Patent Publication Number: US-8525636-B1

Title: Thermistor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
     Not applicable. 
     INCORPORATION-BY-REFERENCE OF MATERIALS SUBMITTED ON A COMPACT DISC 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present application relates to a thermistor, and more particularly to a thermistor with high hold current value at high temperature. 
     2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98. 
     Because the resistance of conductive composite materials having a positive temperature coefficient (PTC) characteristic is very sensitive to temperature variation, it can be used as the material for current sensing devices, and has been widely applied to over-current protection devices or circuit devices. The resistance of the PTC conductive composite material remains extremely low at normal temperature, so that the circuit or cell can operate normally. However, when an over-current or an over-temperature event occurs in the circuit or cell, the resistance instantaneously increases to a high resistance state (e.g., at least 10 2 Ω), so as to suppress over-current and protect the cell or the circuit device. 
     PTC conductive composite material may include one or more crystalline polymers and conductive filler. The polymer may be usually polyolefin such as polyethylene, while the conductive filler may be carbon black. However, PTC conductive composite material having carbon black cannot obtain high hold current due to insufficient thermal mass and high resistance. When temperature rises, the hold current of the PTC conductive composite material will be tremendously decreased. Therefore, it cannot meet the requirements for protection to large-current secondary batteries. 
     BRIEF SUMMARY OF THE INVENTION 
     To overcome the shortcomings of low hold current of the PTC device having carbon black, in accordance with the present application, different conductive fillers and compositions are used to sustain high hold current at high temperature. The thermistor of the present application can be associated with secondary battery circuit design and satisfy the need to protect large-current secondary batteries. 
     In accordance with an embodiment of the present application, a thermistor includes a first electrically conductive member, a second electrically conductive member and a polymeric material layer. The polymeric material layer is laminated between the electrically conductive members, and exhibits PTC behavior. The polymeric material layer includes at least one crystalline polymer and at least one conductive filler dispersed in the crystalline polymer. The resistivity of the conductive filler is less than 500 μΩ-cm, and the conductive filler may include 72-96% by weight of the polymeric material layer. The thermistor has a device effective area, and a value of the hold current thereof at 60° C. divided by the device effective area is around 0.16-0.8 A/mm 2 . The ratio of the hold current of the thermistor at 60° C. to the hold current of the thermistor at 25° C. is around 40-95%. 
     In an embodiment, the thermal cut-off (TCO) temperature corresponding to the hold current of the thermistor at 60° C. is less than 95° C. Nevertheless, in practice, the above TCO temperature is not restricted to the basis of the hold current at 60° C. The TCO temperature may be alternatively defined as follows: The TCO temperature on a basis of the hold current of the thermistor at a temperature T is less than T+35° C., where T≧60° C. 
     In an embodiment, one or more thermistors of the present application may be coupled to a protective circuit module (PCM) and secondary batteries in series or in parallel by spot-welding, reflow, supersonic welding or laser welding. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The present application will be described according to the appended drawings in which: 
         FIG. 1  shows a thermistor in accordance with an embodiment of the present application; 
         FIG. 2  shows the hold current of a thermistor vs. temperature relationship; 
         FIG. 3  shows an embodiment of a thermisitor associated with secondary batteries in accordance with the present application; and 
         FIG. 4  shows another embodiment of thermistors associated with secondary batteries in accordance with the present application. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The making and using of the presently preferred illustrative embodiments are discussed in detail below. It should be appreciated, however, that the present application provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific illustrative embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. 
     The composition and weight (unit: gram) of the polymeric material layer of the thermistor of the present application is shown in Table 1. LDPE-1 is low-density crystalline polyethylene of a density 0.924 g/cm 3  and a melting point 113° C. HDPE-1 is high-density crystalline polyethylene of a density 0.943 g/cm 3  and a melting point 125° C. HDPE-2 is high density crystalline polyethylene of a density 0.961 g/cm 3  and a melting point 131° C. The conductive filler may use titanium carbide (TiC), tungsten carbide or molybdenum carbide (Mo 2 C). In an embodiment, the conductive filler includes 72-96%, and particularly 75-94% or 78-92%, by weight of the polymeric material layer. The conductive filler may include 74%, 78%, 80%, 85% or 90% by weight of the polymeric material layer. LDPE-1 includes less than 18%, or particularly less than 10%, 8%, 5% or 3%, by weight of the polymeric material layer. The HDPE-1 and HDPE-2 in total is 3-25%, and particularly 5-20%, by weight of the polymeric material layer. In an embodiment, HDPE-1 of lower density includes less than 20% by weight of the polymeric material layer, while HDPE-2 of higher density includes less than 20% by weight of the polymeric material layer. The melting points of HDPE-1 and HDPE-2 are both larger than 115° C. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                 Carbon 
                 Conductive 
               
