PPTC material with a low percolation threshold for a conductive filler and components

Abstract

Component (100, 400, 500, 600) with a polymer with a positive temperature coefficient (PPTC) comprising: a PPTC body (104, 402, 502), a first electrode (102) arranged on a first side of the PPTC body and a second electrode (106) arranged on a second side of the PPTC body (104), wherein the PPTC body (104) is formed from a PPTC material containing a polymer matrix and a conductive filler material, wherein the PPTC material has a volume fraction and a median diameter of the conductive particles such that the PPTC material has a resistivity of about 0.15 ohm-cm; where the volume fraction and the median diameter of the conductive particles are selected from the group consisting of: 27% volume fraction of conductive particles with a median diameter of 0.55 micrometers; 37% volume fraction of conductive particles with a median diameter of 1.0 micrometer; 39% volume fraction of conductive particles with a median diameter of 1.57 micrometers; 41.2% volume fraction of conductive particles with a median diameter of 2.15 micrometers; 42.5% volume fraction of conductive particles with a median diameter of 3.21 micrometers; 45.5% volume fraction of conductive particles with a median diameter of 4.82 micrometers, and wherein the conductive filler material comprises tungsten carbide.

Description

FIELD OF DISCLOSUREEmbodiments relate to the field of circuit protection devices including fuse elements.BACKGROUND OF THE DISCLOSUREPositive temperature coefficient polymer (PPTC) devices can be used as over-current or over-temperature protection devices as well as current or temperature sensors, among various applications. In over-current or over-temperature protection applications, a PPTC device may be considered a resettable fuse configured to have low resistance when operated under predetermined conditions such as low current. The resistance of the PPTC device may be changed by direct heating due to temperature increases in the vicinity of the PPTC device or by resistive heating generated with a current flowing through the PPTC device. For example, a PPTC device may include a composite PPTC material formed from a polymeric material and a conductive filler material, wherein the PPTC material transitions from a low resistance state to a high resistance state due to thermally induced changes in the polymeric material such as a melting transition or a glass transition. At a transition temperature, sometimes called "trigger temperature", which trigger temperature may range from room temperature to far above room temperature, the polymer material may expand and disrupt the electrically conductive network of conductive filler particles in the PPTC material, making the PPTC material much less electrically conductive. This change in resistance, which resistance may be reversible as the PPTC material cools back to room temperature, gives PPTC materials a fuse-like character.The cost and mass of a PPTC material is generally determined by the amount (e.g., percent by volume) of conductive filler material in the PPTC material. In almost all applications, it is desirable to minimize the cost and mass of PPTC devices while maintaining the desired operating characteristics, such as trip temperature. With these and other considerations in mind, the present disclosure is provided.EP 0 038 717 A2 discloses electrical devices comprising two flat electrodes and a conductive PTC polymer element.DE 694 16 128 T2 discloses circuit protection devices with conductive polymer compositions.CN 2 470 923 Y shows a variable heat sensitive electrical device.CN 1 05 139 984 A shows a PTC protection element capable of maintaining a very high current.The object set is achieved according to the invention by the characterizing features of claim 1. Further embodiments of the invention are set forth in the dependent claims and subordinate claims.SummaryThis Summary is provided to introduce in simplified form a selection of concepts that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended as an aid in determining the scope of the claimed subject matter.A PPTC device according to an exemplary embodiment of the present disclosure may include a PPTC body, a first electrode disposed on a first side of the PPTC body, and a second electrode disposed on a second side of the PPTC body, wherein the PPTC body is formed from a PPTC material including a polymer matrix and a conductive filler material, wherein the conductive filler material defines 20-39% of the volume of the PPTC material.Another PPTC device according to an exemplary embodiment of the present disclosure may include a PPTC body, a first and second metallic foil layer respectively disposed on opposite sides of the PPTC body and extended from a first and second metallic trace on opposite ends of the PPTC body, wherein the first metallic foil layer is extended to but does not contact the second metallic trace, and wherein the second metallic foil layer is extended to but does not contact the first metallic trace. The PPTC device may further include electrically insulating insulating insulating layers covering the first and second metal foil layers and metallic electrodes disposed on the insulating layers in electrical contact with the metallic traces. The PPTC body may be formed from a PPTC material containing a polymer matrix and a conductive filler material, wherein the conductive filler material defines 20-39% of the volume of the PPTC material.A PPTC material according to an exemplary embodiment of the present disclosure may include a polymer matrix and a conductive filler material, wherein the conductive filler material defines 20-39% of the volume of the PPTC material and is formed from particles having a median diameter of 50 nanometers to 20 micrometers.Brief Description of the DrawingsFIGS. 