US3462522A - Deposition of pyrolytic material - Google Patents

Deposition of pyrolytic material Download PDF

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US3462522A
US3462522A US598633A US3462522DA US3462522A US 3462522 A US3462522 A US 3462522A US 598633 A US598633 A US 598633A US 3462522D A US3462522D A US 3462522DA US 3462522 A US3462522 A US 3462522A
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mandrel
article
pyrolytic
coat
deposition
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US598633A
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Thomas J Clark
Bruce L Ettinger
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General Electric Co
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General Electric Co
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    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
    • C04B35/52—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbon, e.g. graphite
    • C04B35/522—Graphite
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/20—Graphite
    • C01B32/21—After-treatment
    • C01B32/23—Oxidation

Definitions

  • This invention relates to a method and apparatus for forming graphite articles by pyrolysis of a carbonaceous gas, and more particularly to an improved mandrel on which to deposit pyrolytic graphite and to a method for preparing same.
  • Pyrolytic graphite because of its extremely high temperature resistance and its nuclear and other desirable properties has a broad field of utility such, for example, as for lamp filaments, furnace linings, nuclear reactor moderators, rocket nozzles and re-entry heat shields.
  • Pyrolytic graphite is manufactured by pyrolysis, or thermal decomposition, of a carbonaceous gas. Any of a wide variety of carbonaceous gases may be used, though in practice methane, either alone or in combination with hydrogen, is preferred.
  • Manufacture generally requires that the pyrolytic graphite article be of tubular shape or circular crosssection.
  • thecarbonaceous gas is passed through a tubular mandrel, the mandrel being a a sufliciently high temperature to cause pyrolysis of the carbonaceous gas.
  • This pyrolysis results in the deposition of the pyrolytic graphite on the interior wall of the tubular mandrel.
  • the graphite deposits in laminae and the process is continued until the desired thickness is accomplished.
  • the mandrel having the deposited article thereon is then cooled. Finally, the formed article is separated from the mandrel.
  • the deposition of the pyrolytic graphite is generally carried out at temperatures from 1200 C. to 2500 C., preferably from 1800 C. to 2300 C.
  • the mandrels used in the process must be able to withstand such elevated temperatures for extended periods of time. Ordinarily these mandrels are fabricated from electrographite. This material not only has the necessary high temperature resistance and a satisfactory coeflicient of thermal expansion but is also relatively inexpensive, which means that the mandrel can be broken and destroyed, for purposes of obtaining a good separation of the pyrolytic article from the mandrel, without serious cost disadvantage.
  • Electrographite itself is made from a mixture of coke particles and a pitch binder by baking the mixture at about 750 C. to 1200 C. and then graphitizing it at about 2500 C.
  • the resultant electrographite is polycrystalline, and contains numerous voids composed mostly of tortuous inner-connected pores between the various graphitized coke particles which are bonded together by the graphitized pitch. These voids explain why electrographite weighs only about from 70% to 80% of its theoretical density.
  • electrographite when electrographite is machined into tubular mandrel form, its inte- 3,462,522 Patented Aug. 19, 1969 rior wall, upon which the pyrolytic graphite article will be deposited, is not continuous, but rather consists of machined coke particles separated by pores.
  • the texture of the machined coke particles depends upon the local random orientation of the cleavage planes of the crystallites that are present in the coke particles. If, by chance, the coke particle is machined along a cleavage plane, the surface will be relatively smooth; however, as is much more probable, the machining will take place at some angle to the local cleavage planes which results in a relatively irregular surface.
  • a mild oxidation treatment carried out by passing an oxidizing gas, such as oxygen or carbon dioxide, through a tubular electrographite mandrel before the pyrolytic article is deposited results in the interior wall of the mandrel being far better adapted to receive the deposition of a pyrolytic graphite article than it was prior to the oxidation treatment.
  • This mild oxidation treatment leads to an improved mandrel release after deposition, superior surface smoothness of the deposited article, and minimizes the formation of large isolated nodules in the deposited article.
  • the mandrel consists of the oxidation-treated electrographite surface With the thin pre-coat of pyrolytic graphite thereover.
