This invention relates to high-voltage electric line insulators, specifically suspension insulators of the cap-and-pin type. Background of the Invention

Electrical insulators commonly known as suspension insulators can be used individually, but usually form part of a string to support an electrical conductor from a supporting structure. Generally such a suspension insulator comprises two metal hardware members secured to opposite surfaces of a suitably contoured porcelain insulator shell, one hardware member being embedded by means of cement in a cavity in the porcelain insulator shell. The hardware members, typically an upper cap and a lower pin, each are secured by a layer of cement or other suitable material. By this arrangement the metal hardware members are separated and insulated from each other. This traditional combination of metal, porcelain and cement yields a heavy unit, generally weighing eight to thirty pounds.

Prior art suspension insulators, which include a one-piece ceramic head and shed, are easy to break during manufacture, transport, or installation. During operation the insulators suffer from vandalism, especially in those areas in which hunting is prevalent. U.S. Patent 4,689,445 shows a cap-and-pin insulator which has a ceramic shed with a designed failure mode. The ceramic shed is made to fracture along specific fault lines, to maximize the insulation properties of the damaged unit.

Glass or porcelain line insulators are at risk for surface arcing phenomenon, especially in highly polluted or coastal areas. This phenomenon is related to a damp layer of conductive polluting substance on the surface of the insulator. Leakage current dries the layer in some high-current density zones, and conditions promote the generation of electric arcs which short-circuit the dry zones. Numerous solutions have been proposed to mitigate the surface arcing phenomenon. They are generally based on the principle of providing a

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modified distribution of the electric field to make it less favorable to the generation of surface .arcs.

In polluted areas there is an additional problem encountered in the region of the metal pin. Due to the action of the pollution, the stretching of the metal pin when it is under tension, and the leakage current which flows through the metal cap and pin, corrosion takes place. This can lead to failure of the metal pin, and cause the line to drop. One approach has been to thicken the pin member in the areas in which most corrosion occurs. See, for example, U.S. Patent 4,443,659. An alternate approach has been to apply a zinc sleeve to the galvanized pin. These thickening and plating methods merely postpone the corrosion of the pin member, but do not solve the corrosion problem.

Because the prior art has not found an adequate solution to the surface arcing problem and the corrosion of the metal pin, there is a need to wash or clean the surface of line insulators in coastal or polluted areas. This is a process which requires the use of specialized equipment and trained staff, and includes a risk of flashover and damage to the ceramic sheds.

It would be desirable to provide a cap-and-pin type insulation unit which is lighter than those of the prior art, resists the electrical surface phenomena associated with the prior art, and provides improved mechanical properties and pin protection properties, while providing excellent insulation properties.

Objects of the Invention

It is therefore an object of the invention to provide a line insulator with improved pin corrosion protection.

It is therefore an object of the invention to provide a line insulator with improved resistance to surface arcing phenomenon.

It is another an object of the invention to provide a line insulator which is relatively lightweight.

It is another an object of the invention to provide a line insulator which is resistant to breakage.

It is yet another an object of the invention to provide a line insulator which is simple in design and relatively easy to manufacture.

It is an object of this invention to provide methods and apparatus to accomplish the foregoing.

These and other objects will be apparent from the following description and the claims appended hereto.

Summary of the Invention

The improved insulator comprises, adhered in sequential order, a first metal cap, a first porcelain head, a metal pin enclosed by sealing material, a second porcelain head, and a second metal cap. Generally cement is used to adhere the first and second metal cap to the first and second porcelain head, respectively, and to adhere the ends of the metal pin within the recesses in the first and second porcelain heads. The sealing material is a silicone sealant, a sealing gel, or a sealing mastic. A shed is present. Preferably a polymeric shed is attached to and bridges the first and second ceramic head, and encloses the sealing material.

The improved electrical line insulator of this invention comprises (a) a first porcelain head having a first metal cap secured thereto; (b) a second porcelain head having a second metal cap secured thereto; (c) a metal pin situated with a first end secured within a recess of the first porcelain head and with a second end secured within a recess of the second porcelain head; (d) a sealing material enclosing the metal pin body; and (e) a shed attached to at least one of the first porcelain head and the second porcelain head. The shed can comprise a ceramic shed, but preferably comprises a polymeric shed, especially a shed of a non-tracking polymer.

