Absolute vacancy concentrations in noble metals and some of their alloys

Abstract

Absolute vacancy concentrations are being determined by Differential Dilatometry (DD) measuring simultaneously the relative change in length by a laser/optical slit technique and in lattice parameter by a Debye-Scherrer technique in one and the same specimen. Accuracy has been improved to some parts in 10 6. Precautions are taken to prevent selective evaporation of constituents making such measurements in alloys feasible up to 1150°C presently. The temperature range, in which vacancy formation can be reliably observed, can be extended further to lower temperatures by adding results of positron lifetime measurements, which give absolute vacancy concentrations only, if the trapping rate of vacancies for positrons is well known. Since the DD measurements and the ones employing positron annihilation show some regions of overlap, the trapping rate may be determined combining both techniques. The results are discussed in terms of mono/divacancy formation for the pure metals Cu, Ag and Al and of the “complex model” for some Cu and Ag alloys, from which reliable data for the enthalpy and entropy of binding may be deduced. The Gibbs free energy of binding is being compared to literature data and models published so far. In this context, the following question may be discussed again: which quantities govern the binding effect most? Excess charge, misfit or others?