Values Of Thermoelectric Power Research Articles

Electrical and optical properties of thin β-FeSi 2 films on Al 2O 3 substrates

Polycrystalline β-FeSi 2 thin films of about 600 nm were prepared by simultaneous electron beam evaporation of Si and Fe ( 1.6 < Si Fe < 2.4 ) onto Al 2O 3-ceramic and saphire substrates. The films were deposited at 100°C and subsequently annealed between 500 and 950°C. Films crystallized below 870°C predominantly consist of the semiconducting phase β-FeSi 2 with a p-type conductivity. At higher temperatures only the metallic monosilicide ϵ-FeSi could be detected probably due to a chemical reaction of Fe, Si and the substrate material. The influence of the crystallization temperature and of deviations from the stoichiometric composition Si Fe = 2 on the grain size and some electrical and optical properties are studied. A Si Fe ratio between 2.1 and 2.2 is correlated with maximum grain sizes and thermoelectric power values and minimum conductivities. A minimum optical subband absorption below the gap energy is also characteristic for this composition. The Hall voltage between room temperature and 10 K was too low to be resolved. Therefore Hall mobilities are estimated to be rather small (≤ 0.1–0.2 cm 2/Vs) and high defect densities and carrier concentrations are concluded. A photoconductive effect could not be demonstrated without any doubt. Due to these results the suitability of such films for photovoltaic applications seems to be questionable up to now.

Influence of lead content on temperature dependence of thermoelectric power of 2223 (BiPb) sintered cuprates

The temperature dependence of the thermoelectric power (TEP) of Bi 2 − x Pb x Sr 2Ca 2 Cu 3O 10 + δ pellets with 0 < x < 0.5 is reported between T CO and 300 K. The thermoelectric power values of all the samples were found to be positive and to increase almost linearly ( dS dT = −0.030 μ VK −2 ) with decrease in temperature down to ≈120 K, before falling rapidly to zero at T CO . The results are analysed on the basis of different theoretical models and it is concluded that the measured TEP data could be explained extremely well (within 1%) using the modified two-band model proposed by Forro et al.

Resistivity and thermoelectric power of [formula omitted] superlattice with 2-unit-cell-thick YBa 2Cu 3O 7−δ layer in one period

Measurements of the resistivity and thermoelectric power (TEP) for a series of YBa 2Cu 3O 7−δ PrBa 2Cu 3O 7−δ ( YBCO PrBCO ) superlattice samples with 2-unit-cell-thick YBCO layer and 2, 4, 6 or 8-unit-cell-thick PrBCO layer in one period are reported. For the sample with 2-unit-cell-thick PrBCO layers, the resistivity of the YBCO layers is larger than that of bulk YBCO samples. The contribution of stoichiometric diffusion and interface scattering is estimated. Its TEP is positive with positive temperature coefficient, different from the typical negative one in bulk YBCO samples with positive TEP values. The property of the YBCO layers changes with increasing thickness of the PrBCO layers. For the sample with 8-unit-cell-thick PrBCO layers, the contribution of the PrBCO layers has to be considered.

Characterization of transparent conducting CdO films deposited by spray pyrolysis

Transparent conducting cadmium oxide (CdO) films have been deposited by spray pyrolysis. The film thicknesses have been determined using Rutherford backscattering spectrometry. X-ray diffraction measurements show that the films are polycrystalline with a preferential orientation along the (111) diffraction plane and the lattice parameter has been calculated. The dislocation density and strain have also been evaluated. The films possess a transmittance of about 75% in the visible and near-infrared region. The refractive index is found to vary between 1.68 and 2.84 in the wavelength range 500-1500 nm. The values of indirect and direct bandgaps obtained are 1.98 and 2.32 eV respectively. Hall effect measurements have been carried out in the temperature range 304-349 K. Resistivity, carrier concentration and mobility of the films at room temperature have been evaluated as 6.6*10-5 Omega m, 1.4*1025 m-3 and 0.68*10-2 m2 V-1 s-1 respectively. Thermoelectric power values from thermoelectric power measurements carried out in the temperature range 304-376 K have been found to be about 19.7-89 mu V K-1. Laser damage studies performed at a wavelength of 1.06 mu m indicate that the films possess a damage threshold density of about 2.37*104 J m-2.

