A lead-free piezoelectric ceramic material of bismuth ferrite-barium titanate and a preparation method thereof

Abstract

The present invention belongs to the technical field of piezoelectric ceramics, and discloses a bismuth ferrite-barium titanate lead-free piezoelectric ceramic material and a preparation method thereof. The composition of the piezoelectric ceramic is 0.7Bi 1.02 FeO3 - 0.3BaTiO3 - x% LiF + y% M + z% SiO2; M represents a Mn-containing metal compound; x, y, and z respectively represent the molar fractions of LiF, M, and SiO2, 0.25 ≤ x ≤ 0.75, 0.35 ≤ y ≤ 0.75, 0 ≤ z ≤ 0.3. The present invention also discloses a preparation method of the piezoelectric ceramic. The present invention regulates the oxygen vacancy concentration in the crystal lattice, strengthens the crystal lattice distortion, improves the microstructure, and significantly improves both the piezoelectricity and the Curie temperature. The piezoelectric ceramic material of the present invention has a wide sintering temperature range (880 - 1020 °C), a high piezoelectric constant (d 33 = 180 - 208 pC / N), and a high Curie temperature (T C = 550 °C).

Description

Technical Field

[0001] The present invention belongs to the technical field of piezoelectric ceramics, and particularly relates to a bismuth ferrite-barium titanate lead-free piezoelectric ceramic material and a preparation method thereof. Background Technique

[0002] In order to meet the requirements of low-carbon environmental protection and high-temperature applications such as aerospace, geothermal drilling, petrochemical, and automotive fields, high-temperature lead-free piezoelectric materials and devices have obtained great development. Among the numerous high-temperature lead-free piezoelectric ceramic material systems developed in recent years, more research has been carried out on perovskite-structured sodium bismuth titanate, sodium potassium niobate, and bismuth ferrite-based piezoelectric materials. Among them, the Curie temperature of bismuth ferrite (~830 °C) is significantly higher than that of the other two material systems. Introducing barium titanate to form a bismuth ferrite-barium titanate (BF-BT) solid solution can further stabilize the perovskite structure. And there is a morphotropic phase boundary in the BF-BT solid solution. When the BT content is between 0.25 - 0.35, the rhombohedral phase and the pseudo-cubic phase coexist, having good piezoelectric and ferroelectric properties. Therefore, it is also the composition range with more research. However, due to the Bi volatilization and Fe 3+ valence change during the high-temperature sintering process, the leakage current of bismuth ferrite-based materials is large, the loss is high, and it is difficult to polarize, which limits its application in high-temperature piezoelectric devices. The improvement of the piezoelectric properties of ceramics is usually accompanied by a decrease in the Curie temperature. There is a mutual restriction between high piezoelectric properties and the Curie temperature. Therefore, pursuing a high Curie temperature and high piezoelectric properties has always been an important research direction for this type of ceramics.

[0003] Chinese Patent CN110128127B regulates the phase structure and microstructure of ceramics by introducing Bi(Ti 0.5 Zn 0.5 )O3, and uses one or a combination of Ba(W 0.5 Cu 0.5 )O3, Ba(Cu 1 / 3 Nb 2 / 3 )O3, B2O3, Li2CO3, V2O5 as sintering aids to obtain a bismuth ferrite-barium titanate-based lead-free piezoelectric ceramic with high piezoelectric properties and high-temperature stability: 0.75BiFeO3 - 0.25BaTiO3 + 0.10Bi(Ti 0.5 Zn 0.5 )O3 + 0.02Li2CO3 + 0.01MnCO3. The best performance is obtained by sintering the ceramics at 970 °C: d 33 = 187 pC / N, k p = 0.31%, T C = 558 °C, T d = 530 °C, tanδ = 2.99%. In addition to the traditional solid-phase sintering process, its preparation method also includes synthesizing Bi(Ti 0.5 Zn0.5 )Processes such as O3, exhaust gassing with pure oxygen, and high-temperature quenching.

[0004] Chinese Patent Application CN110563456A uses the method of partially replacing Fe with Sc and partially replacing Ti with Zr. While reducing Fe 3+ reduction, it increases the distortion degree of BaTiO3. Then, combined with the method of adding MnO2 after pre-sintering, without the additional production conditions of high pressure and quenching, it realizes the material with high Curie temperature and high piezoelectric properties obtained by traditional ceramic sintering process: (1 - x)BiFe 0.985 Sc 0.015 O3 - xBaZr 0.2 Ti 0.8 O3 + 1mol% MnO2. The highest piezoelectric performance index is: x = 0.25, d 33 ≈105 pC / N, k p ≈0.28, T C ≈600 °C; but when x = 0.30, T C = 550 °C, d 33 then drops to only about 20 pC / N. Compared with materials with the same Curie temperature, its piezoelectric performance needs to be further improved.

