Improved thermoelectric figure of merit of self‐doped Ag8–xGeTe6 compounds with glass‐like thermal conductivity

Nonstoichiometric Ag_8–xGeTe₆ (x = 0, 0.01, 0.02, 0.04) compounds with complex crystal structure are demonstrated to exhibit very low thermal conductivities of <0.28 W/m K, comparable with the calculated theoretical minimum thermal conductivity κ_min. Ag deficiency leads to the improved electrical properties and a maximum thermoelectric figure of merit ZT of 0.85 has been obtained at 623 K for Ag_7.99GeTe₆, about 30% increase compared to that of stoichiometric Ag₈GeTe₆. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature-dependent thermal expansion of cast and hot-pressed LAST (Pb–Sb–Ag–Te) thermoelectric materials

The thermal expansion for two compositions of cast and hot-pressed LAST (Pb–Sb–Ag–Te) n-type thermoelectric materials has been measured between room temperature and 673 K via thermomechanical analysis (TMA). In addition, using high-temperature X-ray diffraction (HT-XRD), the thermal expansion for both cast and hot-pressed LAST materials was determined from the temperature-dependent lattice parameters measured between room temperature and 623 K. The TMA and HT-XRD determined values of the coefficient of thermal expansion (CTE) for the LAST compositions ranged between 20 × 10^?6 K^?1 and 24 × 10^?6 K^?1, which is comparable to the CTE values for other thermoelectric materials including PbTe and Bi₂Te₃. The CTE of the LAST specimens with a higher Ag content (Ag_0.86Pb₁₉Sb_1.0Te₂₀) exhibited a higher CTE value than that of the LAST material with a lower Ag content (Ag_0.43Pb₁₈Sb_1.2Te₂₀). In addition, a peak in the temperature-dependent CTE was observed between room temperature and approximately 450 K for both the cast and hot-pressed LAST with the Ag_0.86Pb₁₉Sb_1.0Te₂₀ composition, whereas the CTE of the Ag_0.43Pb₁₈Sb_1.2Te₂₀ specimen increased monotonically with temperature.     

Evidence for insulator–metal transition in amorphous chalcogenide Se–Ge–Te films

The temperature dependence of the dc conductivity and thermoelectric power was determined for five different amorphous chalcogenide Se–Ge–Te films, with Ge?=?3.0–22?at.%, Se?=?0–97?at.% and Te?=?0–97?at.%. The films were prepared by thermal evaporation of GeSe₄, GeTe₄ and GeSe₂Te₂ quenched bulk materials. Values of the activation energy calculated from the temperature dependence of both electrical conductivity and thermoelectric power showed a decrease with increasing Ge content in the Se–Ge films as well as with replacement of Te for Se in the Se–Ge–Te films. The results showed an Anderson transition, with the conductivity showing insulating behaviour on the Ge–Se side to metallic behaviour at the binary composition Ge–Te. The radius of localization was obtained for the different compositions investigated. The wave function associated with the charge carriers at the composition Ge_3.3Te_96.7 is non-localized. A minimum metallic conductivity of 237?±?5?(Ω?cm)^?1 was found.     

Non-saturating linear magnetoresistance in phase separated amorphous Ag10Ge15Te75 films

We report non-saturating linear magnetoresistance (LMR) in silver-poor composite thin films of Ag₁₀Ge₁₅Te₇₅ (AGT). The LMR increases with decreasing temperature and reaches a value of 110% at 200 K under a magnetic field of 70 kOe. The observed magnetoresistance is proportional to carrier mobility due to identical temperature dependencies. The microstructures of AGT films annealed at different temperature are investigated by TEM observations. The observed LMR effect is resulting from the inhomogeneous distribution of conductivity caused by highly conductive Ag₂Te and GeTe₄ precipitates, which generate a current distortion in the surrounding amorphous silver-poor Ag–Ge–Te matrix.     

