Preparation method of ZnO-based piezoresistive ceramic based on heat treatment assisted cold sintering

Abstract

The application discloses a kind of based on heat treatment auxiliary cold sintering ZnO base pressure-sensitive ceramic preparation method, including with deionized water and citric acid preparation citric acid solution;Initial ZnO powder is mixed with metal oxide dopant to constitute precursor powder, then precursor powder is mixed with citric acid solution, obtains mixed slurry after grinding dispersion;Mixed slurry is placed in steel mould, pressure is applied, and cold sintering is carried out;The sample after cold sintering is heat treated, and finally high-performance ZnO base pressure-sensitive ceramic is obtained.The application prepares ceramic with relative density up to 99% under the condition of ≤950 DEG C, the prepared ceramic also has excellent nonlinear coefficient up to 92, and 1600V / mm ultra-high breakdown strength, far more than ZnO base pressure-sensitive resistance prepared by solid phase method.Compared with traditional high-temperature sintering process, the application not only reduces sintering temperature, saves energy, and can significantly improve electrical performance.

Description

Technical Field

[0001] This invention relates to a method for preparing ZnO ceramics, and particularly to a method for preparing ZnO-based varistors by heat treatment-assisted cold sintering. Technical Background

[0002] ZnO-based varistors have wide applications in high-voltage resistors and gas sensors; however, their fabrication faces numerous technical challenges. Traditional high-temperature sintering processes typically require temperatures around 1100 °C, leading to high energy consumption and excessive grain growth, significantly reducing material density and mechanical properties. Furthermore, multi-component formulations of ZnO-based ceramics, such as Bi₂O₃, Mn₂O₃, NiO, SnO₂, In₂O₃, CaO, and Dy₂O₃, are prone to adverse chemical reactions or excessive volatilization at high temperatures, affecting their varistor performance and long-term stability. Currently, common problems with commercial varistors include low varistor coefficients (typically between 30 and 50) and low breakdown strength (approximately 400 V / mm), limiting their further development in high-performance applications. Therefore, exploring new fabrication processes and optimizing ZnO-based ceramic formulations are crucial for improving their performance.

[0003] To address the shortcomings of traditional high-temperature sintering processes and improve the overall performance of ceramics, cold sintering technology has gradually gained attention. Cold sintering achieves initial densification of ceramics at relatively low temperatures (typically ≤300 ℃) by applying pressure. Compared to high-temperature sintering, cold sintering significantly reduces energy consumption (only about 1% of traditional sintering) and effectively controls grain growth, reduces component volatilization, and maintains the compositional uniformity of the ceramic. For example, by controlling the sintering temperature between 120 ℃ and 300 ℃, cold sintering can achieve a ceramic density of over 90%. However, despite its excellent performance in temperature reduction and energy saving, the ceramic materials prepared by cold sintering still have shortcomings in density and electrical properties. This is because at low temperatures, most inorganic dopants cannot achieve effective synergistic sintering, and there is a lack of sufficient nucleation driving force during the dissolution-precipitation process, leading to the formation of a large number of amorphous phases at grain boundaries, which seriously affects the electrical properties of electrical ceramic materials. Secondly, the "dissolution-precipitation" process in the cold sintering step forms an interface layer that encapsulates the grains, hindering grain boundary formation and thus deteriorating the varistor properties. While traditional solid-state methods produce ZnO varistor ceramics with good density, the high-temperature environment (≥1100℃) required for sintering leads to grain coarsening, reducing the proportion of highly insulating grain boundaries and making it difficult to achieve high breakdown strength. Therefore, further optimization of the formulation and process parameters, particularly in improving the varistor coefficient and breakdown strength of the ceramic, is crucial based on existing cold sintering processes. This also provides a new research direction and practical path for developing high-performance ZnO-based varistor ceramics. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing high-performance ZnO-based varistors based on heat treatment-assisted cold sintering. Compared to the traditional solid-state method with a sintering temperature as high as 1100 °C, this invention can significantly reduce the sintering temperature to below 900 °C, and the prepared ZnO-based varistor material has a relative density as high as 99%, while also possessing excellent electrical properties (varistivity as high as 92, breakdown strength of 1600 V / mm). This method not only has significant advantages in energy saving and emission reduction, but also significantly improves the electrical performance of the ceramic, surpassing the performance of general commercial resistors.

