Small-size low-resistance high-molecular thermistor chip, manufacturing method and high-molecular PTC (Positive Temperature Coefficient) device

Abstract

The invention discloses a small-size low-resistance polymer thermistor chip, a manufacturing method and a polymer PTC (Positive Temperature Coefficient) device, and relates to the technical field of thermistor devices. The preparation method comprises the following steps: melting and mixing carbon chain coated and grafted tungsten carbide powder and polyethylene to obtain core material particles; carrying out hot rolling and film covering on the core material particles and the coarsened electrode film to form a thermistor chip plate; irradiating the thermistor chip board to obtain a small-size low-resistance polymer thermistor chip; the carbon chain coated and grafted tungsten carbide powder contains carbon black, a grafting modifier and tungsten carbide compound powder, and is subjected to irradiation pretreatment after modification. Carbon-coated grafted modified tungsten carbide powder is adopted as a conductive filler, carbon black changes the surface polarity of tungsten carbide and increases the contactable point locations of tungsten carbide particles, and irradiation pretreatment can solve the problem that internal conductive particles move due to polarity agglomeration after the thermistor expands, reduce circuit conduction gaps of the conductive particles and improve the conductivity of the thermistor. And the resistance drift problem of the small-size low-resistance polymer PTC is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of thermistor devices, and in particular to a small-size low-resistance polymer thermistor chip, a manufacturing method and a polymer PTC device. Background Art

[0002] Polymer thermistor (PPTC for short) is a polymer material with positive temperature coefficient thermistor characteristics, which is widely used in the field of current limiting protection and thermal protection. In recent years, with the increase in the capacity of power supply batteries for portable and wearable electronic products, the working standby time has increased, and the charging and discharging current has increased, while the overall size of the product has become smaller and smaller. The size requirements of the corresponding thermal protection and overcurrent protection devices must also be smaller and smaller, most of which are as small as 1206, 0805, 0603 and even 0402. The 1206, 0805, 0603, and 0402 protection devices made of the first generation of carbon black polymer PTC chips in the early days have high resistance and high energy consumption, and the device allows a small current to pass through, which can no longer meet the higher power charging and discharging requirements of the new generation of electronic products. The second generation of low-resistance polymer PTC products with cemented carbide as conductive particles have the characteristics of ultra-low resistance, high PTC strength and high heat dissipation, which just solves the problem of large impedance and small current of the first generation of carbon polymer PTC. It is a new functional material that has emerged in recent years. Among them, cemented carbide is the most common conductive filler of tungsten carbide. Tungsten carbide low-resistance polymer PTC device is a composite material with positive temperature coefficient thermistor effect obtained by melt mixing tungsten carbide with polyethylene, polyvinylidene fluoride and other polymer resins. It is a kind of electronic component made of chips and packaging using this kind of composite material. Because of its low resistance and small size, it is particularly suitable for use in Bluetooth headsets, health wearable monitoring products, and portable mobile electronics.

[0003] Although low-resistance polymer PTC devices are increasingly used as protective devices in portable electronic products, wearable products, fast charging and other electronic products, in actual applications, it is often found that low-resistance polymer thermistor devices have inexplicable resistance drift after welding, high temperature and high humidity, and cyclic use. The magnitude of such vicious drift is as high as 10 times or even 1000 times, and the device resistance changes from milliohm level to ohm level, resulting in product function degradation and false protection, affecting customer use. The above-mentioned undesirable phenomenon is particularly prone to occur in small-sized devices such as SMD surface mount products 1206, 0805, 0603, 0402, etc. The smaller the device, the higher the probability and defective ratio, which is the main obstacle to the promotion and application of ultra-low resistance polymer PTC devices.

[0004] Tungsten carbide low-resistance polymer PTC devices are a typical type of ultra-low-resistance polymer PTC devices, and are also the main application on the market. The main components of its chip are tungsten carbide and polyethylene. Because tungsten carbide filler belongs to hard alloy, it is generally highly polar, while polyethylene belongs to non-polar polymer substrate. The expansion coefficients of non-polar substrate and polar conductive filler are also quite different. When the two are melted and mixed, the conductive particles are prone to move or agglomerate. After molding, the conductive particles are prone to micro-movement when the chip undergoes PTC transformation or high and low temperature cycles during thermal protection. The change in the position of the conductive particles leads to gaps between the conductive paths after high and low temperature cycles. When the gap between the conductive particles that cannot be restored is too large, the device is prone to exponential increase in resistance.

