相变微胶囊复合材料的多层级力学性能测试和强化机理研究

Micro-encapsulated phase change composites (MEPCCs) are one of the most promising materials for energy saving. To increase the thermal energy storage capacity of MPCCs, the microcapsules content must be increased, which however leads to a remarkable decrease of the mechanical strength of MEPCCs. Only the strengthening mechanism of MEPCCs is revealed, can the mechanical strength be strengthened while keep high thermal energy storage capacity. The primary aims of the present project include: (1) Establishment of testing methods for characterizing the multi-scale mechanical properties of MEPCCs. The MEPCCs have three levels of microstructures, i.e. the shell of microcapsule, the microcapsule and the joint area of microcapsule and matrix. Due to the small scale and complex shape, the shell of microcapsule will be tested by nano-indentation using a Berkovich indenter, the microcapsule will be tested by micro-compression using a flat-ended indenter, and the joint area will be tested by nano-indentation using a Berkovich indenter. According to these tests, the corresponding deformation models of thin shell during indentation, micro-ball during compression and joint area during indentation will be developed via theoretical analysis and finite element analysis. Based on the developed deformation models, a nano-indentation method for characterization of viscoelastic-plastic properties of microcapsule shell, a micro-compression method for determining rupture strength of microcapsule and a nano-indentation method for extracting elastic-plastic parameters of the joint area will be established. (2) Investigation of strengthening mechanism of MEPCCs. Using the newly proposed testing methods to determine the corresponding mechanical properties of each level of microstructures, then analyze how these microstructures and their mechanical properties affect the macro-mechanical strength of MEPCCs from the bottom up, and finally reveal the strengthening mechanism of MEPCCs. With the successful accomplishment of the proposed project, the compression deformation model of micro-ball will be developed, and a series of testing methods for characterization of micro-scale mechanical properties will be established, which will be helpful for the application of MEPCCs.

相变微胶囊复合材料是新兴节能材料。目前应用瓶颈是：为提高节能效果，需增加微胶囊含量，但会显著降低破坏强度。只有揭示材料破坏强度的强化机理，才能调和节能物性与力学性能的矛盾。本申请项目旨在：针对微胶囊的壁材、单体、囊基结合区三个微观层级，利用纳米压入仪的高分辨测量能力，分别选用玻氏压头(锥形压入壁材)、平底压头(显微压缩单体)、玻氏压头(锥形压入囊基结合区)，借助解析分析结合有限元修正，分别发展薄壁球壳压入变形、薄壁微球压缩变形、薄壁微球和基体复合压入变形的模型，依次建立壁材黏弹-塑性参数的纳米压入、单体破坏强度的显微压缩、囊基结合区弹塑性参数的纳米压入测试方法。再逐级测定各层级的力学性能，自下而上分析不同层级微观结构和力学性能对宏观破坏强度的影响规律，揭示强化复合材料宏观破坏强度的微观机理。本研究可发展薄壁微球压缩变形的模型理论，丰富微尺度力学性能的测试方法，促进此类节能材料的工程应用。

相变微胶囊复合材料是高效的吸热储能和节能控温材料。当前的应用限制是：增加微胶囊含量提高储能节能效果的有效途径，但复合材料的破坏强度随微胶囊含量增加而显著降低。为精准改进相变微胶囊复合材料的制备工艺，制备储能效率高和力学性能好的相变微胶囊复合材料，需要先揭示影响相变微胶囊复合材料宏观力学性能的微观机理。.本申请项目主要针对微胶囊的壳材、单体、囊基结合区三个微观层级，根据研究对象的几何和力学特征，分别建立起球形压入薄壁球壳的接触变形模型、平板压缩薄壁微球的接触变形模型与起裂准则、锥形压入囊基结合区的有限元分析模型。利用纳米压入仪的高分辨测量能力，基于提出的接触变形模型，依次建立微胶囊壳材弹性模量的纳米压入测试方法、微胶囊单体破坏强度的显微压缩测试方法、囊基结合区弹性模量分布和压入硬度分布的纳米压入测试方法。通过逐级测定各层级的力学性能，自下而上分析不同层级微观结构和力学性能对宏观破坏强度的影响规律，揭示出强化微胶囊复合材料宏观破坏强度的微观机理。.微胶囊复合材料强化机理主要涉及两方面：(1)增强囊基结合强度。囊基结合强度弱，其破坏模式以囊基脱粘为主，通过化学改性在微胶囊与基体间引入共价键或氢键等形成交联网络，增强囊基结合强度的同时减少囊基界面中的微观缺陷，从而抑制微裂纹的形核长大，提高宏观力学性能。(2)增强微胶囊破裂强度。当囊基结合强度高于微胶囊破坏强度时，复合材料的破坏模式由囊基脱粘转变为微胶囊破裂为主，增强微胶囊单体破坏强度可进一步提高复合材料宏观力学性能。基于强化机理，采用交替提升囊基结合强度和微胶囊破坏强度的策略，有望制备储能控温和力学性能均优异的相变复合材料，在建筑、航空等领域用于智能控温和防结冰。