Electronic Component Of Thermal Conductivity Research Articles

Feasibility of high performance in p‐type Ge1−xBixTe materials for thermoelectric modules

AbstractGeTe is a promising candidate for the fabrication of high‐temperature segments for p‐type thermoelectric (TE) legs. The main restriction for the widespread use of this material in TE devices is high carrier concentration (up to ∼ 1021 cm−3), which causes the low Seebeck coefficient and high electronic component of thermal conductivity. In this work, the band structure diagram and phase equilibria data have been effectively used to attune the carrier concentration and to obtain the high TE performance. The Ge1−xBixTe (x = 0.04) material prepared by the Spark plasma sintering (SPS) technique demonstrates a high power factor accompanied by moderate thermal conductivity. As a result, a significantly higher dimensionless TE figure of merit ZT = 2.0 has been obtained at ∼ 800 K. Moreover, we are the first to propose that application of the developed Ge1−xBixTe (x = 0.04) material in the TE unicouple should be accompanied by SnTe and CoGe2 transition layers. Only such a unique solution for the TE unicouple makes it possible to prevent the negative effects of high contact resistance and chemical diffusion between the segments at high temperatures.

Effect of heat treatment on the galvanomagnetic properties of bulk nanostructured samples of Bi85Sb15 solid solution

Extruded bulk nanostructured samples of Bi85Sb15 solid solution from particles with average sizes ∼ 2.105; 950; 650; 380; 30 and 15 nm were obtained and investigated their galvanomagnetic properties in the range of ∼ 77-300 K. Were investigated samples that have not passed heat treatment, and the same samples that have passed heat treatment. It was found that the electrical and thermal properties of Bi85Sb15 solid solution samples significantly depend on the size of nanoparticles and post-extrusion heat treatment. Heat treatment leads to a decrease in the concentration of current carriers and to an increase in the mobility of current carriers and the total thermal conductivity of the samples under study, which is mainly due to the electronic component of thermal conductivity. The change in thermal parameters is satisfactorily explained by changes in occurring in the structure of samples during extrusion and heat treatment and correlates well with changes in the electrical parameters of these processes.

Влияние фотонной обработки на структуру и субструктуру термоэлектрического материала Bi2Te3 – хSeх

X-ray diffractometry, scanning electron microscopy and transmission electron microscopy techniques were used to study the phase composition and morphology as well as structure and substructure of semiconductor plates based on Bi2Te3 –xSex before and after photon treatment (PT) by radiation of high-power xenon lamps (spectral range λ = 0.2–1.2 μm) in an Ar atmosphere. It was found that the PT results in recrystallization of the near-surface layers (~2 μm) of the Bi2Te3 – xSex solid solution with the formation of inhomogeneous structure in the form of microcrystals surrounded by nanocrystalline grains. It also changes the chemical composition of the near-surface layer (~5 μm) associated with an increase in the concentration of selenium. More than a 40-fold increase in the area of intergrain boundaries and the formation of nanocontacts along the boundaries of the microcrystalline phase make a significant contribution to the scattering of phonons, and a change in the concentration of tellurium and selenium leads to a decrease in the electronic component of thermal conductivity. Substructural changes correlate with a significant (up to 5%) reduction in the thermal conductivity of samples of Bi2Te3 – xSex solid solution with a surface layer modified as a result of PT.

Electronic structure and thermophysical properties of U3Si2: A systematic first principle study

Uranium silicides are considered to be prominent accident tolerant fuel for the light water reactors due to their high metal density and high thermal conductivity. Among the uranium silicon binary compounds, U3Si2 is found to be advantageous. Here, we present a systematic first principles study of electronic structure and thermophysical properties of U3Si2 within the framework of quasi-harmonic approximation. The relativistic effects have been treated through incorporating the spin-orbit interactions for all the calculations. The optimized cell volume from PBE method is found to be slightly underestimated, however, from the PBE+U method, it is found to be slightly overestimated as compared to the experimental volume, all the electronic structure results are comparable to the reported results. Volume dependant phonon frequencies have been calculated using the density functional perturbation theory to incorporate the effect of anharmonicity indirectly through quasi-harmonic approximation. Various thermophysical properties like free energy, thermal expansion, heat capacity, bulk modulus, etc. have been evaluated. The vibrational contribution alone to molar specific heat is found to be underestimated as compared to the experimentally reported results. Interestingly, incorporation of the electronic contribution is found to improve the results significantly. Electronic component of thermal conductivity has been calculated using the Boltzmann transport theory. The computed results are comparable to the available experimental results, which supports the reliability of the present study. All these predicted properties are very much important to gain knowledge about the U3Si2 based fuel.