[0005] In order to solve the above defects of low-resistance polymer thermistors, mainstream manufacturers in the industry have proposed many methods for surface modification of tungsten carbide. However, most of the commonly used surface modifiers will affect the conductivity of the conductive particles, and the incompatibility or insulation between the modifier and the polymer substrate will often affect the PTC effect of the device, limiting the use of commonly used modifiers such as silane coupling agents and titanates; some other modifiers undergo decomposition of the surface modified structure during secondary high-temperature melting and mixing, resulting in poor modification effect. In order to reduce the gap caused by this movement, the industry has also proposed a method of using the outer layer of the device to fix the epoxy resin layer to limit the expansion and moisture erosion of the thermistor, so as to improve the resistance stability of the ultra-low resistance polymer thermistor. This method also has a certain effect. However, due to the outer layer of epoxy glue sealing method, it is often only applicable to products with slightly larger devices. Small-sized devices are easily subject to process restrictions due to the small operating space available for wrapping and the high requirements for device size accuracy. It is often difficult to achieve all-around wrapping and sealing. Even if the outer layer of the small-sized device is wrapped and coated, its manufacturing yield is low. Summary of the invention

[0006] The technical problem to be solved by the present invention is that small-sized low-resistance devices are prone to resistance drift in high and low temperature environments or during thermal expansion, and a small-sized low-resistance polymer thermistor chip and a manufacturing method thereof are provided.

[0007] In order to solve the above problems, the present invention proposes the following technical solutions:

[0008] In a first aspect, the present invention provides a method for preparing a small-size low-resistance polymer thermistor chip, comprising the following steps:

[0009] The carbon chain-coated grafted tungsten carbide powder and polyethylene are melt-mixed at a mass ratio of 10:1-17:1 to obtain core material particles;

[0010] The core material particles and the roughened electrode film are subjected to thermal rolling and lamination to form a thermistor chip plate;

[0011] Irradiating the thermistor chip plate with an irradiation dose of 5 kGy-40 kGy to obtain a small-sized low-resistance polymer thermistor chip;

[0012] The carbon chain coated grafted tungsten carbide powder contains carbon black, a grafted modifier and tungsten carbide composite powder, and is irradiated and pretreated after modification.

[0013] A further technical solution is that, in terms of mass percentage, the carbon chain-coated grafted tungsten carbide powder contains 80.0-98.0% of tungsten carbide compound powder, 1.5-15.0% of grafted modifier, and 0.1-5.0% of carbon black.

[0014] A further technical solution is that the particle size D50 of the carbon chain-coated grafted tungsten carbide powder is ≤30 μm.

[0015] A further technical solution is that the tungsten carbide composite powder includes at least two of small-particle tungsten carbide powder of 0.1-1.0 μm, medium-particle tungsten carbide powder of 2.0-4.5 μm, and large-particle tungsten carbide powder of 5.0-10.0 μm.

[0016] A further technical solution is that in the tungsten carbide composite powder, medium-sized tungsten carbide powder accounts for 60-90%, small-sized tungsten carbide powder accounts for 10-25%, and large-sized tungsten carbide powder accounts for 5-15%.

[0017] A further technical solution is that the carbon black is superconducting carbon black. Commercially available superconducting carbon black includes Mitsubishi 3050B, Cabot XC72, Sterling C, and AC-80 high structure carbon black. Mitsubishi 3050B conductive carbon black and AC-80 superconducting carbon black are preferably used.

[0018] A further technical solution is that the melt index of the graft modifier is greater than 8 g / min, and the grafting rate is greater than 0.8 wt%.

[0019] A further technical solution is that the grafting modifier is maleic anhydride grafted polyethylene wax, and the grafting rate is greater than 0.8wt%.

