WO1993004485A1

WO1993004485A1 - Boron nitride insulated electrical components and method for making same

Info

Publication number: WO1993004485A1
Application number: PCT/US1992/006843
Authority: WO
Inventors: Collins Cannon; William J. Koch
Original assignee: American Technology, Inc.
Priority date: 1991-08-13
Filing date: 1992-08-13
Publication date: 1993-03-04
Also published as: CA2115437A1; EP0600005A1; AU2498292A; BR9206360A; JPH07500942A

Abstract

A mineral insulated electrical component, such as a cable (18), heater, or sensor, is provided with ceramic insulation formed primarily from boron nitride. The boron nitride insulation is constructed from a ceramic preform (10) composed of up to 99 % boron nitride, with the remaining portion being a sintering aid, which is preferably a refractory dielectric, such as silica. The addition of silica allows the boron nitride to be fired into a crushable ceramic. The crushable ceramic allows the fabrication of a boron nitride-silica insulated electrical component, such as a cable or sensor, possessing high electrical bandwidth and high resistance to environmental stresses, such as nuclear radiation, high temperature, pressure and corrosive chemical environments.

Description

BORON NITRIDE INSULATED ELECTRICAL COMPONENTS

AND METHOD FOR MAKING SAME

This invention is generally directed to mineral insulated electrical components, such as cables, heaters, and sensors, and, more particularly, to boron nitride insulation used in such cables, heaters, and sensors, methods for producing the boron nitride insulation, a boron nitride preform, and a boron nitride insulated cable.

Mineral insulation is often used in the fabrication of electrical components, such as sensors, heaters, and cables. Mineral insulation is known to provide good resistance to environmental stresses, such as thermal and nuclear radiation. Heretofore, significant shortcomings have existed in the use of conventional mineral insulation. These shortcomings are evident in the problems associated with mineral insulated cables, but exist in many other types of electrical components, such as heaters and sensors. Accordingly, while the problems described below are illustrated in conjunction with mineral insulated cables, they are common to most other applications of mineral insulation, including heaters and sensors.

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 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 do not transmit high frequency signals well, 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 the strong bonding of its crystal structure. The insulation in soft cables, on the other hand, has low strength bonds holding the structure together. Thus, any stress, such as nuclear radiation or heat, disturbs the bonding in the insulation and can 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 such as MgO (magnesium oxide) or A1₂0₃ (aluminum oxide) . 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 high frequency applications. Magnesium and aluminum oxide are also unsuitable for high performance applications because they tend to react with the wire and sheath of the cable at high temperatures.

In an attempt to produce mineral insulated cables for high frequency applications, some manufacturers have used silica (Si0₂) as the insulation material. The dielectric constant of silica is 3.8, giving it better electrical properties over magnesia and alumina oxide in some applications.

Silica, however, has proved somewhat problematic. During the construction of silica-based hard cables, for example, silica is typically mixed with water into a paste-like consistency, and then extruded onto the wires and into the sheath in this relatively damp 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 contaminants remaining in the cable can cause intergranular attack or corrosion and ultimate failure of either the housing, or more likely, the conducting wire.

Additionally, while silica is generally very stable, reaction between the sheath and impure silica is sometimes observed. This reaction can also result in the metal conductors being corroded and ultimately failing. Also, silica is highly abrasive, which precludes the manufacture of long cable lengths. During construction of long cables, manufacturers commonly employ drawing or swaging techniques to progressively reduce the diameter and increase the length of the cable. Unfortunately, using this process with abrasive silica damages the conductor wires by abrasion. Typically, an abraded wire has impaired electrical properties, and does not conduct as efficiently as an unabraded wire.

Boron nitride (BN) , in contrast to soft organic insulation, has a crystal structure that is highly resistant to environmental stress, such as temperature, and yet, has excellent electrical and heat transfer properties in addition to its relatively low dielectric constant (approximately 4.0). Boron nitride is also soft and lubricous and has excellent flowability during drawing or swaging. Further, BN is very stable and does not tend to react with the sheath or wire of the hard cable. Heretofore, however, BN has not been successfully employed in mineral insulated cables because BN cannot, by itself, be formed into a ceramic preform or chemically stable granulated powder suitable for cable fabrication.

Heater cables are a specialized type of mineral insulated cable, and are subject to severe environmental stresses attendant with their normal operation (i.e., heat) . Thus, prior art heater cables suffer from many of the generic problems associated with mineral insulated cables generally, as well as additional, specialized problems.

