Friday, December 24, 2010

A proper conductive polymer mix?



In which your narrator begins to test a new conductive polymer mix.

Do you want to read more?

One of the long term grails of the reprap project is to be able to print circuitry. Currently, Rhys Jones, a PhD candidate working for Adrian Bowyer at the University of Bath, is devoting a lot of his time to addressing this issue.

Heretofore, we've looked at very low temperature eutectic alloys like Rose's and Wood's metal and confected methods of extruding it into preprinted channels.  More recently, Rhys has demonstrated that ordinary solder can be successfully printed onto ABS.

We've looked at various concoctions of carbon, silver and other conductive materials with polymers at arm's length.  Always, though, the conductivity of these mixes has been too low for them to be useful in printed circuits.  Rhys, however, has looked at a two step process which may get us by that.

The main problem with printing PCBs with a reprap machine is that PCBs are not that hard or expensive to have made using conventional, proved materials.  This makes coming up with a method of printing them a bit tricky because it not only has to work, but it also has to yield a product that is not all that much difficult to use than conventional boards.

Recently, I ran across a high-tech company, Integral Technologies, located in Bellingham, Washington.  They've come up with a different approach to creating a conductive polymer mix which uses conductive fibers mixed with conventional polymers.  Probably the most exciting is their mix of very fine stainless steel fibres mixed with a blend of polycarbonate and ABS.

The volume resistivity for this material on the data sheet runs to 0.06 Ohm-meter.  This is far too high for a useful conductive plastic.  I rather shrugged the material off until I got a look at a few non-technical demonstrations of the material.    This one shows two small bars of the material used to conduct 110 v current running an electric drill.

Interestingly, Bill quoted me a price range of between $9-45/lb depending on the composition. More interestingly, the PC/ABS mixes which resemble what we are already used to using in Repraps are at the cheap end of the price spectrum.



As well, they demonstrate measured resistance across an extruded plate of their material using an analog multimeter.




There was obviously a mismatch between what their data sheets said and what the non-tech demos indicated.  I explored this question with Bill Robinson at Integral.  It boils down to the "skinning effect" mentioned in the first clip.  Simply explained, it means that the metal fiber doesn't tend to show up at the surface of an extrusion.  This means that the material is partially insulated at the surface of an extruded surface.

That left the issue of how, in the second demo clip, ordinary multimeter probes registered effectively zero resistance when measuring resistance across a extruded plate of their plastic.

Bill sent me some demo plates of the material a few days ago.  I have a digital multimeter rather than the analog Radio Shack multimeter.




Multimeters are not particularly good at accurately measuring low resistances.  Anything reading below about 5 ohms for most multimeters is not something you want to bet your life on.

I tried duplicating the measurements in the clip and kept running into skinning effects.  In places I could get readings less than 1 ohm, but others I would be in the mega ohm range.  I made a cut across one end of one of the samples.  You could barely see the fiber ends.  Integral is using VERY fine fibres.

I then drilled 1.5 mm holes through the plaques and put the probes through those.  I was able the get more consistent and lower resistance measurements.  Mechanical contact between the probes and the fibre ends is how good conductivity is achieved.




The best I was able to get consistently was 0.6-0.8 ohms over about 100 mm of the material.

Bill at Integral suggested that if we used a 0.5 - 1.0 mm extrusion nozzle we out to be able to avoid jamming of our extrusion nozzle with the stainless steel fibres.  Making strip extrusion as we can with a Reprap printer should tend to align the fibres much better than a simple injection moulding would do.  The problem as I see it will be making a conductive connection between two traces and making a connection between a trace and a component.

The question now is whether what we know about this material now justifies having a sample turned into fibre for further testing.

15 comments:

whosawhatsis? said...

This test is seems a little absurd. If you want to test resistance, you should not use a plate and just touch the surface anywhere. What you need is a strip of the material, with a known (and preferably very small, to get resistance up into the measurable range) cross-sectional area and a known length, and you need to touch your probes to the ends. Resistance is proportional to the length and inversely proportional the to the cross-sectional area.

