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The Use of Ammonium Carbamate as a High Energy Density Thermal Energy Storage Material Joel E. Schmidt1,2, Douglas S. Dudis1 and Douglas J. Miller3 1

Air Force Research Laboratory, Materials and Manufacturing Directorate, Thermal Sciences and Materials Branch, Wright Patterson Air Force Base, OH 45433 2 University of Dayton, Department of Chemical and Materials Engineering, Dayton, OH 45409 3 Department of Science and Mathematics, Cedarville University, Cedarville, OH 45314

ABSTRACT Phase change materials (PCMs) often have higher specific energy storage capacities at elevated temperatures. Thermal management (TM) systems capable of handling high heat fluxes in the temperature range from 20-100°C are necessary but lacking. State of the art PCMs in this temperature range are usually paraffin waxes with energy densities on the order of a few hundred kJ/kg or ice slurries with energy densities of the same magnitude. However, for applications where system weight and size are limited, it is necessary to improve this energy density by at least an order of magnitude. The compound ammonium carbamate, [NH4][H2NCOO], is a solid formed from the reaction of ammonia and carbon dioxide which endothermically decomposes back to CO2 and NH3 in the temperature range 20-100°C with an enthalpy of decomposition of ~2,000 kJ/kg. Various methods to use this material for TM of low-grade, high-flux heat have been evaluated including: bare powder, thermally conductive carbon foams, thermally conductive metal foams, hydrocarbon based slurries, and a slurry in ethylene glycol or propylene glycol. A slurry in glycol is a promising system medium for enhancing heat and mass transfer for TM. Progress on material and system characterization is reported. INTRODUCTION Increasing power loads for various electronic devices have created a demand for novel thermal management (TM) technologies which allow these devices to operate in their ideal temperature ranges and ensure device efficiency and lifetime. Most electronic devices need to operate between 20 °C and 100 °C. It can be possible to cool these devices with air or conventional liquid coolants, but this can be energy intensive, require a large TM system, or even be impossible with high thermal fluxes [1]. One class of materials that is being explored to stabilize these devices is graphitic foams impregnated with paraffin wax phase change materials (PCMs) [1-6]. However, there exists a need to mange high heat fluxes on the order of 100 W/cm2 to 500 W/cm2 for near room temperature continuous and intermittent cooling [7,8]. Managing high-flux, low-grade heat is very difficult for many systems and is especially problematic in environments where weight, device orientation, recharge time and heat rejection are all problems which must be addressed. Therefore, the need exists to investigate novel methods for the TM of high heat fluxes at or near room temperature. Ammonium carbamate (AC) is proposed to serve as a high specific capacity thermal energy heat sink for the management of low-grade, high-flux hea