Thermal Properties of Gold-Silicon Binary Alloy near the Eutectic Composition

Abstract

We have measured the specific heat C_p and the enthalpy of Au‐18.6 at.% Si alloy in some of its various forms. C_p of the liquid form falls continuously with increasing temperature from 8.38±0.05 at 563°K, corresponding to an undercooling of 73°K, to 7.90±0.05 cal/gfw·°K, and it exhibits no singular behavior at the melting temperature T_m. It exceeds the heat capacity of the eutectic solid by 1.87 cal/gfw·°K at T_m. This high specific heat in the liquid state and its decrease with increasing temperature is attributed principally to the configurational entropy, as has been suggested for the pure liquid metals. C_p of the amorphous alloy, formed by rapidly chilling the liquid, was measured for the temperature range from 208° to 278°K. It rises slowly to 225°K, then falls, and then rises again with increasing temperature. It exceeds C_p of the eutectic solid by 0.2 to 0.4 cal/gfw·°K. C_p of the metastable intermetallic crystalline phase, formed when the liquid is less rapidly chilled, falls between the values for the eutectic and the amorphous solids. The intermetallic phase melts at 631°K, which is only a little lower than the melting point, 636°K, found for the eutectic mixture. Its heat of fusion ΔH_f is 1.59 kcal/gfw, while that of the eutectic mixture is found to be 2.35±0.1 kcal/gfw. The heat and excess entropy of mixing of the liquid elements to form the liquid alloy at 1336°K are evaluated to be −1.6±0.1 kcal/gfw and 0.39±0.17 cal/gfw·°K, respectively. The especially deep‐lying eutectic exhibited by this system is attributed to the tendency for unlike atoms to associate when in the metallic state, in contrast with the strong tendency for phase separation in the solid state owing to the high stability of silicon in its covalent crystalline form.