[0020] A further technical solution is that the preparation method of the carbon chain coated grafted tungsten carbide powder is as follows:

[0021] According to the ratio, the tungsten carbide composite powder and the grafting modifier are first melted and mixed for grafting modification. After the modification is completed, carbon black is added and mixed evenly. The material is cooled and irradiated. The irradiated particles are ball-milled to obtain carbon chain-coated grafted tungsten carbide powder.

[0022] The present invention grafts and modifies tungsten carbide based on the principle of ultra-low resistance polymer PTC up-drifting, and simultaneously grafts conductive carbon black on the surface to form carbon chains, thereby changing the surface polarity of tungsten carbide. The grafted conductive carbon chains are fixed on the surface of the modified tungsten carbide through radiation cross-linking, thereby increasing the contact points of the tungsten carbide particles.

[0023] A further technical solution is that in the preparation of the carbon chain-coated grafted tungsten carbide powder, the irradiation dose is 80 kGy-160 kGy.

[0024] A further technical solution is to irradiate the thermistor chip plate with an irradiation dose of 5 kGy-40 kGy.

[0025] It is understood that the irradiation is carried out under an electron accelerator or a cobalt Co60 radiation source.

[0026] In a second aspect, the present invention provides a small-sized low-resistance polymer thermistor chip, which is prepared by the preparation method described in the first aspect.

[0027] A further technical solution is that the roughened electrode film is an electrolytic copper film with a thickness of 35-70 μm. Preferably, the electrolytic copper film has high temperature resistance and high ductility, and the surface of the electrolytic copper film can be nickel-plated.

[0028] In a third aspect, the present invention provides a polymer PTC device, characterized in that it includes the small-sized low-resistance polymer thermistor chip described in the second aspect, or the small-sized low-resistance polymer thermistor chip prepared by the preparation method described in the first aspect.

[0029] Furthermore, according to the different packaging forms of the PTC device, the small-size low-resistance polymer thermistor chip is processed into PTC device products with different packaging sizes.

[0030] Compared with the prior art, the technical effects achieved by the present invention include:

[0031] The invention provides a method for preparing a small-size low-resistance polymer thermistor chip. Carbon-coated grafted modified tungsten carbide powder is used as a conductive filler. Carbon black forms a conductive carbon chain on the surface of the grafted tungsten carbide. The surface polarity of the tungsten carbide is changed by irradiation pretreatment, and the contact points of the tungsten carbide particles are increased. The low-resistance polymer PTC device made by the method can solve the movement of the internal conductive particles of the thermistor due to polar agglomeration after expansion. The grafted carbon chain can reduce the circuit conduction gap of the conductive particles, reduce the probability of a "gap" caused by the displacement of the tungsten carbide particles, and reduce the resistance drift of the low-resistance polymer PTC.

[0032] The small-size low-resistance polymer thermistor chip provided by the present invention can solve the resistance drift problem of a small-size ultra-low-resistance polymer thermistor device with a low voltage below 12V, thereby improving the resistance stability of the small-size low-resistance polymer thermistor. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying any creative work.

[0034] Figure 1 This is a schematic diagram of the structure of a small-sized low-resistance polymer thermistor chip provided by the present invention; in the figure; a front roughened electrode film A1, a back roughened electrode film A2, and a polymer PTC core material B.

[0035] Figure 2 The structural diagram of a PTC product in the form of a surface mount package processed by the small-size low-resistance polymer thermistor chip provided by the present invention; in the figure, a front roughened electrode film A1, a back roughened electrode film A2, a polymer PTC core material B, a front FR4 board insulating outer layer C1, a back FR4 board insulating outer layer C2, a left-end tinned terminal D1, and a right-end tinned terminal D2.

[0036] Figure 3 This is an electron microscope image of the internal conductive particle arrangement and morphology structure of the small-sized low-resistance polymer PTC chip in Example 1.

[0037] Figure 4 : This is an electron microscope image of the internal conductive particle arrangement and morphology structure of the low-resistance polymer PTC chip in reference example 1#. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Similar component numbers in the drawings represent similar components. Obviously, the embodiments described below are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] It should be understood that when used in this specification and the appended claims, the terms "include" and "comprises" indicate the presence of described features, integers, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or combinations thereof.

