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Old 4th March 2005, 10:01 PM   #21
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Ummmm... Sy, more and less than I wanted to know...

Could you translate that a bit for those of us who are not experts in the field? In practical terms what are the steps? You said something about "humidity" with respect to "protonating" the polyanaline?? Acids are wet, no?

And, the Sony paper seems to imply that the conductive layer is part of the underlying plastic substrate after treatment - no?

Where does one, if one wanted to, find a bottle of an appropriate polyaniline?? Ok, I know the chemical supply places have gone nutty wacky these days and buying anything from them is a PIA...
Just curious about the process in practical terms... since they seemed to say this stuff was stable and *less* suceptable to the effects of humidity than graphite...

...barking up the tree...



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Old 4th March 2005, 11:07 PM   #22
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I don't think it's easily purchaseable; normally, labs synthesize it using aniline, HCl, and a perchlorate. I think there was a German company, Zipperling, which was selling dispersions of the free base (nonconductive form). Good luck getting that to stick to a polyester film without some heroic effort.

OK, the basic way it works, once you have the polymer, is that the conductivity is "turned on" by exposing the partially oxidized polymer to an acid solution (by "partially oxidized," I mean the oxidation stae corresponding to equilibrium in air). It's a process analogous to doping of a conventional semiconductor. The polymer is dried and what you have left is a conductor. Now, the conductivity of that material will be moderate (on the order of 10mho/cm), but it will vary by a factor of 2 or 3 depending on the relative humidity of the air around it- i.e., the conductivity is not terribly constant over time. And if you limit the doping to try and keep the conductivity relatively low, it will drift around by an order of magnitude. The twists I invented deal with stabilizing that conductivity and giving the material more structural integrity, but they involve some very non-diy synthesis steps and are more suitable for their intended application (controlling the RF characteristics of fiber reinforced composites). I'm very skeptical about the claims of insensitivity to humidity.

When I worked for MacDiarmid, we used to joke that conductive polymers were the technology of the future and always will be. There are MUCH easier ways of getting stable, reliable coatings for ESLs; I suspect that Sony (like many other big companies) bases bonuses for technologists partially on number of patents, thus something with very little practical purpose can get patented. I know, because I've patented some perfectly useless things just for that reason.
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Old 5th March 2005, 12:53 AM   #23
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Sy,

Your knowledge is valuable and rare here.

So, OK, I'll bite:

Quote:
There are MUCH easier ways of getting stable, reliable coatings for ESLs;
Would you share a few? Do you know of coatings that are consistent across the diaphragm to prevent hot spots (charge concentration), coatings that stay on, that aren't very humidity sensitive? That aren't deadly toxic?

Graphite rubbing is too inconsistent and risks stretching or tearing the film. Sputtering with metals is not easy for DIYers. Dissolved nylon leaves variable patches. Etc., etc.

What do you recommend?
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Old 5th March 2005, 01:06 AM   #24
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The problem is on the other end, the film itself; making a good, stable conductive coating is trivial. Any industrial coating operation on polyester will pretreat the surface; sometimes by flame, sometimes by corona, sometimes by plasma, in order to raise the surface energy and enhance the adhesion. There are better and worse ways for different materials. It can even be done chemically, but that's a very, very hazardous procedure. The adhesion requirements are extreme for long-term stability: after all, you shake the coated film around rapidly for years while zapping it with high voltage arcs now and then.

Films that have higher surface energy carry their own problems like being very hygroscopic or not being able to shrink or to hold tension. I've been using Clysar (which I surface treated before coating), but I've got to hit it with a heat gun every few weeks.

If my primary goal were to get a pair of good-sounding speakers in my living room rapidly and reliably, I'd use the rubbed-graphite method. If I were feeling experimental, wasn't in a huge hurry, and didn't have access to cool plastic finishing toys and a chemistry lab, I'd experiment with the nylon coatings that Quad used. Maybe toss in some quaternary ammonium salts (antistats) for giggles. There's at least one web site I've seen that discusses the formulation- I can't remember what it was, but I'll bet Sheldon Stokes has a link to it.

edit: One question occurs to me- if the resistance is nice and high, will variation necessary cause hot spots?
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Old 5th March 2005, 04:13 AM   #25
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Default Charge distribution

When there is no music playing, i.e. the diaphragm is not moving, the charges on it will distribute themselves regardless of the conductivity of the diaphragm, as long as there is some conductivity. If you're dumping extra electrons onto the diaphragm, they all push at each other and try to settle in the arrangement that has lowest energy. That means there will be greater charge accumulating near the edges of the diaphragm (or the conductive area, if it is smaller than the diaphragm) than at the center. At the center of the diaphragm free electrons are being pushed from all directions but near the edges all the push is coming from one side, so it is impossible to have uniform charge distribution on the diaphragm.

