BF-BT-BKT-BNT lead-free piezoelectric ceramic with high-temperature thermal stability and preparation method thereof

Abstract

The application relates to a BF-BT-BKT-BNT lead-free piezoelectric ceramic with high-temperature thermal stability and a preparation method thereof, and aims to provide a lead-free piezoelectric ceramic with high-temperature thermal stability and high piezoelectric performance in a high-temperature environment. The application utilizes the relaxation characteristics of BNT and the A-site composite valence cations of BNT and BKT to break the long-range order of BF-BT, so that BF-BT-BNT-BKT ceramic with a composition close to MPB, a tetragonal phase and a composite domain structure is obtained. When T dr =382 DEG C, in-situ d 33 =687 pC / N, while T c >450 DEG C, the Curie temperature is relatively high.

Description

Technical Field

[0001] This invention relates to the preparation technology of lead-free piezoelectric ceramics, and in particular to a BF-BT-BKT-BNT lead-free piezoelectric ceramic with high-temperature thermal stability and its preparation method. Background Technology

[0002] Piezoelectric ceramics are a class of functional materials capable of converting mechanical energy into and vice versa. Due to their excellent piezoelectric effect and electro-mechanical coupling properties, they are widely used in sensors, actuators, ultrasonic equipment, and other fields. Traditional piezoelectric ceramics are mostly composed of lead zirconate titanate (PZT). Although PZT materials possess excellent piezoelectric properties, their high lead content poses a serious environmental pollution problem. Therefore, the development of lead-free piezoelectric ceramic materials has become a current research focus.

[0003] High-temperature piezoelectric ceramics possess the characteristic of maintaining stable piezoelectric properties under high-temperature environments, making them promising candidates for applications in aerospace, automotive, and oil exploration. However, achieving high-temperature stability while maintaining good piezoelectric performance remains a significant challenge. Existing research indicates that the thermal stability of piezoelectric ceramics is related not only to the Curie temperature but also to the phase structure and domain structure. Higher tetragonality correlates with higher temperature stability, and the domain structure significantly influences temperature stability, particularly ceramics with composite domain structures (containing microdomains, nanodomains, and polar nanoregions) exhibiting higher thermal stability. Furthermore, finer domain structures result in higher piezoelectric activity. BiFeO3-BaTiO3 (BF-BT) based lead-free piezoelectric ceramics have attracted widespread attention due to their high Curie temperature and perovskite structure, theoretically possessing high piezoelectric performance.

[0004] Existing technology shows that BiFeO3-BaTiO3 (BF-BT) ceramic, as a lead-free piezoelectric ceramic with a perovskite structure, has a high T0 of 830℃. C and 90-100μC / cm 2 spontaneous polarization intensity P s This is key to breakthroughs in high-temperature piezoelectric applications, but the temperature stability and piezoelectric properties of this type of ceramic still lag significantly behind lead-based ceramics, particularly in depolarization temperature T. d Its application at high temperatures is limited by the fact that piezoelectric properties and Curie temperature cannot be achieved simultaneously. Summary of the Invention

[0005] To address the shortcomings of the existing technology, this invention aims to provide a lead-free piezoelectric ceramic with high-temperature thermal stability and high-voltage electrical properties, and a method for its preparation. Specifically, this invention provides a BiFeO3-BaTiO3-Bi ceramic with high-temperature thermal stability. 0.5 Na 0.5TiO3-Bi 0.5 K 0.5 TiO3 (BF-BT-BNT-BKT) lead-free piezoelectric ceramics and their preparation method: By controlling the phase structure and domain structure, high-performance lead-free ceramics that maintain piezoelectric properties even at high temperatures are prepared. The piezoelectric ceramics prepared using this invention have a Curie temperature Ti. c >470℃, such as Figure 2 As shown.

