US3811940A

US3811940A - High frequency heating method for vapor deposition of coatings onto filaments

Info

Publication number: US3811940A
Application number: US00225350A
Authority: US
Inventors: F Douglas; T Gregory; R Stielau
Original assignee: United Aircraft Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1969-10-09
Filing date: 1972-02-10
Publication date: 1974-05-21
Anticipated expiration: 1991-05-21

Abstract

A method for the pyrolytic deposition of a coating on a filament axially moving through a pyrolytic vapor deposition chamber comprising establishing a radio frequency electric field to heat a selected length of the filament and cause deposition thereon and regulating the density of the electric field to maintain a temperature profile of desired uniformity along the selected length.

Description

Unite States Patent Douglas et al.

1451 May21, 1974 K. Gregory, East Granby; Robert W. Stielau, Portland, all of Conn.

Assignee: United Aircraft Corporation, East Hartford, Conn.

Filed: Feb. 10, 1972 Appl. No.: 225,350

Related US. Application Data Continuation of Ser. No. 865,157, Oct. 9, 1969, abandoned.

[56] References Cited UNITED STATES PATENTS 3,410,7l5 ll/l968 Hough ll7/23l 3,572,286 3/I97l Forney ll7/l06 3,607,063 9/197] Douglaset al. 2l9/l0.6l 3,661,639 5/l972 Caslaw 117/23] Primary Examiner-Alfred L. Leavitt Assistant Examiner-J. Massie Attorney, Agent, or Firm-John D. Del Ponti [5 7] ABSTRACT A method for the pyrolytic deposition of a coating on a filament axially moving through a pyrolytic vapor deposition chamber comprising establishing a radio frequency electric field to heat a selected length of the Us Cl 117/228 117/933 117/106 filament and cause deposition thereon and regulating l t Cl 3 the density of the electric field to maintain a temperan t f d d 'f l t l t Field of Search 117/106, 107, 9313, 93.2; 2: 2, 6 0 cm um y aong he-se ec ed 219/1055, 10.61

9 Claims, 2 Drawing Figures /Z /i I /flj area, Z

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PATENTEU MY 2 1 i974 HIGH FREQUENCY HEATING METHOD FOR VAPOR DEPOSITION OF COATINGS ONTO FILAMENTS This is a continuation of application Ser. No. 865,157, filed Oct. 9, 1969, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to means for treating filamentary material and more particularly relates to means for continuously heating moving filamentary material by radio frequency energy.

In recent years, considerable effort has been expended in the preparation of low density, high modulus fibers for use as reinforcement materials in lightweight composite structures. In particular, filaments of boron, produced by a method which involves the chemical reduction of a boron halide onto a direct current resistively heated tungsten substrate, have found wide application in meeting the stringent demands of the aerospace industry. A suitable system for vapor depositing boron coatings by direct current heating is shown for example in copending U.S. application Ser. No. 618,511 filed Feb. 24, 1967, by Rice and sharing the same assignee as the present application.

Although direct current heating systems have proven effective for some applications, they have proven limiting or ineffective for others. In particular, in continuous processes wherein conductive filaments are passed through a reaction chamber containing the vaporized coating material, it is known that during vapor deposition the traversing resistively heated wire changes impedance and causes an uneven dissipation of power therein. The resulting temperature variations in the wire lead to disparate deposition rates along a length of the fiber. In some cases, such as in the deposition of titanium diboride, the impedance change is so drastic as to cause the fiber to be vaporized. In other cases, such as in the continuous production of fibers having electrically insulating coatings, as for example, coatings of aluminum oxide, electrical contact with the fiber is cut off as soon as the insulating coating has formed.

Although in recent years a variety of compensating techniques, such as reactor staging or the incorporation of independent heat dissipation means, have been relatively successful in minimizing temperature variation in certain coating applications, notably boron over tungsten, several unattractive features have remained. It is known for example, that even in those processes wherein quality .composite fibers are producible in conjunction with the direct current heating mode, those fibers produced possess attractive characteristics only up to a maximum diameter. Typically, for example, when boron is vapor deposited on V2 mil tungsten to effect a diameter greater than approximately 4 mils, the fiber experiences substantial tensile strengths up to 4 mils with significant increasing losses thereafter. Additionally, in a similar process wherein boron is deposited on a resistively heated 1 mil carbon monofilament, a maximum diameter greater than approximately 2.2 mils has not been obtainable without a concomitant loss of uniformity and the generation of periodic nodes of increased diameter. These nodes appear to be crystalline; and hence, represent weak spots in the filament.

