CA2239990A1 - Electrical device - Google Patents
Electrical device Download PDFInfo
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- CA2239990A1 CA2239990A1 CA002239990A CA2239990A CA2239990A1 CA 2239990 A1 CA2239990 A1 CA 2239990A1 CA 002239990 A CA002239990 A CA 002239990A CA 2239990 A CA2239990 A CA 2239990A CA 2239990 A1 CA2239990 A1 CA 2239990A1
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- Prior art keywords
- filler
- resistive element
- composition
- polymeric component
- gel
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Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01C—RESISTORS
- H01C7/00—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material
- H01C7/18—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material comprising a plurality of layers stacked between terminals
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01C—RESISTORS
- H01C7/00—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material
- H01C7/02—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material having positive temperature coefficient
- H01C7/027—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material having positive temperature coefficient consisting of conducting or semi-conducting material dispersed in a non-conductive organic material
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01C—RESISTORS
- H01C7/00—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material
- H01C7/10—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material voltage responsive, i.e. varistors
- H01C7/12—Overvoltage protection resistors
Abstract
An electrical device (1) in which a first resistive element (7) which is composed of a first electrically non-linear composition is in electrical contact, and preferably in physical and electrical contact, with a second resistive element (13) which is composed of a second composition which has a resistivity of less than 100 ohm-cm. The first composition has a resistivity of more than 109 ohm-cm and contains a first particulate filler (11). The second composition contains a second particulate filler (19) which (a) is magnetic and electrically conductive, and (b) is aligned in discrete regions (17) in the second polymeric component (15). The device also contains first and second electrodes (3, 5) which are positioned so that current can flow between the electrodes through the first and second resistive elements. Devices of the invention have relatively low breakdown voltages and can survive high energy fault conditions.
Description
CA 02239990 1998-06-0~
W O 97/21230 PCT~US96119319 ~,~,F,CTRTCA~, nli',VICF, R~CKGROUND OF THE INVF~TION
Field of the Tnvention This invention relates to electrical devices comprising electrically non-linear compositions.
Tntroduction to the Invention Devices comprising electrically non-linear compositions are known for protectingelectrical eql-irment and cil~;ui~ly. The compositions used in such devices often exhibit non-linear electrical resistivitv, decreasing in resistivity from an in~ tin~ state, i.e. more than 106 ohm-cm, to a conc1~1ctin~ state when exposed to a voltage that exceeds a threshold value. This value is l~nown as the breakdown voltage. Compositions exhibiting non-linear electrical behavior are disclosed in U.S. Patents Nos. 4,977,357 (Shrier), 5,294,374 (Martinez et al), and 5,557,250 (Debbaut et al), in Tnt~rn~tional Publication No.
W095/33278 (Raychem Cul~oldlion), and in Tnfern~tional Application No.
PCT/US96/09113 (Raychem Corporation).
Electrical devices prepared from these conventional compositions have been described. See, for example, Tntçrn~tional Publication No. W095/33278 which discloses an electrically non-linear resistive element suitable for repeated use as the secondary protection in a telecommunications gas tube d~palaLus. That resistive element compri~çc a composition in which a particulate filler such as alllminllm is dispersed in a polymeric matrix . The composition has an initial resistivity Pi at 25~C of at least 109 ohm-cm and, even after exposure to a standard impulse breakdown test in which a high energy impulse is applied across the element five times, has a final resistivity pf at 25~C of at least 109 ohm-cm. However, such devices, when exposed to a high energy fault condition, will short out and are thus not reusable. Furthermore, the scatter in the breakdown voltage on successive test events is relatively broad.
Tntf~ ti~nal Application No. PCT/US96/09113 discloses a device which is designed to protect electrical components as a primary protection device rather than as a secondary protection device. In this device, a resistive element is positioned between two CA 02239990 1998-06-0~
electrodes and is composed of a polymeric component in which a first magnetic, electrically conductive particulate filler and a second magnetic particulate filler with a resistivity of at least 1 x 104 ohm-cm are aligned in discrete regions exten~linp from the first to the second electrode. In order to increase the electrical stability of the device, a S conductive interme~ t~ layer, e.g. a conductive adhesive or a conductive polymer layer, is positioned between the resistive element and an electrode. This intermediate layer has a resistivity sllhst~ntiAlly lower than that of the resistive element. While such devices have improved stability over conventional devices, they require relatively high breakdown voltages, exhibit relatively high scatter, and are not able to with~t~n~ the high 10 power conditions nioc~ ry for some applications.
SUMMARY OF THF INV~TION
In order to provide m;lxi~ , protection, it is ~Lc;rel-ed that the breakdown voltage 15 of the device be relatively low, e.g. Iess than 500 volts, so that the device will operate under fault conditions in which the applied voltage is relatively low. It is also preferred that the breakdown voltage be relatively constant after multiple fault conditions. In order to effectively and repeatedly provide protection, it is pl~r~led that the device have a relatively stable insulation resistance, i.e. an insulation resi~tAnce of more than 1 x 109 20 ohms after exposure to a breakdown voltage is usually required. Furthermore, it is desirable that the device have the cArAhility to with~tAn~l high energy fault conditions such as a li~htnin~-type surge, i.e. a 10 x 1000 microsecond current waveform and a peak current of 60A. We have now found that a device which comprises at least two layers of different materials can exhibit each of these features. In a first aspect this invention ~5 provides an electrical device which comprises (A) a first resistive element which is composed of a first electrically non-linear composition which (i) has a resistivity at 25~C of more than 108 ohm-cm and (ii) comprises (1) a first polymeric component, and (2) a first particulate filler dispersed in the first polyrneric component;
(B) a second resistive element which (i) is in electrical contact, and preferably in physical and electrical contact, with the first element, and (ii) is CA 02239990 1998-06-0~
W O 97/21230 PCTrUS96/19319 composed of a second composition which has a resistivity of less than 1 0û
ohm-cm and which comprises (1) a second polymeric component, and (2) a second particulate filler which (a) is m~gnt-tic and electrically conductive, and (b) is aligned in discrete regions in the second polymeric component; and (C) first and second electrodes which are positioned so that current can flow between the electrodes through the first element and the second element.
RRTFF DF~CE~TPTION OF TH~ DRAWTNGS
The invention is illustrated by the drawings in which Figure 1 is a schematic cross-sectional view of an electrical device according to the first aspect of the invention;
Figure 2 is a cross-sectional view of a test fixture used to test a device of the invention; and Figures 3, 4, 5a to 5d, and 6 are graphs of breakdown voltage as a function of test cycle number for devices of the invention.
DFTATT F.n pFSCRTPTION OF THF INVF.NTION
The electrical device of the invention comprises at least two resistive elementswhich, in the ~ler~ d embodiment, are in physical and electrical contact with each other.
In this specification, the term "electrical contact" means having electrical c(mtimlity and includes configurations in which there may not be direct physical contact. It is plc;r~lled 30 that the two resistive elements be electrically connected in series, so that electrical current flows through the first resistive element and then the second resistive element. The first resistive element is composed of a first composition which exhibits electrically non-linear behavior. In this specification the terrn "non-linear" means that the composition is subst~nli~lly electrically non-conductive, i.e. has a resistivity of more than 10 ohm-cm, 35 and preferably more than 1 o8 ohm-cm, when an applied voltage is less than the impulse breakdown voltage, but then becomes electrically conductive, i.e. has a resistivity of subst~ntis-lly less than 10 ohm-cm, when the applied voltage is equal to or greater than CA 02239990 1998-06-0~
WO 97~1230 PCT~US96/19319 the impulse breakdown voltage. For many applications, it is ~>ler~ll.,d that the first composition have a resistivity in the "non-con~ ctin~" state of more than 1 o8 ohm-cm, particularly more than 109 ohm-cm, especially more than 101~ ohm-cm, and a resistivity in the "conducting" state of less than 103 ohm-cm.
The second resistive element is composed of a second composition which, when cured, is electrically conductive, i.e. has a resistivity of less than 10 ohm-cm, preferably less than 10 ohm-cm, particularly less than 100 ohm-cm, more particularly less than 10 ohm-cm, especially less than 1 ohm-cm, most especially less than 0.5 ohm-cm. The10 second composition may exhibit positive temperature coefficient (PT~) behavior, i.e. an inclease in resistivity over a relatively narrow L~ ldl~c; range.
The first composition comprises a first polymeric component in which is dispersed a first particulate filler and an optional third particulate filler. The second composition 15 comprises a second polymeric component which contains a second particulate filler and an optional fourth particulate filler. The first and second polymeric components may be the same or different and may be any a~ opllate polymer, e.g. a thertnoplastic m~t~ri~l 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, 20 acrylates, polyurethanes, polyesters, and liquid ethylene/propylene/diene monomers); a grease; or a gel. It is ~e~lred that both the first and the second polymeric components be a curable polymer, i.e. one that undergoes a physical and/or ~hemi- ~l change on exposure to an ~lopliate curing condition, e.g. heat, light, radiation (by means of an electron beam or gamma irradiation such as a Co source), microwave, a chemical component, or 25 a telllpt;l~L-Ile change.
