The nature of the checical bonding in the metallic hydride is still 

 not well understood. There are three different models to explain the 

 bonding of the hydrogen in these hydriues. However, none of the three 

 models is completely satisfactory. The first model assumes that the 

 hydride is an alloy of hydrogen and t!ie metal, where it exists essen- 

 tially as a proton in the hydride lattice. The secoad model is based on 

 a predominantly covalent bond between the stetal and hydrogen. The third 

 model is essentially the converse of the first; the hydrogen exists as 

 anions formed by removal of electrons from the metal to give a partially 

 ionic bond. 



Hydrogen dissolves in, or forms a compound with, metals either 

 exothermally or endothermally. Those which react exothermally form a 

 hydride phase by direct combination of metal and hydrogen. This class 

 includes (1) the alkali and the alkaline-earth metals, which were not 

 considered to be suitable for hydrogen storage applications under con- 

 sideration because the hydrogen pressure at temperatures to 800 F is 

 below atmospheric; (2) the titanium and vanadium subgroup metals; (3) 

 the rare earth (including scandium and yttrium) ; and (4) the actinide 

 series metals and palladiua. Most other cetals (e.g., Fe, Co, Ni, Cu, 

 Ag, and Pt) dissolve hydrogen endothemicaily . At low concentrations, 

 the solubility of hydrogen in all metals so far studied is proportional 

 to the square root of the hydrogen pressure. In exothermic absorbers at 

 low temperatures and high pressures, the amount of hydrogen dissolved 

 approaches compositions corresponding to stoichiometric hydride compounds. 

 The crystal lattice of the endothermic absorbers is seemingly unaffected 

 by the solution of hydrogen, whereas a change in lattice is common for 

 the exothermic absorbers. 



Recently, intermetallic compounds have been discovered which 

 readily absorb and release large quantities of hydrogen per cubic foot 

 of the compound. In the following survey, hydrides of the pure metals 

 are first reviewed, followed by the hydrides of intermetallic compounds. 



LITERATURE SURVEY 

 General 



A search of available documents and summaries of programs underway 

 at the present time under federal government sponsorship was carried out 

 through the data bank search services of the Defense Documentation 

 Center (DDC) and the National Technical Information Service (NTIS) . 

 Additional information was obtained during personal visits to the 

 Brookhaven National Laboratory [11], and the Denver Research Institute 

 [12]. These searches supplemented tie more traditional literature 

 search methods of reviewing technical publications and journals. 



One of the reports obtained through NTIS turned out to be a bibli- 

 ography of 358 publications on metal hydrides [13]. Based upon the 

 titles, 108 of these references were considered to be of possible value 



