Supercooling Phenomena Research Articles

912 過冷却度の制御による相変化材の蓄熱機能改善(OS12-(1) 流体工学・熱流体,オーガナイズドセッション)

When a phase change material (PCM) is stored in the supercooled state and is gotten to start solidification to demand, the heat of solidification of the PCM can be stored at low heat loss for a long term. Thus the influence of the mass of PCM that is filled up in a capsule on the maximum degree of supercooling was investigated using several promising PCMs to evaluate the functional utility as PCMs. As a result, it was found that the maximum degree of supercooling of erythritol or threitol was large and depended on the mass strongly, so that the two materials are useful for using supercooling phenomenon. However, it was found to be difficult to use the supercooled phenomenon of polyethylene glycol effectively because the maximum degree of supercooling of the material is small and little depends on the mass.

Effects of poly-vinyl alcohol on supercooling phenomena of water

The effects of a polymer additive on the supercooling of water were investigated experimentally. Poly-vinyl alcohols (PVAs) were used as the additives, and samples were prepared by dissolving the PVA in water. Since the characteristics of PVA are decided by its degrees of polymerization and saponification, these were varied along with the concentration as the experimental parameters. Moreover, the effect of purity of the water was also considered. Each sample was cooled and the temperature at the instant when ice appeared was measured. Since the freezing of supercooled water is a statistical phenomenon, many experiments were carried out and the average degree of supercooling was obtained. It was found that PVA affects the nucleation of ice in supercooled water and the degree of supercooling increases with the addition of PVA even for water with low purity. The average degree of supercooling increases with an increase in the degree of saponification of PVA.

Thermal diffusivity of eutectic of alkali chloride and ice in the freezing–thawing process by temperature wave analysis

Thermal diffusivity of eutectic formed in aqueous alkali chloride solutions (NaCl, KCl, RbCl and CsCl) was determined by temperature wave analysis (TWA) during the freezing–thawing process of aqueous alkali chloride solution. An obvious change of thermal diffusivity in freezing–thawing processes of eutectic was observed with the super-cooling phenomena for each alkali chloride solute. The rate of thermal diffusivity decrease during the eutectic melting in comparison to the total decrease from the solid to the liquid state was observed larger than that of the heat of fusion measured by DSC, especially in the dilute concentration. It was confirmed that thermal diffusivity was sensitively influenced by the highly structure of the coexistence of eutectic crystallization and ice.

Silicide Formation from Laser Thermal Processing of Ti/Co Bilayers

A bilayered CoTi silicide structure consisting of an amorphous CoTi silicide and a highly textured CoTi silicide was found after pulsed excimer laser annealing of titanium/cobalt/silicon stack at high fluence of 0.6 J/cm 2 . The highly textured CoTi silicide is monocrystalline and fully coherent with the Si(111) plane of the substrate but has a large amount of microstructural defects. The constitutional supercooling phenomenon is the solidification mechanism responsible for the highly textured CoTi silicide. The incomplete crystallization shown by the presence of the amorphous CoTi silicide is attributed to a high concentration of titanium impurity.

Phase Transitions of Superfluid 3He in 0.8-μm Slab Geometry

We have studied superfluid 3He in 0.8 μm slab geometry by cw-NMR method. The static magnetic field for NMR was 22 mT and was perpendicularly applied to the surface. At pressures from 5 bar to 27 bar, we observed the negative shifts of NMR frequencies below the transition temperatures. Moreover we observed the fast order phase transition with the supercooling phenomena at lower temperatures, but NMR frequencies did not show a jump as they do at the A-B transition in the bulk liquid. Although the phase at low temperatures seemed to be B-phase, NMR frequencies still had the negative shifts.

