Mapping the Creation of Nanoporous Ultra-High-Temperature Ceramics

Nanoporous materials have found use in a broad variety of applications ranging from sensing and actuation to catalysis and coatings. In the coatings space, nanoporous materials have generally been used as templates for thin networks, where the template is later dissolved away. However, nanoporous materials could themselves be used as part of a high-temperature coating, though the ligament coarsening rate must be controlled to retain integrity of the nanoporous structure at temperature. While nanoporous metals undergo curvature-accelerated surface smoothening, which leads to thickening of ligaments, increasing pore size, and decreasing surface-to-volume ratio, it has been proposed that this morphology evolution may be frozen by creating a nanoporous carbide/nitride ceramic due to the strong covalent bonding limiting surface mobility. The present work examines the conversion of nanoporous refractory metals to nanoporous ultra-high-temperature ceramics <i>via</i> gas-phase conversion at elevated temperature. Nanoporous Ta precursors were exposed to carburizing (methane-containing) and nitriding (ammonia-containing) environments over a range of temperatures for different lengths of time to examine the interplay between ceramic conversion and ligament coarsening rates. Kinetic models were developed for both degree of ceramic conversion and degree of coarsening to quantify the process for creation of a nanoporous ultra-high-temperature ceramics.