CN114591657B

CN114591657B - Polymer PTC material with filler concentration gradient distribution and preparation method and application thereof

Info

Publication number: CN114591657B
Application number: CN202210130079.4A
Authority: CN
Inventors: 方斌; 胡伟; 张震; 王保余; 杨向民; 梁莹; 唐桤泽
Original assignee: East China University of Science and Technology
Current assignee: East China University of Science and Technology
Priority date: 2022-02-11
Filing date: 2022-02-11
Publication date: 2023-03-10
Anticipated expiration: 2042-02-11
Also published as: CN114591657A

Abstract

The invention discloses a macromolecule PTC material with conductive filler concentration gradient distribution, a preparation method and application thereof, the macromolecule PTC material mainly comprises a macromolecule resin matrix, a first conductive filler and a second conductive filler, wherein the total volume fraction of the three components is 20-70%, 25-70% and 5-10% in sequence; and the conductive filler is distributed in a gradient manner in the vertical direction. The preparation method mainly comprises the following steps: (1) Preparing a high-molecular resin matrix and a conductive filler dispersion liquid by ball milling; (2) coating the dispersion liquid on a metal foil, standing and settling; and (3) drying, folding in half and hot-press forming. The invention can effectively reduce the interface resistance and improve the pressure-resistant and flow-resistant characteristics and PTC strength by controlling the sedimentation of the conductive filler and adjusting the distribution of the conductive filler in the vertical direction in the polymer matrix, and provides a feasible scheme for efficiently preparing high-performance polymer PTC materials and elements.

Description

Polymer PTC material with filler concentration gradient distribution and preparation method and application thereof

Technical Field

The invention belongs to the field of polymers, and relates to a polymer PTC material with filler concentration gradient distribution, and a preparation method and application thereof.

Background

The polymer-based thermistor mainly comprises crystalline polymer resin and conductive filler, and the resistivity of the thermistor shows nonlinear change along with the change of temperature. The resistivity of the high-molecular Positive Temperature Coefficient (PTC) thermistor gradually increases along with the rise of the temperature, and when the temperature reaches the vicinity of the turning temperature point of the material, the resistivity increases by multiple orders of magnitude in a very small temperature interval. The polymer PTC thermistor is connected in series in the circuit, when the circuit generates larger current, the temperature of the thermistor rises, the resistance of the thermistor increases instantly, and the current in the circuit is cut off instantly, so that the function of protecting the circuit is achieved; then, as the temperature is reduced, the resistance value can be restored to a normal value, and the self-recovery characteristic is realized.

For polymer PTC, the room temperature resistivity is relatively high, which is difficult to satisfy the use of high-power equipment and the electric energy loss in the circuit is large. Meanwhile, the polymer PTC has poor pressure resistance and flow resistance characteristics during operation. British Scientific reports (2014, volume 4, page 6684) report that a polymer PTC material with carbon nanotubes as conductive filler effectively reduces the room temperature resistivity, but has poor voltage and current resistance. The high-molecular thermistor material is difficult to have low resistivity at room temperature and excellent pressure-resistant and current-resistant characteristics at the same time. Therefore, it is significant to develop a preparation method capable of improving the voltage-resistant and current-resistant characteristics while reducing the room temperature resistivity of the polymer PTC material.

Disclosure of Invention

The invention aims to solve the technical problem that the room temperature resistivity of the polymer PTC material is reduced and the pressure-resistant and flow-resistant properties are improved, so that the polymer PTC material has low room temperature resistivity and excellent pressure-resistant and flow-resistant properties.

In order to solve the problems, the invention provides a polymer PTC material with gradient distribution of conductive filler concentration and a preparation method thereof, which can effectively reduce the polymer PTC interfacial resistance and improve the PTC strength and the pressure and flow resistance characteristics.

The invention provides a polymer PTC material with conductive filler concentration gradient distribution, which comprises a polymer resin matrix, a first conductive filler and a second conductive filler.

Wherein the polymer resin matrix comprises one or more of polyethylene and its copolymer, polypropylene and its copolymer, polyvinylidene fluoride (PVDF) and its copolymer, polyhexafluoropropylene and its copolymer, polyamide and its copolymer, polyurethane and its copolymer, fluororubber and its copolymer, etc.; preferably, polyvinylidene fluoride.