[0005] Based on the conventional process, Chinese Patent Application CN113461419A combines the annealing treatment after sintering and the quenching treatment after silver firing, namely water cooling, air cooling or oil cooling, to significantly improve the piezoelectric performance of BF - BT ceramics. This patent application uses annealing treatment to improve the microstructure, density and charge defect concentration of ceramics, and uses the quenching treatment after silver firing to cause lattice distortion during the rapid cooling process at high temperature, further improving its piezoelectric performance. After sintering 0.7Bi 1.02 FeO3 - 0.3BaTiO3 ceramics at 980 °C for 3 h, anneal at 600 - 900 °C for 2 h, and then use air cooling quenching after applying silver electrodes. The piezoelectric performance of the ceramics annealed at 800 °C is the best: d 33 = 210 pC / N, k p = 0.34, but the Curie temperature is relatively low (T C = 490 °C); the Curie temperature of the ceramics annealed at 900 °C is the highest and the comprehensive performance is better: T C = 531 °C, d 33 = 193 pC / N, k p = 0.32.

[0006] The electrical properties of BF-BT ceramics are closely related to the synthesis process and material composition. Although the electrical properties of BF-BT ceramics can be improved by using the above-mentioned processes such as high temperature and high pressure, annealing, and quenching, it will increase the additional production cost at the same time. In addition, microcracks are likely to occur in the ceramic samples after quenching, reducing the material strength. Usually, BF-BT ceramics are sintered at temperatures above 970 °C, and the sintering temperature range is narrow, reducing the process stability; at the same time, compared with lead-based piezoelectric ceramics, the piezoelectric properties of BF-BT still need to be improved, and the improvement of piezoelectric properties often leads to a decrease in the Curie temperature. These have greatly restricted the practical application of BF-BT piezoelectric ceramic materials. Summary of the Invention

[0007] Aiming at the deficiencies of the prior art, the present invention provides a BF-BT lead-free piezoelectric ceramic material and a preparation method thereof. The piezoelectric ceramic material obtained by the present invention not only has high piezoelectric properties and a high Curie temperature, but also has a low sintering temperature and a wide sintering temperature range, and its piezoelectric properties are superior to other materials with the same Curie temperature in this system.

[0008] The present invention utilizes the low melting point characteristic of LiF and the acceptor / donor co-doping of Li + / F- ions to improve the sintering characteristics, regulate the oxygen vacancy concentration in the lattice, strengthen the lattice distortion, and increase the Curie temperature; while the composite doping of MnO2 and SiO2 after pre-sintering mainly acts on the grain boundaries, further improving the density and insulation resistance of the ceramics, and improving the microstructure, thereby realizing the optimization of structure and performance, improving the contradiction between piezoelectricity and Curie temperature, and significantly improving both piezoelectricity and Curie temperature.

[0009] The object of the present invention can be achieved by the following technical solutions:

[0010] A bismuth ferrite-barium titanate lead-free piezoelectric ceramic material, the composition of which is 0.7Bi 1.02 FeO3-0.3BaTiO3-x%LiF+y%M+z%SiO2; where M represents one of the Mn ion metal compounds MnO2, Mn2O3 or MnCO3; x, y, and z respectively represent the molar fractions of LiF, M, and SiO2, 0.25 ≤ x ≤ 0.75, 0.35 ≤ y ≤ 0.75, 0 ≤ z ≤ 0.3.

[0011] Preferably, x = 0.50, 0.35 ≤ y ≤ 0.75, 0 ≤ z ≤ 0.3.

[0012] The present invention also provides a preparation method of a bismuth ferrite-barium titanate lead-free piezoelectric ceramic material, including the following steps:

[0013] (1) Select Bi2O3, Fe2O3, BaCO3, TiO2 and LiF as raw materials, according to the chemical formula 0.7Bi 1.02Weigh the raw materials according to the stoichiometric ratio of FeO3 - 0.3BaTiO3 - x%LiF, conduct primary ball milling, drying, and pre - sintering to obtain intermediate powder, where x represents the molar fraction of LiF, and 0.25 ≤ x ≤ 0.75.

[0014] (2) Weigh 0.7Bi according to the chemical formula of the piezoelectric ceramic material 0.7Bi 1.02 FeO3 - 0.3BaTiO3 - x%LiF + y%M + z%SiO2 in stoichiometric ratio, weigh the ceramic intermediate powder of 0.7Bi 1.02 FeO3 - 0.3BaTiO3 - x%LiF, as well as powders of M and SiO2, conduct secondary ball milling, granulation, molding, sintering, electrode deposition, and poling to obtain the piezoelectric ceramic, where M represents one of the Mn - ion metal compounds MnO2, Mn2O3, or MnCO3; y and z represent the molar fractions of M and SiO2 respectively, and 0.35 ≤ y ≤ 0.75, 0 ≤ z ≤ 0.3.

[0015] In step (1), the medium for the primary ball milling is anhydrous ethanol, and the ball milling time is 4 - 8 h.

[0016] In step (1), the pre - sintering is carried out by heating from room temperature to 720 - 780 °C (preferably 750 °C) at a heating rate of 3 - 5 °C / min and holding for 4 - 6 h.

[0017] In step (2), the medium for the secondary ball milling is anhydrous ethanol, and the ball milling time is 6 - 12 h.

[0018] In step (2), the granulation uses an aqueous solution of polyvinyl alcohol with a mass concentration of 5 - 12% as the binder.

[0019] In step (2), the molding is to press into a ceramic wafer at a pressure of 50 - 150 MPa.

