Thermoelectric properties of Ge-doped Bi85Sb15 alloys at low temperatures

Bulk polycrystalline Bi₈₅Sb_15−xGe_x (x=0, 0.5, 1, 1.5, 2) composites were prepared by mechanical alloying followed by pressureless sintering. The thermoelectric properties were studied in the temperature range of 77–300 K. The results indicate that increasing the Ge concentration causes the Seebeck coefficient to change sign from negative to positive. Moreover, it is found that the maximum value of the Seebeck coefficient can be precisely controlled with the Ge concentration. The maximum dimensionless figure of merit reaches 0.07 at 140 K. These results suggest that the preparation of p-type Bi–Sb alloys is possible by using the Ge-doping approach.     

High temperature thermoelectric properties of Ca1−xBixMn1−yV yO3−δ (0≤x=y≤0.08)

CaMnO_3?δ with complex additives Bi₂O₃–V ₂O₅ were prepared by the solid-state reaction. The crystal structures of the Ca_1?xBi_xMn_1?yV _yO_3?δ (0≤x=y≤0.08) solid solutions were determined by means of the powder X-ray diffraction (XRD) using Rietan 2000 program and the high temperature thermoelectric properties were also investigated. Perovskite-type Ca_1?xBi_xMn_1?yV _yO_3?δ solid solutions are n-type semiconductors. The lattice parameters increase with increasing dopant level. The high temperature thermoelectric properties are improved due to Bi₂O₃–V ₂O₅ simultaneous doping. A maximum ZT value reaches to 0.21 for electron-doped Ca_0.96Bi_0.04Mn_0.96V _0.04O_3?δ at 1050 K, which is about twice as high as that of CaMnO_3?δ. The thermal shock resistance at temperatures between 20 and 450 ^°C is also highly improved.     

Relationship between average and local crystal structure and the ferroelectric properties of a Sr-Bi-Ta-Si-O ferroelectric material

We investigated the relationship between the average and local crystal structures and the ferroelectric properties of Bi₂SiO₅, Bi₄Si₃O₁₂, or Bi₂O₃ added Sr_1−xBi_2+xTa₂O₉ (x=0, 0.2) produced by a solid-state reaction. By measuring the P-E hysteresis, we found that Sr_1−xBi_2+xTa₂O₉ (x=0, 0.2) has higher P_r and E_c than SrBi₂Ta₂O₉ (SBT). P_r increased and E_c decreased by adding Bi₂SiO₅, Bi₄Si₃O₁₂, or Bi₂O₃ to Sr_1−xBi_2+xTa₂O₉. The average crystal structures were determined by the Rietveld method. On the other hand, the local structure is important, because the ferroelectric property is related to the distortion, and ferroelectric complex oxides have domains. We also investigated the local crystal structure using atomic pair distribution function (PDF) analysis. Based on the results, the bond angle variance, σ², of each TaO₆ octahedron increased by substituting Si for the Ta site. The tilting angle, α_a,α_b, of each TaO₆ octahedron increased relative to that of the average structure, and the symmetry of the TaO₆ octahedron in the local structure deteriorated in comparison to that of the average structure. This distortion and symmetry of TaO₆ contributes to the remanent polarization.     

Synthesis, structural and physical properties of new Terbium containing Tb-Hg-Sr-Ca-Cu-O superconducting system

Samples with various nominal compositions in the Tb-Hg-Sr-Ca-Cu-O system were prepared and studied by EDX, powder X-ray diffraction including the Rietveld refinement, electrical resistivity, magnetic susceptibility and thermoelectric power measurements. EDX and powder X-ray diffraction studies showed that Tb is required for the stabilization of the 1212, (Hg_1−yTb_y)Sr₂TbCu₂O_6+δ; y≈0.5 phase. Electrical resistivity and magnetic susceptibility measurements indicated that substitution of Tb by Ca is necessary to induce superconductivity in the 1212, (Hg_0.5Tb_0.5)Sr₂(Tb_1−xCa_x)Cu₂O_6+δ samples. The Rietveld refinements of the X-ray data of two samples with x=0.0 and 0.5 were carried out on the basis of tetragonal symmetry (space group P4/mmm) and the results indicated that the phase with x=0.5 has less puckered Cu-O planes than the Ca-free (Hg_0.5Tb_0.5)Sr₂TbCu₂O_6+δ phase. Syperconductivity is observed only for samples with x>0.2 and T_c increases with increasing Ca content, x. The results of thermoelectric power measurements suggest that the samples with x<0.8 are located in the underdoped region and the x=0.8 sample is optimally doped and exhibits the highest T_c of 88 K.     

