OVER-CURRENT PROTECTION DEVICE

Abstract

An over-current protection device includes first and second electrode layers and a PTC material layer laminated therebetween. The PTC material layer includes a polymer matrix, a conductive filler, and a titanium-containing dielectric filler. The polymer matrix has a fluoropolymer. The titanium-containing dielectric filler has a compound represented by a general formula of MTiO.sub.3, wherein the M represents transition metal or alkaline earth metal. The total volume of the PTC material layer is calculated as 100%, and the titanium-containing dielectric filler accounts to for 5-15% by volume of the PTC material layer.

Claims

1. An over-current protection device, comprising: a first electrode layer; a second electrode layer; and a positive temperature coefficient (PTC) material layer laminated between the first electrode layer and the second electrode layer, the PTC material layer comprising: a polymer matrix comprising a fluoropolymer; a conductive filler dispersed in the polymer matrix, thereby to forming an electrically conductive path in the PTC material layer; and a titanium-containing dielectric filler dispersed in the polymer matrix, wherein the titanium-containing dielectric filler has a compound represented by a general formula (I):
MTiO.sub.3   (I) wherein the M represents transition metal or alkaline earth metal, and the total volume of the PTC material layer is calculated as 100%, and the titanium-containing dielectric filler accounts for 5-15% by volume of the PTC material layer.

2. The over-current protection device of claim 1, wherein the fluoropolymer has a first dielectric constant and the titanium-containing dielectric filler has a second dielectric constant, and a value obtained by dividing the second dielectric constant by the first dielectric constant is in a range from 16 to 667.

3. The over-current protection device of claim 2, wherein the titanium-containing dielectric filler is selected from the group consisting of BaTiO.sub.3, SrTiO.sub.3, CaTiO.sub.3, and any combination thereof.

4. The over-current protection device of claim 3, wherein BaTiO.sub.3 accounts for 9.5-10.5% by volume of the PTC material layer.

5. The over-current protection device of claim 3, wherein SrTiO.sub.3 accounts for 11.5-12.5% by volume of the PTC material layer.

6. The over-current protection device of claim 3, wherein CaTiO.sub.3 accounts for 8.8-9.8% by volume of the PTC material layer.

7. The over-current protection device of claim 3, wherein the titanium-containing dielectric filler has a median diameter ranging from 8 μm to 10 μm.

8. The over-current protection device of claim 2, wherein the fluoropolymer is selected from the group consisting of polyvinylidene fluoride, poly(tetrafluoroethylene), poly(vinylidene fluoride), ethylene-tetra-fluoro-ethylene, tetrafluoroethylene-hexafluoro-propylene copolymer, ethylene-tetrafluoroethylene copolymer, perfluoroalkoxy modified tetrafluoroethylenes, poly(chlorotri-fluorotetrafluoroethylene), vinylidene fluoride-tetrafluoroethyene copolymer, tetrafluoroethylene-perfluorodioxole copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, and any combination thereof.

9. The over-current protection device of claim 8, wherein the conductive filler comprises a conductive ceramic filler and carbon black, and the conductive ceramic filler is selected from the group consisting of tungsten carbide, titanium carbide, vanadium carbide, zirconium carbide, niobium carbide, tantalum carbide, molybdenum carbide, hafnium carbide, titanium boride, vanadium boride, zirconium boride, niobium boride, molybdenum boride, hafnium boride, zirconium nitride, and any combination thereof.

10. The over-current protection device of claim 9, wherein the over-current protection device has an electrical resistivity ranging from 0.0136 Ω.Math.cm to 0.0211 Ω.Math.cm.

11. The over-current protection device of claim 10, wherein the over-current protection device has a trip threshold value making the over-current protection device change from an electrically conductive state to an electrically non-conductive state, and the trip threshold value ranges from to 0.198 A/mm.sup.2 to 0.233 A/mm.sup.2.

12. The over-current protection device of claim 1, wherein the PTC material layer has a top-view area ranging from 25 mm.sup.2 to 72 mm.sup.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present application will be described according to the appended drawings in which:

[0020] FIG. 1 shows an over-current protection device in accordance with an embodiment of the present invention; and

[0021] FIG. 2 shows the top view of the over-current protection device shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0022] 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.

