Effects of electrode materials on freezing of supercooled water in electric freeze control

In order to clarify effects of electric charge on freezing of supercooled water, experiments were carried out with various kinds of electrodes in supercooled water. Water sample was kept in a test tube and cooled down at a constant cooling rate. When the water sample was maintained under a supercooling state, an electric charge was applied to the water sample with a small electric current. The degree of supercooling was measured continuously. Then the degree of supercooling at freezing was determined. Six kinds of materials were used for electrodes. Those materials were Aluminum, Copper, Argentum, Aurum, Platinum and Carbon. It was found that the effects of electric charge were distinct according to the material used for electrodes. The degree of supercooling at freezing was the lowest in the case of Aluminum. On the other hand, the highest value of the degree of supercooling at freezing was obtained in the case of carbon. The reason for the difference in the degree of supercooling at freezing by six materials was discussed.     

Effect of bubble nuclei on freezing of supercooled water

In order to clarify effects of bubble nuclei on freezing of supercooled water, various kinds of experiments were carried out with invisible sizes of bubbles in supercooled water. Water samples were kept in a test tube and cooled down at a constant cooling rate until the water solidified. The degree of supercooling at freezing was then measured. Two kinds of water surfaces were applied. One was exposure to the atmosphere, and the other was covered with silicone oil. Three kinds of pressure conditions were applied. The first type was atmospheric pressure. The second type was compression up to 6.0 atm. The third type was evacuated down to 0.02 atm. Two holding time periods before starting the experiments were applied. One was 30 min and the other was 24 h. It was found that the degree of supercooling at freezing is high in the case of the free surface compared to the one with oil–water surface. The reason suggested was that the bubbles in the water can be released from the surface in the case of normal atmospheric exposure and trapped at the oil–water interface in the case of water covered with oil. Hence, it was clarified that the freezing of supercooled water is affected by the existence of bubble nuclei.     

The solidification phenomenon of the supercooled water containing solid particlesPhénomène de solidification dans l'eau surfondue contenant des particules solides

The effect of solid particles on the freezing of the supercooled water was investigated. Silver iodide was selected as the solid particle, and several types of contaminants were prepared. The size of the particle and wetability of the surface were varied to three, and two types, respectively. The amount of the particle was also varied. The sample water was put into a test tube and cooled at a constant cooling rate until the water solidified. The relationship between the degree of supercooling at freezing and various variables were investigated. Then, an analytical method to predict the results was introduced. It was found that the results agreed quite well with the experiments. Hence, it was concluded that the freezing temperature does not depend upon the size of the solid particle nor its amount, but is decided by the total area exposed to the water.     

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.     

The experimental study on freezing of supercooled water using metallic surface

Freezing of supercooled water on a metallic plate was studied, experimentally. First of all, a gold plated surface was selected as a metallic surface because the surface has very little change in the characteristics of the surface against time. The experiments on freezing under various cooling rates were carried out and the probability of freezing per unit surface area per unit time interval was calculated. It was found that the probability was independent of the cooling rate. Secondly, in order to clarify the effect of oxidation on freezing of supercooled water, an electrolytically polished copper surface was selected and a time variation of the probability of freezing was investigated. A large number of experimental data is required to obtain an accurate value for the probability, but it is impossible to perform it before the characteristics of the surface changes. Hence, in order to cover a wide range of the degree of supercooling at freezing in a short period of time, experiments were carried out under various cooling rates. It was found that the oxidation of the surface restrained the supercooled water on the surface from freezing. By comparing the results with the one for a gold plated surface, a parameter was obtained to express the characteristics of the surface.     

