High temperature transport properties of Ag-added (Ca0.975La0.025)3Co4O9 ceramics

Ag-added (Ca_0.975La_0.025)₃Co₄O₉ ceramics were fabricated using spark plasma sintering from the precursor powder synthesized by a polyacrylamide gel method. The results indicated that Ag precipitated as a second phase in Ca₃Co₄O₉ matrix. The addition of Ag was effective in enhancing the electrical conductivity and had a slight effect on Seebeck coefficient. In addition, the temperature dependence of electrical conductivity showed that the hole hopping conduction mechanism was dominant for the Ag-added (Ca_0.975La_0.025)₃Co₄O₉ ceramics. The activation energy remained unchanged with the increasing Ag content. The thermoelectric power factor of Ag-added (Ca_0.975La_0.025)₃Co₄O₉ ceramics reached about 5×10⁻⁴ Wm⁻¹ K⁻² at 700 °C, suggesting a promising thermoelectric oxide candidate at high temperatures.     

晶粒尺寸对CoSb3化合物热电性能的影响

系统地研究了晶粒尺寸对CoSb₃化合物热电性能的影响规律，结果表明晶粒尺寸对CoSb₃化合物的晶格热导率κ_p、电导率σ、能隙宽度E_g和Seebeck系数α有显著影响.当晶粒尺寸由微米尺度减小到纳米尺度时，晶格热导率κ_p显著降低，Seebeck系数α有较大幅度的增加，能隙宽度E_g变宽，电导率σ有一定程度的下降.平均晶粒尺寸为200nm的CoSb₃化合物在温度为700K时，ZT值达到0.43，比平均晶粒尺寸为5000nm的试样增加了4倍.     

Theoretical investigation of the thermoelectric transport properties of BaSi2

The full-potential linear augmented plane wave method based on density functional theory is employed to investigate the electronic structure of BaSi 2 . With the constant relaxation time and rigid band approximation,the electrical conductivity,Seebeck coefficient and figure of merit are calculated by using Boltzmann transport theory,further evaluated as a function of carrier concentration. We find that the Seebeck coefficient is more anisotropic than electrical conductivity. The figure of merit of BaSi 2 is predicted to be quite high at room temperature,implying that optimal doping may be an effective way to improve thermoelectric properties.     

Thermoelectric properties of SnzCo8Sb24 skutterudites

Sn-filled CoSb₃ skutterudite compounds were synthesized by the induction melting process. Formation of a single δ-phase of the synthesized materials was confirmed by X-ray diffraction analysis. The temperature dependences of the Seebeck coefficient, electrical resistivity and thermal conductivity were examined in the temperature range of 300-700 K. Positive Seebeck and Hall coefficients confirmed p-type conductivity. Electrical resistivity increased with increasing temperature, which shows that the Sn-filled CoSb₃ skutterudite is a degenerate semiconductor. The thermal conductivity was reduced by Sn-filling because the filler atoms acted as phonon scattering centers in the skutterudite lattice. The lowest thermal conductivity was achieved in the composition of Sn_0.25Co₈Sb₂₄.     

Effect of Zn substitution for Co on the transport properties of Y BaCo4O7

Electrical resistivity and Seebeck coefficients of Y BaCo_4−xZn_xO₇ (x=0.0,0.5,1.0,2.0) were investigated in the temperature range 350-1000 K. It was found that the electrical resistivity and activation energy increase with increasing Zn concentration, while Seebeck coefficients do not increase but decrease when electrical resistivity increases. We explained the increase of electrical resistivity and the drop of Seebeck coefficients for Zn-substituted samples by the decrease of carrier mobility, rather than of carrier concentration. The effect of oxygen absorption and desorption on the electrical resistivity and Seebeck coefficients was also investigated. An abrupt change of transport properties happens at about 650 K for x=0.0 and 0.5 samples measured in oxygen. For x=1.0 and 2.0 samples, however, such change disappears and the transport behavior in oxygen is almost same as that in nitrogen due to the significant suppression of oxygen diffusion caused by the higher Zn concentration in these samples.     

