Constitutional and structural studies of the intermetallic phase,ZrCu

Constitutional and structural studies have been carried out on the equiatomic percent alloy ZrCu, using magnetic susceptibility measurements, metallography and X-ray diffraction. The high-temperature magnetic susceptibility measurements and the metallographic studies indicate the presence of two solid state transformations in this alloy, a martensitic transformation with M _s=440±5 K and a eutectoid transformation with T _E=985±5 K. Thus, if the high-temperature ZrCu phase is metastably retained to lower temperatures then the martensitic transformation is observed, whereas prolonged annealing just below T _E produces a eutectoid mixture; the metallographic studies indicate that this mixture is lamellar in nature. The X-ray diffraction studies indicate that the high-temperature ZrCu phase has the b c c CsCl-type structure.     

Direct Measurements of Magneto-caloric Effect of Gd5Si2Ge2 Alloys in Low Magnetic Field

Direct measurements of the magneto-caloric effect of Gd₅Si₂Ge₂ alloys under heat treatment conditions are investigated by measuring adiabatic temperature change (ΔT _ad) in a magnetic field of 1.5 T using a homemade magnetocaloric effect measuring apparatus. The crystal structure, microstructure as well as the chemical composition of the alloys are measured using X-ray diffraction (XRD), scanning electron microscope (SEM) with energy dispersive X-ray Detector (EDX). It is found that the microstructure of the alloys could be fully homogenized and the impurities in the alloys could be remarkably removed via appropriate heat treatment. As a result, the maximum adiabatic temperature change (ΔT _ad) of the alloy annealed at 1573 K increases by 105% from 1.9 to 3.9 K for a magnetic field change from 0 to 15 kOe when compared to the as arc-melted material, while the magnetic ordering temperature slightly reduced.     

Thermomagnetic studies of (Co0.93Fe0.07)75−x Cr x Si15B10 metglas

Thermomagnetization measurements have been carried out on the Co-Fe-Cr-Si-B metglas up to 7.5 at% Cr. For the Co₇₀Fe₅Si₁₅Si₁₀ metglas, application of magnetic field has been found to influence the process of crystallization. Thermal annealing of this alloy at temperaturesT _c<T _a<T _cr leads to a decrease in the value ofT _c due to chemical short-range ordering (CSRO) and topological short-range ordering (TSRO). Introduction of chromium into the above alloy reduces the saturation magnetization and Curie temperature of the alloy at the rate of 5.55 emu gm^–1 and 41 K per at% Cr, respectively. These are explained due to the antiparallel coupling of the 3 d electrons of iron and cobalt atoms with those of chromium and to a deterioration in the strength of ferromagnetic exchange interactions between Co-Co, Co-Fe and Fe-Fe magnetic pairs.     

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.     

Low-temperature properties of Ga0.5Mo2S4

Specific heat measurements from 2 to 27 K and crystal structure and magnetic properties for 4.2–300 K are reported for the spinel Ga_0.5Mo₂S₄. AtT _p =42 K this ternary chalcogenide undergoes a phase transition. At liquid helium temperatures it has a rhombohedral crystal structure with lattice parametera=9.275 Å and rhombohedral angle =90.45°. It follows from the magnetization measurements that ferromagnetic ordering takes place at about 20 K, and close to the Curie point an anomaly in the temperature dependence ofC _p has been found. The magnetic behavior of Ga_0.5Mo₂S₄ at low temperatures can be explained by a small concentration of magnetic ions and far-distant-neighbor superexchange interactions partly weakened by vacancies located in 4(c) positions. Magnetocaloric measurements indicate the dominance of ferromagnetic interactions.     

The effect of nitrogen on viscosity of La-Si-Mg-O-N glasses by compressive creep and dilatometry

Viscosity as a function of temperature and nitrogen content and the glass transition temperature (T_g) were investigated in 20La-60Si-20Mg-O-N glasses with nitrogen contents varying from 0 to 28 eq.% (e/o) using compressive creep and dilatometric experiments. Although T_g obtained from dilatometry were 6–12°C lower than the lower limit of the transition temperature range from creep, both methods revealed identical and linear dependencies of T_g on N content. The average activation energy was 1184 ± 36 kJ/mol and viscous flow remains the deformation mechanism in all glasses regardless of nitrogen content. Addition of 28 e/o N in oxide glass resulted in an increase of T_g by 94–105°C and an increase in viscosity of around 5 orders of magnitude. These changes are greater than those reported in similar Al-containing glasses. Linear increase of T_g and compactness of the glass with nitrogen content result from enhanced cross-linking of the glass network.     

