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Low cost conductive epoxy?

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LAN_403

IntraWinding

Fri Aug 14 2009, 12:43PM

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

Hi, My first post at this great site

I want to join several low cost Lithium coin cells together into a battery. I think the best way is with a small blob of electrically conductive epoxy between each cell. I only need a little and what I've seen for sale is expensive so I been trying to make my own.

Traditionally conductive paints and glues are loaded with silver but nickel is also used so I picked up 50g of fine Nickel powder on eBay and mixed as much as I could get into a little epoxy. After setting the result was disappointing: with closely spaced crocodile clips I measured about 10K ohms.

I think the problem may be that that commercial electrically conductive paints and glues use fine flaked metal rather than the granular powder I have. It might also be oxide or other coating on the very dark coloured nickel particles, but I haven't seen any change in that colour when I added some to very dilute Sulphuric Acid.

I'm looking for ideas what to do next if anyone can help?

Thanks,

Alan

Weston

Fri Aug 14 2009, 08:33PM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

A lot of conductive glues use graphite powder. Here is an instructable for something that should be easy to make

Conundrum

Fri Aug 14 2009, 09:15PM

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

graphite lubricant sold at your locksmith might work, be sure to get the fine sort.
-A

MinorityCarrier

Sat Aug 15 2009, 12:38AM

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

You may run into a problem with the native oxide that has grown on the metal surfaces of the battery cells. Device battery contacts typically exert some pressure to punch through this oxide and establish good ohmic contact. An applied carbon paste may not succeeed in making good ohmic contact unless the metal oxide is removed first and the carbon paste is quickly applied, and does not chemically react with the metal to regrow an insulating layer.

It is toxic to use, and I can't recommend it, but I used mercury whetting when I was younger to ensure good contact in circumstances similar to what you want to do. Used a rubber pencil eraser to clean off the oxide and simultaneously get the mercury to whet the metal. How I survived my youth is a mystery...

klaksÉ™n

Sat Aug 15 2009, 05:10AM

Registered Member #2060 Joined: Fri Apr 03 2009, 01:49PM
Location:
Posts: 4

Automotive rear window defroster repair pen - conductive 'ink' used to bridge gaps in the resistance elements... $8-10

IntraWinding

Sat Aug 15 2009, 09:40AM

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

Thanks for the replies. I have some 'Aquadag', which is conductive colloidal carbon, but I'd really like to get better conductivity.

I've got some Silver conductive ink, but it won't dry sandwiched between two metal plates and anyway, it'd be much nicer to combine gluing the cells with their electrical connection.
I'll remember to make a point of cleaning the contact surfaces before joining them to avoid oxide problems, but I'm steering away from mercury. The worst is when you spill some in your home and never know if you've managed to clear it all up.

Your ideas got me thinking. Googling came up with this:

Section on conductives in a book on adhesives:
http://books.google.com/books?id=IIdNnG6AnB0C&pg=PA368&dq#v=onepage&q=&f=false

So it sounds like silver is really the way to go and I found some really good stuff on making silver powder.

From just Silver Nitrate and Glycerol!
Preparation of silver powder through glycerol process
http://docs.google.com/gview?a=v&q=cache:N0TestecVSgJ:www.ias.ac.in/matersci/bmsjun2005/213.pdf+%22PVP+or+seed+particle+(nucleating%22&hl=en

I also found this:
Preparation of Ultrafine Silver Powder Using Glycerol as Reducing Agent
http://209.85.229.132/search?q=cache:TEqu_JVwonQJ:www.material.chula.ac.th/Journal/v18-2/1-5%2520Nisaratanaporn,%2520E.pdf

But I'd really like to get a look at this article if anyone can help:
Laboratory preparation and evaluation of conductive silver powder pigments
http://www.emeraldinsight.com/Insight/viewContentItem.do?contentType=Article&hdAction=lnkpdf&contentId=1690387

If you can't get any of these links to work just manually copy and paste them into the address bar of a new browser window and hit return.

Steve Conner

Sat Aug 15 2009, 09:43AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

How complicated do you need to make this?! Bung the cells into a plastic tube with a spring at the end, flashlight style. If you can't find a tube the right size, get the next size up and pad it with rolled-up paper.

mikeselectricstuff

Sat Aug 15 2009, 03:25PM

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

conductive epoxy does work very well for this application - it's expensive but you only need a teeny amount.
An alternative might be some sort of compressable metal link - e.g. something thin, crinkly and springy, with normal epoxy to provide the adheion - you'd need to keep it under compression while the epoxy sets.

IntraWinding

Sat Aug 15 2009, 06:42PM

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

Hey! I'm doing all this for fun even if it is bonkers

Actually, I'm a perfectionist and I also like to collect new skills (sometimes).

I've ordered some Glycerol and I already have Silver Nitrate (I made that from AR Nitric Acid & an ounce Silver ingot years ago

) and I have a really nice magnetic stirrer hotplate with temperature control probe I got real cheap off eBay, so it's no real effort to give it a try. And after I'll hopefully have another useful product to add to my chemical collection as well as the ability to neatly make any size battery I want in future.

According to the paper I linked to I need to use a molar ratio of Silver Nitrate to Glycerol of between 0.01 to 0.1 for Silver particles sized 1.5u - 11u. It's a long time since I did that sort of calculation, but I reckon that means for a 5g batch of Silver Nitrate I need between 27g and 270g of Glycerol, which is about 21ml to 214ml. What size particle should I aim for? I figure I'll have trouble getting the silver to settle out after the reaction, even if I add lots of water to lower the viscosity (Glycerine's viscous), so larger particles will be better, but perhaps the smaller particles conduct better? As a compromise I'm aiming for 5u particles and looking at a rough sketch graph I made of particle size verses concentration (I reckon it's a curve, not a straight line) I'm going to try 5 grams of Silver Nitrate with 80 grams of Glycerol. If it still won't settle out I reckon I can add some more Silver Nitrate and heat it all up again to grow the particles a bit bigger, so long as I just test a drop in water for settling first. I'll post the results.

Conundrum

Sun Aug 16 2009, 08:43AM

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

obtain some of that emc conductive foam from a dead TFT monitor, that works.
-A

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