CURRENT LIMITING DEVICE AND METHOD OF MANUFACTURING THEREFOR

Abstract: A current limiting device utilizes an electrically conductive composite material structure and an inhomogeneous distribution of resistance structure. The electrically conductive composite material strucutre comprises alternating regions of an insulating flexible material and a composite material.

Description: CURRENT LIMITING DEVICE AND METHOD OF MANUFACTURING THEREFOR 5 FIELD OF THE INVENTION This invention relates to current limiting devices for general circuit protection including electrical distribution and motor 10 control applications. In particular, the invention relates to current limiting devices that are capable of limiting the current in a circuit when a high current event or high current condition occurs. DESCRIPTION OF RELATED ART 15 There are numerous devices that are capable of limiting the current in a circuit when a high current condition occurs. One known limiting device includes a filled polymer material that exhibits what is commonly referred to as a PTCR (positive-temperature 20 coefficient of resistance) or PTC effect. U. S. Patent No. 5,382,938, U.S. Patent No. 5,313,184, and European Published Patent Application No. 0,640,995 A1 each describes electrical devices relying on PTC behavior. The unique attribute of the PTCR or PTC effect is that at a certain switch temperature the PTCR material undergoes a 25 transformation from a basically conductive material to a basically resistive material. In some of these prior current limiting devices, the PTCR material (typically polyethylene loaded with carbon black) is placed between pressure contact electrodes. U.S. Patent No. 5,614,881 , to Duggal et al., issued March 30 25, 1997, the entire contents of which are herein incorporated by reference, discloses a current limiting device. This current limiting device relies on a composite material and an inhomogeneous distribution of resistance structure. Current limiting devices are used in many applications to 35 protect sensitive components in an electrical circuit from high fault currents. Applications range from low voltage and low current electrical circuits to high voltage and high current electrical distribution systems. An important requirement for many applications is a fast current limiting response time, alternately known as switching time, to minimize the peak fault current that develops. In operation, current limiting devices are placed in a circuit to be protected. Under normal circuit conditions, the current limiting device is in a highly conducting state. When a high current condition occurs, the PTCR material heats up through resistive heating until the temperature is above the "switch temperature." At this point, the PTCR material resistance changes to a high resistance state and the high current condition current is limited. When the high current condition is cleared, the current limiting device cools down over a time period, which may be a long time period, to below the switch temperature and returns to the highly conducting state. In the highly conducting state, the current limiting device is again capable of switching to the high resistance state in response to future high current condition events. Known current limiting devices comprise electrodes, electrically conductive composite material, a low pyrolysis or vaporization temperature polymeric binder and an electrically conducting filler, combined with an inhomogeneous distribution of resistance structure. The switching action of these current limiting devices occurs when joule heating of the electrically conducting filler in the relatively higher resistance part of the composite material causes sufficient heating to cause pyrolysis or vaporization of the binder. During operation of known current limiting devices, at least one of material ablation and arcing occur at localized switching regions in the inhomogeneous distribution of resistance structure. The ablation and arcing can lead to at least one of high mechanical stresses, which are often in the form of moment mechanical stresses, subjected on the conductive composite material. These high mechanical stresses often lead to the mechanical failure of the composite material. Therefore, electrically conductive composite materials for use in current limiting devices should possess desirable and constant properties. These properties are suitable for high current multiple use current polymer limiting devices that avoid a build up of undesirable high moment mechanical stresses. SUMMARY OF THE INVENTION Accordingly, it is desirable to provide a quick, reusable current limiting device, where the current limiting device overcomes the above noted, and other, disadvantages of the related art. It is further desirable to provide a current limiting device, where the composite material possesses desirable properties suitable for a multiple use current polymer limiting device, so as to avoid a build up of high moment mechanical stresses. It is also desirable to provide a current limiting device comprising at least two electrodes; an electrically conducting composite material structure between the electrodes; interfaces between the electrodes and electrically conducting composite material structure; an inhomogeneous distribution of resistance at the interfaces whereby, during a high current event, adiabatic resistive heating at the interfaces causes rapid thermal expansion and vaporization and at least a partial physical separation at the interfaces; and means for exerting compressive pressure on the electrically conducting composite material structure, wherein the