Low-temperature thermoelectric properties of Bi85Sb15−xNbx alloys prepared by high-press sintering

The nanocrystalline materials with the general formula Bi₈₅Sb_15−xNb_x (x=0, 0.5, 1, 2, 3) were prepared by mechanical alloying and subsequent high-pressure sintering. Their transport properties involving electrical conductivity, Seebeck coefficient and thermal conductivity have been investigated in the temperature range of 80-300 K. The absolute value of Seebeck coefficient of Bi₈₅Sb₁₃Nb₂ reaches a maximum of 161 μV/K at 105 K, which is 69% larger than that of Bi₈₅Sb₁₅ at the same temperature. The power factor and figure-of-merit are 4.45×10⁻³ WK⁻²m⁻¹ at 220 K and 1.79×10⁻³ K⁻¹ at 196 K, respectively. These results suggest that thermoelectric properties of Bi₈₅Sb₁₅ based material can be improved by Nb doping.     

In situ energy dispersive X-ray diffraction study of iron disilicide thermoelectric materials

The dynamic phase transformation and structure of rapidly solidified Fe_1−xCo_xSi₂ (0.02?x?0.06) thermoelectric materials were in situ investigated under high temperatures and high pressures by energy dispersive X-ray diffraction using synchrotron radiation. The FeSi₂ alloys which solidified as α-Fe₂Si₅ and ε-FeSi eutectic structures, were transformed to the semiconducting β-FeSi₂ phase upon heating by the main reaction α+ε→β and the subsidiary reaction α→β+Si. The low heating rates and Co contents were found to be beneficial for the β phase formation. The decomposition temperature of β→α+ε was weakly dependent on heating rate, but significantly suppressed by the high pressures.     

Thermoelectric properties of p‐type Mg2Si0.6Ge0.4 fabricated by bulk mechanical alloying and hot pressing

Bulk mechanical alloying (BMA) followed by hot pressing (HP) was used to prepare Mg₂Si_0.6Ge_0.4 thermoelectric material with high densification. Starting from the elemental power mixture, the Mg₂Si_0.6Ge_0.4 solid solution was solid‐state synthesized via BMA. In fact, the peaks for the cubic‐structured Mg₂Si_0.6Ge_0.4 solid solution phase were detected after 300 cycles in BMA. The single phase of Mg₂Si_0.6Ge_0.4 was synthesized at 600 cycles in BMA. Mg₂Si_0.6Ge_0.4 showed p‐type semiconduction without doping. Effects of hot pressing conditions on thermoelectric properties were investigated. With increasing hot pressing temperature from 673 to 773 K and pressure from 500 MPa to 1 GPa, the electrical conductivity increased and the Seebeck coefficient decreased. The maximum figure of merit was obtained with the processing parameter of 600 cycles BMA and hot pressing at 773 K, 1 GPa for 1 h. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermoelectric properties of Bi-Sb semiconducting alloys prepared by quenching and annealing

Bi_100−xSb_x (x=8-17) alloys were prepared by direct melting of constituent elements, which was followed by quenching and annealing. The synthesis of high-homogeneity alloys was confirmed by X-ray diffraction, differential thermal analyses and electron microprobe analysis. The semiconducting and thermoelectric properties of the samples were investigated by measuring Hall coefficient, electrical resistivity and Seebeck coefficient in the temperature range from 20 to 300 K for both the as-quenched and annealing samples. The properties change gradually with the Sb concentration x, which is attributed to the variation of the energy gap. The Hall mobility was enhanced by annealing, which leads to a small electrical resistivity and a large Seebeck coefficient. Consequently, large values of about 8.5 mW/mK² for the power factor were obtained in the annealed alloys of x=8,12, and 14.     

Thermoelectric performance of ternary Bi-Sb-Ag alloys prepared by mechanical alloying and subsequent pressureless sintering

The alloys with the general formula of Bi₈₅Sb_15−xAg_x (x=0, 1, 3, 5, 7) were prepared by mechanical alloying and subsequent pressureless sintering (Bi₈₅Sb₁₅ alloy was used for comparison). Their transport properties involving electrical conductivity, Seebeck coefficient, and thermal conductivity had been investigated in the temperature range of 80-300 K. The maximum absolute value of Seebeck coefficient (120 μV/K) was found at 160 K in the alloy Bi₈₅Sb_15−xAg_x (x=3). The figure-of-merit of alloy Bi₈₅Sb_15−xAg_x (x=1) reached a maximum value of 2.16×10⁻³ K⁻¹ at 219 K, which is as large again as that of the reference sample Bi₈₅Sb₁₅.     

