KNN-based lead-free piezoelectric ceramic, preparation method and application thereof

Abstract

The application discloses KNN-based lead-free piezoelectric ceramics and a preparation method and application thereof, and belongs to the technical field of piezoelectric ceramics. 0.52 Na 0.48 ) （1‑a‑b） Ce a Li b Nb （1‑c） Ta c O3, wherein 0.005<=a<=0.02, 0.01<=b<=0.08; 0.01<=c<=0.1; a sintering aid is added to the KNN-based lead-free piezoelectric ceramic, and the sintering aid is a niobium borate-based glass powder. The three elements Li, Ta and Ce are cooperatively doped, and the electromechanical coupling coefficient is significantly improved; the Ce element is doped into the piezoelectric ceramic matrix to avoid local composition segregation, compensate the temperature coefficient, and inhibit the lattice mutation caused by the phase transition near the Curie temperature; the piezoelectric performance of the piezoelectric ceramic material can be improved, and the temperature stability of the piezoelectric ceramic can also be improved.

Description

Technical Field

[0001] This application relates to a KNN-based lead-free piezoelectric ceramic, its preparation method and application, belonging to the field of piezoelectric ceramic technology. Background Technology

[0002] Piezoelectric ceramics, as a functional material capable of converting mechanical energy into electrical energy, have broad application prospects in various fields such as sensors, actuators, ultrasonic equipment, and energy harvesting. Among them, potassium sodium niobate (KNN)-based ceramics have become a core candidate material to replace traditional lead-based piezoelectric ceramics due to their advantages such as being lead-free and environmentally friendly, having excellent piezoelectric properties, and being inexpensive, and have received widespread attention from industrial production and academic research.

[0003] KNN-based ceramics belong to the perovskite crystal structure, and their piezoelectric properties and temperature stability are mainly determined by the crystal structure, phase boundary state, and distribution of lattice defects. However, pure KNN-based ceramics face significant technical challenges in practical preparation and application: First, phase boundary control is difficult; simple single-element doping cannot simultaneously improve the electromechanical coupling coefficient and temperature stability, often resulting in improved piezoelectric properties but decreased temperature stability. Second, lattice integrity is poor; problems such as lattice distortion and compositional segregation easily occur during preparation, especially near the Curie temperature, where phase transitions can easily lead to abrupt lattice changes, further reducing the temperature stability and reliability of piezoelectric ceramics. Third, oxygen vacancy defects are difficult to precisely control; excessive or insufficient oxygen vacancies will disrupt lattice stability, affecting the long-term use of piezoelectric ceramics.

[0004] To address the aforementioned technical challenges, existing technologies often employ element doping to optimize the performance of KNN-based ceramics. For instance, Chinese patent CN 117902897 B discloses a lead-free piezoelectric ceramic and its preparation method and application. However, although adding gallium oxide for doping can improve grain density and provide good dielectric properties, the patent does not address the temperature stability of the piezoelectric ceramic. Consequently, the piezoelectric ceramic in the patent may not be suitable for wide-range driver scenarios.

[0005] Therefore, there is an urgent need to develop a piezoelectric ceramic with a reasonable doping system and suitable new sintering aids to solve the technical problems that need to be solved in the current field of KNN-based piezoelectric ceramics. Summary of the Invention

[0006] To address the aforementioned issues, a KNN-based lead-free piezoelectric ceramic is provided, employing synergistic doping of Li, Ta, and Ce elements to significantly enhance the piezoelectric electromechanical coupling coefficient. The incorporation of Ce into the piezoelectric ceramic matrix can compensate for the temperature coefficient and suppress lattice abrupt changes caused by phase transitions near the Curie temperature. This not only improves the piezoelectric properties of the piezoelectric ceramic material but also enhances its temperature stability.

[0007] One aspect of this application provides a KNN-based lead-free piezoelectric ceramic with the chemical formula (KNN). 0.52 Na 0.48 ) （1-a-b） Ce a Li b Nb （1-c） Ta c O3, wherein 0.005≤a≤0.02, 0.01≤b≤0.08; 0.01≤c≤0.1; sintering aids are added to the KNN-based lead-free piezoelectric ceramic, wherein the sintering aids are niobium borate glass powder.

