WO1993005521A1

WO1993005521A1 - Silica based mineral insulated cable and method for making same

Info

Publication number: WO1993005521A1
Application number: PCT/US1992/007664
Authority: WO
Inventors: Williams J. Koch; Collins P. Cannon
Original assignee: American Technology, Inc.
Priority date: 1991-09-12
Filing date: 1992-09-10
Publication date: 1993-03-18
Also published as: AU2583392A

Abstract

A mineral insulated cable (18) is provided with a ceramic insulator formed primarily from fused silica. The silica insulator is constructed from a ceramic preform (10) composed primarily of substantially pure fused silica. The preform (10) is swageable and allows the fabrication of a silica insulated cable (18) possessing very high electrical bandwidth, high electrical resistivity, and high resistance to environmental stresses, such as, radiation, high temperature, pressure, and corrosive chemical environments.

Description

SILICA BASED MINERAL INSULATED CABLE AND METHOD FOR MAKING SAME

This invention is generally directed to mineral insulated cables and, more particularly, to a silica insulated cable, the preform used as the mineral insulation in such cables, and a. method for producing the silica preform.

Generally, there are two types of cables for use in transmitting electrical signals, soft and hard. Soft cables typically include an inner conductive wire surrounded by an organic insulator, such as rubber or plastic. These organic cables have the advantages of being relatively inexpensive to manufacture, flexible and therefore inexpensive to install, and good multi- frequency conductors, owing to the relatively low dielectric constant of the organic insulator. That is, soft cables are typically capable of transmitting electrical signals within a broad spectrum of frequencies, which is commonly referred to as having a broad bandwidth. In particular, soft cables are relatively good at transmitting high frequency signals.

Hard cables, on the other hand, typically have less of the electrical transmission advantages possessed by soft cables, but are advantageously resistant to damage and disruption from environmental stresses, such as nuclear radiation, temperature, or pressure, which is not true of soft cables. Typically, a hard cable includes an inner conductive wire surrounded by a mineral insulator, such as a ceramic, and placed within a metallic sheath. The mineral insulator resists the invasive effects of environmental stress, such as radiation, owing to its crystal structure with its very strong covalent or ionic type atomic bonds. The insulation in soft cables, on the other hand_r has low strength organic chemical bonds holding the structure together. Thus, any stress, such as nuclear radiation or heat, can disturb the bonding of the atoms in the insulation and severely deteriorate its insulative properties.

While hard cables are advantageously resistant to environmental stress because of the structure of the inorganic mineral insulator, they are correspondingly disadvantaged by this same inorganic structure. That is, hard cables typically do not have good electrical characteristics at high frequency, owing to the relatively high dielectric constant of a typical mineral insulator. Rather, hard cables are generally limited to the transmission of low frequency and D.C. signals. Hard cables typically do not have a high bandwidth.

In the past, mineral insulation for cables has typically been formed from MgO or A1₂0₃. These two ceramics have high dielectric constants, each in excess of 9.0. This, along with the general method of manufacture, makes them unsuitable for any applications requiring high bandwidth. In an attempt to produce a mineral insulated cable that possesses a high bandwidth, some manufacturers have used amorphous silica (Si0₂) , ground quartz, ground cristobalite or ground tridimite as the insulation material. The dielectric constant of silica is 3.8. Silica, however, has proved somewhat problematic.

For example, during the construction of silica-based hard cables, the amorphous silica or ground quartz,

(typically 97 to 98% pure) , is mixed with water into a paste-like consistency, and then extruded onto the wires and into the sheath in this relatively wet form. The entire cable is then heated for a relatively long period of time to drive off most of the water. Water trapped well inside the cable is very difficult to remove. Therefore, expensive furnaces capable of drawing a hard vacuum have been used to aid in removing the water. However, as is to be expected, even this expensive process does not remove all of the water from the cable, but rather, trace water and other impurities remain therein. The trace water and impurities in the raw material remaining in the cable can cause intergranular attack or corrosion and ultimate failure of either the housing, or more likely, contaminate the wire.

