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Thursday, 9 February 2012

Managing Heat Transfer at Material Interfaces

Feb 9, 2012

Credit: ASME

Managing heat transfer at material interfaces is a major challenge for engineers who work on devices ranging from jet engines to personal electronics to nanoscale transistors. The physical and chemical properties that control thermal transport at interfaces between dissimilar materials are poorly understood. A deeper knowledge of how these properties impact heat transfer could lead to increased performance and energy efficiency in military and commercial materials and devices such as high-power electronics, thermoelectric generators, thermal interface materials, and thermal barrier coatings.

Through a Multidisciplinary University Research Initiative program sponsored by the Air Force Office of Scientific Research (AFOSR), Arlington, VA, Kumar Jata of AFOSR, along with a team of researchers from the University of Michigan, Brown University, Stanford University, and University of California at Santa Cruz, is studying the fundamental processes by which heat is transferred across material interfaces. Their ultimate goal is to develop a set of design rules for custom-engineering interfaces that have specific thermal properties and higher performance.

"Recent progress in nanoscience such as advancements in molecular beam epitaxy and self-assembled monolayers has enabled the precise control of interface physical and chemical structure," says Dr. Kevin Pipe, an associate professor of mechanical engineering at the University of Michigan, Ann Arbor, MI, and leader of the research team. "However, the fundamental physics that link this nanoscale structure with thermal transport is not yet well developed, inhibiting the engineering of interfaces with radically enhanced thermal properties."

Measuring Thermal Resistance

The physical and chemical nature of material interfaces can decrease a composite material's thermal conductivity by scattering the acoustic waves that are the primary carriers of heat in solids. Interface properties that contribute to this scattering include roughness or irregularities and weak chemical bonding, as well as different acoustic properties (acoustic mismatch). This scattering creates a thermal resistance at the interface.

Dr. Pipe's team uses advanced techniques based on ultrafast lasers that emit pulses of light less than 50 fs in duration. When these pulses hit a sample, they generate acoustic waves that scatter off of interfaces within the material. "By examining the echo patterns from these acoustic waves, we learn how they interact with different kinds of interfaces," says Dr. Pipe. The measurements utilize a new optical cavity technique, developed by researchers Dr. Humphrey Maris, professor of physics, and Dr. Arto Nurmikko, professor of engineering and physics, at Brown University, Providence, RI. The technique dramatically enhances the signal-to-noise ratio of the measured echoes.

Abstract acoustic waves. Credit:ASME

The team has further extended this method by combining ultrafast laser pulses with x-ray diffraction through techniques developed by Dr. David Reis, an associate professor of photon science and applied physics at Stanford University, Stanford, CA. In these studies, picosecond x-ray pulses are used to look directly at atomic motion near an interface as heat flows across it. "The x-ray pulses create a stroboscopic movie that allows us to delineate the particular vibrational modes of the atoms that carry heat energy across an interface," he says.

Additional measurement advancements the team has made include a thermal imaging camera with 800-ps time resolution and 250-nm spatial resolution. Developed by Dr. Ali Shakouri, professor of electrical engineering at the University of California at Santa Cruz, CA, the camera allows the researchers to directly map the temperature near an interface on the same spatial and time scales at which heat flows across it.

One particular area that Dr. Pipe and his team have explored is the propagation of heat across interfaces between soft and hard materials, which are commonly found in thermal interface composites. Dr. Pipe and Dr. Max Shtein, associate professor of materials science and engineering at the University of Michigan, have shown that the degree to which organic materials and metals mix at the nanoscale when they form interfaces within a composite can change the composite's thermal conductivity by more than a factor of three. In fact, if very little mixing occurs, the resistance of these interfaces to heat transfer can reduce the thermal conductivity of the composite below that of either constituent.

Controlling Thermal Boundary Resistance

"For some time now, people have been able to measure the thermal boundary resistance of an interface, which quantifies how well heat moves across it," says Dr. Pipe. "However, only recently have people been able to make precise nanoscale modifications to an interface that allow us understand how to control this resistance." These precise modifications are key to generating experimental data that can be compared with fundamental models.

Being able to control thermal boundary resistance is an important capability for either efficiently transferring or blocking heat flow between materials, critical factors in performance and reliability. Inefficient heat flow is a barrier to the development of higher-power lasers and transistors. On the other hand, being able to block heat transfer can dramatically improve the efficiency of thermoelectric energy conversion for compact power sources.

Using their discoveries, Dr. Pipe and his colleagues plan to re-engineer the structure and chemistry of various material interfaces at the nanoscale to better regulate their heat transfer. A broad range of military and commercial products can be enhanced with better thermal interface control, including heat sinks for high-power electronics, thermal barrier coatings for aerospace components, and thermoelectric materials for power generation.

Source: ASME


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