NTC (Negative Temperature Coefficient) thermistor capable of resisting low temperature of-100 DEG C and preparation method thereof

Abstract

The invention discloses an NTC (Negative Temperature Coefficient) thermistor resistant to a low temperature of-100 DEG C and a preparation method thereof, and relates to the technical field of thermistors. The preparation method comprises the following steps: firstly, adding metal oxide mixed powder into a dispersing agent, and pressing into an NTC thermistor matrix by an isostatic pressing process; reacting silicon carbide, silver nitrate and sodium chloride to obtain silver-loaded silicon carbide, and reacting the silver-loaded silicon carbide and a silane coupling agent to obtain modified silicon carbide; the preparation method comprises the following steps: reacting polytetrafluoroethylene with acrylic acid under irradiation to obtain modified polytetrafluoroethylene; carrying out condensation reaction on the modified silicon carbide and the modified polytetrafluoroethylene to prepare conductive aerogel; and wrapping the outer layer of the NTC thermistor substrate with conductive aerogel, and then preparing the NTC thermistor by adopting a microwave sintering process. The low-temperature-resistant conductive aerogel is used for replacing conductive silver paste, and good performance stability can still be kept at the low temperature.

Description

Technical Field

[0001] The present invention relates to the technical field of thermistors, in particular to an NTC thermistor capable of withstanding low temperatures of -100°C and a preparation method thereof. Background Art

[0002] As a key component of temperature sensors, thermistors are widely used in numerous fields. NTC thermistors, due to their negative temperature coefficient (NTC) characteristic—where resistance decreases as temperature increases—play a vital role in temperature measurement and control. Certain applications, such as meteorological monitoring equipment in extremely cold regions, aerospace instruments operating in cryogenic environments, and ultra-low-temperature experimental equipment, require stable operation under these conditions, placing extremely high demands on the low-temperature performance of NTC thermistors.

[0003] Traditional NTC thermistors are typically designed for ambient or slightly wider temperature ranges. When temperatures drop to -100°C, existing NTC thermistor materials can experience unstable performance, reduced accuracy, and slow response speeds. Some existing low-temperature-resistant thermistors either have complex and expensive manufacturing processes, limiting their large-scale application; or their key performance indicators, such as accuracy and stability at low temperatures, still fail to meet practical requirements. Therefore, developing an NTC thermistor that can stably withstand temperatures of -100°C, exhibit good performance, and have a relatively simple and feasible manufacturing process is of great practical significance and application value. Summary of the Invention

[0004] The object of the present invention is to provide an NTC thermistor resistant to low temperatures of -100°C and a preparation method thereof, so as to solve the problems existing in the prior art.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions: the NTC thermistor with low temperature resistance of -100°C is prepared by adding a mixed powder of metal oxides to a dispersant and pressing it into an NTC thermistor matrix using an isostatic pressing process, and then wrapping the outer layer of the NTC thermistor matrix with a conductive aerogel and sintering it using microwaves; the conductive aerogel is prepared by a condensation reaction of modified silicon carbide and modified polytetrafluoroethylene; the modified silicon carbide is prepared by reacting silicon carbide, silver nitrate, and sodium chloride to obtain silver-loaded silicon carbide, and then reacting the silver-loaded silicon carbide with a silane coupling agent; and the modified polytetrafluoroethylene is prepared by reacting polytetrafluoroethylene and acrylic acid under irradiation.

[0006] Furthermore, an NTC thermistor resistant to low temperatures of -100°C includes the following preparation steps: (1) The metal oxide mixture obtained by mixing MnO2, Co3O4, and NiO in a molar percentage of (48%~55%): (25%~30%): (15%~23%) was placed in an agate mortar for the first ball milling. The metal oxide mixture, zirconia balls, and anhydrous ethanol were mixed in a mass ratio of 1:2~3:1, and wet-milled in a clockwise and counterclockwise alternating cycle with an alternating interval of 30 min. The powder was ball milled for 4 h to obtain a powder. The powder, dispersant, and deionized water were mixed in a mass ratio of 1: (0.01~0.02): (1.5~2), placed in a drying oven, and dried at 110~120℃ and 7~10Pa for 1~2 h. The powder was ball milled for a second time at a speed of 500 r / min for 8 h using a planetary ball mill. The powder was passed through a 100-mesh sieve to obtain a precursor powder. (2) The precursor powder was placed in a hydraulic press and held under pressure for 1 minute. Then, an isostatic press was used with a pressure of 300 MPa and a pressure of 5 to 10 kN to obtain a ceramic body. The ceramic body was placed in a microwave sintering furnace and heated to 500 °C at a rate of 10 °C / min. After sintering for 30 minutes, the body was taken out to obtain the NTC thermistor matrix. (3) Silicon carbide, 10% nitric acid aqueous solution, silver nitrate, sodium chloride, deionized water and anhydrous ethanol were mixed and stirred in a mass ratio of 1: (15-20): (0.1-0.2): (0.1-0.2): 30: 100, stirred at 70-80 ° C, 300-500 r / min for 10-12 h, filtered, and the filter cake was washed with anhydrous ethanol and deionized water for 3-5 times each, placed in a drying oven, and dried at 60-7 0°C, 5-8 Pa, and drying for 12-14 hours to obtain silver-loaded silicon carbide; mixing the silver-loaded silicon carbide, silane coupling agent, and ethanol in a mass ratio of 1: (0.04-0.05): (20-30) and stirring evenly, stirring and refluxing at 90-100°C, 300-500 r / min, and filtering, washing the filter cake with anhydrous ethanol 2-3 times, placing it in a drying oven, and drying it at 70°C, 7-10 Pa, for 4 hours to obtain modified silicon carbide; (4) Under nitrogen atmosphere, polytetrafluoroethylene, acrylic acid, ammonium ferrous sulfate hexahydrate, 50% sulfuric acid aqueous solution and ethanol are mixed in a mass ratio of 1: (0.5-0.7): (0.01-0.02): (2-2.5): (5-10), irradiated with γ-rays at a dose of 10-20 kGy for 10-15 h at 25-30 °C, filtered, and the filter cake is washed 2-3 times with deionized water, placed in a drying oven, and dried at 60 °C, 5-7 Pa for 2 h to obtain modified polytetrafluoroethylene; (5) Modified silicon carbide, modified polytetrafluoroethylene, dicyclohexylcarbodiimide, triethylamine and N,N-dimethylformamide were mixed in a mass ratio of 1: (0.3~0.5): (1~1.2): (2~3): (10~20), stirred at 25~30℃, 300~500r / min for 10~12 hours, filtered, and the solid was placed in a drying oven at 60~70℃, 6~10Pa for 4~6 hours to obtain to functional silicon carbide; a silicon source, a 0.5 mol / L hydrochloric acid aqueous solution, and anhydrous ethanol are mixed in a ratio of 1: (0.05-0.08): (4-6), stirred at 40-50°C and 300-500 r / min for 1-2 hours, 0.3-0.8 times the amount of functional silicon carbide as the silicon source is added, and stirring is continued for 2-3 hours to obtain a wet gel, which is placed in a high-pressure reactor and replaced with liquid carbon dioxide for 12 hours to obtain a conductive aerogel; (6) A layer of conductive aerogel is evenly spread on the bottom of the groove of the mold, and a 0.05 mm thick adhesive is coated. An NTC thermistor substrate with a thickness of 0.5 to 0.8 mm is placed in the mold. Then, an adhesive with a thickness of 0.05 mm and a conductive aerogel are sequentially spread on the NTC thermistor substrate. The mold is placed in a microwave sintering furnace for calcination, cooled naturally, and the mold is taken out and demolded to obtain an NTC thermistor; the adhesive is polyimide glue.

