Vacancy-silicon binding energy in aluminium by positron annihilation

Measurements have been made of the positron annihilation coincidence counting rate at the peak position in aluminum and in an Al-0.27 at% Si alloy as a function of temperature from 25 degrees C to 600 degrees C. The results were analysed according to the positron trapping model to yield the vacancy formation energy, Ef, and the vacancy-silicon binding free energy. The thermal expansion of the sample and the temperature dependence of the Si solubility were taken into account, and the trapping rate assumed to be temperature independent. For pure aluminum it is found that Ef=0.67+or-0.03 eV. For the Al-Si alloy a least squares analysis based on the Lomer expression for the vacancy concentration of a dilute alloy shows that the vacancy-silicon binding free energy is 0.12 eV. This is smaller than the binding free energy of 0.20 eV found by Burke and King (1970) from length and lattice parameter thermal expansion measurements.