| Heat may be essential for life, but in some cases - such as protecting the space shuttle or improving the efficiency of a jet engine - materials with low thermal conductivities are needed to prevent passage of too much heat. As reported in the Feb. 13 issue of the journal Science, researchers have created a better thermal insulator by controlling material structure at the nanoscale. "We explored ways to control thermal properties in materials by introducing structure on nanometer length scales," said David Cahill, a professor of materials science and engineering and a Willett Faculty Scholar at the University of Illinois at Urbana-Champaign. "By making nanolaminates of dissimilar materials, we found that we could significantly decrease the thermal conductivity because heat cannot be carried efficiently across the material interfaces." Cahill, graduate student Ruxandra Costescu and colleagues at the University of Colorado at Boulder first synthesized thin-film nanolaminates composed of alternating layers of tungsten and aluminum oxide using atomic layer deposition and magnetron sputter deposition. Cahill and Costescu then measured the thermal conductivity of the nanolaminates using a technique called time-domain thermoreflectance. "The reflectivity of a metal is a very subtle function of its temperature," Cahill said. "By measuring how fast the reflectivity, and therefore the temperature, changes over time, we can determine the thermal conductivity." To measure the temperature of such small samples, the researchers use an ultra fast, mode-locked laser that produces a series of subpicosecond pulses. The laser output is split into a "pump" beam and a "probe" beam. The pump beam heats the sample and the probe beam measures the reflectivity, and hence the temperature. "By making the individual layers only a few nanometers thick, we produced a nanolaminate material that had a thermal conductivity three times smaller than a conventional insulator," Cahill said. "The high interface density produced a strong impediment to heat transfer." Heat flow from one material to another is limited at the interface, Cahill said. Heat is carried by vibrations of atoms in the lattice, and some of these lattice vibrations are scattered at the interface and don't get transmitted across the interface. "In our nanolaminates, vibrations in one material don't communicate well with those in another," Cahill said. "The heavy tungsten atoms are vibrating fairly slowly, but the light aluminum oxide atoms are vibrating quickly. The differences in elastic properties and densities of vibrational states inhibit the transfer of vibrational energy across the interface." The experimental results suggest that materials engineered with high interface densities may provide a route for the production of thermal insulators with ultra-low thermal conductivities. The researchers' findings also have some surprising implications for nanomaterials that are intended to perform as high thermal conductors in applications such as dissipating heat from electronic circuits or sensors. For example, carbon nanotubes -- which have been shown to have extremely high thermal conductivities - will not perform well as fillers in composite materials designed to improve thermal transport. "Nanotubes do not couple well thermally to the surrounding material," Cahill said. "As a result, the heat transport across the nanotube-matrix interfaces will be very limited." |