Slope Of Temperature Dependence Research Articles

In-situ study of pearlite nucleation and growth during isothermal austenite decomposition in nearly eutectoid steel

The evolution of the microstructure during the isothermal austenite/pearlite transformation in a nearly eutectoid steel was studied by the three-dimensional neutron depolarization technique, which simultaneously provides information about the pearlite fraction, the average pearlite colony size, and the spatial distribution of the pearlite colonies during the transformation. The in-situ measurements show that the pearlite nucleation rate increases linearly with time with a temperature-dependent slope. The in-situ measured average pearlite growth rate is accurately described by the Zener-Hillert theory, which assumes that volume diffusion of carbon is the rate-controlling mechanism. The measured overall transformation rate deviates from the predictions of the theory developed by Kolmogorov, Johnson, Mehl, and Avrami.

Dynamic and structural behavior of different rigid nonpolarizable models of water

The local structure and the dynamical behavior of water have been analyzed in two different regimes (the dense isochore of 0.995 g/cm2 and the supercritical isotherm of 673 K) through four rigid nonpolarizable models of water. An important change in the slope of temperature dependence of the self-diffusion coefficient at ρ=0.995 has been observed at T≈450 K, showing two main regions that are related to a change on the activation energy of the process (originated for a change of the structure of the first solvation shell from a tetrahedral to dodecahedral arrangement). The local orientational structure of water has been analyzed through the use of tetrahedral order parameters q. A direct relation between q and D has been observed for all models showing some kind of master curve up to 450 K at the 0.995 g/cm3 isochore. The structure of the system at short and large radial distances has been analyzed through a decomposition of the site–site radial distribution functions in terms of spherical harmonics, and a three-dimensional picture of the total pair distribution function has been reconstructed from this set of spherical harmonic projections.

Modified Model of Photon Echoes in Low-Temperature Glasses: Effect of Minimal Distance between Two-Level Systems and Chromophore

A modified theoretical model of photon echoes (PE) in low-temperature doped glasses based on the tunneling two-level system (TLS) and stochastic sudden jump models has been developed. The distinguishing feature of the model is its adaptation for Monte Carlo technique. The developed approach is more general than usual PE theories and allows us to take into account the different microscopic parameters of spectral dynamics in doped amorphous solids: in particular, the minimal radius of interaction between TLS and chromophore molecules, the dispersion of this coupling constant, and the changes of matrix properties nearby chromophore (shell-effect). The developed theory was applied for calculations of two-pulse PE decays for an organic amorphous system with typical parameters. It was found that the slope of temperature dependence of the homogeneous line width is decreased with an increase in the above-mentioned minimal radius. This effect could be one of the main reasons for the discrepancy between some PE ex...

Anisotropy of martensitic transformations in modeling of shape memory alloy polycrystals

Based on the knowledge of the anisotropy associated with the martensitic transformations obtained from tension/compression experiments with oriented CuAlNi single crystals, a simple constant stress averaging approach is employed to model the SMA polycrystal deformation behaviors. Only elastic and inelastic strains due to the martensitic transformation, variant reorientations in the martensite phase and martensite to martensite transformations in thermomechanical loads are considered. The model starts from theoretical calculation of the stress-temperature transformation conditions and their orientation dependence from basic crystallographic and material attributes of the martensitic transformations. Results of the simulations of the NiTi, NiAl, and Cu-based SMA polycrystals in stress–strain tests are shown. It follows that SMA polycrystals, even with randomly oriented grains, typically exhibit tension/compression asymmetry of the shape of the pseudoelastic σ − ε curves in transformation strain, transformation stress, hysteresis widths, character of the pseudoelastic flow and in the slope of temperature dependence of the transformation stresses. It is concluded that some macroscopic features of the SMA polycrystal behaviors originate directly from the crystallography of the undergoing MT's. The model shows clearly the crystallographic origin of these phenomena by providing a link from the crystallographic and material attributes of martensitic transformations towards the macroscopic σ − ε − T behaviors of SMA polycrystals.

