Metallic Type Of Conductivity Research Articles

The microstructure of and charge transfer in thin films based on metal-polymer nanocomposites

Charge transfer in nanostructured metal-polymer composites was studied. The frequency dependences of film conductance and susceptance were obtained at various metal concentrations. The susceptance of samples above the percolation threshold was negligibly small, which corresponded to the purely metallic conductivity type. For samples below the percolation threshold, susceptance and conductance were comparable in magnitude, which was evidence of an important role played by susceptance mechanisms. At low frequencies, the samples behaved as quasi-linear RC circuits and both the active and reactive impedance components increased linearly as the frequency grew. At high frequencies, the dispersion of susceptance, which was inversely proportional to frequency, was observed. The conclusion was drawn that the hopping conductivity mechanism through polymeric matrix surface states prevailed in films below the percolation threshold. At high frequencies, when the applied voltage period was shorter than the characteristic time of surface state recharging, these states began to be eliminated from charge transfer processes. It was suggested that a decrease in the reactive impedance component with an increase in frequency might be the reason for the dispersion observed experimentally.

Oxygen nonstoichiometry and transport properties of strontium substituted lanthanum cobaltite

Oxygen nonstoichiometry, structure and transport properties of the two compositions (La 0.6Sr 0.4) 0.99CoO 3− δ (LSC40) and La 0.85Sr 0.15CoO 3− δ (LSC15) were measured. It was found that the oxygen nonstoichiometry as a function of the temperature and oxygen partial pressure could be described using the itinerant electron model. The electrical conductivity, σ, of the materials is high ( σ > 500 S cm − 1 ) in the measured temperature range (650–1000 °C) and oxygen partial pressure range (0.209–10 − 4 atm). At 900 °C the electrical conductivity is 1365 and 1491 S cm − 1 in air for LSC40 and LSC15, respectively. A linear correlation between the electrical conductivity and the oxygen vacancy concentration was found for both samples. The mobility of the electron-holes was inversely proportional with the absolute temperature indicating a metallic type conductivity for LSC40. Using electrical conductivity relaxation the chemical diffusion coefficient of oxygen was determined. It was found that accurate values of the chemical diffusion coefficient could only be obtained using a sample with a porous surface coating. The porous surface coating increased the surface exchange reaction thereby unmasking the chemical diffusion coefficient. The ionic conductivity deduced from electrical conductivity relaxation was determined to be 0.45 S cm − 1 and 0.01 S cm − 1 at 1000 and 650 °C, respectively. The activation energy for the ionic conductivity at a constant vacancy concentration ( δ = 0.125) was found to be 0.90 eV.

Influence of cobalt substitution on structure and electric conduction of CuCr 2Se 4

Seleno-spinels with the nominal composition Cu 1− x Co x Cr 2Se 4 (for x = 0.1–0.9) and CuCr 2− y Co y Se 4 (for y = 0.1–1.9) were prepared as polycrystalline samples using ceramic method. Change of crystal structure, from cubic to monoclinic symmetry, was observed in the first system with the increase in cobalt amount. In the second system monoclinic phase as well as single-phase CuCr 2− y Co y Se 4 compounds were not obtained. The X-ray analysis was used in order to make phase analysis of the obtained samples to determine their crystal structure and lattice parameters. The scanning electron microscopy test confirmed the chemical composition and shown the variation in local distribution of cations. The porosity of the samples was observed. Electric resistance of the compressed powdered samples was measured at frequency 100 Hz within temperature 100–500 K range. The samples showing the spinel structure, with low Co-substitution, x = 0.1–0.5, exhibit thermally activated conductance with activation energy 90–220 meV above room temperature. The increase in the Co-concentration, x = 0.6–1.0, lowers the activation energy to about 40 meV. These samples show the metallic-type conductivity within intermediate temperature range, 200 ≲ T ≲ 400 K. The small polaron-type conductance, with low value of the activation energy, E a ≲ 15 meV, was deduced below 300 K for the all Cu 1− x Co x Cr 2Se 4 ( x = 0.1–0.9) samples. It was found that the electric transport properties correspond both to the chemical composition and to the crystal structure.

