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NASA Science News for April 14, 2003

Glass from Space

NASA-supported researchers have discovered that glass formed in space
has remarkable properties. 

April 14, 2003: It's easy: mix together some materials like sand,
limestone and soda. Heat them above 2000o F. Then cool the
incandescent liquid carefully so that crystals cannot form. 

That's how you make glass.

Craftsmen on Earth have followed this basic recipe for millennia. It
works. "Now we know it works even better in space," says glass and
ceramics expert Delbert Day, who has been experimenting with glass
melts on space shuttles over the past twenty years. Day is the
Curators' Professor Emeritus of Ceramic Engineering at the University
of Missouri-Rolla. 

Going into those first experiments, he says, he expected to end up
with a purer glass. That's because on Earth, the melts--the molten
liquid from which glass is formed--must be held in some kind of
container. That's a problem. "At high temperatures," says Day, "these
glass melts are very corrosive toward any known container." As the
melt attacks and dissolves the crucible, the melt--and thus 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. 

With that acoustic levitator, explains Day, "we could melt and cool
and melt and cool a molten droplet without letting it touch
anything." As Day had hoped, containerless processing produced a
better glass. To his surprise, though, the glass was of even higher
quality than theory had predicted. 

When most people think of glass, they think of that transparent stuff
in window panes. But glass doesn't have to be transparent nor is it
always found in windows. Among researchers there's a different
definition: "glass" is a solid material with an amorphous internal
structure. The atoms in solids are usually arranged in regular,
predictable patterns, like bricks fitted into a wall. But if the
atoms are just jumbled together in a disorganized way, like bricks
dumped on the ground... that's glass. 

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. 

But why is that important? What's wrong with glass made of silica?

For windows silica is just fine. But glass made from other chemical
compositions offers a panoply of unexpected properties. For example,
there are "bioactive glasses" that can be used to repair human bones.
These glasses eventually dissolve when their work is done. On the
other hand, Day has developed glasses which are so insoluble in the
body that they are being used to treat cancer by delivering high
doses of radiation directly to a tumor site. 

Another example: 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. 

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. 

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. 

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. 

Credits & Contacts
Author: Karen Miller, Dr. Tony Phillips
Responsible NASA official: Ron Koczor 
Production Editor: Dr. Tony Phillips 
Curator: Bryan Walls 
Media Relations: Steve Roy

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