Effect of a pulsed magnetic field on the transverse resistance of a layered superconductor

The field dependences of the transverse resistance of a single crystal of the layered superconductor Bi2Sr2CaCu2Oy (BSCCO-2212) with Tc0≃92 K are studied in magnetic fields up to 50 T in the perpendicular orientation H⊥(ab). It is established that in the resistive region the resistance is a power-law function of the field, and the temperature dependence of the barrier height for flux creep is obtained. It is found that in a wide temperature range, 50–125 K, the transverse magnetoresistance of the crystal in the normal state and under conditions of superconductivity suppression by a strong magnetic field is negative and can be approximated by a linear law with a temperature-dependent slope.

Magnetotransport properties of NiFe–Ag granular alloys: Origin of the thermal behavior

The effect of the temperature and magnetic field on the giant magnetoresistivity (GMR) of two FeNi–Ag granular alloys of composition Fe11.4Ni6.4Ag82.2 and Fe7.6Ni16.4Ag76.0 is discussed. Both samples were prepared by rf magnetron sputtering. Parts of them were rapidly annealed at 600, 650, and 750 °C. All samples displayed giant magnetoresistivity which decays from its maximum value with a Tm behavior, with m≈0.8–0.9, suggesting that the decrease in the maximum magnetoresistivity is due to the reduction in the particle magnetization associated with the spin wave excitation, which is a different mechanism to the electron-magnon interaction responsible for the T dependence of GMR in magnetic multilayers. Magnetoresistivity ρM decreases with temperature sharing essentially the same temperature decrease as the square of the macroscopic magnetization M in the whole magnetic field range studied, which is due to the reduction in the particle magnetization and to superparamagnetic effects. The effect of the width of the particle size distribution and interparticle interactions on the linear relation ρM vs M2 are discussed. Care should be taken when representing ρM/ρ(T,H=0) vs (M/Ms)2 because the strong temperature-dependent slope shown in these plots is mainly due to the temperature dependence of both the resistivity ρ(T,H=0) and Ms, and it is not an intrinsic T dependence of GMR in granular alloys. Experimental results suggest that in granular materials, magnetoresistivity is dominated by magnetic moments at the surface of the particles, which also play a very important role in the demagnetization processes, and small magnetic particles.

TiO2 Thin film oxygen sensors

TiO2 thin film was made on an alumina rod through sol-gel process by dip-coating of a titania sol derived from titanium butanol alkoxide solution in ethanol. The dependence of the electric resistance (Rx) of TiO2 film on the oxygen partial pressure (PO2) and temperature (T) was determined. The TiO2 thin film oxygen sensor possesses a linear relations between lg(Rx) and lg(PO2) with temperature-dependent slopes. The response time of TiO2 thin film sensor was investigated and discussed

Absorption edge in liquid iodine under pressure

Absorption coefficient measurements of the optical bands of liquid iodine near the triple point and of the band edge along the melting curve to 1.52 GPa and 683 K are reported. The intense absorption observed in the near ultraviolet is discussed in terms of intramolecular absorption bands. The absorption edge is dominated in the 4000 to 12000 cm −1 range by exponential tails of the ‘Urbach’ type with strongly temperature dependent slopes and amplitudes. The possibility of connecting the evolution of this spectral feature with the conductivity jump recently observed in liquid iodine around 3 GPa is discussed.

An investigation of response time of TiO 2 thin-film oxygen sensors

TiO 2 thin-film oxygen sensors have been made by a sol-gel process. The TiO 2 thin film is formed on an alumina rod by dip-coating of a titania sol derived from titanium butanol alkoxide solution in ethanol. The dependence of the resistance ( R) of the TiO 2 film on the oxygen partial pressure ( P O2) and temperature ( T) has been determined. The TiO 2 thin-film oxygen sensor possesses linear relations between 1g( R) and 1g( P O2) with temperature-dependent slopes. The response time of the TiO 2 thin-film sensor is shorter for an atmosphere change from oxygen rich to lean than that from oxygen lean to rich. The response time decreases with increasing temperature. A platinum coating on the TiO 2 surface reduces the response time.