A method of manufacturing an improved electrical line insulator of the invention comprises (a) securing a first metal cap to a first porcelain head, (b) securing a second

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metal cap to a second porcelain head, (c) securing a first end of a metal pin within a recess . of the first porcelain head, and a second end of the metal pin within a recess of the second porcelain head; and (d) coating any exposed body of the metal pin of step (c) with a sealing material selected from the group consisting of a silicone sealant, a sealing gel, and a sealing mastic. A preferred embodiment further includes (e) attaching one or more polymeric shed to at least one of the first and second porcelain head.

Brief Description of the Drawings

Figures 1 through 5 show various cap-and-pin type line insulators of this invention. Figure 6 shows a string of insulator units.

Description of the Invention Including Best Mode

A cap-and-pin type electrical insulator with improved pin protection characteristics, and improved insulation, breakage and weight parameters is disclosed. Cap-and-pin insulators are generally used in the transmission electricity in the 15kV to 735kV range. The insulators are commonly used in series, that is, more than one insulator unit is provided, and the insulator units are joined to one another to provide a string of insulating units.

The improved electrical line insulator of this invention comprises (a) a first porcelain head having a first metal cap secured thereto; (b) a second porcelain head having a second metal cap secured thereto; (c) a metal pin situated with a first end secured within a recess of the first porcelain head and with a second end secured within a recess of the second porcelain head; (d) a sealing material enclosing the metal pin body; and (e) a shed attached to at least one of the first and second porcelain head. The shed can comprise a ceramic shed, but preferably comprises a polymeric shed, especially a shed comprising a non-tracking polymer.

Similar numbers refer to similar function throughout the Figures. The Figures are drawn for clarity and are not drawn to scale.

Figure 1 shows an insulator unit 110 of this invention. The insulator unit 110 is shown in partial cross-section. The insulator unit comprises a first metal cap 112, a first porcelain head 114, a metal pin 116, a second porcelain head 118, and a second metal cap 120. A shed 122 portion, preferably one or more polymeric shed, is attached to each of the first porcelain head 114 and the second porcelain head 118, and encloses the sealing material 124.

When assembled in a series, the first metal cap 112 is attached to the second metal cap of the insulator unit above it (not shown). Similarly, the second metal cap 120 is attached to the first metal cap of the insulator unit below it (not shown). Suitable assemblies are well known in the art. Conveniently the caps are manufactured from cast iron. For convenience, the caps are preferably configured in conformance with industry standards, so that an insulator unit of this invention can easily replace a worn or broken unit in the field. As shown in Figure 1, a standard ball-and-socket type linkage can be used. The first metal cap 112 provides the socket fitting 126, and the second metal cap 120 provides the ball fitting 128. Tongue-and-clevis embodiments are shown in Figures 2, 4, and 5. Other attachment schemes, including alternate pins or linkages, can be used.

The first metal cap 112 is joined to the first porcelain head 114 by the first cap securing means 130. The second metal cap 120 is joined to the second porcelain head 118 by the second cap securing means 132. The first cap securing means 130 and the second cap securing means 132 can be the same material, or they can be different. Preferably, each of the first cap securing means 130 and the second cap securing means 132 is neat Portland cement. However, either or both of the first cap securing means 130 and the second cap securing means 132 can be a high mechanical strength cement or polymer concrete. Suitable cap securing means are well known in the art.

The first porcelain head 114 and the second porcelain head 118 can each comprise, for example, porcelain or other ceramic material, a glass or other vitreous material, or other materials presently used as electrical insulation material in high voltage insulators. Porcelain is a preferred insulating material in some applications because of its superior resistance to

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damage by electrical discharges, to weathering, and to chemical attack. It is not an ' expensive material to manufacture into an insulator, such as a porcelain head. It is to be understood that the term "porcelain" is used for convenience of terminology, and is intended to include porcelain as well as these alternate materials.

In a preferred embodiment, the first porcelain head 114 and the second porcelain head

118 are each a metal oxide dielectric dense body as described in PCT WO90/03955, the disclosure of which is incorporated herein by reference. These ceramics can be fired at relatively low temperatures, which simplifies the manufacturing process. The ceramics exhibit good mechanical properties. Especially preferred are porcelain heads made of mullite, mullite-silica, or silica.

The first porcelain head 114 and the second porcelain head 118 can be made of the same or similar material, or they can be different. For example, one porcelain head can a mullite material, and the other porcelain head can comprise a silica material.