[0040] It should also be understood that the terms used in this description of the embodiments of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the embodiments of the present invention. As used in the description of the embodiments of the present invention and the appended claims, unless the context clearly indicates otherwise, the singular forms of "a", "an" and "the" are intended to include plural forms.

[0041] See also Figure 1 , which is a structural schematic diagram of a small-size low-resistance polymer thermistor chip provided by an embodiment of the present invention. As can be seen from the figure, it includes a polymer PTC core material B and a roughened electrode film A1 bonded to the front side of the polymer PTC core material, and a roughened electrode film A2 bonded to the back side of the polymer PTC core material B.

[0042] In one embodiment, the roughened electrode film may be bonded to both sides of the polymer PTC core material by placing the polymer PTC core material in an extrusion calender and extruding it together with the roughened electrode film to form a sandwich structure of film-material-film stacking.

[0043] The polymer PTC core material of the embodiment of the present invention is obtained by compounding carbon chain-coated grafted tungsten carbide powder and polyethylene, and its precursor is core material particles. The technical solution of the present invention is described below with specific embodiments.

[0044] Example 1

[0045] Roughened electrode film raw material: 40μm thick high-temperature and high-ductility copper foil (Taiwan, China) with nickel plating on the surface, smooth surface (S surface) Ra0.11~0.13μm, rough surface (M surface) Rz 9.0~12.5μm, and elongation at 180℃ 5.4%~7.3% as the electrode film material.

[0046] Conductive filler: The conductive filler of Example 1# is carbon chain-coated grafted tungsten carbide powder with a particle size D50≤30μm; the conductive filler of Reference Example 1# is untreated tungsten carbide powder with a particle size of 2.0-3.5μm provided by a manufacturer in Zhuzhou.

[0047] The tungsten carbide composite powder of Example 1# is composed of: 400g of tungsten carbide fine powder with a particle size of 0.3μm, 1500g of tungsten carbide medium powder with a particle size of 2.0-3.5μm, and 300g of tungsten carbide coarse powder with a particle size of 5-8μm, all of which are provided by a manufacturer in Zhuzhou.

[0048] Graft modifier raw materials: German-made liconene PE Ma4351 GR maleic anhydride grafted polyethylene wax is used as a graft modifier, the melting point of the modifier is 120-126°C, the viscosity at 140°C is 200-500mPa.s, the acid value is 42-49mg (KOH / g), and the grafting rate is about 7%.

[0049] Carbon black raw material: superconducting carbon black AC-80.

[0050] Preparation of conductive filler (carbon chain coated grafted tungsten carbide powder) in Example 1: weigh 50g of graft modifier, weigh 2200g of tungsten carbide powder of different particle sizes, mix them to form tungsten carbide composite powder, mix the tungsten carbide composite powder with l iconene PE Ma4351 The GR graft modifier and the GR graft modifier are mixed together and put into the XSM-1 rubber and plastic experimental internal mixer at a room temperature below 40°C, and mixed at a high speed of 80r / min-120r / min for 20 minutes. The material temperature is controlled to be less than 100°C during the mixing process. After the mixing is completed, the speed is reduced to 10-20r / min, and then the cavity temperature is raised to 175°C. After the material temperature is higher than 175°C, it is mixed for more than 10 minutes to make the tungsten carbide powder react with the graft modifier. Finally, 5g of conductive carbon black AC-80 is slowly added in batches. After the addition is completed, the mixture is mixed for more than 5 minutes to obtain carbon chain coated graft modified tungsten carbide powder; the carbon chain coated graft modified tungsten carbide powder is cooled, crushed and vacuum packaged, the packaging thickness is less than 30mm, and then irradiated with 160kgy under Co60 cobalt source irradiation. The irradiated powder particles were placed in a MSK-SFM-1S modified tungsten carbide ball mill with 10 mm tungsten carbide grinding balls for ball milling and finally sieved with a 2500 mesh vibrating screen to obtain carbon chain-coated grafted tungsten carbide powder with a particle size D50≤30 μm.

[0051] The conductive filler of reference example 1# is tungsten carbide powder, which is not subjected to grafting modification and carbon coating treatment.

