WO1996041355A1

WO1996041355A1 - Electrically non-linear composition and device

Info

Publication number: WO1996041355A1
Application number: PCT/US1996/009113
Authority: WO
Inventors: William H. Simendinger, Iii; Rudolf R. Bukovnik; Chi Suk Yom
Original assignee: Raychem Corporation
Priority date: 1995-06-07
Filing date: 1996-06-06
Publication date: 1996-12-19
Also published as: TW302486B; AR002199A1

Abstract

An electrically non-linear composition in which a curable polymeric component (9) contains a first particulate filler (13) which is magnetic and electrically conductive, a second particulate filler (15) which is magnetic and electrically non-conductive, and an optional third particulate filler which is electrically non-conductive and non-magnetic. The first and second fillers are aligned in discrete regions (11) through the polymeric component. When the composition is used in an electrical device (1), the discrete regions extend from a first electrode (3) to a second electrode (5).

Description

ELECTRICALLY NON-LINEAR COMPOSITION AND DEVICE

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to electrically non-linear compositions and to devices comprising such compositions.

Introduction to the Invention

Electrically non-linear compositions are commonly used to protect electrical equipment and circuitry. Such compositions often exhibit non-linear electrical resistivity, decreasing in resistance when exposed to a voltage that exceeds a threshold value. This value is known as the breakdown voltage. Compositions exhibiting non-linear electrical behavior are disclosed in U.S. Patent Nos. 4,977,357 (Shrier) and 5,294,374 (Martinez et al), in International Application No. PCT/US95/06867 (Simendinger et al, filed May 30, 1995), and in U.S. Patent Application No. 08/046,059 (Debbaut et al, filed April 10, 1993). It is common to use such compositions in devices which provide secondary or backup protection for other protection devices, i.e. primary protection devices. Use in a secondary role is required because conventional compositions generally are not able to accommodate the energy levels required to act as primary protection in a way that devices such gas discharge tubes do. Furthermore, such compositions often exhibit a decrease in breakdown voltage on successive impulses, making them unstable for repeated use.

SUMMARY OF THE INVENTION

We have now discovered that an electrically non-linear composition with high energy-carrying capability and improved stability during breakdown can be prepared by selecting a combination of particulate fillers, dispersing the fillers in a polymeric component, and then aligning the fillers in discrete regions throughout the polymeric component. In a first aspect, this invention provides an electrically non-linear composition which comprises

(1) a polymeric component, (2) a first paniculate filler which is magnetic and electrically conductive, and

(3) a second particulate filler which is magnetic and has a resistivity of at least 1 x 10⁴ ohm-cm,

said first and second fillers being aligned in discrete regions in the polymeric component.

Compositions of the first aspect of the invention can be used to prepare electrical devices which themselves act to protect electrical components, e.g. act as a primary protection device in a telecommunications circuit rather than a backup protection device, and thus replace crowbar devices such as gas discharge tubes and thyristors. Thus, in a second aspect, this invention provides an electrical device which comprises

(A) a resistive element which comprises a composition of the first aspect of the invention;

(B) a first electrode which has a first resistivity and is electrically connected to the resistive element; and

(C) a second electrode which has a second resistivity and is electrically connected to the resistive element so that current can flow through the element and between the electrodes;

said first and second fillers being aligned in discrete regions extending through the resistive element from the first electrode to the second electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated by the drawings in which Figure 1 is a schematic cross-sectional view of an electrical device of the invention;

Figure 2 is a schematic cross-sectional view of another electrical device of the invention;

Figure 3 is a schematic cross-sectional view of a test fixture used to test a device of the invention; and Figures 4, 5, 6a, and 6b are graphs of breakdown voltage as a function of test number for devices of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The composition of the invention exhibits electrically non-linear behavior. In this specification the term "non-linear" means that the composition is substantially electrically

6 non-conductive, i.e. has a resistivity of more than 10 ohm-cm, and preferably more than 10⁸ ohm-cm, when an applied voltage is less than the impulse breakdown voltage, but then becomes electrically conductive, i.e. has a resistivity of substantially less than 10 ohm-cm, when the applied voltage is equal to or greater than the impulse breakdown voltage. For many applications, it is preferred that the composition have a resistivity in the "non-conducting" state of more than 10 ohm-cm, e.g. 10 ohm-cm, and a resistivity in the "conducting" state of less than 10 ohm-cm.

