Values Of Thermoelectric Power Research Articles

Contents and Chemical Science

The substitution of cobalt and nickel for manganese in the perovskite manganate LaBiMn2O6+d; has been realized, leading to the perovskites LaBiMn2-x(M)xO6+d, with M = Co, Ni and x =0-2/3. In contrast to the literature those phases are found to be orthorhombic with Pnma symmetry. More importantly, it is shown that ferromagnetism is enhanced, TC being increased from 80 K for the parent compound (x = 0) to 97 K for Ni-phase, and to 130 K for the Co-phase. Moreover, a strong competition between ferromagnetism and a glassy-ferromagnetic state in the case of nickel or a spin-glass behaviour in the case of cobalt is observed. These phenomena are interpreted in the frame of a phase separation scenario, where the ferromagnetic Mn4+/Ni2+ and Mn4+/Co2+ interactions reinforce the Mn3+/Mn4+ interactions. These compounds are found to be insulating with a relatively large positive value of thermoelectric power.

Enhancement of ferromagnetism by Co and Ni substitution in the perovskite LaBiMn2O6+δ

The substitution of cobalt and nickel for manganese in the perovskite manganate LaBiMn2O6+δ has been realized, leading to the perovskites LaBiMn2−MxO6+δ, with M = Co, Ni and 0 ≤ x ≤ 2/3. In contrast to the literature these phases are found to be orthorhombic with Pnma symmetry. More importantly, it is shown that ferromagnetism is enhanced, TC being increased from 80 K for the parent compound (x = 0) to 97 K for the Ni phase, and to 130 K for the Co phase. Moreover, strong competition between ferromagnetism and a glassy-ferromagnetic state in the case of nickel or spin-glass behaviour in the case of cobalt is observed. These phenomena are interpreted in the frame of a phase separation scenario, where the ferromagnetic Mn4+/Ni2+ and Mn4+/Co2+ interactions reinforce the Mn3+/Mn4+ interactions. These compounds are found to be insulating with a relatively large positive value of thermoelectric power.

Resistance anomaly and structural disorder in NiSi 2 under high pressure

Results of X-ray diffraction, electrical resistance, thermoelectric power measurements and electronic band structure calculations on NiSi 2 under high pressure are reported. The thermoelectric power (TEP) changes sign near 0.5 GPa (from +30 to −20 μV/K). As the pressure is increased, the value of TEP increases further in magnitude and near 7 GPa it becomes −50 μV/K. The pressure vs. resistance curve measured up to 30 GPa using diamond anvil (DAC)-based technique exhibits a broad hump near 12 GPa and exhibits hysteresis on pressure release. The ADXRD patterns up to 42 GPa show a gradual irreversible loss of long-range order in NiSi 2 with the diffraction lines progressively broadening under pressure. The FWHM of the diffraction lines show a rapid increase in the half-widths close to 0.5 GPa and also near 12 GPa. The computed band structure at a compression (without any disorder) corresponding to 12 GPa, exhibits an electronic topological transition (ETT). The rapid increase in disorder above 12 GPa implies that the ETT may be facilitating the structural disorder. It is suggested that the pressure drives the material through a region of entropic and energetic barriers and induces disorder in the material.

Automated portable high-pressure setup for study of phase transitions in solids

Abstract The automated portable high-pressure setup developed for combined measurements of several properties of solids at ultrahigh pressure is discussed. The setup produces a high quasi-hydrostatic pressure P (0–30 GPa) and permits to perform electrical, thermal and dilatometric measurements simultaneously. The setup registers changes both in electron and crystal structures of sample under nearly continuous variation of pressure. The results obtained demonstrate the advantages of the setup in studying both phase transformations and multiphase states in solids. The novel features of phase transitions in Si and ZnTe have been found. For silicon and zinc telluride, both the signs and values of thermoelectric power S (Seebeck coefficient) of the several high-pressure phases have been established in the pressure range of 0–20 GPa. In ZnTe, a novel transient high-pressure phase has been observed of electron type of conductivity between the high-pressure trigonal (cinnabar) and orthorhombic ( Cmcm ) ones. Multiphase states of the semiconductors in vicinity of the structural transitions were analysed in the terms of the multiphase model of “orientated inclusions” permitting to vary configuration and concentration of phase inclusions.

