ELECTRICAL-RESISTANCE CHANGE DURING HYDROGEN CHARGING AND DISCHARGINGIN NI-67-XCUXZR33 GLASSY ALLOYS

The hydrogen charging and discharging processes are investigated in Ni 67-xCuxZr33 glassy alloys by in situ resistance measurements in order to clarify the surface and bulk contribution to the activation process . The process has been monitored by conventional DC method using a com puter-controlled system, exposing the samples to hydrogen at pressure from 0.1-8.6 MPa at temperatures of 293 K and 393 K. The H-contents (H /M) were determined by weight and by pulse NMR measurements. The NMR d ata are obtained by the Carr-Purcell-Meiboom-Gill (CPMG) method. The c harging is extremely long and the resulting H-distribution is inhomoge nous during the first cycle, especially at 293 K. This fact shows the significance of the surface-activation during the H-uptake from the ga s phase. The duration of the second saturation decreases significantly , and the resistance-time curves obtained: on two parallel samples are very 'similar. The saturation value for the normalized resistance (R/ R-0) remains almost the same during several cycles. Increasing the tem perature of the charging (393 K), the equilibrium value of the resista nce decreases. The resistance of the samples at the end of the dischar ging process is always higher than before charging. This indicates the partial irreversibility of the H-absorption at constant temperature. This observation is correlated with the NMR results.