EPR study of V2O5–P2O5–Li2O glass system

glass system, with 0 < x 50 mol%, was prepared and investigated by EPR method. For low content of V₂O₅ all the spectra present a hyperfine structure typical for isolated V⁴⁺ ions. With the increasing of V₂O₅ content, the EPR absorption signal showing hyperfine structure is superposed by a broad line without hyperfine structure characteristic for clustered ions. At high V₂O₅ content, the vanadium hyperfine structure disappears and only the broad line can be observed in the spectra. Spin Hamiltonian parameters g , g , A , A , dipolar hyperfine coupling parameters, P, and Fermi contact interaction parameters, K, have been calculated.The composition dependence of line widths of the first two absorptions from the parallel band and of the broad line characteristic to the cluster formations was also discussed.     

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.     

Temperature-dependent current–voltage and capacitance–voltage characteristics of the Ag/n-InP/In Schottky diodes

The current–voltage (I–V) and capacitance–voltage (C–V) characteristics of Ag/n-InP/In Schottky diodes have been studied in a wide temperature range by steps of 10 K. A decrease in the apparent barrier height (BH), , an increase in the ideality factor, and a nonlinearity in the activation energy plot with a decrease in temperature have been seen. The experimental values of BH and ideality factor for the devices were calculated as 0.61 eV and 1.18 at 320 K, 0.48 eV and 1.52 at 200 K, and 0.20 eV and 3.89 at 70 K, respectively. An abnormal decrease in the experimental BH and an increase in the ideality factor n with a decrease in temperature have been explained by the barrier inhomogeneities at the metal–semiconductor (MS) interface. From the temperature dependent I–V characteristics of the Ag/n-InP contact, that is, and A* as 0.79 and 0.55 eV, and 7.96 and , respectively, have been calculated from a modified vs. 1/T plot for the two temperature regions. The Richardson constant values are in close agreement with the value of known for n-type InP. Moreover, the difference between the apparent BHs obtained from the I–V and C–V characteristics has been attributed to the existence of Schottky BH inhomogeneity.     

Activation kinetics of the As acceptor in HgCdTe

The amphoteric model of As in HgCdTe is the basis of an investigation into how the transfer is achieved under Hg saturation and so obtain a method for calculating optimum annealing times for the transfer. It is concluded that Schaake’s assumption that the transfer, or activation, process is diffusion limited, rather than reaction-rate limited, is a better fit to experimental data. The identification of the Te self-diffusivity in HgCdTe as due to Te _i defects is considered to be incorrect. As a result, Schaake’s activation model can only underestimate the optimum anneal times for activation if the experimentally observed Te self-diffusivity is used. An equation based on experimental activation data is given that permits an estimate of the optimum anneal time to be obtained in HgCdTe layers.     

A model of non-Ohmic conduction in ZnO varistors

A simple phenomenological approach to non-Ohmic conduction in zinc oxide based varistors is suggested. The decrease of the barrier height on voltage is considered as a reason of varistor effect. This model gives the relationship between the current density j and the average electric field E in the form , where the nonlinearity factor α is proportional to the rate of change of the barrier height on voltage . The nonlinearity factor α or the normalized nonlinearity coefficient (E ₁ is the electric field at fixed current density) can be used instead of the traditional but empirical nonlinearity coefficient . Fairly reasonable agreement between suggested model and experimental results is found. On leave from Dniepropetrovsk National University, Dniepropetrovsk, Ukraine; Fax: +52-953-5320214, ext. 106     