[0007] Furthermore, the first ball milling method in step (1) is as follows: the metal oxide mixture: zirconium oxide balls: anhydrous ethanol are mixed in a mass ratio of 1: (2-3): 1, and the mixture is wet-milled in an alternating clockwise and counterclockwise direction with an alternating interval of 30 minutes.

[0008] Furthermore, the dispersant in step (1) is: polyether P123.

[0009] Furthermore, the parameters of the hydraulic press in step (2) are: the mold is 10 mm, and the pressure is 5 to 10 kN.

[0010] Furthermore, the silane coupling agent in step (3) is one or more of (3-aminopropyl)trimethoxysilane, γ-aminoethylaminopropyltrimethoxysilane, and 3-aminopropyltriethoxysilane.

[0011] Furthermore, the silicon carbide described in step (5) is: RS07.

[0012] Furthermore, the silicon source in step (5) is: ethyl orthosilicate.

[0013] Furthermore, the parameters of the high-pressure reactor in step (5) are: temperature of 35-40°C and pressure of 6-10 MPa.

[0014] Furthermore, the setting parameters of the microwave sintering furnace described in step (6) are: a heating rate of 5°C / min, heating to 100°C, 200°C, and 300°C respectively, and keeping each temperature zone for 30 minutes, 50 minutes, and 1 to 2 hours respectively.

[0015] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention uses a microwave sintering process, and the temperature difference between different parts of the electrode is small, which can obtain a uniform fine-grained structure. The small grain size enables electrons to transmit quickly in the material and has a shorter response time at low temperatures.

[0016] (2) The present invention uses a low-temperature-resistant conductive aerogel to replace the conductive silver paste, providing a richer conductive channel and improving the conductive properties of the thermistor. The conductive aerogel has excellent low-temperature resistance. The molecular chain can still maintain its mobility at extremely low temperatures, avoiding brittle fracture, not absorbing moisture, and avoiding performance degradation due to ice formation. The stress transfer between interfaces is good, which reduces the shrinkage of the thermistor caused by stress concentration in low-temperature environments, and can still maintain good electrical performance stability at low temperatures. DETAILED DESCRIPTION

[0017] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described 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 making creative efforts are within the scope of protection of the present invention.

[0018] Example 1; (1) A metal oxide mixture obtained by mixing MnO2, Co3O4, and NiO in a molar percentage of 51%:26%:23% was placed in an agate mortar and ball-milled for the first time. The metal oxide mixture, zirconium oxide balls, and anhydrous ethanol were mixed in a mass ratio of 1:2:1, and wet-milled in a clockwise and counterclockwise alternating cycle with an alternating interval of 30 minutes. The powder was ball-milled for 4 hours to obtain a powder. The powder, polyether P123, and deionized water were mixed in a mass ratio of 1:0.01:1.5, placed in a drying oven, and dried at 110°C and 7Pa for 1 hour. The mixture was ball-milled for a second time at a speed of 500 r / min for 8 hours using a planetary ball mill, and passed through a 100-mesh sieve to obtain a precursor powder. (2) The precursor powder was placed in a hydraulic press with a mold of 10 mm, the pressure was set to 5 kN and the pressure was maintained for 1 min, and then the isostatic press was used with a pressure of 300 MPa and the pressure was maintained for 4 min to obtain a ceramic body. The ceramic body was placed in a microwave sintering furnace and heated to 500 °C at a rate of 10 °C / min. After sintering for 30 min, it was taken out to obtain the NTC thermistor matrix; (3) Silicon carbide, 10% nitric acid aqueous solution, silver nitrate, sodium chloride, deionized water and anhydrous ethanol were mixed and stirred in a mass ratio of 1:15:0.1:0.1:30:100, stirred at 70 ° C, 300 r / min for 10 hours, filtered, and the filter cake was washed with anhydrous ethanol and deionized water three times each, placed in a drying oven, and dried at 60 ° C, 5 Pa for 12 hours to obtain silver-loaded silicon carbide; silver-loaded silicon carbide, γ-aminoethylaminopropyltrimethoxysilane and ethanol were mixed and stirred in a mass ratio of 1:0.04:20, stirred and refluxed at 90 ° C, 300 r / min for 3 hours, filtered, and the filter cake was washed with anhydrous ethanol twice, placed in a drying oven, and dried at 70 ° C, 7 Pa for 4 hours to obtain modified silicon carbide; (4) In a nitrogen atmosphere, polytetrafluoroethylene, acrylic acid, ammonium ferrous sulfate hexahydrate, a 50% sulfuric acid aqueous solution and ethanol were mixed in a mass ratio of 1:0.5:0.01:2:5, irradiated with γ-rays at an irradiation dose of 10 kGy for 10 h at 25 ° C, filtered, and the filter cake was washed twice with deionized water, placed in a drying oven, and dried at 60 ° C, 5 Pa for 2 h to obtain modified polytetrafluoroethylene; the molecular weight of the polytetrafluoroethylene is 10 6 g / mol; (5) Modified silicon carbide, modified polytetrafluoroethylene, dicyclohexylcarbodiimide, triethylamine and N,N-dimethylformamide were mixed in a mass ratio of 1:0.3:1:2:10, stirred at 25°C and 300 r / min for 10 hours, filtered and the solid was taken out, placed in a drying oven, and dried at 60°C and 6Pa for 4 hours to obtain functional silicon carbide; tetraethyl orthosilicate, 0.5 mol / L hydrochloric acid aqueous solution and anhydrous ethanol were mixed in a ratio of 1:0.05:4, stirred at 40°C and 300 r / min for 1 hour, and functional silicon carbide 0.3 times of tetraethyl orthosilicate was added, and continued to stir for 2 hours to obtain a wet gel, which was placed in a high-pressure reactor at a temperature of 35°C and a pressure of 6 MPa, and replaced with liquid carbon dioxide for 12 hours to obtain a conductive aerogel; (6) A layer of conductive aerogel was evenly spread on the bottom of the groove of the mold, and a 0.05 mm thick adhesive was applied. A 0.5 mm thick NTC thermistor substrate was placed in the mold. Then, a 0.05 mm thick adhesive and conductive aerogel were sequentially spread on the NTC thermistor substrate. The mold was placed in a microwave sintering furnace and calcined at a heating rate of 5°C / min to 100°C, 200°C, and 300°C, respectively. Each temperature zone was kept at this temperature for 30 min, 50 min, and 1 h, respectively. The temperature was naturally cooled, the mold was removed, and demolding was performed to obtain an NTC thermistor. The adhesive was a polyimide adhesive, model YJ-22.

