Increase In Thermopower Research Articles

Magnetic field-dependent ordinary Hall effect and thermopower of VO2 thin films

We investigated the magnetic field-dependent Hall effect and the thermopower in VO2 thin films at various temperatures by using physical property measurement systems. From the ordinary Hall effect measured at 300–370 K, it was found that the Hall voltage decreased with increasing magnetic field, attributed to the weakening of strong electron correlation, and dominant charge carriers were changed implying the existence of mixed phases near the critical temperature of VO2. A gradual thermopower increase and its sign inversion with increasing temperature gradient were observed at 320–350 K, which seems to stem from percolation processes during the phase transition in VO2.

Optical and low-temperature thermoelectric properties of phase-pure p-type InSe thin films

Polycrystalline phase-pure p-type InSe thin films were deposited on glass substrates by reactive evaporation at an optimized substrate temperature of 473 ± 5 K and pressure of 10−5 mbar. The as-prepared InSe thin films were analyzed by X-ray diffractometry, energy-dispersive X-ray spectroscopy, atomic force microscopy, UV–Vis–NIR spectroscopy, electrical conductivity and Hall measurements. The lattice parameters, particle size, dislocation density, number of crystallites per unit area and the lattice strain of the prepared InSe thin films were calculated and found as a = 4.00 ± 0.002 A and c = 16.68 ± 0.002 A, 48 ± 2 nm, 4.34 × 1010 lines cm−2, 15.37 × 1010 cm−2 and 1.8 × 10−3, respectively. The as-deposited InSe thin films showed a direct allowed transition with an optical band gap of 1.35 ± 0.02 eV and high absorption coefficient of about 105 cm−1. The oscillator energy (E o) and dispersion energy (E d) were calculated using the single-oscillator Wemple and DiDomenico model. The p-type conductivity and photosensitivity of the as-prepared InSe thin films confirmed their potential application in photovoltaic devices. The mean free path, relaxation time, density of states, Fermi energy and effective mass of holes in the film were determined by correlating the results of thermopower and Hall measurements. The sudden and sharp increase in thermopower from 80 to 37 K was explained as due to the effect of phonon drag on charge carriers.

Characterization of Lorenz number with Seebeck coefficient measurement

In analyzing zT improvements due to lattice thermal conductivity (κL) reduction, electrical conductivity (σ) and total thermal conductivity (κTotal) are often used to estimate the electronic component of the thermal conductivity (κE) and in turn κL from κL = ∼ κTotal − LσT. The Wiedemann-Franz law, κE = LσT, where L is Lorenz number, is widely used to estimate κE from σ measurements. It is a common practice to treat L as a universal factor with 2.44 × 10−8 WΩK−2 (degenerate limit). However, significant deviations from the degenerate limit (approximately 40% or more for Kane bands) are known to occur for non-degenerate semiconductors where L converges to 1.5 × 10−8 WΩK−2 for acoustic phonon scattering. The decrease in L is correlated with an increase in thermopower (absolute value of Seebeck coefficient (S)). Thus, a first order correction to the degenerate limit of L can be based on the measured thermopower, |S|, independent of temperature or doping. We propose the equation: L=1.5+exp−|S|116 (where L is in 10−8 WΩK−2 and S in μV/K) as a satisfactory approximation for L. This equation is accurate within 5% for single parabolic band/acoustic phonon scattering assumption and within 20% for PbSe, PbS, PbTe, Si0.8Ge0.2 where more complexity is introduced, such as non-parabolic Kane bands, multiple bands, and/or alternate scattering mechanisms. The use of this equation for L rather than a constant value (when detailed band structure and scattering mechanism is not known) will significantly improve the estimation of lattice thermal conductivity.

Improvement of thermoelectric efficiency of the polyaniline molecular junction by the doping process.

Thermoelectric properties of a polyaniline molecular junction with face centered cubic electrodes are investigated using the Green function formalism in a linear response regime in the presence of the doping process. Doping causes the increase of thermopower and the figure of merit (ZT) and the decrease of electrical conductance as found experimentally in the work of Li et al., (Synthetic. Metals, 2010, 160, 1153-1158). We also find that the ZT increases with the molecular length in short polyanilines. [Golsanamlou et al., Phys. Chem. Chem. Phys., 2000, 35, 3523].

