Temperature Dependence Of Thermoelectric Power Research Articles

Dielectric Relaxation and Electrical Conduction Properties of La_2NiO_(4+δ) Ceramics

Thermoelectric power, dc conductivity, and the dielectric relaxation properties of are reported in the temperature range of 77 K - 300 K and in a frequency range of 20 Hz - 1 MHz. Thermoelectric power was positive below 300K. The measured thermoelectric power of decreased linearly with temperature. The dc conductivity showed a temperature variation consistent with the variable range hopping mechanism at low temperatures and the adiabatic polaron hopping mechanism at high temperatures. The low temperature dc conductivity mechanism in was analyzed using Mott`s approach. The temperature dependence of thermoelectric power and dc conductivity suggests that the charge carriers responsible for conduction are strongly localized. The relaxation mechanism has been discussed in the frame of the electric modulus and loss spectra. The scaling behavior of the modulus and loss tangent suggests that the relaxation describes the same mechanism at various temperatures. The logarithmic angular frequency dependence of the loss peak is found to obey the Arrhenius law with activation energy of ~ 0.106eV. At low temperature, variable range hopping and large dielectric relaxation behavior for are consistent with the polaronic nature of the charge carriers.

Effect of Al3+ substitution on the transport properties of copper ferrite

This study presents the compositional and the temperature-dependent thermoelectric power and dc resistivity measurements on polycrystalline spinel ferrite system: CuAlxFe2−xO4 (x = 0.0, 0.2, 0.4 and 0.6). The probable conduction mechanisms in the system are Fe2+ ↔ Fe3+ + e− (n-type) at the octahedral sites and Cu2+ ↔ Cu1+ + e+ (p-type) at the tetrahedral sites. The cationic concentrations of Cu1+, Cu2+, ferrous and ferric ions have been deduced from Seebeck coefficients which, in turn, are used to determine the actual cation distribution, oxygen deficiency and to explain the variation and origin of dc resistivity with Al concentration. The Fermi energy, charge carrier concentration and mobility of charge carrier have been determined to explain the charge transport phenomena.

Band spectrum transformation and temperature dependences of thermoelectric power of Hg1-xRxBa2Ca2Cu3O8+δ system

Temperature dependences of thermoelectric power S(T) at $T>T_c$ of the Hg-based high temperature superconductors Hg$_{1-x}$R$_x$Ba$_2$Ca$_2$Cu$_3$O$_{8+\delta}$ (R=Re, Pb) have been analyzed with accounting for strong scattering of charge carriers. Transformation of parameters of a narrow conducting band in the region of the Fermi level was studied. The existence of correlation between the effective bandwidth and the temperature of a superconductive transition $T_c$ is shown.

Enhanced thermoelectric power near the quantum phase transition in the itinerant-electron ferromagnet MnSi

The itinerant-electron ferromagnet MnSi is a well-known example that shows a transition from Fermi-liquid $\ensuremath{\Delta}\ensuremath{\rho}\ensuremath{\propto}{T}^{2}$ to non-Fermi-liquid (NFL) $\ensuremath{\Delta}\ensuremath{\rho}\ensuremath{\propto}{T}^{3/2}$ behavior when the spiral ferromagnetic transition ${T}_{\text{c}}\ensuremath{\approx}29\text{ }\text{K}$ at ambient pressure is suppressed to zero by the application of hydrostatic pressures above ${P}_{\text{c}}\ensuremath{\approx}14.6\text{ }\text{kbar}$. Several experimental probes have been employed to reveal the intriguing properties near ${P}_{\text{c}}$. In this paper, we report the temperature dependence of thermoelectric power $S(T)$ under hydrostatic pressures up to 20 kbar on a single crystal of MnSi. At pressures close to ${P}_{\text{c}}$, we observed at low temperatures an unusual enhancement of $S(T)$, which can be described well with the relationship $S/T\ensuremath{\propto}\text{ }\text{ln}(1/T)$, a formula that has been proposed to describe a system as a quantum critical point (QCP) is approached. The relationship has previously been observed in systems close to a magnetic QCP, for example, ${\text{La}}_{1.6\ensuremath{-}x}{\text{Nd}}_{0.4}{\text{Sr}}_{x}{\text{CuO}}_{4}$ $(x=0.24)$. The enhancement of thermoelectric power in MnSi occurs at a temperature above the NFL phase and over a broad pressure range around ${P}_{\text{c}}$.

