Thermophysical properties of uranium dioxide

An inconsistency between commonly used values of the silicon intrinsic carrier concentration, the effective densities of states in the conduction and valence bands, and the silicon band gap is resolved by critically assessing the relevant literature. As a result of this assessment, experimentally based values for the valence‐band ‘‘densities‐of‐states’’ effective mass are determined in the 300–500 K range and are shown to be in good agreement with recent theoretical calculations. At 300 K, experimentally based values of 3.1×1019 cm−3 for the valence‐band effective densities of states and 1.08×1010 cm−3 for the intrinsic carrier concentration are determined. Although in good agreement with theoretical calculations, these are significantly higher and lower, respectively, than commonly used values in the past. These results have important implications in the calculation of other silicon material and device parameters.

Intrinsic concentration, effective densities of states, and effective mass in silicon

An account is given of experiments in which the shear flow stress of mild steel was measured at temperatures from 195 to 713°K and strain rates from 10−3 to 4 × 104 sec−1 The experimental results obtained at room temperatures are compared with those of earlier tension tests The rate sensitivity of the flow stress, (∂τ/∂ In γ) T , is found to be a decreasing function of temperature, except at the highest strain rates; at these rates a large increase in the rate sensitivity is observed, the flow stress at constant temperature varying approximately linearly with strain rate The data are interpreted in terms of thermal activation rate theory and the theory of the damping of dislocation motion by phonon viscosity

The temperature and strain-rate dependence of the shear strength of mild steel

Good thermoelectric materials are often complex compounds. Here, the authors reveal that elemental tellurium has a high thermoelectric figure of merit between 300 and 700 K when doped with As, with the potential advantages of easy preparation and relative isotropy.

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Tellurium as a high-performance elemental thermoelectric.

The high density of heat generated in power electronics and optoelectronic devices is a critical bottleneck in their application. New materials with high thermal conductivity are needed to effectively dissipate heat and thereby enable enhanced performance of power controls, solid-state lighting, communication, and security systems. We report the experimental discovery of high thermal conductivity at room temperature in cubic boron arsenide (BAs) grown through a modified chemical vapor transport technique. The thermal conductivity of BAs, 1000 ± 90 watts per meter per kelvin meter-kelvin, is higher than that of silicon carbide by a factor of 3 and is surpassed only by diamond and the basal-plane value of graphite. This work shows that BAs represents a class of ultrahigh-thermal conductivity materials predicted by a recent theory, and that it may constitute a useful thermal management material for high-power density electronic devices.

https://science.sciencemag.org/content/sci/early/2018/07/03/science.aat8982.full.pdf

High thermal conductivity in cubic boron arsenide crystals.

Results are presented of measurements of the thermal conductivity (90-1328\ifmmode^\circ\else\textdegree\fi{}K), the electrical resistivity (300-1273\ifmmode^\circ\else\textdegree\fi{}K), and the Seebeck coefficient (350-1273\ifmmode^\circ\else\textdegree\fi{}K) for single-crystal and large-grained polycrystalline specimens of 99.99+% pure silicon. The thermal conductivity above 387\ifmmode^\circ\else\textdegree\fi{}K was measured by an absolute radial-heat-flow technique; below 350\ifmmode^\circ\else\textdegree\fi{}K, by an absolute longitudinal technique. Some intermediate thermal-conductivity measurements from 300-400\ifmmode^\circ\else\textdegree\fi{}K were made on the polycrystalline material using a comparative longitudinal-heat-flow apparatus. The estimated errors of these three thermal-conductivity methods were \ifmmode\pm\else\textpm\fi{}2%\ifmmode\pm\else\textpm\fi{}2\ifmmode^\circ\else\textdegree\fi{}K, \ifmmode\pm\else\textpm\fi{}1.2%\ifmmode\pm\else\textpm\fi{}0.1\ifmmode^\circ\else\textdegree\fi{}K, and \ifmmode\pm\else\textpm\fi{}4.0%\ifmmode\pm\else\textpm\fi{}1\ifmmode^\circ\else\textdegree\fi{}K, respectively. The estimated error for the electrical-resistivity measurements was \ifmmode\pm\else\textpm\fi{}1.4%\ifmmode\pm\else\textpm\fi{}2\ifmmode^\circ\else\textdegree\fi{}K, and for the Seebeck measurements \ifmmode\pm\else\textpm\fi{}1.6%\ifmmode\pm\else\textpm\fi{}2\ifmmode^\circ\else\textdegree\fi{}K. The thermal-conductivity values were compared with conflicting data from the literature, and they corroborate the higher-temperature results obtained by Glassbrenner and Slack. Therefore, we agree with their conclusion that the electronic contribution is reasonably close to theoretical estimates which include a large ambipolar-diffusion term. The temperature dependence of the lattice thermal resistance has been compared to various theoretical models but no approach seems to explain the data in detail. An abrupt slope change in the thermal resistivity at about 670\ifmmode^\circ\else\textdegree\fi{}K is a major cause of the difficulty.

Thermal Conductivity, Electrical Resistivity, and Seebeck Coefficient of Silicon from 100 to 1300°K

The effects of temperature, purity, magnetic state, and crystal structure on the thermal conductivity, electrical resistivity, and Seebeck coefficient of iron were obtained from measurements on Armco iron (99.5% pure, ρ300/ρ4.2=11.0) and a high‐purity iron (99.95% pure, ρ300/ρ4.2=26.2). The most probable determinate errors of the measurements were thermal conductivity ±1.5%, electrical resistivity ±0.1%, and Seebeck coefficient ±0.9%; and larger absolute errors. Where theory permits, the thermophysical properties of iron are discussed in terms of contributing transport mechanisms. The thermal conductivity of iron can be calculated to ±1.5% between 0° and 910°C from electrical‐resistivity measurements and the lattice portion of the thermal conductivity determined in this study.