Additionally, while silica is generally very stable, reaction between the sheath and the impure silica is sometimes observed. This reaction can also result in the metal conductors being corroded and ultimately failing.

The present invention is directed to overcoming one or more of the problems discussed above by providing a hard cable that is both resistant to environmental stresses and is capable of transmitting relatively high- frequency signals. This invention eliminates effects of residual moisture and other impurities, and gives electrical and dielectric properties not possible with organic insulation or all other forms of presently used silica as amorphous silica, ground quartz, ground cristobalite or ground tridimite. In one aspect of the present invention, a ceramic preform for use as an insulator in a mineral insulated cable is provided. The ceramic preform is formed primarily from fused silica.

In another aspect of the present invention, a paste suitable for firing into a ceramic preform for use as an insulator in a mineral insulated cable is provided. The paste is formed from a mixture of fused silica and a binder solution.

In still another aspect of the present invention, a method for forming a ceramic preform to be used as a insulator in a mineral insulated cable is provided. The method includes the steps of: preparing a binder solution; combining fused silica and the binder solution into an extrudable paste; extruding the paste into a preselected geometric configuration; and firing the extruded paste.

In another aspect of the invention, a mineral insulated cable is provided. The cable includes at least one conductive wire, a sheath positioned about and spaced from the conductive wire, and an insulator positioned between the conductive wire and the sheath, the insulator being comprised of fused silica.

Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawing in which:

Fig. 1 illustrates a perspective cut-away view of a coaxial mineral insulated cable; and

Fig. 2 illustrates a perspective cut-away view of a triaxial mineral insulated cable. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that this specification is not intended to limit the invention to the particular forms disclosed herein, but on the contrary, the intention is to cover all modification, equivalents, and alternatives falling within the spirit and scope of the invention, as defined by the appended claims.

Referring now to the drawings. Fig. 1 shows a cut¬ away perspective view of a ceramic preform 10, a center conductor electrical wire 12 and a metallic sheath 16, which, when assembled, form a hard cable 18. The metallic sheath 16 is preferably formed from a nickel iron chrome alloy, but can be constructed from a more refractory metal, if desired. The electrical wire 12 is formed from any suitable conductive material, however, copper is preferred. The ceramic preform 10 is preferably formed from a compound that includes a significant amount of silica. The specific compound and method used in constructing the preforms 10 is discussed in greater detail below. However, for a proper understanding of the instant invention, it is useful to first appreciate the construction and assembly process of a hard cable.

During assembly, the wire 12 is manually threaded through a series of relatively short preforms 10 (i.e., two inches each) until a desired length is reached (i.e. forty feet) . Thereafter, the wire 12 and preforms 10 are inserted into a sheath 16 to form the hard cable 18.

At this point, the assembled hard cable 18 is passed through conventional metal drawing dies. The drawing dies forcibly reduced the diameter of the sheath 16, and thereby lengthen the cable by a small amount. Thus, by passing the cable 18 through a series of progressively smaller dies, the cable 18 is consecutively reduced in diameter and increased in length until a desired diameter and/or length is reached. It is preferable to heat treat (anneal) the cable between successive draws to remove work hardening from the metal sheath.

It should be appreciated that the ceramic preforms 10 positioned within the cable 18 are relatively hard, but are crushed lightly during the drawing operation. This crushing action, however, advantageously allows the ceramic to fill any voids within the cable 18 and thereby improve the cable's electrical properties.

The instant invention also finds beneficial application in multi-conductor cables, i.e. cables where the number of electrical conductor wires present within the interior of the ceramic insulation varies from one to upwards of 200. In cable geometries of this type, it is generally preferable to position the wires so that insulation thickness between adjacent wires and between wires and the sheath are equal.

