High Temperature Electrical Conductivity Research Articles

High temperature electrical conductivity of epitaxial Gd-doped CeO 2 thin films

Single-crystalline epitaxial Gd-doped CeO 2 (GCO, Gd 0.2Ce 0.8O 1.9) thin films were grown on (001) NdGaO 3 (NGO) and LaAlO 3 (LAO) substrates through pulsed-laser deposition. The electrical conductivity was investigated by AC impedance spectroscopy. The temperature dependence of the electrical conductivity gives an activation energy of 0.74 eV for GCO/NGO and a conductivity which is very close to that of the bulk polycrystalline material. For GCO films deposited on LAO, the conductivity of the substrate makes a significant contribution to the total measured conductivity.

High temperature electrical conductivity in ZnS:Al and in CdSe:Al

High temperature electrical conductivity (HTEC) as a function of temperature and component vapour pressure has been investigated in ZnS:Al and in CdSe:Al single crystals using a two zone resistance furnace and a vacuum sealed quartz ampoule with four tungsten or graphite electrodes. The activation energies of HTEC isobars and the slope values of HTEC isotherms in the large temperature region (500–1200 °C) and in a large component vapour pressure range were determined. The region of compensation of electrons with onefold ionized substitutional Al exists up to change with the region of self-compensation of native defects at high Cd vapour pressures in HTEC isotherms of CdSe:Al. These two electroneutrality conditions were never achieved in ZnS:Al crystals under investigation. The common feature for ZnS:Al isotherms at zinc vapour pressure and for CdSe:Al isotherms at selenium vapour pressure is the existence of self-compensation of Al defects. The equilibrium constant of the association of aluminium with cadmium vacancies in CdSe:Al was determined. The increasing role of holes appears similarly at low Zn vapour pressure values in ZnS:Al and at high selenium vapour pressure values in CdSe:Al.

High-temperature oxidation resistance and surface electrical conductivity of stainless steels with filtered arc Cr–Al–N multilayer and/or superlattice coatings

The requirements of low cost and high-temperature corrosion resistance for bipolar interconnect plates in solid oxide fuel cell (SOFC) stacks has directed attention to the use of metallic alloys with oxidation-resistant coatings. Candidate coatings must exhibit chemical and thermal-mechanical stability and high electrical conductivity during long-term (>40,000 h) exposure to SOFC operating conditions. The high-temperature oxidation resistance and surface electrical conductivity of 304, 440A and Crofer-22 APU steel coupons, with and without multilayer and/or superlattice coatings from a Cr–Al–N system, were investigated as a function of exposure in an oxidizing atmosphere at high temperatures. The coatings were deposited using large area filtered arc deposition (LAFAD) technology [V.I. Gorokhovsky, R. Bhattacharya, D.G. Bhat, Surf. Coat. Technol. 140, (2001), 82-92] and subsequently annealed in air at 800 °C for varying times. Area-specific resistance (ASR) was measured in air as a function of time and temperature using a four-point technique with Pt paste for electrical contact between facing oxidized coupon surfaces. The surface composition, structure and morphology of the sample coupons were characterized using RBS, nuclear reaction analysis, XPS, SEM and AFM techniques. The structure of the CrN/(CrN/AlN) multilayered superlattice coatings was characterized by transmission electron microscopy (TEM). By altering the architecture of the coating layers, both surface electrical conductivity and oxidation resistance [R.J. Smith, C. Tripp, A. Knospe, C.V. Ramana, A. Kayani, Vladimir Gorokhovsky, V. Shutthanandan, D.S. Gelles, J. Mater. Eng., (2004), in press] improved significantly for some of the coated samples tested up to ∼100 h. An order of magnitude decrease in the ASR parabolic growth rate constant was observed in two Crofer 22APU coupons coated with ∼1.5 μm CrN/AlN superlattice coatings with different nanometrical bilayer periods.

Phase equilibrium and electrical conductivity of SrCo 0.8Fe 0.2O 3− δ

The phase diagram of the SrCo 0.8Fe 0.2O 3− δ compound has been determined at high temperatures (823⩽ T⩽1223 K) and in the oxygen partial pressure range (10 −5⩽ pO 2⩽1 atm) by thermogravimetric measurements of the equilibrium pO 2, high temperature X-ray diffraction and electrical conductivity measurements. The cubic perovskite phase SrCo 0.8Fe 0.2O 3− δ is stable in a broad range of oxygen content, while the orthorhombic brownmillerite phase SrCo 0.8Fe 0.2O 2.5 stabilizes within a small range around 3− δ=2.5 at temperatures below 1073 K. Equilibrium pO 2 measurements under isothermal conditions show chemical hysteresis at the perovskite to brownmillerite transition. The hysteresis loop decreases its amplitude in pO 2 with decreasing temperature. This behavior is discussed considering the evolution from coherent intergrowth interfaces with elastic strain energy to incoherent interfaces without elastic strain energy as T decreases. The thermodynamic quantities h O 2 oxide and s O 2 oxide for the perovskite phase decrease when increasing the oxygen defects concentration. The electrical conductivity ( σ) of the cubic phase exhibits a thermally activated behavior at high temperature. The variation of σ with the oxygen content is non-linear and the activation energy varies from 0.4 to 0.28 eV as the oxygen content increases from 2.4 to 2.6. These results are interpreted in the frame of the small polaron model.

