An ultra-low resistance paste for stainless steel heating elements and a method for preparing the same

Abstract

The application belongs to the technical field of electronic paste, and discloses a super-low-resistance paste for a stainless steel heating element and a preparation method thereof. ZnO nanowires are added to the resistance paste to fill the gaps between silver particles, so that the square resistance is reduced to less than 200 mΩ / □. On the other hand, the Bi-Mo glass system is used as a sintering aid, and compared with the traditional glass, the Bi-Mo glass system has a lower change rate of high-temperature sintering, higher resistance stability and higher mechanical strength. The application has significant advantages in conductivity, thermal stability and mechanical properties through the synergistic innovation of ZnO nanowire bridging and the Bi-Mo glass system, and is especially suitable for the field of high-precision electronic devices.

Description

Technical Field

[0001] This invention belongs to the field of electronic paste technology, and particularly relates to an ultra-low resistance paste for stainless steel heating elements and its preparation method. Background Technology

[0002] With the rapid development of flexible electronic devices and high-precision heating equipment, stainless steel heating elements have become a key carrier material due to their excellent mechanical strength and corrosion resistance. As a core functional material, the conductivity and thermal stability of electronic pastes directly determine the component's performance. Currently, mainstream commercial resistive pastes are mainly silver-based, achieving conductivity through the composite of micron-sized silver particles and glass powder. However, significant technical bottlenecks still exist in practical applications.

[0003] First, traditional silver pastes reduce sheet resistance by increasing the silver content (typically ≥80wt%), but the "point contact" conductivity between silver particles results in persistently high interfacial resistance. Although some studies have attempted to construct three-dimensional conductive networks using silver nanowires (such as CNTs or AgNWs), the actual sheet resistance remains difficult to exceed 200mΩ / □ due to issues such as the tendency of nanomaterials to agglomerate and poor adhesion to the substrate. More importantly, the Ostwald curing phenomenon of silver particles during high-temperature sintering further deteriorates the continuity of the conductive pathway.

[0004] Secondly, existing glass systems (such as Pb-B-Si and Zn-B-Si systems) have two shortcomings: (1) the reheating rate is large, which leads to cracks in the slurry layer after repeated high-temperature operation; (2) the coefficient of thermal expansion (CTE) of the glass phase and the stainless steel matrix is ​​mismatched, and interface peeling is prone to occur under cyclic conditions of -40~300℃. Studies have shown that the resistivity drift rate of traditional glass powder is as high as 8~12% after aging at 300℃ for 1000h, which seriously affects the device life.

[0005] Furthermore, existing technologies typically require increasing the glass phase content to improve slurry adhesion, but this significantly sacrifices conductivity; conversely, reducing the glass content leads to insufficient mechanical strength of the sintered body. Existing commercially available slurries exhibit conductive layer fracture when the bending radius is <8mm, making it difficult to meet the requirements of flexible devices.

[0006] The industry urgently needs an innovative solution that can achieve the following objectives simultaneously: (1) constructing a long-lasting conductive network through new fillers to reduce sheet resistance; (2) developing a glass system with low reheat change rate; and (3) maintaining resistance stability under high-temperature cyclic conditions.

[0007] In recent years, some studies have attempted to improve conductive networks through modification with nano-copper powder or graphene composites. However, the former faces resistance drift due to high-temperature oxidation, while the latter suffers from uneven current distribution due to the randomness of layer orientation. A more fundamental contradiction is that existing technologies mostly focus on optimizing single performance aspects, failing to systematically address the synergistic requirements of "low resistance, high stability, and high strength." This directly restricts the reliability of devices in high-end applications such as precision temperature control equipment and PTC systems in new energy vehicles. Therefore, to address these issues, it is necessary to develop high-performance ultra-low resistance slurries for stainless steel heating elements. Summary of the Invention

[0008] To meet the core requirements of stainless steel heating elements for low resistance, high adhesion, and low-temperature sintering, this invention provides an ultra-low resistance slurry for stainless steel heating elements and its preparation method. The specific technical solution provided by this invention is as follows.