The electrically non-linear composition comprises a polymeric component which acts as a matrix to contain the first, second, and optional third paniculate fillers. The polymeric component may be any appropriate polymer, e.g. a thermoplastic material such as a polyolefin, a fluoropolymer, a polyamide, a polycarbonate, or a polyester; a thermosetting material such as an epoxy; an elastomer (including silicone elastomers, acrylates, polyurethanes, polyesters, and liquid ethylene/propylene/diene monomers); a grease; or a gel. It is preferred that the polymeric component be a curable material, i.e. one that undergoes a physical and/or chemical change on exposure to an appropriate curing condition, e.g. heat, light, radiation, microwave, or a chemical component. The polymeric component is generally present in an amount of 30 to 99.8%, preferably 35 to 95%, particularly 40 to 90% by volume of the total composition.

For many applications it is preferred that the polymeric component comprise a polymeric gel, i.e. a substantially dilute crosslinked solution which exhibits no flow when in the steady-state. The crosslinks, which provide a continuous network structure, may be the result of physical or chemical bonds, crystallites or other junctions, and must remain intact under the use conditions of the gel. Most gels comprise a fluid-extended polymer in which a fluid, e.g. an oil, fills the interstices of the network. Suitable gels include those comprising silicone, e.g. a polyorganosiloxane system, polyurethane, polyurea, styrene- butadiene copolymers, styrene-isoprene copolymers, styrene-(ethylene/propylene)-styrene (SEPS) block copolymers (available under the tradename Septon™ by Kuraray), styrene- (ethylene-ρropylene/ethylene-butylene)-styrene block copolymers (available under the tradename Septon™ by Kuraray), and/or styrene-(ethylene/butylene)-styrene (SEBS) block copolymers (available under the tradename Kraton™ by Shell Oil Co.). Suitable extender fluids include mineral oil, vegetable oil, paraffinic oil, silicone oil, plasticizer such as trimellitate, or a mixture of these, generally in an amount of 30 to 90% by volume of the total weight of the gel without filler. The gel may be a thermosetting gel, e.g. silicone gel, in which the crosslinks are formed through the use of multifunctional crosslinking agents, or a thermoplastic gel, in which microphase separation of domains serves as junction points. Disclosures of gels which may be suitable as the polymeric component in the composition are found in U.S. Patent Nos.4,600,261 (Debbaut),

4,690,831 (Uken et al), 4,716,183 (Gamarra et al), 4,777,063 (Dubrow et al), 4,864,725 (Debbaut et al), 4,865,905 (Uken et al), 5,079,300 (Dubrow et al), 5,104,930 (Rinde et al), and 5,149,736 (Gamarra); and in International Patent Publication Nos. WO86/01634 (Toy et al), WO88/00603 (Francis et al), WO90/05166 (Sutherland), WO91/05014 (Sutherland), and WO93/23472 (Hammond et al).

In order to accommodate the necessary loading of the particulate fillers, and to allow alignment of the fillers in the polymeric component, it is preferred that the polymeric component, prior to any curing, have a viscosity at room temperature of at most 200,000 cps, preferably at most 100,000 cps, particularly at most 10,000 cps, especially at most 5,000 cps, more especially at most 1,000 cps. This viscosity is generally measured by means of a Brookfield viscometer at the cure temperature, T_c, if the polymeric component is curable, or at the mixing temperature at which the particulate fillers are dispersed and subsequently aligned if the polymeric component is not curable.