The first porcelain head 114 and the second porcelain head 118 can have similar configurations, as shown, or they can each be different from the other. An insulator unit incorporating two very different insulator head units is shown in Figure 4. For purposes of simplicity it may be preferred that each of the first porcelain head 114 and the second porcelain head 118 be manufactured of the same material, and in similar shapes.

The first porcelain head 114 and the second porcelain head 118 are each generally a cup-shaped member. The specific configuration of the porcelain heads can be varied as desired. For example, the walls of the "cup" can be extended as desired to provide a platform for the adhesion of the polymeric shed, as shown in Figure 3. As shown in Figure 1, the porcelain heads can each have straight sides, ending in a flattened or curved lip.

The first porcelain head 114 is joined to the metal pin 116 by the first pin securing means 134. The second porcelain head 118 is joined to the metal pin 116 by the second pin securing means 136. The first pin securing means 134 and the second pin securing means

136 can be the same material, or they can be different. Preferably, each of the first pin securing means 134 and the second pin securing means 136 is neat Portland cement. However, either or both of the first pin securing means 134 and the second pin securing means 136 can be, for example, a cement or polymer concrete. Suitable pin securing means are well- known in the art.

The metal pin 116 is generally a steel pin having two enlarged and rounded ends. Each of the ends is configured to integrate with the metal cap member. The design parameters are described in Insulators for High Voltages, by J.S.T. Looms (1988, Peter Peregrinus Ltd., publishers), the entire contents of which are incorporated herein by reference. The metal pin 116 is generally from about less than about 8 inches in length to more than about 12 inches in length. The diameter of the metal pin 116 will be from about less than about 1/2 inch to more than about 1 inch along the narrow body portion of the pin; and from less than about 1 inch to more than about 2 inches at the widest portion of each end.

Surrounding the metal pin body is a sealing material 124. The sealing material 124 acts to enclose the metal pin shaft and protect it from the environment. The sealing material 124 is a material which is non-conductive, and is preferably inexpensive, light, has good adhesion properties, and is non-flowing at ambient temperatures. Examples of such materials includes known high voltage silicone, mastics, and gels. Preferred sealing materials 124 include a silicone, such as Dow Corning 738 RTV, or General Electric RTV-31.

The sealing material 124 is preferably not exposed to the environment. It is enclosed by the ceramic heads and by the polymeric shed (as shown in Figure 1), or by the ceramic heads and a polymeric belt (as shown in Figure 5).

When the insulator unit is in use, the unit is generally under a stretching tension. The interactiøn of the metal caps 112 and 120 with the ends of the metal pin 116 is well documented. The body of the metal pin 116 stretches and becomes somewhat thinned. In cap-and-pin insulators of the prior art, thinning in the area of the pin securing means may

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have provided an entry for water or other contaminants and led to the corrosion of the pin at •the pin/securing means interface. In an insulator unit of the subject invention, the pin is surrounded by the sealing material 124. Even though the pin thins, the sealing material 124 and the external shed 122 keep water from the pin/securing means interface. Preferably the sealing material 124 and/or the shed 122 have sufficient elastic properties that they give gently without cracking when tension on the unit is increased or released.

As shown in Figure 1 , a shed 122 is attached to the porcelain heads. The shed 122 of Figure 1 comprises two separate shed portions 122a and 122b, each shed including one fin 138. The sheds are overlapped to provide resistance to weathering and to the effects of electrical activity.

In general, the shed 122 can comprise a ceramic material which is a continuation of the ceramic material of the porcelain head, such as the sheds found in traditional insulator units. Preferably, the shed 122 comprises a high-voltage polymer. Multiple shed units 122 can be present. Multiple fins 138 can be present, on one or more shed 122. When two or more sheds 122 or fins 138 are present, they can be substantially the same, or they can be different in shape or in composition. A small sampling of possible shed and fin configurations and materials are represented in the Figures. In preferred configurations, the shed is a polymeric shed including one or more fins, as shown in Figures 1, 2 and 3.

