Abstract:
A current control device is described wherein a pressure conduction composite is compressed and decompressed to alter its conductivity and thereby current conduction through the device. The pressure conduction composite is composed of a nonconductive matrix, a conductive filler, and an additive. The invention consists of electrodes and pressure plates contacting the composite. Electrically activated actuators apply a force onto pressures plates. Actuators are composed of a piezoelectric, piezoceramic, electrostrictive, magnetostrictive, and shape memory materials, capable of extending and/or contracting thereby altering pressure and consequently resistivity within the composite. In an alternate embodiment, two or more current control devices are electrically coupled parallel to increase power handling.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a divisional application of co-pending application Ser. No. 10/674,712, filed Sep. 29, 2003, which is a divisional application of co-pending application Ser. No. 10/072,587, filed Feb. 8, 2002, and claims the benefit of U.S. Provisional Application No. 60/267,306, filed on Feb. 8, 2001. The subject matters of the prior applications are incorporated in their entirety herein by reference thereto. 
     
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002] This invention was made with government support under Contract No. N00024-01-C-4034 awarded by the United States Navy. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0003]    1. Field of the Invention  
           [0004]    The present invention generally relates to a current control device for regulating current flow. The invention specifically described is a device wherein current flow is regulated by compression and expansion of a composite.  
           [0005]    2. Related Arts  
           [0006]    Mechanical circuit breakers are best described as a switch wherein a contact alters the electrical impedance between a source and a load. Mechanical breakers are typically composed of a snap-action bimetal-contact assembly, a mechanical latch/spring assembly, or an expansion wire. Such devices are neither gap-less nor shock resistant, therefore prone to chatter and subject to arcing. Chatter and arcing pose substantial problems in many high-voltage applications.  
           [0007]    Variably conductive composites are applicable to current control devices. Compositions include positive temperature coefficient resistive (PTCR), polymer current limiter (PCL), and piezoresistive formulations. PTCR and PCL applications and compositions and piezoresistive compositions are described in the related arts.  
           [0008]    Anthony, U.S. Pat. No. 6,157,528, describes and claims a polymer fuse composed of a PTCR composition exhibiting temperature-dependent resistivity wherein low resistivity results below and high resistivity results above a transition temperature.  
           [0009]    PTCR composites are composed of a conductive filler within a polymer matrix and an optional nonconductive filler. Chandler et al., U.S. Pat. No. 5,378,407, describes and claims a PTCR composite having a crystalline polymer matrix, a nickel conductive filler, and a dehydrated metal-oxide nonconductive filler. Sadhir et al., U. S. Pat. No. 5,968,419, describes and claims a PTCR composite having an amorphous polymer matrix, a thermoplastic nonconductive filler, and a conductive filler. During a fault, the composite heats thereby increasing volumetrically until there is sufficient separation between particles composing the conductive filler to interrupt current flow. Thereafter, the composite cools and shrinks restoring conduction. This self-restoring feature limits PTCR compositions to temporary interrupt devices.  
           [0010]    PCL composites, like PTCR compositions, are a mixture of a conductive filler and a polymer. However, PCL composites are conductive when compressed and interrupt current flow by polymer decomposition. For example, Duggal et al., U.S. Pat. No. 5,614,881, describes a composite having a pyrolytic-polymer matrix and an electrically conductive filler. During a fault, temperature within the composite increases causing limited decomposition and evolution of gaseous products. Current flow is interrupted when separation occurs between at least one electrode and conductive polymer. Gap dependent interrupt promotes arcing and arc related transients. Furthermore, static compression of the composites increases time-to-interrupt by damping gap formation. Neither PTCR nor PCL applications provide for the dynamically-tunable compression of a composite in response to electrical load conditions.  
           [0011]    Piezoresistive composites, also referred to as pressure conduction composites, exhibit pressure-sensitive resistivity rather than temperature or decomposition dependence. Harden et al., U.S. Pat. No. 4,028,276, describes piezoresistive composites composed of an electrically conductive filler within a polymer matrix with an optional additive. Conductive particles comprising the filler are dispersed and separated within the matrix, as shown in FIGS. 1A and 1C. Consequently, piezoresistive composites are inherently resistive becoming less resistive and more conductive when compressed. Compression reduces the distance between conductive particles thereby forming a conductive pathway, as shown in FIGS. 1B and 1D. The composite returns to its resistive state after compressive forces are removed. However, piezoresistive compositions resist compression.  