[0019] Example 2; (1) A metal oxide mixture obtained by mixing MnO2, Co3O4, and NiO in a molar percentage of 48%:30%:22% was placed in an agate mortar for the first ball milling, the metal oxide mixture: zirconium oxide balls: anhydrous ethanol were mixed in a mass ratio of 1:2.5:1, and wet-milled in a clockwise and counterclockwise alternating cycle with an alternating interval of 30 min. The powder was ball milled for 4 h to obtain a powder, the powder, polyether P123, and deionized water were mixed in a mass ratio of 1:0.015:1.7, placed in a drying oven, and dried at 110°C and 8 Pa for 2 h. The mixture was ball milled for a second time at a speed of 500 r / min for 8 h using a planetary ball mill, and passed through a 100-mesh sieve to obtain a precursor powder; (2) The precursor powder was placed in a hydraulic press with a mold of 10 mm, the pressure was set to 8 kN and the pressure was maintained for 1 min, and then the isostatic press was used with a pressure of 300 MPa and the pressure was maintained for 4 min to obtain a ceramic body. The ceramic body was placed in a microwave sintering furnace and heated to 500 °C at a rate of 10 °C / min. After sintering for 30 min, it was taken out to obtain the NTC thermistor matrix; (3) Silicon carbide, 10% nitric acid aqueous solution, silver nitrate, sodium chloride, deionized water and anhydrous ethanol were mixed and stirred in a mass ratio of 1:17:0.15:0.15:30:100, stirred at 75 ° C, 400 r / min for 11 hours, filtered, and the filter cake was washed with anhydrous ethanol and deionized water 4 times each, placed in a drying oven, and dried at 70 ° C, 8 Pa for 13 hours to obtain silver-loaded silicon carbide; silver-loaded silicon carbide, γ-aminoethylaminopropyltrimethoxysilane and ethanol were mixed and stirred in a mass ratio of 1:0.045:25, stirred at 100 ° C, 400 r / min, and refluxed for 4 hours, filtered, and the filter cake was washed with anhydrous ethanol 3 times, placed in a drying oven, and dried at 70 ° C, 10 Pa for 4 hours to obtain modified silicon carbide; (4) In a nitrogen atmosphere, polytetrafluoroethylene, acrylic acid, ammonium ferrous sulfate hexahydrate, a 50% sulfuric acid aqueous solution and ethanol were mixed in a mass ratio of 1:0.6:0.015:2.3:8, irradiated with γ-rays at an irradiation dose of 15 kGy for 12.5 h at 25 ° C, filtered, and the filter cake was washed twice with deionized water, placed in a drying oven, and dried at 60 ° C, 5 Pa for 2 h to obtain modified polytetrafluoroethylene; the molecular weight of the polytetrafluoroethylene is 10 6 g / mol; (5) Modified silicon carbide, modified polytetrafluoroethylene, dicyclohexylcarbodiimide, triethylamine and N,N-dimethylformamide were mixed in a mass ratio of 1:0.4:1.1:2.5:15, stirred at 30°C and 400 r / min for 11 hours, filtered and the solid was taken out, placed in a drying oven, and dried at 70°C and 70 Pa for 5 hours to obtain functional silicon carbide; tetraethyl orthosilicate, 0.5 mol / L hydrochloric acid aqueous solution and anhydrous ethanol were mixed in a ratio of 1:0.07:5, stirred at 45°C and 400 r / min for 1 hour, and functional silicon carbide 0.5 times of tetraethyl orthosilicate was added, and continued to stir for 2 hours to obtain a wet gel, which was placed in a high-pressure reactor at a temperature of 40°C and a pressure of 10 MPa, and replaced with liquid carbon dioxide for 12 hours to obtain a conductive aerogel; (6) A layer of conductive aerogel is evenly spread on the bottom of the groove of the mold, and a 0.05 mm thick adhesive is coated. An NTC thermistor substrate with a thickness of 0.6 mm is placed in the mold. Then, an adhesive with a thickness of 0.05 mm and a conductive aerogel are sequentially spread on the NTC thermistor substrate. The mold is placed in a microwave sintering furnace and calcined at a heating rate of 5 ° C / min. The temperature is raised to 100 ° C, 200 ° C, and 300 ° C respectively. Each temperature zone is kept warm for 30 min, 50 min, and 2 h respectively. The temperature is naturally cooled, the mold is taken out, and the mold is demoulded to obtain an NTC thermistor. The adhesive is a polyimide glue, model YJ-22.

