Pressure-composition isotherms of hydrogen and deuterium in vanadium films measured with a vibrating quartz microbalance

Abstract

Thin films of vanadium produced by vapour deposition on vibrating quartz crystals were used to determine thermodynamic parameters of hydrogen occlusion from the gas phase in the pressure range 10-10<or=p<or=40 bar and temperature range -20<or=T<or=100 degrees C. The partial molar enthalpies and entropies of solution in the dilute Sieverts' law regime were found to be Delta HHx=-26 kJ (mol H)-1, Delta HDx=-28 kJ (mol H)-1 and Delta SH,Dx nc/R=-6. The pressure-composition isotherms indicated various phase transitions in agreement with the known phase diagram. Values of the enthalpies of transition Delta H (in kJ (mol H)-) and the entropies of transition Delta S/R were -35 and -5 for VHx<<1( alpha )-VH0.5( beta ); -36 and -6 for VDx<<1( alpha )-VD0.5( beta ); -37 and -7 for VD0.5( beta )-VD0.7( alpha '); -21 and -8.5 for VH( beta ')-VH2( epsilon ); and -23 and -8 for VD( alpha ')-VD2( epsilon ). The hydriding-dehydriding kinetics were dominated by surface effects.