  • the mild oxidation treatment is accomplished by passing the oxidizing gas, such as carbon dioxide or oxygen (either as such or as air), over the surface of the electrographite mandrel while the mandrel is at a temperature suflicient to cause the desired oxidation reaction, generally from about 400 C. to 1300" C.
  • the precise temperature used will depend on the oxidizing gas employed. For example, with a relatively mild oxidizing gas such as carbon dioxide, a relatively high temperature can be used, on the order of 800 C. to 1300 C., whereas with a stronger oxidizing gas such as oxygen, 21 lower temperature, from 400 C. to 800 C., is desirable.
  • the oxidation should be halted before the mandrel suffers more than about a 1% weight loss, since continued oxidation could result in a weakened mandrel due to oxidation of the graphitized binder.
  • This oxidation treatment is preferably carried out immediately prior to the deposi- 3 tion of the pyrolytic graphite, be it the thin pre-coat in accordance with the preferred embodiment of the invention or, where the thin pre-coat is not used, the pyrolytic graphite article desired to be manufactured. Otherwise damage of the prepared surface may occur by accidental contact during handling or storage.
  • the temperature of the mandrel is, of course, raised to that desired for the pyrolytic graphite deposition, be it the pre-coat or, where the pre-coat is not used, the article desired to be manufactured.
  • the flow of carbonaceous gas should be interrupted prior to commencement of the deposition of the pyrolytic graphite article of manufacture.
  • the temperature used for deposition of the article may be different than that used for the deposition of the pre-coat.
  • the mild oxidation treatment will result in easier release of the mandrel from the deposited article and superior surface smoothness of the deposited article. In addition, and most significantly, the formation of large isolated nodules in the deposited article is minimized.
  • an even more desirable surface will result from the addition of a thin pre-coat, less than .002 inch, of pyrolytic graphite on the treated surface.
  • This thin layer is deposited by passing through the mandrel a measured amount of carbonaceous gas, such that the resulting pyrolytic graphite deposit on the inner tubular wall of the mandrel does not exceed .002 inch.
  • This thin layer fills the pores of the treated electrographite surface and, in addition, provides filleting which results in a smoother surface than that achieved by the mild oxidation treatment alone.
  • the pyrolytic graphite article is then deposited in the usual manner. Upon separation of the mandrel from the desired deposited pyrolitic graphite article, even if part of the thin layer sticks to the deposited article, its thickness is so minute as to be negligible.
  • the thin pre-coat less than .002 inch, of pyrolytic graphite is used in combination with a mandrel surface which has been treated by the mild oxidation treatment, we have found that, even used alone, such a thin pre-coat not only serves as a good surface for receiving the pyrolytic graphite article but also enables maintenance of good surface finish and close tolerances in the pyrolytic graphite article of manufacture since even if portions of the thin layer adhere to the article, the effect on dimensions is negligible.
  • the pre-coat can be pyrolytic graphite and the article deposited on the pre-coat can be an alloy of pyrolytic graphite such as boron alloy of pyrolytic graphite.
  • the deposited article of manufacture can be boron nitride, with the pre-coat being either pyrolytic graphite or boron nitride.
  • a method for manufacturing an article of pyrolytic material by the pyrolytic deposition from a gas on an electrographite mandrel comprising subjecting the surface of said mandrel to an oxidizing gas to cause oxidation of surface portions thereof, pyrolytically depositing on said surface portions a pre-coat of pyrolytic material having a thickness of less than .002 inch, interrupting the pyrolytic deposition, and then pyrolytically depositing said article on said pre-coat.
  • a method for manufacturing an article of pyrolytic material by the pyrolytic deposition of a gas on an electrographite mandrel comprising subjecting the surface of said mandrel to an oxidizing gas to cause oxidation of surface portions thereof, and then pyrolytically depositing said article on said surface portions.
  • a method for manufacturing an electrographite mandrel having a surface adapted to receive a pyrolytic article comprising subjecting the surface of said mandrel to an oxidizing gas to cause oxidation of surface portions thereof and thereby impart thereto a uniform textured surface.
  • a mandrel adapted for the pyrolytic deposition of a pyrolytic material including an electrographite member, surface portions of said member having been treated with an oxidizing gas to cause oxidation of said surface portions and thereby impart thereto a uniform textured surface.
  • the mandrel as defined by claim 9 additionally including a layer of pyrolytic material having a thickness of less than .002 inch deposited on said oxidized surface portions of said mandrel.

