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In this study, the strain‐induced effects on thermal and electrical transport properties of LaCuOSe are studied by combining first‐principles calculations with semi‐classical Boltzmann transport theory. The results show that external strain affects the crystal and electronic structures of LaCuOSe significantly. It is interesting to find that the electrical conductivity of n‐type LaCuOSe is improved by the tensile strain. In addition, the tensile strain also has positive effects on the Seebeck coefficient of p‐type doped LaCuOSe. Consequently, the power factor of p‐type doped LaCuOSe is enhanced by 18.2% under 10% tensile strain at 800 K. However, due to the obviously affected mechanical properties of LaCuOSe under external strain, the decreased Grüneisen constant and reduced Debye temperatures are resulted, and the lattice thermal conductivity of LaCuOSe at 300 K increases from 2.91 to 5.95 Wm/K with the tensile strain increasing from 0% to 10%. The sharply increased thermal conductivity finally results in the optimal ZT value of 0.076 and 0.417 for ideal p‐ and n‐type LaCuOSe at 800 K being reduced to 0.048 and 0.286 under 10% tensile strain, respectively. These results suggest that the thermoelectrical properties of LaCuOSe need to be further improved for its application under stress. [ABSTRACT FROM AUTHOR]

The freeze‐casting method shows great potential for environmentally friendly and industrial scale‐up application in thermoelectric materials due to low cost, low toxicity, and pore control. The porous Ca3Co4O9 ceramics with anisotropic thermoelectric performance were fabricated by the freeze‐casting method. High porosities, hierarchical pores, and three‐dimensional (3D) networks significantly contributed to the suppressed thermal conductivity. An ultralow total thermal conductivity of 0.21 W/(m·K) was obtained. The optimized freeze‐casting conditions gave rise to a Seebeck coefficient of 232.3 µV/K. In addition, the ZT value of 0.39 was obtained at 800°C due to the manipulation of hierarchically connected pores and small portions of isolated hexagonal pores. This work provides a new route to form the 3D pores network in oxide thermoelectric ceramics, leading to a decrease in thermal conductivity and optimized thermoelectric performance parallel to the freeze‐casting direction. [ABSTRACT FROM AUTHOR]

We investigated the Ruddlesden‐Popper series CaO(CaMnO3)m with m = 1, 2, 3, ∞, to study the impact of the varying amounts of CaO layers on their thermoelectric properties. Previous studies showed that highly dense samples are difficult to obtain due to the refractory nature of these materials. In this study, we managed to obtain dense pellets during a classical hot‐pressing step, if and only if the samples were subjected to extended ball‐milling prior to pressing, resulting in crystallite sizes of 30–35 nm after hot‐pressing. The sample with the largest amount of CaO layers (m = 1) had the lowest electrical and thermal conductivity, and the highest Seebeck coefficient, as predicted. Ultimately the perovskite CaMnO3 (m = ∞, no CaO layers) exhibited the best thermoelectric properties. [ABSTRACT FROM AUTHOR]

Layered oxychalcogenides have received extensive attention in the fields of magnetism, superconductivity, lithium battery, and luminescence due to their unique electronic, magnetic properties, and layered structure, among which layered oxyselenides have excellent and promising thermoelectric performance, such as BiCuSeO and Bi2LnO4Cu2Se2. Here, we successfully synthesized Sr2MO2Cu2Se2 (M = Co, Ni, Zn) and Sr2FeO3CuSe and investigated the thermoelectric properties at a wide temperature range (298–923 K). They have a relatively high Seebeck coefficient (>300 μV K−1) in medium to high temperature range and possess a low thermal conductivity. The power factor and ZT reach 65 μW m−1 K−1 and 0.07 at 923 K for intrinsic Sr2NiO2Cu2Se2, and a higher performance is expected to be achieved by strategies like carrier concentration optimization and band structure engineering. [ABSTRACT FROM AUTHOR]