[0023] FIG. 1 shows one basic aspect of an over-current protection device of the present invention. The over-current protection device 10 includes a first electrode layer 12, a second electrode layer 13, and a positive temperature coefficient (PTC) material layer 11 laminated between the first electrode layer 12 and the second electrode layer 13. The first electrode layer 12 and the second electrode layer 13 may be composed of the nickel-plated copper foils. The PTC material layer 11 includes a polymer matrix, a conductive filler, and a titanium-containing dielectric filler.

[0024] In the PTC material layer 11, the polymer matrix has a fluoropolymer, and the conductive filler is evenly dispersed in the polymer matrix, thereby forming an electrically conductive path in the PTC material layer 11. In an embodiment, the fluoropolymer is selected from the group consisting of polyvinylidene fluoride, poly(tetrafluoroethylene), poly(vinylidene fluoride), ethylene-tetra-fluoro-ethylene, tetrafluoroethylene-hexafluoro-propylene copolymer, ethylene-tetrafluoroethylene copolymer, perfluoroalkoxy modified tetrafluoroethylenes, poly(chlorotri-fluorotetrafluoroethylene), vinylidene fluoride-tetrafluoroethylene copolymer, tetrafluoroethylene-perfluorodioxole copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, and any combination thereof.

[0025] In addition, according to the present invention, the titanium-containing dielectric filler is added into the PTC material layer 11. The titanium-containing dielectric filler is well-mixed and dispersed in the polymer matrix evenly, thereby enhancing the voltage endurance capability of the over-current protection device 10. The fluoropolymer has a first dielectric constant, and the titanium-containing dielectric filler has a second dielectric constant. A value obtained by dividing the second dielectric constant by the first dielectric constant is in a range from 16 to 667. Compared with the fluoropolymer, the filler with a higher dielectric constant may possess is better capability for electric polarization. That is, the filler with a higher dielectric constant has a better capability to restrict movement of electric charges, and thus lowers tendency of negative charges to attack the fluoropolymer. In other words, as long as a specific amount of the titanium-containing dielectric filler is added into the polymer matrix, the added titanium-containing dielectric filler may help in trapping negative charges after trip of device and consequently lowering the energy accumulated at the fluoropolymer, thereby avoiding degradation of the fluoropolymer. In this way, the integrity of the composite system is well stabilized, and thus the over-current protection device can withstand more times of voltage of same value applied to the device. Besides, the less degradation of the fluoropolymer takes place, the less hydrofluoric acid is produced. This prevents environmental pollution, device corrosion, or other adverse factors that affect performance of the over-current protection device.

[0026] Moreover, according to the present invention, the total volume of the PTC material layer is calculated as 100%, and the titanium-containing dielectric filler accounts for 5-15% by volume of the PTC material layer. The amount of the titanium-containing dielectric filler could not be too much or too less. If the volume percentage of the titanium-containing dielectric filler is less than 5%, the issue of negative coefficient temperature (NTC) behavior after trip of device would occur. The NTC behavior after trip of device refers to a situation in which a high electrical resistance (after trip of device) gradually decreases with gradual elevation of temperature. If the volume percentage of the titanium-containing dielectric filler is more than 15%, the wettability of the mixture of the polymer and fillers is insufficient, therefore the polymer, the titanium-containing dielectric filler, and/or the conductive filler are unable to be blended to form a uniformly mixed mixture. Accordingly, in some embodiments, the titanium-containing dielectric filler accounts for 5.2-14.5%, 5.2-10.1%, or 10.1-14.5% by volume of the PTC material layer. Besides, the titanium-containing dielectric filler may include one or more materials with perovskite structure, in which the one or more compounds has a general formula represented by MTiO.sub.3. The “M” in this general formula can be transition metal or alkaline earth metal. The transition metal may be manganese (Mn), and the alkaline earth metal may be beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), or radium (Ra). For example, calcium (Ca), strontium (Sr), or barium (Ba) can be selected as the alkaline earth metal, and the titanium-containing dielectric filler could be selected from the group consisting of BaTiO.sub.3, SrTiO.sub.3, CaTiO.sub.3, and any combination thereof. In an embodiment, BaTiO.sub.3 accounts for 9.5-10.5% by volume of the PTC material layer. In an embodiment, SrTiO.sub.3 accounts for 11.5-12.5% by volume of the PTC material layer. In an embodiment, CaTiO.sub.3 accounts for 8.8-9.8% by volume of the PTC material layer.

