Showing papers on "Seebeck coefficient published in 1995"

Low temperature properties of the filled skutterudite CeFe4Sb12

Abstract: CeFe4Sb12 is a member of a class of advanced thermoelectric materials. In order to evaluate this material’s potential for such applications, we have measured a variety of its properties at low temperature, including thermal conductivity, thermoelectric power, electrical resistivity, Hall coefficient, and magnetic susceptibility.

Thermopower of Sr1−xLaxTiO3 ceramics

Abstract: The thermopower η of Sr1−xLaxTiO3 ceramics was investigated up to x=0.5 and in the temperature range between 150 K and 1200 K. In addition, the carrier concentration n was determined by Hall measurements and by a chemical Ti3+‐analysis. For low temperatures and high n, η depends linearly on temperature and on n−2/3, as expected from a degenerate quasi free electron gas. In the case of high temperatures and low n, the absolute value of η rises with 1.5⋅ln10⋅k/e per decade of temperature and with ln10⋅k/e per decade of carrier concentration, as expected from a classical broad‐band semiconductor obeying the Boltzmann statistics. In the range of degeneration an effective mass meff of 4.2 electron masses can be deduced without the assumption of a transport factor Ae. In the classical range Ae=3 can be evaluated, requiring only a temperature and lanthanum independent meff. Thus, the thermopower of Sr1−xLaxTiO3 ceramics can be described by a constant effective mass and a constant transport factor within a wide range of temperature and lanthanum content. Furthermore, the transition from degeneration to classical behavior can be described as a function of temperature and electron density, e.g., at room temperature it takes place at about x≊0.2 (i.e., n≊3.4⋅1021/cm3).

Thermoelectric figure of merit of quantum wire superlattices

Abstract: The electrical conductivity, the thermoelectric power, and the electrical contribution to the thermal conductivity of quantum wire superlattices have been studied. The effects of tunneling through the barriers due to finite potential off‐sets and of the thermal currents through the barrier layers are shown to be essential to describe properly the thermoelectric figure of merit of realistic quantum wire superlattices. The figure of merit exhibits a maximum as a function of superlattice period which, for large barrier off‐sets, is found to be substantially enhanced over that for the bulk material and also larger than that for quantum well superlattices.

Fast thermoelectric response of normal state YBa2Cu3O7−δ films

Abstract: Normal state YBa2Cu3O7−δ films, epitaxially grown ‘‘off‐c axis’’ with tilt angles up to 20° are shown to be fast thermoelectric detectors for radiation from UV to far infrared wavelengths. Upon radiation heating of the tilted films a thermoelectric voltage arises due to the anisotropy of the thermopower in YBa2Cu3O7−δ. The response time is limited by the decaying temperature gradient and thus by heat diffusion. For thin films we have measured a response time from ≲1 ns in the UV to ∼5 ns in the far infrared. Because of the wavelength dependent reflectivity, the responsivity of the films varies between 0.5 V/MW and 20 V/MW. Thus, thin tilted YBa2Cu3O7−δ films can be used as fast room temperature detectors over a wide spectral range.

High p, T physical property studies of Earth's interior: Thermoelectric power of solid and liquid Fe up to 6.4 GPa

Abstract: High p,T experiments provide indirect access to the deep interior of Earth. High p, T research at the University of Western Ontario using large volume cubic anvil presses has focused on in-situ measurements of physical properties of mantle and core related substances as they relate to Earth's interior physical processes. Thermoelectric power measurements of solid and liquid Fe, using twisted Fe/Pt–10%Rh junctions, have been made at p up to 6.4 GPa. By isolating the sign determining energy derivative of the density of states at the Fermi energy, ∂N(eF)/∂e, in the expression for the Seebeck coefficient of Fe, these measurements show that ∂N(eF)/∂e is positive for liquid Fe at p below approximately 4.0 GPa but ∂N(eF)/∂e is negative for p above 4.0 GPa. The solid phase polymorphism which Fe exhibits in this p region, and its influence on the liquid state, is suggested as the explanation for the short range bcc-like and fcc-like properties of the liquid. This finding corroborates our earlier measurements on th...