Heater cables of varying geometries, i.e., flat, spiral, straight, round, etc., are found in a wide variety of commercial and industrial applications, and are typically used to transfer heat. That is, a heater cable can be used to generate and transmit heat, such as in stoves or ovens. Separately, cables and components can be used to receive and detect heat, such as in thermocouples or temperature sensors.

For example, one type of heat generating heater cable is found in electrical stoves and ovens in the form of heating elements. That is, a conventional electrical heating element is constructed from a metallic jacket coaxially positioned about an internal electrical wire, with the space therebetween filled with a non-conducting mineral insulator, such as a ceramic. The ceramic provides electrical isolation between the jacket and wire, but allows for thermal transmission. Heat generated by the electrical wire is transported through the ceramic insulator and jacket to the external environment. In this manner, electricity is converted to heat and transferred to the surrounding environment, such as in an oven.

Likewise, one type of similarly insulated cable is found in industrial and commercial applications for detecting temperature, and is typically referred to as a thermocouple. Thermocouples are substantially similar in construction to heater cables used in stoves and ovens. For example, an external jacket is coaxially positioned about a pair of dissimilar wires and a ceramic insulator. The dissimilar wires of the thermocouple have an electrical property that varies with temperature. When joined at the hot end, the dissimilar metals set-up an E.M.F. (voltage) differential which varies with temperature change. Thus, heat in the external environment is transported through the jacket and ceramic insulator to the wires, thereby affecting the electrical parameter (e.g., voltage) of the wire, and providing an effective electrical indication of the temperature of the external environment.

Attendant with these devices are a variety of support or connection components, such as inserts, spacers, and other internal components that are ordinarily also formed from a mineral insulator, such as a ceramic. The problems described below in conjunction with the cables are ordinarily also found in these various support components.

Both types of these present devices suffer from a variety of problems, including, inter alia , lack of temperature uniformity, high input power requirements, and, generally, a short life span. These problems tend to originate in the ceramic insulator, and are exacerbated by the high temperature environment in which they are necessarily and continuously placed.

The ceramic insulator in conventional heater cables is typically formed from a variety of ceramics, such as magnesium oxide (MgO) , which have poor thermal conductivity, tend to react with both the jacket and the internal wire, and hydrate easily.

Poor thermal conductivity has a direct, adverse impact on both the power requirements and the life span of the heater cable. The principal factor in the measurement of heater efficiency or thermal conductivity -1-

is the heat transfer coefficient. The higher the heat transfer coefficient, the better the heater will perform. With a higher heat transfer coefficient, not only is the heater more efficient, but the heater has far better temperature uniformity. A higher heat transfer coefficient means that the heat is more rapidly transferred to the outer sheath, and, as a result, the temperature of the internal wire will be much lower. This lower temperature insures less reaction between the insulator, the internal wire, and the jacket. Longer heater cable life and lower power requirements result from a ceramic insulator that has good thermal conductivity (i.e., superior thermal conductivity to that of MgO) . Moreover, the poor thermal conductivity of MgO results in a "thermal lag" between excitation of the wire and heat being radiated through the jacket. Accordingly, there is an undesirable "warming up" period for the heating elements. Similarly, in thermocouples this "thermal lag" results in erroneous readings of external temperature when the external temperature is varying.

Reaction of the MgO with the heater jacket and internal wire is a clear cause of increased power requirements and reduced lifespan of MgO insulated heater cables. Ordinarily, as an oxide, MgO has a tendency to react with the jacket and internal wire; however, when heated, this problem is exacerbated, as the heat partially reduces the MgO, and the resulting magnesia is highly reactive with the internal wire and jacket. Some relief has been obtained by venting the jacket of the cable to expose the MgO to the external atmosphere. This venting has a tendency to keep the MgO in its oxide form, and thereby reduce reaction with the jacket and internal wire. Unfortunately, MgO has a strong tendency to hydrate or absorb water, which necessarily results in corrosion of the jacket and internal wire. Thus, while venting the jacket tends to limit reduction of the MgO, and reduce reaction between the MgO, jacket, and internal wire, it also induces a different type of reactive influence, corrosion. Moreover, hydration reduces the effectiveness of the MgO as an electrical insulator, and may lead to ^• voltage breakdown between the jacket and wire. These areas of voltage breakdown produce hot spots, resulting in uneven heating, and eventual failure of the cable.