Measuring the resistance between two probes touching arbitraty points on a plate is almost meaningless, especially when the measured resistance is so close to zero. I wouldn't even try to extrapolate from a measurement of less than 10 ohms unless I had special equipment for measuring very low resistances (and just because the resistance is low, it doesn't mean the resistivity is low).

If you can get a sample in the form of a strip (or better, a filament similar to either the feedstock or the output of a typical reprap extruder), that should allow you to do some useful testing. Failing that, you could try cutting a strip from your sample to test (the thinner the better as long as you can measure it accurately, but uniform thickness is probably more important), though the alignment of the fibers would make this both a less favorable and a less useful measurement than if you had an extruded filament.

Actually, come to think of it, if you could measure the volume of your strip (by water displacement, for instance), you could divide by the (easily measurable) length to get the average cross-sectional area, which should work, as what you want to know is the average resistance per unit of length of the strip.

Thin extrusions will also be an issue as the cross-sectional area of the extrusion approaches that of the fibers and as the ratio of these values approaches the volume ratio of the two materials, as the fewer fibers there are in a cross-section, the more likely it will be that there will be some point in the length of the extrusion with no fibers, and thus no conductivity.

Forrest Higgs said...

I've cut strips and tested them. Unfortunately, the connections between fibres are obviously not random and the samples are not isotropic. After testing several strips, I'm getting about 8 ohms for a 100 mm strip about 3 mm wide.

I should have mentioned that. I suspect that an extruded strip from a Reprap extruder would do much better than that since the fibres would tend to be lying in the same direction as the printed trace.

whosawhatsis? said...

3mm wide and how thick? That is starting to sound more like a resistor than a conductor. More fibers parallel to the extrusion should help a bit, and filament stretching after leaving the nozzle would help with this, but it would also make sections with no fibers more likely.

It would also be interesting to look at the fluid dynamics going on inside a nozzle with these fibers, especially if they will conduct heat better than the surrounding plastic, but I suspect that there are enough unknowns to make a large-sample empirical test much more useful.

Forrest Higgs said...

@whosawhatsis? "I suspect that there are enough unknowns to make a large-sample empirical test much more useful"

It's not that expensive to have some filament made up for more meaningful tests. I'm going to wait to see what Adrian has to say before doing that, however.

Bogdan Kecman said...

The 0.5-1mm nozzle would be good for trough hole components but we are slowly moving away from them... as for the smd I am afraid that stretching the filament would create traces of variable resistance - and that is a big issue with pcb design..

As for the skinning effect, maybe laying down components and then printing over them would allow for contact to form between components and trace avoiding skinning effect and that only the outer skin is then insulated?!

Forrest Higgs said...

I don't think there is any way we can do SMD with this technology.

Steve Baker said...

The thing that bothers me is that you have a randomly oriented bunch of little conductive threads in a matrix of insulating plastic. So the way that you'll get low resistivity is if (by luck) there is a continuous path of strands that happen to touch weaving through the plastic. Whenever there is a short gap, the resistance goes up.

Now, in a 3 dimensional volume, there might be many such paths, resulting in you being able to rely upon a more or less continuous metal-to-metal contact through the material through a twisty maze of connections. But the thinner and more two-dimensional it becomes, the less the scope for those paths to exist...and the more it starts to look like a one-dimensional "wire", the fewer still paths there can be. The "orienting" behavior of the reprap extruder might easily make that worse still.

I wonder whether there is any mileage to be made in sticking a massive magnetic field across the material as the plastic hardens to encourage the little bits of metal to form chains like iron filings following the "field lines" around a magnet? Stick a neodymium magnet (north-pole-downwards) on the extruder head and another (south-pole-uppermost) underneath the print bed so that the little wires align themselves AND connect up as they are extruded.

-- Steve

Forrest Higgs said...