致密度对填充skutterudite化合物La0.75Fe3CoSb12热电性能的影响

研究了致密度对填充skutterudite化合物La_0.75Fe₃CoSb₁₂热电性能的影响.La_0.75Fe₃CoSb₁₂表现为p型传导，载流子迁移率随着致密度的增加而升高.由于样品中空洞的散射作用，致使电阻率ρ随着致密度的降低而升高，同时造成热导率κ的下降，但塞贝克系数α与致密度关系不大，致密度造成电阻率ρ升高的比率与热导率κ下降的比率相当，致密度不同的样品具有相当的ZT 关键词： 致密度 填充式skutterudite化合物 热电性能     

过量Zn对β-Zn4Sb3热电性能影响的研究

采用真空熔融缓冷方法制备了单相β-Zn₄Sb₃以及含有过量Zn的β-Zn₄Sb₃块体热电材料.在300—700K的温度范围内测试了材料的电导率、Seebeck系数和热导率，研究了β-Zn₄Sb₃化合物中过量Zn的分布状态及其对材料热电性能的影响规律.结果表明：过量的Zn作为第二相较均匀的分布在β-Zn₄Sb₃的晶界上，随着Zn含量增加，材料电导率和热导率上升，Seebeck系数下降，Zn第二相的引入能有效提高材料的功率因子，Zn过量2at%的材料在700K时其ZT值达到1.10. 关键词： 4Sb₃')" href="#">β-Zn₄Sb₃ 电导率 Seebeck系数     

The under-pressure behaviour of mechanical,electronic and optical properties of calcium titanate and its ground state thermoelectric response

Abstract

In this study, the elastic, electronic, optical and thermoelectric properties of CaTiO₃ perovskite oxide have been investigated using first-principles calculations. The generalised gradient approximation (GGA) has been employed for evaluating structural and elastic properties, while the modified Becke Johnson functional is used for studying the optical response of this compound. In addition to ground state physical properties, we also investigate the effects of pressure (0, 30, 60, 90 and 120 GPa) on the electronic structure of CaTiO₃. The application of pressure from 0 to 90 GPa shows that the indirect band gap (Γ-M) of CaTiO₃ increases with increasing pressure and at 120 GPa it spontaneously decreases transforming cubic CaTiO₃ to a direct (Γ-Γ) band gap material. The complex dielectric function and some optical parameters are also investigated under the application of pressures. All the calculated optical properties have been found to exhibit a shift to the higher energies with the increase of applied pressure suggesting potential optoelectronic device applications of CaTiO₃. The thermoelectric properties of CaTiO₃ have been computed at 0 GPa in terms of electrical conductivity, thermal conductivity and Seebeck coefficient.     

Large polarization-dependent photovoltages in ceramic BaTiO3 + 5 wt.% CaTiO3

Large photovoltages proportional to length and to remanent polarizations of 40 (V/cm)/(μC/cm²) have observed in wafers of ceramic BaTiO₃ + 5 wt.% CaTiO₃. The photovoltages existed only below the Curie temperature, decreasing with increasing temperature in a linear fashion until close to the Curie temperature.     

Power factors of late rare earth-doped Ca3Co2O6 oxides

Polycrystalline samples of (Ca_1−xR_x)₃Co₂O₆ with R = Gd, Tb, Dy and Ho at x=0-0.1 were synthesized and the effects of rare earth substitution on their thermoelectric properties were investigated. In the high-temperature region, the rare earth substitution resulted in an increase in the Seebeck coefficients (S), and the S values increased with decreasing ionic radius of rare earth elements in the order Gd³⁺>Tb³⁺>Dy³⁺>Ho³⁺. In contrast, the influence of rare earth substitution on the electrical resistivity was small. The high-temperature power factor was thereby improved by the late rare earth substitutions, particularly those with Ho³⁺ for Ca²⁺. For the Ho-doped samples (x≤0.05), the power factor was significantly improved by increasing Ho concentration.     

Hall mobility of amorphous Ge2Sb2Te5

The electrical conductivity, Seebeck coefficient, and Hall coefficient of three-micron-thick films of amorphous Ge₂Sb₂Te₅ have been measured as functions of temperature from room temperature down to as low as 200 K. The electrical conductivity manifests an Arrhenius behavior. The Seebeck coefficient is p-type with behavior indicative of multi-band transport. The Hall mobility is n-type and low (near 0.07 cm²/V s at room temperature).     