[0020] Example 3; (1) A metal oxide mixture obtained by mixing MnO2, Co3O4, and NiO in a molar percentage of 53%:25%:22% was placed in an agate mortar and ball-milled for the first time. The metal oxide mixture, zirconium oxide balls, and anhydrous ethanol were mixed in a mass ratio of 1:3:1, and wet-milled in a clockwise and counterclockwise alternating cycle with an alternating interval of 30 minutes. The powder was ball-milled for 4 hours to obtain a powder. The powder, polyether P123, and deionized water were mixed in a mass ratio of 1:0.02:2, placed in a drying oven, and dried at 120°C and 10Pa for 2 hours. The mixture was ball-milled for a second time at a speed of 500 r / min for 8 hours using a planetary ball mill, and passed through a 100-mesh sieve to obtain a precursor powder. (2) The precursor powder was placed in a hydraulic press with a mold of 10 mm, the pressure was set to 10 kN and the pressure was maintained for 1 min, and then the isostatic press was used with a pressure of 300 MPa and the pressure was maintained for 4 min to obtain a ceramic body. The ceramic body was placed in a microwave sintering furnace and heated to 500 °C at a rate of 10 °C / min. After sintering for 30 min, it was taken out to obtain the NTC thermistor matrix; (3) Silicon carbide, 10% nitric acid aqueous solution, silver nitrate, sodium chloride, deionized water and anhydrous ethanol were mixed and stirred in a mass ratio of 1:20:0.2:0.2:30:100, stirred at 80°C, 500 r / min for 12 hours, filtered, and the filter cake was washed with anhydrous ethanol and deionized water 5 times each, placed in a drying oven, and dried at 70°C, 8Pa for 14 hours to obtain silver-loaded silicon carbide; silver-loaded silicon carbide, γ-aminoethylaminopropyltrimethoxysilane and ethanol were mixed and stirred in a mass ratio of 1:0.05:30, stirred at 100°C, 500 r / min, and refluxed for 4 hours, filtered, and the filter cake was washed with anhydrous ethanol 3 times, placed in a drying oven, and dried at 70°C, 10Pa for 4 hours to obtain modified silicon carbide; (4) In a nitrogen atmosphere, polytetrafluoroethylene, acrylic acid, ammonium ferrous sulfate hexahydrate, a 50% sulfuric acid aqueous solution and ethanol were mixed in a mass ratio of 1:0.7:0.02:2.5:10, irradiated with γ-rays at an irradiation dose of 20 kGy at 30°C for 15 h, filtered, and the filter cake was washed three times with deionized water, placed in a drying oven, and dried at 60°C and 7 Pa for 2 h to obtain modified polytetrafluoroethylene; the molecular weight of the polytetrafluoroethylene was 10 6 g / mol; (5) Modified silicon carbide, modified polytetrafluoroethylene, dicyclohexylcarbodiimide, triethylamine and N,N-dimethylformamide were mixed in a mass ratio of 1:0.5:1.2:3:20, stirred at 30°C and 500 r / min for 12 hours, filtered and the solid was taken out, placed in a drying oven, and dried at 70°C and 10Pa for 6 hours to obtain functional silicon carbide; tetraethyl orthosilicate, 0.5 mol / L hydrochloric acid aqueous solution and anhydrous ethanol were mixed in a ratio of 1:0.08:6, stirred at 50°C and 500 r / min for 2 hours, and functional silicon carbide 0.6 times of tetraethyl orthosilicate was added, and continued to stir for 3 hours to obtain wet gel, which was placed in a high-pressure reactor at a temperature of 40°C and a pressure of 10 MPa, and replaced with liquid carbon dioxide for 12 hours to obtain conductive aerogel; (6) A layer of conductive aerogel was evenly spread on the bottom of the groove of the mold, and a 0.05 mm thick adhesive was applied. A 0.8 mm thick NTC thermistor substrate was placed in the mold. Then, a 0.05 mm thick adhesive and conductive aerogel were sequentially spread on the NTC thermistor substrate. The mold was placed in a microwave sintering furnace and calcined at a heating rate of 5°C / min to 100°C, 200°C, and 300°C, respectively. Each temperature zone was kept at this temperature for 30 min, 50 min, and 2 h, respectively. The temperature was naturally cooled, the mold was removed, and demolding was performed to obtain an NTC thermistor. The adhesive was a polyimide adhesive, model YJ-22.