Wherein the polymer resin matrix accounts for 20-70% of the total volume fraction of the polymer PTC material; preferably, it is 65%.

The first conductive filler is ceramic conductive filler and comprises one or more of titanium carbide, tungsten carbide, tantalum carbide, titanium nitride, titanium diboride, aluminum nitride, magnesium nitride, boron nitride and the like; preferably, titanium carbide.

Wherein, the first conductive filler accounts for 25 to 70 percent of the total volume fraction of the polymer PTC material; preferably, it is 25%.

The second conductive filler is a carbon conductive filler and comprises one or more of carbon black, acetylene black, short carbon fibers, long-cut carbon fibers, single-walled carbon nanotubes, multi-walled carbon nanotubes, graphite and the like; preferably, it is carbon black.

Wherein the second conductive filler accounts for 5-10% of the total volume fraction of the polymer PTC material; preferably, it is 10%.

The high polymer PTC material with the gradient distribution of the concentration of the conductive filler has lower room temperature resistivity and good pressure and flow resistance characteristics, the conductive filler is unevenly distributed in the PTC material through solution sedimentation, the concentration of the conductive filler in the center of the high polymer PTC material is lowest, the concentration of the conductive filler is gradually increased along the vertical direction by taking the center of the high polymer PTC material as a starting point, the concentration of the conductive filler on the upper surface and the lower surface of the high polymer PTC material is highest, the high-concentration conductive filler on the upper surface and the high-concentration conductive filler on the lower surface generate a large number of contact points with metal foils, a current transmission path is provided, the interface resistance can be effectively reduced, the concentration of the conductive filler in the center area of the high polymer PTC material is low, the conductive path formed by the conductive filler is easily damaged by the thermal expansion of a matrix, the resistance is sensitive to the change of the temperature, and when the temperature in the circuit is increased to the vicinity of the melting point of the matrix, the resistance of the high polymer PTC material is rapidly increased, and the current is rapidly reduced, so the pressure and flow resistance characteristics are improved.

In the invention, the center of the polymer PTC material refers to a plane which is positioned at the midpoint of the polymer PTC material in the vertical direction.

In the invention, the concentration of the conductive filler is increased progressively towards two ends by taking the center of the polymer PTC material as a starting point; the incremental increase means that the concentration of the conductive filler is gradually increased along the vertical direction by taking the center of the polymer PTC material as a starting point and taking the upper surface and the lower surface of the polymer PTC material as an end point.

According to the invention, through the density difference between the conductive filler and the solution, the gradient distribution of the conductive filler in the polymer solution is innovated for the first time, the conductive filler is uniformly distributed in the dispersion liquid after ball milling, the dispersion liquid is coated on the metal foil, in the standing process, the settling speed of the first conductive filler with large particle size in the dispersion liquid is high, the first conductive filler with small particle size is settled to the interface of the bottom metal foil at first, the settling speed of the first conductive filler with small particle size is low and the first conductive filler is still positioned on the upper surface part of the dispersion liquid, at the moment, the solvent is dried quickly, so that the distribution of the conductive filler is fixed, and the gradient distribution of the conductive filler can not only reduce the room temperature resistance of the polymer PTC material, but also improve the flow resistance.

Compared with the traditional melting and mixing method, the solution sedimentation method provides a low-viscosity environment, the structural design is carried out according to the density difference between the solution and the conductive filler, and the conductive filler is uniformly distributed in the high-molecular solution by ball milling, so that the problem that the conductive filler is difficult to disperse uniformly in the low-viscosity environment is solved. Controlling the sedimentation degree of the conductive filler by designing a sedimentation time gradient and changing the viscosity of the polymer solution; excessive settlement of the conductive filler is avoided by rapidly drying the solvent, and the distribution state of the conductive filler is fixed.

The invention also provides a preparation method of the polymer PTC material with the concentration gradient distribution of the conductive filler, which is a dispersion liquid sedimentation method and comprises the following preparation steps:

(1) Preparing dispersion liquid containing a high molecular resin matrix, a first conductive filler and a second conductive filler by ball milling;

(2) Coating the dispersion liquid prepared in the step (1) on a conductive metal foil, standing and settling;

(3) And folding and hot-press forming after drying to obtain the polymer PTC material.

