Development Of High Thermal Conductivity Microwave Composite Substrate And Design Of Microstrip Antenna

High impact polystyrene（HIPS）is a polystyrene-based copolymer with good impact and processing properties.It is widely used in electronic and electrical products,medical equipment and other fields.At the same time,HIPS has excellent dielectric properties and is an ideal choice for microstrip antenna dielectric materials.However,the poor thermal conductivity of HIPS resin limits its application in high-power antenna fields such as satellite communications,aerospace,and automotive radar.Therefore,it is of great significance to develop HIPS-based high thermal conductivity composites and verify the antenna application,so that it has a wider application prospect.Firstly,due to the high thermal conductivity and low dielectric loss of alumina（Al₂O₃）,it is utilized as a thermal conductive filler in this research.Alumina powder is incorporated into the HIPS resin matrix at various weight percentages of 0wt%,10wt%,30wt%,50wt%,70wt%,and80wt%through extrusion granulation to produce composite particles.Subsequently,these composite particles were hot-pressed to form composite substrates.The microstructure,density,thermal conductivity,dielectric properties,and thermal expansion coefficient of the HIPS/Al₂O₃thermal conductive composite substrate were evaluated.The influence of the alumina filler loading ratio on the composite substrate properties is also examined.The results demonstrated that the thermal conductivity of the composite substrate increased with increasing alumina filler loading ratio.At an alumina loading ratio of 80wt%,the thermal conductivity of the HIPS/Al₂O₃ composite substrate increased to 1.07 W/m·K,which is 8.2 times that of pure HIPS.Additionally,the dielectric constant of the composite substrate at 10 GHz is 4.74,and the dielectric loss is 7.21×10^-4,which is lower than that of pure HIPS.Furthermore,the thermal expansion coefficient of the composite substrate is 37.87 ppm/℃,which meant that the thermal expansion coefficient is effectively reduced compared to pure HIPS.Secondly,hexagonal boron nitride（h-BN）powder is selected to fill HIPS resin to fabricate HIPS/h-BN composite substrates in this study.By controlling the filling ratio of hexagonal boron nitride fillers with different particle sizes（5μm,25μm）,the impact of the hexagonal boron nitride particle size on the composite substrate properties is investigated,and the optimal particle size of hexagonal boron nitride for preparing thermal conductive composite substrates is determined.The results indicated that h-BN₂₅ with a larger particle size（25μm）is more advantageous for enhancing the thermal conductivity and reducing the dielectric loss of the composite substrate.By increasing the filling ratio of h-BN₂₅ to 70 wt%,the thermal conductivity of the HIPS/h-BN₂₅ microwave composite substrate in the in-plane and inter-plane directions increased to 7.43 W/m·K and 2.55 W/m·K,respectively,57 times and 20 times higher than that of pure HIPS.Additionally,the dielectric loss of the composite substrate at 10 GHz is decreased from 7.3×10^-4 for pure HIPS to 5.3×10^-4,and the thermal expansion coefficient is also reduced from 93.8 ppm/℃for pure HIPS to 18.7 ppm/℃,exhibiting significant performance improvement.Based on the aforementioned investigation,the HIPS/h-BN₂₅ composite substrate,which is fabricated using 70 wt%h-BN₂₅ ceramic powder,is chosen as the antenna substrate for the microstrip antenna centered at a frequency of 1.575 GHz.To design the GPS microstrip antenna,HFSS is employed for simulation and optimization.The optimized antenna dimensions were determined to be 46×46 mm,with a feed point position of（6 mm,0）.