Enhanced power factor of Ag0.208Sb0.275Te0.517 prepared by HPHT

In this Letter, p-type thermoelectric materials Ag_0.208Sb_0.275Te_0.517 with enhanced power factor were prepared by high pressure and high temperature (HPHT) method. The samples are near single phase AgSbTe₂ with a very small quantity of impurities including Ag₂Te and Te. The concentration of impurities decreases with an increase of synthetic pressure. The synthetic pressure-dependent transport properties including electrical resistivity, Seebeck coefficient and power factor were studied at room temperature. We find the power factor of the sample prepared at the synthetic pressure of 2.1 GPa reaches a maximum value of 14.6 μW cm⁻¹ K⁻², which is about two times higher than that of the same sample prepared at normal pressure (6.9 μW cm⁻¹ K⁻²).     

Enhancement of the power factor of Pb1–xSnxTe (0.00 ≥ x ≥ 0.08) alloys

Thermoelectric properties of Pb_1–x Sn _x Te (0.00 ≥ x ≥ 0.08) alloys synthesized by melting-quenching-annealing method have been investigated. The sample structure and phases have been investigated by Raman spectroscopy and X-ray diffraction, while the morphology and stoichiometry have been studied by SEM and EDX. The nanomaterial exists in a single phase and has a face-centred cubic (fcc) lattice of rock-salt type in the whole range of x values in Pb_1–x Sn _x Te alloys. The effect of tin substitution on the lattice vibration and chemical bonding nature in the lead telluride has been investigated by Raman spectroscopy at room temperature. The Seebeck coefficient and electrical resistivity have been measured in the temperature range of 100–400 K. The electrical resistivity measurements reveal that the compounds have extrinsic to intrinsic conduction transition and the electrical temperature transition shifts to higher values with increasing the Sn content. For all studied compounds, the Seebeck coefficient is positive indicating predomination of positive charge carriers over the entire temperature range. The thermoelectric power factor was enhanced to 2.03 mWm^?1 K^?2 for the sample with 4% Sn content at room temperature.     

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)     

Thermoelectric properties in p-type nanostructured Ge-doped Sb100GeTe150 alloy

Ge-doped Sb₁₀₀GeTe₁₅₀ alloy were prepared using spark plasma sintering technique, and its thermoelectric properties were evaluated over the temperature range 318–492 K. Through XRD analysis, we observed the same single phase as Sb₂Te₃ and weakened diffraction peaks. Rietveld refinement reveals that there is 0.96 at.% Ge that occupies in the Sb sites, leading to the lattice distortion in the Sb–Te crystal. High-resolution TEM images show that there are many nanodomains randomly distributed in the matrix with a large amount of amorphous phase adjoined. Measurements indicated that the Seebeck coefficients (α) increase and the electrical and thermal conductivities decrease with temperature in the entire temperature range. The maximum α value reaches 135 μV/K at 492 K, and the thermal conductivities are about 0.3 W/mK lower than those of present Sb₂Te₃ for the corresponding temperatures. The highest thermoelectric figure of merit ZT for the nanostructured alloy Sb₁₀₀GeTe₁₅₀ is 0.84 at 492 K, whereas that of the currently prepared Sb₂Te₃ is 0.74 at the corresponding temperature.     

Electrical properties of the Ag6PSe5X (X=Cl, Br, I) argyrodites

The total electrical conductivities at room temperature of Ag₆PSe₅Cl, Ag₆PSe₅Br, and Ag₆PSe₅I were found to be 2.0×10⁻⁴, 5.6×10⁻⁴, and 6.8×10⁻⁴ S/cm, respectively. In the chloride and iodide compounds, the electronic contribution comprises approximately 1% of the total conductivity, although it exceeds 10% of the total conductivity in the bromide compound. Ag₆PSe₅Cl and Ag₆PSe₅Br exhibit purely Arrhenius behavior throughout the temperature range 150-300 K. Ag₆PSe₅I exhibits a second-order anomaly in electrical conductivity at 324 K.     

