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.