In the step (1), the polymer resin matrix comprises one or more of polyethylene and its copolymer, polypropylene and its copolymer, polyvinylidene fluoride and its copolymer, polyhexafluoropropylene and its copolymer, polyamide and its copolymer, polyurethane and its copolymer, fluororubber and its copolymer, etc.; preferably, polyvinylidene fluoride.

In the step (1), the solvent used for preparing the dispersion is one or a mixture of more of N, N-dimethylformamide, N-dimethylacetamide, N-methylpyrrolidone, diphenylamine, toluene, xylene, diethylene glycol ethyl ether, m-cresol, phenol and the like; preferably, it is N, N-dimethylformamide.

In the step (1), the mass ratio of the polymer resin matrix to the solvent is 1 (4-9); preferably 1:7.

In the step (1), the first conductive filler is one or more of titanium carbide, tungsten carbide, tantalum carbide, titanium nitride, titanium diboride, aluminum nitride, magnesium nitride, boron nitride and the like; preferably, titanium carbide.

In the step (1), the second conductive filler is one or a mixture of more than one of carbon black, acetylene black, short carbon fiber, long-cut carbon fiber, single-walled carbon nanotube, multi-walled carbon nanotube, graphite and the like; preferably, it is carbon black.

In the step (1), the volume ratio of the polymer resin matrix to the first conductive filler to the second conductive filler is 20-70%: 25-70%: 5 to 10 percent; preferably, 65%:25%:10 percent.

In the step (1), the rotation speed of the ball milling is 40-50 rpm, and the ball milling time is 5-10 h; preferably, the rotation speed of the ball milling is 40rpm, and the time of the ball milling is 6h.

In the step (2), the conductive metal foil comprises one of copper foil, silver foil, nickel-plated copper foil, tin-plated copper foil and the like; preferably a nickel-plated copper foil.

In the step (2), one surface of the conductive metal foil is a rough surface, the other surface of the conductive metal foil is a smooth surface, and the uniformly mixed dispersion liquid is coated on the rough surface.

In the step (2), the time gradient of standing and sedimentation is 20-60 min, and the sedimentation temperature is 20-50 ℃; preferably, the time gradient for settling at rest is 30min and the temperature for settling is 25 ℃.

In the step (3), the drying temperature is 100-170 ℃, and the drying time is 6-12 h; preferably, the drying temperature is 130 ℃ and the drying time is 8h.

And (3) folding the metal foil covered with the core layer in half in a hot pressing mode to enable the core layer to be contacted, and placing the metal foil in a flat vulcanizing machine for hot pressing.

Wherein the hot pressing temperature is 160-210 ℃, the hot pressing pressure is 10-20 Mpa, and the hot pressing time is 15-30 min; preferably, the hot pressing temperature is 190 deg.C, the hot pressing pressure is 15MPa, and the hot pressing time is 20min.

The invention also provides the polymer PTC material with the concentration gradient distribution of the conductive filler, which is prepared by the method.

The invention also provides the application of the polymer PTC material with the conductive filler concentration gradient distribution in the fields of electronic components, communication circuits, sensors, energy-saving lamp electronic ballasts, automobiles and the like.

In the present invention, the polymer PTC material and the core layer have the same meaning.

The innovation point of the invention is that the solution method is adopted to dissolve the high molecular resin to form the high molecular solution, the low-viscosity solution environment is favorable for the dispersion of the conductive filler, and simultaneously, the invention also provides conditions for the sedimentation of the conductive filler with high density. Compared with a melt mixing method, the dispersion liquid sedimentation method provided by the invention has the advantages of low equipment cost, low preparation temperature (25 ℃), easy uniform mixing of conductive fillers and the like.

