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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]

The thermal properties were investigated for hot-pressed zirconium diboride containing solid solution additions of tantalum, molybdenum, rhenium, vanadium, and chromium. The nominal additions were equivalent to 3 at.% of each metal with respect to zirconium. Using 0.5 wt% carbon as a sintering aid, powders were hot-pressed to near full density at 2150°C. Rietveld refinement of X-ray diffraction data was used to measure lattice parameters and to ensure that the additives formed solid solutions. Thermal conductivities were calculated from measured thermal diffusivities and temperature-dependent values for density and heat capacity. Thermal conductivities at 25°C ranged from 88 W·(m·K)−1 for nominally pure ZrB2 down to 28 W·(m·K)−1 for (Zr,Cr)B2. Electron contributions to thermal conductivity were calculated from electrical resistivity measurements using the Wiedemann-Franz law. Decreases in phonon and electron conduction correlated with the size of the metallic additive, indicating that changes in atom size in the Zr lattice positions reduced thermal transport. [ABSTRACT FROM AUTHOR]

The thermal properties were investigated for hot-pressed zirconium diboride-transition-metal boride solid solutions. The transition-metal additives included hafnium, niobium, tungsten, titanium, and yttrium. The nominal additions were equivalent to 3 at.% of each metal with respect to zirconium. Powders were hot-pressed to nearly full density at 2150°C using 0.5 wt% carbon as a sintering aid. Thermal diffusivity was measured using the laser flash method. Thermal conductivity was calculated from the thermal diffusivity results using temperature-dependent values for density and heat capacity. At 25°C, the thermal conductivity ranged from 88 to 34 W·(m·K)−1 for specimens with various additives. Electrical resistivity measurements and the Wiedemann-Franz law were used to calculate the electron contribution of the thermal conductivity and revealed that thermal conductivity was dominated by the electron contribution. The decrease in thermal conductivity correlated with a decrease in unit cell volume, indicating that lattice strain may affect both phonon and electron transport in ZrB2. [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]