[0020] In step (2), for the sintering, heat is raised to 800 °C at a heating rate of 3 - 5 °C / min, and then raised to 880 - 1020 °C at a heating rate of 1 - 3 °C / min, with a holding time of 2 - 4 h.

[0021] In step (2), for the poling, the obtained ceramic wafer is immersed in silicone oil at 100 - 120 °C for poling, and the poling voltage used is 3 - 5 kV / mm, and the poling time is 10 - 20 min.

[0022] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0023] (1) The lead-free piezoelectric ceramics of bismuth ferrite-barium titanate prepared by the present invention simultaneously have high piezoelectricity and high Curie temperature. By selecting appropriate values of x, y, z and process parameters (here the process parameters refer to the conditions of pre-sintering, sintering and poling), the piezoelectric ceramics of this system can have a maximum value of d C at the Curie temperature T 33 = 550 °C reaching 208 pC / N, which is higher than other materials with the same Curie temperature in this system.

[0024] (2) The lead-free piezoelectric ceramics prepared by the present invention have the advantages of low sintering temperature and wide firing temperature range. In the sintering range of 880 - 1020 °C, the piezoelectric coefficient d 33 values of the obtained ceramics are all higher than 180 pC / N, indicating that the piezoelectric properties of the ceramics are not sensitive to the change of sintering temperature, significantly improving the stability of the ceramic preparation process, which is extremely beneficial to the large-scale production of piezoelectric ceramics of this system.

[0025] (3) The lead-free piezoelectric ceramics prepared by the present invention adopt a mature traditional ceramic sintering process, do not require additional production conditions such as high temperature and high pressure, atmosphere, annealing, quenching, etc., have low cost, and have good repeatability and stability, and have strong practical value. Brief Description of the Drawings

[0026] Figure 1 shows the variation of the piezoelectric coefficient d 33 of the piezoelectric ceramic materials prepared in Example 1, Example 2, Example 3 and Example 4 of the present invention with the sintering temperature; Ts represents the sintering temperature;

[0027] Figure 2 is the scanning electron microscope photograph (SEM) of the piezoelectric ceramic material prepared in Example 4 of the present invention;

[0028] Figure 3 shows the variation of the piezoelectric coefficient d 33 of the piezoelectric ceramic materials prepared in Example 5, Example 6, Example 7 and Example 8 of the present invention with the sintering temperature; Ts represents the sintering temperature;

[0029] Figure 4 is the scanning electron microscope photograph (SEM) of the piezoelectric ceramic material prepared in Example 5 of the present invention. Detailed Description of the Invention

[0030] The present invention will be further described below in conjunction with specific embodiments, but the scope of protection required by the present invention is not limited thereto. It should be understood that the embodiments described herein are only used to explain the present invention and are not used to limit the scope of the present invention.

[0031] The values of x, y, z in Examples 1 - 8 are listed in Table 1.

[0032] Example 1

[0033] Prepare bismuth ferrite-barium titanate lead-free piezoelectric ceramic material with x = 0.25, y = 0.35, and z = 0.

[0034] Select Bi2O3, Fe2O3, BaCO3, TiO2, and LiF with purity higher than 99% as raw materials, and weigh the raw materials according to the stoichiometric ratio of the chemical formula 0.7Bi 1.02 FeO3 - 0.3BaTiO3 - 0.25%LiF. Use anhydrous ethanol as the ball-milling medium, zirconia balls with a diameter of 1 mm as the grinding balls, and the ball-milling tank is made of polytetrafluoroethylene. Ball-mill and mix according to the weight ratio of raw materials, grinding balls, and ethanol of 1:3:1.2 for 4 h, then discharge and dry at 85 °C. Pass the dried powder through an 80-mesh sieve, and then heat from room temperature to 780 °C at a heating rate of 5 °C / min for pre-sintering and hold for 6 h. Grind the pre-sintered intermediate and pass it through an 80-mesh sieve, then weigh it, and weigh 0.35% of MnO2 powder according to the stoichiometric ratio of the chemical formula 0.7Bi 1.02 FeO3 - 0.3BaTiO3 - 0.25%LiF + 0.35%MnO2 + 0%SiO2. Use anhydrous ethanol as the ball-milling medium for secondary ball-milling, mixing, and pulverizing for 6 h, then dry and pass through a sieve. Then add an aqueous solution of polyvinyl alcohol with a mass concentration of 10% for granulation, and press into a ceramic thin sheet with a diameter of 10 mm and a thickness of 1.0 mm under a pressure of 150 MPa. Subsequently, in the sintering furnace, heat the ceramic thin sheet from room temperature to 600 °C in 5 h and hold for 2 h for debinding. Heat the debound ceramic green body to 800 °C at a heating rate of 5 °C / min, and then heat to 900 °C, 920 °C, 940 °C, 960 °C, 980 °C, and 1000 °C at a heating rate of 3 °C / min for sintering, hold for 4 h, and then naturally cool to room temperature. After the surface of the sintered ceramic sheet is polished flat with sandpaper, silver is applied by screen printing process and sintered at 600 °C for 10 min. Finally, immerse the ceramic sheet in silicone oil at 100 °C and polarize it at a DC voltage of 4 kV / mm for 20 min. Place the polarized sample at room temperature for 24 hours and then conduct electrical property tests.

