相变材料的过冷现象及其抑制方法的研究进展

Progress in Supercooling and Suppression Methods of Phase Change Materials

过冷是材料液-固相变过程中为提供离子扩散、晶体生长及晶面扩大所需能量而产生的一种亚稳态。过冷是结晶过程的推动力,但大的过冷度导致相变材料结晶温度降低、结晶时间延迟,使得储存的潜热不能及时释放,储-放热温度不匹配,降低了热能利用效率。过冷度大已成为限制相变储热技术规模化应用的重要影响因素之一。大量实验结果表明:过冷现象与熔体晶核的生成与长大速率、环境温度、接触面的粗糙程度、熔体温度等因素有关,但其产生的内在机制尚不明确,影响规律和调控手段仍需借助实验探索。目前,主要采用外加添加剂或在胶囊化、流体化过程中加入添加剂的添加晶种法,以及通过搅拌、超声振荡和鼓泡等外部刺激的动力学成核法诱发过冷液体结晶。利用外加成核剂、纳米颗粒以及部分未熔化母相晶体作为晶核诱发非均匀成核是抑制过冷现象最常用、最有效的方法。为提高添加剂的分散性和抑制水合无机盐类相变材料的相分离现象,在添加成核剂的同时往往需要加入一定量的增稠剂,但成核剂和增稠剂的添加量需要优化。胶囊化可以改变相变材料的结晶特性,目前的研究一致认为在胶囊化前添加成核剂有利于改善相变材料的过冷度。动力学成核常采用的方法是超声振荡法,其通过空化作用使晶体持续破碎并与熔体混合而提高晶核的分散性和加速结晶过程。在超声的同时添加纳米颗粒也有利于抑制过冷度。本文简述了过冷现象和典型的过冷曲线,分析了影响过冷度的因素,并着重介绍了外加添加剂法、胶囊化法、功能流体法和超声振荡法等抑制过冷度的方法。

S upercooling is a metastable state produced in the process of liquid-solid phase transition to provide energy for ionic diffusion, crystal growth and crystal surface expansion. Although it is the driving force of crystallization, the excessive supercooling degree leads to the decrease of crystallization temperature and extension of crystallization time, which further makes the stored energy released in unsuited temperature and reduces the thermal energy utilization efficiency.The excessive supercooling degree has become one of the most important factors that restrict the large-scale application of phase change thermal storage technology. Numerous experimental results demonstrate that supercooling phenomenon is closely related to the formation and growth rate of melt crystal nucleus, environment temperature, roughness of contact surface, melt temperature, etc. Unfortunately, the intrinsic mechanism of the supercooling phenomenon is still problematic, and yet it is also necessary to experimentally explore the influence law and the regulation means. Currently, manual seeding including adding nucleating agents or nanoparticles, introducing additives before capsulation or fluidization, as well as dynamic nucleation including mechanical vibration, stirring and ultrasonic irradiation, are the most effective ways to induce crystallization. Heterogeneous nucleation induced by adding nucleating agents, nanoparticles and partially unmelted parent phase crystals is the most commonly used and effective method to eliminate supercooling. In order to improve the dispersibility of additives and prevent phase separation of hydrated inorganic salt, a certain amount of thickening agents is often required to be added apart from nucleating agent, but the addition amounts of the two need to be optimized. Capsulation can change the crystallization characteristics of phase change materials. Previous research has reached a consensus that dispersing nucleating agents prior to capsulation will improve the supercooling degree of phase change materials. Ultrasonic oscillation, the commonly used dynamic nucleation method, can accelerate the crystallization process and improve the dispersion of the crystal nucleus by breaking crystals and mixing with the melt. The addition of nanoparticles along with applying ultrasonic process are also conducive to suppressing supercooling. This paper briefly described the supercooling phenomenon and typical supercooling curves, as well as the factors influencing supercooling degree. In addition, it summarizes the methods of suppressing supercooling, including adding additives method, capsulation method, functional fluid method and ultrasonic vibration method.