Description

United States Patent 3,462,522 DEPOSITION OF PYROLYTIC MATERIAL Thomas J. Clark, Troy, and Bruce L. Ettinger, Washington, Mich., assignors to General Electric Company, a
corporation of New York No Drawing. Filed Dec. 2, 1966, Ser. No. 598,633
Int. Cl. B29c 13/04; B28d 7/38; C01b 31/00 US. Cl. 264-39 10 Claims ABSTRACT OF THE DISCLOSURE Pyrolytic articles are deposited on a mandrel surface by initially oxidizing the surface with an oxidizing gas, heating the mandrel and depositing by vapor deposition a very thin pre-coat of pyrolytic material over the oxidized surface, interrupting the deposition and finally depositing by vapor deposition the desired pyrolytic article.
Background of the invention This invention relates to a method and apparatus for forming graphite articles by pyrolysis of a carbonaceous gas, and more particularly to an improved mandrel on which to deposit pyrolytic graphite and to a method for preparing same.
Pyrolytic graphite because of its extremely high temperature resistance and its nuclear and other desirable properties has a broad field of utility such, for example, as for lamp filaments, furnace linings, nuclear reactor moderators, rocket nozzles and re-entry heat shields. Pyrolytic graphite is manufactured by pyrolysis, or thermal decomposition, of a carbonaceous gas. Any of a wide variety of carbonaceous gases may be used, though in practice methane, either alone or in combination with hydrogen, is preferred. Manufacture generally requires that the pyrolytic graphite article be of tubular shape or circular crosssection. To make a tube of pyrolytic graphite thecarbonaceous gas is passed through a tubular mandrel, the mandrel being a a sufliciently high temperature to cause pyrolysis of the carbonaceous gas. This pyrolysis results in the deposition of the pyrolytic graphite on the interior wall of the tubular mandrel. The graphite deposits in laminae and the process is continued until the desired thickness is accomplished. The mandrel having the deposited article thereon is then cooled. Finally, the formed article is separated from the mandrel.
Since the deposition of the pyrolytic graphite is generally carried out at temperatures from 1200 C. to 2500 C., preferably from 1800 C. to 2300 C., the mandrels used in the process must be able to withstand such elevated temperatures for extended periods of time. Ordinarily these mandrels are fabricated from electrographite. This material not only has the necessary high temperature resistance and a satisfactory coeflicient of thermal expansion but is also relatively inexpensive, which means that the mandrel can be broken and destroyed, for purposes of obtaining a good separation of the pyrolytic article from the mandrel, without serious cost disadvantage.
Electrographite itself is made from a mixture of coke particles and a pitch binder by baking the mixture at about 750 C. to 1200 C. and then graphitizing it at about 2500 C. The resultant electrographite is polycrystalline, and contains numerous voids composed mostly of tortuous inner-connected pores between the various graphitized coke particles which are bonded together by the graphitized pitch. These voids explain why electrographite weighs only about from 70% to 80% of its theoretical density. Moreover, because of these voids, when electrographite is machined into tubular mandrel form, its inte- 3,462,522 Patented Aug. 19, 1969 rior wall, upon which the pyrolytic graphite article will be deposited, is not continuous, but rather consists of machined coke particles separated by pores.
The texture of the machined coke particles depends upon the local random orientation of the cleavage planes of the crystallites that are present in the coke particles. If, by chance, the coke particle is machined along a cleavage plane, the surface will be relatively smooth; however, as is much more probable, the machining will take place at some angle to the local cleavage planes which results in a relatively irregular surface.
These surface irregularities can affect the structure of the deposited article and such irregularities frequently generate large isolated nodules in the deposited article. Such nodules may act as local stress raisers and failure of the deposited article has often been found to be associated with their existence. In order to provide a more uniformly textured mandrel surface than machined electrographite, it has been proposed to deposit on the interior wall of the mandrel a relatively thick pre-coat of pyrolytic graphite. However, the use of such a pyrolytic graphite pre-coat has not proved to be satisfactory since frequently part of this pre-coat sticks to the deposited article upon breakage and separation of the mandrel from the article, causing the article to have a spalled surface and often to be out of tolerances. Such defects generally cannot be corrected by machining since machining of the deposited article is frequently detrimental to the properties in other respects. Hence the art is in need of an improved mandrel for the manufacture of pyrolytic graphite, and a method for making such mandrels. The present invention fulfills this need.
Summary of the invention Briefly, what we have found is that a mild oxidation treatment carried out by passing an oxidizing gas, such as oxygen or carbon dioxide, through a tubular electrographite mandrel before the pyrolytic article is deposited results in the interior wall of the mandrel being far better adapted to receive the deposition of a pyrolytic graphite article than it was prior to the oxidation treatment. This mild oxidation treatment leads to an improved mandrel release after deposition, superior surface smoothness of the deposited article, and minimizes the formation of large isolated nodules in the deposited article. In addition, we have discovered that these features can be even further enhanced by depositing a very thin precoat of pyrolytic graphite, i.e., a layer less than .002 inch in thickness, over the oxidized interior wall of the mandrel. Hence, in this preferred embodiment, the mandrel consists of the oxidation-treated electrographite surface With the thin pre-coat of pyrolytic graphite thereover.
Description of the preferred embodiment The mild oxidation treatment is accomplished by passing the oxidizing gas, such as carbon dioxide or oxygen (either as such or as air), over the surface of the electrographite mandrel while the mandrel is at a temperature suflicient to cause the desired oxidation reaction, generally from about 400 C. to 1300" C. The precise temperature used will depend on the oxidizing gas employed. For example, with a relatively mild oxidizing gas such as carbon dioxide, a relatively high temperature can be used, on the order of 800 C. to 1300 C., whereas with a stronger oxidizing gas such as oxygen, 21 lower temperature, from 400 C. to 800 C., is desirable. The oxidation should be halted before the mandrel suffers more than about a 1% weight loss, since continued oxidation could result in a weakened mandrel due to oxidation of the graphitized binder. This oxidation treatment is preferably carried out immediately prior to the deposi- 3 tion of the pyrolytic graphite, be it the thin pre-coat in accordance with the preferred embodiment of the invention or, where the thin pre-coat is not used, the pyrolytic graphite article desired to be manufactured. Otherwise damage of the prepared surface may occur by accidental contact during handling or storage.
