The thermal properties of thin films, such as conductivity and diffusivity, are key to controlling heat flow, with applications in protecting spacecraft from high temperatures and diffusing heat from lasers. However, thin films’ thermal properties diverge from bulk materials, and their scale complicates thermal measurements.
My research at Trinity College Dublin focused on implementing the 3ω technique for measuring thermal properties of both thin films and bulk materials. The technique uses a metal strip on one’s sample as both a heater and thermometer. By passing an alternating current through the strip, it heats up with a temperature rise dictated by the thermal properties of the sample. The temperature oscillations cause small, proportional changes in the strip’s resistance. The net effect is to produce two voltage harmonics across the sample, one at the frequency of the alternating current and a much smaller one at three times the frequency, or “3ω”, which can be used to deduce the magnitude of temperature oscillations.
Given the small size of the 3ω voltage, I used a lock-in amplifier and a bridge circuit to separate the frequency contributions, taking care to isolate the sample from mechanical oscillations. Comparing heat equation solutions to the temperature readings, I fit the thermal conductivity of quartz and silicon samples. For future measurements, I designed new contacts and metal strip designs with geometries that have greater sensitivity to thermal anisotropies in the samples. In all, I’m very happy to have contributed to the foundations of thermal measurements at Trinity College Dublin.