Electrodag question

Aquadag?

Contact is usually mechanical, a spring or spring-loaded wire. It can also be painted over a metallic contact, or connected to with conductive epoxy.

John

Acheson deflocculated colloidal graphite - very finely divided graphite suspended in water with a little something to stabilise the suspension.

It is used in elecron microscopy to put a thin conducting layer on insulating specimens which would otherwise charge up under the electron beam, so you can appreciate that the graphite particles are distinctly sub-micron, and the conducting layer hasn't got any mechanical strength or resilience.

We also had stocks of "silver dag" which is an aqueous suspension of equallly finely divided silver particles where higher conductivity seemed necessary.

-- Bill Sloman, Nijmegen

I've heard DAG expanded as 'dispersed aqueous graphite', but since the good stuff isn't aqueous, I don't know if that's where the name came from. I think it was first used as a coating for phototubes, because it eliminates one of the major sources of dark counts in photon counting--light that rattles round in the glass envelope (as in an optical fibre) until it reaches the photocathode. The dag absorbs the light before it gets to there. It also helps eliminate gain variations in photomultipliers due to static charges.

For SEM, dag is usually used as an adhesive rather than a coating, to stick irregular or delicate specimens to the SEM stub. If you have an insulating sample and you can't manage the charging by changing the accelerating voltage, you use a little table top sputtering unit to put graphite or gold on top if you need it. The coatings are very much thinner than you can achieve with a Q-tip and dag. I bought a 1-litre can of dag in about 1988, and have been using it ever since--a little goes a really long way.

Cheers,

Phil 'photon counter and part time electron microscopist' Hobbs

Usually that is a form of carbon painted or sprayed on a surface; a wire or metal spring touching that surface is the noemalwayof making an electrical connection. Yes, very reliable; takes abit of work to srape some off.

Thanks all, Robert had the answer closest to my application. I should have been more explicit. My company is switching to it for EMC shielding (vs. some metallic coating). I am not directly involved but an engineer expressed concern about using screws with star washers; breaking the coating apart and subsequent fractures. From the comments and some review I presume I should recommend pressed flat contacts or springs? It is a case and has to be mechanical secured to a base as well as electrically connect to some exterior mechanical protrusions for ESD; darn marketing glitter. Any more comments would be welcome. Ray

Managed to dig out the name:Electrodag=AE 502. Upon thought I think that our present scheme of using screws and star washers stands a reasonable chance of breaking a brittle coating; or causing a fracture that would spread with time and vibration to a continuity break.

Use belleville washers and locking screws. :-)

Good Luck! Rich

I think your idea of a screw and a flat washer is good. If there is some concern of breaking the electrodag coating, then use a large washer and low torque; mechanical tighness or support done elsewhere *or* (say) four screws and washers for both mechanical and electronic connection. The "worry" comes from what vibration, dropping, etc will do in moving things.

Possible; use flat washer instead and not high torque; mechanical fastening should be seperate.