[0021] Example 4; (1) A metal oxide mixture obtained by mixing MnO2, Co3O4, and NiO in a molar percentage of 55%:26%:19% was placed in an agate mortar and ball-milled for the first time. The metal oxide mixture, zirconium oxide balls, and anhydrous ethanol were mixed in a mass ratio of 1:2:1, and wet-milled in a clockwise and counterclockwise alternating cycle with an alternating interval of 30 minutes. The powder was ball-milled for 4 hours to obtain a powder. The powder, polyether P123, and deionized water were mixed in a mass ratio of 1:0.01:1.5, placed in a drying oven, and dried at 110°C and 7Pa for 1 hour. The mixture was ball-milled for a second time at a speed of 500 r / min for 8 hours using a planetary ball mill, and passed through a 100-mesh sieve to obtain a precursor powder. (2) The precursor powder was placed in a hydraulic press with a mold of 10 mm, the pressure was set to 5 kN and the pressure was maintained for 1 min, and then the isostatic press was used with a pressure of 300 MPa and the pressure was maintained for 4 min to obtain a ceramic body. The ceramic body was placed in a microwave sintering furnace and heated to 500 °C at a rate of 10 °C / min. After sintering for 30 min, it was taken out to obtain the NTC thermistor matrix; (3) Silicon carbide, 10% nitric acid aqueous solution, silver nitrate, sodium chloride, deionized water and anhydrous ethanol were mixed and stirred in a mass ratio of 1:15:0.1:0.1:30:100, stirred at 70 ° C, 300 r / min for 10 hours, filtered, and the filter cake was washed with anhydrous ethanol and deionized water three times each, placed in a drying oven, and dried at 60 ° C, 5 Pa for 12 hours to obtain silver-loaded silicon carbide; silver-loaded silicon carbide, γ-aminoethylaminopropyltrimethoxysilane and ethanol were mixed and stirred in a mass ratio of 1:0.04:20, stirred and refluxed at 90 ° C, 300 r / min for 3 hours, filtered, and the filter cake was washed with anhydrous ethanol twice, placed in a drying oven, and dried at 70 ° C, 7 Pa for 4 hours to obtain modified silicon carbide; (4) In a nitrogen atmosphere, polytetrafluoroethylene, acrylic acid, ammonium ferrous sulfate hexahydrate, a 50% sulfuric acid aqueous solution and ethanol were mixed in a mass ratio of 1:0.5:0.01:2:5, irradiated with γ-rays at an irradiation dose of 10 kGy for 10 h at 25 ° C, filtered, and the filter cake was washed twice with deionized water, placed in a drying oven, and dried at 60 ° C, 5 Pa for 2 h to obtain modified polytetrafluoroethylene; the molecular weight of the polytetrafluoroethylene is 10 6 g / mol; (5) Modified silicon carbide, modified polytetrafluoroethylene, dicyclohexylcarbodiimide, triethylamine and N,N-dimethylformamide were mixed in a mass ratio of 1:0.3:1:2:10, stirred at 25°C and 300 r / min for 10 hours, filtered and the solid was taken out, placed in a drying oven, and dried at 60°C and 6Pa for 4 hours to obtain functional silicon carbide; tetraethyl orthosilicate, 0.5 mol / L hydrochloric acid aqueous solution and anhydrous ethanol were mixed in a ratio of 1:0.05:4, stirred at 40°C and 300 r / min for 1 hour, and functional silicon carbide 0.8 times of tetraethyl orthosilicate was added, and continued to stir for 2 hours to obtain wet gel, which was placed in a high-pressure reactor at a temperature of 35°C and a pressure of 6 MPa, and replaced with liquid carbon dioxide for 12 hours to obtain conductive aerogel; (6) A layer of conductive aerogel was evenly spread on the bottom of the groove of the mold, and a 0.05 mm thick adhesive was applied. A 0.5 mm thick NTC thermistor substrate was placed in the mold. Then, a 0.05 mm thick adhesive and conductive aerogel were sequentially spread on the NTC thermistor substrate. The mold was placed in a microwave sintering furnace and calcined at a heating rate of 5°C / min to 100°C, 200°C, and 300°C, respectively. Each temperature zone was kept at this temperature for 30 min, 50 min, and 1 h, respectively. The temperature was naturally cooled, the mold was removed, and demolding was performed to obtain an NTC thermistor. The adhesive was a polyimide adhesive, model YJ-22.

[0022] Example 5; (1) A metal oxide mixture obtained by mixing MnO2, Co3O4, and NiO in a molar percentage of 50%:29%:21% was placed in an agate mortar for the first ball milling, and the metal oxide mixture: zirconium oxide balls: anhydrous ethanol were mixed in a mass ratio of 1:2.5:1, and wet-milled in a clockwise and counterclockwise alternating cycle with an alternating interval of 30 min. The powder was ball milled for 4 h to obtain a powder, and the powder, polyether P123, and deionized water were mixed in a mass ratio of 1:0.015:1.7, placed in a drying oven, and dried at 110°C and 8 Pa for 2 h. The mixture was ball milled for a second time at a speed of 500 r / min for 8 h using a planetary ball mill, and passed through a 100-mesh sieve to obtain a precursor powder; (2) The precursor powder was placed in a hydraulic press with a mold of 10 mm, the pressure was set to 8 kN and the pressure was maintained for 1 min, and then the isostatic press was used with a pressure of 300 MPa and the pressure was maintained for 4 min to obtain a ceramic body. The ceramic body was placed in a microwave sintering furnace and heated to 500 °C at a rate of 10 °C / min. After sintering for 30 min, it was taken out to obtain the NTC thermistor matrix; (3) Silicon carbide, 10% nitric acid aqueous solution, silver nitrate, sodium chloride, deionized water and anhydrous ethanol were mixed and stirred in a mass ratio of 1:17:0.15:0.15:30:100, stirred at 75 ° C, 400 r / min for 11 hours, filtered, and the filter cake was washed with anhydrous ethanol and deionized water 4 times each, placed in a drying oven, and dried at 70 ° C, 8 Pa for 13 hours to obtain silver-loaded silicon carbide; silver-loaded silicon carbide, γ-aminoethylaminopropyltrimethoxysilane and ethanol were mixed and stirred in a mass ratio of 1:0.045:25, stirred at 100 ° C, 400 r / min, and refluxed for 4 hours, filtered, and the filter cake was washed with anhydrous ethanol 3 times, placed in a drying oven, and dried at 70 ° C, 10 Pa for 4 hours to obtain modified silicon carbide; (4) In a nitrogen atmosphere, polytetrafluoroethylene, acrylic acid, ammonium ferrous sulfate hexahydrate, a 50% sulfuric acid aqueous solution and ethanol were mixed in a mass ratio of 1:0.6:0.015:2.3:8, irradiated with γ-rays at an irradiation dose of 15 kGy at 25°C for 10 to 15 hours, filtered, and the filter cake was washed twice with deionized water, placed in a drying oven, and dried at 60°C and 5 Pa for 2 hours to obtain modified polytetrafluoroethylene; the molecular weight of the polytetrafluoroethylene is 10 6 g / mol; (5) Modified silicon carbide, modified polytetrafluoroethylene, dicyclohexylcarbodiimide, triethylamine and N,N-dimethylformamide were mixed in a mass ratio of 1:0.4:1.1:2.5:15, stirred at 30°C and 400 r / min for 10-12 hours, filtered and the solid was taken out, placed in a drying oven, and dried at 70°C and 70 Pa for 5 hours to obtain functional silicon carbide; tetraethyl orthosilicate, 0.5 mol / L hydrochloric acid aqueous solution and anhydrous ethanol were mixed in a ratio of 1:0.07:5, stirred at 45°C and 400 r / min for 1 hour, and functional silicon carbide 0.5 times of tetraethyl orthosilicate was added, and continued to stir for 2 hours to obtain a wet gel, which was placed in a high-pressure reactor at a temperature of 40°C and a pressure of 10 MPa, and replaced with liquid carbon dioxide for 12 hours to obtain a conductive aerogel; (6) A layer of conductive aerogel is evenly spread on the bottom of the groove of the mold, and a 0.05 mm thick adhesive is coated. An NTC thermistor substrate with a thickness of 0.6 mm is placed in the mold. Then, an adhesive with a thickness of 0.05 mm and a conductive aerogel are sequentially spread on the NTC thermistor substrate. The mold is placed in a microwave sintering furnace and calcined at a heating rate of 5 ° C / min. The temperature is raised to 100 ° C, 200 ° C, and 300 ° C respectively. Each temperature zone is kept warm for 30 min, 50 min, and 2 h respectively. The temperature is naturally cooled, the mold is taken out, and the mold is demoulded to obtain an NTC thermistor. The adhesive is a polyimide glue, model YJ-22.

