Session: 11-07-01 Measurements of Thermophysical Properties

Paper Number: 73381

Start Time: Tuesday, 03:50 PM

73381 - Infrared Radiometry Based Steady-State Method for Thermal Conductivity Measurement 

Knowledge of thermal conductivity is critical for the design of thermal systems. The measurement techniques for thermal conductivity can be divided into two categories:  steady-state methods and transient methods. Steady-state methods commonly measure the time-independent temperature difference created by constant heat flux from a heating source. On the contrary, transient methods are usually based on measuring the time-dependent temperature dissipation induced by a pulse or periodic heating source.

For traditional steady-state techniques, including absolute techniques, comparative techniques, radial heat flow methods, and parallel thermal conductance techniques, the thermal conductivity can be extracted using a variation of Fourier's law straightforwardly. However, traditional steady-state techniques often use contact heaters and thermal couples, introducing undesired thermal contact resistance and parasitic heat losses, including radiation and convection heat loss through thermal couple wires. Moreover, those techniques are designed for bulk thermal conductivity measurement with low spatial resolution and require a large sample size. The time needed to reach a steady-state condition may take hours.      

Transient methods based on photothermal techniques, such as Frequency-Domain Thermoreflectance (FDTR) and Time-domain Thermoreflectance (TDTR), employ pump/probe lasers for heating and measurement. They have been proven to be robust, enabling non-contact measurement for both bulk and local (~100nm) thermal properties. However, the pulsed or modulated heat transport is governed by the transient heat conduction equation. The results are usually thermal diffusivity or thermal effusivity based on the experimental conditions. Moreover, TDTR and FDTR data processing is based on fitting experimentally measured phase lag to the heat transfer model. Thus, the phase lag needs to be measured accurately, which requires an excellent separation between frequency-dependent electronic phase shifts of the instrument and desired probe signal. 

This study aims to develop a steady-state infrared radiometry technique (SSIR). It combines the benefits of the transient pump technique, the steady-state method, and infrared radiometry. The method uses a continuous wave (CW) laser as a pump source and a high-speed infrared camera to capture temperature-time maps based on emissivity change as the probe system. Instead of the thermoreflectance measurement, the advanced infrared camera makes the theoretical model and the experiment greatly simplified.  

Comparing to transient thermoreflectance measurement techniques, SSIR has several advantages. First, TDTR and FDTR require exceedingly high optical precision. The pump and probe laser beams' optical path should be identical, from reaching the sample surface to the photodetector. For the SSIR, the probe optics are integrated into the infrared camera system, and no optical table is needed. As such, the cost of the experiment system is also reduced. Second, in steady-state measurement, the temperature profile is only sensitive to the sample's thermal conductivity and not sensitive to the metallic transducer properties. This feature can significantly reduce the uncertainty induced by input transducer and sample properties in transient methods. Third, Fourier signal analysis and corresponding instruments like a lock-in amplifier are not needed because the control variable is the power of the pump source. The thermal conductivity is determined directly from Fourier's law.

Principle of system 

A constant-wave laser modulated by a square wave at a very low frequency is used for heating. The sample reaches "on" state and "off" state of steady-state temperature. A high-speed IR video camera records the corresponding temperature-time data. The material with unknown thermal conductivity and a standard sample are coated with a metallic transducer film to guarantee identical laser energy absorption. Varying the output power of the pump laser can control the steady-state temperature rise. The ratio of heat flux (power of pump laser) to the steady-state temperature rise has a fixed correlation for samples with an identical metallic film. Thus, by comparing the ratio of an unknown material with the known correlation measured from the standard sample, unknown material's thermal conductivity can be derived.

Presenting Author: Dihui Wang University of Pittsburgh

Authors:

Dihui Wang University of Pittsburgh
Heng Ban University of Pittsburgh