Electrical transport and semiconductor-semimetal transition in LaVO4

The electrical conductivity (), and thermoelectric power (S) (300 to 1200 K), magnetic susceptibility () (300 to 900 K) and DTA and TGA (300 to 1200 K) together with X-ray diffraction studies are reported. At room temperature the lattice is orthorhombic. The DTA study shows a broad peak around 1173 K indicating a possible structural phase transition of the compound. Practically no weight loss was observed in TGA from 300 to 1200 K. The electrical conductivity exhibits an exponential increase up to 855 K and between 855 and 885 K, it then jumps by a factor of 5×10⁴ and remains practically constant up to 1200 K, indicating a transition from semiconducting to the metallic state around 870 K. S values remain positive throughout the temperature range studied indicating electrons to be the main charge carrier. In the semiconducting state (300 to 855 K) the plot of S against T ^–1 is linear with a slope of 0.04 eV against the activation energy of 0.83 eV. It has been shown that conduction is due to hopping of holes from V⁴⁺ defect centres to V⁵⁺ normal sites. The defects exist due to a small oxygen deficiency. The semiconductor semi-metal transition appears due to the overlapping of vanadium 3d and oxygen 2p bands around 870 K.     

Experimental Results on Tungsten-Wire Explosions in Air at Atmospheric Pressure—Comparison with a One-Dimensional Numerical Model

Experimental results on exploding tungsten wires in air at atmospheric pressure at current densities 10⁷ A·cm^–2 and a current rise 10¹⁰ A·s^–1 are presented. Besides the current through the probe and the voltage across it, the diameter of the wire material and its surface temperature have been measured. The final aim of this investigation is the determination of the thermophysical properties of a high-melting liquid metal up to its critical point. Here a first step should be made to demonstrate the reliability of the method and to justify the crucial assumptions. To determine the limits for the applicability of a homogeneous approach used so far, a one-dimensional numerical model in Z-pinch geometry has been used which gives the time evolution of the profiles of temperature, density, and pressure across the wire. The model describes well the main features observed in these experiments. A physical explanation for the maximum in the time dependences of the surface temperature is proposed. This behavior is related to special thermodynamic properties of a two-phase (liquid–gas) mixture forming in a peripheral layer around the liquid metal. The temperature limit is determined for which there are no remarkable gradients of temperature and density across the wire. The specific heat, the thermal expansion coefficient, and the electrical as well as thermal conductivity of liquid tungsten can now, in principle, be obtained. The parameters of the critical point of the liquid-vapor phase transition can also be estimated.     

The fatigue behaviour of dispersed-oxide-strengthened lead in air and in vacuum

Specimens of dispersed-oxide-strengthened lead containing 4.5 wt % oxide were fatigued at room temperature ( 0.5 T _m where T _m is the melting point in degrees K) in air and in vacuum (< 2×10^–4 torr). Metallography of the damage during fatigue and after fatigue fracture showed that the improved fatigue resistance of dispersed-oxide-strengthened lead (DS lead) over that of pure lead was due mainly to the mechanical strengthening effects of the dispersed oxide rather than an increase in the resistance to atmospheric corrosion fatigue. The ratio of the fatigue life of DS lead in vacuum to that in air was 8.5 at a strain of ± 0.145%. In specimens fatigued in air, failure occurred at grain boundaries and in those fatigued in vacuum it occurred by a mixture of intercrystalline and transcrystalline modes.     

Introducing Rare Earth Dopants for Controlled Conductivity in Thermoelectric Cobaltites