electrically conducting composite material structure comprises alternating regions of an insulating flexible material and a composite material. It is also desirable to provide a method for forming the current limiting device and the conducting composite material. Additionally, it is desirable to provide a conducting composite material comprising alternating regions of an insulating flexible material and a composite material. These and other advantages and salient features of the invention will become apparent from the following detailed description, which, when taken in conjunction with the annexed drawings, disclose preferred embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS While the novel features of this invention are set forth in the following description, the invention will now be described from the following detailed description of the invention taken in conjunction with the drawings, in which: Figure 1 is a schematic representation of a current limiting device, as embodied by the invention; Figure 2 is a schematic representation of a further current limiting device, as embodied by the invention; Figure 3 is a top perspective view of a composite material structure for a current limiting device, as embodied by the invention; Figure 4 is a side sectional view of a composite material structure for a current limiting device, as embodied by the invention; Figure 5 is a side sectional view of a current limiting device with a composite material, as embodied by the invention; Figures 6a - 6d are side sectional views of a method of forming a composite material structure for a current limiting device, as embodied by the invention; Figure 7 is a flow chart illustrating the steps in the method of forming in Figs. 6a-6d; and Figs. 8-10 are top perspective views of composite material structures for a current limiting device, as embodied by the invention. DETAILED DESCRIPTION OF THE EMBODIMENTS A current limiting device, as described in US Patent No. 5,614,881 to Duggal et al, which is fully incorporated by reference, comprises an electrically conductive composite material positioned between electrodes, so that there is an inhomogeneous distribution of resistance throughout the current limiting device. The electrically conductive composite material comprises at least a conductive filler and an organic binder. The current limiting device in US Patent No. 5,614,881 further comprises means for exerting compressive pressure on the electrically conductive composite material of the current limiting device. Fig. 1 illustrates a current limiting device, described in US Patent No. 5,614,881 . The current limiting device comprises a high current multiple use fast-acting current limiting device 1 . In Fig. 1 , the current limiting device 1 described in US Patent No. 5,614,881 , comprises electrodes 3 and an electrically conductive composite material 5 with inhomogeneous distributions 7 of resistance structure under compressive pressure P. However, the scope of the invention includes a high current multiple use current limiting device with any suitable construction where a higher resistance is anywhere between the electrodes 3. For example, the higher resistance may be between two composite materials 55 in the high current multiple use current limiting device, as illustrated in Fig. 2. To be a reusable current limiting device, the inhomogeneous resistance distribution is arranged so at least one thin layer of the current limiting device is positioned perpendicular to the direction of current flow, and has a higher resistance than the average resistance for an average layer of the same size and orientation in the device. In addition, the current limiting device is under compressive pressure in a direction perpendicular to the selected thin high resistance layer. The compressive pressure may be inherent in the current limiting device or exerted by a resilient structure, assembly or device, such as but not limited to a spring. In operation, a current limiting device is placed in the electrical circuit to be protected. During normal operation, the resistance of the current limiting device is low, i.e., in this example the resistance of the current limiting device would be equal to the resistance of the electrically conductive composite material plus the resistance of the electrodes plus the contact resistance. When a high current event or short circuit occurs, a high current density starts to flow through the current limiting device. In initial stages of the short circuit or high current event, the resistive heating of the current limiting device is believed to be adiabatic. Thus, it is believed that the selected thin, more resistive layer of the current limiting device heats up much faster than the remainder of the current limiting device. With a properly designed thin layer, it is believed that the thin layer heats up so quickly that one of thermal expansion of and gas evolution from the thin layer causes a separation within the current limiting device at the thin layer. As described above, there is a possibility that a build up of moment mechanical stresses occurs in a operation of a current limiting device. The build up of moment mechanical stresses is due, in part to at least one of material ablation and arcing occurring at localized switching regions in the inhomogeneous distribution of resistance structure. The ablation and arcing can lead to high mechanical stresses, which are often in the form of moment mechanical stresses, that are subjected on the conductive composite material. These moment mechanical stresses often lead to the mechanical failure of the composite