[0006] This invention introduces relaxor ferroelectrics with mixed valence cations at two A sites into BiFeO3-BaTiO3 ceramics, and improves the sintering performance and density of the system by adding sintering aids, thereby obtaining a dense BiFeO3-BaTiO3 piezoelectric ceramic with high real-time piezoelectric properties. The piezoelectric ceramic prepared using this invention exhibits a piezoelectric constant dp at 180°C. 33 It can reach 300 pC / N, and the piezoelectric constant d is achieved when operating at 360°C. 33 Maintain above 600 pC / N, such as Figure 1 As shown.

[0007] The technical solution to achieve the objective of this invention is: A high-temperature thermally stable BF-BT-BKT-BNT lead-free piezoelectric ceramic, comprising (1) Select (1-x)BiFeO3-xBaTiO3 with a high Curie temperature and a composition ratio close to that of the trigonal-tetragonal MPB structure as the matrix material, 0 < x ≤ 0.05; (2) Select 0.8 (Bi) 0.5 Na 0.5 TiO3+0.2(Bi) 0.5 K 0.5 TiO3 has a trigonal-tetragonal MPB structure, which serves as the phase structure and domain structure regulating component. (3) Construct (1-x)[(1-y)BiFeO3-yBaTiO3]+x[0.8(Bi 0.5 Na 0.5 TiO3+0.2(Bi) 0.5 K 0.5 The composition of TiO3 was used to construct the quaternary system BF-BT-BNT-BKT. (4) BNT is both a phase structure regulating component and a relaxation property that can be used to regulate the domain structure of ceramics, thereby obtaining BF-BT-BNT-BKT ceramics with composite domain structure, which can significantly improve the electrical properties and thermal stability of BF-BT based ceramics.

[0008] Specifically, a lead-free piezoelectric ceramic with high-temperature thermal stability, BF-BT-BKT-BNT, has the following general formula: (1-x)[(1-y)BiFeO3-yBaTiO3]+x[0.8(Bi 0.5 Na 0.5 TiO3+0.2(Bi) 0.5 K 0.5 The equation is: [TiO3] + mMnO3 + wLi2CO3, where x, y, m, and w represent the mole fractions of the components, and 0 < x ≤ 0.05, 0.30 < y < 0.40, 0 < m ≤ 0.01, and 0 < w ≤ 0.01.

[0009] The preparation method of the BF-BT-BKT-BNT lead-free piezoelectric ceramic with high-temperature thermal stability includes the following steps: 1) Using analytically pure Bi₂O₃, Fe₂O₃, BaCO₃, and TiO₂ as raw materials, according to (1-y)Bi 1.05 BF-BT base material was synthesized by the ratio of FeO3-yBaTiO3, where y represents the mole fraction of the component, 0.30 < y < 0.40; Bi element was in excess at 5.0% mol to compensate for the volatilization of bismuth element during sintering. The mixed powder was ball-milled with anhydrous ethanol as the medium for 24 h, then dried in an oven at 100 °C for 12 h, passed through a 150 mesh sieve, placed in a high-alumina crucible, compacted and covered, and then placed in a muffle furnace for pre-firing at 780 °C for 4 h for later use. 2) Using Bi₂O₃, TiO₂, Na₂CO₃, and K₂CO₃ as raw materials, according to 0.8(Bi 0.5 Na 0.5 TiO3+0.2(Bi) 0.5 K 0.5 The TiO3 was prepared according to the specified ratio. The mixed powder was ball-milled for 24 hours with anhydrous ethanol as the medium. After being taken out, it was dried in an oven at 100℃ for 12 hours, passed through a 150-mesh sieve, placed in a high-alumina crucible, compacted and covered, and then placed in a muffle furnace at 760℃-780℃ for 4 hours to synthesize BNT-BKT powder. 3) Combine the BF-BT pre-calcined powder synthesized in step 1) and the BNT-BKT pre-calcined powder synthesized in step 2), along with Li2CO3 and MnO2, according to (1-x)[(1-y)BiFeO3-yBaTiO3]+x[0.8(Bi 0.5 Na 0.5 TiO3+0.2(Bi) 0.5 K 0.5)TiO3]+mMnO3+wLi2CO3 formulation, where 0 < x ≤ 0.05, 0.30 < y < 0.40, 0 < m ≤ 0.01, 0 < w ≤ 0.1. Mix and ball-mill for 24 h, take out and dry in an oven at 100 °C for 12 h, and sieve through a 150-mesh sieve; 4) Add a PVA solution with a concentration of 8% for granulation, and press into tablets in a tablet press at a pressure of 10 MPa; 5) Put the formed green tablets into a muffle furnace and slowly heat them to 600 °C at a heating rate of 30 °C / h, and keep them at this temperature for 3 h to remove the binder; then heat them to 970 - 1010 °C at a heating rate of 2 °C / min for sintering, keep them at this temperature for 240 min, and then cool them to room temperature with the furnace; 6) Grind the sintered ceramic tablets, then coat the surface of the samples with silver paste, and burn the silver at 550 °C for 30 min; 7) Polarize the piezoelectric ceramic tablets coated with silver electrodes in silicone oil, with a polarization electric field of 7000 V / mm, a polarization temperature of 120 °C, and a time of 15 min. Keep the voltage unchanged, and take them out after the temperature drops to room temperature.

