Abstract:
A flexible sheet of positive temperature coefficient (PTC) material formed of a polymer resin and a conductive filler, the sheet of PTC material having a thickness in a range of 10 μm to 100 μm. A method for forming the flexible sheet of positive temperature coefficient material may include preparing a PTC ink from a polymer resin, a conductive filler, and a solvent, applying the PTC ink to a substrate, pulling a blade over the PTC ink to create a uniformly thick layer of the PTC ink on the substrate, and allowing the PTC ink to dry so that the solvent evaporates and leaves a solid layer of PTC material on the substrate.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 62/358,952, filed Jul. 6, 2016, which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
     Field 
       [0002]    The present invention relates generally to positive temperature coefficient (PTC) materials. More specifically, the present invention relates to an ultrathin, flexible sheet formed PTC material. 
       Description of Related Art 
       [0003]    PTC devices are typically used in electronic devices to provide protection against overcurrent and/or overtemperature conditions. PTC material in such devices is selected to have a relatively low resistance within a normal operating temperature range of the electronic, and a high resistance above the normal operating temperature of the electronic device. For example, a PTC device may be connected in electrical series between a battery and a load so that current flowing from the battery to the load flows through the PTC device. The temperature of the PTC device gradually increases as current flowing through the PTC device increases. When the temperature of the PTC device reaches an “activation temperature,” the resistance of the PTC device increases sharply. This in-turn sharply reduces the current flowing through the PTC device, thereby protecting the battery and the load from an overcurrent or overtemperature condition. 
         [0004]    Existing PTC devices normally include a core material having PTC characteristics surrounded by a package. Conductive pads or conductive leads may be provided on the outside of the package and may be electrically coupled to opposite surfaces of the core material so that current flows through a cross-section of the core material. 
         [0005]    Existing PTC materials and devices typically have a thickness of about 200 μm or more which is constrained by conventional manufacturing methods. At such thicknesses, typical PTC materials and devices are too rigid for use in conjunction with applications that may benefit from a PTC material or device having flexible and malleable properties. Such applications include overcurrent and overtemperature protection for batteries in cellular telephones and wearable electronic devices, for example. 
         [0006]    It is with respect to these and other considerations that the present improvements may be useful. 
       SUMMARY 
       [0007]    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. 
         [0008]    An exemplary embodiment of a flexible sheet of positive temperature coefficient material (PTC) in accordance with the present disclosure may include a layer of PTC material formed of a polymer resin and a conductive filler, the layer of PTC material having a thickness in a range of 10 μm to 100 μm. 
         [0009]    An exemplary embodiment of a method for forming a flexible sheet of PTC material in accordance with the present disclosure may include preparing a PTC ink from a polymer resin, a conductive filler, and a solvent, applying the PTC ink to a substrate, and allowing the PTC ink to dry so that the solvent evaporates and leaves a solid layer of PTC material on the substrate. 
         [0010]    An exemplary embodiment of an electronic device in accordance with the present disclosure may include a protected component and a flexible positive temperature coefficient (PTC) device including a flexible sheet of PTC material coupled to an exterior surface of the protected component, the flexible PTC device electrically connected to the protected component and adapted to arrest or mitigate electrical current flowing through the protected component. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1A  is a top view illustrating an exemplary embodiment of a flexible sheet of positive temperature coefficient (PTC) material in accordance with the present disclosure; 
           [0012]      FIG. 1B  is a top view illustrating another exemplary embodiment of a flexible sheet of PTC material in accordance with the present disclosure; 
           [0013]      FIG. 2  is a flow diagram illustrating an exemplary method for forming a flexible sheet of PTC material in accordance with the present disclosure; 
           [0014]      FIG. 3  is a cross sectional view illustrating an exemplary embodiment of a PTC device in accordance with the present disclosure; 
           [0015]      FIG. 4  is a schematic view illustrating an exemplary embodiment of an electronic device in accordance with the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Exemplary embodiments of a flexible sheet of positive temperature coefficient (PTC) material, a device incorporating the flexible sheet, and methods for making the flexible sheet in accordance with the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings. The flexible sheet, device, and methods may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will convey certain exemplary aspects of the flexible sheet, device, and methods to those skilled in the art. 
