First principle investigation of the electronic and thermoelectric properties of Mg_2C

In this paper, electronic and thermoelectric properties of Mg_2C are investigated by using first principle pseudo potential method based on density functional theory and Boltzmann transport equations. We calculate the lattice parameters,bulk modulus, band gap and thermoelectric properties(Seebeck coefficient, electrical conductivity, and thermal conductivity) of this material at different temperatures and compare them with available experimental and other theoretical data. The calculations show that Mg_2C is indirect band semiconductor with a band gap of 0.75 eV. The negative value of Seebeck coefficient shows that the conduction is due to electrons. The electrical conductivity decreases with temperature and Power factor(PF) increases with temperature. The thermoelectric properties of Mg_2C have been calculated in a temperature range of 100 K–1200 K.     

Effect of pressure on electronic and thermoelectric properties of magnesium silicide: A density functional theory study

We study the effect of pressure on electronic and thermoelectric properties of Mg_2Si using the density functional theory and Boltzmann transport equations. The variation of lattice constant, band gap, bulk modulus with pressure is also analyzed. Further, the thermoelectric properties(Seebeck coefficient, electrical conductivity, electronic thermal conductivity) have been studied as a function of temperature and pressure up to 1200 K. The results show that Mg_2Si is an n-type semiconductor with a band gap of 0.21 eV. The negative value of the Seebeck coefficient at all pressures indicates that the conduction is due to electrons. With the increase in pressure, the Seebeck coefficient decreases and electrical conductivity increases. It is also seen that, there is practically no effect of pressure on the electronic contribution of thermal conductivity.The paper describes the calculation of the lattice thermal conductivity and figure of merit of Mg_2Si at zero pressure. The maximum value of figure of merit is attained 1.83 × 10~(-3) at 1000 K. The obtained results are in good agreement with the available experimental and theoretical results.     

Fast preparation and thermal transport property of TiCoSb-based half-Heusler compounds

TiCoSb-based half-Heusler compounds with the substitution of Zr for Ti have been prepared quickly by combining high-energy ball milling method with spark plasma sintering technique, and their thermal transport properties have been investigated. With the increase of the concentration of Zr, the thermal conductivity of Til-xZrxCoSb compounds decreases significantly. Compared with the thermal conductivity of TiCoSb compound, that of Ti0.5Zr0.5CoSb decreases by 200% at 1000 K.[第一段]     

Unexpected low thermal conductivity and large power factor in Dirac semimetal Cd_3As_2

Thermoelectrics has long been considered as a promising way of power generation for the next decades. So far,extensive efforts have been devoted to the search of ideal thermoelectric materials, which require both high electrical conductivity and low thermal conductivity. Recently, the emerging Dirac semimetal Cd3As2, a three-dimensional analogue of graphene, has been reported to host ultra-high mobility and good electrical conductivity as metals. Here, we report the observation of unexpected low thermal conductivity in Cd3As2, one order of magnitude lower than the conventional metals or semimetals with a similar electrical conductivity, despite the semimetal band structure and high electron mobility. The power factor also reaches a large value of 1.58 m W·m-1·K-2at room temperature and remains non-saturated up to 400 K.Corroborating with the first-principles calculations, we find that the thermoelectric performance can be well-modulated by the carrier concentration in a wide range. This work demonstrates the Dirac semimetal Cd3As2 as a potential candidate of thermoelectric materials.     

La2-xNdxCuO4+δ(0.1≤x≤1.2)体系中滞弹性弛豫与相变内耗研究

通过对La_2-xNd_xCuO_4+δ(0.1≤x≤1.2)体系中滞弹性弛豫与相变内耗性能的研究发现,当0.1≤x≤1.0时,在250K左右存在一个与间隙氧有关的弛豫内耗峰,并且当0.1≤x≤0.4时,弛豫内耗峰峰高随着x值的增大而升高,此时体系为正交结构;当0.5≤x≤1.0时,体系在宏观上呈现四方结构,此时内耗峰峰高随着x< 关键词： 2-xNd_<i>xCuO_4+δ')" href="#">La_2-xNd_<i>xCuO_4+δ 间隙氧 弛豫内耗峰 相变内耗峰     

Thermoelectric enhancement in triple-doped strontium titanate with multi-scale microstructure

