The effect of hydride formation and decomposition cycles on hysteresis in VxNb1−x-H systems

Abstract

The pressure-composition ( P — C ) isotherms were determined at 318, 353 and 370 K for the first 10 hydriding-dehydriding cycles in V x Nb 1− x -H, ( x = 0.1, 0.2, 0.3) alloys. The magnitude of the hysteresis decreased from the first cycle up to the third cycle, but no further decrease was recorded after the third cycle. The magnitude of the hysteresis decreases with increasing temperature and with decreasing x (in V x Nb 1− x -H). The results are explained in terms of strain energy barriers. The elastic strain energy involved in the hydrogen atoms entering the interstitial sites in the bcc metal lattice, for V 0.3 Nb 0.7 , V 0.2 Nb 0.8 and V 0.1 Nb 0.9 was calculated to be 16.28 kJ (mole H 2 ) −1 , 7.80 kJ) (mol H 2 ) −1 and 7.52 kJ (mol H 2 ) −1 , respectively. These values are in excellent agreement with the observed free-energy differences obtained from absorption and desorption P — C isotherms, thus suggesting that hysteresis is due to elastic strain energy. The results for the V x Nb 1− x -H bcc solid solution alloys were compared with those for two Zr(Fe x Cr 1− x ) 2 intermetallic compounds. The changes in hysteresis upon cycling the intermetallic compound are explained in terms of attrition (i.e decreasing particle sizes) of the Zr(Fe x Cr 1− x ) 2 .