An investigation on electrical properties of microwave treated natural ilmenite (FeTiO3)

It has been observed that microwaves of 2.45 GHz heat dielectric materials. On subjecting natural ilmenite to microwave irradiation, the mineral is observed to heat, with a surface temperature in proportion to the irradiation time. With irradiation times from 40 to 240 seconds (increased in steps of 40 sec), the surface temperature measured on the samples were between 280 and 520 K. Electrical measurements made on the samples before and after irradiation show that the electrical properties are modified by the microwave irradiation. The real conductivity (σ′), dielectric constant (ɛ′) and the dielectric loss (ɛ″) plotted against frequency generally showed universal dielectric behaviour [1 and references therein] similar to that observed in other systems studied in the literature but using conventional heating techniques. Plots of σ′, ɛ′ and ɛ″ against the surface temperature of the sample showed frequency independent peaks around 460 K. The experimental dielectric loss (ɛ″) results fit a peak function of the form:

Viscoelastic properties of composites of calcium alginate and hydroxyapatite

Calcium alginate was reinforced with hydroxyapatite (HA) particles, whose dimensions were a few micrometres, with mass fraction, m _f, values in the range 0–0.8. Cylindrical samples of these composite materials were subjected to cyclic compression in the frequency range f = 0.001–20 Hz; in these tests a sinusoidal load of amplitude 1 N was applied either side of a static compression of 2 N. Storage and loss moduli, E′ and E′′, respectively, were found to be independent of particle size; however, E′ increased with frequency consistent with the materials undergoing a glass transition. Above frequencies of about 0.05 Hz, for all materials. For each frequency, the dependence of the moduli on log₁₀ f could be represented by a third order polynomial; these equations can be used to calculate E′ and E′′ for a range of compositions. Approximate values of are predicted by a Reuss model.     

Electrical conductivity and dielectric properties of cadmium thiogallate CdGa<Subscript>2</Subscript>S<Subscript>4</Subscript> thin films

Cadmium thiogallate CdGa₂S₄ thin films were prepared using a conventional thermal evaporation technique. The dark electrical resistivity calculations were carried out at different elevated temperatures in the range 303–423 K and in thickness range 235–457 nm. The ac conductivity and dielectric properties of CdGa₂S₄ film with thickness 457 nm has been studied as a function of temperature in the range from 303 to 383 K and in frequency range from 174 Hz to 1.4 MHz. The experimental results indicate that σ_ac(ω) is proportional to ω ^s and s ranges from 0.674 to 0.804. It was found that s increases by increasing temperature. The results obtained are discussed in terms of the non overlapping small polaron tunneling model. The dielectric constant (ε′) and dielectric loss (ε″) were found to be decreased by increasing frequency and increased by increasing temperature. The maximum barrier height (W _m) was estimated from the analysis of the dielectric loss (ε″) according to Giuntini’s equation. Its value for the as-deposited films was found to be 0.294 eV.     

Preparation and electromagnetic characteristics of silica coated Fe–Ni–Mo alloy flakes

The Fe–Ni–Mo alloy flakes were firstly prepared from water atomized powders. Subsequently, by a facile sol–gel technique using tetraethylorthosilicate (Si(OC₂H₅)₄) as a precursor, we successfully synthesized silica coated Fe–Ni–Mo alloy flakes. These products were characterized by Scanning Electron Microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Furthermore, their electromagnetic parameters, complex permittivity and complex permeability within 2–12 GHz were measured. The results show that both and values of the silica coated flakes substantially decrease with the increase of coating times. Meanwhile, the μ′ values of silica coated samples are larger than those of the as-milled sample at the high frequencies. However, the maximum μ′ exhibits a decline while resonance frequency shifts to high values with the increase of coating times. The calculated reflection loss (RL) curve reveals that a proper thickness of coating can redound to improve the absorbing performances of silica coated Fe–Ni–Mo alloy flakes in high frequency range.     