[0009] Firstly, this invention provides an ultra-low resistance slurry for stainless steel heating elements, comprising: by mass ratio, 80-86% conductive phase, 8-12% sintering aid, 1-3% resistance modifier, and the balance being an organic carrier;

[0010] The sintering aid is a Bi-Mo glass system, and the composition of the sintering aid is: 60% Bi2O3, 30% MoO3, and 10% SiO2 by mass ratio.

[0011] The resistance modifier is ZnO nanowires.

[0012] Furthermore, for the aforementioned ultra-low resistance slurry for stainless steel heating elements, the ZnO nanowires have a diameter of 50~100nm and a length of 1~2μm.

[0013] Furthermore, for the aforementioned ultra-low resistance slurry for stainless steel heating elements, the conductive phase is flake silver powder, wherein the flake silver powder has a D50 ≤ 2 μm and an aspect ratio > 10.

[0014] Furthermore, for the aforementioned ultra-low resistance slurry for stainless steel heating elements, the organic carrier comprises, by mass ratio, 2% ethyl cellulose, 70% terpineol, 25% tributyl citrate, and 3% Span-80, with the sum of the mass ratios of each component being 100%.

[0015] Furthermore, the ultra-low resistance paste for stainless steel heating elements described above is suitable for austenitic stainless steel or ferritic stainless steel substrates.

[0016] Secondly, the present invention provides a method for preparing the above-mentioned ultra-low resistance slurry for stainless steel heating elements, the preparation method comprising:

[0017] 1) Glass powder preparation: Bi2O3 60%, MoO3 30% and SiO2 10% were ball-milled and mixed, melted at 800℃ for 2h, water-quenched and then ball-milled to D50≤1μm;

[0018] 2) Slurry mixing: The conductive phase, glass powder, and ZnO nanowires are added to the organic carrier in proportion, and rolled with three rollers to a fineness of ≤5μm to obtain the slurry.

[0019] Furthermore, in step 2) of the above preparation method, the viscosity is 25~35 Pa·s when rolling is completed.

[0020] Thirdly, the present invention claims protection for a stainless steel heating element, the stainless steel heating element comprising: a stainless steel substrate, an insulating dielectric layer, a resistive paste layer, and electrodes; the resistive paste layer is formed by printing and sintering the aforementioned ultra-low resistivity paste.

[0021] Furthermore, for the aforementioned stainless steel heating element, the stainless steel substrate is an austenitic stainless steel substrate or a ferritic stainless steel substrate.

[0022] Furthermore, for the aforementioned stainless steel heating element, the printing process is screen printing.

[0023] Compared with the prior art, the present invention, "An ultra-low resistance slurry for stainless steel heating elements and its preparation method," has at least the following beneficial effects or advantages:

[0024] This invention adds ZnO nanowires to a resistive paste, which fill the gaps between silver particles, thereby improving the conductivity of the paste and reducing the sheet resistance to less than 200 mΩ / □.

[0025] On the other hand, the present invention uses the Bi-Mo glass system as a sintering aid. Compared with traditional glass, the re-firing resistance change rate is only 6.8%, which is much lower than Comparative Example 1 and Comparative Example 2, indicating that the Bi-Mo glass system improves the high-temperature stability of the material.

[0026] The temperature coefficient of resistance (TCR) (1400 ppm / ℃) of the embodiments of the present invention is significantly lower than that of Comparative Examples 1 or 2, indicating that the low-resistivity slurry has higher stability under temperature changes. The adhesion of the resistive slurry of the embodiments of the present invention reaches level 0 (the optimal level), which is better than that of Comparative Examples 1 or 2.

[0027] This invention achieves ultra-low resistance characteristics of the resistive paste through ZnO nanowire bridging and improvements to the Bi-Mo glass system, while effectively improving the thermal stability and mechanical properties of the resistive paste, making a significant contribution to the preparation of stainless steel heating elements. Detailed Implementation

[0028] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0029] Example 1

[0030] This embodiment describes the preparation of the ultra-low resistance slurry.

[0031] 1. Silver powder pretreatment

[0032] Select flake silver powder (D50≤2μm, aspect ratio>10), first ultrasonically clean it with 0.1M citric acid solution for 10min to remove surface oxides, and then vacuum dry it at 60℃ for later use.

