

using the metal hydride to store energy derived from wind, sun, and sea 

 might stress items 1, 2, and 3 so as not to waste any hard-won energy. 

 A combat-zone application might stress item A because of the need for 

 mobility »md invisibility. In the writing of this report curtain hydrides 

 were screened because they were either too stable, they tr^re overly 

 dangerous, or their operating temperatures and pressures were much 

 greater than other available hydrides. The metallic hydrides considered 

 are listed In the text. Intermetallic hydrides that were considered are 

 listed in Table 2. 



Properties of Hydrides 



Important properties of each metal-hydrogen system may now be 

 examined to see if they fulfill the above requirements. 



As indicated earlier, hydrides of interest for our purpose are 

 exothermic; i.e., heat is evolved when hydrogen is absorbed. These 

 hydrides are almost always reversible, and the hydrogen can be recovered 

 by lowering the pressure below, or raising the temperature above, the 

 pressure and temperature required for the absorption process. 



The behavior of a metal-hydrogen system may be described using the 

 pressure-teoperature-composition characteristics. The characteristics 

 of a typical (albeit slightly idealized) metal-hydrogen/metal-hydride 

 system are shown in Figure 2. Starting at the lower left-hand corner 

 with the pure metal, as hydrogen is taken up by the metal and the atomic 

 ratio H/M increases, the equilibrium pressure increases steeply until 

 point A is reached. Up to iMs point the solid consists of a solution 

 of hydrogen In metal rather than a compound. As the concentration is 

 further increased by supplying more hydrogen, a second phase appears, 

 having the composition B; and the addition of further hydrogen does not 

 cause a further increase in pressure until all the solid phase has 

 attained this composition. Above this plateau region, further enrich- 

 ment of the solid by hydrogen requires a steep increase in pressure. 

 The curves labeled T , T_, and T in Figure 2 show the effect of tempera- 

 ture on the pressure-composition relationship. 



At a given temperature, therefore, each hydride is In equilibrium 

 with a definite pressure of hydrogen, (called its decomposition pressure) 

 which also depends upon the quantity of hydrogen in the metal. In the 

 two-phase region, between A and B, at a given temperature, the decompo- 

 sition pressure is independent of hydrogen concentration in accordance 

 with the phase rule. In the single-phase region, below point A and 

 above point B, the decomposition pressure varies with both temperature 

 and hydrogen concentration. These relationships hold irrespective of 

 the nature of the solid present. 



10 



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