For many applications it is pl~ Ç~ d that the first and/or the second 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 cont;nllous 30 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-ç~t~n~ 1 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, polyulc;Lllane, polyurea, styrene-b-lt~-lTene copolymers, styrene-isoprene copolymers, 35 styrene-(ethylene/propylene)-styrene (SEPS) block copolymers (available under the trs~ rne SeptonTM by Kuraray), styrene-~ethylene-propylene/ethylene-butylene)-styrene block copolymers ~available under the tr~(len~mc SeptonTM by Kuraray), and/or styrene-CA 02239990 l998-06-0~
W O97/21230 PCT~US96/19319 (ethylene/butylene)-styrene (SEBS) block copolymers (available under the tr~t1en~me KratonTM by Shell Oil Co.). Suitable ~rt.qnclçr 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 5 may be a thermosetting gel, e.g. silicone gel, in which the crosslinks are formed through the use of multifunctional crossl;nkin~ agents, or a thermoplastic gel, in whichmicrophase separation of domains serves as junction points. Disclosures of gels which may be suitable as the first and/or the second polymeric component in the composition are ~ound in U.S. PatentNos. ~,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 Tnt~rn~tional Patent Publication Nos. WO86/01634 (Toy et al), W088/00603 (Francis et al), WO90/05166 (Slltherl~n~l), WO91/05014 (Sutherland), and W093/23472 (Hammondet al).
The first polymeric component generally comprises 30 to 99%, preferably 30 to 95%, particularly 35 to 90%, especially 40 to 85% by volume of the total first composition. The second polymeric component generally comprises 50 to 99.99%, preferably 55 to 99.9%, particularly 60 to 99.9%, especially 65 to 99.9%, e.g. 70 to 99%, 20 by volume of the total second composition.
Dispersed in the first polymeric component is a first particulate filler which may be electrically conductive, nonconductive, or a mixture of two or more types of fillers as long as the resulting composition has the ~p~ul.~iate electrical non-linearity. In this 25 specification the term "electrically conductive" is used to mean a filler which is conductive or semiconductive and which has a resistivity of less than 10 ohm-cm and is preferably much lower, i.e. less than 1 ohm-cm, particularly less than 10-1 ohrn-cm, especially less than 10-3 ohm-cm. It is generally preferred that the filler be conductive or semiconductive. Conductive fillers generally have a resistivity of at most 10 ohm-cm;
30 semicon~ ctive fillers generally have a resistivity of at most 10 ohm-cm, although their resistivity is a function of any dopant material, as well as temperature and other factors and can be subst~nti~lly higher tha~ 10 ohm-cm. Suitable fillers include metal powders, e.g. ali....;..-l..., nickel, silver, silver-coated nickel, pl~tinllm, copper, tantalum, t mg~tPn, gold, and cobalt; metal oxide powders, e.g. iron oxide, doped iron oxide, doped Li 35 dioxide, and doped zinc oxide; metal carbide powders, e.g. silicon carbide, ~iL~ ium carbide, and tantalum carbide; metal nitride powders; metal boride powders; carbon black or graphite; and alloys, e.g. bronze and brass. It is also possible to use glass or ceramic CA 02239990 1998-06-0~
W O 97/21230 PCT~US96/19319 particles, e.g. spheres, coated with any conductive material. Particularly ~lere"~,d as fillers are alllmimlm, iron oxide (Fe304), iron oxide doped with titanium dioxide, silicon carbide, and silver-coated nickel. If the first polymeric component is a gel, it is important that the selected filler not in~ rel~ with the crosslinkin~ of the gel, i.e. not "poison" it.
The first filler is generally present in an amount of I to 70%, preferably 5 to 70%, particularly 10 to 65%, especially 15 to 60% by volume of the total first composition.
The volurne lo~-ling, shape, and size of the filler affect the non-linear electrical properties of the first composition, in part because of the spacing between the particles.
10 Any shape particle may be used, e.g. spherical, flake, fiber, or rod, although particles having a subst~nti:~lly spherical shape are preferred. Useful first compositions can bc prepared with particles having an average size of 0.010 to l O0 microns, preferably 0.1 to 75 microns, particularly 0.5 to 50 microns, especially l to 20 microns. A mixture of dirrt;lelll size, shape, and/or type particles may be used. The particles may be magnetic or 15 nonm~gnetic. F.~mples of compositions suitable for use in the first composition are found in Tnt~rn~tional Publication No. W095/3327g.
The second composition comprises a second particulate filler which is present at0.01 to 50%, preferably 0.1 to 45%, particularly 0.1 to 40%, especially 0.1 to 35%, e.g. l 20 to 30%, by volume of the total second composition. The second filler is both electrically conductive and magnetic. The term "magnetic" is used in this specification to mean ferromagnetic, fe~rim~gnetic, and paramagnetic materials. The filler may be completely m~gn~?tic, 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 25 non-magnetic coating, e.g. a silver-coated nickel particle. Suitable second fillers include nickel, iron, cobalt, ferric oxide, silver-coated nickel, silver-coated ferric oxide, or alloys of these materials. Any shape particle may be used, although approximately spherical particles are p~cr~ ed. In general, the l~lhrr~ ~ particle size of the second filler is less than 300 microns, preferably less than 200 microns, particularly less than 150 microns, 30 especially less than 100 microns, and is preferably in the range of 0.05 to 40 microns, particularly 1 to l O microns. Because processing techniques, e.g. coating the primary particle, may result in agglomeration, it is possible that the second filler, as mixed into the second polymeric component, may have an agglomerate size of as much as 300 microns.
For some applications, a mixture of different particle sizes and/or shapes and/or rn~tt riz 35 may be desirable.
CA 02239990 1998-06-0~
The second particulate filler is aligned in discrete regions or domains of the second polymeric component, e.g. as a column that extends through the second polymeric component from one side to the other, in particular from one side of the second resistive element (generally in contact with an electrode) to the first resistive element. Such 5 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 polymeric component, the ~lip;nment is m~int~ine(l in the cured polymeric component.
The resulting al;gnment provides anisotropic conductivity. Any type of magnetic field that is capable of supplying a field strength sufficient to align the particles may be used.
10 A conventional magnet of any type, e.g. ceramic or rare earth, may be used, although for ease in m~mlf~rture, it may be ~r~r~ d to use an electrom~gnet with suitably formed coils to generate the desired magnetic field. It is often preferred that the uncured polymeric component 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 15 means of ultraviolet light, it can be sufficient that there be only one magnet that is positioned on one side of the polymeric component. The polymeric component is generally separated from direct contact with the magnets by means of an electrically in~ ting spacing layer, e.g. a polycarbonate, polytetrafluoroethylene, or silicone sheet, or by means of first and second electrodes. It is important that the amount of second filler 20 present produces a resistive element which has conductivity only through the thickness of the resistive element, not between adjacent columns, thus providing anisotropic conductivity.
In order to improve the electrical performance of devices of the invention, it is 25 ~lef~--ed that the first composition and the second composition comprise at least one additional particulate filler, i.e. a third particulate filler for the first composition and a fourth particulate filler for the second composition. This additional particulate filler may be the same for both the first and second compositions, or it may be dirr~,lellt. In addition, the additional particulate filler may comprise a mixture of two or more dirrt;~el~l m~t,ori~
30 which may be the same or different, and in the same concentration or different concentrations, for the first and second compositions. The third particulate filler is present in an amount of 0 to 60%, preferably 5 to 50%, particularly 10 to 40% by total volume of the first composition. The fourth particulate filler is present in an amount of 0 to 60%~ preferably 5 to 50%, particularly 10 to 40% by total volume of the second 35 composition. Particularly ~-ere--~d for use as the third or fourth particulate fillers are arc ~Uppl~ s~ing agents or flame retardants, and oxidizing agents. ~ompositions withparticularly good performance under high current conditions, e.g. 250A, have been ~ = =
W O 97~1230 PCTnUS96/19319 ~L~h~d when the third and/or the fourth particulate filler comprises a mixture of (i) an arc su~ g agent or flame retardant, and (ii) an oxi~i7ing agent. It is pLer~ d that the oxi~li7ing agent be present in an amount 0. l to 1.0 times that o f the arc ~ es~ g agent or flame retardant. The oxidizing agent is generally present at 0 to 20%, preferably 5 to 15% by total volume of the first composition, and/or at 0 to 20%, preferably 5 to l 5%
by total volume of the second composition. Particularly good results are achieved when the oxi-li7ing agent is coated onto the arc ~u~lc;ssillg agent or flame .eLar~lal~l prior to mixin~ Suitable arc :ju~lessillg agents and flame retardants include zinc borate, m~gnf siurn hydroxide, alumina trihydrate, al--mimln~ phosphate, barium hydrogen10 phosphate, calciurn phosphate (tribasic or dibasic), copper pyrophosphate, iron phosphate, lithium phosphate, m~gn~ium phosphate, nickel phosphate, zinc phosphate, calciumoY~l~te~ iron al) oxalate, m~ng~nese oxalate, strontium oxalate, and al~ .", trifluoride trihydrate. It is important that any decomposition products of the arc suppressing agent be electrically nonconductive. Suitable oxi-li7ing agents include potassium 15 perm~ng~n~t~, ammonium perslllf~te, m~gn~sium perchlorate, m:~ng~nese dioxide, bismuth subnitrate, m~gn~ium dioxide, lead dioxide (also called lead peroxide), and bariurn dioxide. While we do not wish to be bound by any theory, it is believed that the presence of the arc ~u~rt;s~illg agent or flame retardant, and the oxidizing agent controls the plasma chemistry of the plasma generated during an electrical discharge, and provides 20 discharge products that are nonconductive.
For some applications, it is ~l~;r~ ll. d that the third and/or fourth particulate fillers comprise a surge initiator. Surge initiators have a low decomposition t~ LIlre, e.g.
150 to 200~C, and act to decrease the breakdown voltage of the composition and provide 25 more repeatable breakdown voltage values. Suitable surge initiators include oxalates, carbonates, or phosphates. The surge initiator may also act as an arc ~u~plessallt for some compositions. If present, the surge initiator generally comprises 5 to 30%, preferably 5 to 25% by total volume of the composition.
