Laser flash analysis

Laser Flash Apparatus

Uses	to measure thermal diffusivity, thermal conductivity, specific heat,

The laser flash analysis or laser flash method is used to measure thermal diffusivity of a multiplicity of different materials. An energy pulse heats one side of a plane-parallel sample. The temperature rise on the backside due to the energy input is time-dependent detected. The higher the thermal diffusivity of the sample, the faster the energy reaches the backside. A state-of-the-art laser flash apparatus (LFA) to measure thermal diffusivity over a broad temperature range, is shown on the right hand side.

In a one-dimensional, adiabatic case the thermal diffusivity $a$ is calculated from this temperature rise as follows:

$a = 0.1388 \cdot \frac{d^2}{t_{1/2}}$

Where

• $a$ is the thermal diffusivity
• $d$ is the thickness of the sample
• $t_{1/2}$ is the time to the half maximum

Measurement principle

LFA measurement principle: An energy / laser pulse (red) heats the sample (yellow) on the bottom side and a detector detects the temperaure signal versus time on the top side (green).

The laser flash method was developed by Parker et al. in 1961.^[1] In a vertical setup a light source (e.g. laser, flashlamp) heats the sample from the bottom side and a detector on top detects the time-dependent temperature rise. For measuring the thermal diffusivity, which is strongly temperature-dependent, at different temperatures the sample can be placed in a furnace at constant temperature.

Perfect conditions are

• homogenous material,
• a homogenous energy input on the front side
• a time-dependent short pulse - in form of a Dirac delta function

Several improvements on the models have been made. In 1963 Cowan takes radiation and convection on the surface into account.^[2] Cape and Lehman consider transient heat transfer, finite pulse effects and also heat losses in the same year.^[3] Blumm and Opfermann improved the Cape-Lehman-Model with high order solutions of radial transient heat transfer and facial heat loss, non-linear regression routine in case of high heat losses and an advanced, patented pulse length correction.^[4][5]

See also

• Thermal diffusivity
• Thermal conductivity
• Thermal conductivity measurement
• Thermal physics

References

1. W.J. Parker; R.J. Jenkins; C.P. Butler; G.L. Abbott (1961). "Method of Determining Thermal Diffusivity, Heat Capacity and Thermal Conductivity". Journal of Applied Physics. 32 (9): 1679. Bibcode:1961JAP....32.1679P. doi:10.1063/1.1728417.
2. R.D. Cowan (1963). "Pulse Method of Measuring Thermal Diffusivity at High Temperatures". Journal of Applied Physics. 34 (4): 926. Bibcode:1963JAP....34..926C. doi:10.1063/1.1729564.
3. J.A. Cape; G.W. Lehman (1963). "Temperature and Finite-Pulse-Time Effects in the Flash Method for Measuring Thermal Diffusivity". Journal of Applied Physics. 34 (7): 1909. Bibcode:1963JAP....34.1909C. doi:10.1063/1.1729711.
4. U.S. Patent 7,038,209
5. J. Blumm; J. Opfermann (2002). "Improvement of the mathematical modeling of flash measurements". High Temperatures – High Pressures. 34: 515. doi:10.1068/htjr061.