Comparison of the Thermal Conductivity, Electrical Resistivity, and Seebeck Coefficient of a High‐Purity Iron and an Armco Iron to 1000°C

Measurements are reported for the thermal conductivity, λ(80° to 400°K), electrical resistivity, ρ(4.2° to 400°K), and absolute Seebeck coefficient, Q(6° to 400°K), of pressed and sintered uranium mononitride. The measurements between 77° and 400°K were made using an absolute longitudinal heat flow apparatus. These data and literature values for the thermal conductivity and electrical resistivity at higher temperatures were used to separate the electronic and lattice portions of the thermal conductivity. The results indicate that the lattice conductivity peaks in the range 250° to 300°K and that the high-temperature limit of the Lorenz function may be greater than the Sommerfeld value of 2.443 × 10-8 (V2°K-2). The electrical resistivity and the absolute Seebeck coefficient exhibit sharp slope changes near the Neel temperature, TN(∼50° to 60°K). The Seebeck coefficient has a minimum at 34°K and then rises to a local maximum at 10°K. This low-temperature peak is probably due to magnon drag. The temperature dependence of the electrical resistivity is dominated by the magnetic contribution which increases as T2.38±0.08 between 10° and 52°K. The magnetic contribution is constant at high temperatures with an estimated value of 142 μΩ-cm.

Thermal Conductivity, Electrical Resistivity, and Seebeck Coefficient of Uranium Mononitride

A radial heat flow technique was used to measure the thermal conductivity, k, of polycrystalline UO2in the range -57° to 1100°C. The technique yielded results with a probable accuracy of ±1.5% and a precision of ±0.1% in the range 50° to 1100°C. Meaningful measurements were limited to 1100°C by Pt-90 Pt10Rh thermocouple instability, although the apparatus was structurally sound to 1400°C. The thermal conductivity data up to 1000°C could be explained on the basis of heat transport by phonons. The thermal resistance, l/k, exhibits a linear temperature dependence from 200° to 100°C, which is expected for an insulator well above the Debye temperature. The slope of the l/k-temperature plot is 0.0223 cm w−1 which is independent of impurity content and is associated with three phonon umklapp processes. The intercept is sensitive to impurity content as indicated by the fact that it was decreased by a decrease in the oxygen/uranium ratio. Between 1000° and 1100°C, there is a slight departure of l/k from linearity which may be due to the onset of an electronic contribution. Near room temperature, UO2 has a maximum in k which is apparently caused by the rapid decrease in specific heat below this temperature.

Thermal Conductivity of Uranium Dioxide from ‐57° to 1100°C by a Radial Heat Flow Technique

The specific heat of uranium mononitride was measured at temperatures from 1.3 to 4.6\ifmmode^\circ\else\textdegree\fi{}K and was found to be of the form $\ensuremath{\gamma}T+\ensuremath{\beta}{(\frac{T}{{\ensuremath{\Theta}}_{D}})}^{3}$, where $\ensuremath{\gamma}$ is 24.79\ifmmode\pm\else\textpm\fi{}0.05 mJ${(\mathrm{g}\mathrm{at}.\mathrm{wt})}^{\ensuremath{-}1}$${(\mathrm{deg})}^{\ensuremath{-}2}$ and the Debye $\ensuremath{\Theta}$ is 324\ifmmode\pm\else\textpm\fi{}7\ifmmode^\circ\else\textdegree\fi{}K. The large value of $\ensuremath{\gamma}$ (35 times the value for copper) apparently results from the Fermi surface intersecting narrow $5f$ electronic bands. The above values refer to a UN sample heat-treated in an atmosphere of nitrogen for 6.5 h at 2250\ifmmode^\circ\else\textdegree\fi{}C. The same sample, before this heat treatment, had a measured $\ensuremath{\gamma}=24.07\ifmmode\pm\else\textpm\fi{}0.08$ mJ ${(\mathrm{g}\mathrm{at}.\mathrm{wt})}^{\ensuremath{-}1}$ ${(\mathrm{deg})}^{\ensuremath{-}2}$ and $\ensuremath{\Theta}=322\ifmmode\pm\else\textpm\fi{}13\ifmmode^\circ\else\textdegree\fi{}$K. The shift in $\ensuremath{\gamma}$ may be due to a small change in stoichiometry caused by the heat treatment. Although the Debye temperature obtained from the present work, 324\ifmmode^\circ\else\textdegree\fi{}K, is larger than previous specific-heat estimates of 276-289\ifmmode^\circ\else\textdegree\fi{}K, it is lower than the value of 364\ifmmode^\circ\else\textdegree\fi{}K obtained from velocity-of-sound measurements. No changes were observed in the specific heat upon application of magnetic fields up to 35 kG, suggesting the absence of any appreciable magnetic contribution to the total specific heat for temperature below 4.7\ifmmode^\circ\else\textdegree\fi{}K.

W. Fulkerson

Papers

Thermal Conductivity, Electrical Resistivity, and Seebeck Coefficient of Silicon from 100 to 1300°K

Comparison of the Thermal Conductivity, Electrical Resistivity, and Seebeck Coefficient of a High‐Purity Iron and an Armco Iron to 1000°C

Thermal Conductivity, Electrical Resistivity, and Seebeck Coefficient of Uranium Mononitride

Thermal Conductivity of Uranium Dioxide from ‐57° to 1100°C by a Radial Heat Flow Technique

Specific Heat of Uranium Mononitride from 1.3 to 4.6°K