US3872278A

US3872278A - Method for heat treatment of substrates

Info

Publication number: US3872278A
Application number: US389502A
Authority: US
Inventors: Abraham A Boom
Original assignee: Celanese Corp
Current assignee: SUBJECT TO AGREEMENT RECITED SEE DOCUMENT FOR DETAILS; BASF SE; BASF Corp
Priority date: 1971-09-30
Filing date: 1973-08-20
Publication date: 1975-03-18
Anticipated expiration: 1992-03-18

Abstract

A method for the heat treating of substrates in which the substrate is passed through a passage surrounded by a gas impervious envelope and in which a high frequency electrical signal is applied to an electrode exteriorly to the gas impervious envelope and to the substrate so that a gaseous plasma is generated within the envelope but not within the central passage.

Description

United States Patent 1191 Boom 1 1 Mar. 18, 1975 METHOD FOR HEAT TREATMENT OF 3.090.737 5/1963 Swzlrtz 219/1081 x 3,146,336 8/1964 Whitucre 219 121 P SUBSTRATES I 3,182,982 5/1965 Ruff 219/155 X Inventor: Abraham Boom, Martmsvllle, 3,203,768 8/1965 Tiller ct 111. 219/1043 X NJ. 3,383,163 5/1968 Menilshi 219/121 P X 3,405,301 10/1968 H1 ilkllwil et a1. 315 111 X [73] Asslgneei Celanese Corlmmtmn, New York 3,571,551 3 1971 OQZSZIWLII'U et 111 219/1061 X 3,572,286 3/1971 FOI'I'ICy 219/1061 X 3,636,300 l/1972 Gunnell ct 211. 219/121 P [22] 1973 3,671,195 6/1972 Bersin 315/111 X [21] Appl. No.: 389,502

Related us. Application 1361a 2'1"? gf 'l f c g fl [62] Division Of Ser. N6. 185,014, Sept. 30, 1971, Pat. eterso [57] ABSTRACT [52] US. Cl 219/121 P, 315/204 A method f heat treating f substrates in which [511 1111. C1. 823k 9/00 the Substrate i passed through a passage Surrounded [58 Fleld 0f Search 219/121 P, 121 R, 155, by a gas impervious envelope and in which a high 219/1043 10611 10811 204; quency electrical signal is applied to an electrode ex- 315/111, 108; 313/158, 157, 162, 3 teriorly to the gas impervious envelope and to the substrate so that a gaseous plasma is generated within the 156] References C'ted envelope but not within the central passage.

UNlTED STATES PATENTS 6 Cl 5 D F 2,282,317 5/1942 136116611 219/155 X raw'ng R.F. SOURCE PATENTEDNARI 8 1975 RF. SOURCE METHOD FOR HEAT TREATMENT OF SUBSTRATES This is a division of application Ser. No. 185,014, filed Sept. 30, 1971-, now US. Pat. No. 3,780,255.

BACKGROUND OF THE INVENTION The present invention relates to a method and apparatus for highly concentrated electrical heating and more specifically to a method and apparatus for efficiently heating a substrate through the generation of a plasma in the vicinity of the substrate.

It is often desirable to heat various substrates at elevated temperatures to obtain desired substrate characteristics or to aid in the coating of the substrate. For example, in the manufacture of carbonaceous fibrous materials, carbon graphite fibers may be treated at elevated temperatures to modify the surface or overall characteristics of the fiber. I

In the past, substrates have been heated in various manners to provide the desired modification of the substrate characteristics. For example, resistance heating, i.e., passing an electrical current through the fiber, has been frequently used to obtain the elevated temperatures required. However, the current flow and therefore the cost of heating fibers by resistance heating may necessarily be excessively high in order to reach the temperatures required.

Other conventional electrical methods for heating substances may include indirect heating through the use of resistively or inductively heated elements in an oven or other enclosed or semi-enclosed space. The efficiency of these methods may also suffer due to the necessity of heating the element from which heat is transferred to the substrate.

