Component Of Thermopower Research Articles

Magnetic and Electrical Characteristics of Nd3+ and Mn2+-Co-doped Calcium Molybdato-Tungstates Single Crystals.

Single crystals of Ca1-3x-yMny□xNd2x(MoO4)1-3x(WO4)3x molybdato-tungstates (□ denotes vacant sites) with a scheelite-type structure have been successfully grown by the Czochralski technique in an inert atmosphere. This paper presents the results of structural, optical, magnetic, and electrical properties, as well as the broadband dielectric spectroscopy measurements of single crystals with different Nd3+ ion concentrations, i.e., when x = 0.0050 or x = 0.0098, and with constant content of Mn2+ ions, i.e., when y = 0.0050. Our magnetic studies have shown that substitution of diamagnetic Ca2+ ions in the CaMoO4 matrix with paramagnetic Nd3+ ones with a content not exceeding 0.02 and having a screened 4f-shell revealed a significant effect of orbital diamagnetism and Van Vleck's paramagnetism. Both single crystals have revealed residual electrical conductivity without an intrinsic region and a change of sign of the Seebeck coefficient at ca. 230 K. Dielectric spectroscopy measurements have shown constant values of relative permittivity (εr ∼ 8) and loss tangent (tan δ ∼ 0.01) both up to 400 K and up to 1 MHz, as well as the Fermi energy (∼0.04 eV) and the Fermi temperature (∼500 K) determined for both crystals from the diffusion component of thermopower. These results suggest the presence of shallow acceptor and donor levels in the studied crystals.

Magnetic and Electrical Characteristics of Nd3+-Doped Lead Molybdato-Tungstate Single Crystals.

Single crystals of Pb1-3x▯xNd2x(MoO4)1-3x(WO4)3x (PNMWO) with scheelite-type structure, where ▯ denotes cationic vacancies, have been successfully grown by the Czochralski method in air and under 1 MPa. This paper presents the results of structural, optical, magnetic and electrical, as well as the broadband dielectric spectroscopy measurements of PNMWO single crystals. Research has shown that replacing diamagnetic Pb2+ ions with paramagnetic Nd3+ ones, with a content not exceeding 0.01 and possessing a screened 4f-shell, revealed a significant effect of orbital diamagnetism and Van Vleck's paramagnetism, n-type electrical conductivity with an activation energy of 0.7 eV in the intrinsic area, a strong increase of the power factor above room temperature for a crystal with x = 0.005, constant dielectric value (~30) and loss tangent (~0.01) up to room temperature. The Fermi energy (~0.04 eV) and the Fermi temperature (~500 K) determined from the diffusion component of thermopower showed shallow donor levels.

Thermoelectric power of CuCr xV ySe 4p-type spinel semiconductors

Structural, electrical and magnetic measurements of polycrystalline CuCr x V y Se 4 spinels with x=1.79, 1.64 and 1.49 and y=0.08, 0.22 and 0.45, respectively, are presented. The compounds under study crystallize in regular system of a normal spinel type MgAl 2O 4 structure with the space group symmetry Fd3 m. The chromium spins are coupled ferromagnetically and show both strong long- and short-range magnetic interactions evidenced by the large values of the Curie ( T C) and Curie–Weiss ( θ CW) temperatures, decreasing from T C=407 K and θ CW=415 K for y=0.08, via T C=349 K and θ CW=367 K for y=0.22 to T C=283 K and θ CW=293 K for y=0.45, respectively. In all the studied spinels a change of the electrical conductivity character from the semiconductive into the metallic one above 230 K was observed. A detailed thermoelectric power analysis showed a domination of diffusion thermopower component, maximum of phonon drag component at 230 K, a decrease of impurity component with increasing V content, as well as the weak magnon excitations at 40 K.

Influence of spin arrangement on thermopower in Zn xCu yCr zSe 4 spinels

The thermopower analysis carried out on Zn x Cu y Cr z Se 4 spinel single crystals show that the ferrimagnetic spin arrangement in the range 0.49 ≤ y ≤ 0.72 makes easier the carrier diffusion from one side and hinders both the spin wave propagation and the transfer of the phonon momentum to the electron gas from the other. The opposite behaviour for the ferromagnetic spin arrangement in the range 0.81 ≤ y ≤ 0.95 was observed. In particular, the maxima contributions to the total thermopower give a magnon component (9 μV/K) of ferromagnetic samples in the temperature range 25–100 K, a phonon component (20 μV/K) of ferromagnetic samples in the temperature range 150–200 K and a diffusion component (47 μV/K) of ferrimagnetic samples in the temperature range 250–400 K. The impurity component of thermopower does not significantly depend on a kind of spin coupling and it increases with increasing non-stoichiometry of a sample ( x + y + z ≠ 3). Its maximum value does not exceed 2.5 μV/K at 400 K.