Electronic conductivity of solid and liquid (Mg, Fe)O computed from first principles

Ferropericlase (Mg, Fe)O is an abundant mineral of Earth's lower mantle and the liquid phase of the material was an important component of the early magma ocean. Using quantum-mechanical, finite-temperature density-functional theory calculations, we compute the electronic component of the electrical and thermal conductivity of (Mg0.75, Fe0.25)O crystal and liquid over a wide range of planetary conditions: 0–200 GPa, 2000–4000 K for the crystal, and 0–300 GPa, 4000–10,000 K for the liquid. We find that the crystal and liquid are semi-metallic over the entire range studied: the crystal has an electrical conductivity exceeding 103 S/m, whereas that of the liquid exceeds 104 S/m. Our results on the crystal are in reasonable agreement with experimental measurements of the electrical conductivity of ferropericlase once we account for the dependence of conductivity on iron content. We find that a harzburgite-dominated mantle with ferropericlase in combination with Al-free bridgmanite agrees well with electromagnetic sounding observations, while a pyrolitic mantle with a ferric-iron rich bridgmanite composition yields a lower mantle that is too conductive. The electronic component of thermal conductivity of ferropericlase with XFe=0.19 is negligible (<1 W/m/K). The electrical conductivity of the crystal and liquid at conditions of the core-mantle boundary are similar to each other (3×104 S/m). A crystalline or liquid ferropericlase-rich layer of a few km thickness thus accounts for the high conductance that has been proposed to explain anomalies in Earth's nutation. The electrical conductivity of liquid ferropericlase exceeds that of liquid silica by more than an order of magnitude at conditions of a putative basal magma ocean, thus strengthening arguments that the basal magma ocean could have produced an ancient dynamo.

OBTAINING A COPPER SELENIDE BASE MATERIAL BY POWDER METALLURGY METHODS

Copper selenide is a promising material for power generation in medium−temperature range 600—1000 K. A number of features of the Cu—Se system, i.e. the existence of a phase transition in Cu2Se compound, the high speed of Cu ion diffusion and the high vapor pressure of Se at high temperatures, necessitate massive experimental investigations aimed to develop and optimize a method for obtaining a copper selenide base bulk material. In this work the effect of mechanochemical synthesis mode and subsequent compaction method on the thermoelectric properties and structure of copper selenide were studied. The source material was obtained by mechanochemical synthesis. The hot pressing and spark plasma sintering methods were used for obtaining the bulk samples. The structure and phase composition were studied by X−ray diffraction and scanning electron microscopy. We show that increasing the time of mechanochemical synthesis to 5 hours leads to copper depletion of the powders and the formation of nonstoichiometric phase Cu1,83Se which persists after spark plasma sintering. Comparison of the structure and properties of the material obtained by spark plasma sintering and hot pressing showed that the material obtained by hot pressing has a greater degree of the grain defects. The highest thermoelectric efficiency ZT = 1.8 at 600 °C was observed in the material obtained by spark plasma sintering. We show that the main factor affecting the value of the thermoelectric efficiency ZT of the studied materials is the low thermal conductivity. The difference in thethermal conductivities of the materials obtained by different methods is attributed to the electronic component of thermal conductivity.

Obtaining Material Based on Copper Selenide by the Methods of Powder Metallurgy

Copper selenide is a promising material for power generation in a medium-temperature range 600–1000 K. A number of features of the Cu–Se system, namely, the existence of phase transition in a Cu2Se compound, the high speed of the diffusion of Cu ions, and the high vapor pressure of Se at elevated temperatures, make it necessary to carry out a series of experimental investigations to develop and optimize the methodology for obtaining the bulk material based on copper selenide. The influence of mechanochemical synthesis regimes and subsequent compaction method on the thermoelectric properties and structure of copper selenide is studied. The source material is obtained by mechanochemical synthesis. The methods of hot pressing (HP) and spark plasma synthesis (SPS) are used to obtain the bulk samples. The investigation of the structure and phase composition is performed by the X-ray diffraction and scanning electron microscopy. It is shown that increasing the duration of the mechanochemical synthesis up to 5 h leads to the depletion of copper in powders and to the formation of nonstoichiometric β-phase Cu1.83Se, which persists after SPS. A comparison of the structure and properties of the material obtained by SPS and HP showed that the material obtained by HP has a greater degree of grain defects. The highest thermoelectric efficiency ZT = 1.8 at a temperature of 600°C is achieved for the material obtained by SPS. It is shown that low thermal conductivity is the main factor affecting the value of the thermoelectric efficiency ZT of the studied materials. The difference in the values of thermal conductivity of the materials obtained by different methods is related to the electronic component of thermal conductivity.

