Because of their similarity, the rare-earth metals would be expected 

 to have similar hydriding characteristics. With the exception of 

 europium and ytterbiua, all the rare-earth cetals fora dlhydrides that 

 readily absorb additional hydrogen to fora trihydrides. Ytterbium forms 

 trihydride vith difficulty under high pressures whereas europium does 

 not form a trihydride at all [36]. 



Based upon the available experimental data in References 29 and 37 

 through 43, however, the rare-earth hydrides are not suitable for 

 hydrogen storage applications because, to extract hydrogen out of their 

 hydrides, the system pressure has to be lowered below the atmospheric 

 pressure even at temperatures over 1 ,000 F. 



Titanium, Zirconium, and Hafnium. Titanium hydride can be prepared 

 by passing hydrogen over titanium at temperatures over 630 F--the teepera- 

 ture at which the hydride begins Co form [<t4]--and cooling quickly. 

 Sponge and ingot titanium absorb, respectively, a maximum of 42.96 and 

 38.14 liters of hydrogen per 100 grams of the metal. The compound obtained 

 desorbs hydrogen at 570 F when heated in vacuum. McQuillan [45] and 

 others [46, 47, 48] have studied the titanium-hydrogen system exten- 

 sively and developed the pressure-temperature-coraposition diagrams which 

 show that the dissociation pressure of the hydride is below atmospheric 

 pressure even at temperatures approaching 1,600 F. 



Zirconium reacts with hydrogen readily forming hydrides with coepo- 

 siticn varying between ZrHj_33 and ZrR-. Because of the good thermal 

 stability of zirconium hydride, it has been extensively studied for 

 nuclear reactor applications, particularly as a moderator. For the 

 purpose of this survey, only two references [49, 50] need be cited in 

 which the pressure-tenperature-composition curves have been reported at 

 pressures up to slightly above atmospheric pressure. These curves show 

 conclusively that temperatures of over 1,800 F are required to liberate 

 any hydrogen from zirconium hydride. This observation make the use of 

 zirconium for hydrogen storage applications impractical. For essen- 

 tially the same reason, hafnium [22] is also not useful for hydrogen 

 storage applications. 



Copper, Silver, and Gold; Zinc, Cadmium and Mercury. Solubility of 

 hydrogen in copper, silver, and gold is very low, and increases slightly 

 with temperature. The solubility of hydrogen, in at xas of hydrogen per 

 atom of.metal, even under the most favorable conditions does noc exceeu 

 8 x 10 in copper, 0.42 x \0 mi> in silver, and even less in gold [19]. 

 The hydrides of zinc, cadmium, and mercury cannot be prepared from 

 direct reaction of hydrogen with the metal because the solubility is 

 very low. None of these metals, therefore, is suitable for hydrogen 

 storage applications. 



Boron, Aluminum, Gallium, Indium, and Thallium. The solubility of 

 hydrogen in boron is negligible. Boranes are therefore prepared through 

 chemical reactions in solution. Various hydrides of boron are either 

 gaseous at room temperature or are low-boiling-point liquids. Boron 



17 



