Determination of self-diffusion coefficients by quasielastic neutron scattering measurements of levitated Ni droplets

Conventional techniques to measure diffusion coefficients in liquid metals and alloys are hampered by buoyancy-driven convective fluid flow and chemical reactions of the liquids with container material. To overcome these obstacles we combined containerless processing via electromagnetic levitation with quasielastic neutron scattering. This combination allowed us to study the atomic self-motion in liquid nickel within a broad temperature range from $200\mathrm{K}$ above to more than $200\mathrm{K}$ below the melting point, in the metastable regime of an undercooled melt. Other than in liquid Sn the temperature dependence of the Ni self-diffusion coefficient is well described with an Arrhenius law.

Figure 1

Scattering law $S(q,\omega )$ of liquid Ni: (a) at fixed momentum transfer $q$ for two different temperatures; more than $200\mathrm{K}$ above melting point (open circles) and more than $200\mathrm{K}$ below melting point (full circles), (b) at $1514\mathrm{K}$ (undercooled) for two $q$ values, 0.6 and $1.7{\mathrm{\AA }}^{-1}$, respectively. The lines are fits with a Lorentz function that has been convoluted by the instrumental energy resolution function.

Figure 2

$q$ dependent linewidth of the scattering function $S(q,\omega )$ for different temperatures. Lines are fits with Eq. (2) in the $q$ range of $0.6–1.5{\mathrm{\AA }}^{-1}$.

Figure 3

Ni self-diffusion coefficients $D$ as a function of inverse temperature in a range spreading from more than $200\mathrm{K}$ above to more than $200\mathrm{K}$ below the melting point. The full circles are the data points determined by the levitation experiments of this work, and the open circles are data points from the experiments of Ref. 14. The line is an Arrhenius fit of the experimental data resulting in an activation energy of $0.47\mathrm{eV}∕\mathrm{at.}$. The dashed line represents the best fit with a $T^2$ law.