| Using a technique called time-domain thermoreflectance, a graduate student measures the thermal conductivity of thin-film nanolaminates. |
Researchers at the University of Illinois at Urbana-Champaign created a better thermal insulator by controlling material structure at the nanoscale.
Says David Cahill, a professor of materials science and engineering, "Making nanolaminates of dissimilar materials let us significantly cut thermal conductivity because heat cannot pass efficiently across the material interfaces."
The nanolaminates consist of alternating layers of tungsten and aluminum oxide. Researchers fabricate them using atomic layer deposition and magnetron sputter deposition. "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," says Cahill. Heat flow from one material to another is limited at the interface. Atoms vibrating in the lattice carry heat, and some of the lattice vibrations are scattered at the interface and are not transmitted.
The thermal conductivity of the nanolaminates is then measured using a technique called time-domain thermoreflectance.
Researchers measure the temperature of the small samples with a fast, mode-locked laser that produces subpicosecond pulses. Laser output is split into two beams; one that heats the sample, the other measures reflectivity.
The research also revealed some surprising implications for nanomaterials serving as thermal conductors for dissipating heat from electronics. As an example, carbon nanotubes, which are thermally conductive, will not perform well as fillers in composite materials designed to be thermally conductive. "Nanotubes do not couple well thermally to the surrounding material," says Cahill. "As a result, heat transport across the nanotube-matrix interfaces will be very limited."