N-type diamond with high room temperature electrical conductivity by deuteration of boron doped diamond layers

The conversion of the conductivity of B-doped homoepitaxially grown diamond layers from p- to n-type upon deuteration and its reconversion to p-type following annealing is extensively studied. Several B doped samples have been converted to n-type when exposed to a deuterium plasma at approximately 500 °C. The n-type features are related to D uptake of the samples. The donors, thus formed, have an ionization energy of approximately 0.34 eV below the conduction band edge and a high RT (room temperature) mobility (up to 430 cm 2/V s), as determined by Hall effect measurements as a function of temperature. In the B-doped layers that do convert to n-type upon deuteration, the D to B concentration ratio is found to be in the range of one to two. Higher D uptake (D to B ratio ≫2) must be connected to B-doped layers containing a large amount of growth defects, which trap the D. The exact nature of the donor is yet unknown. However, the reversibility of the effect and its relationship with the D concentrations suggest that deuterium is involved in the formation of some complex with B or with some other defects.

High-temperature electrical conductivity of codoped ZnS and CdSe

Systems of binary compounds containing two types of foreign atoms are complicated. In this work we are interested in high-temperature electrical conductivity (HTEC) behaviour of ZnS and CdSe crystals codoped with two impurities (Cu and Cl or In). We tried to find common behaviour and common features in HTEC isobars of these systems. We were also interested in the temperature regions of forming associates between these dopants. At a temperature T=500°C, thermodynamic equilibrium between defects in ZnS:Cu:Cl is achieved. For HTEC isobars, three temperature regions with different dominating defects for ZnS:Cu:Cl, CdSe:Cu:In and CdSe:Cu:Cl can be observed. The low-temperature region characterizes the reactions between associates of dopants and single defects with an activation energy ΔE=1.5eV for ZnS:Cu:Cl and ΔE=1.05eV for CdSe:Cu:In. In the HTEC isobar region, the donor action of both dopants is observed with close to zero or even negative activation energy of HTEC isobars.

Thermoelectric properties of photo- and thermal CVD boron and boron phosphide films

The electrical properties of n-BP films newly prepared by thermal CVD in the B 2H 6–PH 3–H 2 system were improved by a deuterium lamp excitation. High-temperature electrical conductivity and thermoelectric power of amorphous boron and polycrystalline boron phosphide films grown on silica glass were measured to evaluate the thermoelectric figure-of-merit ( Z). In particular, the Z-value for photo-thermal BP films was higher (10 −4/K) than that of boron films, indicating that they are promising for high-temperature thermoelectric materials.

High temperature electrical conductivity in donor‐doped II–VI compounds

Dopants of group III and group VII elements act as donors (D) in II–VI materials. Here we report the results of our investigations of donors in some II–VI donor-doped compounds (CdSe:Al, CdSe:Ga, CdSe:In, ZnSe:In, ZnS:Al, ZnS:Ga and ZnS:In) using the method of high temperature electrical conductivity (HTEC). Our interests lie in the activation energies determined from HTEC isobars and slopes of HTEC isotherms in the temperature region from 600 to 1200 °C where the donor action of dopant appears. We assumed the existence of near-zero activation energies of free carrier concentrations determined from HTEC isobars Arrhenius plots for the donor action regions. In some cases, this is realized as the result of formation of compensation through electroneutrality condition n = [D•]. In this region, the best results to obtain high n-type conductivity in frozen-in crystals can be achieved. Unexpected negative HTEC activation energies with non-zero slopes of HTEC isotherms were obtained for the systems ZnSe:In, ZnS:Al and ZnS:Ga in the donor action region. The appearance of negative activation energies of HTEC and the models for high temperature defect equilibrium of these systems are analyzed.

Nickel/Ceramic Composites for Current Collection Terminals

Ni/spinel composite materials have been produced by electro-deposition of Ni from a suitable aqueous electrolyte into the pores of a porous spinel pre-form. The Ni grows uni-directionally to substantially fill the pores in the pre-form and produces a bi-continuous composite with interpenetrating ceramic and metallic phases and no open porosity. The thermal expansion coefficient of the composite is close to that of the ceramic and the electrical conductivity is controlled by the metallic phase. The high temperature electrical conductivity shows an initial degradation with time, but soon becomes stable in isothermal exposure and on thermal cycling.