[0052] Preparation of low-resistance polymer PTC chip: Untreated 2.0-3.5μm tungsten carbide powder was used as the conductive filler of reference example 1#, and prefabricated carbon chain-coated grafted tungsten carbide powder was used as the conductive filler of embodiment 1#. Polyethylene and conductive filler were melt-blended at a mass fraction of 1:16.5, and attached to the electrode film material on an extrusion calender. Figure 1 The low-resistance polymer PTC chip of the structure shown is finally irradiated on an electron accelerator with an irradiation dose of 20 kgy to obtain the chips described in reference example 1# and embodiment 1#.

[0053] The reference example 1# chip and the embodiment 1# chip are processed into attached Figure 2 The surface mount PTC product shown has a 0603 size (length 1.65 mm, width 0.85 mm), a device model with a voltage of 6 V and a non-actuating current of 2 A.

[0054] The electrical performance and reliability of the PTC products obtained from the 1# embodiment group and the 1# reference example were compared and tested, and the data are shown in the following table.

[0055] Table 1 Performance test results of PTC products obtained in Example 1# and Reference Example 1#

[0056]

[0057]

[0058] The following are the definitions and test methods of the test items;

[0059] Nominal zero-power resistance (R0): The initial resistance value of the device (R0).

[0060] Post-soldering resistance (Rh): The resistance value of the device after it is mounted on the PCB and soldered using a standard reflow soldering curve and left to cool for 24 hours at 25±2℃.

[0061] -40℃~85℃ high and low temperature cycle: The device stays in a low temperature environment of -40℃, and then moves to a high temperature environment of 85℃ within 1 minute for one cycle. The dwell time at each temperature does not exceed 30 minutes. The resistance after conversion n times is recorded as (Rn).

[0062] Double 85 experiment: Place it at 85℃ and 85% humidity for 168h. After the experiment, place it at 25±2℃ for 24h and then test the resistance value, record it as Rs. Then perform a welding and record it as Rsh.

[0063] Resistance value after thermal protection: After the device is kept at 125℃ thermal protection transition temperature for 24 hours, and then kept at 25±2℃ for 24 hours, its resistance value Rb is measured.

[0064] Resistance value after switching action cycle: The device is subjected to 100 cycles of 6V / 50A switching protection, with a power-on time of 5S, an interval time of 15S, and a cooling time of 60S. After the test is completed, the resistance value Rv(100) is measured after standing at 25±2℃ for 24h.

[0065] The results in Table 1 show that compared with the reference example 1#, the initial resistance R0 of the low-resistance polymer thermistor in the embodiment 1# is larger, but the change rate during welding is small, especially in the environment of 85% humidity + 85°C temperature, it shows the advantage of controllable change rate of resistance Rsh after welding, and there is no resistance drift of 10 times, 100 times or even more than 1,000 times. This shows that the small-sized 0603 surface mount device obtained by using the carbon-coated grafted modified tungsten carbide powder as the conductive filler of the present invention has a significantly smaller change rate than the reference group after high and low temperature cold and hot cycles, high temperature and high humidity, and action cycles. It was observed under a scanning electron microscope. Figure 3 In the embodiment 1, the low-resistance polymer chip made of carbon-coated grafted modified tungsten carbide powder has improved compatibility and entanglement between conductive particles, and no stacking distance between particles is observed. Figure 4 In the 1# reference example chip, there are obvious gaps between the conductive particles stacked under an electron microscope.

[0066] Example 2

[0067] Roughened electrode film raw material: 40μm thick high-temperature and high-ductility copper foil (Taiwan, China) with nickel plating on the surface, smooth surface (S surface) Ra0.11~0.13μm, rough surface (M surface) Rz 9.0~12.5μm, and elongation at 180℃ 5.4%~7.3% as the electrode film material.

[0068] Conductive filler: The conductive filler of Example 2# is carbon chain-coated grafted tungsten carbide powder with a particle size D50≤30μm; the conductive filler of Reference Example 2# is untreated tungsten carbide powder with a Fisher particle size of 2.29μm and a medium particle size (D50) of 2.423μm provided by a factory in Jiujiang, Jiangxi.

