From: "Mike" <induction@comcast.net>
References: <c5pj8n$47fi$1@saturn.cs.uml.edu> <c7nofq$1vnn$1@saturn.cs.uml.edu>
Subject: Re: Bizarre Plasma Cleaning of Platinum Parts
Date: Mon, 10 May 2004 20:55:22 -0400
Newsgroups: sci.physics.plasma


HI PJ and ZZ,
Yes, we had a like problem but in a large 2 Ft wide,
6 feet high glow discharge tube, right now used for Sprite research but also
many other projects. Our problem was, as we use the Anode or positive
terminal as the vacuum table (earth grounded to protect us and the computer
interfaced metering) and this takes the positive column to easy view and
probes via 7 stopper holes. We found that the suction port hole was a real
hot spot, more so when we ran it at 2600 volts and 10 amps DC.(Stainless got
blue after) So we added 1 inch metal spacers, a stainless false bottom on
top of the spacers, gave enough side space to still pump down fast and
solved the problem. The plasma loved camping in that 6 inch port and
effected how the tube would fire if we went beyond low level striations. We
found that the false bottom became the more desired target and the plasma
lost interest in the hollow area beneath it. Switching polarity made no real
difference, so cathode or anode, if it was hollow and in the path, it got
both hot and bright. Pressure range from .002 Torr to .500 Torr, typical
..050 Torr working.
Mike, Brockton, Ma

"PJ" <atmospheric_plasma@hotmail.dotcom> wrote in message
news:c7nofq$1vnn$1@saturn.cs.uml.edu...
> ZZ
> You are seeing something called the hollow cathode effect... Basically at
> certain pressure regimes you get electron trapping inside the cylindrical
> parts... This greatly enhances ionization within the cylindrical part.
> Your parts are probably getting red hot... The platinum is most likely
> forming an oxide that is being sputtered and evaporated onto your titanium
> fixture. This effect is common at this pressure. You might try operating
> at a different pressure to reduce this. If you hit the "perfect" pressure
> you might get them all to light up!
>
> PJ
>
> "Chucker" <charles.e.wood@medtronic.com> wrote in message
> news:c5pj8n$47fi$1@saturn.cs.uml.edu...
> >
> > Hi, I have a thought experiment regarding something I'm seeing when I
> > plasma clean platinum parts. I'm currently using a
> > desktop plasma cleaner at the following settings to prepare platinum
> > parts for platinization:
> >
> > Time: 15 minutes
> > Gas: Oxygen
> > Flow: 25ml/min
> > Microwave Power: 250 Watts
> > Pressure: 40 Pa (0.30 Torr)
> > Fixture material: Titanium
> > Part material: 90/10 Platinum/Iridium
> > Part shape: Hollow cylindrical part about 0.1" diameter and 0.5" long
> >
> > 100 parts are held against individual V-grooves with a neoprene
> > O-ring. There are 2 rings, an inner ring holding 40 parts and
> > an outer ring holding 60 parts.
> > The weird thing I'm seeing is that a couple of parts will glow orange
> > on the outside ring, burning through the O-ring and
> > discoloring the titanium fixture a bluish-gold at the particular
> > v-groove. My initial reaction is the glow is due to heat.
> >
> > What could be causing this? I'm a mechanical engineer by training so
> > I'm way out of my element. Could it be the platinum is
> > acting as a catalyst? Does it have something to do with the microwave
> > energy? How about a secondary plasma? Is a current be developed
> > between the fixture and part, generating heat?
> >
> > Any thoughts would be appreciated.
> >
> >