WO1993017982A1

WO1993017982A1 - Aluminum nitride insulated electrical components and method for making same

Info

Publication number: WO1993017982A1
Application number: PCT/US1993/002055
Authority: WO
Inventors: William J. Koch; Collins P. Cannon
Original assignee: American Technology, Inc.
Priority date: 1992-03-09
Filing date: 1993-03-09
Publication date: 1993-09-16

Abstract

A mineral insulated electrical component, such as a cable (18) or sensor is provided with a ceramic insulator formed primarily from aluminum nitride. The aluminum nitride insulator is constructed from a ceramic preform (10) or sintered, granulated powder composed of up to 99 % aluminum nitride, with the remaining portion being selected from a group of sintering aids, including yttrium oxide. The addition of yttrium oxide allows the aluminum nitride to be fired into a stabilized, swageable ceramic, or further into granulated, stabilized powder. The ceramic allows the fabrication of aluminum nitride-yttrium oxide insulated heater cables, insulated thermocouples, insulated electrical cables, or the like.

Description

ALUMINUMNITRIDEINSULATEDELECTRICAL COMPONENTSANDMETHODFORMAKINGSAME

I, 5 1. TECHNICALFIELD

This invention is generally directed to mineral insulated electrical components, such as heater cables and sensors, and, more particularly, to aluminum nitride 10 insulated cables and sensors, the aluminum nitride insulation used as the thermal conductor and electrical insulator in such components, and methods for producing the aluminum nitride insulation.

15

2. DESCRIPTIONOFTHERELATEDART

Heater cables of varying geometries, i.e., flat, spiral, straight, round, etc., are found in a wide variety 20 of commercial and industrial applications, and are typicall 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 a detect heat, such as in thermocouples or temperature ^■» 25 sensors.

X 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 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 a heater, electrical, or thermocouple cable that is resistant to reaction between the ceramic insulator, jacket, and internal wire, is capable of efficiently and effectively transporting heat through the ceramic insulator, and is not hydroscopic.

SUMMARYOF INVENTION

In one aspect of the present invention, a ceramic insulator for use in a mineral insulated electrical component is formed from aluminum 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 a mixture of aluminum nitride, a sintering aid, and a binder solution.

In still another aspect of the present invention, a method or forming a ceramic preform to be used as an insulator in a mineral insulated electrical component is provided. The method includes steps of: preparing a binder solution; preparing a ceramic powder by combining aluminum nitride and a sintering aid; combinirig the powder and binder solution into a paste; and firing the paste. In still another aspect of the present invention, a mineral insulated electrical component is provided. The electrical component includes a conductive wire, a sheath, and an insulator positioned therebetween. The insulator is comprised of a mixture of aluminum nitride and a sintering aid.

BRIEF DESCRIPTION OFTHE DRAWINGS

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

FIG. 1 illustrates a perspective cutaway view of a coaxial mineral insulated heater cable;

FIG. 2 illustrates a perspective cutaway 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 therein be described in detail. It should be understood, however, that this specification is not intended to limit 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.

DESCRIPTIONOFTHEPREFERRED EMBODIMENTS

Referring now to the drawings, and, in particular, to Fig. 1, there is shown an exploded perspective view of a ceramic insulator 10, a center conductor or electrical wire 12, and a metallic sheath 16, which, when assembled, form a 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, such as conventional thermocouple wires, or Nichrome where the cable 18 is to be used as a heater. The ceramic insulation 10 is preferably formed from a compound that includes a significant amount of aluminum nitride (A1N) . Preferably, the sheath 16 is hermetically sealed after being backfilled with nitrogen or an inert gas. This provides a stable internal environment, which advantageously minimizes exposure of the A1N to oxygen. 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 AIN 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 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 one type of construction and assembly process of a heater cable.

In a first method of constructing the heater cable 18, the wire 12 is manually threaded through a series of relatively short preforms 10 (i.e., two to four inches each) until a desired length is reached (i.e., ten feet).

Thereafter, the wire 12 and the preforms 10 are inserted into the sheath 16 to form the 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.

Thermocouples and small diameter mineral insulated electric cables are commonly constructed using the above- described ceramic preform technique. Mineral insulated heaters and larger cables are, however, commonly constructed using a powder fill method, as described below.

In the powder fill method, the AIN insulation 10, which has had a sintering aid added thereto and has 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 and other types of insulated heater 5 cables, and, therefore, need not be discussed in detail

" herein. For purposes of understanding this invention, it is sufficient to recognize that the powdered, sintered, and stabilized AIN is typically placed in the sheath 16 about the wire 12 by a mechanism such as a funnel loading or screw

10 type auger while the sheath is vibrated in order to compact the powder and prevent 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 AIN to

15 provide increased thermal conductivity, and to further enhance the heat transfer coefficient, the insulation resistance, and the voltage breakdown values.