               
                 Material 
                 LDPE-1 
                 HDPE-1 
                 HDPE-2 
                 TiC 
                 WC 
                 Mo 2 C 
                 Black 
                 filler (wt %) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Em. 1 
                 4 
                 13.4 
                 — 
                 112- 
                 — 
                 — 
                 — 
                 86% 
               
               
                 Em. 2 
                 3.6 
                 12.8 
                 — 
                 120- 
                 — 
                 — 
                 — 
                 88% 
               
               
                 Em. 3 
                 3 
                 9 
                 4 
                 125 
                 — 
                 — 
                 — 
                 89% 
               
               
                 Em. 4 
                 2 
                 11.4 
                 3 
                 130 
                 — 
                 — 
                 — 
                 89% 
               
               
                 Em. 5 
                 — 
                 11.5 
                 3.5 
                 128 
                 — 
                 — 
                 — 
                 90% 
               
               
                 Em. 6 
                 2.7 
                 21.5 
                 — 
                 85 
                 — 
                 — 
                 — 
                 77% 
               
               
                 Em. 7 
                 1.8 
                 — 
                 21.2 
                 100 
                 — 
                 — 
                 — 
                 81% 
               
               
                 Em. 8 
                 2 
                 — 
                 17.7 
                 — 
                 — 
                 193 
                 — 
                 91% 
               
               
                 Em. 9 
                 3.1 
                 17.3 
                 — 
                 — 
                 280 
                 — 
                 — 
                 93% 
               
               
                 Em. 10 
                 2.1 
                 — 
                 16 
                 — 
                 300 
                 — 
                 — 
                 94% 
               
               
                 Em. 11 
                 11.5 
                 — 
                 6 
                 90 
                 — 
                 — 
                 — 
                 83% 
               
               
                 Em. 12 
                 21.9 
                 — 
                 — 
                 100 
                 — 
                 — 
                 — 
                 82% 
               
               
                 Comp. 1 
                 7.7 
                 19.2 
                 — 
                 — 
                 — 
                 — 
                 22 
                 45% 
               
               
                 Comp. 2 
                 10.9 
                 — 
                 14.3 
                 — 
                 — 
                 — 
                 31.6 
                 56% 
               
               
                   
               
            
           
         
       
     