Diffusivity and solubility of oxygen in solid palladium

The solid solubility c _O of oxygen in palladium in equilibrium with gaseous oxygen has been determined from absorption-desorption experiments for temperatures T of 1123 and 1173 K and oxygen partial pressures between 2.7 × 10³ and 4.0 × 10⁴ Pa. The relationship between c _O, and T is given by , where R = 8.314 JK⁻¹ mol⁻¹ is the universal gas constant, ΔH _s = −13.55 kJ/mol denotes the heat of solution of oxygen in palladium and the constant a amounts to or . The diffusion coefficient D _O of oxygen in solid palladium has been determined by incomplete isothermal internal oxidation of Pd–Fe alloys using the data on the oxygen solubility in palladium. The temperature dependence of D _O obeys the Arrhenius equation D _O = D ⁰ exp(−E _d/RT) with pre-exponential factor D ⁰ = 2.33 × 10⁻⁷ m²/s and activation energy of diffusion of oxygen in palladium, E _d = 102.76 kJ/mol     

Phase conversions in calcium manganites with changing Ca/Mn ratios and their influence on the electrical transport properties

The phase-interconversions between the spinel-, brownmillerite-, defect rocksalt and perovskite-type structures have been investigated by way of (i) introducing deficiency in A-sites in Ca_xMn_2−xO₃ (0.05 ≤ × ≤ 1) i.e., by varying Ca/Mn ratio from 0.025 to 1 and (ii) nonstoichiometric CaMnO_3−δ (CMO) with 0.02 ≤ δ ≤ 1. The temperature dependence of resistivity (ρ–T) have been investigated on nonstoichiometric CaMnO_3−δ (undoped) as well as the CMO substituted with donor impurities such as La³⁺, Y³⁺, Bi³⁺ or acceptor such as Na¹⁺ ion at the Ca-site. The ρ–T characteristics of nonstoichiometric CaMnO_3−δ is strongly influenced by oxygen deficiency, which controls the concentration of Mn³⁺ ions and, in turn, affects the resistivity, ρ. The results indicated that the substitution of aliovalent impurities at Ca-site in CaMnO₃ has similar effects as of CaMnO_3−δ (undoped) annealed in atmospheres of varying partial pressures whereby electron or hole concentration can be altered, yet the doped samples can be processed in air or atmospheres of higher . The charge transport mechanisms of nonstoichiometric CaMnO_3−δ as against the donor or acceptor doped CaMnO₃ (sintered in air,  ~ 0.2 atm) have been predicted. The ρ (T) curves of both donor doped CaMnO₃ as well as non-stoichiometric CaMnO_3−δ, is predictable by the small polaron hopping (SPH) model, which changes to the variable range hopping (VRH) at low temperatures whereas the acceptor doped CaMnO₃ exhibited an activated semiconducting hopping (ASH) throughout the measured range of temperature (10–500 K).     

Reference Viscosities of H<Subscript>2</Subscript>, CH<Subscript>4</Subscript>, Ar,and Xe at Low Densities

The zero-density viscosity of hydrogen, methane, and argon was determined in the temperature range from 200 to 400 K, with standard uncertainties of 0.084% for hydrogen and argon and 0.096% for methane. These uncertainties are dominated by the uncertainty of helium’s viscosity , which we estimate to be 0.080% from the difference between ab initio and measured values at 298.15 K. For xenon, measurements ranged between 200 and 300 K and the zero-density viscosity was determined with an uncertainty of 0.11%. The data imply that xenon’s viscosity virial coefficient is positive over this temperature range, in contrast with the predictions of corresponding-states models. Furthermore, the xenon data are inconsistent with Curtiss’ prediction that bound pairs cause an anomalous viscosity decrease at low reduced temperatures. At 298.15 K. the ratios , and were determined with a relative uncertainty of less than 0.024% by measuring the flow rate of these gases through a quartz capillary while simultaneously measuring the pressures at the ends of the capillary. Between 200 and 400 K, a two-capillary viscometer was used to determine with an uncertainty of 0.024% for H₂ and Ar, 0.053% for CH₄, and 0.077% for Xe. From was computed using the values of calculated ab initio. Finally, the thermal conductivity of Xe and Ar was computed from and values of the Prandtl number that were computed from interatomic potentials. These results may help to improve correlations for the transport properties of these gases and assist efforts to develop ab initio two- and three-body intermolecular potentials for these gases. Reference viscosities for seven gases at 100 kPa are provided for gas metering applications.     

