Thermotransport of hydrogen and deuterium in vanadium, niobium and tantalum alloys
Heats of transport have been determined for thermotransport of hydrogen and deuterium in pure vanadium, niobium and tantalum; in vanadium alloyed with either niobium, titanium or chromium; and in niobium-tantalum alloys. In all cases, thermotransport was toward colder regions and was significantly greater for deuterium than for hydrogen. A mas spectrometric technique was used to simultaneously measure heats of transport for hydrogen and deuterium in a single specimen containing both isotopes. This technique greatly increased the precision with which isotope effects in the heat of transport could be measured. The predominant effect of alloying was to dramatically increase thermotransport; however, thermotransport decreased as niobium was added to tantalum. The heat of transport did not vary linearly with alloy composition, but rather, the changes were always greater at low concentrations of the substitutional element. Close correlations between the heat of transport and the activation energy for diffusion of hydrogen indicate that the temperature dependence of atomic jump frequencies is the predominant factor controlling thermotransport with some gradient-related bias in the direction of atomic jumps possibly caused by an electronic mechanism. In tantalum the biasing mechanism could account for up to half of the observed thermotransport; however, in most of the systems studied the biasing is quite small. Heats of transport for hydrogen in pure vanadium, niobium and tantalum are 1.4, 11.9 and 28.5 kJ/mol, respectively, and corresponding values for deuterium are 4.8, 18.0 and 34.8 kJ/mol;('1)DOE Report IS-T-939. This work was performed under Contract W-7405-eng-82 with the Department of Energy.