Niobium carbide as a technology demonstrator of ultra-high temperature ceramics for fully ceramic microencapsulated fuels

Abstract

The consolidation of Fully Ceramic Microencapsulated (FCM) fuels has been extended from matrices based on SiC to ultra-high temperature ceramics (UHTCs). Specifically, sintering conditions of NbC1-x were compatible with hosting of microencapsulated fuel. NbC powder in as-received, chemically treated, and composite forms was consolidated. Elemental analysis, shrinkage of powder compacts, contents of ejected vapor, density, microstructure, and NbC lattice constants were analyzed. As-received NbC showed more shrinkage due to the presence of a liquid phase compared with the chemically treated powder. Removal of impurity metals was observed from chemical treatment and during sintering of as-received powders. An increase in true density during sintering was attributed to removal of compounds with lower density than NbC. Chemically treated powder showed reduced densification rate and absence of a liquid phase after sintering. Smaller grain sizes were observed in the NbC composite. The implications for NbC and other binary carbides as matrices for FCM fuels are discussed.