Thermopower Changes Research Articles

Thermopower of composites containing metallic cobalt nanoparticles embedded in the Al2O n dielectric matrix

The concentration and temperature dependences of the thermopower of composites containing Co nanoparticles embedded in the Al2O n dielectric matrix are investigated. Below the percolation threshold, i.e., in the tunneling conduction region, the absolute values of the thermopower of the composites under investigation are less than those above the percolation threshold. It is revealed that, in the tunneling conduction region, the slope of the temperature dependences of the thermopower changes at a temperature of ∼205 K. This can indicate that the thermopower is sensitive to a change in the mechanism of conduction from the Mott law ln(σ) ∝ (1/T)1/4 to a power relation that corresponds to the model of inelastic resonant tunneling through a chain of localized states in the dielectric matrix. The introduction of oxygen in the course of sputtering brings about a decrease in the absolute values of the thermopower; however, the character of variation in the concentration and temperature dependences of the thermopower remains unchanged.

Structure and physical properties ofYCoO3at temperatures up to1000K

The crystal structure of perovskite $\mathrm{Y}\mathrm{Co}{\mathrm{O}}_{3}$ has been studied by neutron powder diffraction up to high temperatures. The orthorhombic $Pbnm$ symmetry is confirmed in the whole temperature range. A significant isotropic enlargement of $\mathrm{Co}{\mathrm{O}}_{6}$ octahedra is evidenced above $600\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ leading to unit cell expansion and increased octahedral tilting. Supported by complementary physical measurements, the origin of anomalous expansion is identified with a gradual transition of ${\mathrm{Co}}^{3+}$ ions from the diamagnetic low-spin $(S=0)$ ground state to excited magnetic states with spin $S=1$ or 2. The magnetic transition is closely followed by a broad resistivity transition of the insulator-metal type, centered at $750\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The changes in magnetic susceptibility, electric resistivity, thermopower and thermal conductivity associated with transitions in $\mathrm{Y}\mathrm{Co}{\mathrm{O}}_{3}$ are discussed in comparison with similar data on related perovskite $\mathrm{La}\mathrm{Co}{\mathrm{O}}_{3}$.

Oscillatory thermopower of carbon chains: First-principles calculations

We investigate the thermoelectric transport through carbon chains connected by two Al leads. Using a Landauer-Buttiker-like formula, we calculate the thermopower and thermoconductance of $\mathrm{Al}\text{\penalty1000-\hskip0pt}{\mathrm{C}}_{n}\text{\penalty1000-\hskip0pt}\mathrm{Al}$ from first principles. We find that the charge transfer plays an important role in the thermoelectric transport. Because of the charge transfer, the thermopower changes sign for even-odd number of carbon atoms. The thermopower and electric conductance as a function of the gate voltage also exhibit oscillatory behaviors with a phase difference of $\ensuremath{\pi}∕2$.

Thermopower due to defects in Ni, Pd and Pt

The thermopower induced by tensile plastic deformation at a near-room temperature ismeasured to determine the change in thermopower per unit dislocation densityΔSd/Nd in nickel (99.98%). The dislocation density is determined indirectly by measurement of theflow stress. The partial-wave method is used to calculate the contribution of dislocationsand vacancies to the thermoelectric power of nickel, platinum and palladium. The valueΔSd/Nd calculated for nickel agrees quite satisfactorily with measured ones. These results are thefirst quantitative estimation of the defect contribution to thermopower of transition metals.

P-Type electron transport in La 1−xSr xFeO 3−δ at high temperatures

Electrical conductivity, thermopower and oxygen content were measured for La 1− x Sr x FeO 3− δ ( x=0.2, 0.5, 0.9) within the oxygen partial pressure range 10 −4–0.5 atm and at temperatures 750–950 °C. The dominating charge carriers under these experimental conditions are electron holes. The results of oxygen nonstoichiometry measurements are used to estimate the concentration of holes and to analyze data on conductivity and thermopower. The changes in thermopower are described by the model assuming that the number of sites accessible for migration of holes is independent on oxygen content. The mobility of electron holes is calculated, and it is found to be virtually independent on temperature in the compositions with x<0.5 while compositions with x>0.5 exhibit thermally activated mobility and mobility values about 0.1 cm 2 V −1 s −1 or smaller. It is suggested that the main contribution to the composition dependent variations in electron hole mobility are associated with Coulomb interactions at small x's, whereas at high degrees of doping the mobility of holes is most strongly affected by the increasing amount of oxygen vacancies.