The conversion of waste heat into electrical energy plays a key role in our current challenge to develop alternative energy technologies to reduce our dependence on fossil fuels. Thermoelectric (TE) materials are the first choice to handle this subject. In TE materials, cobaltites are the material of interest, due to their nontoxic properties. Cobaltites exhibits large TE power, low resistivity, and relatively small thermal conductivity at room temperature. TE material (BiCa_2?x R _x CoO _y ) where R is for rare earth like Nd, (x = 0.0, 0.2) was synthesized in nanoregime by simplified sol–gel method. Simplified sol–gel method was chosen because it gives maximum phase purity and tunable parameters for desired properties. The mechanism used to enhance thermoelectric properties is to reduce thermal conductivity of the material and increase electrical conductivity of the material using nanostructures. Material characterizations were done for structural studies using X-ray diffraction (XRD). XRD revealed monoclinic crystal structure. Temperature-dependent electrical conductivity showed an increase with increase in temperature. Thermal conductivity was also measured. Both electrical resistivity and thermal conductivity decrease as a result of neodymium doping which enhanced its importance as thermoelectric material. The different parameters were correlated to understand the conduction mechanism.     

Chitosan-based electrolyte for secondary lithium cells

The system chitosan : ethylene carbonate : LiCF₃SO₃ was prepared by the solution cast technique. To verify that the conductivity of the material is due to the salt, the electrical conductivity at room temperature of the chitosan acetate film and that of the chitosan acetate films containing different amounts of ethylene carbonate added to it were measured. The order of magnitude of the electrical conductivity was 10^–10 S cm^–1. Films containing fixed content of chitosan and plasticizer but different amounts of salt were then prepared in the same manner and the highest electrical conductivity obtained was 1.3 × 10^–5 S cm^–1 at room temperature. These results indicate that the conductivity is due to the salt. Conductivity-temperature studies show that the ln T versus 10³/T graphs obey Arrhenius rule implying that the conductivity occurs by way of some thermally assisted mechanism. Polarization current measurement shows that the lithium ion transference number is 0.09. A LiMn₂O₄/chitosan-LiCF₃SO₃/C cell was fabricated which cycled between 1.5 to 2.5 V with fading capacity. This could be the result of LiF formation due to interaction between the salt and the fluorine in the binding agent.     

Evolution of the electrical resistance of \bf{\hbox {YBa}_2\hbox {Cu}_3\hbox {O}_{7-\varvec\delta }} single crystals in the course of long-term aging

Investigated are the changes in the basal-plane electrical resistivity of an optimally doped \(\hbox {YBa}_2\hbox {Cu}_3\hbox {O}_{7-\delta }\) single crystal in the course of long-term aging (17 years) at room temperature in air. In consequence of aging the sample has decomposed into three phases with different temperatures of the superconducting transition, while the transition widths of these phases have increased significantly. The temperature dependence of the electrical resistivity has retained a metallic character. The fluctuation conductivity near the critical temperature is described well by the 3D Aslamazov–Larkin model. In the course of aging significant changes in the scattering characteristics have been observed, whereas the Debye temperature has changed slightly and the transverse coherence length has remained constant.     

Preparation and characterization of pulse-plating electrodeposited CuInSe2 thin films

CuInSe₂ thin films were electrodeposited on an indium-tin-oxide glass substrate by a pulse-plating technique from an aqueous electrolyte solution, containing CuSO₄, InCl₃ and SeO₂ with the pH adjusted to 1.5 at room temperature. The deposited and heat-treated films were characterized structurally using X-ray diffraction (XRD), scanning electron microscopy (SEM) and auger electron spectroscopy (AES) analysis. The optical and the electrical properties of the films were also examined. When the film was submitted to heat-treatment above 300 C in air for 1 h, the sample showed a chalcopyrite structure and n-type conductivity with a carrier concentration of ~ 10¹⁷ cm^–3 and a band-gap (direct allowed transition) energy of 0.99 eV.     

Conductivity study of solid polyelectrolytes based on hydroiodide salt of poly(4-vinyl pyridine-co-butylmethacrylate), poly(4-vinyl pyridine-co-butylacrylate)

The chain flexibility of poly(4-vinylpyridine) was tried to increase by lowering its glass transition temperature ( T _g ) and by increasing its amorphous region by copolymerizing with butyl methacrylate and butylacrylate which act as internal plasticizer. The copolymers were prepared in five different feed molar ratios to optimize the required properties such as higher room temperature conductivity and better film-forming capacity. The conductivity and conduction behaviour of the copolymers, as well as their hydroiodide salts have been reported. There was about 10³–10⁴-fold increase in room temperature conductivity of these plasticized polyelectrolytes.     