material. Therefore, electrically conductive composite materials for use in current limiting devices should possess desirable and constant properties and comprise a structure that avoids a build up of moment mechanical stresses. Fig. 3 illustrates a composite material structure 10, which is for use in a current limiting device, as embodied by the invention, to avoid a build up of moment mechanical stresses in a current limiting device. The composite material structure 10 in Fig. 3 comprises alternating regions of an insulating flexible material 12 and a composite material 14. The composite material 14 of the composite material structure 10 has a composition similar to that disclosed in U.S. Patent No. 5,614,881. Accordingly, the composite material 14 comprises a polymer material filled with a conducting filler, and an inhomogeneous distribution of resistance. The composite material 14 comprises a low pyrolysis or vaporization temperature binder and an electrically conducting filler combined with an inhomogeneous distribution of resistance structure. The binder should be chosen such that significant gas evolution occurs at low approximately (<800°C) temperature. The inhomogeneous distribution structure is typically chosen so that at least one selected thin layer of the current limiting device has much higher resistance than the rest of the current limiting device. For example, the electrically conductive composite 14 may comprise an elastomer, such as silicone, as the binder material and a metal, such as silver, as the filler material and have a resistivity of about 0.004 ohm-cm. The silver-filled curable silicone material (elastomer) is made, for example, by mixing two parts, A & B. The A part comprises a vinyl silicone organopolysiloxane fluid having terminal dimethylvinylsiloxy units and dimethylsiloxy units with a viscosity of 400 cps at 25°C (23g), the following silver particles from Ames Goldsmith Corp. Ag 4300 (46.6g), Ag 1036 (37.3g) and Ag 1024 (37.3g), and a silicone hydride siloxane fluid having terminal trimethyl siloxy units to provide a fluid with about 0.8% by weight chemically combined hydrogen attached to silicon (1 g). The B part comprises vinyl silicone organopolysiloxane fluid having terminal dimethylvinylsiloxy units and dimethylsiloxy units with a viscosity of 400 cps (2g), dimethyl maleate (14 mL) and Karstedt's platinum catalyst (83 mL of a 5% platinum solution in xylene) [for details see U.S. Patent 3,775,452, B.D. Karstedt (1973)]. The A component (40g) and B component (0.44g) are mixed and then poured into a mold, and then cured in a Carver press at 150°C, 30 minutes at 5000 pounds pressure. Alternately, the composite material 14 may comprise a thermoset binder, such as an epoxy binder (Epoxy-Technology Inc. N30 material) and a metal, such as nickel powder, as the conducting filler material. When cured at 150°C for about 1 hour, this material has a resistivity of about 0.02-0.03 ohm-cm. Further, the composite material 14 may comprise a thermoset binder, such as, an epoxy binder with a metal filler, such as silver, as the conducting filler. Such a composite is prepared using silver particles from Ames Goldsmith Corp. Ag 4300 (5.6g), Ag 1036 (4.2g), Ag 1024 (4.2g) and a two component commercial epoxy (Epotek 301 ) obtained from Epoxy Technology Inc. The epoxy resin (2.3g) is mixed with the hardener (0.6g) and then the silver particles are added and the mixture is placed in a Teflon® mold and cured at 60C for 1 hour Another example of an epoxy binder with a metal, such as silver can be prepared using Ablebond® 967-1 (Commercial Conducting Adhesive Material from Ablestik Electronic Materials & Adhesives (a subsidiary of National Starch and Chemical Company) placed in a Teflon® mold and cured at 80C for about 2 hours. Alternately, the composite material 14 may comprise an elastomer binder, such as a silicone binder with a two component metal conducting filler, such as silver and aluminum, as the conducting filler. This composition is prepared by mixing two parts, A and B. The A part comprises a vinyl silicone organopolysiloxane fluid having terminal dimethylvinylsiloxy units and dimethylsiloxy units (400 cps, 23g), 37.3g of aluminum powder, silver particles from Ames Goldsmith Corp. Ag 4300 (46.6g), Ag 1036 (37.3g) and Ag 1024 (37.3g), and a silicone hydride siloxane fluid having terminal trimethyl siloxy units to provide a fluid with about 0.8% by weight chemically combined hydrogen attached to silicon (1g). The B part comprises vinyl silicone organopolysiloxane fluid having terminal dimethylvinylsiloxy units and dimethylsiloxy units with a viscosity of 400 cps (2g), dimethyl maleate (14 mL) and Karstedt's platinum catalyst (83 mL of a 5% platinum solution in xylene). The A component (40g) and B component (0.44g) are mixed and then poured into a mold and then cured in a Carver press at about 150°C for about 30 minutes at about 5000 pounds pressure. As a further alternative, the composite material 14 may comprise an elastomer binder, such as a silver-filled, curable silicone is made from two parts, A and B. The A part comprises a vinyl silicone organopolysiloxane fluid having terminal dimethylvinylsiloxy units and dimethylsiloxy units (400 cps, 33g), silver particles from Ames Goldsmith Corp. Ag 4300 (46.6g), Ag 1036 (37.3g) and Ag 1024 (37.3g), alpha quartz (Minusil, 23g) and a silicone hydride siloxane fluid having terminal trimethyl siloxy units to provide a fluid with about 0.8% by weight chemically combined hydrogen attached to silicon (2g). The B part comprises vinyl silicone organopolysiloxane fluid having terminal dimethylvinylsiloxy