Phase Change Materials for Vehicle and Electronic Transient Thermal Systems

Abstract

Most vehicle operating environments are transient in nature, yet traditional subsystem thermal management addresses peak load conditions with steady-state designs. The large, overdesigned systems that result are increasingly unable to meet target system size, weight and power demands. Phase change thermal energy storage is a promising technique for buffering thermal transients while providing a functional thermal energy reservoir. Despite significant research over the half century, few phase change material (PCM) based solutions have transitioned out of the research laboratory. This work explores the state of phase change materials research for vehicle and electronics applications and develops design tool compatible modeling approaches for applying these materials to electronics packaging. This thesis begins with a comprehensive PCM review, including over 700 candidate materials across more than a dozen material classes, and follows with a thorough analysis of transient vehicle thermal systems. After identifying promising materials for each system with potential for improvement in emissions reduction, energy efficiency, or thermal protection, future material research recommendations are made including improved data collection, alternative metrics, and increased focus on metallic and solid-state PCMs for high-speed applications. Following the material and application review, the transient electronics heat transfer problem is specifically addressed. Electronics packages are shown using finite element based thermal circuits to exhibit both worsened response and extreme convective insensitivity under pulsed conditions. Both characteristics are quantified using analytical and numerical transfer function models, including both clarification of apparently nonphysical thermal capacitance and demonstration that the convective insensitivity can be quantified using a package thermal Elmore delay metric. Finally, in order to develop design level PCM models, an energy conservative polynomial smoothing function is developed for Enthalpy and Apparent Capacity Method phase change models. Two case studies using this approach examine the incorporation of PCMs into electronics packages: substrate integrated Thermal Buffer Heat Sinks using standard finite element modeling, and direct on-die PCM integration using a new phase change thermal circuit model. Both show effectiveness in buffering thermal transients, but the metallic phase change materials exhibit better performance with significant sub-millisecond temperature suppression, something improved cooling or package integration alone were unable to address.