Porous calcium bismuth titanate piezoelectric ceramic and preparation method thereof

Abstract

The invention belongs to the field of piezoelectric ceramics, and particularly relates to a porous calcium bismuth titanate piezoelectric ceramic and a preparation method thereof. The raw material composition of the porous calcium bismuth titanate piezoelectric ceramic is Ca < 0.94-x > Bi < 4.06 > Ti < 4-x > O < 15-x > mol% CaTiO3, wherein x is greater than or equal to 0 and less than or equal to 1; preferably, 0.34 < = x < = 0.94. The invention provides an innovative pore-forming method, namely pore-forming is carried out by utilizing the density difference before and after the reaction, excellent electrical properties are effectively maintained while the large porosity is obtained, and a new method is provided for obtaining the porous piezoelectric ceramic with the large porosity and the excellent electrical properties at the same time; the porous calcium bismuth titanate piezoelectric ceramic with high piezoelectric coefficient, high electromechanical coupling coefficient and high porosity can be obtained.

Description

Technical Field

[0001] This invention belongs to the field of piezoelectric ceramics, specifically relating to a porous bismuth calcium titanate piezoelectric ceramic and its preparation method. Background Technology

[0002] Piezoelectric materials are functional materials capable of converting electrical energy into mechanical energy and vice versa, and are widely used in fields such as electronic communications, medical devices, and aerospace. Piezoelectric materials are mainly classified into piezoelectric single crystals, piezoelectric ceramics, piezoelectric polymers, and piezoelectric composites. Among them, piezoelectric ceramics occupy a large market share and have a very broad prospect due to their advantages such as low cost, excellent piezoelectric properties, and simple preparation process. Bismuth layered piezoelectric ceramic materials are widely studied due to their high Curie temperature, low aging rate, and high resistivity. Porous piezoelectric ceramics are ceramic materials with high porosity, possessing many superior properties compared to traditional dense piezoelectric ceramics. The high-capacity air phase gives them good thermal and sound insulation properties, and their low dielectric constant and high piezoelectric voltage constant also make porous piezoelectric ceramics widely applicable in various electronic devices, making them important materials for the fabrication of underwater acoustic transducers, ultrasonic transducers, sensors, and other devices.

[0003] Currently, common preparation methods for porous piezoelectric ceramics can be divided into template methods and foaming methods. Template methods use a template as the source of bubble shape and structure, including sacrificial template methods and replication template methods. Foaming methods form bubbles in situ through specific reactions. The simplest method in the sacrificial template method is the pore-forming agent method, which involves mixing piezoelectric ceramic powder with a pore-forming agent (such as polymethyl methacrylate, stearic acid, and starch), then dry-pressing it into powder, and finally removing the pore-forming agent at high temperature, leaving pores in the piezoelectric ceramic. Foaming methods utilize the addition of specific bubble-generating substances to the ceramic composition, trapping the bubbles within the ceramic matrix before they burst or disappear, and finally sintering or drying to obtain porous ceramics. These common pore-forming methods generally require the introduction of organic materials to create pores. These materials may generate defects and microcracks during removal, affecting electrical properties and typically reducing the piezoelectric coefficient and electromechanical coupling coefficient. Pore-forming methods without introducing organic materials, such as extrusion molding, cannot form pores at the microscale. Therefore, there is an urgent need for a new pore-forming method to prepare porous piezoelectric ceramics with excellent electrical properties without introducing other organic substances. Summary of the Invention

[0004] To address the issue that existing porous piezoelectric ceramics cannot simultaneously meet specific performance and porosity requirements, this invention provides a porous bismuth calcium titanate piezoelectric ceramic and its preparation method. This invention proposes an innovative pore-forming method, utilizing the density difference before and after the reaction to create pores. This method effectively maintains excellent electrical properties while achieving high porosity, providing a new approach to obtaining porous piezoelectric ceramics with both high porosity and excellent electrical properties. It can yield porous bismuth calcium titanate piezoelectric ceramics with high piezoelectric coefficient, high electromechanical coupling coefficient, and high porosity.

[0005] In a first aspect, the present invention provides a porous bismuth-calcium titanate piezoelectric ceramic. The raw material composition of the porous bismuth-calcium titanate piezoelectric ceramic is Ca. 0.94-x Bi 4.06 Ti 4-x O 15 -x mol% CaTiO3, where 0≤x≤1; preferably, 0.34≤x≤0.94.

[0006] Preferably, the porous bismuth calcium titanate piezoelectric ceramic has a pore size of 2–8 μm and a porosity of 12.4–25.1%.

[0007] Preferably, the porous bismuth-calcium titanate piezoelectric ceramic has a room-temperature piezoelectric coefficient of 19.8–20.5 pC / N, a Curie temperature >775℃, and a piezoelectric voltage coefficient of 18 × 10⁻⁶. -3 ~30.4×10 -3 Vm / N, thickness electromechanical coupling coefficient k t The radial electromechanical coupling coefficient k ranges from 20.2% to 29.7%. p The percentage ranges from 10.1% to 13.2%.