The incipient kinetics of hydride growth on cerium surfaces

The incipient hydriding stage of cerium is characterized by nucleation and growth of the hydride phase on the surface of the metal. The kinetics of this initial growth process were studied utilizing a hot-stage microscope (HSM) equipped with a video camera. Growth velocities of nuclei were determined as a function of reaction temperature and hydrogen pressure over a grid of temperatures and pressures which enabled the maintenance of near isothermal conditions (25–100°C, 10–1000 mbar H2). A linear pressure dependence of the growth velocities was obtained, with temperature-dependent slopes. An Arrehenius plot of these slopes yielded a straight line with apparent activation energy of 6.3 kcal (g atomic H)−1 (0.27 eV). The results were compared with those of the previously studied massive hydriding stage, where the reaction is characterized by a contracting envelope morphology with a hydride-metal boundary moving at a constant (temperature-pressure dependent) velocity. Under similar experimental conditions, the velocities of the initial nuclei growth process are significantly higher than the corresponding boundary velocities at the massive stage. Different types of controlling mechanism are proposed to account for these observed differences between the incipient and the massive hydriding stages.

Near-edge optical absorption in liquid iodine under high pressure.

Absorption coefficient measurements near the optical band edge of liquid iodine along the melting curve up to 1.52 GPa and 683 K are reported. An externally heated diamond anvil cell has been used to pressurize the sample. The spectra are dominated in the 4000--12000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range by exponential tails of the Urbach type with strongly temperature-dependent slopes and amplitudes. The possibility of connecting the evolution of this spectral feature with the conductivity jumps recently observed in liquid iodine around 3--4 GPa is discussed.

Model for c-axis transport in high-Tc cuprates.

We present a model for the c-axis resistivity ${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$ in the cuprates which incorporates interplanar disorder. Higher-order perturbative calculations demonstrate that this special disorder stabilizes a low-T metallic state and in the dynamical limit leads to temperature-dependent slopes d${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$/dT which are negative (positive) for low (high) hole concentrations. Predictions are presented for correlations which associate a nonlinear planar resistivity with the magnitude of negative d${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$/dT.

Ultrasonic Study on Anisotropic Elastic Properties of Oxide Superconductors

To investigate the interaction of a crystalline lattice with high-temperature superconductivity as well as with magnetic flux lines, we measured temperature dependence of elastic constants and ultrasonic attenuation in single-crystalline La1.86Sr0.14CuO4 in magnetic fields of 0-14 T. In zero field, the elastic response to the superconducting transition was found to be remarkably anisotropic. The elastic constant C33 shows abrupt softening just below Tc. In contrast, C11 revealed no abrupt softening but change in the slope of temperature dependence. In magnetic field, elastic constants exhibit either an additional hardening due to the stiffness of the flux line lattice or an apparent softening probably due to the depression of superconductivity.

Energy distribution and thermal broadening of band tail states in doped and undoped hydrogenated amorphous silicon

Total yield photoelectron spectroscopy is used to determine the energy distribution and temperature dependence of the occupied and total density of states (DOS) in doped and undoped a-Si:H. We find that n-type films possess an exponential conduction band tail above the Fermi level E F with a strongly temperature dependent slope. Over the temperature range investigated, 270K to 550K, the energy distribution of thermally occupied states above E F displays no peak, contrary to what has been previously suggested. The valence band tail remains exponential over the entire temperature range with a much weaker temperature dependence. Undoped a-Si:H films show similar behaviour as n-type material and their yield spectra tend to preclude the existence of a distinct defect sub-band above E F.

Hall-Coefficient Sign Inversion, Electrical Conductivity, and Its Anisotropy in Highly Intercalated NaxInSe Single Crystals

Electrical conductivity along and across the layers, its anisotropy, and Hall effect in indium selenide intercalated with high contents of sodium (NNa = 1020 to 1022 cm−3) are studied. It is observed that (i) negative slope of temperature dependence of conductivity changes into positive one when the sodium content reaches values greater than 4 × 1020 cm−3; (ii) electrical conductivity anisotropy increases under intercalation due to a more pronounced decrease of conductivity across the layers than along them; (iii) for samples NaxInSe with NNa ⪆ 5 × 1021 cm−3 Hall coefficientsign inversion takes place. A model proposed for the explanation of the observed results is discussed. [Russian Text Ignored].

