Patent Publication Number: US-10777340-B2

Title: PPTC material with mixed conductive filler composition

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 62/557,310, filed Sep. 12, 2017, the entirety of which is incorporated by reference herein. 
    
    
     FIELD OF THE DISCLOSURE 
     Embodiments relate to the field of circuit protection devices, including fuse devices. 
     BACKGROUND OF THE DISCLOSURE 
     Polymer positive temperature coefficient (PPTC) devices may be used as overcurrent or over-temperature protection devices, as well as current or temperature sensors, among various applications. In overcurrent or over-temperature protection applications, a PPTC device may be considered a resettable fuse, designed to exhibit low resistance when operating under predetermined conditions, such as low current. The resistance of the PPTC device may be altered by direct heating due to temperature increases in the environment of the PPTC device, or via resistive heating generated by electrical current passing through the PPTC device. For example, a PPTC device may include a composite PPTC material formed of a polymer material and a conductive filler, wherein the PPTC material transitions from a low resistance state to a high resistance state due to thermally-induced changes in the polymer material, such as a melting transition or a glass transition. At a transition temperature, sometimes called a “trip temperature,” where the trip temperature may range from room temperature to well above room temperature, the polymer material may expand and disrupt the electrically conductive network of conductive filler particles in the PPTC material, rendering the PPTC material much less electrically conductive. This change in resistance imparts a fuse-like character to PPTC materials, which resistance may be reversible when the PPTC material cools back to room temperature. 
     The cost, weight, and performance characteristics of a PPTC material are largely influenced by the type and quantity of conductive filler in the PPTC material. It is with respect to these and other considerations that the present disclosure is provided. 
     SUMMARY 
     This Summary is provided to introduce a selection of concepts in a simplified form 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 in accordance with 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 of a PPTC material that includes a maximum of 65% by volume of a conductive filler, wherein 10%-39% by volume of the PPTC material is a conductive ceramic filler and wherein the rest of the conductive filler includes at least one of carbon and a metallic filler. 
     Another PPTC device in accordance with an exemplary embodiment of the present disclosure may include a PPTC body, first and second metallic foil layers disposed on opposing sides of the PPTC body, respectively, and extending from first and second metallic traces at opposing ends of the PPTC body, respectively, wherein the first metallic foil layer extends toward, but does not contact, the second metallic trace, and wherein the second metallic foil layer extends toward, but does not contact, the first metallic trace. The PPTC device may further include electrically insulating insulation layers covering the first and second metallic foil layers, and metallic electrodes disposed on the insulation layers in electrical contact with the metallic traces. The PPTC body may be formed of a PPTC material that includes a maximum of 65% by volume of a conductive filler, wherein 10%-39% by volume of the PPTC material is a conductive ceramic filler and wherein the rest of the conductive filler includes at least one of carbon and a metallic filler. 
     A PPTC material in accordance with an exemplary embodiment of the present disclosure may include a maximum of 65% by volume of a conductive filler, wherein 10%-39% by volume of the PPTC material is a conductive ceramic filler and wherein the rest of the conductive filler includes at least one of carbon and a metallic filler. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  and  FIG. 1B  illustrate a PPTC device according to embodiments of the present disclosure; 
         FIG. 2  illustrates a detailed cross-sectional view of the PPTC device shown in  FIG. 1A  showing a percolation network in the PPTC material of the PPTC device including two different types of conductive filler; 
         FIG. 3  illustrates exemplary resistance behavior for a PPTC material according to embodiments of the present disclosure; 
         FIG. 4  illustrates a PPTC device according to embodiments of the present disclosure; and 
         FIGS. 5A and 5B  illustrate PPTC devices according to various additional embodiments of the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The present embodiments will now be described more fully 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. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey their scope to those skilled in the art. In the drawings, like numbers refer to 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 one another. Also, the terms “on,” “overlying,” “disposed on,” and “over”, may mean that two or more elements are not in direct contact with one another. For example, “over” may mean that one element is above another element while not contacting one another and may have another element or elements in between the two elements. Furthermore, the term “and/or” may mean “and,” it may mean “or,” it may mean “exclusive-or,” it may mean “one,” it may mean “some, but not all,” it may mean “neither,” 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 that includes at least two different types of conductive fillers. In one example, a PPTC material in accordance with the present disclosure includes a maximum of 65% by volume of a conductive filler, wherein 10-39% by volume of the PPTC material is a conductive ceramic filler and wherein the rest of the conductive filler includes at least one of carbon and a metallic filler. 
     In various embodiments, a PPTC device may be constructed as shown in  FIG. 1A  and  FIG. 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  that are arranged on a first side and a second side of the PPTC body  104 , respectively.  FIG. 1B  illustrates a configuration of the PPTC device  100  after a first terminal  108  is joined to the first electrode  102  and a second terminal  110  is joined to the second electrode  106 . The first terminal  108  may be joined to the first electrode  102  (e.g., by soldering, welding, etc.) to form a first interface  112 , and the second terminal  110  may be joined to second electrode  106  to form a second interface  114 . 