[0008] Secondly, this invention provides a method for preparing porous bismuth-calcium titanate piezoelectric ceramics. By using Ca... 0.94- x Bi 4.06 Ti 4-x O 15 The powder and the first sintered CaTiO3 ceramic powder are processed according to the porous bismuth titanate calcium piezoelectric ceramic Ca 0.94- x Bi 4.06 Ti 4-x O 15 The materials were weighed and mixed at a stoichiometric ratio of -x mol% CaTiO3, and then subjected to a second sintering. The CaTiO3 ceramic powder and Ca 0.94-x Bi 4.06 Ti 4-x O 15The volume of the bismuth calcium titanate ceramic generated after the second sintering of the powder is smaller than the volume of the CaTiO3 ceramic powder that has been sintered in the first sintering, resulting in a portion of the original volume of the CaTiO3 ceramic powder after the first sintering not being occupied, thus obtaining the porous bismuth calcium titanate piezoelectric ceramic.

[0009] Bismuth calcium titanate ceramics are produced by reacting CaTiO3 with Ca... 0.94-x Bi 4.06 Ti 4-x O 15 The reaction typically begins at 700℃ and completes at around 1000℃. The theoretical density of the sintered CaTiO3 (reactant) ceramic is 4.1 g / cm³. 3 And CaTiO3 and Ca 0.94-x Bi 4.06 Ti 4-x O 15 The theoretical density of the resulting bismuth calcium titanate ceramic is 6.9 g / cm³. 3 Based on this, the present invention designs the raw material composition of bismuth calcium titanate piezoelectric ceramics and utilizes the density difference before and after the reaction to create pores. This allows for the preparation of porous piezoelectric ceramics without the need for any additional organic compounds, thus eliminating the issue of whether organic matter is completely removed. This results in superior electrical properties, providing a new approach to achieving porous piezoelectric ceramics that balance porosity and electrical performance.

[0010] Preferably, the preparation method includes: performing a first sintering of CaCO3 and TiO2 to obtain CaTiO3 ceramic powder; using Bi2O3, CaCO3, and TiO2 as raw materials, according to Ca... 0.94-x Bi 4.06 Ti 4-x O 15 Weigh the above raw materials according to the stoichiometric ratio and synthesize Ca. 0.94-x Bi 4.06 Ti 4-x O 15 Powder; CaTiO3 ceramic powder and Ca 0.94-x Bi 4.06 Ti 4-x O 15 Powder according to porous bismuth titanate calcium piezoelectric ceramic Ca 0.94-x Bi 4.06 Ti 4-x O 15 The materials were weighed and mixed at a stoichiometric ratio of -x mol% CaTiO3, and then subjected to a second sintering to obtain the porous bismuth-calcium titanate piezoelectric ceramic.

[0011] Preferably, the first sintering temperature is 1400–1600℃, and the first sintering time is 2–6 hours; the synthesis temperature is 700–900℃, and the synthesis time is 2–4 hours; the second sintering temperature is 1100–1200℃, and the second sintering time is 1–3 hours.

[0012] Preferably, the preparation method further includes: coating the porous bismuth calcium titanate piezoelectric ceramic with platinum, drying and calcining it to cure it with platinum, and then polarizing it.

[0013] Preferably, the platinum curing conditions are: heating to 800-900°C at a heating rate not exceeding 2°C / min and holding at that temperature for 60 minutes or less, then cooling to room temperature; the polarization conditions are: polarization at 14-16 kV / mm at 160-200°C for 10-20 minutes.

[0014] Preferably, the preparation method further includes: before the second sintering, adding CaTiO3 ceramic powder and Ca... 0.94- x Bi 4.06 Ti 4-x O 15 Powder according to porous bismuth titanate calcium piezoelectric ceramic Ca 0.94-x Bi 4.06 Ti 4-x O 15 The mixture of -x mol% CaTiO3 was weighed and mixed, and a binder was added to granulate it. The mixture was then pressed and extruded to obtain a green blank, which was subsequently sintered.

[0015] Preferably, the amount of the binder added is 4-8 wt% of the mixture; more preferably, the binder is a polyvinyl alcohol aqueous solution with a mass fraction of 6-8%; the descaling conditions are to heat to 700-800°C at a heating rate not exceeding 2°C / min and hold at that temperature for no more than 3 hours. Attached Figure Description

[0016] Figure 1 X-ray diffraction patterns of porous piezoelectric ceramics prepared in Examples 1-3 and Comparative Examples 1-2; Figure 2 Scanning electron microscope (SEM) images of the porous piezoelectric ceramics prepared in Examples 1-3 and Comparative Examples 1-2; Figure 3 Porosity and piezoelectric coefficient of porous piezoelectric ceramics prepared in Examples 1-3 and Comparative Examples 1-2; Figure 4 The electromechanical coupling coefficient diagrams are shown for the porous piezoelectric ceramics prepared in Examples 1-3 and Comparative Examples 1-2. Figure 5 The resistivity of the porous piezoelectric ceramics prepared in Examples 1-3 and Comparative Examples 1-2 varies with temperature. Detailed Implementation

[0017] The technical solutions in the embodiments of the present invention will be further described with reference to the accompanying drawings and the following embodiments. The following embodiments are for illustrative purposes only and are not intended to limit the invention. All other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of the present invention.

