Hydrogen absorption-desorption characteristics, kinetics of hydrogen absorption and thermodynamics of dissolved hydrogen in Zr0.1Tb0.9Fe1.5Co1.5

The hydrogen absorption and desorption pressure-composition isotherms of th e PuNi3, type rhombohedral Zr0.1Tb0.9Fe1.5Co1.5 are investigated by using a Sievearts' type apparatus based on pressure reduction method in the ranges 75 less than or equal to T(degrees C) less than or equal to 150 and 0.01 l ess than or equal to P(bar) less than or equal to 30. The hydrogen solubili ty properties are compared with those of Zr0.1Tb0.9Fe3 to study the effect of Co substitution at the Fe site. The P-C isotherms show the alpha-->(alph a+beta) transition at an average hydrogen concentration (y = n(H)/n(fu)) of 0.2 and (alpha + beta) --> beta transition at an average value of y = 2.5. Further, the plateau pressure of Zr0.1Tb1.5Fe1.5Co1.5-H at any given tempe rature increases when compared to that of Zr0.1Tb0.9Fe3-H. It is found that hysteresis is quite small in the temperature regions investigated. At a hy drogen concentration of 3.1 hydrogen atoms per formula unit, the host latti ce is accompanied by an expansion of about 16% without any change in crysta l structure as evidenced by powder X-ray diffraction studies of Zr0.1Tb0.9F e1.5Co1.5H3.1. The thermodynamical parameters viz., the relative partial mo lar enthalpy (Delta H-H) and the relative partial molar entropy (Delta S-H) of dissolved hydrogen are found to be in the ranges -(17-23)kJ(mol H)(-1) and -(48-66)kJ(-1)(mol H)(-1) respectively for absorption and, +(19-23)kJ(m ol H)(-1) and +(46-72)JK(-1)(mol H)(-1) for desorption. From the dependence of Delta H-H on y, the different phases [alpha, (alpha+), beta] and phase boundaries [alpha -->(alpha + beta)], [(alpha + beta) --> beta] of the allo y-hydrogen system are identified. The different phases identified by the th ermodynamic studies agree well with those seen in the hydrogen absorption i sotherms of the respective alloy. These phases are further confirmed throug h powder X-ray diffraction studies. The kinetic studies show that the rate of hydrogen absorption is relatively slow when compared with Zr0.1Tb0.9Fe3- H. (C) 2000 Elsevier Science S.A. All rights reserved.