At least two different types of particulate fillers are present in the polymeric component. The first particulate filler is both magnetic and electrically conductive. In this specification the term "electrically conductive" is used to mean a filler which is

2 conductive or semiconductive and which has a resistivity of less than 1 x 10 ohm-cm and is preferably much lower, i.e. less than 1 ohm-cm, particularly less than 1 x 10^"1 ohm-cm, especially less than 1 x 10^"3 ohm-cm. In this specification, the term "magnetic" is used to include ferromagnetic, ferrimagnetic, and paramagnetic materials. The filler may be completely magnetic, e.g. a nickel sphere, it may comprise a non-magnetic core with a magnetic coating, e.g. a nickel-coated ceramic particle, or it may comprise a magnetic core with a non-magnetic coating, e.g. a silver-coated nickel particle. Suitable first fillers include nickel, iron, cobalt, ferric oxide, silver-coated nickel, silver-coated ferric oxide, or alloys of these materials. If the polymeric component is a gel, it is important that the selected filler not interfere with the crosslinking of the gel, i.e. not "poison" it. The first filler is generally present in an amount of 0.1 to 30%, preferably 1 to 25%, particularly 2 to 20% by volume of the total composition.

The second particulate filler is magnetic and has a resistivity in the magnetic direction of at least 1 x 10 ohm-cm. For many applications, the second filler is non¬ conductive, i.e. has a resistivity of more than 1 x 10 ohm-cm. Suitable second fillers include garnets, i.e. materials with a formula M₃Fe₅O₁₂, where M is Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu; and ferrites, i.e. materials with a formula MFe₂O₄, where M is Zn, Mn, Fe, Co, Ni, Cu, and Mg. Particularly preferred are second fillers that comprise a hexagonal ferrite material, i.e. materials having a formula MFe₁₂O₁₉, where M is Ba, Pb, and Sr; a formula of M₂BaFe₁₆O₂₇, where M is Mg, Mn, Fe, Co, Ni, Cu, Zn, NiFe, ZnFe, and MnZn; or a formula M₂Ba₂Fe₁₂O₂₂, where M is Mg, Mn, Fe, CO₂, Ni, Cu, and Zn; as well as other ferrites. Of particular interest is BaFeι₂O₁₉. Such hexagonal ferrites are electrically nonconductive in the magnetic direction, although they can have some electrical conductivity in the basal plane. The second filler is generally present in an amount of 0.1 to 30%, preferably 1 to 25%, particularly 2 to 20% by volume of the total composition.

The composition may also comprise one or more optional third particulate fillers which are electrically non-conductive, i.e. have a resistivity of more than 1 x 10 ohm-cm, and non-magnetic. Suitable third fillers include silica, alumina, alumina trihydrate, magnesium hydroxide, zinc borate, antimony trioxide, halogenated compounds such as decabromodiphenyl oxide, and phosphorus-containing compounds. The third filler is present in an amount of 0 to 60%, preferably 5 to 50%, particularly 10 to 40% by volume of the total composition. Compositions with particularly good performance under high current conditions, e.g. at 250A, have been achieved when the third component comprises an arc suppressing agent or flame retardant, e.g. zinc borate, and an oxidizing agent, e.g. magnesium perchlorate or potassium permanganate. It is preferred that the oxidizing agent be present in an amount 0.1 to 1.0 times that of the arc suppressing agent or flame retardant. Particularly good results are achieved when the oxidizing agent is coated onto the arc suppressing agent or flame retardant prior to mixing. While we do not wish to be bound by any theory, it is believed that the presence of the zinc borate and the oxidizing agent controls the plasma chemistry of the plasma generated during an electrical discharge, and provides discharge products that are nonconductive. The volume loading, shape, and size of the fillers affect the non-linear electrical properties and the breakdown voltage of the composition, in part because of the spacing between the particles. Any shape particle may be used, e.g. spherical, flake, fiber, or rod. Useful compositions can be prepared with a first filler having particles with an average size of 0.1 to 300 microns, preferably 0.5 to 200 microns, particularly 1 to 200 microns, especially 1 to 100 microns. It is preferred that the second filler have an average particle size of 0.1 to 100 microns, preferably 0.1 to 50 microns. It is especially preferred that the second filler have a smaller, and often substantially smaller, e.g. 10 times smaller, average particle size than the first filler so as to give good packing during alignment. For most applications, it is preferred that there be as large a number of aligned columns per unit volume as possible to increase and/or maintain voltage breakdown stability, so a relatively small particle size filler for both the first and second fillers is preferred. A mixture of different size, shape, and/or type particles may be used for the first, second, and third fillers.

