Abstract:
Niobia was reduced in 4% H2-96% N2 to form various Magneli shear structured phases with composition NbO2.5-_. Phases were confirmed using powder X-ray diffraction and imaged using SEM which, combined with EDS, revealed phase separation in the compacts. Dopants of Fe2O3 and Cr2O3 were added in 2 and 4 atomic percent amounts. Properties, such as electrical conductivity, Seebeck coefficient, and thermal conductivity, of various structure types and dopant levels were measured from 250-950_C in an inert atmosphere. The effect of dopant level, sintering time, and annealing on electrical conductivity and Seebeck coefficient were studied. It was found that Nb12O29 can be formed over a range of temperatures by sintering in 4% H2 but that refinement is necessary to produce phase purity. Control of sintering parameters was found to be important in controlling the phase identity and purity formed, the grain size, and surface reduction. Based on electrical conductivity data a hypothesis is put forward; that oxygen vacancies controlled conduction at low temperature and chromium substitutions on niobium sites controlled at higher temperatures, though more study is needed to confirm this. Lastly, phase stability of these structures was demonstrated up to 1000_C in an inert atmosphere demonstrating that these materials are viable for thermoelectric applications.