Thermopower and electrical resistivity behavior near the martensitic transition in Gd5(SixGe1−x)4 magnetocaloric compounds

Recently, the Gd5(SixGe1−x)4 system exhibits fascinating physical properties, namely giant magnetocaloric, magnetoresistance and magnetostriction effects near a first-order (martensitic like; T=TS) magneto-structural transition. We report the thermopower behavior S(T) between 10 and 300K for compounds with x=0, 0.10 and 0.45, belonging to three distinct regions of the magnetic/structural phase diagram. Large thermopower changes are observed near TS for x=0.10 (TS=78K; AFM-Orthorhombic II/FM-Ortho.I) and for x=0.45 (TS=242K; PM-Monoc/FM-Ortho.I) with a Gaussian distribution in dS/dT, characteristic of a first-order phase transition. Near the purely magnetic transitions (PM/AFM) for x=0(TN=122K) and x=0.10(TN=127K) we observe dS/dT peaks governed by spin fluctuation effects. A comparative analysis between dS/dT and the resistivity derivative dρ/dT near TS is made. At low temperatures S(T) behaves similarly in the ferromagnetic x=0.10 and 0.45 compounds (S≈AT+BT2), the anomalous B term being related to the mean internal field (spin wave excitations). For x=0 (AFM phase at low T) one has S=AT (no B term; zero mean internal field).

The size change of ferromagnetic domains induced by the effect of cation disorder in oxideperovskites

A-site cation disorder effects on the structural, electrical transport and magnetic properties have been studiedin the Nd0.7(Ca,Sr,Ba)0.3MnO3 series with a fixed A-site mean radius Å by resistivity, dc susceptibility, infrared spectra and thermopower measurements. Thedisparity of transition temperature obtained from resistivity, magnetization and thermopowerincreases with increasing amounts of A-site cation disorder quantified by the size varianceσ2. The insulator-like ferromagnetic behaviour and the sign change of thermopower induced byσ2 have been observed below the Curie temperatureTC.As σ2 increases, the size of the ferromagnetic domains decreases, and the carriers seemto be trapped strongly in the interfaces between ferromagnetic domains; suchbehaviours may help to understand the insulator-like ferromagnetic state induced byσ2 belowTC.

Giant magnetothermopower associated with large magnetoresistance in Ag2−δTe

We have probed the temperature and magnetic-field dependence of the thermopower and resistance of a p-type silver chalcogenide, Ag2−δTe. The application of a magnetic field causes not only a large magnetoresistance but also a giant magnetothermopower effect. The maximum change of thermopower is as high as 470 μV/K in a 7 T magnetic field. Both the magnetoresistance and the magnetothermopower show a pronounced peak and nearly linear behavior near the sign change of the thermopower. Bandcrossing and quantum confinement due to disorder appear to play key roles in the heightened response to field.

Phonon drag in disordered films and structures

Employing the quantum transport equation, we investigate the effect of electronic disorder on the phonon-drag thermopower. We consider the electron–phonon interaction via the deformation potential, which is strongly renormalized due to elastic electron scattering. The scattering potential of impurities, boundaries and defects is modeled by quasistatic scatterers and vibrating scatterers, which move in the same way as host atoms. In thin films, micro and nanostructures of the phonons relax mainly in a substrate, and the phonon-drag thermopower substantially depends on the character of electron scatterers. Vibrating scatterers decrease thermopower, while quasistatic scatterers (e.g. rigid boundaries) increase it. These changes in thermopower correlate to the disorder-induced modification of the electron–phonon relaxation rate.

Thermoelectric Device of Na(Co, Cu)2O4 and (Ba, Sr)PbO3.

The thermoelectric device of oxide p-n coupled element of NaCo1.7Cu0.3O4 and Ba0.2Sr0.8PbO3 has been fabricated using direct joining between p-type and n-type. Because of unwanted reaction between the two p- and n-type oxides, the diffusion barrier of Au foil was used and the device was stable and showed no appreciable contact resistance at the junction after the thermal cycle at 1000 K in air. The TE maximum power from the device was 12.5 mW when the temperatures of the hot and cold side of the device were 1045 K and 539 K, respectively. The device showed neither appreciable contact resistance at the junction nor changes in thermopower, after the high-temperature durability test in air.

