectric properties of dielectric substrates for 5G focuses on how to reduce theε_rand dielectric loss（tanδ）of the matrix material.Among them,boric acid（H₃BO₃）is of great interest because of its relatively lowε_r（≈2.84）and high Q value（≈8,000 GHz）when sintered into porcelain at ultra-low temperature.If this problem can be effectively solved by doping and compounding,the boric acid matrix material will become a key material for new dielectric substrates that can meet all the high performance requirements at the same time.Therefore,in this paper,low-loss polymers polyethylene（PE）,polytetrafluoroethylene（PTFE）,polyethylene terephthalate（PET）,soluble polytetrafluoroethylene（PFA）and H₃BO₃matrix were selected to form a polymer-ceramic composite system,aiming to obtain a lowε_r（<3）,tanδ（<0.004）andτ_f（≈0 ppm/°C）while improving the moisture resistance properties of boric acid composite ceramics.The main research contents and results are as follows:1.Study on（1-x）H₃BO₃-x PE（x=0.0～0.5）system:through XRD and Raman analysis shows that pure H₃BO₃ceramics can be densely formed into ceramics at near room temperature and its main phase is triclinic phase,while the polymer-ceramic system shows the coexistence of H₃BO₃and PE phases at different content ratios;combined with SEM and EDS analysis,it was found that the pores of the system slightly increased with the increase of PE doping,but PE gradually and uniformly adhered to the surface of the H₃BO₃matrix.As x increases,ε_rdecreases from 2.83 to 2.14,Q×fdecreases from 116,000 GHz to37,000 GHz,andτ_fincreases from-147 ppm/°C to+27 ppm/°C.Therein,when x=0.3,the better microwave dielectric properties of the component systems are obtained at 100°C:ε_r≈2.38,Q×f≈46,000 GHz（15.7 GHz）andτ_f≈+9 ppm/°C.In addition,by selecting H₃BO₃,0.7H₃BO₃-0.3PE,and 0.6H₃BO₃-0.4PE samples for hydrolysis experiments,it can be seen that the pure H₃BO₃ceramics showed the phenomenon of surface powder layer detachment after hydrolysis,while the x=0.4 samples were intact and the Q×fvalue could still be maintained around 40,000 GHz,which shows that PE addition can improve the deliquescence resistance of H₃BO₃materials to a certain extent.2.Study on（1-x）H₃BO₃-x PTFE/x PET/x PFA（x=0.0～0.5）system:combined with XRD and Raman test results,it can be seen that the three different polymer addition systems are H₃BO₃and PTFE/PET/PFA phase coexistence state;by using geometric method to analyze the bulk density of different polymer-ceramic systems,it is known that（1-x）H₃BO₃-x PTFE polymer-ceramic bulk density increases with the increase of x value,but the bulk density of（1-x）H₃BO₃-x PET and（1-x）H₃BO₃-x PFA decreased accordingly;according to the SEM observation results,the higher the PTFE doping content,the greater the（1-x）H₃BO₃-x PTFE polymer-ceramic pores.While the pores of the PET/PFA-added samples increased,which was due to the difference in the bulk density of the three polymers.For microwave dielectric properties,combined with the analysis results of microstructure characteristics,the component 0.5H₃BO₃-0.5PET sintered at 90°C for 1h has a low dielectric constant（ε_r≈2.75）,a high quality factor（Q×f≈5,238 GHz）and a near-zero temperature coefficient of resonant frequency（τ_f≈+4 ppm/°C）as well as strong deliquescence resistance.3.Based on the H₃BO₃-PE composite material under the component of strong resistance to deliquescence,a band-pass substrate integrated waveguide filter（center frequency f₀≈16.3 GHz）is designed:by constructing a filter model that can meet the performance requirements,combined with the TE excitation measured（1-x）H₃BO₃-x PE（x=0.4）composite substrate electromagnetic wave flow direction,using three-dimensional electromagnetic simulation,the structural changes and electromagnetic field distribution of this type of substrate-integrated waveguide filter under the conditions of high frequency and high power are obtained.The simulation results show that the filter composite substrate has good microwave dielectric properties,mechanical properties and strong anti-interference ability,which verifies the feasibility of the composite material as a high-performance dielectric substrate.