US 3813296 A
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May 28, 1974 0. D. MCSTRACK ET AL 3,813,296
INSULATING comma FOR ELECTRICAL DEVICES Filed Nov. 23. 1971 United States Patent 3,813,296 INSULATING COATING FOR ELECTRICAL DEVICES ABSTRACT OF THE DISCLOSURE An electrical insulating coating for a lightning arrester valve block includes a major amount of silicon carbide particles which have a bulk resistivity substantially higher than the resistivity of the electrically conductive portion of the valve block. The coating is prepared by admixing the silicon carbide particles, preferably green silicon carbide, with ceramic forming materials and suflicient water to form a mixture which can be applied as a coating to the device in a convenient manner. After the resistance device has been coated with this mixture, it is fired at an elevated temperature to mature the ceramic-forming ingredients and thereby form a hard, highly-adherent coating.
BACKGROUND OF THE INVENTION This invention relates to electrical resistance devices and, more particularly, to lightning arrester valve blocks and an electrical insulating coating therefor.
Many electrical resistance devices require an electrical insulating coating to insure their satisfactory performance for the intended purpose, e.g. valve blocks used in lightning arresters. Lightning arresters are used in electrical distribution systems as protection against excessive voltage surges. Such arresters typically include a series gap which normally insulates the arrester from the system, but sparks over when the voltage across 'the gap exceeds an excessive level. Valve blocks are used as a means for limiting the follow current subsequent to a drop in the impressed voltage so the series gaps are capable of interrupting the follow current and remove the arrester from the system. To perform this function, the valve block must have the capacity to discharge high currents at excessive voltages while increasing its resistance to the discharges at normal or line voltages.
The most commonly used valve blocks are cylindrically shaped and are comprised of silicon carbide particles bound in intimate contact with each other in a matrix,
such as a water glass matrix. During'voltage surges the' silicon carbide particles are subjected to high voltage stresses which can cause ionization of the air in contact with the particles. If the insulation strength of the air between the particles is exceeded, an arc discharge will occur. An accumulation of these discharge arcs can cause a complete block fiashover, i.e. arcs travelling down the outersurface of the block. Once the block has flashed over, the valve blocks are ineffective in limiting the follow. current and the series gaps cannot interrupt the follow current. The result islightning arrester failure.
Various insulating coatings applied over the outside surface of the valve block have been used to prevent this ilashover. In order to provide the required protection, the insulating coating must be completely bonded to the valve block body so that the area between the body and the coating isfree of voids through which arcs can travel. Typically, prior art insulating coatings are formed by applying a moistened mixture of a binder (eg a water soluble silicate) and a filler (e.g. talc, powdered mica or the like) over the outer surface of the valve body and then baked to remove moisture and .set the coating.
Insulating coatings of this type are reasonably adequate for valve blocks which are composed of silicon carbide particles bound in a compression-molded, water glass matrix. However, they are not completely satisfactory for many valve blocks because they do not adequately adhere to the valve block body and voids exist. For instance, with improved valve blocks comprised of silicon carbide particles bound together within a ceramic matrix the above-described prior art insulating coatings are not satisfactory because of their poor adherence.
SUMMARY OF THE INVENTION An object of this invention is to provide a smooth, hard, highly-adherent, electrical insulating coating for electrical resistance devices and a method for making same.
Another object of this invention is to provide an electrical insulating coating for a valve block composed of silicon carbide particles bound together in a ceramic matrix which can be applied to an uncured valve block body and both the coating and body cured to a hardened form in a single heating step.
The electrical insulating coating of this invention is comprised of a major portion of silicon carbide particles, having a bulk resistivity substantially higher than the resistivity of the electrically conductive portion of the body of the electrical resistance device. Because of the sub stantial dilferential between the resistivities of the silicon carbide in the coating and the electrically conductive portion of the resistance device body, the device conducts an electrical current and the coating is electrically insulating.