Strontium titanate(SrTiO_3) is a thermoelectric material with large Seebeck coefficient that has potential applications in high-temperature power generators.To simultaneously achieve a low thermal conductivity and high electrical conductivity,polycrystalline SrTiO_3 with a multi-scale architecture was designed by the co-doping with lanthanum,cerium,and niobium.High-quality nano-powders were synthesized via a hydrothermal method.Nano-inclusions and a nano/microsized second phase precipitated during sintering to form mosaic crystal-like and epitaxial-like structures,which decreased the thermal conductivity.Substituting trivalent Ce and/or La with divalent Sr and substituting pentavalent Nb with tetravalent Ti enhanced the electrical conductivity without decreasing the Seebeck coefficient.By optimizing the dopant type and ratio,a low thermal conductivity of 2.77 W·m~(-1)·K~(-1) and high PF of 1.1 mW·m~(-1)·K~(-2) at 1000 K were obtained in the sample co-doped with 5-mol% La,5-mol% Ce,and 5-mol% Nb,which induced a large ZT of 0.38 at 1000 K.     

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.     

An Alternating-Current Voltage Modulated Thermal Probe Technique for Local Seebeck Coefficient Characterization

Nanostructured and nanocomposite thermoelec- tric materials have recently attracted a great deal of attention due to the optimization of thermal and electrical transports for high thermoelectric performance The initial ideas for the applica- tions of nano-structures in thermoelectric materials are that the lattice thermal conductivity can be de- pressed by the scattering of nano-particles or nano- boundaries as well as the enhanced electron density of states at the Fermi level. The latter is expected to enhance Seebeck coefficients due to the fact that the low energy carriers can be filtered by nano-sized grain boundaries. Lowered thermal conductivity and enhanced thermoelectric figure of merit have been ob- served in lots of bulk materials with nanostructures or nano-impurities. However, the thermal and electrical transports in these nano-materials are usually mea- sured by normal commercial systems, in which only the statistical values of the transports are obtained. The characterization of local thermoelectric parame- ters still remains a challenging task at the submicro, even nanometer level as a powerful tool for Scanning probe microscopy nanostructure imaging and local properties characterization, has become a promis- ing technique for measuring local thermal and electri- cal properties, like scanning tunneling microscopy, scanning thermal microscopy, and scanning Joule expansion microscopy. Recent work has demon- strated simultaneously determined the thermal con- ductivity and Seebeek coefficient of Bi2Se3 thin film by a microprobe technique.     

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.     

Probing the thermoelectric transport properties of n-type Bi2Te3 close to the limit of constitutional undercooling

Bulk n-type Bi2Te3 single crystals with optimized chemical composition were successfully prepared by a high temperature-gradient directional solidification method. We investigate the influence of alloy microstructure, chemical composition, and growth orientation on the thermoelectric transport properties. The results show that the composition of single-crystal Bi2Te3 alloy, along the c axis direction, could be slightly tuned by changing the growth rate of the crystal. At a rate of 18 mm/h, the formed Bi2Te3 crystal exhibits good thermoelectric properties. At 300 K, a maximum Seebeck coefficient of -245 μV/K and an electrical conductivity of 5.6 × 10 4 S/m are acquired. The optimal power factor is ob- tained as 3.3 × 10 -3 W/K2m, with a figure of merit of 0.74. It can be attributed to the increased tellurium allocation in the Bi2Te3 alloys, as verified well by the density functional theory caLculations.     

In-Situ High Pressure Raman Spectrum and Electrical Property of PbMoO4

In-situ high pressure Raman spectra and electrical conductivity measurements of scheelite-structure compound PbMoO4 are presented. The Raman spectrum of PbMoO4 is determined up to 26.5 GPa on a powdered sample in a diamond anvil cell （DAC） under nonhydrostatic conditions. The PbMoO4 gradully experiences the trans- formation from the crystal to amorphous between 9.2 and 12.5 GPa. The crystal to amorphous transition may be due to the mechanical deformation and the crystalographic transformation. Furthermore, the electrical conductivity of PbMoO4 is in situ measured accurately using a microcircuit fabricated on a DAC based on the van der Pauw method. The results show that the electrical conductivity of PbMoO4 increases with increases of pressure and temperature. At 26.5 GPa, the electrical conductivity value of PbMoO4 at 295K is 1.93 - 10-4 S/cm, while it raises by one order of magnitude at 430K and reached 3.33 - 10-3 S/cm. However, at 430K, compared with the electrical conductivity value of PbMoO4 at 26.5 GPa, it drops by about two order magnitude at 7.4 GPa and achieves 2.81 × 10^-5 S/cm. This indicates that the effect of pressure on the electrical conductivity of PbMoO4 is more obvious than that of temperature.     