Both the first composition and the second composition may comprise additional components including antioxidants, radiation crosclinkin~ agents (often referred to as prorads or cro~linking enh~nrers), stabilizers, dispersing agents, coupling agents, acid scavengers, or other components. These components generally comprise at most 10% by volume of the total composition in which they are present.
W O 97/21230 PCT~US96119319 ~,~,F,CTRTCA~, nli',VICF, R~CKGROUND OF THE INVF~TION
Field of the Tnvention This invention relates to electrical devices comprising electrically non-linear compositions.
Tntroduction to the Invention Devices comprising electrically non-linear compositions are known for protectingelectrical eql-irment and cil~;ui~ly. The compositions used in such devices often exhibit non-linear electrical resistivitv, decreasing in resistivity from an in~ tin~ state, i.e. more than 106 ohm-cm, to a conc1~1ctin~ state when exposed to a voltage that exceeds a threshold value. This value is l~nown as the breakdown voltage. Compositions exhibiting non-linear electrical behavior are disclosed in U.S. Patents Nos. 4,977,357 (Shrier), 5,294,374 (Martinez et al), and 5,557,250 (Debbaut et al), in Tnt~rn~tional Publication No.
W095/33278 (Raychem Cul~oldlion), and in Tnfern~tional Application No.
PCT/US96/09113 (Raychem Corporation).
Electrical devices prepared from these conventional compositions have been described. See, for example, Tntçrn~tional Publication No. W095/33278 which discloses an electrically non-linear resistive element suitable for repeated use as the secondary protection in a telecommunications gas tube d~palaLus. That resistive element compri~çc a composition in which a particulate filler such as alllminllm is dispersed in a polymeric matrix . The composition has an initial resistivity Pi at 25~C of at least 109 ohm-cm and, even after exposure to a standard impulse breakdown test in which a high energy impulse is applied across the element five times, has a final resistivity pf at 25~C of at least 109 ohm-cm. However, such devices, when exposed to a high energy fault condition, will short out and are thus not reusable. Furthermore, the scatter in the breakdown voltage on successive test events is relatively broad.
Tntf~ ti~nal Application No. PCT/US96/09113 discloses a device which is designed to protect electrical components as a primary protection device rather than as a secondary protection device. In this device, a resistive element is positioned between two CA 02239990 1998-06-0~
electrodes and is composed of a polymeric component in which a first magnetic, electrically conductive particulate filler and a second magnetic particulate filler with a resistivity of at least 1 x 104 ohm-cm are aligned in discrete regions exten~linp from the first to the second electrode. In order to increase the electrical stability of the device, a S conductive interme~ t~ layer, e.g. a conductive adhesive or a conductive polymer layer, is positioned between the resistive element and an electrode. This intermediate layer has a resistivity sllhst~ntiAlly lower than that of the resistive element. While such devices have improved stability over conventional devices, they require relatively high breakdown voltages, exhibit relatively high scatter, and are not able to with~t~n~ the high 10 power conditions nioc~ ry for some applications.
SUMMARY OF THF INV~TION
In order to provide m;lxi~ , protection, it is ~Lc;rel-ed that the breakdown voltage 15 of the device be relatively low, e.g. Iess than 500 volts, so that the device will operate under fault conditions in which the applied voltage is relatively low. It is also preferred that the breakdown voltage be relatively constant after multiple fault conditions. In order to effectively and repeatedly provide protection, it is pl~r~led that the device have a relatively stable insulation resistance, i.e. an insulation resi~tAnce of more than 1 x 109 20 ohms after exposure to a breakdown voltage is usually required. Furthermore, it is desirable that the device have the cArAhility to with~tAn~l high energy fault conditions such as a li~htnin~-type surge, i.e. a 10 x 1000 microsecond current waveform and a peak current of 60A. We have now found that a device which comprises at least two layers of different materials can exhibit each of these features. In a first aspect this invention ~5 provides an electrical device which comprises (A) a first resistive element which is composed of a first electrically non-linear composition which (i) has a resistivity at 25~C of more than 108 ohm-cm and (ii) comprises (1) a first polymeric component, and (2) a first particulate filler dispersed in the first polyrneric component;
(B) a second resistive element which (i) is in electrical contact, and preferably in physical and electrical contact, with the first element, and (ii) is CA 02239990 1998-06-0~
W O 97/21230 PCTrUS96/19319 composed of a second composition which has a resistivity of less than 1 0û
ohm-cm and which comprises (1) a second polymeric component, and (2) a second particulate filler which (a) is m~gnt-tic and electrically conductive, and (b) is aligned in discrete regions in the second polymeric component; and (C) first and second electrodes which are positioned so that current can flow between the electrodes through the first element and the second element.
RRTFF DF~CE~TPTION OF TH~ DRAWTNGS
The invention is illustrated by the drawings in which Figure 1 is a schematic cross-sectional view of an electrical device according to the first aspect of the invention;
Figure 2 is a cross-sectional view of a test fixture used to test a device of the invention; and Figures 3, 4, 5a to 5d, and 6 are graphs of breakdown voltage as a function of test cycle number for devices of the invention.
DFTATT F.n pFSCRTPTION OF THF INVF.NTION
The electrical device of the invention comprises at least two resistive elementswhich, in the ~ler~ d embodiment, are in physical and electrical contact with each other.
In this specification, the term "electrical contact" means having electrical c(mtimlity and includes configurations in which there may not be direct physical contact. It is plc;r~lled 30 that the two resistive elements be electrically connected in series, so that electrical current flows through the first resistive element and then the second resistive element. The first resistive element is composed of a first composition which exhibits electrically non-linear behavior. In this specification the terrn "non-linear" means that the composition is subst~nli~lly electrically non-conductive, i.e. has a resistivity of more than 10 ohm-cm, 35 and preferably more than 1 o8 ohm-cm, when an applied voltage is less than the impulse breakdown voltage, but then becomes electrically conductive, i.e. has a resistivity of subst~ntis-lly less than 10 ohm-cm, when the applied voltage is equal to or greater than CA 02239990 1998-06-0~
WO 97~1230 PCT~US96/19319 the impulse breakdown voltage. For many applications, it is ~>ler~ll.,d that the first composition have a resistivity in the "non-con~ ctin~" state of more than 1 o8 ohm-cm, particularly more than 109 ohm-cm, especially more than 101~ ohm-cm, and a resistivity in the "conducting" state of less than 103 ohm-cm.
The second resistive element is composed of a second composition which, when cured, is electrically conductive, i.e. has a resistivity of less than 10 ohm-cm, preferably less than 10 ohm-cm, particularly less than 100 ohm-cm, more particularly less than 10 ohm-cm, especially less than 1 ohm-cm, most especially less than 0.5 ohm-cm. The10 second composition may exhibit positive temperature coefficient (PT~) behavior, i.e. an inclease in resistivity over a relatively narrow L~ ldl~c; range.
The first composition comprises a first polymeric component in which is dispersed a first particulate filler and an optional third particulate filler. The second composition 15 comprises a second polymeric component which contains a second particulate filler and an optional fourth particulate filler. The first and second polymeric components may be the same or different and may be any a~ opllate polymer, e.g. a thertnoplastic m~t~ri~l 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, 20 acrylates, polyurethanes, polyesters, and liquid ethylene/propylene/diene monomers); a grease; or a gel. It is ~e~lred that both the first and the second polymeric components be a curable polymer, i.e. one that undergoes a physical and/or ~hemi- ~l change on exposure to an ~lopliate curing condition, e.g. heat, light, radiation (by means of an electron beam or gamma irradiation such as a Co source), microwave, a chemical component, or 25 a telllpt;l~L-Ile change.
For many applications it is pl~ Ç~ d that the first and/or the second 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 cont;nllous 30 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-ç~t~n~ 1 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, polyulc;Lllane, polyurea, styrene-b-lt~-lTene copolymers, styrene-isoprene copolymers, 35 styrene-(ethylene/propylene)-styrene (SEPS) block copolymers (available under the trs~ rne SeptonTM by Kuraray), styrene-~ethylene-propylene/ethylene-butylene)-styrene block copolymers ~available under the tr~(len~mc SeptonTM by Kuraray), and/or styrene-CA 02239990 l998-06-0~
W O97/21230 PCT~US96/19319 (ethylene/butylene)-styrene (SEBS) block copolymers (available under the tr~t1en~me KratonTM by Shell Oil Co.). Suitable ~rt.qnclçr 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 5 may be a thermosetting gel, e.g. silicone gel, in which the crosslinks are formed through the use of multifunctional crossl;nkin~ agents, or a thermoplastic gel, in whichmicrophase separation of domains serves as junction points. Disclosures of gels which may be suitable as the first and/or the second polymeric component in the composition are ~ound in U.S. PatentNos. ~,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 Tnt~rn~tional Patent Publication Nos. WO86/01634 (Toy et al), W088/00603 (Francis et al), WO90/05166 (Slltherl~n~l), WO91/05014 (Sutherland), and W093/23472 (Hammondet al).
The first polymeric component generally comprises 30 to 99%, preferably 30 to 95%, particularly 35 to 90%, especially 40 to 85% by volume of the total first composition. The second polymeric component generally comprises 50 to 99.99%, preferably 55 to 99.9%, particularly 60 to 99.9%, especially 65 to 99.9%, e.g. 70 to 99%, 20 by volume of the total second composition.