[0035] Example 2

[0036] Prepare bismuth ferrite-barium titanate lead-free piezoelectric ceramic material with x = 0.50, y = 0.35, and z = 0.

[0037] Select Bi2O3, Fe2O3, BaCO3, TiO2, and LiF with purity higher than 99% as raw materials, and weigh the raw materials according to the stoichiometric ratio of the chemical formula 0.7Bi 1.02Weigh the raw materials according to the stoichiometric ratio of 0.7BiFeO3 - 0.3BaTiO3 - 0.50% LiF. Using absolute ethanol as the ball - milling medium, zirconia balls with a diameter of 1 mm as the grinding balls, and a polytetrafluoroethylene ball - milling tank, ball - mill and mix according to the weight ratio of raw materials, grinding balls, and ethanol of 1:3:1.2 for 6 h, then discharge the material and dry it at 85 °C. Pass the dried powder through an 80 - mesh sieve, and then pre - sinter it from room temperature to 750 °C at a heating rate of 3 °C / min, and hold for 4 h. Grind the pre - sintered intermediate and pass it through an 80 - mesh sieve, then weigh it, and according to the chemical formula 0.7Bi 1.02 Weigh 0.35% of MnO2 powder according to the stoichiometric ratio of 0.7BiFeO3 - 0.3BaTiO3 - 0.50% LiF + 0.35% MnO2 + 0% SiO2. Use absolute ethanol as the ball - milling medium for secondary ball - milling, mixing, and pulverizing for 8 h, then dry and sieve it. Then add an aqueous solution of polyvinyl alcohol with a mass concentration of 10% for granulation, and press it into a ceramic wafer with a diameter of 10 mm and a thickness of 1.0 mm under a pressure of 100 MPa. Subsequently, in a sintering furnace, heat the ceramic wafer from room temperature to 600 °C over 5 h and hold for 2 h for debinding. Heat the debound ceramic green body to 800 °C at a heating rate of 5 °C / min, and then heat it to 880 °C, 900 °C, 920 °C, 940 °C, 960 °C, 980 °C, 1000 °C, 1020 °C at a heating rate of 2 °C / min for sintering, hold for 2 h, and then naturally cool to room temperature. After the surface of the sintered ceramic wafer is polished flat with sandpaper, silver is applied by screen - printing technology and fired at 600 °C for 10 min while holding. Finally, immerse the ceramic wafer in silicone oil at 120 °C and polarize it at a DC voltage of 4 kV / mm for 20 min. Place the polarized sample at room temperature for 24 h and then conduct electrical property tests.

[0038] Example 3

[0039] Prepare a lead - free piezoelectric ceramic material of bismuth ferrite - barium titanate with x = 0.75, y = 0.35, and z = 0.

[0040] Select Bi2O3, Fe2O3, BaCO3, TiO2, and LiF with a purity higher than 99% as raw materials, and weigh the raw materials according to the stoichiometric ratio of 0.7Bi 1.02 Weigh the raw materials according to the stoichiometric ratio of 0.7BiFeO3 - 0.3BaTiO3 - 0.75% LiF. Using absolute ethanol as the ball - milling medium, zirconia balls with a diameter of 1 mm as the grinding balls, and a polytetrafluoroethylene ball - milling tank, ball - mill and mix according to the weight ratio of raw materials, grinding balls, and ethanol of 1:3:1.2 for 8 h, then discharge the material and dry it at 85 °C. Pass the dried powder through an 80 - mesh sieve, and then pre - sinter it from room temperature to 720 °C at a heating rate of 3 °C / min, and hold for 4 h. Grind the pre - sintered intermediate and pass it through an 80 - mesh sieve, then weigh it, and according to the chemical formula 0.7Bi 1.02Weigh 0.35% of MnCO3 powder according to the stoichiometric ratio of FeO3 - 0.3BaTiO3 - 0.75% LiF + 0.35% MnCO3 + 0% SiO2. Using anhydrous ethanol as the ball - milling medium, conduct secondary ball - milling and mixing for pulverization for 8 h, then dry, sieve, and then add an aqueous solution of polyvinyl alcohol with a mass concentration of 5% for granulation. Press into a ceramic thin sheet with a diameter of 10 mm and a thickness of 1.0 mm under a pressure of 50 MPa. Subsequently, in a sintering furnace, heat the ceramic thin sheet from room temperature to 600 °C over 5 h and hold for 2 h for debinding. Heat the debound ceramic green body to 800 °C at a heating rate of 4 °C / min, and then heat to 900 °C, 920 °C, 940 °C, 960 °C, 980 °C, 1000 °C at a heating rate of 2 °C / min for sintering, hold for 2 h, and then naturally cool to room temperature. After the surface of the sintered ceramic sheet is polished flat with sandpaper, silver it using the screen - printing process and sinter the silver at 600 °C for 10 min. Finally, immerse the ceramic sheet in silicone oil at 100 °C and polarize it for 20 min under a DC voltage of 3 kV / mm. Place the polarized sample at room temperature for 24 hours and then conduct electrical property tests.

