Dependence Of Thermopower Research Articles

Electron transport and thermoelectric properties of layered perovskiteLaBaCo2O5.5

We have investigated systematically the physical transport properties of layered 112-typecobaltite by means of electrical resistivity, magnetoresistance and thermopowermeasurements. In order to understand the complex transport mechanism ofLaBaCo2O5.5, the data have been analysed using different theoretical models. The compoundshows an electronic transition between two semiconducting states around 326 K,which coincides with the ferromagnetic transition. Interestingly, the system alsodepicts a significant magnetoresistance (MR) effect near the ferro/antiferromagneticphase boundary and the highest value of MR is close to 5% at 245 K under ± 7 T. The temperature dependence of thermopower,S(T), exhibits p-typeconductivity in the 60 K≤T≤320 K range and reaches a maximum value of around303 µV K−1 (at 120 K). In the low temperature antiferromagnetic region the unusualS(T) behaviour, generally observed for the cobaltite seriesLnBaCo2O5.5 (Ln = rare earth), is explained by the electron magnon scattering mechanism.

Morphology dependence of thermopower and conductance in semiconducting oxides with space charge regions

The mechanism of the gas concentration dependent conductivity of semiconducting metal oxides is often described by one-dimensional analytic models. The thermopower of materials with space charge regions is mostly not taken into account. Here, a numerical method is presented that allows to calculate the effective thermopower and effective resistance of semiconducting gas sensitive oxides with space charge regions. This involves both the numerical solution of the nonlinear Poisson–Boltzmann equation leading to a potential distribution and two potential equations for the calculation of the electrical conductivity and the thermovoltage. Varied morphological parameters were the grain sizes and the neck radii. The results were plotted in the form of Jonker diagrams and compared with homogeneous metal oxides. Intrinsic materials and slightly donor-doped materials turned out to be ideal candidates for direct thermoelectric gas sensors. First tests with Fe 2O 3 as an intrinsic material confirmed this assumption.

Pressure and Magnetic Field Effects on Electrical Resistivity and Thermopower of Y1-xTbxCo2 System

The electrical resistivity ρ and thermopower S of the Laves phase Y 1- x Tb x Co 2 alloy system have been measured in magnetic fields up to 10 T and under pressures up to 8 GPa in the temperature range from 1.5 to 300 K. The residual resistivity of the alloys peaks at the critical composition x c ≈0.2 of the onset of 4f-moment long-range magnetic order in the Tb sublattice. In the magnetically ordered region of the phase diagram x > x c , an anomalous large positive magnetoresistivity and a pressure effect on resistivity are observed. The variation of the resistivity with alloy composition, magnetic field, and pressure are attributed to the scattering of conduction electrons due to magnetic static disorder within the partially ordered Co 3d itinerant subsystem. The evolution of the temperature dependence of thermopower with pressure are in agreement with this conduction mechanism.

Metal—Insulator Transition in Ca-Doped Sr14-xCaxCu24O41 Systems Probed by Thermopower Measurements

High quality Sr14-xCaxCu24O41 single-crystals are successfully grown by floating-zone technique, and the transport properties are studied. The temperature dependence of resistivity along the c-axis direction is semiconductorlike for x ⩽ 10 and it can be fitted by the thermal activation equation ρ = ρ0exp(Δ/kBT) with kB being the Boltzmann constant and Δ the activation energy. A break in the slope of thermopower (S) versus the inverse temperature (1/T) corresponding to the formation of charge-density waves (CDW) is first observed for x ⩽ 6. The temperature dependence of thermopower becomes metallic for x ⩾ 8 while the resistivity is still semiconductorlike. We propose that the insulation behaviour of the resistivity in the Ca doping range 8 ⩽ x ⩽ 11 could result from the localization of the charge carriers due to the disorder induced by Ca doping and a revised electronic phase diagram is derived based on our observations.

Kinetic effects in manganites La1 − x Ag y MnO3 (y ≤ x)

We have measured the resistivity, magnetoresistance, and thermopower of ceramic manganite samples La{sub 1-x}Ag{sub y}MnO{sub 3} (y {<=} x) doped with silver as functions of temperature (4.2-350 K) and magnetic field (up to 26 kOe). A metal-insulator phase transition is observed in all investigated samples at temperatures close to room temperature. The behavior of the resistivity and thermopower in the high-temperature paramagnetic region is interpreted using the concept of small radius polaron; the activation energy decreases with increasing doping level. The resistivity in the low-temperature ferromagnetic region is approximated by the expression {rho}{sub FM}(T) = {rho}{sub 0} + AT{sup 2} + BT{sup 4.5} presuming the existence of electron-electron and electron-magnon interactions. A resistivity minimum and a strong magnetoresistive effect are observed at low temperatures. The latter effect is associated with scattering of charge carriers at grain boundaries, which are antiferromagnetically ordered relative to one another. The temperature dependence of thermopower in the magnetically ordered phase is described in the framework of a model taking into account the drag of charge carriers by magnons.