units and dimethylsiloxy units with a viscosity of 400 cps (10g), dimethyl maleate (70 mL) and Karstedt's platinum catalyst, as mentioned above (415 mL of a 5% platinum solution in xylene). The A component (40g) and B component (0.5g) are mixed and then poured into a mold and then cured in a Carver press at about 150°C for about 30 minutes at about 5000 pounds pressure. In a still further alternative, the composite material 14, the composite material 14 may comprise a reinforced elastomer binder, such as a curable silicone reinforced with fumed silica, with a two component metal filler, such as, silver and aluminum is made with an A part and a B part. The A part comprises an elastomer binder, such as a vinyl silicone organopolysiloxane fluid having terminal dimethylvinylsiloxy units and dimethylsiloxy units (400 cps, 23g), a silicone hydride siloxane fluid having terminal trimethyl siloxy units to provide a fluid with about 0.8% by weight chemically combined hydrogen attached to silicon (2g), doubly treated fumed silica (300 rτ.2/g, treated with cyclo-octamethyltetrasiloxane and with hexamethyldisilazane, 1.2g),aluminum powder (37.3g), silver particles from Ames Goldsmith Corp. Ag 4300 (46.6g), Ag 1036 (37.3g), Ag 1024 (37.3g). The B part comprises the vinyl silicone organopolysiloxane fluid having terminal dimethylvinylsiloxy units and dimethylsiloxy units (400 cps, 2g), dimethylmaleate (14 mL) and Karstedt's platinum catalyst (83 mL). A curable formulation is prepared by combining the A part (40g) and the B part (0.44g) and then hand mixing and placing in a mold. Cure is accomplished in a Carver press at 5000 pounds pressure and 150°C for 30 min. In another alternative for the composite material 14 an elastomer binder, such as a nickel filled silicone, is made from two parts, A and B. The A part comprises a vinyl silicone organopolysiloxane fluid having terminal dimethylvinylsiloxy units and dimethylsiloxy units (400 cps, 25g), nickel powder (INCO type 123, 100g) and a silicone hydride siloxane fluid having terminal trimethyl siloxy units to provide a fluid with about 0.8% by weight chemically combined hydrogen attached to silicon (2g). The B part comprises the vinyl silicone organopolysiloxane fluid having terminal dimethylvinylsiloxy units and dimethylsiloxy units with a viscosity of 400 cps (10g), dimethyl maleate (70 mL) and Karstedt's platinum catalyst (415 mL of a 5% platinum solution in xylene). The A component (40g) and B component (0.5g) are mixed and then poured into a mold and then cured in a Carver press at 150°C, 30 minutes at 5000 pounds pressure. In a further alternative the composite material 14 may comprise thermoplastic binder, such as polytetrafluoroethylene binder, with a semiconductor conducting filler, such as Carbon Black. An example is GS-2100-080-5000-SC (Commercial Conductive Fluoropolymer from W. L. Gore & Associates, Inc.). Finally, in another alternative, the composite material 14 may comprise a thermoplastic binder, such as Poly(ethylene glycol) with a metal filler, such as silver, as the conducting filler. A silver particle mixture comprising the following particles from Ames Goldsmith Corp., Ag 4300 (2.8g), Ag 1036 (2.1g), Ag 1024 (2.1g) is heated to about 80C and poured into molten Poly(ethylenegiycol) (MW8000) at about 80°C and mixed. The material is then poured into a Teflon® mold and allowed to harden at room temperature. A binder material having a low pyrolysis or vaporization temperature (<800_C) such as: a thermoplastic (for example, polytetrafluoroethylene, poly(ethyleneglycol), polyethylene, polycarbonate, polyimide, polyamide, polymethylmethacrylate, polyester etc.); a thermoset plastic (for example, epoxy, polyester, polyurethane, phenolic, alkyd); an elastomer (for example, silicone (polyorganosiloxane), (poly)urethane, isoprene rubber, neoprene, etc.); an organic or inorganic crystal; combined with an electrically conducting filler such as a metal (for example, nickel, silver, copper, etc.) or a semiconductor (for example, carbon black, titanium dioxide, etc.) with a particulate or foam structure; combined with a metal or semiconductor electrode pressure contacted to the electrically conducting composite material, could also perform effectively in the current iimiter of the present invention. Third phase fillers could be used to improve specific properties of the composite such as the mechanical properties; dielectric properties; or to provide arc-quenching properties or flame- retardant properties. Materials which could be used as a third phase filler in the composite material include: a filler selected from reinforcing fillers such as fumed silica, or extending fillers such as precipitated silica and mixtures thereof. Other fillers include titanium dioxide, lithopone, zinc oxide, diatomaceous silicate, silica aerogel, iron oxide, diatomaceous earth, calcium carbonate, silazane treated silicas, silicone treated silicas, glass fibers, magnesium oxide, chromic oxide, zirconium oxide, alpha-quartz, calcined clay, carbon, graphite, cork, cotton sodium bicarbonate, boric acid, alumina-hydrate, etc.. Other additives may include: impact modifiers for preventing damage to the current limiter such as cracking upon sudden impact; flame retardant for preventing flame formation and/or inhibiting flame formation in the current limiter; dyes and colorants for providing specific color components in response to customer requirements; UV screens for preventing reduction in component physical properties due to exposure to sunlight or other forms of UV radiation. As illustrated in Fig. 3, composite material structure 