The present invention is directed to overcoming or minimizing one or more of the problems discussed above by providing an electrical component, such as a cable, heater, or sensor, that is both resistant to environmental stresses and is suitable for high frequency and high performance applications, and in the case of cables is capable of transmitting relatively high- frequency signals. Further, the invention also allows fabrication of cables in relatively long lengths, in an economical manner, and eliminates effects of residual moisture. Thus it overcomes the drawbacks of silica insulated cable and MgO insulated heater cables.

In one aspect of the present invention, a ceramic preform for use as an insulator in a mineral insulated electrical component is provided. The ceramic preform comprises a mixture of boron nitride and a sintering aid.

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 electrical component is provided. The paste is formed from a mixture of boron nitride, a sintering aid, and a binder solution. In still another aspect of the present invention, a method for forming a ceramic preform to be used as an insulator in a mineral insulated electrical component is provided. The method includes the steps of: preparing a binder solution; preparing a ceramic powder by combining boron nitride and a sintering aid; combining the ceramic powder and binder solution into an extrudable paste; extruding the paste into a preselected geometric configuration; and firing the extruded paste.

In yet another aspect of the invention, a mineral insulated electrical component is provided. The electrical component 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 boron nitride^"and a refractory dielectric.

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;

Fig. 2 illustrates a perspective cut-away view of a triaxial mineral insulated cable; and

Fig. 3 illustrates a cross-sectional view of a coaxial mineral insulated thermocouple.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawing 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 modifications, equivalents, and alternatives falling within the spirit and scope of the invention, as defined by the appended claims.

Referring now to the drawings, and, in particular, to Fig. l, there is shown an exploded perspective view of a ceramic insulator 10, a center conductor electrical wire 12, and a metallic sheath 16, which, when assembled, form a mineral insulated or hard cable 18. The metallic sheath 16 is preferably formed from a nickel iron chrome alloy, or a more refractory metal, if required. The electrical wire 12, is formed from any suitable conductive material, however, copper is preferred for electrical transmission cables, and conventional thermocouple wires, or michrome wires where the cable 18 is to be used as a heater. The ceramic insulator 10 is preferably formed from a compound that includes BN with a sintering aid added thereto. Preferably, the sintering aid is a refractory dielectric. The term refractory dielectric is generally defined as a high temperature (or heat resistant) electrical insulator, and generally includes mineral compounds such as oxides, nitrides, etc. , or, more particularly, silica (Si0₂) , kaolin, pyrophyllite, bentonite, talc, or other similarly good dielectric insulator material. Accordingly, while any of the above-listed refractory dielectric sintering aids are suitable for use in the instant invention, fumed silica is preferred.

The hard cable 18 represents a first embodiment of the instant invention, with other embodiments, such as a sensor, illustrated and discussed subsequently herein. The ceramic insulation 10 can be deposited in the metallic sheath 16 using a variety of processes without departing from the spirit and scope of the instant invention. Two preferred processes are described subsequently herein. The first process involves firing the BN-refractory dielectric into ceramic preforms and threading the preforms onto the wire 12. Alternatively, the fired or sintered insulation 10 can also be placed in the sheath 16 in powdered or granulated form, using conventional loading, vibrating, and ramming techniques to ensure that voids are not created.

The specific compound and method used in preparing the insulation 10 and depositing it within the sheath 16 are discussed in greater detail below. However, for a proper understanding of the instant invention, it is useful to first appreciate the construction and assembly processes of a hard cable.

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

At this point, the assembled hard cable 18 is passed through conventional metallic drawing dies. The drawing dies forcibly reduce 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 drawing dies, the cable 18 is gradually 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 16.

It should be appreciated that the ceramic preforms 10 positioned within the cable 18 are relatively hard, but are crushed 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.

An alternative to the use of preforms 10 in constructing cable 18 is the use of a powder fill method. Larger cables are commonly constructed using such a powder fill method, as described below.

