Electrical resistor with a polycrystalline ceramic cover and a process for its manufacture

Claim 2

An electrical resistor comprising a ceramic support, a resistive element on said support, and conductors connected to said resistance element and extending from said support, and an inorganic insulating and protective cover chemically and physically bonded to all the parts constituting the resistor including those portions of said conductors adjacent said support, said cover comprising a polycrystalline ceramic obtained by controlled in situ crystallization of a vitreous material in such a manner as to obtain microcrystals uniformly dispensed and distributed in the vitreous mass.

Claim 3

An electrical resistor as claimed in claim 1, in which the vitreous ceramic insulating and protective cover consists for 95% of a vitreous phase and for 5% of a crystalline phase.

Claim 4

An electrical resistor as claimed in claim 1, in which the vitreous ceramic insulating and protective cover consists for 95% of a crystalline phase and for 5% of a vitreous phase.

Claim 5

An electrical resistor as claimed in claim 1, in which the vitreous ceramic insulating and protective cover has a coefficient of expansion lying between 1.10 6 and 8.10 6 and a softening point lying between 600* and 800*C.

Claim 6

In the method of making electrical resistor components which comprises the steps of: a. forming a length of electrically resistive metal around a ceramic support member and electrically connecting conductor elements to the opposite ends of said length of electrically resistive metal to provide an uncovered resistor component sub-assembly; and then b. applying a protective covering around at least that portion of the resistor component sub-assembly which encompasses said electrically resistive metal and baking such protective covering at an elevated temperature to provide an integrated assembly; the improvement which comprises: in step (b), providing said covering in the form of a vitreous ceramic frit which consists essentially of a mixture of ceramic oxides having a softening point and at least one crystalline phase within the temperature range of 600*-850*C, and wherein said baking is carried out in the temperature range of 600*-850*C for a time sufficient to convert at least 5% of said frit to crystalline phase.

Claim 7

In the method of making electrical resistor components which comprises the steps of: a. forming a length of electrically resistive metal around a ceramic support member and electrically connecting conductor elements to the opposite ends of said length of electrically resistive metal to provide an uncovered resistor component sub-assembly; and then b. applying a protective covering around at least that portion of the resistor component sub-assembly which encompasses said electrically resistive metal and baking such protective covering at an elevated temperature to provide an integrated assembly; the improvement which comprises: in step (b), providing said covering in the form of a vitreous ceramic frit which consists essentially of a mixture of ceramic oxides having a softening point and at least one crystalline phase within the temperature range of 600*-850*C, and wherein said baking is carried out in the temperature range of 600*-850*C for a time sufficient to convert at least 5% of said frit to crystalline phase.

Claim 8

The method of claim 5 in which the vitreous ceramic covering is applied to the electrically resistive metal by pulversing it and then mechanically pressing the powder, possibly in the presence of known organic or inorganic binders such as methylcellulose, clay or the like and baking the covered Electrically resistive metal at 600*-800*C for a time of 1 - 120 minutes, so as to form in-situ the vitreous ceramic covering which constitutes the electrical insulation and protection of the electrically resistive metal.

Claim 9

The method of claim 5 in which the vitreous ceramic covering is provided in the form of powder which is dried, mixed with suitable known crystallisation agents and/or nucleants and binders to form a paste, the paste is drawn into tubes of suitable length, the tubes are mounted over the electrically resistive metal, and the assembly is baked at said temperatures of 600*-850*C for 1-120 minutes so as to form in-situ the vitreous ceramic covering which constitutes the electrical insulation and protection for the electrically resistive metal.

Claim 10

The method of claim 5, in which the original electrically resistive metal is immersed in a suspension in water of a fine powder consisting of the vitreous ceramic covering material containing nucleants and/or crystallisation promoters and is baked at said temperatures between 600* and 850*C for 1-120 minutes so as to obtain in-situ on the electrically resistive metal the formation of the vitreous ceramic covering, the thermal cycle and application of the syspension being repeated at least twice so as to form on the electrically resistive metal a uniform thickness of vitreous ceramic material.

Claim 11

The method of claim 5, in which the original electrically resistive material is in the form of a wire which is sprayed with an aqueous suspension of a fine powder consisting of the vitreous ceramic covering material containing nucleants and/or crystallisation promoters and is baked at said temperatures between 600* and 850*C for 1-120 minutes so as to obtain the vitreous ceramic covering in-situ on the wire, the thermal cycle and spray application of the aqueous suspension on the wire being repeated at least twice so as to form a uniform thickness of vitreous ceramic covering on the wire.

Claim 12

In the method according to claim 5 wherein said frit consists essentially of the following weight percentages:

Claim 13

In the method according to claim 5 wherein said frit consists essentially of the following:

Claim 14

In the method according to claim 5 wherein said frit consists essentially of the following:

Claim 15

In the method according to claim 10 wherein said frit is prepared by melting the ingredients at a temperature of 1200*-1300*C, pouring the melt into water and then wet grinding the material to form the frit.