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Iron pyrites negative resistance?

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Conundrum

Thu Jun 04 2009, 09:04AM

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

Hi all.

I just managed to obtain a nice sample (50g or so) of iron pyrites crystals.

As expected it is very fragile but does indeed have lots of small crystals just under the surface.
So my plan is to use the approach here

and connect to the bulk crystal using BiSnPb alloy (as i have several pounds of it from a failed project)
then use a conventional cat's whisker to probe the crystal and find an active region.

Another interesting idea might be to shield the encapsulated bulk with wax etc then electroplate or silver paint a contact onto an active region for a more useful device. Theory:- can the crystal generate an SCR-like recoverable response for building a power oscillator if a "gate" electrode is added?

-A

Tonskulus

Thu Jun 04 2009, 09:40AM

Registered Member #1223 Joined: Thu Jan 10 2008, 04:32PM
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Posts: 133

Interesting. I have about 2kg of iron pyrite pieces. Or I think its iron pyrite, it looks like standard pyrite but it can be lifted with magnet. So there is some iron too. I really have to try that experiment :) So we can compare our results later.

Proud Mary

Thu Jun 04 2009, 12:58PM

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

According to Camm, F.J. The Wireless Constructor's Encylopaedia, London, 1934, p. 135

A gold point is the best sort of whisker to use with a pyrite crystal.

A copper or brass whisker is best with a galena crystal.

Molybdenite works best with a flat silver strip as 'whisker'

Silicon crystals work best with gold or steel.

And lastly, carborundum crystals with a steel whisker work much the best of all, but must be biased with a 4 1/2 volt battery.

According to Camm, crystals work well only in spots; the surface should be freshly broken, and then kept clean with carbon disulphide.

Proud Mary

Fri Jun 05 2009, 09:27AM

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

Tonskulus wrote ...

Interesting. I have about 2kg of iron pyrite pieces. Or I think its iron pyrite, it looks like standard pyrite but it can be lifted with magnet. So there is some iron too. I really have to try that experiment :) So we can compare our results later.

These natural ores are usually much the best as detector crystals than refined products, because of the rich amount of impurities - dopants - in the crystal, just as in a modern semiconductor.

Your piece of pyrite is probably mixed up with magnetite, and other ferro-magnetic metals, and possibly (non-magnetic) arsenic (arseno-pyrite) too - an important semi-conductor when in the form of gallium arsenide

Break off a small pieces of the ore that is very golden - that is your pyrite. A piece the size of a small pea is about the right size for a traditional crystal detector.

Tonskulus

Fri Jun 05 2009, 03:46PM

Registered Member #1223 Joined: Thu Jan 10 2008, 04:32PM
Location:
Posts: 133

Maybe my phyrite is something else or bad quality. I couldnt obtain any oscillation with these..
btw I have some real antique adjustable chrystal diodes. There is about 5mm diameter piece of some mineral, maybe galena or similar.
I took some pics of my phyrite?:

Proud Mary

Fri Jun 05 2009, 04:59PM

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

It looks like iron pyrite, Tonskulus, and should be quite heavy in the hand. There is also a pyrite ore of copper, called chalcopyrite, and an arsenical pyrite - arsenopyrite, but then it would not be magnetic. All these pyrites usually contain very small quantities of silver, gold, lead and tin, which must act as semiconductor dopants.

Why not smash off a small piece with a hammer, and try it out with a gold point - I would think the cheapest piece of gold plated jewelry would be fine - the kind of women's hoop earring you can get for a dollar would be fine, so you could straighten it out and make a point (but don't strip off the micron gold plate by mistake, as I think they have a nickel substrate underneath.)

The impedance of the junction will be very high, so you should only expect micro-amp sized currents to flow through it. This is why you have to use high impedance headphones >2000R, and a big antenna and a very good earth to get good results from a crystal set.

Your original crystal may very well be galena. From my old books, I read that galena was one of the most popular crystals - perhaps because it was cheapest, and easier to break up than pyrite and some of the other minerals.

A man has made a negative resistance oscillator out of pyrite, and also of galvanized iron (i.e. zinc plated steel sheet) here:

Tonskulus

Sat Jun 06 2009, 02:01PM

Registered Member #1223 Joined: Thu Jan 10 2008, 04:32PM
Location:
Posts: 133

Many modern semiconductors has also gold plated wires, like CPU's and similar computer stuff. Those are much more available than women jewelery :)

And yes, my phyrite pieces are very heavy. Those two pieces are actually from one big, I hammered it to a smaller pieces.

Proud Mary

Sat Jun 06 2009, 04:36PM

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

Well, you've got the raw materiala for the experiment, so tell us how it goes.

Conundrum

Mon Apr 10 2017, 07:01AM

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

Apparently two crystals work better, ie zincite and pyrite. IIRC this setup was patented as the "Perikon detector" and probably would work better than a simple point contact. Something about the combination, it does mention that the zincite breaks down if overloaded so they used a rotary setup.

A salvaged CPU pin mounted on some sprt of spring would make a useful whisker, although like PM mentioned having the crystal under oil or mild solvent would prevent it deteriorating with time and a solid mount such as the inside of a defunct reed switch filled with Wood's metal or an equivalent might make a reliable base contact and crystal casing.
Wonder if a sulphide containing liquid like Thiourea would work?

For oscillators you really need precise contacts, if the material breaks down oscillator stops working.
This can be a serious problem with radio circuits as no heterodyne generally means no output whatsoever.

Proud Mary

Mon Apr 10 2017, 11:39AM

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

In his Wireless Constructor's Encyclopaedia (1932) the redoubtable Fred Camm lists the following combinations of crystals and cats whiskers:

Galena - copper or brass, "but sometimes silver is better."

Molybdenite - flat silver strip.

Silicon - a gold or steel whisker.

Iron pyrites - a gold point.

Camm finishes his list by telling us that "The carborundum and steel crystal detector is undoubtedly the best, but this necessitates the use of an applied potential in the form of a 4 1/2 volt dry cell connected across the crystal and contact. This, of course, must be used in conjunction with a potentiometer."

Camm is very clear that the crystal surface should be freshly exposed at the point of contact and that handling the crystal so as to contaminate it with sweat deposits etc will stop it from working.

You defintely can make negative resistance oscillators using some of these combinations which I suppose is due to quantum tunneling and such, but I don't understand that part of it too well.

I think I'd go straight for Camm's "best" biased carborundum and steel wire detector. I guess you adjust the bias voltage across the junction until it starts working. I've got a big piece of crystalline silicon carbide and could break off a piece if you want to give it a go, AndrÃ©.

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