The coating is prepared by admixing the silicon carbide particles with ceramic forming ingredients and suflicient water to form a mixture which can be applied in a convenient manner as a coating to the outer surface of the electrical resistance device, such as dipping, brushing, rolling and the like. The thus-coated device is fired at an elevated temperature to mature the ceramic-forming ingredients of the coating mixture and form a hard, highlyadherent coating.
Although the insulating coating of this invention can ,be used advantageously for many electrical resistance de-- of the device simultaneously fired to maturity in one step.
BRIEF DESCRIPTION OF THE DRAWING FIG.1 is a perspective view of a lightning arrester valve block coated with an insulating coating of this invention. FIG. 2 1S a cross-sectional view of the valve block of FIG. 1 taken along plane designated 22.
DESCRIPTION OF THE PREFERRED EMBODIMENT Although the insulating coating of this invention can be used in any electrical resistance device made from a material capable of withstanding elevated firing temperatures, it will be described for use on a lightning arrester valve block comprised of silicon particles bound together within a ceramic matrix.
As shown in FIGS. 1 and 2, valve block 10 includes a body 12 comprised of a major portion of silicon carbide particles 14 tightly held together in intimate contact within a ceramic-matrix 16.
A hard, tightly-adherent insulating coating, 18 comprised of a major portion of silicon carbide particles 20, e.g. about 70-85 weight percent, dispersed within a ceramic matrix 22 circumscribes the outer curvate surfaces of body 12. Metallic electrode 24, 24 are provided on the opposite ends of body 12.
The silicon carbide particles used for making coating 18 are preferably green silicon carbide having a bulk resistivity substantially higher than that of the silicon carbide particles of body 12. With this higher resistivity, coating 18 is electrically insulating with respect to the valve body 12. Generally, the bulk resistivity of the silicon carbide particles used in a coating should be about times or more higher than that of silicon carbide particles of the valve body. For example, if the bulk resistivity of silicon carbide particles 14 of valve block body 12 is about 3X10 ohm-cm., the bulk resistivity of the silicon carbide particles 20 used in coating 18 can be about 5X10 ohm-cm. The particle size of the silicon carbides used in the coating to a large degree determines the smoothness and porosity of the final coating, with smaller particles producing a smoother, less porous coating. Generally, the size of the starting particles used in making the coating can be in the range of about 160 to about 320 grit (US. Bureau of Standards) with a particle size of about 240 grit being the most preferred. As described hereinafter, the silicon carbide particles and other ingredients used for forming the coating mixture are preferably admixed in suitable milling apparatus, such as a ball mill, so that the size of the silicon carbide particles are further reduced to provide a very smooth, relatively nonporous final coating.
Ceramic matrix 22 of coating 18 includes a kaolincontaining clay as the major ingredient. The clay acts primarily as a binder during the forming step. Preferably,
the clay is one containing about 12 to about 16 weightpercent naturally-occurring micaceous materials, such as English china clay. Ceramic matrix 22 also includes a fiuxing material which combines with the clay to form theceramic matrix, such as feldspar (either existing as part of the clay or added as needed to obtain a good bond with the clay).
The silicon *carbide particles and the ceramic forming ingredients, i.e. clay andfluxing material, "are admixed with a sufficient amount of water to form a paste-like coating mixture which can be applied as a coating to the outer'surface of body 12 in a convenient manner, such as dipping, brushing, rolling and the like. The amount of water used is the primary control of the viscosity and mixing characteristics of the mixture. Preferably, the coating mixture also includes a thickening agent, especially where larger amounts of water are used to enhance mixing, and a plasticizer. The thickening agent used is one capable of increasing the viscosity of the mixture for easy application and, preferably, also is capable of decreasing the. surface tension of the mixture so that entrapped air can befleasily removed. Aluminum stearate is an example of a material capable of performing both functions. The presence of entrapped air in the coating mixture is highly undesirable because of the potential voids produced thereby in the final coating. Suchvoids represent potential discharge are passages in the coating which can result in the failure of the coating and/or valve block assembly. If desired, separate additives can be used, one of which is capable of increasing the viscosity of the mixture and the other acting to decrease the surface tension of the mixture.
