A multiplateau extension of the lacher theory

243

Journal of the Less-Common Metals, 74 (1980) 243 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

A MULTIPLATEAU

EXT~NSrON

OF THE LACHER

THEORY*

HENRY A. KIERSTEAD Argonne

National

Laboratory,

Argonne,

Ill. 60439

(U.S.A.)

An extension of Lather’s theory of hydrogen absorption isotherms will be presented which is applicable to isotherms with more than one plateau. In this model hydrogen absorption occurs on several different kinds of sites. Each type of site is characterized by four parameters: ni, A%, AS; and Tie ni is the number of sites of the ith kind, AHi* is the heat of absorption on sites of the ith kind when half of these sites are occupied, AS: is the corresponding entropy of absorption and Ti is the critical temperature. The fractional occupation 19~of type i sites at pressure P and temperature T is given by In P = 2 In {@i/(1 - 8,)) + 8(% - @i)TifT + (AH; and the total concentration x = n*& + n,e, The theory hydrides.

of hydrogen

- TASP)/RT

is given by

+ ns8a f . . .

will be illustrated

by fitting the isotherms

of ErFe,

and DyFes

*Abstract of a paper presented at the International Symposium on the Properties and Applications of Metal Hydrides, Colorado Springs, Colorado, U.S.A., April 7 - 11, 1980; J. Less-Common Met., in the press.