[0018] Existing porous piezoelectric ceramic materials cannot simultaneously meet the requirements of transducers, sensors, and other devices in terms of porosity and electrical properties. To address this, this invention proposes an innovative pore-forming method that utilizes the density difference before and after the reaction to create pores. This method achieves high porosity while effectively maintaining excellent electrical properties, providing a new approach to obtaining porous piezoelectric ceramics with both high porosity and superior electrical performance. The following exemplifies the method for preparing porous bismuth-calcium titanate piezoelectric ceramics without the need for organic additives.

[0019] By Ca 0.94-x Bi 4.06 Ti 4-x O 15 The powder and the first sintered CaTiO3 ceramic powder are processed according to the porous bismuth titanate calcium piezoelectric ceramic Ca 0.94-x Bi 4.06 Ti 4-x O 15 The materials were weighed and mixed at a stoichiometric ratio of -x mol% CaTiO3, and then subjected to a second sintering. The CaTiO3 ceramic powder and Ca 0.94-x Bi 4.06 Ti 4-x O 15 The volume of the bismuth calcium titanate ceramic formed after the second sintering (reaction) of the powder is smaller than the volume of the CaTiO3 ceramic powder after the first sintering. This results in a portion of the original volume of the CaTiO3 ceramic powder from the first sintering not being occupied, thus yielding the chemical composition Ca. 0.94 Bi 4.06 Ti4O 15 Porous bismuth calcium titanate piezoelectric ceramics.

[0020] The porous bismuth calcium titanate piezoelectric ceramic is composed of Ca... 0.94-x Bi 4.06 Ti 4-x O 15-x mol% CaTiO3. Where 0≤x≤1. As an example, but not limited to, x can be 0.34, 0.64, 0.94, etc. If x exceeds the above range, there will be no excess calcium titanate for priority first sintering, and more pore structures cannot be generated, and the porosity will not be optimized. Preferably, 0.34≤x≤0.94. The larger the value of x, the more low-density CaTiO3 ceramic powder is introduced, and the volume of unoccupied CaTiO3 ceramic powder increases, thereby forming more pores. Since this method does not introduce organic matter, it can avoid defects and microcracks caused by the volatilization of organic matter, and can ensure that the breakdown field strength of the porous ceramic prepared by this method will not decrease significantly. At the same time, this method can avoid the phenomenon of electric field concentration to a certain extent, thereby reducing the voltage division effect of pores and ensuring that the polarization voltage can act on the piezoelectric phase component in the ceramic as much as possible rather than the pores (since the pores are filled with air, the relative permittivity of air is 1, which is much smaller than the permittivity of the piezoelectric phase prepared by this invention, and the low-capacitance material will share more voltage). The porous bismuth-calcium titanate piezoelectric ceramics obtained in this invention within the above-mentioned x value range all have a chemical composition of Ca. 0.94 Bi 4.06 Ti4O 15 .

[0021] Taking the comparative example, the synthesis pathway of bismuth calcium titanate is as follows: (1) CaCO3=CaO+CO2; (2) 2.03Bi2O3+3.06TiO2=Bi 4.06 Ti 3.06 O 12 ; (3)CaO+TiO2=CaTiO3; (4)0.94CaTiO3+Bi 4.06 Ti 3.06 O 12 =Ca 0.94 Bi 4.06 Ti4O 15 It can be seen that CaTiO3 and Bi 406 Ti 3.06 O 12 The reaction produces the final product, bismuth calcium titanate. The theoretical density of bismuth calcium titanate ceramic is 6.9 g / cm³. 3 The density is approximately 4.1 g / cm³, while the theoretical density of CaTiO3 ceramics is 4.1 g / cm³. 3 This invention involves first sintering the CaTiO3 in the reactants, and then using the sintered CaTiO3 ceramic powder as a raw material for the reaction, thus obtaining porous piezoelectric ceramics. This is because the sintered CaTiO3 ceramic powder has a low density and occupies a large volume, while the CaTiO3 ceramic powder reacts with Ca... 0.94-x Bi 4.06 Ti 4-x O 15 The Ca produced after the reaction0.94 Bi 4.06 Ti4O 15 Ceramics have a relatively high density and occupy a small volume, resulting in a volume difference after the reaction, thus exhibiting porous ceramic characteristics. This invention utilizes the density difference between reactants and products to prepare porous piezoelectric ceramics. The porous piezoelectric ceramics prepared using this method achieve both high porosity and excellent electrical properties, strongly promoting the application of porous piezoelectric ceramics in transducers and sensors.

[0022] The following exemplarily illustrates the preparation process of the porous bismuth calcium titanate piezoelectric ceramic of the present invention, specifically including batching, mixing, synthesis, fine grinding, granulation, molding, plasticizing, and sintering. Solid-state sintering is employed for preparation.

[0023] According to Ca 0.94-x Bi 4.06 Ti 4-x O 15 (0≤x≤1; preferably, 0≤x≤0.94; more preferably, 0.34≤x≤0.94) Bi₂O₃, CaCO₃, and TiO₂ powders are weighed according to stoichiometry, and after mixing and synthesis, Ca is obtained. 0.94-x Bi 4.06 Ti 4-x O 15 Powder.

[0024] The mixture is prepared by wet planetary ball milling. In this wet planetary ball milling, the mass ratio of raw material: anhydrous ethanol: milling media is 1:0.5–0.9:1.2–1.8. The mixing time can be 2–6 hours. The milling media can be agate balls. The milled mixture is then dried in a constant temperature oven at 50–100°C.