The instant invention also finds beneficial application in triaxial-type hard cables. Triaxial-type hard cables are substantially similar to that shown in Fig. 1, but include an additional metallic sheath 20 positioned between the wire 12, and the sheath 16 (see Fig. 2) . Insulation 22, 24 is provided between the sheaths 16, 20 and also between the additional sheath 20 and the wire 12. In such a triaxial-type hard cable, the insulation 24 placed between the wire 12 and additional sheath 20 is preferably fused silica, whereas the insulation 22 placed between the sheaths can take any of a variety of forms, including, but not limited to, mineral insulators, such as fused silica.

Fused silica (Si0₂) , in contrast to soft organic insulation or amorphous silica, ground quartz, cristobalite or tridimite insulation, strongly resists environmental stresses, such as nuclear radiation, temperature, etc. , and yet has excellent electrical properties in addition to its low dielectric constant (approximately 3.8). Further, fused Si0₂ is very chemically stable and does not tend to react with the sheath or wire of the hard cable. Fused silica possesses a lower dielectric constant and considerably higher voltage breakdown resistance than any of the other four forms of silica (i.e., amorphous silica, quartz, cristobalite or tridimite) . The thermal conductivity is very low and uniform across a temperature range from room temperature to its melting point at about 1732°C. The coefficient of thermal expansion has similar uniformly low values.

The resistance to voltage breakdown of fused silica is very high and nearly double that of the other four types of silica. For example, fused silica has a resistance to voltage breakdown of about 500 volts per mill, in a 1/8 inch thick test sample. This is second only to Boron Nitride in voltage breakdown resistance.

Preparation of Fused Silica Swageable Ceramic Powder

The first step in manufacturing a fused silica preform involves the preparation of a swageable ceramic powder, consisting of pure ground fused silica. Fused silica is a chemically fabricated material. The process for manufacturing fused silica involves heating high purity alpha quartz to a temperature above the melting point of silica, 1732°C. This temperature causes the alpha quartz grains to fuse together and form a molten high viscosity liquid. The liquid is quenched by pouring the white hot material into distilled water. Quenching, of course, causes the liquid material to solidify, but in a shattered form.

The resultant shattered material is then ball milled using silica balls in a silica lined mill to an average particle size of 8 to 10 microns. This resulting ball milled silica is very fine, with a powder like consistency.

At this point complete laboratory tests are made to confirm: average particle size and purity of the resultant fused silica powder. The impurities are measured in parts per million using spectrographic analysis. In the alternative, acceptable fused silica powder is available from, for example, C-E Minerals, Greeneville, Tennessee as electronic grade 44B.

Preparation of Binder Solution

A binder solution is combined with the ground fused silica powder to form a paste that is suitable for extruding and will substantially maintain the extruded shape. Accordingly, the next step involves the preparation of the binder solution. A variety of well known binders could be readily used, but preferably, an ash free binder is employed. One example of such an ash free binder is discussed below. The following list of items should be combined in the order listed and then heated to approximately 85°C. Substantial stirring is preferred during the heating process. Triple Distilled Water, 3001 grams

Glycerin, 567.5 grams Maycon-10, 56.75 grams Triton X-100, 5.7 grams

When the solution reaches 85°C, it should be removed from the heat, and 567.5 grams of Methocel #A4C is stirred into the solution slowly. After approximately two minutes of stirring, the solution will appear milky in color. Thereafter, 1800 grams of triple distilled water is stirred into the milky solution. Preferably, the triple distilled water is approximately room temperature. This entire solution is preferably covered and aged overnight. Shelf life of the solution is estimated to be approximately six to eight weeks if kept cool (i.e. 7 to 10°C) .

Preparation of Extrusion Paste

With the binder solution and ceramic powder prepared, they may be combined to form an extrusion paste. Approximately 530 grams of the fused silica powder and 389 grams of the binder solution are added to a sigma blade type stainless steel mixer, such as is available from Paul 0. Abbe', Inc., of Little Falls, New Jersey. Preferably, the total working chamber, including all moving parts, of the mixer are coated with titanium nitride. A titanium nitride coated cover plate of the mixer should be locked in place to prevent spillage.