PHONON DRAG, CARRIER DIFFUSIVE THERMOELECTRIC POWER AND SEMICONDUCTING RESISTIVITY BEHAVIOR OF Zn NANOWIRES

In this paper, we undertake a quantitative analysis of observed temperature-dependent thermoelectric power (S) of 4 nm Zn /Vycor composite nanowires by developing a model Hamiltonian that incorporates scattering of acoustic phonons with impurities, grain boundaries, charge carriers and phonons. Mott expression is used to determine the carrier diffusive thermoelectric power [Formula: see text]. The [Formula: see text] shows linear temperature dependence and the computed [Formula: see text] when subtracted from the experimental data is interpreted as phonon drag thermoelectric power [Formula: see text]. The model Hamiltonian within the relaxation time approximation sets the limitations of the scattering of acoustic phonons with impurities, grain boundaries, charge carriers and phonons for thermoelectric power in the nanowires. It is shown that for acoustic phonons the scattering and transport cross sections are proportional to fourth power of the phonon in the Rayleigh regime. The resultant thermoelectric powers is an artefact of various operating scattering mechanisms and are computed for the first time to our knowledge for Zn nanowires consistent with the experimentally reported behavior. The semiconducting nature of resistivity is discussed with small polaron conduction (SPC) model which consistently retraces the temperature-dependent resistivity behavior of Zn /Vycor composite.

Thermoelectric power in graphene

Considering electron–impurity and electron–phonon scattering, we present abalance-equation-based theoretical examination of thermoelectric power (TEP)in a two-dimensional single-layer graphene away from the carrier neutralitypoint. Both the boundary scattering and phonon–phonon interaction in phononrelaxation processes are taken into account. We find that, at temperaturesT > 10 K, the contribution to TEP mainly comes from diffusive processes and the phonon-drageffect can be ignored. However, at K, the phonon–phonon interaction leads to a phonon-drag peak in the temperaturedependence of TEP. We also compare our results with experiments.

Diameter Dependence of the Transport Properties of Antimony Telluride Nanowires

We report measurements of electronic, thermoelectric, and galvanomagnetic properties of individual single crystal antimony telluride (Sb(2)Te(3)) nanowires with diameters in the range of 20-100 nm. Temperature-dependent resistivity and thermoelectric power (TEP) measurements indicate hole dominant diffusive thermoelectric generation with an enhancement of the TEP for smaller diameter wires up to 110 microV/K at T = 300 K. We measure the magnetoresistance in magnetic fields both parallel and perpendicular to the nanowire [110] axis, where strong anisotropic positive magnetoresistance behavior was observed.

Temperature dependence of thermoelectric power and thermal conductivity in ferromagnetic shape memory alloyNi50Mn34In16in magnetic fields

We report a study of the temperature dependence of thermoelectric power $S$ and thermal conductivity $\ensuremath{\kappa}$ in the ferromagnetic shape memory alloy ${\text{Ni}}_{50}{\text{Mn}}_{34}{\text{In}}_{16}$ in presence of zero and 20 kOe magnetic field. A peak is observed in both $S$ and $\ensuremath{\kappa}$ just below the paramagnetic to ferromagnetic transition temperature $({T}_{C})$ of this alloy. Further down the temperature another sharp change is observed both in $S$ and $\ensuremath{\kappa}$, and this latter change is associated with the austenite to martensite phase transition in this alloy. The low temperature peak (below $\ensuremath{\sim}50\text{ }\text{K}$), which is usually observed in the temperature dependence of $S$ and $\ensuremath{\kappa}$ in metallic systems, is absent in the present alloy system. The measured thermal transport properties of ${\text{Ni}}_{50}{\text{Mn}}_{34}{\text{In}}_{16}$ are qualitatively different from those reported earlier for the well known magnetic shape memory alloy system Ni-Mn-Ga. We interpret the measured thermal transport properties using the changes in the electronic structure near the Fermi level. Apart from electronic structure, the scattering of conduction electrons and phonons by the twin boundaries introduced into the system during the martensitic transition also plays an important role in the temperature dependence of $S$ and $\ensuremath{\kappa}$.