Electronic structure and thermoelectric properties of p-type Ag-doped Mg2Sn and Mg2Sn1-xSix (x = 0.05, 0.1)

An experimental and theoretical study of p-type Ag-doped Mg2Sn and Mg2Sn1-xSix (x = 0.05, 0.1) is presented. Band structure calculations show that behavior of Ag in Mg2Sn depends on the site it occupies. Based on Bloch spectral functions and density of states calculations, we show that if Ag substitutes for Sn, it is likely to form a resonant level; if it substitutes for Mg, a rigid-band-like behavior is observed. In both cases, the doped system should exhibit p-type conductivity. Experimentally, thermoelectric, thermomagnetic, and galvanomagnetic properties are investigated of p-type Mg2Sn1−xSix (x = 0, 0.05, 0.1) samples synthesized by a co-melting method in sealed crucibles. Ag effectively dopes the samples p-type, and thermoelectric power factors in excess of 20 μW cm−1K−2 are observed in optimally doped samples. From the measured Seebeck coefficient, Nernst coefficient, and mobility, we find that the combination of acoustic phonon scattering, optical phonon scattering and defect scattering results in an energy-independent scattering rate. No resonant-like increase in thermopower is observed, which correlates well with electronic structure calculations assuming the location of Ag on Mg site.

Anomalous increase of thermopower in epitaxial graphene

Thermoelectric effect in epitaxial graphene formed on the surface of a semiconductor is considered in the context of the Davydov model. The approach based on the Kubo formula for the conductivity and differential thermopower is used. It is shown that near the edges of the semiconductor bandgap, the thermopower of epitaxial graphene increases by more than four times as compared to the thermopower near the Dirac point. A possible explanation of this effect is given.

Enhanced Thermoelectric Performance Induced by Cr Doping at Ca‐Sites in Ca3Co4O9 System

Thermoelectric performance of Ca3Co4O9 system has been enhanced through the misplaced substitution with Cr doping at Ca‐sites. The study of Cr doping at Co‐sites has also been performed for comparing. Base on the analysis of the structural data, we conclude that the doped Cr ions should be in the form of Cr+6, and enter into Ca‐sites of Ca2CoO3 layers in Ca3−xCrxCo4O9 lattice and Co‐sites of CoO2 layers in Ca3Co3.90Cr0.10O9 lattice, respectively. As Cr ions are doped into Ca‐sites in Ca3−xCrxCo4O9 lattice, both resistivity and thermopower increase obviously, while thermal conductivity decreases. Among all samples, Ca2.95Cr0.05Co4O9 shows the maximum ZT value, which is enhanced by 12% than that of Ca3Co4O9. The results show that the misplaced substitution with a proper Cr doping at Ca‐sites is beneficial to the enhancement of the thermoelectric performance in Ca3Co4O9 system compared to the traditional idea: Cr doping at Co‐sites, which is suggested to originate from the combined action of the carrier concentration, the electronic correlation, and the lattice disharmony.

Thermoelectric properties of Al substituted misfit cobaltite Ca3(Co1−x Al x )4O9 at low temperature

Thermoelectric properties of Al substituted compounds Ca3(Co1−x Al x )4O9 (x = 0, 0.03, 0.05), prepared by a sol-gel process, have been investigated in the temperature range 305–20 K. The results indicate that after Al substitution for Co in Ca3(Co1−x Al x )4O9, the direct current electrical resistivity and thermopower increase due to the reduction of carrier concentration. Experiments show that Al substitution results in decreased lattice thermal conductivity. The figure of merit of temperature behavior suggests that Ca3(Co0.97Al0.03)4O9 would be a promising candidate thermoelectric material for high-temperature thermoelectric application.

Thermoelectric properties of an ultra-thin topological insulator

Thermoelectric coefficients of an ultra-thin topological insulator are presented here. The hybridization between top and bottom surface states of a topological insulator plays a significant role. In the absence of a magnetic field, the thermopower increases and thermal conductivity decreases with an increase in the hybridization energy. In the presence of a magnetic field perpendicular to the ultra-thin topological insulator, thermoelectric coefficients exhibit quantum oscillations with inverse magnetic field, whose frequency is strongly modified by the Zeeman energy and whose phase factor is governed by the product of the Landé g-factor and the hybridization energy. In addition to the numerical results, the low-temperature approximate analytical results for the thermoelectric coefficients are also provided. It is also observed that for a given magnetic field these transport coefficients oscillate with hybridization energy, at a frequency that depends on the Landé g-factor.

