Thermoreflectance of carbon nanofibers: joule heating experiment and calibration

Abstract

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.Thermoreflectance (TR) is a novel, non-contact technique that uses the change in surface reflectivity over optical wavelengths of light to deduce temperature. This reflectivity is also wavelength dependent and material dependent. By calibrating a sample to determine the TR coefficient k, the relative change in intensity per degree change in temperature, the difference between heated and cool images is used to measure the relative temperature change. A two-dimensional, steady state TR method is used to examine the thermoreflectance behavior of carbon nanofibers (CNFs). Signal mixing between the CNF and the substrate at the sub-micron level is minimized by use of gold at its TR cross-point. A TR signal is created by the CNF as it is subject to Joule heating by passing a constant current through it. The calibration coefficient is measured though uniform heating of the sample. Initially, imaging during the heating process suffered from image shifting caused by air currents. Once the visible shifting is removed, the TR coefficient kcomp derived from the heating experiment was 7.93 x 10-5/K. To validate these results, the TR signal from a CNF undergoing Joule heating and the predicted temperature from a heat-transfer model were used to produce a second calibration, yielding a TR coefficient kJoule of 2.45 x 10-5/K. The discrepancy between the two TR coefficients suggests that further experiments are needed to determine more accurately the CNF TR coefficient.