[0025] The synthesis involves holding the mixture at 700–900°C (preferably 800–900°C) for 2–4 hours. Preferably, the temperature is increased to 800–900°C at a rate not exceeding 2°C / min, held for 1–3 hours, and then cooled to room temperature in the furnace to obtain Ca. 0.94- x Bi 4.06 Ti 4-x O 15 Powder.

[0026] According to the stoichiometric ratio of CaTiO3, CaCO3 and TiO2 are weighed and sintered at high temperature (first sintering) to obtain CaTiO3 ceramic powder with a certain strength.

[0027] The CaTiO3 ceramic powder can be obtained by heating it to 1400–1600℃ (e.g., 1550℃) in a high-temperature furnace at a heating rate not exceeding 2℃ / min and holding it at that temperature for 2–6 hours, then cooling it to room temperature in the furnace. Alternatively, it can be obtained by crushing it after sintering.

[0028] CaTiO3 ceramic powder and Ca 0.94-x Bi 4.06 Ti 4-x O 15 Powder according to porous bismuth titanate calcium piezoelectric ceramic Ca 0.94- x Bi 4.06 Ti 4-x O 15 Weigh and mix materials according to the stoichiometric ratio of -x mol% CaTiO3 to obtain a mixture. The CaTiO3 ceramic powder constitutes a significant portion of the Ca content. 0.94-x Bi 4.06 Ti 4-x O 15 The molar ratio of the powder is 0–0.94%. CaTiO3 ceramic powder can be added to Ca... 0.94- x Bi 4.06 Ti 4-x O 15 in powder.

[0029] The mixture is subjected to a second ball milling (also known as fine grinding) and then dried. In the second ball milling, the mass ratio of raw material: anhydrous ethanol: milling media can be 1:0.5–0.9:1.2–1.8. The fine grinding time can be 4–8 hours. The milling media can be agate balls. After the second planetary ball milling, the mixture is dried at 50–100°C.

[0030] A binder is added to the mixture for granulation, and after aging, it is pressed into shape and plasticized to obtain a green blank.

[0031] The binder includes, but is not limited to, a 6-8% (w / w) aqueous solution of polyvinyl alcohol (PVA). For example, a 7% (w / w) aqueous solution of polyvinyl alcohol can be used. Preferably, the amount of binder added is 4-8 wt% of the mixture.

[0032] The compression molding method is not limited; any molding method commonly used in the field can be used. The extrusion molding involves heating the powder to 700–800°C at a heating rate not exceeding 2°C / min and holding it at that temperature for no more than 3 hours. For example, the granulated powder is compressed into discs with a diameter of 13 mm and a thickness of 0.5 mm using a pressure of 100–200 MPa. The compressed discs are then heated to 700–800°C at a heating rate not exceeding 2°C / min and held at that temperature for no more than 3 hours, and then cooled to room temperature in the furnace.

[0033] The green blank is sintered. The sintering process involves heating to 850–950°C at a rate not exceeding 3°C / min, then heating to 1100–1200°C at a rate not exceeding 2°C / min, holding at that temperature for 1–3 hours, and then cooling to room temperature in the furnace. For example, the green blank after descaling is placed in a five-sided heating furnace for sintering. To reduce the volatilization of bismuth oxide at high temperatures, CaBi4Ti4O is used. 15 Ceramic powder was used as a filler and then sintered to obtain porous bismuth calcium titanate piezoelectric ceramic sheets.

[0034] The porous bismuth-calcium titanate piezoelectric ceramic is subjected to platinum coating, drying, and platinum calcination for curing, followed by polarization. For example, the sintered ceramic sheet is processed to the required size, cleaned, screen-printed with platinum paste, dried, calcined, and then electrodes are applied and polarized to obtain the porous piezoelectric ceramic. The platinum calcination involves heating to 800–900°C at a rate not exceeding 2°C / min and holding at that temperature for no more than 60 minutes, followed by cooling to room temperature. The polarization can be performed at 14–16 kV / mm at 160–200°C (e.g., 200°C) for 10–20 minutes.

[0035] The porous piezoelectric bismuth calcium titanate ceramic exhibits a single bismuth layer structure.

[0036] The porous bismuth calcium titanate piezoelectric ceramic exhibits a disordered pore structure. The piezoelectric ceramic itself exhibits anisotropic grain growth, and this characteristic of different growth directions leads to significant non-uniformity in the pore direction, i.e., a disordered pore structure. The pore size of the porous bismuth calcium titanate ceramic is approximately 2–8 μm; the porosity can be 9.1–25.1%.

[0037] The porous bismuth-calcium titanate piezoelectric ceramic has a room temperature piezoelectric coefficient of 19.8–20.5 pC / N, a Curie temperature >775℃, and a piezoelectric voltage coefficient of 18 × 10⁻⁶. -3 ~30.4×10 -3 Vm / N, thickness electromechanical coupling coefficient k t The radial electromechanical coupling coefficient k ranges from 20.2% to 29.7%. p The piezoelectric content is 10.1% to 13.2%. Preferably, the porous piezoelectric bismuth-calcium titanate ceramic has a piezoelectric coefficient of 20 to 20.5 pC / N, a Curie temperature >775℃, and a piezoelectric voltage constant of 21.4 × 10⁻⁶. -3 ~30.4×10 -3 Vm / N, the thickness electromechanical coupling coefficient is 25.2% to 29.7%, and the radial electromechanical coupling coefficient is 10.5% to 13.2%.

