An alternative approach for optimal carrier concentration towards ideal thermoelectric performance

Optimization of the carrier concentration of any thermoelectric material is a prime factor for the enhancement of the thermoelectric figure of merit. An alternative approach for achieving optimal carrier concentration is presented here. We introduce impurity levels of ytterbium (Yb) near the valence band edge of Pb_1–xSn_x Te. The temperature‐dependent redistribution of electrons between the Yb‐impurity levels and the valence band is found to optimize the excess hole concentration at low temperature and negating the effect of intrinsic conduction at higher temperature leading to significantly improved thermoelectric performance in Pb_1–xSn_x Te. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ab initio study of thermoelectric properties of Cu<Subscript>3</Subscript>PSe<Subscript>4</Subscript> and Cu<Subscript>3</Subscript>PS<Subscript>4</Subscript>: alternative materials for thermoelectric applications

This letter discusses the thermoelectric properties of Cu₃PSe₄ and Cu₃PS₄ compounds, using the Ab initio calculations. These compounds are predicted to be good thermoelectric materials thanks to the nature of their band edge states. Seebeck coefficient of Cu₃PSe₄ exhibits a maximum value of 1256 µV/K at roopm temperature, whereas it is 2389 µV/K for Cu₃PS₄. Furthermore, the electrical conductivity is significantly enhanced with doping level while the electronic thermal conductivity is weakly increased. Besides, the factor of merit of these compounds shows a value around the unity only at low doping levels. Hence, this predicts that these compounds may present excellent thermoelectric properties, therefore they could be considered as alternatives for thermoelectric applications.     

Temperature dependence of the Raman spectra of Bi2Te3 and Bi0.5Sb1.5Te3 thermoelectric films

The temperature dependence of the Raman spectra of Bi₂Te₃ and Bi_0.5Sb_1.5Te₃ thermoelectric films was investigated. The temperature coefficients of the E_g(2) peak positions were determined as –0.0137 cm^–1/°C and –0.0156 cm^–1/°C, respectively. The thermal expansion of the crystal caused a linear shift of the Raman peak induced by the temperature change. Based on the linear relation, a reliable and noninvasive micro‐Raman scattering method was shown to measure the thermal conductivity of the thermoelectric films. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Bi-Sb-Te基热电薄膜温差电池离子束溅射制备与表征

本文采用离子束溅射Bi/Te和Sb/Te二元复合靶,直接制备n型Bi₂Te₃热电薄膜和p型Sb₂Te₃热电薄膜.在退火时间同为1 h的条件下,对所制备的Bi₂Te₃薄膜和Sb₂Te₃薄膜进行不同温度的退火处理,并对其热电性能进行表征.结果表明,在退火温度为150 ℃时,制备的n型Bi₂Te₃关键词： 薄膜温差电池 2Te₃薄膜')" href="#">Sb₂Te₃薄膜 2Te₃薄膜')" href="#">Bi₂Te₃薄膜 离子束溅射     

First principles calculations of Sr-based fluoroperovskite compounds under applied pressure

The structural, electronic and thermoelectric properties of SrXF₃ (X?=?Li, Na, K, Rb) compounds are performed using first principle calculations. The mBJ-GGA method has been considered to obtain accurate band gaps. The present compounds are found to be thermodynamically stable under 0?GPa and 10?GPa. This stability has been determined using the standard enthalypy of formation. The band structures of the compounds display direct band-gap (Γ-Γ). The band gap has slightly increased for almost studied compounds under 10?GPa. The Boltzmann transport calculations are used to calculate and explain the thermoelectric properties as a function of temperature within the range 20–1500?K. The majority charge carriers of SrXF₃ compounds are holes rather than electrons. Under 10 GP pressure the SrLiF₃ compound is shifted from n-type to p-type doping, whereas SrKF₃ and SrRbF₃ are shifted from p-type to n-type. SrNaF₃ has p-type doping character under 0?GPa and 10?GPa. The Seebeck coeffiecient is found to decrease, whereas σ/τ and S² σ/τ increase for higher temperature. According to the figure of merit and the high S² σ/τ values for SrXF₃, promising thermoelectric applications are expected for the present compounds.     