10, as embodied by the invention, comprises an insulating flexible material 12. The insulating flexible material 12 comprises a plurality of generally equi-spaced generally parallel segments, otherwise known as, strips 13. The strips 13 are illustrated in Fig. 3 to intersect at generally orthogonal angles, so as to define generally rectangular pieces 15, here square, of composite material 14. However, the generally rectangular geometrical configuration illustrated in Fig. 3 is merely exemplary of the numerous geometrical configurations within the scope of the invention. As discussed hereinafter, the invention contemplates a composite material structure 10 with any geometrical configuration of composite material 14 and insulating flexible material 12. Fig. 4 illustrates a side sectional view of a composite material structure 10, as embodied by the invention. The insulating flexible material 12 is positioned between the generally rectangular pieces 15 of the composite material 14. The composite material 14 extends above the insulating flexible material 12 at both upper and lower portions of the composite material structure 10. As described below, the altitude of the composite material 14 is a result of a process used to form the composite material structure 10 (described hereinafter). The composite material structure 10 may have the insulating flexible material 12 positioned between pieces of the composite material 14 at any appropriate altitude with respect to the composite material 14, as long as the composite material structure 10 reduces a build up of moment mechanical stresses. A current limiting device 20 incorporating the composite material structure 10, as embodied by the invention, is illustrated in Fig. 5. In Fig. 5, the current limiting device 20 comprises electrodes 23 and an electrically conductive composite material structure 10, as embodied by the invention. Inhomogeneous distributions (not illustrated but similar to those described in US Patent No. 5,614,881 ) of resistance structure are included, and the current limiting device 20 is under compressive pressure P. However, the scope of the invention includes a high current multiple use current limiting device with any suitable construction where a higher resistance is anywhere between the electrodes 23. The advantages of electrically conductive composite material structure 10, as embodied by the invention arise, at least in part, due to an increased and enhanced flexibility of the electrically conductive composite material structure 10. The increased flexibility is imparted by the flexible material regions defined by the insulating flexible material 12. The enhanced and increased flexibility is advantageous, for example, when a switching event occurs. During a switching event, many different localized ablation events occur in the current limiting device 20. These localized ablation events cause a distribution of localized stresses over the composite material structure 10. If relatively wide area electrodes are used in the current limiting device 20, large lever moments on the composite material structure 10 in the current limiting device 20 may occur during switching. These large lever moments often result in material fracture of the composite material structure 10. With the composite material structure 10, as embodied by the invention, large lever moment stresses are avoided and do not build up because there are at least one, and preferably a plurality of, regions of insulating flexible material 12. The regions of insulating flexible material 12 are interspersed between stress points, which are also separated by a relatively large distance. These regions of insulating flexible material 12 permit displacement of the composite material structure 10. The displacement of the composite material structure 10 relieves stresses that build up during switching. The insulating flexible material 12 is formed of a flexible material, such as but not limited to, both natural and synthetic rubbery materials, such as for example, a silicone rubber, an elastomer, such as but not limited to silicone (polyorganosiloxane), (poly)urethane, isoprene rubber, and neoprene. The rubbery materials of the insulating flexible material 12 have a sufficient dielectric strength to prevent an arc from striking directly between the two electrodes 23. In order to prevent an arc, the insulating flexible material 12 is also in close physical contact with the surrounding composite material 14. Alternately, the insulating flexible material 12 can be adhered to the surrounding composite material 14. Further, it is alternately desirable to add a predetermined quantity of fillers to the insulating flexible material 12. The addition of fillers can enhance the dielectric strength or thermal conductivity of the insulating flexible material 12 and the resulting flexible regions. Spacing between the flexible regions of the insulating flexible material 12 is dependent on design, ultimate intended use and environmental factors, such as for example a desired fracture toughness and thickness of the electrically conducting composite material 14 utilized in a current limiting device 20. Also, spacing between the flexible regions of the insulating flexible material 12 depends on the ultimate intended use and environment of the current limiting device 20. In general, a less tough and thinner electrically conducting composite material 14 requires a smaller distance between flexible regions of the insulating flexible material 12, than would a tougher and thicker electrically conducting composite material 14. Figs. 6a-6d illustrate one method for