Effects of Mechanical Alloying and Grinding on the Preparation and Thermoelectric Properties of β-FeSi 2 ( Overview )

Abstract: Improving effects of mechanical alloying (MA) and grinding (MG) on the preparation processes as well as on the thermoelectric properties of semiconducting β-FeSi 2 were reviewed. MA and MG strongly accelerates the preparation reaction to form β-phase from elemental powders and from the mixture of α-Fe 2 Si 5 and e-FeSi. The electrical resistivity significantly decreased due to the dispersion of fine particles of metallic phase, e.g. e-FeSi phase. As a result, the power factor Q 2 /ρ was significantly improved. Figure of merit Z = Q 2 /ρκ was also improved by MA and MG due to the decrease in the thermal conductivity K. This is due to a decrease in grain size of the β-phase which causes an increase in phonon scattering at the grain boundaries. MA and MG are promissing techniques for the acceleration of β-FeSi 2 formation as well as the improvement of the thermoelectric properties of β-FeSi 2 .

Si80Ge20 thermoelectric alloys prepared with GaP additions

Abstract: Controlled amounts of GaP and P were added to a Si0.8Ge0.2 matrix by a powder‐metallurgical technique in order to evaluate the optimum composition for thermoelectric applications. Bulk determination of the gallium and phosphorus content in fully dense, hot pressed compacts was performed by inductively coupled plasma atomic emission spectroscopy. The transport properties of the compacts were characterized by Hall effect measurements at room temperature and by measurements of electrical resistivity, Seebeck coefficient, and thermal diffusivity to 1000 °C. Considerable variation in the electrical transport properties were found to accompany changes in the Ga/P ratio, in the total amount of dopant, and changes in other preparation conditions. Alloys with gallium phosphide additions exhibit carrier concentrations higher than those obtained in alloys doped only with phosphorus. Alloys with a nominal phosphorus content greater than 2.0 at. % were found to be overdoped and those containing less than 0.6 at. % pho...

Superconductors in a temperature gradient

Abstract: In the mixed state of a type II superconductor quasiparticles and magnetic flux quanta respond to a temperature gradient by thermal diffusion, in this way generating the Seebeck and Nernst effects, respectively. Our understanding of the Seebeck effect originates from an extension of the two-fluid counterflow concept, originally introduced by Ginzburg, to the situation where vortices (with a normal core) are imbedded in the superconducting phase. This mechanism results in an intimate connection between the Seebeck coefficient and the electric resistivity due to vortex motion. In all thermal diffusion processes it is the transport entropy of the diffusing species that determines the driving force, and the physics of this quantity is illustrated. Our discussion of the experimental side concentrates on the recent work performed with the cuprate superconductors. The characteristic broadening of the resistive transition in the mixed state, found in these materials due to their high anisotropy and the peculiar vortex structure (pancake vortices), results in a similar broadening of the temperature regime where the Seebeck and Nernst effects appear. In the cuprate superconductors fluctuation effects are highly pronounced because of the large anisotropy, small coherence length, and high critical temperature of these materials. Here the Nernst effect yields particularly useful information since it nearly vanishes in the normal state, and complicated subtraction procedures are unnecessary. As in all transport phenomena, the Hall angle also appears in the thermal diffusion processes, and a summarizing discussion is given. The observation of an unusually large Hall angle for the thermal diffusion of vortices still remains puzzling. A tentative explanation is based on the thermal generation of unbound vortex-antivortex pairs.

Electrical and thermal transport properties of (La,Ca)(Cr,Co)O3

Abstract: DC electrical conductivity and Seebeck data as functions of temperature and oxygen activity were used to explain the electrical and thermal transport properties of (La,Ca) (Cr,Co)O3. The electrical conductivity data of La(Cr,Co)O3 suggested that it depends upon Co content. At Co concentrations less than 20 mol%, Co acted as traps for the carriers and decreased the electrical conductivity of the compositions at temperatures less than 900 °C. As the Co concentration increased to above 20 mol%, electrical conductivity increased significantly due to a connecting path of available Co sites. Additional Ca substitution for La increased the electrical conductivity in accordance with Verway's principle. Defect models derived from the electrical conductivity data were found to relate the carrier concentrations to the Ca, Co and oxygen vacancy concentrations. The measured Seebeck coefficients were found to be positive even for the most reducing conditions, indicating that (La,Ca)(Cr,Co)O3 compositions with Co ≤ 0.3 and Ca ≤ 0.3 were as stable as LaCrO3. The Heikes formula was adopted to interpret the Seebeck coefficient results. These results indicated that electrical conduction in La(Cr,Co)O3 and (La,Ca)(Cr,Co)O3 occurs via the non-adiabatic and adiabatic small polaron mechanisms, respectively.