[0033] 2. Glass powder preparation

[0034] Ingredients: Weigh the raw materials according to 60wt% Bi2O3, 30wt% MoO3 and 10wt% SiO2, add them to a planetary ball mill (agate balls, 5mm in diameter) for ball milling, and set the ball-to-material ratio to 5:1 (ball mass: material mass).

[0035] Melting: The ball-milled powder is heated to 800°C at 5°C / min in a high-temperature tube furnace and held for 2 hours to completely melt it. The molten glass is then quickly poured into 25°C deionized water (quenching rate > 30°C / s) and water-quenched to obtain glass fragments.

[0036] Ball milling: Glass powder with D50≤1μm was obtained by wet ball milling with alcohol at 300rpm for 24h and dried for later use.

[0037] 3. Preparation of ZnO nanowires

[0038] The hydrothermal synthesis of ZnO nanowires includes the following steps:

[0039] 1) Preparation of zinc salt solution: Weigh 15g of zinc nitrate hexahydrate [Zn(NO3)2·6H2O], dissolve it in 50mL of deionized water, and stir until homogeneous;

[0040] 2) Preparation of the precipitant: Slowly add 5 mol / L NaOH solution, stirring vigorously until the pH reaches 9-11. The initial flocculent precipitate gradually dissolves into a clear solution. Add 0.5 g of polyvinylpyrrolidone (PVP K60) to increase the aspect ratio.

[0041] 3) Loading the reactor: Transfer the mixed solution to a high-pressure reactor lined with polytetrafluoroethylene. The filling degree should be strictly controlled at 60-80% to avoid boiling over. Seal the reactor lid, check the airtightness, and rotate the bolts to ensure uniform stress.

[0042] 4) Hydrothermal reaction: The reactor is heated to 150°C at a rate of 5~10°C / min, and reacted at 150°C for 12 hours to obtain uniform nanowires;

[0043] 5) After the reaction is complete, allow it to cool naturally to room temperature. Open the vessel lid, centrifuge at 8000-10000 rpm for 10 minutes to separate the precipitate, and discard the supernatant.

[0044] 6) Wash the precipitate three times alternately with deionized water and anhydrous ethanol to remove residual ions;

[0045] 7) ZnO nanowires were obtained by drying in a 90℃ oven for 2 hours. The ZnO nanowires had a diameter of 50-80 nm and a length of 1-2 μm.

[0046] 4. Organic carrier preparation

[0047] By mass ratio, 2 wt% of ethyl cellulose (0.2 g) was dissolved in 70 wt% of terpineol (7 g), and stirred at 60°C for 2 h until transparent (ethyl cellulose was completely dissolved). Then, 25 wt% of tributyl citrate (2.5 g) and 3 wt% of Span-80 (0.3 g) were added, and stirring was continued under heating until the mixture was homogeneous.

[0048] 5. Slurry mixing

[0049] Initial mixing: Add silver powder (83wt%), glass powder (10wt%), and ZnO nanowires (2wt%) to the organic carrier (5wt%), and premix in a planetary mixer for 30 minutes at 500 rpm.

[0050] Three-roll dispersion: Rolled to a fineness of ≤5μm using three rolls, the viscosity was measured to be 28±2Pa·s (25℃, Brookfield viscometer) after rolling.

[0051] Example 2

[0052] This embodiment describes the preparation of the ultra-low resistance slurry.

[0053] The preparation method of the ultra-low resistance slurry described in this embodiment is the same as that in embodiment 1, except that in step 5, silver powder (86%), glass powder (8%), and ZnO nanowires (1%) are added to the organic carrier (5%).

[0054] The ultra-low resistance slurry prepared in this embodiment has a viscosity of 32±2 Pa·s (25℃, Brookfield viscometer).

[0055] Example 3

[0056] This embodiment describes the preparation of the ultra-low resistance slurry.

[0057] The preparation method of the ultra-low resistance slurry described in this embodiment is the same as that in embodiment 1, except that in step 5, silver powder (80%), glass powder (12%), and ZnO nanowires (3%) are added to the organic carrier (5%).