     In an embodiment, the manufacturing process of the thermistor is described as follows. The raw material is fed into a blender (HAAKE 600) at 160° C. for two minutes. The procedure of feeding the raw material includes adding the crystalline polymers with the amounts according to Table 1 into the blender; after blending for a few seconds, then adding the conductive ceramic carbide filler, e.g., titanium carbide, tungsten carbide or molybdenum carbide, with particle size distribution between 0.1 μm and 50 μm. The rotational speed of the blender is set at 40 rpm. After blending for three minutes, the rotational speed is increased to 70 rpm. After blending for 7 minutes, the mixture in the blender is drained and thereby a conductive composition with PTC characteristic is formed. 
     The above conductive composition is loaded symmetrically into a mold with outer steel plates and a 0.33 mm and 0.2 mm thick middle, wherein the top and the bottom of the mold are disposed with a Teflon cloth. The mold loaded with the conductive composition is pre-pressed for three minutes at 50 kg/cm 2 , 180° C. Then the generated gas is exhausted and the mold is pressed for 3 minutes at 100 kg/cm2, 180° C. Next, the press step is repeated once at 150 kg/cm 2  and 180° C. for three minutes to form a PTC material layer. In an embodiment, the thickness of the PTC material layer is greater than 0.1 mm, or preferably greater than 0.2 mm or 0.3 mm. 
     The PTC material layer is cut into many pieces each with an area of 20×20 cm 2 . Then two metal foils physically contact the top surface and the bottom surface of the PTC material layer, in which the two metal foils are symmetrically placed upon the top surface and the bottom surface of the PTC material layer. Next, Teflon cloths and the steel plates are placed on the metal foils and are pressed to form a multi-layered structure. Next, the multi-layered structure is punched or cut to form a thermistor  10  with an area less than 25 mm 2 , or preferably less than 20 mm 2 . 
     More specifically, the thermistor  10  includes a first electrically conductive member  11 , a second electrically conductive member  12 , and a polymeric material layer  13 , those are equivalent to the aforesaid metal foils and the PTC material layer. The weight ratio of the polymeric material layer  13  to the crystalline polymer with same volume (i.e., the density ratio) is between 2.5 and 12, or between 3 and 10 in particular. 
     In an embodiment, two metal electrode sheets  22  are jointed to the electrically conductive members  11  and  12  by solder reflow to form an axial-leaded over-current protection device  20 . 
     In addition to the materials listed in Table 1, the polymeric material layer may use crystalline polyolefines (e.g., high-density polyethylene (HDPE), medium-density polyethylene, low-density polyethylene (LDPE), polyvinyl wax, vinyl polymer, polypropylene, polyvinyl chlorine and polyvinyl fluoride), copolymer of olefin monomer and acrylic monomer (e.g., copolymer of ethylene and acrylic acid or copolymer of ethylene and acrylic resin) or copolymer of olefin monomer and vinyl alcohol monomer (e.g., copolymer of ethylene and vinyl alcohol), and may include one or more crystalline polymer materials. The LDPE can be polymerized using Ziegler-Natta catalyst, Metallocene catalyst or the like, or can be copolymerized by vinyl monomer and other monomers such as butane, hexane, octane, acrylic acid, or vinyl acetate. 
     The conductive filler may be metal powder or conductive ceramic carbide powder. The metal powder may include nickel, cobalt, copper, iron, tin, lead, silver, gold, platinum, vanadium or the alloy thereof. The conductive ceramic carbide powder may include titanium carbide, tungsten carbide, vanadium carbide, boron carbide, zirconium carbide, niobium carbide, tantalum carbide, molybdenum carbide, hafnium carbide or the mixture thereof, and may be of various shapes, e.g., spherical, debris, flake, or polygonal shape. The particle size of the conductive ceramic carbide filler is essentially between 0.1 μm and 50 μm. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 I-hold @  
                 I-hold @ 
               
               
                   
                 Device 
                   
                   
                   
                 60° C./device  
                 60° C./I- 
               
               
                   
                 effective 
                 I-hold @ 
                 I-hold @ 
                 I-hold @ 
                 effective area 
                 hold @  
               
               
                   
                 area (mm) 
                 25° C. (A) 
                 60° C. (A) 
                 70° C. (A) 
                 (A/mm 2 ) 
                 25° C. 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Em. 1 
                 2.8 × 3.5 
                 3.5 
                 2.4 
                 1.7 
                 0.244 
                 68% 
               
               
                 Em. 2 
                 2.8 × 3.5 
                 4.4 
                 3.3 
                 2.7 
                 0.336 
                 75% 
               