The thermodynamic characteristics of point defects and the mechanism of charge transfer in lanthanum cobaltite doped with strontium and nickel

The specific conductivity and thermal electromotive force (EMF) coefficient of lanthanum cobaltite doped with strontium and nickel La0.9Sr0.1Co0.9Ni0.1O3−δ were measured over the temperature and pressure ranges 1023–1223 K and 10−6−1 atm. The oxide was found to exhibit metallic-type conductivity (800 Ω−1 cm−1 ≤ σ ≤ 1150 Ω−1 cm−1) and small positive thermal EMF coefficient values. A joint analysis of the thermodynamic characteristics of formation of point defects and external T and pO2 parameter dependences of conductivity and thermal EMF showed that charge was transferred in the oxide by small-radius polarons according to the hopping mechanism. The role of polarons was played by electrons MeCo′ and holes MeCo• localized on 3d transition metals. The isothermal dependences of the concentrations and mobilities of charge carriers on the degree of oxygen nonstoichiometry of the oxide studied were calculated.

Electrical Properties of Niobium-Doped Titanium Dioxide. 1. Defect Disorder

The present work reports the electrical conductivity and thermoelectric power for Nb-doped TiO(2) at elevated temperatures (1073-1298 K) in the gas phase of controlled oxygen activity, 10(-14) Pa < p(O(2)) < 75 kPa. It is shown that in reduced conditions the Nb-doped TiO(2) exhibits metallic-type conductivity. This finding paves the way for the development of high-performance photoelectrodes with substantially reduced internal energy losses during charge transport. The present work also determined the equilibrium constant for the formation of oxygen vacancies and titanium vacancies for Nb-doped TiO(2).

The microstructure of and charge transfer in thin films based on metal-polymer nanocomposites

Charge transfer in nanostructured metal-polymer composites was studied. The frequency dependences of film conductance and susceptance were obtained at various metal concentrations. The susceptance of samples above the percolation threshold was negligibly small, which corresponded to the purely metallic conductivity type. For samples below the percolation threshold, susceptance and conductance were comparable in magnitude, which was evidence of an important role played by susceptance mechanisms. At low frequencies, the samples behaved as quasi-linear RC circuits and both the active and reactive impedance components increased linearly as the frequency grew. At high frequencies, the dispersion of susceptance, which was inversely proportional to frequency, was observed. The conclusion was drawn that the hopping conductivity mechanism through polymeric matrix surface states prevailed in films below the percolation threshold. At high frequencies, when the applied voltage period was shorter than the characteristic time of surface state recharging, these states began to be eliminated from charge transfer processes. It was suggested that a decrease in the reactive impedance component with an increase in frequency might be the reason for the dispersion observed experimentally.

On the high conductivity of nonconjugated polymers

The mechanism of charge transfer in a metal-electroactive polymer-metal structure has been experimentally studied near the threshold of the uniaxial-pressure-induced transition into a high-conductivity state in the polymer. The dynamics of the I-V curve is investigated as a function of the applied pressure. The data obtained are analyzed in terms of the model of injection currents using the concepts of possible scanning of a quasi-Fermi level near an injection level. Our estimates suggest that a narrow band made of deep trap states located near the Fermi level forms in the polymer film in the pretransition pressure range. In the immediate vicinity of the transition range, a narrow band of coherent charge transfer appears from these states; this band can be responsible for the high metal-type conductivity of thin polymer films, which has been repeatedly observed by many researchers.

Optical and electrophysical properties of Ag–PEPC nanocomposites

Absorption and luminescence spectra as well as the temperature dependences of electrical conductivity of polyepoxypropylcarbazole (PEPC) with Ag nanoparticles are studied. Polyepoxypropylcarbazole results in an additional stabilization of Ag nanoparticles. They cause essential PEPC photoluminescence quenching. An additional maximum in the luminescence spectrum appeared to be decreasing in time, probably due to some photochemical reaction. Metallic type of conductivity appeared at intermediate temperatures even with rather low concentration of nanoparticles.