Reinvestigation of the thermoelectric properties of Ag8GeTe6

High‐density polycrystalline samples (above 98% of the theoretical density) of Ag₈GeTe₆ were prepared by solid‐state reactions of Ag₂Te, GeTe, and Te, followed by hot‐pressing. The thermoelectric properties were measured at temperatures ranging from room temperature to around 700 K. The thermal conductivity values were extremely low (0.25 Wm^–1 K^–1 at room temperature), and consequently Ag₈GeTe₆ exhibited a relatively high thermoelectric figure of merit, ZT = 0.48 at 703 K. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

DFT investigation on the electronic and thermoelectric properties of ternary semiconductor AgBiS2 for energy conversion application

Via the FP-APW+lo method, we have performed a systematic theoretical study of the structural, electronic and thermoelectric properties of β-AgBiS₂ compound. The estimated structural properties such as cell parameters a and c, c/a ratio and internal parameters are in reasonable agreement with the earlier measured one. From band structure calculations we have found that β-AgBiS₂ is semiconductor with a band gap of 1.23 eV using the TB-mBJ approximation. In addition, the analysis of the total and partial DOS shows a considerable hybridization between Ag ‘d’ states and S ‘p’, Bi ‘s’ states indicating that both Ag-S and Bi-S have covalent character. The main thermoelectric properties such as electrical conductivity, thermo-power, electronic thermal conductivity, power factor and figure of merit are calculated and discussed. We observed that ZT increases when temperature is augmented and reached its maximum of 0.95 and 0.85 at 2 × 10¹⁹ cm⁻³ for p and n-type doping, respectively. Thus, β-AgBiS₂ compound has interesting thermoelectric properties in both p and n-type doping.     

Thermoelectric power and electrical conductivity of layer compounds n-GaS and n-GaSe

The effective density of states N_c of n-GaS and n-GaSe are calculated from the thermoelectric power, the conductivity and the Hall mobility. From the results on GaS, N_c is found to be 10²¹ cm^-3 at room temperature. The N_c value of the upper conduction band in GaSe appears to be approximately 10²² cm^-3 at room temperature.     

Electrical transport measurements in a quasi-onedimensional semiconductor ZrS3

We have used the technique of chemical vapour transport to prepare needle shaped single crystal of ZrS₃. Results of the measurements of d.c. resistivity. Hall coefficient and thermoelectric power of the temperature range 100–500 K are reported. All the samples exhibited semiconducting behaviour with a room temperature resistivity of about 15 Ω-cm and an activation energy of 0.20±0.02 eV. Room temperature thermoelectric power is -850 μVK^?1 and the dominant carriers are electrons. The thermoelectric power varies as (1/T), a behaviour associated with a typical semiconductor. Mobility at low temperatures is limited by ionized impurity scattering and is given by μ₁ = 6.5 × 10^?2T^3/2 cm²V^7-1 sec^?1. At high temperatures, phonon scattering is dominant and the mobility is given by μ₂ = 1.35 × 10⁺⁵T^?32 cm²V^?1 sec^?1.     

Transition of synthetic chromium oxide gel to crystalline chromium oxide: a hot‐stage Raman spectroscopic study

Chromium oxide gel material was synthesised and appeared to be amorphous in X‐ray diffraction study. The changes in the structure of the synthetic chromium oxide gel were investigated using hot‐stage Raman spectroscopy based upon the results of thermogravimetric analysis. The thermally decomposed product of the synthetic chromium oxide gel in nitrogen atmosphere was confirmed to be crystalline Cr₂O₃ as determined by the hot‐stage Raman spectra. Two bands were observed at 849 and 735 cm⁻¹ in the Raman spectrum at 25 °C, which were attributed to the symmetric stretching modes of O Cr^III OH and O Cr^III O. With temperature increase, the intensity of the band at 849 cm⁻¹ decreased, while that of the band at 735 cm⁻¹ increased. These changes in intensity are attributed to the loss of OH groups and formation of O Cr^III O units in the structure. A strongly hydrogen‐bonded water H O H bending band was found at 1704 cm⁻¹ in the Raman spectrum of the chromium oxide gel; however, this band shifted to around 1590 cm⁻¹ due to destruction of the hydrogen bonds upon thermal treatment. Six new Raman bands were observed at 578, 540, 513, 390, 342 and 303 cm⁻¹ attributed to the thermal decomposed product Cr₂O₃. The use of the hot‐stage Raman spectroscopy enabled low‐temperature phase changes brought about through dehydration and dehydroxylation to be studied. Copyright © 2010 John Wiley & Sons, Ltd.     

