Expendable High Energy Density Thermal Management Material: Ammonium Carbamate

Abstract

I NCREASING power loads for various electronic devices have created a demand for novel thermal management (TM) technologies that allow these devices to operate in their ideal temperature ranges in order to ensure device efficiency and lifetime. Most electronic devices need to operate between 20 and 100 C. Cooling these devices with air or conventional liquid coolants can be energy intensive, require a large TM system, or even be impossible when dealing with high thermal fluxes [1]. Traditionally, cooling in these devices is accomplished using the sensible heat associated with a temperature change of a fluid such as air or water. Air cooling systems are the simplest but cannot handle high thermal loads. The higher specific heat of water allows it to handle greater thermal loads than air. Nevertheless, the specific heat of water is low relative to that of its phase changes, which cannot be used for the temperature range of interest. Since latent heats accompanying phase changes are fundamentally higher than sensible heats on a mass basis, phase change materials (PCMs) have garnered intense interest for TM. Additionally, as PCMs offer the ability to maintain a constant temperature, they are being explored for the temperature range of interest. One class of PCMs being explored for this temperature range is graphitic or metallic foams impregnated with paraffin wax [1–6]. Waxes are advantageous since their melting temperatures can be tuned between 5 and 95 C, but they suffer from low specific energy densities on the order of 200 kJ=kg for the purematerial. The systemlevel properties (specific energy density and thermal power) are much lower since the low thermal conductivity of waxes, which is around 0:2 W=m K, necessitates system-level architectures that enhance the heat transfer rate in order to achieve a practical thermal power rating [7]. Much of the work in using waxes as PCMs has concentrated on incorporating them into thermally conductive carbon or metal matrices in order to improve the thermal conductivity. This has shown success but lowers the system-specific properties to less than half those of the neat waxes [1,2,4,8]. TM systems based on chemical reactions, rather than sensible or latent heats, are another class of materials being explored. In principle, chemical reaction systems can yield the highest gravimetric TM capacities since the energies associated with changes in chemical bonding are intrinsically greater than those of physical phase changes. One class of chemical reaction systems involves a solid that exothermically reacts with a gas to form a complex. The complex can then be disassociated in an endothermic process, which can serve as a heat sink.Ammoniates are representative of this type of system. They work by using ammonia gas and a salt (normally period 4, 5, and 6 metal chlorides) to form complexes with ammonia [9–11]. The problem with these systems is that they require high pressures and large amounts of the salts, which lead to low specific properties and makes them unsuitable for applications in which specific energy and specific thermal power are key drivers at the system level. Metal hydride-based chemical reaction systems function by using hydrogen gas that exothermically reacts with metals to form a metalhydrogen complex from which the hydrogen can then be endothermically released. Metal hydrides can have very high gravimetric thermal energy densities, and there are many possible metal-hydrogen storage material systems that have been reported elsewhere [12,13]. Much of the research into metal hydrides is being conducted for the purpose of hydrogen storage in fuel cell vehicles. This application requiresminimization of the thermal load associated with the sorption process. However, metal hydride systems with a high thermal load may be good candidates for TM. The Mg=MgH2 system is well characterized with a thermal storage density of 1850 kJ=kg, while LiAlH4 has been reported to have the extremely Presented as Paper 2011-3950 at the 42nd AIAA Thermophysics Conference, Honolulu, HI, 27–30 June 2011; received 21 June 2011; revision received 11 August 2011; accepted for publication 5 September 2011. This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 percopy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0887-8722/12 and $10.00 in correspondence with the CCC. Research Engineer, Materials and Manufacturing Directorate, Thermal Sciences and Materials Branch, 2977 Hobson Way. Principle Research Chemist, Materials and Manufacturing Directorate, Thermal Sciences and Materials Branch, 2977 Hobson Way (Corresponding Author). Professor of Chemistry, Department of Science and Mathematics, 251 N. Main Street. JOURNAL OF THERMOPHYSICS AND HEAT TRANSFER Vol. 26, No. 2, April–June 2012