Application Note: Evaporating Gold (Au) in an e-Gun™ source
For Gold, the best liners are Coated Graphite and Intermetallic (Ti doped BN). You should not sweep during coating. You can sweep during material conditioning, but once the material is fully melted, just place the beam in the center of the pocket. This will give the best stability. You can overpower/overheat the material when using a liner. Watch for oscillation in the light emission from the source. If you get a strobe light effect, the material is unstable, and spitting will occur (reduce power). It is best to have a couple of welding lenses to look at the melt with. A #5, #8 and #10 lens will allow viewing of practically any material during evaporation. For this process, a #8 should work fine.
Using a CG liner, blow the liner off with an inert gas prior to loading. Load the material to no more than 80% volume of the liner.
Coated graphite:
*Chemical resistivity. Materials such as aluminum will not react as fast, forming carbides.
*Particulate reduction. Graphite is messy and the coating “seals” the graphite. Evaporating for example- Gold. Straight graphite you will often get a piece or flake of graphite floating around in the melt. Not a real problem as it won’t evaporate.
*Reduced surface area. Glassy carbon -CG liners have a very smooth surface. This reduces contact and reduces mechanical stress from the material forming to the surface. As the material expands and contracts from heat the smooth surface helps reduce breakage.
Insulating liners:
BN and Al2O3 liners are insulating. They must be soaked at low power for several minutes prior to evaporation. When you first apply beam current the beam will strike the center of the crucible and then build a surface charge that causes the electrons to “skip” to the edges of the liner (blue luminescent you are seeing). As the beam continues to hit the center and skip to the edges it will gradually create heat on the liner. Once the liner is heated adequately it will begin to conduct electrically and the beam will stabilize.
Run the source at low power 30-40 mA for 3 to 10 minutes -or- until you see the blue haze stop and the beam stabilize in the center of the crucible.