Doping dependence of thermopower and thermoelectricity in strongly correlated materials

The search for efficient semiconductor thermoelectrics has been greatly aided by theoretical modeling of electron and phonon transports in bulk materials and nanocomposites. Recent experiments have studied thermoelectric transport in thermoelectrics that are “strongly correlated” and derived by doping Mott insulators. Here a unified theory of electrical and thermal transport in the atomic and “Heikes” limit is applied to understand recent experiments on sodium cobaltate and other doped Mott insulators at room temperature and above. For optimal electron filling, a broad class of narrow-bandwidth correlated materials is shown to have thermoelectric power factors as high as in the best semiconductors.

Hopping Conductivity in CuIr2S4 Spinel Compound: I. Empirical Model for Electronic Configuration and Mechanism of Metal–Insulator Transition

Two types of spinel compound CuIr 2 S 4 samples were investigated: sample 1 was synthesized under protective Ar atmosphere to avoid contamination by oxygen, whereas sample 2 was exposed to oxygen during preparation procedure. According to the results of scanning electron microscopy, the oxygen concentration in sample 2 is 5 times higher than in sample 1. However, the metal–insulator transition (MIT) temperature is the same for both samples irrespective of oxygen concentration. The conductivity σ in the insulating phase (LIP) is proportional to exp [-( T * / T ) 1/2 ] at T ≤120 K for sample 1 and at T ≤50 K for sample 2, while in the metallic phase the resistivity ρ is proportional to the same function of σ in LIP for both samples. Temperature independent diamagnetic susceptibility and temperature dependence of thermopower suggest that this compound is not a normal activation-type semiconductor. The electronic configuration of CuIr 2 S 4 is estimated to a first approximation from the experimental results. ...

Quantum-fluctuation effects on the thermopower of a single-electron transistor

We study thermal conductance and thermopower of a metallic single-electron transistor beyond the limit of weak tunnel coupling. Employing both a systematic second-order perturbation expansion and a non-perturbative approximation scheme, we find, in addition to sequential and cotunneling contributions, terms that are associated with the renormalization of system parameters due to quantum fluctuations. The latter can be identified by their logarithmic temperature dependence that is typical for many-channel Kondo correlations. In particular, the temperature dependence of thermopower, which provides a direct measure of the average energy of transported particles, reflects the logarithmic reduction of the Coulomb-blockade gap due to quantum fluctuations.

Laser-modulated ac measurement of high temperature thermopower

An experimental set-up for ac-measurement of thermopower between room temperature and 1200K by using modulated laser-heating has been developed. The method shows many advantages compared with the conventional dc-measurements: convenience, applicability to small samples, possibility to measure the anisotropy of single crystals and capability to continuously trace the temperature dependence of thermopower. The thermopower of a very short piece of pure Pt was measured with the present set-up to test the reliability and accuracy of our new method.

In-Situ Measurement of Electrical Character of PbTe at HighPressure and High Temperature

There is a widespread interest in lead telluride (PbTe) as a goodthermoelectric material. We report the temperature dependence ofthermopower S(T) and resistance R(T) for PbTeat the different pressures of from 1.8 GPa to 5 GPa obtained by usingthe cubic anvil high pressure apparatus. With increasing pressure,R(T) and S(T) decrease. The effect of pressure on R(T) is largerthan that on S(T). The power factor that is determined by thermopowerand resistivity increases with increasing pressure. This method is anefficient tool for synthesizing good thermoelectric materials at highpressure and high temperature.

Thermopower of ap-typeSi/Si1−xGexheterostructure

We report thermopower measurements in zero and low magnetic fields for a p-type ${\mathrm{S}\mathrm{i}/\mathrm{S}\mathrm{i}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ heterostructure. The diffusion components of both the longitudinal and transverse components are reasonably well described by the Mott approach, including the quantum oscillations at low magnetic fields. The magnetic-field dependence of thermopower shows that the diffusion contribution at zero field deviates from the linear temperature dependence that would be expected for a degenerate system, probably as a result of the nearby metal-insulator transition. Phonon drag also does not behave as expected. Instead of exhibiting an approximate ${T}^{6}$ dependence at low temperatures appropriate to screened, hole-phonon, deformation-potential scattering, an approximate ${T}^{4}$ dependence is observed. This is consistent with previous observations on the energy-loss rates in SiGe hole systems. The experimental data on drag are in good agreement with numerical calculations by assuming either hole-phonon scattering by an unscreened deformation-potential interaction or by assuming a screened piezoelectric plus screened deformation-potential coupling.