Variation in conductivity at some parts of the diaphragm simply affects the time it takes for the charges to "settle". High conductivity lets it happen faster than low conductivity. In the end the charges will end up distributed the same way whether the film is high or low conductivity. I have actually seen audio voodoo claims of it taking weeks for the charge on some commercial ESLs to distribute itself over the diaphragm.

When the diaphragm starts moving a high conductivity coating will allow the charges to move around which is generally undesirable because it leads to distortion and reduced output at low frequencies.

I think you could use a high conductivity diaphragm if you could set up a magnetic field that is parallel to the surface of the diaphragm. As the diaphragm deflects, the high conductivity coating would allow the charges (electrons) to move- they will move toward the center of the diaphragm when the diaphragm is maximally deflected, and then spread themselves back out when the diaphragm is centered between the stators. The moving electrons constitute a current which would interact with the magnetic field to move the diaphragm even as the electric field was losing its ability to do so (maybe).

I don't know if the magnitude of the current would be enough to generate any substantial mechanical force. I'll have to think about it for a while.

I think that the current could get pretty high at high frequencies, so if one were to drive the high conductivity diaphragm speaker with a high amplitude, high frequency signal the current might heat up the diaphragm. The amount of heat might be enough to cause the diaphragm to melt. More thought is required....

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Old 5th March 2005, 02:52 PM   #26
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Some interesting thoughts, for sure.

A few comments come to mind: There is one common misunderstanding about ESLs that may apply here. In frequency bands much above where the diaphragm stiffness reactance equals the air load mass reactance (plus air load radiation resistance), the motion of the diaphragm is substantially piston like, not drum-head like, moving as a flat plane across most of its area. Almost all stretching and bending takes place at the edges (something to consider when designing edge terminations and frame shapes, but that’s another matter). This behavior applies to most of the useable frequency range of most ESLs. Below this transition frequency, the diaphragm will tend to move in a bowed fashion, approaching a catenary curve (assuming that charge migration doesn’t affect the movement, nor do resonant standing waves, assumptions that are probably not realistic in most cases).

Anyway, in the useable upper bands, there shouldn’t be much charge migration or current running across the surface of the diaphragm even in a relatively conductive one, since each point on the diaphragm is equidistant from the stators. (If very low bass is allowed to act on the diaphragm, the centers will be bowed as a result of the summation of a catenary shape with the overlying planar pistonic motion; there is a lesson or implication to design here.)

BTW, the Quad 63/988/989 allows bending at the higher frequencies in each of the two center “half-bull’s-eye” diaphragms at each annular ring transition, and not just at the frame edges. The center spot gets the most full-range voltage and therefore the most motion, and this is where arcing often takes place first.
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Old 5th March 2005, 03:17 PM   #27
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Brian, excellent and thoughtful comments. That's very interesting information about the Quad.

The sad news is that all ESLs suffer from periodic wardrobe malfunctions when the diaphragm and stator become better acquainted than the designer intended. And for many of us, this happens a lot more often than we'd like to admit... Each one of those ominous ratatats I hear means the poor diaphragm is whacking into a (relative) brick wall a couple hundred times. Ooops!
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Old 5th March 2005, 10:39 PM   #28
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"...fascinating, Captain..." - Mr. Spock

Thanks to all who have participated so far in this thread.

Illuminating.

Anyone know what the stuff used to coat the film in "High R" windows is? What its resistivity might be??

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Old 5th March 2005, 11:19 PM   #29
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Normally, coatings of that sort are sputtered or chemical vapor deposited metals. Expensive machinery for THAT! And to anticipate your next question, the coating on touchscreens is indium tin oxide.

Window coatings could possibly be practical (touch-screen ITO is too conductive), but they only seem to come on thick, heavy film.
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Old 7th March 2005, 12:41 AM   #30
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Wink What about this?

Please refer to Licron 1755.

http://www.techspray.com/staticcontrol.htm

It's readily available, virtually insensitive to humidity changes, easily applied straight from the spray can, one light coat will give you 500M/sq and adheres very good to plastic substrates.

It is based on Tin-Oxide.

Regards

Note:
Graphite is much cheaper for a quick and DIRRRRTY job
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