[0010] Let it stand at room temperature in air for 24 h and then measure the performance.

[0011] In order to achieve the improvement of piezoelectric properties and thermal stability while maintaining a high Curie temperature, the present invention selects (Bi 0.5 Na 0.5 )TiO3 (BNT) with a high Curie temperature and a rhombohedral phase structure and (Bi 0.5 K 0.5 )TiO3 (BKT) with a tetragonal structure, and combines them with BiFeO3 - BaTiO3 to construct a BF - BT - BNT - BKT lead-free piezoelectric ceramic near the rhombohedral - tetragonal phase structure, biased towards the tetragonal structure side, with a composite domain structure. From the perspective of phase structure and domain structure regulation, the temperature stability and piezoelectric properties are simultaneously improved, and a high-performance lead-free piezoelectric ceramic with a Curie temperature T c [[ID=2:4]]> 474 °C, an in-situ depolarization temperature T dr > 350 °C, a maximum operating temperature exceeding 350 °C, and a piezoelectric property d 33 > 650 pC / N at a temperature T > 350 °C is obtained.

[0012] The positive effects produced by the present invention are: (1) Phase structure regulation: Using BF - BT near the MPB as the matrix, introducing BNT - BKT near the MPB. Both are near the rhombohedral - tetragonal MPB quasi-homogeneous phase boundary, and the phase structure of the ceramic is regulated by changing the ratio of BF and BT; (2) By using the quaternary system, the MPB of the binary BF-BT system can be extended from point to surface, thereby enabling the control of ceramic properties over a wider range and making it easier to obtain high voltage electrical properties. (3) Domain Structure Regulation: Utilizing the relaxation characteristics of BNT and the A-site composite cations of BNT and BKT, the long-range order of BF-BT is disrupted to obtain BF-BT-BNT-BKT ceramics with a tetragonal phase and composite domain structure near the MPB composition. Transmission electron microscopy analysis shows that this ceramic system has a large number of polar nanoregions (PNRs). The presence of PNRs allows the polar ferroelectric phase and the non-polar paraelectric phase to coexist over a wide temperature range, resulting in a dispersed phase transition and effectively improving the temperature stability and piezoelectric properties of the material. Figure 3 As shown, a large number of polar nanoregions (PNRs) were observed using transmission electron microscopy, which effectively improved the piezoelectric properties. (4) High-temperature piezoelectric properties: Through reasonable composition design and optimized sintering process, the BF-BT-BNT-BKT ceramic material prepared by this invention maintains excellent thermal stability in high-temperature environments, and can exhibit d properties at temperatures up to 382℃. 33 With a piezoelectric property of 687 pC / N, it can be applied to high-temperature applications. Attached Figure Description

