Molly Wire Was:Towers to the Stars

From: Stirling Westrup (sti@cam.org)
Date: Thu Mar 02 2000 - 09:35:32 MST

hal@finney.org wrote:

> > Pg. 386 of NM says the Young's Modulus is ~400 GPa for saphire and 1050
> > GPa for diamond. But what we really need are some comparisons between
> > the compressive and tensile strength.
>
> I don't have figures, but diamond anvil compression cells are quoted as
> being good up to 100 kbar, which would be about 10^8 g/cm^2, so you might
> use that as a ballpark upper limit for the weight that can be supported by
> a diamond column.

I've been doing some research into the Gibson idea of molecular cutting
("molly") wire, and have been trying to research theoretical tensile
strengths of materials. I came across a couple of tables that gave
artificial diamond a (normal) tensile strength of around 2 GPa. Recent
experiments on multi-walled bucky tubes have given a measured strength of
the researchers think that single-walled tubes will have 'several hundred'
GPa. Other researchers have apparantly posted numbers around 200 GPa,
which have been quoted in some newsgroups.

Anyway, does anyone know the thickness of the end of a *really* sharp
knife blade? I ask this because 200 GPa is a suspension strength of
2,000,000 kg/cm^2 in 1g which means that a line able to lift 20 kg will
have a thickness of around 30 um. This hardly seems sharp enough, given
the ease of breaking such a wire. If I can figure out how thin molly wire
should be (I've always assumed 75 nm or so, which would give a test of
about .1 gram, which seems too low) I could come up with a required
tensile strength, and compare with theory, to do a reality check. Of
course, any pointers on figuring out how much force it takes a blade with
given parameters to pass through a substance with given parameters would
help greatly in modelling this stuff. If its sharp enough to hardly feel
resistance when passing through materials, maybe the wire can get away
with having only a few grams test.

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