Effect of double exchange on thermoelectric power of CuxCoyCrzSe4

Abstract A correlation between the thermoelectric power and the double exchange and superexchange magnetic interactions in the ferromagnetically ordered Cu x Co y Cr z Se 4 p-type spinel conductors (with low Co content y up to 0.11) and in the antiferromagnetically ordered n-type semiconductor (with Co content y = 0.23) in the temperature range from 5 to 400 K is considered, respectively. The magnetic and thermopower investigations showed that a bandwidth of the 3d t 2g band due to Cr 3+ and Cr 4+ ions narrows with increasing cobalt content. For p-type conductors (with Co content y up to 0.11) all thermopower components are positive while for n-type semiconductor ( y = 0.23) the phonon drag component is positive and the diffusion and impurity components are negative. In the latter case the linear dependence between thermopower ( S ) and the normalized electrical resistivity (ln ρ / ρ 0 ) suggests a polaron conduction at high temperatures. The carrier diffusion dominates in single crystals with high cobalt content and the impurity component increases with increasing non-stoichiometry of a sample. For samples with low cobalt content y up to 0.11 the strong ferromagnetic coupling connected mainly with double-exchange mechanism makes easier the magnon excitations. Their intensities increase with increasing sample stoichiometry ( i.e. when x + y + z → 3).

Electrical resistivity and thermopower of intercalation compounds Bi xTiS 2

The temperature dependence of DC electrical resistivity and thermopower of bismuth intercalated compounds Bi x TiS 2 ( x = 0 – 0.25 ) were investigated in the temperature range from 7 to 300 K. The resistivity and thermopower decreased as intercalated Bi content increased. However, the thermopower component originating from phonon-drag effect was found to have enhanced.

Thermoelectric power of mixed electronic-ionic conductors III. Case of calcium titanate

The primary purpose of the present work is the determination of the thermopower component related to electronic charge carriers for undoped calcium titanate. The second purpose of this work is establishment of the relationship between this thermopower component and the electronic component of electrical conductivity. An essential part of the present study includes the determination of the thermopower components corresponding to different charge carriers (electrons, electron holes and ions). The determination procedures are based on the following three models: Symmetrical model. This model assumes consistency between thermopower and electrical conuctivity in terms of the n-p transition (this model assumes that minimum of electrical conductivity corresponds to the electronic component of the thermopower equal zero). It was shown that this model does not apply for CaTiO3. The Heikes model. This model is based on Heikes formula and also hopping mechanism of the transport of electrons. It was shown that thermopower of CaTiO3 cannot be described by this model and, consequently, thermopower vs. electrical conductivity cannot be considered within the Jonker formalism. General model. This model is based on a general thermopower equation for mixed conductors without any simplifying assumptions. Application of this model indicates that the electronic component of thermopower is not consistent with the minimum of electrical conductivity.

Thermoelectric power of mixed electronic-ionic conductors II. Case of titanium dioxide

The purpose of the present work is the determination of the thermopower components corresponding to different charge carriers (electrons, electron holes and ions) for TiO2 and the use of these data for evaluation of the effect of symmetry between these two properties. The procedure of the determination of these components was based on the following two approximations: The first approximation is based on a symmetrical model assuming a consistency between thermopower and electrical conductivity within the n-p transition (minimum of electronic component of the electrical conductivity corresponds to zero value of the electronic component of thermopower). The second approximation is based on the apparent asymmetry between thermopower and electrical conductivity within the n-p transition as determined from the first approximation.

Thermoelectric power of mixed electronic-ionic conductors I. Basic equations

The present work considers thermopower of oxide materials within n-p transition regime. Specifically, basic equations describing the effect of thermocell reactions on both ionic and electronic component of thermoelectric power are derived. The proposed formalism considers the impact of gas/solid reactions on the relationship between thermopower and electrochemical potential within a system involving a metal oxide of nonstoichiometric composition and a metal (such as Pt) that is applied as a measuring electrode. The derived theoretical model allows the determination of the thermopower components corresponding to different charge carriers, including ions, electrons and electron holes, for metal oxides. The proposed model may be used for derivation of defect chemistry models based on thermopower data that are free of the ionic component.

