Adding cerium oxide to phosphate glass rather than the commonly used silicate
glass may make glasses that block ultraviolet light and have increased radiation
damage resistance while remaining colorless, according to Penn
State researchers. These cerium-containing phosphate glasses have many commercial
applications for use in windows, sunglasses and solar cells.
"We wanted to get larger amounts of cerium into glass, because of its
beneficial properties, and then investigate the properties of the glasses,"
said 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 today's (Dec. 15)
issue 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,"
The researchers could get more cerium into phosphate glass compared to silicate
because of the different bonding networks silica and phosphorus form when made
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 -- cerium (III) and cerium (IV) -- are shifted to absorb less blue light
in phosphate glasses.
"A good example is in solar cells," said 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 3000 degrees Fahrenheit melting the contents to
"After it's all melted, we pull it out of the furnace and pour it into
a graphite mold," said 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," said Rygel. "You
can see this by looking at what happens to a Coke bottle over time. It darkens
because of radiation exposure."
The proposed mechanisms for cerium's ability to block radiation are all based
on cerium's two states and their ratio within the glass. Because of these implications
Rygel wanted to know what percentages of each existed within her glasses.
Using X-ray photoelectron spectroscopy Rygel could determine whether the cerium
in the glass was mostly in the cerium (III) or cerium (IV) oxidation state,
or a ratio of the two. She found that all of her glasses contained approximately
95 percent cerium (III).
The National Science Foundation and the U.S. Air Force Research Laboratory
supported this work.