         [0017]    Referring to  FIG. 1   a,  a top view illustrating a flexible sheet  102  formed of a layer of PTC material in accordance with an exemplary embodiment of the present disclosure is shown. The flexible sheet  102  may have a thickness that is significantly less than that of conventional sheets of PTC material, thus providing the flexible sheet  102  with a flexibility and a malleability that allow the flexible sheet  102  to be wrapped about, and to conform to, the surfaces of other structures and devices (e.g. batteries) while preserving the PTC characteristics of the flexible sheet  102 . In a non-limiting example, the flexible sheet  102  may have a thickness in a range of approximately 10 μm to approximately 100 μm. In another non-limiting example, the flexible sheet  102  may have a thickness in a range of approximately 10 μm to approximately 50 μm. In a specific example, the flexible sheet  102  may have a thickness of 25 μm. In another specific example, the flexible sheet  102  may have a thickness of 15 μm. 
         [0018]    The flexible sheet  102  may be a perforated, net-like structure formed of a plurality of nodes  108  that are interconnected by a plurality of threads  110  to define a plurality of openings  106 . The openings  106 , which may improve the flexibility of the flexible sheet  102  relative to a solid, non-perforated sheet, may be formed in the flexible sheet  102  after the flexible sheet  102  is manufactured in the manner described below, such as by punching, cutting, drilling, etching, etc. the flexible sheet  102 . The nodes  108  and threads  110  are depicted as being rectangular and linear, respectively, but this is not critical. In various alternative embodiments, the nodes  108  may be circular, triangular, tetrahedral, irregular, etc., and the threads may be curved, zig-zag shaped, etc. without departing from the present disclosure. In some embodiments, greater than 25% of the flexible sheet  102  may be open space defined by the openings  106 . In other embodiments, greater than 50% of the flexible sheet  102  may be open space defined by the openings  106 . In other embodiments, greater than 75% of the flexible sheet  102  may be open space defined by the openings  106 . 
         [0019]    Referring to  FIG. 1   b,  a top view illustrating a flexible sheet  120  formed of a layer of PTC material in accordance with an exemplary embodiment of the present disclosure is shown. The flexible sheet  120  may be substantially similar to the flexible sheet  102  described above, but may be formed as a solid layer of PTC material having no perforations or openings formed therein. Like the flexible sheet  102 , the flexible sheet  120  may have a thickness that is significantly less than that of conventional sheets of PTC material, thus providing the flexible sheet  120  with a flexibility and a malleability that allow the flexible sheet  120  to be wrapped about, and to conform to, the surfaces of other structures and devices (e.g. batteries) while preserving the PTC characteristics of the flexible sheet  120 . In a non-limiting example, the flexible sheet  120  may have a thickness in a range of approximately 10 μm to approximately 100 μm. In another non-limiting example, the flexible sheet  120  may have a thickness in a range of approximately 10 μm to approximately 50 μm. In a specific example, the flexible sheet  120  may have a thickness of 25 μm. In another specific example, the flexible sheet  120  may have a thickness of 15 μm. 
         [0020]    Referring to  FIG. 2 , a flow diagram illustrating an exemplary method for forming the above-described flexible sheets  102 ,  120  of PTC material is depicted. The method described below facilitates the ultra-thin form factor of the flexible sheets  102 ,  120 , which in-turn provides the flexible sheets  102 ,  120  with flexibility and malleability not found in conventional PTC materials and devices. 
         [0021]    In step  200  of the exemplary method shown in  FIG. 2 , a polymer resin and a conductive filler may be dissolved in a solvent to produce a fluidic, “PTC ink.” The polymer resin, which may be provided in pelletized or powdered form, may include particles of semi-crystalline polymer. Examples of semi-crystalline polymers that may be used include, but are not limited to, polyvinylidene difluoride, polyethylene, ethylene tetrafluoroethylene, ethylene-vinyl acetate, ethylene butyl acrylate, and other materials having similar characteristics. The conductive filler may include conductive particles of metal, metal ceramic, carbon, or various other materials selected for specific conductive properties. Specific examples of conductive fillers include, but are not limited to, tungsten carbide, nickel, and titanium carbide. The solvent may be or may include dimethylformamide, N-Methyl-2-pyrrolidone, tetrahydrofuran, tricholorobenzene, dichlorobenzene, dimethylacetamide, dimethyl sulfoxide, cyclohexane, toluene, or a different solvent capable of dissolving or suspending the polymer resin and conductive filler. 
