Showing papers on "Seebeck coefficient published in 2000"

An Oxide Single Crystal with High Thermoelectric Performance in Air

Abstract: An oxide single-crystalline whisker with high thermoelectric properties at temperatures (T) higher than 600 K in air has been discovered. This whisker is assigned to Ca2Co2O5 phase (abbreviated to Co-225 whiskers) and has a layered structure in which Co–O layers of two different kinds alternate in the direction of the c-axis. Seebeck coefficient of the whiskers is higher than 100 µVK-1 at 100 K and increases with temperature up to 210 µVK-1. Temperature dependence of electric resistivity shows a semiconducting-like behavior. These results indicate that the electric carriers are transported via hopping conduction. Using thermal conductivity of a Co-225 polycrystalline sample, figure of merit (ZT) of the Co-225 whiskers is estimated 1.2–2.7 at T≥873 K. This compound is characterized with regard to low mobility and high density of carriers, which contradicts the conventional materials with high thermoelectric properties.

Thermoelectric quantum-dot superlattices with high ZT

Abstract: Following the experimentally observed Seebeck coefficient enhancement in PbTe quantum wells in Pb1−xEuxTe/PbTe multiple-quantum-well structures which indicated the potential usefulness of low dimensionality, we have investigated the thermoelectric properties of PbSexTe1−x/PbTe quantum-dot superlattices for possible improved thermoelectric materials. We have again found enhancements in Seebeck coefficient and thermoelectric figure of merit (ZT) relative to bulk values, which occur through the various physics and materials science phenomena associated with the quantum-dot structures. To date, we have obtained estimated ZT values approximately double the best bulk PbTe values, with estimated ZT as high as about 0.9 at 300 K.

Low-Temperature Thermoelectric Properties of the Composite Crystal [Ca 2CoO 3.34] 0.614[CoO 2]

Abstract: Electric resistivity, thermoelectric power and thermal conductivity of a polycrystalline sample of the composite crystal [Ca2CoO3.34]0.614[CoO2], also known as Ca3Co4O9, have been measured below 300 K. Metallic conductivity accompanied by large thermoelectric power has been observed down to 50 K. At 300 K, the sample exhibits a thermoelectric power of S = 133 µVK-1, resistivity of ρ= 15 mΩcm and thermal conductivity of κ= 9.8 mWK-1cm-1. The resulting dimensionless figure of merit becomes ZT300 = 3.5×10-2, which is comparable to the value reported for a polycrystalline sample of NaCo2O4, indicating that the title compound is a potential candidate for a thermoelectric material.

Preparation and thermoelectric properties of A8IIB16IIIB30IV clathrate compounds

Abstract: Polycrystalline samples of clathrate compounds Ba8Ga16Si30, Ba8Ga16Ge30, Ba8Ga16Sn30, and Sr8Ga16Ge30 were prepared by direct melting and characterized using X-ray powder diffraction and differential thermal analysis. The Ge- and Si-based clathrates melt congruently, whereas Ba8Ga16Sn30 melts incongruently. At room temperature the Ge- and Si-based clathrates possess a moderate negative Seebeck coefficient and a high electron concentration in the range of 7×1020–9×1020 cm−3 while Ba8Ga16Sn30 exhibits substantially lower electron concentration of 2.2×1019 cm−3. The Seebeck coefficient and electrical resistivity were measured over the range 100–870 K. The temperature dependence of transport properties of the clathrates is typical for heavily doped semiconductors. The transport properties were analyzed using a standard semiconductor transport model. There is a good agreement between the assumed model and experimental temperature dependence of the Seebeck coefficient in the extrinsic conductivity range for all...

Thermoelectric properties of Bi2Sr2Co2Ox polycrystalline materials

Abstract: Bi2Sr2Co2O9 (BC-2202) polycrystalline materials with a layered structure have been prepared by partial melting. The chemical compositions of the samples are Bi2Sr2Co2Ox (2202), Bi1.8Sr2Co2Ox (Bi-1.8), and Bi2Sr1.8Co2Ox (Sr-1.8). All three samples are p-type conductors. The electric properties, namely, the Seebeck coefficient (S) and electric resistivity (ρ), of the samples are dependent on chemical composition. The S values increase with temperature at T>673 K and, at 973 K, reach 100, 110, and 150 μV K−1 for the 2202, the Bi-1.8, and the Sr-1.8 samples, respectively. Thermal conductivity (κ) for all samples is lower than for ordinary conducting oxides. The figure of merit (Z) increases with temperature for all samples. Z values at 973 K are 0.77×10−4, 0.61×10−4, and 2.0×10−4 K−1 for the 2202, Bi-1.8, and Sr-1.8 samples, respectively. The thermoelectric properties depend on the chemical composition of the BC-2202 phase. The BC-2202 material thus appears to be a promising thermoelectric material due to its...

