US 3293072 A
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Description (OCR text may contain errors)
1966 H. D. DOOLITTLE ETAL 3,
CERAMIC'METALLIZING TAPE Filed June 29. 1961 FIG. 2 FIG. I
I? v TANK CARRIER METALLIZING FILM F'LM DRYER )5 l4 }0 Fl G. 3
METALLIZI'NG FILM INV EN TORS HOWARD D. DOOLITTLE y KITT $.ETTRE FIG.6
United States Patent 3,293,072 CERAMIC-METALLIZING TAPE Howard Daniel Doolittle and Kitty S. Ettre, Stamford,
Conn., assignors, by mesne assignments, to Vltta Qorporation, Wilton, Conn., a corporation of Connecticut Filed June 29, 1961, Ser. No. 120,617 Claims. (Cl. 117138.8)
This invention relates to improvements in means and methods of metallizing ceramic surfaces, and has particular reference to a novel ceramic metallizing film and method of applying same to a ceramic surface.
These means and methods relate specifically to a reliable hard-solder type of seal utilizing a metallizing film consisting of a self-supporting plastic tape which contains selected metallizing powders in a binder of selected type.
Prior art types of ceramic seals are usually produced by utilizing some variation of the sintered metal powder process sealing technique. According to such procedure, a thin layer of liquid material containing a mixture of suitable powdered metals or metal compounds is applied to a ceramic surface and sintered at elevated temperatures to form a bond with the ceramic surface. The layer is then plated and may be subsequently brazed to the metal surface.
The application of the thin metal powder layer on the ceramic surface was accomplished by painting, spraying, printing, or silk screening, and for this purpose the metal powder is suspended in an organic solvent containing a binder and a plasticizer, and the resultant liquid mixture is applied directly to the ceramic surface.
These methods, however, have many disadvantages, a primary one of which is that the concentration and viscosity of the suspension changes from time to time due to evaporation of the organic solvents, and this makes reproducibility of the applied layers variable not only from day to day, but also from hour to hour during production. Further, the drying of the coating on the ceramic surfaces presents a problem. It has been found that the composition of the liquid, its concentration, and the viscosity of the fluid are of considerable importance in obtaining a uniform coating of the right thickness on the ceramic surface. Butt seals, particularly, are extremely sensitive to any variations in the thickness, density, and smoothness of the metallizing layers.
Known ceramic metallizing methods utilize liquid suspensions which, if mass production is necessary, require relatively expensive machinery for performing the techniques. Threfore, most mass production is based on hand painting or spraying operations where uniformity and reproducibility of thickness and smoothness are the greatest problems.
Accordingly, a primary object of this invention is to provide a prefabricated self-supporting metallizing film which may be applied to a ceramic surface for the purpose of sealing the surface to a surface of a metal element or another ceramic element.
Another object is to provide a metallizing tape or film which may be formed in large quantities with a uniformly smooth surface, which may be provided in uniform controlled densities and thicknesses, and which has a high degree of bond strength and a high degree of vacuum-retention characteristics.
Another object is to provide a tape or film of the above character wherein the plastic binder material therein will be substantially completely eliminated at elevated temperatures without leaving voids which will destroy vacuum tightness of the seal, which will not leave any undesirable residue on the ceramic, and will not result in shrinkage, cracks or peeling of the layer after heating.
Another object is to provide economical means and method of sealing ceramic members to metal or ceramic surfaces.
Other objects and advantages of the invention will become apparent from the following description taken in connection with the accompanying drawings, wherein FIG. 1 is a front elevational view of two members joined together by a butt seal formed in accordance with this invention;
FIG. 2 are fragmentary front elevational views partly in section illustrating the means and method of sealing ceramic and metal parts together in telecoped relation;
FIG. 3 is a schematic view illustrating a method of making a self-supporting metallizing film in accordance with this invention;
FIG. 4 is a fragmentary perspective view of a section of a film made in accordance with this invention;
FIG. 5 illustrates the removability of the metallizing film from its carrier film; and
FIG. 6 is 'a front elevational view of a semiconductor device utilizing the invention.
