Calculating Thermal Diffusivity from Laser-Flash Experiments

By Christyn Wood, Queensland University of Technology

What could be worse than running barefoot over scorching asphalt or forgetting the oven mitts when pulling out a freshly baked cake? In these unpleasant situations, we usually place the fault on the object being ‘too hot’, however the cause is actually a certain property of the material, called the thermal diffusivity. The thermal diffusivity describes how fast a heated or cooled material returns to thermal equilibrium, where a material with high thermal diffusivity transfers­­­ heat in a short time span, allowing the heat to travel deeper into the applied material. This explains why metal, with a larger thermal diffusivity, causes a deeper burn than plastic when pulled from boiling water.

Laser-flash experiments are commonly used to determine the thermal diffusivity of solid materials. This experiment involves applying an energy heat-pulse to the surface of the material, and recording the temperature rise at the opposite surface. Figure 1 visualises the rear-surface temperature rise (blue line) along with the eventual temperature limit of the material, called the steady-state temperature (red line). The original method for calculating the thermal diffusivity relies on a simplified mathematical model that allows the thermal diffusivity to be expressed in terms of the time when the temperature reaches half of its steady-state. This research considered a more realistic mathematical model and developed a new formula for the thermal diffusivity in terms of the area enclosed between the steady-state temperature and the rear-surface temperature rise curve over time (Figure 1).

Figure 1

To verify the resulting formula, we generated synthetic experimental data to mimic a realistic laser-flash experiment (see Figure 2). The error between the experimental and theoretical thermal diffusivity values is shown in Figure 3, the blue and red histogram displaying the error from our method and the half-rise time method respectively.

Figures 2 and 3

 

Christyn Wood was a recipient of a 2018/19 AMSI Vacation Research Scholarship.

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