Is the intrinsic thermoelectric power of carbon nanotubes positive

Abstract: The thermoelectric power (TEP) of single-walled carbon nanotubes (SWNTs) is extremely sensitive to gas exposure history. Samples exposed to air or oxygen have an always positive TEP, suggestive of holelike carriers. However, at fixed temperature the TEP crosses zero and becomes progressively more negative as the SWNTs are stripped of oxygen. The time constant for oxygen adsorption/desorption is strongly temperature dependent and ranges from seconds to many days, leading to apparently "variable" TEP for a given sample at a given temperature. The saturated TEP can be accounted for within a model of strong oxygen doping of the semiconducting nanotubes.

Effect of Sb doping on the thermoelectric properties of Ti-based half-Heusler compounds, TiNiSn1−xSbx

Abstract: Half-Heusler alloys (MgAgAs type) with the general formula MNiSn where M is a group IV transition metal (Hf, Zr, or Ti) are currently under investigation for potential thermoelectric materials. These materials exhibit a high negative thermopower (−40 to −250 μV/K) and low electrical resistivity values (0.1–8 mΩ cm) both of which are necessary for a potential thermoelectric material. Results are presented in this letter regarding the effect of Sb doping on the Sn site (TiNiSn1−xSbx). The Sb doping leads to a relatively large power factor of (0.2–1.0) W/m K at room temperature for small concentrations of Sb. These values are comparable to that of Bi2Te3 alloys, which are the current state-of-the-art thermoelectric materials. The power factor is much larger at T≈650 K where it is over 4 W/m K making these materials very attractive for potential power generation considerations.

Synthesis and Thermoelectric Properties of the New Oxide Materials Ca3-xBixCo4O9+δ (0.0 < x < 0.75)

Abstract: A new series of oxides Ca3-xBixCo4O9+δ, (x = 0.0−0.75) with Ca2Co2O5-type structures were synthesized, and their structures, electrical properties, Seebeck coefficients, and thermal conductivities were measured. The values of Seebeck coefficients of the new oxides are all positive, showing that they are p-type conductors. Both the electrical conductivity and Seebeck coefficients increase with the increasing Bi contents which can be attributed to the increase of carrier mobility due to the larger size of Bi ion. The electrical conductivity, Seebeck coefficient, and the calculated value of the power factor of Ca3-xBixCo4O9+δ (x = 0.5) are 105 S cm-1, 160 μV K-1, and 2.7 × 10-4 W K-2 m-1 at 700 °C, respectively. The thermal conductivity of Ca3-xBixCo4O9+δ (x = 0.5) at room temperature is 1.14 W m-1 K-1 and increase slightly with the increasing temperature. At 700 °C, the figure of merit of Ca3-xBixCo4O9+δ (x = 0.5) is 2.0 × 10-4 K-1.

Thermoelectric and transport properties of β-Ag2Se compounds

Abstract: The Hall coefficient, electrical resistivity, and Seebeck coefficient of n-type specimens of β-Ag2Se, the low temperature polymorph of silver selenide, were measured over the temperature range from 70 to 300 K. The results showed maxima in both Hall coefficient and the electrical resistivity just below the onset of the intrinsic conduction range. This anomaly was qualitatively explained by the deviation of the Coulomb scattering from the usual assumption of independence due to the degenerate nature of the samples. The estimated energy gap for different samples of about 160 meV seems to confirm the existence of the second low temperature phase β2. This second phase is a probable reason for the relatively high thermoelectric figure of merit observed.

Thermoelectric properties of semimetallic (Zr, Hf)CoSb half-Heusler phases

Abstract: Unlike semiconducting TiCoSb, ZrCoSb and HfCoSb half-Heusler phases are semimetallic below room temperature and exhibit small Seebeck coefficients of ∼−10 μV/K at 300 K. However, upon substituting (doping) the Co and Sb sites with Pt and Sn, respectively, much larger thermopowers (S) are obtained. For ZrCoSb, S reaches −110 and +130 μV/K while resistivity ρ decreases from ∼5×104 μΩ cm in the undoped phase to 1–2×103 μΩ cm in the substituted phases at 300 K. The lowest thermal conductivity obtained in the substituted alloys is ∼3.0 W/m K at 300 K, which is among the lowest reported for this class of structural phases. There are indications that the thermoelectric properties have not been optimized in these multinary alloys.