Modelling the freezing of butter

Butter is a water-in-oil emulsion so its behaviour during freezing is very different from that of most food products, for which water forms a continuous phase. The release of latent heat during freezing is controlled as much by the rate of crystallization of water in each of the water droplets as by the rate of heat transfer. Measurements of the freezing of butter show that the release of latent heat from the freezing water depends on the degree of supercooling, which, in turn, depends on the cooling medium temperature, the size of the butter item, the packaging and the type of butter. Four modelling approaches were tested against the experimental data collected for a 25 kg block of butter. A “sensible heat only model” accurately predicted the butter temperature until temperatures at which water freezing becomes significant were reached. An “equilibrium thermal properties model” predicted a temperature plateau near the initial freezing point of the butter in a manner that was inconsistent with the measured data. A third model used a stochastic approach to ice nucleation based on supercooling using classical homogeneous nucleation theory. The predicted temperatures showed that supercooling-driven nucleation alone is not sufficient to predict the freezing behaviour of butter. A fourth approach took account of time-dependent nucleation and ice crystal growth kinetics using classical Avrami crystallization theory. The relationship between the ice crystal growth rate and the supersaturation was assumed to be linear. The model predicted the experimental data accurately, particularly by predicting the slow rebound in the temperature following supercooling that is found when freezing butter under some conditions.     

Study on anisotropy of growth rate of ice crystal in supercooled water

The purpose of this research was to investigate the ice growth of a single crystal in three dimensions. Three-dimensional pattern of ice crystal growth in supercooled water was observed using Mach–Zehnder spectro-interferometer. Temperature was varied from −0.3 to −1.6 °C. It was found that the ice crystal began to grow as a single crystal at the tip of the capillary tube and propagated freely in supercooled water. Time variation of the shape of dendrite on a–c plane was obtained. It was found that half parabola fits the shape very closely, and the coefficient of squared term, a, of a quadratic function was calculated. The coefficient, a varied in time but at quasi steady state it was found to be depending mostly upon the degree of supercooling. Furthermore, the growth velocity in c-axis at the flat surface was calculated from the thickness measured. It was found that the velocity in c-axis is independent of the degree of supercooling but depends upon time, in other words, the thickness in c-axis.     

Freezing phenomena of supercooled water under impacts of ultrasonic waves

In order to clarify effects of ultrasonic waves on supercoold water, ultrasonic waves were applied to supercooled water. Frequency of the ultrasonic waves applied was 45 kHz, and the intensities of the waves were 0.13 and 0.28 W cm⁻², respectively. Four cases of experimental conditions were selected; a case of a free surface, a case of an oil-water surface, a case of a free surface with a dipped metal bar and a case of an oil–water surface with a dipped metal bar. For each experimental condition, water was cooled at a constant cooling rate and the ultrasonic wave was applied from underneath until the water in a test tube solidified. It was found that in the case of 0.28 W cm⁻², the ultrasonic wave had distinct effects on freezing of supercooled water for all experimental conditions. On the other hand, in the case of 0.13 W cm⁻², the ultrasonic wave had an effect on freezing of supercooled water only under existence of a free surface and a metal bar.     

有关因素对冰浆制取过程中过冷度影响的实验研究

实验研究了搅拌速度、液浴温度、添加醇类物质和成核剂等因素对冰浆形成过程中过冷度及结冰特征的影响。结果表明,在制冰浆过程中,对水溶液进行搅拌有利于提高水过冷现象的稳定性。在搅拌速度0~100r/min时,不容易出现过冷现象而直接结冰;过冷度随搅拌速度增大而增大,在搅拌速度为200r/min时达到最大值4.35℃,之后过冷度急剧下降。在液浴温度降低情况下水过冷度先明显变大,随后变化不明显。添加乙二醇后,有效的改善了冰晶的细腻程度,在低浓度区(乙二醇浓度6%),过冷度呈小幅上升趋势;在中高浓度区(乙二醇浓度6%),过冷度随着乙二醇浓度的增加急剧的减小。添加成核剂后过冷度明显下降,过冷解除时间提前,与硫酸铁和二氧化硅相比,硼砂成核性较差,但是形成的冰晶具有较好的特性。     

Effect of ultrasound irradiation on some freezing parameters of ultrasound-assisted immersion freezing of strawberries

In this study, the effect of ultrasound irradiation temperature and ultrasound intensity on the freezing and nucleation in strawberry samples was studied. The application of ultrasound irradiation at different temperatures was able to induce nucleation at lower degree of supercooling compared to the control samples. The achieved degree of supercooling in the ultrasound irradiated strawberries was linearly correlated to the ultrasound irradiation temperature. At the ultrasound irradiation temperature of −1.6 °C, the characteristic freezing time (CFT) was significantly shorter than that in the control sample (p < 0.05). The application of ultrasound at higher intensities was found to effectively shorten the CFT. The degree of supercooling in ultrasound irradiated samples was not linearly correlated to ultrasound intensity. In conclusion, the combination of ultrasound irradiation temperature and intensity can be effective in controlling nucleation and freezing processes of perishable fruits such as strawberry.     