           [0012]    Pressure-based interrupt facilitates a more rapid regulation of current flow as compared to PTCR and PCL systems. Temperature dependent interrupt is slowed by the poor thermal conduction properties of the polymer matrix. Decomposition dependent interrupt is a two-step process requiring both gas evolution and physical separation between electrode and composite. Furthermore, decomposition limits the life cycle of a composition.  
           [0013]    Active materials, including but not limited to piezoelectric, piezoceramic, electrostrictive, and magnetostrictive, are ideally suited for the controlled compression of piezoresistive composites thereby achieving rapid and/or precise changes to resistivity. Active materials facilitate rapid movement by mechanically distorting or resonating when energized. High-bandwidth active materials are both sufficiently robust to exert a large mechanical force and sufficiently precise to controllably adjust force magnitude.  
           [0014]    As a result, an object of the present invention is to provide a current control device tunably and rapidly compressing a pressure-dependent conductive composite. A further object of the present invention is to provide a device that eliminates arcing thereby facilitating a complete current interrupt. It is an additional object of the present invention to provide a device that quenches transient spikes associated with shut off.  
         SUMMARY OF THE INVENTION  
         [0015]    The present invention is a current control device controlling current flow via the tunable compression of a polymer-based composite in response to electrical load conditions. The invention includes a pressure conduction composite compressed by at least one pressure plate. In several embodiments, the composite is compressed by a conductive pressure plate. In other embodiments, the composite is compressed by a nonconductive pressure plate and current flow occurs between two electrodes contacting the composite. The composite is variably-resistive and typically composed of a conductive filler, examples including metals, metal-nitrides, metal-carbides, metal-borides, metal-oxides, within a nonconductive matrix, examples including polymers and elastomers. Optional additives typically include oil, preferably silicone-based.  
           [0016]    A compression mechanism applies, varies, and removes a compressive force acting on the composite. Compression mechanisms include electrically driven devices comprised of actuators composed of an active material extending and/or contracting when energized. Active materials include piezoelectric, piezoceramic, electrostrictive, and magnetostrictive. Piezo-controlled pneumatic devices are also appropriate. Actuator movement adjusts the pressure state within the composite thereby altering resistivity within the confined composite.  
           [0017]    Several advantages are offered by the present invention. Compression-based control of a pressure-sensitive conduction composite provides a nearly infinite life cycle. A gap-less interrupt eliminates arcing and arc quenching requirements. The present invention lowers fault current thereby avoiding stress related chatter. Parallel arrangements of the present invention offer power handling equal to the sum of the individual units. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:  
         [0019]    [0019]FIG. 1 is a schematic diagram showing exemplary microstructures for composites before and after compression.  
         [0020]    [0020]FIG. 2 is a flowchart of composite manufacturing method.  
         [0021]    [0021]FIG. 3 is a side elevation view of a pressure switch with conductive pressure plates.  
         [0022]    [0022]FIG. 4 is a side elevation view of a pressure switch with nonconductive pressure plates.  
         [0023]    [0023]FIG. 5 is a side elevation view of a current controller comprised of four pressure switches wherein pressure plates are pushed by actuators.  
         [0024]    [0024]FIG. 6 is a side elevation view of a current controller comprised of four pressure switches wherein pressure plates are pulled by actuators.  
         [0025]    [0025]FIG. 7 shows a parallel arrangement of current controllers comprising a single unit.  
         [0026]    [0026]FIG. 8 is a top elevation view of pressure switch showing cylindrical pores oriented through electrodes.  
         [0027]    [0027]FIG. 9 is a section view of pressure switch showing cylindrical holes through switch thickness.  
         [0028]    [0028]FIG. 10 is a section view of pressure switch showing cylindrical holes within composite.  
         [0029]    [0029]FIG. 11 is a section view of pressure switch showing cylindrical holes filled with a temperature sensitive material.  
         [0030]    [0030]FIG. 12 is a side elevation view of temperature activated switch.  
         [0031]    [0031]FIG. 13 is a side elevation view of temperature activated switch. 