At the completion of the oxidation treatment the temperature of the mandrel is, of course, raised to that desired for the pyrolytic graphite deposition, be it the pre-coat or, where the pre-coat is not used, the article desired to be manufactured. Where the pre-coat is used, at the completion of the desired pre-coat depositions, the flow of carbonaceous gas should be interrupted prior to commencement of the deposition of the pyrolytic graphite article of manufacture. The temperature used for deposition of the article may be different than that used for the deposition of the pre-coat.
As has been alluded to above, the mild oxidation treatment will result in easier release of the mandrel from the deposited article and superior surface smoothness of the deposited article. In addition, and most significantly, the formation of large isolated nodules in the deposited article is minimized.
Although the precise reasons for the improvements are not fully known, we theorize that oxidation occurs on the mandrel surface more rapidly at exposed crystallite edges than at crystallite faces when such edges and faces are exposed, as by machining. This result would theoretically occur owing to the different nature of the chemical bonding in the two directions of the anisotropic crystallites which make up the graphitized coke particles. It is probably as a result of this same structural anisotropy that cleavage occurs between the planes more readily than across the planes of the carbon atoms during machining with the result that the crystallite edges form hill-like structures at the exposed surface. Upon oxidation, these crystallite edges are oxidized more rapidly than the faces, resulting in a more uniformly textured surface than that of an untreated machined surface of electrographite.
Moreover, as already indicated, an even more desirable surface will result from the addition of a thin pre-coat, less than .002 inch, of pyrolytic graphite on the treated surface. This thin layer is deposited by passing through the mandrel a measured amount of carbonaceous gas, such that the resulting pyrolytic graphite deposit on the inner tubular wall of the mandrel does not exceed .002 inch. This thin layer fills the pores of the treated electrographite surface and, in addition, provides filleting which results in a smoother surface than that achieved by the mild oxidation treatment alone. After the thin pyrolytic pre-coat is deposited, the pyrolytic graphite article is then deposited in the usual manner. Upon separation of the mandrel from the desired deposited pyrolitic graphite article, even if part of the thin layer sticks to the deposited article, its thickness is so minute as to be negligible.
Although it is much preferred if the thin pre-coat, less than .002 inch, of pyrolytic graphite is used in combination with a mandrel surface which has been treated by the mild oxidation treatment, we have found that, even used alone, such a thin pre-coat not only serves as a good surface for receiving the pyrolytic graphite article but also enables maintenance of good surface finish and close tolerances in the pyrolytic graphite article of manufacture since even if portions of the thin layer adhere to the article, the effect on dimensions is negligible.
Also, whereas the invention has been taught specifically with reference to the deposition of pyrolytic graphite both for the pre-coat and for the article of manufacture, it will be understood that other pyrolytic material can be used for both or either the pre-coat and the article. For example, the pre-coat can be pyrolytic graphite and the article deposited on the pre-coat can be an alloy of pyrolytic graphite such as boron alloy of pyrolytic graphite. As a further example, the deposited article of manufacture can be boron nitride, with the pre-coat being either pyrolytic graphite or boron nitride.
What is claimed is:
1. A method for manufacturing an article of pyrolytic material by the pyrolytic deposition from a gas on an electrographite mandrel comprising subjecting the surface of said mandrel to an oxidizing gas to cause oxidation of surface portions thereof, pyrolytically depositing on said surface portions a pre-coat of pyrolytic material having a thickness of less than .002 inch, interrupting the pyrolytic deposition, and then pyrolytically depositing said article on said pre-coat.
2. The method as defined by claim 1 wherein the quantity of oxidizing gas utilized is insufficient to oxidize more than 1% by weight of said mandrel.
3. The method as defined by claim 1 wherein said material of said article and said pre-coat is graphite, and wherein the gas is carbonaceous.
4. A method for manufacturing an article of pyrolytic material by the pyrolytic deposition of a gas on an electrographite mandrel comprising subjecting the surface of said mandrel to an oxidizing gas to cause oxidation of surface portions thereof, and then pyrolytically depositing said article on said surface portions.
5. The method as defined by claim 4 wherein the quantity of oxidizing gas utilized is insufficient to oxidize more than 1% by weight of said mandrel.
6. The method as defined in claim 4 wherein said material is graphite and where the gas is carbonaceous.
7. A method for manufacturing an electrographite mandrel having a surface adapted to receive a pyrolytic article comprising subjecting the surface of said mandrel to an oxidizing gas to cause oxidation of surface portions thereof and thereby impart thereto a uniform textured surface.
8. The method as defined by claim 7 additionally including the step of pyrolytically depositing on said surface a layer of pyrolytic graphite having a thickness of less than .002 inch.
9. A mandrel adapted for the pyrolytic deposition of a pyrolytic material including an electrographite member, surface portions of said member having been treated with an oxidizing gas to cause oxidation of said surface portions and thereby impart thereto a uniform textured surface.
10. The mandrel as defined by claim 9 additionally including a layer of pyrolytic material having a thickness of less than .002 inch deposited on said oxidized surface portions of said mandrel.
References Cited UNITED STATES PATENTS 2,201,049 5/1940 Moore 2491l5 XR 3,206,331 9/1965 Diefendorf 117-146 XR 3,367,826 2/1968 Heestand et a1. 117--46 XR DAVID KLEIN, Primary Examiner US. Cl. X.R.
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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3683996A (en) * 1970-02-26 1972-08-15 Adam Dunlop Method of carbonizing refractory moulds
US4034031A (en) * 1974-10-23 1977-07-05 U.S. Philips Corporation Method of manufacturing grid electrodes for electron tubes
US4124666A (en) * 1976-07-03 1978-11-07 Bayer Aktiengesellschaft Method of keeping nozzle bodies or breaker plates clean during extrusion of polymer melts
US4229391A (en) * 1976-12-22 1980-10-21 Ravenhead Brick Company Limited Process for the manufacturing of colored clay products
US4332751A (en) * 1980-03-13 1982-06-01 The United States Of America As Represented By The United States Department Of Energy Method for fabricating thin films of pyrolytic carbon