Thermoelectric properties of Sn-filled skutterudites

Abstract: Thermal conductivity, resistivity, Seebeck coefficient, and structure measurements of CoSb3 with tin interstitially placed in the voids are reported. These tin-filled skutterudites were synthesized under high pressure and temperature conditions; they cannot be synthesized under “normal” synthesis approaches. The tin atoms exhibit very large atomic displacement parameters indicating a large “rattling” motion inside their atomic “cages.” The disorder induced by the Sn atoms is a very good phonon scattering mechanism. The thermal conductivity of these compounds is very low with a temperature dependence that is atypical of simple solids. The tin-filled compounds exhibit n-type semiconducting behavior with relatively high Seebeck coefficients for compounds whose electronic properties have not been optimized. The potential for thermoelectric applications is discussed.

Chemical Design and Thin Film Preparation of p-Type Conductive Transparent Oxides

Abstract: Chemical design to find a new transparent conductive oxide having p-type conductivity has been proposed. Following the chemical design, we have selected CuGaO2 and CuAlO2 as candidate materials. CuGaO2 thin films were prepared on silica glass substrates by RF sputtering method. The optical band gap of the film was estimated to be ∼3.4 eV. Positive sign of Seebeck coefficient demonstrated the p-type conductivity of the film. The dc conductivity of the film was 5.6 × 10−3S·cm-1 and the activation energy was 0.22 eV at room temperature. Because of rough texture of the film, the observed conductivity was not an intrinsic property of the material. Further, CuAlO2 thin films were prepared by laser ablation. The film deposited in O2 atmosphere of 1.3 Pa at 690°C showed higher optical transmission in visible and near-infrared regions than previously reported. Contribution of Cu 3d components to upper edge of valence band in CuGaO2 and CuAlO2 were confirmed by photoemission spectroscopic measurements.

Thermoelectric Properties of Bi2.3-xPbxSr2.6Co2OySingle Crystals

Abstract: A set of single crystals of the layered cobaltite Bi2.3-xPbxSr2.6Co2Oy (0 ≤x≤0.44) were prepared by a flux technique. The resistivity ρ and the thermopower S along the in-plane direction were measured, and the thermoelectric properties were compared with those of other oxides. The Pb substitution causes a decrease in ρ and an increase in S below 100 K, which improves the power factor S2/ρ by 3–4 times. The transport parameters exhibit an anomaly near 25–50 K, such as a plateau in S and an upturn in ρ, which we ascribe to the reduction of the density of states owing to a pseudogap.

Thermoelectric properties of pure and doped FeMSb (M=V,Nb)

Abstract: The thermoelectric properties of the pure and doped half-Heusler compounds FeVSb and FeNbSb are reported. The electrical resistivities are between 0.2 and 20 mΩ cm at room temperature. Thermoelectric power measurements indicate that FeVSb is an n-type material with moderate Seebeck coefficients near −70 μV/K at 300 K. The thermal conductivity at room temperature is large, approximately 0.1 W/cm K, and increases with decreasing temperature. Chemical substitutions, which have a dramatic effect on the transport properties, were performed in an effort to enhance the thermoelectric performance. Band-structure calculations are presented for the pure materials.

Transport properties of polypyrrole–ferric oxide conducting nanocomposites

Abstract: Conducting nanocomposite samples were prepared combining colloidal ferric oxide particles with conducting polypyrrole. Three composite samples (prepared keeping colloidal content fixed and varying the content of the conducting polypyrrole) and the pure polymer were used in the present investigation. Temperature dependent dc and ac conductivity and thermoelectric power for the samples have been measured. The dc conductivity results were analyzed by Mott’s variable range hopping mechanism. The variation of ac conductivity with the frequency shows very little change in total conductivity up to a critical frequency, followed by a sudden jump with discontinuity and then increases monotonically following a power law. The frequency exponent decreases with temperature as predicted by the correlated barrier hopping theory. Above 50 K the ac component of the conductivity increases almost linearly as predicted by the quantum mechanical tunneling model. It is found that all the features of ac conductivity cannot be reconciled into an existing single theory. The thermoelectric power is positive, low, and varies linearly with temperature, indicating a metallic character and the presence of polarons and/or bipolarons as the cationic charge carriers in the composites. The overall nature of the S(T) curves suggests that in addition to a contribution from hopping a linear metallike component is also active for the thermopower.

