Hydrogen absorbing alloy and process for preparing same

A process for preparing a hydrogen absorbing alloy represented by the general formula R.sub.1-x A.sub.x (Ni.sub.5-y B.sub.y).sub.z wherein R is Mm (misch metal) or La, A is at least one element selected from the group consisting of Ce, Nd, Pr, Sm and Y, B is at least one element selected from the group consisting of Al, Sn, V, Cr, Mn, Fe, Co and Cu, x is in the range of 0.ltoreq.x.ltoreq. 0.5, y is in the range of 0<y.ltoreq.1.0 and z is in the range of 0.8.ltoreq.z.ltoreq.1.2, and B is present in a measurable amount,

• said process comprising the steps of:
• cooling a molten alloy at a controlled rate of 10.sup.5 to 10.sup.6 K/sec;
• and
• subjecting the resultant alloy to a heat treatment, so that a plateau region between a hydrogen solid solution region (.alpha. phase region) and a metal hydride region (.beta. phase region) is flattened to a level of up to 0.04 with respect to a standard deviation .sigma. of a normal cumulative distribution function .psi., whereupon said plateau region is evaluated by the normal cumulative distribution function .psi. wherein the hydrogen content of the alloy is taken as frequency and the logarithm of a ratio of equilibrium hydrogen pressure P to unit pressure Po is taken as a random variable,
• said normal cumulative distribution function .psi. being obtained by integrating a normal probability density function .phi.:
• ##EQU10## provided that distribution of ln(P/Po) is considered to be the normal probability density function .phi. of the standard deviation .sigma. and a mean value m, and is expressed by the following equation:
• ##EQU11## where t=(ln(P/Po)-m)/.sigma., and m=ln(Pm/Po), said hydrogen content being determined by the following equation wherein said normal cumulative distribution function is scaled with the width X.sub..beta. -X.sub..alpha. of the plateau region:
• hydrogen content=(X.sub..beta. -X.sub..alpha.).psi.(ln(P/Po);
• m,.sigma.).