Newly synthesized fused heterocyclic compounds in thin films with semiconductor properties

Temperature-dependent electrical conductivity and thermoelectric power of some recently synthesized fused heterocyclic compounds (nonsymmetrical bisindolizines), are studied. Thin-film samples ( d = 0.10–0.20 μm) deposited from dichloromethane solutions onto glass substrates were used. Organic films with reproducible electronic transport properties can be obtained if, after deposition, they are submitted to a heat treatment within temperature range 300–600 K. The studied compounds show typical p-type semiconductor behavior. The activation energy of the electrical conduction ranges between 0.90 and 1.17 eV, while the ratio of charge carrier mobilities was found in the range 0.94–0.97. Some correlations between semiconducting parameters and molecular structure of the organic compounds have been discussed. In the higher temperature range (373–601 K), the electronic transport in examined compounds can be interpreted in terms of the band gap representation model, while in the lower temperature range, the Mott's variable-range hopping conduction model was found to be appropriate.

Interpretation of thermoelectric power behaviour of Zinc nanowire composites: Phonon-scattering mechanism

We develop a theoretical model for quantitative analysis of temperature-dependent thermoelectric power ( S) of Zn nanowires. In doing so, we first use the Mott expression to compute the electron diffusive thermoelectric power ( S c diff. ) using Fermi energy as electron-free parameter, S c diff. shows linear temperature dependence. Further, the S c diff. contribution is subtracted from the experimental data and the difference ( S experimental – S c dif ) is characterized as phonon drag thermoelectric power ( S ph drag ) which is obtained within the relaxation time approximation where the thermoelectric power is limited by the scattering of phonons with impurities, grain boundaries, charge careers and phonons in the nanowires. The S ph drag shows anomalous temperature-dependent behaviour, which is an artifact of various operating scattering mechanisms. The observed anomalies are well accounted in terms of interaction among the phonons–impurity, phonon-grain boundaries, phonon–electron and the umklapp scattering. It is also shown that for phonons the scattering and transport cross-sections are proportional to ω 4 in the Rayleigh regime where ω is the frequency of the phonons. Numerical analysis of thermoelectric power from the present model shows similar results as those revealed from experiments.

The impurities of iron and cobalt in mercury selenide: Localization effects of hybridized electronic states in the temperature dependences of thermoelectric power

The temperature dependences of thermoelectric power coefficient have been studied in mercury selenide with small concentrations of iron and cobalt impurities at low temperatures. The considerable anomalies (sharp non-monotonic regions of the temperature dependences), related to the influence of the impurities, have been revealed. On the base of the comparison with the data of the other experiments, obtained before, and interpretation of the anomalies, proposed by us, it has been shown that in crystals with iron impurities the specific behaviors above are explained by manifesting the hybridized electronic states on donor impurities. As a result of discussing the data, obtained on the crystal with cobalt impurities, it was established that the revealed anomalies of thermoelectric power are the conclusive evidences of hybridizing the states on donor cobalt impurities.

Thermoelectric power and Hall coefficient measurements onBa(Fe1−xTx)2As2(T=Coand Cu)

Temperature dependent thermoelectric power (TEP) data on Ba(Fe1-xTMx)2As2 (TM = Co and Cu), complemented by the Hall coefficient data on the samples from the same batches, have been measured. For Co-doping we clearly see a change in the temperature dependent TEP and Hall coefficient data when the sample is doped to sufficient $e$ (the number of extra electrons associated with the TM doping) so as to stabilize low temperature superconductivity. Remarkably, a similar change is found in the Cu-doped samples at comparable e-value, even though these compounds do not superconduct. These changes possibly point to a significant modification of the Fermi surface / band structure of Ba(Fe1-xTM_x)2As2 at small electron doping, that in the case of Co-doping is just before, and probably allows for, the onset of superconductivity. These data further suggest that suppression of the structural / magnetic phase transition and the establishment of a proper e-value are each necessary but, individually, not sufficient conditions for superconductivity.

