High voltage, high temperature wire

[0001] 1. Field of the Invention

[0002] The invention relates generally to insulated conducting wires (ICW) having a gap between a microwire and a cladding, and processes for making the same.

[0003] 2. Description of the Prior Art

[0004] The Taylor process is a method of a making a very fine wire, referred to as a microwire. The microwire is encased in a cladding. The process is described in Donald et al., The preparation, properties and applications of some glass-coated metal filaments prepared by the Taylor-wire process, J. Mat. Sci., 31, 1139 (1996). The metal to be made into a microwire is contained in a glass tube which is closed at one end. The metal is then melted and the end of the glass tube softened and drawn down to produce a fine glass-encapsulated metal filament. A limitation of this method is that the metal microwire is in contact with the glass. This prevents the wire from being useful at high temperatures because the metal can diffuse into the glass, reducing the insulating, electrical, and mechanical properties of the glass. A further limitation is that the glass must be drawn at above the melting point of the metal.

[0005] There are known inorganic cladded metal wires which are capable of conducting electricity under high voltages and high temperatures such as 1000 V and 500 C. For example, a Pyrex coated stainless steel wire, typically used in photocopy machines. Another example is a glass coated gold wire that can be made as small as 600 nm in diameter. However, such wires are not flexible and tend to be brittle, primarily due to the insulating coating. They cannot be fabricated in long lengths either. Flexibility is a required characteristic for certain applications such as decoy towlines. The best flexible ICW is able to perform up to only 400 C. and contains a flexible organic insulating coating which decomposes over time at high temperatures. An example of such an ICW is a copper wire coated with polyimide.

[0006] There remains a need for a flexible wire at least 30 centimeters in length that is capable of conducting at 1000 V and 500 C.

[0007] It is an object of the invention to provide an ICW having a gap between a metal microwire and an inorganic cladding.

[0008] It is a further object of the invention to provide a high temperature, high voltage ICW.

[0009] It is a further object of the invention to provide a high temperature, high voltage ICW that is flexible.

[0010] It is a further object of the invention to provide a high temperature, high voltage ICW that can be made in long lengths.

[0011] It is a further object of the invention to provide a high temperature, high voltage ICW suitable for use in a decoy towline.

[0012] It is a further object of the invention to provide methods of making the above ICW's.

[0013] These and other objects of the invention are accomplished by an ICW comprising: a metal microwire having an outer diameter; and an inorganic cladding having an inner diameter; wherein the microwire is positioned within the cladding; wherein the outer diameter is less then the inner diameter, such that there is a gap between the microwire and the cladding.

[0014] The invention further comprises a process of making an ICW, comprising the steps of: providing a metal microwire having an outer diameter; providing an inorganic tube having an inner diameter larger than the outer diameter; drawing the tube through a heating zone at draw process parameters such that the inner diameter of the drawn portion of the tube is reduced; inserting the microwire into the drawn portion of the tube, whereby the drawn portion of the tube becomes a cladding around the microwire; and adjusting the draw process parameters such that the inner diameter of the cladding becomes larger than the outer diameter of the microwire.

[0015] FIG. 1 is a schematic cross-sectional representation of an embodiment of the ICW of the invention;

[0016] FIG. 2 is a schematic illustration of an embodiment of an apparatus capable of making the a ICW of the invention; and

[0017] FIG. 3 is a graph of measured leakage current v. voltage for an embodiment of the ICW.

[0018] The method of the invention comprises several steps. In the step of providing a metal microwire, the microwire is a very thin metal wire. Such microwires are known in the art. Microwires inherently have an outer diameter. The maximum outer diameter of microwires is generally considered to be about 1000 m. Microwires with outer diameters as small as 1-10 m or less are also known. Microwires are commercially available in a variety of metals and diameters, although the invention is not limited to commercially available microwires.