[0058] The ultra-low resistance slurry prepared in this embodiment has a viscosity of 26±2 Pa·s (25℃, Brookfield viscometer).

[0059] Comparative Example 1

[0060] The preparation method of this comparative example is the same as that of Example 1 (preferred example), except that the same mass of silver powder is used instead of ZnO nanowires.

[0061] Comparative Example 2

[0062] The preparation method of this comparative example is the same as that of Example 1 (preferred example), except that: conventional glass (by mass ratio, 55% Bi2O3, 30% SiO2, 13% B2O3, 2% CaO) is used instead of the Bi-Mo glass system of the present invention.

[0063] Example 4

[0064] This embodiment describes the performance of the slurries prepared in the examples and comparative examples.

[0065] 1. Pretreatment of stainless steel substrate

[0066] An austenitic stainless steel substrate (model 304, size 25.4mm×25.4mm×0.2mm) was sandblasted with Al2O3 sand with a particle size of 50μm. After sandblasting, the surface roughness Ra of the substrate was controlled to be 1.0~1.5μm. Subsequently, it was ultrasonically cleaned with acetone, ethanol and deionized water in sequence, and dried with nitrogen to obtain a clean stainless steel substrate.

[0067] 2. Printing and Sintering

[0068] First, a dielectric paste was printed on a stainless steel substrate using a screen printing process (325-mesh polyester screen). After drying at 150°C for 10 minutes, the paste was printed again, dried at 150°C for 10 minutes, and then printed a third time, dried at 150°C for 10 minutes. Then, the temperature was raised to 550°C at a uniform rate of 5°C / min, and sintered at this temperature for 30 minutes to produce an insulating stainless steel substrate with a film thickness ≥60μm. Next, an electrode paste was printed on the prepared insulating stainless steel substrate using a screen printing process. After drying at 150°C for 10 minutes, the paste was sintered at 550°C for 30 minutes to produce a silver conductor electrode. Finally, a resistance paste was printed using a screen printing process. After drying at 150°C for 10 minutes, the paste was sintered at 550°C for 30 minutes. After cooling in the furnace, the sample was removed to produce a test sample.

[0069] 3. Testing

[0070] The slurries prepared in the examples and comparative examples were subjected to substrate processing, printing and sintering according to the above method to obtain test samples of stainless steel substrate thick film electric heating elements, and performance tests such as sheet resistance and re-firing resistance change rate were performed respectively.

[0071] 1) Sheet resistance: tested using the four-probe method.

[0072] 2) Resistivity change rate after re-firing: After printing the resistive paste, keep it at 700℃ for 15 minutes, cool it to room temperature and measure the resistance. Record this resistance as the first sintering resistance R1. Keep it at 700℃ for 15 minutes again, cool it to room temperature and measure the resistance. Record this resistance as the second sintering resistance R2. The formula for calculating the resintering resistance change rate ρ is as follows.

[0073]

[0074] Where R1 is the resistance for the first sintering and R2 is the resistance for the second sintering.

[0075] 3) Temperature coefficient of resistance (TCR): The resistance R of the test sample was measured at 25℃ and 125℃ respectively. 25 R 125 The TCR calculation formula is as follows.

[0076]

[0077] Among them, R 25 The resistance at 25℃ is R. 125 It is a resistor at 125℃.

[0078] 4) Adhesion test: Use a blade to draw a 1mm×1mm grid on the coating surface (deep to the substrate); apply 3M 610 tape and quickly peel it off; rate by the percentage of area that peels off.

[0079] Level 0: No detachment;

[0080] Level 1: Detachment area ≤ 5%;

[0081] Level 2: 5-15% of the area detached;

[0082] Level 3: 15-35% of the area detached;

[0083] Level 4: Desquamation area 35-65%;

[0084] Level 5: Area of ​​detachment > 65%.