High pressure synthesis of EuGa2-xSi4+x[x = 1.3 (1)

The title compound was prepared using a pressure of 8 GPa and a temperature of 1000 °C followed by slow cooling under pressure. After releasing pressure the sample was examined by x-ray powder diffraction, metallographical and EDX analysis. In the sample after the preparation, the compound EuGa2–xSi4+x is found to coexist with EuGa4–xSix (EuGa4 type) and Si. EuGa2–xSi4+x is the second representative of the EuGa2Ge4 structure type [1] and crystallizes in space group Cmcm with a = 4.348(1) A, b = 10.457(1) A and c = 11.938(2) A (V= 542.8 A). Gallium and germanium build up a three-dimensional network of four-bonded atoms with europium located in large voids. By replacing germanium with silicon the volume decreases by about 12 %. This change in volume is pronouncedly anisotropic; the length of the a axis increases by 4.6 % whereas the b and c axis shorten by 7.2 % and 9.3 %, respectively. Due to a partial replacement of gallium by silicon, the electron balance can be written as Eu[Ga(3b)]0.7[Si(4b)]5.3·1.3e. Thus, we expect metal-type conductivity. Magnetic susceptibility, electrical resistance and thermoelectric properties of the silicon compound are currently under investigation.

Electronic Structure of the Ti1‐xScxNiSn and Zr1‐xScxNiSn Solid Solutions.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Electronic limitations of lithium diffusibility. From layered and spinel toward novel olivine type cathode materials

The ability and efficiency of lithium intercalation into transition metal compounds have been found to depend strongly on their electronic structure. This work is a brief review of physicochemical properties of intercalated transition metal compounds with layered, spinel or olivine type structure in order to correlate their microscopic electronic properties, i.e. the nature of electronic states with the efficiency of lithium intercalation process that is controlled by the chemical diffusion coefficient of lithium. The data concerning cell voltages and character of discharge curves for various materials are correlated with the nature of chemical bonding and electronic structure following it. The nature of the metallic type conductivity of doped phospho-olivine is discussed and some fundamental arguments against the bulk nature of the observed high electronic conductivity are presented.

Galvanomagnetic phenomena in layered organic conductors (Review)

The experimental research on galvanomagnetic phenomena in layered organic conductors at high magnetic fields is discussed in terms of the theoretical ideas about charge transfer phenomena in conductors with a metallic type of conductivity and a quasi-two-dimensional electron energy spectrum of arbitrary form. Attention is devoted mainly to the problem of recovering the dispersion relation of the conduction electrons in layered organic charge-transfer complexes from experimental studies of their magnetoresistance and quantum oscillation phenomena at low temperatures.

Electronic structure of the Ti 1− xSc xNiSn and Zr 1− xSc xNiSn solid solutions

The electronic properties of the Ti 1− x Sc x NiSn and Zr 1− x Sc x NiSn solid solutions were investigated using the Korringa–Kohn–Rostoker Green's function method within the coherent potential approximation. The calculations confirm the metallic type conductivity of the Sc content compounds as evidenced from electrical resistivity measurements and show that the main source of the electron conductivity originates from the d-electrons of Ti(Zr) and Sc.

Selective Deposition of Ruthenium Films by Digital CVD

A novel process for selective and conformal deposition of ruthenium thin films is demonstrated by a liquid‐source digital‐CVD technique using alternate exposures of Ru(THD)2COD and oxygen. The films are selectively deposited at 280–320 °C on the exposed area of hydroxyl‐terminated ALD‐HfO2 surfaces patterned with photoresist (Figure). In general, ruthenium films deposited are found to be dense and polycrystalline, have a typical resistivity of 20.6 μΩ cm, and exhibit a positive temperature coefficient of resistivity, indicating metallic‐type conduction through electrically continuous grains.

Field Electron Emission from Single‐Walled Carbon Nanotube Layers

Field emission characteristics of single‐walled carbon nanotube layers have been investigated at room and low temperatures. For these layers the emission current density of 10 mA/cm2 was obtained at the average field E av = 1.6–3.8 V/µm. Current–voltage characteristics in Fowler–Nordheim coordinates have a break at emission current about 10−8 A. Cooling of samples only insignificantly changed the form of current–voltage characteristics. This indicates, that investigated single‐walled nanotubes have the metal type conductivity.