The use of a polymeric shed 122 in combination with a first and second porcelain head 114 and 118 provides a variety of advantages over traditional materials. The units provide an appreciable reduction of weight when contrasted to units having traditional porcelain sheds. Standard insulator units weigh from about eight pounds to more than thirty pounds. An insulator unit of this invention which includes polymeric sheds will weigh in the range of less than about six pounds to about ten pounds, and provide approximately two times the insulative capacity as a standard unit of the prior art. The polymeric shed 122 is significantly less subject to breakage in manufacture, shipping, use, and cleaning than porcelain sheds. The polymeric shed 122 is not subject to fracture from vandalism and, if

damaged, provides an improved insulator when contrasted to a porcelain shed. Additionally, the porcelain head portions are largely enclosed within a metal cap 112 or 120, or covered by the polymeric shed 122, so that they are protected from damage.

The polymeric shed 122 is advantageously electrically substantially non-tracking. It should have good weather resistant properties when it is to be used out of doors, and may comprise a thermoplastic material, which may or may not be cross-linked; a thermoset material; or an elastomeric material. The polymeric shed generally comprises one or more anti-tracking high voltage insulating materials, such as those described in U.S. Patents 4,399,064 and 4,521 ,549, the disclosure of each of which is incorporated herein by reference. The polymeric shed is preferably a polyolefin or other olefin polymer, obtained from two or more monomers, especially terpolymers, polyacrylates, silicone polymers and epoxides, especially cycloaliphatic epoxides. Among epoxide resins of the cycloaliphatic type there may be especially mentioned those sold commercially by CIBA (A.R.L.) Limited, under the names CY 185 and CY 183. Particularly suitable polymers include polyethylene, ethylene/ethyl acrylate copoiymers, ethylene/vinyl acetate copolymers, ethylene/propylene copolymers, ethylene/propylene non-conjugated-diene terpolymers, chlorosuiphonated polyethylene, polypropylene, polydimethyl siloxane, dimethyl siloxane/methyl vinyl siloxane copolymers, fluoro silicones, e.g., those derived from 3,3,3-trifluoropropyl siloxane, carborane siloxanes, e.g. "Dexsil" polymers made by Olin Mathieson, polybutyl acrylate/acrylonitrile copolymers, butyl acrylate/acrylonitrile copolymers, butyl acrylate/glycidyl methacrylate copolymers, polybutene, butyl rubbers, ionometric polymers, e.g. "Surlyn" materials sold by DuPont, or mixtures of any two or more of the above. More preferably the polymeric shed is an ethylene/vinyl acetate copolymer.

Generally, the polymeric shed 122 is moulded or push-fitted onto the porcelain head or porcelain heads. An adhesive layer 140, as described below, which will bond to both the porcelain head and to the polymeric shed, is preferably present.

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Alternatively, the polymeric shed can be recovered (for example, by heat) onto the porcelain head. A recoverable article is an article in which the dimensional configuration can be made to change when subjected to an appropriate treatment. The article can be heat- recoverable, such that the dimensional configuration can be made to change when subjected to a heat treatment. Usually these articles recover, on heating, towards an original shape from which they have previously been deformed, but the term "heat recoverable", as used herein, also includes an article which, on heating, adopts a new configuration, even if it has not been previously deformed. In their most common form, such articles comprise a heat- shrinkable sleeve made from a polymeric material exhibiting the property of elastic or plastic memory as described, for example, in U.S. Patents 3,086,242 and 3,597,372. High voltage heat-shrinkable polymers are described in U.S. Patents 4,399,064 and 4,521,549.

The original dimensionally heat-stable form can be a transient form in a continuous process in which, for example, an extruded tube is expanded, while hot, to a dimensionally heat-unstable form. In other applications, a preformed dimensionally heat stable article is deformed to a dimensionally heat unstable form in a separate stage. The polymeric material can be cross-linked at any stage in its production that will enhance the desired dimensional recoverability. One manner of producing a heat-recoverable article comprises shape in the polymeric material into the desired heat-stable form, subsequently cross-linking the polymeric material, heating the article to a temperature above the crystalline melting point or, for amorphous materials the softening point, as the case may be, of the polymer, deforming the article and cooling the article while in the deformed state so that the deformed state of the article is retained. In use, since the deformed state of the article is heat-unstable, application of heat will cause the article to assume its original heat-stable shape. In other articles, as described for example in British Pat. 1 ,440,524, an elastomeric member such as an outer tubular member is held in a stretched state by a second member, such as an inner tubular member. Upon heating the inner tubular member weakens and allows the elastomeric member to recover.