[0041] Example 4

[0042] Prepare lead - free piezoelectric ceramic material of bismuth ferrite - barium titanate with x = 0.50, y = 0.35, z = 0.15.

[0043] Select Bi2O3, Fe2O3, BaCO3, TiO2, and LiF with a purity higher than 99% as raw materials, and weigh the raw materials according to the stoichiometric ratio of 0.7Bi 1.02 Weigh the raw materials according to the stoichiometric ratio of FeO3 - 0.3BaTiO3 - 0.50% LiF. Using anhydrous ethanol as the ball - milling medium and zirconia balls with a diameter of 1 mm as the grinding balls, and the ball - milling tank is made of polytetrafluoroethylene. Ball - mill and mix according to the weight ratio of raw materials, grinding balls, and ethanol of 1:3:1.2 for 6 h, then discharge and dry at 85 °C. Sieve the dried powder through an 80 - mesh sieve, and then heat from room temperature to 750 °C at a heating rate of 3 °C / min for pre - sintering and hold for 4 h. Grind the pre - sintered intermediate through an 80 - mesh sieve and weigh it, and according to the chemical formula 0.7Bi 1.02Weigh 0.35% of MnO₂ and 0.15% of SiO₂ powder according to the stoichiometric ratio of 0.7BiFeO₃ - 0.3BaTiO₃ - 0.50% LiF + 0.35% MnO₂ + 0.15% SiO₂. Use anhydrous ethanol as the ball-milling medium, and perform secondary ball-milling and mixing for pulverization for 10 h, then dry, sieve, and then add an aqueous solution of polyvinyl alcohol with a mass concentration of 10% for granulation, and press into a ceramic thin sheet with a diameter of 10 mm and a thickness of 1.0 mm under a pressure of 100 MPa. Subsequently, in a sintering furnace, the ceramic thin sheet is heated from room temperature to 600 °C in 5 h and kept warm for 2 h for debinding. The debound ceramic green body is heated to 800 °C at a heating rate of 5 °C / min, and then heated to 880 °C, 900 °C, 920 °C, 940 °C, 960 °C, 980 °C, 1000 °C, 1020 °C at a heating rate of 2 °C / min for sintering, keep warm for 2 h, and then naturally cool to room temperature. After the surface of the sintered ceramic sheet is polished flat with sandpaper, it is silvered by screen printing process and fired at 600 °C for 10 min while keeping warm. Finally, the ceramic sheet is immersed in silicone oil at 120 °C and polarized for 10 min under a DC voltage of 4 kV / mm. The polarized sample is placed at room temperature for 24 hours and then the electrical properties are tested.

[0044] Example 5

[0045] Prepare a lead-free piezoelectric ceramic material of bismuth ferrite - barium titanate with x = 0.50, y = 0.35, z = 0.30.

[0046] Select Bi₂O₃, Fe₂O₃, BaCO₃, TiO₂ and LiF with a purity higher than 99% as raw materials, and weigh the raw materials according to the stoichiometric ratio of 0.7BiFeO₃ - 0.3BaTiO₃ - 0.50% LiF. Use anhydrous ethanol as the ball-milling medium, zirconia balls with a diameter of 1 mm as the grinding balls, and the material of the ball-milling tank is polytetrafluoroethylene. Ball-mill and mix according to the weight ratio of raw materials, grinding balls, and ethanol of 1:3:1.2 for 6 h, then discharge and dry at 85 °C. Sieve the dried powder through an 80-mesh sieve, and then pre-burn from room temperature to 750 °C at a heating rate of 3 °C / min and keep warm for 4 h. Grind the pre-burned intermediate and sieve it through an 80-mesh sieve and then weigh it. According to the chemical formula 0.7Bi 1.02 Weigh the raw materials according to the stoichiometric ratio of 0.7BiFeO₃ - 0.3BaTiO₃ - 0.50% LiF. Use anhydrous ethanol as the ball-milling medium, zirconia balls with a diameter of 1 mm as the grinding balls, and the material of the ball-milling tank is polytetrafluoroethylene. Ball-mill and mix according to the weight ratio of raw materials, grinding balls, and ethanol of 1:3:1.2 for 6 h, then discharge and dry at 85 °C. Sieve the dried powder through an 80-mesh sieve, and then pre-burn from room temperature to 750 °C at a heating rate of 3 °C / min and keep warm for 4 h. Grind the pre-burned intermediate and sieve it through an 80-mesh sieve. 1.02Weigh 0.35% of MnO₂ and 0.30% of SiO₂ powders according to the stoichiometric ratio of 0.7BiFeO₃ - 0.3BaTiO₃ - 0.50% LiF + 0.35% MnO₂ + 0.30% SiO₂. Using absolute ethanol as the ball-milling medium, conduct secondary ball-milling and mixing for pulverization for 12 h, then dry, sieve, and then add an aqueous solution of polyvinyl alcohol with a mass concentration of 12% for granulation. Press into a ceramic thin sheet with a diameter of 10 mm and a thickness of 1.0 mm under a pressure of 80 MPa. Subsequently, in a sintering furnace, heat the ceramic thin sheet from room temperature to 600 °C in 5 h and keep it warm for 2 h for debinding. Heat the debound ceramic blank to 800 °C at a heating rate of 5 °C / min, and then heat it to 880 °C, 900 °C, 920 °C, 940 °C, 960 °C, 980 °C, 1000 °C at a heating rate of 2 °C / min for sintering, keep it warm for 3 h, and then naturally cool to room temperature. After the surface of the sintered ceramic sheet is polished flat with sandpaper, silver is applied using the screen printing process and sintered at 600 °C for 10 min while keeping warm. Finally, immerse the ceramic sheet in silicone oil at 120 °C and polarize it at a DC voltage of 5 kV / mm for 10 min. Place the polarized sample at room temperature for 24 hours and then conduct electrical property tests.