Heat transfer in paraffin oil/water emulsion involving supercooling phenomenon

An emulsion of a mixture of alkane in water has been elaborated for thermal energy storage and transport. Investigations on heat transfer inside a sample of the emulsion in an agitated tank during a cooling phase are presented. The variation of the temperature as a function of time shows a stabilisation of temperature below the melting temperature which is interpreted as a supercooling phenomenon. The results are found in good agreement with experimental data obtained by using a calorimetric method. A theoretical expression is proposed to simulate the thermal behaviour which permits predicting the duration of the plateau of crystallisation. The results compare very well with experimental data, thus giving a new impulse to the design of new complex fluids for both storage and heat transport.

Temperature Dependence of Thermophysical Properties of Disodium Hydrogenphosphate Dodecahydrate

The temperature dependence of thermophysical properties was evaluated for disodium hydrogenphosphate dodecahydrate, which is used in long-term, supercooled thermal energy storage (Super-TES). Super-TES stores thermal energy at temperatures lower than the melting point of the phase-change material, which reduces heat loss from the storage system. Although the degree of supercooling depends not only on the intensive material properties, such as phase-change temperatures, but also on extensive properties, such as the total material volume, except for distilled water and metals there is insufficient information in the open literature on supercooling phenomena to develop heat storage devices. A promising material for Super-TES applications is disodium hydrogenphosphate dodecahydrate. The thermophysical properties of disodium hydrogenphosphate dodecahydrate are evaluated, and its suitability for Super-TES applications is clarified. We found that in the liquid phase the hydrate density and viscosity decrease monotonically with increasing temperature. We also found that the specific heat in the liquid phase (3.45 kJ/kg . K) and the thermal conductivity in the solid phase (1.01 W/m . K near the melting point) were twice as large as reported in the open literature. This indicates that this hydrate is useful for storing both latent and sensible heat, for reducing heat loss from energy storage systems, and for efficiently exchanging energy between heat sources and the hydrate.

Transport of cold thermal energy with a slurry as secondary biphasic refrigerant

A dispersion of phase-change materials is proposed as an experimental biphasic refrigerant. The phase-change material is water stabilized by a tri-dimensional network of polymer, obtained by using a polymerizing process. The particle contains 90 per cent of water and has the consistency of a viscoelastic solid above the phase-change temperature. The measured values of the physical properties of the biphasic refrigerant, that is, specific heat, latent heat, density and viscosity, are presented and discussed for the considered temperature range. A supercooling phenomena is observed if the size of the particles becomes lower than 1 mm. The degree of supercooling however does not exceed 3°C. The various advantages of this slurry are presented and the comparison with a common single-phase carrier fluid has proved how great its properties are as a secondary refrigerant. This material appears well adapted for a wide variety of schemes where the production involves low-temperature intervals, such as the refrigeration industry. Copyright © 2001 John Wiley & Sons, Ltd.

Supercooling across first-order phase transitions in vortex matter

Hysteresis in cycling through first-order phase transitions in vortex matter, akin to the well-studied phenomenon of supercooling of water, has been discussed in literature. Hysteresis can be seen while varying either temperature T or magnetic field H (and thus the density of vortices). Our recent work on phase transitions with two control variables shows that the observable region of metastability of the supercooled phase would depend on the path followed in H-T space, and will be larger when T is lowered at constant H compared to the case when H is lowered at constant T. We discuss the effect of isothermal field variations on metastable supercooled states produced by field-cooling. This path dependence is not a priori applicable to metastability caused by reduced diffusivity or hindered kinetics.

Short-range order in liquid matter probed by high-temperature x-ray absorption measurements

X-ray absorption spectroscopy can be applied to the study of liquid matter yielding quantitative structural information on the short-range order. Recent experimental advances and a new single-energy detection technique suitable for monitoring phase transition and supercooling phenomena are described. Examples of the change in absorption at the melting of Ge and Ag are presented. The general theoretical framework for the interpretation of the spectra is reviewed.