The mixer is first operated until the contents appear uniform (approximately three minutes) . Approximately 190 grams of triple distilled water are added and mixed for an additional three minute period. Thereafter, any additional distilled water is added using a 10% binder 90% triple distilled water solution mixture, up to 80 grams maximum. The resulting mixture should be sufficiently stiff to have no deformation of the hole or outer diameter of the preform 10 after extrusion or drying.

Thereafter, 490 grams of fused silica powder is added to the mixture. After all of the fused silica powder has been added, and mixed for 3-minutes, the batch is divided in half, and each half batch is mixed for approximately three to four minutes. Each half batch is then placed in a plastic bag and aged for an approximate minimum time of ten hours at a temperature of 7 to 10°C.

At this time, it is desirable to check the moisture content of the paste. Preferably, the moisture content should be approximately 27±1 percent. The moisture content can be determined either by using a weight loss method or by using a moisture balance, such as a Mettler LP16-M Moisture Determination System available from Fisher Scientific Co. of St. Louis, Missouri, as item number 01-913-93B. The weight loss method of determining moisture content involves accurately weighing the paste before and after drying at about 110°C for about four hours. The difference in weight corresponds to the amount of moisture lost therefrom.

The paste should preferably be extruded within one week after preparation.

Extrusion

The paste should be kneaded by hand on a clean plexiglass sheet for approximately two minute to give it uniform plasticity and to form it into a shape for loading into a tungsten carbide extrusion chamber. The extrusion process may begin using, for example, a 40-ton extruder having an extrusion chamber and plunger or ram head formed from tungsten carbide. Such an extruder is available from Loomis Products, Co. of Levittown, Pennsylvania.

With the extruder turned on, its ram plunger should slowly be moved downward into the extrusion chamber with the vacuum pressure also turned on. When the gauge begins to show pressure, the ram plunger should be stopped and held in place while the vacuum is operated for approximately four minutes before further operating the extruder.

It should be appreciated that the extruder includes a pin used to form the opening in the preform 10 so that the wire 12 may be threaded therethrough. This pin is adjustable and should preferably be properly positioned or centered prior to actually extruding the paste.

Preferably, the extrusions are formed in approximately 30-inch lengths using tungsten carbide tooling. Extrusion pressure should be three to four tons, as registered on the pressure gauge. The extrusions are preferably placed on 1/4" wide V shaped grooves in a plaster board (6" wide x 30" long x 3/4" total thickness) that has been previously dusted with GP- 38 graphite powder, available from Union Carbide Coatings Corp. , of Danbury, Connecticut.

Drying

Immediately during extrusion, the damp extrusions should preferably be placed in a drying chamber. The extrusions should be dried on the graphite dusted plaster boards at approximately 100°F maximum in a closed chamber (minimum of approximately 25 hours) . Thereafter, this initial drying period is followed by overnight drying in an air circulating dryer at a temperature range of about 125 to 150°C.

Cutting

After drying, the extrusions have a wood-like consistency and can be readily cut to a desired length for firing. For example, the extrusions can be cut to a desired firing tile length using a back and top block, straight edge, and a safety-type stainless steel razor blade or Ex-acto™ knife.

Loading

The cut extrusions should be loaded onto a mullite grain bonded hi-alumina type V-grooved setter tray, such as is available from Applied Ceramics, Inc., of Atlanta, Georgia as part number TAC 218. Preferably, the V- grooves should be filled to approximately 1/8 inch over their depth, with a plurality of the extrusions stacked therein.

Preferably, the setter trays are stacked four high and two wide into a periodic furnace, such as a 3000 series furnace with Kanthal Super 33 heating elements, available from CM Furnaces, Inc. of Bloo field, N.J.