         [0022]    In optional step  210  of the exemplary method, an additive, such as an antioxidant, an adhesion promoter, an anti-arcing material, or a different additive, may be added to the PTC ink to improve various characteristics of a flexible sheet (e.g.,  102 ,  120  described above) that will be formed from the PTC ink. Such characteristics may include, but are not limited to, polymer stability, voltage capability, or film adhesion. 
         [0023]    In step  220  of the exemplary method, the PTC ink may be applied to a substantially flat surface or substrate. For example, the PTC ink may be poured or sprayed onto a substantially flat surface. In step  230  of the method, a blade may be pulled over the PTC ink, with an edge of the blade disposed parallel to, and spaced a short distance (e.g., 15 μm) above, the surface to produce a uniform layer of PTC ink having a desired thickness. The thickness of the uniform layer of PTC ink may be in a range of approximately 10 μm to approximately 100 μm, for example. 
         [0024]    In step  240  of the exemplary method, the PTC ink may be allowed to dry, whereby the solvent may evaporate out of the ink, leaving an ultrathin, solid layer of PTC material similar to the flexible sheet  120  described above. In an optional step  250  of the method, the layer of PTC material may be punched, etched, cut, drilled, etc. to form an ultrathin, perforated sheet of PTC material similar to the flexible sheet  102  described above. In optional step  260  of the method, one or more additional layers of material, including, but not limited to, a flexible supporting film and/or a flexible insulating substrate, may be applied to the layer of PTC material as may be appropriate for a particular application. 
         [0025]    Referring to  FIG. 3 , a cross sectional view of an exemplary PTC device  300  that includes an ultrathin, flexible sheet  302  of PTC material in accordance with the present disclosure is shown. The flexible sheet  302  may be substantially identical to either of the perforated or solid flexible sheets  102 ,  120  described above. Particularly, the flexible sheet  302  may be flexible and malleable and may have a thickness in a range of approximately 10 μm to approximately 100 μm. In a specific example, the flexible sheet  302  may have a thickness of 25 μm. In another specific example, the flexible sheet  302  may have a thickness of 15 μm. 
         [0026]    The PTC device  300  may further include first and second layers of flexible, conductive foil or metallized polyamide material  308 ,  310  (hereinafter “the first conductive foil  308 ” and “the second conductive foil  310 ,” respectively) that may be coupled to opposing planar surfaces of the flexible sheet  302 . The first and second conductive foils  308 ,  310  may be formed of copper, nickel, or the like, for example. The conductive foils  308 ,  310  may act as thermal contact layers that provide the PTC device  300  with enhanced thermal conductivity. The conductive foils  308 ,  310  may also extend from the flexible sheet  302  and may serve as electrical leads for facilitating electrical connection of the PTC device  300  between an source of electrical energy and a load as further described below. In various embodiments, one or both of the conductive foils  308 ,  310  may be provided with an electrically insulating substrate or covering  312  coupled thereto for preventing inadvertent electrical contact with surrounding devices or structures. 
         [0027]    Referring to  FIG. 4 , a schematic illustration of an exemplary electronic device  400  implementing the above-described PTC device  300  is shown. The electronic device  400  may include one or more components (hereinafter “the protected component”) that may be protected by the PTC device  300 . In the exemplary embodiment shown in  FIG. 4 , the protected component is a battery or pack of batteries  402  (hereinafter collectively referred to as “the battery  402 ”). The present disclosure is not limited in the regard, and it is contemplated that the protected component may alternatively be, or may alternatively include, any of a variety of electrical power sources and/or electrical devices that may benefit from overcurrent or overtemperature protection. 
         [0028]    In various examples, the battery  402  may be a Li-ion battery, a Li-Polymer battery, a Ni-MH rechargeable battery, or the like. While the battery  402  is shown as being generally rectangular, that battery may, in various embodiments, have a variety of other shapes (e.g., cylindrical, irregular) as may be suitable for a particular application. Additionally, the battery  402  may, in various embodiments, be flexible, such as may be suitable for use in flexible, wearable electronic devices. 