     According to embodiments of the present disclosure, the PPTC body  104  may be formed from of a PPTC material having a relatively low percolation threshold as further detailed below. The first electrode  102  and the second electrode  106  may be formed of known metals, such as a 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 known materials, such as 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 of a composite PPTC material that includes a polymer matrix and a conductive filler. The polymer matrix may be, or may include, a semi-crystalline polymer such as a polyvinylidene fluoride (PVDF) polymer, an ethylene vinyl acetate (EVA) polymer, a 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 of the PPTC material defines a maximum of 65% by volume of the PPTC material, wherein 10%-39% by volume of the PPTC includes one or more conductive ceramic materials. Such materials may include, but are 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, hafnium boride, or mixtures thereof. The remainder of the conductive filler (i.e., 0-55% by volume of the PPTC material) may include at least one of carbon and a metallic filler, wherein exemplary metallic fillers include, but are not limited to, nickel, tungsten, copper, and copper alloy. 
     An exemplary PPTC material in accordance with the present disclosure may include 39% conductive ceramic filler by volume and 10% metallic filler by volume. Another exemplary PPTC material in accordance with the present disclosure may include 10% conductive ceramic filler by volume and 39% metallic filler by volume. Another exemplary PPTC material in accordance with the present disclosure may include 10% conductive ceramic filler by volume, 25% carbon by volume, and 25% metallic filler by volume. 
     In various embodiments, the median diameter of the particles of conductive filler in the PPTC material may be in a range of about 50 nanometers to 20 micrometers. It has been found that using conductive ceramic particles of such relatively small size can achieve a given resistivity in a PPTC material using a smaller quantity of conductive filler by volume relative to particles of larger size that are traditionally used in conventional PPTC materials. The cost and weight of the PPTC material of the present disclosure may therefore be lower than those of traditional PPTC devices while achieving similar operational characteristics such as resistivity and trip temperature. 
     Turning now to  FIG. 2 , there is shown a detailed cross-sectional view of the PPTC device  100  shown in  FIG. 1A  illustrating particles of the conductive filler of the PPTC material, which may include a conductive ceramic filler and at least one additional conductive filler, which may be carbon or a metallic filler. 
     Turning now to  FIG. 3  there is shown a graph plotting the resistance behavior as a function of temperature of a PPTC device, arranged according to embodiments of the disclosure. In this example, the conductive filler of the PPTC material in the PPTC device includes 18% by volume tungsten carbide and 35% by volume carbon black, wherein the polymer of the PPTC material is ETFE. As shown, an abrupt increase in resistance takes place at 210-220° C. Accordingly, the PPTC material of  FIG. 3  may be deemed to exhibit a trip temperature of about 215° C. 
     The hold current density of the PPTC materials of the present disclosure may be designed to exhibit a value between 0.05 to 0.4 A/mm 2  by appropriate choice of volume fraction of conductive filler and type of conductive filler, where hold current density is calculated as a ratio of the hold current of a PPTC material at 25° C. to the area of the PPTC through which current travels between opposing electrodes. 
     The configuration of a PPTC device may vary according to different embodiments of the present disclosure.  FIG. 4  presents a top plan view of a PPTC device  400 , shown as radial lead PPTC device, including a bottom lead  404  and a top lead  406 , attached to opposite surfaces of a PPTC body  402 . The PPTC body  402  may have first and second electrodes (not separately shown) attached to the top surface and bottom surface thereof, respectively, as generally described above. The PPTC device  400  may be encapsulated by an encapsulant layer  410 , such as an epoxy. The PPTC body  402  may be formed of a PPTC material formulated generally as described above. 
       FIG. 5A  and  FIG. 5B  depict side cross-sectional views of embodiments of a single-layer surface mount PPTC device  500  and a double-layer surface mount PPTC device  600 , respectively, according to exemplary embodiments of the present disclosure. These devices may include PPTC bodies  502 , and first and second metallic foil layers  504   a ,  504   b  disposed on opposing sides of the PPTC bodies  502  and extending longitudinally from first and second metallic traces  506   a ,  506   b  at opposing longitudinal ends of the PPTC bodies  502 , wherein the first metallic foil layers  504  extend toward, but does not contact, the second metallic traces  506   b , and wherein the second metallic foil layers  504   b  extend toward, but does not contact, the first metallic traces  506   a . The devices may further include electrically insulating insulation layers  510  covering the metallic foil layers  504   a ,  504   b , and metallic electrodes  512  disposed on the outermost insulation layers  510  in electrical contact with the metallic traces  506   a ,  506   b . In these devices, the PPTC bodies may be formed of a PPTC material formulated generally as described above. 
     While the present embodiments have been disclosed with reference to certain embodiments, numerous modifications, alterations and changes to the described embodiments are possible while not departing from the sphere and scope of the present disclosure, as defined in the appended claims. Accordingly, the present embodiments are not to be limited to the described embodiments, and may have the full scope defined by the language of the following claims, and equivalents thereof.