Sciencemadness Discussion Board

PVD at home

Baphomet - 12-5-2008 at 21:47

Hi guys

I am embarking on another out-there attempt at recreating hitech at home. This time it is Physical Vapour Deposition - which would be rather neat to try out as I know someone who uses the equipment at work for plating, and they said it is great.

What I have envisaged so far is a small crucible at the bottom of a cone-shaped container used to direct the particles. No vacuum at this stage but who knows, maybe later.

The crucible would have two graphite electrodes separated by a spark gap for the generation of heat to vapourise the feedstock.

The base of the crucible has another cathode (launch cathode) on a different circuit, and maybe 10 - 15cms away is a plate that is the opposite anode for that circuit - this is what attracts the particles to the target.

Now already I have a lot of questions:
- Is a transformer sufficient for the circuit separation? Last thing I want is for arcing between heating anode and the launch cathode!
- Perhaps a special design is required to prevent arcing within the transformer itself?
- I'm assuming that at least rudimentary results will be achievable, or is vacuum a neccessity for some reason?
- Is terra cotta a suitable refractory material or am I living in dreamland?

If you have answers or any ideas please chime in as I am all ears. Thanks!

Twospoons - 12-5-2008 at 22:19

What do you want to deposit?
Instead of a crucible and arc electrodes you could use a tungsten or tantalum boat : http://www.tungsten.com/mtsprod.html which is heated simply by passing a current through it. Good for low boiling materials like Al.
If you don't run under vacuum you run into three problems: reaction of your coating material with air; rapid condensation of your vapor due to conduction cooling; and very short mean free path for the vapor molecules which will make coating larger areas very difficult.

Pulling even a poor vacuum will improve your chances considerably. Refrigeration service pump go down to a few millitorr, are not that expensive, and would be a good starting point for this exercise.

[Edited on 13-5-2008 by Twospoons]

Baphomet - 12-5-2008 at 22:45

Good idea! I like it since the crucible could fit in a vac chamber and it is electrically conductive too

Do we really need to boil the material? I think hight heat is enough is it not? I am depositing random metals at the start (iron, copper, zinc etc. though I will try aluminium first)

Now I just need to figure out how to get the power cables into the vac unit :P

Or... helium filled chamber instead?

Thanks for your help.

not_important - 12-5-2008 at 23:05

What Twospoons said. Reducing the pressure also increases the boil-off rate at a given temperature.

For some evaporated coating you need to clean the surface in an inert gas, or even under vacuum.

Terra-cotta is pretty low melting, someone I know used it as the basis for a glaze. Typical firing temperatures for common ceramics are:


terra-cotta - cone 06 to 2

Stoneware - cone 6 to 10

Porcelain - cone 7 to 12

High mullite ceramics - cone 8 to 14


and to turn cone numbers into degrees
http://www.bigceramicstore.com/Information/ConeChart.html

cones are not precise, fixed temperature points; they react to the firing environment in the same way the ceramic body does. Rate of temperature change and gases in the kiln atmosphere affect when a ceramic will be properly fired, not some exact temperature reading.

Quote:
Do we really need to boil the material? I think hight heat is enough is it not? I am depositing random metals at the start (iron, copper, zinc etc. though I will try aluminium first)


You need the material being evaporated to have a high enough vapour pressure to give reasonable transfer rates. For some materials this means boiling, although ay a low pressure. You have to get enough energy into the target to kick loose atoms or molecules ofit; heat, energetic electrons, plasmas, are all ways to do this.



[Edited on 13-5-2008 by not_important]

12AX7 - 13-5-2008 at 10:28

Maybe your first exploration should start with CuO + Al. Packed tightly, I've seen this thermite mixture burn smoothy like a fountain, producing a spray of copper vapor. I once found a metallized acorn after burning a charge...

Tim

Twospoons - 13-5-2008 at 15:39

Quote:
Originally posted by Baphomet
Now I just need to figure out how to get the power cables into the vac unit :P


Sparkplugs. They are cheap, pressure / vacuum tight, have a nice insulator section for HV, come with a standard threaded end to screw into your equipment.

kilowatt - 13-5-2008 at 21:25

Quote:

Is terra cotta a suitable refractory material or am I living in dreamland?

Maybe; people have done some surprising things with flower pot crucibles (like cast iron), but much better alumina and zirconia crucibles are available on the internet from several companies, just do a google search and shop around. I got a 500mL alumina crucible for about $50 and a 50mL zirconia one from $40. They are really worth the money especially for something like this, but you have to be careful with materials that can reduce or dissolve them. Graphite crucibles (which are quite cheap and might be a good place to start) and tungsten crucibles are available too, or you could make your own graphite crucible from a solid piece. Keep in mind that graphite is fairly volatile at over 2000°C especially in a vacuum, and both it and tungsten can easily burn away in air at such temperatures. There are refractory coatings like ITC-213 that can protect graphite or metal parts from oxidation, but the composition is proprietary.

tentacles - 13-5-2008 at 21:30

Tungsten and tantalum boats aren't too expensive on ebay, do some searches..