In accordance with the objects of this invention, we have utilized casting or laminating techniques for producing a very satisfactory film for use in ceramic-to-metal, ceramicato-ceramic or sapphire-to-metal seals. A slurry containing the selected metal powder or compound, binder, plasticizer, and solvent is deposited upon a carrier film in a uniformly thick andsmooth layer, in a manner to be described hereinafter.
In the metallizing of ceramics and in the sintering of metallized layers, the role of the binder and plasticizer is of considerable importance. Cellulose nitrates have been commonly used as binders for metallizing liquids deposited by painting and spraying methods. Although nitrocellulose may be used for a metallizing tape or film of the presently described type, nitrocellulose does not support, as a binder, large amounts of the selected metal powder. This deficiency results in somewhat low density metallizing and deterioration of the metallized layer after sintering, which, however, may not be undesirable in some cases. Furthermore, nitrocellulose film also is inclined to become brittle and to harden slightly during aging or under influence of light, affecting the storing qualities of the prepared film.
We have found that of the common organic resins having best film-forming characteristics, polyvinyl alcohol and polymethacrylates give particularly excellent results as binders. For example, using poly-n-butyl methacrylate as a binder, it is possible to prepare high density metallizing film with as little as about 5% binder, and the resultant film will be substantially unaffected by sunlight or aging. Not more than about 10% binder is preferred for best results. Polymethacrylates have good molding qualities and a relatively high modulus of elasticity. Further-more, they permit good adhesion of the metal powder to ceramics at room or elevated temperatures. We found, in this respect, that the transition, at elevated temperatures, from film adherence to chemical bond with the ceramic surface is influenced to a considerable degree by the binder and also by the plasticizer.
It is desirable in many cases that the binder completely vaporize during processing so as to leave no solid organic residue in the coating. This is particularly true in connection with the manufacture of electron discharge devices or other devices operated in a vacuum where the residue might become a deleterious contaminant. In such cases, the polymethacrylates are particularly desirable as binders for the metal compounds because during the sintering process the polymethacrylate leaves as gaseous products, with little or no solid organic residue remaining in the coating.
A plasticizer is used as :part of the binder to give the film a softening effect without affecting other properties of the film. It is preferable here that the plasticizer, similarly to the resin, should completely evaporate or decompose without leaving any harmful residue when the film is to be used in vacuum devices. In accordance with this invention, plasticizers such as sucrose-.acetate-isobutyrate, dibutyl-phthalate, anddiet-hyl-oxalate may be used successfully with poly-n-butyl methacrylate, and glycerine with polyvinyl alcohol or other binder. The plasticizers specified above decompose completely, have an excellent softening effect upon the resin, and do not affect the films mechanical properties.
The carrier film may be any selected material such as paper, Mylar brand of polyethylene terephthalate polyethylene or other flexible sheet material to which the mixture will adhere. Polyethylene is the most suitable for many reasons, being flexible, smooth, strong, capable of being made in very thin sheets, clean, and capable of being easily separated from the layer of metallized film, and furthermore, will not contaminate the metallizing film.
The solvent is preferably acetone, but amylacetate or ethylacetate may be used.
It is particularly pointed out that the term metal powder is used herein to include any of the metal compounds which are known to be useful in the formation,
on the surface of a ceramic element, of a bonded layer which may be subsequently brazed to a metal element, the brazing step, however, being unnecessary when sealing ceramics-to-ceramics. For example, different combinations of molybdenum and manganese may be used or they may be combined with other metal powders such as titanium or iron. The titanium may be mixed as a hydride with molybdenum and manganese, or may itself be compounded with nickel, silver or copper. Also the term metal powder is intended to encompass a com- 'bination of oxides of the selected metals, such as molybdenum oxide and lithium oxide, or of glass powder com bined with metal powder. One example of a good metal powder is a compound containing 80% molybdenum, and 20% manganese; another is a compound of 80% molybdenum, 16% manganese, and 4% titanium. Also, a combination of 80% molybdenum and 20% glass may be used. The actual amounts of the ingredients may be varied within wide ranges.