Thermal and Electrical Properties of Gadolinium Sulfides at High Temperatures

A study is made into the temperature dependence of the thermal conductivity, electrical conductivity, thermoelectromotive force, thermal expansion coefficient, and heat capacity in the temperature range from 300 to 1200 K for polycrystalline gadolinium sulfides GdS _y (y= 1.495–1.487) produced both by recrystallization pressing and by crystallization from a melt. The role of the mechanisms of heat and charge transfer is estimated depending on the composition. The reasons for changes in their electrical and thermal properties are analyzed. The thermoelectric efficiency is calculated. It is demonstrated that Z 0.6 × 10^–3K^–1at T 1000 K.     

Synthesis and properties of Na<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>CoO<Subscript>2</Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0.55, 0.89) oxide thermoelectrics

Na_xCoO₂ (x = 0.55, 0.89) sodium cobaltites have been prepared by solid-state reactions; their structural parameters have been determined; their microstructure has been studied; and their thermal (thermal expansion, thermal diffusivity, and thermal conductivity), electrical (electrical conductivity and thermoelectric power), and functional (power factor, thermoelectric figure of merit, and self-compatibility factor) properties have been investigated in air at temperatures from 300 to 1100 K. The results demonstrate that, with increasing sodium content, the electrical conductivity and thermoelectric power of the materials increase and their thermal conductivity decreases. As a result, the power factor and thermoelectric figure of merit of the Na_0.89CoO₂ ceramic at a temperature of 1100 K reach 0.829 mW/(m K²) and 1.57, respectively. The electron and phonon (lattice) contributions to the thermal conductivity of the ceramics have been separately assessed, and their linear thermal expansion coefficients have been evaluated.     

Grain-size effects on the structural, electrical properties and ferroelectric behaviour of barium titanate-based glass–ceramic nano-composite

Grain-size effects on the structural and electrical properties as well as ferroelectric behaviour of 10BaTiO₃–70V₂O₅–20Bi₂O₃ glass–ceramic nano-composite have been studied by scanning electron micrographs (SEM), X-ray diffraction (XRD), differential scanning calorimeter (DSC), dc conductivity (σ) and dielectric (ε) measurements over a wide temperature range. The present glass has been transformed into glass–ceramic nano-composite by annealing at temperatures close to crystallization temperature (T_cr). The XRD and SEM observations have shown that by heat treating at T_cr, the sample under study undergoes structural changes: from amorphous to partly crystalline for 1 and 8 h and to colossal crystallization for 24 h. After heat treated at T_cr for 1 and 8 h, the samples under load consist of small nano-crystallites (average size ca. 20–50 nm) embedded in glassy matrix. However, when the glass heat treated at T_cr for 24 h, the microstructure of the sample changes considerably. It is found that the glass–ceramic nano-composite obtained by heat treated at T_cr for 1 and 8 h exhibit giant improvement of electrical conductivity that is up to four order of magnitude. The electrical conductivity increases with increasing grain-size. The major role in the conductivity enhancement of this glass–ceramic nano-composite is played by the developed interfacial regions “conduction tissue” between crystalline and amorphous phases, in which the concentration of V⁴⁺–V⁵⁺ pairs responsible for electron hopping, is higher than inside the glassy matrix. The heat treated at T_cr for 24 h leads to decrease of the electronic conductivity. This phenomena lead to disappearance of most “conduction tissue” for electrons and substantial reduction of electronic conductivity. The experimental results were discussed in terms of a model proposed in this contribution which is based on a “core–shell” concept. The glass heat-treated at different times (1, 8 and 24 h) exhibited broad dielectric anomalies in the vicinity of the ferroelectric-to-paraelectric transition temperature. The Curie temperature (T_c), corresponding to ferroelectric phase transition increases with increasing grain-size. The observation of the glass–ceramic nano-composite being studied here can be used to control BaTiO₃ grain-size and hence transition temperature by proper adjustment of annealing times.     