The properties of AlGaN films and AlGaN/GaN heterostructures grown on (11\bar{2}0) sapphire substrates

Al _x Ga_1-x N films and Al _x Ga_1-x N/GaN heterostructures were prepared on the ( ) sapphire substrates at elevated temperatures by alternate supply of trimethylgallium (TMG)/trimethylaluminum (TMA) and ammonia (NH₃) in an inductively heated quartz reactor. X-ray studies reveal the monocrystalline nature of these Al-containing structures. The results of absorption measurements of the Al _x Ga_1-x N films exhibit clear cut-off energies of the films. Based on the investigations of transmission electron microscopy (TEM), Al _x Ga_1-x N films and Al _x Ga_1-x N/GaN structures were found to deposit on the ( ) sapphire substrates with < 0001 > _AlGaN and being parallel to and , respectively.     

Thermoelectric properties of the welded Cu/Bi<Subscript>0.88</Subscript> Sb<Subscript>0.12</Subscript> /Cu composites in the temperature region from 193 K to 298 K

The resultant thermoelectric properties of Cu/T/Cu composites welded with alloy were measured in the temperature range from T = 193 K to 298 K and compared with those calculated as a function of x by treating these composites as an electrical and thermal circuit, where x is the ratio of thickness of Bi–Sb alloy to the interval between two thermocouples. Consequently, the resultant electrical resistivities ρ of composites coincided closely with the calculated ρ values as a function of x, while the resultant Seebeck coefficients α were enhanced significantly in the range from x=0.076 to 0.61. In the x range from 0.16 to 1.0, the x-dependence of the resultant thermoelectric power factor P was found to be explained roughly at every temperature by the simple model proposed here when an enhancement factor in α was taken into the calculation. However, the maximum resultant P appeared at a small x of 0.076. The resultant P at x = 0.076 increases with a decrease of T and reached a surprisingly great value of 128.3 mW/K² m at 193 K, which is 15.5 times larger than 8.29 mW/K² m obtained for Bi_0.88Sb_0.12 alloy. On the other hand, its resultant ZT also increases monotonically with a decrease of T and has a great value of 0.54 at 193 K, which is 31% higher than 0.41 at 193 K for Bi–Sb alloy. The significant enhancement in the resultants P and ZT at low temperatures is owing predominantly to the increase in α due to the boundary effect.     

The effect of Ti<Superscript>4+</Superscript> ions and gamma radiation on the structure and electrical properties of Mg ferrite

Ferrite samples of the general formula Mg_1+x Ti _x Er _y Fe_2−2x−y O₄; 0.1 ≤ x ≤ 0.9, y = 0.025 were prepared using the standard ceramic method. The final sintering temperature was 1,200 °C with heating rate 4 °C/min during 100 h. X-ray diffraction analysis was carried out to assure the formation of the spinel structure. The effect of Ti⁴⁺ ion concentration on the structural and the electrical properties of the investigated samples is studied. It change the iron ion concentration from 2 to 2−2x thereby decreasing the number of ferrous ions on octahedral sites, with a consequent decrease the dielectric constant. The most important result of γ-irradiation on the electrical properties is the change of ratio on the octahedral site leading to increase the conductivity as well as the dielectric constant. The variation of the thermoelectric power with a temperature is performed, the common feature of all compositions is the fluctuation of Seebeck coefficient between positive and negative over the whole range of temperature. This indicates that the charge carriers are electrons and holes, depending on both the temperature range and the additive in the ferrite samples.     

