Inviscid melt spinning: As-spun crystalline alumina fibers
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N. E. Weston Consultant, formerly Micron, Inc., Analytical Services Laboratory, Wilmington, Delaware 19805
S. A. Dunn University of Wisconsin, Department of Chemical Engineering, Madison, Wisconsin 53706 (Received 29 March 1990; accepted 19 July 1990)
Inviscid melt spinning yielded the first crystalline alumina fibers directly from the melts. In this experimental process, a liquid jet having a melt viscosity of 104 for fiberglass) is extruded into propane and is thus chemically stabilized (vs rapidly quenched) before it forms Rayleigh waves and breaks up into droplets. This letter describes a 65.5% alumina-zirconia fiber, an 81.5% and a 90.6% alumina-calcia fiber, a 98.6% alumina-magnesia fiber, and a 100% alumina fiber. The 5-allomorph was identified as the crystalline phase of the melt spun 100% alumina fibers, compared to the a-allomorph reported for FIBER FP, a slurry spun and sintered 100% alumina fiber.
I. INTRODUCTION
A liquid jet of a low viscosity (inviscid) melt has a short lifetime before it forms Rayleigh waves and breaks up into droplets which will freeze to yield shot. Metal jets can be stabilized to form fibers, ribbons, or flakes by either rapid solidification technology or inviscid melt spinning. Ashbrook1 reviewed the rapid solidification processes. These methods and that by Tatsumisago et al.2 are suitable for metals. They use high quench rates (106 to 109 K/s) to stabilize the molten jets before they break up. The resulting fibers or flakes are mostly amorphous and the fibers have ribbon cross sections. In 1978, Cunningham et al? reviewed inviscid melt spinning, a process that uses a chemically reactive medium (propane, O2, H 2 S, or NH 3 ) but moderate quench rates (102 to 104 K/s) to stabilize the molten metal jets (Fig. 1). The resulting metal fibers are crystalline and have round cross sections. Recent advances4 reflect renewed interest in inviscid melt spinning of metal fibers using oxygen as the chemically reactive medium. An inviscid melt spinning process was fully developed for metal fibers, but an attempt was also made to spin a range of 22 compositions with 50.5 to 100% alumina.3 In mostly single experiments, the oxide powders were entered into graphite or tungsten crucibles and placed in a furnace tube. The surrounding air was replaced with argon. The powder was gradually heated to the spin temperature. The melt viscosities at this tem-
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http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 11, Nov 1990
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perature can be deduced from the literature5 to range from 8.7 to 0.5 poise, respectively (versus >104 for fiberglass). Prior to spinning, argon was reintroduced both above and below the crucible. Propane was then introduced below the crucible and the argon pressure was increased above the melt to extrude a jet from an orifice in the crucible bottom into gaseous propane at 1 atm pressure. The pyrolytic reaction which chemically stabilized the liquid alumina jets will be discussed in a separate paper. A
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