Aluminum metal and silica glass react chemically as they are heated and drawn, producing a fiber with a core of pure, crystalline silicon — the raw material of computer chips and solar cells — and a coating of silica.
It turned out that the chemical reaction in the fiber was a well-known one: At the high temperatures used for drawing the fiber, about 2,200 degrees Celsius, the pure aluminum core reacted with the silica, a form of silicon oxide. The reaction left behind pure silicon, concentrated in the core of the fiber, and aluminum oxide, which deposited a very thin layer of aluminum between the core and the silica cladding.
Now, Hou says, “We can use this to get electrical devices, like solar cells or transistors, or any silicon-based semiconductor devices, that could be built inside the fiber.” Many teams have tried to create such devices within fibers, he says, but so far all of the methods tried have required starting with expensive, high-purity silicon.
“Now we can use an inexpensive metal,” Hou says. “It gives us a new approach to generating a silicon-core fiber.”
“We want to use this technique to generate not only silicon inside, but also other materials,” Hou says. In addition, the team is working to produce specific structures, such as an electrical junction inside the material as it is drawn. “We could put other metals in there, like gold or copper, and make a real electrical circuit,” he says.
Fink adds that this is “a new way of thinking about fibers, and it could be a way of getting fibers to do a lot more than they ever have.” As mobile devices continue to grow into an ever-larger segment of the electronics business, for example, this technology could open up new possibilities for electronics — including solar cells and microchips — to be incorporated into fibers and woven into clothing or accessories.
“Optical fibers are central to modern communications and information technologies, yet the materials and processes employed in their realization have changed little in 40 years,” says John Ballato, director of the Center for Optical Materials Science and Engineering Technologies at Clemson University in South Carolina, who was not involved in this research. He says, “Of particular importance here is that the starting and ending core composition are entirely different. Previous work focused on chemical reactions and interactions between core and clad phases, but never such a wholesale materials transformation.”
Henry Du, a professor of chemical engineering and materials science at Stevens Institute of Technology in Hoboken, NJ, who also was not associated with this research, says “This work is simply beautiful.” He adds that “this new strategy will enable the fabrication of new classes of functional fibers that would otherwise be difficult, if not impossible, using the traditional approach.”
Traditional fibre-optic drawing involves a thermally mediated geometric scaling where both the fibre materials and their relative positions are identical to those found in the fibre preform. To date, all thermally drawn fibres are limited to the preform composition and geometry. Here, we fabricate a metre-long crystalline silicon-core, silica-cladded fibre from a preform that does not contain any elemental silicon. An aluminium rod is inserted into a macroscopic silica tube and then thermally drawn. The aluminium atoms initially in the core reduce the silica, to produce silicon atoms and aluminium oxide molecules. The silicon atoms diffuse into the core, forming a large phase-separated molten silicon domain that is drawn into the crystalline silicon core fibre. The ability to produce crystalline silicon core fibre out of inexpensive aluminium and silica could pave the way for a simple and scalable method of incorporating silicon-based electronics and photonics into fibres.
SOURCES – MIT Technology Review