Tracer diffusion coefficients of hydrogen at high concentration in b.c.c. host metal lattices

Abstract

Measurements of tracer (self) diffusion coefficients of hydrogen in the body-centred cubic (b.c.c.) phase of TiH x for x = 0.57 and 0.70 are reported for the temperature range 620 K to 830 K. Experimental results obtained from the NMR pulsed field gradient spin echo technique, fit the relation D = D 0 exp( − E a k BT ) , with E a = 155 ± 10 and 147 ± 10 (meV atom − 1) and D 0 = 7.8 ± 1 and (6.3 ± 1) × 10 − 4 (cm 2 s − 1), respectively, for x = 0.57 and 0.70. Diffusion parameters are compared with those of other b.c.c. transition metal hydrides. All have high diffusion coefficients for hydrogen which are decreased as the hydrogen concentration is increased. Although an increase in hydrogen concentration causes an increase in E a over most of the concentration range, the maximum value of E a does not exceed 190 meV atom − 1 for any system considered. In contrast, it is noted that most facecentred cubic (f.c.c.) transition metal hydrides have values of E a that are higher by a factor of three or more and diffusion coefficients that are smaller by two or more orders of magnitude.