Thermophysical Properties Research Laboratory, Inc.
Phone : 765-463-1581
(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:
where Cp 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, Cp, 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 (t1/2) using the relation:
where t1/2 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:
where kx is a constant corresponding to x percent rise (constant values are available in ASTM E 1461-07) and
tx 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.