CVD-diamond-based thermocouple for high sensitive temperature measurements

The possibility to realize a high sensitive thermocouple by means of boron doped chemical vapour deposition (CVD) diamond was investigated. The thermoelectric power of p-type diamond, grown by plasma enhanced CVD was studied for films of electrical resistivity in the 0.2–40 Ω cm range in order to asses the dependence of thermocouple sensitivity on the doping level. The p-type diamond films were prepared by CH3OH + B2O3 vapour addition to a 1% CH4–H2 gas mixture during the growth. The conductive films were then tested tracing the I–V characteristic in order to study the conduction properties of the films. An appropriate experimental setup was built to evaluate the thermoelectric properties of the grown samples for different temperatures imposed between two ends of the samples. Firstly, the output voltage was measured maintaining a reference temperature of 273 K at one end and varying the second temperature between 275.5 and 360.5 K. A constant value of the temperature drop of 5 K was then used for an accurate evaluation of the thermoelectric properties of the diamond films for different value of the average temperature. The measurements provided values of thermoelectric power in the range 0.3–0.6 mV/K while conductivity increases. These values showed different decreasing behaviour with increasing temperature for different resistivity of the sample. In particular, more relevant changes in thermoelectric power were measured for high resistive samples.

Thermoelectric Properties of CaxCo2O4 Aligned Crystals

Agglomerates of aligned crystals of CaxCo2O4 with a layer of CoO2 were grown using a chloride flux technique, and their thermoelectric properties in air were determined. The agglomerates take the form of a very thin flakelike cluster of crystals with a typical size of almost 3 × 2 × 0.07 mm. The values of thermoelectric power along the ab-plane are larger than 200 μV K−1 at temperatures above 873 K and reach almost 300 μV K−1 at 973 K. The temperature dependence of the electrical resistivity along the ab-plane shows bends around 450 and 825 K, and the ln ρab−T−1 curve followed an Arrhenius-type behavior below 450 K. Temperature dependence of thermal conductivity indicated that stacking faults along the c axis induce phonon scattering like that in a misfit-layered structure. The effect of the CoO2 layer on thermoelectric performance is discussed in comparison with related compounds.

Structural, optical and electrical properties of chemically sprayed CdO thin films

The cadmium oxide (CdO) thin films have been deposited onto amorphous and fluorine doped tin oxide (FTO) glass substrates using spray pyrolysis technique. The aqueous solution containing precursor of Cd has been used to obtain good quality deposits at optimized preparative parameters. The films have been characterized by techniques such as X-ray diffraction (XRD), optical absorption, electrical resistivity and thermoelectric power (TEP) measurements. The XRD study reveals that the films are polycrystalline with cubic structure. Optical absorption studies reveal that the value of absorption coefficient is in the order of 10 4 cm −1, indicating direct band to band transition with band gap energy 2.26 eV, close to its value of intrinsic band gap energy. The electrical characterization shows that the electrical resistivity ( ρ) is of the order 10 −3 Ω cm and it decreases with increase in temperature indicate the samples are semiconducting in nature. The value of activation energy is found to be 0.077 eV. TEP measurement shows the thermoelectric voltage for CdO films is positive towards the hot end, indicating n-type behavior of sample. The value of TEP increases with increase in temperature. The Hall effect measurement study reveals that the carrier concentration ( n), Hall coefficients ( R H) and carrier mobility ( μ H) are of the order of 10 23 cm −3 , 10 −8 cm 3/C and 10 −4 cm −2/V s, respectively.