Thermoelectric power, Hall effect, and mobility ofn-typeBaTiO3

The resistivity and the Hall and Seebeck effects of polycrystalline n-type ${\mathrm{Ba}}_{0.996}{\mathrm{Y}}_{0.004}{\mathrm{TiO}}_{3}$ and undoped, reduced single crystal ${\mathrm{BaTiO}}_{3}$ were studied in the 100--300 K interval. Both Hall coefficient and resistivity showed similar temperature activated dependence for the single crystal in the rhombohedral phase and for the polycrystal in the rhombohedral and orthorhombic phases. Low-temperature Hall mobility in the ${\mathrm{BaTiO}}_{3}$ single crystal decreased from $12{\mathrm{cm}}^{2}{\mathrm{V}}^{\ensuremath{-}1}{\mathrm{s}}^{\ensuremath{-}1}$ at 120 K to $2{\mathrm{cm}}^{2}{\mathrm{V}}^{\ensuremath{-}1}{\mathrm{s}}^{\ensuremath{-}1}$ at 220 K, implying a phonon scattering of charge carriers. The temperature dependence of the Hall mobility of the polycrystal showed a weak temperature dependence in the 100--220 K range with a maximum of $1.2\ifmmode\pm\else\textpm\fi{}0.3{\mathrm{cm}}^{2}{\mathrm{V}}^{\ensuremath{-}1}{\mathrm{s}}^{\ensuremath{-}1}$ at 170 K, probably indicating a transition from ionized impurity scattering at low temperature to a phonon scattering at higher temperatures. Temperature dependence of thermopower and Hall coefficient confirmed that the increase in conductivity of the single crystal below 200 K was caused by an increase in the concentration of the charge carriers. The results suggest that below 300 K, electron transport in ${\mathrm{BaTiO}}_{3}$ occurs via the conduction band rather than by a small polaron hopping.

Spin entropy as the likely source of enhanced thermopower in Na(x)Co2O4.

In an electric field, the flow of electrons in a solid produces an entropy current in addition to the familiar charge current. This is the Peltier effect, and it underlies all thermoelectric refrigerators. The increased interest in thermoelectric cooling applications has led to a search for more efficient Peltier materials and to renewed theoretical investigation into how electron-electron interaction may enhance the thermopower of materials such as the transition-metal oxides. An important factor in this enhancement is the electronic spin entropy, which is predicted to dominate the entropy current. However, the crucial evidence for the spin-entropy term, namely its complete suppression in a longitudinal magnetic field, has not been reported until now. Here we report evidence for such suppression in the layered oxide Na(x)Co2O4, from thermopower and magnetization measurements in both longitudinal and transverse magnetic fields. The strong dependence of thermopower on magnetic field provides a rare, unambiguous example of how strong electron-electron interaction effects can qualitatively alter electronic behaviour in a solid. We discuss the implications of our finding--that spin-entropy dominates the enhancement of thermopower in transition-metal oxides--for the search for better Peltier materials.

Tunneling thermopower in magnetic granular alloys

The nature of the field dependence of thermopower in the Co-Al-O and Fe-Al-O magnetic granular alloys with tunneling conduction is shown to be related to tunneling thermopower. The tunneling thermopower is small and depends approximately linearly on temperature and squared magnetization, and its field dependence is described by a relation of the type S(H)/T=a+bρ(0)/ρ(H), where ρ is the alloy electrical resistivity and the parameters a and b are field-independent.

Thermoelectric properties of Bi2Sr2Co2O9 whiskers under hydrostatic pressure

We report the temperature dependence of thermopower (S) in the ab plane of Bi2Sr2Co2O9 (BC-222) single crystalline whiskers under high pressure up to 1.2 GPa. Special modifications were made to our high-pressure transport measurement system to improve the measurement accuracy of S for whiskers with high resistance. Pressure improved the contact resistance dramatically. We observed a slight decrease of S and four-wire electrical conductivity (σ) under high pressure. As a result, the power factor (S2σ) was decreased less than 20% by pressure. From the temperature dependence of σ, the band gap of BC-222 whiskers increase very slightly under high pressure.

