Tungsten carbide surface passivation modification method and high-stability PPTC (Polymeric Positive Temperature Coefficient) preparation method

Abstract

The invention discloses a tungsten carbide surface passivation modification method and a high-stability PPTC (Polymeric Positive Temperature Coefficient) preparation method, and belongs to the field of fuses. Carrying out suction filtration and washing on the ground tungsten carbide powder; the tungsten carbide powder is dried, and surface-passivated tungsten carbide powder is obtained; the PPTC is prepared by blending the surface-passivated tungsten carbide and HDPE, and the resistance of the PPTC is detected. Tungsten carbide powder is physically ground in an acetone solution, when sufficient energy is provided for grinding, hydroxyl on the surface of tungsten carbide can be attracted to be subjected to protonation dehydrogenation, and an irreversible dehydration process can be generated on the surface of tungsten carbide, so that the surface activity and hydrophilicity of tungsten carbide are effectively reduced, and the purpose of inhibiting the oxidation process is achieved. The PPTC prepared by the preparation method disclosed by the invention is relatively small in resistance change after accelerated oxidation and high in stability, and can be widely applied to the technical field of high-stability electronic devices.

Description

Technical Field

[0001] The invention belongs to the field of fuses and relates to a preparation technology of a resettable fuse, and in particular to a surface passivation modification method of tungsten carbide and a preparation method of a high-stability PPTC. Background Art

[0002] PPTC (Polymer Positive Temperature Coefficent) is a polymer positive temperature coefficient thermistor. PPTC is mainly used for short circuit and overload protection of electronic equipment. Compared with traditional fuses, it has a recoverable function, so it is also called a resettable fuse. Tungsten carbide powder has excellent stability, conductivity and hardness. It is mainly used in electronic materials, electronic products, mechanical parts and other fields. PPTC can be made by blending high conductivity tungsten carbide powder with high-density polyethylene (HDPE).

[0003] Tungsten carbide powder hardly oxidizes in a low-temperature dry atmosphere, but in an atmosphere with high humidity, due to the presence of hydrophilic groups such as hydroxyl groups on the surface of tungsten carbide, these hydrophilic groups easily capture moisture in the air in a humid environment, thereby accelerating the oxidation process; therefore, tungsten carbide powder needs to be stored in a cool and dry environment; when tungsten carbide powder is prepared into PPTC, since PPTC is an electronic device used for a long period of time and the environment is complex, it is difficult to be in a low-temperature dry environment for a long time. In fact, the use of PPTC in the prior art is greatly affected by natural weather, resulting in unstable performance of PPTC. Therefore, a passivation technology is urgently needed to passivate tungsten carbide and inhibit its oxidation, thereby improving the stability and service life of PPTC. Summary of the invention

[0004] In view of the shortcomings of the prior art, one of the purposes of the present invention is to provide a method for passivation modification of the surface of tungsten carbide, which utilizes the addition of acetone to physically grind to cause an irreversible dehydration process, thereby effectively reducing the hydrophilicity of its surface and achieving the effect of inhibiting the oxidation process.

[0005] Another object of the present invention is to provide a method for preparing a high-stability PPTC, wherein the PPTC is prepared by blending surface-passivated tungsten carbide with polyethylene plastic, and passivation is used to inhibit its oxidation, thereby improving the environmental adaptability and service life of the PPTC.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is as follows: In one aspect, the present invention provides a method for surface passivation modification of tungsten carbide, comprising the following steps: Acetone was mixed with tungsten carbide powder for physical grinding; The ground tungsten carbide powder is filtered and washed; The tungsten carbide powder is dried to obtain tungsten carbide powder with a passivated surface.

[0007] Acetone is composed of two methyl groups and one carbonyl group. The carbonyl group has a strong polarity and can attract the hydroxyl groups on the surface of tungsten carbide to undergo protonation and dehydrogenation. In the present invention, acetone and tungsten carbide powder are mixed and then physically ground. During the contact between acetone and tungsten carbide, the carbonyl group can attract the hydroxyl groups on the surface of tungsten carbide. When the hydroxyl groups on the surface of tungsten carbide provide sufficient energy through grinding, an irreversible dehydration process will occur on the surface, thereby effectively reducing the hydrophilicity of the surface, achieving the effect of inhibiting the oxidation process, so that the tungsten carbide powder of the present invention can be stably exposed in a humid environment for a long time.