[0008] Under this setting, Li + Replace K + Post-contraction of the crystal lattice enhances piezoelectric activity; it also broadens the room-temperature stable phase region, suppressing abrupt changes in low-temperature performance; and increases the Curie temperature, enhancing high-temperature stability. The addition of Ce element firstly suppresses K / Na volatilization, reducing dielectric loss; secondly, Ce³… + Segregation at grain boundaries inhibits abnormal grain growth, refines grains, and improves density and mechanical strength; thirdly, Ce doping enhances polarization response, adapting to the requirements of high-sensitivity devices. 5+ It can replace Nb 5+ It can not only regulate the crystal phase composition and reduce the polarization reversal energy barrier, significantly improving the piezoelectric activity and planar electromechanical coupling coefficient; it can also diffuse the phase transition temperature, making the rate of change of the piezoelectric and dielectric parameters of the material with temperature significantly reduced; the output accuracy, sensitivity and resonant frequency of the device are not easily drifted under ambient temperature fluctuations and working heat conditions.

[0009] Optionally, the K source is K2CO3, the Na source is Na2CO3, the niobium source is Nb2O5, the Li source is Li2CO3, the Ta source is Ta2O5, and the Ce source is CeO2.

[0010] Optionally, 0.0075≤a≤0.0125, 0.02≤b≤0.05, 0.02≤c≤0.06.

[0011] Within the aforementioned doping range, Ce doping achieves grain refinement and moderate enrichment of grain boundaries, forming a high-resistivity grain boundary structure, reducing leakage current and dielectric loss. It can achieve effective A-site substitution and moderate lattice distortion, constructing a diffuse phase transition and improving temperature stability; while avoiding excessive lattice distortion and impurity phase generation due to excessive Ce, thus preventing a decrease in piezoelectric activity.

[0012] The amount of Li doping can replace the A site, moderately shrink the lattice, enhance oxygen octahedral distortion, and effectively improve the piezoelectric constant and planar electromechanical coupling coefficient. If the doping amount is too low, there is no obvious piezoelectric gain. If the doping amount is too high, the lattice collapses easily, the Curie temperature drops significantly, and the temperature stability deteriorates.

[0013] Ta doping can smoothly regulate lattice energy levels, further broaden the diffusion phase transition temperature range, significantly weaken the temperature drift of piezoelectric and dielectric parameters, reduce domain wall flipping energy barriers, moderately reduce the coercive field, and make it easier to polarize to saturate at room temperature, thereby improving electromechanical conversion efficiency. However, excessive doping of Ta can easily lead to lattice mismatch and internal micropores, reducing insulation and withstand voltage performance.

[0014] Optionally, the chemical formula of the KNN-based lead-free piezoelectric ceramic is (K 0.4992 Na 0.4608 Ce 0.01 Li 0.03 )Nb 0.96 Ta 0.04 O3.

[0015] Optionally, the niobium borate glass powder comprises 20-40 mol% Nb₂O₅, 15-30 mol% B₂O₃, 10-20 mol% K₂CO₃, 10-20 mol% Na₂CO₃, and 5-15 mol% ZnO. The method for preparing the niobium borate glass powder is as follows: Nb2O5, B2O3, K2CO3, Na2CO3, and ZnO are weighed and mixed according to the molar ratio, melted at 1200-1300°C for 50-90 min, then rapidly water-quenched, and pulverized to D50=0.5-1μm by high-energy ball milling, and then dried and sieved.

[0016] Preferably, the concentrations are: Nb₂O₅ 25-35 mol%, B₂O₃ 18-25 mol%, ZnO 8-12 mol%, and Na₂CO₃ + K₂CO₃ 20-30 mol%.

[0017] At the above-mentioned addition amounts, niobium borate glass powder melts during sintering, forming a liquid phase that wets solid particles. Through viscous flow and particle rearrangement, the densification process is significantly accelerated. Simultaneously, it effectively reduces the sintering temperature of KNN-based ceramics and effectively suppresses alkali metal volatilization. Nb₂O₅ is highly structurally and elementally matched with KNN crystals. Nb₂O₅ can enter the glass network in the form of [NbO₆] octahedrons or [NbO₄] tetrahedra, forming a transition layer at the glass-ceramic interface. During heat treatment or sintering, niobium borate glass powder tends to form a perovskite phase. B₂O₃, in the form of [BO₃] trigonometric bodies or [BO₄] tetrahedra, intersperses in the gaps of the niobium-oxygen network, reducing the softening point of the niobium borate glass powder. K₂CO₃ and Na₂CO₃ can dynamically compensate for K₂O₃. + Ions and Na + Ions compensate for alkali metal ions that volatilize at high temperatures, maintaining a stable stoichiometric ratio.

