Conductometric Study of Some Metal Halides in Glycerol + Water Mixtures

Electrolytic conductivities of potassium halides, KX (X = Cl⁻, Br⁻, I⁻) have been investigated in 10, 20, and 30 mass% glycerol + H₂O mixtures at 298.0, 308.0, and 318.0 K. The conductance data have been analyzed by the Fuoss-conductance–concentration equation in terms of the limiting molar conductance (Λ⁰), the association constant (K _A ), and the distance of closest approach of ion (R). The association constant (K _A ) tends to increase in the order: 10 mass% < 20 mass% < 30 mass% glycerol + water mixtures, while it decreases with temperature. Thermodynamic parameters ΔH ⁰, ΔG ⁰, and ΔS ⁰ are obtained and discussed. Also, Walden products (Λ⁰η) are reported. The results have been interpreted in terms of ion–solvent interactions and structural changes in the mixed solvents.     

Structural, optical and photoelectrochemical properties of brush plated CdSe x Te1 − x thin films

CdSe _x Te_{1 – x} films have been deposited by the brush plating technique for the first time, on titanium and conducting glass substrates at room temperature. These films were annealed in argon atmosphere at 475°C for 15 min. Their structural, optical and photoelectrochemical (PEC) properties are presented and discussed. The power conversion efficiency has been found to be 9.0% at 60 mW cm^–2white light illumination. A peak quantum efficiency of 0.7 has been obtained for the films of composition CdSe_0.7Te_0.3. Donor concentration of 10¹⁷cm^–3and electron mobility of 60 cm²V^–1sec^–1were obtained.     

Thermal Conductivity Measurements of Liquid Mercury and Gallium by a Transient Hot-Wire Method in a Static Magnetic Field

The transient hot-wire method, incorporating a static magnetic field, has been developed to measure thermal conductivities of liquid mercury and gallium. Prior to the measurements, the effect of an alumina-coated hot wire on the measurements has been evaluated. Natural convection in the liquid metals has been effectively suppressed by the Lorentz force acting on the liquid metals in a static magnetic field. The thermal conductivities of liquid mercury and gallium have been determined to be 7.9 W.m ⁻¹.K ⁻¹ at 291 K and 24 W.m ⁻¹.K ⁻¹ at 302.9 K, respectively.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

EPR and photoluminescence properties of combustion-synthesized ZnAl<Subscript>2</Subscript>O<Subscript>4</Subscript>:Cr<Superscript>3+</Superscript> phosphors

An efficient red emitting ZnAl₂O₄:Cr³⁺ powder phosphor material was prepared at furnace temperatures as low as 500 °C by using the combustion method. The prepared powders were analyzed by X-ray diffraction and scanning electron microscopy techniques. The optical properties were studied using photoluminescence technique. The EPR spectra exhibit an intense resonance signal at g = 3.74 which is attributed to Cr³⁺–Cr³⁺ pairs, and the weak resonance signal of at g = 1.97 is attributed to Cr³⁺ single ion transition. The spin population (N) has been evaluated as a function of temperature. The excitation spectrum exhibits two broad bands in the visible region which are characteristic of Cr³⁺ ions in octahedral symmetry and the emission spectrum exhibits zero-phonon line frequencies along with vibronic frequencies. The crystal field parameter (Dq) and Racah parameters (B and C) have been evaluated and discussed.     

Apparent new phase of monolayer3He and4He films adsorbed on Grafoil as determined from heat capacity measurements

Heat capacity measurements in monolayer³He and⁴He films adsorbed on Grafoil at densities higher than the one corresponding to the substrate lattice registered phase show a series of sharp, narrow peaks at 1 K for densities between 0.072 and 0.077Å ^–2. The exact nature of the transition cannot be determined from this measurement alone, but several possibilities are discussed. It has been determined that the melting line of two-dimensional solid films starts atn=0.078Å ^–2 for both isotopes. Extensive heat capacity measurements at and above this density are presented for³He, and some new measurements for⁴He are shown to complement measurements reported elsewhere. The solid³He measurements are compared to predictions of recent models for melting in two dimensions.Work supported by National Science Foundation Grant # DMR72-03003A04.     

