Dynamic Electronic Structure in Transition Metal Oxides

The isomer shift of La1-xCaxFeO3 decreases gradually from the value for trivalent iron to that for tetravalent iron, as the Ca content x increases from 0 to 1. This indicates that iron in La1.x-CaxFeO3 has an intermediate valence state. The intermediate valence state of iron increases gradually from trivalence to tetravalence. This can be interpreted as being due to electron delocalization among iron ions on identical octahedral sites. The Mossbauer spectra at various temperatures of La0.5Ca0.5FeO3 and their theoretical treatment show that electron delocalization slows down with decreasing temperature.     

Very Low Temperature Electronic Transport in Al-Pd-Re Quasicrystalline Alloys

Electrical resistivities of two icosahedral (I) Al-Pd-Re alloys have been measured between room temperature and mK temperatures. One quasicrystalline (QC) polygrain Al-Pd-Re sample exhibited insulating behavior in its resistivities, increasing by a factor of r=R(4 K)/R(300 K)=7.76; its room temperature resistivity was 9,890 μΩ cm. A “phenomenological” expression fitted the conductivity data well between 300 K to 0.5 K. Below 0.4 K a crossover to an activated variable-range hopping law was observed. Low temperature magnetoresistance ratio data and fits using the wave function shrinkage theory are presented. A second QC Al-Pd-Re sample had a small resistance temperature ratio r=2.12. The room temperature resistivity was extremely large, ρ(300 K)≈40,980 μΩ cm. Its conductivity could be described well using a simple temperature power law between 300 K to 20 K. Below 20 K there was a crossover to a new behavior. Below 1 K, the conductivity could be fitted using a very weakly insulating power law where σ(T)≈11.37T ^0.032 in (Ω cm)⁻¹, suggesting that this sample is located just below the metal-insulator transition. The magnetoconductivity data could not be fitted successfully using the 3D weak localization (WL) theory and inserting into it physical and realistic fitting magnitudes for the inelastic magnetic field B _in.      

Synthesis and Properties of Layered Oxides LiCo_(1-x)Ni_xO_2

The mechanism of LiCo1-xNixO2(x=0, 0.5, 1) synthesis from carbonates or hydrates was studied by thermogravimetric analysis and X-ray diffraction to identify the medium and final products. The synthesis process from carbonates can be divided into two steps. Below 300℃cobalt carbonate and/or nickel carbonate decompose forming oxides. Over 300℃ Li2CO3 decomposes and reacts with Co and/or Ni oxides gradually, as a result Co++ and/or Ni++ areoxidized to Co+++ and/or Ni+++, finally LiCo1-xNixO2 forms. The proportion of cobalt tonickel in the starting mixture and atmosphere during synthesis affects the structure of products.LiCo1-xNixO2(x≤0.5) can be synthesized in air or oxygen and characterized by solid solutionof LiCoO2 and LiNiO2. LiNiO2 can be obtained only from hydrates and in oxygen atmosphere.LiNiOz and LiCo0.5Ni0.5O2 have higher first charge capacity than that of LiCoO2. Their discharge capacity reaches a level with that of LiCoO2 and has reasonable reversibility.     

Crystal and Electronic Structure of High-Temperature Superconductive Layered Cuprates in Temperature Interval 100–300 K

For two HTSC materials, Y_1?x Ca _x Ba₂Cu₃ O _6.7 and Tl_0.8Hg_0.2Ba₂Ca₂Cu₃ O _8.15, their crystal structure has been studied earlier in the temperature interval 100–300 K. It is shown that for both compounds, the height of CuO₅ pyramids has a minimum in temperature interval from ～ 150 to ～ 250 K. The electronic structure is calculated using first principles based on the obtained data of crystal structure. It is revealed that a decrease in the CuO₅ pyramid height leads to the appearance of the electronic state density peak with an energy of 0.3–0.4 eV under Fermi level. Localized states of barium and apical oxygen make the main contribution to this peak. By reduction the temperature below ～ 150 K, these localized electronic states disappear, and under continuous cooling, the superconductivity appears.     

Depletion and “Fines” Effects on Transport in Hard-Sphere Colloids

The effect of depletion forces induced by the presence of much smaller B spheres (diameter ratio 10/1) in dense hard A spheres has been investigated. Simulations for systems of 10⁴ spheres with mole fraction of larger spheres x _A = 0.05 and with total sphere volume fraction from 0.35 to 0.655 have been carried out. The depletion force steeply increases in the A–A pair distribution function at contact, and creates a lower first coordination and more open network A-structures. There is no evidence of demixing. It is found that in the amorphous solid region there is an enhanced diffusive mobility of the larger spheres, relative to that of pure spheres, at the same densification rates. This observation resembles the fines effect that occurs in colloidal suspensions and granular fluids.     