The polymeric shed portion has at least one external fin 138, and an inner surface of predetermined normal configuration and diameter. The polymeric shed can be molded in place, for example by injection molding; it can be adhered to the porcelain head using an adhesive layer 140; or, preferably, a combination of methods can be used.

The adhesive layer 140 forms a bond between the porcelain head(s) and the polymeric shed 122, and comprises, for example, a high-voltage mastic or bonding agent. The adhesive can be any of several known adhesive compounds. Preferably the adhesive causes a permanent bond, and adheres to both the porcelain material of the porcelain head and to the polymeric material of the polymeric shed. Standard high voltage mastics can be used. The adhesive is preferably a member of one of three families: the silane coupling agents, the organic titaπate coupling agents, and the organic zirconate coupling agents. The silane agent tri-methoxy silane propy I m ethacryiate , for example, can be used.

The adhesive layer 140 generally does not adhere to the sealing material 124. However, the adhesive layer 140 can be present at the interface of the polymeric shed 122 and the sealing material 124 for ease of manufacturing. Preferably, when the adhesive layer

140 does not adhere to the sealing material 124, the adhesive layer 140 is inert with respect to the sealing material 124.

Figure 2 shows an insulator unit 210 of this invention. The insulator unit 210 is shown in partial cross-section. The insulator unit comprises a first metal cap 212, a first porcelain head 214, a metal pin 216, a second porcelain head 218, and a second metal cap 220. A shed

222 portion having three fins is attached to both the first porcelain head 214 and the second porcelain head 218 by the adhesive layer 240, and encloses the sealing material 224. The polymeric shed 222 is located at the edge of each of the first porcelain head 214, and the second porcelain head 218, such that areas of each porcelain head is exposed. The first metal cap 212 and second metal cap 220 are provided with a tongue-and-clevis attachment means, wherein adjoining units are connected with a pin 242 to link units.

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In polluted conditions, two types of electrical discharge activity will take place on the surface of an insulator. The first type takes place randomly over the entire surface area, and, although the surface is eroded, this activity is not very intense and generally does not seriously damage the insulation. The polymeric sheds used herein preferably comprise a shed made of an anti-tracking high voltage insulating material such as that of U.S. Patents 4,399,064 and 4,521 ,549, the disclosure of each of which is incorporated by reference. This polymeric shed is less subject to fouling in coastal or polluted regions than porcelain sheds.

The second type of activity is that which becomes rooted or anchored, for example causing sparking at a boundary of the insulation with a metal fitting or beneath a shed, and taking place preferentially over a particular portion of the insulating surfaces. This latter activity is more intense than the former, and is often the limiting factor in the lifetime of the insulator.

To combat this sparking, a portion of the first porcelain head 214, is exposed between the first metal cap 212 and the polymeric shed 222, at region 214e. Similarly, a portion of the second porcelain head 218, is exposed between the second metal cap 220 and the polymeric shed 222, at region 218e. This configuration prevents metal sparking, which can occur in the immediate vicinity of metal (such as the metal caps), from damaging the polymeric shed. Instead, the metal spark is directed primarily onto the exposed surface of the porcelain heads 214e and 218e, and not onto the surface of the more vulnerable polymeric shed. The advantages of an exposed porcelain surface are discussed in U.S. Patent

4,845,318, which is incorporated herein by reference.

Figure 2 shows a single polymeric shed 222 having three fins 238a, 238b and 238c of different size. These fins provide creepage distance for any surface electrical activity. Desired creepage distances for a given insulation range are described, for example, in Insulators for High Voltages, by J.S.T. Looms, cited above.

Figure 3 shows an alternate configuration of an insulator unit 310 of this invention. The first metal cap 312 and second metal cap 320 of adjoining units are connected in a ball-

and-socket manner, with the first metal cap 312 providing the socket 326, and second metal cap 320 providing the ball 328. The insulator unit 310 is shown in partial cross-section.

The insulator unit comprises a first metal cap 312, a first porcelain head 314, a metal pin 316, a second porcelain head 318, and a second metal cap 320. The rim edges of each of the first porcelain head 314 and the second porcelain head 318 extend to form a lip. The polymeric shed 322 portion is molded above each of the lip portions, and encloses the sealing material 324. In alternate embodiments, not shown, the rim of one or more porcelain head exhibits additional ridges, rims, variations, and the like, to increase the surface area to which the polymeric shed 322 can be attached. In the embodiment shown, the polymeric shed 322 overlaps each of the first metal cap 312 and the second metal cap 320. The metal caps and the polymeric shed 322 can be joined by molding the polymeric shed around the metal caps, without the use of an adhesive. Preferably, however, an adhesive is used.