Mixed pronton–electron conducting properties of Yb doped strontium cerate

The electrical conductivity and hydrogen permeation properties of membranes were studied as a function of temperature and gradient. The bulk conductivity of was an order of magnitude higher than the grain boundary conductivity over the temperature range 100–250 °C in feed gas of 4% H₂/balance He (pH₂O = 0.03 atm). The significantly lower grain boundary conductivity indicates that larger-grained materials might be more suitable for proton transport. The hydrogen flux through the membranes is proportional to thickness down to 0.7 mm. The hydrogen permeation flux increases with an increase in gradient where the increase in hydrogen flux was explained by an increase in electron conduction as a function of temperature. The ambipolar conductivity calculated from hydrogen permeation fluxes shows the same and dependence as electron concentrations. The hydrogen and oxygen potential dependence of the ambipolar conductivity (, ) was understood from the defect structure. From this, it was confirmed that hydrogen permeation might be limited by electron transport at wet reducing atmosphere. From the temperature dependence of the electronic conductivity, the activation energy calculated at wet reducing conditions is 0.63 eV.     

The effect of internal particle heat conduction on heat transfer analysis of turbulent gas–particle flow in a dilute state

In many cases the conduction mechanism inside a particle can not be ignored (large particles, low thermal conductivity and high porosity) during turbulent gas–particle flows. However, the accurate solution might be difficult to apply. Therefore, we first develop here the ability to conduct accurate solution and then we define the criterion for which the internal conductivity might be ignored. A combination between commercial C.F.D. code and user defined programs was developed to predict numerically the gas–particle velocity and temperature profiles. The selected criterion (defined at the outlet of the pipe’s cross-section), referred to the relation between the computational desirable average temperature difference without ignoring internal heat conductivity and the average particles temperature by ignoring internal heat conductivity, determines whether to consider the heat conduction mechanism in numerical simulations or to ignore it. It was found that the average particles temperature for T _p =  f(r) is lower than the case when T _p =  constant. Also, it was found that the non-dimensional temperature difference criterion is a continuous function of [Bi ×  (d _p/D)] for a specific geometry, various pipe and particle diameters, various particles’ thermal conductivities, constant heat flux and Re number. The numerical code enables to extend the classical criterion for Bi number of solids to various gas–particle systems and different operational conditions.     

Compositional,temperature and frequency dependence of dielectric behaviour of zinc substituted copper- ferri-chromates

The comprehensive study on compositional, temperature and frequency dependent dielectric properties of Zn_xCu_1−xFeCrO₄ (x = 0.0, 0.2, 0.4 and 0.6 ) was carried out by means of a. c. resistivity (ρ_ac), dielectric constant (ε′) and loss tangent (tanδ) measurements in the frequency range from 100 Hz to 1 MHz at different temperatures ranging from 40 °C to 500 °C. An abnormal behaviour of ε′ as a function of temperature is explained on the basis of contribution of two types of charge carriers in the polarization process. It is found that magnetic ordering does not have marked influence on the dielectric properties. The probable conduction mechanism in the present system is due to electron transition such as Fe²⁺ Fe³⁺ and Cu²⁺ Cu¹⁺ rather than ionization or polaron hopping mechanism.     

Three oxidation states and atomic-scale <Emphasis Type="Italic">p–n</Emphasis> junctions in manganese perovskite oxide from hydrothermal systems

Perovskite oxides have provided magical structural models for superconducting and colossal magnetoresistance, and the search for nano-scale and/or atomic-scale devices with particular property by specific preparations in the same systems has been extensively conducted. We present here the three oxidation states of manganese (Mn³⁺, Mn⁴⁺, Mn⁵⁺) in the perovskite oxide, La_0.66Ca_0.29K_0.05MnO₃, which most interestingly shows the rectifying effect as atomic-scale p–n junctions (namely FY-Junctions) of single crystals and films. The family of cubic perovskite oxides were synthesised by the so-called hydrothermal disproportionation reaction of MnO₂ under the condition of strong alkali media. The new concept of the atomic-scale p–n junctions, based on the ideal rectification characteristic of the p–n junctions in the single crystal, basically originates from the structural linkages of [Mn³⁺–O–Mn⁴⁺–O–Mn⁵⁺], where Mn³⁺ and Mn⁵⁺ in octahedral symmetry serve as a donor and an acceptor, respectively, corresponding to the localized Mn⁴⁺ .     