[0005] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows:

[0006] A method for preparing ZnO-based varistors based on heat treatment-assisted cold sintering, the method comprising the following steps:

[0007] S1: Prepare a citric acid solution using deionized water and citric acid, with a concentration of 0.5-4 mol.

[0008] S2: The initial ZnO powder is mixed with a metal oxide dopant to form a precursor powder, wherein the metal oxide dopant accounts for 3-9.2 wt% of the total mass of the initial ZnO powder.

[0009] S3: The precursor powder prepared in S2 is then mixed with citric acid solution, and after grinding and dispersing, a mixed slurry is obtained, wherein the amount of citric acid solution added is 50-100 wt% of the precursor powder mass.

[0010] S4: Place the mixed slurry into a steel mold, apply a pressure of 50-300 MPa, and cold sinter at 150-300 ℃ for 0.5-2 hours.

[0011] S5: The cold-sintered sample is subjected to heat treatment at a temperature range of 800-950 ℃ for 0.5-4 h to obtain high-performance ZnO-based varistors.

[0012] As an improvement, the metal oxide dopant is one or more of Bi2O3, Mn2O3, NiO, SnO2, In2O3, and CaO metal oxides.

[0013] As an improvement, when the metal oxide dopant is composed of multiple metal oxides, the proportion of each metal oxide is 0.3-3 wt% of the total mass of the initial ZnO powder.

[0014] Compared with the prior art, the present invention has at least the following advantages:

[0015] The present invention provides a method for preparing high-performance ZnO-based varistors based on heat treatment-assisted cold sintering. The process is simple and energy-efficient, effectively reducing the energy consumption of traditional high-temperature sintering processes.

[0016] The heat treatment-assisted process of this invention can eliminate internal stress and residual amorphous phase in cold-sintered samples, and transform the interface layer left after cold sintering into high-resistivity grain boundaries, thus significantly enhancing the grain boundary characteristics of the ceramic. This not only helps maintain the microstructural stability of the ceramic, but also greatly enhances its electrical properties. The ZnO-based varistor ceramics prepared using this process far surpass varistor ceramics prepared by single cold sintering or traditional solid-state sintering in terms of both non-ohmic properties and electrical breakdown strength.

[0017] (3) The present invention effectively avoids abnormal growth of ceramic grains by combining cold sintering and heat treatment at a temperature about 300 ℃ lower than the traditional high-temperature sintering temperature, significantly reduces porosity, and achieves densification of ceramic relative density greater than 99%.

[0018] (4) The optimized formulation process proposed in this invention is equally important for improving the nonlinear coefficient (up to 92) and breakdown strength (up to 1600 V / mm) of ZnO-based varistors. The ZnO resistor sheet prepared by the heat treatment-assisted cold sintering process has broad industrial application prospects. Attached Figure Description

[0019] Figure 1 The flowchart illustrates the heat treatment-assisted cold sintering process for preparing ZnO-based varistors as an implementation method.

[0020] Figure 2 The images show the grain morphology under different heat treatment temperatures.

[0021] Figure 3 These are images of grain boundary morphology before and after heat treatment.

[0022] Figure 4 This is a graph showing the actual density and relative density under different heat treatment temperatures.

[0023] Figure 5 For different heat treatment temperature conditions J - E curve. Detailed Implementation

[0024] The present invention will now be described in further detail.