In addition to the particulate fillers, the composition may comprise other conventional additives, including antioxidants, dispersing agents, coupling agents, stabilizers, pigments, crosslinking agents, and inhibitors. These components generally comprise at most 10% by volume of the total composition and must be selected so as not to interfere with any required curing of the polymeric component.

The compositions of the invention may be prepared by any suitable means, e.g. melt-blending, solvent-blending, or intensive mixing. Because it is preferred that the polymeric component have a relatively low viscosity, particularly prior to curing, the fillers can be mixed into the polymeric component by hand or by the use of a mechanical stirrer. Mixing is conducted until a uniform dispersion of the filler particles is achieved. The composition may be shaped by conventional methods including extrusion, calendaring, casting, and compression molding. If the polymeric component is a gel, the gel may be mixed with the fillers by stirring and the composition may be poured or cast onto a substrate or into a mold to be cured.

In order to achieve good electrical properties, it is necessary to align the first and second components in discrete regions in the polymeric component, e.g. as a column that extends through the polymeric component from one side to the other, or, when electrodes are present, as a column that extends through the polymeric component from the first electrode to the second electrode to form a resistive element. Such domains can be formed in the presence of a magnetic field that causes the magnetic first and second filler particles to align. When such alignment occurs during curing of the resin, the alignment is maintained in the cured resin. Any type of magnetic field that is capable of supplying a field strength sufficient to align the particles may be used. We have found that for uncured resins having a viscosity of less than 10,000 cps, magnetic field strengths of between 80 and 1200 gauss are strong enough. A conventional magnet of any type, e.g. ceramic or rare earth, may be used, although for ease in manufacture, it may be preferred to use an electromagnet with suitably formed coils to generate the desired magnetic field. It is often preferred that the uncured resin be positioned between two magnets during the curing process, although for some applications, e.g. a particular device geometry, or the need to cure by means of ultraviolet light, it can be sufficient that there be only one magnet that is positioned on one side of the resin. The resin is generally separated from direct contact with the magnets by means of an electrically insulating spacing layer, e.g. a polycarbonate, polytetrafluoroethylene, or silicone sheet, or by means of first and second electrodes.

The compositions of the invention have excellent stability as measured both by resistivity and breakdown voltage. The compositions are electrically insulating and have

9 10 an initial resistivity Pj at 25°C of at least 10 ohm-cm, preferably 10 ohm-cm,

11 12 particularly 10 ohm-cm, especially 10 ohm-cm. The initial resistivity value pj is such that when the composition is formed into a standard device as described below, the initial

9 10 insulation resistance Ri is at least 10 ohms, preferably at least 10 ohms, particularly at

11 9 least 10 ohms. An Rf value of at least 10 ohms is preferred when the compositions of the invention are used in telecommunications apparatus. After being exposed to the standard impulse breakdown test, described below, the final resistivity p_f at 25°C is at

9 3 2 least 10 ohm-cm, and the ratio of pj to p_f is at most 1 x 10 , preferably at most 5 x 10 ,

2 1 1 particularly at most 1 x 10 , especially at most 5 x 10 , most especially at most 1 x 10 . The final insulation resistance Rf for a standard device after exposure to the standard

8 9 impulse breakdown test is at least 10 ohms, preferably at least 10 ohms, particularly at

10 least 10 ohms.