Electronic, structural, and thermal properties of a nanocable consisting of carbon and BN nanotubes

The band structure and thermal behavior of a coaxial C/BN nanocable (5,5)C@(17,0)BN consisting of a carbon nanotube and a boron nitride nanotube have been studied using a tight-binding approximation based on density functional theory. The system is stable up to T∼3500–3700 K. As the temperature increases, deformations of the BN tube begin earlier than those of the carbon tube. The near-Fermi states of the nanocable are formed by the overlapping π-π* bands of the carbon tube, and the outer BN nanotube (the nanocable sheath) is an insulator with a bandgap of ∼4 eV. The electronic properties of the nanocable (the metallic-type conductivity of the C tube and the insulating character of the BN tube) are retained over the entire temperature interval.

Influence of Si(111)–Cr surface phases in the formation and conductivity of Fe and Yb monolayers at room temperature on Si(111)

Conductivity through the disordered iron and ytterbium two-dimensional layers during their formation on Si(111)-7×7, Si(111)7×7-Cr and Si(111)-(√3×√3)/30°-Cr surface phases was investigated. It was found in the case of Si(111)-7×7 surface that at submonolayer [1 monolayer (ML)=7.8×10 18 at./m 2 ] metal coverage iron and ytterbium atoms intermix with silicon surface atoms resulting in formation of silicide islands and silicide alloy layers which display the properties of a degenerated semiconductor. Deposition of small amounts (within one monolayer) of iron or ytterbium atoms onto the Si(111)-[(√3×√3)/30°]-Cr surface phase results in growth of iron or ytterbium metal islands without formation of silicides. This fact indicates that this surface phase represents a diffusion barrier which blocks intermixing. It has been shown that metallic-type conductivity through the nearly continuous layer occurs at the coverage of 8.5 ML for Fe and 6.0 ML for Yb. For deposition of Fe or Yb atoms onto the Si(111)7×7-Cr surface phase, the 3D island growth mode without metallic-type layer conductivity was found within a coverage range extending up to 20–30 monolayers.

Characterization of the Structural, Optical, and Dielectric Properties of Oxynitride Perovskites AMO2N (A = Ba, Sr, Ca; M = Ta, Nb)

The syntheses, crystal structures, electrical properties, and optical absorbance spectra of six perovskite oxynitrides, AMO2N (A = Ba, Sr, Ca; M = Ta, Nb) have been investigated. The average crystal structure of BaTaO2N is a cubic perovskite, with a Ta−O/N distance of 2.056 Å. SrTaO2N and CaTaO2N are distorted by octahedral tilting, showing noticeably smaller Ta−O/N distances of approximately 2.02 Å. Electron diffraction studies of BaTaO2N are consistent with the simple cubic perovskite crystal structure determined using X-ray powder diffraction methods. Each of the niobium oxynitrides is isostructural with its tantalum analogue, though the Nb−O/N distances are observed to be slightly longer. The optical band gaps are estimated from diffuse reflectance spectra as follows: BaTaO2N, 1.8 eV; SrTaO2N, 2.1 eV; CaTaO2N, 2.4 eV; BaNbO2N, 1.8 eV; SrNbO2N, 1.9 eV; CaNbO2N, 2.1 eV. Impedance spectroscopy was carried out on sintered pellets of the ATaO2N and BaNbO2N to investigate the dielectric and electrical transport properties. The BaNbO2N sample shows metallic-type conductivity apparently from a slight reduction that occurs during sintering. In contrast, the tantalum compounds are semiconductors/insulators with conductivities of ∼10-5 S/cm (A = Ba, Sr) and ∼10-8 S/cm (A = Ca). Interpretation of the impedance data for BaTaO2N and SrTaO2N reveals that these two compounds have unexpectedly high bulk dielectric constants, κ ≈ 4900 and 2900, respectively, at room temperature. The dielectric constants of both compounds are frequency dependent and show a relatively weak linear dependence upon temperature with no sign of a phase transition over the temperature range 300−180 K.

Magnetotransport studies of Ga(Mn,Fe)N bulk crystals

Magnetoresistance and Hall effect in (Ga,Mn)N, (Ga,Mn,Si)N, and (Ga,Fe)N bulk crystals has been studied in the temperature range from 4.2 K to 300 K. No anomalous Hall effect contribution to the Hall voltage vs. magnetic curve has been found. The negative magnetoresistance has been found only in samples with metallic type of conductivity and is interpreted as resulting from weak localization of current carriers. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Transport Properties and Electronic Structure of M2CuSb3 Compounds with M: Ti, Zr, Hf.

AbstractFor Abstract see ChemInform Abstract in Full Text.