@Steve Given how viscous molten plastic is, I suspect you'd need a field as powerful as you find in a MRI machine to make that happen. :-(

whosawhatsis? said...

In addition to that, remember that the power of the magnetic force is inversely proportional to the square of the distance from the magnet. This means that unless the magnets are kept far clear of the barrel (requiring them to be MUCH stronger) the attraction to the magnet will far outweigh the aligning force causing them to form a chain. The fibers will be pulled through the plastic toward the magnet. If the magnetism is strong enough to overcome the viscosity of the plastic at all, its effect will be to clump the fibers around the magnet. This will reduce the number of fibers making it through the nozzle, but worse even worse, when this clump gets larger it will no longer be able to overcome the viscosity and will break loose, causing the nozzle to jam. Not fun.

bluebackflyer said...

So, the only way this works is if the little bits of wire happen to touch each other, forming one long wire, held in place by the surrounding plastic. The idea being that there are enough wires floating around that they pretty much form a sheet of conductive metal, but with the malleability of plastic.

I don't think it makes any sense to extrude this stuff. If you do succeed in extruding it all you're doing is recreating a wire, but with the added problem of not being sure whether or not any particular length of it is going to be insulated properly, or suddenly turn into a resistor. Just use a wire.

It seems like the most straight forward application for this stuff is to replace copper on the top of an un-milled PCB. The idea being that it would be easier to work than metal, so flat sheets of it could be mass produced more quickly/cheaply. Then you'd just mill the surface into a conductive pattern like before.

I mean, when it comes to laying down a wire someone already did that with a mechanical pencil. Push it out hot and it will embed itself into the plastic, so you can use uninsulated wire. Just build a pair of snips into the hot end and you're good to go. No worrying about whether or not short little lengths of wire are going to line up properly.

KMnx said...

whosawhatsis?,

Ohms law doesn't apply to nanofibers because the electron pathway does not follow a mean free path, although i don't recall any place in this blog mentioning that the "stainless steel" filaments were nano-scale.

Regardless, it is true that the conductivity of this material is dependent on the randomly aligned wires touching to form a conductive network. I hate to say it but it is quite hard to align small fibers in the direction you want to in this type of set up. Some things you could do would be to "press" the printed strip down to increase alignment, but that looks unlikely. The magnetic field works in fluid environments of aligning nano filaments but often not in these more viscus settings.

Personally, the best option i would submit would be to functionalize the filaments with a type of chemical that would form connections with other filaments end to end upon application of a certain light frequency or heat level. There is much to say about getting these polymers conductive.

oiaohm said...

This material will be unstable in conductivity.

To make something dependable it has to be a solid material that is completely conductive. Part Conductive will not cut it.

http://www.sensorsmag.com/wireless-applications/news/water-based-conductive-printing-ink-introduced-rfid-antennas-1944

These inks do exist but affordable ways to get hands on them is the issue. Yes circuit boards backing being blocks of wood is possible.

oiaohm said...

http://www.instructables.com/id/Conductive-Glue-And-Conductive-Thread-Make-an-LED/step1/Make-Conductive-Glue-Conductive-Paint-and-Conduc/

These experments also might be useful for the research path.

KMnx said...

Also, just a quick side note:

Using conductive filaments in a polymer matrix is not necessarily the best solution to conductive polymers. In truth, polymers can conduct electricity. Via stress induced crystallization, some polymers can be shown to have "slight" conductivity, but the processes can be increased with increased symmetric and crystallinity of the polymer strands. This can be accomplished by very clever polymer design and processing, often relying on a specific cross linking strategy that leads to highly crystalline strand alignment.

Joel said...

A four-point probe would eliminate the skinning effect.

Pass a controlled (or measured) current through it with two probes, and measure the voltage across two distinct probes. Even though some current will take a long path around the voltage-sensing probes, in practice it's easy to get a very good approximation of the size of this effect, so long as the distance between probes is small relative to any distance to the edge of the sheet.

(The same problem arises in semiconductor manufacturing, by the way.)