ZBLAN continues to show promise
ZBLAN continues to show promise
February 5, 1998: Thin fibers of an exotic glass called ZBLAN are clearer when made in near weightlessness than on Earth under gravity's effects, according to a researcher at NASA's Marshall Space Flight Center.
"What I found is that when you heat the fiber up to its crystallization temperature in 1-g [1 gravity, on the ground], it crystallizes very rapidly, in about 20 seconds," said Dr. Dennis Tucker, a materials scientist in Marshall's Space Sciences Laboratory. "In low-g conditions at the crystallization temperature on the KC-135, fibers did not crystallize."
ZBLAN glass fibers are valuable for advanced communications, medical, and manufacturing technologies using lasers. The most widely used optical fibers are made from silica and have a number of limitations, including the narrow optical "window," the small band of wavelengths they transmit.
Tucker's work on exciting results from earlier aircraft and rocket tests as he develops Space Shuttle experiments scheduled for 2000 and 2001. Space-based processing holds the promise of helping us understand how to make ZBLAN fibers without crystallizing, the main roadblock in their wider use today.
His results were recently published in the Journal of Materials Research.
The image at right shows the surfaces of 0-g and 1-g ZBLAN fibers (links to 1003x1531-pixel, 400K JPG).
The research has drawn the participation of the Bell Labs division of Lucent Technologies, Infrared Fiber, and Galileo Corp. NASA/Marshall also is working with the Center for Materials Development in Space at the University of Alabama in Huntsville.
ZBLAN is part of the family of heavy-metal fluoride glasses. Ordinary glass is based on silica, molecules of silicon dioxide (like sand or quartz), plus other compounds to get different qualities (most eyeglasses, though, are made of special plastics). ZBLAN is fluorine combined with metals: zirconium, barium, lanthanum, aluminum, and sodium (Zr, Ba, La, Al, Na, hence the name).
Most glass-making research has focused on the silica family, partly because it is easiest to make, especially for optical fibers that carry large volumes of information transmitted by lasers. Silica is good at transmitting visible light reasonably well (hence its use in lenses and windows), is moderately good with near-infrared light, and turns black in the deeper infrared spectrum.
For about 20 years, optical scientists have known those exotic blends like ZBLAN can transmit better than silica glass. In fact, a perfect ZBLAN glass should transmit light near the theoretical best allowed by matter.
"This stuff transmits from near-ultraviolet to near infrared," Tucker explained. Further, its attenuation coefficient (left) - a measure of how much of a signal it absorbs - can be as low as 0.001 decibels per kilometer (dB/km) of fiber, far less than silica's 0.2 dB/km. However, production problems now limit the best ZBLAN fibers to a noisy 10 dB/km or higher.
The graph at right illustrates ZBLAN's potential performance. In this case, lower is better (links to 1225x777-pixel, 256K JPG).
The challenge has been getting to the theoretical minimum absorption (that is, the minimum loss of signal). ZBLAN tends to crystallize, so long stretches cannot be made for communications fibers. Even when short stretches are made for other purposes, internal crystals act as partial mirrors, reflecting some of the light and bending the rest. Even a few crystals inside an optical fiber can seriously degrade its performance.
Sophisticated taffy pull
So, unlike NASA's other microgravity experiments, the objective with ZBLAN is to prevent crystallization. Scientists want to find a way to make ZBLAN as an amorphous material, something without an internal shape.
Optical fiber manufacture is a highly sophisticated taffy pull, as much art as science. A cylindrical block of material - called a preform or boule - is heated until it softens and starts to sag. A glass rod touches the end of the preform and pulls until a long thin thread is constantly emerging from the preform. The fiber cools, is coated with to keep it from scratching itself, and is wound on a large spool.
The whole operation is a delicate balancing act to pull the glass at just the right speed, and to cool it just fast enough, so that the fiber thickness is consistent.
Silica glass is comparatively easy to use because it has a wide gap between its glass transition temperature and its crystallization temperature. ZBLAN has a narrow gap, only 124 deg C.
Tucker and his colleagues believe that on Earth, gravity causes small convection cells. The heavier molecules - zirconium fluoride and lanthanum fluoride - sink and the lighter ones - barium fluoride, aluminum fluoride, and sodium fluoride - rise. As like molecules concentrate, they link up to form crystal seeds.
Earlier experiments aboard an unmanned rocket and aboard NASA's KC-135 low-g aircraft have yielded encouraging results. Short ZBLAN fibers inside a quartz ampoule were heated to 400 deg. C and then quickly cooled. These tests were run aboard the Consort 1 rocket flight, sponsored by the University of Alabama in Huntsville, in 1996 and aboard the KC-135 in 1997.
When examined by a scanning electron microscope, fibers processed on the ground resembled tree bark (right; links to 1953x1541-pixel, 448K JPG). Those processed in low-g, though, show no signs of crystallization.
To expand on this work, Tucker designed and built a fiber-pulling apparatus that could be operated aboard the NASA KC-135 low-g aircraft. A preform - which can cost about $2,000 for a unit half the size of a large writing pen - was heated, and during the low-g periods at the top of the aircraft's trajectory, Tucker would pull fibers. He see the fibers coming out clear during the low-g arc and, as the aircraft started its pull-up maneuver, see them turn milky white, a sign of crystallization.
Initial post-flight tests show that the low-g fibers are remarkably clear, Tucker said. The picture at far left shows a defect-free ZBLAN fiber pulled during a low-g arc aboard the KC-135; at right is a crystallized fiber pulled from the same apparatus under 1-g (links to 1053x1179-pixel, 591K JPG). More flights are planned aboard the KC-135 to run tests where a laser shines through the preform and fiber to measure changes as they occur.
Next step, space
The next step is to fly a test models of two experiments planed for the shuttle. On the first, aboard STS-107 in the fall of 2000, Tucker will fly an apparatus to process eight ZBLAN preforms (another eight will be processed on the ground for comparison). Each will be heated to 800 deg. C for 5 minutes to dissolve any crystals, cooled at 15 deg. per minute to get below the crystallization temperature, then held for 5 minutes to let the remaining heat anneal or cure stresses induced by the rapid quenching until they reach the glass transition temperature. After the mission, the preforms will be pulled into fibers to be used in tests to determine whether orbital preparation alone is enough to ensure production of clear fibers.
"It may be that we don't need to do fiber pulling in space," Tucker said. "It may be enough to just send several hundred of these up at once, heat and anneal them, and then pull the fibers on Earth."
Still, a second experiment is being developed to pull a 2 km fiber from a single preform on a shuttle flight in 2001.
For more information, two recent papers are:
'Study of the Effect of Gravity on ZBLAN Glass as a Commercial Program." Gary L. Workman and Dennis Tucker. AIAA 36th Annual Aerospace Sciences Meeting, Jan 12-15, 1998. AIAA 98-0812.
"Effects of gravity on processing heavy metal fluoride fibers." Dennis S. Tucker, Gary L. Workman, and Guy A. Smith. Journal of Materials Research, 12:9, 2223-2225, September 1997.
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