[0025] This invention employs a heat treatment-assisted cold sintering technology, achieving efficient low-temperature sintering of ZnO-based varistor ceramics by introducing a specific concentration and amount of citric acid solution during the cold sintering process. This process can initially achieve ceramic densification at 300 °C by applying appropriate pressure and controlling the time, and further improve the material's density and electrical properties during heat treatment at 800 °C to 950 °C. Compared to the traditional high-temperature sintering temperature of up to 1100 °C, this method significantly reduces the sintering temperature by approximately 300 °C, resulting in substantial energy savings. The core of this technology lies in the local wetting effect of citric acid, which promotes uniform dispersion of ZnO-based powder and gradually achieves grain densification at low temperatures. Subsequently, heat treatment further optimizes the ceramic's microstructure, giving it excellent varistor coefficient and breakdown strength.

[0026] This invention utilizes heat treatment-assisted cold sintering technology to prepare high-performance ZnO-based varistor ceramics. Results show that ZnO-based ceramics can achieve initial densification under conditions of 300 °C and 100 MPa using citric acid solutions with a concentration of 4 mol% and a mass fraction of 50-100 wt%, with relative densities exceeding 88% after cold sintering. After heat treatment at 800 °C to 950 °C, the relative density of the ceramics can be further increased to over 95%, and the relative density of the cold-sintered samples assisted by heat treatment at 870 °C reaches 99%, with a grain size of only 1.75 μm, both superior to samples prepared by traditional solid-state methods. Electrical performance tests show that the ZnO-based varistor ceramics prepared by heat treatment-assisted cold sintering technology exhibit a breakdown strength as high as 1600 V / mm and a nonlinear varistor coefficient as high as 92, far exceeding current commercial levels.

[0027] Example 1: A method for preparing ZnO-based varistors based on heat treatment-assisted cold sintering, the process steps of which include:

[0028] Prepare a 0.5 mol% citric acid solution using deionized water and glacial citric acid.

[0029] Cold sintering: The initial ZnO powder was mixed with 50 wt% citric acid solution, and after grinding and dispersion, a mixed slurry was obtained. The mixed slurry was then placed in a steel mold, a pressure of 50 MPa was applied, and cold sintering was carried out at 300 ℃ for 2 hours.

[0030] Heat treatment assistance: The cold-sintered sample is transferred into a muffle furnace and heated to 800 ℃ at a heating rate of 1 ℃ / min, and held for 0.5 hours to obtain ZnO varistor ceramic.

[0031] The test results are shown in the table below:

[0032] Relative density (%) Grain size (μm) Breakdown strength (V / mm) Nonlinear coefficients 93 1.48 3200 0.03

[0033] Example 2: A method for preparing ZnO-based varistors based on heat treatment-assisted cold sintering, the process steps of which include:

[0034] (1) Prepare a 1 mol% citric acid solution with deionized water and glacial citric acid.

[0035] (2) Cold sintering: The initial ZnO powder is mixed with 50 wt% citric acid solution, and after grinding and dispersing, a mixed slurry is obtained. Then the mixed slurry is placed in a steel mold, a pressure of 50 MPa is applied, and cold sintering is carried out at 200 ℃ for 1 hour.

[0036] (3) Heat treatment assistance: The cold sintered sample is transferred into a muffle furnace and heated to 800 ℃ at a heating rate of 1 ℃ / min. The holding time is 0.5 hours, and ZnO varistors are obtained.

[0037] (4) The test results are shown in the table below:

[0038] Relative density (%) Grain size (μm) Breakdown strength (V / mm) Nonlinear coefficients 96 1.92 2600 0.03

[0039] Example 3: A method for preparing ZnO-based varistors based on heat treatment-assisted cold sintering, the process steps of which include:

[0040] (1) Prepare a 1 mol% citric acid solution with deionized water and glacial citric acid.

[0041] (2) Cold sintering: The initial ZnO powder was mixed with Bi2O3 with a doping amount of 3 wt%, and then mixed with 70 wt% citric acid solution. After grinding and dispersing, a mixed slurry was obtained. The mixed slurry was then placed in a steel mold, a pressure of 300 MPa was applied, and cold sintering was carried out at 150 °C for 2 hours.

