Phase equilibria and kinetics of evolution of dilute solutions of hydrogen in niobium

Ultrahigh vacuum techniques have been used to measure the equilibrium pressure of hydrogen dissolved in niobium in dilute solutions over the pressure, concentration and temperature ranges, p e =10 -7 -10 -2 Torr, H/Nb=0-0.035, and T=351-487 K. Sievert's law which indicates ideal solution behaviour was observed for hydrogen concentrations below about H/Nb=0.023; the solubility data are described by the expression square root p e =c(395.4+or-33.8)exp(-(4.345+or-0.161)*10 3 /T) where c=H/Nb*100. The standard partial molar entropy and enthalpy of solution are Delta s 0 =49.6+or-2.3 J K -1 g-atom -1 and Delta h 0 =-36.12+or-1.34 kJ g-atom -1 . The value of Delta s 0 is compared with theoretical entropies calculated from configurational and vibrational contributions. The rate of evolution of hydrogen from a niobium filament has been studied over the pressure and temperature ranges 10 -7 -10 -3 Torr and 465-553 K. The rate limiting process is identified as the recombination of hydrogen atoms on the metal surface rather than diffusion within the bulk provided the overall concentration is less than H/Nb approximately=0.02. The activation energy for the process is 67.56+or-1.26 kJ mol -1 .