Characterization and Controllable Nucleation of Supercooled Metallic Phase Change Materials

Date of Award

2023

Degree Name

Ph.D. in Mechanical Engineering

Department

Department of Mechanical and Aerospace Engineering

Advisor/Chair

Jamie Ervin

Abstract

Paraffin phase change materials (PCMs) are often used for thermal energy storage due to their high gravimetric latent heat values and low cost. However, they are not well suited for high heat transfer rate applications where their low thermal conductivities limit use. Additionally, applications in which there are volume restrictions may drive material selection toward metallic phase change materials, where the volumetric latent heat can be higher than those of paraffins with comparable melting points. Gallium, for example, has over twice the volumetric latent heat and a thermal conductivity that is two orders of magnitude greater than that of octadecane. The use of gallium is not without issue. Gallium is known to experience supercooling which is often viewed as a detrimental property. Thus, an improved understanding of supercooled gallium nucleation is useful. One purpose in this study is to explore the effects of the thermal history and mass on the supercooling of gallium. In this work, differential scanning calorimetry and a thermal cycling chamber were used to characterize these effects. Material overheating was found to have the largest impact. Additionally, an active control methodology was created to successfully activate solidification without significantly affecting the bulk material temperature. While the previously mentioned active control methodology demonstrated successful nucleation of supercooled gallium using a vortex-tube/cold-finger design, the time (>10 min) and level of support equipment required (e.g., compressor) allows for improved design. Thus, a thermoelectric cooling approach, using electric current to rapidly remove heat from the supercooled gallium, was investigated. In this work, a two-stage thermoelectric cooler and cold-finger design was implemented to decrease the required nucleation time, and increase the effectiveness of the nucleation process. An order of magnitude decrease in time needed for nucleation (~10 s) was achieved for all overheat conditions with significant improvements to nucleation effectiveness. The ever-increasing power throughput and ever-decreasing size of modern electronics, specifically power electronics, requires more advanced packaging techniques and materials to maintain thermal limits and sustain mechanical life. Specific applications with known operating conditions for these components can realize added benefits through a tailored thermal-mechanical-electrical optimized assembly, potentially utilizing niche material classes. Without losing any expected functionality, solid-liquid PCMs, could be incorporated into the device structure to reduce peak temperature and/or suppress high-cycle fatigue problems commonly found at device attachment interfaces. The purpose of this study was to investigate, through model-based design and analysis, the impact of using PCMs at two strategic locations in the standard device stack. The results suggest noteworthy life improvement (40%) is possible when optimizing for a given melting point.

Keywords

Mechanical Engineering, gallium, phase change materials, supercooling, nucleation, thermal management, electronics cooling

Rights Statement

Copyright © 2023, author

Link to Full Text

Share

COinS