Phase transition studies in superionic material 60 AgI-20 AgSO4-20 AgPO3by dielectric method

Abstract Dielectric Constant (ϵ′), thermoelectric power (S) and electrical conductivity (σ) were determined for a superionic material 60 AgI-20 AgSO-20 AgPO3 over a temperature range of 30°C-210C, usingan indigenously fabricated equipment. It was observed that the dielectric constant kept on decreasing and reached a value of almost zero around 148°C and then started shooting up as the temperature of the material was increased. The reason for this interesting behaviour was been analysed in terms of phase-transition. The values of thermoelectric power and electrical conductivity increased with the rise of temperature throughout the range of temperatures studied and no abnormality in these values was observed around 148°C.

Effect of the lattice strain on the thermoelectric power in a β-quenched Zr-1.14 wt.%Cr-0.1 wt.%Fe alloy

A Zr-1.14 wt.%Cr-0.1 wt.%Fe alloy was β-quenched from 1050°C to produce a supersaturated α′ phase and then reheated for varying times at temperatures of 350°C, 550°C, or 760°C to produce precipitation of the Zr(CrFe) 2 phase. Thermoelectric power (TEP) measurements and X-ray line broadening analysis were used to determine the TEP value, ΔS ( μV/ K), and the lattice strain, ε (microstrain), respectively. It is shown that both increasing Zr(CrFe) 2 precipitation from the α′ matrix, which decreases the (Cr + Fe) content in the matrix, and increasing lattice strain, ε, result in an increase in the TEP value.

The kinetics of amorphous-nanocrystalline transformation for a finemet alloy

The kinetics of the nanocrystalline structure formation from its amorphous precursor was investigated for a Finemet alloy with the composition Fe 73.5Nb 3Cu 1Si 13.5B 9. The time- and temperature-dependent transformation was followed in parallel by differential scanning calorimetry (DSC), thermoelectric power (TEP) and Vickers microhardness (HV) measurements on ribbon samples 10 mm wide. After annealing the samples in a tin bath at 500, 525, 550 and 575 °C for different annealing times between 5 s and 2 h, the TEP and HV data were measured at room temperature. Using the conventional Johnson-Mehl-Avrami method, for the apparent activation energy we deduced values from 3.58 to 3.93 eV from the TEP data and values from 3.84 to 4.08 eV from the HV data which agree well with the value we obtained from DSC measurements using the Kissinger method (4.1 ± 0.4 eV). Our DSC measurements on partially crystallized samples revealed a third intermediate exothermal peak beside the well-known two peaks. This was attributed to the segregation of a silicide compound out of the α-Fe(Si) solid solution.

Thermoelectric power studies on the BPSCCO system with silver addition

The effect of silver addition on the superconducting properties (resistance and thermoelectric power) of the Bi1.6Pb0.4Sr1.6Ca2Cu3Oy superconductor has been studied for silver concentrations of 0, 5, 10, 20 and 50 wt%. Silver is found to be nonreactive in so far as the onset Tc is found to remain unchanged at 110 K. Silver, however, diffuses through the grain boundaries of the rather low-density, 2223 material weakening the intergrain coupling. Tc0 (zero resistivity transition) is thus reduced with the addition of silver. The normal state thermoelectric power (TEP) of these specimens has been fitted to a formula derived from marginal Fermi liquid hypothesis. The normal state value of TEP decreases with increasing silver concentration. Ts0(zero TEP temperature) is almost the same for all these specimens and is higher than Tc0, confirming a lower percolation threshold for TEP than the percolation threshold for conductance.

Structure and properties of the La-doped Bi-2201 system

In this article, the influence of La substitution for Sr on structure and physical properties of the 2201 phase is studied. First, the crystal microstructure of the Bi2.1Sr1.9-xLaxCuOy (0 ≤ x ≤ 1.0) is characterized by means of x-ray diffraction and electron diffraction analyses, it is discovered that in the La-doped 2201 system with increasing La content the 2201 phase undergoes tetragnoal-orthorhombic-monoclinic structural transition. With this phase transition, the modulation vector of the 2201 phase transforms from incommensurate to commensurate, and the period of the modulation wave decreases. Secondly, the superconductivity of this system is analyzed systematically. The results of the resistivity measurement show that the appropriate amount of La doping can raise the superconducting transition temperature (Tc) of the 2201 phase, with x=0.2, Tc reaches as high as 25 K. However the excessive doping of La (x ≥ 0.3) leads the 2201 phase to transit from metal to insulator and the superconductivity disappears. The measurement of thermoelectric power (TEP) of this system shows that with increasing La content the TEP value also increases gradually. But the interesting point is that the transition from positive to negative in TEP occurs for x=0.2 sample. In addition, the electronic structure and oxygen content of this system are also studied. Based on these, we analyze the relation between the microstructure, electronic structure of the La-doped 2201 phase and its superconductivity, indicating that the type of element substitution with higher valence, together with the microstructural distortion induced by the substitution affects the electronic states of Cu 3d seriously, that is, changes the degree of coulomb correlation of Cu 3d electrons, thus resulting in the transition of the type of carrier of this system and the decrease of hole carrier concentration, as well as the corresponding variation of the superconductivity of La-doped 2201 phase.