High-temperature electrical conduction of plasma-treated FEP films

Ar plasma treatment effect on the electrical conduction of FEP was examined at high temperatures. Conduction current in the treated FEP was much larger than that in the untreated FEP at high temperatures. The authors proposed a model to explain the observed high conductivity of the plasma-treated FER In this model, charge carrier generation centers are produced by the plasma treatment and these centers are thermally dissociated to positive mobile ions with an activation energy of 0.7 eV. These positive charge carriers are responsible for the high conductivity in the plasma-treated FEP at high temperatures.

High Temperature Electrical Conductivity in Cu-Doped ZnS

High temperature electrical conductivity (HTEC) was measured and high temperature defect equilibrium (HTDE) was analysed in ZnS:Cu to study the conditions for making p-type material. In the high zinc vapour pressure (p Zn ) region, the Cu solubility limit determines the composition of phases coexisting with ZnS crystal. The negative slope of the HTEC isotherm in this region is caused by the chemical outdiffusion of Cu. The high p Zn and the high sulfur vapour pressure (p S2 ) regions are characterized by dominant donors. At high p S2 antistructural disorder and its associates are introduced into the HTDE model. At low p Zn or p S2 values, the reverse sign of the isotherm slope can be explained by two-carrier conduction, and in this region p-type HTEC may be formed.

Transport Properties and Viscosity of Liquid CdTe Doped with In, Ge, and Sn

Data are presented on the high-temperature electrical conductivity, thermoelectric power (up to 1825 K), and viscosity (up to 1400 K) of undoped and doped (In, Ge, and Sn) CdTe melts. All of the materials were found to retain semiconducting properties upon melting, with a gradual increase in the contribution of metallic bonding, especially pronounced for the CdTe + 2 mol % Sn melt. The results are interpreted in terms of the double-structured melt model, which considers the coexistence of densely packed metallic regions and crystal-like CdTe clusters. The transition to metallic behavior of conductivity is accounted for by a gradual increase in the volume fraction of the densely packed, metallic phase. The doping effects on the conductivity and thermoelectric power of liquid CdTe are interpreted in terms of s–p hybridization.

High temperature electrical conductivity of undoped ZnS

High temperature defect equilibrium (HTDE) in ZnS was analysed to study the conditions of making p-type material. High temperature electrical conductivity (HTEC) under defined component vapour pressure was investigated in undoped ZnS in the temperature range from 600°C to 1140°C. HTEC measurements were carried out using a two-zone resistance furnace and a vacuum sealed quartz ampoule with four sealed tungsten or graphite electrodes. All ZnS HTEC isotherms can be divided into three regions. The high zinc vapour pressure ( p Zn) and the high sulphur vapour pressure ( p S 2 ) regions are characterized by dominant native donors. In the ZnS HTDE model, antistructure disorder is dominating at high p S 2 values, similar to CdS and CdTe HTDE models. Changes occur at low p Zn or at low p S 2 values. The increase of HTEC at low p Zn or at low p S 2 values is explained by the existence of the p-type area in this region. This p-type area appears in the total range of temperature investigated.

High temperature electrical conductivity in Ga and In solubility limit region in ZnS

High temperature electrical conductivity (HTEC) and solubility measurements of Ga and In were performed in ZnS. To investigate Ga and In solubility in ZnS under the controlled vapour pressure of Zn, a special construction of quartz ampoules were used. Ga and In solubility isotherms in ZnS are given. At high temperatures and at low concentrations, Ga and In act as a single donor. At high zinc pressures, outdiffusion of gallium and indium occurs. The change of the solubility of Ga or In is reflected in ZnS HTEC isotherms. If doped at the same doping conditions, ZnS:Ga and ZnS:In had the same HTEC absolute values despite the differences in Ga and In solubilities.

High temperature electrical conductivity in the Ag solubility limit region in ZnS and in CdSe

To determine analytically Ag solubility in ZnS or in CdSe monocrystals, Ag was diffused into the monocrystal at a fixed temperature T and under the defined component pressure p. In the region of sulfide phases, in ZnS the Ag solubility is complicated. It depends on activities of Ag2S, Ag and incorporation mechanisms. In the equilibrium of ZnS with metallic alloys, an increase of pZn causes the total outdiffusion of Ag. The high temperature electrical conductivity of silver doped CdSe single crystal was measured over the range from 500°C to 900°C. In the range, where conductivity decreases with the increase of pCd, the results may be explained by the decrease of solubility of Ag in CdSe. These results were in agreement with the data obtained by the analytical method.