The compositions of the invention may be used to prepare an electrical device which comprises a resistive element which is in physical and electrical contact with at least one, and preferably two, electrodes, i.e. first and second electrodes. Each of the electrodes is electrically connected to the resistive element so that when the device is connected to a source of electrical power, current can flow through the element. The type of electrode is dependent on the shape of the element, but is preferably laminar and in the form of a metal foil, metal mesh, or metallic ink layer. The first electrode has a first resistivity and the second electrode has a second resistivity, both of which are generally less than 1 x 10^"2 ohm-cm, preferably less than 1 x 10^"3 ohm-cm, particularly less than 1 x 10^"4 ohm-cm. Particularly suitable metal foil electrodes comprise microrough surfaces, e.g. electrodeposited layers of nickel or copper, and are disclosed in U.S. Patents Nos. 4,689,475 (Matthiesen), and 4,800,253 (Kleiner et al), and in International Application No. PCT/US94/07888 (Chandler et al, filed June 7, 1995.

Depending on the type of the resin and the electrode, it may be desirable to cure the resin directly in contact with the electrodes. Alternatively, it is possible to cure the resin partially or completely before attaching the electrodes to the cured resin. The latter technique is especially appropriate for use with mesh or other foraminous electrode materials. In order to control the thickness of the resistive element, the uncured resin may be poured or otherwise positioned within a mold of specified thickness, and then cured. Particularly good electrical stability for devices of the invention may be achieved if at least one and preferably both of the electrodes is both electrically conductive and has at least some portion which is magnetic. Electrodes of this type include nickel, nickel- coated copper, and stainless steel. It is preferred that the entire surface of the electrode comprise the magnetic material. Similar electrodes and techniques may be used to prepare electrical devices as described in U.S. Patent Application No. 08/482,064 (Munch et al, filed June 7, 1995).

The polymeric component may be cured by any suitable means, including heat, light, microwave, electron beam, or gamma irradiation, and is often cured by using a combination of time and temperature suitable to substantially cure the resin. The curing temperature T_c may be at any temperature that allows substantial curing of the resin, i.e. that cures the resin to at least 70%, preferably at least 80%, particularly at least 90% of complete cure. When the curable polymeric component is a thermosetting resin which has a glass transition temperature Tg, it is preferred that the curing be conducted at a curing temperature T_c which is greater than Tg. A catalyst, e.g. a platinum catalyst, may be added to initiate the cure and control the rate and/or uniformity of the cure. When the polymeric component is a gel, it is preferred that, when cured without any filler, the gel be relatively hard, i.e. have a Voland hardness of at least 100 grams, particularly at least 200 grams, especially at least 300 grams, e.g. 400 to 600 grams, in order to minimize disruption of the aligned particles when exposed to a high energy condition. In addition, it is preferred that the cured gel have stress relaxation of less than 25%, particularly less than 20%, especially less than 15%. The Voland hardness and stress relaxation are measured using a Voland-Stevens Texture Analyzer Model LFRA having a 1000 gram load cell, a 5 gram trigger, and a 0.25 inch (6.35 mm) ball probe, as described in U.S. Patent No. 5,079,300 (Dubrow et al). To measure the hardness of a gel, a 20 ml glass scintillating vial containing 10 grams of gel is placed in the analyzer and the stainless steel ball probe is forced into the gel at a speed of 0.20 mm/second to a penetration distance of 4.0 mm. The Voland hardness value is the force in grams required to force the ball probe at that speed to penetrate or deform the surface of the gel the specified 4.0 mm. The Voland hardness of a particular gel may be directly correlated to the ASTM D217 cone penetration hardness using the procedure described in U.S. Patent No. 4,852,646 (Dittmer et al).