[0027] It is noted that dispersity of the titanium-containing dielectric filler in the polymer matrix and processability during blending operation should be taken into consideration, and therefore the titanium-containing dielectric filler has a median diameter ranging from 8 μm to 10 μm, wherein the median diameter is denoted by D(0.5). In the present invention, D(0.5)=8−10 μm means that on the premise that total number of filler particles is calculated as 1(100%), the number of the filler particles with particle diameter smaller than 8-10 μm is equal to or less than 0.5(50%). As noted above, in consideration of processability for blending the polymer matrix with the titanium-containing dielectric filler particles, median diameter D(0.5) of the titanium-containing dielectric filler has to be appropriately controlled. If the median diameter D(0.5) is smaller than 8 μm, aggregation of small-sized filler particles would dominantly occur, and also, the small-sized filler particles having light weight before blending will easily suspend in the air. If the median diameter is larger than 10 μm, sedimentation of large-sized filler particles would dominantly occur, and hence the large-sized filler particles will deposit in certain regions of the PTC material layer after blending. In the latter case, the titanium-containing dielectric filler particles would stick together, causing agglomeration of these large-sized particles. Since, in the present invention, amount and particle size of the titanium-containing dielectric filler are properly adjusted, the over-current protection device 10 has an electrical resistivity ranging from 0.0136 Ω.Math.cm to 0.0211 Ω.Math.cm. Additionally, the device can pass a cycle life test undergoing 100 cycles while maintaining low electrical resistivity.

[0028] Please refer to FIG. 2, it shows the top view of the over-current protection device 10 as shown in FIG. 1. The over-current protection device 10 has a length A and a width B, and the top-view area “A×B” of the over-current protection device 10 is substantially equivalent to the top-view area of the PTC material layer 11. The PTC material layer 11 may have a top-view area ranging from 25 mm.sup.2 to 72 mm.sup.2 based on different products having different sizes. In some embodiments, the top-view area “A×B” may be 5×5 mm.sup.2, 5×7 mm.sup.2, 7.62×7.62 mm.sup.2, 8.2×7.15 mm.sup.2, or 7.62×9.35 mm.sup.2.

[0029] It is noted that FIG. 1 merely and exemplarily shows one basic structure of the over-current protection device. In practice, various designs may be applied thereto. For instance, area of device exposed to the environment could be decreased, insulation of device from the environment could be enhanced, or external leads can be installed onto the device, etc. In an embodiment, a solder paste is coated on the outer surfaces of the first electrode layer 12 and second electrode layer 13, and two copper electrodes with a thickness of 0.5 mm are respectively welded on the solder paste on the first electrode layer 12 and second electrode layer 13 as external leads, and then the assembled device is subjected to a reflow soldering process at 300° C. so as to form a PTC device of an axial-type or a radial-leaded type. In other embodiments, printed circuit board (PCB) manufacturing processes may be adopted to form a PTC device of surface-mountable device (SMD) type, by which insulating layers are formed on the first electrode layer 12 and second electrode layer 13, respectively, and followed by forming external electrode layers on the insulating layers. Then, a patternization process is conducted on the external electrode layers so as to form external electrodes, and conductive connecting holes are made for electrical connection between the first electrode layer 12/second electrode layer 13 and the external electrodes. The PTC devices according to the present invention are not limited to the above mentioned types, such as axial-type, radial-leaded type, or SMD type. The above over-current protection device 10 is intended to be illustrative in the present disclosure only.

[0030] As described above, the over-current protection device 10 of the present invention has low electrical resistivity and excellent voltage endurance capability. It could be verified according to the experimental. data in Table 1 to Table 3 as shown below.

