## Conductive pastes take 453

As ever, I'm constantly on the look out for nice conducting materials. I recently stumbled across a special powdered Nickel. What’s particularly interesting about it is that it is dendritic i.e. the particles are spiky rather than spherical. Essentially this means that when mixed with a polymer, the metal particles are more likely to touch each other so it therefore gives a much lower resistance.

Apparently this stuff is typically used making conductive paints, so its readily available. I managed to get some filamentary powder thats about 2.35um in cross section and 15um long . In small quantities this costs about £95/kg

A colleague and I crudely made a thin film that measures 100x100x1mm out of this powder mixed with liquid latex in a 1:1 ratio in terms of weight. The paste seemed sufficiently viscous that we could extrude it with a paste extruder. We embedded 9 electrodes in the film so that we could get repeatable readings. The total resistance between opposite electrodes came in at 0.8ohms, which is about a 9cm length spanning the film. I guess this is sufficiently low to be useful for a lot of low current applications.

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I was working with a polymer filled with stainless steel fibres recently. You have to have a trace of a measured cross section so that you can derive the ohm-cm for the material. When I did that with my material, it turned out to not be at all suitable. :-(

Forrest.

I've just cut a small strip of the film.

The cross section is about 1.3x1.3mm and its about 18mm in length. The resistance came in at 9.5ohms

I reckon this comes to a resistivity of 8.9 x 10-4 ohm.

Given that we could still up the concentration of the nickel (the sample above wasn't saturated) a bit. I'm still hopeful.

P.S Adrian and I once calculated the optimum resistivity to be 2 x 10^-5 oh meters. At that level, and the resolution we can achieve we thought we could do both useful resistors and conductive track by changing the cross sectional area.

Is this the carbonyl nickel described on this wikipedia page? It says the LD50 of the stuff is 3ppm... I hope it's not the same stuff :O

I would use graphite with cellulosic resin as the binder with isopropanol as the carrier. This is what's normally used in professionally printed circuits that aren't worthy of silver or platinum (both of which work a lot better but \$\$\$).

Carbonyl iron is used in some paint applications, but it's not conductive enough really. Nickel carbonyl is relatively dangerous stuff - and fairly explosive without the proper shielding gas.

Powdered nickel would be fairly easy to deal with and fairly conductive as well (probably not as nice as graphite.)

Its not the same stuff Rob.
The health and safety data sheet for the stuff I have is available here:
http://nickel.vale.com/products/_pdf/msds/Nickel%20%20200_EU_EN.pdf

I've been in touch with the manufacturer.

The powder I am using is just a dendritic nickel. It is is however made from Nickel carbonyl gas at high pressures.

So it's a catalyst and other fun, but much less dangerous stuff.