Further shiny things

Crystalline silicon carbide

This extraordinary object looks as if it came from outer space.

Silicon carbide crystals

It's around 11 centimetres long (4.3 inches). It weighs a bit more than 170 grams (six ounces). Its overall colour is a sort of greenish black. But it's entirely composed of darkly reflective crystalline facets, ranging in size from microscopic to about 8mm (5/16ths of an inch) in length.

Close-up of silicon carbide crystals

On close inspection, the mass of crystals comes in a dark rainbow of different colours; yellows, blues, purples and reds.

Extreme close-up of silicon carbide crystals

Just like solid chromium, this stuff doesn't look real. Like the chromium, it looks more like some sort of movie prop. But my chromium lumps look like rocks spray-painted silver for an Original Series Star Trek episode in which those silver rocks were the most valuable object to appear. This stuff isn't nearly as shiny, but is much flashier, if you get my meaning. It's detailed. It's high-definition. It looks like a prop from a sci-fi movie we won't have the technology to make for another ten years.

You know what it looks like? It looks like black kryptonite, that's what it looks like.

(If you ask me, it looks better than the actual black kryptonite prop from Smallville. I presume you all share my incredulity that it took them until 2004, the 66th year of Superman's existence, to add the seemingly obvious black to the host of other kryptonite colours. Oh, and this Flickr user had the same thought, about what looks to me like the same material.)

Or possibly this stuff is what was left over after Gus Gorman boiled some home-made kryptonite in ammonia and ether and then smoked it.

This mass of black crystals is actually a chunk of crystalline silicon carbide (SiC), which I purchased quite cheaply on eBay (see below. There's video, too!).

This means it must be man-made, because natural, "native", silicon carbide does exist, but it's fantastically rare. It takes a lot of energy to persuade silicon and carbon to form a molecule.

Native silicon carbide is known as "moissanite", and so are simulated diamonds made from high-purity SiC. Far, far more SiC is made for use as an industrial abrasive or super-hard coating for tools, though, and some such process probably made this lump of the stuff.

Silicon carbide is so widely used as an abrasive because its Mohs hardness is as high as 9.5, between corundum (sapphire, ruby, and a component of emery) at 9, and diamond at 10.

Those numbers are misleading, because the Mohs scale is ordinal; it tells you what's harder than what, but not by how much. There are different ways of measuring the hardness of a material - compare and contrast the Knoop, Vickers and Rockwell tests, for instance. Whatever method you use, if you do a relative-hardness test, pretty much everything looks sick compared with diamond.

Assign a relative-hardness score of 10 to diamond and, depending on what test you use, corundum may score as high as 2.63 or as low as 2.2, and silicon carbide may score as high as 4.63 or as low as 2.5. For further comparison, quartz, generally regarded as a pretty hard material, scores down around one, if diamond is 10.

(In view of this, the fact that humans are now finally more-or-less managing to make exotic materials that are actually harder than diamond is quite amazing. The most widely used new super-hard material is diamond-like carbon, which as the name suggests isn't actually "better" than diamond, and two of the other candidates are actually just novel forms of diamond. Only minuscule amounts of the best non-diamond candidate have been made to date - with some debate over whether any of it has actually been made at all. But one way or another, we're doing it, and the achievement is a lot more impressive than a mere Mohs hardness of "11" would suggest.)

I don't know exactly how my lump of SiC was made, but I suspect it was an unwanted byproduct of some industrial process, perhaps one or another kind of vapour deposition. The carbide is meant to coat drills or saws or something, but it deposits elsewhere on the equipment too. When some lucky duck gets to clean out the machinery, stuff like this crystal mass ends up in their bucket.

Most industrial waste is not particularly decorative, but every now and then, something extraordinary comes along.

I also don't know how pure this carbide is. High-purity silicon carbide can be black, just like this material, but there may be various impurities in there too.

The surface is definitely not pure SiC; the rainbow reflections are created by a very thin layer of silicon dioxide on the surface. This interacts with light in the same way as various other super-thin coatings, like the surface of anodised titanium, the "rainbow of temper" on steel, or a soap bubble, for that matter.

Silicon carbide is very hard, but rather brittle. If you buy a chunk mail order like I did, you're going to get a few broken-off crumbs in the box along with the main piece, unless the seller packed the carbide in thick cotton wool. And if they did pack it in cotton wool, you're going to spend forever picking cotton shreds off the pointy crystals. Just hitting the thing with a blowtorch might be a faster solution. Or it might heat-shatter.

You don't really have to treat SiC like the egg of a tiny bird, though. When I deliberately broke off a little crystal stuck to the main mass by a couple of millimetres of hair-thin filament, I was surprised to see the filament bend a good five degrees before it snapped. And tiny crumbs coming off even a small SiC lump won't make any obvious difference to its appearance.

One thing you probably don't need to worry about your silicon carbide doing is melting. The melting point of pure silicon carbide is 2730°C, 3003K, or 4946°F in the old money. So you may be able to melt it with an oxy-acetylene torch; the theoretical perfect-combustion temperature for that is around 3500°C. Oxy-hydrogen might manage it, too. MAPP-gas and oxygen probably won't cut it, though, and no cheap butane torch will come within a hundred miles.

Silicon carbide was the material used for the very first light-emitting diode, way back in 1907, though this discovery was largely ignored at the time. That could be why nobody managed to make an LED bright enough to be useful for anything until the Sixties. Henry Round's original discovery was still scientifically important, though, and I swear I managed to get a tiny spot of my chunk of SiC to light up under a pin connected to minus 12 volts. But once I set my camera up, it refused to do it again, no matter what I poked with the pin or where I attached the positive cable's alligator clip.