Thermospin effects in parallel coupled double quantum dots in the presence of the Rashba spin-orbit interaction and Zeeman splitting

The thermoelectric and the thermospin transport properties,including electrical conductivity,Seebeck coefficient,thermal conductivity,and thermoelectric figure of merit,of a parallel coupled double-quantum-dot Aharonov-Bohm interferometer are investigated by means of the Green function technique.The periodic Anderson model is used to describe the quantum dot system,the Rashba spin-orbit interaction and the Zeeman splitting under a magnetic field are considered.The theoretical results show the constructive contribution of the Rashba effect and the influence of the magnetic field on the thermospin effects.We also show theoretically that material with a high figure of merit can be obtained by tuning the Zeeman splitting energy only.     

Significant role of nanoscale Bi-rich phase in optimizing thermoelectric performance of Mg_3Sb_2

Mg_3Sb_(1.5)Bi_(0.5)-based alloys have received much attention, and current reports on this system mainly focus on the modulation of doping. However, there lacks the explanation for the choice of Mg_3Sb_(1.5)Bi_(0.5) as matrix. Here in this work,the thermoelectric properties of Mg_3Sb_(2-x)Bi_x(0.4 ≤ x ≤ 0.55) compounds are systematically investigated by using the first principles calculation combined with experiment. The calculated results show that the band gap decreases after Bi has been substituted for Sb site, which makes the thermal activation easier. The maximum figure of merit(ZT) is 0.27 at 773 K,which is attributed to the ultra-low thermal conductivity 0.53 W·m~(-1)·K~(-1) for x = 0.5. The large mass difference between Bi and Sb atoms, the lattice distortion induced by substituting Bi for Sb, and the nanoscale Bi-rich particles distributed on the matrix are responsible for the reduction of thermal conductivity. The introduction of Bi into Mg_3Sb_2-based materials plays a vital role in regulating the transport performance of thermoelectric materials.     

Pb1－xMnxSe薄膜的光学特性

采用分子束外延的方法在BaF₂(111)衬底上制备出了高质量的Pb_1-xMn_xSe(0≤x≤0.0681)薄膜.X射线衍射结果表明，Pb₁-xMn_xSe薄膜为立方相NaCl型结构，没有观察到MnSe相分离现象，薄膜的取向为平行于衬底(111)晶面.晶格常数随着Mn含量的增加逐渐减小，Mn含量由Vegard公式得到.通 关键词： 1-xMn_xSe外延薄膜')" href="#">Pb_1-xMn_xSe外延薄膜 透射光谱 带隙 折射率     

Different effects of grain boundary scattering on chargeand heat transport in polycrystalline platinum and goldnanofilms

The in-plane electrical and thermal conductivities of several polycrystalline platinum and gold nanofilms with different thicknesses are measured in a temperature range between the boiling point of liquid nitrogen (77K) and room temperature by using the direct current heating method. The result shows that both the electrical and thermal conductivities of the nanofilms reduce greatly compared with their corresponding bulk values. However, the electrical conductivity drop is considerably greater than the thermal conductivity drop, which indicates that the influence of the internal grain boundary on heat transport is different from that of charge transport, hence leading to the violation of the Wiedemann--Franz law. We build an electron relaxation model based on Matthiessen's rule to analyse the thermal conductivity and employ the Mayadas & Shatzkes theory to analyse the electrical conductivity. Moreover, a modified Wiedemann--Franz law is provided in this paper, the obtained results from which are in good agreement with the experimental data.     

Utilizing of high-pressure high-temperature synthesis to enhance the thermoelectric properties of Zn0.98Al0.02O with excellent electrical properties

The temperature in the high-pressure high-temperature(HPHT) synthesis is optimized to enhance the thermoelectric properties of high-density Zn O ceramic, Zn_0.98Al_0.02O. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy show that HPHT can be utilized to control the crystal structure and relative density of the material.High pressure can be utilized to change the energy band structure of the samples via changing the lattice constant of samples, which decreases the thermal conductivity due to the formation of a multi-scale hierarchical structure and defects. The electrical conductivity of the material reaches 6×10⁴ S/m at 373 K, and all doped samples behave as n-type semiconductors. The highest power factor(6.42 μW·cm^-1·K^-2) and dimensionless figure of merit(z T = 0.09) are obtained when Zn_0.98Al_0.02O is produced at 973 K using HPHT, which is superior to previously reported power factors for similar materials at the same temperature. Hall measurements indicate a high carrier concentration, which is the reason for the enhanced electrical performance.     