It is accordingly an object of the present invention to provide a novel method and apparatus for electrically generating high temperatures.

It is another object of the present invention to provide a novel method and apparatus for generating high temperatures in a relatively confined heating zone for the treatment of substrates. It is a further object of the present invention to provide a novel method and apparatus for electrically heat treating substrates wherein the substrate is heated through a combination of direct and indirect heating, for example, radiantly, resistively, inductively and through conduction.

It is yet another object of the present invention to provide a novel balun output transformer structure for selectively coupling RF power to the heating chamber.

These and other objects and advantages of the present invention will become apparent to one skilled in the art to which this invention pertains from a perusal of the following detailed description when read in conjunction with the appended drawings.

THE DRAWINGS FIG. 1 is a schematic representation of a heating chamber constructed in accordance with the principles of the present invention;

FIG. 2 is a view in cross section of the heating chamber of FIG. 1, taken along the line 2-2;

FIG. 2A is a schematic representation of a second embodiment of a heating chamber constructed in accordance with the principles of the present invention;

FIG. 3 is a functional diagram of the RF source of FIG. 1; and,

2 FIG. 4 is a perspective view of the output transformer of FIG. 3.

DETAILED DESCRIPTION Referring to FIGS. 1 and 2 wherein a preferred embodiment of the heating chamber constructed in accordance with the present invention is illustrated, a plasma chamber 10 is formed within a central passage 14 extending into a substantially gas impervious, generally electrically nonconductive or insulative envelope 12. The substrate to be heated provides a central electrode 16 which extends through the central passage 14 and is isolated from the chamber 10 by the radially inward wall of the envelope 12. An electrode 18 is disposed radially outward of the envelope l2 and is separated at least in part from the centrally disposed electrode 16 by at least a portion of the envelope 12, thereby defining an area within the envelope 12, i.e., at least a portion of the chamber 10, which is disposed between the electrodes l6 and 18.

High frequency electrical potential is applied between the

electrodes

16 and 18 from a suitable source such as a variable frequency and amplitude radio frequency (RF) source 20 to thereby subject the chamber defined by the envelope 12 between the

electrodes

16 and 18, to a selectable time varying electrical field. A suitable fill tube 22 may be provided communicating with the chamber 10 through the envelope 12 and having a valve or other suitable closure means 24 therein to selectively control the gas pressure and gas constituency within the envelope 12.

With continued reference to FIGS. 1 and 2, the envelope 12 defining the chamber 10 preferably comprises an outer elongated hollow glass cylindrical member 26, an inner elongated hollow glass cylindrical member 28, and apertured

end plates

30 and 32 sealed therebetween in a suitable conventional manner. The cylindrical member 28 illustrated is substantially coextensive with the member 26 and is disposed in telescoping relationship thereto coaxially within the member 26 to define a chamber annular in cross section as is shown in FIG. 2.

As was previously mentioned, the substrate to be heated preferably forms the central electrode 16. The substrate may be passed through the central passage 14 from a feed reel 36, over suitable guides such as the rollers 38, and onto a take-up reel 40. Either or both of the rollers 38 may be connected to one output terminal of the RF source, for example, by grounding the rollers 38 and one output terminal of the RF source as is illustrated in FIG. 1.

The outer electrode 18 is preferably a hollow cylindrical electrically conductive member circumferentially disposed round at least a portion of the insulative member 26 and may be, for example, a metallic foil conformed to the radially outer surface of the envelope. The central electrode 16 preferably extends axially into the central passage 14 sufficiently so that an elongated annular portion of the chamber 10 is located bet ee t e e ec rode and1 The application of a potential from the RF source 20 between the

electrodes

16 and 18 creates an electric field between these electrodes, as is indicated by the lines 34 in FIG. 2. The electrode configuration, i.e., the relative positions of the electrodes and the relative dimensions thereof, cause the electric field to be more concentrated or dense in the vicinity of the central 3 electrode 16 near the axis of the annular chamber 10.