Microstructure, Magnetic and Electronic Transport Properties of Co–TiO2 Nanocomposite Films in Metal Matrix

In this work, Co–TiO₂ metallic composite films with a novel nanostructure have been electrodeposited in potentiostatic regime onto copper substrates, from a solution based on cobalt sulfate containing suspended TiO₂ nanoparticles, with magnetic stirring of the electrolyte. The effect of deposited film thickness on the morphology, microstructure, and composition of the films was investigated by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and energy dispersive spectroscopy. Functional properties (magnetic and electronic transport) of films with different thicknesses were studied in a view to find out the possibility for some technological applications. Nanocomposite Co–TiO₂ films contain three main phases: hcp Co crystalline grains (9–10 nm average size), TiO₂ nanoparticles (28 nm average diameter) embedded in Co metallic matrix and Co(OH)₂ adsorbed on the crystallite frontiers. The films display hysteresis (coercive field of 7.8÷11.9 kA/m) and significant values of magnetoresistance (with a maximum of ?59 % in the case of 0.07 μm film thickness). These properties can be qualitatively explained both by the elastic spin-dependent scattering of the conduction electrons at the interface between the magnetic Co matrix grains and the nonmagnetic regions, and by occurrence of antiferromagnetic coupling between Co crystallites, favored by inclusion in film of TiO₂ nanoparticles.     

Electronic Transport Properties of Speer Carbon Resistors—Experimental Results and Semi-empirical Fits

Speer carbon composition resistors, in particular the 470 and 220 1/2 W grade 1002 resistors, have been used as secondary thermometers at temperatures below 4 K for many years. Their zero field resistances have been measured between 300 K and 4 K using a dip probe. Above 10 K, the resistance behavior can be explained using a simple temperature power law, R(T) R₀/T^0.16. The resistance measurements have been extended to 0.02 K using dilution refrigerators. Between 4 K and 0.3 K, the resistances exhibited activated laws having hopping exponents y 0.5. Below 0.3 K, the 470 resistors exhibit a crossover to a weaker activated law. Crossover resistance expressions suggest that the resistances follow a Mott variable-range hopping (VRH) law below 0.05 K. The low temperature magnetoresistance (MR) data showed changes of less than ±12 % of the zero field resistance values in fields up to 10 T. Fits using the wave function shrinkage and the forward interference models gave only fair agreement with the MR data.     

Magnetic Phase Transition and Electronic Anomalies in La1.96−x Y0.04Sr x CuO4 around x∼1/8

La-NMR, Hall coefficient and sound velocity have been measured in order to investigate the correlation between the magnetic ordering, the prominent change of transport properties, the suppression of T _c and the structural transformation around x1/8 in La _1.96–x Y _0.04 Sr _x CuO ₄. By the substitution of 0.04/2 Y for La-sites, the structural transformation to the low temperature tetragonal phase (LTT) is caused at 57±14 K for samples around x0.115. The. prominent decrease of the Hall coefficient followed by the sign reversal and the magnetic ordering are observed around x0.115 below temperatures T _s and T _N, respectively. T _s vs x shows a bell shaped curve with the maximum value of 65 K at x00115 where most prominent suppression of T _c appears. T _N shows similar x dependence to T _s with the maximum value of 40 K at x0.115. The change of the electronic state below T _s and the suppression of T _c become more prominent and the magnetic ordering is observed more wide range of x under the LTT phase.     

Synthesis of Molybdenum- and Vanadium-Containing Mixed Oxides in Polymer–Salt Systems

Rare-earth alkaline-earth mixed oxides containing transition metals (Mo, V) were prepared via pyrolysis in polymer–salt systems. The products were characterized by thermal analysis, resistance measurements, dilatometry, optical microscopy, and x-ray diffraction. The introduction of polyvinyl alcohol into the system containing lanthanum or strontium nitrate and ammonium molybdate was found to have a significant effect on the thermal decomposition process, testifying to changes in the bonding configurations of the constituent components in the systems studied, capable of forming stable gels, which are then used as precursors to synthesize oxide materials. The temperatures of different stages of dehydration were shown to be lower in the polymer-containing systems. The effect of solution acidity was assessed by examining thermal decomposition in systems containing a polymer and Mo or W salts and acidified with nitric acid. The reaction of nitrates (oxidants) with the polymer was accompanied by an exotherm at 170°C, corresponding to the melting of ammonium nitrate, resulting from an exchange reaction. The exothermic reaction was found to reduce the decomposition temperatures of the salts involved. The use of polymer–salt systems allowed the mixed oxides SrMoO₄ and La₂(MoO₄)₃ to be synthesized at lower temperatures in comparison with the coprecipitation of poorly soluble compounds. The method was also shown to be suitable for preparing perovskite oxides in the La_{1 – x} Sr _x Co_{1 – z} M _z O_{3 ± y} (M = Mo, V) systems.     