Thermoelectric properties of Li-doped Sr_(0.7)Ba_(0.3)Nb_2O_(6-δ) ceramics

Thermoelectric properties of Li-dopedSr_(0.7)Ba_(0.3)Nb_2O_(6-δ)ceramics were investigated in the temperature range from 323 K to 1073 K. The electrical conductivity increases significantly after lithium interstitial doping. However, both of the magnitudes of Seebeck coefficient and electrical conductivity vary non-monotonically but synchronously with the doping contents, indicating that doped lithium ions may not be fully ionized and oxygen vacancy may also contribute to carriers. The lattice thermal conductivity increases firstly and then decreases as the doping content increases, which is affected by competing factors.Thermoelectric performance is enhanced by lithium interstitial doping due to the increase of the power factor and the thermoelectric figure of merit reaches maximum value(0.21 at 1073 K) in the sample Sr_(0.70)Ba_(0.30)Li_(0.10)Nb_2O_6.     

Thermoelectric properties of lower concentration K-doped Ca_3Co_4O_9 ceramics

The tuning of electron and phonon by ion doping is an effective method of improving the performances of thermoelectric materials. A series of lower concentration K-doped Ca_(3-x)K_xCo_4O_9(x = 0, 0.05, 0.10, 0.15) polycrystalline ceramic samples are prepared by combining citrate acid sol-gel method with cold-pressing sintering method. The single-phase compositions and plate-like grain morphologies of all samples are confirmed by x-ray diffraction and field emission scanning electron microscope. The effects of lower concentration K doping on the thermoelectric properties of the material are evaluated systematically at high temperatures(300–1026 K). Low concentration K doping causes electrical conductivity to increase up to 23% with little effect on the Seebeck coefficient. Simultaneously, the thermal conductivity of K-doped sample is lower than that of the undoped sample, and the total thermal conductivity reaches a minimum value of approximately1.30 W·m~(-1)·K~(-1), which may be suppressed mainly by the phonon thermal conduction confinement. The dimensionless figure-of-merit ZT of Ca_(2.95)K_(0.05)Co_4O_9 is close to 0.22 at 1026 K, representing an improvement of about 36% compared with that of Ca_3Co_4O_9, suggesting that lower concentration K-doped Ca_3Co_4O_9 series materials are promising thermoelectric oxides for high-temperature applications.     

A STUDY ON SYNTHESIS OF La1-x SrxCrO3 BY SOL-GEL AND THEIR ELECTRICAL PROPERTIES

La_1-x Sr_xCrO₃ nanoparticles were prepared by the sol-gel method. The sintering temperature of the samples could be decreased to about 400 K. The crystal structures of La_1-x Sr_xCrO₃ at room temperature are all of the othorhombic perovskite GdFeO₃-type (x≤0.4). And the electrical conductivity increases with x, but when x>0.3 it decreases. The higher the sintering temperature, the better the electrical conductivity, because the grain size of the examples increases with increasing sintering temperature.     

Thermoelectric properties of Sr_(0.61)Ba_(0.39)Nb_2O_(6-δ) ceramics in different oxygen-reduction conditions

The thermoelectric properties of Sr0.61Ba0.39Nb2O6-δceramics, reduced in different conditions, are investigated in the temperature range from 323 K to 1073 K. The electrical transport behaviors of the samples are dominated by the thermal-activated polaron hopping in the low temperature range, the Fermi glass behavior in the middle temperature range,and the Anderson localized behavior in the high temperature range. The thermal conductivity presents a plateau at hightemperatures, indicating a glass-like thermal conduction behavior. Both the thermoelectric power factor and the thermal conductivity increase with the increase of the degree of oxygen-reduction. Taking these two factors into account, the oxygen-reduction can still contribute to promoting the thermoelectric figure of merit. The highest ZT value is obtained to be ～0.19 at 1073 K in the heaviest oxygen reduced sample.