[0019] Examples of microwires suitable for the invention can have outer diameters in the range of, but not limited to, about 1 m to about 250 m. The microwire can comprise any solid metal. The microwire can also comprise an alloy or other combination such as a coating of one metal on another metal or a composite of two or more dissimilar metals. One possible composite structure is a core-clad structure. This may be made by inserting a rod of one metal into tube of another metal. The microwire than comprises the core-clad composite. An advantage of the core-clad composite is that the core can be a cheap metal such as silver or copper. The cladding can be a more expensive metal that resists oxidation, such as platinum or a metal with a higher melting point such as tungsten, molybdenum, iridium, or rhenium. Suitable metals for the microwire can include, but are not limited to, one or more metals selected from the group consisting of copper, silver, gold, platinum, tungsten, molybdenum, rhenium, rhenium/platinum alloy, high temperature metals, and alloys and composites thereof.

[0020] In the step of providing an inorganic tube, the tube is a hollow cylinder. The cylinder inherently has an inner diameter. Inorganic tubes are commercially available in a variety of inorganic materials and diameters, although the invention is not limited to commercially available tubing. The tube may also comprise different materials in different areas of the tube. For example, the tube may have a starter zone at one end that comprises a material useful for beginning the drawing process, while the rest of the tube comprises the desired insulating material. The tube may also be a graded seal material, where there is a gradient of the composition of the tube along the length of the tube. In these cases, the softening point of the tube may vary along the length of the tube. This may allow or require changing the draw process parameters during the course of the drawing.

[0021] Examples of tubes suitable for the process of the invention can have inner diameters in the range of, but not limited to, about 0.25 in to about 1.5 in. The inner diameter is larger than the outer diameter of the microwire. The thickness of the tube is chosen by reference to the desired thickness of the cladding. The ratio of the inner and outer diameters of the cladding may be about the same as the ratio of the inner and outer diameters of the tube. The major limitation on the size of the tube is the ability of the drawing equipment to reduce inner diameter of the tube to the appropriate size. The tube comprises an inorganic material which has a softening temperature that can be, but is not limited to, lower than the melting point of the metal. Suitable inorganic materials can include, but are not limited to, one or more materials selected from the group consisting of fused silica, fused quartz, alumina, a glass, and combinations thereof. Fused silica in highly pure form is a particularly strong material with excellent dielectric, thermal, and mechanical properties.

[0022] In the step of drawing the tube, the tube is drawn through a heating zone at draw process parameters such that the inner diameter of the drawn portion of the tube is reduced to about the same size as the outer diameter. The draw process parameters can include, but are not limited to, heating zone temperature and profile, drawing speed, feed speed, atmosphere, and tube material. The tube is softened by the heating zone, and stretched during the drawing process. This makes the tube narrower and reduces the inner diameter. An inert atmosphere may be used to avoid metal oxidation and reduce the number of hydroxyl groups that could weaken the properties of the cladding material.

[0023] After a portion of the tube has been drawn, the microwire is inserted into the tube. The insertion is done such that continued drawing of the tube will also pull the microwire. The insertion may comprise contacting the microwire to the inside of the drawn tube by adjusting the draw process parameters to further reduce the inner diameter of the drawn portion of the tube. The inserting step may also be performed before or simultaneously with the drawing step, although the microwire may not be immediately pulled with the tube. The microwire may also have a leader portion on the end inserted into the tube. The leader may be a wire that can withstand higher temperatures than the microwire. The use of a leader may allow for heating the tube to a higher temperature such that it can be baited-off rapidly. Once the process parameters have been set, the leader can end and the microwire can be drawn into the tube. The leader may be attached to the microwire by any means, and may even be attached while the process is running. The portion of the drawn tube that surrounds the microwire is then referred to as a cladding.

[0024] In the adjusting step, the draw process parameters are then adjusted such that the inner diameter of the cladding becomes larger than the outer diameter of the microwire. This results in a gap between the microwire and the cladding. The size of the gap can be controlled by appropriate adjustment of the draw process parameters. If the inner diameter was about the same as the outer diameter in the drawing step or the inserting step, then the drawing speed would be reduced in the adjusting step. The reduction in drawing speed increases the inner diameter of the cladding. If the gap is too large during the drawing step, the drawing speed would be increased. If the gap is already the desired size, then the adjusting step can comprise maintaining the draw process parameters as they were during the drawing step or the inserting step. The cladding can have an inner diameter in the range of, but not limited to, about 2 m to about 290 m. The adjusting step may also comprise a change of tube material, as when the tube is not homogenous, such as a graded seal.