The cation distribution of the negative temperature coefficient (NTC) thermistor spinel NiMn2O4 was studied in the temperature range from 55 to 900°C, using a combined in situ neutron diffraction and thermopower study. Rietveld refinements of in situ neutron diffraction data reveal a temperature dependence of the degree of inversion with an inversion parameter of 0.70(1) at 900°C and 0.87(1) at 55°C. Thermopower measurements were evaluated using a modified Heikes formula, and the [Mn4+]/[Mn3+] ratio on octahedral sites of the spinel structure was calculated considering spin and orbital degeneracy. The inversion degree and disproportionation parameter, determined independently as function of temperature, were used to calculate the cation distribution of NiMn2O4 in the whole temperature range. At high temperature, within the stability range of the spinel, the cation distribution is characterized by a moderate degree of inversion with a concentration of NiB2+${\mathrm{Ni}}_{\mathrm{B}}^{2+}$ = 0.71(2) and small tendency toward disproportionation with a high concentration of MnB3+${\mathrm{Mn}}_{\mathrm{B}}^{3+}$ = 0.98(2). At 55°C, cation inversion is enhanced as illustrated by a large concentration of NiB2+${\mathrm{Ni}}_{\mathrm{B}}^{2+}$ = 0.87(2), and a small MnB3+${\mathrm{Mn}}_{\mathrm{B}}^{3+}$ = 0.24(2) content indicates extensive Mn‐disproportionation. Finally, conclusions regarding possible ageing mechanisms of this important NTC thermistor material are drawn. [ABSTRACT FROM AUTHOR]

Oxide‐based ceramics offer promising thermoelectric (TE) materials for recycling high‐temperature waste heat, generated extensively from industrial sources. To further improve the functional performance of TE materials, their power factor should be increased. This can be achieved by nanostructuring and texturing the oxide‐based ceramics creating multiple interphases and nanopores, which simultaneously increase the electrical conductivity and the Seebeck coefficient. The aim of this work is to achieve this goal by compacting electrospun nanofibers of calcium cobaltite Ca3Co4−xO9+δ, known to be a promising p‐type TE material with good functional properties and thermal stability up to 1200 K in air. For this purpose, polycrystalline Ca3Co4−xO9+δ nanofibers and nanoribbons were fabricated by sol–gel electrospinning and calcination at intermediate temperatures to obtain small primary particle sizes. Bulk ceramics were formed by sintering pressed compacts of calcined nanofibers during TE measurements. The bulk nanofiber sample pre‐calcined at 973 K exhibited an improved Seebeck coefficient of 176.5 S cm−1 and a power factor of 2.47 μW cm−1 K−2 similar to an electrospun nanofiber‐derived ceramic compacted by spark plasma sintering. [ABSTRACT FROM AUTHOR]

We investigate phase stability, microstructure, and thermoelectric transport of polycrystalline bulk Ca3−xRxMn2O7 samples prepared by standard solid‐state reaction, where R = Y or La and 0 ≤ x ≤ 0.33. Ab‐initio calculations predict that Y‐doping at Ca‐sites should reduce the potential energy barrier for electron transport, as opposed to La‐doping. We find that Y‐doping prompts transformation from Ca3Mn2O7 to Ca2MnO4, whereas La‐doping is accompanied by no phase transformation. La‐doping significantly hinders grain growth, for example, the average grain size decreases from 4.44 ± 0.24 to 1.20 ± 0.03 μm for x = 0 (undoped) and x = 0.33 upon La‐doping, respectively. Electrical conductivity and Seebeck coefficients are measured for the temperature range of 300–1300 K, and analyzed in terms of the small polaron hopping model. We find that Y‐doping reduces the activation energy for conduction compared to La‐doping, for example, 43 and 63 meV, respectively. This suggests that Y reduces the energy barrier for polaron transport, in accordance with computational predictions. This trend is further supported by calculations of selected electronic, structural, and vibrational properties, highlighting the intriguing correlation between electronic transport governed by small polarons and elastic properties, thereby shedding light on charge transport and thermoelectric properties of such layered perovskites. [ABSTRACT FROM AUTHOR]

GeTe 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. [ABSTRACT FROM AUTHOR]

Here we report the study temperature dependencies of the Seebeck coefficient, the electrical resistivity (T = 300–750 K), the total thermal conductivity (T = 300–973 K), and the thermoelectric figure of merit (T = 300–750 K) of ceramic samples of γ‐Ln0.8Yb0.2S1.5‐y (Ln = Gd, Dy) solid solutions. It was found that Yb3+ ions in γ‐Ln0.8Yb0.2S1.5‐y act as the promoters of higher crystallite nucleation rate during the formation of solid solutions. This results in the sample dispersion increase and the formation of the additional phonon scattering centers (dislocations and strain stresses along the crystallites semi‐coherent boundaries). These features of the real structure determined the low value of thermal conductivity of γ‐Ln0.8Yb0.2S1.5‐y solid solutions. The lowest electrical resistivity 20 μΩ m at 750 K and the thermal conductivity 0.58 W/m K at 973 K, the highest Seebeck coefficient 125 μV/K at 700 K and the maximum thermoelectric efficiency, ZT = 0.60 (at 770 K) were obtained for γ‐Dy0.8Yb0.2S1.5‐y. [ABSTRACT FROM AUTHOR]