If the intensity of the electric field is sufficient, the gas in the chamber will be excited sufficiently to create a gaseous plasma in the chamber. The plasma generally comprises highly reactive species such as ions, electrons and neutral fragmented particles in highly excited states. Since the exciting of the gas by the electric field creates the plasma, the plasma concentration or density generally conforms to the electric field concentration or density. Thus, the concentration or density of the plasma generated within the gas impervious envelope 12 varies between the outer cylindrical member 26 and the inner cylindrical member 28 in a manner related to the electric field concentration or density.

The relationship between the gas conditions within the envelope l2 and the gas conditions exteriorly thereof is desirably such that the plasma may be confined to the chamber 10. The electric potential applied to the

electrodes

16 and 18 may thus be lower and the current density will be correspondingly less. This desirable relationship may be obtained by utilizing selected gases at predetermined pressures within the chamber 10, while exposing the electrodes outside the envelope 12 to the atmosphere.

By way of example, a monatomic inert gas, such as argon or helium at atmospheric or slightly less than atmospheric pressure may be utilized in the chamber 10. When the RF signal is applied to the

electrodes

16 and 18, a plasma will be more readily generated within the chamber 10 than exteriorly thereof. With the potential of the RF signal applied to the electrodes set at a value above the potential required to generate a plasma within the chamber 10, but below the potential required to generate a plasma in the vicinity of the

electrodes

16 and 18 externally of the chamber 10, the current which flows between the

electrodes

16 and 18 will depend primarily upon the capacitive coupling between the electrodes rather than on the ion flow within the plasma.

When an RF signal of sufficient amplitude is applied to the substrate forming the electrode 16 and the electrode 18, a gaseous plasma, concentrated about the inner cylindrical member 28, is generated within the chamber 10. The temperature of the plasma is extremely high due to the scattering of the energy gained from the electrical field set up between the

electrodes

16 and 18, and the temperature may be controlled within practical limits in direct relation to the field intensity.

The current flowing through the substrate causes resistive heating of the substrate apparently due to the intense magnetic and electrical fields in the plasma.

The heating of the substrate as described above results in highly efficient use of the energy supplied by the RF source 20. Thus, the required substrate temperature may be achieved more efficiently than by other conventional heating methods and the substrate temperature may be easily controlled in a number of ways, for example, by controlling the amplitude ofthe RF signal, varying the diameter of the inner cylindrical member 28, or varying the distance between the electrodes along the length of the envelope 12 as shown in FIG. 2A. 1

As was previously described, the RF source 20 of FIG. 1 preferably supplies a high frequency RF signal at selectable power levels to the heating apparatus of FIG. 1. As is illustrated in FIG. 3, the RF source 20 may include a high power RF oscillator 42 connected to the load (e.g., the

electrodes

16 and 18 of FIG. 1) through a balun transformer 44. For example, the balun transformer 44 may form a portion of the tank circuit of the oscillator 42. Maximum power transfer between the oscillator 42 and the load is thus obtained when an impedance match exists between the oscillator tank circuit and the load impedance reflected back to the tank circuit. Since it may be desirable to vary the oscillator output power and frequency to suit the requirements of the heating apparatus, it may be necessary to vary the oscillator output impedance to retain the desired impedance match.

Impedance matching for maximum efficiency and control of the power transfer to the load is preferably accomplished by providing a balun transformer arrangement as is illustrated in FIG. 4.

With reference now to FIG. 4, the balun transformer 44 of FIG. 3 preferably includes a primary coil 46 wound in a helical groove 47 on an electrically insulative core 48. A secondary coil 50 is wound in a helical groove 51 on an electrically insulative core 52 disposed coaxially with respect to the core 48.

The

coils

46 and 50 may be, for example, helically wound, hollow copper tubes generally conforming to the shapes of the helical grooves in the

respective cores

48 and 52. The coil 46 may be secured to a pair ofoutput terminal blocks 53 and the coil 50 may be terminated with a suitable transmission line connector 55 such as a 50 ohm connector.

