Antennas are used to transmit communication signals for many applications including for the navigation of aircraft. To protect the antennas from environmental conditions electromagnetic transparent structures called radomes are used. Advancements in technology have led to the development of hypersonic flight vehicles. These aircraft travel at speeds of Mach 5 and greater subjecting them to extreme environmental conditions. These aircraft require precise navigation making it important to have radome materials that can withstand the extreme conditions of high-speed flight while maintaining transparency to the incoming and outgoing signals of the antenna. Silicon nitride is a ceramic material of interest for high temperature radomes due to its mechanical properties, temperature stability, and satisfactory dielectric properties. Incorporating porosity into silicon nitride further enhances the transmission performance making porous silicon nitride a leading candidate material for high temperature radomes. In this dissertation slip casting with pressureless sintering is proposed as a route to fabricate porous silicon nitride ceramics for radomes. Modification of sintering aids and sintering temperatures are explored as a method to control the amount of porosity. Mechanical properties and dielectric properties of these materials are investigated.

 

An optimized suspension for slip casting was developed by investigating the rheological properties, zeta potential, and sedimentation behavior. The optimized suspension was used to fabricate silicon nitride samples with yttria and alumina sintering aids with an average density of 93% with a strength of 659 MPa. Dielectric constant and loss tangent were measured on samples with 4-17% porosity ranging from 5.99-7.61 and <0.03, respectively. To fabricate samples with higher levels of porosity and lower dielectric constants, slip cast samples with 5-15 wt% Yb2O3 were sintered at temperatures of 1650-1850°C resulting in densities of 62-79% and strengths of 267-445 MPa. The dielectric constants of these materials were measured ranging from 4.13-5.80 with loss tangents <0.01. The processing method presented in this dissertation shows potential for producing porous silicon nitride for high temperature radome applications with controlled porosity and relatively high strengths as well as the ability to create radome shapes and layered structures.