Hydrogen absorption–desorption characteristics, kinetics of hydrogen absorption and thermodynamics of dissolved hydrogen in Zr 0.1Tb 0.9Fe 1.5Co 1.5

Abstract

The hydrogen absorption and desorption pressure–composition isotherms of the PuNi 3 type rhombohedral Zr 0.1Tb 0.9Fe 1.5Co 1.5 are investigated by using a Sieverts’ type apparatus based on pressure reduction method in the ranges 75≤ T(°C)≤150 and 0.01≤ P(bar)≤30. The hydrogen solubility properties are compared with those of Zr 0.1Tb 0.9Fe 3 to study the effect of Co substitution at the Fe site. The P–C isotherms show the α→(α+β) transition at an average hydrogen concentration ( y= n H/ n fu) of 0.2 and (α+β)→β transition at an average value of y=2.5. Further, the plateau pressure of Zr 0.1Tb 0.9Fe 1.5Co 1.5–H at any given temperature increases when compared to that of Zr 0.1Tb 0.9Fe 3–H. It is found that hysteresis is quite small in the temperature regions investigated. At a hydrogen concentration of 3.1 hydrogen atoms per formula unit, the host lattice is accompanied by an expansion of about 16% without any change in crystal structure as evidenced by powder X-ray diffraction studies of Zr 0.1Tb 0.9Fe 1.5Co 1.5H 3.1. The thermodynamical parameters viz., the relative partial molar enthalpy (Δ H H) and the relative partial molar entropy (Δ S H) of dissolved hydrogen are found to be in the ranges −(17–23) kJ(mol H) −1 and −(48–66) JK −1(mol H) −1 respectively for absorption and, +(19–23) kJ(mol H) −1 and +(46–72) JK −1(mol H) −1 for desorption. From the dependence of Δ H H on y, the different phases [α, (α+β), β] and phase boundaries [α→(α+β)], [(α+β)→β] of the alloy–hydrogen system are identified. The different phases identified by the thermodynamic studies agree well with those seen in the hydrogen absorption isotherms of the respective alloy. These phases are further confirmed through powder X-ray diffraction studies. The kinetic studies show that the rate of hydrogen absorption is relatively slow when compared with Zr 0.1Tb 0.9Fe 3–H.