Influence of gas atmospheres on the first-stage sintering of high-purity niobium powders

Niobium and tantalum surfaces easily absorb oxygen. With decreasing particle size, the content of oxygen increases. The role of this surface oxygen and oxygen in the sintering atmospheres on the first-stage sintering is not well established. Therefore the sintering behavior of high-purity niobium powders was studied by annealing cylindrical powder compacts (particle size less than 63 microns) in the temperature range from 1000 C to 1600 C in ultra-high vacuum and under low oxygen partial pressures, as well as in inert gas atmospheres with low oxygen contents. The specific surface of the samples was determined by metallographic methods, adsorption, and capacitance measurements. Low oxygen partial pressures (0.0001 Pa) lead to a slight enhancement of the surface diffusion which is controlling first-stage sintering. High heating rates (T greater than 3000 K/min) to temperatures above the melting point of Nb2O5 (Tm 1495 C) enhances the neck growth due to the formation of a liquid oxide phase on the surface of the powder particles. 23 references