Firing

The fused silica extrusions are fired in the furnace in an air atmosphere. Preferably, the furnace is heated to about 400°C over a three hour period, and then held at this temperature for one hour. Subsequently, the temperature of the furnace is raised to about 1275°C at a rate of about 150°C per hour. From this temperature, the rate of increase is slowed to about 100°C per hour until a peak temperature of about 1400°C is reached. This peak temperature is held for an additional two hours. The furnace is then turned off, and the fired extrusions are allowed to cool overnight (minimum) in the closed furnace.

The ceramic preforms 10 are now ready to be cut to specified length, usually two inches. The preforms 10 are preferably cut to a flat end cut using a back and top block straight edge and a four inch diameter diamond impregnated 1/32" wide cutting wheel. Physical Tests

To ensure that the preforms 10 are of suitable quality, several tests are performed. The modulus of rupture (MOR) is measured using a Chatillon model DFGHS- 100 digital compressive force gauge on a model LTCM-4 mechanical test stand. The test stand is preferably equipped with a controlled motorized lifting table and a tungsten carbide base block having its knife-edge force- points positioned one-inch apart and its tungsten carbide chisel edge tip positioned on the bottom of the digital force gauge. Preferably, the MOR of the preforms 10 should be approximately 1000 ± 200 PSI.

The outer diameter of the preforms 10 should be measured with, for example, a spring loaded electronic digital read-out micrometer accurate to ± 0.0001" min. Preferably, the outer diameter of the preforms 10 is approximately 0.125 ± 0.002 inch.

The inner diameter of the openings should be measured using, for example, spring steel gauge pins accurate to ± 0.0001 inch. The gauge pins are available in intervals of 0.001 inch.

Also, camber is measured and preferably should be within a tolerance of about 0.005 inch/inch or mm/mm. Although particular detailed embodiments of the apparatus have been described herein, it should be understood that the invention is not restricted to the details of the preferred embodiment. Many changes in design, configuration, and dimensions are possible without departing from the spirit and scope of the instant invention.

Claims

CLAIMS :

1. A ceramic preform for use as an insulator in a mineral insulated cable, comprising a mixture including a substantial portion of fused silica.

2. A ceramic preform, as set forth in claim 1, wherein said fused silica comprises approximately 100 percent by weight of the mixture.

3. A ceramic preform, as set forth in claim 1, wherein said preform has a substantially cylindrical configuration and is swageable.

4. A ceramic preform, as set forth in claim 3, wherein said preform includes at least one bore hole extending longitudinally through said cylindrical configuration.

5. A paste suitable for firing into a ceramic preform for use as an insulator in a mineral insulated cable, comprising a mixture including fused silica and a binder solution.

6. A paste, as set forth in claim 5, wherein said binder solution includes distilled water and glycerin.

7. A paste, as set forth in claim 5, wherein said binder solution includes distilled water, Methocel™, and glycerin.

8. A paste, as set forth in claim 5_r wherein said fused silica is substantially pure.

9. A method for forming a ceramic preform to be used as a insulator in a mineral insulated cable, comprising the steps of:

preparing a binder solution:

combining fused silica and said binder solution into an extrudable paste;

extruding said paste into a preselected geometric configuration; and

firing said extruded paste.

10. A method, as set forth in claim 9, wherein said step of preparing a binder solution includes the step of combining distilled water and glycerin.

11. A method, as set forth in claim 9, wherein said step of preparing a binder solution includes the step of combining distilled water, Methocel™, and glycerin.

12. A method, as set forth in claim 9, wherein said step of firing said extruded paste includes the steps of:

heating said extruded paste in an air atmosphere at a first preselected temperature and for a first preselected period of time sufficient to burn off said binder solution; and

heating said extruded paste at a second, higher preselected temperature for a second preselected period of time.

13. A mineral insulated cable, comprising:

at least one conductive wire;

a sheath positioned about and spaced from said conductive wire; and

an insulator positioned between said conductive wire and said sheath, said insulator being comprised of fused silica.

14. A mineral insulated cable, as set forth in claim 13, wherein said fused silica comprises approximately 100 percent by weight of said insulator.