The invention regulates and controls the sedimentation degree of the conductive filler by controlling the standing sedimentation time of the dispersion liquid on the metal foil, respectively coats the uniformly mixed dispersion liquid on the copper foil, stands and settles on the horizontal plane for different times, immediately dries the solvent after the sedimentation is finished, fixes the distribution of the conductive filler, and the conductive filler in the polymer PTC material is distributed in a gradient manner in the vertical direction, and the concentration of the conductive filler is gradually increased towards two ends by taking the center of the polymer PTC material as a starting point. The first conductive filler is ceramic conductive filler with high density, the sedimentation in a polymer solution is obvious, the sedimentation speed of the conductive filler with large particle size is high, the conductive filler with small particle size is firstly sedimentated to the bottom, and the sedimentation speed of the conductive filler with small particle size is slow, so that the concentration of the first conductive filler on the upper surface of the core layer is low, and the concentration of the first conductive filler on the lower surface of the core layer is high after sedimentation. The second conductive filler is carbon conductive filler with low density, is not easy to settle in a high molecular solution, and is distributed in the core layer uniformly.

The PTC chip with the gradient distribution of the conductive filler concentration is obtained by folding the metal foil covered with the core layer in half and carrying out hot pressing on the core layer in contact with the metal foil, wherein the upper layer and the lower layer of the chip are the metal foils, and the middle layer is the core layer with the gradient distribution of the conductive filler. In the vertical direction, the closer to the metal foil, the higher the content of the first conductive filler, and the high concentration of the conductive filler at the interface reduces the interface resistance between the core layer and the metal foil. The closer to the center of the core layer, the lower the content of the first conductive filler, and the higher the resistance, and the high resistance in the center of the core layer enables the voltage resistance and the current resistance of the material to be improved. The PTC material with the filler concentration gradient distribution prepared by the invention has the structure that the room temperature resistivity of the PTC material can be reduced, and the pressure-resistant and flow-resistant characteristics are improved.

Drawings

FIG. 1 is an SEM photograph of a section of a polymer PTC material prepared in example 1 of the present invention, which has settled for 0min;

FIG. 2 is an SEM photograph of a section of a polymer PTC material prepared in example 1 of the present invention, which has been settled for 30min;

FIG. 3 is an SEM photograph of a section of the polymer PTC material prepared in example 1 of the present invention after being settled for 60min;

FIG. 4 is an SEM photograph of a section of a polymer PTC material prepared in example 1 of the present invention, which has been settled for 90 min;

FIG. 5 is an SEM photograph of a cross-section of a polymer PTC material prepared in example 1 of the present invention after being settled for 120min.

Fig. 6 is a process flow chart of the present invention for preparing a polymer PTC material with a gradient distribution of conductive filler concentration.

Detailed Description

The invention provides a preparation method of a polymer PTC material with gradient distribution of conductive filler concentration. In order to more clearly illustrate the present invention, the present invention is further described below in conjunction with preferred embodiments. It is to be understood by persons skilled in the art that the following detailed description is illustrative and not restrictive, and is not to be taken as limiting the scope of the invention. The method mainly comprises the following steps:

example 1

(1) 3.22g of polyvinylidene fluoride (PVDF) powder is added into 23.59g of N, N-dimethylformamide, and the mixture is magnetically stirred for 20min under the water bath heating condition of 50 ℃ until the PVDF is completely dissolved, so that a colorless and transparent PVDF solution is obtained.

(2) Then 0.46g of Carbon Black (CB) powder was added to the PVDF solution, and magnetic stirring was carried out for 10min.

(3) 3.22g of titanium carbide (TiC) powder is continuously added into the PVDF solution containing the carbon black, and magnetic stirring is carried out for 10min.

(4) Adding the PVDF solution mixed with TiC and carbon black into a ball milling tank, adding a proper amount of zirconia ball milling beads with different particle sizes, and carrying out ball milling for 5 hours at the rotating speed of 42 rpm.

(5) 4.73g of the TiC dispersed solution subjected to ball milling is dropwise added on the rough surface of the nickel-plated copper foil with the thickness of 6 x 8.5cm, and the dispersion liquid is scraped and coated flatly by a coating device.

(6) And horizontally standing the copper foil coated with the dispersion liquid for 0min, 30min, 60min, 90min and 120min respectively.

(7) And (3) putting the copper foil after standing into a forced air drying oven at 130 ℃ for drying for 8h.