[0025] Once the adjustment has been done, the draw process parameters can be maintained while more microwire and tube is fed into the heating zone. The result can be an ICW comprising a metal microwire and an inorganic cladding. The microwire is positioned within the cladding and the outer diameter of the microwire is less then the inner diameter of the cladding. There is a gap between the microwire and the cladding. The gap can be all the way around the microwire such that there is little to no contact between the microwire and the cladding. The size of the gap may be, but is not limited to, from about 1 m to about 40 m.

[0026] The drawing temperature is not necessarily restricted by the melting point of the metal in the microwire. The microwire should not melt during the process. This is easily done at temperatures below the melting point of the metal. This can also be done at higher temperatures by pulling the microwire through the heating zone fast enough that is does not have time to melt before returning to lower temperatures. When the microwire comprises a core-clad composite, it may only be necessary to avoid melting the cladding, while allowing the core to temporarily melt during the drawing process.

[0027] FIG. 1 schematically illustrates the cross-section of an embodiment of the wire. The microwire 10 is surrounded by the cladding 20. There is a gap 30 between the microwire 10 and the cladding 30.

[0028] FIG. 2 schematically illustrates an embodiment of an apparatus known as a draw tower capable of making the wire of the invention. The microwire 10 is supplied on a spool 40 at the top of the draw tower. The tube 20 is drawn through a furnace 50 and narrowed to a drawn tube 70. The microwire 10 passes through an x-y translator 60 to correctly position it in the center of the drawn tube 70. An insulating plate 80 protects the spool 40 from the heat of the furnace 50. A tensioning device 90 on the spool 40 keeps the microwire 10 straight and taught. Arbitrarily long wires, such as 200 meters, 400 meters, or longer, can be made with a draw tower.

[0029] The ICW can also be made in shorter lengths with lab-scale apparatus. This may be done by simply inserting the microwire into an inorganic cladding that already has the desired final inner diameter. No drawing or heating is necessary for this method. The ICW of the invention may also be made by other methods.

[0030] The ICW may be suitable as a general purpose ICW. At least some embodiments of the insulated conducting wire have properties that are not available in any other insulated conducting wire. Due to the small diameter, the ICW may be flexible. As used herein, the term flexible refers to the ability to bend the ICW according to methods known in the art. It is also possible to manufacture the ICW in long lengths of at least 30 cm and up to 400 meters and longer. The wire may be capable of conducting current while subjected to a potential of at least about 1000 V and a temperature of at least about 500 C. without dielectric breakdown. The wire may withstand even more severe conditions such as 5000 V at 840 C., 10,000 V at 650 C., and 1000 V at 1500 C.

[0031] The presence of the gap may be responsible for the high temperature, high voltage properties of the ICW. The following descriptions of the mechanism by which the gap improves the performance of the ICW are proposed mechanisms. The proposed mechanisms do not limit the scope of the invention. The gap may help to offset the effects of differential thermal expansion of the microwire and the cladding. If the microwire and the cladding were in contact, both contact and thermal expansion could cause fracture or failure of the cladding. When there is a gap between them, each can expand without being constrained by the other.

[0032] The gap may also avoid contact and diffusion of metal onto and/or into the cladding during the drawing process. The drawing process may take place at temperatures high enough that any metal in contact with the cladding may stick onto and/or diffuse into the cladding. Such metal contamination can adversely effect the physical and electrical properties of the cladding. The cladding may lose strength as well as dielectric properties. Since the gap is present during the drawing process, there is no metal in contact with the cladding while at elevated temperatures and therefore minimal to no metal diffusion. Prevention of contamination is particularly important where the cladding is fused silica, as its mechanical and dielectric-properties are very sensitive to contamination.

