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Unformatted text preview: 104 Vacancy Experiments It is instructive to review the classical experimental techniques that have been used to determine the thermodynamic properties of vacancies. We wish the measure the formation enthalpy, h v , the formation entropy, s v , and the formation volume, v v for vacancies. It is plain from the equilibrium expression x v = exp s v k exp h v kT = exp s v k exp e v kT exp p ext v v kT that the enthalpy, h v , can be determined from the temperature dependence of x v , that the volume, v v , can be determined from the pressure dependence of x v , but that a determination of the entropy, s v , requires a measurement of the absolute concentration of vacancies at some temperature and pressure. Measurement of h v by the quench-resistivity technique of J.S. Koehler The temperature dependence of x v can be found by quenching metal wires from various high temperatures and measuring the electrical resistivity to measure the vacancy concentration. The experiment is as follows: Anneal metal wires at various high temperatures to establish various equilibrium vacancy concentrations, x v T ( ) . Cool the wires rapidly (quench) to a very low temperature, say the temperature of liquid nitrogen, where the vacancies are essentially immobile. This allows the vacancies created at the various high temperatures to be retained in the wire. Essentially all of the vacancies...
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- Spring '08