DXF-200 Getting Started Guide
Page 31
Transparent or Translucent Materials
The method requires that all of the energy from the flash be absorbed on one face of the sample. For mate-
rials that are transparent or translucent, the energy from the light pulse will travel to different depths in the
sample (depending on opacity) or may completely pass through in a completely transparent sample. To
prevent this, a very thin and opaque layer of high thermal diffusivity material must be deposited on the
faces of the samples. For best results, one should select a coating that has a high reflectivity (low emissiv-
ity), such as gold or platinum. Conversely, this type of coating is not well suited to absorb the energy pulse,
and therefore it is customary to put a second, highly absorbing graphite coat on top of it.
It is critical to keep both coatings at minimal thickness. This reduces the contribution of the layers to the
total transmission time as compared to the contribution of the sample.
Best results are obtained with vacuum sputtering of a gold, silver, nickel, or platinum layer with < 1 µm
thickness. This coating is then over-coated using an aerosol spray coating of graphite.
Please note that:
•
In all cases the coating material must be selected to safely withstand any temperature within the test
parameters, and the graphite coating must be applied in a thin layer.
•
Avoid scratching the coated surfaces after curing; even tiny amounts of porosity will severely affect
the data. Prevent any dust from settling on the coated surfaces.
•
Some materials that generally require coating for opacity are: glass, quartz, alumina, zirconia, silicon
carbide, silicon nitride, calcium fluoride, zinc selenide, etc.
High Reflectance Samples
In instances where the material has a highly reflective surface, it may be difficult to absorb sufficient
energy from the flash to produce a good signal on the opposite face. A thin coating of graphite spray usu-
ally remedies this situation.
For specific heat capacity testing it is imperative that both the unknown sample and the reference have
identical emissivities over the spectral range of the High-Speed Xenon Source. To ensure this, always coat
both the sample and the reference with graphite coating.