[0023] Example 6; (1) A metal oxide mixture obtained by mixing MnO2, Co3O4, and NiO in a molar percentage of 52%:28%:20% was placed in an agate mortar and ball-milled for the first time. The metal oxide mixture, zirconium oxide balls, and anhydrous ethanol were mixed in a mass ratio of 1:3:1, and wet-milled in a clockwise and counterclockwise alternating cycle with an alternating interval of 30 minutes. The powder was ball-milled for 4 hours to obtain a powder. The powder, polyether P123, and deionized water were mixed in a mass ratio of 1:0.02:2, placed in a drying oven, and dried at 120°C and 10Pa for 2 hours. The mixture was ball-milled for a second time at a speed of 500 r / min for 8 hours using a planetary ball mill, and passed through a 100-mesh sieve to obtain a precursor powder. (2) The precursor powder was placed in a hydraulic press with a mold of 10 mm, the pressure was set to 10 kN and the pressure was maintained for 1 min, and then the isostatic press was used with a pressure of 300 MPa and the pressure was maintained for 4 min to obtain a ceramic body. The ceramic body was placed in a microwave sintering furnace and heated to 500 °C at a rate of 10 °C / min. After sintering for 30 min, it was taken out to obtain the NTC thermistor matrix; (3) Silicon carbide, 10% nitric acid aqueous solution, silver nitrate, sodium chloride, deionized water and anhydrous ethanol were mixed and stirred in a mass ratio of 1:20:0.2:0.2:30:100, stirred at 80°C, 500 r / min for 12 hours, filtered, and the filter cake was washed with anhydrous ethanol and deionized water 5 times each, placed in a drying oven, and dried at 70°C, 8Pa for 14 hours to obtain silver-loaded silicon carbide; silver-loaded silicon carbide, γ-aminoethylaminopropyltrimethoxysilane and ethanol were mixed and stirred in a mass ratio of 1:0.05:30, stirred at 100°C, 500 r / min, and refluxed for 4 hours, filtered, and the filter cake was washed with anhydrous ethanol 3 times, placed in a drying oven, and dried at 70°C, 10Pa for 4 hours to obtain modified silicon carbide; (4) In a nitrogen atmosphere, polytetrafluoroethylene, acrylic acid, ammonium ferrous sulfate hexahydrate, a 50% sulfuric acid aqueous solution and ethanol were mixed in a mass ratio of 1:0.7:0.02:2.5:10, irradiated with γ-rays at an irradiation dose of 20 kGy at 30°C for 15 h, filtered, and the filter cake was washed three times with deionized water, placed in a drying oven, and dried at 60°C and 7 Pa for 2 h to obtain modified polytetrafluoroethylene; the molecular weight of the polytetrafluoroethylene was 10 6 g / mol; (5) Modified silicon carbide, modified polytetrafluoroethylene, dicyclohexylcarbodiimide, triethylamine and N,N-dimethylformamide were mixed in a mass ratio of 1:0.5:1.2:3:20, stirred at 30°C and 500 r / min for 12 hours, filtered and the solid was taken out, placed in a drying oven, and dried at 70°C and 10Pa for 6 hours to obtain functional silicon carbide; tetraethyl orthosilicate, 0.5 mol / L hydrochloric acid aqueous solution and anhydrous ethanol were mixed in a ratio of 1:0.08:6, stirred at 50°C and 500 r / min for 2 hours, and functional silicon carbide 0.3 times of tetraethyl orthosilicate was added, and continued to stir for 3 hours to obtain a wet gel, which was placed in a high-pressure reactor at a temperature of 40°C and a pressure of 10 MPa, and replaced with liquid carbon dioxide for 12 hours to obtain a conductive aerogel; (6) A layer of conductive aerogel was evenly spread on the bottom of the groove of the mold, and a 0.05 mm thick adhesive was applied. A 0.8 mm thick NTC thermistor substrate was placed in the mold. Then, a 0.05 mm thick adhesive and conductive aerogel were sequentially spread on the NTC thermistor substrate. The mold was placed in a microwave sintering furnace and calcined at a heating rate of 5°C / min to 100°C, 200°C, and 300°C, respectively. Each temperature zone was kept at this temperature for 30 min, 50 min, and 2 h, respectively. The temperature was naturally cooled, the mold was removed, and demolding was performed to obtain an NTC thermistor. The adhesive was a polyimide adhesive, model YJ-22.