Study of ac electrical conductivity and dielectric properties of ortho-hydroxy acetophenone azine thin films

AC electrical conductivity measurements were carried out in the temperature range 290–473 K over the frequency range 0.1–20 kHz of vacuum deposited Ortho-hydroxy acetophenone azine films. It was found that the ac electrical conductivity increases with frequency according to the relation σ_ac(ω) = A ω^s. The values of the dielectric constant, , slightly changed in higher frequencies irrespective of temperature change, whereas its value increases in higher temperature with the decrease in frequency. Also, the dielectric loss, ɛ′′, and tan δ, has been found to increase with raise in temperature and decrease in frequency. The obtained experimental data has been analyzed with reference to various theoretical models. The analysis shows that the correlated barrier hopping (CBH) model is the most appropriate mechanism for the ac electrical conduction in these films.     

Structural,thermal and transport studies on Ag1 − x Cuxl (0.05 ⩽ x ⩽ 0.25) solid electrolyte

Preparation and characterization studies on polycrystalline samples of Ag_{1 – x}Cu_xl wherex=0.05, 0.1, 0.15, 0.2 and 0.25, respectively, have been reported. Samples were analysed using powder X-ray diffraction (XRD) and differential scanning calorimetric (DSC) techniques in order to identify the compositions and phase transition temperatures. A.c. electrical conductivity studies were carried out on pelleted specimens of various compositions in the frequency range 65.5 kHz to 1 Hz and over the temperature range 293–412 K. DSC results obtained in the temperature range 373–473 K have shown that the ß- to -phase transition temperature is enhanced from 426 K to 438 K whenx is increased from 0.05 to 0.25. XRD results have indicated that there is a shift ind-spacing when the Cul content is increased, suggesting changes in the crystal structure. Typical XRD patterns recorded for the composition Ag_0.95Cu_0.05l at three different temperatures (room temperature, 373 and 473 K, respectively) have confirmed that both face-centred cubic and hexagonal phases would be present at room temperature and at 373 K as well, whereas at 473 K the structure would be purely body-centred cubic in nature. A.c. impedance analysis of the above samples appears to suggest that their electrical conductivity, predominantly due to the migration of Ag⁺ ions, lies in the order of 10^–4S cm^–1 at room temperature.     

Transport properties of CdGa2Se4 thin films

The d.c. conductivity was determined for CdGa₂Se₄ thin films in the temperature range 300–625 K for as-deposited and heat-treated films. The conductivity at room temperature of films of thickness 326 nm prepared at a temperature of 573 K was found to be 10^–12 ( cm)^–1. The dependence of the electrical conductivity on the annealing temperature in a vacuum of 1 Pa for a thin film of thickness 140 nm, shows that the electrical conductivity increases with increasing annealing temperature. However, the activation energies E and E decrease with increasing annealing temperature. The data of these annealed films are in agreement with the Meyer–Neldel rule. The thermoelectric power measurements indicate p-type conduction in the as-deposited films as well as for the heat-treated films. The p-type conduction is due to the cadmium deficiency as indicated by EDX. The difference between the value of the activation energy calculated from the thermoelectric power E _S and that obtained from the conductivity E indicates the presence of long-range potential fluctuations.     

Electrical conductivity and defect structure of polycrystalline tin dioxide doped with antimony oxide

Electrical conductivity () of tin dioxide doped with antimony has been measured as functions of temperature and oxygen partial pressure (p0_2> ). Variation of electrical conductivity is explained by assuming that the antimony oxide forms a substitutional solid solution and doubly ionized oxygen vacancies are predominant defects. Above –10^–5 atm oxygen partial pressure antimony ions are present predominantly in the pentavalent state in tin dioxide lattice. However, it is converted to the trivalent state below this oxygen partial pressure accompanied by a sudden rise in conductivity.     