The core 52 may be fixedly connected to a shaft 54 which extends through a central passage in the core 48 so that the core 48 is freely rotatable on the shaft 54. A shoulder 56 may be provided on the shaft 54 to in sure a fixed spacing between the

cores

48 and 52, and the end 58 of the shaft 54 may be threaded and may protrude out of the core 48 so that a nut 60 may be utilized to prevent removal of the shaft 54 from the central passage in the core 48.

An insulative knob 62 having a position indicator 64 thereon may be connected to the core 52 to facilitate the rotation of the core 52 and to provide an indication of the relative positions of the

cores

48 and 52. As the knob 62 is rotated, the secondary coil 50 moves axially along the core 52 varying the spacing between the coils and thereby varying the mutual inductance between the coils. Thus, at a particular frequency setting, the coil spacing may be varied until an impedance match and/or a desired output power is obtained as may be indicated on a suitable wattmeter (not shown).

It should be noted that the axial spacing between the

cores

48 and 52 remains substantially constant as the core 52 is rotated. Thus, the minimum spacing between the coils cannot be decreased below a predetermined distance, preventing accidental arcing between the coils. Moreover, the grooves which receive the coils aid in preventing arcing by interposing a material having a high dielectric strength than that of air at least partially between the

coils

46 and 50 and between adjacent coil windings.

While only the core 52 rotates in the embodiment of FIG. 5, it is apparent that the axial spacing between the coils may be varied in other manners. For example, the

cores

48 and 52 may be connected for axial rotation together with the cores being grooved in opposite directions, Le, a left-handed thread or groove on the core 48 and a right-handed thread or groove on the core 52.

Thus, the coils may both be movable axially in response to the rotation of the cores.

GENERAL SUMMARY OF ADVANTAGES lt is apparent from the foregoing description that the present invention is particularly advantageous for the efficient and controlled electrical heating of substrates such as conductive fibers or wire. The substrate is heated both directly and indirectly, thereby making the most efficient use of the electrical power supplying the heating energy. Moreover, the generated energy of the plasma acting indirectly on the substrate through thermal conduction and radiation is concentrated in the vicinity of the substrate and also acts indirectly on the substrate to improve the heating efficiency.

The balun transformer used in conjunction with the present invention provides a convenient way to maximize power transfer and to control the power applied to the electrodes between which the plasma is generated. Moreover, the entire transformer core assembly may be easily and inexpensively constructed by conventional molding techniques and the coils maybe constructed from commercially available tubing and commercially available fittings. Also, accidental arcing between the transformer windings is prevented by the novel structure of the transformer. The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed is:

l. A method for heat treating substrates comprising the steps of:

providing electrically insulative means defining a gas impervious envelope having a central passage extending therethrough;

passing the substrate through said central passage to thereby create relative movement between the substrate and said central passage; and,

generating a gaseous plasma within said envelope without generating a plasma in said central passage, said substrate being heated at least indirectly by said plasma.

2. The method of claim 1 wherein said gaseous plasma is generated by applying a high frequency electrical signal to said substrate and an electrode disposed exteriorly of said envelope, said substrate thereby being resistively heated by current flow therethrough.

3. The method of claim 2 wherein said electrode is cylindrical and is concentric with said substrate, the surface area of said electrode exceeding the surface area of said substrate whereby said plasma is concentrated in the vicinity of said substrate.

4. The method of claim 2 wherein the current through said substrate includes a current transverse to the axis of the envelope.

5. The method of claim 4 wherein the amplitude of the transverse current is an order of magnitude greater than the current between the substrate and the electrode.

6. The method of claim 5 wherein the transverse current is generally confined to the exterior surface of the substrate and the substrate is conductively heated inwardly from the surface thereof.

Claims

1. A method for heat treating substrates comprising the steps of: providing electrically insulative means defining a gas impervious envelope having a central passage extending therethrough; passing the substrate through said central passage to thereby create relative movement between the substrate and said central passage; and, generating a gaseous plasma within said envelope without generating a plasma in said central passage, said substrate being heated at least indirectly by said plasma.