The plasticizer functions to keep the coating elastic during drying and thereby eliminates drying cracks. Any conventional plasticizer capable of performing this, function can be used. When the thickening agent used is insoluble in water, such as is the case with aluminum stearate, a plasticizer which is capable of dissolving the thickening agent, such as a glycerin, should be used.
The silicon carbide particles, ceramic forming ingre- 4' dients, water and other additives (if used) are mixe together in any convenient manner to form a homogeneous admixture thereof. Preferably, they are admixed in suitable milling apparatus, such as a ball mill or the like, so that the size of the silicon carbide particles are reduced to improve the smoothness and decrease the porosity of the final coating. After mixing, the resulting coating mixture is degassed, such as in a vacuum, to remove the air entrapped during mixing and thereby minimize potential voids in the coating.
Valve block body 12 is coated with a sufficient amount of the coating mixture, preferably by dipping for a few seconds, to provide the desired thickness of the insulating coating, The thickness of the coating can be varied to provide the necessary insulative protection. Generally, a final coating thickness of about 15 to about 40 mils is adequate for most valve blocks. The outer surface of the valve block body can be treated prior to application ofthe coating mixture to improve the adhesion, such as by mild abrasion, application of a mild etching solution or the like.
The coated valve block body is dried to remove the moisture from the coating mixture, e.g. heated at to C. for about 1 to 2 hours, and is then fired at an elevated temperature to mature the ceramic matrix. As will be appreciated by those skilled in the art, the firing temperature used depends upon the amount of silicon carbide and ceramic forming ingredients used. For example, increased amounts of fiuxing material in the coating mixture requires lower firing temperatures to mature the ceramic matrix. Increased amounts of clay also require lower firing temperatures but have a proportionately less effect on the firing temperature than the fluxing material. Increased amounts of silicon carbide require higher firing temperatures. A representative firing and cooling schedule is as follows: heat at a rate of about 150 C./hr. to a peak temperature of about 1200 to 1300 C., soak at this peak temperature for about 2 to about 8 hours, and then cool to room temperature at a rate of about 200 C./hr. After cooling, the coated valve blocks are cleaned and metallic electrodes are applied to the opposite ends by a conventional technique, such as by flame spraying.
The amounts of silicon carbide and ceramic forming ingredientsused in a coating mixture is adjusted so that the coefiicient of thermal expansion of the final coating approximates the coefficient of thermal expansion of the valve block body. This prevents the coating from cracking during the firing step. Preferably, the amount of the ingredients are adjusted so that the coeffcient of thermal expansion of the coating is slightly lower than that of the valve body. This produces a final coating which is under slight compression and thereby fits very tightly over the outside surface of the valve block body.
As a guide, it has been found that coating'mixtures made in accordance with the following formulations are acceptable for use on ceramic bound valve block bodiesz Weight percent, based on total Ingredient: weight of mixture Green silicon carbide 40-60 English china clay 7-11 Feldspar 5-7 Aluminum stearate 0.3-0.5 Glycerin 5-15 Water The above formulations are respresentative of those which can be used in making coating mixtures in accordance with this invention and the ingredients and amounts of the ingredients should be regarded as preferred. As will be readily apparent to those skilled in stituted for the English china clay and feldspar. Also,
thickening agents other than aluminum stearate and plasticizers other than glycerin can be used.
The following specific example is presented to illustrate this invention and is not to be construed as to be restrictive or limiting thereof.