Synthesis and properties of CeN thin films deposited by arc ion plating

Cerium nitride films were deposited by ion plating in an electron-beam sustained Ce arc discharge in a nitrogen atmosphere. The crystal structure was strongly affected by the arc discharge current and the substrate temperature. The lattice spacing of CeN film is 0.5020 nm with a density of 7.82 g cm^–3. This film showed a paramagnetic property at 10 K in a magnetic field of 20 kOe. The Knoop hardness for CeN film is over 1600. The electrical resistivity was 4.6 × 10^–4 cm with p-type conductivity. The carrier concentration of the CeN film increased after exposure to the air, which suggested that the valence of Ce in CeN is probably 4+.     

Electrical conductivity of zinc ferrites near stoichiometry and manganese-zinc ferrites under vacuum or in the presence of oxygen

The electrical conductivity of zinc ferrites near stoichiometry and of manganese-zinc ferrites has been investigated as a function of temperature under vacuum and in the presence of oxygen. Under vacuum, the conductivity of these ferrites with iron excess is explained by the hopping mechanism, and with ZnO excess by the development of vacancies in octahedral sites of cation-deficient spinel. Activation energies and the transition temperatures are presented. During the oxidation in oxygen of Mn-Zn ferrites, the profile of the log σ= f(T) curves shows that the mechanism of electrical conduction in the temperature range 100 to 350 °C can be explained in terms of the oxidation of Fe²⁺ to Fe³⁺ ions at octahedral sites. For the temperature range 300 to 450 °C, the conductivity involves the hopping of electrons from octahedral sites of Mn³⁺ ions to octahedral sites of Mn⁴⁺ ions. Above 550 °C the oxidation of Mn²⁺ ions leads to a marked change in conductivity with the generation of new phases.     

Gamma-ray dosimetric properties of molybdenum phosphate glasses

In the present work, a study of the temperature dependence of the d.c. electrical conductivity and conduction activation for a series of MoO₃-P₂O₃ glass systems has been carried out. The conductivity measurements of the unirradiated glass specimens proved to be mainly dependent on both temperature and transition metal ion content in the glass matrix. The results of the present investigation have shown that the conduction mechanism would be due to the electron exchange between the low and high valency states of the MoO₃ oxide (Mo" and Mo⁶⁺). The radiation-induced conductivity of the glass system studied, produced by gamma rays, has also been measured experimentally. The d.c. electrical conductivity has proved to be dose dependent, which showed a decrease with increasing -dose. The results reflect some evidence of the-ray dosimetric potential of the glass specimens studied.     

Electrical conductivity of Cs<Subscript>3</Subscript>PO<Subscript>4</Subscript> doped with trivalent cations

Cs_{3 ? 3x} M _x PO₄ (M = Sc, Y, La, Sm, Nd) solid electrolytes have been synthesized, their phase composition has been determined, and their electrical conductivity has been measured as a function of temperature. In all of the systems, we have identified cesium orthophosphate based solid solutions. Above ～550°C, the solid solutions are isostructural with the high-temperature, cubic phase of Cs₃PO₄. They offer high cesium ion conductivity owing to the formation of cesium vacancies via 3Cs⁺ → M³⁺ substitutions and the decrease in phase transition temperature. The conductivity of the synthesized solid solutions, (4.8?5.6) × 10^?3 S/cm at 300°C and (1.6?1.9) × 10^?1 S/cm at 800°C, is at the level of earlier studied Cs_{3 ? 2x} M _x ^II PO₄ solid electrolytes.     

Anomalous resistivity of sodium-lithium-niobium-tantalum ceramics

The electric resistance R of the samples of a Na_0.88Li_0.12Nb_0.5Ta_0.5O₃ ferroelectric ceramic composition with the electron conductivity increased by thermal reduction in vacuum exhibit anomalous temperature variation of the posistor type in the temperature region of 350–400°C, where the nonreduced samples exhibit transition to the state with superionic conductivity. The R(T) anomaly was observed during the electrical measurements both in air and in vacuum. The effect depends on the degree of material reduction and the measurement frequency, reaching 4–5 orders of magnitude for the dc measurements in samples with maximum conductivity. It is suggested that the R(T) anomaly is localized in the near-electrode layer and is related to a mutual influence of the electron and ion transport processes.