Cooking up clear, UV-proof glasses


By adding an oxide of the rare earth metal cerium to phosphate glass, researchers are developing glasses that are colorless and can block ultraviolet light and resist radiation damage. The glasses could have many commercial applications for use in windows, sunglasses, and solar cells.

PENN STATE (US)—Adding cerium oxide to phosphate glass rather than the commonly used silicate glass may make eye glasses that remain colorless, block ultraviolet light, and have increased resistance to radiation damage.

These cerium-containing phosphate glasses have many commercial applications for use in windows, sunglasses, and solar cells, according to Penn State researchers.

“We wanted to get larger amounts of cerium into glass, because of its beneficial properties, and then investigate the properties of the glasses,” says Jen Rygel, graduate student in materials science and engineering.

Cerium exists in two states in glasses—cerium (III) and cerium (IV)—both states strongly absorb ultraviolet light. For years cerium has been added to silicate glass to enhance its ultraviolet absorbing capacity. The problem has always been that silicate glass can only dissolve so much cerium before it becomes saturated and can hold no more.

Also, with high concentrations of cerium, silicate glass begins to turn yellow—an undesirable characteristic for such things as windows or sunglasses.

Phosphate glasses have a more flexible structure then silicate glasses, which allow higher percentages of cerium to be incorporated before it begins to color.

Rygel, working with Carlo Pantano, distinguished professor of materials science and engineering, and director of Penn State’s Materials Research Institute, synthesized and compared 11 glasses with varying concentrations of cerium, aluminum, phosphorus, and silica.

They found that they could make phosphate glasses with 16 times more cerium oxide than silicate glasses while maintaining the same coloration and ability to absorb ultraviolet light. They published their work in the Dec. 15 issue of the Journal of Non-Crystalline Solids.

“We were able to get a lot more cerium into our phosphate glass without sacrificing the optical transmission—they both still looked clear,” says Rygel.

The researchers could get more cerium into phosphate glass compared to silicate because of the different bonding networks silica and phosphorus form when made into glasses.

One explanation for why phosphate glass can incorporate more cerium than silicate glass without yellowing may be that the absorbing ranges for the two cerium states are shifted to absorb less blue light in phosphate glasses.

“A good example is in solar cells,” says Rygel. “The wavelengths that solar cells use aren’t ultraviolet, and actually ultraviolet radiation can cause damage to the electronics of a solar cell. If you add cerium to the glass you can prevent the ultraviolet from getting down to the photovoltaic cells, potentially increasing their lifetime.”

To synthesize their glasses the researchers used a procedure called open-crucible melting. Raw materials such as phosphorus pentoxide, aluminum phosphate, cerium phosphate, and silicon dioxide were combined in a crucible and heated in a high-temperature furnace to a temperature of 3,000 degrees Fahrenheit melting the contents to a liquid.

“After it’s all melted, we pull it out of the furnace and pour it into a graphite mold,” says Rygel. “The glass is then cooled down slowly so it doesn’t break due to thermal stress.”

Cerium additions do not just block ultraviolet light. Increasing a glass’ cerium concentration can also increase its resistance to radiation damage from x-rays and gamma rays by capturing freed electrons.

“Radiation can kick electrons free from atoms,” says Rygel. “You can see this by looking at what happens to a Coke bottle over time. It darkens because of radiation exposure.”

The National Science Foundation and the U.S. Air Force Research Laboratory supported this work.

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