Co+Ga liquid system: enthalpy of formation

Abstract

The molar enthalpy of formation of [Co+Ga] liquid alloys corresponding to the reaction: aCodq+bCadq→Co1 has been measured with a fully automated high temperature calorimeter, in the temperature range 1524 <TK < 1694 K and the molar fraction range 0 <xCo < 0.71. The integral molar enthalpy of formation of the liquid [Co+Ga] system (in kilojoules per mole) is given by ΔminH°m=(xCo)(1−xCo)[−44.069−106.71xCo+148.61x2Co−23.859x1Co]The enthalpies are negative with a minimum ΔmaxH°m = − 15.9 kJ mol−1 at xCo = 0.45, independent of the temperature. Limiting partial molar enthalpies of cobalt and gallium were also evaluated: Δh°m (Co liquid in α liquid Ga) = − 44 kJ mol−1 and Δh°m (Ga liquid in α liquid Co) = − 26 kJ mol−1The enthalpy of formation of the [Co+Ga] liquid system is less exothermic than those of the neighbouring alloy [Ni+Ga] (ΔmaxH°m = −34 kJ mol−1 with xCo = 0.55). This result is in agreement with the difference between the equilibrium phase diagram. These results exhibit a discrepancy with the values predicted by using the Miedema model. Moreover, for several (transition metal + In or Ga) binary liquid alloys, the trend of the Fermi enthalpy function vs. molar fraction has been given.