         [0029]    The PTC device  300  may be coupled to the battery in a conforming relationship with an exterior surface thereof. The flexible sheet  302  is schematically shown as covering a portion of a front surface of the battery  402 , but in various alternative embodiments that flexible sheet may cover a majority of, or an entirety of, an exterior surface of the battery  402 . In some examples, the flexible sheet  302  may cover less than 25% of an exterior surface of the battery  402 . In other examples, the flexible sheet  302  may cover greater than 25% of an exterior surface of the battery  402 . In other examples, the flexible sheet  302  may cover greater than 50% of the exterior surface of the battery  402 . In further examples, the flexible sheet  302  may cover greater than  75 % of the exterior surface of the battery  402 . In some examples, the flexible sheet  302  may be wrapped, bent, or shaped around edges, corners, contours, and other surface features of the battery  402  in a conforming or substantially conforming relationship therewith (e.g., in flat or continuous contact therewith). In various examples, the PTC device may be coupled to the battery  402  using ultrasonic welding, laser or plasma cleaning and clamping, conductive epoxy, various other conductive or non-conductive adhesives, etc. The present disclosure is not limited in this regard. 
         [0030]    The PTC device  300  may be configured to sense an overcurrent or overtemperature condition in the battery  402  and to arrest or mitigate current flowing to or from the battery upon the occurrence of such a condition. For example, the PTC device  300  may be connected in electrical series between the battery  402  and an electrical load  404  (e.g., an electrical circuit in a wearable electronic device, a cellular telephone, etc.) that is powered by the battery  402 . For example, the first conductive foil  308  of the PTC device  300  may be connected to a positive or negative terminal  406  of the battery  402 , and the second conductive foil  310  of the PTC device  300  may be connected to the electrical load  404 . Thus, the PTC device  300  may operate to prevent or mitigate damage to the load  404  and/or the battery  402  that might otherwise result from an overcurrent or overtemperature condition in the electronic device  400 . For example, upon the occurrence of an overcurrent or overtemperature condition in the electronic device  400 , the temperature of the battery  402  (or a portion of the battery  402 ) may increase, causing the temperature of the adjacent flexible sheet  302  to increase. When the temperature of the flexible sheet  302  reaches an “activation temperature,” the resistance of the PTC material which from which the flexible sheet  302  is formed may increase sharply. This in-turn arrests or mitigates the current flowing through the flexible sheet  302  and between the battery  402  and the load  404 , thereby protecting the battery  402  and the load  404  from the overcurrent condition. When overcurrent condition subsides, the temperatures of the battery  402  and the flexible sheet  302  may decrease below the activation temperature, and current may again be allowed to flow between the battery  402  and the load  404  as during normal operation. 
         [0031]    Those of ordinary skill in the art will appreciate numerous advantages provided by the above-described ultrathin, flexible sheets  102 ,  120 , and  302  as implemented in a PTC device (e.g., the above-described PTC device  300 ). For example, since the flexible sheets  102 ,  120 , and  302  are flexible and malleable, the flexible sheets  102 ,  120 , and  302  may flex, bend, expand, and contract to accommodate flexing, bending, thermal expansion, and thermal contraction of an underlying battery. Furthermore, since the flexible sheets  102 ,  120 , and  302  can be wrapped about, or otherwise disposed on, an entire exterior surface (or a majority of an exterior surface) of a battery, the flexible sheets  102 ,  120 , and  302  may be effective for sensing temperature increases over a much greater surface area of a battery relative to point sensors that are used in conventional overcurrent/overtemperature protection devices and that only sense temperatures on small, discrete points or portions on an exterior of a battery. Still further, since the flexible sheets  102 ,  120 , and  302  are extremely thin relative to conventional PTC materials and devices, the flexible sheets  102 ,  120  may be implemented in applications that require small, slim, or low-profile form factors (e.g., wearable devices, cell phones, etc.) while still providing robust overcurrent/overtemperature protection. 
         [0032]    As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
         [0033]    While the present disclosure makes reference to certain embodiments, numerous modifications, alterations and changes to the described embodiments are possible without departing from the sphere and scope of the present disclosure, as defined in the appended claim(s). Accordingly, it is intended that the present disclosure not be limited to the described embodiments, but that it has the full scope defined by the language of the following claims, and equivalents thereof.