[0018] Optionally, the amount of niobium borate glass powder added is 0.1-3 mol.

[0019] Preferably, the amount of niobium borate glass powder added is 0.5-2 mol.

[0020] Within the aforementioned range, the niobium borate glass powder can lower the sintering temperature of KNN ceramics and suppress alkali metal volatilization. At the same time, the doped ions can effectively pin defect dipoles, reduce dielectric loss, and refine grains, thereby enabling piezoelectric ceramics to have a higher Curie temperature while maintaining certain piezoelectric properties.

[0021] If the addition amount is less than 0.1 mol%, the doping effect is not obvious; when the addition amount is greater than 3 mol%, non-functional second phase is easily precipitated, leading to increased dielectric loss and deterioration of piezoelectric performance.

[0022] Another aspect of this application provides a method for preparing KNN-based lead-free piezoelectric ceramics, comprising the following steps: (1) weighing raw materials according to the chemical formula ratio, placing them in a ball mill, adding anhydrous ethanol, then performing ball milling once, stirring evenly, drying, and sieving to obtain a mixture; (2) The mixture is processed at a pre-calcination temperature, cooled, and then sintered with sintering aids for secondary ball milling, drying, and sieving to obtain the pre-made material; (3) Add adhesive to the prefabricated material, granulate it and press it into shape to obtain a green body; (4) The green blank is first degreased and then sintered and cooled to room temperature to obtain sintered material; (5) The sintered material is pretreated, and the electrode is prepared and then polarized to obtain KNN-based lead-free piezoelectric ceramic.

[0023] The above preparation method can achieve microscopic uniform dispersion of raw material components and sintering aids, complete solid-phase pre-reaction, refine powder particles and enhance reaction activity; through the segmented heat treatment method of debinding and sintering, the binder can be slowly removed, avoiding green body cracking and porosity defects; the sintered material obtained after sintering has uniform sintering shrinkage and is not easily deformed, and the resulting ceramic matrix has fine and uniform grains and high density. Through polarization treatment, good electrode contact and uniform electric field distribution can be ensured, so that the piezoelectric ceramic domains are fully oriented and the piezoelectric activity is maximized.

[0024] Optionally, the pre-firing temperature in step (2) is 850-950℃, the heating rate is 5-10℃ / min, and the pre-firing holding time is 4-5h.

[0025] Within the above range, while ensuring that the raw materials undergo sufficient solid-phase reaction, form pure KNN main crystal phase and uniform solid solution of doped ions, it avoids problems such as powder hard agglomeration, premature grain coarsening and impurity phase residue caused by excessively high pre-calcination temperature, excessively rapid heating or improper holding time. It can fully decompose carbonates and water-containing impurities in the raw materials and maintain the high reactivity of the pre-calcined powder.

[0026] Optionally, the temperature of the degreasing treatment in step (4) is 500-600℃, the heating rate is 2-3℃ / min, and the holding time is 2-3h.

[0027] When the degreasing parameters are within the above range, the binders and organic components inside the green body can be removed slowly and completely, avoiding rapid vaporization of organic matter that could cause bubbling, cracking, delamination, and residual carbon in the green body, thus ensuring that the green body has an intact structure and is clean and free of impurities and defects after degreasing.

[0028] Optionally, the sintering temperature in step (4) is 1120-1200℃, the heating rate is 3-5℃ / min, and the holding time is 3-5h.

[0029] Within the above-mentioned range, the parameters of the sintering treatment can match the liquid-phase sintering characteristics of the compounded sintering aids, effectively promoting ceramic densification, uniform grain development, and full solid solution of doped ions, avoiding abnormal grain coarsening and excessive enrichment of grain boundary phases; it can stably form a ceramic matrix with fine and uniform grains, high density, and intact grain boundary structure, and perfectly construct the diffusion phase transition effect and high-resistivity grain boundary insulation structure, so that the material has excellent piezoelectric activity, electromechanical coupling performance, low dielectric loss, high breakdown field strength, and wide temperature stability.