[0013] Figure 1 The Td of the lead-free piezoelectric ceramic sample BF-BT-BKT-BNT of this invention 33 In-situ depolarization curve; Figure 2 The dielectric temperature spectrum of the lead-free piezoelectric ceramic sample BF-BT-BKT-BNT of this invention; Figure 3 This is a transmission electron microscope (TEM) image of the lead-free piezoelectric ceramic sample BF-BT-BKT-BNT of this invention. Detailed Implementation

[0014] The present invention will be further illustrated below through embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0015] Example 1: A lead-free piezoelectric ceramic with high-temperature thermal stability, BF-BT-BKT-BNT, has the following chemical formula: 0.9684[0.68BiFeO3-0.32BaTiO3]+0.0316[0.8(Bi 0.5 Na 0.5TiO3+0.2(Bi) 0.5 K 0.5 )TiO3]+0.005MnO3+0.008Li2CO3.

[0016] The preparation method includes the following steps: (1) Using analytical grade Bi2O3, Fe2O3, BaCO3 and TiO2 as raw materials, according to 0.68Bi 1.05 BF-BT base material was synthesized by a ratio of FeO3-0.32BaTiO3; Bi element was in excess at 5.0%mol to compensate for the volatilization of Bi element during sintering. The mixed powder was ball-milled with anhydrous ethanol as the medium for 24 hours, dried in an oven at 100℃ for 12 hours, passed through a 150-mesh sieve, placed in a high-alumina crucible, compacted and covered, and then placed in a muffle furnace for pre-firing at 780℃ and held for 4 hours for later use. (2) Using Bi2O3, TiO2, Na2CO3, and K2CO3 as raw materials, according to 0.8 (Bi 0.5 Na 0.5 TiO3+ 0.2(Bi) 0.5 K 0.5 The TiO3 was prepared according to the specified ratio. The mixed powder was ball-milled for 24 hours with anhydrous ethanol as the medium. It was then dried in an oven at 100℃ for 12 hours, passed through a 150-mesh sieve, placed in a high-alumina crucible, compacted and covered, and then placed in a muffle furnace at 760℃-780℃ for 4 hours to synthesize BNT-BKT powder. (3) The BF-BT pre-calcined powder synthesized in step (1) and the BNT-BKT pre-calcined powder synthesized in step (2), along with Li2CO3 and MnO2, are mixed according to the following formula: 0.9684[0.68BiFeO3-0.32BaTiO3]+0.0316[0.8(Bi 0.5 Na 0.5 TiO3+0.2(Bi) 0.5 K 0.5 The ingredients were prepared by mixing TiO3, 0.005MnO3, and 0.008Li2CO3, ball milling for 24 hours, drying in an oven at 100°C for 12 hours, and then passing through a 150-mesh sieve. (4) Add 8% PVA solution to granulate, and compress into tablets at a pressure of 10 MPa in a tablet press; (5) The formed sheet is placed in a muffle furnace and slowly heated to 600°C at a heating rate of 30°C / h, and held for 3 h to remove the glue; then it is heated to 970°C at a heating rate of 2°C / min for sintering, and held for 240 min before being cooled to room temperature in the furnace. (6) Polish the sintered ceramic sheet, then coat the sample surface with silver paste, and burn the silver at 550℃ for 30 min; (7) The piezoelectric ceramic sheet after being plated with silver electrodes is polarized in silicone oil with a polarization electric field of 7000V / mm, a polarization temperature of 120℃, and a time of 15min. The voltage is kept constant, and the temperature is reduced to room temperature before it is taken out.

[0017] Performance was measured after the sample was left to stand in air at room temperature for 24 hours.

[0018] The performance results are as follows: .

[0019] Example 2: A lead-free piezoelectric ceramic with high-temperature thermal stability, BF-BT-BKT-BNT, has the following chemical formula: 0.9682[0.68BiFeO3-0.32BaTiO3]+0.0318[0.8(Bi 0.5 Na 0.5 TiO3+0.2(Bi) 0.5 K 0.5 )TiO3]+0.005MnO3+0.006Li2CO3.