Partial Molar Volumes and Viscosity B-Coefficients of Nicotinamide in Aqueous Resorcinol Solutions at <Emphasis Type="Italic">T</Emphasis> = (298.15, 308.15, and 318.15) K

Partial molar volumes and viscosity B-coefficients for nicotinamide in (0.00, 0.05, 0.10, 0.15, and 0.20) mol·dm⁻³ aqueous resorcinol solutions have been determined from solution density and viscosity measurements at (298.15, 308.15, and 318.15) K as a function of the concentration of nicotinamide (NA). Here the relation , has been used to describe the temperature dependence of the partial molar volume . These results and the results obtained in pure water were used to calculate the standard volumes of transfer and viscosity B-coefficients of transfer of nicotinamide from water to aqueous resorcinol solutions to study various interactions in the ternary solutions. The partial molar volume and experimental slopes obtained from the Masson equation have been interpreted in terms of solute–solvent and solute–solute interactions, respectively. The viscosity data have been analyzed using the Jones–Dole equation, and the derived parameters B and A have also been interpreted in terms of solute–solvent and solute–solute interactions, respectively, in the ternary solutions.The structure making or breaking ability of nicotinamide has been discussed in terms of the sign of . The activation parameters of viscous flow for the ternary solutions studied were also calculated and explained by the application of transition state theory.     

Superconducting properties of Ce-substituted TaSr2(Gd,Ce)2 {\rm Cu}_2{\rm O}_y

A layered cuprate TaSr₂ Cu₂ for x = 0.4–0.8 has been successfully synthesized by solid state reaction. X-ray powder diffraction analysis indicates that nearly all the peaks from the samples can be indexed to a single phase of Ta-1222. The sample with nominal composition TaSr₂Gd_1.6Ce_0.4Cu₂ showed a tetragonal structure with lattice parameters a = 3.858 Å, c = 28.81 Å  with space group most likely I4/mmm. The TaSr₂ Cu₂ compound exhibits a narrow superconducting region near x = 0.6 with T_c-onset = 30 and T_c-zero=10 K as determined by d.c. electrical resistance versus temperature measurements.     

Studies on a.c. properties of Ca<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiO<Subscript>3</Subscript> perovskites

A.c. measurements were preformed on bulk samples of Ca_1−x Sr _x TiO₃ (CST) perovskites with x = 0, 0.1 and 0.5 as a function of temperature range 300–450 K and frequency range 10³–10⁵ Hz . The experimental results indicate that the a.c. conductivity σ_a.c.(ω), dielectric constant ε′ and dielectric loss ε′′ depend on the temperature and frequency. The a.c. conductivity as a function of frequency is well described by a power law Aω ^S with s the frequency exponent. The obtained values of s > 1 decrease with increasing temperature. The present results are compared to the principal theories that describe the universal dielectric response (UDR) behavior.     

Effects of oxygen partial pressure control on the microstructure and PTCR properties of Ho doped BaTiO<Subscript>3</Subscript>

Effects of oxygen partial pressure ( ) control on the electrical properties and microstructural development of (Ba_1-xHo_x)_0.997TiO₃ were studied. An oxidation condition ( ∼ 1.0 atm) was maintained during the heating process, and then the specimen was sintered in a reducing atmosphere ( < 10⁻⁹ atm) at 1350 °C, followed by the annealing process at 1000 °C and = 1 atm. The switching temperature (T_S) from the oxidation atmosphere to the reducing condition was changed from 1100 to 1350 °C. A significant decrease in the room-temperature resisitivity (ρ₂₅) was observed as T_S was increased. The temperature coefficient of resistance (TCR) was independent of the change in T_S, and closed pores decreased with increasing T_S.     