[0038] In summary, this invention utilizes the density difference between reactants and products to prepare porous piezoelectric ceramics. First, CaTiO3 in the reactants is sintered, and then the sintered CaTiO3 ceramic powder is used as a raw material for the reaction. Because the sintered CaTiO3 ceramic powder has a low density, it occupies a large volume, and the CaTiO3 ceramic powder reacts with Ca... 0.94-x Bi 4.06 Ti 4-x O 15 The Ca produced after the reaction 0.94 Bi 4.06 Ti4O 15 Ceramics have a relatively high density and occupy a small volume, resulting in a volume difference after the reaction, thus exhibiting porous ceramic characteristics. The porous piezoelectric ceramics prepared using this method achieve both high porosity and excellent electrical properties, strongly promoting the application of porous piezoelectric ceramics in transducers and sensors. For example, the porous Ca prepared by the method described in this invention... 0.94 Bi 4.06 Ti4O 15 Ceramics and dense Ca 0.94 Bi 4.06 Ti4O 15 Ceramics (piezoelectric coefficient 19.8 pC / N, piezoelectric voltage constant 18 × 10⁻⁶) -3 Compared to porous piezoelectric ceramics prepared by other pore-forming methods (with a thickness electromechanical coupling coefficient of 20.2% and a radial electromechanical coupling coefficient of 10.1%), the piezoelectric properties are improved, and the electrical properties do not decline as they do in porous piezoelectric ceramics prepared by other pore-forming methods. This further demonstrates the superiority of the pore-forming method of this invention.

[0039] The following examples further illustrate the present invention in detail. Similarly, the following embodiments are only for further explanation of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention are within the scope of protection of the present invention.

[0040] Example 1

[0041] The porous bismuth calcium titanate Ca 0.94 Bi 4.06 Ti4O 15 Methods for preparing piezoelectric ceramics include:

[0042] Step 1. Using Bi₂O₃, CaCO₃, and TiO₂ powders as raw materials, according to Ca... 0.94-x Bi 4.06 Ti 4-x O 15The raw materials were prepared according to a stoichiometric ratio of (x = 0.34), and then sequentially placed into a ball mill jar for ball milling and mixing. The mass ratio of raw materials, anhydrous ethanol, and milling media (agate balls) was 1:0.65:1.5. The mixture was wet-milled using a planetary ball mill for 4 hours to ensure homogeneity. The mixed raw materials were dried at 80℃, passed through a 40-mesh sieve, and pressed into blocks using a press. These blocks were then placed in an alumina crucible and heated to 850℃ in a high-temperature furnace at a heating rate of 2℃ / min, and held at that temperature for 2 hours. After cooling in the furnace, the desired Ca was obtained. 0.94- x Bi 4.06 Ti 4-x O 15 Block.

[0043] Step 2. Weigh the raw materials CaCO3 and TiO2 according to the stoichiometric ratio of CaTiO3, heat them directly in a high-temperature furnace to 1550℃ at a heating rate of 2℃ / min and hold for 4 hours. After cooling to room temperature in the furnace, take them out and crush them to obtain CaTiO3 ceramic powder.

[0044] Step 3. Take the Ca synthesized in Step 1 0.94-x Bi 4.06 Ti 4-x O 15 After the bulk material is mechanically crushed and passed through a 40-mesh sieve, it is mixed with the sintered CaTiO3 ceramic powder according to the Ca... 0.94-x Bi 4.06 Ti 4-x O 15 -x mol% CaTiO3 (x = 0.34) were mixed in a stoichiometric ratio and finely ground using a wet planetary ball mill. The mixture was ball-milled for 6 hours at a mass ratio of raw material: anhydrous ethanol: milling media of 1:0.6:1.5 to ensure homogeneity, resulting in a finely ground powder. Then, a binder comprising 6 wt.% of the finely ground powder was added for further grinding and granulation. The binder was a 7 wt.% PVA aqueous solution. The granulated powder was pressed into large blocks and allowed to stand for 24 hours for aging. After aging, the powder was crushed and ground in a mortar and passed through a 40-mesh sieve to obtain a granular powder with a certain degree of flowability. The powder was then pressed into circular blanks with a diameter of 13 mm using a mold in a press. These blanks were then heated to 750°C in a low-temperature furnace at a heating rate of 2°C / min and held for 2 hours to obtain a raw blank.

[0045] Step 4. Use CaBi4Ti4O 15Ceramic powder is placed as filler in a sealed alumina crucible (with the ceramic blank above the filler), and then placed in a high-temperature furnace and heated to 900°C at a heating rate of 3°C / min, and then heated to 1150°C at a heating rate of 2°C / min. The temperature is held for 2 hours and then cooled to room temperature with the furnace to obtain the desired ceramic sheet.

[0046] Step 5. The obtained ceramic sheet is processed to a thickness of 0.5 mm, then ultrasonically cleaned, dried, and coated with platinum on both sides using a screen. The temperature is then increased to 850 °C at a rate of 2 °C / min and held for 30 minutes to cure the platinum slurry. Finally, polarization is performed. The polarization conditions are: applying a 15 kV / mm electric field to silicone oil at 200 °C for 10 minutes to obtain the porous bismuth-calcium titanate piezoelectric ceramic.