Dispersed in the first polymeric component is a first particulate filler which may be electrically conductive, nonconductive, or a mixture of two or more types of fillers as long as the resulting composition has the ~p~ul.~iate electrical non-linearity. In this 25 specification the term "electrically conductive" is used to mean a filler which is conductive or semiconductive and which has a resistivity of less than 10 ohm-cm and is preferably much lower, i.e. less than 1 ohm-cm, particularly less than 10-1 ohrn-cm, especially less than 10-3 ohm-cm. It is generally preferred that the filler be conductive or semiconductive. Conductive fillers generally have a resistivity of at most 10 ohm-cm;
30 semicon~ ctive fillers generally have a resistivity of at most 10 ohm-cm, although their resistivity is a function of any dopant material, as well as temperature and other factors and can be subst~nti~lly higher tha~ 10 ohm-cm. Suitable fillers include metal powders, e.g. ali....;..-l..., nickel, silver, silver-coated nickel, pl~tinllm, copper, tantalum, t mg~tPn, gold, and cobalt; metal oxide powders, e.g. iron oxide, doped iron oxide, doped Li 35 dioxide, and doped zinc oxide; metal carbide powders, e.g. silicon carbide, ~iL~ ium carbide, and tantalum carbide; metal nitride powders; metal boride powders; carbon black or graphite; and alloys, e.g. bronze and brass. It is also possible to use glass or ceramic CA 02239990 1998-06-0~
W O 97/21230 PCT~US96/19319 particles, e.g. spheres, coated with any conductive material. Particularly ~lere"~,d as fillers are alllmimlm, iron oxide (Fe304), iron oxide doped with titanium dioxide, silicon carbide, and silver-coated nickel. If the first polymeric component is a gel, it is important that the selected filler not in~ rel~ with the crosslinkin~ of the gel, i.e. not "poison" it.
The first filler is generally present in an amount of I to 70%, preferably 5 to 70%, particularly 10 to 65%, especially 15 to 60% by volume of the total first composition.
The volurne lo~-ling, shape, and size of the filler affect the non-linear electrical properties of the first composition, in part because of the spacing between the particles.
10 Any shape particle may be used, e.g. spherical, flake, fiber, or rod, although particles having a subst~nti:~lly spherical shape are preferred. Useful first compositions can bc prepared with particles having an average size of 0.010 to l O0 microns, preferably 0.1 to 75 microns, particularly 0.5 to 50 microns, especially l to 20 microns. A mixture of dirrt;lelll size, shape, and/or type particles may be used. The particles may be magnetic or 15 nonm~gnetic. F.~mples of compositions suitable for use in the first composition are found in Tnt~rn~tional Publication No. W095/3327g.
The second composition comprises a second particulate filler which is present at0.01 to 50%, preferably 0.1 to 45%, particularly 0.1 to 40%, especially 0.1 to 35%, e.g. l 20 to 30%, by volume of the total second composition. The second filler is both electrically conductive and magnetic. The term "magnetic" is used in this specification to mean ferromagnetic, fe~rim~gnetic, and paramagnetic materials. The filler may be completely m~gn~?tic, 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 25 non-magnetic coating, e.g. a silver-coated nickel particle. Suitable second fillers include nickel, iron, cobalt, ferric oxide, silver-coated nickel, silver-coated ferric oxide, or alloys of these materials. Any shape particle may be used, although approximately spherical particles are p~cr~ ed. In general, the l~lhrr~ ~ particle size of the second filler is less than 300 microns, preferably less than 200 microns, particularly less than 150 microns, 30 especially less than 100 microns, and is preferably in the range of 0.05 to 40 microns, particularly 1 to l O microns. Because processing techniques, e.g. coating the primary particle, may result in agglomeration, it is possible that the second filler, as mixed into the second polymeric component, may have an agglomerate size of as much as 300 microns.
For some applications, a mixture of different particle sizes and/or shapes and/or rn~tt riz 35 may be desirable.
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The second particulate filler is aligned in discrete regions or domains of the second polymeric component, e.g. as a column that extends through the second polymeric component from one side to the other, in particular from one side of the second resistive element (generally in contact with an electrode) to the first resistive element. Such 5 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 polymeric component, the ~lip;nment is m~int~ine(l in the cured polymeric component.
The resulting al;gnment provides anisotropic conductivity. Any type of magnetic field that is capable of supplying a field strength sufficient to align the particles may be used.
10 A conventional magnet of any type, e.g. ceramic or rare earth, may be used, although for ease in m~mlf~rture, it may be ~r~r~ d to use an electrom~gnet with suitably formed coils to generate the desired magnetic field. It is often preferred that the uncured polymeric component 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 15 means of ultraviolet light, it can be sufficient that there be only one magnet that is positioned on one side of the polymeric component. The polymeric component is generally separated from direct contact with the magnets by means of an electrically in~ ting spacing layer, e.g. a polycarbonate, polytetrafluoroethylene, or silicone sheet, or by means of first and second electrodes. It is important that the amount of second filler 20 present produces a resistive element which has conductivity only through the thickness of the resistive element, not between adjacent columns, thus providing anisotropic conductivity.
In order to improve the electrical performance of devices of the invention, it is 25 ~lef~--ed that the first composition and the second composition comprise at least one additional particulate filler, i.e. a third particulate filler for the first composition and a fourth particulate filler for the second composition. This additional particulate filler may be the same for both the first and second compositions, or it may be dirr~,lellt. In addition, the additional particulate filler may comprise a mixture of two or more dirrt;~el~l m~t,ori~
30 which may be the same or different, and in the same concentration or different concentrations, for the first and second compositions. The third particulate filler is present in an amount of 0 to 60%, preferably 5 to 50%, particularly 10 to 40% by total volume of the first composition. The fourth particulate filler is present in an amount of 0 to 60%~ preferably 5 to 50%, particularly 10 to 40% by total volume of the second 35 composition. Particularly ~-ere--~d for use as the third or fourth particulate fillers are arc ~Uppl~ s~ing agents or flame retardants, and oxidizing agents. ~ompositions withparticularly good performance under high current conditions, e.g. 250A, have been ~ = =
W O 97~1230 PCTnUS96/19319 ~L~h~d when the third and/or the fourth particulate filler comprises a mixture of (i) an arc su~ g agent or flame retardant, and (ii) an oxi~i7ing agent. It is pLer~ d that the oxi~li7ing agent be present in an amount 0. l to 1.0 times that o f the arc ~ es~ g agent or flame retardant. The oxidizing agent is generally present at 0 to 20%, preferably 5 to 15% by total volume of the first composition, and/or at 0 to 20%, preferably 5 to l 5%
by total volume of the second composition. Particularly good results are achieved when the oxi-li7ing agent is coated onto the arc ~u~lc;ssillg agent or flame .eLar~lal~l prior to mixin~ Suitable arc :ju~lessillg agents and flame retardants include zinc borate, m~gnf siurn hydroxide, alumina trihydrate, al--mimln~ phosphate, barium hydrogen10 phosphate, calciurn phosphate (tribasic or dibasic), copper pyrophosphate, iron phosphate, lithium phosphate, m~gn~ium phosphate, nickel phosphate, zinc phosphate, calciumoY~l~te~ iron al) oxalate, m~ng~nese oxalate, strontium oxalate, and al~ .", trifluoride trihydrate. It is important that any decomposition products of the arc suppressing agent be electrically nonconductive. Suitable oxi-li7ing agents include potassium 15 perm~ng~n~t~, ammonium perslllf~te, m~gn~sium perchlorate, m:~ng~nese dioxide, bismuth subnitrate, m~gn~ium dioxide, lead dioxide (also called lead peroxide), and bariurn dioxide. While we do not wish to be bound by any theory, it is believed that the presence of the arc ~u~rt;s~illg agent or flame retardant, and the oxidizing agent controls the plasma chemistry of the plasma generated during an electrical discharge, and provides 20 discharge products that are nonconductive.
For some applications, it is ~l~;r~ ll. d that the third and/or fourth particulate fillers comprise a surge initiator. Surge initiators have a low decomposition t~ LIlre, e.g.
150 to 200~C, and act to decrease the breakdown voltage of the composition and provide 25 more repeatable breakdown voltage values. Suitable surge initiators include oxalates, carbonates, or phosphates. The surge initiator may also act as an arc ~u~plessallt for some compositions. If present, the surge initiator generally comprises 5 to 30%, preferably 5 to 25% by total volume of the composition.
Both the first composition and the second composition may comprise additional components including antioxidants, radiation crosclinkin~ agents (often referred to as prorads or cro~linking enh~nrers), stabilizers, dispersing agents, coupling agents, acid scavengers, or other components. These components generally comprise at most 10% by volume of the total composition in which they are present.
3~
The first and second compositions may be plG~d by any suitable means, e.g.
melt-blending, solvent-blending, or intensive mixing. Because it is pler~ lled that the first CA 02239990 l998-06-0~
and second polymeric components 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 conclllct~d until a uniform dispersion of the filler particles is achieved. The composition may be shaped by conventional methods including extrusion, 5 calçn~1~ring, 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 subskate or into a mold to be cured.
In order to accommodate the necessary loading of the particulate fillers, and to10 allow ~ nment of the fillers in the polymeric component, it is ~-~;r~ d that the first and second polymeric components, prior to any curing and without any filler, have a viscosity at room Le,l,~;.dLul~ 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 15 te-n~c;.dLIlre~ Tc~ if the polymeric cu~pollent is curable, or at the mixing te.llp~.aL~e at which the particulate fillers are dispersed and subsequently aligned if the polymeric component is not curable.