[0085] 5) Bending Performance Test: Three stainless steel heating element test samples prepared according to the examples or comparative examples were selected for each group, ensuring the sample surface was free of scratches, bubbles, and other defects. The cylindrical shaft bending method was used, selecting cylindrical shafts with diameters of 5mm and 8mm. The bending rate was controlled at 10mm / min, and the bending angle was 180°. A reciprocating bending test was performed, with a total of 100 bending cycles. Performance Testing and Recording: Before bending, the initial sheet resistance R0 of the sample was measured using the four-probe method, and the data was recorded. After 100 bending cycles, R0 was measured and recorded. 100 Shear resistance change rate .

[0086] 6) Long-term high-temperature aging test: Select stainless steel heating element test samples prepared in the examples or comparative examples, three samples per group. Clean the surface of the samples with anhydrous ethanol and dry them before testing. Place the samples in a high-temperature constant temperature chamber, set the aging temperature to 300℃, and the aging time to 1000h. The temperature fluctuation of the constant temperature chamber is controlled within ±2℃ to avoid uneven local temperature. Performance monitoring and recording: Test the initial sheet resistance before aging. After aging, the final resistance was tested. Calculate the sheet resistance drift rate during high-temperature aging. %.

[0087] 7) Corrosion Resistance Test: Select stainless steel heating element test samples prepared in the examples or comparative examples, three samples per group. Lightly sand the edges of the samples with 800-grit sandpaper (to avoid edge effects), then ultrasonically clean with acetone and ethanol sequentially for 5 minutes, and dry with nitrogen. Prepare a 5% (w / w) NaCl solution, and adjust the pH to 6.5-7.2 with hydrochloric acid or sodium hydroxide. Set the test conditions: test temperature 35±2℃, salt spray deposition rate 1.0-2.0 mL / (h·80cm²). 2 The test lasts for 100 hours. Performance testing and recording: Before testing, the sample should be free of rust and scratches. Test and record the initial sheet resistance. After the test, remove the sample, rinse it with running deionized water for 5 minutes, wipe it dry with anhydrous ethanol, and then dry it at 60℃ for 30 minutes before testing the sheet resistance. ; Calculate the change rate of sheet resistance in salt spray testing %.

[0088] 8) Thermal Shock Test: Select stainless steel heating element test samples prepared in the examples or comparative examples, with 3 samples per group, ensuring that the resistance paste layer of the sample is tightly bonded to the stainless steel substrate and free of initial defects; use a two-chamber thermal shock test chamber, setting two temperature zones: low temperature zone: -40±2℃, holding time 30min; high temperature zone: 300±2℃, holding time 30min; temperature zone transition time: ≤20s; total number of cycles: 100; performance testing and recording: test and record the initial sheet resistance. After the loop ends, test the final resistance. Calculate the rate of change of sheet resistance after 100 thermal shocks. %.

[0089] The test results for each indicator are shown in Table 1.

[0090] Table 1. Test results of slurry properties

[0091]

[0092] The performance test results of Example 1 and Comparative Examples 1 and 2 in Table 1 show that:

[0093] 1) It has ultra-low resistance characteristics: The sheet resistance of Example 1 is as low as 163mΩ / □, which is significantly lower than that of Comparative Example 1 without ZnO nanowires and Comparative Example 2 using a conventional glass system. This shows that the technical solution of filling the gaps between silver particles with ZnO nanowires and constructing a bridging conductive network can effectively reduce the interface resistance caused by the "point contact" between silver particles, achieve ultra-low resistance characteristics, and achieve the goal of reducing the sheet resistance to less than 200mΩ.

[0094] 2) Performance stability: In thermal stability tests, the sheet resistance change rate of Example 1 after reheating was only 6.8%, far lower than that of Comparative Examples 1 and 2; the sheet resistance drift rate after high-temperature aging was only 3.2%, significantly better than that of Comparative Examples 1 and 2; and the sheet resistance change rate after thermal shock was only 3.5%, significantly lower than that of Comparative Examples 1 and 2. This indicates that the Bi-Mo glass system can suppress the Ostwald curing of silver particles at high temperatures, while optimizing the matching of thermal expansion coefficients with the stainless steel substrate, solving the defects of traditional glass systems such as "large reheating change rate and easy peeling under high and low temperature cycles". In addition, Example 1 achieved an adhesion level of 0, and the sheet resistance change rate after 100 hours of neutral salt spray testing was only 3.3%, far lower than that of Comparative Examples 1 and 2, further proving that the Bi-Mo glass system can enhance the bonding strength between the paste and the substrate and form a dense protective layer to isolate corrosive media.

