Over-current protection device

Abstract

An over-current protection device includes an electrode layer and a heat-sensitive layer. The heat-sensitive layer is laminated between a top metal layer and a bottom metal layer of the electrode layer, and includes a polymer matrix and an electrically conductive filler. The polymer matrix includes a polyolefin-based polymer. The electrically conductive filler includes a metal compound filler dispersed in the polymer matrix, thereby forming an electrically conductive path in the heat-sensitive layer. In addition, the metal compound filler has a core metal and a carbon layer. The carbon layer covers more than

Description

Technical Field

[0001] This invention relates to an overcurrent protection element, and more specifically, to an overcurrent protection element with low volume resistivity and stable resistance variation. Prior Technology

[0002] Conductive composite materials with a known positive temperature coefficient (PTC) exhibit highly sensitive resistance changes to specific temperature variations, making them suitable materials for current sensing elements. They are currently widely used in overcurrent protection components and circuit components. Specifically, PTC conductive composite materials maintain extremely low resistance at normal temperatures, allowing circuits or batteries to operate normally. However, when an overcurrent or overtemperature event occurs in the circuit or battery, its resistance instantaneously increases to a high resistance state (at least 10⁴ Ω), a phenomenon known as a "trip," which interrupts the overcurrent and protects the battery or circuit components.

[0003] In terms of basic structure, overcurrent protection elements consist of a PTC material layer and metal electrodes attached to both sides. The PTC material layer includes at least a matrix and conductive filler. The matrix is ​​composed of a polymer, while the conductive filler is composed of carbon black and / or metal compound fillers dispersed within the polymer. As mentioned above, the positive temperature coefficient characteristic of overcurrent protection elements mainly comes from the PTC material layer; therefore, various electrical characteristics of overcurrent protection elements can be further adjusted by modifying the PTC material layer.

[0004] To achieve lower volume resistivity, conductive fillers often incorporate a certain proportion of metal compound fillers. However, some high-hardness metal compound fillers are prone to lattice defects during processing due to friction or collision, resulting in high and unstable resistance when used as conductive fillers. For example, tungsten carbide often undergoes high-temperature and high-shear machining processes, which easily create defects on its surface, forming tungsten or ditungsten carbide (W₂C). It should be noted that tungsten or ditungsten carbide is easily oxidized to form tungsten oxide (WO₃), which, when adsorbing moisture, forms tungstic acid (WO₃‧nH₂O), making the powder acidic and more susceptible to forming hydrogen bonds with water molecules. Therefore, when tungsten carbide is used as a conductive filler, it is often affected by ambient humidity, exhibiting high or unstable resistance. Furthermore, when overcurrent protection components are manufactured as surface-mount devices (SMD), the insulating layer is often primarily composed of epoxy resin. Epoxy resin is particularly susceptible to moisture, which can cause the board to burst during the thermal process, resulting in bubbles or peeling at the interface between the insulation layer and the metal or composite material layer.

[0005] In view of this, there is an urgent need to develop a new overcurrent protection element that has low volume resistivity and excellent resistance stability. Summary of the Invention

[0006] According to one embodiment of the present invention, an overcurrent protection element includes an electrode layer and a thermistor layer. The electrode layer has an upper metal layer and a lower metal layer. The thermistor layer is stacked between the upper metal layer and the lower metal layer. The thermistor layer has a positive temperature coefficient and includes a polymer substrate and a conductive filler. The polymer substrate includes a polyolefin polymer. The conductive filler includes a metal-containing material dispersed in the polymer substrate, thereby forming a conductive channel in the thermistor layer. The metal-containing material has a core metal and a carbon layer covering the core metal. The carbon layer covers more than 60% of the surface area of ​​the core metal, with the total surface area of ​​the core metal being 100%.

[0007] According to some embodiments, the carbon layer completely covers the core metal, such that the number of carbon layers is at least one.

[0008] According to some embodiments, the pH value of the metal-containing material is between 6 and 9.

[0009] According to some embodiments, the maximum particle size of the metal-containing material is greater than 8 μm.

[0010] According to some embodiments, the maximum particle size of the metal-containing material is between 8 μm and 13.5 μm.

[0011] According to some embodiments, based on the volume of the thermistor layer as 100%, the polyolefin polymer accounts for 50% to 60%, while the conductive filler accounts for 40% to 50%.

[0012] According to some embodiments, the aforementioned core metal is tungsten carbide.

[0013] According to some embodiments, the polyolefin polymers are selected from the group consisting of low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear low-density polyethylene, polypropylene, and polybutene.

[0014] According to some embodiments, the thermistor layer does not contain flame retardants.

[0015] According to some embodiments, the polymer substrate does not contain fluoropolymers. Fluoropolymers are selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, ethylene-tetrafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, perfluoroalkyl oxygen-modified tetrafluoroethylene, poly(chlorotrifluorotetrafluoroethylene), difluoroethylene-tetrafluoroethylene polymer, tetrafluoroethylene-perfluorodioxane copolymer, difluoroethylene-hexafluoropropylene copolymer, difluoroethylene-hexafluoropropylene-tetrafluoroethylene trimer, and combinations thereof.