Characterization of Cd1−x Zn x Se thin films deposited at low temperature by chemical route

Optoelectronic technologically important pseudo-binary Cd_1−x Zn _x Se thin films with a variable composition (0 < x < 1) has been developed by chemical bath deposition method. The objective to study growth kinetics, physical, microscopic, compositional, optical, electrical and structural changes. Cd_1−x Zn _x Se have been deposited on non-conducting glass substrate in tartarate bath containing Cd⁺² and Zn⁺² ions with sodium selenosulphate with an aqueous alkaline medium at 278 K. The quality and the thickness of the films are depends upon deposition temperature, deposition time and pH, etc. X-ray diffraction (XRD), atomic absorption spectroscopy, optical absorption, scanning electron microscopy and thermoelectric technique characterized the films. The XRD study indicates the polycrystalline nature in single cubic phase over whole range of composition. Analysis of absorption spectra gave direct type band gap, the magnitude of which increases non-linearly as zinc content in the film is increased and dc electrical conductivity at room temperature was found to decreases from 10⁻⁷ to 10⁻⁸ (Ω cm)⁻¹. All the films show n-type conductivity. The promising features observed are the formation of continuous solid solutions in a single cubic phase.     

Melting curve and related thermodynamic properties of He3 below 1°K

Measurements of the melting curve of He³ have been made between 16.6° mK and 0.7°K. The liquid and solid molar volumes at melting have been extended from 0.33°K to below 0.03°K, and the molar volume change on melting V _m obtained. An increase in V _m from 1.20 cm³/mole at 0.33°K to 1.28 cm³/mole at 0.06°K is found, with an extrapolation to 1.27 cm³/mole atT=0. Examination of the thermodynamic consistency of the data indicates that our melting curve slopes are accurate, within experimental error, down to a temperature of 0.04°K. The demagnetization apparatus is described, with considerable attention devoted to thermometry, thermal equilibrium, and experimental techniques. Construction details of the high-sensitivity capacitive strain gauge are given.Research supported by the National Science Foundation.     

The de Haas-van Alphen effect in Sb(Sn) and Sb(Te) alloys

Measurements have been made of the de Haas-van Alphen effect in Sb(Sn) alloys containing up to 0.58 at % Sn and Sb(Te) alloys with up to 0.26 at % Te. The maximum electron period in the bisectrix-trigonal plane increased from 14.66 × 10^–7 G^–1 in pure Sb to 68.0 × 10^–7 G^–1 in the most concentrated Sn-doped sample, whereas the maximum hole period decreased from 16.33 × 10^–7 to 7.4 × 10^–7 G^–1 . The Te doping had the opposite effect—increasing the number of electrons and decreasing the number of holes. The results of measurements of effective mass and Fermi surface area are found to be consistent with a rigid-band model of these dilute alloys. Both electron and hole bands are strongly nonparabolic and a two-band model is used to estimate that the gap below the electron pocket at L is 110 ± 25 meV. The Fermi levels of electrons and holes in Sb are estimated to be 150 ± 10 and 180 ± 40 meV, respectively. It is predicted that the electron pockets will be completely emptied for an alloy with 0.78 ± 0.07 at % Sn. Pseudopotential calculations suggest that the rigid-band model is a reasonable approximation in these dilute alloys.     

Phase Transition of Superfluid 3He in Aerogel

We have studied phase transition of superfluid ³He in 97.5% porosity aerogel by NMR method. Above 1.0 MPa, superfluid phase transition has been observed. The transition temperature T _c ^a is strongly suppressed from its bulk value. The Pressure-Temperature diagram suggests that superfluid phase will not appear below near 0.8 MPa. The A-B phase transition has been observed above 1.3 MPa, below which a state of superfluid phases remains to be identified. The temperature dependence of NMR frequency shifts Δf in the A-like and the B-like phases are almost linear at pressures below 2.4 MPa. We obtained the differential coefficient of NMR frequency shifts ∂(Δf)/∂(T/T _c ^a ) at 0.9T _c ^a as a function of pressure, and it suggests that superfluid phase will not appear below near 0.8 MPa which is the same pressure estimated by P-T diagram.     