Raman spectroscopic characterization of CH4 density over a wide range of temperature and pressure

The positions of the CH₄ Raman ν₁ symmetric stretching bands were measured in a wide range of temperature (from −180 °C to 350 °C) and density (up to 0.45 g/cm³) using high‐pressure optical cell and fused silica capillary capsule. The results show that the Raman band shift is a function of both methane density and temperature; the band shifts to lower wavenumbers as the density increases and the temperature decreases. An equation representing the observed relationship among the CH₄ ν₁ band position, temperature, and density can be used to calculate the density in natural or synthetic CH₄‐bearing inclusions. Copyright © 2014 John Wiley & Sons, Ltd.     

Nanostructured thermoelectric oxides with low thermal conductivity

Complex metal oxides, such as e.g. perovskite‐type phases are developed as potential functional materials to improve the efficiency of thermoelectric converters. Among those, cobaltates with p‐type conductivity and n‐type manganates are considered for the realisation of a ceramic thermoelectric converter. Sintered pellets with the composition AMO_3–δ (A = Ln, RE; M = Co, Mn, Ni, Ti) and “Ca₃Co₄O₉ derivates” were synthesized and characterised concerning their thermoelectric properties in a broad temperature range. It was found that the Seebeck coefficient and the electrical conductivity do not depend on the dimensions of the crystallites, while the heat conductivity can be substantially lowered by decreasing the size of the crystalline domains in these systems. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Evaluation of Thermoelectric Properties of Ag0.366Sb0.558Te

Ternary AgSbTe₂ materials are frequently reported to show a promising thermoelectric performance, due to the intrinsically low lattice thermal conductivity and complex valence band structure. However, stoichiometric AgSbTe₂ is found to be thermodynamically unstable and would partially decompose into Ag₂Te and Sb₂Te₃ during thermal cycling. Instead, Ag_0.366Sb_0.558Te is the composition for stabilizing the single-phase according to the Ag₂Te-Sb₂Te₃ phase diagram, while the thermoelectric transport properties have rarely been reported and are the focus of this work. Sn/Sb substitution is found to effectively increase not only the carrier concentration from ≈5 × 10¹⁹ cm⁻³ to ≈4 × 10²¹ cm⁻³, but also the density-of-states effective mass, leading to an enhanced Seebeck coefficient along with a decreased carrier mobility. Single parabolic band (SPB) model with acoustic phonon scattering enables a good understanding on the charge transport. The increased carrier concentration effectively suppresses the bipolar effect at high temperatures. As a result, a peak zT of ≈1.3 and an average of ≈0.9 are achieved.     

Thermo‐Raman spectroscopy of synthetic nesquehonite — implication for the geosequestration of greenhouse gases

Pure nesquehonite (MgCO₃·3H₂O)/Mg(HCO₃)(OH)·2H₂O was synthesised and characterised by a combination of thermo‐Raman spectroscopy and thermogravimetry with evolved gas analysis. Thermo‐Raman spectroscopy shows an intense band at 1098 cm⁻¹, which shifts to 1105 cm⁻¹ at 450 °C, assigned to the ν₁CO₃²⁻ symmetric stretching mode. Two bands at 1419 and 1509 cm⁻¹ assigned to the ν₃ antisymmetric stretching mode shift to 1434 and 1504 cm⁻¹ at 175 °C. Two new peaks at 1385 and 1405 cm⁻¹ observed at temperatures higher than 175 °C are assigned to the antisymmetric stretching modes of the (HCO₃)⁻ units. Throughout all the thermo‐Raman spectra, a band at 3550 cm⁻¹ is attributed to the stretching vibration of OH units. Raman bands at 3124, 3295 and 3423 cm⁻¹ are assigned to water stretching vibrations. The intensity of these bands is lost by 175 °C. The Raman spectra were in harmony with the thermal analysis data. This research has defined the thermal stability of one of the hydrous carbonates, namely nesquehonite. Thermo‐Raman spectroscopy enables the thermal stability of the mineral nesquehonite to be defined, and, further, the changes in the formula of nesquehonite with temperature change can be defined. Indeed, Raman spectroscopy enables the formula of nesquehonite to be better defined as Mg(OH)(HCO₃)·2H₂O. Copyright © 2008 John Wiley & Sons, Ltd.     

Electrical and thermoelectrical properties of selenium-tellurium liquid alloys

A study has been made of the electrical conductivity and thermoelectric power of liquid alloys Te_1-x Se _x with 0≤-x≤-0.5. The temperature range extends from undercooling to about 900°C for electrical conductivity and 750°C for thermoelectric power. A partial conservation after melting of covalent bonds between the atoms of the chains leads to a liquid model in which Gubanov's theory predicts an energy band gap. The experimental results in the intrinsic semiconductor range give the band gap and the mobility ratio values. The thermal gap changes from 1.2 to 3 ev between pure tellurium and the alloy with 70 at. % selenium. There is a large increase in hole mobility with atomic % selenium. For x≥0.2 the low temperature results of the electrical conductivity can be explained by the existence of localized states in the band gap. The high temperature measurements show a trend to the metallic state, but this state cannot be reached at one atmosphere pressure even for tellurium.     

