Thermodynamic analysis of the formation and decomposition of intermetallic hydrides

Abstract

8 H 4 intermetallic hydrides are thermodynamically analyzed in identical conditions. These reactions are competitive and the interface between them is determined by boundary composition A x B y H n b . For LaNi 5 , TiFe, Mg 2 Ni, and ZrMn 2.8 , intermetallides, ranges of stable hydrogenation are equal to 17, 27, 5, and 17% of the maximum hydrogen capacity, and the others are ranges of metastable hydrides, over which decomposition reactions are limited kinetically. The improvement in kinetics of intermetallide hydrogenation prevents the decomposition of intermetallic hydrides. This is promoted by mechanical or catalytic activation of hydrogenation and by the use of heat-conducting nonvolatile metal matrix composites based on intermetallides.Hydrogen accumulation by intermetallides has been well examined in terms of intermetallide-hydrogen phase equilibria, hydrogen capacity, and hydrogenation kinetics. With the current level of knowledge, one can optimally select the composition of an intermetallic hydrogen sorbent to promote required characteristics of hydride devices: hydrogen accumulators, thermal sorption compressors, hydride refrigerators, etc. However, the chemical stability of intermetallic hydrides during multiple hydrogenation-dehydrogenation cycles associated with the decomposition of starting intermetallic hydrides and the formation of new hydride and metal phases is still a challenge. This usually leads to a significant increase in desorption temperature and a decrease in equilibrium hydrogen pressure in a hydride device. It is hardly possible to recover the original intermetallic composition and, thus, the decomposition of intermetallic hydrides needs to be prevented.The objective of this paper is to examine the thermodynamics of formation and decomposition of intermetallic hydrides and to recommend methods to prevent their decomposition.The research focused on direct hydrogenation of LaNi 5 , TiFe, Ti 2 Ni, TiNi, Ti 2 Cu, TiCu, Ti 3 Al, Mg 2 Ni, Mg 2 Cu, and ZrMn 2.8 intermetallides and subsequent decomposition of their hydrides.A general hydrogenation reaction is as follows:where A is a hydride-forming metal; B is a nonhydrogenated metal; H is hydrogen; x is the number of hydrideforming metal atoms in an intermetallide; y is the number of nonhydrogenated metal atoms in the intermetallide; k

Synthesis and Characterization of TiFe0.7−xMn0.3Vx (x = 0.05, and 0.1) and Ti1−yTayFe0.7Mn0.3 (y = 0.2, and 0.4) Nanostructured Metal Hydrides for Low Temperature Applications

“…This serves making a "pure", "clear" material without hydrogen for next maximum absorption/desorption measurements. Although, a hydrogen content (about 1wt.% of hydrogen) remains inside to our specimens before the beginning of hydrogen absorption/desorption measurements and attests the creation of a stable hydride which need high temperatures to absorb whole hydrogen contents (800-1000 o C) [25]. These hydrogen contents owing to the stable hydrides and firmly increase when the temperature drops to 50 o C for hydrogen absorption/desorption cycles.…”