In preparing the metallizing mixture, it must be compounded so as to provide the required density of metal powder. One film may be produced wherein the mixture is a film-forming product containing about 92% by weight of the selected metal powder, about 5% by weight of poly-n-butyl methaicrylate, and about 3% by weight of plasticizer. This composition was ball milled, using acetone as a solvent, and then spread evenly upon a sheet of polyethylene.
The method of forming the metallized film is clearly depicted in FIG. 3 wherein it can be seen that a carrier film is initially carried by a spool 11 from which it passes over a guide roller 12 and beneath a tank 13 which contains the slurry. The carrier film is adapted to move in the direction indicated by arrows, and as it moves a supply 14 of slurry is deposited upon its upper surface and is carried beneath a flat profile blade 15 which smoothes the'mixture. Blade 15 is adjustable perpendicularly to the surface of the film so as to provide the layer of the mixture with controlled uniform thickness, After the carrier film is thus provided with a uniformly thick and smooth layer 16, it is dried in any suitable manner such as by dryer 17, whereupon the film-fonming layer becomes self-supporting and removably attached to the carrier film 10. The resultant laminated tape or film then passes to a spool 18 on which it is wound for storing.
We have found that one example of such film can easily be provided having a thickness of 50- microns plus or minus 2 /2 microns, with a density of 5.0 g./cm. Surface roughness of the film was less than 2 microns. This film is easily separated as a self-supporting film 16 from the carrier film 10 on which it was formed, as seen in FIG. 5.
The metallizing tape described above can be applied to a ceramic surface by first separating it from the carrier film and using it as a self-supporting film or it can first be applied to the ceramic surface after which the carrier film may be removed. The metallizing film itself may be secured to the ceramic surface by thermal sealing or by the use of a suitable solvent to form the bond therebetween.
In the thermal sealing technique, the metallizing film is removed from the carrier film and applied to the surface area to be coated. Sealing is then accomplished by applying a pressure of about 50 pounds per square inch with the ceramic heated to about 100 C.
When using a solvent, (xylene or methyl-cellusolve acetate, for example), the separation of the metallizing film from its carrier is not necessarily done prior to making the seal because the tape adheres to the ceramic after the solvent has dried, and is released from the carrier automatically. Both polar and non-polar organic solvents are suitable for use with this technique, which is adaptable to extremely fast manual production or fully automated production.
Average tensile strength value of articles sealed by use of a film manufactured according to the above is about two times greater than hand-painted or sprayed seals.
In connection with ceramic-to-metal seals, 2 further improvement is achieved by depositing the plating material directly in the metallizing film. This can be done by adding the plating material, for example, nickel, copper, or combinations thereof, in controlled amounts such as from 2% to 20% directly into the slurry from which the film is formed. In this way the resultant film is provided with the plating constituent which is required in order to braze the metallized surface to a metal surface, thus eliminating the separate plating and firing steps.
Another improvement is achieved by providing the metallizing film slurry with metal oxides mixed with the selected binder material, and adding to themixture an amount of brazing solder material in oxide form. For example, the metallizing materials may be about of molybdenum oxide and 20% manganese oxide mixed with poly-nbutyl methacrylate, to which is added about 10% by weight of silver as the solder material. By use of oxides instead of metal powders for the metallizing material, the coated ceramic surfaces may be sintered at temperatures lower than usual sintering temperatures.
In preparing 'a ceramic surface for subsequent sealing to a metal surface, the ceramic surface is moistened with the solvent and the metallizing film is applied to the moistened surface. After the solvent is dried, the filrm is firmly adhered to the surface and cannot be easily damaged by handling or storage. Special protective precautions are unnecessary. A ceramic surface prepared in this manner may be subsequently sintered, plated with nickel or copper, or combination of both, and brazed or otherwise sealed to a metal surface in a conventional manner, such as by use of a silver or silver alloy solder.
