Figure 1 shows an apparatus for growing a single-crystal fiber by solidification from a floating zone of laser-heated molten material on the tip of a feed rod. The apparatus can be used to produce single-crystal fibers of various highly pure ceramic and metal compositions, controlled crystal orientations, and small, uniform diameters. Such fibers are needed for experimental research on fiber reinforcements for metal-matrix/fiber and intermetallic-matrix/fiber composite materials. Fiber compositions that have been produced thus far include sapphire (Al2O3) with and without ternary additions, ZrO2, and yttrium aluminum garnet (YAG). Typical fiber diameters have ranged from 100 to 250 µm.
Preparation of a feed rod begins with mixing of metallic or ceramic powders with an organic binder. The powders are formulated with a modified stoichiometric composition; that is, the composition is chosen to obtain the desired fiber crystal composition, taking account of anticipated losses of various constituents through differential vaporization from the melt. The mixture of powders and organic binder is extruded to produce the feed rod. The organic binder is typically a commercial water-soluble cellulose ether product formulated to obtain the desired extrusion properties and to vaporize during subsequent laser heating, leaving behind little or no residue.
The feed rod is mounted vertically on a vertical-translation mechanism inside the vacuum chamber. A seed crystal (which could be a piece of previously grown fiber) is placed in the desired orientation by use of x-ray diffraction for measurement and a goniometer for adjustment. The oriented seed crystal is mounted in the desired orientation on the tip of a pull rod that is collinear with the feed rod and is connected to another, independently controllable vertical-translation mechanism inside the vacuum chamber.
The laser beam is split into two beams aimed at the floating-zone melt location from opposite sides. The tip of the feed rod and the seed crystal on the tip of the pull rod are slowly brought toward each other and into the laser-heated floating zone, causing them to begin to melt (see Figure 2). Eventually, the molten tips touch and wet each other. Once a stable molten zone with a relatively uniform temperature profile has been established, growth of a single-crystal fiber can begin.
To effect this growth, the feed rod is translated toward the laser-heated zone at one speed while the pull rod is translated away from the laser-heated zone at a different speed. The ratio between the speeds is chosen to obtain the desired change from the diameter of the feed rod to the diameter of the fiber. Ordinarily, one seeks to produce a fiber narrower than the feed rod, so that the pull rod must be translated more rapidly. The translation can be either downward as in Figure 2, or else upward.
A technique called "melt modulation" is used to maintain stability and symmetry in the molten zone. Melt modulation is effected by optomechanically scanning the opposing laser beams back and forth across the feed-rod/fiber axis to obtain more nearly even heating. Melt modulation gives rise to small vibrations that help to stabilize the molten zone. The vibrations also increase thermal agitation and mixing, thereby helping to make the temperature more nearly uniform throughout the melt. The vibrations also help to shake bubbles out of the melt; without the vibrations, small bubbles tend to coalesce into one large bubble in the molten zone, with consequent disruption of crystal growth. The frequency of vibration can be adjusted to avoid mechanical resonances and minimize vibration of the growing crystal. Typically, the optimum frequency lies between 30 and 50 Hz.
This work was done by Frank Ritzert and Leonard Westfall ofLewis Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Materials category, or circle no. 132 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Lewis Research Center
Commercial Technology Office
Attn: Tech Brief Patent Status
Mail Stop 7 - 3
21000 Brookpark Road
Refer to LEW-16539.