               
                 Em. 3 
                 2.8 × 3.5 
                 4.4 
                 3.0 
                 — 
                 0.306 
                 68% 
               
               
                 Em. 4 
                 2.8 × 3.5 
                 4.7 
                 3.5 
                 2.8 
                 0.357 
                 74% 
               
               
                 Em. 5 
                 2.8 × 3.5 
                 5.2 
                 4.6 
                 4.0 
                 0.469 
                 88% 
               
               
                 Em. 6 
                 2.8 × 3.5 
                 2.5 
                 1.8 
                 — 
                 0.183 
                 72% 
               
               
                 Em. 7 
                 2.8 × 3.5 
                 2.9 
                 2.3 
                 — 
                 0.234 
                 79% 
               
               
                 Em. 8 
                 3 × 5 
                 4.3 
                 2.8 
                 — 
                 0.186 
                 65% 
               
               
                 Em. 9 
                 3 × 5 
                 5.4 
                 3.7 
                 — 
                 0.246 
                 68% 
               
               
                 Em. 10 
                 3 × 5 
                 6.8 
                 5.1 
                 — 
                 0.340 
                 75% 
               
               
                 Em. 11 
                 3 × 5 
                 4.5 
                 2.2 
                 — 
                 0.146 
                 48% 
               
               
                 Em. 12 
                 3 × 5 
                 3.8 
                 1.6 
                 — 
                 0.106 
                 42% 
               
               
                 Comp. 1 
                 4.2 × 4.4 
                  0.55 
                 0.1 
                 — 
                 0.005 
                 18% 
               
               
                 Comp. 2 
                 4.2 × 4.4 
                 1.3 
                 .33 
                 — 
                 0.017 
                 25% 
               
               
                   
               
            
           
         
       
     
     The thermistor  10  undergoes hold current (I-hold) test at 25° C., 60° C. and 70° C., respectively, and the results of twelve embodiments (Em. 1-Em. 12) and two comparative examples (Comp. 1 and Comp. 2) are shown in Table 2. The hold current is the maximum current at which the thermistor  10  does not trip at a predetermined temperature. The volume resistivity (ρ) of the polymeric material layer  13  can be calculated by formula (1) below, where R, A, and L indicate the resistance, the area, and the thickness of the polymeric material layer  13 , respectively. In this embodiment, the thermistor  10  has a device effective area that is the effective cross-sectional area of current path of the polymeric material layer  13 , e.g., the area A. For example, the device effective area of Em. 1 is 2.8×3.5=9.8 mm 2 . 
     
       
         
           
             
               
                 
                   ρ 
                   = 
                   
                     
                       R 
                       × 
                       A 
                     
                     L 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     At 60° C., the value of I-hold divided by device effective area is around 0.16-0.8 A/mm 2 , particularly 0.18-0.75 A/mm 2  or 0.2-0.7 A/mm 2 . Moreover, at 60° C., the value of I-hold divided by device effective area may be 0.3 A/mm 2 , 0.4 A/mm 2 , 0.5 A/mm 2  or 0.6 A/mm 2 . Theoretically, the smaller the device effective area, the larger the hold current is. Therefore, the device area of the thermistor  10  is preferably smaller than 25 mm 2 . The values of I-hold/device effective area of Comp. 1 and Comp. 2 using carbon black are smaller than 0.05 A/mm 2 , and thus they are not suitable for large-current applications. 
     The ratios of I-hold@60° C. to I-hold@25° C. of all embodiments are about 40%-95%, and particularly 60-90%, or 70-85%. Such ratios may include 45%, 50%, 55%, 65% or 70%. However, the ratios of I-hold@60° C. to I-hold@25° C. of Comp. 1 and Comp. 2 are below 30%. In other words, in comparison with those using carbon black, the hold current of the thermistor  10  of the present application is decreased slowly as temperature rises, as shown in  FIG. 2 . As a result, the thermistor  10  of the present application can sustain relatively high hold current at high temperature. 
     The thermistor  10  may further undergo thermal cut-off (TCO) temperature test. The thermistor  10  is placed in an oven and the hold current at a temperature is applied to the thermistor  10 . TCO temperature is a temperature at which the thermistor is tripped and the current flowing therethrough is tremendously diminished during the process that temperature in the oven gradually increases. In an embodiment, TCO temperatures are tested on a basis of hold current at 60° C., and the results are shown in Table 3 below. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                 Device effective  
                 I-hold @ 
                 TCO @ 60° C.  
               