The electrical device of the invention comprises at least one first resistive element 20 which is preferably in electrical and physical contact with at least one second resistive element. It is plc;r~led that the first and second elements be in direct physical and electrical contact with one another, but it is possible that only some part of the first and second elements is in direct physical contact, or that there is an intermediate layer, e.g. a metal sheet, between the two elements. While a single first resistive element and a single 25 second resistive element can be used, it is also possible that two first resistive elements may be positioned on opposite sides of a second resistive element, or two second resistive elements may be positioned on opposite sides of a first resistive element. The direction of conductivity of the second resistive element is perpendicular to the plane of the first resistive element. Depending on the method of pl~ lg the resistive elements, they may 30 be of any thiel~nt-~ or geometry, although both the first and the second resistive elements are of generally laminar configuration. In a pl~r~l.ed configuration, the first resistive element has a thickness of 0.25 to 1.0 mm, while the second resistive element has a thickness of 1.0 to 2.0 mm. The first and second resistive elements may be attached by any suitable method, e.g. a physical ~tt~ehment method such as a clamp, or an ~ ehment 35 resultin~; from physical or chemical bonds. In some cases, if the first and second compositions are curable, the first and second resistive elements may be cured in contact with one another, as long as it is possible to plo~.ly align the second particulate filler.
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WO 97/21Z30 PCT~US96/19319 The electrical device comprises first and second electrodes which are positionedso that, when the device is connected to a source of eleckical power, current can flow between the eleckodes through the first and second resistive elements. Generally the first eleckrode is ~tt~h~d to the first resistive element, and the second electrode to the second resistive element, but if the device comprises a center first resistive element sandwiched between two second resistive element~, the first eleckode may be positioned in contact with one second resistive element and the second eleckrode may be positioned in contact with the other second resistive elçmçnt Similarly, if the device comprises a center second resistive element bet~,veen two first resistive elements, the first and second eleckrodes may be positioned in contact with the two first resistive elements. The electrodes and the resistive elements are configured so that the first and second resistive elements are eleckically in series. The type of electrode is dependent on the shape of the first and second elements, but is preferably laminar and in the form of a metal foil, metal mesh, or metallic ink layer. The first eleckode has a first resistivity and the second electrode has a second resistivity, both of which are generally less than 1 x 1 o~2 ohm-cm, preferably less than ~ x 10-3 ohm-cm, particularly less than I x 10 '~ ohm-cm. Particularly suitable metal foil eleckodes comprise microrough surfaces, e.g. eleckodeposited 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 Tntern~tional Publication No. WO95/34081 (Raychem Corporation).
Depending on the type of the polymeric components and the electrodes, it may be desirable to cure the first and second compositions directly in contact with the electrodes.
~ltern~tively, it is possible to cure the compositions partially or completely before cchin~ the electrodes to the cured compositions. The latter technique is especially ~I ~r~liate for use with mesh or other f<~r~minl-us electrode mzlteris~l~ In order to control the thickness of the first and second resistive elements, the uncured composition may be poured or otherwise positioned within a mold of specified thickness, and then cured. For some applications, i~ v~d electrical stability for the device 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 m~n~tic. Electrodes of this type include nickel, nickel-coated copper, and stainless steel. It is ~l~f~ ed 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 International Application No. PCT/US96/09103 (Raychem Corporation~.
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W O 97/2123~ PCT~S96/19319 11 , The first and second polymeric components may be cured by any suitable means, including heat, light, microwave, electron bearn, or gamma irradiation, and are often ~ cured by using a combination of time and temperature suitable to subst~nti~lly cure the polymeric components. The curing tt;~ cl~Lu~e Tc may be at any temperature that allows substantial curing of the polymeric colllpullelll~ i.e. that cures the polymeric component 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 Tc 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 minimi7~ disruption of the aligned particles when exposed to a high energy condition. In addition, it is pl~r~lled 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 scintill~ting vial C(~t~ lg 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 mrn/second to a penetration ~ t~n~e of 4.0 rnm. 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 (I)ittmer et al).
The device of the invention is nonro~ ctive, i.e. has an insulation rcsi~t~nce at 25~C of more than 1 o6 ohms, preferably more than 1 o8 ohms, particularly more than 109 ohms, especially more than 101~ ohms. The r~ci~t~nce ofthe second resistive element at 30 25~C, if measured on its own, not in contact with the first resistive element, is at most 1000 ohms, preferably at most 100 ohms, particularly at most 10 ohms, especially at most 1 ohm.
Electrical devices of the invention, when tested according to the Standard Impulse 35 Breakdown Voltage Test, described below, preferably exhibit low breakdown voltage and m~int~in a high insulation resistance. Thus the breakdown voltage when tested at either 60A or 250A is at most 1000 volts, preferably at most 800 volts, particularly at most 700 CA 02239990 1998-06-0~
WO 97/21~30 PCT~US96/19319 volts, especially at most 600 volts, more especially at most 500 volts, e.g. 200 to 500 volts, and the final insulation re~i~t~nce is at least lOg 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 varies from the average breakdown S 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 fi~al breakdown voltage Vsf which is from 0.7ovsi to 1.30VSi, preferably from 0.80VSi to 1.20VSi, particularly from 0.85VSi to 1.15VSi, especially from O.90Vsi to l.10VSi.
The first resistive element acts as a "switch" due to its non-linear nature, andcontrols the breakdown voltage of the device. However, if exposed to a very high energy pulse, e.g. a 10 x 1000 microsecond current waveform and a peak current of 300A, a small region in the first resistive element will short out if not in contact with the second resistive element. The second resistive element acts as a "point-plane" electrode. Each of the domains, generally in the form of columns, behaves as a microfuse which can be destroyed by the breakdown event. As a result, even if an affected portion of the first resistive element shorts out, a corresponding domain in the second resistive element will be destroyed, and will disconnect the shorted section of the first resistive element from the circuit. The device thus returns to a nonconductive state after the breakdown event.
In addition, the electric field is concentrated at the tip of each domain or column, thus increasing the repeatability of the breakdown voltage on successive electrical events.
The invention is illustrated by the drawing in which Figure I shows in cross-section electrical device 1. First electrode 3 is in contact with first resistive element 7, while second electrode 5 is in contact with second resistive element 13. First resistive element 7 is made of first polymeric component 9 which acts as a matrix in which is dispersed first particulate ~lller 11. Second resistive element 13 is made of second polymeric component 15 through which is dispersed in discrete domains aligned chains 17. Each chain 17 contains particles of second particulate filler 19.
The invention is illustrated by the following examples, each of which was testedusing the Standard lmpulse Breakdown Test.
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W O97/21230 PCTnJS96/19319 Stslnl1~rd Pevice Both the first composition and the second composition were prepared by mixing the clesi~n~ted co~ ollents with a tongue depressor or mechanical stirrer to wet and disperse the particulate filler. Each composition was deg~se~l in a vacuum oven for one minute. The second composition was 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 ~U~pOl l~d 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 10 pounds (available from McMaster-Carr) were positioned over the metal sheets, sandwiching the composition. The second composition was then cured at 100~C for 15 minute~. The top magnet, the top metal sheet, and the top release sheet were removed, additional spacers were added to give a thickness of 1.5 mm, and the first composition was poured onto the surface of the cured second composition.
The top release sheet and the top metal sheet were replaced and a weight (which may be the top magnet) was placed on top of the top metal sheet. The arrangement was then cured at 100~C for an additional 15 minutes to give a l~min~te of the first and second compositions. A disc 20 (as shown in Figure 2) with a diameter of 15.9 mm and a thickness of 1.5 mm was cut from the cured l~min~t~ The disc 20 consisted of a second resistive element 21 with a thickness of 1.0 mm from the cured second composition and a first resistive element 22 with a thickness of 0.5 mm from the first composition.
Molybdenum electrodes 23, 25 having a diameter of 15.9 mm and a thickness of 0.25 mm (0.010 inch) were attached to the top and bottom surfaces of disc 20 to form a standard device 27.
St~n~l~rd Tmrulse Rreakdown Test A standard device 27 was inserted into the test fixture 29 shown in Figure 2. Two copper cylinders 31,33, ~ loxilllately 19 mm (0.75 inch) in diameter, were mounted in a polycarbonate holder 35 such that the end faces 37,39 were parallel. One end 37 was fixed and immobile; the other end 39 was free to travel while still m~ g the parallel end-face geometry. Movement of cylinder 33 was controlled by barrel micrometer 41 mounted through mounting ring 43. Device 27 was mounted between cylinders 31,33,and micrometer 41 was adjusted until contact with zero compressive pressure was made to both sides of device 27. Pressure was then applied to device 27 by further moving cylinder 33 (via micrometer 41) to cOlll~ ,S the sample 10% (generally 0.1 to 0.3 mm).
Electrical leads 45,47 were conn~cte~l from copper cylinders 31,33 to the testing CA 02239990 1998-06-0~
W O 97/2123~ PCT~US96/19319 equipment (not shown). Prior to testing, the insulation resi~t~n-~e R; for the device was measured at 25~C with a biasing voltage of 50 volts using a Genrad 1864 Megaohm meter; the initial resistivity Pi was calcnl~t~l Electrical connection was then made to a Keytek ECAT Series 100 Surge Generator using an E514A IQxlO00 waveform generator.
For each cycle a high energy impulse with a 10 x 1000 ~LS current wavefonn (i.e. a rise time to maximum current of 10 ~ls and a half-height at 1000 ~LS) and a peak current of 60A was applied. 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 or one hundred cycles for the standard test was measured and the final resistivity pf was calculated.