[0047] Example 6

[0048] Prepare a lead-free piezoelectric ceramic material of bismuth ferrite - barium titanate with x = 0.50, y = 0.50, z = 0.30.

[0049] Select Bi₂O₃, Fe₂O₃, BaCO₃, TiO₂, and LiF with a purity higher than 99% as raw materials, and weigh the raw materials according to the chemical formula 0.7Bi 1.02 Weigh the raw materials according to the stoichiometric ratio of 0.7BiFeO₃ - 0.3BaTiO₃ - 0.50% LiF. Using absolute ethanol as the ball-milling medium and zirconia balls with a diameter of 1 mm as the grinding balls, and the ball-milling tank is made of polytetrafluoroethylene. Ball-mill and mix according to the weight ratio of raw materials, grinding balls, and ethanol of 1:3:1.2 for 8 h, then discharge and dry at 85 °C. Sieve the dried powder through an 80-mesh sieve, and then heat it from room temperature to 750 °C at a heating rate of 3 °C / min for pre-sintering and keep it warm for 4 h. Grind the pre-sintered intermediate through an 80-mesh sieve and then weigh it. According to the chemical formula 0.7Bi 1.02Weigh 0.50% of MnO₂ and 0.30% of SiO₂ powder according to the stoichiometric ratio of FeO₃ - 0.3BaTiO₃ - 0.50%LiF + 0.50%MnO₂ + 0.30%SiO₂. Use anhydrous ethanol as the ball - milling medium, carry out secondary ball - milling and mixing for pulverization for 12 h, then dry, sieve, and then add an aqueous solution of polyvinyl alcohol with a mass concentration of 12% for granulation. Press it into a ceramic thin sheet with a diameter of 10 mm and a thickness of 1.0 mm under a pressure of 80 MPa. Subsequently, in the sintering furnace, heat the ceramic thin sheet from room temperature to 600 °C in 5 h and keep it warm for 2 h for debinding. Heat the debound ceramic green body to 800 °C at a heating rate of 5 °C / min, and then heat it to 880 °C, 900 °C, 920 °C, 940 °C, 960 °C, 980 °C, 1000 °C at a heating rate of 2 °C / min for sintering, keep it warm for 3 h, and then naturally cool to room temperature. After polishing the surface of the sintered ceramic sheet smoothly with sandpaper, silver it by screen - printing process and sinter the silver at 600 °C for 10 min. Finally, immerse the ceramic sheet in silicone oil at 120 °C and polarize it for 10 min under a DC voltage of 5 kV / mm. Place the polarized sample at room temperature for 24 hours and then conduct electrical property tests.

[0050] Example 7

[0051] Prepare lead - free piezoelectric ceramic material of bismuth ferrite - barium titanate with x = 0.75, y = 0.35, z = 0.15.

[0052] Select Bi₂O₃, Fe₂O₃, BaCO₃, TiO₂ and LiF with a purity higher than 99% as raw materials, and weigh the raw materials according to the stoichiometric ratio of 0.7Bi 1.02 Weigh the raw materials according to the stoichiometric ratio of FeO₃ - 0.3BaTiO₃ - 0.75%LiF. Use anhydrous ethanol as the ball - milling medium, zirconia balls with a diameter of 1 mm as grinding balls, and the material of the ball - milling tank is polytetrafluoroethylene. Ball - mill and mix according to the weight ratio of raw materials, grinding balls, and ethanol of 1:3:1.2 for 8 h, then discharge and dry at 85 °C. Sieve the dried powder through a 80 - mesh sieve, and then heat it from room temperature to 750 °C at a heating rate of 3 °C / min for pre - sintering and keep it warm for 4 h. Grind the pre - sintered intermediate and sieve it through an 80 - mesh sieve, then weigh it, and according to the chemical formula 0.7Bi 1.02Weigh 0.35% of MnO₂ and 0.15% of SiO₂ powders according to the stoichiometric ratio of FeO₃ - 0.3BaTiO₃ - 0.75% LiF + 0.35% MnO₂ + 0.15% SiO₂. Using absolute ethanol as the ball-milling medium, conduct secondary ball-milling and mixing for pulverization for 8 h, then dry, sieve, and then add an aqueous solution of polyvinyl alcohol with a mass concentration of 8% for granulation, and press into a ceramic thin sheet with a diameter of 10 mm and a thickness of 1.0 mm under a pressure of 50 MPa. Subsequently, in a sintering furnace, heat the ceramic thin sheet from room temperature to 600 °C at a rate of 5 °C / min and hold for 2 h for debinding. Heat the debound ceramic green body to 800 °C at a heating rate of 5 °C / min, and then heat to 880 °C, 900 °C, 920 °C, 940 °C, 960 °C, 980 °C, 1000 °C at a heating rate of 2 °C / min for sintering, hold for 2 h, and then cool naturally to room temperature. After the surface of the sintered ceramic sheet is polished flat with sandpaper, silver is applied by screen printing technology and fired at 600 °C for 10 min. Finally, immerse the ceramic sheet in silicone oil at 100 °C and polarize it at a DC voltage of 4 kV / mm for 10 min. Place the polarized sample at room temperature for 24 hours and then conduct electrical property tests.