Thermoelectric power of Pr 2−yCe yCuO 4−ò superconductors with various content of Ce

The temperature dependence of the thermoelectric power (TEP) of superconducting ceramic Pr 2−yCe yCuO 4−ò samples with various Ce content (0.13 < y < 0.17) has been investigated. The TEP value is small and has a weak temperature dependence within the range of 80–300 K. With increase in Ce content the TEP sign reverses from negative to positive. The comparison of the data obtained with TEP of La 2−xSr xCuO 4−ò compound points to the significantly less value of both the effective mass of carriers and the density of their states in Pr 2−yCe yCuO 4−ò. The Emery model for HTSC explains satisfactorily the experimental data for Pr 2−yCe yCuO 4−ò and La 2−xSr xCuO 4−ò and gives the correct sign and the order of TEP and effective mass value in these compounds.

Thermoelectric power of amorphous Ce-Cu alloys

Thermoelectric power (TEP) of amorphous Ce-Cu alloys with compositions of CeCu 6, CeCu 2 and CeCu has been measured from 2 to 300 K. The value of TEP is very small and insensitive with respect to the temperature, making a drastic contrast to that of the counterparts in the crystalline states. The role of a random atomic potential in the amorphous alloys is discussed.

Anomalous thermoelectric power of the alkali metal doped and the pyrolyzed fullerene

We report the temperature dependent thermoelectric power (TEP) and conductivity of the alkali-metal doped bulk fullerene, Na x C 60, K x C 60 and the pyrolyzed fullerene. For K x C 60, their TEP data can be classified into two kinds according to their signs and temperature dependences. In the case of the samples with negative TEP, their TEP data show quasi-linear temperature dependence with S RT ≅ −110 μV/K. However, for the samples with positive TEP, the TEP data show big anomalies. The TEP value of the pyrolyzed C 60 is about S RT ≅ +3.8 μV/K and upon cooling, the TEP decreases quasi linearly with the change of slope at T = 180K. Our preliminary analysis indicates that the positive TEP data of doped C 60 are originated from the collectively bridged C 60 molecules while the negative TEP data are due to the electrons donated from the alkali metals in doped C 60 fullerene.

Electrical transport of the pyrolyzed materials; polyacenic and polyacrylonitrile

The electrical conductivity and thermoelectric power (TEP) of the pristine pyrolyzed polyacrylonitrile(Pyro PAN) as well as the pristine and the FeCl 3 doped polyacenic materials have been measured. The results for Pyro PAN are somewhat complicated, which depend on the pyrolysis conditions and aging time in the air after the pyrolysis. Some samples show the nearly zero TEP value and the temperature independent TEP in a wide region of temperature is observed, which may indicate the strongly correlated system. But the change of TEP in time sequence means that the system is metastable. The values3of room temperature conductivity vary from ∼ 0.1 to 90 (S/cm), and the weak temperature dependence monotonously decreasing upon cooling with σ(30K)/ σ(290K) = 0.7 ∼ 0.8 is observed. The conductivity measurement of the pyrolyzed polyacenic materials of T p = 500° C indicates that the conduction mechanism of this material can be explained by the variable range hopping model. And the conductivity of T p = 700° C sample indicates that it is much more carbonized.

Thermoelectric study of Y-Ba-Cu-O thin film on MgO substrate prepared by resistive evaporation

Thermoelectric measurements were carried out on Y-Ba-Cu-O thin film deposited on MgO substrate by resistive evaporation technique. A pulverized mixture of Y, BaF 2 and Cu weighed in the atomic proportion was evaporated from resistively heated source onto a MgO substrate kept at 400°C using a simple vacuum system. The substrate temperature was then raised to 700°C for insitu heat treatment. Oxygen was injected through a nozzle placed close to subtrate surface, thus raising the pressure to about 7 Pa during the heat treatment, which lasted for about 15 minutes. The film was then gradually cooled down to room temperature and the pressure raised to atmospheric pressure. The films thus obtained were measured and the values of thermoelectric power measurements in the plane of the film were found to be close to the typical thermoelectric power values of crystalline Y-Ba-Cu-O superconductors. As expected, vanishing values of the thermoelectric power have been observed below 80 K. If the relation observed for sintered Y-Ba-Cu-O is applied for thin films, it suggests an extremely low oxygen deficiency.