The temperature dependences of the electrical conductivity σ(T), Seebeck coefficient σ(T), and heat capacity Cp(T) of polycrystalline samples of Bi2Te3, Bi2Te3+1%CuI, and Bi2Te3+1%(CuI+1/2Pb) are investigated in the temperature range below room temperature. Based on the temperature dependences of all investigated physical properties, it is discovered that phase transition occurs at 120–200 K. Investigation of single crystals shows that anomalies in the electrical resistivity (ρ(T)=1/σ(T)) occur only across the crystal growth axis (across the well‐conducting Bi–Te plane). Investigation of the low‐temperature dependence of electrical conductivity shows that all polycrystalline samples exhibit quasi‐two‐dimensional electron transport. Additionally, quasi‐two‐dimensional transport is detected in single crystals based on anisotropy analysis ρ⊥(T)/ρ‖(T):10 (where ρ‖(T) is the resistivity along the crystal growth axis, and ρ⊥(T) is resistivity across the crystal growth axis) and temperature dependence ρ(T):T2 below 50 K. The Fermi energy EF is estimated using the temperature dependence of S(T). It is discovered that an increase in EF at T > 200 K is associated with the phase transition. For single‐crystal samples, the maximum thermoelectric figure of merit ZT, as observed along the crystal growth axis, increases with doping. A maximum ZT value of ∼1.1 is observed for the Bi2Te3+1%(CuI+1/2Pb) sample at room temperature (T=300K). [ABSTRACT FROM AUTHOR]

The thermoelectric properties of tungsten‐titanium phosphate glass‐ceramics, which were described in previous articles, were measured as a function of composition, ceram temperature, and measurement temperature. The glass‐ceramics comprise tungsten monophosphate crystals, (PO2)4(WO3)2 m, in a matrix of TiP2O7. The glass‐ceramics behave as n‐type semiconductors with nearly metallic behavior. Conduction occurs along percolating networks of primarily m6 and m7 crystals. The highest electrical conductivities and (absolute) Seebeck coefficients were measured on a glass‐ceramic containing large, interconnecting, prismatic, m7 crystals. At 777°C, the electrical conductivity reached nearly 5500 S/m and the Seebeck coefficient was −60 µV/K. Thermal conductivity was in the 2.5‐3 W/m.K range, and the maximum ZT obtained was 0.007. This ZT is two orders of magnitude lower than those of the best bulk, polycrystalline, n‐type, oxide thermoelectric materials. The exceptional property of the tungsten‐titanium phosphate glass‐ceramics is their high electrical conductivity when compared with oxide glasses. [ABSTRACT FROM AUTHOR]

Gallium (Ga) doping together with low dimensionality has been a promising approach to improve thermoelectric performance of zinc oxide (ZnO) materials, due to the increase of carrier concentration and suppression of phonon transport. So far, the highest power factor of Ga‐doped ZnO (GZO) thin films has reached 280 μW m−1 K−2, which is still limited for practical applications. In this work, we have simultaneously optimized the electrical conductivity and Seebeck coefficient of GZO thin films using the combination of oxygen defects and sandwich structure (GZO‐ZnO‐GZO). Benefiting from energy filtering effect at the interface between GZO and ZnO layers and high oxygen vacancy concentration, the density of states (DOS) effective mass has been increased together with a relatively high carrier concentration. As a result, an improved power factor value of 434 μW m−1 K−2 at 623 K has been achieved, which is comparable to the best values reported for ZnO‐based films. This method of combining defect engineering and sandwich structure design shows great potential in enhancing the thermoelectric performance of ZnO‐based thin films or other oxide materials. [ABSTRACT FROM AUTHOR]