Critical Current Densities and Vortex Dynamics in Ba(Fe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript>As<Subscript>2</Subscript> Single Crystals

Superconducting properties are evaluated for high-quality single crystals of Ba(Fe_1−x Co _x )₂As₂ in a wide range of doping levels. The critical current density, J _c , in an optimally-doped crystal (T _c ∼24 K) shows a fishtail effect with its value over 10⁵ A/cm² even at 5 T below 10 K. Magneto-optical imaging has clarified rather homogeneous supercurrent flow in the crystal, in spite of a large amount of impurities. In the heavy-ion irradiated sample, the presence of columnar defects are confirmed and J _c has been enhanced by a factor of five at low temperatures, reaching 6×10⁶ A/cm² at 2 K under zero field. Flux creep rate in the heavy-ion irradiated sample has been reduced in accordance with the enhancement of J _c .     

Oxygen self-diffusion in rutile under hydrothermal conditions

Oxygen self-diffusion coefficients have been determined for synthetic and natural rutile single crystals under hydrothermal conditions at 100 MPa total pressure and in the temperature range 873–1373 K. The diffusion coefficients are lower than the results from dry gas exchange studies would predict. Between 973 and 1373 K the results can be characterized by two linear Arrhenius relationships. D=1.14×10^–11 exp(–168.8 kJ mol^–1/RT) m^–2s^–1 for the natural rutile, and D=2.41×10^–12 exp(–172.5 kJ mol^–1/RT) m²s^–1 for the synthetic crystal. The results have been interpreted in terms of a defect model involving the dissolution of water in rutile as substitutional hydroxyl defects on oxygen lattice sites, with a solution enthalpy in the range 81–106 kJmol^–1.     

Grain growth in copper and alpha-brasses

The wires of 99.999% copper and alpha-brasses containing 12, 20, 30 and 35 at % Zn have been annealed in vacuum for 30 to 240 min at 873, 923, 973 and 1023 K. The grain-growth data obtained are well encompassed by the relationD ², —D ₀ ² , =Kt exp(-H/kT), whereD is the instantaneous mean grain diameter at the time,t, of isothermal anneal andD ₀ refers to the initial mean grain diameter. In alpha-brasses the activation energy for grain-boundary self-diffusion,H, and the pre-exponential factor,K, depends on the zinc concentration,c, asH = (H ₀ — 1.1c) eV andK =K ₀ exp(-10.7c) cm² sec^–1. The values ofH ₀ andK ₀, referred to the base metal are respectively 0.87 eV and 3.0 × 10^–4 cm² sec^–1, which are in good agreement with those (0.85 eV and 3.6 × 10^–4 cm² sec^–1) found for copper.     

Cyclic fatigue-crack propagation in a silicon carbide whisker-reinforced alumina composite: role of load ratio

The characteristics of subcritical crack growth by cyclic fatigue have been examined in a silicon carbide whisker-reinforced alumina composite, with specific reference to the role of load ratio (ratio of minimum to maximum applied stress intensity, R=K _min/K _max); results are compared with similar subcritical crack-growth data obtained under constant load conditions (static fatigue). Using compact-tension samples cycled at ambient temperatures, cyclic fatigue-crack growth has been measured over six orders of magnitude from ∼10⁻¹¹–10⁻⁵ m cycle⁻¹ at load ratios ranging from 0.05–0.5. Growth rates (da/dN) display an approximate Paris power-law dependence on the applied stress-intensity range (ΔK), with an exponent varying between 33 and 50. Growth-rate behaviour is found to be strongly dependent upon load ratio; the fatigue threshold, ΔK _TH, for example, is found to be increased by over 80% at R=0.05 compared to R=0.5. These results are rationalized in terms of a far greater dependency of growth rates on K _max(da/dN ∞ K _max ³⁰ ) compared to ΔK(da/dN ∞ ΔK ⁵), in contrast to fatigue behaviour in metallic materials where generally the reverse is true. Micromechanisms of crack advance underlying such behaviour are discussed in terms of timedependent crack bridging involving either matrix grains or unbroken whiskers.     