Relations Between Transport Coefficients in Lennard–Jones Fluids and in Liquid Metals

Several simple approximate hard-sphere relations for transport coefficients are compared with the results of molecular dynamics (MD) simulations performed on Lennard–Jones (LJ) fluids. Typically the individual transport coefficients: self-diffusion coefficients, D, shear viscosity, _s, bulk viscosity, _B, and thermal conductivity, , agree within a factor of two of the exact results over the fluid and liquid parts of the phase diagram, which seems reasonable in view of the approximations involved in the models. We have also considered the ratio, /_s, and the product, D_s, for which simple analytic expressions exist in the hardsphere models. These two quantities also agree within a factor of two of the simulation values and hard sphere analytic expressions. Using time correlation functions, Tankeshwar has recently related the ratio /D to thermodynamic quantities, in particular, to the differences in specific heats, C _p – C _V, and to the isothermal compressibility, T. Using D and thermodynamic values taken solely from LJ MD simulations, his relation was tested and found to give typically better than ~20% agreement at liquid densities, deteriorating somewhat as density decreases into the gas phase. Finally liquid metals are considered. In this case, is dominated by its electronic contribution, which is related approximately to the electrical conductivity by the Wiedemann–Franz Law. Some theoretical results for the electrical conductivity of Na are referenced, which allow a semiquantitative understanding of the measured thermal conductivity of the liquid metal. Shear viscosity is also discussed and, following the work of Tosi, is found to be dominated by ionic contributions; Nevertheless, at the melting temperature of Na, a relation emerges between thermal conductivity, electrical resistivity and shear viscosity.     

Dielectric and Magnetic Anomalies in α-MnS Single Crystals at Applied Magnetic Fields

Journal of Superconductivity and Novel Magnetism - Temperature and magnetic field dependences of the magnetic susceptibility and dielectric permittivity of the α-MnS single crystal with NaCl...     

Anomalies of Physical Properties inα-Bi2O3 — a Phase Transition Governed by the Electronic Mechanism?

A number of physical properties of -Bi₂O₃ was studied: the thermal expansion, thermally stimulated conductivity, electrical resistivity, and dielectric permeability. X-ray diffraction, DTA, and differential scanning calorimetry measurements were also carried out. Several anomalies were observed: a small maximum and drastic rise of the single crystal dielectric permeability, a sharp drop of the electric resistivity, a jump and change in sign of the thermally stimulated current, and an exothermal maximum in differential heat capacity. At the same time, the monoclinic lattice symmetry of -Bi₂O₃ was retained over the temperature range where these anomalies were found.     

Anomalies in electrical and dielectric properties of nanocrystalline Ni–Co spinel ferrite

Nanocrystalline Ni-substituted cobalt ferrite sample is prepared by chemical co-precipitation method. X-ray diffraction and scanning electron microscopy techniques are used to obtain structural and morphological characterizations. Nanocrystalline nature is clearly seen in SEM picture. Variation of electrical resistivity as a function of temperature in the range 300–900 K is investigated. ln ρ versus 1/T plot shows four break resulting into five regions in 300–900 K temperature range of measurements. The magnetic transition temperature of the sample is determined from resistivity behavior with temperature. The activation energy in different regions is calculated and discussed. Variation of dielectric constant (?′) with increasing temperature show more than one peak; one at around 773 K and other around 833 K, which is unusual behavior of ferrites. The observed peaks in ?′ variation with temperature show frequency dependence. Electrical and dielectric properties of Ni_0.4Co_0.6Fe₂O₄ sample show unusual behavior in the temperature range 723–833 K. To our knowledge, nobody has discussed anomalous behavior in the temperature range 723–833 K for Ni_0.4Co_0.6Fe₂O₄. The possible mechanism responsible for the unusual electrical and dielectric behavior of the sample is discussed.     

Plasmon–Phonon Pairing Mechanism and Superconducting State Parameters in Layered Mercury Cuprates

An effective two-dimensional dynamic interaction is developed which incorporates screening of holes by plasmons and by optical phonons to discuss the nature of the pairing mechanism leading to superconductivity in layered mercury cuprates. The system is treated as an ionic solid containing layers of charge carriers and a model dielectric function is set up which fulfils the appropriate sum rules on the electronic and ionic polarizabilities. The static limit of the model dielectric function is used to calculate the effective hole-hole coupling strength. The values of the electron-phonon coupling strength and of the Coulomb interaction parameter indicate that the superconductor is in the strong coupling regime with effective screening of the charge carriers. The superconducting transition temperature of optimally doped HgBa₂CuO₄₊ is estimated as 120 K from Kresin's strong coupling theory and the energy gap ratio is substantially larger than the BCS value. The value of the isotope exponent is severely reduced below the BCS value. The implications of the model and its analysis are discussed.     