非化学计量比AgSbTe2+x化合物制备及热电性能

采用高纯元素直接熔融、淬火并结合放电等离子烧结方法制备了非化学计量比AgSbTe_2+x(x=0—0.05)系列样品,研究了不同Te含量在300—600 K范围内对样品热电性能的影响规律.结果表明:随着Te含量的增加,Ag⁺离子空位浓度增加,空穴浓度和电导率大幅度提高,Seebeck系数减小.热导率随Te过量程度的增加略有增加,但所有Te过量样品的晶格热导率均介于0.32—0.49 W/mK之间,低于化学计量比样品的值,接近理论最低晶格热导率.AgS 关键词： 2')" href="#">AgSbTe₂ 非化学计量比 热电性能 热导率     

Structural and transport properties of Li-intercalated vanadium pentoxide nanocrystalline films

The X-ray diffraction (XRD), transmission electron microscopy, density, electrical and thermoelectric power (TEP) properties of nanocrystalline Li _x V₂O₅ ? nH₂O xerogel films (0 ≤ x ≤ 22 mol.%) were investigated. The films were produced by the sol–gel technique (colloidal route), which was used to enable high-purity, uniform preparation. The relative intensity of the (002) XRD line increased with increasing Li content. The particle size was found to be about 6.0 nm. Electrical conductivity and thermoelectric power were measured parallel to the substrate surface in the temperature range 300–480 K for the as-prepared films. The electrical conductivity showed that all the samples were semiconductors and that conductivity increased with increasing Li content. The conductivity of the present system was primarily determined by hopping carrier mobility, which was found to vary from 6.81 × 10^?6 to 0.33 × 10^?6 cm² V^?1 s^?1 at 380 K. The carrier density was evaluated to be 8.73 × 10¹⁹–1.118 × 10²¹ cm^?3. The conduction was confirmed to obey non-adiabatic small polaron hopping. The thermoelectric power, or Seebeck effect, increased with increasing Li content. The results obtained indicate an n-type semiconducting behavior within the temperature range investigated.     

Deuteron induced damage in alkali halides

Abstract

The thermoelectric and galvanomagnetic properties in the CdxHg1_xTe solid solution samples (x = 0.12; 0.15) irradiated at 300 K by electrons (3.5 MeV; by integrated fluxes up to 1.46 × 10¹⁸e/cm²) have been investigated in a wide temperature range 4.2–300 K and in magnetic fields 0–22 kOe.

It has been discovered that the electron irradiation of Cd_xHg_1–xTe crystals leads to an increase of the Hall coefficient and a decrease of the thermo e.m.f. sign inversion temperature. The concentrations and mobilities of electron and hole have been calculated on a basis of quantitative analysis of galvanomagnetic data. It has been shown that the kinetic coefficient behavior upon irradiation is due to the Te vacancy-based type radiation defect origin.     

Synthesis and thermoelectric properties of Mn-doped AgSbTe2 compounds

Polycrystalline p-type Ag 0.9 Sb 1.1 x Mn x Te 2.05（x = 0.05,0.10,and 0.20） compounds have been prepared by a combined process of melt-quenching and spark plasma sintering.The sample composition of Ag 0.9 Sb 1.1 x Mn x Te 2.05 has been specially designed in order to achieve the doping effect by replacing part of Sb with Mn and to present the uniformly dispersed Ag 2 Te phase in the matrix by adding insufficient Te,which is beneficial for optimizing the electrical transport properties and enhancing the phonon scattering effect.All the samples have the NaCl-type structure according to our X-ray powder diffraction analysis.After the treatment of spark plasma sintering,only the sample with x = 0.20 has a small amount of MnTe 2 impurities.The thermal analysis indicates that a tiny amount of Ag 2 Te phase exists in all these samples.The presence of the MnTe 2 impurity with high resistance and high thermal conductivity leads to the deteriorative thermoelectric performance of the sample with x = 0.20 due to the decreased electrical transport properties and the increased thermal conductivity.In contrast,the sample with x = 0.10 exhibits enhanced thermoeletric properties due to the Mn-doping effect.A dimensionless thermoelectric figure of merit of 1.2 is attained for the sample with x = 0.10 at 573 K,showing promising thermoelectric properties in the medium temperature range.     

