I 



Magnesium on the other hand appears suitable for hydrogen storage. 

 Its dissociation pressure at 547 F is 14.7 psia. To obtain high disso- 

 ciation pressures, the tea^rature of the hydride will have to be 

 raised. For instance, temperatures of 705°F and 800°F will provide 

 dissociation pressures of 10 and 30 atmospheres, respectively [26], The 

 stoichiometric composition of the magnesiua hydride that i3 formed 

 corresponds to KgH2, and alasost all of the hydrogen is available as the 

 pressure is reduced and the temperature of the hydride is increased. 



Magnesiua is -".nexpensive , about $1.00 per pound and widely avail- 

 able in nature, Magnesium hydride has a density of 1.42 ga/cc and con- 

 tains 7.652 by weight of hydrogen, 932 of which is readily available. 

 The vclumetric density of hydrogen in the magnesium hydride, assuming 

 602 void fraction, is 485 cubic feet per jubic foot of the hydride. The 

 heat of dissociation required to release hydrogen from magnesium hydride 

 is about 252 of the heat of combustion of hydrogen [11J, which is excel- 

 lent in view of the fact that this is much less than the heat that will 

 normally be wasted as exhaust in any combustion process involving 

 hydrogen. 



The density of hydrogen in magnesium hydride may be compared with 

 the density of liquid hydrogen at boiling point. If 602 void fraction 

 is assumed in the hydride, the density of recoverable hydrogen in it is 

 about 2.71 pounds per cubic foot, which is approximately 612 of the 

 density of liquid hydrogen of 4.42 pounds per cubic foot. In view of 

 the fact that no cryogenic temperatures are involved and handling boil- 

 offs is not required the density of hydrogen in magnesium hydride is 

 excellent. 



The major disadvantage in the use of magnesium for hydrogen storage 

 appears to be in its very slow reaction with hydrogen. This problem has 

 been overcome by J. J. Reilly and R. H. Wisuall, Jr. [32], by the use of 

 nickel as a catalyst. Since the addition of nickel to magnesium makes 

 it an intermetallic compound, it will be discussed in detail In the 

 Hydrides of Intermetallic Compounds section of the report. 



Scandium, Yttriua, and the Rare Earth Elecents. Scandium hydrogen 

 and yttriua-hydrogen systecs are unsuitable for hydrogen storage. The 

 scandium-hydrogen system has been studied in detail by Beck [33] and 

 Lieberman and Wahlbeck [34] and was found to be very stable, with disso- 

 ciation pressure below the atmospheric pressure even when the tempera- 

 ture was raised to over 1,800 F. Similarly, the dissociation pressure 

 for the yttrium-hydrogen system has also been found to be below atmos- 

 pheric [25, 35] at temperatures to 1,800°F. 



Studies of the rare-earth/hydrogen systems have been carried on 

 more or less continuously since the beginning of this century. However, 

 since the rare earths are somewhat difficult to separate: from one 

 another, much of the work was done on mixtures of rare-e«»rth metals. 

 These mixtures have been described as lanthanum "raischmettil, * ' cerium 

 mischmetal, etc., depending on the particular rare-earth metal that 

 predominates the composition. 



16 