The instant invention also finds beneficial application 20 in multi-conductor cables, i.e. cables where the number of electrical wires present within the interior of the ceramic insulation varies from one to upwards of two-hundred. In cable geometries of this type, it is generally preferable to position the wires so that the insulation thickness between 25 adjacent wires and between the wires and the sheath are substantially equal. The instant invention also finds beneficial applications in other swageable components or hard-body internal heater parts, such as spacers, inserts, mandrels, and other internal parts used in heater design.

The instant invention also finds beneficial application in triaxial-type hard cables. Triaxial 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 cable, the insulation 24 placed between the wire 12 and the additional sheath 20 preferably includes a substantial amount of AIN, 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 AIN.

Fig. 3 illustrates a slightly different use of an AIN insulator from the above-described use in heater cables. That is, the insulator is useful in electrical sensors, such as thermocouples. AIN insulation is useful in sensors for the same reasons that it is in heater cables. While many types of sensors are envisioned as being benefited by the use of AIN insulation, its application to thermocouples is illustrative of the application to sensors in general. Fig. 3 illustrates a cross sectional view of a 5 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 10 50 are substantially identical to those discussed above in conjunction with the cable 18. Further, the methods described below for constructing the AIN insulation are equally useful in constructing the mineral insulator 58.

15 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.

20

The use of aluminum nitride (AIN) in electric heater cables of all dimensions and configurations will greatly improve the temperature uniformity, decrease the input power requirements, and greatly extend the life of electric heater

25 cables over conventional ceramic insulated electric heater cables, including MgO. The four principal factors that make AIN a far superior heater cable insulator is its unusually high thermal conductivity, its inertness, or tendency not to react with respect to most metals, its tendency not to hydrate, and its unusually high voltage breakdown resistance. These four characteristics of AIN are diametrically opposed to the properties of MgO.

As discussed above, MgO tends to react with the wire 12 and sheath 16 of the cable 18, and attracts water, which also reacts with the wire 12 and cable 16. AIN, on the other hand, is inert to most metals, and, therefore, tends not to react with the heater wire, or even the metal jacket, particularly at elevated temperatures. Further, sintered and stabilized AIN is not hydroscopic, and, therefore, does not attract water that will react with the wire 12 and sheath 16. Accordingly, for both of these reasons, the life expectancy of sintered and stabilized AIN insulated heater cables is determined to be significantly longer than conventional MgO insulated heaters or thermocouples.

As discussed above, the poor heat transfer characteristics of MgO also adversely impact the performance and life span of conventional heater cables. The heat transfer coefficient values and thermal conductivity for pure dense AIN versus pure dense MgO are shown in Table l.

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The heat transfer coefficient for AIN at 1000°C is, as shown in Table 1, about 17 times that of MgO. It should be noted that the values given in Table 1 represent fully dense MgO and AIN. Typically, the ceramic insulator materials used in heater cables are not fully dense. The fired density of both types of ceramic swageable preforms or compacted, granulated powder is approximately 70% to 90% of full density, which tends to lower the heat transfer coefficient values of both materials slightly. However, even in the worst case, it is expected that the heat transfer coefficient for AIN is more than ten times that of MgO.

As one example of constructing a AIN-sintering aid insulator, the chemical makeup and method used for constructing a AIN-yttrium oxide preform 10 is described below. It should be appreciated that other AIN-sintering aid insulators could be formed using processes similar to those described below. The chemical makeup and method used for constructing the AIN-yttrium oxide preform 10 is as follows.

Preparation of Binder Solution

A binder solution is combined with the an AIN powder 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 gram 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 851.2 grams of Methocel #A4C is stirred into the solution slowly. After approximately two minutes of stirring, the solution appears thick and milky in color. Thereafter, 1800 grams of triple distilled water is stirred into the thick milky solution. Preferably, the triple distilled water is at approximate room temperature. This entire solution is preferably capped and aged overnight. Shelf life of the binder solution is estimated to be approximately six to eight weeks if kept cool (i.e. 7 to 10°C) .

Preparation of Aluminum Nitride Swaσeable Ceramic Powder

A swageable ceramic powder, consisting of a substantial amount of aluminum nitride is combined with the binder solution to form an extrusion paste. Preferably, the main components of the ceramic powder include AIN and a sintering aid of yttrium oxide (Y₂0₃) , calcium oxide (CaO) , magnesium oxide (MgO) , or other similarly good dielectric insulator material, such as Li₂0, BeO, BaO, La₂0₃, Ce0₂, Sm₂0₃, Ti0₂, TiN, BN, Sic, Si₂N₄, BaTi0₃, C, Si0₂, or the like.

