Compositions – Electrically conductive or emissive compositions – Elemental carbon containing
Reexamination Certificate
1999-12-01
2001-05-15
Gupta, Yogendra (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Elemental carbon containing
C252S510000, C423S460000, C423S44500R, C501S099000, C201S022000
Reexamination Certificate
active
06231787
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a resistive film made from a paste comprising a powdery, vitreous carbon as an electrically conductive component in admixture with an electrically non-conductive component. This invention also relates to a method for producing a vitreous carbon in powder form by pyrolyzing a resin in inert atmosphere.
Vitreous carbon has a Mohs' hardness of 6. Therefore, reducing it to a powder entails significant effort.
Production of powdery, vitreous carbon is disclosed in German patent document DE 27 18 308 A1, in which acrylamides mixed with water-soluble salts are pyrolyzed, and a vitreous carbon component is recovered following pyrolysis by dissolving a salt component in water. The vitreous carbon is then dried, and a powder produced in this manner is further pulverized as needed. The method is very time-consuming.
Use of an precursor polymer having a three-dimensional, cross-linked structure for producing a vitreous carbon is disclosed in
Plastverarbeiter,
vol. 41, no. 6, pages 16-21 (1990). After shaping via casting or molding, the polymer is cured and additionally machined down. No mention is made of possible pulverization of the vitreous carbon.
German patent document DE 30 02 112 A1 discloses a paste for producing polymer-film integrated circuits having predetermined electrical conductivity. A predetermined electrical resistivity of the polymer-film integrated circuits to be produced is achieved by mixing electrically conductive and electrically non-conductive film components. A desired resistivity value is thus produced by adding the electrically non-conductive component.
It is generally known that the electrical conductivity of a paste, or of a film made of a paste, formed as a mixture of an electrically conductive component and an electrically non-conductive component, is largely determined by the specific electrical conductivity and the concentration, i.e. a packing density, of the conductive component in the film system. Electrical resistivity in the film increases nearly exponentially when the conductive component reaches a critical minimum concentration in the film. When the electrically conductive component is increased, electrical resistivity stabilizes once an optimal concentration is achieved. When a specific resistivity value is established through proportional increases in the electrically non-conductive component, electrical and mechanical stability may diminish as a result, owing to lack of homogeneity of the film.
When this paste is used for a resistive film, on a potentiometer, the electrically conductive component, for example carbon, causes a typical electrical micro-heterogeneity of the surface of the film, and thus causes increased contact resistivity at the film's wearing surface.
European patent document EP 0 399 295 A1 discloses a use of a vitreous carbon as an electrically conductive component in a resistive film. An electrical resistivity level is established by modifying a concentration or packing density of an electrically conductive component in the resistive film with respect to that of an electrically non-conductive component, such as a binding agent. In order to maintain desirable properties, such as mechanical and electrical stability, an optimum concentration or packing density of electrically conductive particles of the vitreous carbon, i.e. the mixture ratio of the two components, can vary only within a limited range.
It is an object of this invention to provide a method for producing powdery, vitreous carbon that saves money and time. A further object of this invention is to furnish a paste having predetermined electrical conductivity, and to disclose a resistive film having a high degree of abrasion resistance as well as mechanical and electrical stability.
SUMMARY OF THE INVENTION
This object is achieved through a method for producing powdered, vitreous carbon by pyrolysis of a resin in an inert atmosphere, wherein a polymer is cured to a three-dimensional, cross-linked resin and ground to a powder prior to the pyrolysis. This object is also achieved through a paste made of a mixture of electrically non-conductive and electrically conductive components. In the paste, different electrical conductivity is established, at a constant mixture ratio, by using a powdery, vitreous carbon produced by pyrolysis as an electrically conductive component. The electrical resistivity of the vitreous carbon is determined by the selective use of final pyrolysis temperature and pyrolysis duration. A resistive film may be prepared from such paste.
According to principles of the invention, a preferably aromatic polymer is cured to a three-dimensional, cross-linked resin, and is reduced to powder in this state prior to pyrolysis. Pyrolysis of the resulting powder is performed in an inert atmosphere, such as nitrogen, so that powdered, vitreous carbon (also called glass carbon powder) results. The powdery, vitreous carbon has various electrical resistivities that can be predetermined by using varying pyrolysis temperatures or pyrolysis times in each pyrolysis process. In so doing, the dependence of electrical resistivity on pyrolysis temperature and pyrolysis duration is utilized, as described in further detail in the periodical
Chemie
-
Ingenieur
-
Technik,
vol. 42, nos. 9-10, p. 663 (1970). By performing pyrolysis of the polymer in powder form, the duration of pyrolysis can be reduced significantly, introducing time savings into this method. The powdery, vitreous carbon resulting from pyrolysis, with its specified electrical resistivity, is then dispersed into a binding agent and ground in a mill along with the binding agent, to a grain size of <10 &mgr;m. Then the viscosity of the paste obtained in this manner with vitreous carbon is established by adding a quantity of solvent as a thinning agent, for producing a resistive film.
A resistive film having maximum mechanical and electrical stability, with a set but constant packing density of vitreous carbon, in which resistivity ranging from 0.2 KOhm to 1 MOhm per square unit of area can be predetermined, can be manufactured using the paste produced in this manner.
In the method of the present invention, the polymer is preferably an aromatic polymer. The polymer may be a natural or a synthetic polymer. Electrically conductive pigments, such as graphite, carbon black, and/or nickel, may be mixed into the polymer prior to curing. Preferably, curing of the polymer to the three-dimensional, cross-linked resin takes place at a temperature of 150° C. for a period of 30 minutes. Subsequently, the cured resin is ground to a particle size, preferably, of <145 &mgr;m. In accordance with the present invention, a final pyrolysis temperature and a duration of pyrolysis are set so that a predetermined electrical resistivity is established in the vitreous carbon.
The present invention also provides paste as described above. In the paste of the invention, the powdery, vitreous carbon may be dispersed with a binding agent as an electrically non-conductive component and milled to a powder, preferably having a particle size of <10 &mgr;m, at the same time. The paste is made ready for screen printing by addition of a solvent. In the resistive film of the invention, resistivity per square unit of surface area may be predetermined by selection of a vitreous carbon with a specific resistivity, or by selection of the paste produced using the vitreous carbon.
As indicated, the electrical conductivity of the vitreous carbon can be further influenced by mixing in additional electrically conductive pigments, such as graphite or carbon black.
The conductivity of the glass carbon powder can also be influenced by addition of a catalyst, such as an iodine compound, the catalyst being similarly mixed into the resin to be pyrolyzed prior to the three-dimensional cross-linking. Simultaneous dispersing and grinding of the vitreous carbon and the binding agent during paste production reduces the time required for this process, and, through further milling of the paste, makes
Ambros Peter
Johannes Hugo
Kissner Franz
Birch & Stewart Kolasch & Birch, LLP
Gupta Yogendra
Hamlin Derrick G.
Preh-Werke GmbH & Co. KG
LandOfFree
Method for manufacturing powdery, vitreous carbon useful for... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for manufacturing powdery, vitreous carbon useful for..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for manufacturing powdery, vitreous carbon useful for... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2558818