Active Thermal Switching of Smart Composite Materials

Case Western Reserve University, Cleveland OH

Investigators

Abstract

Active Thermal Switching of "Smart" Composite Materials All 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.