Glass From Space

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.

more 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.

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.