High-temperature thermodynamics of the solid solutions of hydrogen in bcc vanadium, niobium, and tantalum

Abstract

The high-temperature thermodynamic properties of V+H2 and Nb+H2 solid solutions have been studied by a combined calorimetric and equilibrium method at 264, 300, and 437° (V), and at 356 and 440° (Nb). The results demonstrate experimentally that the relative partial enthalpies and entropies of hydrogen in these solid solutions are very nearly independent of temperature in this temperature range. The observed enthalpies are compared with data derived from equilibrium work, and with our earlier data at 430° for Ta+H2. From the equilibrium pressures and the partial enthalpies, we have calculated the partial entropies, also the partial excess entropies of hydrogen. The limiting values (at NH=0) of the excess entropies are compared with values derived from the sum of the contributions from the volume expansion and from vibrational and electronic heat capacity terms. These calculated values are systematically about 0.5–1.0 cal/deg · g-atom lower than the experimental ones. This difference is too small to allow for any site degeneracy term in the excess entropies.