超声对Ti/SnO2+Sb2O3/PbO2电极材料结构及电极性能的影响

用SEM和XRD技术测定和分析了不同超声波作用时间下Ti／SnO2 Sb2O3／PbO2电极材料的表面形貌和结构，应用循环伏安法研究了超声波作用时间对Ti／SnO2 Sb2O3／PbO2电极在1．0mol／LH2SO4溶液中的伏安特性的影响．实验结果表明：超声波作用于电极材料会对电极表面的形貌和结构产生影响；电极经超声处理20min后，其循环伏安峰电流与未经超声波作用时相比增加了近1倍；电极的电化学活性表面积随超声波作用时间的延长呈先增大后减小的趋势，在本实验中超声波作用时间为20min时，伏安电荷最大，即活性表面积最大.     

Freezing of a water droplet due to evaporation—heat transfer dominating the evaporation–freezing phenomena and the effect of boiling on freezing characteristics

One of the authors has proposed a novel transport/storage system for the waste cold from the gasification process of liquefied natural gas (LNG), which consists of an evaporator, a cold trap, and a pipeline. In order to estimate the performance of this system, one should know the pressure in the evaporator, in which evaporation–freezing of a PCM occurs, and in the cold trap, as well as the pressure drop of the pipeline due to the flow of low pressure vapor of the PCM. In this paper, the cooling/freezing phenomena of a water droplet due to evaporation in an evacuated chamber was experimentally examined, and the heat transfer dominating the evaporation-freezing phenomena was investigated in order to estimate the pressure in the evaporator. From the results, it was shown that the water droplet in the evacuated cell is effectively cooled by the evaporation of water itself, and is frozen within a few seconds through a remarkable supercooling state, and that the cooling rate of the water droplets were dominated by heat transfer within the droplet under the abrupt evacuation condition. The later result means that, in order to obtain an ice particle by evaporation–freezing, the surroundings of the water droplet should be evacuated at the pressure as low as the saturate pressure of water at the maximum supercooling temperature of the droplet.     

Influence of surface roughness on the supercooling degree: Case of selected water/ethanol solutions frozen on aluminium surfaces

This paper presents a study on the impact of the roughness of a metallic surface on the magnitude of the supercooling during freezing of an aqueous solution. Aqueous solutions of ethanol (5%, 10% and 15% w/w) were used as model solutions. Five tubes of aluminium (internal diameter 8 mm) were machined to obtain a roughness between 0.63 and 13.3 μm. These tubes were immersed in a refrigerated bath with a programmable temperature scan. Thermocouples located at the inner surface of the tubes and in the solution were used to measure the magnitude of supercooling. Crystallisations were monitored and supercooling released calculated for each experiments. Our experimental results reveal that roughness is the influencing parameter of the supercooling released: larger the roughness, lower the supercooling. Moreover, a power law correlation between the roughness and supercooling was deduced.     

Reduction of supercooling of water by TiO2 nanoparticles as observed using differential scanning calorimeter

Aqueous dispersions of Titania nanoparticles were cooled using differential scanning calorimetry (DSC). The nanoparticles reduce the supercooling of water by acceleration of nucleation. The experimental results demonstrated the important role in water crystallisation by the internal surface of the sample holder, and that stable nanofluids are required for the measurements to investigate the effect of nanoparticles on freezing. But for dispersions at low concentrations, such as 0.05?wt% and 0.1?wt%, the starting temperatures of freezing were found to be unusual at the cooling rates of 3°C/min and 4.5°C/min, namely they were even lower than that of the deionised (DI) water. It is found that surface controlled nucleation (SCN) is dominant at low cooling rates, while volumetric controlled nucleation (VCN) is dominant at high cooling rates.     