[0030] Optionally, the polarization process in step (5) includes the following steps: S1: Apply an electric field strength of 3-3.5kV / mm at a polarization temperature of 100-120℃ and polarize for 15-30min; S2: Apply an electric field strength of 4-5kV / mm at a temperature of 80-100℃ and polarize for 10-20 minutes.

[0031] In the polarization process, high-temperature low-field pre-polarization is first performed by polarizing at 100-120℃ and 3-3.5kV / mm for 15-30 minutes. The appropriate temperature is used to reduce the domain flipping energy barrier, so that the domains inside the ceramic are initially oriented and arranged, and the internal stress of the material is slowly released, avoiding microcracks and electrical breakdown defects caused by direct high-voltage polarization. Then, the temperature is lowered to 80-100℃ and the electric field is increased to 4-5kV / mm for 10-20 minutes to perform medium-temperature high-field enhanced polarization, which further forces the domain orientation and locks the arrangement state, so as to achieve high polarization saturation.

[0032] The polarization treatment method in this application can significantly improve the piezoelectric constant and electromechanical coupling coefficient, give full play to the piezoelectric activity of the material, effectively protect the high-resistivity structure of the grain boundary, reduce the risk of leakage and breakdown during the polarization process, reduce internal microcrack defects, and make the overall polarization of the piezoelectric ceramic uniform.

[0033] In another aspect of this application, the KNN-based lead-free piezoelectric ceramic prepared by the above-described KNN-based lead-free piezoelectric ceramic preparation method is used in sensors, actuators, and ultrasonic-related equipment.

[0034] The beneficial effects of this application include, but are not limited to: 1. According to the KNN-based lead-free piezoelectric ceramic of this application, the various properties of the piezoelectric ceramic are effectively improved by the synergistic doping of Li, Ta and Ce elements. Li element is doped at the A site and Ta element is doped at the B site. The two elements synergistically regulate the phase boundary state and significantly improve the piezoelectric electromechanical coupling coefficient. Ce element is doped into the piezoelectric ceramic matrix to balance lattice distortion, compensate temperature coefficient, and suppress lattice abrupt changes near the Curie temperature, thereby improving the piezoelectric performance and temperature stability of the piezoelectric ceramic.

[0035] 2. According to the KNN-based lead-free piezoelectric ceramic of this application, niobium borate glass powder melts during sintering to form a liquid phase that wets solid particles. Through viscous flow and particle rearrangement, the density of the piezoelectric ceramic is improved, and the sintering temperature of the KNN-based ceramic can be effectively reduced, effectively suppressing the volatilization of alkali metals.

[0036] 3. According to the KNN-based lead-free piezoelectric ceramic of this application, the piezoelectric ceramic of this application uses KNN as the matrix and does not add lead, which is in line with the trend of environmental protection industry and avoids the hazards of traditional lead-based ceramics; at the same time, the raw materials, sintering aids and preparation process do not require special reagents and complex equipment, which can significantly reduce the preparation cost.

[0037] 4. The KNN-based lead-free piezoelectric ceramic according to this application has both excellent piezoelectric properties and temperature stability, and can be widely used in sensors, actuators, ultrasonic-related equipment and other fields, and is suitable for complex working conditions such as high and low temperatures and strong vibrations. Attached Figure Description

[0038] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings: Figure 1 This is a SEM image (magnification 3000×) of the KNN-based piezoelectric ceramic involved in Embodiment 3 of this application. Detailed Implementation

[0039] The present application is described in detail below with reference to the embodiments, but the present application is not limited to these embodiments.

[0040] Unless otherwise specified, the raw materials used in the embodiments and comparative examples of this application were all purchased commercially.

[0041] Unless otherwise specified, the methods used in the embodiments and comparative examples of this application are conventional methods in the prior art.