[0024] Comparative Example 1: The only difference between Comparative Example 1 and Example 2 is that the microwave sintering process is not used, and step (2) is changed to: the precursor powder is placed in a hydraulic press with a mold of 10 mm, the pressure is set to 8KN and the pressure is maintained for 1 minute, and then the isostatic press is used with a pressure of 300 MPa and the pressure is maintained for 4 minutes to obtain a ceramic embryo, the ceramic embryo is placed in a calcining furnace, and the temperature is increased to 750°C at a rate of 10°C / min, and it is taken out after sintering for 2 hours to obtain an NTC thermistor substrate; step (6) is changed to: at the bottom of the groove of the mold A layer of conductive aerogel was evenly spread, and a binder with a thickness of 0.05 mm was coated. An NTC thermistor substrate with a thickness of 0.6 mm was placed in a mold. Then, a binder with a thickness of 0.05 mm and a conductive aerogel were sequentially spread on the NTC thermistor substrate. The mold was placed in a calcining furnace and calcined. The temperature was raised to 200° C. at a rate of 10° C. / min and kept warm for 1 h. The temperature was raised from 200° C. to 320° C. and kept warm for 2 h. The temperature was naturally lowered, the mold was removed, and the mold was demolded to obtain an NTC thermistor. The remaining steps were the same as in Example 2.

[0025] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that step (3) is different. Step (3) is changed to: silicon carbide, γ-aminoethylaminopropyltrimethoxysilane, and ethanol are mixed and stirred in a mass ratio of 1:0.045:25, stirred and refluxed at 100°C and 400 r / min for 4 hours, filtered, and the filter cake is washed with anhydrous ethanol three times, placed in a drying oven, and dried at 70°C and 10Pa for 4 hours to obtain modified silicon carbide; the remaining steps are the same as in Example 2.

[0026] Comparative Example 3; The difference between Comparative Example 3 and Example 2 is that there is no step (4), and step (5) is changed to: modified silicon carbide, polytetrafluoroethylene, and N,N-dimethylformamide are mixed in a mass ratio of 1:2.5:15, stirred at 30°C and 400r / min for 10 hours, filtered, and the solid is taken, placed in a drying oven, and dried at 70°C and 70Pa for 5 hours to obtain functional silicon carbide; tetraethyl orthosilicate, 0.5mol / L hydrochloric acid aqueous solution, and anhydrous ethanol are mixed in a ratio of 1:0.07:5, stirred at 45°C and 400r / min for 1 hour, 0.5 times the functional silicon carbide of tetraethyl orthosilicate is added, and stirring is continued for 2 hours to obtain a wet gel, which is placed in a high-pressure reactor at a temperature of 40°C and a pressure of 10MPa, and replaced with liquid carbon dioxide for 12 hours to obtain a conductive aerogel; the remaining steps are the same as those in Example 2.

[0027] Comparative Example 4; The difference between Comparative Example 4 and Example 1 is that polytetrafluoroethylene is not present, so Comparative Example 4 does not have step (4), and step (5) is modified as follows: modified silicon carbide, dicyclohexylcarbodiimide, triethylamine and N,N-dimethylformamide are mixed in a mass ratio of 1:1:2:10, stirred at 25°C and 300 r / min for 10 hours, filtered and solid is taken, placed in a drying oven, and dried at 60°C and 6Pa for 4 hours to obtain functional silicon carbide; tetraethyl orthosilicate, 0.5 mol / L hydrochloric acid aqueous solution and anhydrous ethanol are mixed in a ratio of 1:0.05:4, stirred at 40°C and 300 r / min for 1 hour, functional silicon carbide 0.3 times of tetraethyl orthosilicate is added, and stirring is continued for 2 hours to obtain wet gel, which is placed in a high-pressure reactor at a temperature of 35°C and a pressure of 6 MPa, and replaced with liquid carbon dioxide for 12 hours to obtain conductive aerogel; the remaining steps are the same as in Example 1.

[0028] Effect Examples Aging performance test Testing Method: NTC thermistors were prepared according to the methods described in the Examples and Comparative Examples. The sintered wafers were removed from the microwave sintering furnace and their resistivity was measured. The resistance at room temperature (25°C) and at -100°C were measured in a liquid helium refrigerated thermostat. The B value (B value) was calculated from these two resistance values. The B value represents the resistance drift rate. The NTC thermistors from the Examples and Comparative Examples were aged at -100°C for 1500 hours and then tested. The resistance drift rates before and after aging were calculated based on the standard resistance at 25°C before aging. Resistance drift rate = resistance after aging - standard resistance / standard resistance × 100%. The results are shown in Table 1.

[0029] Table 1

[0030] The experimental data of Comparative Example 1 and Example 2 demonstrate the necessity of the microwave sintering process of the present invention; heat is transferred from the external heat source to the sample surface through thermal radiation and convection, and then enters the interior of the sample through heat conduction; this results in a high surface temperature and a low internal temperature of the sample; in order to ensure that the internal temperature reaches a sufficient sintering temperature, the overall temperature, especially the surface temperature, must be raised to 750°C. The microwave effect generated is that the microwave energy directly penetrates the material and is absorbed by the entire material, converted into molecular kinetic energy, and significantly reduces the temperature gradient between the interior and the surface of the sample; the interior and the exterior are heated to the target temperature almost simultaneously. Temperature, avoiding the problem of surface overheating, so Comparative Example 1 needs to increase the corresponding temperature to achieve the same effect as Example 1; the experimental data of Comparative Example 2 and Example 2 verify the indispensability of silver-modified silicon carbide (rather than ordinary silicon carbide) for low-temperature conductive paths; the experimental data of Comparative Example 3 and Example 2 prove the role of dicyclohexylcarbodiimide (DCC) and triethylamine in the cross-linking reaction, indicating that the lack of a cross-linking agent will significantly reduce the durability of the material; the experimental data of Comparative Example 4 and Example 2 verify the necessity of polytetrafluoroethylene (PTFE) modification for the aerogel structure.

[0031] From the data results of Examples 1 to 6 and Comparative Examples 1 to 4 and Table 1, it can be seen that the NTC thermistors prepared by the present invention have low resistivity and B value, small resistance value drift rate, and are within 1.0% after aging treatment, and have excellent low-temperature stability. In contrast, since Comparative Examples 1 to 4 do not adopt this preparation process, their performance test results are significantly lower than those of Examples 1 to 6.

