Method for improving stability of PPTC by using metastable oxide and PPTC

Abstract

The application discloses a method for improving PPTC stability by using metastable oxide and PPTC, and belongs to the field of thermistors and circuit protection. The method comprises the following steps: mixing tungsten carbide powder and metastable oxide powder, vacuum drying to obtain dry modified tungsten carbide powder; and blending the dry modified tungsten carbide powder with polyvinylidene fluoride (PVD F ) to prepare PPTC. The application utilizes the instability of metastable oxides such as ferrous oxide (FeO) to make it react with oxygen in the environment in preference to tungsten carbide, thereby playing the roles of "sacrificial protection" and "oxygen scavenger", and the generated stable oxide further forms a barrier layer. It is verified by experiments that the PPTC prepared by the application has a significantly reduced resistance change rate after aging in a double-85 high temperature and high humidity environment, and has higher environmental stability.

Description

Technical Field

[0001] This invention belongs to the field of electronic component manufacturing technology, and particularly relates to a material modification technology for a self-resetting fuse, specifically a method for improving the stability of PPTC using metastable oxides and PPTC. Background Technology

[0002] Polymer positive temperature coefficient thermistors (PPTC) are widely used for overcurrent protection in electronic equipment. Tungsten carbide (WC) is often used as a conductive filler and blended with high-density polyethylene, PVDF and other materials to prepare PPTC due to its excellent conductivity.

[0003] However, tungsten carbide powder is prone to oxidation in high temperature and high humidity environments. In humid air, water molecules adsorbed on the metal surface form micro batteries, which accelerates the oxidation reaction and causes the formation of a non-conductive oxide layer (such as WO3) on the tungsten carbide surface. This causes the resistance of PPTC to increase significantly over time, affecting the reliability of the device.

[0004] Existing solutions mainly include surface coating (such as coating with barrier resin) or physical passivation (such as grinding and dehydroxylation with organic solvents); however, the surfactant coating layer may fall off during processing, and physical passivation mainly targets surface hydroxyl groups, and the use of organic solvents is not environmentally friendly; therefore, it is of great significance to develop a method that can actively consume oxygen and form a protective mechanism. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method and PPTC for improving the stability of PPTC using metastable oxides. This invention creatively introduces metastable oxides (such as ferrous oxide) into the tungsten carbide system, utilizing their high reactivity with oxygen to preferentially consume oxygen invading the polymer matrix, thereby protecting the tungsten carbide matrix from oxidation.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] On one hand, the present invention provides a method for improving the stability of PPTC using metastable oxides, comprising the following steps:

[0008] Step 1: Mix and disperse the metastable oxide powder and tungsten carbide powder to obtain a mixed powder;

[0009] Step 2: Vacuum dry the mixed powder to obtain dried modified tungsten carbide powder;

[0010] Step 3: Blend the dried modified tungsten carbide powder with the polymer matrix to prepare PPTC.

[0011] The principle of this invention is as follows: ferrous oxide (FeO) is a metastable oxide that readily reacts with oxygen when heated in air to form stable iron oxide (4FeO + O2 → 2Fe2O3, the following discussion takes the formation of iron oxide as an example) or iron(II,III) oxide; in the PPTC system, the dispersed FeO acts as a "sacrificial anode" or "chemical oxygen scavenger"; when humid air or oxygen permeates into the PPTC matrix, FeO preferentially combines with oxygen over tungsten carbide (WC).

[0012] This mechanism provides dual protection:

[0013] First, the competitive reaction mechanism: FeO has a preferential oxidation and reaction thermodynamic basis; the sacrificial protection of FeO is not achieved through a single direct oxidation pathway, but follows a dynamic, multi-level consumption pathway composed of multiple chemical reactions under different environmental conditions; this pathway not only ensures efficient oxygen capture, but also enables it to maintain excellent protective performance in high temperature and high humidity environments.

[0014] Pathway 1: Oxygen reaction pathway. Direct oxidation by oxygen is the main way FeO is consumed in an aerobic environment. FeO reacts with O2 to generate stable Fe2O3.

[0015] Pathway 2, the electrochemical pathway, further highlights the proactive role of FeO in electrochemical corrosion; under high temperature and high humidity conditions, a miniature galvanic cell forms between FeO and WC particles; due to FeO's lower oxidation potential (or higher reduction potential), it preferentially acts as the anode for oxidation and dissolution.

[0016]

[0017] Subsequently, Fe 2+ Ions act as electron donors, reacting with O2 or H2O in the environment and being oxidized to Fe. 3+ Ultimately, Fe2O3 is formed; simultaneously, WC, acting as the cathode, undergoes its own oxidation reaction (e.g., WC + 2H2O → WO3 + 4H2O). + +4e - Due to the lack of sufficient electron acceptors, it is effectively suppressed; this competition at the electrochemical level ensures that the corrosion current preferentially flows to FeO, making it the main object of consumption, thereby protecting the integrity of WC.

