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Last 10 Posts [ In reverse order ]
Capracus Posted on Aug 27 2013, 09:10 AM
  Space Elevators On Hold At Least Until Stronger Materials Are Available, Experts Say
Capracus Posted on Aug 24 2013, 07:54 AM
  A New Space Propulsion Concept - A One-Kilometer-Long Electric Sail Tether Produced!
Capracus Posted on Jul 25 2013, 10:38 AM
  Now, device to help clean up space junk
Capracus Posted on Jun 28 2013, 03:20 AM
  About Space Tethers
Capracus Posted on Dec 20 2012, 12:07 PM
  Practical materials
Enthalpy Posted on Aug 5 2009, 05:09 PM
  Pure single-crystalline silicon, even in bulk form, has a fabulous yield strength combined with a low density BUT is not used in mechanical applications because it is so brittle. To my knowledge, nobody has achieved an alloy nor anything that keeps silicon's strength and makes it tough.

Combining silicon with carbon is a working solution. At least two ways are used commonly:

SiC is a ceramic, very stable against heat and chemical agents, very strong in compression, but still a bit brittle. It is used mainly as an abrasive, but gets usable for mechanical parts if included as a powder in a metal: search for "Metal Matrix Ceramic" or Cermet.

Silicon is poured (mp~+1400C) on carbon fibres. Their interface is excellent because it builds the stable SiC, so the composite of carbon fibres and silicon matrix doesn't delaminate. I played with some: they are light, hard, very strong in compression but not magic in tension nor bending. Used as brakes (Porsche) against metal for low-wear, high-temp, low dilation, and good friction coefficient when cold.
Meem Posted on Jul 5 2009, 01:27 AM
  According to some of the information on this Stanford work, it doesn't seem to be very brittle, at least that was the impression I had. lol the link was dead ... wierd.
Silicon placed in a battery swells as it absorbs positively charged lithium atoms during charging, then shrinks during use (i.e., when playing your iPod) as the lithium is drawn out of the silicon. This expand/shrink cycle typically causes the silicon (often in the form of particles or a thin film) to pulverize, degrading the performance of the battery.
Cui's battery gets around this problem with nanotechnology. The lithium is stored in a forest of tiny silicon nanowires, each with a diameter one-thousandth the thickness of a sheet of paper. The nanowires inflate four times their normal size as they soak up lithium. But, unlike other silicon shapes, they do not fracture. Research on silicon in batteries began three decades ago. Candace Chan, a graduate student of Cui, explained: "The people kind of gave up on it because the capacity wasn't high enough and the cycle life wasn't good enough. And it was just because of the shape they were using. It was just too big, and they couldn't undergo the volume changes."

Then, along came silicon nanowires. "We just kind of put them together," Chan said.

For their experiments, Chan grew the nanowires on a stainless steel substrate, providing an excellent electrical connection. "It was a fantastic moment when Candace told me it was working," Cui said.

Cui said that a patent application has been filed. He is considering formation of a company or an agreement with a battery manufacturer. Manufacturing the nanowire batteries would require "one or two different steps, but the process can certainly be scaled up," he added. "It's a well understood process."

The breakthrough is described in detail in a paper, "High-performance lithium battery anodes using silicon nanowires," published online Dec. 16 in Nature Nanotechnology, written by Cui, his graduate chemistry student Candace Chan and five others.

Also contributing to the paper in Nature Nanotechnology were Halin Peng and Robert A. Huggins of Materials Science and Engineering at Stanford, Gao Liu of Lawrence Berkeley National Laboratory, and Kevin McIlwrath and Xiao Feng Zhang of the electron microscope division of Hitachi High Technologies in Pleasanton, Calif.

I mean .. this stuff sounds like the answer to a lot of problems. I don't know. Very interesting stuff. I think all the money for "drill-here, drill-now" should be going to this, get us off that middle east teet, and let them make "blood money" somewhere else. Off topic, I am so glad that Palin quit. If she runs in 2012, I will be moving to Germany and work for my wife's uncle or something. Faster than you can say, "You betcha."
Ron Posted on Jul 5 2009, 01:10 AM
  HI Paul,
Have you ever heard Les Claypool do the entire Animals album (with the 'fearless flying frog brigade')? You must listen to it, especially if you like The Colonel!
As far as Carbon non-tubes go, you really need to see the structure. If you think about how carbon locks together (think of carbonizing Iron to make steel), You'll see that it's structure is perfect for making very strong bonds with itself.
I'm not great at chemistry, but I know electronics, and what I know of Si compound, they can be extremely brittle. I'm not really sure why you don't like the idea of a pure carbon nano-tube. Check out some of the research. It's quite far along.
Meem Posted on Jul 4 2009, 12:08 AM
  Heya Ron,

Maybe the pure Si would not work, the pure carbon seemed to have it's failure as well, maybe a "weave" of the two? A stiff and rigid crystalline structure seems to discount Green's ideas about flux. Run with me here, because I am not that mathematically literate. So I will try to get at what I ... "think" I am saying. Take the change in building mechanics in earth quake prone areas. Once all buildings had a solid foundation, and they still are solid but, today most are designed to have flux incorporated into the base structure. I would think in the tethers case, something would have to allow for flux all along it. If Si is not able to be used to help the situation and only carbon is applicable, it probably needs better shielding from cosmic rays, which can be adjusted to compensate for flux in cosmic rays.

(edit) is it possible to make a ... "carbon-silicon" polymer? Where's a chemist when you need one! OR whoever the molecular structure specialists are!
susceptible to surges because of it's higher electrical resistivity

I don't know, what about the role of capacity though? maybe the carbon wire could be wrapped around a silicon core, something like coaxial I suppose. Is that stupid? I know that's a loaded question around these parts but, honesty over pride wink.gif

I'll have to look that about this structure in Bolivia, though it sort of sounds familiar already. I think maybe a while ago, I can vaguely remember something about really old stones and the speculations on how they could have been cut.

(p.s.) what kind of person doesn't like to hear Pink Floyd, even if they "overhear" someone "singing it in the shower?" laugh.gif

Later on,
Ron Posted on Jul 3 2009, 11:10 PM
  Hi Meem,
Sorry to interrupt your self-conversation! I've been trying to find any references to compare the 2 (Si or C nano-wires), but, from what I'm finding there is no direct comparisons for this type of application. Si seems to be used for electronics. This would make sense to me because Carbon nano-tubes are made for their tensile strength, where Si is more applicable to semi-conduction. You know that carbon crystal structure lends itself to strength because of the way they bond. And, in a nano-tube, that strength is increased greatly.
Also, you wouldn't have the problems like you saw with Si because carbon would be much less susceptible to surges because of it's higher electrical resistivity.
I've only looked into this briefly, but I just wanted to keep you from getting to good at talking to yourself!

On a side note, I watched a program on ancient structures and the ancient alien connections today. There is a site in Bolivia that I'd never heard of before that is extremely interesting. It appears that this megalithic site is as much as 17,000 yrs old and extremely sophisticated. The structures are not intact, but the individual blocks and the way they are cut is impressive. I'll try to find more on it.
Talk to ya later,
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