THERMAL METROLOGY AND CHARACTERIZATION OF HIGH THERMAL CONDUCTIVITY POLYMER FIBERS AND FABRICS

Recent technological advances in the field of electronics and the accompanying trend of device miniaturization with enhanced functionality has led to growing interest in new methods of electronic device integration. As a result, flexible, wearable, and portable electronic devices have emerged as a way of providing a multifunctional infrastructure to facilitate various consumer needs, creating new challenges for materials development. Polymers possess a unique combination of desirable properties such as mechanical compliance, durability, low density and chemical stability which makes them ideally suitable as substrate materials to cater to such diverse applications. However, the low thermal conductivity of polymers hinders their heat spreading capability in thermal management applications for flexible and wearable devices. In recent years, there has been a growing interest in ultra-high molecular weight polyethylene (UHMW-PE) materials with aligned polymer chains due to their remarkably high thermal conductivity that is similar to some metals. These are commercially manufactured in large volumes as fibers using gel-spinning and ultra-drawing processes that impart a high degree of crystallinity and orientation to the polymer chains. As a result, these materials develop exceptionally high mechanical strength, elastic modulus, and thermal conductivity compared to conventional polymers. Therefore, UHMW-PE materials have found applications in commercial products like motorcycle gear and ballistic vests, but have not been commercially deployed for heat spreading and thermal management applications. While there has been much fundamental work on the development of high thermal conductivity fibers, effective translation of the high conductivity from individual fibers to macroscale (wearable) flexible fabrics has not been previously explored. The objective of this thesis is to obtain a fundamental understanding of the thermal transport properties of fabric materials constructed from the high conductivity polymer fibers, and assess their applicability for potential heat spreading applications.

In the present work, commercially available high thermal conductivity fibers made of UHMW-PE are utilized to fabricate plain-weave fabrics prototypes, and the thermal properties of individual fibers, yarns, and woven fabrics are measured using a novel in-plane thermal measurement method. The characterization technique leverages infrared (IR) microscopy for a non-contact temperature sensing and is generally scalable for thermal characterization of the in-plane thermal-conductivity of materials across different length scales. Effective thermal conductivities on the order of ~10 Wm^-1K^-1 are achieved along the in-plane dominant heat transport direction of the woven fabric, which is exceptionally high (~2-3 orders of magnitude) compared to conventional clothing and textile-based materials. The thermal conductivity and mechanical flexibility of the UHMW-PE fabrics are benchmarked with respect to conventional materials and the effect of bend-stressing and thermal annealing of the fabrics is characterization using the developed metrology.

Additionally, a laser-based IR thermal metrology technique leveraging both non-contact heating and temperature sensing is conceptualized and validated using a numerical thermal modeling approach. The proposed technique provides an approach to estimate the in-plane heat spreading properties of anisotropic materials with direction-depended thermal properties based on quantifying the surface temperature map of a sample subjected to periodic heating. Numerical simulations are leveraged to demonstrate the applicability of this method to enable measurement of a wide range of thermal properties indicating great potential to develop this further as a standardized robust method for in-plane anisotropic thermal characterization of materials such as fabrics and films.

This work sheds light on the high thermal conductivity of UHMW-PE materials that can be achieved using a scalable manufacturing process and describes the thermal metrology approaches to enable their characterization, thereby providing a foundation for the conceptualization and design of flexible substrate based thermal solutions in future wearable/flexible electronic devices.