In addition to the limitations on the kinds of coatings and substrates useable in a DC heating system, as well and toxic effects of mercury, as well as presenting handling difficulties during filamentary threading of the reactor.

SUMMARY OF THE INVENTION The present invention relates to the treatment of filamentary materials, particularly for pyrolytic deposition techniques, in an improved process and apparatus whereby filaments can be heated in substantial disregard of their level of resistivity. It contemplates means for exercising close temperature control over a wire which is otherwise prone to overheating due to changes in wire composition or character.

In accordance with one aspect of this invention a suitable. wire, such as tungsten, carbon, or the like, is drawn through a reactor containing a decomposable material-containing gas, such as a boron halide admixed with hydrogen. A selected length of the wire is heated by energy in the radio frequency range to create a hot zone having a temperature sufficiently high to effect material deposition thereon as it passes through the reactor. The RF energy is transmitted to the fiber by an RF energy coupler which is designed in general, for control of hot Zone characteristics and, in particular, to compensate for impedance changes and obtain a desirable power distribution along a selected length of the fiber. The coupler is impedance matched to both the fiber and the oscillator and further, is tuned to the frequency of the oscillator to maintain a hot zone in the selected length of desired temperature profile to effect rapid and uniform deposition on the fiber.

By means of the present invention, a simple and inexpensive process has been discovered which not only overcomes the maximum diameter-fiber degradation problems associated with prior art deposition processes but also vastly expands the rangeof materials useable as coatings and substrates. The present process has for example, been successful in depositing boron on A mil tungsten to a final diameter of over 15 mils with tensile strengths of 375,000-400,000 psi. Further, boroncarbon fibers having uniform diameters greater than 4.0 (up to 6.8 mils) mils have been produced without the desirable nodal effect. Additional applications of the invention include the satisfactory production of aluminum oxide and titanium diboride fibers. It will be appreciated that, as a consequence of the teachings herein, vapor deposition of materials onto a filamentary substrate is achieved in a manner heretofore unknown.

BRIEF DESCRIPTION OF THE DRAWINGS An understanding of theiinvention will become'more apparent to those skilled in the art by reference to the following detailed description when viewed in light of the accompanying drawings, wherein:

FIG. 1 is an elevational view, in section, of apparatus employing a quarter wavelength, phased, tuned coaxial cavity coupler; and

FIG. 2 is an elevational view, in section, of apparatus employing a modification of the quarter wavelength tuned coaxial cavity coupler shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, wherein like numerals indicate like parts, an apparatus suitable for carrying out the method is schematically shown in FIG. 1, which is a view partly'in section.

The apparatus comprises a vertical reaction chamber 10, which may be made of glass or quartz, fitted at both ends with appropriate closure means 12 through which suitable gas inlet 14 and outlet 16 pass for communication with the chamber 10. The closure means 12 permit axial passage ofa wire substrate 18 through the reactor while containing suitable fluid, such as an inert gas or liquid mercury or the like, to seal it from the atmosphere. Although wire traverse is depicted as downward in the drawings, direction of movement is not critical and upward movement will produce satisfactory results.

The reaction chamber is received within a quarter-wave phased, tuned coaxial cavity coupler 20 which actually comprises upper and lower coaxial cavities 22 and 24 respectively in symmetrical relation. Each coupler includes an outer tubular, electrically conductive, enclosure 26, a quarter wavelength long, which supports, by annular flange 28, an inner cylindrical inductortube 30, also a quarter wavelength long. As shown in the drawing, power is fed in by a coaxial lead from a radio frequency power source 32 capable of generating frequencies within the range of l to 500 megahertz (MHz). A variable capacitor 34 is provided between the enclosure 26 and tube 30 for tuning purposes and it will be appreciated that impedance matching may be accomplished by selective placement of the input tap from the power source 32 and proper tuning of the variable capacitor 34. The phased coaxial cavity coupler has been found to be extremely advantageous in meeting a wide range of requirements for various fibers and it is believed that the coaxial cavity concept is primarily responsible for the dramatic results achieved since such a configuration is basically independent of the fiber characteristics while producing a high density, axial oscillating electric field. By placing two such configurations in line with the proper phasing to obtain a phased, coaxial cavity, there is created a highly uniform axial electric field distribution of high density.

