Researchers developed a new class of ceramics that are so pure and perfectly transparent they can be used as a substitute for crystals in solid-state lasers.
To most people, the word "ceramics" refers to clay flower pots or porcelain tea cups. But not all ceramics block or scatter light. In fact, researchers are developing a brand new class of advanced ceramics that are so pure and perfectly transparent, they can be used as a substitute for traditionally grown crystals in solid-state lasers.
"Ceramics can eliminate most of the inherent defects of melt-grown crystals," said Gary L. Messing, a distinguished professor of ceramic science and engineering at Pennsylvania State University. "That means we may be able to make ceramics with superior optical properties."
Messing heads a research team that recently reported making a 99.999 “perfect” ceramic crystal.
Advanced ceramics have unique mechanical, electrical, optical or thermal properties that make them useful in all sorts of applications. They are already used in catalytic converters, space shuttle tiles, electronic components in desktop computers, and in medical prostheses. But many applications are limited by tiny pores that scatter light and make the material opaque or translucent. Pores can also make the ceramic too brittle and ruin the flow of electrons or heat.
To solve the problem, Messing and his group are developing processes that produce extremely pure ceramics with almost no defects or pores.
To make such dense ceramics, the scientists use synthetic powders, because they are much purer than clays and other materials mined from the earth. "We start with a very fine powder, form that powder into a desired shape, and then heat the formed powder to create a solid, dense body," said Messing.
This heating process, called sintering, happens at temperatures below the material's melting point. So without liquefying the material, sintering allows atoms in the powder to move around and fill in the spaces between the individual grains.
Some scientists predict that properties of advanced ceramics may be enhanced by reducing grains to nanometer size. It turns out ceramic grains grow the same way soap bubbles do. The larger grains—or bubbles—slowly consume the smaller ones.
"If we understand the processes that lead to grain growth better, we can refine the grain size to the nanoscale and maximize mechanical, electrical and optical properties of the ceramic material." said Messing.
To test their new method, Messing and his group experimented with a type of laser important in industrial and military applications. Making to the so-called Nd:YAG crystal the usual way requires temperatures greater than 3,500 degrees Fahrenheit and weeks or months of growing time.
According team member Adam J. Stevenson, switching to ceramic processing could reduce the temperatures by almost 500 degrees and reduce the time to just days.
To make the ceramics, the group started with powders and mixed them with liquids and polymers to make a material similar in consistency to paint. They cast long, thin sheets of the material and cut and stacked them to form thicker squares about 2.5 inches square and a half-inch thick.
After applying heat to sinter the squares, the ceramics became transparent. But the material still contained enough pores to degrade a laser's performance. By combining heat with pressure, the researchers eliminated the few remaining pores.
After the process, a ceramic material looks like a mosaic of tiny crystals, almost like a puzzle, when viewed under a scanning electron microscope. Messing says the grain boundaries are so small, they have virtually no effect on light traveling through the material.
Using techniques to control the positions of the ions inside the ceramic, scientists may be able to create new designs for high power lasers. And ceramic processing could allow complex shaped parts, using extrusion or slip casting, for novel laser designs-something they could never achieve with melt-grown crystals.
"Our goals are to make perfect materials and to lay out the science of transparent ceramics in such a way that it can easily be applied to other systems in the future," Stevenson said.
The research was funded by the National Science Foundation and reported in the Oct. 17, 2008, issue of the journal Science.
—By Holly Martin/NSF
This report is provided by the National Science Foundation, an independent federal agency that supports fundamental research and education across all fields of science and engineering, in partnership with U.S. News and World Report. For more information, go to www.nsf.gov.
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