Metal hydrides intermetallic compounds

Keywords Hydrogen, metal hydrides, intermetallic compounds, phase equilibriums, model of non-ideal lattice gas. 

A high-temperature chemical heat pump system has been designed at JAERI to heat up waste heat of about 500 °C to above 1000 °C by using Ti/Cr metal hydride intermetallic compounds (Fig. 4-11). This method would improve the total energy efficiency of the hydrogen production system combined with the process heat provision by the HTTR. The system consists of a low-temperature and a high-temperature vessel with the metal hydride material. The exothermal process of hydrogen absorption by the metal is able to heat up helium to approx. 1000 °C as was demonstrated in a test system [36]. 

Tarasov, B.P., (2002) Preparations and properties of ultradispersed hydrogen-sorbing metals and intermetallic compounds, in Veziroglu, T.N., Zaginaichenko, S. Yu., Schur, D.V., et al. (eds.), Hydrogen Materials Science and Chemistry of Metal Hydrides (NATO Science Series. II), Kluwer Academic Publishers, Dordrecht-Boston-London, V. 71, pp. 275-281. 

The hydrides formed in reaction (a) may be classified as (1) saline or ionic hydrides, (2) metallic hydrides and (3) covalent hydrides. The saline hydrides include the hydrides of the alkali and alkaline-earth metals, except BeHj, which is covalent. Transition metals form binary compounds with hydrogen that are classified as metallic hydrides including rare-earth and actinide hydrides. Intermetallic compound hydrides, such as TiFeHj and LaNijH, may be thought of as pseudobinary metallic hydrides. 

FIGURE 1 Pressure-composition isotherms left) and van t Hoff plot right) tor the reaction of a metal or intermetallic compound with hydrogen. ph2. hydrogen pressure Ch. hydrogen concentration in the solid phase T, absolute temperature a, solid solution phase of hydrogen in the metal or Intermetallic compound metal hydride phase Peq, equilibrium pressure of metal hydride formation To, critical temperature AH, enthalpy of hydride formation. [From Schlapbach, L., Meli, R, and Zuttel, A. (1995). Intermetallic hydrides and their applications. In Intermetallic Compounds Principles and Practice (J. H. Westbrook and R. L. Fleischer, eds.), Vol. 2, pp. 475-488. Reproduced with permission from John Wiley Sons, New York.]... 

In order for an intermetallic compound to react directly and reversibly with hydrogen to form a distinct hydride phase, it is necessary that at least one of the metal components be capable of reacting directly and reversibly with hydrogen to form a stable binary hydride. 

If the metal atoms are not mobile (as is the case in low—temperature reactions) only hydride phases can result in which the metal lattice is structurally very similar to the starting intermetallic compound because the metal atoms are essentially frozen in place. In effect the system may be considered to be pseudo-binary as the metal atoms behave as a single component. 

Metals, intermetallic compounds, and alloys generally react with hydrogen and form mainly solid metal-hydrogen compounds (MH ). Hydrides exist as ionic, polymeric covalent, volatile covalent and metallic hydrides. Hydrogen reacts at elevated temperatrrres with many transition metals and their alloys to form hydrides. Many of the MH show large deviations from ideal stoichiometry (n= 1, 2, 3) and can exist as multiphase systems. 

X.L. Yeh, K. Samwer, W.L. Johnson, Formation of an amorphous metallic hydride by reaction of hydrogen with crystalline intermetallic compounds - a new method of synthesizing metallic glasses, Appl. Phys. Lett. 42 (1983) 242-244. 

The complex hydride Mg CoH is very similar to Mg FeH. In the binary system of Mg-Co there is no solubility of Co in either solid or liquid Mg and no inter-metallic compound, Mg Co, exists in equilibrium with other phases. However, in contrast to the Mg-Fe system, the intermetallic compound MgCo exists in equili-brium in the Mg-Co binary system (e.g., [14, p. 251]). The theoretical hydrogen capacity of Mg CoH is only 4.5 wt% which is obviously lower than that of Mg FeHg due to the presence of the heavier Co element and one less H atom in the hydride formula. 

Metals, intermetallic compounds and alloys generally react with hydrogen and form mainly solid metal-hydrogen compounds. Hydrides exist as ionic, polymeric covalent, volatile covalent and metallic hydrides. 

Especially interesting are the metallic hydrides of intermetallic compounds, in the simplest case the ternary system because the variation of the elements allows... 

More general is the rule of reversed stability (Miedema model) the more stable an intermetallic compound, the less stable is the corresponding hydride, and the other tvay around [36]. This model is based on the fact that hydrogen can only participate on a bond with a neighboring metal atom if the bonds between the metal atoms are at least partially broken (Figures 5.25 and 5.26). 

Many metals, alloys and intermetallic compounds (Me) react reversibly with gaseous H2 to form a metal hydride, MeHx, at practical temperatures and pressures. This simple reaction, neglecting the solid solution phase, may be written as ... 

AB5 alloys are intermetallic compounds with hexagonal crystalline lattice. Constituting compounds are inter alia rare earth metals. These compounds are capable to form hydrides as the dissolve hydrogen. 

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