(8) And folding the dried metal foil in half, and carrying out hot pressing by contacting the core layer, wherein the hot pressing temperature is 190 ℃, the hot pressing pressure is 15MPa, and the hot pressing time is 20min.

(9) After hot pressing, a PTC sheet core material with two smooth surfaces of copper foil and a middle combination of a rough surface and a core layer is formed, and the core material is punched into 3 x 4mm chips.

(10) And welding nickel electrodes on the two smooth surfaces of the chip by a soldering tin dip-welding method, wherein the welding temperature is 260 ℃, and the welding time is 1min.

The resistivity and compressive strength were as follows:

TABLE 1

Settling time/min	0	30	60	90	120
						Specific resistance/(Ω. Cm)	225	70	49	298	324
Withstand voltage/V	16	28	32	24	24

Example 2

(1) 2.89g of PVDF powder was added into 16.43g of N, N-dimethylformamide, and the mixture was magnetically stirred under the water bath heating condition at 60 ℃ for 20min until PVDF was completely dissolved, thereby obtaining a colorless transparent PVDF solution.

(2) Then 0.32g of Carbon Black (CB) powder was added to the PVDF solution, and magnetic stirring was carried out for 10min.

(3) And continuously adding 8.6g of titanium carbide (TiC) powder into the PVDF solution containing the carbon black, and magnetically stirring for 10min.

(4) Adding the PVDF solution mixed with TiC and carbon black into a ball milling tank, adding a proper amount of zirconia ball milling beads with different particle sizes, and carrying out ball milling for 7 hours at the rotating speed of 50 rpm.

(5) And 4.45g of the TiC dispersed liquid subjected to ball milling is dropwise added onto the rough surface of the nickel-plated copper foil of 6 x 8.5cm, and the dispersion liquid is scraped and coated flatly by a coating device.

(6) And horizontally standing the copper foil coated with the dispersion liquid for 0min, 20min, 40min, 60min and 80min respectively.

(7) And (3) placing the copper foil after standing into a forced air drying oven at 120 ℃ for drying for 10h.

(8) And folding the dried metal foil in half, and carrying out hot pressing by contacting the core layer, wherein the hot pressing temperature is 190 ℃, the hot pressing pressure is 20MPa, and the hot pressing time is 15min.

(10) And welding nickel electrodes on the two smooth surfaces of the chip by a soldering tin dip-welding method, wherein the welding temperature is 280 ℃, and the welding time is 20s.

The resistivity and compressive strength were as follows:

TABLE 2

Settling time/min	0	20	40	60	80
						Specific resistance/(Ω. Cm)	32	18	9	6	29
Withstand voltage/V	9	12	15	15	12

Example 3

(1) 2.35g of PVDF powder is added into 11g of N, N-dimethylformamide, and the mixture is magnetically stirred for 15min under the water bath heating condition of 55 ℃ until PVDF is completely dissolved, so that a colorless and transparent PVDF solution is obtained.

(2) Then 0.29g of Carbon Black (CB) powder was added to the PVDF solution, and magnetic stirring was carried out for 10min.

(3) And (3) continuously adding 14.2g of titanium carbide (TiC) powder into the PVDF solution containing the carbon black, and magnetically stirring for 10min.

(4) Adding the PVDF solution mixed with TiC and carbon black into a ball milling tank, adding a proper amount of zirconia ball milling beads with different particle sizes, and carrying out ball milling for 8 hours at the rotating speed of 45 rpm.

(5) And dropwise adding 4.25g of the ball-milled TiC dispersion liquid onto the rough surface of the nickel-plated copper foil of 6 x 8.5cm, and scraping and coating the dispersion liquid by using a coating device to be smooth.

(6) And horizontally standing the copper foil coated with the dispersion liquid for 0min, 40min, 80min, 120min and 160min respectively.

(7) And (3) putting the copper foil after standing into a forced air drying oven at 135 ℃ for drying for 6h.

(8) And folding the dried metal foil in half, and carrying out hot pressing by contacting the core layer, wherein the hot pressing temperature is 200 ℃, the hot pressing pressure is 18MPa, and the hot pressing time is 18min.