Synthesis and thermoelectric properties of YbSb2Te4

The study of the ternary phase diagram Yb–Sb–Te has led to the synthesis of YbSb₂Te₄ as a pure phase by way of high energy ball milling followed by annealing, whereas typical high temperature powder metallurgy leads to multiphase sample with impurities of the very stable YbTe. The Hall mobility, Seebeck coefficient, electrical resistivity and thermal conductivity of the layered compound YbSb₂Te₄ were measured in the range of 20–550 °C. The thermoelectric figure of merit peaks at 525 K and reaches 0.5. Of particular interest is the very low lattice thermal conductivity (as low as a glass) which makes YbSb₂Te₄ and related compounds promising thermoelectric materials. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synergistically Enhancing Thermoelectric Performance of n-Type PbTe with Indium Doping and Sulfur Alloying

To achieve high-performance n-type PbTe-based thermoelectric materials, this work provides a synergetic strategy to improve electrical transport property with indium (In) element doping and reduces thermal conductivity with sulfur (S) element alloying. In n-type PbTe, In doping can tune the carrier density in the whole working temperature range, causing the carrier density to increase from 2.18 × 10¹⁹ cm⁻³ at 300 K to 4.84 × 10¹⁹ cm⁻³ at 823 K in Pb_0.98In_0.005Sb_0.015Te. The optimized carrier density can further modulate electrical conductivity and Seebeck coefficient, finally contributing to a substantial increase of power factor, and a maximum power factor increase from 19.7 µW cm⁻¹ K⁻² in Pb_0.985Sb_0.015Te to 28.2 µW cm⁻¹ K⁻² in Pb_0.9775In_0.0075Sb_0.015Te. Based on the optimally In-doped PbTe, S alloying is introduced to suppress phonon propagation by forming a complete solid solution, which could effectively reduce lattice thermal conductivity and simultaneously benefit carrier mobility to maintain high power factor. With S alloying, the minimum lattice thermal conductivity decreases from 0.76 Wm⁻¹ K⁻¹ in Pb_0.985Sb_0.015Te to 0.42 Wm⁻¹ K⁻¹ in Pb_0.98In_0.005Sb_0.015Te_0.88S_0.12. Combining the advantages of both In doping and S alloying, the peak ZT value and averaged ZT (ZT_ave) (300–873 K) are boosted from 1.0 and 0.60 in Pb_0.985Sb_0.015Te to 1.4 and 0.87 in Pb_0.98In_0.005Sb_0.015Te_0.94S_0.06.     

Magnetic and structural studies of the alloy system REIn0.5Ag0.5

The structural and magnetic properties of the alloy system REIn_0.5Ag_0.5 [RE = Gd, Tb, Dy, Ho, Er, Tm and Yb] are reported. All these alloys (except that of Yb) crystallize in a cubic CsCl type structure at room temperature. Low temperature X-ray diffraction data does not reveal any structural phase transformation down to 8 K. On the basis of magnetic susceptibility data at a different temperature (3–300 K) and applied magnetic field (2 × 10⁵ to 8 × 10⁶ A m^-1, it has been concluded that GdIn_0.5Ag_0.5 is ferromagnetic (T_c = 118 K), TbIn_0.5Ag_0.5 and DyIn_0.5Ag_0.5 are meta magnetic (T_N = 66 and 30 K, respectively) and alloys involving Ho, Er, Tm and Yb are ferrimagnetic with Néel temperatures (T_N) equal to 24, 22, 21 and 20 K, respectively. The evaluated effective magneton number (p) is found to be slightly larger compared to theoretical values for tripositive ions of Gd, Tb and Dy and a bit smaller for Ho, Er, Tm and Yb. The results have been qualitatively explained using appropriate theories.     