[0069] The tungsten carbide composite powder of Example 2# is composed of: 300g of tungsten carbide fine powder with a particle size (D50) of 0.902μm, 2000g of tungsten carbide medium powder with a particle size (D50) of 2.423μm, and 600g of tungsten carbide coarse powder with a particle size (D50) of 5.06μm, all of which are provided by a factory in Jiujiang, Jiangxi. The tungsten carbide medium powder of this embodiment is the same as the powder model used in Reference Example 2#.

[0070] Graft modifier raw materials: German-made liconene PE Ma4351 GR maleic anhydride grafted polyethylene wax is used as a graft modifier, the melting point of the modifier is 120-126°C, the viscosity at 140°C is 200-500mPa.s, the acid value is 42-49mg (KOH / g), and the grafting rate is about 7%.

[0071] Carbon black raw material: superconducting carbon black Japan Mitsubishi 3050B.

[0072] Preparation of conductive filler (carbon chain coated grafted tungsten carbide powder) in Example 2#: Weigh 50g of grafted modified tungsten carbide powder and weigh 2900g of the above tungsten carbide powders of different particle sizes to form a tungsten carbide composite powder. The GR graft modifier and the GR graft modifier are mixed together and put into the XSM-1 rubber and plastic experimental internal mixer at a room temperature below 40°C, and mixed at a high speed of 80r / min-120r / min for 20 minutes. During the mixing process, the material temperature is detected and controlled to be less than 100°C. After the mixing is completed, the speed is reduced to 10-20r / min, and then the cavity temperature is raised to 175°C. After the material temperature is higher than 175°C, it is mixed for more than 10 minutes to make the tungsten carbide powder react with the graft modifier. Finally, 8g of conductive carbon black AC-80 is slowly added in batches. After the addition is completed, the mixture is mixed for more than 5 minutes to obtain carbon chain coated graft modified tungsten carbide powder; the carbon chain coated graft modified tungsten carbide powder is cooled, crushed and vacuum packaged, the packaging thickness is less than 30mm, and then irradiated with 160kgy under Co60 cobalt source irradiation. The irradiated powder particles were placed in a MSK-SFM-1S modified tungsten carbide ball mill with 10 mm tungsten carbide grinding balls for ball milling and finally sieved with a 2500 mesh vibrating screen to obtain carbon chain-coated grafted tungsten carbide powder with a particle size D50≤30 μm.

[0073] The conductive filler of reference example 2# is the above-mentioned tungsten carbide powder without grafting modification and carbon coating treatment.

[0074] Preparation of low-resistance polymer PTC chip: Untreated tungsten carbide powder with a medium particle size (D50) of 2.423 μm was used as the conductive filler of reference example 2#, and prefabricated carbon chain-coated grafted tungsten carbide powder was used as the conductive filler of embodiment 2#. Polyethylene and conductive filler were melt-blended at a mass fraction of 1:15, and attached to the electrode film material on an extruder calender. Figure 1 The low-resistance polymer PTC chip shown is finally irradiated on each chip plate in an electron accelerator with an irradiation dose of 16 kgy to obtain the chips described in Reference Example 2# and Embodiment 2#.

[0075] The reference example 2# chip and the embodiment 2# chip are processed into attached Figure 2 The surface mount PTC product shown has a size of 1206 (length 3.25 mm, width 1.65 mm), a device model of 6 V voltage, and a non-actuating current of 6.5 A.

[0076] The electrical performance and withstand voltage rating of the PTC products obtained in the 2# embodiment group and the 2# reference example were compared and tested, and the data are shown in the following table. Table 2 Performance test results of the PTC products obtained in the 2# embodiment group and the 2# reference example

[0077]

[0078]

[0079] The following are the definitions and test methods of the test items;

[0080] Nominal zero power resistance (R 0 ):The initial resistance value of the device (R0).

[0081] Post-soldering resistance (Rh): The resistance value of the device after it is mounted on the PCB and soldered using a standard reflow soldering curve and left to cool for 24 hours at 25±2℃.

[0082] -40℃~85℃ high and low temperature cycle: The device stays in a low temperature environment of -40℃, and then moves to a high temperature environment of 85℃ within 1 minute for one cycle. The dwell time at each temperature does not exceed 30 minutes. The resistance after conversion n times is recorded as (Rn).