1A and 1B illustrate a PPTC device according to embodiments of the present disclosure, FIGS. 2A and 2B illustrate the effect of varying the size of conductive particles in PPTC materials on the resistivity and percolation threshold of such materials, FIG. 3 illustrates an example resistivity for a PPTC material according to embodiments of the present disclosure, FIG. 4 illustrates a PPTC device according to an embodiment of the present disclosure; and FIGS. 5A and 5B illustrate PPTC devices according to various further embodiments of the present disclosure.Detailed DescriptionThe present embodiments will now be described in more detail hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. The embodiments are not to be construed as limited to the embodiments set forth herein. These embodiments are provided rather than certain exemplary aspects of the present disclosure for those skilled in the art. In the drawings, like reference numerals designate like elements throughout.In the following description and / or claims, the terms "on," "overlying," "disposed on," and "over" may be used in the following description and claims. "on," "overlying," "disposed on," and "over" may be used to indicate that two or more elements are in direct physical contact with each other. Likewise, the terms "on", "overlying", "disposed on", and "over" may mean that two or more elements are not in direct contact with each other. For example, "over" may mean that one element is over another element while not contacting each other and may have another element or elements between the two elements. Further, the term "and / or" may mean "and", it may mean "or", it may mean "exclusive or", it may mean "a", it may mean "some but not all", it may mean "neither (of both)", and / or it may mean "both", although the scope of the claimed subject matter is not limited in this respect.In various embodiments, novel device structures and materials are provided for forming a PPTC device where the PPTC device includes a PPTC material having a relatively low "percolation threshold", where "percolation threshold" is defined as a minimum percentage of a conductive ceramic fill material in the PPTC material of the volume necessary to achieve a desired resistivity. In one example, a PPTC material according to the present disclosure may have a resistivity of about 0.15 ohmcm with a percolation threshold in a range of 20-39%.In various embodiments, a PPTC device may be constructed as shown in FIGS. 1A and 1B. FIG. 1A illustrates a side cross-sectional view of a PPTC device 100 where a PPTC body 104 is disposed between a first electrode 102 and a second electrode 106 disposed on a first side and a second side of the PPTC body, respectively. FIG. 1B illustrates a configuration of the PPTC device 100 after a first terminal 108 is connected to the first electrode 102 and a second terminal 110 is connected to the second electrode 106. The first terminal 108 may be connected to the first electrode 102 using any suitable electrically conductive means of attachment (e.g., by soldering, welding, a conductive epoxy, etc.) to form a first interface 112, and the second terminal 110 may be similarly connected to the second electrode 106 to form a second interface 114.According to embodiments of the present disclosure, the PPTC body 104 may be formed from a PPTC material having a relatively low percolation threshold, as will be discussed in more detail below. The first electrode 102 and the second electrode 106 may be formed of various metals including, but not limited to, copper foil. In some embodiments, the copper foil may be nickel plated. The first terminal 108 and the second terminal 110 may also be formed of various materials including, but not limited to, copper or brass. The embodiments are not limited in this context.In some embodiments of the present disclosure, the PPTC body 104 may be formed from a PPTC material composite that includes a polymer matrix and a conductive filler material. The polymer matrix may be or contain a semicrystalline polymer such as a polyvinylidene fluoride (PVDF) polymer, an ethylene vinyl acetate (EVA) polymer, a high density polyethylene [high density polyethylene] (HDPE) polymer, an ethylene tetrafluoroethylene (ETFE) polymer or a perfluoroalkoxy (PFA) polymer. The embodiments are not limited in this context.According to some embodiments of the present disclosure, the conductive filler material of the PPTC material may be formed from particles of an electrically conductive ceramic material including, but not limited to, titanium carbide, tungsten carbide, vanadium carbide, zirconium carbide, niobium carbide, tantalum carbide, molybdenum carbide, titanium boride, vanadium boride, zirconium boride, niobium boride, molybdenum boride or hafnium boride, or mixtures thereof.The percolation threshold of the PPTC material may be in a range from about 20% to about 39%. That is, the volume fraction of the conductive fill material in the PPTC material may range from about 20% to about 39%. Those skilled in the art will appreciate that a percolating threshold in the range indicated is less than the percolating thresholds of conventional PPTC materials, which generally have percolating thresholds above 40%. The relatively low