Effect of Synthesis Process and Substrate on Electrical and Thermal Transport Properties of Bi-2212

Resistivity, thermoelectric power, and thermal conductivity have been measured for a Bi-2212 system synthesized from a glassy precursor, either with a commercially Al₂O₃ substrate or with a homemade BaZrO₃ substrate. Those measurements show that the BaZrO₃ substrate gives better superconducting properties to the Bi-2212 than the Al₂O₃ substrate. The effect of (1.0 T) weak magnetic field has been searched for. The thermal magnetoconductivity and the contributions of the magnetic field to the thermoelectrical power are studied and compared through fine measurements. An electronic contribution seems to appear already well above the critical temperature and to exist up to 200°K. The onset temperature is thus deduced.     

Magnetic and structural phase transitions of MnBi under high magnetic fields

Abstract

High-field x-ray diffraction and magnetization measurements and differential thermal analysis (DTA) were carried out for polycrystalline MnBi with an NiAs-type hexagonal structure to investigate its magnetic and structural phase transitions. The lattice parameter a rapidly decreases below the spin reorientation temperature T_SR(=90 K) in a zero magnetic field. The parameter c decreases gradually with decreasing temperature and exhibits an anomaly in the vicinity of T_SR. By applying a magnetic field of 5 T, the parameter a increases by ～0.05% when T<T_SR and varies smoothly when 8≤T≤300 K. DTA data show that the magnetic phase transition temperature from the ferromagnetic state to the paramagnetic state increases linearly at a rate of 2 KT^?1 with increasing magnetic field up to 14 T.     

Magnetic properties of D-Er2Si2O7 at low temperatures

The low field magnetic susceptibility and the high field magnetic moment of D-Er₂Si₂O₇ at low temperatures are measured by an inductance method. The susceptibility results show that D-Er₂Si₂O₇ undergoes a transition from a paramagnetic to an antiferromagnetically ordered state at T _N=1.9 +-0.1 K. The magnetic moment data is analysed on the basis of a four-sublattice antiferromagnetic spin model. The g-values obtained from magnetic susceptibility and moment experiments are compared with those of optical measurements.     

Oxidation kinetics of SiC deposited from CH3SiCl3/H2 under CVI conditions

Oxidation tests were performed on SiC deposits prepared from CH₃SiCl₃/H₂ under chemical vapour infiltration conditions, at temperatures ranging from 900–1500 °C under a flow of pure oxygen at 100 kPa (passive oxidation regime). The kinetics of growth of the silica layer were established from thickness measurements performed by spectroreflectometry. They obey classical parabolic laws from which rate constants are calculated. Within 1000–1400 °C, the oxidation process is thermally activated with an apparent activation energy of 128 kJ mol^–1. Above 1400 °C and below 1000 °C, an increase in the activation energy is observed which is thought to be related to a change in the mechanism of the oxygen transport across the silica layer forT>1400 °C and tentatively to stress effects forT<1000 °C. The kinetics data are compared to those measured on silicon single crystals (used as a standard) and to other reported data on SiC.     