TABLE-US-00001 TABLE 1 (volume percentage, vol %) Group PVDF PTFE BN Mg(OH).sub.2 BaTiO.sub.3 SrTiO.sub.3 CaTiO.sub.3 CB WC E1 47.5 5.3 5.2 2 40 E2 43.4 5.3 10.1 2 40 E3 40.0 3.5 14.5 2 40 E4 41.5 5.3 12 2 40 E5 43.5 5.3 9.3 2 40 C1 43.5 5.3 10 2 40 C2 45.0 6 2 47 C3 42.1 5.3 8.6 4 40

[0031] Table 1 shows the composition to form a PTC material layer 11 by volume percentages in accordance with embodiments (E1-E5) of the present disclosure and comparative examples (C1-C3). The first column in Table 1 shows test groups E1-C3 from top to bottom. The first row in Table 1 shows various materials used for the PTC material layer 11 from left to right, that is, polyvinylidene difluoride (PVDF), polytetrafluoroethene (PTFE), boron nitride (BN), magnesium hydroxide (Mg(OH).sub.2), barium titanate (BaTiO.sub.3), strontium titanate (SrTiO.sub.3), calcium titanate (CaTiO.sub.3), carbon black (CB), and tungsten carbide (WC). It should be noted that E1-E5 and C1-C3 all use “PVDF system” (i.e., a system adopts PVDF as its major polymer material in the polymer matrix) with PTC characteristic to conduct tests. Besides, each test group has approximately same volume percentage value in polymer matrix (PVDF and PTFE), and also in conductive filler (CB and WC). However, different inner filler materials (i.e., blank, BN, Mg(OH).sub.2, BaTiO.sub.3, SrTiO.sub.3, and CaTiO.sub.3) are used to compare experimental results of these test groups. The above-said “blank” means that the PTC material layer 11 includes only the conductive filler (e.g., CB and WC) as its filler, and no additional inner filler is added thereinto. For the polymer matrix used in the tests, PVDF (commercialized under the brand name Kynar® 761A) has a melting point temperature of 165° C., and PTFE (commercialized under the brand name Zonyl™ PTFE MP1000) has a melting point temperature of 315° C. For the conductive filler, tungsten carbide (WC) has a relatively high volume percentage in the PTC material layer 11 in order to lower electrical resistance of the device, and carbon black (CB) is included therein with a relatively less amount in order to increase voltage endurance and electrical characteristic stability of the device. Besides, according to the present invention, the fluoropolymer is not limited to PVDF or PTFE, and the conductive filler is not limited to tungsten carbide (WC) or carbon black (CB). According to the present invention, all the aforementioned fluoropolymers and conductive fillers may be implemented in the tests and may produce similar technical effect, and thus details using these materials are not further described herein.

[0032] In the embodiments E1-E5, at least one compound with perovskite structure (referred to as “perovskite-based compound” hereinafter) is included in the PTC material layer besides the polymer matrix and conductive filler. The perovskite-based compounds in the experiment are BaTiO.sub.3, SrTiO.sub.3, and CaTiO.sub.3 with different volume percentages, respectively. In the embodiments E1 to E3, BaTiO.sub.3 is selected with 5.2 vol %, 10.1 vol % and 14.5 vol % by volume percentage order from low to high. SrTiO.sub.3 in the embodiment E4 is selected with 12 vol %, and CaTiO.sub.3 in the embodiment E5 is selected with 9.3 vol %, so as to further demonstrate that the perovskite-based compounds can effectively improve voltage endurance as long as its volume percentage falls within the range of 5-15 vol %. It is noted that the volume percentage of perovskite-based compounds in the PTC material layer should not be too low or too high. If the volume percentage of the perovskite-based compound is below 5 vol %, the experimental result would be like that of “blank” where the PTC material layer includes conductive filler (e.g., CB and WC) only. In other words, if the volume percentage of the perovskite-based compound in the PTC material layer 11 is lower than 5 vol %, the PTC material layer 11 would be like a PTC material layer with no inner fillers added thereinto, and the over-current protection device may have lots of undesirable electrical characteristics, such as NTC behavior after trip of device. If the volume percentage of the perovskite-based compound in the PTC material layer 11 is above 15 vol %, the issue of inconvenience in processability would be raised. That is, the wettabillty of the mixture (polymer and perovskite-based compound) is insufficient due to excessive powder of perovskite-based compound, and hence the polymer and the perovskite-based compound, and/or the conductive filler are unable to be blended to form a uniformly mixed mixture.

[0033] In the comparative examples C1-C3, for comparison, Mg(OH).sub.2, blank (i.e., no inner filler), and BN are selected besides the polymer matrix and conductive filler. Mg(OH).sub.2 and BN are the fillers used in conventional over-current protection devices to function as flame retardant. Besides, Mg(OH).sub.2 may function as a filler for neutralization between acid and base. Mg(OH).sub.2 could react with hydrofluoric acid, which is produced from degradation of fluoropolymer, to generate magnesium fluoride (MgF.sub.2) and water, thereby reducing hazards caused by hydrofluoric acid. As to BN, BN can mitigate NTC behavior after trip of device. In other words, both inner fillers (i.e., Mg(OH).sub.2 and BN) can stabilize the aforementioned PVDF system, and are usually used in conventional over-current protection devices for extending device longevity.