fabricating a composite material structure 10. Fig. 7 is a flow chart illustrating the steps set forth in Figs. 6a-6d. In step S1 , a single piece of a relatively inflexible electrically conductive composite material 100, illustrated in Fig. 6a, is provided. In step S2, the single piece of a relatively inflexible electrically conductive composite material 100 is cut with an appropriate cutting tool, for example, but not limited to, a dicing saw. The relatively inflexible electrically conductive composite material 100 is cut to form depressions, otherwise known as, cut regions 102. The cut regions 102 are formed at a cut side 103 of the relatively inflexible electrically conductive composite material 100 to form the segments, otherwise known as strips, 12 with an appropriate pattern for a desired final composite material structure 10. The cut relatively inflexible electrically conductive composite material 100 is illustrated in Fig. 6b. The cut relatively inflexible electrically conductive composite material 100 comprises an un-cut side 104, which is opposite openings of the cut regions 102 of the cut relatively inflexible electrically conductive composite material 100. An uncured insulating flexible material 121 is prepared in step S3. Selected cut regions 102 of the cut relatively inflexible electrically conductive composite material 100 are then filled with the uncured insulating flexible material 121 , as illustrated in Fig. 6c, in step S4. The uncured insulating flexible material 121 can be any appropriate uncured material that will form an insulating flexible material 12, as embodied by the invention, when cured. For example, the uncured insulating flexible material 121 may comprise uncured silicone liquid. The uncured insulating flexible material 121 in the selected cut regions 102 of the cut relatively inflexible electrically conductive composite material 100 is then cured into a flexible solid cured insulating material 121, in step S5. The cured insulating material 121 will shrink, to a certain degree, and its top level 122 will be below a top level of the cut side 103 of the cut relatively inflexible electrically conductive composite material 100. The cut relatively inflexible electrically conductive composite material 100 is then machined, cut or otherwise in step S6 to remove portions of the originally un-cut side 104, which are opposite openings of the cut regions 102 of the cut relatively inflexible electrically conductive composite material 100. Thus, the flexible solid cured insulating material 121 is exposed from both sides of the cut relatively inflexible electrically conductive composite material 100, without extending to the sides, as illustrated. If step S6 is omitted, the composite material structure 10 will still possess an enhanced fracture toughness. The enhanced fracture toughness is due in part to the flexible solid cured insulating material 121 in the flexible solid cured insulating material 121 , which still provides a suitable degree of flexibility. As discussed above, the geometrical shape and configuration of a composite material structure 10 as illustrated in Fig. 3, is merely exemplary of the numerous geometrical configurations within the scope of the invention. The scope of the invention comprises composite material structures 10 with any geometrical configuration of composite material 14 and insulating flexible material 12. Figs 8-10 illustrate several of the many different possibilities of the geometrical configuration of composite material structures 10. In Fig. 8, the composite material structure 10 comprises arcuate geometrical configurations of composite material 14 and insulating flexible material 12. Fig. 9 illustrates triangular geometrical configurations of composite material 14 and insulating flexible material 12. Further, Fig. 10 illustrates a mixed geometrical configuration of composite material 14 and insulating flexible material 12. As stated above, these are merely exemplary and do not limit the invention in any manner. It is believed that the advantageous results of the invention are obtained because, during a high current event, adiabatic resistive heating of the thin layer lends to rapid thermal expansion and gas evolution from the binding material in the high current multiple use current limiting device. This rapid thermal expansion and gas evolution lead to a partial or complete physical separation of the current limiting device at the selected thin layer, and produce a higher over-all device resistance to electric current flow. Therefore, the current limiting device limits the flow of current through the current path. When the high current event is cleared externally, it is believed that the current limiting device regains its low resistance state due to the compressive pressure built into the current limiting device allowing thereby electrical current to flow normally. The current limiting device, as embodied by the invention, is reusable for many such high current event conditions, depending upon such factors, among others, as the severity and duration of each high current event. In a current limiting device, as embodied by the invention, it is believed that the vaporization and/or ablation of the composite material causes a partial or complete physical separation at the area of high resistance, for example the electrode/material interface. In this separated state, it is believed that ablation of the composite material occurs and arcing between the separated layers of the current limiting device can occur. However, the overall resistance in the separated state is much higher than in the non-separated