Investigation of the temperature dependence of electron and phonon Raman scattering in single crystal YBa2Cu3O6.952

Detailed Raman-scattering measurements have been performed on high-quality YBa₂Cu₃O_6.952 single crystal (T _c =93 K, T _c =0.3 K). A sharp (FWHM 7.2 cm^–1 at 70 K and 10.0 cm^–1 at 110 K) 340 cm^–1phonon mode has been observed inB _1g polarization. An electronic scattering peak at 500 cm^–1 in theB _1g polarization extends down to 250 cm^–1. These FWHM values determine the upper limit of the homogeneous linewidth of the phonon and electronic excitations. The start of the electronic spectral function renormalization and of the 340 cm^–1 mode anomalies (frequency softening, linewidth sharpening, and intensity increase) have been observed to occur approximately 40 K aboveT _c . The 340 cm^–1 mode Fano shape analysis has been performed and the temperature dependences of the Fano shape parameters have been estimated. All 340 cm^–1 mode anomalies have been explained by the electronic spectral function renormalization.This work was supported by Swedish Natural Sciences Research Council (G.B. and L.B.) and by the National Science Foundation (DMR 91-20000) through the Science and Technology Center for Superconductivity (G.B. and M.V.K.).     

Characteristics of surface states at the insulator-semiconductor interface of ZnS:Mn thin-film electroluminescent emitters

The energy distribution of the density of occupied surface states (N _ss) at the cathode insulatorphosphor interface in ZnS:Mn electroluminescent thin-film (ELTF) emitters has been modeled on the basis of experimental data. Changes in this distribution depending on the parameters of exciting voltage pulses have been studied. It is established that the energy distribution of N _ss shifts toward deeper levels upon a decrease in the frequency of the exciting signal and the resulting increase in the pause between the adjacent switch-on states. This behavior corresponds to a cascade relaxation mechanism of electrons trapped on the surface states. Maximum values of the N _ss (∼2.5 × 10¹³ cm⁻²) and the specific density of surface states per unit energy (2 × 10¹⁴–10¹⁵ cm⁻² eV⁻¹) are determined for the cathode insulator-phosphor interface from which electrons are tunneling. Positions of the equilibrium (∼1.25 eV below the conduction-band bottom) and the quasi-equilibrium (0.6–1.25 eV) Fermi levels during the ELTF emitter operation are estimated.     

3He Anomalously High Spin Diffusion in Solid Para-Hydrogen

NMR measurements of ³He spin diffusion coefficient in solid para-H₂ are carried out at the temperatures 0.45–1.5 K. The crystals have been grown under constant pressure 20–130 bar. The ³He concentrations in the initial para-H₂–³He gas mixtures were 0.1% and 0.3%. It is found out that the decay of echo amplitude vs both magnetic field gradient G and time interval τ between RF pulses is of non-exponential character, typical of one-dimensional diffusion in restricted geometry. The values of true spin-diffusion coefficient D _S measured are found to be ∼10⁻⁴ cm²/s at 20 bar. At 108 bar D _S value is one order of magnitude less. D _S does not depend on temperature. Such spin diffusion coefficient values seem to be anomalously high in comparison with well-known values of D _S =10⁻⁵ cm²/s for bulk liquid ³He at 27 bar and D _S =10⁻⁸ cm²/s for bulk solid ³He at 108 bar. The special experiments with the crystal annealing make it clear that the high spin diffusion here is connected with fast diffusion along dislocation lines.     