Tuning of the upper and lower cavities provides a means of varying the spatial distribution of the electric field in the chamber 10 so that hot zones can be obtained and shaped in the coaxially located fiber. The length of the cavity section determines the operating frequency while the distance between upper and lower cavities determines the length of the hot zone to a maximum value dependent on operating frequency and elwtrisdc nsluct v ty h fiber sfiset d- Qf was operated .for boron depositlon with an eleven inch course, since the field is spatially distributed, no mechanical contact with the fiber is required to set up the electric field within the fiber.

As stated hereinbefore, the generation of heat in a fiber by the RF power depends upon the conversion of electromagnetic energy to thermal energy through the electrical resistance of the fiber material. Placing the fiber in a region where an oscillating axial electric field has been established produces oscillating currents in the fiber which dissipate the RF power as heat according to: power per unit length FR, where I is the rms value of the current due to the RF electric field, and R is the resistance per unit length of the fiber.

In order to prevent the end heating efiect and thus confine the energy to a selected area in the fiber, the outer end of each outer enclosure 26 is provided with a resonant energy trap 36, tunable by a variable capacitor 38. In essence, the resonators present avery low impedance, thus reflecting escaping energy back into the hot zonef Each cavity 22, 24 provides impedance matching between its oscillator 32 and the fiber 18 while being tunable to the frequency of the oscillator.

During an investigation of the coupler shown in FIG. I, using a k mil tungsten substrate with a gas flow of 2,000 cc/min. (35 percent BCI and percent H the apparatus was operated for boron deposition with a 28 inch hot zone over which temperature variations were less than i 50C. Data is given in the following table:

TABLE I With the same conditions as above, the apparatus by the former by means of flange 46. Power source 48,

variable capacitor 50 and'end decouplers 52 are located as hereinbefore described.

During aninvestigation of the single cavity system at 148 MHz, using a k mil tungsten substrate with a gas flow of 2,000 cc./min. (35 percent BCI 65 percent H a ten inch hot zone was maintained and filamentary boron was produced according to the following table:

Photomicrographs of etched cross sections of the large diameter fibers produced using the apparatus of FIG. 2 did not show a cone structure in the outer portion of the fiber which is typical for comparable boron fibers produced by conventional direct current heating techniques. The RF heated fiber showed a very uniform boron deposit, which may account for the high strengths attained in the large diameter fibers.

It will be appreciated that the procedures abovedescribed are effective to transmit the RF power to the fiber for heating purposes. They have been designed and tested for impedances varying from to 2,000 ohms per inch. Although no quantitative measurements have been made of efficiency in terms of power input converted to heat in the fiber, measurements have been 'made where input and reflected energy are compared.

' tantalum, or metalloids, such as silicon, boron and the like; or carbon. The chemical compounds used in depositing the above elements will typically comprise halides, e.g., chlorides, fluorides, iodides, or bromides of the described metals or metalloids.

A reducing gas such as hydrogen will also ordinarily be included as part of the reactant gas or vapor mixture 4 when halides of the metals or metalloids are employed although inert gases, such as helium, neon, argon, krypton, xenon and the like may also be included, if desired.

Chemical compounds as distinguished from elements, which may be deposited, include nitrides, carbides, oxides, phosphides, borides and sulfides of such elements as silicon, titanium, zirconium, aluminum and the like. Typical of such compounds are titanium diboride, titanium nitride, aluminum oxide, zirconium carbide and the like. To deposit the described chemical compounds, vapors of phosphorous, sulfur, oxygen, carbon, boron, nitrogen and the like, or compounds of such elements, will be included as part of the reactant gas stream.

Aluminum oxide has been deposited on a 1% mil tungsten substrate utilizing aluminum chloride and carbon dioxide with a substrate temperature of 1,200C, and an ambient pressure at less than atmospheric (of the order of 25 inches of water, absolute). Silicon carbide and titanium diboride were also deposited on V2 mil tungsten substrates.

Any suitable wire or filament whether electrically conductive or not, may be used as a substrate for deposition purposes or simply for heat treatment, as in the case of carbon. For example, the wire or filaments may be any of the elements or compounds mentioned above or glass coated with the same and the like. Typical substrate filaments may be composed of tungsten, silicon, silicon carbide, boron, carbon, etc.

