Disappearance of Meissner Effect and Specific Heat Jump in a Multiband Superconductor,Ba<Subscript>0.2</Subscript>K<Subscript>0.8</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript>

We observed the disappearance of the Meissner effect and the specific heat jump in Ba_0.2K_0.8Fe₂As₂. A crossover of the type of superconducting order parameter can be a source of these phenomena. A quantum phase disorder provides a possible explanation.     

Visualization for heat and mass transport phenomena in supercritical artificial air

A laser holography interferometer is applied to investigate heat and mass transport phenomena around the pseudo-critical line of supercritical artificial air with a composition of 79% nitrogen and 21% oxygen. In a previous study, we successfully observed the heat transport phenomenon, the piston effect, around the pseudo-critical line of nitrogen. The same experimental set-up is applied to the supercritical artificial air, which is a compressible binary mixture fluid. We attempt to suppress the generation of natural convection, and successfully observe the heat and mass transport phenomena, which are the soret effect and the piston effect, respectively. Here, we discuss these effects observed in the supercritical artificial air.     

Light-induced refractive-index modifications in dielectric thin films: experimental determination of relaxation time and amplitude

A two-beam setup based on the totally reflecting prism coupler is shown to be a powerful means of characterizing light-induced refractive-index modifications in dielectric thin films. Rise and relaxation times and amplitudes of thin-film refractive-index variations can be measured. Some developments of the electromagnetic theory of prism coupling are presented for Gaussian incident beams. Measurements made on a single Ta(2)O(5) layer deposited on a silica glass are presented. Relaxation times of a few milliseconds reveal the thermal origin of the phenomena. The thermal nonlinear coefficient of this Ta(2)O(5) layer is nearly 10(-15) m(2)/W.     

Athermic relaxation phenomena in polycrystalline copper after cooling to helium temperatures

Long-time heat release relaxation phenomena observed in different types of solids after cooling are attributed to the inelastic relaxation of thermoelastic stresses. Under this assumption an estimate of the relaxing power for polycrystalline copper is obtained that agrees with the observed values.     

Ferromagnetic- and Superconducting-Like Behavior of Graphite

We have identified ferromagnetic- and superconducting-like magnetization hysteresis loops in highly oriented pyrolytic graphite samples below and above room temperature. We also found that both behaviors are very sensitive to low-temperature—as compared to the sample synthesis temperature—heat treatment. The possible contribution of magnetic impurities and why these do not appear to be the reason for the observed phenomena is discussed.     

Multiscale simulations: application to the heat transfer simulation of sliding solids

Molecular dynamics is a powerful tool allowing the simulation of matter behaviour at the atomic scale. Due to computation time, it is clearly not possible to use molecular dynamics to simulate a forming process. However, atomistic simulations can be used to study and understand the physical phenomena that occur during matter deformation. As an example, heat transfer between the contacting solids in forming processes is one of the important physics phenomena that have to be taken into account in order to do realistic simulations. A multiscale analysis of heat transfer is presented. This analysis leads to two kinds of models: a macroscopic model which can be used for the simulation of the process itself and a microscopic model that is used to determine the parameters of the macroscopic model. In this microscopic model, the friction heat generation phenomena has to be described quite accurately. Friction heat is mainly due to plastic and elastic deformation and adhesion. Thus, to understand the underlying friction heat generation phenomena, atomistic simulations using molecular dynamics are carried out. It is shown that friction heat is the transformation of mechanical work given to the system at the macroscopic scale into potential energy during elastic deformation. This potential energy which is stored in the system is finally transformed into atomic kinetic energy (friction heat) during plastic transformation.     

Opportunities for High-Resolution Measurements and Microgravity Research near the Superfluid Transition of Liquid 4He

Six years ago an experiment to determine the specific heat of liquid helium near the superfluid transition in a microgravity environment opened up a new era of very-high resolution measurements. Recently this led to the establishment of the Fundamental Physics Discipline by NASA for the support of microgravity research in low-temperature/condensed-matter physics, laser cooling and atomic physics, and gravitation and relativity. This paper describes very-high resolution and microgravity research in one particular sub-field of low-temperature physics, namely the superfluid transition of ⁴ He, in order to illustrate the diverse research opportunities which exist within the Discipline. The effect of gravity on this system will be illustrated. Projects carried out already and expected to be undertaken in the near future on critical phenomena, surface effects, and non-equilibrium phenomena, will be discussed. These include measurements of the specific heat and the thermal conductivity, both in bulk samples and finite geometries, of the singularity of the boundary resistance between helium and a solid surface, as well as of non-equilibrium effects due to finite heat currents.     

Thermodynamics of deformable magnetic materials with memory

The theory of deformable heat conducting magnetic materials with fading memory is developed. The case of saturated ferrogmagnetic type materials is treated in detail. The restrictions placed on the response functionals by the Clausius-Duhem inequality are derived. Relaxation is discussed and integrated dissipation inequalities are derived. Finally, the theory of deformable paramagnetic materials with memory is briefly developed.     