[0095] 3) Meeting the requirements of flexible applications: In the bending performance test, after repeated bending along 5mm and 8mm axes, Example 1 showed a sheet resistance change rate of only 4.9% and 3.3%, respectively, with no breakage or peeling of the slurry layer; while Comparative Example 1 showed localized breakage after bending along the 5mm axis and edge peeling after bending along the 8mm axis, and Comparative Example 2 showed obvious breakage and large-area peeling after bending along both axis diameters. This difference stems from the synergistic effect of the ZnO nanowires and Bi-Mo glass system of this invention: ZnO nanowires enhance the flexibility of the conductive network, and Bi-Mo glass improves the interfacial bonding force, jointly solving the problem of easy failure of traditional slurry bending and adapting to the application requirements of flexible electronic devices.

[0096] In summary, the technical solution of this invention, through the ZnO nanowire and Bi-Mo glass system, achieves ultra-low resistance, high thermal stability, high mechanical strength, excellent corrosion resistance and flexible adaptability of the slurry for stainless steel heating elements, solving the problem of difficulty in achieving low resistance, high stability and high strength in the prior art, and meeting the performance requirements of stainless steel heating elements in various scenarios such as high-precision electronic devices and PTC for new energy vehicles.

[0097] The above embodiments can well illustrate the technical solution of the present invention, but they are only describing preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, all kinds of changes and improvements made by those skilled in the art to the technical solution of the present invention should fall within the protection scope defined by the present invention.

Claims

1. A low-resistivity slurry for stainless steel heating elements, characterized in that, Its composition is as follows (by mass): 80-86% conductive phase, 8-12% sintering aid, 1-3% resistivity modifier, and the balance being organic carrier; The conductive phase is flake-shaped silver powder; The sintering aid is a Bi-Mo glass system, and the composition of the sintering aid is: 60% Bi2O3, 30% MoO3, and 10% SiO2 by mass ratio. The resistance modifier is ZnO nanowires.

2. The ultra-low resistance slurry for stainless steel heating elements according to claim 1, characterized in that, The ZnO nanowires have a diameter of 50~100nm and a length of 1~2μm.

3. The ultra-low resistance slurry for stainless steel heating elements according to claim 1, characterized in that, The flake-shaped silver powder has a D50 ≤ 2 μm and an aspect ratio > 10.

4. The ultra-low resistance slurry for stainless steel heating elements according to claim 1, characterized in that, The organic carrier consists of the following components by mass ratio: 2% ethyl cellulose, 70% terpineol, 25% tributyl citrate, and 3% Span-80, with the sum of the mass ratios of each component being 100%.

5. The ultra-low resistance slurry for stainless steel heating elements according to claim 1, characterized in that, The ultra-low resistance paste is suitable for austenitic stainless steel or ferritic stainless steel substrates.

6. The method for preparing the ultra-low resistance slurry for stainless steel heating elements according to claim 1, characterized in that, The preparation method includes: 1) Glass powder preparation: Bi2O3 60%, MoO3 30% and SiO2 10% were ball-milled and mixed, melted at 800℃ for 2h, water-quenched and then ball-milled to D50≤1μm; 2) Slurry mixing: The conductive phase, glass powder, and ZnO nanowires are added to the organic carrier in proportion, and rolled with three rollers to a fineness of ≤5μm to obtain the slurry.

7. The preparation method according to claim 6, characterized in that, In step 2), the viscosity is 25~35 Pa·s when rolling is completed.

8. A stainless steel heating element, characterized in that, The stainless steel heating element comprises: a stainless steel substrate, an insulating dielectric layer, a resistive paste layer, and electrodes; the resistive paste layer is formed by printing and sintering the ultra-low resistivity paste as described in claim 1.

9. The stainless steel heating element according to claim 8, characterized in that, The stainless steel substrate is an austenitic stainless steel substrate or a ferritic stainless steel substrate.

10. The stainless steel heating element according to claim 8, characterized in that, The printing process is screen printing.

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