               
                   
                 area (mm) 
                 60° C. (A) 
                 I-hold (° C.) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Em. 1 
                 2.8 × 3.5 
                 2.4 
                 72 
               
               
                 Em. 2 
                 2.8 × 3.5 
                 3.3 
                 77 
               
               
                 Em. 3 
                 2.8 × 3.5 
                 3.0 
                 70 
               
               
                 Em. 4 
                 2.8 × 3.5 
                 3.5 
                 78 
               
               
                 Em. 5 
                 2.8 × 3.5 
                 4.6 
                 76 
               
               
                 Em. 6 
                 2.8 × 3.5 
                 1.8 
                 66 
               
               
                 Em. 7 
                 2.8 × 3.5 
                 2.3 
                 70 
               
               
                 Em. 8 
                 3 × 5 
                 2.8 
                 80 
               
               
                 Em. 9 
                 3 × 5 
                 3.7 
                 81 
               
               
                 Em. 10  
                 3 × 5 
                 5.1 
                 79 
               
               
                 Em. 11 
                 3 × 5 
                 2.2 
                 77 
               
               
                 Em. 12 
                 3 × 5 
                 1.6 
                 76 
               
               
                 Comp. 1 
                 4.2 × 4.4 
                 0.1 
                 71 
               
               
                 Comp. 2 
                 4.2 × 4.4 
                 .33 
                 78 
               
               
                   
               
            
           
         
       
     
     On a basis of hold current at 60° C., TCO temperatures for all embodiments are less than 95° C., or particularly less than 90° C. or 85° C. More specifically, the TCO temperatures on a basis of hold current at 60° C. are in the range of 60° C.-95° C., and 65° C.-85° C. in particular. In practice, TCO temperatures may be measured on a basis of hold current at a temperature greater than 60° C. As such, the TCO temperature on a basis of the hold current of the thermistor at a temperature T is less than T+35° C., wherein T≧60° C. 
     The thermistor  10  can be used for protecting secondary batteries, as the illustrative embodiments below. 
     In  FIG. 3 , the thermistor  10  can be secured on a protective circuit module (PCM)  30  and coupled to the PCM in series. The thermistor  10  is further connected to the secondary batteries  31  and  32  and a load  33  in series to form a loop, so that the thermistor  10  can instantly decrease the current flowing therethrough to protect the secondary batteries  31  and  32  when over-current occurs. In an embodiment, the thermistor  10  may be electrically connected to the secondary batteries  31  and  32  and PCM  30  by spot-welding, reflow, supersonic welding or laser welding. 
     In  FIG. 4 , the secondary batteries  31  and  32  are connected in parallel, and each of them is connected to a thermistor  10 , thereby providing over-current protection to large-current applications. In an embodiment, the thermistors  10  may be electrically connected to the secondary batteries  31 ,  32  and PCM  30  by spot-welding, reflow, supersonic welding or laser welding. 
     In summary, the hold current of the thermistor of the present application is less sensitive to temperature variation. That is, the decrease of the hold current is mitigated as temperature rises; thus the thermistor can sustain high hold current at a high temperature. Therefore, the thermistor of the present application can be introduced to secondary battery circuit design to meet the requirement of over-current protection for large-current secondary batteries. 
     The above-described embodiments of the present invention are intended to be illustrative only. Numerous alternative embodiments may be devised by persons skilled in the art without departing from the scope of the following claims.