

Libowitz [19] plotted the logarithm of the dissociation pressure for a 

 nuraber of aetal hydrides taken froa a number of publications, including 

 References 20 through 26 and found the above relationship to be accurate. 

 Figures 3a, b, and c show several of these plots. 



Ac temperatures below about 700 F, soase hydrides exhibit hysteresis 

 on their pressure-temperature-coaiposition diagrams. For a given fixed 

 temperature, the equilibrium pressure is higher during absorption than 

 during desorption for the same composition. Typically, this behavior 

 is illustrated in Figure 4. Several theories have been offered for 

 hysteresis [19], but none of them have been satisfactory in explaining 

 all the observed phenomenon. 



In order to evolve hydrogen for use as a fuel, the pressure in the 

 fuel supply line must be lowered below r.he dissociation pressure corre- 

 sponding to the temperature of the stored hydride bed. The rate of 

 hydrogen evolution is dependent upon the difference between the actual 

 hydrogen pressure and the equilibrium dissociation pressure. The heat 

 of dissociation must also be provided; and, depending upon the geometry 

 of the heat transfer surface, tbe heat transfer rate can limit the rate 

 of hydrogen evolution. Tae vaste heat of the internal combustion engine 

 provides an excellent source for the heat of dissociation, the exhaust 

 temperature being anywhere betweer. 600 and 1,400 F, depending upon the 

 load and speed. 



Hydrides of Pure Metals 



The grouping of elements below is based upon their position in the 

 periodic table. Because of the cost, availability, and safety consid- 

 erations, radioactive elements will not be considered here for hydrogen 

 storage for the applications under consideration. 



Lithium, Sodium, Pot assium, Rubidium, and Cesium. The alkali 

 metals absorb hydrogan readily and violently. All elements in this 

 group compress upon hydrogenation. The volumetric percentage compres- 

 sion ranges between 21.5% for lithium to 44.6% for cesium. These hydrides 

 are very reactive, the reactivity depending upon the degree of solu- 

 bility of the reaction products in the reaction medium. Tney are very 

 reactive with water, which makes them difficult to handle. All alkali 

 metal hydrides require temperatures above 800 F for hydrogen generation 

 at atmospheric pressure [10, 20, 24]. Because of the high temperature 

 requiiements and high reactivity with water j the alka„ metals are not 

 suitable for hydrogen storage applications. 



Beryllium, Magnesium, Calcium, Strontium, and Barium. Beryllium 

 hydride cannot be obtained by direct synthesis from elements [9], and, 

 therefore, it is not suitable for the application under consideration. 

 The dissociation pressures of the hydrides of calcium [29], strontium 

 [30], and barium [31] are well below atmospheric even when temperatures 

 exceed 1,000 F. These metals, therefore, cannot be considered for 

 hydrogen storage. 



I 



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