[0053] Example 8

[0054] Prepare a lead-free piezoelectric ceramic material of bismuth ferrite - barium titanate with x = 0.50, y = 0.75, and z = 0.15.

[0055] Select Bi₂O₃, Fe₂O₃, BaCO₃, TiO₂, and LiF with a purity higher than 99% as raw materials, and weigh the raw materials according to the stoichiometric ratio of 0.7Bi 1.02 Weigh the raw materials according to the stoichiometric ratio of FeO₃ - 0.3BaTiO₃ - 0.50% LiF. Using absolute ethanol as the ball-milling medium and zirconia balls with a diameter of 1 mm as grinding balls, and the material of the ball-milling tank is polytetrafluoroethylene. Ball-mill and mix according to the weight ratio of raw materials, grinding balls, and ethanol of 1:3:1.2 for 6 h, then discharge and dry at 85 °C. Sieve the dried powder through an 80-mesh sieve, and then heat from room temperature to 750 °C at a heating rate of 3 °C / min for pre-sintering and hold for 4 h. Grind the pre-sintered intermediate through an 80-mesh sieve and weigh it. According to the chemical formula 0.7Bi 1.02Weigh the MnCO₃ and SiO₂ powders according to the stoichiometric ratio of FeO₃ - 0.3BaTiO₃ - 0.50% LiF + 0.75% MnCO₃ + 0.15% SiO₂. Using anhydrous ethanol as the ball - milling medium, conduct secondary ball - milling and mixing for 8 h, then dry, sieve, and add an aqueous solution of polyvinyl alcohol with a mass concentration of 8% for granulation. Press into a ceramic thin sheet with a diameter of 10 mm and a thickness of 1.0 mm under a pressure of 100 MPa. Subsequently, in a sintering furnace, heat the ceramic thin sheet from room temperature to 600 °C over 5 h and hold for 2 h for binder burnout. Heat the debound ceramic green body to 800 °C at a heating rate of 5 °C / min, and then heat to 900 °C, 920 °C, 940 °C, 960 °C, 980 °C, 1000 °C at a heating rate of 2 °C / min for sintering, hold for 2 h, and then cool naturally to room temperature. After the surface of the sintered ceramic sheet is polished flat with sandpaper, silver is applied using the screen - printing process and fired at 600 °C for 10 min. Finally, immerse the ceramic sheet in silicone oil at 120 °C and polarize it at a DC voltage of 4 kV / mm for 10 min. Place the polarized sample at room temperature for 24 h and then conduct electrical property tests.

[0056] Table 1 shows the compositions of the piezoelectric ceramics prepared in Examples 1 - 8 and the test results of the electrical properties of the ceramics sintered at the optimal temperature, including the piezoelectric constant d 33 , the planar electromechanical coupling coefficient k p , the Curie temperature T C , the room - temperature dielectric constant ε r at 1 kHz, and the loss tanδ. It can be seen that LiF doping significantly improves the piezoelectric properties, Curie temperature, and room - temperature dielectric constant of the ceramics. Mn doping slightly reduces the Curie temperature. Example 4 sintered at 920 °C shows the best comprehensive performance, with high piezoelectricity and a high Curie temperature: d 33 = 208 pC / N, k p = 34%, T C = 550 °C, ε r = 872, tanδ = 3.49%. Ts in Table 1 represents the sintering temperature.

[0057] Table 1 Test parameters of the compositions and electrical properties of the piezoelectric ceramics in Examples 1 - 8

[0058]

[0059] In Examples 1 - 4 of the present invention, the piezoelectric constant d 33 of the ceramics obtained at different sintering temperatures is as Figure 1 shown. It can be seen that both Example 2 and Example 4 have relatively high piezoelectric constants in a relatively wide sintering temperature range. Especially for Example 4, in the sintering temperature range of 880 - 1020 °C, the piezoelectric constant d 33= 180 - 208 pC / N, and has a high Curie temperature (T C = 550 °C), and a low optimal sintering temperature (920 °C). Figure 2 Figure Figure 2 is the SEM image of the piezoelectric ceramic material prepared in Example 4 of the present invention after sintering at 920 °C, polishing, and thermal etching. It can be seen from the figure that the ceramic material has a dense and uniform microstructure, and most of the grain sizes are distributed between 6 - 10 μm.