[0047] Example 2

[0048] Example 2 is basically the same as Example 1, except that x = 0.64.

[0049] Example 3

[0050] Example 3 is basically the same as Example 1, except that x = 0.94.

[0051] Comparative Example 1

[0052] The porous bismuth calcium titanate Ca 0.94 Bi 4.06 Ti4O 15 Methods for preparing piezoelectric ceramics include:

[0053] Step 1. Using Bi₂O₃, CaCO₃, and TiO₂ powders as raw materials, according to Ca... 0.94 Bi 4.06 Ti4O 15 The raw materials were prepared according to the stoichiometric ratio and then sequentially placed into a ball mill jar for ball milling and mixing. The mass ratio of raw materials, anhydrous ethanol, and milling media (agate balls) was 1:0.65:1.5. The mixture was wet-milled using a planetary ball mill for 4 hours to ensure homogeneity. The mixed raw materials were dried at 80℃, passed through a 40-mesh sieve, and pressed into blocks using a press. These blocks were then placed in an alumina crucible and heated to 850℃ in a high-temperature furnace at a rate of 2℃ / min, and held at that temperature for 2 hours. After cooling in the furnace, the desired Ca was obtained. 0.94-x Bi 4.06 Ti 4-x O 15 Block.

[0054] Step 2. Take the Ca synthesized in Step 1... 0.94 Bi 4.06 Ti4O 15After the bulk material was mechanically crushed and passed through a 40-mesh sieve, it was finely ground using a wet planetary ball mill. The raw material, anhydrous ethanol, and milling media were mixed at a mass ratio of 1:0.6:1.5 for 6 hours to ensure uniform mixing, resulting in a finely ground powder. Then, a binder comprising 6 wt.% of the finely ground powder was added for further grinding and granulation. The binder was a 7 wt.% PVA aqueous solution. The granulated powder was pressed into large blocks and allowed to stand for 24 hours for aging. After aging, it was crushed and ground in a mortar and passed through a 40-mesh sieve to obtain a granular powder with a certain degree of flowability. Next, the powder was pressed into circular blanks with a diameter of 13 mm using a mold in a press. These blanks were then heated to 750°C in a low-temperature furnace at a heating rate of 2°C / min and held for 2 hours to obtain a raw blank.

[0055] Step 3. Use CaBi4Ti4O 15 Ceramic powder is placed as filler in a sealed alumina crucible (with the ceramic blank above the filler), and then placed in a high-temperature furnace and heated to 900°C at a heating rate of 3°C / min, and then heated to 1150°C at a heating rate of 2°C / min. The temperature is held for 2 hours and then cooled to room temperature with the furnace to obtain the desired ceramic sheet.

[0056] Step 4. The obtained ceramic sheet is processed to a thickness of 0.5 mm, then ultrasonically cleaned, dried, and coated with platinum on both sides using a screen. The temperature is then increased to 850 °C at a rate of 2 °C / min and held for 30 minutes to cure the platinum slurry. Finally, polarization is performed. The polarization conditions are: applying a 15 kV / mm electric field to silicone oil at 200 °C for 10 minutes to obtain the porous piezoelectric ceramic.

[0057] Comparative Example 2

[0058] The porous bismuth calcium titanate Ca 0.94 Bi 4.06 Ti4O 15 Methods for preparing piezoelectric ceramics include:

[0059] Step 1. Using Bi₂O₃, CaCO₃, and TiO₂ powders as raw materials, according to Ca... 0.94 Bi 4.06 Ti4O 15 The raw materials were prepared according to the stoichiometric ratio and then sequentially placed into a ball mill jar for ball milling and mixing. The mass ratio of raw materials, anhydrous ethanol, and milling media (agate balls) was 1:0.65:1.5. The mixture was wet-milled using a planetary ball mill for 4 hours to ensure homogeneity. The mixed raw materials were dried at 80℃, passed through a 40-mesh sieve, and pressed into blocks using a press. These blocks were then placed in an alumina crucible and heated to 850℃ in a high-temperature furnace at a rate of 2℃ / min, and held at that temperature for 2 hours. After cooling in the furnace, the desired Ca was obtained. 0.94-x Bi4.06 Ti 4-x O 15 Block.

[0060] Step 2. Take the Ca synthesized in Step 1... 0.94 Bi 4.06 Ti4O 15 After the bulk material is mechanically crushed and passed through a 40-mesh sieve, it is mixed with the Ca... 0.94 Bi 4.06 Ti4O 15 4.7 wt% of polymethyl methacrylate (PMMA) was mixed and finely ground using a wet planetary ball mill. The mixture was ball-milled for 6 hours at a mass ratio of raw material: anhydrous ethanol: milling media of 1:0.6:1.5 to ensure homogeneity, resulting in a finely ground powder. Then, a binder comprising 6 wt.% of the finely ground powder (a 7 wt.% PVA aqueous solution) was added for further grinding and granulation. The granulated powder was pressed into large blocks and allowed to stand for 24 hours for aging. After aging, the powder was crushed and ground in a mortar, then passed through a 40-mesh sieve to obtain a granular powder with a certain degree of flowability. The powder was then pressed into circular blanks with a diameter of 13 mm using a mold in a press. These blanks were then heated to 750°C in a low-temperature furnace at a rate of 2°C / min and held for 2 hours to obtain a raw blank.