In the powder fill method, the BN-refractory dielectric insulation 10, which has previously been stabilized by firing, is reduced to a powdered or granulated form and then loaded into the sheath 16 using conventional techniques. These conventional techniques are in common usage to produce MgO type ceramic insulated cables, and, therefore, need not be discussed in detail herein. For purposes of understanding this invention, it is sufficient to recognize that the powdered BN- refractory dielectric is typically placed in the sheath 16 about the wire 12 by a mechanism such as a funnel loading or screw type auger as the sheath 16 is vibrated. Vibration aids in compacting the powder and prevents formation of voids in the insulation 10. Ordinarily, the powder is also tamped into the sheath during this process. In most cases it is desirable to swage or draw down the cable 18 to further compact the BN-refractory dielectric to enhance the electrical characteristics of the cable. It should be appreciated that if the preforms 10 are to be crushed or reduced to powder prior to loading into the sheath 16, then no specific shape or form is critical. Moreover, other techniques may be employed to produce a fired mass of the BN-refractory dielectric, which can be readily crushed or reduced to powder.

For example, spray dried BN-refractory dielectric powder granules are loaded into a furnace and fired into a mass of BN-refractory dielectric, according to the steps described below. The sintered mass is then powdered using a ball mill with dehydrated methanol or distilled water as the liquid, and high fired dense alumina balls or cylinders. The coarsely ground powder is then air dried at a temperature of 35 to 50'C to remove the methanol. A titanium nitride coated jaw crusher or a jet mill can be used for large volumes and the resulting granules can be screened to the desired particle size distribution.

Further, in another alternative powder fill method, the BN-refractory dielectric powder is not fired to preform conditions, but rather, is mixed and left in the dry powder state prior to powder loading into the sheath 16.

The instant invention also finds beneficial application in multi-conductor cables (not shown) , i.e. cables where the number of electrical conductor wires present within the interior of the ceramic insulation varies from two to upwards of two hundred. In cable geometries of this type, it is generally preferable to position the wires so that the insulation thicknesses between adjacent wires and between wires and the sheath are substantially equal. The instant invention also finds beneficial application in triaxial-type hard cables. Triaxial-type cables are substantially similar to that shown in Fig. 1, but, as shown in Fig. 2, include an additional metallic sheath 20 positioned between the wire 12 and the sheath 16. 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 the additional sheath 20 is preferably BN-refractory dielectric, whereas the insulation 22 placed between the sheaths 16, 20 can take any of a variety of forms, including, but not limited to, mineral insulators, such as BN-refractory dielectric.

Fig. 3 illustrates a slightly different use of a BN- refractory dielectric insulator from the above-described use in cables. That is, the insulator is useful in not only the transmission cable, but also in an electrical sensor, which may be attached to the cable 18. Hardened sensors are useful for the same reasons that hardened cables are useful in environmentally stressful conditions. In fact, attaching a non-hardened sensor to a hardened cable may not extend the useful life of the system, since the sensor remains susceptible to early failure.

While many types of sensors are envisioned as being benefited by the use of BN-refractory dielectric insulation, its application to thermocouples is illustrative of the application to heaters and sensors in general. Fig. 3 illustrates a cross sectional view of a thermocouple 50 formed from an outer sheath 52, a pair of internal dissimilar wires 54, 56, and a mineral insulator 58. The methods useable for constructing the thermocouple 50 are substantially identical to those discussed above in conjunction with the cable 18. Further, the methods described below for constructing the BN-silica insulation are equally useful in constructing the mineral insulator 58.

It is also envisioned that some applications of mineral insulation to sensors, and even cables in some instances, do not require crushed or powdered insulation. Rather, some electrical components can make use of the ceramic preform 10 in its hard body configuration.

The use of a refractory dielectric in combination with BN greatly improves the resulting hard cables and sensors because BN alone does not hold together after being sintered. The refractory dielectric responds to the sintering process to create a matrix that contains the sintered BN. In this manner BN can be made into a ceramic preform for use in constructing hardened electrical components. Further, the refractory dielectrics have electrical properties that are consistent with those of BN, and, therefore, do not reduce the desirable electrical properties of BN. That is, a BN-refractory dielectric cable will retain the advantage of being able to transmit relatively high frequency signals, and retains it excellent heat transfer properties.

As one example of constructing a BN-refractory dielectric insulator, the chemical makeup and method used for constructing a BN-silica preform 10 is described below. It should be appreciated that other BN-refractory dielectric insulators could be formed using processes similar to those described below. The chemical makeup and method used for constructing BN-silica preforms 10 is as follows.

Preparation of Binder Solution

A binder solution is combined with the BN to form a paste that is suitable for extruding and will substantially maintain the extruded shape. Accordingly, a first step involves the preparation of the binder solution.