以预锂化多壁碳纳米管为负极的锂离子电容器性能

采用内部短路方式对多壁碳纳米管负极进行不同程度的预嵌锂处理,预嵌锂时间为5,30,60min,以预嵌锂多壁碳纳米管极片作为负极,活性炭极片作为正极,组装成锂离子电容器。利用扫描电子显微镜(SEM)、透射电子显微镜(TEM)对多壁碳纳米管及电极极片进行表征分析,采用恒流充放电(GCD)和交流阻抗谱(EIS)研究预嵌锂多壁碳纳米管负极和未预嵌锂处理多壁碳纳米管负极锂离子电容器的性能。电化学测试结果表明,多壁碳纳米管负极预嵌锂大幅提高了电容器充放电性能,对比未嵌锂多壁碳纳米管电容器,在相同的电流密度下(100mA/g),能量密度提高400%。预嵌锂60min,电流密度100mA/g时,其比容量达到57F/g。在电流密度为100～3200mA/g范围内,其最高能量密度与功率密度分别达到90Wh/kg,4130W/kg。1000次充放电循环后,容量保持率维持在85%以上,表现出良好的超级电容器性能。     

Vacuum electric discharge initiated by accelerated nanoparticles

A static breakdown induced by the impact of particles detached from a point anode in a strong electric field, corresponding to the athermal field evaporation threshold, was studied by field ion microscopy. Under these conditions, the particle size threshold for the vacuum discharge initiation decreases by one order of magnitude as compared to the case of flat electrodes and falls within a nanometer range of the average radius of bombarding charged particles. The threshold energies of particles initiating a static electric discharge also exhibit a significant decrease.     

Quantitative evaluation of the effects of poly-vinyl alcohol on supercooling phenomena of water

The effect of poly-vinyl alcohols (PVAs) on the supercooling of water was investigated experimentally for varying sample volumes and degrees of polymerization and saponification. A statistical analysis was carried out on the experimental results and a quantitative evaluation of the inhibition effect of PVA on release of the supercooled state was attempted by introducing a coefficient representing the inhibition effect of PVA. Experimental results and the values calculated from the statistical analysis agreed well and it was found the statistical analysis could be applied to the freezing of supercooled water containing PVA. Moreover, the effects of the PVA on the release of the supercooled state were evaluated quantitatively, using the statistical analysis. The effect of the degree of polymerization is insignificant but it was found that the inhibition effect of PVA increased with increasing degrees of saponification.     

Study on formation of high performance ice slurry by W/O emulsion in ice storage (discussion on characteristics of propagation of supercooling dissolution)

A W/O type emulsion was developed as a new thermal material for ice storage. The water contents of the emulsions were 70, 80 and 90 vol%, and silicone oil was used. An amino group modified silicone oil with 0.9 vol% was used as a surface-active agent. The freezing points of those emulsions were 0 °C. However, due to the emulsion structure, the propagation rate of supercooling dissolution for each emulsion was very slow. Therefore, the propagation rate and maximum supercooling degree were estimated using probability, varying the water content of the emulsion, the method of the ice nucleus charging, and the size and number of ice nuclei. In addition, the influence of various parameters on the propagation rate and maximum supercooling degree was clarified.     

Two-Dimensional Model of Plasma Cathode with an Open Plasma Boundary

The paper deals with the solution of two problems for which a two-dimensional distribution is found of the current density and of the electric field in the hollow-cathode plasma. In the first one of these problems, the anode surface of the cavity represents two concentric flat anodes separated by an insulator. This system is used to study the problem of switching the current over from the external to internal anode, with the anode drop being a nonlinear function of current density. The distribution of the current density and of the anode voltage drop on the surfaces of both anodes is calculated for different potentials of the external and internal anodes. In the second problem, the function of internal anode is served by the open boundary of plasma with zero anode drop. The shape of the plasma boundary is determined depending on the plasma cathode geometry and parameters. The singularities are discussed that characterize the extraction of current from the open plasma boundary for different forms of electric discharges.