Background
Thermal analysis is a subset of materials science that broadly studies how the properties of materials change with temperature over time. One of the properties that can be observed through thermal analysis is thermal conductivity. Thermal conductivity describes a material’s ability to transfer heat through a unit thickness of material. And it can be determined through direct experimental observation or extrapolated from thermal diffusivity, a related thermal property. Thermal diffusivity describes the rate at which thermal energy spreads through a material – in units of square meters per second (m²s⁻¹). The laser flash method is one of the most popular techniques for determining the thermal diffusivity of solids.
Laser Flash
Laser flash is a transient thermal analysis technique; This means that the temperature of the sample under observation is not held constant, but changes over time. And it is typically limited to measuring cross-plane diffusivity – or diffusivity along Z axis. How does it work? Laser flash exposes the front face of a sample to a pulse of radiant energy from a laser or flash lamp. As the sample absorbs the pulse, it experiences an increase in internal temperature, and a thermal gradient is created. The resulting heat fluxes are then recorded at the rear face of the sample, with the observations ending when the sample reaches thermal equilibrium.
Once at equilibrium, the temperature rise over time is used to calculate thermal diffusivity. This calculation’s two key inputs are the sample thickness and the time required for the sample to reach half its maximum temperature. Due to the importance of the thickness measurement, it is critical to consider any thermal expansion of the sample; Laser flash analysis (LFA) is most useful for high-temperature work, thus it is necessary to correct for any expansion of the sample at the desired temperature point. Typically a lab operating LFA would additionally require a dilatometer or TMA for this reason. Similar to the requirement for accurate thermal expansion (CTE) data, obtaining reliable specific heat capacity data for the sample is also necessary. Labs offering LFA require a good quality differential scanning calorimeter (DSC). C-Therm recommends the Rigaku DSCvesta.
LFA calculations can be extended to approximate a sample’s thermal conductivity by taking the product of thermal diffusivity, density and specific heat capacity; Inversely, thermal diffusivity can be calculated by dividing thermal conductivity by density and specific heat capacity.
Practically speaking, laser flash is recommended more for high-temperature testing of metals and ceramics; The samples must be homogeneous and isotropic. LFA offers the benefits of small sample size requirements (typically 6 to 18 mm in diameter) and – despite the considerable machining requirements – a short testing time (generally within 40 to 200 ms), making it suitable for highly conductive materials. One of the greatest benefits of LFA is that it is an absolute method of measurement of thermal diffusivity that is accurate at a very high-temperature range (up to ~2500°C).
Scope of Application
ASTM E1461 is a recommended resource material in considering the capabilities and limitations of LFA. Note that the efficacy of laser flash is dependant on a relatively narrow scope of application. According to the American Society for Testing and Materials – specifically ASTM standard E 1461-01:
- “This test method is applicable only for homogenous solid materials, in the strictest sense;” (Section 1.7); and
- “This test method is applicable to the measurements performed on essentially fully dense materials…Since the magnitude of porosity, pore shape, sizes and parameters of pore distribution influence the behaviour of the thermal diffusivity, extreme caution must be exercised when analyzing data” (Section 1.5)
Data produced outside of these parameters may have value, but its use should be limited to rough comparisons of similar materials as the standard notes:
- “When substantial inhomogeneity and anisotropy is present in a material, the thermal diffusivity data obtained with this method may be substantially in error” (Section 1.71); and
- “[By extension,] special caution is advised when other properties, such as thermal conductivity, are derived from thermal diffusivity obtained by this method” (Section 1.5).
Users are advised to avoid applying LFA on composites, powders, anisotropic materials, and liquids. Based on experimental results, the method is most effective for testing metals and high-temperature ceramics.
Issues & Errors Associated with Laser Flash
The utility of laser flash is ultimately constrained by experimental conditions that adversely affect the precision of observations. More specifically:
- Nonuniform Heating: A laser beam with an asymmetrical energy profile or nonuniform absorption of thermal energy by the sample, can lead to an erroneous characterization of sample diffusivity. This effect is particularly amplified in thinner materials, like thin films;
Heat Loss: The transfer of heat between a sample and its environment is almost inevitable and most often occurs via convection and radiation. The issue with this is that it results in fluctuations across observed temperatures – which in turn introduces added complexity to mathematical modelling efforts; and
Pulse Duration: When a laser pulse is emitted for a finite amount of time – as opposed to being instantaneous, it will take a longer amount of time to observe changes in temperature at the rear face of the sample. This is called the “finite pulse-time effect.”
Closing Thoughts
Laser flash is a powerful method for determining the thermal conductivity of – primarily – isotropic, solid materials via thermal diffusivity. The method is prone to substantial errors when extended to test non-ideal materials, for which it is advised that caution be exercised when interpreting experimental results. Informed testing is the most effective testing. And it’s the best way to ensure that one capitalizes on the utility of laser flash.
Works Referenced
(1) ASTM E1461-13, Standard Test Method for Thermal Diffusivity by the Flash Method. (2013). ASTM International. http://www.astm.org/cgi-bin/resolver.cgi?E1461-01
(2) Lunev, A., Zborovskii, V., & Aliev, T. (2021). Complexity matters: Highly-accurate numerical models of coupled radiative–conductive heat transfer in a laser flash experiment. International Journal of Thermal Sciences, 160. https://www.sciencedirect.com/science/article/pii/S1290072920311406
(3) Park, D.G., Kim, H.M., Baik, S.J.,Yoo, B.O., Ahn, S.B., & Ryu, W.S. (2010). Correction Effect of Finite Pulse Duration for High Thermal Diffusivity Materials. Proceedings of the KNS autumn meeting, (pp. 1CD-ROM).
(4) Tiwari, A., Boussois, K., Nait-Ali, B., Smith, D. S., & Blanchart, P. (2013). Anisotropic thermal conductivity of thin polycrystalline oxide samples. AIP Advances, 3(11), 112129. https://aip.scitation.org/doi/citedby/10.1063/1.4836555
(5) Vozár, L., & Hohenauer, W. (2003). Flash method of measuring the thermal diffusivity. A review. High Temperatures-high Pressures, 253-264. https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.634.6812&rep=rep1&type=pdf
