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From: (Jordin Kare)
Subject: Re: Vacuum balloons (was Re: Gas giant atmospheric habitats?)
Date: 25 May 1994 01:06:38 GMT

In article <> (Kevin R. Boyce) writes:

>Or how about this:  Aerogels are often referred to as "lighter than air". 
>I assume this means that if you pump all the air out, the resulting
>structure is lighter than air at 1 atm, and said to be very strong.  I
>wanted to get a chunk, put it in a bell jar and pump it down, then coat it
>with a layer of Mylar.  Make a great novelty gift, if it really is lighter
>than air.  

Yes, it is.  (Actually, the stuff I've seen this demonstrated with is
Seagel, which is an organic-base foam made from seaweed)  It will in
fact float in air.  Silica-based aerogels get down to close to the
density of air, at which point they are practically invisible; you
mostly see the layer of finger oils and miscellaneous crud that
deposits on the surface when you handle the stuff.  

>Whether it's lighter than hydrogen at 1 atm is another matter.  

Almost certainly not, alas.

	Jordin (Aerogellybeans -- the _really_ light snack) Kare

From: (Jordin Kare)
Subject: Re: SPS structural materials - Aerogels?
Date: Sat, 18 Dec 1999 10:15:13 -0800

In article <839opr$6bq$>, wrote:

> In article <>,
>   Ian Stirling <> wrote:

> }
> } This being so, I can't think of a lightweight way of making an aerogel
> } factory, able to turn out reasonable volumes.
> } (that's the other problem, you have to allow enough time that there
> } is no significant pressure difference through the aerogel, otherwise
> } it's structure will be destroyed, this means slow drying.)
> And long drying times may easily wind up not being a problem for an
> automated, orbital facility... just put the fresh aerogel in a plastic
> bubble with an air scrubber, tether the bubble to the factory's back
> door and very slowly lower the pressure in the bubble.

Unfortunately, most aerogels cannot be dried in this fashion.  The problem
is that, at the cell scale of aerogels, surface tension at any liquid-gas
interface is sufficient to collapse the structure.  Aerogels are made by
*raising* the pressure above the critical pressure of the solvent, then
raising the temperature to extract the solvent as a supercritical fluid.

The drying process is still slow, due to the long diffusion time of the
solvent thru the aerogel matrix, but it's not related to large-scale
pressure gradients across the aerogel.
> And would the microgravity environment help with aerogel production,
> especially with large volume parts? No settling of the matrix material
> onto the bottom of the tank.

Diffusion limits the maximum practical thickness of aerogel parts anyway
(they must have one thin dimension, of order a few cm maximum) so this
isn't a limit even in 1-g.

Jordin Kare

From: (Jordin Kare)
Subject: Re: Tensile strength? Re: SPS structural materials - Aerogels?
Date: Tue, 28 Dec 1999 11:13:28 -0800

In article <>, wrote:

> "Gregg E." <> wrote:

>         Another interesting foam-like material made by Livermore was
> called SEA-gel (I think for Safe Emulsion Agar-gel).  This stuff was
> almost entirely C, H, and O and had densities ranging from .1 gm/cc to
> about half the density of air.  Only the air trapped in it's pores
> kept it from floating away.

The Seagel folks actually made some lighter-than-air samples that *would*
float away; at this point I don't recall details, but I helped run the
LLNL display at one of the technology shows and we had a tank with some
~1" cubes of seagel floating at the top.

Before anyone asks, no, seagel wouldn't make good balloons; the lift per
unit volume is poor compared to helium, hydrogen, or even hot air.

Jordin Kare

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