Tuning the dielectric properties of porous BN/Si3N4 composites by controlling porosity

To solve the shortcomings of traditional layered structure for broadband wave-transmitting applications, a novel porosity gradient structure is proposed. The precise turning of the relative permittivity of each layer and their simultaneous shrinkage during sintering are the keys to design and fabrication. Porous BN/Si3N4 composites with different porosities were prepared by incorporation of pore former, tape casting and gas pressure sintering. Results show that the obtained composites with different porosities exhibit low and similar shrinkage after sintering at 1900 degrees C, which is mainly due to the bridging effect between rapidly growing beta-Si3N4 rod. When the porosity increases from 33.2% to 59.4%, the relative permittivity decreases from 4.00 to 2.38. Moreover, the composites with different porosities have excellent mechanical properties, with bending strength and fracture toughness of 180 +/- 10-36 +/- 4 MPa and 3.2 +/- 0.30-0.85 +/- 0.10 MPa.m(-1/2), respectively, which is attributed to the bridging and pull-out strengthening effects of the elongated beta-Si3N4 grains.