     
    
     REFERENCE NUMERALS  
       [0032]    [0032] 1  Current controller  
         [0033]    [0033] 2  Conductive filler  
         [0034]    [0034] 3  Nonconductive matrix  
         [0035]    [0035] 4  Composite  
         [0036]    [0036] 6  First electrode  
         [0037]    [0037] 7  Second electrode  
         [0038]    [0038] 11  Pressure switch  
         [0039]    [0039] 18  Pressure plate  
         [0040]    [0040] 19  Actuator  
         [0041]    [0041] 22  Force  
         [0042]    [0042] 30  Restoration element  
         [0043]    [0043] 31  Conductor  
         [0044]    [0044] 32  Insulator  
         [0045]    [0045] 33  Insulator  
         [0046]    [0046] 40  Hole  
         [0047]    [0047] 41  Temperature sensitive material  
         [0048]    [0048] 51  Temperature sensitive actuator  
         [0049]    [0049] 52  Wire  
         [0050]    [0050] 53  Wire  
         [0051]    [0051] 54  Nonconducting terminal  
         [0052]    [0052] 55  Rigid element  
         [0053]    [0053] 56  Thermal element  
       DESCRIPTION OF THE INVENTION  
       [0054]    Two embodiments of the present invention are comprised of a rectangular solid composite  4  contacting and sandwiched between two or more plates, namely a planar first electrode  6  and a planar second electrode  7 , as shown in FIG. 3, and a planar first electrode  6  and a planar second electrode  7  and two planar pressure plates  18   a ,  18   b , as shown in FIG. 4. A pressure switch  11  is comprised of a composite  4  and electrodes  6 ,  7  as shown in FIG. 3 or a composite  4  and pressure plates  18   a ,  18   b  as shown in FIG. 4.  
         [0055]    The composite  4  functionally completes the current path between first electrode  6  and second electrode  7  during acceptable operating conditions and interrupts current flow when a fault condition occurs. The composite  4  is either conductive or resistive based on the pressure state within the composite  4 . For example, the composite  4  may be conductive above and nonconductive below a threshold pressure. Alternately, the resistivity of the composite  4  may vary with pressure over a range of resistance values.  
         [0056]    A typical composite  4  is a pressure dependent conductive material, for example a piezoresistive formulation, comprised of a nonconductive matrix  3  and a conductive filler  2 , as schematically shown in FIG. 1. Preferred mixtures have a volume fraction below the percolation threshold wherein conductive filler  2  is randomly dispersed within the nonconductive matrix  3 . During compression, the nonconductive matrix  3  between conductive filler  2  particles is dimensional reduced thereby crossing the percolation threshold.  
         [0057]    The nonconductive matrix  3  is a resistive, yet compressible material including but not limited to polymers and elastomers. Specific examples include polyethylene, polystyrene, polyvinyldifluoride, polyimide, epoxy, polytetrafluorethylene, silicon rubber, polyvinylchloride, and combinations thereof Preferred embodiments are comprised of the elastomer RTV R3145 manufactured by the Dow Corning Company.  
         [0058]    The conductive filler  2  is an electrically conductive material including but not limited to metals, metal-based oxides, nitrides, carbides, and borides, and carbon black. Preferred fillers resist deformation under compressive loads and have a melt temperature sufficiently above the thermal conditions generated during current interrupt. Specific metal examples include aluminum, gold, silver, nickel, copper, platinum, tungsten, tantalum, iron, molybdenum, hafnium, combinations and alloys thereof. Other example fillers include Sr(Fe,Mo)O3, (La,Ca)MnO3, Ba(Pb,Bi)O3, vanadium oxide, antimony doped tin oxide, iron oxide, titanium diboride, titanium carbide, titanium nitride, tungsten carbide, and zirconium diboride.  
         [0059]    [0059]FIG. 2 describes a fabrication method for various composites  4 . Generally, composites  4  are prepared from high-purity feedstock, mixed, formed into a solid, and suffused with oil. One or more plates are adhered to the composite  4 .  
         [0060]    Feedstocks include both powders and liquids. Conductive filler  2  feedstock is typically composed of a fine, uniform powder, one example being 325 mesh titanium carbide. Nonconductive matrix  3  feedstock may include either a fine, uniform powder or a liquid with sufficiently low-viscosity to achieve adequate dispersion of powder. Powder-based formulations are mechanically mixed and compression molded using conventional methods. Polytetrafluorethylene formulations may require sintering within an oven to achieve a structurally durable solid. Powder-liquid formulations, one example being titanium carbide and a silicone-based elastomer, are vulcanized and hardened within a die under low uniaxial loading at room temperature.  
