The thermal decomposition of lithium aluminium hydride

Abstract

Lithium aluminium hydride on heating decomposes in three stages: ( a ) an initial surface reaction during the induction period, followed by ( b ) an interface reaction giving an S-shaped pressure against time curve, corresponding to LiAlH 4 → LiAlH 2 + H 2 , and ( c ) a slow process during which a third hydrogen atom is liberated. The initial reaction is rapid at first and then slows down to a constant rate. It occurs to the extent of about 0⋅7% of the total decomposition and penetrates the surface to a depth of several molecular layers. Its activation energy is 22 kcal. The interface reaction obeys a cube-root plot and has an activa­tion energy of 25 kcal. The electrical conductivity of LiAlH 4 has a positive temperature coefficient with an activation energy of 16 kcal. During the thermal decomposition there is a maximum on the electrical conductivity curve, occurring early in the initial surface reaction. The conductivity at the maximum is ten times greater than the extrapolated from the natural conductivity. It is concluded that conductivity arising during the thermal decomposition is, in the main, due to defects produced during the initial surface reaction; also, that the formation of nuclei is a two-stage process consisting ( a ) of the aggregation of defects at lattice discontinuities followed by ( b ) the collapse of the defect aggregate to give nuclei at which a normal interface reaction proceeds.