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.