Thermoelectric properties of fine-grained sintered (Bi2Te3)25-(Sb2Te3)75 p-type solid solution

Abstract: P-type semiconductor alloy compacts of the composition (Bi2Te3)25-(Sb2Te3)75 with grain size (L) in the range 30 > L > 20 μm, 20 > L > 15 μm, 15 > L > 10 μm, 10 > L > 5 and L < 5 μm were prepared by cold press at a pressure of 77 × 107 Nm−2. The samples were sintered at 673 K. Measurements of the Seebeck coefficient, electrical resistivity and thermal conductivity were carried out. The experimental results show that the Seebeck coefficient increases, but not much from the single crystal. The electrical resistivity increases in particular for the size L < 5 μm with a reduction in grain size. The total thermal conductivity seriously decreases as grain size decreases. It is concluded that the figure of merit of the compacted alloy would be significantly improved through the use of fine-grained powders of size 30−10 μm.

Electrical Conduction of Glasses in the System Fe2O3-Sb2O3-TeO2

Abstract: Fe2O3-Sb2O3-TeO2 glasses were prepared by the pressquenching method and their do conductivities were measured. The glass formation region was found to be 0≤Fe2O3≤15mot%, 0≤Sb2O3≤18mol%, and 78≤TeO2<100mol%. Seebeck coefficient measurements showed that these glasses were n-type semiconductors. The glasses gave conductivities σ ranging from 1.62×10-6 to 1.86×10-7 S·cm-1 at 473K. The dc conductivity increased with an increase in Fe2O3 content. Evaluated carrier mobility and concentration ranged from 7.5×10-10 to 5.3×10-3cm2·V-1·s-1 and from 1.5×1021 to 1.9×1015cm-3 at 473K, respectively. The conduction of these glasses was due to non-adiabatic small polaron hopping.

Highly sensitive method for simultaneous measurements of thermal-conductivity and thermoelectric-power - fe and al examples

Abstract: The main purpose of this work is to present a highly sensitive method for measuring simultaneously thermal conductivity and thermoelectric power in a 4.2–300 K temperature range, and to discuss the errors made in measurements of such quantities due to radiation and convection losses. An experimental setup is presented in detail and discussed. It is emphasized how in this quasi‐steady‐state method, the residual heat flow and temperature difference between various components are taken into account. Measurements of the thermal conductivity and thermoelectric power of Fe are presented for such a setup with and without radiation shields, and for high or low vacuum in the range 70–300 K. The best conditions are then used for measuring these properties in Al. Brief comparison is made to existing data.

Nature and amount of carriers in Ce doped Nd2CuO4 I. High-temperature characterization

Abstract: DC conductivity, ionic transport number, and thermopower have been measured of Nd2−xCexCuO4±δ materials (x = 0, 0.05, 0.10, 0.15, 0.20) at different temperatures [300 ⩽ T ⩽ 900°C] and under different oxygen partial pressures [1 atm ⩽ P(O2) ⩽ 3 × 10−6 atm]. For the most heavily doped samples, the current carriers are electrons essentially produced by the substitutional defect CeNd., while the crystal-gas equilibria involving Oi″ account for a smaller amount of carriers. The behavior of Nd1.95Ce0.05CuO4±δ is intermediate between more doped and undoped materials. When coupled with oxygen non-stoichiometry data, the results indicate that the mobility of the electronic carriers exhibits a transition from small-polaron to large-polaron behavior with increasing doping level.

Thermoelectric Power of (GeTe)1−x(Bi2Te3)x Solid Solutions (0 ≦ x ≦ 0.05) in the Temperature Interval 80 to 350 K

Abstract: The thermoelectric power S of GeTe-rich (GeTe)1-x(Bi2Te3)x solid solutions (0 ≦ x ≦ 0.05) is investigated as a function of composition x and temperature T in the range from 80 to 350 K, where the materials have the crystal and band structure of the rhombohedral α-phase of GeTe. On the basis of the non-parabolic two-band Kane model of IV-VI compounds information on the Fermi energy F (reduced Fermi energy F* = F/k0T, where k0 is the Boltzmann constant) and the degeneracy of the alloys is deduced. A qualitative interpretation of the temperature and composition dependences of S is made. It is assumed that there are two types of carriers (light and heavy holes) and redistribution among the valence bands with the change of temperature. [Russian Text Ignored].