UC San Diego

Perovskite materials, specifically barium titanate (BT) and strontium titanate (ST), hold significant potential for a range of applications, including ferroelectric devices and multilayer capacitors. The particle morphology of these materials can significantly influence their electronic properties, such as their dielectric constant and ferroelectric behavior. Our study aims to understand the formation mechanisms of faceted particles of BT and ST, synthesized using both molten hydrothermal and aqueous hydrothermal methods. Our experimental results obtained through scanning electron microscopy (SEM) reveal successful development of faceted particles for BT using the molten hydrothermal method and for ST using the aqueous hydrothermal method. However, X-ray diffraction results indicate the presence of unreacted SrCO3 and an amorphous structure in the ST sample synthesized by the molten hydrothermal method, pointing towards an in-situ growth mechanism of ST. Transmission electron microscopy (TEM) results suggest that the formation of faceted particles occurs through the aggregation of smaller crystallites in identical crystallographic directions. This research provides valuable insights into the particle morphology control of perovskite materials through different synthesis methods, shedding light on their potential in advanced electronic applications.Tantalum carbide (TaC) and hafnium carbide (HfC) have some of the highest melting temperatures among the transition metal carbides, borides, and nitrides, making them promising materials for high-speed flight and high temperature structural applications. Solid solutions of TaC and HfC are of particular interest due to their enhanced oxidation resistance compared to pure TaC or HfC. This study looks at the effect of Hf content on the oxidation resistance of TaC- HfC sintered specimens. Five compositions (100 vol.% TaC, 80 vol.% TaC + 20 vol.% HfC, 50 vol.% TaC + 50 vol.% HfC, 20 vol.% TaC + 80 vol.% HfC, and 100 vol.% HfC) were fabricated into bulk samples using spark plasma sintering (2173 K, 50 MPa, 10 min hold). Oxidation behavior of a subset of the compositions (100 vol.% TaC, 80 vol.% TaC + 20 vol.% HfC, and 50 vol.% TaC + 50 vol.% HfC) was analyzed using an oxyacetylene torch for 60 s. The TaC-HfC samples exhibited a reduction in the oxide scale thickness and the mass ablation rate with increasing HfC content. The improved oxidation resistance can be attributed to the formation of a Hf6Ta2O17 phase. This phase enhances oxidation resistance by reducing oxygen diffusion and serving as a protective layer for the unoxidized material. The superior oxidation resistance of TaC-HfC samples makes these materials strong contenders for the development of high-speed flight coatings.