Magnon drag contribution to thermopower of Nd0.67Sr0.33MnO3 nanocrystalline manganites

With a view to investigate the influence of nanosize on thermopower behavior of Nd0.67Sr0.33MnO3 manganites, a systematic investigation of their temperature dependent thermoelectric power (TEP) studies has been undertaken. For this purpose, the samples with different particle sizes were prepared by the sol-gel method by sintering at four different temperatures (viz., 800, 900, 1000, and 1100 °C). The average crystallite sizes were calculated using a modified Scherrer’s formula. The ferromagnetic metallic (FMM) part of the TEP data were fitted to an equation containing diffusion, magnon drag, and phonon drag terms. The relative weights of these contributions were estimated and it has been found that the magnon drag contribution predominantly contributes to the TEP in the FMM region. The phonon drag process in the samples of the present investigation is found to be dominant along the direction of cell parameter “a” than the other two directions. Finally, the paramagnetic insulting part of the TEP data has been analyzed by using small polaron hopping mechanism.

Transport properties of manganese perovskites Bi1−xCaxMnO3 (0.7 ⩽ x ⩽ 0.95)

We report the temperature dependence of electrical resistivity and thermoelectric power of the manganese perovskites Bi1−xCaxMnO3 (0.7 ⩽ x ⩽ 0.95). The resistivity for x = 0.75 and 0.7 is well fitted with ρ ∝ exp(l/T)(1/4) below the charge-ordering temperatures (Tco). The temperature dependence of thermoelectric power for samples with x = 0.8, 0.75 and 0.7 shows the rapid decrease below Tco. In the lower temperatures than Tco, it goes through minimum and absolute thermoelectric power decreases and tends to change the sign. The activation energy was estimated as 57, 64, 148 meV for x = 0.8, 0.75, 0.7 from the linear fits in the form S − 1/T plots. The activation energy increases with decreasing the Ca content in the Bi1−xCaxMnO3 system.

Thermoelectric power measurements of wide band gap semiconducting nanowires

We investigated the temperature-dependent thermoelectric power (TEP) of individual wide band gap ZnO and GaN semiconducting nanowires by fabricating the devices with good Ohmic contacts. In the temperature range of 10–300 K, the measured TEP of both nanowires was linearly dependent on temperature, indicating the degenerate doping nature of these nanowires. The room temperature TEP value of ZnO nanowires was as high as −400 μV/K while an order of magnitude smaller TEP value was observed in GaN. The negative sign of TEP values shows that electrons are the majority carriers in these wide band gap nanowires. More importantly, in comparison with gate-dependent transport measurements of the nanowire field effect transistors, analysis of temperature-dependent TEP measurements provides a reliable way of estimating the majority carrier concentration of nanowires, where conventional Hall effect measurements cannot be used.

Structure, microstructure, and high-temperature transport properties of La1−xCaxMnO3−δ thin films and polycrystalline bulk materials

Single-phase samples of La1−xCaxMnO3−δ (LCMO), x≈0.3, prepared by pulsed reactive crossed beam laser ablation on SrTiO3 (STO) substrates, and soft chemistry synthesized powders were studied by various methods. The precise study of the crystal structure and microstructures by a combination of electron diffraction and high-resolution electron microscopy revealed a monoclinic distortion of the GdFeO3-type structure, P21/c space group, in both types of materials, i.e., the thin films and powder compound. The analysis of the LCMO/STO interface showed nonhomogeneous stress states and a composition that results in a different superstructure from the usual detected structure. The temperature-dependent thermoelectric power in the case of thin films presented an anomalous behavior compared to those from the powder compound. A structural transition at high temperature (T≈750 K) influences the thermopower behavior as well as the thermal conductivity values.