Effect of electronic contribution on temperature-dependent thermal transport of antimony telluride thin film

We study the theoretical and experimental characteristics of thermal transport of 100nm and 500nm-thick antimony telluride (Sb2Te3) thin films prepared by radio frequency magnetron sputtering. The thermal conductivity was measured at temperatures ranging from 20 to 300K, using four-point-probe 3-ω method. Out-of-plane thermal conductivity of the Sb2Te3 thin film was much lesser in comparison to the bulk material in the entire temperature range, confirming that the phonon- and electron-boundary scattering are enhanced in thin films. Moreover, we found that the contribution of the electronic thermal conductivity (κe) in total thermal conductivity (κ) linearly increased up to ∼77% at 300K with increasing temperature. We theoretically analyze and explain the high contribution of electronic component of thermal conductivity towards the total thermal conductivity of the film by a modified Callaway model. Further, we find the theoretical model predictions to correspond well with the experimental results.

Unusual behavior of the lattice thermal conductivity and of the Lorenz number in the YbIn1−x Cu4+x system

Samples of various compositions were obtained in the homogeneity range of the Yb-In-Cu system (YbIn1−xCu4+x), from stoichiometric (YbInCu4) to YbIn0.905Cu4.095. Their lattice constant (at 300 K and in the range 20–100 K), total thermal conductivity, and electrical resistivity (from 4 to 300 K) were measured. All the compositions studied exhibited an isostructural phase transition at Tv⋍40–80 K driven by a change in the Yb ion valence state. It was shown that within the YbIn1−xCu4+x homogeneity range, the lattice thermal conductivity κph decreases with increasing x; at T>Tv, κph grows with temperature and the Lorenz number (which enters the Wiedemann-Franz law for the electronic component of thermal conductivity) of the light heavy-fermion system, to which YbIn1−xCu4+x belongs for T<Tv, behaves as it does in classical heavy-fermion systems. Thermal cycling performed through Tv generates stresses in the YbIn1−xCu4+x lattice, which entails an increase in the electrical resistivity and a decrease in the thermal conductivity. “Soft anneal” (prolonged room-temperature aging of samples) makes the effect disappear. A conclusion is drawn as to the nature of the effects observed.

Thermal conductivity and electrical resistivity of cadmium arsenide (Cd3As2) in the temperature range 4.2?40 K

Results on electrical resistivity and thermal conductivity measured in the temperature range 4.2–40 K are presented for single-crystal and polycrystalline samples of Cd3As2. Hall effect has been studied at temperatures of 4.2, 77, and 300 K. The calculated value of the conduction electron concentration was in the range 1.87–1.95 1024m−3. Electrical resistivity of all investigated samples was independent of temperature up to about 10K and increased slowsly at higher temperatures. The thermal conductivity shows a maximum in the region in which the lattice component of thermal conductivity dominates. The strong anisotropy of the lattice component determines the anisotropy of the total thermal conductivity. The electronic component of thermal conductivity does not exhibit any anisotropy and shows a maximum at a temperature of about 300 K.

Anomalous Thermal Conductivity of Cd3As2 and the Cd3As2−Zn3As2 Alloys

The room-temperature lattice thermal conductivity of Zn3As2 is 0.012 W/cm·°C; that of Cd3As2 is 0.014 W/cm·°C or less. Anomalously low thermal conductivities (as much as 30% below calculated values) are found for samples of Cd3As2 and Cd-rich alloys of Cd3As2 with Zn3As2 where the electrical conductivity is high (&amp;gt;103 Ω−1 cm−1). Thermal conductivities for Cd3As2 samples fall into two groups: Samples doped with an element which is expected to enter the anion sublattice have normal thermal conductivities, while undoped samples or those doped with an element which should enter the cation sublattice tend to have anomalously low thermal conductivities. High electron mobilities and general lack of correlation of carrier concentration with thermal conductivity indicate that the anomaly is not in the electronic component of thermal conductivity. Instead, doping experiments, as well as the temperature dependencies of thermal conductivities indicate that the anomaly is entirely in the lattice component and is due to a lattice defect in the anion sublattice, probably an As vacancy. Although the lattice thermal conductivity of some samples of Cd3As2 appears to be only 0.003–0.004 W/cm·°C, their electron mobility is 104 cm2/V·sec. To explain the apparently enormous phonon scattering cross-section of the lattice defect, it is suggested that the phonon scattering may involve the transitions of localized electrons from one type of As site (vacancy) to another (there are three different types of As sites in Cd3As2).

Die Wärmeleitfähigkeit von Germanium bei hohen temperaturen

An account is given of thermal conductivity measurements on germanium monocrystals, both intrinsic and with high hole-conductivity. The author took mobility values, and values for the width of the forbidden zone and its dependence on temperature—all necessary for theoretical discussion of the electronic component of thermal conductivity—from the measurements of other authors, and confirmed them by comparison with his own measurements of thermoelectric force and electrical conductivity. The measured values of the electronic component of thermal conductivity, of both intrinsic and hole-conductive germanium, are about 4 times as high as those determined theoretically.