[0034] The embodiments and comparative examples are manufactured by the same method. First, materials of the test groups are formulated to the compositions with corresponding specific volume percentages (i.e., the percentages of embodiments and comparative examples as shown in Table 1), and the formulated materials are put into HAAKE twin screw blender for blending. The blending temperature is 215° C., the time for pre-mixing is 3 minutes, and the blending time is 15 minutes. The conductive polymer after being blended is pressed into a sheet by a hot press machine at a temperature of 210° C. and a pressure of 150 kg/cm.sup.2. The sheet is then cut into pieces of about 20 cm×20 cm, and two nickel-plated copper foils are laminated to two sides of the sheet with the hot press machine at a temperature of 210° C. and a pressure of 150 kg/cm.sup.2, by which a three-layered structure is formed. Then, the sheet with the nickel-plated copper foils is punched into PTC chips, each of which is the over-current protection device of the present invention. The PTC chips to be tested has a length of 5 mm and a width of 7 mm, which means it has a top-view area of 35 mm.sup.2, and additionally, it has a thickness of 0.65 mm. It is understood that the size of chip to be tested is intended to be illustrative only and is not limited in the present invention. The present invention may be applied to other PTC chips with different sizes, such as 5×5 mm.sup.2, 7.62×7.62 mm.sup.2, 8.2×7.15 mm.sup.2, 7.62×9.35 mm.sup.2, or any common size of the art.

[0035] Then, the PTC chips of the embodiments E1-E5 and comparative examples C1-C3 are subjected to electron beam irradiation of 50 kGy (irradiation dose can be adjusted depending on the requirement). After irradiation, the following measurements are performed by taking five PTC chips as samples for each of E1-E5 and C1-C3, by which voltage endurance of the PTC chips (i.e., the over-current protection device) can be verified. The results are shown in Table 2 below.

TABLE-US-00002 TABLE 2 Endurable R.sub.i ρ I-trip I-trip/area R.sub.100 c power/area Group (Ω) (Ω .Math. cm) (A) (A/mm.sup.2) (Ω) (W/mm.sup.2) R.sub.100 c/R.sub.i E1 0.00357 0.0192 7.55 0.216 0.025 6.47 7.0 E2 0.00273 0.0147 8.10 0.231 0.012 6.94 4.4 E3 0.00253 0.0136 8.17 0.233 0.035 7.00 13.8 E4 0.00354 0.0191 7.25 0.207 0.018 6.21 5.1 E5 0.00392 0.0211 6.93 0.198 0.021 5.94 5.4 C1 0.00448 0.0241 6.33 0.181 FAIL NONE NONE C2 0.00207 0.0111 8.28 0.237 FAIL NONE NONE C3 0.00302 0.0151 7.52 0.215 FAIL NONE NONE

[0036] In Table 2, the first row shows items to be tested from left to right.

[0037] “R.sub.i” refers to initial electrical resistance of the over-current protection device at room temperature. In the embodiments E1-E5, R.sub.i ranges from 0.00253Ω to 0.00392Ω.

[0038] “η” refers to electrical resistivity of the over-current protection device at room temperature. It can be calculated in accordance with the electrical resistance formula R=ρ×L/A. “R” is electrical resistance, “L” is length, and “A” is cross sectional area. In the embodiments E1-E5, ρ ranges from 0.0136 Ω.Math.cm to 0.0211 Ω.Math.cm.

[0039] “I-trip” refers to trip current needed for the over-current protection device at 25° C. In the embodiments E1-E5, I-trip ranges from 6.93 A to 8.17 A.

[0040] “I-trip/area” refers to relative trip current per unit area of the over-current protection device at 25° C. I-trip/area is also called as “trip threshold value,” that is, a current value needed for trip of the over-current protection device per unit area. In the embodiments E1-E5, I-trip/area ranges from 0.198 A/mm.sup.2 to 0.233 A/min.sup.2.

