CONTROL OF HYDROGEN EQUILIBRIUM PRESSURE FOR C-14-TYPE LAVES PHASE ALLOYS

It is very important to control continuously the hydrogen equilibrium pressure P-temperature T characteristics of hydrogen storage alloys to have them applicable in a wide range of temperatures. The relation between the alloy phases and the P-C-T characteristics was determined for C14 (MgZn2) type Laves phase alloys based on Ti(Zr)Mn2 by changing the alloy composition. The P-C-T characteristics, hysteresis and plateau pressure were changed substantially by substituting zirconium for titanium in the A site and other elements such as chromium, copper or nickel in the B site. In this case, the crystal lattice constants and hydrogen equilibrium pressure were changed more effectively by substituting zirconium for titanium in the A site. The range of equilibrium pressures covered was from 10 to 10(8) Pa at 20-degrees-C (this corresponds to the range from -50-degrees-C to +250-degrees-C at 1.013 x 10(5) Pa). The equilibrium pressure changes particularly markedly for lattice constants a = (4.84-5.05) x 10(-10) m and c = (7.94-8.28) x 10(-10) m. These results show that the C14-type Laves phase alloys with excellent flatness of the plateau pressure and small hysteresis, are applicable in these ranges of temperatures and pressures for hydrogen storage. We have confirmed these results by fabricating a prototype heat pump with a hydrogen storage alloy that utilizes high temperature waste heat.