Magnetic susceptibility and effective magnetic moment of the Nd3+ and Co3+ ions in NdCo1 ? x Ga x O3

The magnetic susceptibility of NdCo_{1 − x} Ga _x O₃ (x = 0, 0.1, 0.3, 0.5, 0.7, 0.8, 0.9, 1) has been measured at temperatures from 80 to 950 K. The effective magnetic moments (μ_eff) due to the magnetic moments of the Co³⁺ and Nd³⁺ ions have been determined in the temperature ranges of Curie-Weiss behavior, 130–370 and 600–940 K, and have then been used, together with the effective magnetic moment of Nd³⁺ (3.62μ_B or 4.20μ_B), to evaluate the effective magnetic moment of Co³⁺ in NdCo_{1 − x} Ga _x O₃. For the solid solutions with < 2.83μ_B, we have determined the fractions of intermediate-and low-spin Co³⁺ ions. In the range 2.83μ_B < < 4.90μ_B, we have determined the fraction of high-spin Co³⁺ ions. The results indicate that, in the temperature range 130–370 K, the Co³⁺ ions in NdCo_{1 − x} Ga _x O₃ with x = 0, 0.5, 0.8, and 0.9 are in the intermediate-and high-spin states, and the fraction of high-spin Co³⁺ ions gradually increases from 10% at x = 0 to 67% at x = 0.9. In the solid solutions with x = 0.1, 0.2, 0.3, and 0.7, more than half of the Co³⁺ ions are in the low-spin state, and the rest are in the intermediate-spin state. In the temperature range 600–940 K, the Nd³⁺ ions are in the ground and excited states, with theoretically predicted of of 3.62μ_B and 5.52μ_B, respectively. Because of the significant uncertainty in in this temperature range, has been determined less accurately compared to the range 130–370 K. Original Russian Text ? N.N. Lubinskii, L.A. Bashkirov, A.I. Galyas, S.V. Shevchenko, G.S. Petrov, I.M. Sirota, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 9, pp. 1137–1143.     

The role of protons in ionic diffusion in (Mg, Fe)O and (Mg, Fe)2SiO4

The presence of hydrogen dissolved within iron-magnesium oxides and silicates results in an increase in the rate of Fe–Mg interdiffusion. Experimental data and point defect models suggest that the increased interdiffusivity is due to an increase in the total metal-vacancy concentration through stabilization of proton-vacancy defect associates in a hydrous environment. In the case of (Mg_1–x Fe _x )O, interdiffusion experiments under hydrothermal conditions at a fluid pressure of ∼0.3 GPa yield similar dependencies of interdiffusivity on Fe-content, oxygen fugacity, and temperature as under dry conditions, but interdiffusion coefficients are a factor of ∼3 larger. These data suggest that the increased interdiffusivities in (Mg_1–x Fe _x )O result from incorporation of defect associates formed between a metal vacancy and a single proton, For (Mg_1–x Fe _x )₂SiO₄, interdiffusion under hydrothermal conditions over a range of fluid pressures reveals a significant difference in the dependence of interdiffusivity on Fe content than obtained under dry conditions, combined with a strong dependence on water fugacity. These data indicate that the increased diffusivities in (Mg_1–x Fe _x )₂SiO₄ result from incorporation of defect associates involving a metal vacancy and 2 protons, It is anticipated that, at higher water fugacities, Fe–Mg interdiffusion in both materials will become dominated by these latter defects and that the interdiffusivity will increase linearly with water fugacity but will be independent of oxygen fugacity and iron concentration.

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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.     

Electrocatalytic applications of platinum-decorated TiO<Subscript>2</Subscript> nanotubes prepared by a fully wet-chemical synthesis