[0041] “R.sub.100c” refers to electrical resistance of the over-current protection device after a cycle life test. The cycle life test is performed by 100 cycles of operation, each of which includes applying voltage/current at 30V/30 A for 10 seconds and turning it off for 60 seconds (i.e., on: 10 seconds; off: 60 seconds). Electrical resistance of the over-current protection device after 100 cycles is measured and therefore its R.sub.100c can be obtained. In the embodiments E1-E5, R.sub.100c ranges from 0.012Ω to 0.035Ω.

[0042] “Endurable power/area” refers to power per unit area that the over-current protection device can withstand without burnout. In the embodiments E1-E5, endurable power/area. ranges from 5.94 W/mm.sup.2 to 7.00 W/mm.sup.2. is “R.sub.100/R.sub.i” refers to the ratio in which the electrical resistance of the over-current protection device after the cycle life test is divided by the initial electrical resistance of the over-current protection device at room temperature. In the present invention, this ratio is also defined as “electrical resistance retention ratio.” The smaller the electrical resistance retention ratio is, the less fluctuation of the electrical resistance of the over-current protection device will be. That is, the over-current protection device with small electrical resistance retention ratio has better capability for electrical resistance recovery from trip of device toward the initial electrical resistance. In the embodiments E1-E5, R.sub.100/R.sub.i ranges from 4.4 to 13.8.

[0043] From the above results, it is understood that the embodiments E1-E5 maintain low electrical resistivity while having capability to endure high voltage. More specifically, the embodiments E1-E5 have the electrical resistivity (ρ) ranging from 0.0136 Ω.Math.cm to 0.0211 Ω.Math.cm, and the comparative examples C1-C3 have the electrical resistivity (ρ) ranging from 0.0111 Ω.Math.cm to 0.241 Ω.Math.cm. In this regard, it is known that the embodiments E1-E5 which are formulated to have perovskite-based compounds ranging from about 5 vol % to 15 vol % does not make the PTC chips have increased electrical resistivities but still possess excellent electrical conductivity at room temperature. As to cycle life test, it is clear that the embodiments E1-E5 of the present invention can endure 100 cycles without burnout when the applied voltage/current is set as 30V/30 A. However, for comparative examples C1-C3, R.sub.100c cannot be obtained because the devices are burnt out during the cycle life test. Apparently, perovskite-based compounds' intrinsic properties provide the device with excellent electrical performance in accordance with the specific volume percentages of the present invention.

[0044] As described above, the perovskite-based compounds have better capability to restrict movement of electric charges in PVDF system, and thus lower its influence on PVDF, by which the perovskite-based compounds may function as excellent dielectric inner fillers. For example, as for dielectric constant at room temperature, dielectric constant of PVDF ranges from about 6 to 10, BaTiO.sub.3 ranges from about 2000 to 4000, SrTiO.sub.3 ranges from about 200 to 250, and CaTiO.sub.3 ranges from about 150 to 190. No matter what kind of perovskite-based compounds is used, perovskite-based compounds are high-dielectric materials compared to PVDF and therefore may have desirable characteristics in terms of capacitance. That is, perovskite-based compounds possess better capability for electric polarization when placed in an external electric field, thereby attracting free radicals or other charged matters away from PVDF. During the blending process or period of trip of device, the free radicals or other charged matters are generated from the PTC material layer 11, and these charged matters would attack PVDF and make it degraded thereafter. However, possibilities of degradation of PVDF can be lowered due to the capability for charge attraction possessed by the perovskite-based compounds, by which the integrity of the composite system is well stabilized. Besides, hydrofluoric acid can also be generated upon degradation of PVDF, thereby causing environmental pollution, device corrosion, or other adverse factors that affect performance of the over-current protection device. Compared with Mg(OH).sub.2 of the comparative example C1, the perovskite-based compounds of the present invention are used to prevent hydrofluoric acid from generation, instead of neutralizing hydrofluoric acid after its generation. As to the comparative example C3, BN cannot provide the entire system with assistance to endure high voltage, although it may function as the flame retardant and mitigate NTC behavior after trip of device. It is noted that the embodiments E1-E5 have the same capability as the comparative example C3 in mitigating NTC behavior after trip of device when compared to the comparative examples C1 and C2. However, the experiments are not focused on NTC behavior after trip of device, so NTC behavior after trip of device is not described in detail herein. At least, it is understood that the embodiments E1-E5 not only have the advantage like that of the comparative example C3, but also produce the technical effect to endure repeated shocks of high voltage.