Small polaron hopping conduction in V2O5-Sb-TeO2 glasses

Abstract: The dc electrical conductivity of glasses in the system V2O5–Sb–TeO2 prepared by press quenching was studied at temperatures between 303 and 473 K. The composition range of the glass formation region was found to be 10⩽TeO2⩽100 mol%, 0⩽V2O5⩽70 mol%, and 0⩽Sb⩽20 mol%, respectively. The glasses indicated n-type semiconductors from the measurement of thermoelectric power. The dc conductivities at 473 K for the present glasses were determined to be 6.38×10 −6 –7.13×10 −3 S cm −1 , indicating that the conductivity increased with increasing V2O5 concentration. Sb content also contributed to increase the conductivity and decrease activation energy for electrical conduction. A model of redox reaction during melting was proposed and quantitatively explained the reaction between V2O5 and Sb. A glass of composition 70 V 2 O 5 · 20 Sb · 10 TeO 2 (mol%) having a conductivity of 7.13×10 −3 S cm −1 at 473 K was found to be the highest conductive glass among the previous vanadium–tellurite glasses. From the conductivity-temperature relation, it was found that small polaron hopping model was applicable at the temperature above 1 2 Θ D (ΘD : the Debye temperature); the electrical conduction at T> 1 2 Θ D was due to adiabatic small polaron hopping of electrons between vanadium ions for V2O5⩾50 mol%, and non-adiabatic for 30⩽V2O5 2.90×10 −4 cm 2 V −1 s −1 at 473 K. The carrier density was obtained to be of the order of 10 20 –10 21 cm −3 , and temperature dependence of the carrier density was barely present between 423 and 473 K. The conductivity of the present glasses was primarily determined by hopping carrier mobility.

Physical properties of skutterudites YbxM4Sb12, M = Fe, Co, Rh, Ir

Abstract: A series of compounds YbxM4Sb12, were synthesised by reaction sintering. From Rietveld refinements isotypism was determined in all cases with the LaFe4P12-(skutterudite)-type, space group $${\mathop{\rm Im} olimits} \overline 3 $$ - No. 204. These refinements also served to derive the Yb-content in the samples. There is a systematic trend for the Yb-occupancy in the parent lattice M4Sb12, revealing a gradual decrease of the Yb-content from x =0.8 (M=Fe), x =0.5 (FeCo), x =0.2 (Co), x =0.1 (Rh) to $$x \approx 0.02$$ (Ir). This dependency seems to correlate with the thermal stability of the ternary compounds: a true ternary compound forms for M =Fe, whilst for M = Co stable binary skutterudite compounds MSb3 already exist. Measurements of various bulk properties revealed the absence of any long range magnetic order in this series of compounds. While the samples rich in Yb behave metallic like, the Rh and Ir based skutterudites show a semiconducting-like resistivity which at lower temperatures is characterised by variable range hopping in the presence of strong Coulomb interaction. Although Yb0.1Rh4Sb12 exhibits a Seebeck coefficient up to about 150 μV/K, figures of merit ZT generally are below 0.1 near room temperature, primarily due to the large resistivities of the sintered material.

Thermoelectric properties of (Bi,Pb)–Sr–Co–O oxide

Abstract: Electrical conductivity and thermopower of Bi2Sr3Co2Ox sintered bodies were first investigated in the temperature range 440–1060 K in air for high-temperature thermoelectric application. The samples fabricated by the sinter-forging method increased their bulk densities and electrical conductivity. The value of thermopower and the temperature dependence of electrical conductivity of the sinter-forged samples were close to that of the single crystal. Evaluating the thermal conductivity of the polycrystalline sample, we calculated a thermoelectric figure of merit Z to be 0.107 × 10−3 K−1 at 1060 K.

Study of interface effects in thermoelectric microrefrigerators

Abstract: Interface phenomena play a vital role in thermoelectric (TE) microrefrigerators. The present study employs a phenomenological model to examine the behavior of TE refrigerators as a function of thermal and electrical contact resistance, boundary Seebeck coefficient, and heat sink conductance. We modify the conventional definition of the figure of merit to capture the interface effects. A finite temperature drop across the interface between a metal electrode and a thermoelement is found to strongly influence the boundary Seebeck effect. Interface engineering can potentially improve the overall performance of TE microrefrigerators.

Structural, thermal and electrical properties of Ce-doped SrMnO3

Abstract: The Sr1-xCexMnO1-α system (0≤ x ≤ 0.5) was investigated with respect to its structural, thermal and electrical properties. Although un-doped SrMnO3 has the perovskite structure above 1400°C, the structure is unstable at room temperature. However, partial substitution of Ce for Sr in SrMnO3 stabilizes the perovskite structure down to room temperature. Single phase perovskite is obtained for 0.1≤ x ≤ 0.3 in Sr1-xCexMnO1-α, and it remains stable even following heat treatment at 800°C for 100 h. The dependence of the electrical conductivity on temperature was measured from room temperature to 1000°C in air. Ce doping dramatically enhanced the electrical conductivity of SrMnO3. Sr0.7Ce0.3MnO1-α exhibits a higher conductivity (290 S · cm-1 at 1000°C) than that of La0.8Sr0.2MnO3 (LSM, about 175 S · cm-1) and remains n-type over the whole range of temperature examined. The thermal expansion coefficients in the system were nearly constant with values ranging between 1.24 × 10-6 and 1.01 × 10-6 cm/cm · K for temperatures of 50°C to 1000°C.