Active Thermal Switching of Smart Composite Materials

“智能”复合材料的主动热开关在自然界中发现的所有材料都表现出由其导热性定义的导热能力。例如，聚合物具有低导热性，而金属通常具有高导热性。本研究探索了热开关的创新概念，其中可以设计一种智能材料，使其热导率可以被主动控制。热开关在控制热导率提高设备或系统功能的应用中很有意义。例如，建筑物的加热和冷却，储热，温度自适应纺织品和电子或太阳能电池的热管理都是下一代解决方案需要智能控制传热的应用。该研究的目标是开发一种具有热导率的创新智能材料，该材料可以在遇到热刺激时永久或暂时切换高达三个数量级。换句话说，这种材料的导热性可以根据需要从聚合物转变为金属。正在开发的材料系统由形状记忆聚合物基体和具有在聚合物内部智能重新排列能力的纤维状填料组成。这项研究的目标是开发一种具有热导率的创新智能材料，该材料可以在遇到有或没有额外外力的热刺激时永久或暂时切换多达三个数量级。该材料系统包括形状记忆聚合物作为基体，用于在施加刺激时控制纳米纤维填料的取向。正在使用的方法和途径如下：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，需要高纵横比填料来诱导排列。2. 纳米纤维/形状记忆聚合物复合材料的制造，当纤维“切换”成排列时，具有高导热性，而当纤维排列减少时，具有低导热性。研究人员正在探索两类关键材料：半结晶交联聚合物和玻璃化转变温度高于室温的交联聚合物。在这些材料中，晶体区域充当热可逆转变，将材料固定在其应变临时状态。在将温度提高到玻璃化转变温度以上时，材料增加其弹性，允许复合材料拉伸并诱导纤维对齐。3. 利用x射线衍射技术对复合材料进行表征，以确定填料的分散程度和排列。4. 复合材料的面内导热系数和通过厚度的导热系数以及单个纤维的轴向导热系数的热特性。5. 对单个纤维和复合体系导热性的理论研究。6. 将热传导路径“写入”到复合薄膜上，并随后进行测试，以证明器件级热开关的有效性。

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.