[0008] Preferably, before physical grinding, the tungsten carbide powder and acetone are mixed in a mass ratio of 1: (1.0-1.5); the mixing ratio is selected to meet the grinding requirements and to allow sufficient carbonyl groups to attract the hydroxyl groups on the surface of the tungsten carbide. When the amount of acetone is too low, there are not enough carbonyl groups; when the amount of acetone is too high, due to the specific heat of the liquid and the volatilization effect of acetone, the physical grinding does not have enough energy to cause irreversible dehydration of the hydroxyl groups on the surface of the tungsten carbide.

[0009] Preferably, the physical grinding is ball mill grinding.

[0010] Preferably, the physical grinding time is 0.5-2 hours. If the grinding time is too short, the energy is insufficient to cause irreversible dehydration of the hydroxyl groups on the surface of tungsten carbide. If the grinding time is too long, unnecessary energy consumption is increased and the passivation effect will not be significantly improved.

[0011] Preferably, the ground tungsten carbide powder is washed with water, anhydrous ethanol, and water in sequence to remove residual acetone.

[0012] Preferably, the drying time for the tungsten carbide powder is 25-45 minutes, and the drying temperature is 30-70°C. If the drying time is too short, the drying will be incomplete; if the drying time is too long, useless energy consumption will be increased; if the temperature is too low, the drying effect will be poor; if the temperature is too high, the tungsten carbide powder will be slowly oxidized.

[0013] Preferably, the tungsten carbide powder is a micron-sized tungsten carbide powder produced by a mechanical alloying method, a direct reduction carburization method or a sol-gel method, and the particle size of the tungsten carbide powder is 1um-30um.

[0014] On the other hand, the present invention provides a method for preparing a high-stability PPTC, comprising the following steps: The surface passivated tungsten carbide powder is blended with polyethylene plastic to prepare PPTC.

[0015] Preferably, the polyethylene plastic is HDPE. HDPE has large crystallinity and high strength. Using HDPE as a substrate to prepare PPTC has better performance.

[0016] Preferably, when blending, the mass ratio of polyethylene plastic to surface passivated tungsten carbide powder is 1:15-20; too little tungsten carbide powder content will cause excessively high resistance and poor performance, while too much tungsten carbide powder content will affect the mechanical properties of the composite material. The above mass ratio is a commonly used ratio range that can meet the use requirements.

[0017] On the other hand, the present invention also provides a high-stability PPTC prepared by the above preparation method.

[0018] Since the surface of the tungsten carbide powder used in the present invention has been well passivated, when it is blended with HDPE to prepare PPTC, the early performance is similar to that of the one without passivation modification. However, after long-term use, since the passivated modified tungsten carbide powder has no hydrophilic group, the capture of moisture in the ambient air is inhibited, thus greatly improving the stability of the tungsten carbide to the product.

[0019] Compared with the prior art, the present invention has the following beneficial effects: The present invention uses physical grinding in combination with a specific solvent, and utilizes grinding energy and polar adsorption to cause irreversible dehydration of hydroxyl groups on the surface of tungsten carbide, thereby effectively reducing the hydrophilicity of the surface, inhibiting the oxidation process, and realizing the passivation of the surface of tungsten carbide. The passivation can significantly improve the stability of tungsten carbide. When it is blended with HDPE to prepare PPTC, in an accelerated oxidation experiment under double 85 test conditions in a constant temperature and humidity chamber, after 1008 hours, the resistance is only about half of that without passivation, which can significantly improve the use stability of PPTC and improve its service life and environmental adaptability. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the process of high stability PPTC provided by an embodiment of the present invention; Figure 2 It is the surface morphology of tungsten carbide in the composite material provided by the comparison category; Figure 3 is a surface morphology of tungsten carbide in the composite material provided in Example 1; Figure 4 is a surface morphology of tungsten carbide in the composite material provided in Example 2; Figure 5 is a surface morphology of tungsten carbide in the composite material provided in Example 3; Figure 6 The PPTC structure and resistance test principle diagram provided for the comparative example and embodiment of the present invention. DETAILED DESCRIPTION

[0021] The following embodiments of the present invention are described in further detail in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0022] In order to make the technical scheme and advantages of the present invention clearer and more explicit, the present invention is based on a fixed ratio of HDPE and tungsten carbide, changes acetone and grinding time to form Examples 1-4, and uses unpassivated tungsten carbide as a comparative example to illustrate the technical scheme of the present invention.