state. This high arc resistance is believed due to the high pressure generated at the interface by the gas evolution from the composite binder combined with the deionizing properties of the gas. In any event, the current limiting device of the present invention is effective in limiting the high current event current so that the other components of the circuit are not harmed by the high current event. After the high current event is interrupted, it is believed that the current limiting device returns or reforms into its non-separated state, due to compressive pressure, which acts to push the separated layers together. It is believed that once the layers of the current limiting device have returned to the non-separated state or the low resistance state, the current limiting device is fully operational for future current-limiting operations in response to other high current event conductors. Alternate embodiments of the current limiting device of the present invention can be made by employing a parallel current path containing a resistor, varistor, or other linear or nonlinear elements to achieve goals such as controlling the maximum voltage that may appear across the current limiting device in a particular circuit or to provide an alternative path for some of the circuit energy in order to increase the usable lifetime of the current limiting device. While the embodiments described herein are preferred, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made by those skilled in the art that are within the scope of the invention.

Claims: What is claimed is: 1. A current limiting device comprising: at least two electrodes; an electrically conducting composite material structure between said electrodes; interfaces between said electrodes and electrically conducting composite material structure; an inhomogeneous distribution of resistance at said interfaces whereby, during a high current event, adiabatic resistive heating at said interfaces causes rapid thermal expansion and vaporization and at least a partial physical separation at said interfaces; and means for exerting compressive pressure on said electrically conducting composite material structure, wherein said electrically conducting composite material structure comprises alternating regions of at least one insulating flexible material and at least one composite material. 2. The device according to claim 1 , wherein the at least one composite material comprises a polymer material filled with a conducting filler. 3. The device according to claim 1 , wherein the at least one insulating flexible material comprises at least one of natural and synthetic rubbers. 4. The device according to claim 1 , wherein the at least one insulating flexible material comprises at least one material selected from the group consisting of: silicone rubber; elastomers; polyorganosiloxane; (poly) urethane; isoprene rubber; and neoprene. 5. The device according to claim 1 , wherein the at least one insulating flexible material comprises fillers that enhances thermal conductivity of the insulating flexible material. 6. The device according to claim 1 , wherein the alternating regions of the at least one insulating flexible material and the at least one composite material define at least one insulating flexible material region. 7. The device according to claim 1 , wherein the at least one insulating flexible material is adhered to surrounding composite material. 8. The device according to claim 1 , wherein the at least one insulating flexible material is in close physical contact with surrounding composite material. 9. The device according to claim 1 , wherein the at least one insulating flexible material in the alternating regions of the at least one insulating flexible material and the at least one composite material comprises at least one of strips and segments of insulating flexible material. 10. The device according to claim 9, wherein the at least one of strips and segments comprises orthogonally intersecting insulating flexible material. 11. The device according to claim 9, wherein the at least one of strips and segments comprises arcuate insulating flexible material. 12. The device according to claim 9, wherein the at least one of strips and segments comprises triangular insulating flexible material. 13. The device according to claim 1 , wherein the at least one of strips and segments comprises polygonal insulating flexible material. 14. The device according to claim 1 , wherein the at least one of strips and segments comprises at least one of orthagonally insulating flexible material, arcuate insulating flexible material, triangular insulating flexible material and polygonal insulating flexible material. 15. The device according to claim 1 , wherein the compressive pressure provided by the exerting means is applied in a direction perpendicular to current flow. 16. The device according to claim 1 , wherein during a high current event, adiabatic resistive heating is followed by rapid thermal expansion and vaporization of the composite material, the thermal expansion and vaporization being followed by at least a partial physical separation of layers of the current limiting device. 17. The device according to claim 1 , wherein the overall resistance of the device in the partially or complete separated state is much higher than in the non-separated state so that the current limiting device is effective in limiting a high current event. 18. The device according to claim 1 , wherein upon elimination of the high current event, the exerting means exerts pressure sufficient such that the device returns to the low resistive state. 