Multi-scale investigation of composite materials for enhanced phase change thermal storage systems
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Thermal energy storage technology has received renewed attention in recent years for a variety of important applications including storage for thermal solar systems, building thermal management, and thermal management of high power electronics. Thermal storage is used in transient systems where heat loads are shifted by storing heat, which is later used or dissipated as desired for the application. For example, high power pulsed electronics may incorporate thermal storage to absorb waste heat during a pulse and dissipated during the off-portion of the duty cycle, which has the benefit of reducing heat sink performance requirements. Phase change materials (PCMs) are commonly implemented in thermal storage systems due to large storage energy density by latent heat of fusion. Despite high energy density, the low thermal conductivity of most PCMs limits power capacity, which for many applications is a critical engineering challenge. One of the most commonly implemented methods to address the low thermal conductivity of PCMs is incorporation of a high thermal conductivity filler material. For thermal storage applications, it is important to minimize the fraction of the filler material to minimize the impact on storage density, while simultaneously maximizing the effective thermal conductivity of the composite. Both the structure and the intrinsic material properties of the filler material determine the potential relative enhancement of a PCM composite. This work focuses on means to enhancing the thermal transport in PCMs by incorporation of continuous, high thermal conductivity structures and includes detailed exploration into the thermal properties of graphitic filler materials. Regarding thermal storage units, this work demonstrates the performance enhancement of a thermal storage unit with incorporation of aluminum foam. Regarding graphitic materials, this work includes development of a high quality, ultra-thin graphite foam grown by chemical vapor deposition on sintered nickel powder catalytic templates that increases the effective density and effective thermal conductivity compared to previously reported graphite foams. Lastly, this work investigates the synthesis and measurement of thermal properties of a nano-pillared graphitic structure. The intrinsic thermal conductivity is measured via a micro-thermometry four probe measurement. Also presented is the optimization of this measurement method using Fourier analysis.