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Technetium-99 is a highly radioactive isotope with a long half-life that is common in nuclear waste. It volatizes at a low temperature, which poses a significant challenge to the clean-up and containment processes. Due to difficulties in purifying technetium compounds, their thermophysical properties have not been measured or calculated. Here, first principle methods are used along with the quasi quasi-harmonic harmonic approximation to compute the Debye temperature, volumetric thermal expansion coefficient, bulk modulus, and heat capacity of rutile TcO2 for temperatures ranging from 0 to 1500 K and applied pressures ranging from 0 to 255 GPa. The computed atomic structures agree well with the results from diffraction measurements. The computed thermophysical properties are in the neighborhood of other rutile metal oxides and, in particular, are within approximately 10–13% of rutile ReO2, which is frequently used as a substitute for TcO2 in experimental studies. [ABSTRACT FROM AUTHOR]

The arsenic (As) to selenium (Se) ratio in AsxSe1-x glasses ranging from x=0.2 to 0.5 was varied in order to examine the effect of chemical and topological ordering on the glass' thermal transport behavior. The fundamental thermal properties of glass transition temperature (Tg), thermal conductivity (k), and heat capacity (cp) were experimentally measured using differential scanning calorimetry, transient plane source method, and ultrasonic testing. Based on topological constraint theory, inflections in Tg and k were found at the structural coordination number of 2.4, whereas a slight increase in heat capacity (cp) with increasing was observed. A maximum in total thermal conductivity of 0.232 W/m.K was measured for the composition with x=0.4, which corresponds to the stoichiometric As2Se3. Gas kinetic theory was used to derive an expression for the photon (kp) portion of thermal conductivity, which was calculated by measurements of the glass' absorption coefficient (α) and refractive index (n). Models based on Debye theory were used to derive expressions for specific heat (cv) and the lattice (kl) portion of thermal conductivity. The maximum value for kp was 0.173 W/m.K for the composition with x=0.2, and a minimum value of 0.144W/m.K was measured for the composition with x=0.4. Photonic conduction was found to be the dominant carrier mechanisms in all compositions, comprising 60% to 95% of the measured total thermal conductivity. [ABSTRACT FROM AUTHOR]

It has been known since the 1950s that plutonium oxalate powders change color and lose mass over time when stored at room temperature in air. Despite several studies monitoring these changes, there are still discrepancies in the literature regarding the speciation of intermediate and final products that result from this decomposition. Presented here for the first time is a comprehensive series of time‐resolved powder X‐ray diffraction experiments coupled with solid‐state optical spectroscopy and electron microscopy of aged plutonium (III) and (IV) oxalate powders. These data provide fresh insight into the chemical and structural changes that occur in these solids over time at room temperature and represent new evidence suggesting both plutonium (III) and plutonium (IV) oxalates auto‐decompose to form nanocrystalline plutonium oxide in the solid state. [ABSTRACT FROM AUTHOR]

Melt size‐dependent physical property variation is examined in a multicomponent GeSe2‐As2Se3‐PbSe chalcogenide glass developed for gradient refractive index applications. The impact of melting conditions on small (40 g) prototype laboratory‐scale melts extended to commercially‐relevant melt sizes (1.325 kg) have been studied and the role of thermal history variation on physical and optical property evolution in parent glass, the glass' crystallization behavior and postheat‐treated glass ceramics, is quantified. As‐melted glass morphology, optical homogeneity and heat treatment‐induced microstructure following a fixed, two‐step nucleation and growth protocol exhibit marked variation with melt size. These attributes are shown to impact crystallization behavior (growth rates, resulting crystalline phase formation) and induced effective refractive index change, neff, in the resulting optical nanocomposite. The magnitude of these changes is discussed based on thermal history related melt conditions. [ABSTRACT FROM AUTHOR]

The physical properties of chalcogenide glasses in the (GeSe2-3As2Se3)1-xPbSex [GAP-Se] series (x=0-55 mol%) have been measured as a function of PbSe content and glass morphology. Measurements of density, microhardness, thermal properties (glass transition, stability and conductivity) and IR transmission spectra have been correlated with Pb content illustrating the impact of liquid-liquid phase separation (LLPS) on properties within this multicomponent chalcogenide glass system. The role of Pb as both a modifier and network participant is proposed as a structural interpretation of property variation across the series. Density, microhardness and thermal conductivity showed an increase with PbSe content whereas glass transition temperature (Tg) exhibits a minimum near the center of the immiscibility zone correlated to a decrease in glass stability when a Pb-rich matrix is present. The structural origin of the change in the properties is confirmed using Raman spectroscopy and transmission electron microscopy (TEM) which illustrate the different morphology of phase separation present and how it impacts property evolution. [ABSTRACT FROM AUTHOR]

Sintering mechanisms and kinetics were investigated for ZrB2 ceramics produced using reaction hot pressing. Specimens were sintered at temperatures ranging from 1800°C to 2100°C for times up to 120 min. ZrB2 was the primary phase, although trace amounts of ZrO2 and C were also detected. Below 2000°C, the densification mechanism was grain-boundary diffusion with an activation energy of 241 ± 41 kJ/mol. At higher temperatures, the densification mechanism was lattice diffusion with an activation energy of 695 ± 62 kJ/mol. Grain growth exponents were determined to be ~4.5, which indicated that a grain pinning mechanism was active in both temperature regimes. The diffusion coefficients for grain growth were 1.5 × 10−16 cm4/s at 1900°C and 2.1 × 10−15 cm4/s at 2100°C. This study revealed that dense ZrB2 ceramics can be produced by reactive hot pressing in shorter times and at lower temperatures than conventional hot pressing of commercial powders. [ABSTRACT FROM AUTHOR]

Sintering mechanisms and kinetics were investigated for ZrB2 ceramics produced using reaction hot pressing. Specimens were sintered at temperatures ranging from 1800°C to 2100°C for times up to 120 min. ZrB2 was the primary phase, although trace amounts of ZrO2 and C were also detected. Below 2000°C, the densification mechanism was grain-boundary diffusion with an activation energy of 241 ± 41 kJ/mol. At higher temperatures, the densification mechanism was lattice diffusion with an activation energy of 695 ± 62 kJ/mol. Grain growth exponents were determined to be ~4.5, which indicated that a grain pinning mechanism was active in both temperature regimes. The diffusion coefficients for grain growth were 1.5 × 10−16 cm4/s at 1900°C and 2.1 × 10−15 cm4/s at 2100°C. This study revealed that dense ZrB2 ceramics can be produced by reactive hot pressing in shorter times and at lower temperatures than conventional hot pressing of commercial powders. [ABSTRACT FROM AUTHOR]

The effect of Hf additions on the thermal properties of ZrB2 ceramics was studied. Reactive hot pressing of ZrH2, B, and HfB2 powders was used to synthesize (Zr1− x,Hf x)B2 ceramics with Hf contents ranging from x = 0.0001 (0.01 at.%) to 0.0033 (0.33 at.%). Room-temperature heat capacity values decreased from 495 J·(kg·K)−1 for a Hf content of 0.01 at.% to 423 J·(kg·K)−1 for a Hf content of 0.28 at.%. Thermal conductivity values decreased from 141 to 100 W·(m·K)−1 as Hf content increased from 0.01 to 0.33 at.%. This study revealed, for the first time, that small Hf contents decreased the thermal conductivity of ZrB2 ceramics. Furthermore, the results indicated that reported thermal properties of ZrB2 ceramics are affected by the presence of impurities and do not represent intrinsic behavior. [ABSTRACT FROM AUTHOR]