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ZHU Kong-ying WANG Shuang QI Heng-zhi LI Hui ZHAO Yun-hui YUAN Xiao-yan 《高等学校化学研究》2012,28(3):539-541
Supercooling of the microencapsulated phase change materials(PCMs) during cooling usually happens. This phenomenon can interfere with heat transfer and is necessary to further overcome. In this study, melamine- formaldehyde microcapsules containing two n-alkane PCMs, namely, n-dodecane(C12) or n-tetradecane(C14) were prepared by in situ polymerization. A small amount of n-hexatriacontane(C36) was introduced as an organic gelator into the core of microcapsules to cope with the supercooling problem. Analyses demonstrate that supercooling of the microencapsulated C12 or C14 was significantly suppressed by adding 3%(mass fraction) C36, without changing the spherical morphology and dispersibility. It could be also found that the enthalpy of microencapsulated C12 or C14 containing C36 was similar to that of microencapsulated n-alkanes without C36, whereas the difference between onsets of crystallization and melting(degree of supercooling) is similar to that of those of pure n-alkanes, suggesting the remarkable suppression ability of the organic gelator on supercooling. 相似文献
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Rahul P. Patki 《European Polymer Journal》2008,44(2):534-541
This rapid communication reports a summary of the key findings of crystallization kinetics studies of unfractionated high density (linear) polyethylene at extremely large supercoolings. We report, for the first time, the maximum in crystal growth rate-crystallization temperature data for linear polyethylene, which has been sought by many researchers since the 1950s. The maximum growth rate was found to occur in the range of 70-75 °C with two separate methods. The kinetics studies were performed using a newly developed quench-crystallization technique based on depolarized reflection light microscopy that is capable of achieving enormously higher quench rates than existing methods. Typical onset crystallization temperatures accessed with this technique range from 40 to 90 °C. Bulk growth rates of crystals were obtained as the reciprocal of crystallization half times measured from the change in the depolarized light intensity upon direct crystallization from the melt. Separately, radial growth rates of spherulites were measured over a wide range of supercoolings. Secondary nucleation analysis of the crystal growth rates resulted in single linear fits extending into deep regime III, suggesting no change in mechanism of formation of the crystals at the largest supercoolings. The deeply quenched films, crystallized at temperatures below the maximum, contain non-impinged spherulites, capable of further crystallization. 相似文献
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Circumstances under which a quenched system will freeze in a metastable state are studied in simple systems with long-range order. The model used is the time-dependent pair approximation, based on the most probable path (MPP) method. The time dependence of the solution is shown by means of flow diagrams. The fixed points and other features of the differential equations in time are independent of the choice of the rate constants. It is explained qualitatively how the system behaves under varying descending temperatures: the role of the initial conditions, the dependence on the quenching rate, and the response to precooling. 相似文献
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采用自由基乳液聚合法和原位聚合法制备了包含聚苯胺均匀分散在正十八烷中的悬浮体系的交联聚甲基丙烯酸甲酯为囊壁的相变材料纳胶囊(NanoPCMs). 采用扫描电子显微镜(SEM)、差示扫描量热仪(DSC)、热重分析仪(TG)和广角X射线衍射仪(WAXD)等手段分别考察了成核剂聚苯胺的添加量对相变材料纳胶囊表面形态、结晶性能、热稳定性及结晶行为等的影响. 实验结果表明:包含有聚苯胺的相变材料纳胶囊,成核剂的存在对其形貌、粒径、包覆率和结晶行为影响较小,耐热性略有所降低,添加1.5 g 苯胺时生成的成核剂聚苯胺可以有效改善其过冷结晶行为. 相似文献
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Summary A new nematic liquid crystalline GC stationary phase is introduced. The systematic name is: ethylen-4,4 diphenyl bis (4-methoxybenzoate): EPMB. It has a nematic range 168°C to 302°C. Coated on a support (Chromosorb W AW DMCS 100/120 mesh) it can be supercooled to 65°C and used for temperature programmed GC from 65°C to 260°C. The selectivity, bleeding, supercooling and temperature dependence of the capacity ratio are reported. In comparison with nematic phases with similar working range, EPMB shows equal separating power, but less bleeding and an extended temperature range. The main point of this work is the use of EPMB as an example of a liquid crystalline compound as a stationary phase for the gas chromatographic separation of certain compounds. 相似文献
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In this paper, an accurate experimental investigation of the Mpemba effect (that is, of the fact that initially hot water freezes before the colder one) is carried out, showing that, in the adiabatic cooling of water, relevant roles are played by supercooling, and by phase transitions which take place at 6±1 °C,3.5±0.5 °C and 1.3±0.6 °C. The last transition, which occurs with the non-negligible probability of 0.21 with respect to the total number of runs performed, has not been detected earlier. On the basis of our experimental results, we can present a thorough theoretical analysis of supercooling and of such phase transitions, which are interpreted in terms of the different ordering of molecule clusters in water. 相似文献
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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. 相似文献