Thermopower of CeNi5 with strong f-electron instability: Band effects

Numerical calculations of the thermopower component Sd that is associated with the fine structure of the density of d states near the Fermi level are carried out for CeNi5 and its La analog. The estimates, together with earlier experimental data on the transport coefficients in Ce(MxNi1−x)5 solid solutions with M=Ga, Cu (0≤x≤0.4)), are used to analyze the behavior of the thermopower component Sf originating from the Ce valence instability. It is shown that as Ce crosses over from the state of its usual intermediate valence to the saturated-valence state, the feature near the Fermi energy, which represents a Lorentzian-shaped peak of the density of f states, transforms to a double-humped structure. The possibility of formation of a strongly correlated band of f states in Ce(MxNi1−x)5 in the crossover to the Ce saturated-valence state, accompanied by the opening of a quasi-gap Δ ∼ 400 K, is discussed.

Low-temperature thermopower in nanostructured silver

The variation of thermopower with temperature from 77 to 300 K has been measured in samples of nanostructured silver. It is observed that a dramatic change occurs in the thermopower of the nanostructured silver as grain size is reduced into the nanoscale region. In particular, with the grain size decreasing to the nanometer scale, the slope of the electron-diffusion component of thermopower as a function of temperature increases and the phonon-drag component of thermopower is shifted to lower temperatures. The changes in the thermopower correlate well with changes in the microhardness.

Phonon-drag magnetothermopower oscillations in GaAs/AsxGa1-xAs heterojunctions.

The tensor M\ensuremath{\rightarrow} which determines the heat flux U=M\ensuremath{\rightarrow}E in a weak electric field E is calculated for a two-dimensional electron gas in a perpendicular magnetic field B. The dominant phonon-drag contribution is calculated using Boltzmann transport, allowing for the two-dimensional (2D) character of the electron gas. The Landau levels are taken to have Gaussian line shapes with the rms level width \ensuremath{\gamma}=${\mathit{CB}}^{1/2}$ where C is the only adjustable parameter. Setting C=0.5 meV ${\mathit{T}}^{\mathrm{\ensuremath{-}}1/2}$ gives good agreement with new experimental values of ${\mathit{M}}_{\mathit{y}\mathit{x}}$ obtained for GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterojunctions when B varies between 2 and 10 T and the temperature varies between 1 and 5 K. ${\mathit{M}}_{\mathit{y}\mathit{x}}$ is negative and contains strong magneto-oscillations in phase with the density of states at the Fermi level. The model also predicts ${\mathit{M}}_{\mathit{x}\mathit{x}}$=0, whereas experiment gives peak values of \ensuremath{\Vert}${\mathit{M}}_{\mathit{x}\mathit{x}}$\ensuremath{\Vert} up to 60% of those of \ensuremath{\Vert}${\mathit{M}}_{\mathit{y}\mathit{x}}$\ensuremath{\Vert}. We demonstrate that setting ${\mathit{M}}_{\mathit{x}\mathit{x}}$=0 in the calculation has little effect on the predicted thermopower component ${\mathit{S}}_{\mathit{x}\mathit{x}}$ but yields poor approximations to the experimental values of ${\mathit{S}}_{\mathit{y}\mathit{x}}$.

Magnetothermopower in silicon MOSFETs

The phonon drag and electronic diffusion contributions to the tensor M which determines the heat flux U=M.E is calculated for a two-dimensional electron gas in a perpendicular magnetic field B. The drag component is obtained using Boltzmann transport and diffusion is included using the lowest-order cumulant approximation to describe scattering between Landau states with different orbit centres. The 2D Landau levels have Gaussian model lineshapes with RMS width gamma =CB1/2 which is the only adjustable parameter. At temperature T=5.02 K, drag dominates the predicted Myx(B) values which are in good agreement with new data for Si MOSFETs taking C=0.6 meV T-12/. The calculated Mxx values are in worse accord with the data because the predicted drag contribution Mxxg is zero. Both Mxx and Myx reveal magneto-oscillations originating from fluctuations in the density of states at the Fermi level. The authors show that at T=5.02 K, setting Mxxg=0 has little effect on the accuracy of the magnetothermopower Sxx(B) but yields poor approximations to the off-diagonal term Syx(B). At T=1.47 K, these thermopower components are diffusion-dominated. The predicted values are comparable to experiment although the magneto-oscillations are overemphasized in Syx and underestimated in Sxx.

Thermomagnetic transport coefficients of a periodically modulated two-dimensional electron gas.