Resistivity and thermoelectric power of NaxCoO2 (x=1.0, 0.7, and 0.6) system

Results of thermoelectric power (S) and electrical resistivity (ρ) measurements are reported on NaxCoO2 compounds with x=1.0, 0.7, and 0.6. These are single-phase compounds crystallizing in the hexagonal structure (space-group P63∕mmc) at room temperature. Thermoelectric power values at 300K (S300K) are ≅80, 39, and 37μV∕K for the x=1.0, 0.7, and 0.6 samples, respectively. The samples with x=0.7 and 1.0 are metallic down to 5K, while the x=0.6 sample is semiconducting. The value of ρ300K for the x=1.0 sample is ∼0.895mΩcm and the power factor (S2∕ρ) is ∼7.04×10−3W∕mK2 which qualifies it as a good thermoelectric material. In the x=1.0 sample, S(T) is positive throughout the 300–5-K temperature range and decreases monotonically to zero as temperature T→0. In contrast, S(T) of the x=0.7 and 0.6 samples changes sign and shows negative values between 90 and 16K before approaching zero as T→0. The anomalous S(T) behavior of the x=0.6 and 0.7 samples, which are coincidentally the precursor materials to the reported superconductivity in this class of materials, indicates a dramatic change in the electronic structure of these compounds on lowering the Na content.

Structural, electrical and optical properties of TiO 2 doped WO 3 thin films

TiO 2 doped WO 3 thin films were deposited onto glass substrates and fluorine doped tin oxide (FTO) coated conducting glass substrates, maintained at 500 °C by pyrolytic decomposition of adequate precursor solution. Equimolar ammonium tungstate ((NH 4) 2WO 4) and titanyl acetyl acetonate (TiAcAc) solutions were mixed together at pH 9 in volume proportions and used as a precursor solution for the deposition of TiO 2 doped WO 3 thin films. Doping concentrations were varied between 4 and 38%. The effect of TiO 2 doping concentration on structural, electrical and optical properties of TiO 2 doped WO 3 thin films were studied. Values of room temperature electrical resistivity, thermoelectric power and band gap energy ( E g) were estimated. The films with 38% TiO 2 doping in WO 3 exhibited lowest resistivity, n-type electrical conductivity and improved electrochromic performance among all the samples. The values of thermoelectric power (TEP) were in the range of 23–56 μV/K and the direct band gap energy varied between 2.72 and 2.86 eV.

Valence State of Ru at the Mn Sites in Pr0.5Sr0.5MnO3: Thermoelectric Power and X-Ray Absorption Near-Edge Structure Spectroscopy

The Ru valence at the Mn sites in Pr 0.5 Sr 0.5 MnO 3 has been studied using thermoelectric power (TEP) and Ru K- and L III -edge X-ray absorption near edge structure (XANES) spectroscopy. In comparison with TEP value at 400 K for Pr 1-x Sr x MnO 3 and Pr 0.5 Sr 0.5 Mn 1-x Ru x O 3 , it was found that the Mn-site doped Ru is in the mixed valence state between 4+ and 5+. Ru K-and L III -edge XANES study of Pr 0.5 Sr 0.5 Mn 1-x Ru x O 3 (x = 0.04 and 0.1) also confirmed the mixed valence state of Ru from the fact that XANES spectrum of the Mn-site doped Ru is in between those of tetravalent and pentavalent ruthenates. These results indicate that the change of carrier density with Ru doping is not sufficient to understand the drastic enhancement of ferromagnetism observed in Ru-doped Pr 0.5 Sr 0.5 MnO 3 . The prime role of Ru is discussed in terms of Mn valence change and magnetic interactions between Mn and doped Ru ions.

Modulation of thermoelectric power of individual carbon nanotubes.

Thermoelectric power (TEP) of individual single walled carbon nanotubes (SWNTs) has been measured at mesoscopic scales using a microfabricated heater and thermometers. Gate electric field dependent TEP modulation has been observed. The measured TEP of SWNTs is well correlated to the electrical conductance across the SWNT according to the Mott formula. Strong modulations of TEP were observed in the single-electron conduction limit. In addition, semiconducting SWNTs exhibit large values of TEP due to the Schottky barriers at SWNT-metal junctions.