[0060] In Examples 5 - 8 of the present invention, the piezoelectric constants d of the ceramics obtained at different sintering temperatures 33 are as Figure 3 shown. It can be seen that both Example 5 and Example 6 have high piezoelectric constants in a relatively wide sintering temperature range. In particular, in Example 5, in the sintering temperature range of 880 - 1000 °C, the piezoelectric constant d 33 = 175 - 201 pC / N, and has a high Curie temperature (T C = 545 °C), and a low optimal sintering temperature (920 °C). Figure 4 Figure Figure 4 is the SEM image of the piezoelectric ceramic material prepared in Example 5 of the present invention after sintering at 920 °C, polishing, and thermal etching. By comparison Figure 2 it can be seen that the increase in the SiO2 content significantly reduces the grain size, and the average grain size ≈ 5.8 μm. However, the porcelain body is denser, so it can be polarized at a higher voltage to obtain better piezoelectric properties.

[0061] The microscopic morphologies of the materials observed by scanning electron microscopy in other examples are similar to Figure 2 and Figure 4 and will not be specifically described herein.

[0062] The above - mentioned embodiments are the preferred embodiments of the present invention. However, the embodiments of the present invention are not limited by the described embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. A lead-free piezoelectric ceramic material of bismuth ferrite-barium titanate, characterized in that: Its composition is 0.7Bi 1.02 FeO3 - 0.3BaTiO3 - x% LiF + y% M + z% SiO2; Among them, M represents one or more of Mn-containing metal compounds MnO2, Mn2O3, or MnCO3; x, y, and z respectively represent the molar fractions of LiF, M, and SiO2, where 0.25 ≤ x ≤ 0.75, 0.35 ≤ y ≤ 0.75, and 0 ≤ z ≤ 0.

3.

2. The bismuth ferrite-barium titanate lead-free piezoelectric ceramic material according to claim 1, wherein: x = 0.50, 0.35 ≤ y ≤ 0.75, and 0 ≤ z ≤ 0.

3.

3. The preparation method of the bismuth ferrite-barium titanate lead-free piezoelectric ceramic material according to any one of claims 1 to 2, characterized in that: It includes the following steps: (1) Select Bi2O3, Fe2O3, BaCO3, TiO2 and LiF as raw materials, and weigh the raw materials according to the stoichiometric ratio of 0.7Bi 1.02 FeO3 - 0.3BaTiO3 - x% LiF for primary ball milling, drying, and pre - sintering to obtain intermediate powder, where x represents the molar fraction of LiF, and 0.25 ≤ x ≤ 0.75; (2) Weigh 0.7Bi according to the chemical formula of the piezoelectric ceramic material 1.02 FeO3 - 0.3BaTiO3 - x% LiF + y% M + z% SiO2 in stoichiometric ratio, and weigh 0.7Bi 1.02 FeO3 - 0.3BaTiO3 - x% LiF ceramic intermediate powder, as well as M and SiO2 powders, perform secondary ball milling, granulation, forming, sintering, upper electrode deposition and poling to obtain a piezoelectric ceramic, where M represents one or more of the Mn-containing metal compounds MnO2, Mn2O3 or MnCO3; y and z respectively represent the molar fractions of M and SiO2, and 0.35 ≤ y ≤ 0.75, 0 ≤ z ≤ 0.

3.

4. The preparation method of the bismuth ferrite-barium titanate lead-free piezoelectric ceramic material according to claim 3, wherein: The polarization described in step (2) refers to high-temperature polarization. The temperature of high-temperature polarization is 100 - 120 °C, the polarization voltage is 3 - 5 kV / mm, and the polarization time is 10 - 20 min; Specifically, the obtained ceramic sheet is immersed in silicone oil at 100 - 120 °C for polarization.

5. The preparation method of the bismuth ferrite-barium titanate lead-free piezoelectric ceramic material according to claim 3, wherein: The sintering described in step (2) is to increase the temperature to 800 °C at a heating rate of 3 - 5 °C / min, and then increase the temperature to 880 - 1020 °C at a heating rate of 1 - 3 °C / min, with a holding time of 2 - 4 h.

6. The preparation method of the bismuth ferrite-barium titanate lead-free piezoelectric ceramic material according to claim 3, characterized in that: The pre-sintering described in step (1) is to increase the temperature from room temperature to 720 - 780 °C at a heating rate of 3 - 5 °C / min, with a holding time of 4 - 6 h.

7. The preparation method of the bismuth ferrite-barium titanate lead-free piezoelectric ceramic material according to claim 3, characterized in that: The medium for the first ball milling described in step (1) is anhydrous ethanol, and the ball milling time is 4 - 8 h; The medium for the second ball milling described in step (2) is anhydrous ethanol, and the ball milling time is 6 - 12 h; The granulation described in step (2) uses an aqueous solution of polyvinyl alcohol with a mass concentration of 5 - 12% as the binder; The forming described in step (2) is to press into a ceramic wafer under a pressure of 50 - 150 MPa; The upper electrode refers to preparing an electrode on the sintered ceramic.

8. Use of the bismuth ferrite-barium titanate lead-free piezoelectric ceramic material according to any one of claims 1 to 2, characterized in that: The bismuth ferrite-barium titanate lead-free piezoelectric ceramic material is used to prepare piezoelectric devices.

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