Thermodynamic functions of ScVO<Subscript>4</Subscript> at temperatures from 0 to 350 K

The heat capacity of ScVO₄ has been determined by adiabatic calorimetry at temperatures from 14.52 to 347.13 K, and smoothed heat capacity data have been used to evaluate its thermodynamic functions (entropy, enthalpy increment, and reduced Gibbs energy). At 298.15 K, the thermodynamic functions of scandium orthovanadate are C _p ⁰(298.15 K) = 110.5 ± 0.1 J/(mol K), S ⁰(298.15 K) = 110.9 ± 0.1 J/(mol K), H ⁰(298.15 K) − H ⁰(0) = 18.53 ± 0.01 kJ/mol, and Φ⁰(298.15 K) = −[G ⁰(298.15 K)/298.15] = 48.75 ± 0.12 J/(mol K). The calculated Gibbs energy of formation of scandium orthovanadate from its constituent elements is Δ_f G ⁰(ScVO₄, 298.15 K) = −1644.0 ± 2.2 kJ/mol.     

Formation, densification, and electrical conductivity of air-sinterable (Sm1 − x Ca x )CrO3 prepared through citric acid route

In Ca²⁺-substituted SmCrO₃, single-phase perovskite (Sm_{1 – x} Ca _x )CrO₃, where x = 0 to 0.27, have been formed at low temperatures by a citric acid processing. (Sm_{1 – x} Ca _x )CrO₃ powders consisting of submicrometer-size particles are sinterable; dense materials can be fabricated by sintering for 2 h at 1700°C under atmospheric pressure. The relative densities, grain sizes, and electrical conductivities increase with increased Ca²⁺ content. (Sm_0.73Ca_0.27)CrO₃ materials show an excellent electrical conductivity of 2.6 × 10³ S · m^–1 at 1000°C.     

Phonon-limited mean free path in the Sondheimer oscillation of aluminum

The phonon-limited part of the electron mean free path l _Hp ,in aluminum has been investigated by the Sondheimer size effect for a magnetic field along the three principal crystallographic orientations in the temperature range 1.8–12 K. It is found that l _Hp decreases with temperature approximately as T ^–3 and varies from 1.4 × 10^–3 to 1.3 × 10^–2 cm, with the orbits responsible for the oscillations at 20 K. The results are in agreement with a calculation based on the 4-OPW Fermi surface model and the anisotropic electron-phonon scattering time given by Leung et al.Work supported in part by a Grant-in Aid for Scientific Research from the Ministry of Education.     

Influence of silver doping on the physical properties of Mg ferrites

The physical properties of silver substituted magnesium ferrite MgAg_xFe_2−x O₄ (0.0 ≤ x ≤1) have been studied as a function of silver ion concentration. The samples were prepared using the flash combustion technique. The effect of Ag content on microstructure and magnetic properties has been studied. X-ray diffraction (XRD) analysis reveals the spinel structure besides metallic silver particle at all concentrations. The decrease in the lattice constant with increasing Ag up to the critical concentration (x = 0.2) was discussed. The close agreement of the theoretical and experimental lattice constant ratio from XRD pattern supports the occupancy of silver on the tetrahedral sites. The morphological features were studied using scanning electron microscope. The magnetic susceptibility, Curie temperature (T _C), and effective magnetic moment (μ _eff) were decreased with compositional parameter. The DC electrical conductivity of the investigated samples was measured and the results indicated the increase in conductivity with increasing Ag content from 5.6 × 10⁴ at x = 0 to 12.6 × 10⁴Ω⁻¹ cm⁻¹ at x = 1. The transition from the ferrimagnetic to paramagnetic state is accompanied by an increase in the thermo EMF. Ag–Mg ferrite shows p-type conductivity at all concentrations of Ag particles where the creation of lattice vacancies is due to presence of Ag⁺ ions gives rise to the p-type conductivity.