Effect of electronic disorder on phonon-drag thermopower

Using the quantum-transport equation and Keldysh diagrammatic technique, we investigate the phonon-drag thermopower in a disordered conductor. We consider phonon drag of three-dimensional electrons, which interact with longitudinal phonons via the deformation potential. The scattering potential of impurities, boundaries, and defects is modeled by quasistatic scatterers and vibrating scatterers, which move in the same way as host atoms. In thin films and nanostructures the phonons relax mainly in a substrate, and the phonon-drag thermopower substantially depends on the character of electron scatterers. Vibrating scatterers decrease thermopower, while static scatterers, such as rigid boundaries and heavy impurities, increase it. These changes in thermopower correlate to the disorder-induced modification of the electron-phonon relaxation rate. In bulk conductors, phonon-electron scattering dominates in the phonon relaxation, and the phonon-drag thermopower just slightly varies with electron mean free path.

Geometrical magnetothermopower inn- andp-type InSb

In a previous paper [Heremans et al., Phys. Rev. Lett. 86, 2098 (2001)] we reported how the geometry of a sample of lightly p-type InSb can be manipulated to create a very large change of the thermoelectric power with magnetic field. That effect was called the geometrical magnetothermopower (GMT). In this paper, we review and extend the results in p-type InSb, and present a study of the GMT in n-type InSb. We conclude that, in semiconductors with two types of carriers of different mobilities, the GMT effect makes the thermopower change from being dominated by a high-mobility carrier at zero magnetic field to being dominated by a low-mobility carrier at high field, even when the low-mobility carrier is the minority carrier. We complete the study of the reciprocity relations in macroscopic two-contact thermoelectric systems by adding to the experimental verification of the Kelvin relation the new observation that the GMT is an even function of magnetic field, even in asymmetric geometries. Finally, we model the GMT in both n- and p-type InSb using classical transport theory, and show how this model predicts that the GMT effect can lead to an increase of the thermoelectric power factor, and of the figure of merit, by over 80%.

Comparison of thin film and bulk forms of the transparent conducting oxide solution Cd1+xIn2−2xSnxO4

Physical and structural properties of thin films prepared via rf magnetron sputtering of the transparent conducting oxide spinel Cd1+xIn2−2xSnxO4 are compared to those reported for bulk specimens (prepared via high-temperature solid state reaction at 1175 °C). Optical band gaps measured on thin films of Cd1+xIn2−2xSnxO4 were 3.5, 3.70, and 3.65 eV for x=0.15, 0.45, and 0.70, which where 0.57, 0.94, and 0.95 eV higher than their bulk counterparts. Thin film Seebeck coefficients were −18.0, −15.5, and −15.5 μV/K for x=0.15, 0.45, and 0.70, respectively, which were 27, 24, and 19 μV/K smaller in magnitude than their bulk counterparts. Sn-Mössbauer spectroscopy revealed isomer shifts that averaged 0.2 mm/s for both bulk and thin films specimens. The presence of quadrupole splitting, which averaged near 0.48 mm/s for film specimens and 0.39 mm/s for bulk specimens, suggests that Sn+4 in all specimens is in octahedral coordination. The difference in quadrupole splitting suggests that thin films have a different cation distribution than their bulk counterparts. The effective mass at the base of the conduction band, measured via the method-of-four-coefficients, was found to be 0.25, 0.18, 0.21, and 0.22 me for x equal to 0.15, 0.45, 0.70, and 1.0, respectively. A model that explains the changes in optical gap and thermopower as a result of differences in the fundamental band gap (resulting from a changing cation distribution), conduction band curvature, and carrier density is presented.

Low-temperatureresistance minimum in non-superconducting 3R-Nb1+xS2 and3R-GaxNbS2

We report the structural and electron transport properties of3R-Nb1+xS2 (x⩾0.07) and 3R-GaxNbS2 (0.1⩽x⩽0.33) prepared as polycrystalline pellets as well as singlecrystals grown by vapour transport. We observe a resistance minimumin these compounds between 20 and 60 K, with Tminproportional to x. The resistance scales as ρ/ρmin(T/Tmin) in the range 0.2<T/Tmin<2 fordifferent phases with x⩽0.25 whose resistivity differs by anorder of magnitude. Powder x-ray diffraction also showsprogressively increasing intensity of superlattice lines with cationconcentration. The thermopower changes sign around the resistanceminimum. The explanation of the resistance minimum and thesimultaneous rapid suppression of superconductivity is sought inelectron-electron scattering effects in the presence of cationdisorder in these narrow-band anisotropic materials.

Sign Change of Thermopower at Low Temperatures in Nd0.7(Ca,Sr,Ba)0.3MnO3

The temperature dependence of thermopower and resistivity for the series of Nd0.7(Ca,Sr,Ba)0.3MnO3 with a fixed average A-site cation radius ⟨rA⟩ have been measured. Infrared spectra revealed that the size differences between various A-site ions cause large local distortions of MnO6 octahedra. The thermopower is enhanced above metal-insulator transition temperature Tm and is reduced below Tm by increasing amount of A-site cation disorder. The sign change of thermopower induced by A-site cation disorder has been observed at low temperatures. The observed thermopower behaviors have been explained by Mott's formula and the effect of the spin disorder scattering.