Thermoelectric performance of quaternary Mg2(1+x)Si0.2Ge0.1Sn0.7 (0.06 ≤ x ≤ 0.12) solid solutions with band convergence

The study of Mg₂Si based thermoelectric materials has received widespread attention. In this research, quaternary Mg_2(1+x)(Si_0.2Ge_0.1Sn_0.7)_0.99Sb_0.01 (0.06≤x ≤ 0.12) solid solutions with an optimized Sb doping were prepared by B₂O₃ flux method combined with spark plasma sintering (SPS) technique. The Seebeck coefficient, electrical conductivity and thermal conductivity were measured as a function of Mg excess between 300 K and 780 K. The electron concentration, electrical conductivity and lattice thermal conductivity increase while the Seebeck coefficient decreases with increasing magnesium excess content. The electron effective mass enhancement for x ≥ 0.08 suggests the conduction band convergence of Mg₂Si_0.2Ge_0.1Sn_0.7. Mg_2.16(Si_0.2Ge_0.1Sn_0.7)_0.99Sb_0.01 with a maximum dimensionless figure of merit of 0.94 at 780 K stand out as one of the best materials for intermediate temperature applications, providing a good nontoxic alternative to PbTe.     

Fermi surface and thermoelectric power of (Bi1−x Sbx)2Te3<Ag, Sn> mixed crystals

The Fermi surface anisotropy of (Bi_1?x Sb_x)₂Te₃ single crystals (0.25 ≤ x ≤ 1) was studied by analyzing the angular dependence of the frequency of Shubnikov-de Haas oscillations and the effect of tin and silver doping on the thermoelectric power in these crystals in the temperature range 77 ≤ T ≤ 300 K. It was shown that silver doping of (Bi_1?x Sb_x)₂Te₃ mixed crystals produces acceptors, while silver in Bi₂Te₃ acts as a donor. Tin also exhibits acceptor properties. Both tin and silver doping of p-(Bi_1?x Sb_x)₂Te₃ mixed crystals decrease the thermoelectric power due to an increase in the hole concentration.     

Ab‐initio thermodynamic calculations of Mg2BIV (BIV = Si,Ge, Sn) solid solutions

The thermodynamic properties of ternary Mg₂B^IV (B^IV = Si, Ge, Sn) solid solutions were first calculated by the ab‐initio density functional method. The results showed that there exist composition regions with d²G /dx² < 0 in Mg₂Si_1–x Sn_x and Mg₂Ge_1–x Sn_x systems, implying the possibility of spinodal decomposition of the pseudobinary solid solutions. It is suggested that the spinodal decomposition would be a potential way to obtain Mg₂B^IV based bulk in‐situ nanocomposites with reduced grain sizes and enhanced phonon scattering, and hence an improved thermoelectric figure of merit. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of Bi substitution on the magnetic and dielectric properties of epitaxially grown BaFe0.3Zr0.7O3−δ thin films on SrTiO3 substrates

The structural, dielectric and magnetic properties of single crystalline Ba_1−xBi_xFe_0.3Zr_0.7O_3−δ (x=0.0-0.29) thin films have been studied. The pseudotetragonal epitaxial thin films were obtained by pulsed laser-beam deposition (PLD) on (0 0 1) SrTiO₃ (STO) single-crystal substrates. The Bi substitution for the Ba ions up to an extent of x=0.18 caused a slight improvement in the leakage current properties, as well as an enhancement of the apparent dielectric constant. The saturation magnetization of the films was significantly decreased following Bi substitution. These changes were thought to be related to the increase in oxygen deficiencies in the films. The effect of the Bi substitution on the dielectric and magnetic properties was analyzed in conjunction with the change in valence value of the Fe ions.     

Structural and electrical study of Ce-substituted bismuth vanadate

Samples of Ce^IV-substituted bismuth vanadate, formulated as Bi₄Ce_xV_2−xO_{11−(x/2)−δ}; 0≤x≤0.30, were synthesized by solid-state reactions. The phase structure and electrical conductivity were investigated using X-ray powder diffraction, FT-IR, differential thermal analysis and AC impedance spectroscopy. For a low composition range, two phase transitions, α↔β and β↔γ, were exhibited in which the system mimics in most events the parent compound. Impedance analysis evidenced no relationship between the blocking effect of charge carriers and structural changes at ambient temperatures. However, the temperature dependence of conductivity was correlated with the stability region of various phases within the system.     

Enhanced thermoelectric properties of bismuth intercalated compounds BixTiS2

The thermoelectric properties of Bi intercalated compounds Bi_xTiS₂ have been investigated at the temperatures from 5 to 310 K. The results indicate that Bi intercalation into TiS₂ leads to substantial decrease of its electrical resistivity (one order low for x=0.05 and two orders low for x=0.15, 0.25 at 300 K) and lattice thermal conductivity (22, 115 and 158% low at 300 K for x=0.05, 0.15 and 0.25, respectively). Specially, the figure of merit, ZT, of lightly intercalated compound Bi_0.05TiS₂ has been improved at all temperatures investigated, and specifically reaches 0.03 at 300 K, which is about twice as large as that of TiS₂.     

