We have performed high-pressure strength experiments on tantalum and uranium using a diamond-anvil cell. These experiments determined the flow stresses of tantalum and uranium at room temperature and in the low strain rate limit (ЈϽ10 Ϫ6 sec Ϫ1 ) by using x-ray diffraction to measure the pressure gradients in the samples. We find that the flow stresses increase dramatically with increasing pressure and strain, with the flow stress of Ta reaching 10.3 GPa at a pressure of 85.8 GPa and an estimated strain of Ϸ90%, and the flow stress of U reaching 19.8 GPa at a pressure of 109.0 GPa and an estimated strain of Ϸ70%. With further increases in pressure and strain, the flow stresses decrease. This apparent strain-softening effect has also been observed in static high-pressure flow stress experiments on other materials, and has been suggested to be due to either material damage or preferred orientation of grains induced by large strains.

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Nanocrystalline GaN was synthesized through reactive laser ablation of gallium metal in a N2 atmosphere. X-ray diffraction, selected-area electron diffraction, and transmission electron microscopy measurements show that the GaN crystallites are as small as 2 nm in diameter, and follow a log-normal size distribution with a mean particle diameter of 12 nm. Size-selective photoluminescence and photoluminescence excitation spectroscopy reveal a continuous range of blueshifted band-edge emissions and absorptions starting from the bulk value for gallium nitride and continuing to below 300 nm. These results are consistent with a GaN particle size distribution that encompasses regions above and below the excitonic-Bohr radius of GaN, such that the GaN material shows combined bulk and quantum confined optical properties.

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