Pt-decorated \(\hbox {TiO}_{2}\) nanotubes Pt@TiO₂ are prepared only by applying a set of facile wet-chemical redox reactions to ion track-etched polycarbonate templates. First, a homogeneous layer of Pt nanoparticles is deposited onto the complex template surface by reducing potassium tetrachloroplatinate with absorbed dimethylaminoborane. Second, the template is coated with a conformal \(\hbox {TiO}_{2}\) layer, using a chemical bath deposition reaction based on titanium(III) chloride. After the removal of the template, the rutile-type \(\hbox {TiO}_{2}\) nanotubes remain decorated with Pt nanoparticles and nanoparticle-clusters on their outside. During the process, neither vacuum techniques nor external current sources or addition of heat are employed. The crystallinity, composition, and morphology of the composite nanotubes are analysed by X-ray diffraction, scanning and transmission electron microscopy as well as by energy-dispersive X-ray spectroscopy. Finally, the obtained materials are examplarily applied in the electrooxidation of ethanol and formic acid, and their performances have been evaluated. Compared to conventional carbon black-supported Pt nanoparticles, the Pt@TiO₂ nanotubes show higher reaction rates. Mass activities of 2.36 \(\hbox {A}\hbox { mg}_{\rm Pt}^{-1}\hbox { cm}^{-2}\) are reached in ethanol oxidation and 7.56 \(\hbox {A}\hbox { mg}_{\rm Pt}^{-1}\hbox { cm}^{-2}\) in the formic acid oxidation. The present structures are able to exploit the synergy of Pt and \(\hbox {TiO}_{2}\) with a bifunctional mechanism to result in powerful but easy-to-fabricate catalyst structures. They represent an easily producible type of composite nanostructures which can be applied in various fields such as in catalytics and sensor technology.     

Oxygen transport in La<Subscript>2-<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> NiO<Subscript>4+δ</Subscript>: membrane permeation and defect chemical modelling

We describe a defect and transport model for acceptor doped with the layered K₂NiF₄ structure. The model includes mobile oxygen interstitials in the rocksalt layers and oxygen vacancies in the perovskite layers of the crystal structure. Based on fits of literature data of the deviation from stoichiometry, defect concentrations have been calculated for La_1.5Sr_0.5NiO_4+δ as a function of p(O₂) for 1,073–1,223 K. Application of the transport model to oxygen permeation data obtained in this work clearly indicates that the oxygen transport in La_1.5Sr_0.5NiO_4+δ is primarily governed by migration of mobile oxygen interstitials. According to the model calculations, a significant contribution of vacancy migration to the oxygen permeation process is to be expected only for oxygen partial pressures lower than 10⁻⁵ bar at 1,223 K and even lower partial pressures at 1,073 and 1,173 K. As yet, permeation data are not available for such low partial pressures, which is why oxygen migration via vacancies could neither be confirmed nor ruled out.

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Influence of Ce3+ on dielectric properties of tungsten bronze structured PBN ceramics

The effect of A-site substitution of Ce³⁺ in tungsten bronze structured PBN ceramics with the general formula, Pb_{(x − 3y/2)}Ce _y Ba_(1 − x)Nb₂O₆ and the stoichiometric chemical formula, Pb_{(0.65 − 3y/2)}Ce _y Ba_0.35Nb₂O₆, where y = 0, 2, 4, 6, 8 and 10 mol% ceramic compositions synthesized through solid state reaction method are reported. The X-ray diffraction studies exhibited the presence of an orthorhombic phase, and its intensity increased with the increasing Ce³⁺ content up to y = 6 mol% or A₃ composition. The lattice parameters, unit cell volume and density as a function of Ce³⁺ concentration are discussed. It is observed that increasing Ce³⁺ content in A-site influenced the dielectric properties. The optimum dielectric properties of room temperature dielectric constant (ε_RT) and dielectric maximum () are observed in y = 6 mol% or A₃ composition while Curie temperature (T _c) and dielectric loss (tan δ) constantly decreased from undoped to y = 10 mol%, and thus A₃ composition or 6 mol% Ce modified tungsten bronze structured-PBN could be suitable for capacitor applications.     