The effects of Si doping on the thermoelectric and magnetic properties of Ca0.98Bi0.02Mn1−xSixO3−δ with x=0.00, 0.02 and 0.03

• The Si doping element has remarkable effects on the thermoelectric and magnetic properties of the system. • Ca 0.98 Bi 0.02 Mn 0.98 Si 0.02 O 3− δ exhibits the highest dimensionless figure of merit of 0.045 at 540 K. • The Neel temperature occurred at around 120 K for all the samples. Ca 0.98 Bi 0.02 Mn 1− x Si x O 3− δ ( x = 0.00, 0.02 and 0.03) samples are prepared by solid state reaction. All the samples are single phase with an orthorhombic structure. A metal-like behavior is observed for x = 0.00, and it switches to nonmetal-like behavior for x = 0.03. Co-substituting bismuth for calcium and silicon for manganese could modify the electronic states and hence the transport properties. The thermopower of all the samples is negative, indicating that the predominant carriers are electrons over the entire temperature range. The electrical resistivity, thermopower increase and thermal conductivity decreases with increasing Si content. Among the samples, Ca 0.98 Bi 0.02 Mn 0.98 Si 0.02 O 3− δ exhibits the highest dimensionless figure of merit (0.045), which is about 84% higher than that of the undoped at 540 K. Magnetic moments of doped samples decrease as compared to undoped sample. The Neel temperature occurred at around 120 K for all the samples.

The effect of Fe substitution on the electrical and thermal conductivity and thermopower of Ca3(FexCo1−x)4O9 synthesised by a sol–gel process

The effect of Fe substitution for Co on direct current (DC) electrical and thermal conductivity and thermopower of Ca3(Co1−xFex)4O9 (x = 0, 0.05, 0.08), prepared by a sol–gel process, was investigated in the temperature range from 380 down to 5K. The results indicate that the substitution of Fe for Co results in an increase in thermopower and DC electrical resistivity and substantial (14.9–20.4% at 300K) decrease in lattice thermal conductivity. Experiments also indicated that the temperature dependence of electrical resistivity ρ for heavily substituted compounds Ca3(Co1−xFex)4O9 (x = 0.08) obeyed the relation lnρ ∝ T−1/3 at low temperatures, T < ~55K, in agreement with Mott’s two-dimensional (2D) variable range hopping model. The enhancement of thermopower and electrical resistivity was mainly ascribed to a decrease in hole carrier concentration caused by Fe substitution, while the decrease of thermal conductivity can be explained as phonon scattering caused by the impurity. The thermoelectric performance of Ca3Co4O9 was not improved in the temperature range investigated by Fe substitution largely due to great increase in electrical resistivity after Fe substitution.

Improved Thermoelectric Properties of Se-Doped n-Type PbTe1−x Se x (0 ≤ x ≤ 1)

Enhancement of the thermoelectric figure of merit is of prime importance for any thermoelectric material. Lead telluride has received attention as a potential thermoelectric material. In this work, the effect of Se substitution has been systematically investigated in PbTe1−xSex. The thermoelectric properties of synthesized alloys were measured in the temperature range of 300 K to 873 K. For the particular composition of x = 0.5, α was highest at ~292 μV/K, while k was lowest at ~0.75 W/m-K, resulting in the highest dimensionless figure of merit of ZT ≈ 0.95 at 600 K. The increase in thermopower for x = 0.5 can be attributed to the high distortion in the crystal lattice which leads to the formation of defect states. These defect states scatter the majority charge carriers, leading to high thermopower and high electrical resistivity. The dramatic reduction of the thermal conductivity for x = 0.5 can be attributed to phonon scattering by defect states.

THERMOELECTRIC AND THERMOMAGNETIC PROPERTIES OF GRAPHENE IN THE PRESENCE OF DIFFERENT SCATTERING PROCESSES

Thermoelectric and thermomagnetic properties of graphene are analyzed using Boltzmann transport equation within the relaxation time approximation. Influence of temperature, charge carrier density and magnetic field on the thermopower and figure of merit is taken into account in the presence of different scattering processes. It is observed the magnetic field results in the increase of thermopower and figure of merit in the acoustical phonon scattering process, while they are reduced by charged impurity scattering.

Facile charge carrier adjustment for improving thermopower of doped polyaniline

We report a facile method of improving thermopower of doped polyaniline (PANI). It is believed that the electrical properties were altered by controlling the concentration of charge carriers with camphor sulfonic acid (CSA) and ammonium hydroxide. With CSA doping followed by drying and annealing at temperatures below ∼100 °C, the electrical conductivity was increased up to ∼5700 S/m with a thermopower value, ∼14 μV/K at room temperature. The power factor reached ∼1 μW/m-K2 after optimizing the drying and annealing temperature, which is 5–1000 times larger than those of other doped PANI without stretching. This thermopower of PANI–CSA films was monotonically improved in proportion to the time period of the ammonium hydroxide reaction, resulting in ∼115 μV/K at room temperature, which is 1.2–10 times higher than those of other doped PANI. The intensity changes of Fourier transform infrared spectra before and after the de-doping process with ammonium hydroxide indicates the thermopower increase is likely to be from the reduction in the charge carrier concentration. This method is a facile and convenient way of adjusting carrier concentrations, which are crucial to find the optimum doping level for high performance thermoelectrics. We believe further studies to improve crystallinity of doped PANI may dramatically raise the electrical conductivity in addition to the large thermopower, so as to obtain a high performance PANI-containing polymer composite for thermoelectric applications.