Lattice Anomalies in (La,Sr)2CuO4 Under Epitaxial Strain Probed by Polarized X-Ray Absorption Spectroscopy

Local lattice anomalies in optimally doped T-(La,Sr)₂CuO₄ single crystal like thin films (T _c = 43.4 K) grown by molecular-beam epitaxy have been studied by the in-plane polarized Cu K-edge extended X-ray absorption fine structure (EXAFS). The results indicate temperature-dependent local atomic displacements which are anomalous at the T _c and below a higher temperature T _s as demonstrated by a change in the mean square relative displacement of the Cu–O bond , i.e., a sharp drop at the T _c and a gradual deviation from a noncorrelated Debye-like behavior below T _s where the spatial inhomogeneity appears. We find that the magnitude of the Cu–O displacement changes at the T _c, is enhanced by compressive strain while the tendency of charge segregation is suppressed. The results suggest that the uniaxial pressure effects stabilize the system by decreasing the onset temperature and magnitude of spatial heterogeneity.     

Low Temperature Thermal Conductivity of PVC

At very low temperatures, the tunnelling theory for amorphous solids predicts a thermal conductivity κ α T^m, with m = 2. However, most of the data in the literature in the temperature range 0.1–1 K report an m < 2. We want to show that this discrepancy often disappears for T→ 0 K. Here we report the case of Polyvinyl Chloride (PVC) whose thermal conductivity is known in the 0.2–100 K temperature range. A new technique is described which makes the measurement of the exponent m of the thermal conductivity independent of the spurious thermal power. Such technique is particularly useful for measurements of κ when working with a low power refrigerator. We carried out measurements down to 50 mK, obtaining a thermal conductivity W/cm K for our PVC sample below 120 mK.     

Orientation relations between carbon nanotubes grown by chemical vapour deposition and residual iron-containing catalysts

Orientation relationships between the growth direction of carbon nanotubes and encapsulated residual iron-containing particles have been determined using transmission electron microscopy. The nanotubes that are prepared by Fe-catalysed chemical vapour deposition on sol–gel Fe(NO₃)₃-tetraethyl orthosilicate substrates are the helical multiwall type. Nanoscale particles of both the low-temperature α-Fe (ferrite) and high-temperature γ-Fe (austenite) were found in the cavity of the carbon nanotubes with , and parallel to the tube growth direction, respectively. Cementite Fe₃C, the most abundant Fe-containing phase in present samples was also found to be entrapped in nanotubes with or parallel to the tube axis. The metastable retention of γ-Fe particles at room temperature is ascribed to the strain energy induced at the particle-nanotube interface due to volume expansion upon the γ- → α-Fe phase transformation. The decomposition of initially high aspect-ratio, rod-shape particles into a string of ovulation, while encapsulated in carbon nanotubes is accounted for by the Rayleigh instability. Ovulation leading to reduced particle size has also contributed to increase the surface energy term that counterbalances the total free energy change of phase transformation from γ- to α-Fe and further aids to the metastable retention of γ-Fe.     

Turbidity Data of Weightless SF<Subscript>6</Subscript> Near its Liquid–Gas Critical Point

Light transmission measurements performed in SF₆ close to its liquid–gas critical point are used to obtain turbidity data in the reduced temperature range (T is temperature, T _c is the critical temperature). Automatic experiments (ALICE 2 facility) were made at a near critical density, i.e., , in the one-phase homogeneous region, under the microgravity environment of the Mir Space Station ( is the average density, ρ _c is the critical density). The turbidity data analysis verifies the theoretical crossover formulations for the isothermal compressibility and the correlation length ξ. These latter formulations are also used to analyze very near T _c thermal diffusivity data obtained under microgravity conditions by Wilkinson et al. (Phys. Rev. E 57 436, 1998).     

