Thermophysical Properties of Nominally Phase Pure Boride Ceramics

Download or read book Thermophysical Properties of Nominally Phase Pure Boride Ceramics written by Austin David Stanfield and published by . This book was released on 2021 with total page 121 pages. Available in PDF, EPUB and Kindle. Book excerpt: "This research focusses on the thermophysical properties of nominally phase pure boride ceramics. As interest in ultra high temperature ceramics increases due to a renewed interest in hypersonic flight vehicles and with the expanding materials design space accompanying interest in high entropy materials, it is imperative to understand the intrinsic properties of boride ceramics. By reducing Hf content in ZrB2 from the natural abundance, ~1.75 at% in this case, to ~100 ppm, thermal conductivity increased from 88 W/m•K to 141 W/m•K. Removal of Hf allowed the thermal conductivity of ZrB2 with small transition metal solute additions to be measured without being masked by Hf impurity effects. Additions of Ti and Y reduced thermal conductivity by 20% and 30% respectively. The melting temperatures of two different types of ZrB2 were also studied. A commercially available grade of ZrB2 (~1.75 at% Hf) had a melting temperature of 3280°C while a low Hf (100 ppm) ZrB2 had a melting temperature of 3273°C. The kinetics of the final stage of densification was also studied for nominally phase pure ZrB2. Dislocation motion with an activation energy of 162 kJ/mol was determined to be the dominant mechanism in the absence of competing mechanisms such as grain pinning or solute drag caused by secondary phases and impurity soute atoms. The effect of configurational entropy on the solubility of yttrium in high entropy borides was investigated. No significant difference in yttrium solubility was found between nominally pure ZrB2 and a four component high entropy boride (Ti,Zr,Nb,Hf)B2. Mitigation of impurity atoms and secondary phases minimized extrinsic effect and elucidated intrinsic properties of boride ceramics"--Abstract, page iv.