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 should be stirred into the solution slowly. After approximately two minutes of stirring it should appear milky in color. Thereafter 1800 grams of triple distilled water should be stirred into the milky solution. Preferably the triple distilled water should be at approximately room temperature. This entire solution should be 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 Boron Nitride Swaσeable Ceramic Powder

A swageable ceramic powder, consisting of a substantial amount of BN is combined with the binder solution to form an extrusion paste. Preferably, the main components of the ceramic powder include ultra fine BN and a sintering aid of fumed silica (Si0₂) . Prior to combination with the binder solution, the ultra fine BN and fumed silica powders are preferably ball milled for about sixteen (16) hours to ensure a thorough homogenous ceramic powder. This ball milling process is described below:

HCV grade BN in an amount of about 2500 grams is placed in a clean, dry, low-density, wide-mouth polyethylene (Nalgene) carboy with handles and a white polyethylene leak proof screw cap closure. Preferably, the carboy is of approximately the 10 liter size. Approximately 250 grams of grade M-5 fumed silica is placed in the carboy with the BN. Approximately 2.5 liters of dehydrated, purified methanol, such as is available from Fisher Scientific Co. of St. Louis, Missouri, as item number A411-2, is added to the BN and silica mixture in the carboy.

Approximately three pounds of 3/4-inch diameter, high-fired pure alumina ceramic balls or cylinders are placed into the carboy to provide the mixing action when the carboy is agitated.

The carboy screw cap is tightly secured to the carboy. Additional protection against leakage may be provided by securing the cap with a tape product, such as plastic duct tape.

The loaded, tightly sealed carboy is placed on a roller rack, such as is available from Paul O. Abbe, Inc. of Little Falls, New Jersey. The roller rack rolls or agitates the carboy to cause the balls/cylinders to move about in the carboy, homogenizing the BN and silica. Preferably, the carboy is rolled briefly (i.e., 10 minutes) and then checked for leakage. Assuming no leakage, the carboy is returned to the roller rack and rolled for approximately 10 hours.

The carboy, with the alcohol-BN-silica mix is preferably vented every three hours of mixing. This is accomplished by removing the tape and carefully unscrewing the carboy cap to vent the alcohol fumes.

After 16 hours of mixing, the contents of the carboy are poured into clean plastic or Pyrex® trays. The carboy should be flushed with pure, dehydrated methanol. The trays, which are covered with paper fiber, are placed in a flowing, air-drying oven set at approximately 110 to 120°F. The oven drying process continues for about 24 to 30 hours, or until the alcohol has substantially evaporated.

After the alcohol has substantially evaporated, the trays are removed from the flowing, air drying oven and allowed to cool at room temperature. Preferably, the trays are covered during cooling. When cool, the homogenized BN and silica mixture is removed from the tray and placed into a plastic bag. Preferably, the bag should be held closed with a plastic tie strip for storage. Preparation of Extrusion Paste

With the binder solution and homogenized ceramic powder prepared, they may be combined to form an extrusion paste. Approximately 530 grams of the ceramic powder and 389 grams of the binder solution are added to a sig a blade type stainless steel mixer, such as is available from Paul O. Abbe', Inc., of Little Falls, New Jersey. Preferably the total working chamber including . all moving parts, of the mixer should be 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 appears uniform (approximately three minutes maximum) . The contents are diluted by adding approximately 374 grams of triple distilled water, followed by an additional three minutes of mixing. Additional distilled water may be necessary to achieve a paste consistency capable of smooth extrusion flow. This additional water should be added in combination with additional binder solution. Preferably, the combination should be approximately 10% binder solution and 90% triple distilled water, 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, the mixer should be slowed to low speed and 470 grams of ceramic powder should be added to the mixture. After all of the ceramic powder has been added, 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. After aging the paste, it should be kneaded by hand on a clean plexiglass sheet for approximately two minutes to give it a uniform plasticity and to form it into a shape for loading into an extruder chamber.

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 twenty-four hours after preparation.

Extrusion

With the paste sufficiently aged and prepared for extrusion, the extrusion process may begin using, for example, a 40 ton extruder having a tungsten carbide chamber and plunger head, such as 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 openings 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-inch wide "V" shaped grooves in a plaster board (6" wide x 30" long x 3/4" total thickness) that has been previously dusted with BN powder, available from Union Carbide Coatings Corp.