CaO, MgO, and Y₂0₃ were found to work well as sintering aids to AIN. However, CaO and MgO require slightly more sintering heat than Y₂0₃. Further, while CaO and MgO work well as a sintering aid for AIN without reducing the thermal conductivity of the AIN, Y₂0₃ actually increases the thermal conductivity of AIN. Accordingly, while any of the above listed sintering aids are suitable for use in the instant invention, Y₂0₃ is preferred. It has been observed that the dielectric properties of voltage breakdown and insulation resistance are not significantly affected with from 1 to 5% addition of the sintering aid to the AIN compound.

The raw material powder used in the instant invention was purchased from Advanced Refractory Technologies, Inc. of Buffalo, New York. The powder purchased contained about 5 weight percent of Y₂0₃ homogeneously distributed throughout the AIN by ball milling in a non-aqueous solvent. Yttrium

Oxide was selected as a sintering aid, because not only does it have a high melting point and does not severely lower the melting point of AIN, but it was found to increase the thermal conductivity of AIN over that of AIN without a sintering aid.

Before mixing the powder with the above-described aqueous binder solution, a silicone is added to the AIN powder to prevent the formation of ammonia (NH₃) . Without this additive, hydrogen from the water combines with the nitrogen in the AIN, forming NH₃ and AIOOH, and thereby destroying the desired AIN compound. The above-described aqueous binder solution can then be utilized with this type of powder preparation. To minimize the formation of NH₃, abrasion of the powder is preferably minimized, and the processing time from addition of the binder solution to dried, extruded product is preferably accomplished in less than six hours.

Preparation of Extrusion Paste

Approximately 1900 grams of the AIN powder and about 400 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, NJ. 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 is locked in place to prevent spillage. The contents are mixed for a period of about five to seven minutes. Additional binder solution is preferably added using a 10% binder and 90% triple distilled water solution mixture, up to 80 grams maximum. The resulting extrusion paste should be sufficiently stiff to have no deformation of the hole or outer diameter of the preform 10, after extrusion or drying. After this desired consistency is reached, the paste is preferably mixed for an additional period of about seven to ten minutes, which should then give the mix a dough-like consistency.

Extrusion The extrusion process preferably begins immediately after the mixing process by placing the ceramic paste and binder mixture into an extrusion chamber of, for example, a 40-ton extruder having a tungsten carbide chamber and plunger or ram head, such as is available from Loomis Products, Co. of Lexington, PA.

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

It should be appreciated that the extruder includes a pin or many pairs of pins used to form the openings in the preform 10 so that the wires 12, 14, 54, 56 may be threaded therethrough. These pins are 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 is set at approximately three to four tons, as registered on the pressure gauge. The extrusions are preferably placed on 1/4" wide "V" shaped grooves in a conventional plaster board (6" wide x 30" long x 3/4" total thickness) , commonly called a drying board. The extrusion process may be continued until the extrusion chamber is empty.

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During the extrusion process, the damp extrusions on the drying boards should preferably be placed in a high volume air circulating drying oven which has been previously turned on and the temperature set at 30 to 35°C. The loaded "V" groove drying boards are placed in the drying oven with an inverted drying board placed thereover to cover the extrusions. These top boards are then removed after l to 1.5 hours into the drying cycle. The extrusions are preferably dry after a maximum of about 5 hours total drying time. At this point, the extrusions are relatively hard and have a wood-like consistency.

Cutting

After drying, the hard, wood-like extrusions can be readily cut to a desired length for firing. For example, the extrusions can be cut to a desired firing boat 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 available from Coors Ceramics Co. of Golden, Colorado as part No. DI-32042-00. Preferably, the boats are filled to 1/8" below their top surface, with a plurality of extrusions stacked therein.

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The firing process is preferably begun in a nichrome heating element type belt furnace using an air atmosphere to substantially remove the binders. Preferably, the heating cycle involves bringing the furnace boats to 400° to 500^βC. over a four hour period, holding this temperature for two hours, and then allowing a four hour cool down period.

The loaded, prefired ceramic boats are then placed in an inert or reducing atmosphere and fired at 1450 to 1500°C. Any of a variety of reducing atmospheres can be employed without departing from the spirit and scope of the invention; however, an atmosphere of 90% dry nitrogen with 10% dry hydrogen is preferred. Firing in this atmosphere achieves excellent preform ceramic strength sufficient to allow loading of the preforms onto the conducting wires 12, and produces an excellent AIN ceramic preform.