[0061] Step 3. Use CaBi4Ti4O 15 Ceramic powder is placed as filler in a sealed alumina crucible (with the ceramic blank above the filler), and then placed in a high-temperature furnace and heated to 900°C at a heating rate of 3°C / min, and then heated to 1150°C at a heating rate of 2°C / min. The temperature is held for 2 hours and then cooled to room temperature with the furnace to obtain the desired ceramic sheet.

[0062] Step 4. The obtained ceramic sheet is processed to a thickness of 0.5 mm, then ultrasonically cleaned, dried, and coated with platinum on both sides using a screen. The temperature is then increased to 850 °C at a rate of 2 °C / min and held for 30 minutes to cure the platinum slurry. Finally, polarization is performed. The polarization conditions are: applying a 5 kV / mm electric field to silicone oil at 200 °C for 10 minutes to obtain the porous piezoelectric ceramic.

[0063] The polarized porous piezoelectric ceramic was tested.

[0064] The phase structure of porous piezoelectric ceramics was analyzed using an Aeris X-ray diffractometer from PANalyical. Figure 1 X-ray diffraction patterns of the porous piezoelectric ceramics prepared in Examples 1-3 and Comparative Examples 1-2. Figure 1As can be seen from the above, the porous piezoelectric ceramics prepared by density difference pore formation and the porous piezoelectric ceramics prepared by PMMA pore formation both exhibit a single bismuth layer structure with no obvious impurity phases. The main peak is (119) peak, which is consistent with the characteristic peak of a four-layer bismuth layer structure.

[0065] The microstructure of porous piezoelectric ceramics was analyzed using a Hitachi TM3000 scanning electron microscope. Figure 2 The images show cross-sectional scanning electron microscope (SEM) images of the porous piezoelectric ceramics prepared in Examples 1-3 and Comparative Examples 1-2. Figure 2 As can be seen from the cross-section, the number of pores gradually increases with the increase of CaTiO3 ceramic powder content, exhibiting a disordered pore morphology. Furthermore, no obvious CaTiO3 grains are observed in the cross-sectional image, indicating that CaTiO3 reacts with Ca during the sintering process. 0.94-x Bi 4.06 Ti 4-x O 15 The reaction was observed. However, in Comparative Example 2, the PMMA pore-forming method resulted in piezoelectric ceramic cross-sections with noticeable microcracks.

[0066] The ZJ-3A quasi-static d-type instrument produced by the Institute of Acoustics, Chinese Academy of Sciences is used. 33 The tester measures the d of piezoelectric ceramics at room temperature. 33 The test frequency was 100Hz. Porosity was tested according to GB / T 25995. Figure 3 The diagram shows the porosity and piezoelectric coefficient of the porous piezoelectric ceramics prepared in Examples 1-3 and Comparative Examples 1-2. Figure 3 As can be seen, with the increase of porosity, the piezoelectric coefficient of the pore-forming method of this invention does not show a decreasing trend like that of the PMMA pore-forming method, but remains at the original level, reaching a maximum of 20.5 pC / N. This is because: on the one hand, this invention does not introduce organic materials, avoiding defects and microcracks caused by the volatilization of organic materials, which ensures that the breakdown field strength does not decrease significantly; on the other hand, the pore-forming method of this invention avoids the phenomenon of electric field concentration around the pores to a certain extent, which is conducive to the normal application of polarization voltage to the piezoelectric phase. The combined effect of the above two aspects results in the piezoelectric performance of this invention not showing a decreasing trend like that of piezoelectric ceramics prepared by other pore-forming methods.

[0067] The electromechanical coupling coefficient k of porous piezoelectric ceramics was measured using a Keysight 4990A precision impedance analyzer. t and k p . Figure 4 This is a diagram showing the electromechanical coupling coefficients of the porous piezoelectric ceramics prepared in Examples 1-3 and Comparative Examples 1-2. From... Figure 4As can be seen from the data, with the increase of porosity, both the thickness electromechanical coupling coefficient and the radial electromechanical coupling coefficient of the porous piezoelectric ceramic of the present invention increase, which further demonstrates the superiority of the pore-forming method.

[0068] The DC resistivity of porous piezoelectric ceramics was measured using the HRMS-1000I high-temperature resistance testing system developed by Bailibo, connected to a Keithley 6517B electrometer / high resistance meter. Figure 5 The graph shows the DC resistivity of the porous piezoelectric ceramics prepared in Examples 1-3 and Comparative Examples 1-2 as a function of temperature. Figure 5 As can be seen from the present invention, the DC resistivity of the porous piezoelectric ceramic is significantly improved at high temperature. At 700℃, the DC resistivity is increased by about one order of magnitude. This is because the pores inhibit the migration of charge carriers. After PMMA pores are formed, the DC resistivity decreases significantly, which may be due to the increase in defect concentration caused by the volatilization of organic matter.

[0069] The Curie temperature T of porous piezoelectric ceramics shall be tested according to GB / T 3389.3. c The capacitance C of the porous piezoelectric ceramics was tested using a Keysight 4990A precision impedance analyzer. The performance test results of the porous piezoelectric ceramics in each embodiment and comparative example of this invention are shown in Table 1.

