3 omega measurement for thermal conductivity

The 3-Omega technique is used to very accurately measure the thermal properties of materials. It is most commonly used to measure the through-plane thermal conductivity, but it can also measure the thermal diffusivity and effusivity directly. In general, an electrical current of (angular) frequency ω = 2πf and root-mean square value (RMS) Iω,RMS driven through a metal heater line, causing Joule heating at a frequency 2ω. The periodic heating creates a thermal wave that penetrates the surrounding environment. The amplitude of the temperature oscillation at the source depends on the thermal properties of the environment. The periodic temperature oscillation follows the periodic heating and occurs at a frequency 2ω but delayed in phase ϕ (i.e., the phase lag). This temperature oscillation then causes the resistance of the heater to oscillate at 2ω. Because the current is driven at a frequency ω and the resistance changes at a frequency 2ω, an RMS voltage at 3ω results. The 3ω voltage amplitude is directly measurable and provides information on the thermal environment of the heater.



Figure 1: Standard heater line geometry for a thin film measurement

To measure thin film thermal properties, a heater line is deposited on the sample using photolithography or shadow mask evaporation (Fig 1). Typical heater lines are 1 mm long, 50 μm wide and a few nm thick. Dimensions may vary depending on the material being studied. To measure the thermal properties of fluids, an insulating material, say, glass fiber is circumferentially coated with a thin layer of electrically conducting material, which acts as the heater line (Fig 2). This suspended wire is placed in the fluid medium to obtain its thermal properties.



Figure 2: Suspended wire heater line for 3-Omega measurements in fluids



A microbridge heater for low power gas sensing based on the 3-Omega technique