Effect of Te doping on the thermopower of PbSe1–xTex

The authors have studied the effect of Te doping on structural, morphological and thermopower of PbSe for the composition of PbSe1–xTex (x = 0·0, 0·2, 0·50, 0·75, 0·9 and 1·0). The samples were prepared using melting and rocking. The scanning electron microscopy indicated that all the samples have lamellar morphology. The X-ray diffraction analysis suggested that the lattice parameter increases linearly with dopant concentration (x). The temperature dependence of thermopower measured for samples having different x revealed unusual increase in thermopower for the composition x = 0·5, for example, at 427°C, and the thermopower values for PbSe0·5Te0·5, PbTe and PbSe were 292, 188 and 134 μV/K, respectively. A dramatic thermopower enhancement for PbSe0·5Te0·5 is attributed to scattering of the majority charge carriers by the defect sites.

THE THERMOPOWER IN SEMICONDUCTING THIN FILMS WITH NONPARABOLIC ENERGY BAND

In the present work, the in-plane electron thermopower of semiconducting size-quantized films with nonparabolic energy band in a classically strong magnetic field, which is parallel to the film normal, are investigated. It was shown that, for the degenerate electron gas thermopower is a function of film thickness and electron density: for arbitrary thickness thermopower is oscillating function, with the period as a function of concentration, but with respect to concentration thermopower is monotonically increasing function. It is shown that in the case of ultrathin films (quantum wells) thermopower increases, as thickness decreases. This result is in agreement with the experimental dates on GaAs quantum wells.

Structure and properties of solid solutions of La1 − x Pr x BaCuFeO5 + δ

The effect of replacing lanthanum with praseodymium on the crystal chemistry parameters of solid solutions of La1 − xPrxBaCuFeO5 + δ has been investigated using X-ray powder diffraction analysis and IR spectroscopy. The thermal expansion, electroconductivity, and thermopower of these phases have been studied in air in the temperature range 300–1100 K. The values of linear thermal expansion coefficients (LTEC) of ceramics in different temperature ranges have been determined, and the values of electric transfer parameters in the above oxides have been calculated. It has been established that replacing lanthanum with praseodymium resulted in the compression of the elementary oxide unit La1 − xPrxBaCuFeO5 + δ, decrease in the content of labile oxygen in them (δ), decrease in nonmonotonic electroconductivity, increase in thermopower, decrease in LTEC, and difficulties in charge transfer in these phases.

Effect of annealing on the thermoelectric properties of directionally grown Bi2Sr2Co1.8Ox ceramics

The effect of annealing on directionally solidified Bi2Sr2Co1.8Ox ceramic rods has been studied for different times up to 1008h. Microstructure has shown five different phases in the as-grown materials which have been reduced to two major ones after 1008h thermal treatment, accompanied by an important grain growth. These microstructural changes are reflected on the mechanical properties which are higher than those measured in the as-grown materials in all cases. Moreover, they also produce an important decrease on the resistivity and increase of thermopower, leading to a raise on the power factor on thermally treated samples, about two times, compared to the as-grown samples.

Thermoelectric properties of bismuth telluride nanocomposites with fullerene

A nanocomposite material consisting of Bi2Te3 nanocrystals 30 nm in size coated with a C60 molecule layer is obtained. The Hall effect and thermoelectric properties of the composite are studied. The acceptor effect of fullerene in both p- and n-type Bi2Te3 is detected. Nanocomposite material properties are modified due to the formation of layers of fullerene charged molecules: the nanocomposite thermopower increases, while the thermal conductivity decreases.

Improved thermoelectric properties in directionally grown Bi 2Sr 2Co 1.8O y ceramics by Pb for Bi substitution

Thermoelectric performances of misfit cobaltites can be controlled by grain orientation and/or by cation substitution. Both processes have been simultaneously performed by directional solidification at 0.03 m/h, using the laser floating zone technique, of Bi 2− x Pb x Sr 2Co 1.8O y (with x = 0.0, 0.2, 0.4 and 0.6). The microstructure has shown two different main phases as a function of Pb content, a Co-poor phase for low Pb content (0.0 and 0.2) and a Co-rich one for higher Pb substitution. These microstructural changes are reflected on the thermoelectric properties leading to an important decrease on the resistivity and increase of thermopower for samples with 0.4Pb substitution. Both improvements lead to power factor values higher than usual in textured misfit cobaltites.