Cryoscopic and dynamic study of the molten system fluoroacetamide-sodium trifluoroacetate

The behavior of the molten system acetamide-electrolyte, including the peculiarity of the structure and the consequent relevant properties, such as the megavalue of the dielectric constant, was reported earlier. The probable structure is a micellar system composed of salt micelles dispersed in an amorphous solvent. The present paper reports measurements in a related system: fluoroacetamide-sodium trifluoroacetate. The phase diagram, the ultrasonic absorption of both longitudinal and shear waves, and the longitudinal relaxation time of the nuclear magnetic momentum were measured at different temperatures. The most importnat result was thatT 1 (NMR relaxation time) on the19F nucleus shows that the amide and salt participate in different environments. The amide environment is less rigid; this proves that the system must be considered microheterogeneous. Moreover, the present system exhibited: 1) a less extended concentration area where supercooling phenomena occur; 2) a much broader curve of the real part of the mechanical impedance versus frequency; 3) a higher bulk viscosity in comparison with the CH3CONH2−CF3COONa supercapacitive system.

Ca-Sr-Ga-Nb mixed oxide system for high temperature superconductor substrate applications

Twin-free crystals with relatively low melting temperatures are desirable as substrates for high temperature superconductor (HTSC) oxide substrate materials. In the selection of new oxide substrate compositions, special requirements (e.g. suitable dielectric properties for microwave application and perovskite structure with good lattice matching with YBa 2Cu 3O 7-δ) were considered. In this study the calcium-strontium gallate-niobate (CSGN) system has been investigated in both ceramic and single crystal samples, focusing on their crystal growth. The CSGN compositions were formed by mixing orthorhombic calcium gallate-niobate (CGN) and cubic strontium gallate-niobate (SGN) complex perovskites. CGN-excess mixtures do not create a single phase, as opposed to SGN-excess compositions, where single phase solid solutions are formed. The SGN-excess compositions can produce twin-free CSGN single crystals, which have excellent room temperature dielectric loss (tan δ ≤ 3 x 10 -4) and reasonably low dielectric constant ( K = 41) values at 100 kHz. The growth temperature for a 50 mol% SGN starting composition (CSGN1) was about 1600°C, but for a 65 mol% SGN concentration (CSGN2) it increased to 1750°C. A lattice parameter a = 3.939 Å was obtained for the CSGN1 crystal increasing with SGN concentration. A significant drawback was observed for the Ca-Sr-Ga-Nb oxide system: the effective distribution coefficient of Ca 2+ is very small ( k eff(Ca 2+) ≈ 0.4). This can cause crystal growth difficulties due to constitutional supercooling phenomena.

Effect of Various Parameters on a Supercooling Phenomenon of Still Bulk Water and Water Solution.

Supercooling characteristics on still bulk water and several kinds of water solutions were investigated experimentally. The test fluids, which were sandwiched in between two immiscible liquids in a glass tube, were set in a constant low-temperature room, and its freezing temperature was measured. It was found that the critical degree of supercooling for the water increased with decreasing test fluid volume, V, and was proportional to the logarithm of V. However, the critical degree of supercooling for the water solutions varied with the type of solute, and for each solute, it increased with increasing solute concentration. The critical degree of supercooling for the water solution was larger than that for the water by about 0∼4 K. The increment supercooling, which depended on the type of solute, could be explained by the equilibrium freezing point depression, an increase in the viscosity and solute/water interactions. In addition, the present data for the water solutions were correlated with dimensionless parameters which expressed these effects.

Fundamental Study of Cold Heat Storage System of O/W-Type Emulsion Having Cold Latent Heat Dispersion Material. 1st Report, Estimation of Thermophysical properties.

This paper deals with thermal properties of oil (tetradecane, C14H30, melting point of 278.9K) / water emulsion as a latent heat storage material having a low melting point. The measured results of the physical properties of the test emulsion, i.e., thermal conductivity, specific heat, latent heat and density, were discussed for the temperature region of solid and liquid phases of the dispersion material (tetradecane). It was clarified that Eucken's equation could be applied to the estimation of thermal conductivity of the emulsion. Moreover it was established that tetradecane as the dispersion material exhibited a supercooling phenomenon which influenced the thermal properties. Useful correlation equations of thermal properties for the emulsion were proposed in terms of temperature and concentration ratio of the emulsion constituents.