Thermoelectric power and ac conductivity of A-type Nd2O3

Measurement of thermoelectric power Θ of pressed pellets of A-type Nd₂O₃ from 550 to 1180K and electrical conductivity (σ) at dc, 50 Hz, 1.542 kHz and 3 kHz at different temperatures is reported. It is concluded that electrical conduction at high temperature (T>600K) in this solid is due to positive large polarons in O²⁻ : 2p (valence) band and negative intermediate polarons in Nd³⁺ : 5d (conduction band). The energy band gap of the solid has been found to be 2.44 eV. At low temperatures, conduction by hopping of charge carriers from one impurity centre to another has been predicted.     

Investigation on native defects of α-MgAgSb and its effects on thermoelectric properties using first principles calculations

α-MgAgSb is a promising thermoelectric materials having good performance at medium temperature. Native defects in α-MgAgSb are frequently reported experimentally and are tightly involved in the thermoelectric properties of α-MgAgSb. In this paper, all possible native defects in α-MgAgSb are calculated as well as detailed results are given and discussed. The concentrations of several dominant native defects, for example, V_Ag and Ag_Sb, could reach up to 10^?4 cm^?3 at 540 K. Furthermore, the electronic structure and transport properties of α-MgAgSb with dominant native defects are investigated. Results show that the introduction of Ag_Mg and V_Ag contributes to a much lower inertial mass and slight decrease in Seebeck coefficient. The lattice thermal conductivity is greatly reduced with the introduction of native defects. For α-MgAgSb with V_Ag, the peak ZT could reach up to 1.84 at 420 K. Our calculation demonstrates that defect engineering is an effective strategy to enhance thermoelectric performance of the materials.     

Structural and thermoelectric power properties of Na-doped V2O5·nH2O nanocrystalline thin films

X-ray diffraction (XRD), thermoelectric power (S) and at room temperature electrical conductivity (σ) of Na⁺¹-doped V₂O₅·nH₂O nanocrystalline thin films fabricated by sol gel technique (colloid route) were studied. XRD showed that the Na₂O–V₂O₅·nH₂O thin films are highly oriented nanocrystals. The average value of particle size was found to be about 7.5 nm. The thermoelectric power showed that the thermoelectric power for all present nanocrystalline thin films samples decreased with increasing Na⁺¹ content. However, the electrical conductivity increased with increasing Na⁺¹ content. There is evidence that small polarons are responsible for determining the transport properties of the Na⁺¹ doped V₂O₅·nH₂O nanocrystalline thin films samples. The high value of electrical conductivity and small value of thermoelectric power is ideal for device applications, where device to device variation of the thermoelectric power must be small. This preparation technique was demonstrated to fabricate high quality Na₂O–V₂O₅·nH₂O nanocrystalline thin films for thermoelectric device applications. However, this may be further used for deposition with an ink-jet printer.     

Raman spectroscopic study of the phosphate mineral churchite‐(Y) YPO4·2H2O

Raman spectroscopy has been used to study the rare‐earth mineral churchite‐(Y) of formula (Y,REE)(PO₄) ·2H₂O, where rare‐earth element (REE) is a rare‐earth element. The mineral contains yttrium and, depending on the locality, a range of rare‐earth metals. The Raman spectra of two churchite‐(Y) mineral samples from Jáchymov and Medvědín in the Czech Republic were compared with the Raman spectra of churchite‐(Y) downloaded from the RRUFF data base. The Raman spectra of churchite‐(Y) are characterized by an intense sharp band at 975 cm⁻¹ assigned to the ν₁ (PO₄³⁻) symmetric stretching mode. A lower intensity band observed at around 1065 cm⁻¹ is attributed to the ν₃ (PO₄³⁻) antisymmetric stretching mode. The (PO₄³⁻) bending modes are observed at 497 cm⁻¹ (ν₂) and 563 cm⁻¹ (ν₄). Some small differences in the band positions between the four churchite‐(Y) samples from four different localities were found. These differences may be ascribed to the different compositions of the churchite‐(Y) minerals. Copyright © 2009 John Wiley & Sons, Ltd.