[0045] Please further refer to both Table 1 and Table 2. Table 2 shows that the PTC chip could have excellent electrical resistance retention ratio (R.sub.100c/R.sub.i) after appropriately formulating. The embodiments E1-E3 select BaTiO.sub.3 as their inner fillers, and the volume percentage thereof are 5.2 vol %, 10.1 vol % to 14.5 vol. %, respectively. It is found that as the volume percentage of BaTiO.sub.3 is adjusted to 10.1 vol %, the PTC chip with the inner filler of BaTiO.sub.3 has the lowest excellent electrical resistance retention ratio, which means it has the best capability for electrical resistance recovery. As to the embodiments E4 and E5, their electrical resistance retention ratios (R.sub.100c/R.sub.i) are 5.1 and 5.4, respectively. Although the embodiments E4 and E5 select different perovskite-based compounds, it can still maintain electrical resistance retention ratio approximate to that of the embodiments E2 if the volume percentage of the perovskite-based compound is adjusted to about 10 vol % (e.g., 12 vol % in the embodiment E4, and 9.3 vol % in the embodiment E5). In addition, as described above, BaTiO.sub.3, SrTiO.sub.3, and CaTiO.sub.3 sequentially have different dielectric constants from high to low. Correspondingly, the embodiments E2, E4 and E5 sequentially have different electrical resistance retention ratios (R.sub.100c/R.sub.i) from low to high when the volume percentage of perovskite-based compound is adjusted to be from 9.3 vol % to 12 vol %. Obtaining such result, it could be understood that because of the perovskite-based compound's aforementioned capability for electric polarization, or other physical or chemical characteristics, the volume percentage of BaTiO.sub.3, once adjusted to have a value approximate to those of the other two, can give the best electrical resistance retention ratio.

[0046] Particle size distribution of any perovskite-based compound of the to present invention is under control and in a specific range. Before being formulated to the composition as shown in Table 1, particle size distribution of each perovskite-based compound is measured by the particle size analyzer (commercialized brand name Malvern Mastersizer 2000). The details are shown in Table 3 below.

TABLE-US-00003 TABLE 3 perovskite-based compound D(0.1) D(0.5) D(0.9) BaTiO.sub.3 0.970 μm 8.290 μm 19.300 μm SrTiO.sub.3 0.586 μm 5.960 μm 21.800 μm CaTiO.sub.3 0.950 μm 9.150 μm 27.300 μm

[0047] As shown in Table 3, “D” stands for “Distribution of particle size”, and the number within brackets after “D” refers to the proportion of the particles. Specifically, D(0.1), D(0.5), and D(0.9) represent particle sizes. The total number of particles is calculated as 1, so 0.1, 0.5 and 0.9 refer to 10%, 50% and 90%, respectively. For example, D(0.1) means that 10% of particles are smaller than the values of D(0.1) listed in Table 3. D(0.5) and D(0.9) are interpreted in the same way. Accordingly, D(0.5) stands for the middle value of particle size distribution, that is, the median diameter. In other words, in BaTiO.sub.3, half the filler particles are smaller than 8.290 μm. In SrTiO.sub.3, half the filler particles are smaller than 5.960 μm. In CaTiO.sub.3, half the filler particles are smaller than 9.150 μm. In consideration of processability for blending the polymer matrix with the perovskite-based compounds and/or the conductive fillers, D(0.5) of the perovskite-based compound has to be appropriately controlled. if there are too many small filler particles, aggregation of the small-sized filler particles would dominantly occur. Also, the small-sized filler particles having light weight before blending will easily suspend in the air. If there are too many large-sized filler particles, sedimentation of the large-sized filler particles would dominantly occur, and hence the large-sized filler particles will deposit in certain regions of the PTC material layer after blending. In the latter case, the titanium-containing dielectric filler particles would stick together, causing agglomeration of these large-sized particles. In the present to invention, D(0.5) of BaTiO.sub.3, SrTiO.sub.3, or CaTiO.sub.3 is smaller than 10 μm, and such particle size is neither large nor small, and therefore it is beneficial to processability when blending with polymers and/or conductive fillers.

[0048] 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.