Electrical devices of the invention, when tested according to the Standard Impulse Breakdown Voltage Test, described below, preferably exhibit low breakdown voltage and maintain a high insulation resistance. Thus the breakdown voltage when tested at either 60A or 250 A is at most 1500 volts, preferably at most 1200 volts, particularly at most 1000 volts, especially at most 800 volts, more especially at most 600 volts, e.g. 200 to o

400 volts, and the final insulation resistance is at least 10 ohms, as described above. It is preferred that the breakdown voltage be relatively stable over multiple cycles of the test, i.e. for any given cycle, the breakdown voltage vary from the average breakdown voltage for fifty cycles by ±70%, preferably by ±50%. When the composition of the invention is formed into a standard device as described below and exposed to a standard impulse breakdown test, the device has an initial breakdown voltage Vsi and a final breakdown voltage Vsf which is from 0.70Vsi to 1.30Vsi, preferably from 0.80Vsi to 1.20Vsi, particularly from 0.85Vsi to 1.15Vsi, especially from 0.90Vsi to l.lOVsi-

Improved electrical stability has been achieved when a conductive intermediate layer, e.g. a conductive adhesive or conductive polymer layer, is positioned between the resistive element and one or both of the electrodes. The intermediate layer may be a tie layer that enhances the bond between the electrodes and the resin. Alternatively, the intermediate layer may comprise a conductive polymer layer that has a third resistivity that is higher, e.g. at least 10 times higher, than that of the of the first and second resistivities of the first and second electrodes, respectively, but has a third resistivity that is substantially lower, i.e. at least 1000 times lower, than that of the resistive element. Particularly suitable for use as an intermediate layer is a conductive polymer composition prepared by sintering ultrahigh molecular polyethylene with carbon black or another conductive filler, as described in U.S. Patent Nos. 4,853,165 (Rosenzweig et al) and 5,286,952 (McMills et al). The invention is illustrated by the drawing in which Figure 1 shows in cross- section electrical device 1. First and second electrodes 3,5 sandwich resistive element 7 which is made of polymeric component 9 through which is dispersed in discrete domains aligned chains 11. Each chain contains particles 13 of first filler and particles 15 of second filler.

Figure 2 is a similar electrical device which contains intermediate layer 17 made from a conductive polymer.

The invention is illustrated by the following examples, each of which was tested using the Standard Impulse Breakdown Test.

Standard Device

A composition was prepared by mixing the components with a tongue depressor or mechanical stirrer to wet and disperse the particulate filler. The composition was degassed in a vacuum oven for one minute, poured onto a PTFE-coated release sheet, and covered with a second PTFE-coated release sheet separated from the first sheet by spacers having a thickness of about 1 mm. The outer surfaces of the release sheets were supported with rigid metal sheets and magnets with dimensions of 51 x 51 x 25 mm (2 x 2 x 1 inch) and having a pull force of 4.5 kg (10 pounds) (available from McMaster-Carr) were positioned over the metal sheets. The composition was then cured at 100°C for 15 minutes. A disc 21 (as shown in Figure 3) with a diameter of 19.1 mm (0.75 inch) and a thickness of 1 mm (0.039 inch) was cut from the cured composition and molybdenum electrodes 23, 25 having a diameter of 19.1 mm (0.75 inch) and a thickness of 0.10 mm (0.004 inch) were attached to the surface of the disc to form a standard device 27.

Standard Impulse Breakdown Test

A standard device 27 was inserted into the test fixture 29 shown in Figure 3 and maintained in a standard position between top arm 31 and base 33 by means of tightening set screw 35. Brass plates 37,39 served to make electrical contact to device 27. Electrical leads 41,43 were connected from brass plates 37,39 to the testing equipment (not shown). Prior to testing, the insulation resistance Ri for the device was measured at 25°C with a biasing voltage of 50 volts using a Genrad 1864 Megaohm meter; the initial resistivity pj was calculated. Electrical connection was then made to a Keytek ECAT Series 100 Surge Generator using a E514A 10x1000 waveform generator. For each cycle a high energy impulse with a 10 x 1000 μs current waveform (i.e. a rise time to maximum current of 10 μs and a half-height at 1000 μs) and a peak current of 60A or 250A was applied. (For examples tested at 50A or 300A, a Keytek Model 424T Surge Generator was used.) The peak voltage measured across the device at breakdown, i.e. the voltage at which current begins to flow through the gel, was recorded as the impulse breakdown voltage. The final insulation resistance Rf after fifty cycles for the standard test was measured and the final resistivity pf was calculated.