[0016] According to some embodiments, the overcurrent protection element includes a first resistance change rate of less than 1.2. The overcurrent protection element has an initial resistance value; the overcurrent protection element has a first resistance value after being heat-treated once and cooled to room temperature; and the first resistance value divided by the initial resistance value is the aforementioned first resistance change rate.

[0017] According to some embodiments, the overcurrent protection element includes a second resistance change rate of less than 1.5. The overcurrent protection element has an initial resistance value; the overcurrent protection element has a second resistance value after being heat-treated three times and cooled to room temperature; and the second resistance value divided by the initial resistance value is the aforementioned second resistance change rate.

[0018] According to some embodiments, the overcurrent protection element includes a third resistance change rate of less than 1.8. The overcurrent protection element, after undergoing three heat treatments and then being placed in an environment with a temperature / humidity of 85°C / 85% for 504 hours, possesses a third resistance value; and the third resistance value divided by the second resistance value constitutes the aforementioned third resistance change rate.

[0019] According to some embodiments, the overcurrent protection element includes a fourth resistance change rate of less than 2. The overcurrent protection element, after undergoing three heat treatments and then being placed in an environment with a temperature / humidity of 40°C / 95% for 504 hours, possesses a fourth resistance value; and the fourth resistance value divided by the second resistance value constitutes the aforementioned fourth resistance change rate.

[0020] According to some embodiments, at room temperature, the current that the overcurrent protection element can withstand per unit area is between 0.6 A / mm2 and 1.1 A / mm2, and the power that it can withstand is between 3.6 W / mm2 and 7 W / mm2.

[0021] According to some embodiments, at 85°C, the current per unit area that the overcurrent protection element can withstand is between 0.3 A / mm² and 0.7 A / mm², and the power it can withstand is between 2.2 W / mm² and 5 W / mm².

[0022] According to some embodiments, the top view area of ​​the overcurrent protection element is from 2 mm2 to 81 mm2.

[0023] According to some embodiments, the thickness of the thermistor layer is between 0.12 mm and 0.19 mm. Simple Explanation of the Diagram

[0024] Figure 1 shows the structure of the metallic material; Figure 2 shows the coverage of the carbon layer on the surface of the metal-containing material under various conditions; Figure 3 shows a cross-sectional view of an overcurrent protection element according to an embodiment of the present invention; and Figure 4 shows a top view of the overcurrent protection element in Figure 3. Implementation

[0025] To make the above and other technical contents, features and advantages of the present invention more apparent and understandable, relevant embodiments are provided below, and detailed descriptions are given in conjunction with the accompanying drawings.

[0026] Please refer to Figures 1 and 2. Figure 1 is a schematic diagram of the structure of the metal-containing material in this invention. Figure 2 shows the coverage of the carbon layer on the surface of the metal-containing material under various conditions. In the basic architecture of the overcurrent protection element, there is a composite material layer and metal layers stacked on top of and below it (as shown in Figures 3 and 4, which will be described in detail later). The composite material layer is mainly composed of polymer and conductive filler, while the conductive filler contains the aforementioned metal-containing material, thereby forming a stable conductive channel when the element is not activated. In order to obtain good electrical characteristics for the overcurrent protection element, this invention improves the metal-containing material in the conductive filler. As shown in Figure 1, the metal-containing material of this invention has a core metal 1 and a carbon layer 2 covering the surface of the core metal 1. The core metal 1 is a filler with high metal reactivity (e.g., tungsten carbide), which is easily affected by high temperature and physical shear force in various processing processes, resulting in lattice defects on the surface. By covering it with carbon layer 2, the structural integrity of the core metal 1 can be effectively maintained. Please refer to Figure 2 for a further explanation of the impact of the presence or absence, or quantity, of carbon layer 2 in different steps (i.e., steps S1, S2, S3, and S4), where carbon layer 2 is composed of carbon black particles 2a. In step S1, the core metal 1 is not covered with carbon layer 2. Subsequent high-temperature and high-shear processes will cause defects on its surface, generating a first impurity 3 and a second impurity 4. The first impurity 3 can be tungsten dicarbide (W2C), and the second impurity 4 can be tungsten oxide (WO3). For example, the aforementioned surface defects cause tungsten carbide to decompose during the process, generating tungsten dicarbide, which is then oxidized to form tungsten oxide. In other words, the surface of the core metal 1 without carbon layer 2 is prone to many unintended impurities, which significantly affects the purity of the core metal 1 and consequently the electrical characteristics of the overall component. However, more carbon layer 2 is not necessarily better. Please refer to steps S2, S3, and S4 in Figure 2. In step S2, the core metal 1 is co-fired with a small amount of carbon black, causing the carbon black particles 2a to partially cover the surface of the core metal 1. In this case, a portion of the surface of the core metal 1 is still exposed to the environment, and the protective effect provided by the carbon black is insufficient. In step S3, the core metal 1 is co-fired with a sufficient amount of carbon black, causing the carbon black particles 2a to completely cover the surface of the core metal 1. In this case, the surface of the core metal 1 is completely surrounded by carbon black particles 2a, forming a carbon layer 2, which provides excellent protection. In step S4, the core metal 1 is co-fired with an excessive amount of carbon black, causing the carbon black particles 2a to not only completely cover the surface of the core metal 1 but also form multiple carbon layers 2. In this case, the excessive carbon black coverage can actually increase the resistance value and affect the conductivity.

