Structure, Conductivity, and Thermal Expansion Studies of Redox Stable Rutile Niobium Chromium Titanates in Oxidizing and Reducing Conditions

摘要

Materials from the solid solution with general formula Ti_(1-2x)Cr_x-Nb_xO2 (0 20 S cm~(-1) in 5% H2/Ar. The lowest conductivity was observed for samples low in titanium content, indicating that the reduction of Ti~(4+) to Ti~(3+) is primarily responsible for the high electronic conductivity in these materials. All samples showed very slow reduction kinetics, and true equilibrium was not achieved even after > 50 h in flowing 5% H2/Ar at 900 °C, suggesting that the equilibrium conductivity of these materials at 10~(-20) atm may be significantly higher. Conductivity studies as a function of p(O2) showed complex redox behavior with indications of a number of step transitions and very slow redox kinetics. The thermal expansion coefficient was in the range of 5.6 x 10~(-6) — 8.6 x 10~(-6) K~(-1), which changed little between reduced and fully oxidized samples.