Active Thermal Switching of Smart Composite Materials

“智能”复合材料的主动热开关自然界中发现的所有材料都具有由其导热系数定义的导热能力。例如，聚合物的导热系数较低，而金属通常具有较高的导热系数。这项研究探索了热开关的创新概念，其中智能材料可以被设计成其导热系数可以被主动控制。热开关在热导率控制改善设备或系统的功能的应用中是有意义的。例如，建筑物中的加热和制冷、储热、温度自适应纺织品以及电子或太阳能电池的热管理都是下一代解决方案需要热传递智能控制的应用。这项研究的目标是开发一种创新的智能材料，其导热系数可以在遇到热刺激时永久或临时切换到三个数量级。换句话说，这种材料的导热系数可能会从聚合物的行为转变为按需的金属行为。正在开发的材料系统包括形状记忆聚合物基质和纤维状填充物，这些填充物具有在聚合物中智能重新排列的能力。这项研究的目标是开发一种创新的智能材料，其导热系数可以在遇到热刺激时永久或临时切换三个数量级，无论是否存在额外的外力。该材料系统包括形状记忆聚合物作为基质，以在施加刺激时控制纳米纤维填料的取向。采用了以下方法和途径：1.纤维素纳米晶、氮化硼和碳纳米纤维的制备。选择纤维作为填料类型是因为在形状记忆聚合物收缩/膨胀过程中，需要高纵横比的填料来诱导排列。2.制备纳米纤维/形状记忆聚合物复合材料，该复合材料能够在纤维“切换”成取向时获得较高的导热系数，在减少纤维取向时获得较低的导热系数。目前正在探索两类关键材料：半晶态交联型聚合物和玻璃化转变温度高于室温的交联型聚合物。在这些材料中，晶区充当热可逆转变，将材料固定在其紧张的临时状态。当温度高于玻璃化转变温度时，材料增加了其弹性，允许复合材料被拉伸并诱导纤维排列。3.用X射线衍射法对复合材料进行表征，以确定填料的分散度和取向。4.复合材料的面内和厚度导热系数以及单根纤维的轴向导热系数的热表征。5.对单根纤维和复合体系的导热系数进行了理论研究。6.将热传导路径“写入”到复合薄膜上，并随后进行测试，以演示器件级别的热开关的有效性。

Active Thermal Switching of "Smart" Composite MaterialsAll materials found in nature exhibit an ability to conduct heat defined by their thermal conductivity. Polymers, for example, have a low thermal conductivity, while metals typically possess a high thermal conductivity. This research explores the innovative concept of thermal switching wherein a smart material can be engineered such that its thermal conductivity can be actively controlled. Thermal switching is of interest for use in applications where control of thermal conductivity improves the functionality of a device or system. For example, heating and cooling in buildings, thermal storage, temperature adaptive textiles and thermal management of electronics or solar cells are all applications where intelligent control of heat transfer is required for next generation solutions. The objective of the research is to develop an innovative smart material with a thermal conductivity that can be switched either permanently or temporarily up to three orders of magnitude upon encountering a thermal stimulus. In other words, the material's thermal conductivity may be switched from behaving like a polymer to acting like a metal on demand. The material systems being developed consist of a shape memory polymer matrix and fiber-like fillers that have the ability to intelligently rearrange within the polymer. The goal of this research is to develop an innovative smart material with a thermal conductivity that can be switched either permanently or temporarily by up to three orders of magnitude upon encountering a thermal stimulus with or without the presence of an additional external force. The material system comprises a shape memory polymer as the matrix to control the orientation of nanofiber fillers upon application of a stimulus. The following methods and approaches are being used: 1. Preparation of cellulose nanocrystal, boron nitride and carbon nanofibers. Fibers were chosen as the filler type because during the shape memory polymer contraction/expansion, a high aspect ratio filler is required in order to induce alignment. 2. Fabrication of nanofibers/shape memory polymer composites with the ability to achieve high thermal conductivities when fibers are "switched" into alignment and low thermal conductivities when fiber alignment is reduced. Two key classes of materials are being explored: semi-crystalline crosslinked polymers and cross-linked polymers with a glass transitions temperature above room temperature. In these materials the crystalline regions act as the thermal reversible transitions that fix the material in its strained temporary state. Upon increasing the temperature above the glass transition temperature, the material increases its elasticity, allowing to composite to be stretched and inducing alignment of the fibers. 3. Characterization of the composites to determine degree of dispersion and alignment of fillers using x-ray diffraction techniques. 4. Thermal characterization of the in-plane and through-thickness thermal conductivities of the composites and the axial thermal conductivity of the individual fibers. 5. Theoretical investigations into the thermal conductivity of the individual fibers and composite systems. 6. "Writing" of thermal conduction paths onto composite films and subsequent testing to demonstrate the effectiveness of the thermal switching at the device level.