Showing papers by "David S. Smith published in 2003"

Thermal Resistance of Grain Boundaries in Alumina Ceramics and Refractories

Abstract: The influence of grain boundaries on heat transfer through polycrystalline alumina has been investigated between 20° and 500°C. The thermal conductivities of small-grained porous ceramics and large-grained dense ceramics have been measured using the laser-flash technique. Two methods have been developed to assess the average thermal resistance of a grain boundary. The first method is based on the comparison of room-temperature thermal conductivities for dense ceramics that have various average grain sizes. This method yields a value of 0.9 × 10−8 m2·K·W−1. The second method, particularly suitable for porous ceramics, is based on the extrapolation of the inverse of the thermal conductivity versus temperature to give an intercept with the axis at T= 0 K. This value is attributed to the thermal resistance of grain boundaries. By taking into account the influence of the pore content using an effective medium theory, the average thermal resistance of a grain boundary has been evaluated to be 1.3 × 10−8 m2·K·W−1 in dense alumina and 2.2 × 10−8 m2·K·W−1 in alumina containing a pore volume fraction of 0.3.

Grain-boundary thermal resistance in polycrystalline oxides: alumina, tin oxide, and magnesia

Abstract: The influence of grain boundaries on heat transfer through polycrystalline oxides has been investigated. The approach is based on comparison of macroscopic thermal conductivity values for materials which have different grain sizes. Samples of alumina, magnesia, and tin oxide were made in the form of discs by standard ceramic processing of pressed powders. By control of the firing conditions, strong variations in the grain size and pore content were achieved in all three materials. The thermal conductivity was measured via the thermal diffusivity by the laser-flash technique. The average grain-boundary thermal resistances at room temperature in dense ceramics were deduced to be 0.9 x 10 - 8 m 2 K W - 1 for alumina, 0.7 x 10 - 8 m 2 K W - 1 for magnesia, and 1.2 × 10 - 8 m 2 .K W - 1 for tin oxide. Moreover, the grain-boundary thermal resistance in tin oxide is shown to be virtually constant in the temperature range 20-400°C. An alternative method was used to confirm these values, based on the measurement of the thermal conductivity as a function of temperature. The role of the effective heat-carrying cross section for a grain-boundary plane in a porous ceramic is also discussed.

High porosity SiC ceramics prepared via a process involving an SHS stage

Abstract: The preparation of porous SiC ceramics from stoechiometric mixtures of silicon and graphite has been studied. Products with very high pore contents (≈80%) were obtained using a process which consisted of heating the reactive pellets in purified argon, at 15 °C min −1 , up to 1430 °C and applying a weak d.c. voltage across the sample for 20 s. The resulting electrical current was necessary for the ignition of an SHS reaction simultaneously in the whole sample. The analysis of the sample microstructure evolution all along the process has enabled the identification of the different mechanisms involved in the SiC formation. Before the SHS stage, the formation of silicon carbide, during heating from about 1325 up to 1430 °C, is associated with a large sample expansion, which mainly determined the final pore volume fraction. The pore transfer mechanisms, which occur during the SHS stage at 1430 °C, have a specific influence on the pore development. Since the final pore size distribution is strongly related to silicon grain granulometry, the porosity of the porous SiC ceramic, obtained by this process, can be easily modulated.

Importance of Biologically Active Aurora-like Ultraviolet Emission: Stochastic Irradiation of Earth and Mars by Flares and Explosions

Abstract: (Abridged) We show that sizeable fractions of incident ionizing radiation from stochastic astrophysical sources can be redistributed to biologically and chemically important UV wavelengths, a significant fraction of which can reach the surface. This redistribution is mediated by secondary electrons, resulting from Compton scattering and X-ray photoabsorption, with energies low enough to excite atmospheric molecules and atoms, resulting in a rich aurora-like spectrum. We calculate the fraction of energy redistributed into biologically and chemically important wavelength regions for spectra characteristic of stellar flares and supernovae using a Monte-Carlo transport code written for this problem and then estimate the fraction of this energy that is transmitted from the atmospheric altitudes of redistribution to the surface for a few illustrative cases. Redistributed fractions are found to be of order 1%, even in the presence of an ozone shield. This result implies that planetary organisms will be subject to mutationally significant, if intermittent, fluences of UV-B and harder radiation even in the presence of a narrow-band UV shield like ozone. We also calculate the surficial transmitted fraction of ionizing radiation and redistributed ultraviolet radiation for two illustrative evolving Mars atmospheres whose initial surface pressures were 1 bar. Our results suggest that coding organisms on planets orbiting low-mass stars (and on the early Earth) may evolve very differently than on contemporary Earth, with diversity and evolutionary rate controlled by a stochastically varying mutation rate and frequent hypermutation episodes.