EXAMPLE The following constituents are added to a 5 quart ball mill jar:
%-inch high density alumina grinding media 4500 The ball mill jar is rotated at approximately 50 rpm. for about 6 hours. The resulting coating mixture is then degassed in a vacuum of 29 inches of mercury for 20 minutes, after which time the viscosity of the mixture will be approximately 1800 cps. as measured with a Brookfield viscometer (Type LVF, Spindle 4, 60 rpm). 1% inch diameter dried valve block rods comprised of silicon carbide particles bonded together in a ceramic matrix are then dipped into the coating solution for about seconds to provide a final coating of about 25 mils. The thuscoated rods are then fired to maturity in an oxidizing atmosphere containing water vapor using the following firing schedule: heat at a rate of 100 C./hr. to a peak temperature of about 1230 C., a 2 to 8 hours soak at this peak temperature and a cool at a rate of 150 C./hr. to room temperature. The rods are then cut with a diamond saw into appropriate valve block lengths corresponding to the desired IR drop. After washing, and drying, the opposite ends of the blocks are metalized with 0.008 inch copper electrodes by flame spraying. Valve blocks made in this manner have been subjected to and surpassed all the ANSI electrical test requirements.
1. A method for providing an electrically insulating coating on an electrical resistance device including silicon carbide particles bound together comprising:
admixing silicon carbide particles with ceramic forming materials and sufficient water to form a paste-like coating mixture containing about 40 to about 60 weight percent of said silicon carbide particles with said silicon carbide particles being uniformly distributed throughout, the silicon carbide particles in said mixture having a bulk resistivity substantially higher than the bulk resistivity of the silicon carbide particles of said device so that the coating produced from said coating mixture is electrically insulative with respect to said device and the amounts of said silicon carbide particles and said ceramic forming materials in said coating mixture being adjusted so that the coefiicient of thermal expansion of the coating is such as to prevent cracking of the coating during subsequent firing to mature said ceramic forming materials;
applying said coating mixture to the outer surface of said device;
drying said coated device at an elevated temperature to remove at least a major portion of the moisture from said coating; and
firing said coated device at a temperature within the range of about 1200 to about 1300 C. to mature said ceramic forming materials to a hardened form.
2. The method according to claim 1 wherein said electrical resistance device is a lightning arrester valve block having a body comprised of silicon carhide particles bound together within a ceramic matrix;
said coating mixture is applied to the outer surface of said body when said ceramic matrix of said body is still uncured; and
said ceramic forming materials of said coating mixture and said ceramic matrix of said body are both matured to a hardened form during the firing step.
3. The method according to claim 2 wherein the amounts of said silicon carbide particles and said ceramic forming materials in said coating mixture are adjusted so that the coetficient of thermal expansion of said coating is slightly lower than the coefficient of thermal expansion of said body and the matured coating is under compression with respect to the matured body.
4. The method according to claim 3 wherein said particles in said coating mixture are green silicon carbide and have a particle size within the range of about to about 320 grit.
5. The method according to claim 3 wherein said ceramic forming materials include a major amount of a kaolin-containing clay and a fiuxing material.
6. The method according to claim 5 wherein a thickening agent capable of increasing the viscosity of said mixture and decreasing the surface tension of said coating mixture is admixed with said particles, ceramic forming materials, and water.
7. The method according to claim 6 wherein a plasticizer, which maintains said coating mixture sufficiently elastic so as to prevent cracking of said coating during drying, is admixed with the other ingredients of said coating mixture.
8. The method according to claim 7 wherein said coating mixture is subjected to a vacuum to remove entrapped air therefrom prior to being applied to said device.
9. The method according to claim 8 wherein said clay is English china clay.
10. The method according to claim 9 wherein said fiuxing material is feldspar.
11. The method according to claim 10 wherein said thickening agent is aluminum stearate.
12. The method according to claim 11 wherein said plasticizer is glycerin.
13. The method according to claim 12 wherein the amounts of the ingredients forming said coating mixture are within the following ranges:
14. The method according to claim 13 wherein the bulk resistivity of said particles in said coating mixture is at least 10 times the bulk resistivity of the silicon carbide particles in said valve block body.
References Cited UNITED STATES PATENTS 3,009,886 11/1961 Wejnarth 252-516 3,162,831 12/1964 Heath 252-516 3,180,841 4/1965 Murphy 252-516 3,189,477 6/1965 Shaffer 117-123 A 3,380,936 4/ 1968 Masuyama 252-518 LEON D. ROSDOL, Primary Examiner M. F. ESPOSITO, Assistant Examiner US. Cl. X.R.