Best Glass Made in Space

"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.

... 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 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.

... 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]


... Metallic glasses. Bioactive glasses. Super-clear fiber optics. The possible applications go on and on....



http://science.nasa.gov/headlines/y2003/14apr_zeroglass.htm


Author: Karen Miller, Dr. Tony Phillips