Internal radiative transport in the vertical Bridgman growth of semitransparent crystals

A quasi-steady-state model for the vertical Bridgman growth of semitransparent (partially absorbing and emitting) crystals is developed. For the first time, internal radiative heat transfer through the crystal (considered to be a participatine medium) is calculated rigorously, taking into account the full three-dimensional shape of the crystal, including curvature of the melt/crystal interface. The crystal is assumed to be grey, and the ampoule walls, as well as the crystal/melt interface, are assumed to be grey and diffuse. Heat transfer through the melt is considered to be dominated by conduction. A Galerkin finite element method is employed to determine the position and shape of the interface and the temperature field in the crystal and melt. Results for a model oxide growth system demonstrate the sensitivity of melt/crystal interface shape, position, and interfacial gradients to changes in optical properties. The total heat flow through the melt and crystal also depends strongly on the optical parameters of the system. The phenomenon of radiative supercooling is not observed for the system considered here.

Supercooling phenomena in electrolytic mixtures of 2-ketobutyramide

The liquid-solid equilibrium temperature is determined for binary mixtures of 2-ketobutyramide (acetoacetamide) + electrolyte in order to investigate the occurrence of supercooling phenomena. The composition range where supercooling occurs is given for all the binaries studied. The presence of the keto group on the chain of the amide increases the supercooling phenomena, which are more extended in these systems in comparison with those in the corresponding systems in butyramide.

Crystallization and vitrification in aqueous systems

SUMMARYSome key ideas and experimental findings concerning the probability that crystallization of a liquid or its binary solutions will occur at moderate cooling rates are discussed. The use of cryoprotectants and of pressure to favourably influence these probabilities is rationalized, and some of the newer findings on small sample supercooling phenomena are reviewed.

CHANGES IN THE ULTRASTRUCTURE OF XYLEM PARENCHYMA CELLS OF PEACH (PRUNUS PERSICA) AND RED OAK (QUERCUS RUBRA) IN RESPONSE TO A FREEZING STRESS

An ultrastructural investigation was conducted of xylem parenchyma cells of peach (Prunus persica [L.] Batsch.) cv. Harbrite and red oak (Quercus rubra L.) in response to a freezing stress. Freezing curves of xylem tissues, as determined by differential thermal analysis, were used to predict temperatures at which both living and dead cells would be observed. Tissues were exposed to low temperatures (‐15 to ‐35 C) and fixed in a frozen state at ‐10C and at thawing. Current models of the freezing behavior of supercooled plant cells suggest that xylem parenchyma cells behave as individual water droplets. This implies that cells are unresponsive to the presence of low temperature and extracellular ice until internal nucleation triggers lethal, intracellular freezing. For these reasons, deep supercooling has been described as an avoidance mechanism. Results of this study confirmed earlier reports that xylem parenchyma cells freeze as individuals or in small groups. Individual cells, however, did not exhibit a neutral response. Instead, a range of responses was observed that included internal and external vesiculation, deep invaginations of the plasma membrane, and the formation of electron‐dense deposits external to the plasmalemma. In general, our observations suggested that the cells responded to a dehydrative stress. Results are discussed in context of the biophysical data associated with deep supercooling phenomena and compared to responses of cells that exhibit extracellular freezing.

Electrolytic solutions in succinimide: Cryoscopic behaviour

The liquid—solid equilibria in the binary systems succinimide + (Na, K)/(CNS, ClO 4) are reported. The presence of intermediate compounds is shown. In some concentration ranges these systems (with the exception of KClO 4) present supercooling phenomena if the mixtures are cooled without agitation.

Supercooling phenomena in binary mixtures of trifluoroacetic acid sodium salt and amides

Liquid—solid equilibrium temperatures are measured in binary mixtures of trifluoroacetic acid sodium salt and amides (acetamide, propionamide, n-butyramide, stearamide). In some ranges of concentration, all these systems (with the exception of C 17H 35CONH 2 + CF 3COONa) exhibit supercooling phenomena and the liquid becomes very viscous and does not crystallize.