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Running a Discovery DXF Experiment
Preparing the Sample
The method is applicable for testing homogeneous, solid, and opaque materials. The sample should have
an electrically conducive surface. Materials greatly deviating from the above can be tested with special
sample preparations–for non-electrical contact surfaces (polymers, ceramics, etc) a layer of silver paint
must be applied, or you can sputter the sample with gold, platinum, silver, nickel. It is up to you to deter-
mine if a new material can be tested in an "as received form" or if special preparation is needed.
Sample Dimensions
It is extremely important to machine samples to exact dimensions. A sample must clear and fit the holder
(or the insert) opening. If the sample diameter is equal or slightly bigger than the opening, then there will
be an additional heat transfer from the tightly fitted sample to the holder during the measurement, which
may negatively affect the preciseness of the thermal diffusivity measurement. If the sample diameter is
much smaller than the opening, then the light beam can bypass the sample and interact directly with the
Solid State detector, causing temperature signal distortion.
The thermal diffusivity calculation depends on square of the sample thickness. Uncertainty in the sample
thickness value due to rough, or not completely flat surfaces will result in uncertainty (error) in the thermal
diffusivity determination. It is therefore very important to machine a sample with the tolerance as high as
possible (recommended tolerance ±1 mil). The tolerance on the lateral surfaces can be lower (± 5 mil).
Samples that are too thick are difficult to test due to weak temperature response signal and slow and slug-
gish response to the flash affected by heat losses from the sample. Samples that are too thin may not repre-
sent bulk material, and potentially increase measurement error due to severe finite pulse time effect. The
thickness should be designed the way the sample experimental halftime should be within the range from 40
ms to 2 s. (See Theory of the Flash Method in online help for a more detailed explanation of the relation-
ship between the sample thickness and the thermal diffusivity of the material.) More than one iteration of
trial and error may be needed to find the optimal sample thickness for a particular material. Examples of
thicknesses for different materials:
•
Stainless steel = 1.6 mm
•
Pyroceram = 2 mm
•
Graphite, alumina, and molybdenum = 3.2 mm
•
Copper and aluminum = 5 mm
NOTE: Testing samples of irregular shape or uneven thickness is not recommended, due to possible errors
in the thermal diffusivity and the specific heat determination and/or possible damage to the instrument.
DXF-200 Getting Started Guide
Page 30
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