<p dir="ltr">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. </p><p dir="ltr">First, an aqueous silicon nitride suspension developed for slip casting was optimized by investigating the rheological properties, zeta potential, and sedimentation behavior. It was determined that a suspension with 30 vol% solids, 0.5 wt% dispersant (PEI), and a pH of 7 was the optimized condition that resulted in uniform cast parts. This optimized suspension was used to fabricate silicon nitride samples with yttria and alumina sintering aids. An average density of 93% with an average strength of 659 MPa at room temperature and a strength of 472 MPa maintained up to 1200°C was achieved. Dielectric constant and loss tangent were measured on samples with 4-17% porosity to be 5.85-7.70 and <0.02, respectively. </p><p dir="ltr">To create samples with higher levels of porosity and therefore lower dielectric constants the yttria and alumina sintering aids were replaced with ytterbium oxide. Ytterbium oxide assists in forming porous silicon nitride due to the high melting temperature and high viscosity of the resulting glassy phase. Slip cast samples with 5% Yb₂O₃ were sintered at temperatures of 1700-1850°C resulting in porosities of 21-32% and strengths of 267-445 MPa. The dielectric constants of these materials were measured to be 4.56-5.80 with average loss tangents <0.006. The amount of ytterbium oxide was also studied to determine the effects on density, microstructure, mechanical properties, and dielectric properties. Slip-cast samples with 5-15% Yb₂O₃ were made having average porosities of 23-36% and strengths of 275-421 MPa. The dielectric constants of these materials were measured to be 4.13-4.65 with average loss tangents of <0.007. </p><p dir="ltr">Lastly, slip casting using the previously developed and evaluated suspensions was done to fabricate various radome shapes as well as layered structures. The processing method presented in this dissertation shows the potential for fabricating porous silicon nitride for high temperature radome applications with controlled porosity and relatively high strengths.</p>

Ytterbia (Yb2O3) is an oxide ceramic, whose magnetic properties and crystal structure were studied to some extent in the past. However, the information on Yb2O3's catalytic properties is lacking. Therefore, in this work, the sintering behavior and catalytic properties of Yb2O3 were examined. Yb2O3 ceramic samples were made using pressureless sintering of the commercially available Yb2O3 with 99.99% purity powder. The powder was first uniaxially pressed at 20 MPa in a steel die followed by pressureless sintering at different temperatures of 900 ⁰C to 1600 ⁰C for 4 hours. The densities of these samples were measured, and the sample sintered at 1600 ⁰C had the largest density of 62.88% and 900 ⁰C samples had the lowest of 45%. To improve the density of samples, three samples were uniaxially pressed same as the previous samples; however, they were also cold isostatically pressed at 260 MPa. These samples yielded the highest density of 90% after sintering at 1700 ⁰C for 4 hours. Raman spectra of sintered Yb2O3 were collected, which corresponded very well to those published in the literature. The microstructure of sintered Yb2O3 was studied by scanning electron microscopy. X-ray diffraction (XRD) showed that Yb2O3 crystallize in an Ia cubic structure. X-ray fluorescence (XRF) was performed and it confirmed high counts of Yb and high purity of Yb2O3 powder. The catalytic experiments were performed on highly porous Yb2O3 samples sintered at 900 ⁰C. In the first experiment, Syn gas (H2 + CO) was passed over Yb2O3 at 30 psig at 400 ⁰C with a rate of 3 cc/m. In the second experiment, the temperature was lowered to 250 ⁰C with all other conditions of the experiment maintained the same. As a result of the first reaction, the products included methane, ethane, and ethylene. The products produced in the second trial were ethane, ethylene, propene, butane, butene, cyclobutene, and methyl alcohol.