Nuclear-Magnetic-Resonance Determination of the Activation Volume for Self-Diffusion in Aluminum

The spin-echo technique of NMR has been used to measure the activation volume for self-diffusion in aluminum metal in the temperature range 400-450°C. Measurements were made of the inverse linewidth time $T_2$ at hydrostatic pressure up to 3000 atm at selected isotherms in the above temperature range, the upper temperature being determined by the strength characteristics of the stainless-steel pressure vessel and the lower temperature by the minimum value of $T_2$ that could be measured with confidence. The specimens used were 325 mesh filings from 99.99% pure aluminum rod and single crystal. After subtraction of a temperature-independent contribution, the dipolar contribution to $T_2$, ${\left(T_2^{\prime }\right)}_d$, was found to depend on temperature according to ${\left[\ln {\left(T_2^{\prime }\right)}_d\right]}^{-1\mathrm{}}\propto T^{-}$ in accordance with theory, yielding an activation enthalpy for self-diffusion of 1.3±0.2 eV, in good agreement with previous determinations. The pressure dependence of ${\left(T_2^{\prime }\right)}_d$, deduced from ten independent experimental runs, yielded an activation volume for self-diffusion of 0.71±0.13 in units of the atomic volume of aluminum, with no discernible systemmatic dependence on the temperature (cited uncertainty is the standard deviation). Nine of the ten determinations fall within the range 0.71±0.20. The result is consistent with expectations based on monovacancy diffusion in aluminum.