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Low cost conductive epoxy?

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IntraWinding

Fri Aug 21 2009, 03:58PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

The Glycerol arrived and I gave the reaction a test run.

I used a 250ml conical flask with 80g Glycerol + 5g Silver Nitrate on a magnetic stirrer/hotplate.
The solid dissolved easily with heating but the high viscosity of Glycerol made the stirring very localised to the stirring bar until the viscosity dropped sufficiently with rising temperature.
The temperature was slowly raised.
At about 140C a semitransparent mirrored film of silver deposited on the flask wall. Silver started to precipitate and agglomerate into small balls of 'spongy' silver powder. I could smell a very small amount of Nitrogen Oxides near the flask mouth up to this point but as the temperature increased, enough oxides accumulated to become visibly brown/yellow in the flask. None was visibly released from the flask so providing sufficient ventilation was fairly straightforward.
As the temperature approached 175C the Nitrogen Oxides were no longer visible but the quantity of precipitated silver had increased significantly. The rounded stone shaped balls of silver sponge were kept in motion by the magnetic stirrer.
I used vigorous stirring which may have masked the expected 'vigorous boiling' as none was evident in the region of 175C. I increased the temperature to 180C but still saw no boiling. On reflection there may have been some boiling around 150C - 160C.
At this point I turned off the heater and with continued stirring, placed a large aluminium heat sink next to the flask on the hotplate to cool the flask more quickly.
I washed the silver free of Glycerol etc. with several batches of Iso Propyl Alcohol and cycled it through boiling and cooling with the same a couple of times. I suspect the sponge still holds Glycerol so I've left it soaking under alcohol to try to remove it.

So separating the silver from the reaction was no problem, but in its present form it would still take considerable effort to break it down again into a fine powder. I left a piece heating on a bare light bulb until no more smoke (Glycerol vapour) was emitted and then tried mixing it with one component of some two part epoxy. As expected, even after considerable effort the mix was grainy. I tried grinding it in a small pestle add mortar but the viscosity limited the effectiveness so I added some Methylene Chloride solvent and was then able to thoroughly grind the mix. Without the protective effect of the thick undiluted epoxy however, some of the silver was pressed into solid form and stuck to the pestle and mortar surfaces. After evaporating away the Methylene Chloride I mixed it with the other epoxy component and left it to set.
Unfortunately the result was still an insulator. The epoxy was a dark grey rather than the bright silver I've seen in conductive paint and even a freshly cut surface read over 200G Ohms.

So where next? More research online I guess, unless anyone here knows what to do? I think I need to prevent the fine silver particles from agglomerating, although I may then find it hard to separate the silver out. The only idea I have is to add some sort of surfactant like washing up liquid to the Glycerol/Silver Nitrate mix before heating it. That might keep the particles separate, but it might stop them growing too.

Any good ideas out there?

IntraWinding

Sun Sept 13 2009, 10:53PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

I'm still thinking about this project. I plan two approaches. First I want to see if I can break up the spongy lumps of Silver into very fine powder and later I want to find a different chemical reaction which gives finely divided silver directly.

For now I've made myself a crude Ball Mill. It's a glass jam jar with screw on lid. I've epoxied a fitting to the bottom so I can easily attach it to a low speed geared motor I got from a photocopier years ago.

I put about 1000 3mm stainless steel ball bearings and the Silver sponge with some IPA (Iso Propyl Alcohol) in the jar and rotated it at the speed that causes the balls to make the loudest noise (about 2 rpm). After about 24 hours I poured the lot through a very fine sieve and washed as much through as would go. I put the washings containing very fine silver to one side and returned the rest to the jar. I don't think the ball mill is working optimally because the balls are slipping a lot rather than being dragged up the side of the jar and tumbling down. I think this might be due to a combination of the number of ball bearings, the jar diameter, shape and material and also how lubricating the IPA + Silver is. For the second milling I've used much less IPA and I think it's working better.