High temperature thermoelectric power study on calcium substituted lanthanum manganites

Compositional and temperature dependent thermoelectric power measurements have been performed on the polycrystalline manganite perovskite system: La1−xCaxMnO3+δ (x = 0.00, 0.25, 0.33, 0.50 and 1.00), in the paramagnetic state (T > Tc) (300–673 K). The Seebeck coefficient (α) has been used to calculate the Fermi energy, the activation energy (Eα), the Mn4+-ion concentration and the oxygen content (δ), for different compositions. The observed decrease in Eα with increase in Ca-concentration and a large value of Eα as compared with a relatively high temperature sintered material of the same compositions have been explained in the light of the decrease in the interatomic distances and grain growth with Ca-substitution. It is shown that [Mn4+][Mn3+] ion concentration is not the only factor that governs conduction in the manganite perovskites. The significantly small values of α (−22 µV K−1) at high temperature can be explained on the basis of (Mn3+–Mn3+) charge disproportionation into stable Mn2+–Mn4+pairs in the system rather than on the basis of nominal Mn3+/4+ valence arguments. Finally, nearly temperature independent variation of α, charge-carrier concentration at high temperature and low value of mobility are hallmarks of small polaron hopping.

Thermoelectric power of potassium doped lanthanum manganites at low temperatures

The temperature-dependent resistivity and thermoelectric power of monovalent (K) doped La 1− x K x MnO 3 polycrystalline pellets ( x=0.05, 0.10 and 0.15) between 50 and 300 K are reported. K substitution enhances the conductivity of this system. Curie temperature ( T C) also increases from 260 to 309 K with increasing K content. In the paramagnetic region ( T> T C), the electrical resistivity is well represented by adiabatic polaron hopping, while in the ferromagnetic region ( T< T C), the resistivity data show a nearly perfect fit for all the samples to an expression containing, the residual resistivity, spin-wave and two-magnon scattering and the term associated with small-polaron metallic conduction, which involves a relaxation time due to a soft optical phonon mode. Small polaron hopping mechanism is found to fit well to the thermoelectric power (S) data for T> T C whereas at low temperatures ( T< T C) in ferromagnetic region ( S FM), S FM is well explained with the spin-wave fluctuation and electron–magnon scattering. Both, resistivity and thermopower data over the entire temperature range (50–300 K) are also examined in light of a two-phase model based on an effective medium approximation.

Temperature dependence of the thermoelectric power of La 1−xK xMnO 3 compounds in light of a two phase model

The temperature-dependent resistivity and thermoelectric power (TEP) of monovalent (K) doped La 1− x K x MnO 3 polycrystalline pellets ( x = 0.05 , 0.10 and 0.15) between 50 and 310 K are reported. K substitution enhances the Curie temperature ( T C) from 260 to 309 K. Adiabatic small polaron hopping governs the resistivity as well as the TEP in the paramagnetic phase. In the ferromagnetic region, an expression containing the residual resistivity, electron–electron, and electron–magnon scattering term explains well the thermal variation of the electrical resistivity. The TEP at low temperatures in ferromagnetic region ( S lt) can be well represented with an expression containing the electron–magnon and spin-wave fluctuation scattering and confirms dominance of electron–magnon scattering. Both, resistivity and TEP data between 50 and 310 K are further examined in light of a two-phase model based on an effective medium approximation, in which the conductivity in the transition region is supposed to originate due to the mixing of the metallic region, embedded in a semiconductive polaronic matrix and is characterized by “mixing factor”. Our analysis confirms that the temperature dependence of the TEP of La 1− x K x MnO 3 compounds computed using the expression derived using effective medium approach, gives a fairly good description of the observed thermal variation of TEP in monovalent (K) doped lanthanum manganites between 50 and 310 K.

Thermoelectric power study of [formula omitted][formula omitted

Temperature-dependent thermoelectric power (TEP) was measured in the range of 2 K < T < 300 K for single crystals of Eu 1 - x La x B 6 ( x = 0.0 , 0.1, and 0.2). For Eu 1 - x La x B 6 ( x = 0.0 and 0.1), the TEP shows negative values in the whole temperature range, consistent with the fact that dominant charge carriers in Fermi surface are electrons. However, for Eu 1 - x La x B 6 ( x = 0.2 ) , a sign change of the TEP from negative to positive value is observed at about 28 K, which is consistent with the sign reversal of Hall resistivity of the same compound. We surmise that the anomalous sign reversal seems to be accompanied by a magnetic transition from ferromagnetic to antiferromagnetic with La doping in Eu 1 - x La x B 6 .