A microwave-cavity applicator has been developed for coating multiple fibers by chemical vapor deposition (CVD). A prototype of the applicator was used to deposit silicon carbide onto carbon fibers; the design of the applicator can just as well be optimized for coating fibers made of other materials, and for depositing coating materials other than SiC.
There are two conventional techniques for CVD on fibers. In one technique, a fiber is pulled through a chamber that contains CVD reagent gases. The fiber enters and leaves the chamber through mercury electrodes that also serve as gas seals. The fiber is heated to the reaction temperature by an electrical current applied through the mercury electrode/seals. The disadvantages of this technique are that it is limited to electrically conductive fibers, each reaction chamber can accommodate only one fiber at a time, and the toxicity of mercury poses a hazard. In the other conventional technique, a fiber is heated in a waveguide-type microwave applicator. Although this technique is not limited to electrically conductive fibers and does not involve mercury, it, too, is limited to one fiber per applicator, and the waveguide applicator is intrinsically energy-inefficient because its proper operation depends upon absorption of a substantial portion of the incident microwave power in a dummy load.
The present microwave-cavity applicator overcomes the disadvantages of the applicators used in both conventional techniques: It can be used to coat multiple fibers, the fibers can be electrically conductive or nonconductive, there is no need for mercury, and microwave energy is utilized more efficiently than in the older waveguide/dummy-load microwave-applicator.
The enhancement in energy efficiency is achieved by use of a resonant microwave cavity and by positioning the fibers in the cavity so as to cause only a small deviation of the electromagnetic field from the empty-cavity normal mode while causing the fibers to absorb a large fraction of the microwave power that enters the cavity. The positioning is especially critical for coating fibers of carbon and other lossy materials; typically, such fibers should be placed near an electric-field node.
The figure is a simplified drawing of the applicator. The major structural component is a circular cylindrical microwave cavity. Microwave power is supplied through a coaxial transmission line and coupled into the cavity by a coaxial rod antenna.
The electromagnetic field in the chamber is excited in a close approximation of a TM0N0 mode, where N is a positive integer. (The mode would be purely TM0N0, were it not for the small perturbations introduced by the objects described below.) The TM0N0 mode is axisymmetric; therefore, to expose multiple fibers to identical microwave conditions, one need only take care to position the fibers and other objects symmetrically about the axis of the cavity.
The cavity contains multiple (four in the example of the figure) reaction chambers in the form of tubes made of quartz or other low-loss dielectric material. The reaction chambers are placed at equal angular intervals and nominally at the same radius. In operation, reagent gases are made to flow through the reaction chambers while the fibers are pulled through the reaction chambers and heated to the CVD reaction temperature by the microwave field.
Efficient utilization of the incident microwave power depends critically on good impedance matches from the transmission line through the antenna to the partially filled cavity, then to the fibers. The length of the rod antenna can be adjusted for an impedance match from the transmission line to the chamber. The fibers must be positioned at an optimum radius, which is nearer or farther from an electric-field node, depending on whether the fibers are more or less lossy. Slots in the end plates of the cavity accommodate adjustments in the radial positions of the reaction chambers and thus of the fibers.
This work was done by Henry W. Jackson of Acro Service Corp. and Martin Barmatz and Gordon Hoover of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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Refer to NPO-20458
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Microwave heating of fibers for chemical vapor deposition
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Overview
The document presents a novel approach to coating fibers using microwave heating for chemical vapor deposition (CVD), developed by Martin B. Barmatz, Gordon L. Hoover, and Henry W. Jackson. This innovative technique utilizes a resonant microwave cavity applicator, excited in a TMO1O mode, to economically and efficiently heat and coat fibers, particularly carbon fibers, with silicon carbide (SiC) coatings.
The key innovation lies in the ability to coat multiple fibers simultaneously within the cavity, which significantly reduces manpower requirements and enhances power efficiency compared to traditional CVD methods that typically coat one fiber at a time. In conventional processes, fibers are heated by direct electrical current while being spooled through a chamber containing reagent gases, which is less efficient and more labor-intensive.
The document details the design of the microwave cavity applicator, which features a cylindrical cavity that allows for optimal positioning of the fibers. This positioning is critical, especially for fibers with highly lossy cores, as it ensures that they absorb a large fraction of the microwave power while causing minimal disruption to the cavity's electromagnetic field distribution. The fibers are placed near an electric field node to maximize heating efficiency.
The applicator operates at approximately 2.45 GHz with a power input of around 300 Watts. The fibers, with diameters of about 30 microns and outer diameters of 130 microns, are heated to high temperatures, emitting bright yellow light, which is used to monitor their heating uniformity. An optical system, including a curved glass mirror and a video camera, allows for real-time adjustments to the fiber positions to achieve uniform brightness along their lengths.
The document emphasizes the environmental benefits of this microwave-based method, as it reduces the need for hazardous materials and minimizes energy consumption. The authors highlight the potential for this technology to revolutionize the fiber coating industry by providing a more efficient, cost-effective, and environmentally friendly alternative to existing CVD techniques.
In summary, this innovative microwave cavity applicator represents a significant advancement in fiber coating technology, offering enhanced efficiency and uniformity in the application of coatings, particularly for carbon fibers, while addressing the limitations of traditional methods.