The fine silver loaded washings contain some silver so fine it remains in suspension for ages. It shows the same light scattering properties as the conductive paint I have. Just like aluminium powder in water it shows up eddys in the liquid. This is because the silver consists of flakes rather than spherical particles. The silver still has a very bright and shiny look to it, unlike the Nickel I first experimented with. A laser beam shone through the suspension produces some nice reflected diffraction patterns from individual flecks.

I want to mill more of this silver to a very fine form before I try adding it to epoxy for conductivity tests.

Alan

MinorityCarrier

Mon Sept 14 2009, 05:29PM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

You may want to look at gallium/indium alloys.

Proud Mary

Mon Sept 14 2009, 05:59PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I'm really only a beginner here, but would suggest that:

A: You determine the nature of the metals to be mated.

B: You look up in a table the best species of soldering or brazing to mate your cells.

All this kind of data about metal-to-metal combination is readily available online. Get Googling, lad!

IntraWinding

Tue Sept 15 2009, 11:07PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

Thanks folks.
Very low temperature solder would work if the battery cases can be wetted with it, but making conductive epoxy has become an end in itself now

Alan

mikeselectricstuff

Thu Sept 17 2009, 08:26AM

Registered Member #311 Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253

It is generally possible to solder to batteries but you need an iron like a Metcal that can dump a lot of head in very quickly, and reasonably low-melt solder with some silver content - nothing exotic like indium, just standard leaded SMT solder - certainly not lead-free stuff!.
It is possible that battery life might be affected, and it certainly isn't anything you'd want to do in producton.
Obviously what you can't do by soldering but can easily with epoxy is make a compact stack, alhough you could do it by spot-welding thin strips of metal, like they do in commercial packs.

MinorityCarrier

Fri Sept 18 2009, 05:13AM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

Except for Carbon Zinc and maybe alkaline batteries, it's not a good idea to try to solder batteries, unless you use low temp. (Indium or Bismuth containing) solders. Indium is about as expensive as Silver. Commercially-available solder alloys containing Indium melt below 100C. Look it up. Also, resistance soldering tools are a better choice for soldering batteries.

In:Ga eutectic alloy is a low vapor-pressure, low toxicity liquid metal at room temp, and can wet the battery metal. Put two In:Ga-wetted battery ends together and the In:Ga melds establishing excellent electrical connection. This involves very small amounts of commercially available alloy shipped in a syringe.

Seems like a lot of effort trying to precipitate out Silver crystals and try to mix enough into an epoxy base to try to make a reliable conductive bond. I would have concerns about the reliability. Spot-welding is a better idea though if you can't afford 'exotic' Indium alloys.

IntraWinding

Fri Sept 18 2009, 11:26PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

A compact professional looking stack of button cells is my goal. I know commercial silver loaded products work so perhaps I can make it work too.

Update: Ball milling the spongy silver hasn't worked particularly well. I now have a large sample of nice looking silver 'glitter' but only a small amount of very fine silver particles - enough to test though.

Alan

Proud Mary

Sat Sept 19 2009, 12:15AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

IntraWinding wrote ...

A compact professional looking stack of button cells is my goal.

What about dropping the buttons into a dielectric tube of suitable internal diameter, and holding them down with a compression spring? I think that's what I'd do. For the de-luxe version, you could thread an aluminium tube with a tap, line it with 0.5mm etched PTFE, and screw the ends down, tightening the compression spring, and keeping out air and moisture. Positive and negative could penetrate the end caps by means of PTFE feedthroughs, or even feedthrough capacitors as an additional refinement if you want to start decoupling the supply at the very first opportunity. It's hard to see how such an assembly could come apart without main force, rather than all these gluey mixtures of variable adhesive properties and conductivity, which will just add resistance in series with your supply for no good reason.

Conundrum

Sat Sept 19 2009, 05:39PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

or use some very fine metal wire and a polymorph jig to hold it together...

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