[0042] Example 1 This embodiment relates to a KNN-based lead-free piezoelectric ceramic with the chemical formula KNN. 0.468 Na 0.432 Ce 0.02 Li 0.08 Nb 0.99 Ta 0.01 The preparation method of O3,KNN-based lead-free piezoelectric ceramics includes the following steps: (1) According to the molar ratio of K, Na, Nb, Li, Ta and Ce in the chemical formula, accurately calculate the weighing mass of each raw material, accurately calculate the amount of K2CO3, Na2CO3, Nb2O5, Li2CO3, Ta2O5 and CeO2 to be added, put them into a ball mill, add 2 times the amount of anhydrous ethanol, and ball mill once at a ball mill speed of 350 r / min for 6 h. After that, stir evenly, dry in a constant temperature drying oven at 80℃ for 8 h, and pass through an 80 mesh sieve to obtain the mixture. (2) Place the mixture in a crucible and heat it to 850°C at a heating rate of 5°C / min. Hold it at that temperature for 5 hours. After cooling to room temperature, add 0.5 mol% of niobium borate glass powder sintering aid and ball mill it for a second time at a ball mill speed of 350 r / min for 0.5 hours. Dry it in a constant temperature drying oven at 80°C for 8 hours and pass it through an 80-mesh sieve to obtain the preformed material. The preparation method of niobium borate glass powder includes: weighing and mixing Nb2O5 40 mol%, B2O3 30 mol%, K2CO3 10 mol%, Na2CO3 15 mol%, and ZnO 5 mol%, melting at 1200℃ for 90 min, then rapidly water quenching, pulverizing to D50=0.5 μm by high-energy ball milling, drying and sieving to obtain the niobium borate glass powder; (3) Add 2% polyvinyl alcohol adhesive by weight of the precast material to the precast material, stir and mix evenly, press and shape under 10MPa for 1min to obtain green blank; (4) Place the green billet into a muffle furnace and heat it to 600°C at a heating rate of 2°C / min. Hold it for 2 hours to complete the degreasing process. Then heat it to 1180°C at a heating rate of 5°C / min and hold it for 3 hours to complete the sintering process. Cool it to room temperature to obtain the sintered material. (5) The sintered material is ground and polished, and the surface is wiped with ethanol. Electrodes are then prepared on the upper and lower surfaces of the sintered material and polarized to obtain KNN-based lead-free piezoelectric ceramics. The polarization process includes: S1: Applying an electric field strength of 3kV / mm at a polarization temperature of 120℃ for 30 minutes; S2: Apply an electric field strength of 4kV / mm at a temperature of 90℃ and polarize for 15 minutes.

[0043] Example 2 This embodiment relates to a KNN-based lead-free piezoelectric ceramic with the chemical formula KNN. 0.5122 Na 0.4728 Ce 0.005 Li 0.01 Nb 0.90 Ta 0.10 The preparation method of O3,KNN-based lead-free piezoelectric ceramics includes the following steps: (1) According to the molar ratio of K, Na, Nb, Li, Ta and Ce in the chemical formula, accurately calculate the weighing mass of each raw material, accurately calculate the amount of K2CO3, Na2CO3, Nb2O5, Li2CO3, Ta2O5 and CeO2 to be added, put them into a ball mill, add 2 times the amount of anhydrous ethanol, and ball mill once at a ball mill speed of 350 r / min for 6 h. After that, stir evenly, dry in a constant temperature drying oven at 80℃ for 8 h, and pass through an 80 mesh sieve to obtain the mixture. (2) Place the mixture in a crucible and heat it to 950°C at a heating rate of 10°C / min. Hold it at that temperature for 4 hours. After cooling to room temperature, add 0.3 mol% of niobium borate glass powder sintering aid and ball mill it for a second time at a ball mill speed of 350 r / min for 0.5 hours. Dry it in a constant temperature drying oven at 80°C for 8 hours and pass it through an 80-mesh sieve to obtain the preformed material. The preparation method of niobium borate glass powder includes: weighing and mixing Nb2O5 20 mol%, B2O3 30 mol%, K2CO3 15 mol%, Na2CO3 20 mol%, ZnO 15 mol%, melting at 1300℃ for 50 min, then rapidly water quenching, pulverizing to D50=1.0 μm by high-energy ball milling, drying and sieving to obtain the niobium borate glass powder; (3) Add 2% polyvinyl alcohol adhesive by weight of the precast material to the precast material, stir and mix evenly, press and shape under 10MPa for 1min to obtain green blank; (4) Place the green billet into a muffle furnace and heat it to 500°C at a heating rate of 3°C / min. Hold it for 3 hours to complete the degreasing process. Then heat it to 1120°C at a heating rate of 3°C / min and hold it for 5 hours to complete the sintering process. Cool it to room temperature to obtain the sintered material. (5) The sintered material is ground and polished, the surface is wiped with ethanol, and then the upper and lower surfaces of the sintered material are evenly coated with conductive silver paste, dried in a constant temperature drying oven at 100℃, and then polarized to obtain KNN-based lead-free piezoelectric ceramics. The polarization process includes: S1: Applying an electric field strength of 3kV / mm at a polarization temperature of 120℃ for 30 minutes; S2: Apply an electric field strength of 5kV / mm at a temperature of 80℃ and polarize for 10min.