Proposition of Two Parameters for a Good Characterisation of the Austenitising Condition of Microalloyed Steels

The characterisation of the compositional condition (particle dissolution/precipitation level),and the microstructural condition (austenite grain size and deformation state) are of vital importance for the subsequent phase transformations which occur during continuous cooling after thermomechanical processes of microalloyed steels. Thermoelectric power (TEP) measurement has been revealed as powerful method to analyse dissolution/precipitation processes together with recovery and recrystallisation phenomena. In this sense, this paper deals with characterising the austenitising condition by two fundamental parameters such as the prior austenite grain size (PAGS) and TEP. Likewise, to validate this hypothesis, the austenitising condition after a forging process in a medium carbon microalloyed steel has been simulated by a simple heat treatment which reproduce the same PAGS and TEP values. A comparison between the continuous cooling transformations occurred from both conditions, allow us to confirm that these parameters well characterise the autenite state of a microalloyed steel.

Thermoelectric power studies of (Nd–Eu–Gd)Ba–Cu–O superconductors

Thermoelectric power measurements of a series of (Nd–Eu–Gd)Ba–Cu–O(1 2 3) + x% (Nd–Eu–Gd)Ba–Cu–O(2 1 1), where x ranges from 0 to 50, high temperature superconducting materials prepared by the oxygen controlled melt growth (OCMG) process, were carried out over a temperature range 80–225 K. It has been observed that thermoelectric power values are very low but positive and are found to increase almost linearly with decreasing temperature, reaching a maximum value at temperatures close to the superconducting transition temperatures of the materials. On further cooling the materials, the thermopower is found to fall suddenly to zero at their respective T c values. It has been reported that a number of physical properties and their variation with temperature especially in the normal temperature region (above T c) influence the behavior of HTSC materials in their superconducting state and as such an attempt has been made to understand the variation of thermopower (in the normal temperature region) with temperature. The experimental thermopower data has been fitted to different theoretical models and it has been observed that the narrow band phenomenological model alone is in a position to explain the variation of thermopower with temperature especially in the normal temperature region. The narrow band model has also been used to understand how the band spectrum changes with varying dopent’s concentration. The model has also been used to explain the variation of T c with increasing concentration of NEG-211.

Thermoelectric properties of electrodeposited CuNi alloys on Si

Thin films with the composition of the constantan alloy (a solid solution with 35 to 50 wt. % of Ni in Cu) have a high-thermoelectric power, which allows the fabrication of very sensitive heat-flux sensors based on planar technology. In this article, the thermoelectric properties of CuxNi100−x thin films electrodeposited on silicon were studied as a function of the composition, temperature, and thickness. The electrodeposition of thin layers on silicon is an important step for the integration of thermal sensors with semiconductor technology. The CuxNi100−x alloys were electrodeposited potentiostatically at room temperature, from a citrate electrolyte containing both copper and nickel sulfates. The layer composition was controlled by the applied potential in the range from pure copper (at −0.4 V/SCE) up to a solid solution of about 25 wt. % Cu in Ni (at −1.2 V/SCE). Extremely high values of thermoelectric power were measured for very thin layers of Cu40Ni60 on Si, showing a strong influence of the substrate. By considering the system as a thermoelectric bilayer and extracting the contribution of the semiconductor, thermopower values for the Cu40Ni60 alloys comparable to the expected ones for constantan wires were obtained.

Electronic transport properties in the liquid 3d transitionmetal series

The resistivities and thermopowers of the liquid 3d transition-metal series have been investigated within Ziman's formula based on a spin-polarized approach. This implies the existence of atomic magnetic moments and the use of a two-band conduction model for the resistivity. The calculated resistivities are the lowest obtained until now within the Ziman scheme and better agreement with experiment has been found than with the classical non-polarized calculations. In spite of the improvement with respect to the non-magnetic results, the thermoelectric power values do not attain the degree of agreement observed for the resistivities.

Electronic transport properties in the liquid 3d transition-metal series

Abstract The resistivities and thermopowers of the liquid 3d transition-metal series have been investigated within Ziman's formula based on a spin-polarized approach. This implies the existence of atomic magnetic moments and the use of a two-band conduction model for the resistivity. The calculated resistivities are the lowest obtained until now within the Ziman scheme and better agreement with experiment has been found than with the classical non-polarized calculations. In spite of the improvement with respect to the non-magnetic results, the thermoelectric power values do not attain the degree of agreement observed for the resistivities.

