HYDROGEN STORAGE PROPERTIES OF INDIAN MMNI5 WITH ALUMINUM, MANGANESE AND TIN SUBSTITUTIONS

Hydrogen storage properties of cast and annealed MmNi(5-x)M(x)(M = Al, Mn and Sn; x = 0.4 and 0.8) systems using Indian mischmetal (Mm) of c omposition (in wt %) 43 Ce, 23 La, 18 Nd, 5 Pr, 3 Sm and 8 Fe were stu died from absorption and desorption pressure- composition isotherms. T he temperature range of the study extended from 273 K to 318 K. The co mpositions studied were MmNi(4.6)Al(0.4), MmNi(4.2)Al(0.8), MmNi(4.6)M n(0.4), MmNi(4.2)Mn(0.8) and MmNi(4.6)Sn(0.4). In the solid solution ( alpha) region useful thermodynamic properties like partial molar entha lpy (Delta (H) over bar), partial molar entropy (Delta (S) over bar) a nd the partial molar enthalpy at infinite dilution (Delta (H) over bar )(H)(0) of hydrogen in solution have been obtained from pressure- comp osition (PC) isotherms. The solubility of hydrogen in the systems init ially varies linearly with the square root of partial pressure of hydr ogen. The relative partial molar enthalpy of hydrogen in solution decr eases with increasing hydrogen content for all the systems except MmNi (5-x)Mn(x)(x = 0.4 and 0.8). This behaviour has been explained by cons idering the geometric and electronic contributions to the partial mola r enthalpy. The partial molar enthalpy of hydrogen at any fixed hydrog en/metal (H/M) atom ratio decreases with substitution of Al, Mn and Sn for Ni in MmNi(5). It also decreases with increasing amounts of Al an d Mn substitution. This decrease in enthalpy has been correlated to th e increase in the volume of the unit cell of MmNi(5) with Al, Mn and S n substitutions.