Ce3CuxPt3−xSb4:Modifying the properties of a Kondo insulator by substitutional doping

The temperature dependence of the magnetic susceptibility, thermopower, and electrical resistivity were explored for the solid solution ${\mathrm{Ce}}_{3}{\mathrm{Cu}}_{x}{\mathrm{Pt}}_{3\ensuremath{-}x}{\mathrm{Sb}}_{4}.$ The magnetic susceptibility varies from ${\mathrm{Ce}}^{3+}$ Curie-Weiss behavior in ${\mathrm{Ce}}_{3}{\mathrm{Cu}}_{3}{\mathrm{Sb}}_{4}$ to almost ${\mathrm{Ce}}^{4+}$ behavior (intermediate valence) in ${\mathrm{Ce}}_{3}{\mathrm{Pt}}_{3}{\mathrm{Sb}}_{4}.$ All samples had semiconducting resistivities with decreasing activation energies for higher copper concentrations. The large positive thermopower of ${\mathrm{Ce}}_{3}{\mathrm{Pt}}_{3}{\mathrm{Sb}}_{4}$ is greatly affected by small concentrations of copper. The peak of \ensuremath{\sim}300 \ensuremath{\mu}V/K at approximately 110 K for ${\mathrm{Ce}}_{3}{\mathrm{Pt}}_{3}{\mathrm{Sb}}_{4}$ changes to approximately -170 \ensuremath{\mu}V/K for ${\mathrm{Ce}}_{3}{\mathrm{Cu}}_{0.5}{\mathrm{Pt}}_{2.5}{\mathrm{Sb}}_{4}$ at the same temperature. For all the copper-doped samples, the thermopower changes from positive to negative values on cooling.

Ionic thermopower of composite electrolytes: I. Theory

The ionic thermopower of solid electrolyte composites which are composed of a solid electrolyte and an electrically insulating inert phase is analyzed. For a stoichiometric model system AX with Frenkel disorder in the cation sublattice we derive formal relations within a lattice point defect model for the change in ionic thermopower on dispersing the second phase. It is shown that thermopower measurements may provide information on the defect concentrations in the space charge regions surrounding the inert phase. Available experimental results are discussed.

Systematics and Optimisation of Hole Doping in Bi-2223

The authors systematic changes in thermopower, {Tc}, Raman modes and magnetic irreversibility H{sub irr} as a function of carrier hole concentration p in Bi-2223. They have established that Raman spectroscopy can provide an indicator of doping level: the 630cm{sup {minus}1} mode is found to soften with increasing p. A departure of the thermopower from the universal thermopower-doping relationship displayed by other HTS compounds is explained by a non-uniform charge distribution over the three CuO{sub 2} planes. Scaling of H{sub irr} shows the optimal hole concentration for maximum J{sub c} lies beyond currently achieved levels.

Thermoelectric properties of Sr1-xLaxPbO3 (x0.02)

A set of polycrystalline samples of Sr1-xLaxPbO3 (x<0.02) were prepared with special care taken in controlling the La content to within 0.2%. The measured resistivity () and thermopower (S) change systematically with x, and the x-dependences are explained consistently on the basis of a rigid-band picture in a single parabolic band. The carrier concentration evaluated ranges from 1019 to 1020 cm-3, while the scattering time and the mobility do not appreciably change with the La substitution. The power factor (S2/) takes a maximum at a carrier concentration of 4 × 1019 cm-3, which is nearly the same value as for conventional thermoelectric semiconductors. These data clearly indicate that carrier doping in SrPbO3 is as controllable as that in conventional semiconductors.

Thermoelectric power in quasi-two-dimensional systems with scattering of current carriers by phonons

The thermopower α in electron systems with a quasi-two-dimensional energy spectrum is investigated in the relaxation-time tensor approximation. The longitudinal and transverse components of the thermopower are calculated for scattering of the current carriers by different types of phonons. It is shown that the anisotropy of the thermopower in such systems is substantial. The dependence of a on the ratio of the Fermi level ɛF to the half-width ɛ0 of the one-dimensional conduction band is considered. For scattering by acoustical and nonpolar optical phonons, the thermopower changes sign: α becomes positive for ɛF<ɛ0. Comparison of the theory with published experimental data demonstrates good qualitative agreement.