[0083] Double 85 experiment: Place it at 85℃ and 85% humidity for 168h. After the experiment, place it at 25±2℃ for 24h and then test the resistance value, record it as Rs. Then perform a welding and record it as Rsh.

[0084] Resistance value after thermal protection: After the device is kept at 125℃ thermal protection transition temperature for 24 hours, and then kept at 25±2℃ for 24 hours, its resistance value Rb is measured.

[0085] Resistance value after switching action cycle: The device is subjected to 100 cycles of 6V / 50A switching protection, with a power-on time of 5S, an interval time of 15S, and a cooling time of 60S. After the test is completed, the resistance value Rv(100) is measured after standing at 25±2℃ for 24h.

[0086] The results in Table 2 show that compared with the reference example 2#, the low-resistance polymer thermistor device of the embodiment 2# has no significant difference in resistance Rh change rate after welding, double 85 test, thermal protection and switch action 100 cycles compared with the unmodified reference example 2# in the 1206 size, but the two show obvious differences in high and low temperature cycles. The reference example 2# shows a significant increase in R(100) after 100 high and low temperature cycles, showing drift failure, while the embodiment 2# obtained by the present invention using carbon-coated grafted modified tungsten carbide powder as a conductive filler shows better resistance stability.

[0087] In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0088] The above is a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can easily think of various equivalent modifications or replacements within the technical scope disclosed by the present invention, and these modifications or replacements should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A method for preparing a small-size low-resistance polymer thermistor chip, comprising the following steps: The carbon chain-coated grafted tungsten carbide powder and polyethylene are melt-mixed at a mass ratio of 10:1-17:1 to obtain core material particles; The core material particles and the roughened electrode film are subjected to thermal rolling and lamination to form a thermistor chip plate; Irradiating the thermistor chip plate at a dose of 5 kGy to 40 kGy to obtain a small-sized low-resistance polymer thermistor chip; The carbon chain coated grafted tungsten carbide powder contains carbon black, a grafted modifier and tungsten carbide composite powder, and is irradiated and pretreated after modification.

2. The preparation method according to claim 1, characterized in that Calculated by mass percentage, the carbon chain-coated grafted tungsten carbide powder contains 80.0-98.0% of tungsten carbide composite powder, 1.5-15.0% of grafted modifier, and 0.1-5.0% of carbon black.

3. The preparation method according to claim 1, characterized in that: The particle size D50 of the carbon chain-coated grafted tungsten carbide powder is ≤30 μm.

4. The preparation method according to claim 1, characterized in that: The tungsten carbide composite powder comprises at least two of small-particle-size tungsten carbide powder of 0.1-1.0 μm, medium-particle-size tungsten carbide powder of 2.0-4.5 μm, and large-particle-size tungsten carbide powder of 5.0-10 μm.

5. The preparation method according to claim 4, characterized in that: In the tungsten carbide composite powder, medium-sized tungsten carbide powder accounts for 60-90%, small-sized tungsten carbide powder accounts for 10-25%, and large-sized tungsten carbide powder accounts for 5-15%.

6. The preparation method according to claim 1, characterized in that: The graft modifier has a melt index greater than 8 g / min and a graft rate greater than 0.8 wt %.

7. The preparation method according to any one of claims 2 to 6, characterized in that: The preparation method of the carbon chain coated grafted tungsten carbide powder is as follows: According to the ratio, the tungsten carbide composite powder and the grafting modifier are first melted and mixed for grafting modification. After the modification is completed, carbon black is added and mixed evenly. The material is cooled and irradiated for pretreatment. The irradiated particles are ball-milled to obtain carbon chain-coated grafted tungsten carbide powder.

8. The preparation method according to claim 6, characterized in that: In the preparation of the carbon chain-coated grafted tungsten carbide powder, the radiation pretreatment dosage is 80 kGy-160 kGy.

9. A small-size low-resistance polymer thermistor chip, characterized in that: Prepared by the preparation method according to any one of claims 1 to 8.

10. A polymer PTC device, characterized in that: The invention comprises the small-sized low-resistance polymer thermistor chip as claimed in claim 9, or comprises the small-sized low-resistance polymer thermistor chip prepared by the preparation method as claimed in any one of claims 1 to 8.

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