[0042] (3) Heat treatment assistance: The cold sintered sample is transferred into a muffle furnace and heated to 950 ℃ at a heating rate of 3 ℃ / min. The holding time is 4 hours to obtain ZnO varistors.

[0043] (4) The test results are shown in the table below:

[0044] Relative density (%) Grain size (μm) Breakdown strength (V / mm) Nonlinear coefficients 94 3.65 750 5

[0045] Example 4: A method for preparing ZnO-based varistors based on heat treatment-assisted cold sintering, the process steps of which include:

[0046] (1) Prepare a 2 mol% citric acid solution with deionized water and glacial citric acid.

[0047] (2) Cold sintering: The initial ZnO powder is mixed with 2 wt% Bi2O3 and 1 wt% Mn2O3, and then mixed with 100 wt% citric acid solution. After grinding and dispersing, a mixed slurry is obtained. The mixed slurry is then placed in a steel mold, a pressure of 200 MPa is applied, and cold sintering is carried out at 250 °C for 0.5 hours.

[0048] (3) Heat treatment assistance: The cold sintered sample is transferred into a muffle furnace and heated to 950 ℃ at a heating rate of 10 ℃ / min. The holding time is 4 hours to obtain ZnO varistors.

[0049] (4) The test results are shown in the table below:

[0050] Relative density (%) Grain size (μm) Breakdown strength (V / mm) Nonlinear coefficients 94 2.43 940 9

[0051] Example 5: A method for preparing ZnO-based varistors based on heat treatment-assisted cold sintering, the process steps of which include:

[0052] (1) Prepare a 2.5 mol% citric acid solution with deionized water and glacial citric acid.

[0053] (2) Cold sintering: The initial ZnO powder was mixed with doped Bi2O3, Mn2O3, NiO and SnO2 with a doping amount of 0.3 wt% and 3 wt% respectively. Then it was mixed with 100 wt% citric acid solution, ground and dispersed to obtain a mixed slurry. The mixed slurry was then placed in a steel mold, a pressure of 200 MPa was applied, and cold sintering was carried out at 200 °C for 2 hours.

[0054] (3) Heat treatment assistance: The cold sintered sample is transferred into a muffle furnace and heated to 800 ℃ at a heating rate of 3 ℃ / min. The holding time is 2 hours to obtain ZnO varistors.

[0055] (4) The test results are shown in the table below:

[0056] Relative density (%) Grain size (μm) Breakdown strength (V / mm) Nonlinear coefficients 96 1.13 6800 10

[0057] Example 6: A method for preparing ZnO-based varistors based on heat treatment-assisted cold sintering, the process steps of which include:

[0058] (1) Prepare a 4 mol% citric acid solution with deionized water and glacial citric acid.

[0059] (2) Cold sintering: The initial ZnO powder is mixed with doped Bi2O3, Mn2O3, NiO and SnO2 with a doping amount of 3wt% and 1wt% respectively. Then it is mixed with 100 wt% citric acid solution, ground and dispersed to obtain a mixed slurry. The mixed slurry is then placed in a steel mold, a pressure of 100 MPa is applied, and cold sintering is carried out at 300 ℃ for 1 hour.

[0060] (3) Heat treatment assistance: The cold sintered sample is transferred into a muffle furnace and heated to 850 ℃ at a heating rate of 3 ℃ / min. The holding time is 2 hours to obtain ZnO varistors.

[0061] (4) The test results are shown in the table below:

[0062] Relative density (%) Grain size (μm) Breakdown strength (V / mm) Nonlinear coefficients 97 1.38 3200 20

[0063] Example 7: A method for preparing ZnO-based varistors based on heat treatment-assisted cold sintering, the process steps of which include:

[0064] (1) Prepare a 4 mol% citric acid solution with deionized water and glacial citric acid.