I think having an alligator clip as the positive terminal, rather than for instance sitting your SiC chunk on aluminium foil that's connected to positive, is important - you need pressure on the SiC to get a decent contact, and the positive connector needs to be close to the point you're poking with the negative pin, or the semiconducting SiC won't let any current flow. With the clip close to the pin (less than a centimetre), something above 20 volts always persuades my SiC to allow current to flow, but that doesn't give the LED effect, just little blue sparks. You're looking for something greenish-yellow, as in this Wikipedia picture:

Silicon carbide LED effect

(The picture is from this page, which contains further instructions on how to try this experiment yourself. And then there's this dude, whose carbide lump seems happy to light up all over, damn his eyes.)

Trying, and failing, to make my own carbide-LED picture was quite frustrating. I can see why people in the early days of radio were so happy when they could buy machine-made vacuum-tube diodes so they didn't have to fool around with super-fine wires and lumps of galena any more, poking around all over the crystal like a tiny pirate seeking one buried treasure chest on the whole island of Barbados.

Aaaaanyway, you may be pleased to know that I am now finally going to tell you where I got this stuff, and what it costs.

Getting some

I bought a little chunk of crystalline silicon carbide on eBay a few years ago, from this seller, but they don't have any SiC for sale at the moment. This new bigger chunk was another eBay purchase, for $US28.17 including delivery to me here in Australia, from this seller (who's here on eBay Australia, here on eBay Canada, and here on eBay UK).

As I write this, they've got one more lump of the stuff, closer to spherical than mine and weighing 210 grams.

The inimitable Theodore Gray has a chunk of this stuff too; he bought it on eBay as well, but from a seller who called it "native bismuth". Dark SiC crystals resemble bismuth hopper crystals (see here) in colour, but that's as far as the resemblance goes.

(Theo also has this different-looking SiC sample, which was also sold as bismuth. And then there are these high-purity crystals, transparent green with no oxide layer. Oh, and on the subject, if you get a solid block of carbide but your plutonium hasn't arrived yet, you can pass the time with some microwave metal melting!)

Nobody on eBay seems to be selling silicon-carbide "bismuth" at the moment; there's plenty of "native bismuth" crystals that're obviously actually purified bismuth crystallised by the standard stovetop method, but at least those actually are bismuth, so by eBay fake-minerals standards no great crime is being committed.

And now: Twinkling!

Behold, the silicon carbide lump, and the chromium, and a couple of large oval-cut cubic zirconias ("CZs") into the bargain. They all look impressive in sunlight.

(These videos don't have sound. Feel free to add your own vocal "ting" sound effects to synchronise with the reflections and refractions.)

The smaller CZ is, at about 36 by 29 by 22 millimetres (1.4 by 1.1 by 0.9 inches), comfortably in the Crown Jewels size range. If it were a diamond, it'd be around 155 carats (as a CZ, it's 255 carats - CZ is about 1.6 to 1.7 times as dense as diamond). This is a bit less than the original cut of the Koh-i-Noor, but about 1.5 times the Koh-i-Noor's current size.

The larger CZ is about 52 by 38 by 28 millimetres (2 by 1.5 by 1.1 inches), and weighs 132.5 grams; a diamond the same size would be about 400 carats, far larger than any of the world's famous colourless diamonds, and a little less than the total weight of all of the multicoloured diamonds in the two "Aurora" displays.

I bought both CZs in 2009 from this eBay seller; the smaller one cost me $US19.95 delivered, and the bigger one was $US37.95.

That seller doesn't seem to have a lot of huge CZs on offer today, but if you use the always-entertaining "highest price first" sorting option but set a price ceiling at, say, $100, then in among the eBay listings for bags containing many small CZs, there are plenty of monster white and coloured stones.

(Here's that search on eBay Australia; it's here on eBay UK, and here on eBay Canada. I strongly recommend you buy at least a pocketful of small CZs; they make novel presents, and you can also wrap them in black felt, go to a cafe with a friend, wait for people to look, and then make everyone think some serious state secrets are being sold.)

5 Responses to “Further shiny things”

  1. Popup Says:

    I had no idea that CS was that cheap!
    I might have to buy a grab bag, just for the kids to play with...

    (There was quite a lot of interest in SiC 15 years ago. It was touted as a wonder semiconductor capable of high-power as well as high-frequency operations.)

  2. dmanuel Says:

    Sorry Dan, but the most interesting thing was the link to using a microwave an an induction heater. The plasma from grapes thing is pretty neat too.

    Now I kind of want to hook up three or so magnetrons (because obviously more is better) and melt some metal. Would the magnetrons interact with each other in some weird potentially life ending pyrotechnical display or be fine do you think? Obviously a Faraday cage of some sort would be needed to preserve sperm count.

    Hmm, must do research...

    • dmanuel Says:

      Huh. Tried the grape thing but apparently it's much harder to pull off then it looks. I did get some pretty good sparks, but that's about it.

    • TwoHedWlf Says:

      I'm confused, you mean to say that the magnetrons interacting in some wierd potentially life ending pyrotechnical display is NOT fine?

      • dmanuel Says:

        Well, not when I'm the one standing next to them eagerly waiting from some molten metal to play with, no. From far away, perfectly fine, yes.

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