[0023] Example 1: Figure 1 As shown, the present invention provides a method for preparing a high-stability PPTC, comprising the following steps: Step 1: 100 parts of tungsten carbide powder was mixed with 140 parts of acetone, and then ground in a ball mill for 90 minutes; Step 2: Filter the tungsten carbide powder obtained in step 1, and wash it with water, ethanol, and water for 3 times; Step 3, placing the tungsten carbide powder obtained in step 2 into a vacuum drying oven for drying to obtain surface passivated tungsten carbide; Step 4: HDPE and surface passivated tungsten carbide are mixed at a mass ratio of 1:16 to prepare PPTC with a side length of 10 mm (length and width are both 10 mm) and a thickness of 0.5 mm; Step 5: Place the prepared PPTC in a constant temperature and humidity chamber under double 85 test conditions for accelerated oxidation experiment; Step 6: Test the resistance of the oxidized PPTC. Figure 6 As shown in the figure, the PPTC is clamped between two copper foil electrode plates. The current Ig is measured by connecting the two probes of the ammeter A, and the voltage Ug is measured by connecting the two probes of the voltmeter V. The formula Calculating the resistance of a PPTC ,in is the voltage applied to PPTC by the four-probe method, is the current passing through the PPTC using the four-probe method; the experimental conditions and test results are recorded in Table 1.

[0024] Example 2: Step 1, 100 parts of tungsten carbide powder and 120 parts of acetone are mixed and then ground in a ball mill for 60 minutes; Step 2: Filter the tungsten carbide powder obtained in step 1, and wash it with water, ethanol, and water for 3 times; Step 3, placing the tungsten carbide powder obtained in step 2 into a vacuum drying oven for drying to obtain surface passivated tungsten carbide; Step 4: HDPE and surface passivated tungsten carbide are mixed at a mass ratio of 1:16 to prepare PPTC with a side length of 10 mm (length and width are both 10 mm) and a thickness of 0.5 mm; Step 5: Place the prepared PPTC in a constant temperature and humidity chamber under double 85 test conditions for accelerated oxidation experiment; Step 6, testing the resistance of the oxidized PPTC; The experimental conditions and test results are recorded in Table 1.

[0025] Example 3: Step 1, 100 parts of tungsten carbide powder and 100 parts of acetone are mixed, and then ground in a ball mill for 30 minutes; Step 2: Filter the tungsten carbide powder obtained in step 1, and wash it with water, ethanol, and water for 3 times; Step 3, placing the tungsten carbide powder obtained in step 2 into a vacuum drying oven for drying to obtain surface passivated tungsten carbide; Step 4: HDPE and surface passivated tungsten carbide are mixed at a mass ratio of 1:16 to prepare PPTC with a side length of 10 mm (length and width are both 10 mm) and a thickness of 0.5 mm; Step 5: Place the prepared PPTC in a constant temperature and humidity chamber under double 85 test conditions for accelerated oxidation experiment; Step 6, using a four-probe method to test the resistance of the PPTC after oxidation; The experimental conditions and test results are recorded in Table 1.

[0026] Example 4: Step 1, 100 parts of tungsten carbide powder and 140 parts of acetone are mixed and then ground in a ball mill for 60 minutes; Step 2: Filter the tungsten carbide powder obtained in step 1, and wash it with water, ethanol, and water for 3 times; Step 3, placing the tungsten carbide powder obtained in step 2 into a vacuum drying oven for drying to obtain surface passivated tungsten carbide; Step 4: HDPE and surface passivated tungsten carbide are mixed at a mass ratio of 1:16 to prepare PPTC with a side length of 10 mm (length and width are both 10 mm) and a thickness of 0.5 mm; Step 5: Place the prepared PPTC in a constant temperature and humidity chamber under double 85 test conditions for accelerated oxidation experiment; Step 6, testing the resistance of the oxidized PPTC; The experimental conditions and test results are recorded in Table 1.