Influence of negative pressure on thermoelectric properties of Sb2Te3

We investigated the influence of negative pressure on the electrical conductivity, the Seebeck coefficient, and the power factor of Sb₂Te₃. We performed first-principles calculations with the linearized-augmented plane-wave method considering negative hydrostatic pressure in the range from zero to −2 GPa and doping for electrons and holes up to 10²⁰ cm⁻³. Our results predict a significant increase of the Seebeck coefficient and the power factor under negative pressure for certain doping concentrations.     

Magnetic and transport properties of YbRhIn5 and YbIrIn5 single crystals

Single crystals of YbRhIn₅ and YbIrIn₅ have been grown by flux method. The crystals were characterized by means of X-ray diffraction, magnetic and electrical transport measurements. Both compounds were found to be weak diamagnets with metallic character of the electrical conductivity and the Seebeck coefficient.     

First principles study on the ferroelectricity of the perovskite ABO3 ferroelectrics

In order to understand well the different ferroelectric behaviour of quantum paraelectrics and ferroelectrics and the origin of the ferroelectricity of the solid solution KTa0.5Nb0.5O3(KTN),we calculated the electronic structure of CaTiO3,BaTiO3 and KTN by first principles calculation.From total energy analysis,it is shown that,with increasing cell volume,the crystals (CaTiO3,SrTiO3) will have a ferroelectric instability.For BaTiO3,the ferroelectricity will disappear as the cell volume is decreased.From the density of states analysis,it is shown that the hybridization between B d and O p is very important for the ferroelectric stability of ABO3 perovskite ferroelectrics.This is consistent with the analysis of band structure.     

Annealing effects on the structural and electrical transport properties of n-type Bi2Te2.7Se0.3 thin films deposited by flash evaporation

N-type Bi₂Te_2.7Se_0.3 thermoelectric thin films with thickness 800 nm have been deposited on glass substrates by flash evaporation method at 473 K. Annealing effects on the thermoelectric properties of Bi₂Te_2.7Se_0.3 thin films were examined in the temperature range 373-573 K. The structures, morphology and chemical composition of the thin films were characterized by X-ray diffraction, field emission scanning electron microscope and energy dispersive X-ray spectroscopy, respectively. Thermoelectric properties of the thin films have been evaluated by measurements of the electrical resistivity and Seebeck coefficient at 300 K. The Hall coefficients were measured at room temperature by the Van der Pauw method. The carrier concentration and mobility were calculated from the Hall coefficient. The films thickness of the annealed samples was measured by ellipsometer. When annealed at 473 K, the electrical resistivity and Seebeck coefficient are 2.7 mΩ cm and −180 μV/K, respectively. The maximum of thermoelectric power factor is enhanced to 12 μW/cm K².     

Quantitative analysis of UV excitation bands for red emissions in Pr-doped CaTiO3, SrTiO3 and BaTiO3 phosphors by peak fitting

A quantitative spectral analysis of the ultraviolet (UV) broad excitation bands, which are located in the range 300-400 nm, for red emissions at around 610 nm in Pr-doped CaTiO₃, SrTiO₃:Al and BaTiO₃:Mg phosphors has been carried out using a peak fitting technique. The obtained results demonstrate that the UV broad band of CaTiO₃:Pr consists of four primary excitation bands centered around 330, 335, 365 and 380 nm and those of both SrTiO₃:Al and BaTiO₃:Mg consist of three primary bands centered around 310, 345 and 370 nm. Based on the behavior patterns and the values of the respective primary excitation bands’ parameters, i.e. center gravity (λ_top), maximum height (I_max) and full-width at half-maximum (FWHM), the UV-to-red relaxation processes in these titanate phosphors can be explained to be essentially the same, except for the existence of an additional relaxation pathway via electron-trap states in CaTiO₃:Pr, which gives a characteristic shape of its UV excitation spectrum in the wavelength range of >360 nm.     