Electrical Properties of Cerium‐Doped BaTiO3

The high‐temperature equilibrium electrical conductivity of Ce‐doped BaTiO3 was studied in terms of oxygen partial pressure, P(O2), and composition. In (Ba1−xCex)TiO3, the conductivity follows the −1/4 power dependence of P(O2) at high oxygen activities, which supports the view that metal vacancies are created for the compensation of Ce donors, and is nearly independent of P(O2) where electron compensation prevails at low P(O2). When Ce is substituted for the normal Ti sites, there is no significant change in conductivity behavior compared with undoped BaTiO3, indicating the substitution of Ce as Ce4+ for Ti4+ in Ba(Ti1−yCey)O3. The Curie temperature (Tc) was systematically lowered when Ce3+ was incorporated into Ba2+ sites, whereas the substitution of Ce4+ for Ti4+ sites resulted in no change in this parameter.

Conductivity and stability of SrVO 3 and mixed perovskites at low oxygen partial pressures

The electrical conductivity and chemical stability of ABO 3 (where the A-site contains La or Sr, and the B-site contains Ni, V, Ti, Nb, Mo or W and combinations thereof) have been studied in air and reducing atmospheres. Neither the rare-earth vanadates, such as LaVO 3, nor the alkaline-earth vanadates, with the exception of SrVO 3, are good electronic conductors. SrVO 3 shows excellent electronic conductivity of 1000 S/cm at 800°C and an oxygen partial pressure of 10 −20 atm, but is unstable under more oxidizing conditions. However, strontium-doped lanthanum vanadates are stable over a range of oxygen partial pressures of 10 −14–10 −20 atm. La 0.7Sr 0.3VO 3− δ has a conductivity of 50 S/cm at 800°C and P O 2 =10 −20 atm. However, none of the compounds studied have both adequate high-temperature electrical conductivity in reducing atmospheres and stability to cycling between oxidizing/reducing conditions, to serve as anodes in solid oxide fuel cells.

Quadruple and Quintuple Perovskite-Layered Cuprates (NdDyBa2−xSrxCu2+yTi2−yO11−δ and NdDyCaBa2−xSrxCu2+yTi3−yO14−δ): Their Defect Chemistry and Electrical Properties

The structure–property relationships of chemically substituted quadruple- and quintuple-layered cuprate perovskites, NdDyBa2−xSrxCu2+yTi2−yO11−δ and NdDyCaBa2−xSrxCu2+yTi3−yO14−δ, have been investigated with an emphasis on high-temperature electrical properties. Cu doping for Ti in both systems, especially the quadruple system, improves their electrical properties; however, substantial compensation by oxygen vacancies occurs. On the other hand, isovalent Sr substitution for Ba in these systems significantly reduces the ionic compensation, i.e., the [V··O] concentration, as evidenced by thermogravimetry and electrical measurements. Sr substitution not only reduces the Cu–O bond length in favor of hole formation but also introduces metallic behavior as evidenced by in situ high-temperature (800–400°C) electrical conductivity and Seebeck coefficient measurements plus low-temperature resistivity measurements. A master plot of high-temperature Seebeck coefficient vs hole content of known superconductors shows that the hole content necessary for superconductivity has been achieved in some of the doped quadruple systems, yet they fail to exhibit superconductivity owing to other structural limitations.

Thermoelectric Properties of Boron and Boron Phosphide Films

Amorphous boron and boron phosphide films were prepared on silica glass by a solid source molecular beam deposition (SSMBD) method and a low-pressure chemical vapor deposition method (LPCVD) by adapting gas source molecular beam deposition at a reactant pressure of 1.33 Pa. The films were characterized by X-ray diffraction, X-ray photon electron spectroscopy (XPS), and Hall effect measurements. The experimental results on film growth were correlated with the calculation by an ab initio molecular orbital method. The high-temperature electrical conductivity and thermoelectric power of these films were measured to evaluate the thermoelectric figure-of-merit (Z), which was determined by the electrical conductivity of the films. In particular, the Z value for SSMBD boron and LPCVD boron phosphide films was higher (10−5/K) than those of LPCVD boron films, indicating that they are promising high-temperature thermoelectric materials.

Solid-phase recrystallization of ZnS ceramics in phase transition region

ZnS monocrystals, grown by solid-phase recrystallization (SPR) during hot pressing in the phase transition (PT) region are shown to have been improved by the doping of In. SPR of undoped and Al- or Ga-doped ZnS has not been successful. X-ray Laue patterns of as-grown ZnS monocrystal showed cubic symmetry. Scanning electron microscope investigation of the boundary surface between the monocrystalline and the polycrystalline parts of the as-grown ZnS : In ceramic disk showed an abnormal growth of crystallites into a monocrystal. To explain the role of In, high-temperature electrical conductivity (HTEC) measurements were performed. The temperature region from 1000°C to 500°C on the HTEC isobar of ZnS : In monocrystal was “cleared” from step-like PT changes, obligatorily observed on isobars of undoped and Cu-doped ZnS isobars in this region.