Vacancy Assisted Motion of Charges in Quantum Crystals

From comparison of results of measurements of diffusion coefficients D(T) of the positively and negatively charged complexes (charges), created under irradiation in perfect crystals, grown from pure helium or hydrogen at small pressures, with diffusion coefficients of the isotopic impurities or the self-diffusion coefficients known from NMR studies one can conclude that motion of the more mobile charges through these crystals (of positive charges in HCP ⁴ He, ³ He and D ₂ samples and of negative charges in BCC ⁴ He and ³ He and also in HCP crystals, grown from pure p–H ₂ ) is vacancy assisted. Thus strong departures of the temperature dependencies of diffusion coefficients of the positive charges D ₊(T) in HCP ⁴ He samples and of the negative charges D _–(T) in p–H ₂ samples from the simple Arrhenius type of behavior D= D ₀ exp[–G/T] at low temperatures can be attributed to the change of the mechanism of diffusion of thermal vacancies: from classical thermally activated hopping of the localized vacancies near T _melt to the band motion of delocalized vacancions at T< T _melt/2. To explain the nature of the maxima on the D ₊(T) curves observed in perfect ⁴ He crystals, it may be assumed that the flow conditions of the vacancion gas around a positive charge (a probe particle with an effective radius of a few lattice constants) can change significantly with lowering the temperature: from a hydrodynamic flow of the viscous gas round the probe at the transition temperatures to a kinetic flow of the rarefied vacancion gas at low temperatures. In this case the bandwidth of the vacancions in studied ⁴ He samples is near Q _V 10^–4 K.     

3-D Simulations of Diffusivity Measurements in Liquids with an Applied Magnetic Field

The effect of convective contamination in self-diffusivity experiments of liquid metals is predicted via a three-dimensional (3-D) model that includes an applied magnetic field. A uniform heat flux is applied at the sidewall of the cylindrical ampoule, and heat losses are allowed at the top and bottom walls of the ampoule. A wide range of a uniform, steady, axial magnetic field (from moderate to very strong) is considered in the model. Since the thermal Peclet number, Pe, is very small for the parameters of interest, convective heat transfer is neglected. A large interaction parameter, N, suggests that the flow is inertialess. The temperature and flow problems are solved at steady state while the time-dependent concentration problem is determined for various mass Peclet numbers, P e. In all cases, the output D (i.e., with convective contamination) increases with an increase in the temperature non-uniformity T. The radial and azimuthal velocities are much smaller than the axial velocity in each case. A stronger magnetic field can tolerate a higher temperature non-uniformity T, but T is still less than 0.025 K with a 5 T magnetic field for convective contaminations to be less than 5 of the total mass flux.     

Anomalous Magnetic Behavior in RuSr2(Gd2−x Ce x )Cu2O10

We studied the magnetic properties of RuSr₂(Gd_2–x Ce _x )Cu₂O₁₀. In the temperature dependence of magnetic susceptibility, we observed a ferromagnetic increase and an anomalous oscillating behavior below T _M. In ESR measurements, we observed a single magnetic resonance signal from Ru ions and a similar wavy structure in its resonance field below T _M. These magnetic phenomena possibly originate from an intrinsic Ru-spin motion.     

Thermal and magnetic measurements on YBa2Cu3O7−y

Specific heat measurements in zero and7T magnetic fields from 0·4 to 100 K, and magnetic susceptibility measurements aboveT _c on a series of polycrystalline samples of YBa₂Cu₃O_7−y (YBCO) reveal a number of regularities. The size of ΔC(T _c )/T _c for the samples [ΔC(T _c ) is the jump in the specific heat atT _c ] appears to vary linearly with the low-temperature value of the Debye Θ, with the entropy change between ≈ 85 K andT _c induced by the application of a 7 ^Г magnetic field, and with the variation of the observed low temperatureγ with magnetic field, dγ/dH. On the other hand the temperature-independent part of the magnetic susceptibility aboveT _c appears to be essentially independent of ΔC(T _c )/T _c . These results are consistent with the idea that samples of YBCO belowT _c are a mixture of superconducting and normal phases. Supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Dept. of Energy under Contract DE-AC03-76SF00098, and by an EXXON Education Grant from the Research Corporation.     

Thermoelectric effect in the hollow superconducting cylinder of Pb and Pb-In alloy

The technique for measuring of the thermoelectric effect in superconductors near the critical temperatureT _C has been elaborated. The measured magnetic flux was induced by temperature gradient (ΔT∼1K) between joints. The experimental study of the thermoelectric effect has been carried out in the hollow bimetallic superconducting cylinder consisting of the Pb and Pb-In alloy. The observed temperature dependence of the thermoexcited magnetic flux has been well approximated by the logarithmical function. The magnitude order of the thermopower estimated from the experimental data coincides with the known low temperature thermopowers of metals.     