[0044] Example 3 This embodiment relates to a KNN-based lead-free piezoelectric ceramic with the chemical formula KNN. 0.4992 Na 0.4608 Ce 0.01 Li 0.03 Nb 0.96 Ta 0.04 The preparation method of O3,KNN-based lead-free piezoelectric ceramics includes the following steps: (1) According to the molar ratio of K, Na, Nb, Li, Ta and Ce in the chemical formula, accurately calculate the weighing mass of each raw material, accurately calculate the amount of K2CO3, Na2CO3, Nb2O5, Li2CO3, Ta2O5 and CeO2 to be added, put them into a ball mill, add 2 times the amount of anhydrous ethanol, and ball mill once at a ball mill speed of 350 r / min for 6 h. After that, stir evenly, dry in a constant temperature drying oven at 100℃ for 8 h, and pass through a 200 mesh sieve to obtain the mixture. (2) Place the mixture in a crucible and heat it to 900°C at a heating rate of 8°C / min. Hold it at that temperature for 4.5 hours. After cooling to room temperature, add 0.3 mol% of niobium borate glass powder sintering aid and ball mill it for a second time at a ball mill speed of 350 r / min for 0.5 hours. Dry it in a constant temperature drying oven at 80°C for 8 hours and pass it through an 80-mesh sieve to obtain the preformed material. The preparation method of niobium borate glass powder includes: weighing and mixing Nb2O5 40 mol%, B2O3 15 mol%, K2CO3 20 mol%, Na2CO3 10 mol%, ZnO 15 mol%, melting at 1250℃ for 60 min, then rapidly water quenching, pulverizing to D50=0.6 μm by high-energy ball milling, drying and sieving to obtain the niobium borate glass powder; (3) Add 2% polyvinyl alcohol adhesive by weight of the precast material to the precast material, stir and mix evenly, press and shape under 10MPa for 1min to obtain green blank; (4) Place the green billet into a muffle furnace and heat it to 550°C at a heating rate of 2°C / min. Hold it for 2.5 hours to complete the degreasing process. Then heat it to 1200°C at a heating rate of 4°C / min and hold it for 4 hours to complete the sintering process. Cool it to room temperature to obtain the sintered material. (5) The sintered material is ground and polished, the surface is wiped with ethanol, and then the upper and lower surfaces of the sintered material are evenly coated with conductive silver paste, dried in a constant temperature drying oven at 100℃, and then polarized to obtain KNN-based lead-free piezoelectric ceramics. The polarization process includes: S1: Applying an electric field strength of 3kV / mm at a polarization temperature of 120℃ for 30 minutes; S2: Apply an electric field strength of 4kV / mm at a temperature of 90℃ and polarize for 15 minutes.

[0045] Example 4 The difference between this embodiment and Embodiment 3 is that Nb2O5 is not added to the niobium borate glass powder, while the rest is the same as in Embodiment 3.

[0046] Example 5 The difference between this embodiment and embodiment 3 is that the amount of niobium borate glass powder added is 5 mol%, and the rest is the same as in embodiment 3.

[0047] Example 6 The difference between this embodiment and Embodiment 3 is that the chemical formula is K. 0.50024 Na 0.46176 Ce 0.008 Li 0.03 Nb 0.96 Ta 0.04 O3, the rest is the same as in Example 3.