Drying

Immediately after extrusion, the damp extrusions should preferably be placed in a drying chamber. The extrusions should first be dried on the boards at approximately 100°F maximum in a closed chamber (minimum of approximately 25 hours) . Thereafter, overnight drying at 125 to 150°C in an air circulating dryer is preferred.

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 are preferably loaded into 99.8% pure alumina firing boats with outside dimensions of 6" x 2 3/4" x 3/4" high with a 1/8" wall and bottom thickness, such as are available from Coors Ceramics Co. of Golden, Colorado, as part number DI-32042-00. Preferably, the. boats are filled to 1/8" below their top surface, with a plurality of extrusions stacked therein.

Preferably, the loaded alumina boats are placed into a periodic furnace, such as a 3000 series furnace with Kanthal Super 33 heating elements, available from CM Furnaces, Inc. of Bloomfield, New Jersey.

Firing

The firing process should be initially made in a nichrome heating element belt furnace using an air atmosphere. Preferably, the heating cycle involves bringing the furnace to 450°C over a four hour period, holding the temperature for two hours, and then allowing a four hour cool down period.

The loaded, prefired boats are then placed in an inert or reducing atmosphere and fired to 1350 to 1400°C. Firing in this reducing or inert atmosphere achieves preform ceramic strength sufficient to allow loading of the preforms onto the conducting wire 12. Preferably, the boats are held at this temperature for about two hours.

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-inch wide cutting wheel. Physical Tests

To ensure that the preforms 10 are of suitable quality, several tests are performed. The modules 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 1200 ± 200 PSI.

The outer diameter of the preforms 10 should be measured with, for example, a spring loaded electronic digital read-out micrometer. Preferably, the outer diameter tolerance of the preforms 10 is approximately +1.6% for an outer diameter of 0.200 inches.

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 the following tolerance:

Typical camber: 0.005 inch/inch or mm/mm;

The specific method described above results in a preform 10 that has a ratio of BN to silica of about 10 to 1, or the preform 10 consists of about 90% BN and 10% silica. Other ratios of BN to silica have been made, and appear to exhibit the desirable properties of BN without being adversely affected by the presence of silica. For example, preforms having up to 20% silica have been successfully formed into a paste, extruded, and fired. At the opposite extreme, it is expected that the paste may be successfully formed with as little as 1% silica in combination with 99% BN.

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 for use as an insulator in a mineral insulated electrical component, comprising a mixture of boron nitride and a sintering aid.

2. A ceramic, as set forth in claim 1, wherein said sintering aid is a refractory dielectric.

3. A ceramic, as set forth in claim 2, wherein said refractory dielectric is at least one of the group of silica (Si0₂) , kaolin, pyrophyllite, bentonite, and talc.

4. A ceramic, as set forth in claim 1, wherein said mixture contains at least about 50 percent boron nitride by weight.

5. A ceramic, as set forth in claim 1, wherein said mixture contains at least about 1 percent silica by weight.

6. A ceramic, as set forth in claim 1, wherein said mixture contains at least about 50 percent boron nitride and at least about 1 percent sintering aid by weight.

7. A ceramic, as set forth in claim 1, wherein said mixture includes boron nitride in the range of about 50- 99 percent by weight.

8. A ceramic, as set forth in claim 1, wherein said boron nitride and sintering aid are homogenized.

9. A paste suitable for firing into a ceramic for use as an insulator in a mineral insulated electrical component, comprising a mixture of boron nitride, a sintering aid, and a binder solution.

10. A paste, as set forth in claim 9, wherein said sintering aid is a refractory dielectric.

11. A paste, as set forth in claim 10, wherein said refractory dielectric is at least one of the group of silica (Si0₂) , kaolin, pyrophyllite, bentonite, and talc.

12. A paste, as set forth in claim 9, wherein said binder solution includes distilled water, methocel and glycerin.

13. A paste, as set forth in claim 9, wherein said boron nitride and sintering aid are present therein in a range of ratios of about 99:1 to 1:1.

14. A paste, as set forth in claim 9, wherein said boron nitride and said sintering aid are homogenized.

15. A method for forming a ceramic to be used as a insulator in a mineral insulated electrical component, comprising the steps of:

preparing a binder solution:

preparing a ceramic powder by combining boron nitride and a sintering aid;

combining said ceramic powder and binder solution a paste; and

firing said paste.

16. A method, as set forth in claim 15, wherein said step of preparing a binder solution includes the step of:

combining distilled water, methocel, glycerin, and a wetting agent.