percolation thresholds of the present disclosure are achieved by using relatively small particles of the conductive filler material in the PPTC material. For example, in various embodiments, the median diameter of the particles of conductive filler material in the PPTC material may be in a range from about 50 nanometers to 20 micrometers. It has been found that use of conductive particles of such relatively small size can achieve a given resistivity in a PPTC material using a smaller amount of bulk conductive filler material relative to particles of larger size traditionally used in conventional PPTC materials. The cost and mass of the PPTC material of the present disclosure may therefore be lower than those of traditional PPTC devices while achieving similar operating characteristics such as resistivity and trip temperature.Turning now to FIG. 2A, there is shown a graph of the resistivities of PPTC materials according to the present disclosure as a function of volume fractions of the conductive filler material (tungsten carbide in this example) of different particle sizes in such materials. As can be seen, a PPTC material having a volume fraction of about 27% conductive particles having a median diameter of 0.55 micrometers may have a resistivity of about 0.15 ohmcm, and a PPTC material having a volume fraction of about 41.2% conductive particles having a median diameter of 2.15 micrometers may also have a resistivity of about 0.15 ohmcm. FIG. 2B shows a bar graph illustrating percolation thresholds of various sizes of conductive filler particles (tungsten carbide in this example) necessary to achieve PPTC materials having the same resistivity. For example, a PPTC material with conductive particles having an average diameter of 1 micrometer will have a percolation threshold of about 37% to achieve the same resistivity as a PPTC material with conductive particles having an average diameter of 1.57 micrometers at a percolation threshold of about 39%. Thus, it can be seen that different PPTC materials containing different volume fractions of conductive filler material can be made by varying the sizes of conductive particles in such materials to have similar resistivities.Turning now to FIG. 3, a graph is shown that plots resistance as a function of temperature of a PPTC device constructed in accordance with embodiments of the disclosure. In this example, the PPTC material of the PPTC device has a percolation threshold of 35% of the conductive filler material (tungsten carbide in this example) with particles having a median diameter of 1.57 micrometers. As shown, at 160-165°C, an abrupt increase in resistance occurs. Accordingly, the PPTC material of FIG. 3 may be considered to have a release temperature of about 163° C.The holding current density of the PPTC materials of the present disclosure may be provided to have a value between 0.05 to 0.4 A / mm 2 by appropriately selecting a volume fraction of the conductive filler material and a type of conductive filler material, wherein the holding current density is calculated as a ratio of the holding current of a PPTC material at 25° C. to the area of the PPTC through which the current flows between opposing electrodes.The configuration of a PPTC device may vary according to various embodiments of the present disclosure. FIG. 4 shows a top view of a PPTC device 400, shown as a radial line PPTC device, including a bottom line 404 and a top line 406 attached to opposing surfaces of a PPTC body 402. The PPTC body 402 may include first and second electrodes (not separately shown) attached to the surface and the bottom surface thereof, respectively, as generally described above. The PPTC device 400 may be encapsulated by an encapsulation layer 410 such as an epoxy resin. The PPTC body 402 may be formed from a PPTC material generally formulated as described above with low percolation thresholds, such as in a range of 20-39%.FIGS. 5A and 5B illustrate side cross-sectional views of embodiments of a single-layer, face-mounted PPTC device 500 and a two-layer, face-mounted PPTC device 600, respectively, in accordance with example embodiments of the present disclosure. These devices may include PPTC bodies 502 and first and second metallic foil layers 504 a, 504 bdisposed on opposite sides of the PPTC bodies 502 and longitudinally extended from first and second metallic traces 506 a, 506 bat opposite longitudinal ends of the PPTC bodies 502, wherein the first metal foil layers 504 aare extended to but do not contact the second metallic traces 506 b, and wherein the second metal foil layers 504 bare extended to but do not contact the first metallic traces 506 a. The devices may include electrically insulating isolation layers 510 covering the metallic foil layers 504 a, 504 band metallic electrodes 512 disposed on the outermost isolation layers 510 in electrical contact with the metallic traces 506 a, 506 b. In these devices, the PPTC bodies 502 may be formed from a PPTC material generally formulated as described above, with a low percolation threshold, such as in a range of 20-39%.While the present embodiments have been disclosed with reference to specific embodiments, various modifications, changes and changes in the described embodiments are possible without departing from the scope and spirit of the present disclosure as defined in the appended claims. Accordingly, it is intended that the present embodiments not be limited by the described embodiments, but that they may have the full scope defined by the language of the following claims and equivalents thereof.

