SOLUTION THERMODYNAMICS OF HYDROGEN IN THE MISCHMETAL NI5 SYSTEM WITH ALUMINUM, MANGANESE AND TIN SUBSTITUTIONS

The thermodynamics of hydrogen solubility in MmNi5 (where Mm denotes Indian mischmetal containing (wt.%) 43% Ce, 23% La, 18% Nd, 5% Pr, 3% Sm and 8% Fe) by partially replacing nickel with aluminium, manganese and tin has been studied using pressure-composition-temperature isotherms at 273, 278, 288, 298, 308, and 318 K. The compositions that were studied are MmNi5, MmNi4.6Al0.4, MmNi4.2Al0.8, MmNi4.6Mn0.4, MmNi4.2Mn0.8 and MmNi4.6Sn0.4. The solubility of hydrogen in the systems initially varies linearly with the square root of partial pressure of hydrogen in the solution region. The relative partial molar enthalpy of solution as a function of hydrogen content has been obtained for all the systems. It has been found that the relative partial molar enthalpy of hydrogen in solution decreases with increasing hydrogen content for all the systems except MmNi5-xMnx ( = 0.4 and 0.8). This behaviour has been explained by considering the geometric and electronic contributions to the partial molar enthalpy. The partial molar enthalpy of hydrogen at any fixed ratio H:M decreases with the substitution of aluminium, manganese and tin for nickel in MmNi5. It also decreases with increasing amount of aluminium and manganese substitution. This decrease in enthalpy has been correlated with the increase in the volume of the unit cell of MmNi5 with aluminium, manganese and tin substitutions.