Combinatorial synthesis and high throughput evaluation of thermoelectric power factor in Mg-Si-Ge ternary compounds

Discrete phase libraries of thermoelectric compounds, Mg_xSi_yGe_1−y, were fabricated by a combinatorial pulsed laser deposition followed by annealing as a thin film form on an integrated ceramic substrate. In the substrate are embedded four probe electrical contacts to each sample, lead wires and pads to be accessed by needle probes. Resistivity and Seebeck coefficient were evaluated electrically, while temperature difference was locally given to each sample by a local heater also embedded in the substrate. The sample temperature (300-673 K) was controlled by a heating stage and temperature difference at the two contact points for each sample was evaluated by an infrared camera. The dependences of polarity and absolute values of Seebeck coefficient on the composition agree well with the data in literature.     

Structural, electrical, and thermoelectrical properties of (Bi1 − x Sb x )2Se3 alloys prepared by a conventional melting technique

Polycrystalline solid solutions of (Bi_{1 ? x} Sb _x )₂Se₃ (x = 0, 0.025, 0.050, 0.075, 0.100) were prepared using a facile method based on the conventional melting technique followed by annealing process. X-ray analysis and Raman spectroscopical measurements revealed formation of Bi₂Se₃ in single phase. The electrical and thermoelectric properties have been studied on the bulk samples in the temperature range 100–420 K. The electrical conductivity measurements show that the activation energy and room-temperature electrical conductivity dependences on the Sb content respectively exhibit minimum and maximum values at x = 0.05. The thermoelectric power exhibited a maximum value near the room temperature suggesting promising materials for room-temperature applications. The highest power factor value was found to be 13.53 μW K^?2 cm^?1 and recorded for the x = 0.05 compound.     

Preparation of five-layered Si/SixGe1−x nano-films by RF helicon magnetron sputtering

Five-layered Si/Si_xGe_1−x films on Si(1 0 0) substrate with single-layer thickness of 30 nm, 10 nm and 5 nm, respectively were prepared by RF helicon magnetron sputtering with dual targets of Si and Ge to investigate the feasibility of an industrial fabrication method on multi-stacked superlattice structure for thin-film thermoelectric applications. The fine periodic structure is confirmed in the samples except for the case of 5 nm in single-layer thickness. Fine crystalline Si_xGe_1−x layer is obtained from 700 °C in substrate temperature, while higher than 700 °C is required for Si good layer. The composition ratio (x) in Si_xGe_1−x is varied depending on the applied power to Si and Ge targets. Typical power ratio to obtain x = 0.83 was 7:3, Hall coefficient, p-type carrier concentration, sheet carrier concentration and mobility measured for the sample composed of five layers of Si (10 nm)/Si_0.82Ge_0.18 (10 nm) are 2.55 × 10⁶ /°C, 2.56 × 10¹² cm⁻³, 1.28 × 10⁷ cm⁻², and 15.8 cm⁻²/(V s), respectively.     

Microwave solid synthesis and electrical properties of Aurivillius-type oxide-ion conductor

A new oxide-ion conductor of Aurivillius-type structure, namely BISRVOX (Bi₂Sr_xV_1−xO_{5.5-(3x/2)-δ}, 0≤x≤0.20), was successively synthesized by the microwave-assisted solid state reaction. 25 min of microwave irradiation was found to be quite sufficient to ensure the completion of reaction. Powder X-ray diffraction and differential thermal analysis showed better structural properties for the microwave-prepared samples compared to those obtained from the conventional solid synthesis route. Interestingly, the highly conducting γ-phase was effectively stabilized for x≥0.10. AC impedance spectroscopy evidenced the superiority of the microwave heating over conventional solid synthesis routes in exhibiting high oxide-ion performance.     

Electrical conductivity and structural stability of SrCo1−xFexO3−δ

Perovskite compounds in the system of SrCo_1−xFe_xO_3−δ (x=0.2, 0.4 and 0.6) were synthesized by solid state reaction. SrCo_1−xFe_xO_3−δ shows the p-type small polaron conduction behavior. Electrical conductivity and oxygen vacancy content decrease with increase in Fe content. The incorporation of Fe increases the structural stability of SrCo_1−xFe_xO_3−δ at low temperatures, while decreasing the structural stability at high temperatures. Oxygen partial pressure has a strong influence on electrical conductivity. At low oxygen partial pressure, SrCo_0.8Fe_0.2O_3−δ will transform from cubic to orthorhombic structure. This structure can remain in 5%H₂/Ar only for a short time and then dissociates into Sr₃Fe₂O_6.64 and Co due to the reduction of B-site elements.