LaBiTe3: An unusual thermoelectric material

Using first‐principles calculations and semi‐classical Boltzmann transport theory, the thermoelectric properties of LaBiTe₃ are studied. The band gap and, hence, the thermoelectric response are found to be easily tailored by application of strain. Independent of the temperature, the figure of merit turns out to be maximal at a doping of about 1.6 × 10²¹ cm^–3. At room temperature we obtain values of 0.4 and 0.5 for unstrained and moderately strained LaBiTe₃, which increases to 1.1 and 1.3 at 800 K. A large spin splitting is observed in the conduction band at the T point. Therefore, LaBiTe₃ merges characteristics that are interesting for thermoelectric as well as spintronic devices. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

银团簇上吸附氧原子与氢气作用的研究

The interactions between Ag_nO^-(n=1-8) and H₂ (or D₂) were explored by combination of the mass spectroscopy experiments and density function theory (DFT) calculations. The experiments found that all oxygen atoms in Ag_nO^-(n=1-8) are inert in the interactions with H₂ or D₂ at the low temperature of 150 K, which is in contrast to their high reactivity with CO under the same condition. These observations are parallel with the preferential oxidation (PROX) of CO in excess hydrogen catalyzed by dispersed silver species in the condensed phase. Possible reaction paths between Ag_nO^-(n=1-8) and H₂ were explored using DFT calculations. The results indicated that adsorption of H₂ on any site of Ag_nO^-(n=1-8) is extremely weak, and oxidation of H₂ by any kind of oxygen in Ag_nO^-(n=1-8) has an apparent barrier strongly dependent on the adsorption style of the "O". These experiments and theoretical results about cluster reactions provided molecule-level insights into the activity of atomic oxygen on real silver catalysts.     

Physical property characterization of bulk MgB<Subscript>2</Subscript> superconductor

We report synthesis, structure/micro-structure, resistivity under magnetic field [ρ(T)H], Raman spectra, thermoelectric power S(T), thermal conductivity κ(T), and magnetization of ambient pressure argon annealed polycrystalline bulk samples of MgB₂, processed under identical conditions. The compound crystallizes in hexagonal structure with space group P6/mmm. Transmission electron microscopy (TEM) reveals electron micrographs showing various types of defect features along with the presence of 3–4 nm thick amorphous layers forming the grain boundaries of otherwise crystalline MgB₂. Raman spectra of the compound at room temperature exhibited characteristic phonon peak at 600 cm^-1. Superconductivity is observed at 37.2 K by magnetic susceptibility χ(T), resistivity ρ(T), thermoelectric power S(T), and thermal conductivity κ(T) measurements. The power law fitting of ρ(T) give rise to Debye temperature (Θ_D) at 1400 K which is found consistent with the theoretical fitting of S(T), exhibiting Θ _D of 1410 K and carrier density of 3.81 × 10²⁸/m³. Thermal conductivity κ(T) shows a jump at 38 K, i.e., at T_c, which was missing in some earlier reports. Critical current density (J_c) of up to 10⁵ A/cm² in 1–2 T (Tesla) fields at temperatures (T) of up to 10 K is seen from magnetization measurements. The irreversibility field, defined as the field related to merging of M(H) loops is found to be 78, 68 and 42 kOe at 4, 10 and 20 K respectively. The superconducting performance parameters viz. irreversibility field (H_irr) and critical current density J_c(H) of the studied MgB₂ are improved profoundly with addition of nano-SiC and nano-diamond. The physical property parameters measured for polycrystalline MgB₂ are compared with earlier reports and a consolidated insight of various physical properties is presented.     

Ab initio molecular dynamics study on Ag (n = 4, 5, 6)

Ab initio Molecular Dynamics (MD) method, based on density functional theory (DFT) with planewaves and pseudopotentials, was used to study the stability and internal motion in silver cluster Ag_n, with n =4-6. Calculations on the neutral, cationic and anionic silver dimer Ag₂ show that the bond distance and vibrational frequency calculated by DFT are of good quality. Simulations of Ag₄, Ag₅, and Ag₆ in canonical ensemble reveal distinct characteristics and isomerization paths for each cluster. At a temperature of 800 K, an Ag₄ has no definite structure due to internal motion, while for Ag₅ and Ag₆the clusters maintain the planar structure, with atomic rearrangement observed for Ag₅ but not for Ag₆. At a temperature of 200 K, Ag₄ can exist in two planar structures whilst Ag₅ is found to be stable only in the planar form. In contrast Ag₆ is stable in both planar trigonal and 3D pentagonal structures. Micro-canonical MD simulation was performed for all three clusters to obtain the vibrational density of states (DOS). Received 5 May 1999 and Received in final form 20 August 1999