The thermoelectric properties of a series of the polycrystalline samples of titanium dichalcogenides with partial substitution of Ti for Nb and S for Se were investigated. It was found that sintering of the samples improved the thermoelectric efficiency (ZT), and the maximum ZT was achieved at sintering temperature of 600°C. A further increase in the sintering temperature (850°C and 950°C) led to the recrystallization of the samples, as a result, the Seebeck coefficient sharply decreased and electrical conductivity dramatically increased. The temperature dependences of electrical conductivity σ(T) in the temperature range from 4.2 to 300 K and Seebeck coefficient S(T) in the temperature range from 77 to 300 K were investigated in order to determine the nature of the observed improvements in thermoelectric properties due to double substitutions and sintering. Two‐dimensionalization of electron transport properties of Ti1−xNbxS2−ySey solid solutions was found. The Fermi energy EF2D was estimated using the temperature dependences of Seebeck coefficient. The relationship between the Fermi energy EF2D and figure of merit ZT was established. The effect of sintering on parameters σ(T), S(T), charge carrier concentration (n2D), mobility (µ), and thermal conductivity (k) was found. The optimal value of Fermi energy EF2D in terms of figure of merit ZT = 0.31 at room temperature (T = 300 K) was found for Ti0.98Nb0.02S1.3Se0.7 sample sintered at 600°C. [ABSTRACT FROM AUTHOR]

We investigate the thermoelectric properties of bulk polycrystalline samples of WSe2‐based compounds with partial substitutions in the cationic (W) and the anionic (Se) sublattices in the temperature range from 4.2 to 650 K. The substitution of W for Nb leads to a significant increase in the charge carrier concentration, however, deteriorates the charge carrier mobility. In contrast, the substitution of selenium for sulfur increases the charge carrier mobility, the thermal conductivity, and the Seebeck coefficient but conductivity changes non‐monotonical. We show that the addition of sulfur in anionic sublattice affects the grain sizes in the polycrystalline material. Using substitutions in the anionic and cationic sublattices, we find the optimal ratio of the elements for better thermoelectric efficiency. The W0.98Nb0.02Se1.7S0.3 sample showed the best value of the figure of merit ZT = 0.26 (T = 650 K). [ABSTRACT FROM AUTHOR]

The layered oxyselenides BiCuSeO was recently discovered as potential thermoelectric. Our result reveals that the substitute for atom Bi3+ by Pb2+ & Mg2+ in (Bi2O2)2− play an important role in electrical transport properties. The maximum electrical conductivity obtained is 460 Scm−1 for Bi0.84Mg0.10Pb0.06CuSeO at RT, highly above the 15 Sm−1 for BiCuSeO. In synergy with low thermal conductivity (0.6‐0.4 Wm−1 K−1) and large thermopower (300‐500 μVK−1), the highest ZT is achieved about 0.80 at 873 K for Bi0.88Mg0.06Pb0.06CuSeO. [ABSTRACT FROM AUTHOR]

In this work, the thermoelectric properties of SnTe have been regulated synergistically by introduction of n-type ZnO nanoinclusions. On one hand, the excessive holes in SnTe matrix were compensated properly by n-type ZnO, and the Seebeck coefficient has been improved greatly due to the carrier energy filtering effect by a large quantity of ZnO-SnTe hetero-junction interface barriers; On the other hand, the thermal conductivity gets reduced by the enhanced phonon scattering from multiscale hierarchical architectures. Consequently, a maximum ZT of ~0.9 at 873K has been obtained in the 0.8 wt% sample, which enhances by 112% in comparison with the nanoinclusion-free sample. [ABSTRACT FROM AUTHOR]

We report on measurements of the electrical and optical properties of BaPrO3. The temperature dependences of the electrical conductivity σ and the Seebeck coefficient α of polycrystalline samples were studied over a wide temperature range (300°C-1050°C). At lower temperatures, the observed charge transport can be described as thermally activated hopping of electron-based small polarons with an activation energy of 0.37 eV. An observed change in temperature dependence of both σ and α around 700°C was observed and interpreted as a transition from extrinsic to intrinsic carrier transport. The intrinsic conduction can be modeled with an apparent electrical band gap of ~2 eV. Optical absorption and emission spectroscopy in the UV- VIS- NIR range revealed a series of characteristic absorption thresholds and the type of optical transitions was identified by combining transmittance and diffuse-reflectance spectroscopy methods. An absorption edge of indirect type with onset at 0.6 eV is attributed to small polaron effects. The higher lying absorption thresholds of direct origin positioned at around 1.8 and 3.8 eV are correlated with thermal activation parameters from electrical measurements and discussed in terms of the band gap of BaPrO3. [ABSTRACT FROM AUTHOR]