难混溶合金高温相变储热胶囊的无容器凝固制备及性能研究

High-temperature thermal energy storage (over 800oC ) is an nowaday essential issue for developing the technologies to recover the vastly industrial wast heat and to use the solar thermal energy, which needs to overome the problems associated with the mismatch between energy supply and demand. Metallic phase change mateials (MPCMs) have attracted considerable attention since they possess many merits, such as high storage density, constant endothermic-exothermal temperature, superior thermal conductivity, and so on. However, MPCMs over 800oC are suffering from the difficulty to seek an applicable approach for containing the active melts. In this project, we propose that self-capsuled immiscible alloys with core-shell structure could be an alternative MPCMs, which are formed from the spacial liquid-liquid seperation during solidification. Containerless solidification process using an aerodynamic levitator Cu-base is adopted which is beneficial to the precise control of main solidification parameter (composition, undercooling, cooling rate,etc.), elimination of container' influence, as well as the capability of in-situ observation of the levitated sample during solification.In our previous experiments, the Cu-base capsules obtained by this process have a metallic shell (Cr-base or Fe-base) with higher melting temperature than Cu-base core. The formation of this novel structure is owning to the combination of Stokes and Marangoni motions of secondary droplets in levitated liquid spheres,and the corelation between surface and interfacial energy of two immiscible liquid. We believe that the Cu-base capsules obtained through containerless solidification process is a promising solution to realize the containment of high temperature MPCMs. In this project,the work focuses on the design of Cu-Cr and Cu-Fe compositions,and investigation of the relationship between the structure and performance of as-fabricated capsules. The thermal properties including storage density, thermal stability against static/cyclic heating will also be tested. The structure and thermal performance will be optimized based on the understanding of the broken mechanism of capsules. We believe that theorectical and experimental investigation in this work would be significant for large-scale fabrication of high-quality high temperature MPCMs, as requested by technologies for indrustrial wast heat recovery and solar thermal power generation.

工业高温余热回收技术和大规模太阳能热发电技术面临能量供应非稳态、间歇性、供需不匹配的问题，迫切需要研究和开发高效储热技术，尤其是800oC以上的高温相变储热技术。金属基相变储热材料具有热导率高、储热密度大、工作温度高等优点，但高温液态金属的难封装问题始终制约着该类相变材料的发展和工业应用。本项目提出利用气动悬浮无容器凝固技术，通过调控难混溶合金液-液相分离过程，获得核-壳结构相变储热胶囊的创新思想，以实现Cu基高温相变材料的自封装。通过合理设计Cr-Cu和Fe-Cu合金和添加元素，系统研究凝固参数对核-壳结构的影响规律，深入分析液-液相分离过程中Stokes沉积、液相表/界面张力以及Marangoni对流等因素的协同作用，制备出囊壁为富Cr相或不锈钢、囊芯为富Cu相的完整相变胶囊，测试并优化胶囊热性能，揭示组织结构变化和失效机理，为实现金属相变材料的规模化制备与工业应用提供指导。

面向大规模太阳能热发电和工业余热回收技术提出的更高工作温度、更高转化效率的迫切需求，本项目提出制备800℃以上铜基高温金属相变储热材料的研究目标。由于Cu基合金固有的高熔点和化学活性，采用何种材料和技术对其进行封装是关键科学问题。项目组以难混溶体系和不互溶体系合金为着手点，首先基于项目组在难混溶合金方向的研究积累，利用气动悬浮无容器凝固技术制备出高质量Cu@Fe核壳结构相变胶囊；但考虑到气动悬浮技术在量产方面存在局限性，通过不断实验和理论创新，提出采用间歇式电沉积技术制备Cu@Cr@Ni金属胶囊。主要研究内容：（1）利用气动悬浮方法制备Cu@Fe相变胶囊，研究合金熔体的液相分离过程中多因素的协同作用规律、凝固组织形成机理和合金凝固过程，测试合金胶囊的硬度和储热密度。（2）从Cu-Cr相图出发，设计了双壳层Cu@Cr@Ni相变胶囊，依次在铜球表面电沉积与之互溶度极低的铬和抗氧化性能良好的镍，实现了封装和抗氧化的双重目标；提出了周期间歇式电沉积技术和设备，解决了滚镀厚铬层的技术难题；对Cu@Cr@Ni金属胶囊进行储热性能测试、热循环性能测试、失效机理分析。（3）采用FLUENT软件对Cu@Cr@Ni胶囊在高温工业余热回收方面的储热性能进行模拟研究。研究烟气进口速率和进口温度波动周期的影响，分析其储热特点。（4）制备得到了Cu-Sn包覆Pb的核壳结构合金，研究不同组分Cu-Sn-Pb的凝固组织和凝固过程。（5）将周期间歇式电沉积技术扩展到对微米金属粉的包覆上，制备了W@Cu等一系列的金属包覆粉体，解决了传统粉末烧结制备复合材料成分分布不均匀的难题。获得的重要结果为制备出了目前报道的导热性能和热循环性能最好的金属相变胶囊，在1050～1150℃之间热循环1000次之后，仍然保持完好，为高温金属相变材料的包覆问题提供了崭新的解决思路。而且该制备技术简单，易于规模化生产，有望推进该相变胶囊在高温烟气回收中的应用。共发表论文15 篇，其中SCI收录15篇；申请专利5项，授权2项。