An example of a ceramic-to-ceramic butt seal is shown in FIG. 1 wherein two ceramic cylinders 18 and 19 are sealed together by a metallizing tape seal 20 according to this invention. To accomplish this, the ceramic surfaces which are to be joined are then placed against opposite sides of the metallizing film 20, with the surfaces moistened or not, depending upon the method being used. After sintering, the ceramic members are found to be firmly sealed together. The cylinders 18-19 may be solid or hollow and may be of different diameters.
An illustration of a wrap-around seal is shown in FIG. 2 where a cylindrical ceramic member 21 is provided on its outer surface With a circumferential layer 22 of film produced in accordance with the foregoing description. After the strip of film 22 has been bonded to the ceramic surface and suitably plated, the member 21 is inserted within an encircling metal member 23 and brazed thereto.
A still further illustration of an electronic device embodying the invention is found in FIG. 6 wherein a disclike support 24 of selected material, such as ceramic or sapphire, has a small disc 25 of film bonded to each of its opposed surfaces. After the discs 25 are sintered and plated, the ends of respective metal leads 26 may be brazed thereto as shown.
Vacuum leak testing of electron tubes or other electron discharge devices sealed in the manner taught herein, such as various types of semiconductors, shows considerable improvement over seals produced by prior art painting or spraying methods. This is believed to be accomplished at least partly because of the better control of uniformity in thickness and surface smoothness achieved by the presently described techniques and the adaptability of the film to surfaces of various shapes and sizes.
From the foregoing it will be apparent that novel means and methods have been provided in accordance with the objects of this invention for performing ceramic-to-ceramic or ceramic-to-metal seals. It is to be understood, however, that various changes in the means and methods shown and described may be made by those skilled in the art without departing from the spirit of the invention as expressed in the accompanying claims.
1. A metallizing film comprising:
(A) at least one powdered metal compound suspended (B) a cast film of an organic film-forming polymer (a) selected from the group consisting of polyvinyl alcohol and poly-n-butyl methacrylate, (b) said cast film consisting of not more than about by weight of said polymer and about 90% by weight of powdered metal compound.
2. The metallizing film of claim 1 and;
(C) about 2 to 3% by Weight of a volatile plasticizer in said cast film.
3. The metallizing film of claim 2 wherein said plasticizer is selected from the group consisting of sucroseacetate-isobutyrate, dibutyl-phthalate, diethyl-oxalate and glycerine.
4. The metallizing film of claim 1 wherein said polymer is poly-n-butyl methacrylate.
5. The metallizing film of claim 4 wherein said film further comprises sucrose-acetate-isobutyrate as a plasticizer.
6. The metallizing film of claim 5 wherein said film consists of about 5% of said polymer by weight and about 3 of said plasticizer by weight.
7. The metallizing film of claim 1 wherein said powdered metal compounds are selected from the group consisting of elemental metals, metal oxides and metal hydrides.
8. The metallizing film of claim 1 wherein some of said powdered metal compound consists of glass particles suspended in said film.
9. The metallizing film of claim 1 cast as a releasable, flexible layer on a supporting flexible film.
10. The metallizing film of claim 9 wherein said flexible film is selected from the group consisting of paper, polyethylene terephthalate, and polyethylene.
References Cited by the Examiner UNITED STATES PATENTS v 2,161,888 6/1939 Rearick 156-155 2,660,547 11/1953 Robertshaw 156-155 X 2,729,583 1/1956 Sadowsky 156-155 X 2,796,374 6/1957 Donahue 156-67 2,931,802 4/1960 Touey et al. 260-234 2,977,722 4/1961 Mazzoni 156-155 X 3,023,137 2/1962 Colborne et al 156-67 3,189,504 6/1965 Whittle et al 156-89 OTHER REFERENCES Modern Plastics, vol. 35 No. 1A, September 1957, published by Breskin Publications, Inc., Bristol, Connecticut, pages 381384, 606 and 607.
The Chemistry of Synthetic Resins by Ellis, Reinhold Publishing Corp, New York (1935) vol. 1, page 679.
EARL M. BERGERT, Primary Examiner.
W. B. WALKER, J. P. MELOCHE, Assistant Examiners.