Certain process modifications are recognized as being useful in the present invention. It is recognized for example that multistrand filaments or a plurality of separate filaments can be simultaneously passed through a reactor.

While the present invention has been described with reference to particular materials, embodiments and operating techniques, it will be understood that these examples are illustrative only and that alternative materials, arrangements and operating conditions than those already mentioned will be evident to those skilled in the art. Accordingly, the true scope of the invention will be measured, not by the illustrative material, but rather in the spirit of the invention, by the appended claims.

What is claimed is:

1. In those processes for producing composite fibers by vapor depositing material from a materialcontaining decomposable gas on a moving wire heated by electrical power dissipation therein wherein the impedance of the wire changes during deposition, the improvement which comprises moving said wire sequentially past a plurality of phased electromagnetic couplers each connected to radio frequency source power means to electromagnetically couple said wire to said radio frequency source means and thereby to expose the wire to an axial oscillating electric field having an adjustable density to cause current flow in the wire, said wire having sufficient resistance to dissipate the applied power to cause heating and deposition of said material thereon, and varying the density directly with the impedance changes in the wire to maintain a preselected temperature profile therein.

2. A method for the pyrolytic deposition of a coating upon a filament comprising:

axially passing the filament through a pyrolytic vapor deposition chamber and at least two coaxial cavity coupling means;

establishing a radio frequency electric field-of variable density in said deposition chamber by applying radio frequency power through each of said coupling means to cause current flow in the filament, said filament having sufficient resistance to dissipate the applied power to cause heating of a selected length of the filament and deposition thereon as it passes through said deposition chamher; and

regulating the density of said electric field by tuning the plural cavity coupling means to maintain a substantially uniform temperature .profile along said length.

3. The method according to claim 2 wherein the electric field of variable density is established by generating a radio frequency electric current by at least two power source means, each of said power source means being electrically connected to one of said coaxial cavity coupling means so as to provide a separate radio frequency electric current to each coupling means.

4. The method according to claim 3 wherein each of said coaxial cavity coupling means comprises a tuneable coaxial cavity coupler one quarter wavelength long.

5. The method according to claim 4 wherein the density of the. radio frequency electric field is regulated by varying the distance and phasing between the cavities, impedance matching each coupler to its power source means and the filament, and tuning each coupler to the frequency of its power source means.

6. The method according to claim 5 wherein the coating being pyrolytically deposited is boron and the selected length of filament is heated to within the range of 1,000 to 1,300C i 50C.

7. The method according to claim 6 wherein the filament is tungsten.

Claims

2. A method for the pyrolytic deposition of a coating upon a filament comprising: axially passing the filament through a pyrolytic vapor deposition chamber and at least two coaxial cavity coupling means; establishing a radio frequency electric field of variable density in said deposition chamber by applying radio frequency power through each of said coupling means to cause current flow in the filament, said filament having sufficient resistance to dissipate the applied power to cause heating of a selected length of the filament and deposition thereon as it passes through said deposition chamber; and regulating the density of said electric field by tuning the plural cavity coupling means to maintain a substantially uniform temperature profile along said length.
3. The method according to claim 2 wherein the electric field of variable density is established by generating a radio frequency electric current by at least two power source means, each of said power source means being electrically connected to one of said coaxial cavity coupling means so as to provide a separate radio frequency electric current to each coupling means.
4. The method according to claim 3 wherein each of said coaxial cavity coupling means comprises a tuneable coaxial cavity coupler one quarter wavelength long.
5. The method according to claim 4 wherein the density of the radio frequency electric field is regulated by varying the distance and phasing between the cavities, impedance matching each coupler to its power source means and the filament, and tuning each coupler to the frequency of its power source means.
6. The method according to claim 5 wherein the coating being pyrolytically deposited is boron and the selected length of filament is heated to within the range of 1,000* to 1,300*C + or - 50*C.
7. The method according to claim 6 wherein the filament is tungsten.
8. The method according to claim 6 wherein the filament is carbon.
9. The method of claim 5 wherein the coating being pyrolytically deposited is aluminum oxide and the selected length of filament is heated within the range of 1,000* to 1,300*C + or - 50*C.