Formation and Transport of Atomic Hydrogen in Hot-Filament Chemical Vapor Deposition Reactors

In this paper we focus on diamond film hot-filament chemical vapor deposition reactors where the only reactant is hydrogen so as to study the formation and transport of hydrogen atoms. Analysis of dimensionless numbers for heat and mass transfer reveals that thermal conduction and diffusion are the dominant mechanisms for gas-phase heat and mass transfer, respectively. A simplified model has been established to simulate gas-phase temperature and H concentration distributions between the filament and the substrate. Examination of the relative importance of homogeneous and heterogeneous production of H atoms indicates that filament-surface decomposition of molecular hydrogen is the dominant source of H and gas-phase reaction plays a negligible role. The filament-surface dissociation rates of H2 for various filament temperatures were calculated to match H-atom concentrations observed in the literature or derived from power consumption by filaments. Arrhenius plots of the filament-surface hydrogen dissociation rates suggest that dissociation of H2 at refractory filament surface is a catalytic process, which has a rather lower effective activation energy than homogeneous thermal dissociation. Atomic hydrogen, acting as an important heat transfer medium to heat the substrate, can freely diffuse from the filament to the substrate without recombination.     

Electronic Metal–Support Interaction of Single-Atom Catalysts and Applications in Electrocatalysis

The electronic metal–support interaction (EMSI), which acts as a bridge between theoretical electronic study and the design of heterogenous catalysts, has attracted much attention. Utilizing the interaction between the metal and the support is one of the most essential strategies to enhance electrocatalytic efficiency due to structural and synergetic promotion. To date, as the ideal model for realizing EMSI, many types of single-atom catalysts (SACs) have been developed. The understanding of the electronic interaction on SACs has also been pushed to a higher level. However, systematic theories and operando experiments are seldom reported, and will be necessary to put forward and be carried out, respectively. Herein, the types, characterization, mechanism, and electrocatalytic applications of EMSI are comprehensively summarized and discussed. In addition to the basic information above, the challenges, opportunities, and future development of the EMSI on SACs are also proposed to present an overall view and reference to the later research.     

Migration of Oxides in a Composite Coating and Its Effect on Inhibiting Coating Degradation

Based on the fact that the dispersed rare earth oxide particles in an electrolytic co-depositedNi-R_2O_3 layer migrated and accumulated at the substrate/coating interface immediately afteraluminizing,the mechanism of migration of the oxides and its effect on the behaviour of degradationof the coatings oxidized at 1100℃ were studied in this paper.The Ni-R_2O_3 layers were treated invacuum at 900℃ and aluminized at 1100℃ separately.The results indicated that the migration andaccumulation of R_2O_3 particles were favorable to the gathering of R_2O_3 at higher temperatures,andthe small size oxides were easier to migrate and gather.The drawing effect of grain boundary wasused to explain this behaviour.The microstructures of the coatings oxidized at 1100℃ were ob-served.It was found that the coating degradations were related to the amount and distribution of theaccumulated R_2O_3 at the alloy/coating interface.Three typical kinds of the accumulated layerswere identified.The continuous oxide layers acted as diffusion barrier which could inhibit the coat-ing degradation effectively.     

Review and Outlook of China Electronic and IT Industry Footprints in IEC International Standardization Activities

Year 2006 is the year marking the 100th anniversary of the founding of International Electrotechnical Commission (IEC). IEC is mainly engaged in the international standardization for electrical and electronic technology field     

Composition and Properties of Layered Compounds in the GeTe–Sb2Te3System

The 620-K section of the Ge–Sb–Te phase diagram was mapped out using x-ray diffraction, microstructural analysis, and microhardness measurements. The transport properties of the layered tetradymite-like compounds nGeTe · mSb₂Te₃(n, m= 1–4) were studied in wide temperature ranges (Hall effect and electrical resistivity, from 77 to 800 K, and thermoelectric power, from 90 to 450 K). The results show that the nGeTe · mSb₂Te₃compounds are degeneratep-type semiconductors with a fairly high hole concentration due to the high density of intrinsic point defects. The temperature dependences of the Hall coefficient and resistivity exhibit anomalies related to solid-state phase transitions. The room-temperature lattice thermal conductivity ofnGeTe · mSb₂Te₃is fairly low, in the range 8–10 mW/(cm K).     

Transport and localization in Nd1.82Ce0.18CuO4−δ film

Temperature- and magnetic-field — dependences of the resistivity and Hall effect R_H .