[0018] FeO draws oxygen from the environment, reducing the oxygen concentration around tungsten carbide and inhibiting its oxidation at the source.

[0019] Second, the barrier effect: The Fe2O3 generated after FeO oxidation expands slightly in volume and is chemically stable. It can fill the gaps between particles or adhere to the WC surface, forming a dense and continuous physical barrier layer. This physical barrier composed of Fe2O3 has three functions: hindering diffusion, isolating the interface, and enhancing bonding. The diffusion-hindering effect refers to the extremely low ion and molecular diffusion coefficient of the physical barrier layer, which can effectively block the penetration of O2 and H2O molecules from the environment into the WC particles. It cuts off the supply of reactants required for the WC oxidation reaction, preventing it from continuing. The isolation interface refers to the physical barrier layer being firmly attached to the WC surface, forming a stable interfacial phase between the conductive filler and the polymer matrix. It not only prevents the direct oxidation of WC but also indirectly affects the stability of the matrix by changing the chemical environment of the interface (such as the local pH value). The enhanced bonding means that, unlike simple physical coating, the Fe2O3 layer may form chemical bonds with WC particles, thereby constructing a continuous and robust protective layer. This chemical bonding makes the protective layer less prone to detachment and can exist stably for a long time, providing continuous protection for WC.

[0020] It is evident that the sacrificial protection of FeO is a dynamic transformation process from active consumption to passive protection. FeO first consumes reactive oxygen in the environment through its own oxidation, and then its products form a dense physical barrier on the surface of WC, isolating WC from the corrosive external environment. This dual chemical-physical protection strategy is the key to FeO's ability to achieve long-term and efficient protection of WC.

[0021] Preferably, in step 1, the metastable oxide is ferrous oxide, and the amount added is 0.3-2.0% of the mass of tungsten carbide; that is, the mass ratio of tungsten carbide powder to metastable oxide powder is 100:(0.3-2.0); if the amount of ferrous oxide added is too low, it will not be able to play a long-term deoxygenation role, and if the amount added is too high, it may affect the initial conductivity of PPTC.

[0022] Preferably, in step 1, the mixing equipment used for mixing and dispersing is a high-speed mixer or a magnetic stirrer, and the mixing time is 5-20 minutes.

[0023] Preferably, in step 1, the tungsten carbide powder is micron-sized tungsten carbide powder produced by mechanical alloying, direct reduction carbide, or sol-gel method.

[0024] Preferably, in step 2, the vacuum drying process is carried out at a temperature of 40-80°C for 2-5 hours to prevent FeO from being oxidized during the preparation stage.

[0025] Preferably, in step 3, the polymer matrix is ​​polyvinylidene fluoride (PVDF). PVDF is a highly non-reactive thermoplastic fluoropolymer with a semi-crystalline structure, which has high stability and can meet increasingly stringent requirements.

[0026] Preferably, in step 3, the mass ratio of the modified tungsten carbide powder to the polymer matrix is ​​(7-13):1.

[0027] Preferably, the tungsten carbide powder has a particle size of 1 μm-10 μm, and the metastable oxide has a particle size of less than 0.6 μm.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] This invention constructs a synergistic protection mechanism involving multiple reaction pathways (oxygen reaction, water vapor reaction, and electrochemical reaction) by co-designing metastable oxides (such as FeO) and tungsten carbide (WC), achieving a dynamic transformation from active oxygen consumption to passive barrier. FeO preferentially oxidizes in the environment to form a dense Fe2O3 layer, which actively interrupts the oxidation pathway of WC through the electrochemical galvanic cell effect, and achieves long-term stable chemical-physical dual protection through a physical barrier layer (impeding diffusion, isolating interfaces, and enhancing bonding). This effectively solves the problem of easy failure of traditional PPTC during use, and shows greater anti-aging ability and wider applicability in complex environments than existing technologies. The PPTC prepared by this invention, after undergoing accelerated aging tests at 85°C (85% RH), showed significantly better electrical stability than PPTC prepared from untreated tungsten carbide. Attached Figure Description

[0030] Figure 1 This is a flowchart of the preparation process for improving the stability of PPTC using metastable oxides, as described in this invention.

[0031] Figure 2 The image shows the cross-sectional morphology of the PPTC sheet sample subjected to liquid nitrogen embrittlement, provided in Comparative Example 1.

