Molecular simulation aided design of textured cooling loops minimizing Kapitza resistance for high power density space applications

Advanced thermal management techniques for integrated small scale avionics, micro/nano spacecraft, system-on-a-chip technologies and new computing platforms for AI are indispensable to future space exploration. NASA continues to invest significantly in improved design of high performance thermal control systems to optimize performance, prevent thermal failure, and extend operational lifetimes. Such applications, however, are becoming heavily reliant on ever more powerful computing chips packed into ever smaller spaces. High density integrated circuits are quite prone to nucleation of hot spots which nowadays generate thermal fluxes in excess of 25 to 150 Watts/cm2. It is believed that mitigation and dissipation of such high thermal fluxes are now the limiting factors in information processing. Inability to prevent significant thermal buildup is detrimental to systems operation because it leads to thermal runaway, an unstable process whereby hot spots tend to generate more hot spots, which quickly causes system failure. This phenomenon sets in because regions of an integrated chip experiencing higher operating temperatures tend to reduce local electrical resistances thereby increasing leakage currents, which in turn cause increased Joule heating and therefore higher temperatures. Leakage runaway not only generates excessive waste heat but undermines proper functioning of hardware. Consequent thermal damage, often irreversible, also causes higher thermal noise, which incurs higher bit error rates, corrupts program memory, and loss of transistor control due to thermally energized electrons that jump bandgaps. Designers of high density integrated chips are abandoning conventional gas cooling, which is believed to have reached a plateau, in favor of microscale liquid cooling loops, which require smaller volume and far less power to operate due to the higher density and heat capacity of liquids. Due to the increased surface area per unit volume in miniaturized cooling loops, the properties of the liquid/solid interface play a key role in the heat transfer process. The goal of our project is two-fold. On a fundamental level, we wish to ascertain whether there exists a fundamental limit to heat transfer at liquid/solid interfaces as the system size is decreased. On a practical level, we wish to identify those cooling architectures that maximize or minimize interfacial thermal transport for applications requiring rapid thermal dissipation or longer term thermal storage. Our computational program is designed to provide such answers by focusing on the concept of interfacial commensurability and its interplay with the so-called Kapitza resistance. The Kapitza resistance, a dominant factor in nanoscale systems, is known to produce a thermal insulation layer at the liquid/solid interface caused by the intrinsic mismatch in material properties at that boundary. We implement non-equilibrium molecular dynamics simulations, capable of capturing both molecular scale and continuum level behavior in fluidic systems subject to thermal gradients, to develop novel heat transfer correlations for the Kapitza resistance. We explore how this interfacial resistance is influenced by various geometric and dynamic measures of interfacial commensurability, which in turn is affected by the molecular structure, composition and collective response of the first few liquid monolayers adjacent to the solid. In particular, we quantify the influence of textured solid boundaries and system size effects related to liquid confinement on phonon transport in the presence and absence of fluid flow at liquid/solid interfaces. Our results are expected to yield novel cooling and thermal storage designs for high performance thermal control systems critical to future NASA missions.

Advanced thermal management techniques for integrated small scale avionics, micro/nano spacecraft, system-on-a-chip technologies and new computing platforms for AI are indispensable to future space exploration. Thermal control is a major issue for such technologies. Our results are expected to yield novel cooling and thermal storage designs for high performance thermal control systems critical to future NASA missions.

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