Laser Flash

Laser Flash
(ASTM E 1461-07)

With the recent improvements in rapid data acquisition and laboratory equipment, transient measurement techniques have gained markedly in popularity. In particular one technique, called the flash method, has been used to measure materials whole diffusivities range from 0.001 to 10 (a range of 10 ^ 4) over a temperature range from 80 to 2500K (-315 to 4050 F). The method uses small easy to fabricate samples and results can be obtained within seconds.

The thermal diffusivity (α) is not only important in its own right, but it also offers a convenient, economical and accurate method of determining the thermal conductivity (λ). The relationship between λ and α is given by:

λ=αC_pd

where C_p is the specific heat and d is the density. The specific heat and density are relatively structurally insensitive, obey well-known physical laws and can be measured readily on small samples. It is possible to either measure these properties on the same sample used for diffusivity measurements or duplicate samples or even to calculate their values based on the known values of the constituent elements. Thus it is often much easier to measure α, C_p, and d, and calculate λ, than it is to measure directly. Furthermore, the accuracies are at least comparable to and often exceed so-called "direct" λ measurements. (Actually thermal conductivity can not be measured directly. Experimentation measures heat flux, temperature gradients, and sample geometries to calculate thermal conductivity from steady state experiments).

The flash method was first described in 1960 by Parker, Butler, Jenkins, and Abbott of the U.S. Navy Radiological Defense Laboratory. In this method the front face of a small disk-shaped sample (often about the size of a small coin - see Sample Sizes) is subjected to a very short burst of radiant energy. The source of the radiant energy is usually a laser or a xenon flash lamp and irradiation times are of one millisecond or less. The resulting temperature rise of the rear surface of the sample is measured and thermal diffusivity values are computed from the temperature rise versus time data. Often times of less than one second are involved. The ambient temperature may be controlled by a small furnace tube or chiller. The flash method is shown schematically in Figure 1 using a laser as the energy source. The rear face temperature rise is typically 1 to 2°C.

The thermal mass of the system can be made quite small so that it is possible to quickly change temperature and record data over large temperature intervals rapidly. Thus a large amount of data can be generated in a short period of time. This ability along with the small sample size required caused the method to gain rapidly in popularity. The method has been used to measure the thermal diffusivity of metals, alloys, ceramics, semiconductors, composites, liquid metals, and even amoebas. At the Thermophysical Properties Research Laboratory this method has been used to measure the diffusivity of carbon fibers, fiber reinforced materials, individual layers of layered composites, thermal contact conductances at interfaces, and dispersed composites in addition to more routine measurements.

Diffusivity values may be calculated from halftime (t_1/2) using the relation:

α=0.13879L²/t_1/2

where t_1/2 is the time from the initiation of the pulse until the rear face temperature rise reaches one-half of its maximum value and L is the sample's thickness. Actually one may use any percent rise:

α=k_xL²/t_x

where k_x is a constant corresponding to x percent rise (constant values are available in ASTM E 1461-07) and t_x is the elapsed time to x percent rise. When one has a digital data acquisition system, it is relatively easy to calculate alpha at a number of percent rises. If the values at 25, 50, and 75 percent rise agree with each other within + 2%, the overall accuracy is probably within + 5% at the half-rise time.

The computer output for a typical rise contains several values. The output of the temperature detector is given along with the elapsed time in microseconds that the data were taken. Also initial and cooling data are given. It is possible to compare the experimental values with the theoretical model. This is done by dividing the temperature rise by the maximum rise, thus non-dimensionalizing the ordinate. Times are divided by the halftime to non-dimensionalize the abscissa. The current state-of-the-art concerning thermal diffusivity measurements using the flash technique is discussed in detail in Chapter 8 of the book "Compendium of Thermophysical Property Measurement Methods", edited by K.D. Maglic, A.Cezairliyan, and V.E. Peletsky, 1984 by Plenum Press .

It is possible to satisfactorily correct for radiation heat losses and for situations in which the time duration of the energy pulse is not negligible compared to the transient time. It is also possible to correct for non-uniform heating (at least in selected cases). It has been shown that very heterogeneous dispersed composites can be measured. Techniques for layered samples have also been developed and this has led to the ability to measure liquids, thick films, and contact conductance between layers. This technique also allows us to keep temperature excursions to less than one degree by deliberately applying a layer of material whose properties are known. Thus heat-sensitive materials can be measured or measurements can be made very near phase transitions.

The flash method has been extended to two-dimensional heat flow so that large samples can be measured and the diffusivity in both the axial and radial directions in anisotropic materials can be obtained.