17. A method, as set forth in claim 15, wherein said step of preparing a ceramic powder includes the step of:

combining said boron nitride and said sintering aid in a range of ratios of about 99:1 to 1:1.

18. A method, as set forth in claim 17, wherein said step of preparing a ceramic powder includes the step of:

ball milling said combined mixture of boron nitride and sintering aid.

19. A method, as set forth in claim 17, wherein said step of ball milling includes the step of:

adding a predetermined amount of alcohol to said mixture of boron nitride and sintering aid prior to the step of ball milling.

20. A method, as set forth in claim 15, 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 in a reducing or inert atmosphere at a second, higher preselected temperature and for a second preselected period of time.

21. A mineral insulated electrical component, comprising:

at least one conductive element;

a first sheath positioned about and spaced from said conductive element; and

insulation positioned between said conductive element and said sheath, said insulation being comprised of a mixture of boron nitride and a sintering aid.

22. A mineral insulated electrical component, as set forth in claim 21, wherein said sintering aid is a refractory dielectric.

23. A mineral insulated electrical component, as set forth in claim 22, wherein said refractory dielectric is at least one of the group of silica (Si0₂) , kaolin, pyrophyllite, bentonite, and talc.

24. A mineral insulated electrical component, as set forth in claim 21, wherein said mixture contains at least about 50 percent boron nitride by weight.

25. A mineral insulated electrical component, as set forth in claim 21, wherein said mixture contains at least about 1 percent sintering aid by weight.

26. A mineral insulated electrical component, as set forth in claim 21, wherein said mixture contains at least about 50 percent boron nitride and at least about 1 percent sintering aid by weight.

27. A mineral insulated electrical component, as set forth in claim 21, wherein said mixture includes boron nitride in the range of about 50-99 percent by weight.

28. A mineral insulated electrical component, as set forth in claim 21, wherein said boron nitride and sintering aid are homogenized.

29. A mineral insulated electrical component, as set forth in claim 21, including a second sheath positioned about said conductive element and axially within said first sheath, wherein said first insulator is positioned between said conductive wire and said second sheath, and a second insulator is positioned between said first and second sheaths.

30. A mineral insulated electrical component, as set forth in claim 29, wherein said second insulator is comprised of a mixture of boron nitride and a sintering aid.

31. A mineral insulated sensor, comprising:

a sensing element responsive to a physical characteristic of its environment;

a first sheath positioned about and spaced from said sensing element; and

an insulator positioned between said sensing element and said sheath, said insulator being comprised of a mixture of boron nitride and a sintering aid.

32. A mineral insulated sensor, as set forth in claim 31, wherein said sintering aid is a refractory dielectric.

33. A mineral insulated sensor, as set forth in claim 32, wherein said refractory dielectric is at least one of the group of silica (Si0₂) , kaolin, pyrophyllite, bentonite, and talc.

34. A mineral insulated sensor, as set forth in claim 31, wherein said mixture contains at least about 50 percent boron nitride by weight.

35. A mineral insulated sensor, as set forth in claim 31, wherein said mixture contains at least about 1 percent sintering aid by weight.

36. A mineral insulated sensor, as set forth in claim 31, wherein said mixture contains at least about 50 percent boron nitride and at least about 1 percent sintering aid by weight.

37. A mineral insulated sensor, as set forth in claim 31, wherein said mixture includes boron nitride in the range of about 50-99 percent by weight.

38. A mineral insulated sensor, as set forth in claim 31, wherein said boron nitride and sintering aid are homogenized.

39. A method for forming granulated ceramic insulation to be used as an insulator in a mineral insulated electrical component, comprising the steps of:

preparing a binder solution:

preparing a ceramic powder by combining boron nitride and a refractory dielectric;

combining said ceramic powder and binder solution into a paste;

firing said extruded paste into a ceramic mass; and

reducing said ceramic mass to a granular form.

40. A method, as set forth in claim 39, wherein said step of preparing a binder solution includes the step of:

combining distilled water, methocel, glycerin, and a wetting agent.

41. A method, as set forth in claim 39, wherein said step of preparing a ceramic powder includes the step of:

combining said boron nitride and refractory dielectric in a range of ratios about 99:1 to 1:1. -33-

42. A method, as set forth in claim 39, wherein said step of firing said paste includes the steps of:

heating said paste in air atmosphere at a first preselected temperature for a first preselected period of time sufficient to substantially burn off said organic binders;