The polymeric shed 322 can comprise one or more layer of polymer. A polymer which is not substantially non-tracking 322a can be covered with a polymer which is substantially non-tracking 322b, as shown, to form the polymeric shed 322. This provides a non-tracking surface for the polymeric shed 322, while permitting the use of less expensive insulating polymers in non-critical areas.

As shown, the porcelain heads and the polymeric shed 322 can be joined by molding the polymeric shed around the porcelain head, without the use of an adhesive. Preferably, however, an adhesive is used.

Figure 4 shows an insulator unit 410 of this invention. The first metal cap 412 and second metal cap 420 of adjoining units are connected in a tongue-and-clevis manner, with the first metal cap 412 providing the tongue, and second metal cap 420 providing the clevis. A pin 442 connects adjoining insulator units. The insulator unit 410 is shown in partial cross- section.

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The insulator unit comprises a first metal cap 412, a first porcelain head 414, a metal pin 416, a second porcelain head 418, and a second metal cap 420. The shed 422 sub- units are of two different materials. A porcelain shed 422a is a continuation of the the second porcelain head 418. A polymeric shed 422b is molded to attach to the first porcelain head 414 and the second porcelain head 418. The polymeric shed 422b encloses the sealing material 424. As shown, the polymeric shed 422b is similar in shape to the porcelain shed 322a. In alternate embodiments, not shown, the polymeric shed 422b and the porcelain shed 422a are different in shape.

Figure 5 shows an insulator unit 510 of this invention. The insulator unit 510 is shown in partial cross-section.

The insulator unit comprises a first metal cap 512, a first porcelain head 514, a metal pin 516, a second porcelain head 518, and a second metal cap 520. The porcelain sheds 522a and 522b are a continuation of the the first porcelain head 514 and the second porcelain head 518, respectively. A polymeric shed belt 522c is molded to attach to the first porcelain head 514 and to the second porcelain head 518. The polymeric shed belt 522c encloses the sealing material 524. The adhesive layer 540 adheres the polymeric shed belt 522c to the first porcelain head 514 and to the second porcelain head 518.

Figure 6 shows a string of the cap-and-pin insulators 610 of this invention. The individual insulators 610 are linked with a ball-and-socket arrangement such as that shown in Figure 1 and Figure 3.

The following examples illustrate the invention:

Example 1

Mullite-Silica Head Portion

A bismuth stock solution is prepared by dissolving bismuth nitrate pentahydrate (Bi(Nθ3)3 ^« 5H2θ), 5.82 Kg) in concentrated nitric acid (3.84 L) and then diluting with water to a final volume of 40 L.

A 3 gallon mill jar is charged with 300 burundum cylinders (13/16 x 13/16), clay (1.25 Kg, 46.8 atom % Si and 48.2 atom % Al), and 3 L deionized water. The mixture is ball-milled for 72 hours, after which the clay-water slurry is transferred and diluted with water to a volume of 10 L, giving a slurry composition of 1.25 Kg clay/L slurry.

10 L of the clay slurry is added to a vessel. 10 L deionized water, and 2 L concentrated ammonium hydroxide is added. The mixture is homogenized 15 minutes. Finally, 3.322 L of the bismuth stock solution (5.0 atom % Bi) is added to the mixture, which results in the precipitation of the bismuth species onto the clay. The resultant is homogenized for 10 minutes to yield a precursor material.

The precursor material is collected by suction filtration and dried at 140°C. The dried powder is subsequently calcined to remove residual ammonium nitrate by heating according to the following schedule: 4.5 hr at 30-300°C, then 1 hr at 300°C.

The calcined power is ground, sieved with a <106 micron mesh, and 1.22 Kg of the powder is uniaxially pressed at 10,000 psi into a cupped head mold, and fired for 1.5 hr. at 30- 1 _F 100°C, then 12 hr. at 1 ,100°C.

Example 2

Silica Head Portion

A bismuth stock solution is prepared by dissolving bismuth nitrate pentahydrate (Bi(Nθ3)3 ^« 5H2θ), 5.82 Kg) in concentrated nitric acid (3.84 L) and then diluting with water to a final volume of 40 L.