[0027] Based on the foregoing, the metallic material of the present invention can be subsequently applied to overcurrent protection elements. Please refer to Figures 3 and 4. Figure 3 shows the basic configuration of the overcurrent protection element 10 of the present invention. The overcurrent protection element 10 includes a thermistor layer 11 and an electrode layer. The thermistor layer 11 has an upper surface and a lower surface, while the electrode layer has an upper metal layer 12 and a lower metal layer 13 respectively attached to the upper and lower surfaces of the thermistor layer 11. Thus, the thermistor layer 11 contacts the upper metal layer 12 and the lower metal layer 13, and is stacked between them. In one embodiment, the upper metal layer 12 and the lower metal layer 13 may be composed of nickel-plated copper foil or other conductive metals. In addition, the thermistor layer 11 includes a polymer substrate and a conductive filler. The polymer substrate is an insulator that expands easily when heated, while the conductive filler is a conductor, thereby enabling the thermistor layer 11 to have a positive temperature coefficient characteristic. When the overcurrent protection element 10 is not triggered, the conductive filler is evenly distributed in the polymer substrate and connected in series to form a conductive channel; however, when the overcurrent protection element 10 is affected by high temperature, the volume of the polymer substrate expands and the crystalline region transforms into an amorphous region, causing the conductive channel to be cut off.

[0028] As mentioned above, the conductive filler of the present invention comprises a metallic material. The metallic material has a core metal 1 and a carbon layer 2 covering the core metal 1, wherein the core metal 1 may be tungsten carbide. With the total surface area of ​​the core metal 1 being 100%, the carbon layer 2 covers more than 60% of the surface area of ​​the core metal 1, preferably 70% to 100%. It is understood that the carbon layer 2 covering 100% of the surface area of ​​the core metal 1 means that the carbon layer 2 completely encapsulates the core metal 1. Preferably, the aforementioned carbon layer 2 is a single layer.

[0029] To verify the coating effect of carbon layer 2, the present invention further measures the pH value and particle size of the metal-containing material. In some embodiments, the pH value of the metal-containing material is between 6 and 9, for example, 6, 6.3, 7, 7.2, 7.9, 8, 8.4, or 9. In contrast, the pH value of the metal-containing material without carbon layer 2 is below 3, which is quite acidic. Obviously, the metal-containing material of the present invention is protected by carbon layer 2 and will not generate unintended impurities during the processing, thus maintaining a neutral or slightly alkaline pH value. As for the particle size, in some embodiments, the maximum particle size of the metal-containing material of the present invention is greater than 8 μm, for example, between 8 μm and 13.5 μm. In contrast, the maximum particle size of the metal-containing material without carbon layer 2 is 7.59 μm. It can be seen that the outer layer of the core metal 1 is coated with carbon layer 2, which makes the metal-containing material of the present invention exhibit a larger size.

[0030] Regarding the composition of the PTC material, the polymer substrate of this invention mainly uses polyolefin polymers, while the conductive filler is mainly composed of the aforementioned metal-containing materials. The polyolefin polymers are selected from the group consisting of low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear low-density polyethylene, polypropylene, and polybutene. In one embodiment, based on the volume of the thermistor layer 11 as 100%, the polyolefin polymer accounts for 50% to 60%, preferably 53% to 55%, for example 53%, 53.5%, 54%, 54.5%, or 55%; while the conductive filler accounts for 40% to 50%, preferably 45% to 47%, for example 45%, 45.5%, 46%, 46.5%, or 47%.

[0031] It should be noted that the thermistor layer 11 of the present invention does not contain a flame retardant. Flame retardants are selected from the group consisting of zinc oxide, antimony oxide, aluminum oxide, silicon oxide, calcium carbonate, magnesium sulfate, barium sulfate, magnesium hydroxide, aluminum hydroxide, calcium hydroxide, and barium hydroxide. Traditionally, overcurrent protection elements 10 can have their flammability reduced by flame retardants, allowing them to withstand high currents or high power without burning out. Additionally, in some cases, magnesium hydroxide can be used as an internal filler for acid-base neutralization. However, the present invention does not require flame retardants to withstand high currents or high power without burning out, and there is no need to consider acid-base neutralization. As a result, the composition of the PTC material is simpler; that is, the present invention reduces the complexity of formulation design while maintaining excellent electrical properties. Furthermore, in some embodiments, the polymer substrate of the overcurrent protection element 10 does not use a fluoropolymer. Fluoropolymers are selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, ethylene-tetrafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, perfluoroalkyl oxygen-modified tetrafluoroethylene, poly(chlorotrifluorotetrafluoroethylene), difluoroethylene-tetrafluoroethylene polymer, tetrafluoroethylene-perfluorodioxane copolymer, difluoroethylene-hexafluoropropylene copolymer, difluoroethylene-hexafluoropropylene-tetrafluoroethylene trimer, and combinations thereof. It should be understood that the aforementioned polyolefin polymers have higher crystallinity, and therefore generally exhibit better resistance reproducibility than fluoropolymers. Furthermore, the melting points of polyolefin polymers are often lower than those of fluoropolymers, resulting in correspondingly lower protection temperatures (i.e., lower triggering temperatures). Therefore, in some embodiments, this invention is a dedicated overcurrent protection element 10 for low-temperature protection and high resistance stability, and thus does not include fluoropolymers.

