Preparation And Dielectric Properties Study Of Porous BNNTs-SiO₂-Si₃N₄Composites

The antenna housing has been applied under the complex environment, as an important part of signal transmission of the spacecraft. It is necessary to possess excellent resistance syndrome, mechanical properties, and dielectric properties. Silicon nitride ceramics get a lot of attention due to the enhancement of its reliability and properties, with the progress of S13N4powder produce, molding, sintering, and machining technology. Porous silicon nitride ceramics possess excellent mechanical properties, chemical stabilities, high temperature resistance and dielectric properties, and will be widely applied in wave-transmitting materials field.The paper was based on excellent mechanical and dielectric performance of SisN4, BN and SiO2. Porous BNNTs-SiO2-Si3N4composite were fabricated by pressureless-sintering, where BNNTs was used as wild phase, Al2O3-Y2O3as sintering additives, and stearic acid or diatomite were used as pore-making agent. Flexure strength at room and high temperature, fracture toughness, microstructure, thermal properties and dielectric properties were investigated with the sintering temperature, holding time, content of stearic acid or diatomite, content of BNNTs. The main contents are summarized as follows:(1) Porous BN-SiO2-Si3N4composite were fabricated by pressureless-sintering at1500℃holding1h, diatomite can make good effect on pore-forming. Porous S13N4composites containing with10%diatomite possessed highest porosity of50.7%, increased about25%relative to Si3N4composite without diatomite, and exhibited bending strength and fracture toughness of78.04MPa and1.25MPa-m1/2. The dielectric properties of Si3N4composites were enhanced, dielectric constant and dielectric loss were decreased to be3and3×10-2. It was discovered that the microstructure of BN-SiO2-Si3N4composites was good, the pores were distributed uniformly, and distribution of pore size was mainly about on0.3μm.(2) The mechanical properties and high temperature resistant bending strength of BNNTS-S13N4composite were improved with the addition of BNNTs, but thermal shock resistance was bad. When the composite containing1%BNNTs were sintered at1550℃displayed highest bending strength of276.03MPa (increased about25%), and it possessed fracture toughness of3.29MPa-m1/2, porosity of36.7%, dielectric constant of about4.4, dielectric loss of2-9×10-2.(3) The mechanical and dielectric properties of BNNTs-SiO2-Si3N4composite were enhanced with5%diatomite as pore-forming agent and BNNTs as toughening phase, and porosity was increased at the same time. When1%BNNTs and5%diatomite were added to matrix together, bending strength and fracture toughness of BNNTs-SiO2-Si3N4composite sintered at1500℃holding2h reached166.99MPa and3.54MPa-m1/2,40%and50%increase compared to the composite without BNNTs (120.79MPa and2.51MPa-m1/2). The dielectric constant and dielectric loss can decreased to be3.6-3.9, and5-10×10-2.In this paper, BNNTs-SiO2-Si3N4composite possessing excellent comprehensive properties were prepared with diatomite as pore-forming agent and BNNTs as toughness phase. It is significant that the requirement of high performance of wave-transmitting materials was enhanced, and that the diatomite and BNNTs applied to composite ceramics were explored.