Fx~mples 1 to 15 The first and second resistive elements for Examples 1 to 15 were pl~alcd from compositions using the fnrm~ tions shown in Table I. In each case the silicone gel was formulated using 49.420% 1000 cs divinyl-t~rmin~tecl polydimethylsiloxane (available from United Chemical Technology (UCT)), 49.956% 50 cs silicone oil (polydirnethylsiloxane fluid from UCT), 0.580% tetrakis(dimethyl siloxy silane) (UCT), 0.04% catalyst, and 0.004% inhibitor, all arnounts by weight of the composition. The stoichiometry was adjusted for peak hardness, i.e. 600 grams using a Voland texture analyzer with a 7 mm st~inle~ steel probe. The al.l...i~ ,. was a powder with an average particle size of 15 to 20 microns ( 200 mesh) and a sl-hs1~nti~11y spherical shape, available from Aldrich Chemicals. The nickel, available from Alfa Aesar, had a mesh size of -300 mesh and an average particle size of 3 to 10 microns. The arc SU~plcS:iillg agents, i.e. m~gnç~ium phosphate (Mg3(PO4)2-8H20), zinc phosphate (Zn3(PO4)2 2H20), calciurn phosphate (CaHPO4 2~20), iron oxalate (FeC204 2H20), and zinc borate (3ZnO 2B203), the oxidizing agents, i.e. bismuth subnitrate (4BiNO3(0H~2 BiO(OH)) and lead peroxide (PbO2), and the surge initiators, i.e. calcium carbonate (CaCO3, decomposition temperature 898~C), m~n~slnese oxalate (MnC204 2H20, decompositiontemperature 100~C), and iron oxalate (which also acts as an arc ~u~lessillg agent, decomposition temp~,LdLulc 1 90~C), were available from Alfa Aesar. Standard devices were prepared as above and tested using the Standard Impulse Breakdown Test for either 50 or 100 cycles, as indicated. (Testing for Example 11 was done at lOOA rather than 60A.) In each case, except for colllp~livc Examples 5 and 7, the devices had Ri greater than 109 ohms. For Examples 5 and 7 the value of R; was greater than 1 o8 ohms. The CA 02239990 1998-06-0~
WO 97nl230 PCT~US96/19319 average breakdown voltage over the total number of test cycles and the standard deviation (i.e. a measure of the reproducibility of the breakdown voltage) are shown in Table I.
Examples 1 to 4, which contained an arc ~u~pres~ing agent, showed good low breakdown voltage (i.e. less than 1000 volts, and, for Examples 2 to 4, less than 400 volts), and good reproducibility. Each had an Rf value of greater than 1 o8 ohms. The test results for Example 2 are shown in Figure 3.
Examples S to 1 1 show the effects of the presence of both an arc suppressing agent and an oxidizing agent. Examples 5 and 7, which contained bismuth subnitrate in both the first and second resistive elements had an Rf value of 1 x 107. When bismuthsubnitrate, which becomes conductive when exposed to moisture, was used in the second resistive element only (Example 11), the device had an Rf value of greater than 108 ohms, and excellent reproducibility. Examples 12 to l S show the effects of the presence of a l S surge initiator. Examples 14 and l S, which contained a surge initiator which had a low decomposition temperature, had low breakdown voltages and good reproducibility. Each of Examples 12 to l S had an Rf value of greater than 108 ohms. The test results for Examples 4, 9, 10, and 1 1 are shown in Figure 4. The test results for Examples 12 to l S
are shown in Figures Sa to Sd, respectively. In each of Figures Sa to Sd results are shown for three dirrelt;." samples of each type of device. The values reported in Table I are averages of the three samples for each example.
Monolayer devices which contained only a first resistive element made from a composition co..'.~il.i.~g alllmimlm powder dispersed in a silicone, shown, for example in Intern~tional Publication No. WO9S/33278, the disclosure of which is incorporated herein by reference, had a breakdown voltage of more than 1000 volts when tested using a 10 x 1000 microsecond waveforrn and a current of at most lA. They did not survive fifty cycles when tested at 60A.
Fx~mple 16 Following the procedure of Examples 1 to 15, a first composition was l,l~ed coll~ illg 30% al---l,;ol~ 200 mesh), 10% zinc borate, 10% potassium pçnl~ng~n~te~
and 50% silicone gel (as in Example 1), and a second composition was prepared cont~ining 11.25% nickel with a mesh size of -100 to +200 (available from Alfa Aesar, with an average particle size of about 100 microns), 3.75% nickel with a mesh size of -300~ 20% zinc borate, 10% potassium perm~ngs3n~te, and 55% silicone gel (as in Example 1), all percentages by volume of each total composition. A Standard Device was prepared and tested 50 cycles at 60A with a 10 x 1000 microsecond waveform. The average breakdown voltage was 318 volts, with a standard deviation of 27. Both Rj and Rf were I
x 101 ~ ohms. The test results are shown in Figure 6.
S
Fx~n~le 17 A device was prepared as in Exarnple 16 and tested 50 cycles at 220A with a 10 x1000 microsecond waveform. The average breakdown voltage was 365 volts, with a 10 standard deviation of 32. Both R; and Rf were 1 x 1 ol I ohms. The test results are shown in Figure 6.
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The first and second compositions may be plG~d by any suitable means, e.g.
melt-blending, solvent-blending, or intensive mixing. Because it is pler~ lled that the first CA 02239990 l998-06-0~
and second polymeric components 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 conclllct~d until a uniform dispersion of the filler particles is achieved. The composition may be shaped by conventional methods including extrusion, 5 calçn~1~ring, 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 subskate or into a mold to be cured.
In order to accommodate the necessary loading of the particulate fillers, and to10 allow ~ nment of the fillers in the polymeric component, it is ~-~;r~ d that the first and second polymeric components, prior to any curing and without any filler, have a viscosity at room Le,l,~;.dLul~ 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 15 te-n~c;.dLIlre~ Tc~ if the polymeric cu~pollent is curable, or at the mixing te.llp~.aL~e at which the particulate fillers are dispersed and subsequently aligned if the polymeric component is not curable.
The electrical device of the invention comprises at least one first resistive element 20 which is preferably in electrical and physical contact with at least one second resistive element. It is plc;r~led that the first and second elements be in direct physical and electrical contact with one another, but it is possible that only some part of the first and second elements is in direct physical contact, or that there is an intermediate layer, e.g. a metal sheet, between the two elements. While a single first resistive element and a single 25 second resistive element can be used, it is also possible that two first resistive elements may be positioned on opposite sides of a second resistive element, or two second resistive elements may be positioned on opposite sides of a first resistive element. The direction of conductivity of the second resistive element is perpendicular to the plane of the first resistive element. Depending on the method of pl~ lg the resistive elements, they may 30 be of any thiel~nt-~ or geometry, although both the first and the second resistive elements are of generally laminar configuration. In a pl~r~l.ed configuration, the first resistive element has a thickness of 0.25 to 1.0 mm, while the second resistive element has a thickness of 1.0 to 2.0 mm. The first and second resistive elements may be attached by any suitable method, e.g. a physical ~tt~ehment method such as a clamp, or an ~ ehment 35 resultin~; from physical or chemical bonds. In some cases, if the first and second compositions are curable, the first and second resistive elements may be cured in contact with one another, as long as it is possible to plo~.ly align the second particulate filler.
CA 02239990 1998-06-0~
WO 97/21Z30 PCT~US96/19319 The electrical device comprises first and second electrodes which are positionedso that, when the device is connected to a source of eleckical power, current can flow between the eleckodes through the first and second resistive elements. Generally the first eleckrode is ~tt~h~d to the first resistive element, and the second electrode to the second resistive element, but if the device comprises a center first resistive element sandwiched between two second resistive element~, the first eleckode may be positioned in contact with one second resistive element and the second eleckrode may be positioned in contact with the other second resistive elçmçnt Similarly, if the device comprises a center second resistive element bet~,veen two first resistive elements, the first and second eleckrodes may be positioned in contact with the two first resistive elements. The electrodes and the resistive elements are configured so that the first and second resistive elements are eleckically in series. The type of electrode is dependent on the shape of the first and second elements, but is preferably laminar and in the form of a metal foil, metal mesh, or metallic ink layer. The first eleckode has a first resistivity and the second electrode has a second resistivity, both of which are generally less than 1 x 1 o~2 ohm-cm, preferably less than ~ x 10-3 ohm-cm, particularly less than I x 10 '~ ohm-cm. Particularly suitable metal foil eleckodes comprise microrough surfaces, e.g. eleckodeposited 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 Tntern~tional Publication No. WO95/34081 (Raychem Corporation).
Depending on the type of the polymeric components and the electrodes, it may be desirable to cure the first and second compositions directly in contact with the electrodes.
~ltern~tively, it is possible to cure the compositions partially or completely before cchin~ the electrodes to the cured compositions. The latter technique is especially ~I ~r~liate for use with mesh or other f<~r~minl-us electrode mzlteris~l~ In order to control the thickness of the first and second resistive elements, the uncured composition may be poured or otherwise positioned within a mold of specified thickness, and then cured. For some applications, i~ v~d electrical stability for the device 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 m~n~tic. Electrodes of this type include nickel, nickel-coated copper, and stainless steel. It is ~l~f~ ed 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 International Application No. PCT/US96/09103 (Raychem Corporation~.
CA 02239990 1998-06-0~
W O 97/2123~ PCT~S96/19319 11 , The first and second polymeric components may be cured by any suitable means, including heat, light, microwave, electron bearn, or gamma irradiation, and are often ~ cured by using a combination of time and temperature suitable to subst~nti~lly cure the polymeric components. The curing tt;~ cl~Lu~e Tc may be at any temperature that allows substantial curing of the polymeric colllpullelll~ i.e. that cures the polymeric component 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 Tc 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 minimi7~ disruption of the aligned particles when exposed to a high energy condition. In addition, it is pl~r~lled 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 scintill~ting vial C(~t~ lg 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 mrn/second to a penetration ~ t~n~e of 4.0 rnm. 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 (I)ittmer et al).
