NETZSCH LFA 447 Nanoflash, Руководство по эксплуатации

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Operating the Instrument
LFA 447
TM
01/09 J:\LFA447\CHAPTER4.doc
6
Sample Coating
Transparent Material
The analysis of the flash diffusivity data requires that the energy be absorbed in the surface
of the sample. It also requires that only the sample surface temperature is measured by the
IR detector. Some materials are transparent at visible and near IR wavelengths of the Xenon
flash. A sputter-coated metal film (typically gold) is used on both sides of the sample to
minimize the radiation heat transfer within the sample and to prevent the detector from
viewing temperature changes within the sample. The required film thickness is
approximately 1000 Å, and can be controlled by the settings used during the coating process.
As a guide, most unfilled polymers and many ceramics and glasses will require a metal film or
graphite coating. A missing or insufficient coating shows a high temperature increase in the
detector signal directly after the shot (spike).
Graphite Coating
The samples must always be coated with graphite (it is also necessary if a thin sputtered
metal film is already existing) in order to enhance the absorption of laser energy and the
emission of IR radiation to the detector. The graphite coating greatly increases the resulting
signal-to-noise ratio especially when the surface is highly reflective. Also important for the
determination of the specific heat is the uniform coating of both surfaces of sample and
reference. The measuring conditions for sample and reference must be exactly the same.
The total coating thickness should be approximately 5
µ
m and in most cases negligible
concerning heat transfer through the sample. The graphite coating is somewhat porous, so
that too thin a layer will transmit energy and may cause specific heat errors due to differing
bare surface properties of the samples. The coating can be also be damaged by high flash
power. For thin layers with a high thermal conductivity (e.g. foils, films) the thickness of the
graphite coating should be minimized. In this case test the sample with different coating
thicknesses in order to eliminate the influence of the coating.
Note: It is recommended that the spray be applied in a hood with
adequate air flow to prevent fumes from escaping!