Effect of substructure on the thermoelectric power of deformed ZrCrFe sheet

X-ray line broadening analysis and thermoelectric power (TEP) measurements are used to characterize the substructure changes in cold-rolled sheet of a Zr-1.14 wt.%Cr-0.08 wt.%Fe alloy. Data on the microstrain, ε, and crystallite size, D, were obtained from the x-ray line broadening analysis, and these data were then used to calculate the dislocation density, ϱ. An approximately linear relationship was found between the TEP value and the dislocation density, with the TEP value decreasing with increasing dislocation density.

Thermoelectric power of YBa2Cu3O7−δ superconductive thin films as a function of oxygen contents

Experimental correlations of thermoelectric power (TEP), oxygen deficiency (δ), and c-lattice constant in YBa2Cu3O7−δ (YBCO) c-oriented, polycrystalline, thin-film, high-temperature oxide superconductors are presented. The TEP values in the normal state were observed to be positive for samples with oxygen deficiencies between 0.08 ≤ δ ≤ 1.00 and negative for 0.00 ≤ δ ≤ 0.08. Furthermore, the TEP magnitudes were highly sensitive to the amount of oxygen and were found to increase with increasing oxygen deficiency, ranging from values of −2 μ V/K to more than two orders of magnitude higher. In addition, relatively temperature-independent TEP values were observed for hole-conductive samples with low oxygen deficiencies (δ ≤ 0.30). The lattice constants (c) of the same samples were measured by XRD. These were found to increase with oxygen deficiency and to correlate systematically with the TEP values. We conclude that TEP is a reliable indicator of the oxygen content in YBCO materials.

Electronic Properties of Sn- or Ge-Doped In2O3Semiconductors

The thermoelectric power and Hall effect of In2O3single crystals, either undoped or Sn doped, and of In2O3ceramics, either undoped or Sn or Ge doped, are investigated. All doped samples have negative thermoelectric power values. The metal-type conductivity occurs when the carrier concentration exceeds l019cm–3The correspondence between the values of the thermoelectric power and those of the carrier mobility and carrier concentration is given. Most interestingly this study puts into light the enhanced carrier mobility occurring for Ge-doped In2O3samples compared with ITO samples (Sn-doped In2O3widely used in optoelectronic devices.

Structural, electrical and magnetic studies of infinite-layered Sr 1− xLa xCuO 2 superconductor

Superconductivities were investigated in infinite-layered Sr 1− x La x CuO 2 prepared in a reductive atmosphere under high pressure. Its solid solution limit was estimated to be just above x=0.10. The T c onset values were in a range of 42–44 K independently of x and all samples attained the zero resistivities. The volume fraction of the superconductor showed a maximum at x=0.10. Their oxygen contents were analyzed to be not more than 2. The value of thermoelectric power was negative but its Hall coefficient had a positive value in a temperature range from room temperature to 77 K for the x=0.10 sample. These results indicate that electrons were doped into the conduction plane in the infinite-layered compound.

The use of thermoelectric power measurements to study recovery, recrystallization, and phase transformations in cold worked Zr-1%Nb and zircaloy-4 nuclear fuel sheathing

Thermoelectric power (TEP) measurements are used to study recovery, recrystallization, and other microstructural changes during annealing of cold worked Zr-1wt.%Nb and Zircaloy-4 tubing at temperatures from 300 to 1100 °C. The measured value of thermoelectric power is very sensitive to microstructural evolution in zirconium alloys, and TEP measurements correlate well with both microstructural changes, as seen using optical microscopy, and microhardness measurements.

Electrical transport properties of the potassium doped bulk fullerene, K xC 60

Temperature dependent thermoelectric power (TEP) and conductivity of the potassium doped bulk fullerene, K x C 60, of different potassium doping concentrations are reported. The potassium contents are determined by chemical analyses. The resistivity drop at T=18K has been confirmed. Although, the room temperature resistivity values are more or less the same for various K x C 60 samples (typically σ RT = 10 S/cm), the TEP values are very much different. The sign of TEP is positive and its temperature dependence shows a big transition at around T = 150K. Interesting observation is that the TEP drops to zero at around 60K ∼ 80K for certain samples whose resistivity keep increasing at this temperature region.