[0032] By means of the thermistor layer 11 mentioned above, the overcurrent protection element 10 of the present invention can have several good electrical characteristics. These are described in detail below.

[0033] In this invention, the overcurrent protection element 10 can maintain relatively good resistance stability in response to various environmental changes. Resistance stability can be evaluated by the resistance change rate (i.e., the first resistance change rate, the second resistance change rate, the third resistance change rate, and the fourth resistance change rate hereinafter). The lower the resistance change rate, the better the resistance value recovery capability or resistance recoverability of the overcurrent protection element 10, and the better the resistance reproducibility of the protection element.

[0034] The overcurrent protection element 10 includes a first resistance change rate of less than 1.35, such as 1.35, 1.21, 1.11, 1.03, or 1.09, preferably less than 1.2. The overcurrent protection element 10 has an initial resistance value, and after one heat treatment and cooling to room temperature, it has the first resistance value. The first resistance value divided by the initial resistance value is the aforementioned first resistance change rate. The overcurrent protection element 10 includes a second resistance change rate of less than 1.63, such as 1.63, 1.57, 1.29, 1.27, or 1.21, preferably less than 1.5. The overcurrent protection element 10 has a second resistance value after three heat treatments and cooling to room temperature, and the second resistance value divided by the initial resistance value is the aforementioned second resistance change rate. The overcurrent protection element 10 includes a third resistance change rate of less than 1.8, such as 1.16, 1.11, 1.1, or 1.08. After undergoing three heat treatments and then being placed in an environment with a temperature / humidity of 85°C / 85% for 504 hours, the overcurrent protection element 10 exhibits a third resistance value. The third resistance value divided by the second resistance value represents the aforementioned third resistance change rate. The overcurrent protection element 10 includes a fourth resistance change rate lower than 2, such as 1.21, 1.14, 1.13, 1.11, or 1.1. After undergoing three heat treatments and then being placed in an environment with a temperature / humidity of 40°C / 95% for 504 hours, the overcurrent protection element 10 exhibits a fourth resistance value. The fourth resistance value divided by the second resistance value represents the aforementioned fourth resistance change rate. In comparison, conventional overcurrent protection elements exhibit significantly higher resistance change rates than the overcurrent protection element 10 of this invention.

[0035] Please refer to Figure 4, which is a top view of the overcurrent protection element 10 in Figure 3. The overcurrent protection element 10 has a length A and a width B, and the area "A×B" is also equal to the area of ​​the thermistor layer 11. The thermistor layer 11 may have a top view area ranging from 2 mm² to 81 mm², depending on the product model. For example, the area "A×B" may be 1.5×1.5 mm², 2×2 mm², 2.3×2.3 mm², 2.5×3 mm², 2.8×3.5 mm², 4×4 mm², 5×5 mm², 5.1×6.1 mm², 5×7 mm², 7.62×7.62 mm², 8.2×7.15 mm², 7.3×9.5 mm², 7.62×9.35 mm², or 9×9 mm². Furthermore, the thickness of the thermistor layer 11 of the overcurrent protection element 10 can be from 0.12 mm to 0.19 mm, for example, 0.12 mm, 0.13 mm, 0.14 mm, 0.15 mm, 0.16 mm, 0.17 mm, 0.18 mm, or 0.19 mm. The thickness of the upper metal layer 12 and the lower metal layer 13 can be 0.035 mm (i.e., 1 oz), 0.07 mm (i.e., 2 oz), or other sizes, depending on the requirements. It should be understood that the overcurrent protection element 10 of the present invention has the same effect when applied to the above-mentioned sizes. Moreover, the overcurrent protection element 10 can be processed into industrially common component types as needed, such as surface-mount devices (SMD), axial-leaded devices (ALD), radial-leaded devices (RLD), or other types of components.

[0036] To illustrate the technical content of the present invention in more detail, Tables 1 to 7 below are further discussed using actual verification data.