Example 1

A composition was prepared by mixing 5% by volume Fe₃O₄ (available from Aldrich Chemical Company, with a particle size of 5 microns), 2% by volume BaFeι₂O₁₉ (available from Aldrich, with a mesh size of -325 and a particle size of less than 44 microns), 35% by volume SiO₂ (SI 53-3, available from Fisher, with a mesh size of about 240 and a particle size of about 60 microns), and 58% by volume silicone gel (formulated using 74.85% by weight 50 cs silicone oil, 24.95% by weight 1000 cs divinyl- polydimethylsiloxane, and 0.2% by weight tetrakis(dimethyl siloxy silane)), all volumes by total volume of the composition, with a tongue depressor to wet and disperse the particulate filler. When tested at 300A, the device maintained an insulation resistance of 1 x 10 ' ° ohms after 100 cycles.

Example 2

Following the procedure of Example 1, a device was prepared from a composition containing 6% by volume nickel (particle size of about 44 microns (-325 mesh)), 6% by volume BaFe₁₂O₁₉, 30% by volume SiO₂, and 58% by volume silicone gel. The device had similar results to Example 1.

Examples 3 to 15.

The ingredients shown in Table I were mixed and formed into devices having a thickness of either 0.7 mm or 1.1 mm as in Example 1. Devices were then tested using the Standard Impulse Breakdown Test at 50A for 15 cycles. The results showed that the breakdown voltage was increased with increasing concentration of inert filler or BaFej₂O₁₉, indicating that particle spacing, governed by the presence of the nonconductive fillers, is important in establishing the breakdown voltage. Ex. Volume % Breakdown V Ri Rf

Ni SiO₂ BaFe₁₂O_]9 Gel 0.7mm 1.1 mm (ohm) (ohm)

3 7.5 5 10.0 77.5 418 1098 1.0E9 1.5E7

4 7.5 10 7.5 75.0 486 1007 1.0E8 1.5E8

5 7.5 10 12.5 70.0 529 789 1.0E9 2.0E9

6 7.5 15 10.0 67.5 754 987 2.0E9 2.0E6

7 10.0 5 12.5 72.5 398 610 6.0E8 6.0E5

8 10.0 5 7.5 77.5 399 710 2.0E8 5.0E7

9 10.0 15 7.5 67.5 414 810 8.0E9 8.0E9

10 10.0 10 10.0 70.0 450 640 5.0E8 1.0E9

11 10.0 15 12.5 62.5 572 883 3.5E9 1.5E5

12 12.5 15 10.0 62.5 441 740 4.0E7 1.5E5

13 12.5 5 10.0 72.5 450 663 3.5E8 4.0E6

14 12.5 10 7.5 70.0 507 607 3.0E8 2.0E6

15 12.5 10 12.5 65.0 646 902 3.0E8 1.5E8

Example 16

Following the procedure of Example 1, 5% by volume nickel (available from Alfa Aesar, with a mesh size of -250 mesh and a particle size of less than 53 to 63 microns), 5% by volume BaFe₁ O₁₉, 10% by volume magnesium perchlorate (available from Alfa Aesar), 20% by volume zinc borate (available from Alfa Aesar), and 60% by volume silicone gel (formulated using 50% by weight 50 cs silicone oil, 50% by weight 10,000 cs divinyl-polydimethylsiloxane, and 0.2% by weight tetrakis(dimethyl siloxy silane)) were mixed. Prior to mixing, the magnesium perchlorate was dissolved in water, the zinc borate was added to the solution, and the water was then evaporated to give magnesium perchlorate-coated zinc borate. The composition was formed into devices which were tested using the Standard Impulse Breakdown Test at 60A for fifty cycles. The R_t value was 1 x 10 ohms; the R value was 1 x 10 ohms. The breakdown voltage and peak current are shown in Figure 4.