[0048] Example 7 The difference between this embodiment and Embodiment 3 is that the chemical formula is K. 0.4836 Na 0.4464 Ce 0.01 Li 0.06 Nb 0.96 Ta 0.04 O3, the rest is the same as in Example 3.

[0049] Example 8 The difference between this embodiment and Embodiment 3 is that the chemical formula is K. 0.4836 Na 0.4464 Ce 0.01 Li 0.06 Nb 0.96 Ta 0.04 O3, the rest is the same as in Example 3.

[0050] Example 9 The difference between this embodiment and embodiment 3 is that the polarization treatment in step (5) is to apply an electric field strength of 3.5kV / mm at a polarization temperature of 100℃ for 30min, and the rest is the same as in embodiment 3.

[0051] Comparative Example 1 The difference between this comparative example and Example 3 is that it does not dope with Ce element, but otherwise it is the same as Example 3.

[0052] Comparative Example 2 The difference between this comparative example and Example 3 is that it does not dope with Li element, but otherwise it is the same as Example 3.

[0053] Comparative Example 3 The difference between this comparative example and Example 3 is that it does not contain Ta, but is otherwise the same as Example 3.

[0054] Comparative Example 4 The difference between this comparative example and Example 3 is that the niobium borate glass powder is replaced with Li2O, while the rest is the same as in Example 3.

[0055] Test Example 1 The following tests were performed on the piezoelectric ceramic samples from Examples 1-9 and Comparative Examples 1-4: (1) Ceramic body density test method: To characterize the degree of densification of the material, the relative density (unit %) of the sample was measured using the Archimedes method. The sintered sample was immersed in deionized water, and its dry weight, suspended weight and saturated weight were measured respectively. The bulk density was calculated and compared with the theoretical density to obtain the relative density. This parameter is used to reflect the porosity and densification level of the material. The higher the relative density, the better the material densification, thus verifying the effect of glass additives in promoting sintering densification.

[0056] (2) Test method for electromechanical coupling coefficient Kp: The resonant frequency Fr and anti-resonant frequency Fa of the piezoelectric ceramic sample were tested using a PV90A impedance analyzer, and the electromechanical coupling coefficient Kp was calculated using the formula: Equation (1).

[0057] (3) Piezoelectric constant d 33 Test method: Using ZJ-5AN quasi-static piezoelectric constant d 33 The testing instrument is used to test the piezoelectric ceramic sample.

[0058] (4) Test method for mechanical quality factor Qm: The resonance-anti-resonance method is adopted, and the resonance characteristic curve of the piezoelectric ceramic sample is tested by an Agilent 4294A impedance analyzer. The specific steps are as follows: the polarized piezoelectric ceramic sample is placed in a constant temperature environment of (25±2)℃, the resonant frequency Fs and anti-resonant frequency Fp of the piezoelectric oscillator are measured by an impedance analyzer, and the half-power point frequency F1 (the frequency corresponding to 0.707 times the maximum conductance) and F2 are read. The test method refers to the relevant provisions in GB / T 3389-2008 "Test Method for Performance Parameters of Piezoelectric Ceramic Materials".

[0059] (5) Test method for dielectric constant εr: The capacitance of the piezoelectric ceramic sample was tested at 1 kHz using a TH2810B+ capacitance meter, and the dielectric constant εr was calculated using the following formula: εr=14.4×C×t / Φ 2 Equation (2) In the formula: C is the capacitance, pF; t is the thickness of the piezoelectric ceramic sample, cm; Φ is the diameter of the circular silver electrode of the piezoelectric ceramic sample, cm.

[0060] (6) Test method for dielectric loss tanδ: The loss value of the piezoelectric ceramic sample was tested at 1K Hz using a TH2810B+ type capacitance tester.

[0061] (7) Test method for temperature stability: Heat the piezoelectric ceramic sample to 150℃, keep it at that temperature for 30 min and then test its d33 value. Calculate the rate of change of d33 value relative to room temperature. The smaller the absolute value of the rate of change, the better the temperature stability.