[0032] Figure 3 This is a liquid nitrogen embrittlement cross-sectional morphology image of the PPTC sheet sample provided in Example 1;

[0033] Figure 4 This is a cross-sectional morphology image of the PPTC sheet sample subjected to liquid nitrogen embrittlement provided in Example 2. Detailed Implementation

[0034] The present invention will be further described below with reference to the embodiments.

[0035] Example 1: Step 1: Weigh 100g of micron-sized tungsten carbide powder (model: GWC030) and 0.4g of chemically pure ferrous oxide (FeO) powder, add them to a high-speed mixer, and stir at 1500r / min for 10 minutes to mix them evenly and obtain a mixed powder.

[0036] Step 2: Place the mixed powder in an 80℃ vacuum drying oven and dry for 2 hours to obtain dried modified tungsten carbide powder;

[0037] Step 3: Dry modified tungsten carbide powder and PVDF are blended on a two-roll mill at a mass ratio of 10:1 to prepare PPTC sheets with a thickness of 0.5 mm.

[0038] Step 4: Place the prepared PPTC sheet in a constant temperature and humidity chamber (85℃, 85%RH) for accelerated oxidation experiment for 1008 hours and test the change in resistance.

[0039] Example 2: Step 1: Weigh 100g of micron-sized tungsten carbide powder (model: GWC030) and 1.6g of chemically pure ferrous oxide (FeO) powder, add them to a three-necked flask, and stir on a magnetic stirrer at a speed of 300r / min for 20 minutes to mix them evenly and obtain a mixed powder.

[0040] Step 2: Place the mixed powder in a vacuum drying oven at 70℃ and dry for 2.5 hours to obtain dried modified tungsten carbide powder;

[0041] Step 3: Dry modified tungsten carbide powder and PVDF are blended on a two-roll mill at a mass ratio of 10:1 to prepare PPTC sheets with a thickness of 0.5 mm.

[0042] Step 4: Place the prepared PPTC sheet in a constant temperature and humidity chamber (85℃, 85%RH) for accelerated oxidation experiment for 1008 hours and test the change in resistance.

[0043] Example 3: Step 1: Weigh 100g of tungsten carbide powder (model: GWC030) and 0.8g of chemically pure ferrous oxide (FeO) powder, add them to a three-necked flask, and stir on a magnetic stirrer at a speed of 400r / min for 15 minutes to mix them evenly and obtain a mixed powder.

[0044] Step 2: Place the mixed powder in a vacuum drying oven at 60℃ and dry for 3 hours to obtain dried modified tungsten carbide powder;

[0045] Step 3: Dry modified tungsten carbide powder and PVDF are blended on a two-roll mill at a mass ratio of 10:1 to prepare PPTC sheets with a thickness of 0.5 mm.

[0046] Step 4: Place the prepared PPTC sheet in a constant temperature and humidity chamber (85℃, 85%RH) for accelerated oxidation experiment for 1008 hours and test the change in resistance.

[0047] Comparative Example 1: Step 1: Unmodified tungsten carbide powder and PVDF were blended on a two-roll mill at a mass ratio of 10:1 to prepare PPTC sheets with a thickness of 0.5 mm.

[0048] Step 2: Place the prepared PPTC in a constant temperature and humidity chamber (85℃, 85%RH) for accelerated oxidation experiment for 1008 hours and test the resistance change.

[0049] Comparative Example 2 (Solution in prior art CN104319042 A, which uses zinc oxide as an auxiliary filler): Step 1: Weigh 100g of micron-sized tungsten carbide powder and 0.8g of nano-sized zinc oxide (ZnO) powder, add them to a three-necked flask, and stir on a magnetic stirrer at a speed of 300r / min for 20 minutes to mix them evenly and obtain a mixed powder;

[0050] Step 2: Place the mixed powder in a vacuum drying oven at 70℃ and dry for 2.5 hours to obtain dried modified tungsten carbide powder;

[0051] Step 3: Dry modified tungsten carbide powder and PVDF are blended on a two-roll mill at a mass ratio of 10:1 to prepare PPTC sheets with a thickness of 0.5 mm.

[0052] Step 4: Place the prepared PPTC sheet in a constant temperature and humidity chamber (85℃, 85%RH) for accelerated oxidation experiment for 1008 hours and test the change in resistance.

[0053] Table 1, the unit of resistance is (mΩ);

[0054] .

[0055] As can be seen from the data in Table 1:

[0056] 1. The resistance of Comparative Example 1 (without FeO) increased sharply after 1008 hours, indicating that tungsten carbide underwent severe oxidation under high temperature and high humidity conditions; in Comparative Example 2, the addition of ZnO did not have an antioxidant effect.

[0057] 2. The resistance increase in Examples 1-3 was significantly lower than that in the comparative example; this indicates that the added ferrous oxide effectively suppressed the oxidation of tungsten carbide.

