Vibrational density of states of silicon nanoparticles

The vibrational density of states of silicon nanoparticles in the range from 2.3 to 10.3 nm is studied with the help of molecular-dynamics simulations. From these simulations the vibrational density of states and frequencies of bulk-like vibrational modes at high-symmetry points of the Brillouin-zone have been derived. The results show an increase of the density of states at low frequencies and a transfer of modes from the high-frequency end of the spectrum to the intermediate range. At the same time the peak of transverse optical modes is shifted to higher frequencies. These observations are in line with previous simulation studies of metallic nanoparticles and they provide an explanation for a previously observed discrepancy between experimental and theoretical data [C. Meier et al., Physica E, 32, 155 (2006)].Comment: 7 pages, 5 figure; accepted for publication in Phys. Rev.

Origin of Low Thermal Conductivity in Nuclear Fuels

Using a novel many-body approach, we report lattice dynamical properties of UO2 and PuO2 and uncover various contributions to their thermal conductivities. Via calculated Grueneisen constants, we show that only longitudinal acoustic modes having large phonon group velocities are efficient heat carriers. Despite the fact that some optical modes also show their velocities which are extremely large, they do not participate in the heat transfer due to their unusual anharmonicity. Ways to improve thermal conductivity in these materials are discussed.Comment: 4 pages, 3 figures, 1 tabl

Estimates of Soil Solution Ionic Strength and the Determination of pH in West Australian Soils

The average ionic strength of 20 West Australian soils was found to be 0.0048. The effects of three electrolytes (deionized water, CaCl2 and KNO3), three ionic strengths (0.03, 0.005 and soil ionic strength at field capacity, Is) and two soil liquid ratios (1:5 and 1:10) on the pH of 15 soils were investigated. pH measurements in solutions of ionic strength 0.005 differed the least from measurements made at Is. The differences that occurred in comparisons with distilled water or CaCl2 of ionic strength 0.03 (0.01 M) were much greater (≥0.4 pH units). An extractant with an ionic strength of 0.005 may provide a more realistic measure of pH in the field than distilled water or 0.01 M CaCl2 for West Australian soils