FLUX TRAPPING AND SUPERCONDUCTIVE GLASS STATE IN La2CuO4−y: Ba

The mixed copper-oxide superconductors recently discovered are not classical superconductors but have been shown to exhibit some properties of a superconducting glass. The grains in these ceramics are not uniform bulk materials but rather a composition of coupled superconducting clusters. Evidence for the superconducting glass state was first seen in the susceptibility, chi, and magnetic-moment measurements of powder samples of La/sub 2/CuO/sub 4-y/:Ba from 1.9 to 35 K in magnetic fields up to 1.5 T. The diamagnetism observed in the zero-field-cooled state was considerably larger than when field cooled. Figure 1(b) shows typical sequences of temperature dependences expected of a glass state. After zero-field cooling the sample and switching on a 0.03-T field, chi is measured at point A. On heating sample, point B on curve 1 is reached. On recooling, a nearly temperature-independent susceptibility is measured. With further heating, point B is passed until point C is reached on curve 1, where on recooling, a temperature-dependent slope smaller than that of curve is followed. On further heating, curve 1 becomes reversible past point D. This same value D is reached by field cooling from 35 K. In field cooling past D, curve 2 is followed reversibly on a timemore » scale of two hours.« less

Anisotropic upper critical magnetic field in single crystal Ba2YCu3O7−y

Abstract The upper critical magnetic field Hc2 of Ba2YCu3O7−y has been measured using single crystals with applied fields up to 26T. The results exhibit strong anisotropy with regard to the c-axis and the basal plane. The slope of temperature dependence of Hc2 along the c-axis is -0.56T/K, while that along the basal plane is -3.3 to -2.6T/K. Hc2(0) for the c-axis and basal plane are estimated to be 35T and 160 to 210T, respectively, using the Werthamer-Helfand-Hohenberg theory. From these values, the coherence length along the c-axis is estimated to be 5.1 A, and that in the basal plane is 31 A. The former value is comparable to the lattice constant c.

Intrinsic Urbach Rule and Electron—Phonon Interaction in GaAs and Related III‐V Compounds

AbstractThe optical absorption at the edge of the fundamental absorption band is investigated in highpurity and high‐perfection GaAs crystals. The experimental curves show an exponential decay of the absorption coefficient as a function of the photon energy (Urbach's rule) with temperature dependent slope. Since the impurity contribution can be neglected, the data are analyzed within the framework of the polaron model of phonon‐assisted transitions. The good agreement obtained between experimental and calculated curves makes it possible to conclude that the interaction of electrons with LO phonons plays a dominant role in shaping the intrinsic Urbach tail in GaAs. The same theoretical model is also applied successfully to InP and InAs using previously published experimental data.

Electrical Resistivity and Electron-microscopic Observation of Sintered Chromium Dioxide

Since an establishment of the preparation method of the CrO2 powder, various efforts have been made to obtain a compact body of the oxide to measure its electrical and magnetic properties. Since the oxide decomposes to nonmagnetic Cr2O3 at temperatures above 400°-450°C in the atmospheric pressure, a sintered body could not be obtained by the usual technique.The present paper describes the preparation method of the highly dense sintered bodies of CrO2 by hot-pressing at mechanical pressure of 30000kg/cm2 and at 450°C. A high pressure apparatus used in this experiment is of a modified internal heating type. A starting powder of CrO2 was obtained by the thermal decomposition of CrO3 under an oxygen pressure of about 350kg/cm2 at 400°C. The obtained powder was coldpressed into a disk having the diameter of 6mm and the thickness of 3mm. The pressed body was heated at 450°C for 2hours at various pressures ranging from 10000 to 30000kg/cm2 in the pressing apparatus.An electronmicroscopic observation of fractured surfaces of the samples hot-pressed at pressures less than 15000kg/cm2 showed an intergranular fracture and the original shape of each grain remained unchanged. In the samples pressed at above 25000kg/cm2 a transgranular fracture was observed and therefore the samples conclusively exhibited a strong bonding between grains. The applied pressures higher than 30000kg/cm2 remarkably promoted the sintering of CrO2 even at low temperature as 450°C.The bulk density of the samples pressed at 30000kg/cm2 was higher than 99% of the X-ray density. The electrical properties were quite different from the result obtained by CrO2 powder. The electrical resistivity showed a metal-like behavior, i.e. it increased monotonously with increasing temperature in the range -196°C to 300°C and there existed a small change in a slope of temperature dependence of resestivity at the Curie temperature of CrO2. The result is consistent with the metallic behavior of CrO2 as predicted by a Goodenough model.