Claims

A positive temperature coefficient (PPTC) polymer device (100, 400, 500, 600) comprising: a PPTC body (104, 402, 502), a first electrode (102) disposed on a first side of the PPTC body, and a second electrode (106) disposed on a second side of the PPTC body (104), wherein the PPTC body (104) is formed from a PPTC material including a polymer matrix and a conductive filler material, wherein the PPTC material has a volume fraction and a median diameter of the conductive particles such that the PPTC material has a resistivity of about 0.15 ohm-cm; wherein the volume fraction and the median diameter of the conductive particles are selected from the group consisting of: 27% volume fraction of conductive particles having a median diameter of 0.55 micrometers; 37% volume fraction of conductive particles having a median diameter of 1.0 micrometers; 39% volume fraction of conductive particles having a median diameter of 1.57 micrometers; 41.2% volume fraction of conductive particles having a median diameter of 2.15 micrometers; 42.5% volume fraction of conductive particles having a median diameter of 3.21 micrometers; 45.5% volume fraction of conductive particles having a median diameter of 4.82 micrometers, and wherein the conductive filler material comprises tungsten carbide.The PPTC device (100, 400, 500, 600) of claim 1, wherein the PPTC material has a holding current density of between 0.05 to 0.4 A / mm 2.The PPTC device (100, 400, 500, 600) of claim 1, wherein the PPTC material includes at least one of a polyvinyl fluoride (PVDF) polymer, an ethylene vinyl acetate (EVA) polymer, a high density polyethylene (HDPE) polymer, an ethylene tetrafluoroethylene (ETFE) polymer, and a perfluoroalkoxy (PFA) polymer.The PPTC device (100, 400, 500, 600) according to claim 1, wherein the first electrode and / or the second electrode is / are formed from a copper foil.The PPTC device (100, 400, 500, 600) of claim 4, wherein the copper foil is nickel plated.A material comprising a positive temperature coefficient (PPTC) polymer comprising: a polymer matrix; and a conductive filler material, wherein the PPTC material comprises a volume fraction and a median diameter of the conductive particles such that the PPTC material has a resistivity of about 0.15 ohm-cm; wherein the volume fraction and the median diameter of the conductive particles are selected from the group consisting of: 27% volume fraction of conductive particles having a median diameter of 0.55 micrometers; 37% volume fraction of conductive particles having a median diameter of 1.0 micrometers; 39% volume fraction of conductive particles having a median diameter of 1.57 micrometers; 41.2% volume fraction of conductive particles having a median diameter of 2.15 micrometers; 42.5% volume fraction of conductive particles having a median diameter of 3.21 micrometers; 45.5% volume fraction of conductive particles having a median diameter of 4.82 micrometers, and wherein the conductive filler material comprises tungsten carbide.The PPTC material of claim 6, wherein the PPTC material has a holding current density of between 0.05 to 0.4 A / mm 2.The PPTC material of claim 6, wherein the polymer matrix includes at least one of a polyvinyl fluoride (PVDF) polymer, an ethylene vinyl acetate (EVA) polymer, a high density polyethylene (HDPE) polymer, an ethylene tetrafluoroethylene (ETFE) polymer, and a perfluoroalkoxy (PFA) polymer.A positive temperature coefficient (PPTC) polymer device (100, 400, 500, 600) comprising: a PPTC body (104, 402, 502), first and second metallic foil layers disposed on opposite sides of the PPTC body and extended from first and second metallic tracks at opposite ends of the PPTC body, respectively, wherein the first metallic foil layer is extended to but does not contact the second metallic track, and wherein the second metallic foil layer is extended to but does not contact the first metallic track, electrically insulating insulating insulating layers covering the first and second metallic foil layers, metallic electrodes disposed on the insulating layers in electrical contact with the metallic tracks, wherein the PPTC body is formed from a PPTC material, which contains a polymer matrix and a conductive filler material, wherein the PPTC material has a volume fraction and a median diameter of the conductive particles such that the PPTC material has a resistivity of about 0.15 ohm-cm; wherein the volume fraction and the median diameter of the conductive particles are selected from the group consisting of: 27% volume fraction of conductive particles having a median diameter of 0.55 micrometers; 37% volume fraction of conductive particles having a median diameter of 1.0 micrometers; 39% volume fraction of conductive particles having a median diameter of 1.57 micrometers; 41.2% volume fraction of conductive particles having a median diameter of 2.15 micrometers; 42.5% volume fraction of conductive particles having a median diameter of 3.21 micrometers; 45.5% volume fraction of conductive particles having a median diameter of 4.82 micrometers, wherein the conductive filler material comprises tungsten carbide.The PPTC device (100, 400, 500, 600) of claim 9, wherein the PPTC material has a holding current density of between 0.05 to 0.4 A / mm 2.The PPTC device (100, 400, 500, 600) of claim 9, wherein the PPTC material includes at least one of a polyvinyl fluoride (PVDF) polymer, an ethylene vinyl acetate (EVA) polymer, a high density polyethylene (HDPE) polymer, an ethylene tetrafluoroethylene (ETFE) polymer, and a perfluoroalkoxy (PFA) polymer.The PPTC device (100, 400, 500, 600) according to claim 9, wherein the first metallic foil layer and / or the second metallic foil layer is / is formed from a copper foil.The PPTC device (100, 400, 500, 600) of claim 12, wherein the copper layer is nickel plated.

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