[0020] The preparation method is the same as in Example 1, except that: In step (3), the following formula is used: 0.9682[0.68BiFeO3-0.32BaTiO3]+0.0318[0.8(Bi 0.5 Na 0.5 TiO3+0.2(Bi) 0.5 K 0.5 Ingredients: TiO3] + 0.005MnO3 + 0.006Li2CO3; The sintering temperature in step (5) is 980℃.

[0021] The performance results are as follows: .

[0022] Example 3: A lead-free piezoelectric ceramic with high-temperature thermal stability, BF-BT-BKT-BNT, has the following chemical formula: 0.9675[0.68BiFeO3-0.32BaTiO3]+0.0325[0.8(Bi 0.5 Na 0.5 TiO3+0.2(Bi) 0.5 K 0.5 )TiO3]+0.005MnO3+0.006Li2CO3.

[0023] The preparation method is the same as in Example 1, except that: In step (3), the following formula is used: 0.9675[0.68BiFeO3-0.32BaTiO3]+0.0325[0.8(Bi 0.5 Na 0.5 TiO3+0.2(Bi) 0.5 K 0.5 Ingredients: TiO3] + 0.005MnO3 + 0.006Li2CO3; The sintering temperature in step (5) is 990℃.

[0024] The performance results are as follows: .

[0025] Example 4: A lead-free piezoelectric ceramic with high-temperature thermal stability, BF-BT-BKT-BNT, has the following chemical formula: 0.9673[0.68BiFeO3-0.32BaTiO3]+0.0327[0.8(Bi 0.5 Na 0.5 TiO3+0.2(Bi) 0.5 K 0.5 )TiO3]+0.005MnO3+0.006Li2CO3.

[0026] The preparation method is the same as in Example 1, except that: In step (3), the following formula is used: 0.9673[0.68BiFeO3-0.32BaTiO3]+0.0327[0.8(Bi 0.5 Na 0.5 TiO3+0.2(Bi) 0.5 K 0.5 Ingredients: TiO3] + 0.005MnO3 + 0.006Li2CO3; The sintering temperature in step (5) is 1000℃.

[0027] The performance results are as follows: .

[0028] Example 5: A lead-free piezoelectric ceramic with high-temperature thermal stability, BF-BT-BKT-BNT, has the following chemical formula: 0.9671[0.68BiFeO3-0.32BaTiO3]+0.0329[0.8(Bi 0.5 Na 0.5 TiO3+0.2(Bi) 0.5 K 0.5 )TiO3]+0.005MnO3+0.006Li2CO3.

[0029] The preparation method is the same as in Example 1, except that: In step (3), the following formula is used: 0.9671[0.68BiFeO3-0.32BaTiO3]+0.0329[0.8(Bi 0.5 Na 0.5 TiO3+0.2(Bi) 0.5 K 0.5 Ingredients: TiO3] + 0.005MnO3 + 0.006Li2CO3; The sintering temperature in step (5) is 1010℃.

[0030] The performance results are as follows: .

[0031] The upper and lower limits and ranges of the components listed in this invention, as well as the upper and lower limits and ranges of the process parameters, can all realize this invention, and will not be listed one by one here.

[0032] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A lead-free piezoelectric ceramic with high-temperature thermal stability, characterized in that, The general formula for the composition of the ceramic is: (1- x )[(1- y BiFeO3- y BaTiO3]+ x [0.8(Bi 0.5 Na 0.5 TiO3+0.2(Bi) 0.5 K 0.5 TiO3]+ m MnO3+ w Li2CO3, in which x, y , m and w This represents the mole fraction of the component, and 0 < x ≤ 0.05, 0.30 < y < 0.40, 0 < m ≤ 0.01, 0 < w ≤ 0.

01.

