THE THERMODYNAMIC PROPERTIES OF THE NIOBIUM-HYDROGEN SYSTEM MEASURED BY REACTION CALORIMETRY

Calorimetric enthalpies for reaction of hydrogen with niobium in the alpha-, and (alpha + alpha')-, alpha'-, (alpha' + beta)-, (alpha + beta)-, beta- and (beta + delta)-phase regions are reported. The free energies and entropies are reported for those regions where the equilibrium pressures can be measured. The enthalpies for reaction with (1/2)H2(g) in the plateau regions for (alpha + beta) and (alpha + alpha') are -47.4 and -42.8 kJ/mol H, respectively. The magnitude of the calorimetric (absorption or desorption) enthalpy for the (beta + delta) plateau region is 17.3 kJ/mol H which is about 3 kJ/mol H smaller than the literature value based on a van't Hoff plot of the decomposition pressures. This difference arises because of the large hysteresis in this system and because of the irreproducibility in the plateau pressures; neither factor affects the present calorimetrically determined enthalpies.