From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.tech
Subject: Re: fullerines and space bridge
Date: Sun, 31 Dec 1995 22:55:14 GMT

In article <30E25488.72E5@mail.GANet.NET> "William H. Mook, Jr." <wm0@s1.GANet.NET> writes:
>> >...are these materials so much stronger as to make space bridges
>> >easier to build?
>> 
>> What materials?
>
>Um, bucky balls consist of 60 atoms of Carbon.  Variants have been made...

I'm quite aware of that, but a molecule is not a material.  There are lots
of molecules which are immensely strong; the problem is realizing that
strength (and coupling it with some damage tolerance) in macroscopic
objects.  We *already* have extremely strong carbon-based materials, i.e.
graphite fiber.  There is no immediate indication that fullerenes will
be a magic route to better ones.

>...Cleave the balls in half, and bond a carbon ring to it, now repeat
>the process n times.  You end up with a bucky-tube of macroscopic
>length.  Now, erect a lattice enclosing this tube.  Pretty soon you've
>got a macroscopic fiber of indefinite length and size.

Yes, and if I stick carbon atoms together in a cubic lattice, and keep on
doing this until it's the size of a basketball, I'd have a rather valuable
diamond.  Unfortunately, describing where the atoms go and making them go
there are two different problems.  Nobody knows how to make macroscopic
buckytube fibers.  Perhaps eventually we will; perhaps not.

At the moment, the only macroscopic fullerene-based material with possible
space applications is very different.  There's a process that produces a
buckytube film -- closely-packed tubes standing upright -- on a flat
surface.  Such a film turns out to be an excellent field-emission electron
emitter (presumably because a modest fraction of the tubes have very sharp
ends), which might be relevant to ion propulsion. 

>...If it weren't for the diffusion of defects in macrscopic
>solids from their surface things would be much much stronger.  Lack of defects
>in pure diamond crystals give them their strengths...

Actually, most structural materials are limited not by their inherent
strength -- which is that of molecular bonds, i.e. truly immense -- but by
defect tolerance.  Realistic structural materials *must* tolerate some
defects, because handling will always produce some, on the surface at
least.  A diamond is strong, but that beautiful defect-free crystal
structure is terribly brittle.  When a defect does intrude, the diamond
breaks.  That's unacceptable in a structural material; cracks must spread
slowly enough that they can be found and fixed before the final break
happens.  (This is why things like ships and deep-sea oil rigs are built
mostly out of fairly soft steel which is not terribly strong:  its
"critical crack length", the point at which slow crack growth turns into
an explosively-rapid break, is long.)  If the basic material isn't good 
enough, you need to add rip-stops.

This is why composites do best if their fibers are *not* too strongly
bound to the matrix material that ties them together:  if a matrix crack
approaching a fiber tends to separate the fiber from the matrix instead of
carrying on into the fiber, the crack has to do a lot more work to
propagate further.  That's why glass fiber and epoxy, both of which
crack quite easily, can be combined to produce very tough fiberglass.

(For interested onlookers who want to know more about this stuff without
getting into the gory engineering math, I'd highly recommend two books
by J.E. Gordon:  "The New Science of Strong Materials" and "Structures".
They're written for a popular audience but do such a good job of explaining
the fundamentals that they often show up as introductory textbooks at
engineering schools.  They're about 20 years old and could use minor
updates, but are still quite satisfactory.  Yes, they're still in print,
and in paperback too.)

>One interesting idea is to make tiny little dipoles out of bucky-tubes whose
>ends have charges attached to them...

Since charge is quite mobile in a hexagonal carbon structure, I don't think
this is going to work...  Maybe with sufficient modifications to the basic
buckytube structure it could be done.
-- 
Look, look, see Windows 95.  Buy, lemmings, buy!   |       Henry Spencer
Pay no attention to that cliff ahead...            |   henry@zoo.toronto.edu