Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
1998-07-08
2002-11-26
Hoffmann, John (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C501S035000, C501S036000, C501S152000, C428S364000
Reexamination Certificate
active
06484539
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to novel methods for drawing fibers from liquid melts, including those composed of materials whose liquid state viscosity at the melting temperature is normally too low to accommodate fiber drawing operations. The invention also pertains to methods of drawing fibers from a melt while preventing recrystallization of the melt. Further, the invention pertains to methods of controlling heat transfer at melt surfaces so that a portion of the melt is undercooled to a temperature below the equilibrium melting temperature, and fibers can be drawn from the undercooled portion of the melt. The invention also relates to the production of novel fibers including but not limited to fibers of glass and crystalline materials, glass fibers formed using the methods of the invention, and fibers of high tensile strength compared to fibers of the same composition which are currently commercially available.
BACKGROUND OF THE INVENTION
Melt-Drawn Fibers
The drawing of fibers from liquid melts is well known in the art as an inexpensive method of fiber synthesis. This process is possible if the liquid viscosity is (1) sufficiently high so that tensile forces overcome the surface tension forces of the liquid during the fiber drawing process, and (2) sufficiently low so that the tensile forces induce liquid flow into a thin fiber rather than bulk flow of the liquid.
Drawing is widely used to make fibers from materials that form viscous melts, such as but not limited to oxide compositions that contain a high concentration of silicon dioxide and polymeric materials. The silica glass fibers and polymeric fibers have considerable economic utility as, for example, thermal insulation material, components in composite materials, use in textiles, and for many other applications. Fiber-drawing is used to make glass fibers for applications such as, for example, fiber lasers and fiber-based optical devices. These glass fibers typically contain small concentrations of optically-active dopant elements which are added to the host glass, such as neodymium (Nd) in Nd-glass lasers and erbium (Er) in Er-glass lasers. The magnitude and the uniformity of the dopant concentration in the fibers are limited, however, by the solubility and diffusivity of the dopant oxides in a preform of the host glass material during the doping operation that occurs prior to fiber drawing from the preform.
In some cases, mixtures of several pure materials yield melts from which fibers can be drawn. For example, fluoride glass fibers are made by melt drawing from mixtures of several metal fluorides that exhibit a low melting temperature, thereby forming relatively viscous melts. Fluoride glass fibers provide optical transmission outside the bandwidth of silica-based fibers, and are of interest for applications such as fiber-laser and infrared waveguide applications. However, considerable difficulties generally attend the manufacture and use of most fluoride fibers, such as brittleness, moisture sensitivity and toxicity. In addition, many mixtures of fluoride materials or pure liquid fluorides have insufficient equilibrium melting temperature viscosities for fiber drawing operations. Improved alternatives to these prior art fluoride fibers are needed. As used herein, the “equilibrium melting temperature”, “or the melting temperature” of a melt is the temperature at which at equilibrium, all of the melt components of a system are substantially liquid.
It is the temperature, at the prevailing ambient pressure and for a system whose chemical composition is equal to the chemical composition of the melt of interest, for above which no crystalline phases occur in a system at equilibrium.
Oxide glass fibers are often made from mixtures of silicon dioxide, boron oxide, sodium oxide, and other additives, which mixtures melt at temperatures much lower than the melting temperature of pure silica and result in melts sufficiently viscous for the drawing of fibers.
Fibers known as chalcogenide glass fibers can also be drawn from mixtures of elements such as germanium, arsenic, antimony, selenium, tellurium, and others which form viscous, low melting temperature liquids. Chalcogenide glass fibers have application in the transmission of infrared radiation. However, many chalcogenide materials have equilibrium melting temperature viscosities which are too low to accommodate fiber drawing operations.
Prior art methods also limit the concentrations of additives that may be achieved in glass fibers. For example, optically-active additives for fiber laser and fiber laser amplifier applications are generally introduced into pre-formed host glass fibers by heating the host fiber in the presence of the additive materials, which are generally applied to the outside of the drawn fiber (such as, for instance, by spraying). With this method, the additive concentration in the fiber is limited to the equilibrium solubility of the additive material at the heating temperature. Fibers made by this process, however, suffer from the drawback that the heating temperature, and therefore the additive concentrations, are limited by crystallization of the host fiber.
Drawn or extruded fibers are also used as precursor fibers in chemical or physical processes that change the fiber material into a different chemical form or physical state. For example, sol-gels formed from metal-organic chemicals can be drawn or extruded into fibers of an amorphous material that is subsequently heated to decompose the organic fraction of the fiber and produce polycrystalline oxide fibers. Silicon-containing organic polymer materials can be drawn into fibers and subsequently decomposed to form silicon carbide fibers. Organic polymer fibers made from polyacrylonitrile (PAN) are heated and decomposed at high temperatures to obtain carbon fibers. Pitch compositions obtained from hydrocarbon or coal tars can be drawn into fibers and subsequently decomposed at high temperatures to obtain carbon fibers. Polycrystalline oxide fibers of various materials such as zirconia-silica materials, alumina-boria materials, alumina-silica materials, and yttria-alumina materials have been made from precursor fibers that are formed by drawing or extrusion processes.
Prior art methods of fiber manufacture include drawing fibers from undercooled melts. As used herein, an “undercooled” temperature refers to a temperature which is below the melting temperature of the combined melt components. For instance, the melting temperature of aluminum oxide is 2050 degrees C. An undercooled melt of aluminum oxide would be a liquid melt held at a temperature below this. Fibers containing one or more of calcium oxide, aluminum oxide, silicon dioxide, magnesium oxide, and/or barium oxide have been drawn by conventional means utilizing partial undercooling by melting the starting materials in a platinum crucible or by melting the end of a rod of the starting material, removing the source of heating allowing the melt to cool, contacting the undercooled surface of the liquid with a glass rod, and manually drawing a fiber from the undercooled surface of the liquid by withdrawing the glass rod and attached fiber. Alternatively, the center section of a rod of the starting material is melted, the heating source is removed, and a fiber is formed when the two ends of the rod are manually drawn apart. Reportedly, fibers could be drawn using these methods from melts that contained a maximum of 46.2% aluminum oxide by weight, which is equivalent to a maximum of 35.3 molar % of aluminum oxide, Al
2
O
3
. However, it is reported that melts which contain more than 50% aluminum oxide by weight have much lower viscosities, and fibers of these higher-alumina composition materials could not be drawn from melts using these prior art techniques, even when the drawing portions of the melts were undercooled.
The methods described above are limited in part by the fact that the melt is in contact with a solid rod of the same material or with a platinum container. As the melt cools, crystals propagate from the solid/liq
Felten John J.
Nordine Paul C.
Weber J. K. Richard
Bullwinkel Partners, Ltd
Containerless Research, Inc.
Hoffmann John
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