         [0061]    The solid composite  4  is placed within a liquid bath thereby allowing infiltration of the additive into the solid. Additives are typically inorganic oils, preferably silicone-based. The composite  4  is exposed to the additive bath to insure complete suffusion of the solid, whereby exposure time is determined by dimensions and composition of the composite  4 . For example, a 0.125-inch by 0.200-inch by 0.940-inch composite  4  composed of titanium carbide having a volume fraction of 66 percent and RTV R3145 having a volume fraction of 34 percent was suffused over a 48 hour period.  
         [0062]    Conductive or nonconductive plates are adhered to the composite  4  either before or after suffusion. If prior to suffusion, plates are placed within the die along with the liquid state composite  4 . For example, a silicone elastomer composite  4  is adequately bonded to two 0.020-inch thick brass plates by curing at room temperature typically between 3 to 24 hours or at an elevated temperature between 60 to 120 degrees Celcius for 2 to 10 hours. If after suffusion, silicone adhesive is applied between plate and composite  4  and thereafter mechanically pressed to allow for proper bond formation.  
         [0063]    A porous, nonconductive matrix  3  improves compression and cooling characteristics of the composite  4  without degrading electrical properties. A porous structure is formed by mechanical methods, one example including drilling, after fabrication of the solid composite  4 . Another method includes the introduction of pores during mixing of a powder-based conductive filler  2  with a liquid-based nonconductive matrix  3 . An additional method includes the introduction of pores during compression forming the composite  4 . Also, pores are formed by heating the composite  4  within an oven resulting in localized heating or phase transitions that result in void formation and growth. Furthermore, highly compressible microspheres composed of a low-density, high-temperature foam may be introduced during mixing. Pores are either randomly oriented or arranged in a repeating pattern. Pore shapes include but are not limited to spheres, cylinders, and various irregular shapes. A single pore may completely traverse the thickness of a composite  4 .  
         [0064]    [0064]FIGS. 8-9 show an embodiment wherein a plurality of holes  40  traverse the cross section of a pressure switch  11 . FIG. 10 shows an embodiment wherein holes traverse the composite  4  within the pressure switch  11 .  
         [0065]    [0065]FIG. 11 shows a further embodiment wherein holes  40  are filled with a temperature sensitive material  41 , examples including rods or springs composed of a shape memory alloy. Functionally, the temperature sensitive material  41  is typically a rubbery material below, see FIG. 11 a , and hard above, see FIG. 11 b , a phase transition temperature. More importantly, the temperature sensitive material  41  produces a large force above a transition temperature designed within the material as readily understood within the art. This force is sufficiently capable of moving the pressure plates  18  or electrodes  6 , 7  apart and interrupting current flow. The temperature sensitive material  41  is self restoring thereby facilitating current flow after the surrounding composite  4  has cooled.  
         [0066]    [0066]FIGS. 12-13 show two embodiments wherein at least two temperature sensitive actuators  51  apply a compressive force  22  onto a composite  4  thereby allowing current flow. In FIG. 12, current flows directly through the temperature sensitive actuators  51   a ,  51   b , preferably a shape memory alloy. When a fault occurs the temperature sensitive actuators  51   a ,  51   b  are heated and contract thereby decompressing the composite  4  and interrupting current. The composite  4  is compressed as the temperature sensitive actuator  51  cools. In FIG. 13, current flows through the first electrode  6  and the second electrode  7  when temperature sensitive actuators  51   a ,  51   b  are heated by thermal elements  56   a ,  56   b . Thermal elements  56   a ,  56   b  are deactivated when a fault condition occurs thereby decreasing the length of the temperature sensitive actuators  51   a ,  51   b  and reactivated after the fault condition is corrected thereby increasing the length of the temperature sensitive actuators  51   a ,  51   b  causing compression of the composite  4  and current flow.  
         [0067]    [0067]FIGS. 5-6 show additional embodiments of the present invention comprised of four pressure switches  11   a ,  11   b ,  11   c ,  11   d , a first electrode  6 , a second electrode  7 , two planar conductors  31   a ,  31   b , four insulators  32   a ,  32   b ,  33   a ,  33   b , a restoration element  30 , and a pair of actuators  19   a ,  19   b.    