The fired ceramic preforms 10 are now ready to be cut to specified lengths, usually 2 inches. The preforms 10 are preferably cut to a flat perpendicular end cut using a back and top block straight edge and a four inch diamond impregnated 1/32-inch wide cutting wheel.

Physical Tests

To ensure that the preforms 10 are of suitable quality, several tests are preformed. The modulus of rupture (MOR) is measured using a Chatillon model DFGHS-100 digital compression force gauge on a model LTCM-4 mechanical test strand. The 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 a tungsten carbide chisel edge tip positioned on the bottom of the digital force gauge. Preferably, the MOR of the preforms 10 should be approximately 1800±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 about 0.200 inches. The inner diameter or the hole openings can 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 AIN to Yttrium Oxide of about 19 to 1, or the preform 10 consists of about 95% AIN to 5% Y₂0₃. Other ratios of AIN to Yttrium Oxide appear to exhibit the desired properties of AIN without being adversely affected by the presence of Yttrium Oxide. For example, preforms having up to 20% yttrium oxide have been successfully formed into a paste, extruded, and fired. At the opposite extreme, it is expected that the preform may be successfully formed with as little as 1% yttrium oxide in combination with 99% AIN.

Alternatively, rather than forming the preforms 10 and threading them onto the wire 12, as discussed above, the AIN preforms 10 can first be reduced to a powdered or granulated form and loaded into the sheath 16 using conventional techniques. 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 stabilized AIN, which can be readily crushed or reduced to powder.

For example, spray dried AIN powder granules containing one of the sintering aids discussed above could be treated (coated) with silicone to prevent AIN from combining with atmospheric water, and forming NH₃. Thereafter, the granules are loaded into a furnace and fired into a mass of stabilized AIN, according to the steps described above. The sintered and stabilized mass is then powdered using a ball mill with dehydrated methanol 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.

Although particular detailed embodiments of the apparatus and method 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, composition, configuration and dimensions are possible without departing from the spirit and the scope of the instant invention.

Claims

1. A ceramic insulator for use in a mineral insulated electrical component, comprising a mixture of aluminum 5 nitride and a sintering aid.

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2. A ceramic insulator, as set forth in claim 1 wherein said sintering aid is at least one of the group of 10 yttrium oxide, calcium oxide, and magnesium oxide.

3. A ceramic insulator as set forth in claim 1, wherein said mixture contains at least 50 percent aluminum 15 nitride by weight.

4. A ceramic insulator, as set forth in claim 1, wherein said mixture contains at least 1 percent sintering 20 aid by weight.

5. A ceramic insulator, as set forth in claim 1, wherein said mixture contains at least 50 percent aluminum ,25 nitride and at least one percent yttria by weight.

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

7. A ceramic insulator, as set forth in claim 1, wherein said aluminum nitride and sintering aid are homogenized.

8. A ceramic insulator, as set forth in claim 1, wherein said insulator is a hard body preform.

9. A ceramic insulator, as set forth in claim 8, wherein said hard body preform is swageable.

10. A ceramic insulator, as set forth in claim 1, wherein said insulator is granulated.

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

12. A paste, as set forth in claim 11, wherein said sintering aid is at least one of the group of yttrium oxide, calcium oxide, and magnesium oxide.

13. A paste, as set forth in claim 11, wherein said binder solution includes distilled water, methocel, and glycerin.

14. A paste, as set forth in claim 11, wherein said aluminum nitride and sintering aid are present in a range of ratios 99:1 to 1:1.

15. A paste, as set forth in claim 11, wherein said aluminum nitride and sintering aid are homogenized.

16. A method for forming a ceramic preform 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 aluminum nitride and a sintering aid;

combining said ceramic powder and binder solution into a paste; and

firing said paste into a ceramic mass.

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

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

18. A method, as set forth in claim 16, wherein said step of preparing a ceramic powder includes the step of:

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

19. A method, as set forth in claim 16, wherein said step of firing said extruded paste includes the steps of:

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

heating said extruded paste in a reducing or inert atmosphere at a second, higher preselected temperature and for a second preselected period of time.

20. A method, as set forth in claim 16, including the step of granulating the ceramic mass.

21. A mineral insulated electrical component, comprising:

at least one conductive wire;

a first 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 a mixture of aluminum nitride and a sintering aid.

22. A mineral insulated electrical component, as set forth in claim 21, wherein said sintering aid is at least one of the group of yttrium oxide, calcium oxide, and magnesium oxide.

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

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

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

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

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