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2201049A (en) * 1938-02-01 1940-05-14 Gen Electric Glass fabrication process and mold
US3206331A (en) * 1961-04-25 1965-09-14 Gen Electric Method for coating articles with pyrolitic graphite
US3367826A (en) * 1964-05-01 1968-02-06 Atomic Energy Commission Usa Boron carbide article and method of making

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2201049A (en) * 1938-02-01 1940-05-14 Gen Electric Glass fabrication process and mold
US3206331A (en) * 1961-04-25 1965-09-14 Gen Electric Method for coating articles with pyrolitic graphite
US3367826A (en) * 1964-05-01 1968-02-06 Atomic Energy Commission Usa Boron carbide article and method of making

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3683996A (en) * 1970-02-26 1972-08-15 Adam Dunlop Method of carbonizing refractory moulds
US4034031A (en) * 1974-10-23 1977-07-05 U.S. Philips Corporation Method of manufacturing grid electrodes for electron tubes
US4124666A (en) * 1976-07-03 1978-11-07 Bayer Aktiengesellschaft Method of keeping nozzle bodies or breaker plates clean during extrusion of polymer melts
US4229391A (en) * 1976-12-22 1980-10-21 Ravenhead Brick Company Limited Process for the manufacturing of colored clay products
US4332751A (en) * 1980-03-13 1982-06-01 The United States Of America As Represented By The United States Department Of Energy Method for fabricating thin films of pyrolytic carbon

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