[0062] Table 1

[0063] Depend on Figure 1 As can be seen, the ceramic cross-section in Embodiment 3 of this application exhibits typical polycrystalline sintered body characteristics. The grains are mainly irregular blocky / plate-shaped, with some grains having cleavage planes and step-like growth lines. The fracture mode is a mixture of transgranular fracture and intergranular fracture. The grain size is about 1-5 μm, uniformly distributed, and without obvious abnormally large grains. The grains are tightly bonded, with clear and continuous grain boundaries, and no obvious pores, microcracks, or grain boundary phase enrichment defects. This indicates that the sintering process of this invention achieves high density and uniform microstructure of ceramics, providing a microstructural basis for the material's excellent piezoelectric properties, mechanical stability, and long-term reliability.

[0064] The above description is merely an embodiment of this application, and the scope of protection of this application is not limited to these specific embodiments, but is determined by the claims of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the technical concept and principles of this application should be included within the scope of protection of this application.

Claims

1. A KNN-based lead-free piezoelectric ceramic, characterized in that, The chemical formula is (K 0.52 Na 0.48 ) （1-a-b） Ce a Li b Nb （1-c） Ta c O3, wherein 0.005≤a≤0.02, 0.01≤b≤0.08; 0.01≤c≤0.1; sintering aids are added to the KNN-based lead-free piezoelectric ceramic, wherein the sintering aids are niobium borate glass powder.

2. The KNN-based lead-free piezoelectric ceramic according to claim 1, characterized in that, 0.0075≤a≤0.0125, 0.02≤b≤0.05, 0.02≤c≤0.

06.

3. The KNN-based lead-free piezoelectric ceramic according to claim 2, characterized in that, The chemical formula of the KNN-based lead-free piezoelectric ceramic is K 0.4992 Na 0.4608 Ce 0.01 Li 0.03 Nb 0.96 Ta 0.04 O3.

4. The KNN-based lead-free piezoelectric ceramic according to claim 1, characterized in that, The niobium borate glass powder, by molar percentage, comprises 20-40 mol% Nb₂O₅, 15-30 mol% B₂O₃, 10-20 mol% K₂CO₃, 10-20 mol% Na₂CO₃, and 5-15 mol% ZnO. The method for preparing the niobium borate glass powder is as follows: Nb2O5, B2O3, K2CO3, Na2CO3, and ZnO are weighed and mixed according to the molar ratio, melted at 1200-1300°C for 50-90 min, then rapidly water-quenched, and pulverized to D50=0.5-1μm by high-energy ball milling, and then dried and sieved.

5. The KNN-based lead-free piezoelectric ceramic according to claim 4, characterized in that, The amount of niobium borate glass powder added is 0.1-3 mol.

6. The method for preparing KNN-based lead-free piezoelectric ceramics according to any one of claims 1-5, characterized in that, Includes the following steps: (1) Weigh the raw materials according to the chemical formula and put them into the ball mill. Add anhydrous ethanol, then ball mill once, stir evenly, dry, and sieve to obtain the mixture. (2) The mixture is processed at a pre-calcination temperature, cooled, and then sintered with sintering aids for secondary ball milling, drying, and sieving to obtain the preformed material; (3) Add adhesive to the prefabricated material, granulate it and press it into shape to obtain a green body; (4) The green blank is first degreased and then sintered and cooled to room temperature to obtain sintered material; (5) The sintered material is pretreated, and the electrode is prepared and then polarized to obtain KNN-based lead-free piezoelectric ceramic.

7. The preparation method according to claim 6, characterized in that, The pre-firing temperature in step (2) is 850-950℃, the heating rate is 3-5℃ / min, and the pre-firing holding time is 4-5h.

8. The preparation method according to claim 6, characterized in that, Step (4) The degreasing treatment temperature is 500-600℃, the heating rate is 2-3℃ / min; the holding time is 2-3h; and / or In step (4), the sintering temperature is 1120-1200℃, the heating rate is 3-5℃ / min, and the holding time is 3-5h.

9. The preparation method according to claim 6, characterized in that, The polarization process in step (5) includes the following steps: S1: Apply an electric field strength of 2.5-3.5kV / mm at a polarization temperature of 100-120℃ and polarize for 15-30min; S2: Apply an electric field strength of 4-5 kV / mm at a temperature of 80-100℃ and polarize for 10-20 minutes.

10. The application of the KNN-based lead-free piezoelectric ceramic prepared by the method of any one of claims 1-5 or claims 6-9 in sensors, actuators and ultrasonic-related equipment.

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