The device of the invention is nonro~ ctive, i.e. has an insulation rcsi~t~nce at 25~C of more than 1 o6 ohms, preferably more than 1 o8 ohms, particularly more than 109 ohms, especially more than 101~ ohms. The r~ci~t~nce ofthe second resistive element at 30 25~C, if measured on its own, not in contact with the first resistive element, is at most 1000 ohms, preferably at most 100 ohms, particularly at most 10 ohms, especially at most 1 ohm.
Electrical devices of the invention, when tested according to the Standard Impulse 35 Breakdown Voltage Test, described below, preferably exhibit low breakdown voltage and m~int~in a high insulation resistance. Thus the breakdown voltage when tested at either 60A or 250A is at most 1000 volts, preferably at most 800 volts, particularly at most 700 CA 02239990 1998-06-0~
WO 97/21~30 PCT~US96/19319 volts, especially at most 600 volts, more especially at most 500 volts, e.g. 200 to 500 volts, and the final insulation re~i~t~nce is at least lOg 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 varies from the average breakdown S 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 fi~al breakdown voltage Vsf which is from 0.7ovsi to 1.30VSi, preferably from 0.80VSi to 1.20VSi, particularly from 0.85VSi to 1.15VSi, especially from O.90Vsi to l.10VSi.
The first resistive element acts as a "switch" due to its non-linear nature, andcontrols the breakdown voltage of the device. However, if exposed to a very high energy pulse, e.g. a 10 x 1000 microsecond current waveform and a peak current of 300A, a small region in the first resistive element will short out if not in contact with the second resistive element. The second resistive element acts as a "point-plane" electrode. Each of the domains, generally in the form of columns, behaves as a microfuse which can be destroyed by the breakdown event. As a result, even if an affected portion of the first resistive element shorts out, a corresponding domain in the second resistive element will be destroyed, and will disconnect the shorted section of the first resistive element from the circuit. The device thus returns to a nonconductive state after the breakdown event.
In addition, the electric field is concentrated at the tip of each domain or column, thus increasing the repeatability of the breakdown voltage on successive electrical events.
The invention is illustrated by the drawing in which Figure I shows in cross-section electrical device 1. First electrode 3 is in contact with first resistive element 7, while second electrode 5 is in contact with second resistive element 13. First resistive element 7 is made of first polymeric component 9 which acts as a matrix in which is dispersed first particulate ~lller 11. Second resistive element 13 is made of second polymeric component 15 through which is dispersed in discrete domains aligned chains 17. Each chain 17 contains particles of second particulate filler 19.
The invention is illustrated by the following examples, each of which was testedusing the Standard lmpulse Breakdown Test.
CA 02239990 1998-06-0~
W O97/21230 PCTnJS96/19319 Stslnl1~rd Pevice Both the first composition and the second composition were prepared by mixing the clesi~n~ted co~ ollents with a tongue depressor or mechanical stirrer to wet and disperse the particulate filler. Each composition was deg~se~l in a vacuum oven for one minute. The second composition was 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 ~U~pOl l~d 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 10 pounds (available from McMaster-Carr) were positioned over the metal sheets, sandwiching the composition. The second composition was then cured at 100~C for 15 minute~. The top magnet, the top metal sheet, and the top release sheet were removed, additional spacers were added to give a thickness of 1.5 mm, and the first composition was poured onto the surface of the cured second composition.
The top release sheet and the top metal sheet were replaced and a weight (which may be the top magnet) was placed on top of the top metal sheet. The arrangement was then cured at 100~C for an additional 15 minutes to give a l~min~te of the first and second compositions. A disc 20 (as shown in Figure 2) with a diameter of 15.9 mm and a thickness of 1.5 mm was cut from the cured l~min~t~ The disc 20 consisted of a second resistive element 21 with a thickness of 1.0 mm from the cured second composition and a first resistive element 22 with a thickness of 0.5 mm from the first composition.
Molybdenum electrodes 23, 25 having a diameter of 15.9 mm and a thickness of 0.25 mm (0.010 inch) were attached to the top and bottom surfaces of disc 20 to form a standard device 27.
St~n~l~rd Tmrulse Rreakdown Test A standard device 27 was inserted into the test fixture 29 shown in Figure 2. Two copper cylinders 31,33, ~ loxilllately 19 mm (0.75 inch) in diameter, were mounted in a polycarbonate holder 35 such that the end faces 37,39 were parallel. One end 37 was fixed and immobile; the other end 39 was free to travel while still m~ g the parallel end-face geometry. Movement of cylinder 33 was controlled by barrel micrometer 41 mounted through mounting ring 43. Device 27 was mounted between cylinders 31,33,and micrometer 41 was adjusted until contact with zero compressive pressure was made to both sides of device 27. Pressure was then applied to device 27 by further moving cylinder 33 (via micrometer 41) to cOlll~ ,S the sample 10% (generally 0.1 to 0.3 mm).
Electrical leads 45,47 were conn~cte~l from copper cylinders 31,33 to the testing CA 02239990 1998-06-0~
W O 97/2123~ PCT~US96/19319 equipment (not shown). Prior to testing, the insulation resi~t~n-~e R; for the device was measured at 25~C with a biasing voltage of 50 volts using a Genrad 1864 Megaohm meter; the initial resistivity Pi was calcnl~t~l Electrical connection was then made to a Keytek ECAT Series 100 Surge Generator using an E514A IQxlO00 waveform generator.
For each cycle a high energy impulse with a 10 x 1000 ~LS current wavefonn (i.e. a rise time to maximum current of 10 ~ls and a half-height at 1000 ~LS) and a peak current of 60A was applied. 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 or one hundred cycles for the standard test was measured and the final resistivity pf was calculated.
Fx~mples 1 to 15 The first and second resistive elements for Examples 1 to 15 were pl~alcd from compositions using the fnrm~ tions shown in Table I. In each case the silicone gel was formulated using 49.420% 1000 cs divinyl-t~rmin~tecl polydimethylsiloxane (available from United Chemical Technology (UCT)), 49.956% 50 cs silicone oil (polydirnethylsiloxane fluid from UCT), 0.580% tetrakis(dimethyl siloxy silane) (UCT), 0.04% catalyst, and 0.004% inhibitor, all arnounts by weight of the composition. The stoichiometry was adjusted for peak hardness, i.e. 600 grams using a Voland texture analyzer with a 7 mm st~inle~ steel probe. The al.l...i~ ,. was a powder with an average particle size of 15 to 20 microns ( 200 mesh) and a sl-hs1~nti~11y spherical shape, available from Aldrich Chemicals. The nickel, available from Alfa Aesar, had a mesh size of -300 mesh and an average particle size of 3 to 10 microns. The arc SU~plcS:iillg agents, i.e. m~gnç~ium phosphate (Mg3(PO4)2-8H20), zinc phosphate (Zn3(PO4)2 2H20), calciurn phosphate (CaHPO4 2~20), iron oxalate (FeC204 2H20), and zinc borate (3ZnO 2B203), the oxidizing agents, i.e. bismuth subnitrate (4BiNO3(0H~2 BiO(OH)) and lead peroxide (PbO2), and the surge initiators, i.e. calcium carbonate (CaCO3, decomposition temperature 898~C), m~n~slnese oxalate (MnC204 2H20, decompositiontemperature 100~C), and iron oxalate (which also acts as an arc ~u~lessillg agent, decomposition temp~,LdLulc 1 90~C), were available from Alfa Aesar. Standard devices were prepared as above and tested using the Standard Impulse Breakdown Test for either 50 or 100 cycles, as indicated. (Testing for Example 11 was done at lOOA rather than 60A.) In each case, except for colllp~livc Examples 5 and 7, the devices had Ri greater than 109 ohms. For Examples 5 and 7 the value of R; was greater than 1 o8 ohms. The CA 02239990 1998-06-0~
WO 97nl230 PCT~US96/19319 average breakdown voltage over the total number of test cycles and the standard deviation (i.e. a measure of the reproducibility of the breakdown voltage) are shown in Table I.
Examples 1 to 4, which contained an arc ~u~pres~ing agent, showed good low breakdown voltage (i.e. less than 1000 volts, and, for Examples 2 to 4, less than 400 volts), and good reproducibility. Each had an Rf value of greater than 1 o8 ohms. The test results for Example 2 are shown in Figure 3.
Examples S to 1 1 show the effects of the presence of both an arc suppressing agent and an oxidizing agent. Examples 5 and 7, which contained bismuth subnitrate in both the first and second resistive elements had an Rf value of 1 x 107. When bismuthsubnitrate, which becomes conductive when exposed to moisture, was used in the second resistive element only (Example 11), the device had an Rf value of greater than 108 ohms, and excellent reproducibility. Examples 12 to l S show the effects of the presence of a l S surge initiator. Examples 14 and l S, which contained a surge initiator which had a low decomposition temperature, had low breakdown voltages and good reproducibility. Each of Examples 12 to l S had an Rf value of greater than 108 ohms. The test results for Examples 4, 9, 10, and 1 1 are shown in Figure 4. The test results for Examples 12 to l S
are shown in Figures Sa to Sd, respectively. In each of Figures Sa to Sd results are shown for three dirrelt;." samples of each type of device. The values reported in Table I are averages of the three samples for each example.
