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Thermal Interface Materials for Power System Components


OBJECTIVE: Develop technology to reduce thermal resistance between power components and cold plates; increase mechanical compliance due to thermal expansion coefficients; increase thermal cycles before degradation and ensure ease of workability. DESCRIPTION: The stable and reliable operation of megawatt-class, high-temperature power electronics is critical for military aircraft operations. Emerging multi-switch power modules, based in silicon carbide (SiC) electronics, have increased operating temperatures to over 200 degrees C, leading to very high heat fluxes from the components to thermal management systems (i.e., cold plate). Limitations of existing thermal interface materials, combined with high coefficient of thermal expansion (CTE) mismatches between the component and heat sink materials, results in higher temperature operation with commensurately reduced reliability. New thermal interface material technologies should be factors lower in thermal resistance than greases. At end of life, the thermal resistance of greases ranges from 0.05 degrees Cin2 /W to 0.1 degrees Cin2 /W on switch package size applications (~2 by ~4 inches) which lead to excessively high junction temperatures. A high mechanical compliance and the ability to be reworked/reapplied are also highly desirable. Solutions must also result in minimal change to existing manufacturing and assembly process, and demonstrate high potential for scalability. New thermal interface materials must demonstrate improvements in cyclic durability at operating temperatures between -55 degrees C and 200 degrees C. It is estimated that improvements of 20 to 30 degrees C in peak junction temperature rise may yield a 3 to 5X improvement in cyclic durability, resulting in dramatic improvements to meet the objectives for robust electrical power systems (REPS) and aircraft thermal management. PHASE I: Demonstrate improvements in thermal conductance by a factor of 4 over thermal greases and state-of- the-art (SOTA) interface materials, with the goal of reducing delta-T by 10 degrees C for a heat flux of 50 W/cm2. Thermal resistance should be measured by a standard technique (e.g., ASTM D5470). Demonstrate a functional interface prototype for power module component mounting to a cold plate. PHASE II: Demonstrate improvements in thermal interface conductance by a factor of 10 over current SOTA for future high flux devices with marginal spreading. This would reduce the interface delta-T by about 30 degrees C for a heat flux of 100 W/cm2 throughout a -55 degrees C to 200 degrees C temperature regime. Demonstrate improved cyclic durability in a simulated power system operating environment. Uniformity of properties and performance over an expected power module base plate area of at least 25 cm2 is required. PHASE III: Military application: Power system electronics and avionics systems comprised of high heat flux components. Commercial applications: Challenging applications such as telecommunications relay stations, data farms, and computing centers would benefit from interface materials with lower resistivity. REFERENCES: 1. B.A. Cola et al.,"Photoacoustic Characterization of Carbon Nanotube Array Thermal Interfaces,"J. Appl. Phys., Vol. 101, p. 054313, 2007. 2. B.A. Cola et al.,"Carbon Nanotube Array Thermal Interfaces Enhanced With Paraffin Wax,"Proceedings of ASME Summer Heat Transfer Conference, p. 765, 2008. 3. Goodson Wardle et al.,"Temperature-Dependent Phonon Conduction and Nanotube Engagement in Metalized Single Wall Carbon Nanotube Films,"Nano Letters, Vol. 10, p. 2395, 2010.
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