Mechanism of the change in thermoelectric power of cast duplex stainless steel due to thermal aging

It has been shown that thermoelectric power (TEP) measurement is an effective method for evaluating thermal aging of cast duplex stainless steel. However the mechanism underlying the relationship between thermal aging and TEP has not yet been thoroughly explained. In this study, the relationship between TEP and the microstructural changes caused by thermal aging was investigated to help clarify the mechanism. It is well known that phase separation in the ferrite phase proceeds gradually due to thermal aging and that Cr-rich phases are created in the ferrite phase. Therefore, close attention was paid to the effect of Cr concentration on the TEP. Fe–Cr–Ni alloys with different Cr concentrations in the range from 5 to 75% were prepared and the TEP measured for each. As a result, the TEP decreased with a Cr concentration between 5 and 65% but increased at 65% or higher. The Cr concentration distribution in ferrite phases of thermal aged cast duplex stainless steel was measured and its data were combined with the TEP values of the alloys. We tried to predict changes in the TEP of cast duplex stainless steel due to thermal aging, and eventually confirmed the tendency that the composed TEP increased with thermal aging. Considering the fact that TEP is largely related to the electron density of states at the Fermi level, we measured the electron density of states by means of XPS (X-ray photoelectron spectroscopy), and found that changes in the structure of the spectrum due to thermal aging had the same tendency as the experimentally measured TEP. Consequently, we noted the mechanism that the TEP of cast duplex stainless steel increased with thermal aging and confirmed the effect of the electron density of states on TEP.

Electromagnetic and Thermoelectric Characteristics of NaxCoO2 of Precisely Controlled Na Nonstoichiometry

ABSTRACTElectromagnetic and thermoelectric characteristics of a sodium-cobalt oxide, NaxCoO2, were investigated. A precise control of Na nonstoichiometry was successfully facilitated by an unusual synthesis method named “rapid heat-up” technique. With increasing x, the absolute value of resistivity (ρ) monotonically decreased, while the value of thermoelectric power (S) increased, giving rise to a drastic enhancement in the thermoelectric power factor, i.e. S2 / ρ. Simultaneously enhanced thermoelectric power and reduced resistivity of the present compound are difficult to be understood within the framework of a conventional band picture. Moreover, for samples with the solubility limit Na content, i.e. x = 0.75, we discovered an unconventional electronic transition at Tm = 22 K, which may be induced by the strong-correlation effect of 3d electrons.

Elaboration of Bi 2Se 3 by metalorganic chemical vapour deposition

For the first time, Bi 2Se 3 thin films were elaborated by metalorganic chemical vapour deposition (MOCVD) using trimethylbismuth (TMBi) and diethylselenium (DESe) as metalorganic sources. The MOCVD elaboration of Bi 2Se 3 was carried out in a horizontal reactor for a substrate temperature ( T g) varying from 450°C to 500°C, a total hydrogen flow rate D T=3 l min −1, R VI/V ratio >14 and TMBi partial pressure lower than 1.10 −4 atm. By X-ray diffraction and SEM observation, we noticed the polycrystalline structure of the layers typical preferential c-orientation and confirm the hexagonal structure. The microprobe data indicate that the best stoichiometry of Bi 2Se 3 was achieved. These films always displayed n-type conduction, and the maximum value of thermoelectric power α was found to be close to −120 μV/K.

Thermoelectric power of delafossite-type metallic oxide PdCoO 2

Thermoelectric power of metallic cobalt oxide PdCoO 2, of which Co valence is 3+ in the low spin state and the density of state at the Fermi level is dominated by Pd 4d electrons, has been clarified between 100 and 400 K. It is found that the temperature dependence of thermoelectric power shows a gradual change. The values of thermoelectric power in the measured temperature range are 2–4 μV/K. These values comparable to normal conventional metals may be attributable to the Fermi level just at the bottom of the deep cave of the density of state.