2. The method for preparing the high-temperature thermally stable BF-BT-BKT-BNT lead-free piezoelectric ceramic according to claim 1, characterized in that, Includes the following steps: 1) Using analytically pure Bi₂O₃, Fe₂O₃, BaCO₃, and TiO₂ as raw materials, according to (1- y )Bi 1.05 FeO3- y The BF-BT matrix was synthesized using a specific ratio of BaTiO3, in which... y Indicates the mole fraction of the component, 0.30 < y < 0.40; The excess of Bi element is 5.0%mol to compensate for the volatilization of Bi element during sintering. The mixed powder is ball-milled with anhydrous ethanol as the medium for 24 hours, then dried in an oven at 100℃ for 12 hours, passed through a 150-mesh sieve, placed in a high-alumina crucible, compacted and covered, and then placed in a muffle furnace for pre-firing at 780℃ for 4 hours for later use. 2) Using Bi₂O₃, TiO₂, Na₂CO₃, and K₂CO₃ as raw materials, according to 0.8(Bi 0.5 Na 0.5 TiO3+ 0.2(Bi) 0.5 K 0.5 The TiO3 was prepared according to the specified ratio. The mixed powder was ball-milled for 24 hours with anhydrous ethanol as the medium. After being taken out, it was dried in an oven at 100℃ for 12 hours, passed through a 150-mesh sieve, placed in a high-alumina crucible, compacted and covered, and then placed in a muffle furnace at 760℃-780℃ for 4 hours to synthesize BNT-BKT powder. 3) Combine the BF-BT base powder synthesized in step 1) and the BNT-BKT powder synthesized in step 2), along with Li2CO3 and MnO2, according to (1- x )[(1- y BiFeO3- y BaTiO3]+ x [0.8(Bi 0.5 Na 0.5 TiO3+0.2(Bi) 0.5 K 0.5 TiO3]+ m MnO3+ w Li2CO3 ingredient, of which 0 < x ≤ 0.05, 0.30 < y < 0.40, 0 < m ≤ 0.01, 0 < w ≤ 0.1, mix and ball mill for 24 hours, take out and dry in an oven at 100℃ for 12 hours, and pass through a 150-mesh sieve; 4) Add 8% PVA solution to granulate, and compress into tablets at a pressure of 10MPa in a tablet press; 5) The formed raw sheet is placed in a muffle furnace and slowly heated to 600℃ at a heating rate of 30℃ / h, held for 3h to remove the glue; then heated to 970-1010℃ at a heating rate of 2℃ / min for sintering, held for 240min and then cooled to room temperature in the furnace. 6) Polish the sintered ceramic sheet, then coat its surface with silver paste and fire it at 550℃ for 30 minutes; 7) The ceramic sheet with silver electrodes was polarized in silicone oil with a polarization electric field of 7000V / mm, a polarization temperature of 120℃, and a time of 15 min. The voltage was kept constant, and the temperature was lowered to room temperature before it was taken out to obtain BF-BT-BKT-BNT lead-free piezoelectric ceramic.

3. The BF-BT-BKT-BNT lead-free piezoelectric ceramic with high-temperature thermal stability according to claim 1, characterized in that, The chemical formula of the ceramic is: 0.9684[0.68BiFeO3-0.32BaTiO3]+0.0316[0.8(Bi 0.5 Na 0.5 TiO3+0.2(Bi) 0.5 K 0.5 )TiO3]+0.005MnO3+0.008Li2CO3.