         [0068]    Pressure switches  11   a ,  11   b ,  11   c ,  11   d  are composed of a pressure conduction composite  4  disposed between and adhered to two electrically conducting plates, as described above. A pair of pressure switches  11  are electrically aligned in a serial arrangement about a single electrode, either the first electrode  6  or the second electrode  7 . One electrically conducting plate from each pressure switch  11  directly contacts the electrode. Two such pressure switch  11  and electrode arrangements are thereafter aligned parallel and disposed between, perpendicular to and contacting a pair of conductors  31   a ,  31   b  so that each pressure switch  11  in a serial arrangement contacts a separate conductor  31 . Conductors  31  are composed of materials known within the art and should have sufficient strength to resist deformation when a mechanical load is applied. Thereafter, an insulator  32  is placed in contact with and attached or fixed to each conductor  31 . A typical insulator  32  is a planar element composed of an electrically nonconducting material with sufficient strength to resist deformation when a mechanical load is applied.  
         [0069]    At least one restoration element  30  is disposed between and parallel to the serial arrangement of pressure switches  11  and electrodes  6  or  7 . The restoration element  30  is attached to separate electrically nonconductive insulators  33   a ,  33   b . Thereafter, insulators  33   a ,  33   b  are mechanically attached to, perpendicularly disposed and between the conductors  31   a ,  31   b . Insulators  33   a ,  33   b  electrically isolate the restoration element  30  from conductors  31   a ,  31   b . The restoration element  30  decompresses the composite  4  within each pressure switch  11 , returning it to its original thickness, when the compressive mechanical load is removed from the insulators  32   a ,  32   b . A restoration element  30  may be a mechanical spring or coil, a pneumatic device, or any similar device that provides both extension and contraction.  
         [0070]    In preferred embodiments, an actuator  19  contacts an insulator  32 . In one embodiment, at least one actuator  19  is attached or fixed to each insulator  32  opposite of said conductor  31 , as shown in FIG. 5. A pair of actively opposed yet equal actuators  19   a ,  19   b  apply a mechanical load by pushing onto electrically nonconductive insulators  32   a ,  32   b  to compress the composite  4  within each pressure switch  11   a ,  11   b ,  11   c ,  11   d , as shown in FIG. 5 b . In another embodiment, at least two actuators  19   a ,  19   b  are mechanically attached or fixed to a pair of insulators  32   a ,  32   b , see FIG. 6. Again, a pair of actively opposed yet equal actuators  19   a ,  19   b  apply a mechanical load by pulling on electrically nonconductive insulators  32   a ,  32   b  to compress the composite  4  within each pressure switch  11   a ,  11   b ,  11   c ,  11   d , as shown in FIG. 6 b.    
         [0071]    Variations to the described embodiments also include at least two or more actively opposed actuators  19  mechanically compressing one or more current controllers  1 . FIG. 7 describes a three-by-three arrangement of nine current controllers  1 , however not limited to this arrangement. In such embodiments, current controllers  1  are electrically connected parallel thereby providing a total power handling capability equal to the sum of the power handling of individual units.  
         [0072]    One or more actuators  19  may be employed to drive two or more current controllers  1 . For example, a single actuator  19  or two actively opposed yet equal actuators  19  may apply a mechanically compressive load onto the current controllers  1  so that all are simultaneously compressed and decompressed. Alternatively, one or a pair of actuators  19  may apply a mechanically compressive load onto each individual current controller  1 . In this embodiment, it is possible to simultaneously drive all current controllers  1  or to selectively drive a number of units.  
         [0073]    The embodiments described above may also include a current measuring device electrically coupled before or after the current controller  1 . This device provides real-time sampling of current conditions which are thereafter communicated to the actuators  19 . Such monitoring devices are known within the art.  
         [0074]    An actuator  19  is a rigid beam-like element composed of an active material capable of dimensional variations when electrically activated. For example, the actuator  19  may extend, contract, or extend and contract, as schematically represented by arrows in FIGS. 5-6. Extension of the actuator  19  increases the overall length of the actuator  19 . Actuators  19  are composed of electrically activated devices including piezoelectric, piezoceramic, electrostrictive, and magnetostrictive materials. For example, piezoelectric and piezoceramic materials may be arranged in a planar stack along the actuator  19 . Alternatively, an actuator  19  may be a commercially available high-speed piezo-controlled pneumatic element comprised of a pneumatic diaphragm with pilot operated high-bypass value.  
         [0075]    The description above indicates that a great degree of flexibility is offered in terms of the present invention. Although embodiments have been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.