[0065] (2) Cold sintering: The initial ZnO powder was mixed with doped Bi2O3, 0.3wt% Mn2O3, 0.5wt% NiO, 1wt% SnO2, 0.3wt% In2O3 and 0.7wt% CaO, and then mixed with 100 wt% citric acid solution. After grinding and dispersing, a mixed slurry was obtained. The mixed slurry was then placed in a steel mold, a pressure of 100 MPa was applied, and cold sintering was carried out at 300 ℃ for 2 hours.

[0066] (3) Heat treatment assistance: The cold sintered sample is transferred into a muffle furnace and heated to 870 ℃ at a heating rate of 3 ℃ / min. The holding time is 2 hours to obtain ZnO varistors.

[0067] (4) The test results are shown in the table below:

[0068] Relative density (%) Grain size (μm) Breakdown strength (V / mm) Nonlinear coefficients 99 1.75 1600 92

[0069] Example 8: A method for preparing ZnO-based varistors based on heat treatment-assisted cold sintering, the process steps of which include:

[0070] (1) Prepare a 3 mol% citric acid solution with deionized water and glacial citric acid.

[0071] (2) Cold sintering: The initial ZnO powder was mixed with doped amounts of 3 wt% Bi2O3, 2 wt% Mn2O3, 0.5 wt% NiO, 1 wt% SnO2, 0.3 wt% In2O3 and 0.3 wt% CaO, and then mixed with 100 wt% citric acid solution. After grinding and dispersing, a mixed slurry was obtained. The mixed slurry was then placed in a steel mold, a pressure of 200 MPa was applied, and cold sintering was carried out at 200 ℃ for 2 hours.

[0072] (3) Heat treatment assistance: The cold sintered sample is transferred into a muffle furnace and heated to 900 ℃ at a heating rate of 3 ℃ / min. The holding time is 2 hours to obtain ZnO varistors.

[0073] (4) The test results are shown in the table below:

[0074] Relative density (%) Grain size (μm) Breakdown strength (V / mm) Nonlinear coefficients 97 2.24 1000 60

[0075] Example 9: A method for preparing ZnO-based varistors based on heat treatment-assisted cold sintering, the process steps of which include:

[0076] Prepare a 4 mol% citric acid solution using deionized water and glacial citric acid.

[0077] Cold sintering: The initial ZnO powder was mixed with doped Bi2O3, Mn2O3, SnO2 and 0.7 wt% CaO, and then mixed with 60 wt% citric acid solution. After grinding and dispersing, a mixed slurry was obtained. The mixed slurry was then placed in a steel mold, a pressure of 100 MPa was applied, and cold sintering was carried out at 300 ℃ for 0.5 hours.

[0078] Heat treatment assistance: The cold-sintered sample is transferred into a muffle furnace and heated to 830 ℃ at a heating rate of 1 ℃ / min, and held for 1.5 hours to obtain ZnO varistors.

[0079] The test results are shown in the table below:

[0080] Relative density (%) Grain size (μm) Breakdown strength (V / mm) Nonlinear coefficients 96 1.22 3460 20

[0081] Example 10: A method for preparing ZnO-based varistors based on heat treatment-assisted cold sintering, the process steps of which include:

[0082] Prepare a 4 mol% citric acid solution using deionized water and glacial citric acid.

[0083] Cold sintering: The initial ZnO powder was mixed with doped Bi2O3, Mn2O3, NiO and CaO with a doping amount of 3wt% and 1wt% respectively. Then it was mixed with 80 wt% citric acid solution, ground and dispersed to obtain a mixed slurry. The mixed slurry was then placed in a steel mold, a pressure of 100 MPa was applied, and cold sintering was carried out at 300 ℃ for 2 hours.

[0084] Heat treatment assistance: The cold-sintered sample is transferred into a muffle furnace and heated to 880 ℃ at a heating rate of 7 ℃ / min, and held for 1.5 hours to obtain ZnO varistors.