4. The method for preparing the high-temperature thermally stable BF-BT-BKT-BNT lead-free piezoelectric ceramic according to claim 3, characterized in that, The preparation method includes the following steps: (1) Using analytical grade Bi2O3, Fe2O3, BaCO3 and TiO2 as raw materials, according to 0.68Bi 1.05 BF-BT base material was synthesized using a ratio of FeO3-0.32BaTiO3; The excess of Bi element is 5.0%mol to compensate for the volatilization of Bi element during sintering. The mixed powder is ball-milled with anhydrous ethanol as the medium for 24 hours, dried in an oven at 100℃ for 12 hours, passed through a 150-mesh sieve, placed in a high-alumina crucible, compacted and covered, and then placed in a muffle furnace for pre-firing at 780℃ and held for 4 hours for later use. (2) Using Bi2O3, TiO2, Na2CO3, and K2CO3 as raw materials, according to 0.8 (Bi 0.5 Na 0.5 TiO3+ 0.2(Bi) 0.5 K 0.5 The TiO3 was prepared according to the specified ratio. The mixed powder was ball-milled for 24 hours with anhydrous ethanol as the medium. It was then dried in an oven at 100℃ for 12 hours, passed through a 150-mesh sieve, placed in a high-alumina crucible, compacted and covered, and then placed in a muffle furnace at 760℃-780℃ for 4 hours to synthesize BNT-BKT powder. (3) The BF-BT base powder synthesized in step (1) and the BNT-BKT powder synthesized in step (2), along with Li2CO3 and MnO2, are mixed according to the following formula: 0.9684[0.68BiFeO3-0.32BaTiO3]+0.0316[0.8(Bi 0.5 Na 0.5 TiO3+0.2(Bi) 0.5 K 0.5 The ingredients were prepared by mixing TiO3, 0.005MnO3, and 0.008Li2CO3, ball milling for 24 hours, drying in an oven at 100°C for 12 hours, and then passing through a 150-mesh sieve. (4) Add 8% PVA solution to granulate, and compress into tablets at a pressure of 10 MPa in a tablet press; (5) The molded sheet is placed in a muffle furnace and slowly heated to 600°C at a heating rate of 30°C / h, and held for 3 hours to remove the glue; then it is heated to 970°C at a heating rate of 2°C / min for sintering, and held for 240 minutes before being cooled to room temperature in the furnace. (6) Polish the sintered ceramic sheet, then coat its surface with silver paste and fire it at 550°C for 30 minutes; (7) The ceramic sheet coated with silver electrodes was polarized in silicone oil with a polarization electric field of 7000V / mm, a polarization temperature of 120℃, and a time of 15min. The voltage was kept constant, and the temperature was lowered to room temperature before it was taken out to obtain BF-BT-BKT-BNT lead-free piezoelectric ceramic.

5. The BF-BT-BKT-BNT lead-free piezoelectric ceramic with high-temperature thermal stability according to claim 1, characterized in that, The chemical formula of the ceramic is: 0.9682[0.68BiFeO3-0.32BaTiO3]+0.0318[0.8(Bi 0.5 Na 0.5 )TiO3+0.2(Bi 0.5 K 0.5 [TiO3] + 0.005MnO3 + 0.006Li2CO3.

6. The BF-BT-BKT-BNT lead-free piezoelectric ceramic with high-temperature thermal stability according to claim 1, characterized in that, The chemical formula of the ceramic is: 0.9675[0.68BiFeO3-0.32BaTiO3]+0.0325[0.8(Bi 0.5 Na 0.5 )TiO3+0.2(Bi 0.5 K 0.5 [TiO3] + 0.005MnO3 + 0.006Li2CO3.

7. The BF-BT-BKT-BNT lead-free piezoelectric ceramic with high-temperature thermal stability according to claim 1, characterized in that, The chemical formula of the ceramic is: 0.9673[0.68BiFeO3-0.32BaTiO3]+0.0327[0.8(Bi 0.5 Na 0.5 )TiO3+0.2(Bi 0.5 K 0.5 [TiO3] + 0.005MnO3 + 0.006Li2CO3.

8. The BF-BT-BKT-BNT lead-free piezoelectric ceramic with high-temperature thermal stability according to claim 1, characterized in that, The chemical formula of the ceramic is: 0.9671[0.68BiFeO3-0.32BaTiO3]+0.0329[0.8(Bi 0.5 Na 0.5 )TiO3+0.2(Bi 0.5 K 0.5 [TiO3] + 0.005MnO3 + 0.006Li2CO3.

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