Some effects of an electric field on the plastic deformation of metals and ceramics

 The external parameters generally considered in the plastic deformation of metals and ceramics are the temperature, pressure or stress and time. Usually neglected are the effects of electric and magnetic fields. However, such fields can often have a significant influence, especially when applied concurrently with the more common parameters. Some examples of the effects of an electric field on the plastic deformation of metals and ceramics found by the author and his coworkers are presented. Included are the following: (a) the influence of electropulsing on the flow stress of metals at 78–300 K, (b) the effect of an external electric field (surface charge) on the superplastic deformation of the 74754 Al alloy, (c) the influence of an electric field on the flow stress and ductility of polycrystalline NaCl at 0.28–0.75 T_M and (d) the effect of an electric field on the superplastic deformation of 3Y-TZP. Mechanisms responsible for the observed effects are considered. Received: 1 January 1998 / Accepted: 1 March 1998     

Thermally activated dissipation mechanism in Sn doped CuTl-1223 superconductors

We have carried out magneto-resistance measurements of Sn-doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y S _y O_10−δ (y = 0.5, 1.0, 1.5) superconductors and from there studied the effect of Sn doping on the thermally activated dissipation mechanism. A systematic decrease in T _c(R = 0) and shift of T _c (onset) towards lower temperature is typical feature of Sn-doped samples with the increased strength of external magnetic field. In these samples pronounced broadening of resistive transitions has been observed on the application of external magnetic field. The activation energy of carriers obtained from the Arrhenius plots of log (ρ) versus 1000/T have been found to decrease with the increase in the applied external field as well as increased Sn doping concentration. From log (ρ) versus U _o/T plots we have found that for lower concentration of Sn the thermally activated dissipation can be explained in terms of flux creep. On the other hand for higher concentration of Sn, flux flow mechanism of energy dissipation seems to be dominant.     

Influence of magnetic annealing on high-frequency magnetic properties of FeCoNd films

FeCoNd thin film with thickness of 166 nm has been fabricated on silicon (1 1 1) substrates by magnetron co-sputtering and annealed for one hour under magnetic field at different temperatures (T_a) from 200 °C to 700 °C. The As-deposited and annealed FeCoNd film samples at T_a ≤ 500 °C were amorphous while the ones obtained at T_a ≥ 600 °C were crystallized. We found that the perpendicular anisotropy field gradually decreases as the annealing temperature increases from room temperature to 300 °C. A well induced in-plane uniaxial anisotropy is achieved at the annealing temperature between 400 and 600 °C. The variation of the dynamic magnetic properties of annealed FeCoNd films can be well explained by the Landau-Lifshitz equation with the variation of the anisotropy field re-distribution and the damping constant upon magnetic annealing. The magnetic annealing might be a powerful post treatment method for high frequency application of magnetic thin films.     

Temperature dependence of the density of liquid tin

The two-capillary pycnometer method is used to study the temperature dependence of the density of tin in the temperature range fromT _melt to 770 K. The measurements are carried out in a superhigh vacuum in higher purity samples. The experimental data are described by an equation of the straight line p(T) = 7374.7 -676.5 x 10_-3Trand do not confirm the anomaly (observed previously by some authors) in the form of a minimum on the poly therm of the density of tin in the temperature range from 610 to 700 K     

Ideal Gas Thermodynamic Properties of Propyl tert-Butyl Ethers from Density Functional Theory Results Combined with Experimental Data

Ideal gas thermodynamic properties, S°(T), C _p°(, T), H°(T)–H°(0), _f H°(T), and _f G°(T), are obtained on the basis of density functional B3LYP/6-31G(d,p) and B3LYP/6-311 + G(3df,2p) calculations for two propyl tert-butyl ethers. All torsional motions about C–C and C–O bonds were treated as hindered internal rotations using the independent-rotor model. An empirical approximation was assumed to account for the effect of the coupling of rotor potentials. The correction for rotor–rotor coupling was found by fitting to entropy values determined from calorimetric measurements. Enthalpies of formation were calculated using isodesmic reactions.