[0058] 3. As the amount of ferrous oxide added increases (from 0.4% to 1.6%), the stability of PPTC gradually improves. This is because more FeO provides a more durable deoxygenation capability, and the generated Fe2O3 barrier layer is also more compact. Although the initial resistance increases slightly (because FeO has weaker conductivity than WC), the long-term stability benefits are enormous.

[0059] Figure 2 , Figure 3 , Figure 4 The SEM microstructure images of the PPTC sheet samples from Comparative Example 1, Example 1, and Example 2, after undergoing accelerated oxidation at high temperature and humidity (double 85 test) for 1008 hours and then subjected to liquid nitrogen brittle fracture treatment, characterize the surface morphology of the tungsten carbide particles. As can be seen from the figures, the tungsten carbide particles all oxidize and become rough after accelerated oxidation under the double 85 test conditions in the constant temperature and humidity chamber. Figure 2 Tungsten carbide surface area ratio Figure 3 and 4 The surface of the tungsten carbide in the ferrous oxide modified system is rougher than that in the unmodified system, indicating that the FeO modified tungsten carbide system has good oxidation resistance and the prepared PPTC has stable resistance.

[0060] Based on the resistance variation data in Table 1, we can see that:

[0061] The resistivity of Comparative Example 1 (without FeO) increased sharply to 38.26 after 1008 hours, indicating that tungsten carbide underwent severe oxidation under high temperature and high humidity conditions; its corresponding SEM image ( Figure 2 The images clearly show that the surface of the tungsten carbide particles is very rough, which is a typical morphology of oxidation erosion.

[0062] The resistivity of Examples 1 and 2 (both with added FeO) increased only to 18.73 and 15.32 respectively after the same aging time, indicating a significant improvement in antioxidant performance; the corresponding SEM images ( Figure 3 , Figure 4 The results showed that the tungsten carbide particles modified with ferrous oxide had a smoother surface, which was significantly better than that of Comparative Example 1.

[0063] The data in Table 1 show that the addition of FeO effectively inhibited the increase in resistance and improved the long-term stability of PPTC; the SEM microscopic images directly confirmed this conclusion from the microscopic morphology: the FeO modified system can significantly reduce the oxidation and erosion of the tungsten carbide surface, keep the particle surface relatively smooth, and thus ensure the stability of the material resistance. Figure 2 and Figure 3 , Figure 4 The direct comparison, consistent with the resistance change trend in the table, jointly demonstrates that FeO has a clear effect on the antioxidant modification of tungsten carbide.

[0064] In summary, this invention utilizes the chemical reactivity of metastable oxides (FeO) to successfully solve the oxidation failure problem of tungsten carbide-based PPTC in harsh environments through sacrificial oxidation reaction and product blocking effect.

[0065] The above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand 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 covered within the scope of the claims of the present invention.

Claims

1. A method for improving the stability of PPTC using metastable oxides, characterized in that, Includes the following steps: Metastable oxide powder and tungsten carbide powder are mixed and stirred to obtain a mixed powder, wherein the metastable oxide is ferrous oxide; The mixed powder was subjected to vacuum drying to obtain dried modified tungsten carbide powder; PPTC is prepared by blending the dried modified tungsten carbide powder with a polymer matrix.

2. The method for improving the stability of PPTC using metastable oxides according to claim 1, characterized in that, The mass ratio of tungsten carbide powder to metastable oxide powder is 100:(0.3-2.0).

3. The method for improving the stability of PPTC using metastable oxides according to claim 1, characterized in that, The mixing and dispersing process uses a high-speed mixer or a magnetic stirrer, and the mixing time is 5-20 minutes.

4. The method for improving the stability of PPTC using metastable oxides according to claim 1, characterized in that, The vacuum drying process is carried out at a temperature of 40-80℃ for 2-5 hours.

5. The method for improving the stability of PPTC using metastable oxides according to claim 1, characterized in that, The polymer matrix is ​​polyvinylidene fluoride.

6. The method for improving the stability of PPTC using metastable oxides according to claim 1, characterized in that, The mass ratio of the modified tungsten carbide powder to the polymer matrix is ​​(7-13):

1.

7. The method for improving the stability of PPTC using metastable oxides according to claim 1, characterized in that, The tungsten carbide powder has a particle size of 1μm-10μm, and the metastable oxide has a particle size of less than 0.6μm.

8. The method for improving the stability of PPTC using metastable oxides according to claim 1, characterized in that, The tungsten carbide powder is micron-sized tungsten carbide powder produced by mechanical alloying, direct reduction carbide, or sol-gel method.

9. A highly stable PPTC, characterized in that, It is prepared by the method described in any one of claims 1-8.

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