[0037] Table 1 Before burning Mörby weight ratio Proportion WC CB WC CB WC-1 20:1 95.24% 4.76% 99.69% 0.31% WC-2 10:1 90.91% 9.09% 99.39% 0.61% WC-3 5:1 83.33% 16.67% 98.79% 1.21% WC-4 1:1 50.00% 50.00% 94.23% 5.77% WC-RM - 100.00% 0.00% 100.00% 0.00%

[0038] The core metal 1 used in this invention is tungsten carbide (WC). To coat the carbon layer 2 with tungsten carbide, co-firing with carbon black (CB) in different proportions was attempted. As shown in Table 1, WC-1, WC-2, WC-3, and WC-4 represent the different mixing ratios attempted in this invention. In WC-1, the molar percentages of tungsten carbide and carbon black are 95.24% and 4.76% (approximately 20:1), respectively, while the weight percentages are 99.69% and 0.31%, respectively. In WC-2, the molar percentages of tungsten carbide and carbon black are 90.91% and 9.09% (approximately 10:1), respectively, while the weight percentages are 99.39% and 0.61%, respectively. In WC-3, the molar percentages of tungsten carbide and carbon black are 83.33% and 16.67% (approximately 5:1), respectively, while the weight percentages are 98.79% and 1.21%, respectively. In WC-4, the molar percentages of tungsten carbide and carbon black were 50% and 50% respectively (approximately 1:1), while the weight percentages were 94.23% and 5.77% respectively. WC-RM served as the control group, in which tungsten carbide was not mixed with carbon black at all.

[0039] Table 2 After co-firing Ri (Ω) ρ (Ω·cm) Particle size distribution number of carbon layers pH value D(0.1) D(0.5) D(0.9) Dmax WC-1 0.001158 0.004053 1.48 2.56 4.62 8.71 0.7 6.3 WC-2 0.001008 0.003528 1.54 2.86 5.02 10 1 7.2 WC-3 0.001222 0.004277 1.55 2.98 5.22 11.48 1.6 7.9 WC-4 0.001587 0.005555 1.56 3.58 6.95 13.18 1.8 8.4 WC-RM 0.001745 0.006108 1.36 2.44 4.12 7.59 0 2.7

[0040] As shown in Table 2, each group was heat-treated in a vacuum environment at the same temperature (1250℃) in this experiment. The carbon layer coating was then observed, and particle size distribution and pH value were used to further confirm the presence of the carbon layer. The carbon layer coverage was observed using a transmission electron microscope (TEM), and the coverage area was calculated as 100% of the total surface area of ​​tungsten carbide. For example, in WC-1, carbon black covered 70% of the tungsten carbide surface, so the number of carbon layers was 0.7; in WC-2, carbon black covered 100% of the tungsten carbide surface (as shown in step S3 of Figure 2 above), so the number of carbon layers was 1; in WC-3, carbon black not only completely covered the tungsten carbide surface but also continued to stack on top of the first carbon layer, so the number of carbon layers was 1.6; and WC-4 had a similar situation to WC-3, with 1.8 carbon layers. As for WC-RM, no carbon coating was observed.

[0041] Particle size was measured using a Malvern Mastersizer 2000 particle size analyzer. D represents particle size distribution, and the number in square brackets after D represents the proportion of particles. For example, 0.1, 0.5, and 0.9 represent 10%, 50%, and 90%, respectively. For instance, D(0.1) means that 10% of the particles are smaller than this value, and the same applies to D(0.5) and d(0.9). Dmax refers to the largest particle size among all particles. Additionally, D(0.5) is the median value of the particle size distribution, also known as the median particle size. As shown in Table 2, the maximum particle size (Dmax) in WC-1, WC-2, WC-3, and WC-4 ranges from 8.71 μm to 13.18 μm, all larger than the maximum particle size (Dmax) of WC-RM.

[0042] In addition, as mentioned above, the pH values ​​of each group were also measured in this experiment. 10 grams of each group were added to 10 grams of water and dispersed by ultrasonic agitation for 30 minutes, followed by pH measurement of the pH value. Table 2 shows that the pH values ​​of WC-1, WC-2, WC-3, and WC-4 ranged from 6.3 to 8.4. Clearly, tungsten carbide, protected by carbon black, does not undergo unintended physical / chemical reactions, thus maintaining a neutral or slightly alkaline pH. In contrast, the tungsten carbide in WC-RM is less stable and easily forms acidic oxides (such as tungstic acid), hence its pH value is 2.7.

[0043] Next, this experiment continued to apply the aforementioned tungsten carbide to PTC materials, as detailed in Tables 3 to 7 below.

