Preventing the Contribution of Aperture Stops in the Flash Technique
Thermal diffusivity measurements can be improved through field view optimization in the sensing area.
Now more than ever, in an increasingly competitive, technology-driven environment, efficiently devised heat transfer processes are at the heart of success. Researchers and engineers still want to know how best to optimize the manufacturing of glass bottles, how fast the ceramic components of a catalytic converter heat up, how fast an aluminum ingot solidifies, how to minimize thermal stresses within a steel block, how best to select the correct heat exchanger material for the thermal control of a processor, and more.
Such challenges—and many others—cannot be met without accurate knowledge of the two fundamental thermal properties: diffusivity and conductivity. Moreover, researchers want to know how best to thermally characterize high-conductivity materials at cryogenic and moderate temperatures, as well as ceramics and refractories at elevated temperatures.
One accurate, reliable and elegant solution exists: the flash method. Advantages of this test method include easy sample preparation, fast testing times, versatility, and high accuracy.
The physical significance of thermal diffusivity is associated with the challenge between heat conduction (represented by the thermal conductivity) and energy storage (represented by the heat capacity). In other words, the thermal diffusivity dictates the speed of propagation of heat within a medium as its temperature varies in time. The higher the thermal diffusivity, the faster the propagation of the heat wave.
The physical significance of thermal diffusivity is associated with the challenge between heat conduction (represented by the thermal conductivity) and energy storage (represented by the heat capacity).
Introduced by W.J. Parker nearly half a century ago, the flash method has become widely used in measuring the thermal diffusivity of materials. In this method, a cylindrical sample is subjected to a short, uniform energy pulse on its front face. The resulting temperature excursion of the back face is then used to determine the thermal diffusivity.
The advantage of this method is that only the half time is required. However, it doesn’t consider many factors like heat losses, the non-uniformity of the pulse and the duration. These simplifications result in systematic errors; consequently, many advanced methods have been proposed over the years to account for them.
One common practice in the laser flash technique is to use an aperture stop on the back face on the specimen, facing the IR detector. This helps by limiting fluctuations in the detector signal, blocking stray rays from the light/laser pulse and minimizing the influence of the surroundings.
The aperture stop is in the field of view of the detector. This often results in a significant distortion of the temperature excursion, since the detector not only senses the specimen’s back face, but also any fluctuations from the aperture stop. Consequently, the thermogram would show either a continuously increasing trend, or, as depicted in Figure 1, an extended leveling-off period that cannot be accounted for by any analytical formulation.
Researchers have long attempted to prevent these distortions, particularly when testing small diameter specimens like ceramic pellets or nuclear fuel specimens. One convenient solution consists of a software-controlled, stepper-motor-actuated lens carrier.* By adjusting the lens position relative to the IR detector, the field of view can be adjusted to such an extent that the IR signal originates solely from the sample surface, and not from any surrounding parts.
Both large and small specimens can be tested with an optimal sensing area. Contrary to the previous configuration, the lens has been shifted for an adequate field of view. No contributions from the aperture can be noticed. As expected, the thermogram now conforms to the theoretical model, yielding correct diffusivity values (see Figure 2).
An added benefit of the limited field of view is a significant decrease of the overall irradiation of the detector’s active area. This helps maintain the detector in its linear responsivity range, thereby extending its reliability and lifetime.
*ZoomOptics feature, implemented in the Netzsch LFA 467 HyperFlash