Studies on structural,electrical and dielectric properties of nickel ion conducting polyvinyl alcohol based polymer electrolyte films

Polyvinyl alcohol (PVA) complexed with different weight percent ratios of Nickel Bromide (NiBr₂) salt were prepared by using solution cast technique. X-ray diffraction analysis confirmed the complexation of the salt with the polymer. Differential scanning calorimetry was used to determine the glass transition and melting temperatures of pure PVA and PVA:NiBr₂ complexed films. Electrical conductivity was measured using ac impedance analyzer in the frequency and temperature range 1 Hz–1 MHz and 303–373 K respectively. It was observed that the magnitude of electrical conductivity increases with NiBr₂ salt concentration as well as temperature. Frequency dependence electrical conductivity of the complexed polymer electrolyte films follows the Jonscher’s equation. The dielectric behavior was analyzed using dielectric permittivity\(\left( {{\varepsilon ^\prime}} \right)\) and loss tangent \(\left( {\tan \delta } \right)\) of the samples. Relaxation time was determined from the variation of loss tangent with frequency at different temperatures. The modulus spectra indicated the non-Debye nature of the material.     

Photoconductivity in Thin Film of a-(Ge20Se80)0.90Sn0.10

Steady state and transient photoconductivity measurements have been done on thin film of a-(Ge₂₀Se₈₀)_0.90Sn_0.10 as a function of temperature and intensity. Dark conductivity (σ_d) and photoconductivity ( ) measurements show that the conduction in this glass is through an activated process having single activation energy in the temperature range 283–350 K. The intensity dependence of steady state photoconductivity ( ) follows a power law with intensity (F), , where the power γ has been found between 0.5 and 1.0, suggesting bimolecular recombination. Rise and decay of photocurrent at different temperatures and intensities show that photocurrent (I_ph) rises monotonically to the steady state value and the decay of photocurrent is quite slow. A detailed analysis of photoconductive decay shows that the recombination within localized states may be predominant recombination mechanism in this glassy alloy.     

On perfect nonlinear functions (Π)

Perfect nonlinear functions are of importance in cryptography. By using Galois ring, relative trace and investigating the character values of corresponding relative difference sets, we present a construction of perfect nonlinear functions from to , where m′ is a divisor of 2m, and a construction of perfect nonlinear functions from to where 2m is possibly larger than the largest divisor of n. Meanwhile we prove that there exists a perfect nonlinear function from to if and only if p = 2, and there doesn’t exist a perfect nonlinear function from to if m > n and l(l is odd) is self-conjugate modulo 2 ^k (k ≥ 1).      

Hydrothermal processing of barium strontium titanate sol-gel composite thin films

Stoichiometric barium strontium titanate (BST) films of composition with thickness >2 μm have been fabricated on Si/SiO₂/Pt substrates by hydrothermal sol-gel composite processing. This film deposition technique involves the treatment of a spun-on sol-gel composite film, formed from a suspension of a powder in an aqueous BST sol-gel, at temperatures from 100–200°C at a pressure of 1–15 atm. An initial hydrolysis procedure eliminates dissolution of the dried sol-gel during the hydrothermal treatment. Glancing angle X-ray diffraction shows excellent crystallinity and stoichiometry in the BST films with no evidence of new phases created during processing. Scanning electron micrography and atomic force microscopy show densification of the film structure and the development of a bridging microstructure. Transmission electron micrography indicates that while much of the sol-gel derived matrix phase is amorphous a more crystalline interface occurs with the powder particles. The relative permittivity and loss tangent of the films are measured using a parallel plate capacitor technique in the frequency range 1–100 kHz. At 100 kHz relative permittivities of the films range from ɛ_r = 400–1200 and loss tangents lie in the range 0.05 < tan δ < 0.10, depending on the parameters of film preparation. The film structure and morphology and the electrical studies suggest that the microstructure of the films evolves by deposition of the sol-gel derived BST on the underlying powder, resulting in an electrically interconnected microstructure in which the sol-gel derived material bridges between the high permittivity powder particles.