[0044] Table 3 Group WC source HDPE (vol%) WC (vol%) E1 WC-1 53 47 E2 WC-2 53 47 E3 WC-2 55 45 E4 WC-3 53 47 E5 WC-4 53 47 C1 WC-RM 53 47 C2 WC-RM 55 45

[0045] As shown in Table 3, the composition of the thermistor layer in groups E1 to E5 (i.e., Examples E1 to E5 of the present invention) and groups C1 to C2 (i.e., Comparative Examples C1 to C2) is displayed by volume percentage. The polymer substrate in each group is high-density polyethylene (HDPE), and the conductive filler is tungsten carbide (WC) from different sources. Furthermore, as mentioned above, considering the influence of error and the allowable range of variation, the volume percentage of HDPE can be appropriately adjusted within the range of 50% to 60%, and the volume percentage of tungsten carbide can be appropriately adjusted within the range of 40% to 50%. Within the aforementioned range, the overcurrent protection element can have the same or similar technical effects. The manufacturing process of the overcurrent protection element is described below. Based on the formulation presented in Table 3, the materials in the formulation were added to a twin-screw mixer manufactured by HAAKE Corporation for mixing. The mixing temperature was set to 215°C, the premixing time was 3 minutes, and the mixing time was 15 minutes. After mixing, a conductive polymer is obtained, which is then pressed into a sheet using a hot press at 210°C and 150 kg / cm². The sheet is then cut into squares of approximately 20 cm x 20 cm. Next, two nickel-plated copper foils are pressed onto both sides of the conductive polymer sheet using the same hot press at 210°C and 150 kg / cm², forming a three-layer structure. Finally, this sheet is punched into multiple wafers, which are the overcurrent protection elements. The overcurrent protection element has a length and width of 2.8 mm and 3.5 mm (i.e., a top-view area of ​​9.8 mm²), and an overall thickness of 0.28 mm, with the thermistor layer having a thickness of 0.14 mm. The wafers are then irradiated with a light dose of 15 kGy (the light dose can be adjusted as needed and is not a limitation of this invention), and 15 wafers are taken as test samples for subsequent experiments.

[0046] Table 4 Group Ri (Ω) R1 (Ω) ρi (Ω·cm) ρ1 (Ω·cm) R2 (Ω) R3 (Ω) R1 / Ri R2 / Ri R3 / Ri E1 0.00107 0.00116 0.00375 0.00407 0.00124 0.00136 1.09 1.16 1.27 E2 0.00098 0.00101 0.00343 0.00354 0.00109 0.00119 1.03 1.11 1.21 E3 0.00119 0.00132 0.00417 0.00462 0.00143 0.00153 1.11 1.20 1.29 E4 0.00148 0.00179 0.00518 0.00627 0.00204 0.00233 1.21 1.38 1.57 E5 0.00156 0.00211 0.00546 0.00739 0.00233 0.00255 1.35 1.49 1.63 C1 0.00165 0.00198 0.00578 0.00693 0.00222 0.00251 1.20 1.35 1.52 C2 0.00254 0.00376 0.00889 0.01316 0.00411 0.00501 1.48 1.62 1.97

[0047] Ri refers to the initial resistance value of the overcurrent protection element at room temperature. Furthermore, according to the formula for volume resistivity ρ = R × A / L, where R is the resistance value, L is the thickness, and A is the area, Ri can be substituted into the formula to obtain the volume resistivity (ρi). As shown in Table 4, the volume resistivity of Examples E1 to E5 is all below 0.0055 Ω·cm (i.e., 0.00343 Ω·cm to 0.00546 Ω·cm), and their conductivity is superior to that of Comparative Examples C1 and C2.

[0048] In practical applications, overcurrent protection components are assembled onto a substrate via soldering. This experiment then measures the resistance value after assembly, i.e., R1. More specifically, R1 refers to the resistance value measured after the overcurrent protection component has undergone one reflow soldering process and cooled to room temperature. Furthermore, the resistance change of the overcurrent protection component after assembly can be observed by the ratio of R1 to Ri (i.e., R1 / Ri), thereby evaluating the component's resistance stability. R1 / Ri is the first resistance change rate mentioned earlier. The R1 / Ri of Examples E1 to E5 falls between 1.03 and 1.35, maintaining good resistance stability. It is particularly noteworthy that the R1 / Ri of Examples E1 to E3 is quite close to 1, meaning almost no change, and is significantly lower than the R1 / Ri of Comparative Examples C1 and C2.

[0049] To further highlight the advantages of this invention in resistance stability, this experiment further subjected each group of overcurrent protection elements to two heat treatment (i.e., reflow) tests, and measured the electrical values, R2 and R3, after the second and third heat treatments, respectively. More specifically, R2 refers to the resistance value measured after the overcurrent protection element has undergone two reflow treatments and then cooled to room temperature. R3 refers to the resistance value measured after the overcurrent protection element has undergone three reflow treatments and then cooled to room temperature. As shown in Table 4, even after three heat treatments, the overcurrent protection element of this invention still maintains a resistance change rate far below 2 (i.e., 1.21 to 1.63). It is particularly noteworthy that, in terms of R3 / Ri (i.e., the second resistance change rate mentioned above), Examples E1 to E3 are still far lower than all Comparative Examples C1 and C2, demonstrating its stability.

[0050] Furthermore, components encounter various environmental factors during actual use. Therefore, this experiment placed the aforementioned overcurrent protection components, which had undergone three reflow processes, under different temperatures and humidity levels for testing. See Tables 5 and 6 for details.