(9) And after hot pressing, forming a PTC sheet core material with two smooth surfaces of copper foil and a middle combination of a rough surface and a core layer, and punching the core material into 3 x 4mm chips.

(10) And welding nickel electrodes on the two smooth surfaces of the chip by a soldering tin dip-welding method, wherein the welding temperature is 270 ℃, and the welding time is 40s.

The resistivity and compressive strength were as follows:

TABLE 3

Settling time/min	0	40	80	120	160
						Resistivity/(Ω. Cm)	0.143	0.184	0.249	0.287	0.465
Withstand voltage/V	6	6	9	9	12

Example 4

(1) 1.1g of polyamide granules was added to 8.4g of m-cresol, and the mixture was magnetically stirred under heating in a water bath at 60 ℃ for 6 hours until the polyamide was completely dissolved, to obtain a transparent polyamide solution.

(2) Then 3.4g of titanium nitride (TiN) powder was added to the polyamide solution and ultrasonically dispersed for 10min.

(3) And continuously adding 0.12g of acetylene black into the polyamide solution containing the titanium nitride (TiN), and performing ultrasonic dispersion for 10min.

(4) Adding a polyamide solution mixed with titanium nitride (TiN) and acetylene black into a ball milling tank, adding a proper amount of zirconia ball milling beads with different particle sizes, and carrying out ball milling for 7 hours at the rotating speed of 50 rpm.

(5) And dropwise adding 3.7g of the ball-milled dispersion liquid onto the rough surface of the nickel-plated copper foil of 6 x 8.5cm, and scraping the dispersion liquid by using a film coating device to form a smooth coating.

(7) And (3) placing the copper foil after standing into a 160 ℃ forced air drying oven for drying for 7h.

(8) And folding the dried metal foil in half, and carrying out hot pressing by contacting the core layer, wherein the hot pressing temperature is 200 ℃, the hot pressing pressure is 17MPa, and the hot pressing time is 18min.

The resistivity and compressive strength were as follows:

TABLE 4

Settling time/min	0	40	80	120	160
						Resistivity/(Ω. Cm)	0.254	0.212	0.259	0.289	0.321
Withstand voltage/V	6	9	9	9	9

Example 5

(1) 1.2g of polyamide pellets were added to 10.2g of m-cresol, and the mixture was magnetically stirred under heating in a water bath at 60 ℃ for 6 hours until the polyamide was completely dissolved, to obtain a transparent polyamide solution.

(2) Then 6.1g of titanium nitride (TiN) powder is added into the polyamide solution, and ultrasonic dispersion is carried out for 10min.

(3) And continuously adding 0.17g of acetylene black powder into the polyamide solution containing the titanium nitride (TiN), and performing ultrasonic dispersion for 10min.

(5) And dropwise adding 3.52g of the ball-milled dispersion liquid onto the rough surface of the nickel-plated copper foil of 6 x 8.5cm, and scraping the dispersion liquid by using a film coater to form a smooth coating.

(7) And (3) putting the copper foil after standing into a forced air drying oven at 165 ℃ for drying for 6 hours.

(8) And folding the dried metal foil in half, and carrying out hot pressing by contacting the core layer, wherein the hot pressing temperature is 205 ℃, the hot pressing pressure is 17MPa, and the hot pressing time is 20min.

(10) And welding nickel electrodes on the two smooth surfaces of the chip by a soldering tin dip-welding method, wherein the welding temperature is 260 ℃, and the welding time is 50s.

The resistivity and compressive strength were as follows:

TABLE 5

Settling time/min	0	30	60	90	120
						Specific resistance/(Ω. Cm)	0.153	0.135	0.149	0.172	0.183
Withstand voltage/V	6	6	9	9	9

Comparative example 1

(1) 2.74g of PVDF powder was added into 20.23gN, N-dimethylformamide, and the mixture was magnetically stirred under a water bath heating condition at 50 ℃ for 20min until PVDF was completely dissolved, thereby obtaining a colorless and transparent PVDF solution.

(2) 2.72g of titanium carbide (TiC) powder was added to the PVDF solution, and magnetic stirring was carried out for 10min.

