Strong linearity and effect of laser heating location in transient photo/electrothermal characterization of micro/nanoscale wires

In this work, using Transient Electro-Thermal (TET) and Transient Photo-Electro-Thermal (TPET) methods, a rigorous approach is developed for data processing and thermal characterization of micro/nanoscale wires. Applying a step DC current (TET) or step continuous wave laser beam (TPET), a voltage rise (or drop) occurs through the sample, which represents the temperature evolution in the sample. After taking the natural log of this transient voltage change, the data series greatly resembles a line with respect to time with a coefficient of −π2α/L2 (α: thermal diffusivity, L: sample length). So, instead of typical nonlinear raw data fitting, the linear fitting can be effectively exploited, which makes it possible to obtain the fitting uncertainty (or the uncertainty of thermal diffusivity). However, it is shown that there is a nonlinearity part at the beginning of the logarithmic voltage (temperature) that should be excluded from linear fitting. Furthermore, the effect of laser beam location on the sample in the TPET measurement is investigated. It is unraveled that except the locations close to the sample ends, the irradiated location on the sample makes negligible difference in the result for thermal diffusivity measurement. The thermal diffusivity of our microscale graphene fiber is measured to be 7.46×10−7m2s−1 and 6.93×10−7m2s−1 (averaged over different locations) with the TET and TPET techniques respectively. The uncertainty of fitting is determined to be in the order of ∼10−9m2s−1, confirming excellent linearity and measurement accuracy.