On a thermo-mechanical effect and criterion of crack propagation

A thermo-mechanical effect from partial conversion of fracture work into heat energy during crack propagation is considered with a simple mathematical model. It is assumed that the heat production zone in the vicinity of the crack tip is very small. Thus, the crack propagation process can be viewed as propagation of the crack in elastic material with a point thermal heat source fixed at the tip of the crack. This thermal heat source generates its own temperature and stress fields around the crack tip. As shown in this paper it also generates a negative stress intensity factor that specifies fracture mode I and has to be accounted for in the energetic fracture criterion. The model developed may help to explain many experimental observations such as the increase in the specific surface energy that accompanies an increase in the crack speed and why fracture mode I has a special role in crack propagation phenomena.     

冰浆流体流动与换热研究综述

介绍了冰浆流体的特点、应用以及固液两相流体等效比热的概念及其在冰浆流体中应用的困难;概述目前国内外冰浆流体的传热性能、流变模型和流态以及粘性和流动阻力的研究成果,指出由于固液两相密度的不同导致了在不同流速下冰水分层的现象;同时对冰浆流体的的研究进行了综述,最后提出了关于冰浆流体的进一步研究建议.     

Spin-spin interaction interpretation of results in Zn-Mn

We suggest that a crystal field interpretation of susceptibility paramagnetic resonance and specific heat results in Zn?Mn is not consistent and that ion-ion interactions more readily account for observed behavior. Using a spin-spin interaction model we find the exchange to be both anisotropic and dependent on more than ion first-nearest-neighbor interactions alone. Cluster contributions are found in both the specific heat and susceptibility.     

Transient heat transport in He II

Experimental and theoretical investigations of time dependent heat transport in He II have identified several new phenomena not previously reported. For heat fluxes greater than the steady state peak value, there is an observed time delay between the initiation of heating and the onset of film boiling. The time delay is seen to be nearly equal to the enthalpy rise in the helium divided by the applied heat flux. A well defined relationship is shown to exist between the time delay and the applied heat flux. Simple theoretical analysis of heat conductivity in He II is shown to predict the functional dependence and give approximately the correct proportionality factor. Experimental results are reported over a range of temperatures, 1.6 K to 2.1 K and for two external pressures, near saturated vapour pressure and at 0.125 MPa.     

Thermomechanical behaviour of metals in cyclic loading

Temperature variation induced by repeated mechanical cyclic loading on AISI 1045 mild steel was studied.The experimental results of cyclic loading at low stress levels elucidate the coupling phenomena of thermal/mechanical behaviour which causes cooling and/or heating corresponding to the stressed state. The governing factors are thermoelastic effect and viscous dissipation. The thermoelastic effect causes the specimen temperature to go down and/or up which corresponds to the loading and/or unloading in cycling, where the viscous dissipation effect causes heat to generate inside the sample which steadily heats the specimen. As a result, a trend of increasing specimen mean temperature with periodical local fluctuation on temperature history can be observed. The heating rate, due to viscous dissipation, is increased with increasing strain rate. Cyclic loading at high stress levels results in large amounts of heat generation where thermoplasticity predominates. An abrupt temperature rise in the first few cycles, followed by a slow-down in later cycling, is to be seen. The phenomena and results were discussed. In addition, the effect of heat transfer between the specimen and its surroundings should be considered for both cases if the time is sufficiently long or the temperature gradient evolved is of significance.     

Boiling phenomena in pressurized He II confined to a channel

Results from an experiment to study boiling phenomena in a channel containing pressurized He II are presented as a function of temperature, pressure and orientation with respect to gravity. The experimental apparatus is made of glass to allow visual observations and high-speed motion pictures to be made of boiling events. Visual data are combined with temperature and pressure measurements to characterize the boiling behaviour and to group the phenomena into boiling regimes. Results are presented in the form of heat transfer regime maps, temperature and pressure traces during boiling, and sketches of the helium vapour and liquid in the channel. One unexpected result is the observation of a macroscopic region of He I with He II below and vapour above. The two interfaces are clearly visible. Another unanticipated result is that the boiling phenomena are often characterized by the periodic production, growth and collapse of vapour even though the heat input is constant.     

Caractérisation de l'interface métal-diélectrique par l'étude de l'impédance complexe des structures Al/CdS/Au obtenues par pulvérisation cathodique

The interfacial phenomena in sputtered sandwich films were investigated by studying the complex impedance of Al/CdS/Au structures in which the dielectric thickness was less than 800 Å. The experimental results are interpreted using Maxwell-Wagner theory in which the dielectric film is considered as two layers. Semiconductive regions appear when a specific heat treatment is applied. The activation energy associated with conduction variations as a function of temperature is in the range 0.4–1 eV. Comparative studies with restructuring of the aluminium electrode show a correlation between the surface state of the metal and interfacial properties. We show that specific properties of sputtered photocells may be modified by approaching a limit of temperature of 440 K.     