(3) Adding the PVDF solution mixed with TiC into a ball milling tank, adding a proper amount of zirconia ball milling beads with different particle sizes, and carrying out ball milling for 5 hours at the rotating speed of 42 rpm.

(4) And (3) dropwise adding 6.21g of the ball-milled TiC dispersion liquid onto the rough surface of the nickel-plated copper foil of 6 x 8.5cm, and scraping and coating the dispersion liquid by using a coating device to be smooth.

(5) And horizontally standing the copper foil coated with the dispersion liquid for 0min, 30min, 60min, 90min and 120min respectively.

(6) And (3) putting the copper foil after standing into a forced air drying oven at 130 ℃ for drying for 6h.

(7) And folding the dried metal foil in half, and carrying out hot pressing by contacting the core layer, wherein the hot pressing temperature is 190 ℃, the hot pressing pressure is 10MPa, and the hot pressing time is 15min.

(8) After hot pressing, a PTC sheet core material with two smooth surfaces of copper foil and a middle combination of a rough surface and a core layer is formed, and the core material is punched into 3 x 4mm chips.

(9) And welding nickel electrodes on the two smooth surfaces of the chip by a soldering tin dip-welding method, wherein the welding temperature is 260 ℃, and the welding time is 1min.

The resistivities were as follows:

TABLE 6

Settling time/min	0	30	60	90	120
						Specific resistance/(Ω. Cm)	1833	1145	2349	7867	9759

Comparative example 2

(1) 3.23g of PVDF powder is added into 16.82gN, N-dimethylformamide, and is magnetically stirred for 20min under the water bath heating condition of 50 ℃ until PVDF is completely dissolved, so that a colorless and transparent PVDF solution is obtained.

(2) 9.68g of titanium carbide (TiC) powder was added to the PVDF solution, and the mixture was magnetically stirred for 10min.

(4) And (3) dropwise adding 7.24g of the ball-milled TiC dispersion liquid onto the rough surface of the nickel-plated copper foil with the thickness of 6 x 8.5cm, and scraping and coating the dispersion liquid by using a coating device to be smooth.

(7) And folding the dried metal foil in half, and carrying out hot pressing by contacting the core layer, wherein the hot pressing temperature is 190 ℃, the hot pressing pressure is 15MPa, and the hot pressing time is 20min.

(9) And welding nickel electrodes on the two smooth surfaces of the chip by a soldering tin dip-soldering method, wherein the welding temperature is 260 ℃, and the welding time is 1min.

The resistivities were as follows:

TABLE 7

Settling time/min	0	30	60	90	120
						Specific resistance/(Ω. Cm)	150	63	31	82	97

Comparative example 3

(1) 2.13g of PVDF was added to 16.38gN, N-dimethylformamide, and the mixture was magnetically stirred under heating in a water bath at 50 ℃ for 20min until the PVDF was completely dissolved, to obtain a colorless transparent PVDF solution.

(2) 0.34g of Carbon Black (CB) powder was added to the PVDF solution, and the mixture was magnetically stirred for 10min.

(3) Adding the PVDF solution mixed with the CB into a ball milling tank, adding a proper amount of zirconia ball milling beads with different particle sizes, and carrying out ball milling for 5 hours at the rotating speed of 42 rpm.

(4) And (3) dropwise adding 6.18g of ball-milled CB dispersed liquid onto the rough surface of the nickel-plated copper foil of 6 x 8.5cm, and scraping the dispersed liquid by using a film coating device to form a smooth coating.

The resistivities were as follows:

TABLE 8

Settling time/min	0	30	60	90	120
						Specific resistance/(Ω. Cm)	1056	967	998	1065	1097

Comparative example 4

(1) 2.48g of PVDF was added to 13.69gN, N-dimethylformamide, and the mixture was magnetically stirred under the heating condition of water bath at 50 ℃ for 20min until PVDF was completely dissolved, to obtain a colorless transparent PVDF solution.

(2) 0.53g of Carbon Black (CB) powder was added to the PVDF solution, and the mixture was magnetically stirred for 10min.

(3) Adding the PVDF solution mixed with CB into a ball milling tank, adding a proper amount of zirconia ball milling beads with different particle sizes, and carrying out ball milling for 5 hours at the rotating speed of 42 rpm.

