The intrinsic degradation behavior of (V0.53Ti0.47)(0.925)Fe-0.075 alloy during temperature-induced hydrogen absorption-desorption cycling

Abstract

The intrinsic degradation behavior of an (V0.53Ti0.47)(0.925)Fe-0.075 alloy having a BCC structure and two plateau regions (the low and high plateau region) has been investigated using P-C isotherm measurements, thermal desorption analysis, MID and TEM analysis during temperature-induced hydrogen absorption-desorption cycling (thermal cycling). After 400 thermal cycles between room temperature and 600 degrees C under 10 atm H-2, the total reversible hydrogen absorption capacity decreases by about 40%. In the low plateau region the reversible hydrogen absorption capacity decreases by about 24% after 400 cycles. The majority of the capacity loss takes place during the first 243 cycles. In the high plateau region, however the reversible hydrogen absorption capacity decreases by about 16% gradually with cycling. From the result of the thermal desorption analysis, it is found that the peak obtained at high temperature corresponds to the desorption of hydrogen of the low plateau region which shifts to lower temperatures and the peak area decreases after 223 cycles and then the area is maintained without any clear change up to 400 cycles. The low temperature peak corresponds to desorption of hydrogen from the high plateau; this does not shift but its peak area decreases gradually with cycles. From P-C isotherm measurements it is also found that the degradation behavior,lt each plateau region is different. In addition, XRD analysis shows that the crystal structure of the de-hydrided sample changes from BCC to BCT after degradation; the hydrided sample maintains FCC structure although the peaks are broadened after degradation. From the result of a static isothermal hydrogenation treatment, it is found that the change of crystal structure from BCC to BCT is caused only by temperature effects. The result of TEM analysis shows that peak broadening is caused by the formation of an amorphous phase in the FCC matrix. (C) 1999 Elsevier Science S.A. All rights reserved.