颗粒团簇定向迁移对悬浮液凝固糊状区横向冰晶的影响机制

In recent years, many new advanced materials with excellent properties have been developed by means of suspension solidification, such as metal matrix composites enhanced by nanoparticles and nacre-like structural materials with high strength and high toughness, which has brought engineering applications into the theoretical study of suspension solidification. Recently, it has been found that ice lenses grow laterally in the mushy region at high growth rates, which has morphological similarity with side-branching in alloy solidification. Ice lenses destroy the mechanical properties of freeze-casting porous materials. However, this experimental phenomenon can not be reasonably explained by existing theoretical models. Currently, the applicant has found that the growth rate of ice lenses in the mushy region is similar to that of the thermal regelation of particle clusters through preliminary experiments. The thermal regelation of particles is very similar to the temperature gradient zone melting in alloy solidification. Through in-situ experimental observations of laser confocal fluorescence microscopy and mathematical models, this project will study the thermal regelation of particle clusters, and clarify the influence mechanisms of solutes on thermal regelation of particle clusters, and investigate the formation and growth of transverse ice lenses in mushy regions by using the interaction mechanisms between solutes and particle clusters. We aim to control and eliminate the formation of ice lenses in the mushy region. This project will provide theoretical guidance for optimizing the production process of freeze-casting materials and bring new applications to the field of solidification.

近年来通过悬浮液凝固的方法，人们相继开发出了众多性能优异的新材料，如高强高韧金属基纳米颗粒复合材料及仿贝壳结构材料等，为悬浮液凝固的理论研究带来了工程应用。最近在高生长速度下，发现悬浮液凝固糊状区出现横向生长的冰晶(与合金凝固中侧向分枝形态类似)，严重破坏冷冻铸造法多孔材料的力学性能。然而，这一实验现象无法用现存的理论模型合理解释。当前申请人通过预实验发现，糊状区横向冰晶生长速度与颗粒团簇定向热迁移(与合金凝固中温度梯度区域熔化效应相似)的速度相近。本项目将借助激光共聚焦荧光显微镜原位观测实验及建立数学模型系统研究颗粒团簇定向热迁移的热力学成因及动力学特性，阐明溶质对颗粒团簇定向热迁移的影响机制，并应用溶质与颗粒团簇相互作用模型系统描述悬浮液凝固糊状区横向冰晶形成及生长的规律，以期控制和消除糊状区横向冰晶的形成，为优化冷冻铸造法多孔材料的生产工艺提供理论指导，并为现代凝固学科带来新应用。

近年来，冷冻铸造法已被用来制备多尺度、多级次、各向异性多孔结构。最近在高生长速度下，悬浮液凝固糊状区出现横向生长的冰晶，严重破坏冷冻铸造法多孔材料的力学性能。本项目通过数学建模，将颗粒内部与边界处的液体流动及界面处的潜热释放、热焓吸收等综合考虑，发现糊状区横向冰晶生长速度与颗粒团簇定向热迁移的速度(1 m/s)相近。借助激光共聚焦荧光显微镜原位观测颗粒团簇的定向热迁移及糊状区横向冰晶的生长，发现：颗粒团簇的定向热迁移受颗粒尺寸、温度梯度影响显著。这与合金凝固中温度梯度区域熔化效应相似：随着第二相颗粒由固相区迁移至液相区，得到的固相纯度提高。团簇的定向热迁移速度随温度梯度线性增加。颗粒尺寸越大，渗透率越大，团簇迁移速度越大。由此发现，糊状区的横向冰晶生长是由颗粒团簇在温度梯度下的定向热迁移引起。最后，本项目把此最新结晶理论应用至钙钛矿晶体生长中，制备出了低缺陷态密度(1010 cm−3)、高载流子迁移率(78.1cm2V-1s-1)和4869.0 μC Gyair−1 cm−2@1000V/mm的高灵敏度的二维钙钛矿X射线探测器。最后，该器件具有高空间分辨率的X射线成像和良好的长期稳定性，在医学诊断和无损检测中具有广阔的应用前景。