(4) And (3) dropwise adding 6.34g of ball-milled CB dispersed liquid onto the rough surface of the nickel-plated copper foil of 6 x 8.5cm, and scraping the dispersed liquid by using a film coating device to form a smooth coating.

(8) After hot pressing, PTC sheet core materials with two smooth surfaces of copper foil and the middle of rough surface combined with the core layer are formed, and the core materials are punched into 3 x 4mm chips.

The resistivities were as follows:

TABLE 9

Settling time/min	0	30	60	90	120
						Specific resistance/(Ω. Cm)	3.4	3.7	3.4	3.9	4.6

The protection of the present invention is not limited to the above embodiments. Variations and advantages that may occur to those skilled in the art may be incorporated into the invention without departing from the spirit and scope of the inventive concept, which is set forth in the following claims.

Claims

1. A preparation method of a polymer PTC material with vertical distribution of conductive filler concentration gradient is characterized by comprising the following steps:

(1) Preparing a dispersion liquid containing a high molecular resin matrix, a first conductive filler and a second conductive filler by ball milling; the first conductive filler is ceramic conductive filler and comprises one or more of titanium carbide, tungsten carbide, tantalum carbide, titanium nitride, titanium diboride, aluminum nitride, magnesium nitride and boron nitride;

the second conductive filler is a carbon conductive filler and comprises one or more of carbon black, acetylene black, short carbon fibers, long-cut carbon fibers, single-walled carbon nanotubes, multi-walled carbon nanotubes and graphite;

(2) Coating the dispersion prepared in the step (1) on a conductive metal foil, standing and settling; the time gradient of the standing sedimentation is 20-60 min;

(3) After drying, folding the metal foil coated with the polymer PTC material in half and performing hot press molding to obtain the polymer PTC material; the conductive filler in the polymer PTC material is distributed in a gradient manner in the vertical direction, and the concentration of the conductive filler is gradually increased towards two ends by taking the center of the polymer PTC material as a starting point.

2. The method according to claim 1, wherein in the step (1), the solvent used for preparing the dispersion is one or more selected from the group consisting of N, N-dimethylformamide, N-dimethylacetamide, N-methylpyrrolidone, diphenylamine, toluene, xylene, diethylene glycol ethyl ether, m-cresol, and phenol;

the high polymer resin matrix comprises one or more of polyethylene, polypropylene, polyvinylidene fluoride, polyhexafluoropropylene, polyamide, polyurethane and fluororubber;

the mass ratio of the polymer resin matrix to the solvent is 1 (4-9);

the volume ratio of the polymer resin matrix to the first conductive filler to the second conductive filler is 20-70%: 25-70%: 5 to 10 percent.

3. The preparation method according to claim 1, wherein in the step (1), the rotation speed of the ball mill is 40-50 rpm, and the ball milling time is 5-10 h;

in the step (2), the conductive metal foil comprises one of copper foil, silver foil, nickel-plated copper foil and tin-plated copper foil; one surface of the conductive metal foil is a rough surface, and the other surface of the conductive metal foil is a smooth surface, wherein the rough surface is in direct contact with the dispersion liquid; the temperature of the standing and the sedimentation is 20-50 ℃.

4. The preparation method according to claim 1, wherein in the step (3), the drying temperature is 100-170 ℃, and the drying time is 6-12 h;

the hot pressing comprises the steps of folding the settled metal foil covered with the polymer PTC material in half, enabling the polymer PTC material to be in contact, and placing the metal foil in a flat vulcanizing machine for hot pressing, wherein the hot pressing temperature is 160-210 ℃, the hot pressing pressure is 10-20 Mpa, and the hot pressing time is 15-30 min.

5. Polymeric PTC material having a gradient distribution of the concentration of electrically conductive filler, obtainable by a process according to any one of claims 1-4.

6. The use of a polymeric PTC material according to claim 5 with a gradient distribution of conductive filler concentration in electronic components, communication circuits, sensors, automotive applications.

7. The use according to claim 6, wherein the electronic component is an electronic ballast of an energy saving lamp.