[0033] Certain contaminants in the silica may by useful. Fused silica doped with 4ions such as Ti⁴ and Ce⁴ can have a lower softening point. The doping level may be less than 0.2%. The softening point can be reduced from over 2000 C. to about 1600 C. This may be useful when the microwire comprises molybdenum, as molybdenum can become brittle when heated to 2000 C. These ions have very little mobility so there is minimal reduction in dielectric and mechanical strength. A similar effect may be achieved with phosphate doped silica.

[0034] The properties of the ICW may make it suitable for use in a decoy towline. A decoy towline is used to tow a decoy behind a military aircraft. The towline can comprise a number of components, including conducting wire that should be flexible, light-weight and capable of functioning at high voltages and high temperatures. The towline should have as small a diameter as possible, so that it can be wound onto a spool and take up little space. When the towline passes through the jet engine plume, it is subject to extreme temperatures. The wire of the invention may still continue to perform as needed under these conditions.

[0035] The cladding can be strengthened by placing a coating on its surface. The coating can comprise, but is not limited to, one or more materials selected from the group consisting of polyimide, a polymer, an organic coating, and an inorganic coating. The coating may allow the ICW to be handled after the drawing process without breaking the wire. The ICW may be wound onto a spool, incorporated into a device, or otherwise handled. The coating may both strengthen the ICW and protect the cladding from scratching and from materials that may contaminate the cladding.

[0036] The coating may not be necessary to the electrical properties of the wire. In a decoy towline, the coating may be polyimide. The polyimide can protect the wire when the wire is spooled, unwound, incorporated into the towline, and deployed. Once the towline is deployed and exposed to the jet engine plume, the polyimide may decompose and expose the cladding. However, the polyimide is not needed to obtain the electrical properties needed for the wire in the towline and is not needed for strength once the towline is deployed. Polyimide decomposes cleanly. The decomposition products may not contaminate the cladding.

[0037] Having described the invention, the following examples are given to illustrate specific applications of the invention. These specific examples are not intended to limit the scope of the invention described in this application.

[0038] Copper-silica wireAn ICW composed of a copper microwire and a polyimide coated silica cladding was made by feeding the microwire into a fused silica tube (360 micron outer diameter, 100 micron inner diameter).

[0039] The outer diameter of the microwire was 89 m. The inner diameter of the cladding was 100 m. The cladding was 130 m thick, resulting in an outer cladding diameter of 360 m. The wire was heated to 650 C. and subjected to 10,000 V for 30 minutes without dielectric breakdown. The wire also performed for one hour at 840 C. and 5000 V. The electrical tests were done by wrapping an exposed conducting wire around the ICW and placing both wires in a furnace. A high voltage was applied to the exposed wire while the ICW was grounded. The leakage current through the ICW was measured. A graph of the measured leakage current as a function of applied voltage is shown in FIG. 3. There was no voltage breakdown through the cladding.

[0040] The bend radius of the wire was 0.0625 in. This was demonstrated by bending the wire by 90 around a {fraction (1/8)} inch post. The bent wire did not break.

[0041] Copper-silica wireA wire similar to that of Example 1 was made with a thinner cladding. The outer diameter of the cladding was 200 m. The bend radius of the wire was less than 0.0625 in.

[0042] Platinum-silica wireAn ICW-composed of a platinum microwire and a silica cladding was made by feeding a fused silica tube (25 mm outer diameter, 12.5 mm inner diameter) into a high temperature furnace and heated to greater than about 1600 C. The draw speed was set to reduce the inner diameter of the tube to about 100 m. The platinum microwire was then inserted into the thinnest part of the tube. The draw speed was then slightly increased to reduce the inner diameter. The microwire contacted the tube, such that the tube grabbed the wire and pulled it through the furnace with the tube, The draw speed was then slightly increased to create a gap between the microwire and the cladding. The ICW was 18 inches long. The outer diameter of the microwire was 100 m. The inner diameter of the cladding was 110 m.