Combined effect of partial calcination and sintering condition on low loss Mn-Zn ferrite

In order to achieve highly densified lower loss Mn-Zn ferrite materials, various powder processing routes have been under investigation. In this report, a lower sintering temperature with lower oxygen partial pressure and proper attrition milling duration are suggested. From the previous study, a partial calcination procedure was studied and an optimum partial calcination level was found. Accordingly, Mn_0.71Zn_0.22Fe_2.07O₄ ferrite was prepared by calcination with small amounts of CaCO₃, SiO₂, Nb₂O₅ and SnO₂. The partially calcined ferrites were made by calcining the mixture of the whole amount of ZnO and amount of Fe₂O₃ and Mn₃O₄ and sintering the mixture of the calcined powders and the remaining of Fe₂O₃ and Mn₃O₄. Initially, from the 40 min secondary milling and the partial calcination, lower temperature (1300°C) sintered samples showed a power loss of 340 mW cm^–3 at 90°C. Secondly, several milling durations showed secondary milling had a more profound effect on magnetic properties than primary milling. The 20 min primary and 90 min secondary milling showed lower core loss around 320 mW cm^–3 at 1300°C and 1250°C, suggesting the sintering temperature could be reduced to 1250°C. Thus, the proper sintering condition of lower oxygen partial pressure at 1250°C was studied. As a result, lower loss with higher density was realized.     

Effect of KCl and KNO<Subscript>3</Subscript> on Partial Molal Volumes and Partial Molal Compressibilities of Some Amino Acids at Different Temperatures

Density (ρ) and ultrasonic velocity (u) values of amino acids l-alanine, l-proline, l-valine, and l-leucine in 2M aqueous KCl and 2M aqueous KNO₃ solutions have been measured as a function of amino acid concentration at different temperatures (298.15 K, 303.15 K, 308.15 K, 313.15 K, 318.15 K, and 323.15 K). Using the ρ and u data, partial molal volume () and partial molal isentropic compressibility () values have been computed. The increase in partial molal volume with temperature has been attributed to the volume expansion of hydrated zwitterions. The and values of l-alanine, l-proline, l-valine, and l-leucine in 2M aqueous KCl and KNO₃ solutions have been found to be larger than the corresponding values in water. The larger partial molal volumes of l-alanine, l-proline, l-valine, and l-leucine in 2M aqueous KCl and KNO₃ solutions have been ascribed to the formation of ‘zwitterion-K⁺/Cl⁻/NO₃⁻’ and ‘K⁺/Cl⁻/NO₃⁻–water dipole’ aggregates in solutions. The formation of these entities in solutions causes the release of water associated with zwitterions to the bulk water. The larger partial molal compressibilities of l-alanine-/l-proline-/l-valine-/l-leucine–2M aqueous KCl/KNO₃ solutions than the corresponding values in water have been attributed to the formation of ‘zwitterion–ion’ and ‘ion–water dipole’ incompressible entities in solutions.     

Anisotropic lattice coherency of GaAs nanocrystals deposited on Si(100) surface by molecular beam epitaxy

Lattice coherency and morphology of GaAs nanocrystals grown on Si(100) substrate have been studied by transmission electron microscope in order to see growth mechanism of the nanocrystals. GaAs nanocrystals consisting of four {111} facet planes and a rectangular basal plane with four sides along [01 ] and [011] directions have grown on the Si surface. Either (011) or (01) lattice planes along the minor axis on the rectangular basal plane in the GaAs nanocrystal are completely-coherent with {011} lattice planes in the Si substrate. On the other hand, another (01) or (011) lattice planes along the major axis are partially-coherent with those in the Si substrate. When the lattice planes of the either (011) or (01), which is randomly determined by local atomic structures, become partially-coherent with those in the Si substrate to relax accumulated lattice strain, the growth rate of nanocrystal is remarkably increased along the direction parallel to unstrained (011) or (01) planes which prevents from each area of the strained {011} planes in the nanocrystals increasing. The anisotropic lattice coherency between the GaAs nanocrystals and the Si substrate causes the anisotropic morphology of the GaAs nanocrystals which is elongated the directions parallel to the strained {011} planes.