TeI4掺杂量对n型Bi2Te3基烧结材料热电性能的影响

采用区熔法结合放电等离子体快速烧结(SPS)技术制备了n型Bi₂Te₃基热电材料.在300—500K的温度范围内测量了各热电性能参数，包括电导率(σ)、塞贝克系数(α)和热导率(κ)，研究了掺杂剂TeI₄的含量(质量百分比分别为0，0.05，0.08，0.10，0.13和0.15wt%)对热电性能的影响.结果表明：试样的载流子浓度(n)随TeI₄含量增加而增大，使电导率增大、塞贝克系数的绝对值先增大而后减小，从而导致品质因子(α²σ)呈先增加后降低的变化趋势；同时，由于异质离子(I^-)以及载流子对声子的散射作用增强，可显著降低其晶格热导率.烧结材料的性能优值(ZT=α²σT/κ)对应于TeI₄含量为0.08wt%有其最大值，约为0.92.此外，烧结材料的抗弯强度增加至80MPa左右，从而可以显著改善材料的可加工性以及元器件的使用可靠性. 关键词： 2Te₃')" href="#">Bi₂Te₃ 放电等离子体快速烧结 热电性能     

Transport coefficients and defect structure of Sb2−xAgxTe3 single crystals

Incorporation of Ag in the crystal lattice of Sb₂Te₃ creates structural defects that have a strong influence on the transport properties. Single crystals of Sb_2−xAg_xTe₃ (x=0.0; 0.014; 0.018 and 0.022) were characterized by measurements of the temperature dependence of the electrical resistivity, Hall coefficient, Seebeck coefficient and thermal conductivity in the temperature range of 5-300 K. With an increasing content of Ag the electrical resistance, the Hall coefficient and the Seebeck coefficient all decrease. This implies that the incorporation of Ag atoms in the Sb₂Te₃ crystal structure results in an increasing concentration of holes. However, the doping efficiency of Ag appears to be only about 50% of the expected value. We explain this discrepancy by a model based on the interaction of Ag impurity with the native defects in the Sb_2−xAg_xTe₃ crystal lattice. Defects have a particularly strong influence on the thermal conductivity. We analyze the temperature dependence of the lattice thermal conductivity in the context of the Debye model. Of the various phonon scattering contributions, the dominant influence of Ag incorporation in the crystal lattice of Sb₂Te₃ is revealed to be point-defect scattering where both the mass defect and elastic strain play a pivotal role.     

聚对苯撑/LiNi0.5Fe2O4纳米复合热电材料的制备及其性能研究

本文以流变相反应法原位合成了聚对苯撑/LiNi_0.5Fe₂O₄纳米复合热电材料,并对其热电性能进行表征,研究了放电等离子烧结时保温时间对其热电性能的影响.结果发现,复合材料铁氧体颗粒粒径为100---300nm,其外部被一层聚对苯撑膜包覆.电子在Fe²⁺和Fe³⁺之间的跳跃机理在铁氧体电导中占主导作用,因此聚对苯撑/LiNi_0.5Fe₂O₄复合材料具有n型导电特性.随着保温时间增加,复合材料电导率基本不变,但热导率逐渐增大且Seebeck系数逐渐减小,导致热电优值系数降低.由于结合了有机物高电导率和低热导率以及无机材料高赛贝克系数的优点,所制备的复合材料热电性能较单一材料有较大提高.     

The effect of (00l) crystal plane orientation on the thermoelectric properties of Bi2Te3 thin film

Highly (00l)-oriented pure Bi₂Te₃ films with in-plane layered grown columnar nanostructure have been fabricated by a simple magnetron co-sputtering method. Compared with ordinary Bi₂Te₃ film and bulk materials, the electrical conductivity and Seebeck coefficient of such films have been greatly increased simultaneously due to raised carrier mobility and electron scattering parameter, while the thermal conductivity has been decreased due to phonon scattering by grain boundaries between columnar grains and interfaces between each layers. The power factor has reached as large as 33.7 μW cm⁻¹ K⁻², and the out-of-plane thermal conductivity is reduced to 0.86 W m⁻¹ K⁻¹. Our results confirm that tailoring nanoscale structures inside thermoelectric films effectively enhances their performances.