[0027] Comparative Example 1: Step 1, HDPE and untreated tungsten carbide were mixed at a mass ratio of 1:16 to prepare PPTC with a side length of 10 mm and a thickness of 0.5 mm; Step 2: Place the prepared PPTC in a constant temperature and humidity chamber under double 85 test conditions for accelerated oxidation experiment; Step 3, testing the resistance of the oxidized PPTC; The experimental conditions and test results are recorded in Table 1.

[0028] Table 1, resistance unit is (mΩ); Oxidation time (hr) 0 24 168 336 672 1008 Comparative Example 1 Resistance 2.13 3.66 10.13 17.63 25.69 34.33 Example 1 Resistance 2.06 2.91 6.46 11.22 14.65 18.24 Example 2 Resistance 2.11 3.29 7.33 11.96 16.84 19.73 Example 3 Resistance 2.15 3.21 7.41 12.29 17.56 21.13 Example 4 Resistance 2.10 2.92 6.62 11.67 14.88 18.96 It can be seen from the data in Table 1 that the ground modified tungsten carbide powder obtained in Example 1, Example 2, Example 3, and Example 4 was used to prepare PPTC. After 1008 hours in a constant temperature and humidity chamber, the resistance changes were only 16.18, 17.60, 18.98, and 16.86, while the control increased by 32.2. Compared with the untreated sample, it is shown that the surface passivated tungsten carbide powder prepared in the example has good antioxidant properties and the resistance change of the prepared PPTC is small. Comparing Examples 1-3, as the acetone content increases during grinding, the resistance change is small and not particularly obvious. Comparing Examples 1 and 4, as the grinding time increases, the resistance increase does not change much.

[0029] Figure 2-5 This is the liquid nitrogen brittle section of the PPTC sample after 1008 hours of treatment, and the SEM image after acetone etching, which characterizes the surface morphology of the tungsten carbide particles. It can be seen from the figure that after accelerated oxidation under the double 85 test conditions of the constant temperature and humidity chamber, the tungsten carbide will oxidize and the surface will become rough. Figure 2 Tungsten carbide surface ratio Figure 3-5 Rough, that is, the surface of tungsten carbide in the surface passivation modified system is smoother than that without modification, indicating that the surface passivated tungsten carbide has good oxidation resistance and the prepared PPTC resistor is stable.

[0030] The above embodiments are only used to illustrate the present invention, but not to limit the present invention. Although the present invention is described in detail with reference to the embodiments, it should be understood by those skilled in the art that various combinations, modifications or equivalent substitutions of the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and should be included in the scope of the claims of the present invention.

Claims

1. A method for passivation modification of tungsten carbide surface, characterized in that: The following steps are involved: Acetone was mixed with tungsten carbide powder for physical grinding; The ground tungsten carbide powder is filtered and washed; The tungsten carbide powder is dried to obtain tungsten carbide powder with a passivated surface.

2. The method for surface passivation modification of tungsten carbide according to claim 1, characterized in that: Before physical grinding, tungsten carbide powder and acetone were mixed in a mass ratio of 1: (1.0-1.5).

3. The method for surface passivation modification of tungsten carbide according to claim 1, characterized in that: The physical grinding time is 0.5-2 hours.

4. The method for surface passivation modification of tungsten carbide according to claim 1, characterized in that: The ground tungsten carbide powder is washed in sequence using water, anhydrous ethanol, and water.

5. The method for surface passivation modification of tungsten carbide according to claim 1, characterized in that: The drying time of the tungsten carbide powder is 25-45 minutes, and the drying temperature is 30-70°C.

6. The method for surface passivation modification of tungsten carbide according to claim 1, characterized in that: The tungsten carbide powder is micron-sized tungsten carbide powder produced by mechanical alloying, direct reduction carburization or sol-gel method, and the particle size of the tungsten carbide powder is 1um-30um.

7. A method for preparing a high-stability PPTC, characterized in that: The following steps are involved: The surface passivated tungsten carbide powder prepared by the tungsten carbide surface passivation modification method according to any one of claims 1 to 6 is blended with polyethylene plastic to prepare PPTC.

8. The method for preparing high-stability PPTC according to claim 7, characterized in that: The polyethylene plastic is HDPE.

9. The method for preparing high-stability PPTC according to claim 7, characterized in that: When blending, the mass ratio of polyethylene plastic to surface passivated tungsten carbide powder is 1:15-20.

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