Carbon Coating & Evaporation - Physical Considerations

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2. Physical Considerations

Table of Contents:

1. Introduction
2. Physical Considerations
3. Physical Considerations (Part 2)
4. Turbo Pumped, High Vacuum Systems
5. Surface Treatment

The carbon particle evaporation is dependent on the surface area, temperature, and the length of the filament. The filament itself can take several forms including multistranding and "made up form" carbon fibre, generally the "made up form" is more convenient. Figure 1 shows a range of sample alternatives for use as filaments.

Figure 1:

(i) Fibre (ii) String (iii) Cord.

Note: the carbon fibre provided by Emitech, catalogue number C5461 is very much recommended to give the best performance (string and cord are available if required)

Differing arrangements may be used to cover a range of applications. It is more convenient to have a fixed length filament for both mechanical simplicity, and in determining the requirements of the Power Supply, this also ensures for particular arrangement that thigh repeatability of Carbon Evaporation can be achieved.

It is considered that a filament length of 10mm (Exposed length for evaporation, the actual length being somewhat longer to accommodate clamping) is satisfactory. The power supply requirements can now be determined which are twofold, it being necessary to flash the carbon filament rapidly with full applied power to ensure a good repeatability and also prior to this, to outgas the filament at the higher end of the red heat temperature band, for a time period and with variable current facilities to allow for the differing arrangements of filament.

In the vacuum range we are considering of 0.05 to 0.01 Torr where the mean free path lengths are relatively short, of the order of 1-5mm, then if the specimen is several times this spacing from the filament, the coating will not be as sensitive to vacuum. In general the outgassing would be 0.1 Torr with flash evaporation, at 0.05 Torr or better and it is good practice to repeat parameters to ensure consistent results.

The filament itself tends to act as a point source and in the flash process 'breaks' as part of the process without fully evaporating along its complete length.

The geometric arrangement is indicated in Figure 2, which has a shutter used during the outgassing process to avoid heat input to the specimen. The filament length (10mm) can normally be considered as a point source and theoretically the thickness (t) of coating will vary inversely proportional to the height of the specimen to the filament squared (t proportional to 1/h2). In practice due to the short mean free path this relationship is somewhat less than that indicated.

Figure 2

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