Thermopower and thermal conductivity of superconducting perovskiteMgCNi3

The thermopower and thermal conductivity of superconducting perovskite $MgCNi_3$ ($T_c \approx$ 8 K) have been studied. The thermopower is negative from room temperature to 10 K. Combining with the negative Hall coefficient reported previously, the negative thermopower definetly indicates that the carrier in $MgCNi_3$ is electron-type. The nonlinear temperature dependence of thermopower below 150 K is explained by the electron-phonon interaction renormalization effects. The thermal conductivity is of the order for intermetallics, larger than that of borocarbides and smaller than $MgB_2$. In the normal state, the electronic contribution to the total thermal conductivity is slightly larger than the lattice contribution. The transverse magnetoresistance of $MgCNi_3$ is also measured. It is found that the classical Kohler's rule is valid above 50 K. An electronic crossover occures at $T^* \sim 50 K$, resulting in the abnormal behavior of resistivity, thermopower, and magnetoresistance below 50 K.

Thermoelectric power studies on MgO-stabilized β″-alumina

A study of the temperature dependence of thermopower is known to yield auxiliary information about the electronic conductivity of a mixed conductor. In light of the above, thermoelectric power (TEP) measurements were made on MgO-stabilized β″-alumina over the temperature range from 773 to 1223 K under conditions of different sodium activities in the ambient in order to substantiate the existing information on the electronic conductivity of sodium beta alumina (SBA). A mixture of NaxSimO2m+x/2 and SiO2 in an environment of fixed \(P_{{\rm O}_{\rm 2} } \) served as electrodes reversible to Na+. The heat of transport obtained using the thermopower data at higher temperatures (973–1223 K) was in fair agreement with the activation energy of electrical conduction determined by other studies like impedance measurements and molecular dynamics simulation. It could be inferred from these results that there is negligible electronic conductivity in SBA under the conditions of measurement. The average TEP for SBA was determined to be 700–800 µV/K and the partial molar entropy of Na+ in SBA was found to be ~98 J mol–1 K–1.

Thermopower behavior for the shape memory alloy NiTi

We report thermopower measurements for the nickel titanium shape memory alloy Ni0.507Ti0.493. Our measurements reveal abrupt changes in the temperature dependence of thermopower, which correlate well with the structural phase transition between the austenitic and martensitic phases. These transition effects in thermopower are more clearly defined than in the resistivity, which is also reported. In the martensitic phase, thermopower exhibits standard metallic diffusion behavior with a nonlinearity, which is consistent with either a small peak in the density of states just below the Fermi level, as calculated by Kulkova, Egorushkin, and Kalchikhin [Solid State Commun. 77, 667 (1991)], or else electron–phonon mass enhancement. For thermally or mechanically treated samples, the magnitude of the transition effects in thermopower are reduced.

The detailed transport property of the underdoped Bi-2212 system in the pseudogap state

The detailed character of transport properties in underdoped Bi-2212 systems are analysed in the pseudogap state. In a certain underdoped regime, in-plane resistivity ρ ab ( T) shows a typical S-shaped temperature dependence and a perfect ρ ab(T)=ρ 0 *+β exp (−Δ/T) fit is observed over a wide temperature range from slight above T c up to T * where the pseudogap begins opening. The underdoped crystal, in contrast to the optimum crystal where no pseudogap is observed, in-plane transverse magnetoresistance shows a slower temperature decreasing rate with temperature increasing in the pseudogap state, and a larger value around T *. About the temperature dependence of thermopower S( T), all underdoped samples completely fall into a scaling curve S/ S * vs. T/ T * above T *. Whereas, S( T) below T * deviates from this scaling law. The above transport properties should be intimately related with the detailed prospects of Fermi surface destruction in the pseudogap state below T *.

THE RELATIONSHIP BETWEEN ELECTRICAL RESISTIVITY, THERMOPOWER AND TEMPERATURE FOR LIQUID InSb

The electronic resistivity and thermopower of liquid InSb as a function of temperature have been measured using the d.c. four-probe method and also from the electromotive force produced by the temperature gradient. Abnormal behavior is observed and more accurate data are obtained. For liquid InSb, the temperature dependence of resistivity increases with temperature near the melting point, and almost is constant when the temperature is above 716℃. The abnormal behavior has also been found in the temperature dependence of thermopower almost at the same temperature. Based on other physical properties changing with temperature and the structure of liquid InSb near melting point, the result obtained in this work suggests that the liquid structure of InSb may be changed at 716℃.