We measured thermopower, thermal conductivity and electrical resitivity for Ca₂Sn sample across 4-350 K temperature range. Contrary to expectations from recent DFT based calculations the thermopower is not particularly large, reaching 7 μV at 350 K. The thermoelectric figure of merit renders this material in unmodified form practically unusable for thermoelectric aplications.
A new approach to evaluate the relaxation times of various collision events responsible for thermal transport has been reported through which various deficiencies of earlier models of lattice thermal conductivity have been resolved. These investigations involve the evaluation of the phonon Green functions via a non-perturbative approach. The new expressions of relaxation times expressions for scattering of phonons by boundaries, atomic impurities, phonon-phonon scattering, electron-phonon scattering are the new features of the theory. The lattice thermal conductivity of three samples of GaAs has been analyzed on the basis of modified Callaway model and fairly good agreement between theory and experimental observations has been reported.
In this paper, preferentially oriented (002) ZnO thin films have been grown on Si (100) and glass substrates using radio frequency magnetron sputtering. The dependence of the quality of the ZnO thin films at different substrate temperatures on the growth is studied. A ZnO thin film with c-axis-oriented würtzite structure is obtained at a growth temperature from 200 to 400°C. X-ray diffraction shows that the full width at half maximum θ -2θ of (002) ZnO/Si is located at approximately 34.42°, which is used to infer the grain size that is found to be 17 nm to 19.7 nm. The FWHM is 9.5° to 8° in rocking curve mode, from which the crystalline quality has been determined. The texture degree demonstrates the improvement in quality with the increase of substrate temperature, which is best at 400°C. The band gap extracted by UV transmittance spectrum has been identified as 3.2 eV at 400°C. The electrical characteristics via C-V and I-V measurements on the basis of the heterojunction thermal emission model confirm the domination of high-density grain boundary layer existing at the interface. The transport currents indicate to the presence of space-charge-limited current and trap-charge-limited current mechanisms.
The high temperature properties of CeCuAl_{3} single crystal were investigated by differential scanning calorimetry and high-temperature x-ray diffraction. The structural phase transition takes place around 300 °C. The phase transition changes the structural parameters only, the tetragonal BaNiSn_{3}-type structure is preserved. The significant changes of lattice parameters and especially atomic fraction coordinates with this transition are discussed with respect to the number of heating/cooling cycles and stability in time. Magnetic properties of as-cast and annealed sample are shown for comparison.
Reduction of thermal conductivity remains a main approach relevant to enhancement of figure-of-merit of most thermoelectric materials. Melt spinning combined with spark plasma sintering appears to be a vital route towards fine-grain skutterudites with improved thermoelectric performance. However, upon high-temperature processing the Fe_{4-x}Co_{x}Sb_{12}-based skutterudites are prone to decompose into multiple phases, which deteriorate their thermoelectric performance. In this study we addressed the effects of combined melt spinning and spark plasma sintering on the phase composition and microstructural properties of filled Fe_{4-x}Co_{x}Sb_{12} as well as their influence on thermoelectric characteristics of these compounds. The crystallites of filled Fe_{4-x}Co_{x}Sb_{12} were effectively reduced to sizes below 100 nm upon melt spinning, but also severe decomposition with weakly preserved nominal phase was observed. Spark plasma sintering of melt spun skutterudites resulted in even further reduction of crystallites. Upon short annealing and sintering the n-type materials easily restored into single-phase filled CoSb₃ with nanoscale features preserved, while secondary phases of FeSb₂ and Sb remained in p-type compounds. Relatively high figure-of-merit ZT_{max} of 0.9 at T ≈ 400°C has been gained in nanostructured Yb_{x}Co₄Sb_{12}, however, no significant reduction of thermal conductivity was observed. Abundant impurities in p-type filled Fe_{4-x}Co_{x}Sb_{12} led to drastic drop in their ZT, which even further degraded upon thermal cycling.
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