Experimental study of water vaporization occurring inside the channel of a smooth-plate type heat exchanger connected to an adsorber and comparison with trends observed in absorption configuration

Due to the lack of knowledge about water behavior in conditions that might occur in sorption chillers using water as refrigerant, designing compact heat exchangers remains mainly empirical. The objective of this paper is to gain further understanding of phenomena occurring in compact plate-type evaporator in order to be able to design them properly. In that goal, experiments were conducted under various operating conditions during adsorption cycles. Although interactions take place between the adsorber and the evaporator due to coupling phenomena, the present study focuses only on the evaporator. The vaporization of water occurring inside the channel of a smooth plate-type evaporator during these cycles is observed. Two main distinct phenomena observed along the time of the adsorption cycle are identified, described and discussed. Time-averaged cooling capacity and instantaneous cooling capacity for given operating conditions are plotted. A parallel with result obtained with the same experimental apparatus but in absorption configuration (mimicked by a thermosiphon loop) is done. Model developed in absorption configuration is tested. It is shown that, except during the beginning of the cycle, for the major part of running conditions tested, this model is able to well predict the instantaneous cooling capacity obtained during the adsorption cycle as long as the variation of the height of liquid level and the equilibrium pressure is known. The evolution of the cooling capacity measured in adsorption configuration with dimensionless number developed in absorption configuration is plotted for comparison. These dimensionless numbers were developed in order to optimize the design of compact evaporator used in sorption chillers. It is shown that the same trend could be observed for the two configurations.     

Nucleate boiling of low-concentration alcohol aqueous solutions for the development of thermal management systems in space

To develop high-performance space thermal management systems using boiling phenomena, the heat transfer characteristics of low concentration alcohol aqueous solutions were investigated on ground. For mixtures of 1-Propanol/Water, 2-Propanol/Water and Water/1-Butanol, i.e. substantially positive mixtures at very low concentration range of alcohol, heat transfer enhancement was observed, while only heat transfer deterioration was reported in most of existing studies for nucleate boiling of mixtures. A concept of coexisting heat transfer enhancement due to Marangoni effect additionally to the heat transfer deterioration due to mass transfer resistance was emphasized. The concept seems to be true for mixture nucleate boiling independent of gravity level.     

Diffusive relaxation processes in liquid3He-4He mixtures. II. Superfluid phase

Relaxation measurements in superfluid³He-⁴He mixtures when a heat flux across a horizontal layer is switched on or off provide a quantitative test for the solutions of Khalatnikov's hydrodynamic equations. On the basis of these equations we predict that the relaxation times in a layer of heightd can be expressed by a simple scaled relation that is a function of static and transport properties. Relaxation experiments on five mixtures are in good agreement with these predictions. In an appendix we discuss the viscous penetration depth _p and show that it is small enough for the Khalatnikov expressions to be valid under our experimental conditions.Research supported by grants from the National Science Foundation and the Research Corporation.Alfred P. Sloan Fellow.     

Ordering and precipitation effects in the specific heat and magnetization of copper-iron alloys

There is evidence in the specific heat of dilute copper-iron alloys of magnetic interaction phenomena whose behavior varies according to the range of solute concentration. At temperatures well below T_Kondo and at low concentrations two essentially independent contributions to the specific heat are found. One arises from the thermal excitations in the solid solution and the other from those in a rudimentary fine-scale coherent precipitate.The research was supported by the Science Research Council by provision of a Research Studentship, and by the Ministry of Technology, through the National Physical Laboratory, under Contract No. AD 34/8/04/26, by a Research Studentship (D.J.E.) and a Research Grant.     

Effect of oxygen concentration on diffusion length in Czochralski and magnetic Czochralski silicon

Abstract

This paper reports some new results on the influence of oxygen aggregation phenomena in the 450-850°C range on the diffusion length of minorities in both Czochralski (Cz) and magnetic Czochralski silicon. Thefeatures of the oxygen aggregation phenomena were studied by infrared spectroscopy at 20 K and the effect of these processes on the diffusion length was monitored by surface photovoltage measurements at 300 K. The results show that in Cz silicon the formation of thermal donors at 450°C is not associated with a detectable increase of the recombination activity, but that thermal donors are the precursors of other oxygen species, which grow during any further heat treatment. It was shown, moreover, that selfinterstitials injected from a surface oxide inhibit the oxygen segregation, in good agreement with predictions in the literature concerning the ‘volume exigency’ of oxygen segregation processes. Marked evidence of the influence of the cooling rate on the diffusion length was observed at 850°C, where the lifetime of heat treated Cz silicon was shown to be longer for a slow cooling process.