CO+GA LIQUID-SYSTEM - ENTHALPY OF FORMATION

The molar enthalpy of formation of [Co+Ga] liquid alloys corresponding to the reaction: aCo((liq.)) + bGa((liq.)) --> CoxGa1-x(liq.) with x( Co)=a/(a+b) has been measured with a fully automated high temperature calorimeter, in the temperature range 1524 < T/K < 1694 K and the mola r fraction range 0 < x(Co) < 0.71. The integral molar enthalpy of form ation of the liquid [Co + Ga] system (in kilojaules per mole) is given by Delta(mix)H(m) degrees = (x(Co))(1-x(Co))[-44.069-106.71x(Co) + 14 8.61x(Co)(2)-23.859x(Co)(3)] The enthalpies are negative with a minimu m Delta(mix)H(m) degrees = -15.9 kJ mol(-1) at x(Co)=0.45, independent of the temperature. Limiting partial molar enthalpies of cobalt and g allium were also evaluated: Delta h(m) degrees(Co liquid in infinity l iquid Ga) = -44 kJ mol(-1) and Delta h(m) degrees(Ga liquid in infinit y liquid Co) = -26 kJ mol(-1) The enthalpy of formation of the [Co+Ga] liquid system is less exothermic than those of the neighbouring alloy [Ni+Ga] (Delta(mix)H(m) degrees = -34 kJ mol(-1) with x(Co)=0.55). Th is result is in agreement with the difference between the equilibrium phase diagram. These results exhibit a discrepancy with the values pre dicted by using the Miedema model. Moreover, for several (transition m etal+In or Ga) binary liquid alloys, the trend of the Fermi enthalpy f unction vs. molar fraction has been given.