You make glass by mixing together some materials like sand,
limestone and soda, heat them above 2000o F, then cool the
incandescent liquid carefully so that crystals cannot form.
Craftsmen on Earth have followed this basic recipe for millennia. It
works.
But it works even better in space.
A purer glass is to be expected since, on Earth, the melts--the molten
liquid from which glass is formed--must be held in some kind of
container. At high temperatures, glass melts are very corrosive
toward any known container. The glass becomes contaminated.
In microgravity, though, you don't need a container. In Day's
initial experiments, the melt--a molten droplet about 1/4 inch in
diameter--was held in place inside a hot furnace simply by the pressure
of sound waves emitted by an acoustic levitator.
Containerless processing produces a better glass.
To his surprise, though, the glass was of even higher quality than
theory had predicted.
The window glass that we're so familiar with is made mostly of
silica--a compound of silicon and oxygen. It's essentially melted sand.
But in theory, a melt of any chemical composition can produce a glass
as long as the melt can be cooled quickly enough that the atoms don't
have time to hook themselves up into patterns, or crystals.
In Earth-orbit, it turns out, these molten liquids don't crystallize
as easily as they do on Earth. It's easier for glass to form. So not
only can you make glass that's less contaminated, you can also form it
from a wider variety of melts.
Glass made from other chemical compositions offers a panoply of
unexpected properties. "Bioactive glasses" can be used to repair human
bones. These glasses eventually dissolve when their work is done.
Glass made of metal can be remarkably strong and corrosion-resistant.
And you don't need to machine it into the precise, intricate shapes
needed, say, for a motor. You can just mold or cast it.
Right: Steel balls bounce on flat plates of titanium alloy,
metallic glass, and stainless steel. The ball bouncing on metallic
glass keep going for a remarkably long time. [more]
Also intriguing to space researchers is fluoride glass. A blend of
zirconium, barium, lanthanum, sodium and aluminum, this type of glass
(also known as "ZBLAN") is a hundred times more transparent than
silica-based glass. It would be exceptional for fiber optics.
A fluoride fiber would be so transparent, says Day, that light shone
into one end, say, in New York City, could be seen at the other end as
far away as Paris. With silicon glass fibers, the light signal degrades
along the way.
Unfortunately, fluoride glass fibers are very difficult to produce
on Earth. The melts tend to crystallize before glass can form.
Below: The surfaces of ZBLAN fibers formed in
near-weightlessness (upper panel) and in normal Earth-gravity (lower
panel). [more]
The
reason, says Day, is that gravity causes convection or mixing in a
melt. In effect, gravity "stirs" it, and, in a process known as shear
thinning, the melt becomes more fluid. This same process works in
peanut butter: the faster you stir it, the more easily it moves.
In melts that are more fluid, like those stirred by gravity, the
atoms move rapidly, so they can get into geometric arrangements more
quickly. In thicker, more viscous melts, the atoms move more slowly.
It's harder for regular patterns to form. It's more likely that the
melt will produce a glass.
In microgravity, Day believes, melts may be more viscous than they
are on Earth.
While this theory has not yet been confirmed, some experimental
results suggest that it is correct. NASA researcher Dennis Tucker
worked with fluoride melts on the KC-135, a plane that provides short
bursts of near zero-gravity interspersed with periods of high gravity.
"He did some glass-melting experiments, trying to pull thin fibers
out of melts," recounts Day. "During the low-gravity portion of the
plane's flight, when g was almost zero, the fibers came out
with no trouble. But during the double-gravity portion of the plane's
flight, the fiber that he was pulling totally crystallized."
That result, says Day, could be explained by shear thinning. "A melt
in low gravity doesn't experience much shear. But as you increase g,
there'll be more and more movement in the melt." Shear stresses
increase. The effective viscosity of the melt decreases.
Crystallization becomes more likely.
Right:
(left panel) a defect-free ZBLAN fiber pulled during a low-g arc aboard
the KC-135; (right panel) a crystallized fiber pulled from the same
apparatus under 1-g. [more]
Day is currently planning his next experiment in space--onboard the
International Space Station--which he hopes will confirm his ideas.
He'll be melting and cooling identical glass samples in the same way on
Earth and in microgravity. Then he'll count the number of crystals that
appear in each sample. If shear-thinning exists, he says, there will be
fewer crystals in the space-melted samples than in the ones produced on
Earth.
Eventually, Day hopes to take these lessons learned from space and
apply them to glass production on the ground. Metallic glasses.
Bioactive glasses. Super-clear fiber optics. The possible applications
go on and on.... which makes the value of this research crystal clear.
