# Measurement of Thermal Diffusivity Using the Flash Method

## Thermal Diffusivity

Thermal diffusivity measures the speed of propagation of heat in a medium by conduction when the temperature changes with time. It is a thermophysical property and its value is directly proportional to the speed of propagation of heat. The relationship between thermal diffusivity α, specific heat Cp, thermal conductivity λ, and density ρ can be given by the following equation:

As per the above equation, any brief conductive heat transfer within a medium is affected by thermal diffusivity. Length2/time is its dimension and its unit is m2/s.

## The Flash Method

The Flash Method is the most widely used method for determining thermal diffusivity. It is fast and gives accurate and reproducible results. In this method, the sample is first stabilized at a certain temperature T0. Once this is done, the front face of the sample is irradiated by an instantaneous, high-energy pulse from a laser source or a discharge pump. The increase in temperature with time, DT(t), on the sample’s rear surface is measured. Then the thermal diffusivity can be calculated from the thermogram given in Fig. 1 below.

Figure 1: Schematic of the Flash Method

The typical shapes of the temperature rise curves under different experimental conditions are illustrated in Fig. 2. With increase in heat loss, the temperature on the back surface of the sample goes down after reaching a peak as shown in curves B and C. In case of no heat loss, the rear surface temperature of the sample will attain a peak maximum and stay therefore an indefinite time period as depicted in curve A.

Figure 2: Temperature increase for various experimental conditions

## Original Method by Parker

Parker’s original method assumes that the sample is adiabatic and isotropic. In this method, thermal diffusivity is calculated from t½, the time taken for the thermogram to reach half of the maximum rise in temperature and L, the thickness of the sample material

The applicability of this method is limited as it assumes ideal sample conditions. To render it more suitable for practical experimental conditions, other methods which factor in finite pulse duration, heat losses, non-homogeneous materials and non-uniform heating have been developed over the past few years.

## Advantages of the Flash Method

The key advantages of the Flash Method are as follows:

• Small sample size - diameter 12 mm and thickness a few millimeters
• Measurement is done within a few seconds
• A wide range (10-3 to 10 cm2/s) of thermal diffusivities can be determined using a single device
• Other thermophysical parameters such as thermal conductivity and specific heat can also be measured using this method, under certain conditions
• Samples can be tested up to higher temperatures than would otherwise be possible by steady state methods

## Disadvantages of the Flash Method

Some disadvantages of using the Flash Method are as follows:

• Expensive instruments are often needed due to optical detection, generation of heat pulse, and high speed acquisition of data
• Extra care should be taken in processing thermograms in the case of non-homogeneous and porous materials.

## Anter Thermal Analysis Equipment

Anter Corporation manufactures thermal properties analyzers to measure:

This information has been sourced, reviewed and adapted from materials provided by Anter Corporation.

For more information on this source, please visit Anter Corporation.

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