Crystal growth rates during laser-induced, liquid-mediated crystallization of amorphous Ge were measured with movie-mode dynamic transmission electron microscopy (MM-DTEM), a photo-emission microscopy technique with nanosecond-scale time resolution. Films of 50-nm thick amorphous Ge were crystallized using a 12-ns laser pulse with a Gaussian spatial profile, which established high local temperature gradients in the specimen. Crystallization proceeded by formation of a central zone with a high nucleation rate (Zone I), followed by liquid-mediated outward growth of columnar grains (Zone II), followed by spiraling growth (Zone III) until the crystallization halted in cooler parts of the specimen. Zone II growth was imaged for several laser pulse energies with 20-ns electron pulses with 95 ns between frames. A thin liquid layer between the solid amorphous phase and the advancing crystallization front during Zone II growth was imaged. The Zone II growth rate for each experiment remained nearly constant although the crystallization front passes through a large temperature gradient. Measured growth rates ranged from 5.7 to 13.6 m/s, consistent with transient-liquid-layer mediated growth rather than solid-state growth. In contrast with a previous report, the growth rate did not increase systematically with laser energy or absorbed energy. The new results, together with previously reported data, suggest both sets of experiments were conducted under conditions where the growth rate saturates near its maximum value. A phenomenological model based on the concept of an upper and lower threshold temperatures for the Zone II growth was fitted to the data from these experiments and previous MM-DTEM crystallization experiments.

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Fast reactor technology provides a path to transmute long-lived transuranic actinide isotopes contained in spent nuclear fuels. However, development and demonstration of this technology requires an in-depth understanding of the fuel performance. The Advanced Fuels Campaign (AFC) seeks to demonstrate the technologies needed to transmute actinides by developing candidate nuclear fuels and possible transuranic transmutation compositions. As part of the AFC program, candidate nuclear fuels are subjected to irradiation testing and detailed postirradiation examination (PIE), which provides data on reactor fuel performance and input into future fuel design choices.

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