The impurity contribution to the thermopower and thermal conductivity of a two-dimensional electron gas that is modulated weakly and periodically along the x direction is evaluated in the presence of a perpendicular magnetic field. A type of oscillations, having the same origin as the magnetoresistance oscillations discovered recently, is predicted. The thermopower components ${\mathit{S}}_{\mathit{x}\mathit{x}}$, ${\mathit{S}}_{\mathit{y}\mathit{y}}$, and ${\mathit{S}}_{\mathit{x}\mathit{y}}$ all oscillate in phase in small magnetic fields. The oscillations in the thermal conductivity ${\mathit{K}}_{\mathit{x}\mathit{x}}$ are shifted by \ensuremath{\pi}/2 with respect to those oscillations in ${\mathit{S}}_{\mathit{x}\mathit{x}}$. $K sub xx--- oscillates out of phase with ${\mathit{K}}_{\mathit{y}\mathit{y}}$ and ${\mathit{K}}_{\mathit{x}\mathit{y}}$.

Electronic transport in a superlattice with average periodic order.

We introduce a model to describe scattering from disordered metallic interfacial regions in layered structures, or superlattices. This model is also applicable to interface roughness or boundary scattering in single junctions. It consists of an array of slabs of scattering centers embedded in a host metal. The slabs have thickness t and are separated by a distance d. Each simulates an interfacial region in which the physical arrangement of scattering centers is disordered with positional correlations within a slab being represented by a structure factor S, but atomic positions being uncorrelated between slabs. For a description of the structure factor of a slab with assumed quenched disorder, the pair correlation function for homogeneous liquids is used as an input after extension both to finite geometry and into regions of high density typical of solids. The Boltzmann equation, with the anisotropic structural information incorporated in the collision term, is solved by a variational principle, and yields the in-plane and out-of-plane resistivity and thermopower components as functions of d, t, and the density of scattering centers in a slab. The results show that it is possible to characterize the microscopic structural features of the interfacial region in terms of the transport properties of the system.

Thermopower measurements on arsenic from 2 to 300 K

${S}_{11}$ and ${S}_{33}$ thermopower components of arsenic have been determined in the temperature range 2-300 K. The data on trigonal (${S}_{33}$) samples are the first reported investigations below 80 K. The measurements were made on high-quality melt-grown single crystals with resistivity ratios in excess of $3500({S}_{11})$ and $5300({S}_{33})$, the largest reported to date. While the thermopower ${S}_{11}$ is positive above 15 K, the ${S}_{33}$ component is negative throughout the investigated temperature range. Above 70 K, both ${S}_{11}$ and ${S}_{33}$ thermopowers are purely of diffusion origin. At lower temperatures the thermopowers show pronounced anomalies associated with phonon drag of electrons and holes. With increasing sample perfection both ${S}_{11}$ and ${S}_{33}$ tend to become less negative.

Anisotropic thermopower of (TMTSF) 2PF 6: 1D–2D cross over and SDW ordering

The thermopower of (TMTSF) 2PF 6 has been measured along the a-axis as well as along the b-axis. Marked anisotropy is seen in the whole temperature region studied. Close to 100 K, an anomaly is attributed to cross over from dominating one-dimensionality to dominating two-dimensionality. Precursor effects seen in S a near T c are attributed to SDW fluctuations. Well below T c , both thermopower components exhibit typical semiconducting properties.

The thermoelectric power of nickel and its alloys

Measurements are reported of the temperature variation of the absolute thermoelectric power of pure nickel and its dilute alloys with Pd, Cu, Co, Fe, Mn, Cr, V and Ti. Most of the data have been taken between 2 degrees K and 100 degrees K, although certain measurements have only been made between 2 degrees K and 15 degrees K. The curves taken over the wider temperature range all exhibit pronounced minima at about 40 degrees K, with the depth of the minima considerably greater in the Ni-Cu, Ni-Co and Ni-Fe alloys than in pure nickel. It is argued that this is due to the effect of spin-mixing on the phonon drag component of thermopower. There is a discussion of the likely influence of spin- mixing on the diffusion thermopower.

The low temperature thermoelectric power of some palladium and platinum alloys

Abstract The thermoelectric power, S, of a number of palladium and platinum alloys has been measured in the temperature range 2°K to 120°K. For alloys of palladium or platinum with their neighbouring elements it has been possible to correlate the sign of the diffusion component of thermopower Sd in the residual resistance temperature range with the slope of the density of states versus concentration curves. Furthermore, there is excellent numerical agreement between the observed values of Sd/T and those calculated by a method based on the rigid band model for all the platinum-gold alloys investigated and for the palladium-silver alloys apart from one which contained less than 1 % silver. However, the sign of Sd was not predicted correctly for a palladium-platinum alloy. Each of the specimens exhibits a large positive phonon drag component of thermopower which persists to high temperatures and is not greatly reduced on alloying. These two features as well as the positive sign are the result of phonon-induc...