[0032] The present invention uses a low-temperature-resistant conductive aerogel to replace the conductive silver paste. Silicon carbide, silver nitrate, and sodium chloride are reacted to produce silver-loaded silicon carbide, which provides more conductive pathways for improving the silver-loaded silicon carbide and enhances the conductivity of the thermistor. The silver-loaded silicon carbide and a silane coupling agent are mixed and reacted to produce modified silicon carbide. The siloxy group at one end of the silane coupling agent reacts with the oxidized Si-OH group on the surface of the silver-loaded silicon carbide to form a hydrogen bond, while the -NH2 group at the other end undergoes a condensation reaction with the carboxylic acid group of modified polytetrafluoroethylene, resulting in a high degree of crosslinking between the modified polytetrafluoroethylene and the modified silicon carbide. This chemical crosslinking facilitates interfacial stress transfer, reduces shrinkage of the thermistor due to stress concentration at low temperatures, and improves its electrical performance stability at low temperatures. Polytetrafluoroethylene has excellent low-temperature performance. At extremely low temperatures, its molecular chains can still maintain mobility, avoiding brittle fracture, and do not absorb moisture, thus avoiding performance degradation due to freezing.

[0033] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims, not the foregoing description, and all variations within the meaning and range of equivalents of the claims are intended to be encompassed therein. Any reference in a claim should not be construed as limiting the claim to which it relates.

Claims

1. An NTC thermistor resistant to low temperatures of -100°C, characterized in that: The method comprises the following preparation steps: (1) A metal oxide mixture obtained by mixing MnO2, Co3O4, and NiO in a molar percentage of (48%~55%): (25%~30%): (15%~23%) was placed in an agate mortar for the first ball milling. The metal oxide mixture, zirconia balls, and anhydrous ethanol were mixed in a mass ratio of 1:2~3:1, and wet-milled in a clockwise and counterclockwise alternating cycle with an alternating interval of 30 min. The powder was ball milled for 4 h to obtain a powder. The powder, dispersant, and deionized water were mixed in a mass ratio of 1: (0.01~0.02): (1.5~2), and ball milled for a second time for 8 h at a rotation speed of 500 r / min. The mixture was passed through a 100-mesh sieve to obtain a precursor powder. (2) The precursor powder was placed in a hydraulic press and held under pressure for 1 minute. Then, an isostatic press was used with a pressure of 300 MPa and a pressure of 5 to 10 kN to obtain a ceramic body. The ceramic body was placed in a microwave sintering furnace and heated to 500 °C at a rate of 10 °C / min. After sintering for 30 minutes, the body was taken out to obtain the NTC thermistor matrix. (3) Silicon carbide, 10% nitric acid aqueous solution, silver nitrate, sodium chloride, deionized water and anhydrous ethanol were mixed and stirred in a mass ratio of 1: (15-20): (0.1-0.2): (0.1-0.2): 30: 100, stirred at 70-80 ° C, 300-500 r / min for 10-12 h, filtered, and the filter cake was washed with anhydrous ethanol and deionized water for 3-5 times each, placed in a drying oven, and dried at 60-7 0°C, 5-8 Pa, and drying for 12-14 hours to obtain silver-loaded silicon carbide; mixing the silver-loaded silicon carbide, silane coupling agent, and ethanol in a mass ratio of 1: (0.04-0.05): (20-30) and stirring evenly, stirring and refluxing at 90-100°C, 300-500 r / min, and filtering, washing the filter cake with anhydrous ethanol 2-3 times, placing it in a drying oven, and drying it at 70°C, 7-10 Pa, for 4 hours to obtain modified silicon carbide; Under a nitrogen atmosphere, polytetrafluoroethylene, acrylic acid, ammonium ferrous sulfate hexahydrate, a 50% sulfuric acid aqueous solution, and ethanol are mixed in a mass ratio of 1:(0.5-0.7):(0.01-0.02):(2-2.5):(5-10), irradiated with γ rays at a dose of 10-20 kGy for 10-15 hours at 25-30°C, filtered, and the filter cake is washed 2-3 times with deionized water, placed in a drying oven, and dried at 60°C and 5-7 Pa for 2 hours to obtain modified polytetrafluoroethylene; Conductive gel is prepared by condensing modified silicon carbide and modified polytetrafluoroethylene and then adding silicon source; (4) A layer of conductive aerogel is evenly spread on the bottom of the groove of the mold, and a 0.05 mm thick adhesive is coated. An NTC thermistor substrate with a thickness of 0.5 to 0.8 mm is placed in the mold. Then, a 0.05 mm thick adhesive and a conductive aerogel are sequentially spread on the NTC thermistor substrate. The mold is placed in a microwave sintering furnace for calcination, cooled naturally, and the mold is taken out and demolded to obtain an NTC thermistor; the adhesive is polyimide glue.

2. The -100°C low-temperature-resistant NTC thermistor according to claim 1, characterized in that: The first ball milling method in step (1) is as follows: the metal oxide mixture: zirconium oxide balls: anhydrous ethanol are mixed in a mass ratio of 1: (2-3): 1, and the mixture is wet-milled in an alternating clockwise and counterclockwise direction with an alternating interval of 30 minutes.

3. The -100°C low-temperature-resistant NTC thermistor according to claim 1, characterized in that: The dispersant in step (1) is: polyether P123.

4. The -100°C low-temperature-resistant NTC thermistor according to claim 1, characterized in that: The parameters of the hydraulic press in step (2) are: the mold is 10 mm, and the pressure is 5 to 10 kN.

5. The -100°C low-temperature-resistant NTC thermistor according to claim 1, characterized in that: The setting parameters of the microwave sintering furnace described in step (4) are: a heating rate of 5°C / min, heating to 100°C, 200°C, and 300°C respectively, and keeping each temperature zone warm for 30 minutes, 50 minutes, and 1 to 2 hours respectively.

6. The -100°C low-temperature-resistant NTC thermistor according to claim 1, characterized in that: The silicon carbide described in step (3) is: RS07.

7. The -100°C low-temperature-resistant NTC thermistor according to claim 1, characterized in that: The silicon source described in step (3) is: ethyl orthosilicate.

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