Monolayer devices which contained only a first resistive element made from a composition co..'.~il.i.~g alllmimlm powder dispersed in a silicone, shown, for example in Intern~tional Publication No. WO9S/33278, the disclosure of which is incorporated herein by reference, had a breakdown voltage of more than 1000 volts when tested using a 10 x 1000 microsecond waveforrn and a current of at most lA. They did not survive fifty cycles when tested at 60A.
Fx~mple 16 Following the procedure of Examples 1 to 15, a first composition was l,l~ed coll~ illg 30% al---l,;ol~ 200 mesh), 10% zinc borate, 10% potassium pçnl~ng~n~te~
and 50% silicone gel (as in Example 1), and a second composition was prepared cont~ining 11.25% nickel with a mesh size of -100 to +200 (available from Alfa Aesar, with an average particle size of about 100 microns), 3.75% nickel with a mesh size of -300~ 20% zinc borate, 10% potassium perm~ngs3n~te, and 55% silicone gel (as in Example 1), all percentages by volume of each total composition. A Standard Device was prepared and tested 50 cycles at 60A with a 10 x 1000 microsecond waveform. The average breakdown voltage was 318 volts, with a standard deviation of 27. Both Rj and Rf were I
x 101 ~ ohms. The test results are shown in Figure 6.
S
Fx~n~le 17 A device was prepared as in Exarnple 16 and tested 50 cycles at 220A with a 10 x1000 microsecond waveform. The average breakdown voltage was 365 volts, with a 10 standard deviation of 32. Both R; and Rf were 1 x 1 ol I ohms. The test results are shown in Figure 6.
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Claims (10)
1. An electrical device which comprises (A) a first resistive element which is composed of a first electrically non-linear composition which (i) has a resistivity at 25°C of more than 10 9 ohm-cm and (ii) comprises (1) a first polymeric component, and (2) a first particulate filler dispersed in the first polymeric component;
(B) a second resistive element which (i) is in electrical contact with the first element, and (ii) is composed of a second composition which has a resistivity of less than 100 ohm-cm and which comprises (1) a second polymeric component, and (2) a second particulate filler which (a) is magnetic and electrically conductive, and (b) is aligned in discrete regions in the second polymeric component; and (C) first and second electrodes which are positioned so that current can flow between the electrodes through the first element and the second element.
(B) a second resistive element which (i) is in electrical contact with the first element, and (ii) is composed of a second composition which has a resistivity of less than 100 ohm-cm and which comprises (1) a second polymeric component, and (2) a second particulate filler which (a) is magnetic and electrically conductive, and (b) is aligned in discrete regions in the second polymeric component; and (C) first and second electrodes which are positioned so that current can flow between the electrodes through the first element and the second element.
2. A device according to claim 1 wherein the second resistive element is in physical contact with the first resistive element.
3. A device according to claim 1 wherein at least one of the first component and the second component comprises a curable polymer, preferably a curable polymer which has a viscosity of less than 200,000 cps when uncured.
4. A device according to claim 3 wherein the curable polymer comprises a gel, preferably a thermosetting gel or a thermoplastic gel.
5. A device according to claim 3 wherein the curable polymer comprises a thermosetting resin, preferably a silicone elastomer, an acrylate, an epoxy, or a polyurethane.
6. A device according to any one of the preceding claims wherein the first filler comprises a conductive filler or a semiconductive filler, and is selected from the group consisting of metal powders, metal oxide powders, metal carbide powders, metal nitride powders, and metal boride powders, preferably a filler which comprises alluminum, nickel, silver, silver-coated nickel, platinum, copper, tantalum, tungsten, iron oxide, doped iron oxide, doped zinc oxide, silicon carbide, titanium carbide, tantalum carbide, glass spheres coated with a conductive material, or ceramic spheres coated with a conductive material.
7. A device according to any one of the preceding claims wherein the first filler comprises 1 to 70% by volume of the first composition, and the second filler comprises 0.01 to 50% by volume of the second composition.
8. A device according to any one of the preceding claims wherein the second filler comprises nickel, iron, cobalt, ferric oxide, silver-coated nickel, silver-coated ferric oxide, or alloys of these materials.
9. A device according to any one of the preceding claims which comprises (i) twofirst resistive elements, positioned on opposite sides of the second resistive element, or (ii) two second resistive elements, positioned on opposite sides of the first resistive element.
10. A device according to any one of the preceding claims wherein (A) the first resistive element further comprises a third particulate filler dispersed in the first polymeric component which is an arc suppressant, an oxidizing agent, or a surge initiator; and (i) the first polymeric component is a gel, and (ii) the first particulate filler is a conductive filler or a semiconductive filler, preferably aluminum; and (B) the second resistive element (i) is in physical and electrical contact with the first element, (ii) has a resistance at 25°C of at most 100 ohms, and (iii) further comprises a fourth particulate filler dispersed in the second polymeric component which is an arc suppressant, an oxidizing agent, or a surge initiator; and (a) the second polymeric component is a gel, and (b) the second particulate filler preferably comprises nickel;
said device having a breakdown voltage when measured at 60A in a Standard Impulse Breakdown Test of less than 1000 volts.
said device having a breakdown voltage when measured at 60A in a Standard Impulse Breakdown Test of less than 1000 volts.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US08/568,716 US5742223A (en) | 1995-12-07 | 1995-12-07 | Laminar non-linear device with magnetically aligned particles |
| US08/568,716 | 1995-12-07 |
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| Publication Number | Publication Date |
|---|---|
| CA2239990A1 true CA2239990A1 (en) | 1997-06-12 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002239990A Abandoned CA2239990A1 (en) | 1995-12-07 | 1996-12-05 | Electrical device |
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| Country | Link |
|---|---|
| US (1) | US5742223A (en) |
| EP (1) | EP0865654A1 (en) |
| JP (1) | JP2000501884A (en) |
| AR (1) | AR004845A1 (en) |
| AU (1) | AU1279297A (en) |
| CA (1) | CA2239990A1 (en) |
| TW (1) | TW348255B (en) |
| WO (1) | WO1997021230A1 (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| ATE233014T1 (en) * | 1994-07-14 | 2003-03-15 | Surgx Corp | PROTECTIVE STRUCTURES AGAINST VARIABLE VOLTAGE AND METHOD FOR PRODUCING |
| US6537498B1 (en) | 1995-03-27 | 2003-03-25 | California Institute Of Technology | Colloidal particles used in sensing arrays |
| US5571401A (en) * | 1995-03-27 | 1996-11-05 | California Institute Of Technology | Sensor arrays for detecting analytes in fluids |
| US6232866B1 (en) | 1995-09-20 | 2001-05-15 | The United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration | Composite material switches |
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| US5090917A (en) | 1991-05-10 | 1992-02-25 | Thomas & Betts Corporation | Insulation displacing connector for providing repeatable sealed termination of electrical conductors |
| US5069637A (en) | 1991-06-14 | 1991-12-03 | Jacobson Mfg. Co., Inc. | Insulation displacing electrical connector |
| US5139440A (en) | 1991-06-26 | 1992-08-18 | Reliance Comm/Tec Corporation | Environmentally sealed insulation displacement connector terminal block |
| US5557250A (en) | 1991-10-11 | 1996-09-17 | Raychem Corporation | Telecommunications terminal block |
| US5250228A (en) | 1991-11-06 | 1993-10-05 | Raychem Corporation | Conductive polymer composition |
| DE4142523A1 (en) | 1991-12-21 | 1993-06-24 | Asea Brown Boveri | RESISTANCE WITH PTC BEHAVIOR |
| US5309313A (en) | 1992-02-05 | 1994-05-03 | Amerace Corporation | Fault gas seal for a polymer surge arrester |
| US5294374A (en) | 1992-03-20 | 1994-03-15 | Leviton Manufacturing Co., Inc. | Electrical overstress materials and method of manufacture |
| US5378407A (en) | 1992-06-05 | 1995-01-03 | Raychem Corporation | Conductive polymer composition |
| DE4221309A1 (en) | 1992-06-29 | 1994-01-05 | Abb Research Ltd | Current limiting element |
| DE69324896T2 (en) | 1992-11-24 | 1999-12-02 | Tdk Corp | Chip varistor and method for its manufacture |
| US5340641A (en) | 1993-02-01 | 1994-08-23 | Antai Xu | Electrical overstress pulse protection |
| US5451919A (en) | 1993-06-29 | 1995-09-19 | Raychem Corporation | Electrical device comprising a conductive polymer composition |
-
1995
- 1995-12-07 US US08/568,716 patent/US5742223A/en not_active Expired - Fee Related
-
1996
- 1996-12-03 TW TW085114913A patent/TW348255B/en active
- 1996-12-03 AR ARP960105471A patent/AR004845A1/en unknown
- 1996-12-05 AU AU12792/97A patent/AU1279297A/en not_active Abandoned
- 1996-12-05 CA CA002239990A patent/CA2239990A1/en not_active Abandoned
- 1996-12-05 EP EP96943586A patent/EP0865654A1/en not_active Withdrawn
- 1996-12-05 JP JP9521401A patent/JP2000501884A/en active Pending
- 1996-12-05 WO PCT/US1996/019319 patent/WO1997021230A1/en not_active Application Discontinuation
Also Published As
| Publication number | Publication date |
|---|---|
| US5742223A (en) | 1998-04-21 |
| JP2000501884A (en) | 2000-02-15 |
| EP0865654A1 (en) | 1998-09-23 |
| WO1997021230A1 (en) | 1997-06-12 |
| TW348255B (en) | 1998-12-21 |
| AU1279297A (en) | 1997-06-27 |
| AR004845A1 (en) | 1999-03-10 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| FZDE | Discontinued |