Improving Thermal Diffusivity Measurements by Including Detector Inherent Delayed Response in Laser Flash Method

Laser flash experiments are a widely used to determine thermal diffusivity of matter.
Especially for thin samples or high diffusive materials, the laser pulse-time
delay and its shape may become crucial for precise measurements (“finite pulsetime
effect”). An additional delayed response and modification of the shape of the
signal can be caused by time delays of detectors and electronic components (e.g.,
electronic filters) and may be inherent to Laser Flash Apparatuses (LFA). Similar
to the correction of the finite pulse-time effect, this detector system inherent
delayed response needs to be taken into account, especially for thin or high diffusive
materials. To correct for this additional delay, detector signals of direct laser
pulses were measured and for correction of this systematic errors a transfer function
is derived. It reproduces the detector signal using the pulse shape measured by a
diode within the apparatus and thus takes into account the actual laser pulse shape
and detector behavior. For the used experimental setup, systematic errors caused by
this effect are in the same range than the often considered finite pulse-time effect.
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As the detector related time delay is system inherent, the developed transfer function
can be used to effectively eliminate both, the finite pulse-time delay and the
detector inherent delayed response. Besides experimental determinations, synthetic
temperature response data are used to quantify the systematic errors related to the
additional detector inherent retardation for various thermal diffusivities and sample
thicknesses. By not taking the additional delay into account systematic errors > 10 %
may arise for both, high and low temperature detectors, indium antimonide (InSb)
and MCT (HgCdTe), respectively.