A vessel is charged with colloidal silica (7.521 Kg, 95.7 atom % Si), 2 L deionized water, and 500 mL concentrated ammonium hydroxide. The mixture is homogenized for 5

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min. To this mixture is added 7.5 L of the above bismuth stock solution (4.3 atom % Bi), which results in the precipitation of the bismuth species onto the silica. The mixture is then homogenized for 10 minutes to obtain a precursor material.

The precursor material is collected by suction filtration and dried at 140°C. The dried powder is subsequently calcined to remove residual ammonium nitrate by heating according to the following schedule: 4.5 hr at 30-300°C, then 1 hr at 300°C.

The calcined power is ground, sieved with a <106 micron mesh, and 1.22 Kg of the material is uniaxially pressed at 10,000 psi into a cupped head mold, and fired for 1.5 hr. at 30-1,100°C, then 12 hr. at 1,100°C.

Example 3

Mullite Head Portion

A bismuth stock solution is prepared by dissolving bismuth nitrate pentahydrate (Bi(Nθ3)3*5H2θ), 1.96 Kg) in concentrated nitric acid (1.28 L) and then diluting with water to a final volume of 40 L.

Aluminum nitrate nonahydrate (110.4 g, 67.5 atom % Al) is dissolved in 0.2 N nitric acid (1 L). To this solution is added colloidal silica (14.7 g, 22.5 atom % Si) and 436 mL of the above bismuth stock solution (10 atom % Bi). Concentrated aqueous ammonium hydroxide (2 L) is added to precipitate the precursor material, which is is collected by suction filtration and dried at 140°C. The dried powder is ground, sieved with a <106 micron mesh, and 122 Kg of the material is uniaxially pressed at 25,000 psi into a cupped head mold, and fired for 2 hr. at 1,000°C.

Example 4

Non-Tracking Polymer

A formulation is made as follows, with parts determined by weight. The following materials are mixed in the order given: 30 parts dimethyl silicone elastomer (containing a small amount of methyl vinyl siloxane); 30 parts low density polyethylene; 30 parts ethylene ethyl acrylate; 30 parts alumina trihydrate having a surface area of 16.0 m ² /g; 2 parts polymerized trihydroquinaline oxidant; 5 parts calcined ferric oxide; 1 part triallyl cyanurate; and 1 part 2,5-dimethyl 2,5-di-t-butyl peroxy hexyne-3.

Example 6

Manufacture of Device

97 mL of Portland cement is poured into a first cast iron cap member. A first head portion according to Example 1 is positioned into the wet cement, and the cement is allowed to set. 43 mL of Portland cement is poured into the first head portion, and the first end of a steel pin is positioned within the first head portion. The cement is allowed to set, and the head structure is tested for mechanical strength.

97 mL of Portland cement is poured into a second cast iron cap member. A second head portion according to Example 1 is positioned into the wet cement, and the cement is allowed to set. 43 mL of Portland cement is poured into the second head portion, and the second end of the steel pin (with the first cap and first head portion attached, above) is positioned within the second head portion. The cement is allowed to set, and the head structure is tested for mechanical strength.

A sealing material comprising 738 RTV (Dow Corning) is poured into a mold into which the double-cap-and-head structure has been positioned, and the sealing material is cured at room temperature for 30 minutes. The unit is substantially cylindrical.

A polymer according to Example 4 is injection molded in a shed mold into which the cylindrical structure has been positioned, and the polymer is heated at 190°C for 15 minutes. The insulator unit is tested for electrical properties.

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Example 7

Alternate Devices

The process of Example 6 is repeated, substituting the porcelain head of Example 2 or Example 3 for the porcelain head of Example 1.

The above processes of Examples 6 and 7 are repeated, substituting a mastic, a titanate coupling agent, or a zirconate coupling agent for the silane coupling agent.

The above processes of Examples 6 and 7 are repeated, substituting a high-voltage mastic, an oil, or a gel for the sealing material.

While the invention has been described in connection with specific embodiments thereof, those skilled in the art will recognize that various modifications are possible within the principles described herein. Such modifications, variations, uses, or adaptations of the invention, including such departures from the present disclosure as come within known or customary practice in the art, fall within the scope of the invention and of the appended claims.