[0051] Table 5 Group R3 (Ω) Temperature / Humidity (85℃ / 85%) R96hr (Ω) R336hr (Ω) R504hr (Ω) R504hr / R3 E1 0.00136 0.00147 0.00150 0.00158 1.16 E2 0.00119 0.00124 0.00127 0.00132 1.11 E3 0.00153 0.00158 0.00164 0.00168 1.10 E4 0.00233 0.00240 0.00245 0.00252 1.08 E5 0.00255 0.00259 0.00267 0.00275 1.08 C1 0.00251 0.00402 0.00452 0.00532 2.12 C2 0.00501 0.00797 0.00827 0.00947 1.89

[0052] In Table 5, each group was left to stand for different durations (96 hours, 336 hours, and 504 hours) at a temperature / humidity of 85°C / 85% before being cooled to room temperature. The resistance values ​​(R96hr, R336hr, and R504hr) at these different treatment durations were then measured. R504hr / R3 in Table 5 represents the third resistance change rate mentioned earlier. In Examples E1 to E5, R504hr / R3 ranged from 1.08 to 1.16, quite close to 1, showing almost no change. In contrast, Comparative Examples C1 and C2 had an R504hr / R3 of approximately 2, meaning the resistance change was approximately doubled. Therefore, it can be seen that using tungsten carbide filler coated with a carbon layer enables the overcurrent protection element of this invention to have good resistance stability.

[0053] Table 6 Group R3 (Ω) Temperature / Humidity (40℃ / 95%) R96hr (Ω) R336hr (Ω) R504hr (Ω) R504hr / R3 E1 0.00134 0.00143 0.00145 0.00162 1.21 E2 0.00116 0.00125 0.00128 0.00132 1.14 E3 0.00150 0.00163 0.00167 0.00170 1.13 E4 0.00228 0.00242 0.00246 0.00251 1.11 E5 0.00261 0.00279 0.00284 0.00287 1.10 C1 0.00259 0.00488 0.00531 0.00663 2.56 C2 0.00499 0.00878 0.00918 0.01003 2.01

[0054] In Table 6, each group was subjected to a temperature / humidity of 40°C / 95% and then left to stand for different durations (96 hours, 336 hours, and 504 hours) before being cooled to room temperature. The resistance values ​​(R96hr, R336hr, and R504hr) were then measured at these different treatment durations. R504hr / R3 in Table 6 represents the fourth resistance change rate described above. In Examples E1 to E5, R504hr / R3 ranged from 1.1 to 1.21, still quite close to 1. In contrast, the R504hr / R3 of Comparative Examples C1 and C2 both exceeded 2, meaning the resistance change was more than twice that of Comparative Examples C1 and C2. Even under different temperature / humidity conditions than those in Table 5, the overcurrent protection element of the present invention still exhibits good resistance stability.

[0055] The present invention also verifies other basic electrical characteristics of the overcurrent protection element.

[0056] Table 7 Group R1 (Ω) 23℃ 85℃ Itrip (A) Leakage current (A) Current per unit area (A / mm2) Power per unit area (W / mm2) Itrip (A) Leakage current (A) Current per unit area (A / mm2) Power per unit area (W / mm2) E1 0.00115 9.42 0.1948 0.961 5.767 6.14 0.0911 0.627 3.759 E2 0.00102 9.96 0.1990 1.016 6.098 6.66 0.0930 0.680 4.078 E3 0.00130 8.01 0.1846 0.817 4.904 5.04 0.0897 0.514 3,086 E4 0.00181 7.12 0.1774 0.727 4.359 4.08 0.0852 0.416 2.498 E5 0.00208 6.61 0.1662 0.674 4.047 3.62 0.0799 0.369 2.216 C1 0.00198 7.23 0.1801 0.738 4.427 4.76 0.0885 0.486 2.914 C2 0.00385 5.17 0.1986 0.528 3.165 2.84 0.0751 0.290 1.739

[0057] As mentioned above, R1 refers to the resistance value measured after the overcurrent protection element is assembled. Itrip refers to the minimum current value required to trigger the overcurrent protection element. Furthermore, during operation, the overcurrent protection element may not be able to completely cut off the current, resulting in leakage current. The current per unit area is the value obtained by dividing Itrip by the top-view area of ​​the overcurrent protection element (i.e., 9.8 mm²). In addition, with an applied voltage of 6 V, the minimum power (W) required for triggering per unit area can be further calculated, i.e., the power per unit area. In embodiments E1 to E5 of the present invention, Itrip at 23°C ranges from 6.61 A to 9.96 A, all generally higher than comparative examples C1 and C2. In terms of unit area, the current per unit area and the power per unit area at 23°C in embodiments E1 to E5 also show the same trend. In embodiments E1 to E5 of the present invention, Itrip at 85°C ranges from 3.62 A to 6.66 A, also generally higher than comparative examples C1 and C2. In terms of unit area, the current and power that can be withstood per unit area at 85°C in Examples E1 to E5 also show the same trend. As can be seen from the above, the overcurrent protection element of the present invention requires a higher current value to trigger, thus allowing a larger current to flow before triggering, and can also withstand the application of a larger current and power without burning out.