[0085] The test results are shown in the table below:

[0086] Relative density (%) Grain size (μm) Breakdown strength (V / mm) Nonlinear coefficients 98 1.42 2460 32

[0087] Example 11: A method for preparing ZnO-based varistors based on heat treatment-assisted cold sintering, the process steps of which include:

[0088] Prepare a 4 mol% citric acid solution using deionized water and glacial citric acid.

[0089] (2) Cold sintering: The initial ZnO powder was mixed with 3 wt% Bi2O3, 0.5 wt% NiO, 0.3 wt% SnO2 and 0.7 wt% CaO, and then mixed with 80 wt% citric acid solution. After grinding and dispersing, a mixed slurry was obtained. The mixed slurry was then placed in a steel mold, a pressure of 100 MPa was applied, and cold sintering was carried out at 300 ℃ for 2 hours.

[0090] (3) Heat treatment assistance: The cold sintered sample is transferred into a muffle furnace and heated to 920 °C at a heating rate of 7 °C / min. The holding time is 0.5 hours to obtain ZnO varistors.

[0091] (4) The test results are shown in the table below:

[0092] Relative density (%) Grain size (μm) Breakdown strength (V / mm) Nonlinear coefficients 97 3.12 860 12

[0093] Example 12, a method for preparing ZnO-based varistors based on heat treatment-assisted cold sintering, the process steps of which include:

[0094] (1) Prepare a 4 mol% citric acid solution with deionized water and glacial citric acid.

[0095] (3) Cold sintering: The initial ZnO powder was mixed with doped Bi2O3, Mn2O3, NiO, SnO2, In2O3 and CaO with a doping amount of 3wt% and 1wt% respectively. Then it was mixed with 80 wt% citric acid solution, ground and dispersed to obtain a mixed slurry. The mixed slurry was then placed in a steel mold, a pressure of 100 MPa was applied, and cold sintering was carried out at 300 ℃ for 2 hours.

[0096] (3) Heat treatment assistance: The cold sintered sample is transferred into a muffle furnace and heated to 950 ℃ at a heating rate of 5 ℃ / min. The holding time is 3.5 hours to obtain ZnO varistor ceramic.

[0097] (4) The test results are shown in the table below:

[0098] Relative density (%) Grain size (μm) Breakdown strength (V / mm) Nonlinear coefficients 98 3.42 780 35

Claims

1. A method for preparing ZnO-based pressure sensitive ceramic based on heat treatment assisted cold sintering, characterized in that, The method comprises the following steps: S1: preparing a citric acid solution with deionized water and citric acid, the concentration of the citric acid solution being 0.5-4 mol%; S2: mixing the initial ZnO powder with a metal oxide dopant to form a precursor powder, the metal oxide dopant accounting for 3-9.2 wt% of the total mass of the initial ZnO powder; The metal oxide dopant is one or more of Bi2O3, Mn2O3, NiO, SnO2, In2O3, and CaO metal oxides; S3: mixing the precursor powder prepared in S2 with the citric acid solution, and grinding and dispersing to obtain a mixed slurry, wherein the addition amount of the citric acid solution is 50-100 wt% of the mass of the precursor powder; S4: placing the mixed slurry into a steel mold, applying a pressure of 50-300 MPa, and performing cold sintering at 150-300 ℃, with heat preservation for 0.5-2 h; S5: performing heat treatment on the sample after cold sintering, the heat treatment temperature being in the range of 800-950 ℃, and the heat preservation time being 0.5-4 h, to finally obtain a high-performance ZnO-based pressure-sensitive ceramic.

2. The method for preparing ZnO-based pressure sensitive ceramic based on heat treatment assisted cold sintering according to claim 1, characterized in that, When the metal oxide dopant is composed of multiple metal oxides, the proportion of each metal oxide is 0.3-3 wt% of the total mass of the initial ZnO powder.

3. A ZnO-based piezoelectric ceramic, characterized by: The ZnO-based pressure-sensitive ceramic is prepared by any one of the methods of claims 1-2.

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