19. The device according to claim 1 , wherein during a high current event, a higher over-all device resistance to electric current flow is produced during the high current event. 20. A method of manufacturing an electrically conducting composite material structure, electrically conducting composite material structure comprising an alternating regions of an insulating flexible material and a composite material; the method comprising: providing a relatively inflexible electrically conductive composite material; forming at least one depression in the relatively inflexible electrical conductive composite material; providing a uncured insulating flexible material; depositing the uncured insulating flexible material in the at least one depression in the relatively inflexible electrically conductive composite material; and curing the insulating flexible material. 21. A method according to claim 20, further comprising removing a portion of the relatively and flexible electrically conductive composite material opposite the at least one depression, to expose the cured insulating flexible material from both sides of the relatively inflexible electrically conductive composite material. 22. A method according to claim 20, further comprising shrinking the insulating flexible material during curing, wherein a top surface of the insulating flexible material is below a top surface of the relatively inflexible electrically conductive composite material. 23. A method according to claim 20, further comprising providing the insulating flexible material from least one of natural and synthetic rubbers. 24. A method according to claim 20, further comprising providing the insulating flexible material from at least one material selected from the group consisting of; silicone rubber; elastomers; polyorganosiloxane; (poly) urethane; isoprene rubber; and neoprene. 25. A method according to claim 20, further comprising providing the insulating flexible material comprising fillers that enhances thermal conductivity of the insulating flexible material. 26. A method according to claim 20, further comprising adhering wherein the insulating flexible material to surrounding composite material. 27. A method according to claim 20, further comprising providing the insulafing flexible material in close physical contact with the surrounding composite material. 28. A method according to claim 20, further comprising providing the insulating flexible material in the alternafing regions of insulating flexible material and composite material from at least one of strips and segments of insulafing flexible material. 29. The device according to claim 20, wherein the at least one of strips and segments comprises orthagonally intersecting insulating flexible material. 30. The device according to claim 20, wherein the at least one of strips and segments comprises arcuate insulating flexible material. 31. The device according to claim 20, wherein the at least one of strips and segments comprises triangular insulating flexible material. 32. The device according to claim 20, wherein the at least one of strips and segments comprises polygonal insulafing flexible material. 33. The device according to claim 20, wherein the at least one of strips and segments comprises at least one of orthagonally insulating flexible material, arcuate insulating flexible material, triangular insulating flexible material and polygonal insulating flexible material. κu-zo,tau 21 34. An electrically conducting composite material structure comprising alternating regions of flexible material and composite material. 35. A method of current limiting device, the current limiting device comprising at least two electrodes; an electrically conducting composite material structure between said electrodes; interfaces between said electrodes and electrically conducting composite material structure; an inhomogeneous distribution of resistance at said interfaces whereby, during a high current event, adiabatic resistive heating at said interfaces causes rapid thermal expansion and vaporization and at least a partial physical separation at said interfaces; and means for exerting compressive pressure on said electrically conducting composite material structure, wherein said electrically conducting composite material structure comprises alternating regions of at least one insulating flexible material and at least one composite material; the method comprising: manufacturing the electrically conducting composite material structure comprising an alternating regions of at least one insulafing flexible material and at least one composite material; the manufacturing an electrically conducting composite material structure, comprising: providing a relatively inflexible electrically conductive composite material; forming at least one depression in the relatively inflexible electrical conductive composite material; providing a uncured insulating flexible material; depositing the uncured insulafing flexible material in the at least one depression in the relatively inflexible electrically conductive composite material; and curing the insulafing flexible material providing the at least two electrodes; and providing the electrically conducting composite material structure between the at least two electrodes and placing the at least two electrodes and electrically conducting composite material structure under pressure from the exerting means.

Inventors: DUGGAL ANIL RAJ, LEE MINYOUNG, LEVINSON LIONEL MONTY, DUGGAL, ANIL, RAJ, LEE, MINYOUNG, LEVINSON, LIONEL, MONTY
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1998
Main classification: H01C7
IPC codes: H01C7, H01C17