[0058] The technical content and features of this invention have been disclosed above. However, those skilled in the art may still make various substitutions and modifications that do not depart from the spirit of this invention based on the teachings and disclosures of this invention. Therefore, the scope of protection of this invention should not be limited to what is disclosed in the embodiments, but should include various substitutions and modifications that do not depart from this invention, and is covered by the following claims.

[0059] 1: Core Metal 2: Carbon layer 2a: Carbon black particles 3: First impurity 4: Second impurity 10: Overcurrent protection components 11: Thermistor layer 12: Upper metal layer 13: Lower metal layer A: Length B: Width S1, S2, S3, S4: Steps

Claims

1. An overcurrent protection element, comprising: an electrode layer having an upper metal layer and a lower metal layer; and a thermistor layer stacked between the upper metal layer and the lower metal layer, wherein the thermistor layer has a positive temperature coefficient characteristic and comprises: a polymer substrate comprising a polyolefin polymer; and a conductive filler comprising a metal-containing material dispersed in the polymer substrate, thereby forming a conductive channel of the thermistor layer, wherein the metal-containing material has a core metal and a carbon layer covering the core metal, wherein, based on the total surface area of ​​the core metal being 100%, the carbon layer covers more than 60% of the surface area of ​​the core metal, thereby the overcurrent protection element comprises a first resistance change rate of less than 1.2, wherein: The overcurrent protection element has an initial resistance value; the overcurrent protection element has a first resistance value after being heat-treated once and cooled to room temperature; and the first resistance value divided by the initial resistance value is the first resistance change rate.

2. The overcurrent protection element according to claim 1, wherein the carbon layer completely covers the core metal, such that the number of carbon layers is at least one.

3. The overcurrent protection element according to claim 2, wherein the maximum particle size of the metallic material is greater than 8 μm.

4. The overcurrent protection element according to claim 3, wherein the maximum particle size of the metallic material is between 8 μm and 13.5 μm.

5. The overcurrent protection element according to claim 1, wherein, based on the volume of the thermistor layer as 100%, the polyolefin polymer accounts for 50% to 60%, and the conductive filler accounts for 40% to 50%.

6. The overcurrent protection element according to claim 1, wherein the core metal is tungsten carbide.

7. The overcurrent protection element according to claim 1, wherein the polyolefin polymer is selected from the group consisting of low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear low-density polyethylene, polypropylene and polybutene.

8. The overcurrent protection element according to claim 1, wherein the thermistor layer does not contain a flame retardant.

9. The overcurrent protection element according to claim 1, wherein the polymer substrate does not contain a fluoropolymer, wherein the fluoropolymer is selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, ethylene-tetrafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, perfluorocarbon oxygen-modified tetrafluoroethylene, poly(chlorotrifluorotetrafluoroethylene), difluoroethylene-tetrafluoroethylene polymer, tetrafluoroethylene-perfluorodioxane copolymer, difluoroethylene-hexafluoropropylene copolymer, difluoroethylene-hexafluoropropylene-tetrafluoroethylene trimer, and combinations thereof.

10. The overcurrent protection element according to claim 1, wherein the overcurrent protection element includes a second resistance change rate of less than 1.5, wherein: The overcurrent protection element has an initial resistance value; the overcurrent protection element has a second resistance value after being heat-treated three times and cooled to room temperature; and the second resistance value divided by the initial resistance value is the rate of change of the second resistance.

11. The overcurrent protection element according to claim 10, wherein the overcurrent protection element includes a third resistance with a rate of change of less than 1.8, wherein: The overcurrent protection element, after being heat-treated three times and then placed in an environment with a temperature / humidity of 85℃ / 85% for 504 hours, has a third resistance value; and the third resistance value divided by the second resistance value is the rate of change of the third resistance.

12. The overcurrent protection element according to claim 10, wherein the overcurrent protection element includes a fourth resistance change rate of less than 2, wherein: After being heat-treated three times and then placed in an environment with a temperature / humidity of 40℃ / 95% for 504 hours, the overcurrent protection element has a fourth resistance value. And the fourth resistance value divided by the second resistance value is the rate of change of the fourth resistance.

13. The overcurrent protection element according to claim 1, wherein at room temperature, the current per unit area of ​​the overcurrent protection element is between 0.6 A / mm2 and 1.1 A / mm2, and the power per unit area is between 3.6 W / mm2 and 7 W / mm2.

14. The overcurrent protection element according to claim 1, wherein at 85°C, the current per unit area of ​​the overcurrent protection element is between 0.3 A / mm² and 0.7 A / mm², and the power per unit area is between 2.2 W / mm² and 5 W / mm².

15. The overcurrent protection element according to claim 1, wherein the top view area of ​​the overcurrent protection element is 2 mm2 to 81 mm2.

16. The overcurrent protection element according to claim 1, wherein the thickness of the thermistor layer is between 0.12 mm and 0.19 mm.

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