Example 17

Devices were made using a composition similar to that in Example 16 except that -100 to +200 mesh nickel (available from Alfa Aesar, with an average particle size of about 100 microns) was used. The Standard Impulse Breakdown Test was done at 250A for 50 cycles. The R_j value was 1 x 10⁹ ohms; the R_f value was 1 x 10⁹ ohms. The breakdown voltage and peak current are shown in Figure 5.

Example 18

A device similar to that shown in Figure 2 was prepared using a composition made from 9% by volume nickel (-100 to +200 mesh), 6% by volume BaFe₁₂Oι₉, 30% by volume zinc borate, and 55% by volume silicone gel (formulated using 50% by weight 50 cs silicone oil, 50% by weight 1000 cs divinyl-polydimethylsiloxane, and 0.2% by weight tetrakis(dimethyl siloxy silane)) to form the resistive element. Intermediate layers were formed from a conductive polymer composition prepared by drying blending 95% by volume ultrahigh molecular weight polyethylene having a molecular weight of about 4.0 million (Hostalen GUR-413, available from Hoechst) with 5% by volume carbon black (Ketjenblack EC 300, available from Akzo Chemie). The mixture was extruded through a ram extruder to produce a sintered rod and the rod was skived to produce a flexible tape 0.030 inch (0.76 mm) thick and 4.0 inch (102 mm) wide having a resistivity of about 2.5 ohm-cm. Two pieces of the tape were cut to the dimensions of the resistive element and were placed directly in contact with the element to sandwich it. Figures 6a and 6b show the breakdown voltage for the Standard Impulse Breakdown Test at 60A and 250A, respectively, for 50 cycles. The device had substantially less scatter in breakdown voltage than devices shown in Figures 4 and 5.

Claims

What is claimed is:

1. An electrically non-linear composition which comprises

( 1 ) a polymeric component,

(2) a first particulate filler which is magnetic and electrically conductive, and

(3) a second particulate filler which is magnetic and has a resistivity of at least 1 10 ohm-cm,

2. A composition according to claim 1 which further comprises a third particulate filer which is electrically non-conductive and non-magnetic, and preferably comprises silica, alumina, alumina trihydrate, magnesium hydroxide, or zinc borate.

3. A composition according to claim 1 or claim 2 wherein the polymeric component comprises a curable polymer, preferably a gel which is a thermosetting gel or a thermoplastic gel.

4. A composition according to claim 3 wherein the curable polymeric component comprises a thermosetting resin, preferably a silicone elastomer, an acrylate, an epoxy, or a polyurethane.

5. A composition according to any one of the preceding claims wherein the first filler comprises nickel, iron, cobalt, ferric oxide, silver-coated nickel, silver-coated ferric oxide, or alloys of these materials.

6. A composition according to claim 1 wherein the second filler has a hexagonal crystalline structure, and preferably comprises barium iron oxide.

7. A composition according to claim 2 which further comprises an oxidizing agent, preferably potassium permanganate or magnesium perchlorate.

8. An electrical device which comprises

(A) a resistive element which comprises the composition of claim 1 ; (B) a first electrode which has a first resistivity and is electrically connected to the resistive element; and

9. A device according to claim 8 wherein at least one of the first and second electrodes comprises a region composed of a material which is magnetic and electrically conductive, and preferably the region comprises nickel or stainless steel.

10. A device according to claim 8 which further comprises at least one intermediate layer which

(1) has a third resistivity higher than the first and the second resistivities, and

(2) is positioned between the resistive element and an electrode,

and preferably wherein the intermediate layer comprises a conductive polymer composition.

11. A device according to claim 8 which has a breakdown voltage when measured at 60A in a Standard Impulse Breakdown Test of at most 1200 volts.