Self-Diffusion in Molten Lithium. Isotopic mass dependence

Abstract

Abstract The liquid state self-diffusion coefficients of Li6 and Li7 in isotopically enriched Li6 and Li7 metal and in several isotopic alloys have been measured by the nuclear magnetic resonance spin echo, pulsed magnetic gradient technique. At the melting point 180.5 °C, the self diffusion co­efficients are measured as Dm6= (6.8 + 0.7) · 10-5 cm2/sec for Li6 in 99% Li6, and Dm7 = (5.8 ± 0.6) -10-5 cm2/sec for Li7 in 99.9% Li7. The ratio of measured values (Dm6/Dm7) = 1.18 ± 0.07, which is greater than the square root of the mass ratio, (m7/m6) 1/2 = 1.08. The isotopic ratio of self diffusion coefficients is observed to be less than the ratio of the mutual diffusion coefficients in almost pure Li6 and almost pure Li7, (Dᴍm6/Dᴍm7) = 1.35, and the viscosity ratio, (η7/η6) = 1.44. The dependence of D upon isotopic alloy concentration appears to be relatively weak and linear in all but the very low concentrations. As a test of the experimental method, D was measured in liquid Na, H2O , and D2O . In liquid sodium at the melting point, D = (3.7 + 0.3) • 10-5 cm2/sec. The Na measurement and the values of D in water are about 5% below other published data, but they are in agreement within experimental uncertainties. Results of this experiment contain syste­matic corrections amounting to 15 - 20% caused principally by the finite size of the droplets of the liquid metal samples. The corrections as well as the limitations of the technique in measurement of self diffusion coefficients in metals are discussed