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The Laser-driven CVD Process
Free Form Fibers employs advanced laser-driven chemical vapor deposition (LCVD) technology to manufacture high performance ceramic fibers. This new flexible, low cost process is an efficient adaptation of CVD that enables the deposition of solid material from a gas phase in the form of free-standing fibers.
Previously the scale of LCVD-deposited material processing was limited by equipment (laser) size, a low production rate, and poor manufacturing efficiency. Free Form Fibers has overcome these limitations and developed an efficient and environmentally benign production process for continuous, monolithic fibers of almost any material having a CVD gas precursor. The unique characteristics of this manufacturing approach, in particular the compact confinement and efficient utilization of input energy and chemical precursors, lead to low cost, high energy efficiency, a small physical and energy system footprint, and enhanced environmental benefits including little effluent waste, all significant advantages over competing fiber production processes.
The essential LCVD process is simple in concept: imagine a small chamber into which you’ve introduced precursor gases that include the atoms you’re interested in depositing, such as silane and ethylene (as silicon and carbon sources, respectively) in the case of silicon carbide (SiC). Imagine also that you have introduced a fiber into the chamber through a tiny hole in the top. Finally, imagine that you focus a small laser beam on the tip of that fiber through a window in the bottom of the chamber. Even at low laser power (measured in milliwatts) the focused laser energy will heat the fiber tip to the point that the gas near the hot tip will dissociate and the atoms (silicon and carbon in this case) will deposit as SiC on the fiber tip. The fiber will grow towards the laser source and if you pull the fiber out of the reactor at the growth rate, you can pull a solid fiber out of the gas for as long as there is gas present and the laser is on.
This simple technique produces very pure fiber materials and side-steps the performance-robbing impurities associated with previous attempts at producing high performance fiber using the classical spinneret approach. LCVD is low cost, highly effective, and very flexible – to make fiber of a different material you simply change the precursor gases. You can even blend in different elements over the length of a fiber, or change the fiber diameter on the fly during growth.
LCVD is also very fast as compared with the standard CVD process used in computer chip fabrication. Whereas standard CVD film growth rates are measured on the order of 0.1 um/sec, LCVD fiber growth rates are on the order of 0.1 mm/sec or even higher, fully 1,000 to 10,000 times the rate most CVD experts are used to seeing.
The ability to form multiple fiber filaments is possible through the Fiber Laser Printer™ concept using massive optical parallelization. Hundreds of parallel laser beams are focused, each on its own fiber tip, and the process proceeds as with a single fiber. In this case, though, a uniform parallel ribbon of fibers is produced that can be coated with other materials in downstream LCVD reactors.
Multiple coatings can be applied in successive coating reactors with very high precision and uniformity. In fact, multiple coatings were demonstrated on a NASA project by first depositing a boron nitride (BN) coating on SiC fibers, with a subsequent SiC coating over the BN.