[0070] Table 1 Performance Test Table for Porous Piezoelectric Ceramic Materials

[0071] As shown in Table 1, the porosity of porous piezoelectric ceramics gradually increases with the increase of CaTiO3 ceramic powder content, reaching a maximum of 25.1%. Meanwhile, at a constant Curie temperature, the piezoelectric coefficient and electromechanical coupling coefficient of the porous piezoelectric ceramics remain high, showing no decrease but rather an increase. Furthermore, the increase in porosity leads to a sharp decrease in capacitance, which is beneficial for improving the piezoelectric voltage constant and device sensitivity. However, for porous piezoelectric ceramics obtained by PMMA pore formation, their electrical properties deteriorate significantly, with a piezoelectric coefficient of only 12.6 pC / N.

Claims

1. A porous bismuth calcium titanate piezoelectric ceramic, characterized in that, The raw material composition of the porous bismuth titanate calcium piezoelectric ceramic is Ca. 0.94-x Bi 4.06 Ti 4-x O 15 -x mol% CaTiO3, where 0≤x≤1; preferably, 0.34≤x≤0.

94.

2. The porous bismuth calcium titanate piezoelectric ceramic according to claim 1, characterized in that, The porous bismuth-calcium titanate piezoelectric ceramic has a pore size of 2–8 μm and a porosity of 12.4–25.1%.

3. The porous bismuth-calcium titanate piezoelectric ceramic according to claim 1 or 2, characterized in that, The porous bismuth-calcium titanate piezoelectric ceramic has a room temperature piezoelectric coefficient of 19.8–20.5 pC / N, a Curie temperature >775℃, and a piezoelectric voltage coefficient of 18 × 10⁻⁶. -3 ~30.4×10 -3 Vm / N, thickness electromechanical coupling coefficient k t The radial electromechanical coupling coefficient k ranges from 20.2% to 29.7%. p The percentage ranges from 10.1% to 13.2%.

4. The method for preparing porous bismuth-calcium titanate piezoelectric ceramics according to any one of claims 1 to 3, characterized in that, By Ca 0.94-x Bi 4.06 Ti 4-x O 15 The powder and the first sintered CaTiO3 ceramic powder are processed according to the porous bismuth titanate calcium piezoelectric ceramic Ca 0.94-x Bi 4.06 Ti 4-x O 15 The materials were weighed and mixed at a stoichiometric ratio of -x mol% CaTiO3, and then subjected to a second sintering. The CaTiO3 ceramic powder and Ca 0.94-x Bi 4.06 Ti 4-x O 15 The volume of the bismuth calcium titanate ceramic generated after the second sintering of the powder is smaller than the volume of the CaTiO3 ceramic powder that has been sintered in the first sintering, resulting in a portion of the original volume of the CaTiO3 ceramic powder after the first sintering not being occupied, thus obtaining the porous bismuth calcium titanate piezoelectric ceramic.

5. The method for preparing porous bismuth calcium titanate piezoelectric ceramic according to claim 4, characterized in that, The preparation method includes: performing a first sintering of CaCO3 and TiO2 to obtain CaTiO3 ceramic powder; using Bi2O3, CaCO3, and TiO2 as raw materials, according to Ca... 0.94-x Bi 4.06 Ti 4-x O 15 Weigh the above raw materials according to the stoichiometric ratio and synthesize Ca. 0.94-x Bi 4.06 Ti 4-x O 15 Powder; CaTiO3 ceramic powder and Ca 0.94-x Bi 4.06 Ti 4-x O 15 Powder according to porous bismuth titanate calcium piezoelectric ceramic Ca 0.94-x Bi 4.06 Ti 4- x O 15 The materials were weighed and mixed at a stoichiometric ratio of -x mol% CaTiO3, and then subjected to a second sintering to obtain the porous bismuth-calcium titanate piezoelectric ceramic.

6. The method for preparing porous bismuth calcium titanate piezoelectric ceramic according to claim 5, characterized in that, The first sintering temperature is 1400–1600℃, and the first sintering time is 2–6 hours; The synthesis temperature is 700–900℃, and the synthesis time is 2–4 hours; The second sintering temperature is 1100-1200℃, and the second sintering time is 1-3 hours.

7. The preparation method according to any one of claims 4 to 6, characterized in that, The preparation method further includes: coating the porous bismuth calcium titanate piezoelectric ceramic with platinum, drying and calcining it with platinum, and then polarizing it.

8. The preparation method according to claim 7, characterized in that, The platinum curing conditions are as follows: heating to 800-900℃ at a heating rate not exceeding 2℃ / min and holding for less than 60 minutes, then cooling to room temperature; the polarization conditions are as follows: polarization at 160-200℃ for 10-20 minutes at 14-16kV / mm.

9. The preparation method according to any one of claims 4 to 8, characterized in that, The preparation method further includes: before the second sintering, adding CaTiO3 ceramic powder and Ca... 0.94-x Bi 4.06 Ti 4-x O 15 Powder according to porous bismuth titanate calcium piezoelectric ceramic Ca 0.94-x Bi 4.06 Ti 4-x O 15 The mixture of -x mol% CaTiO3 was weighed and mixed, and a binder was added to granulate it. The mixture was then pressed and extruded to obtain a green blank, which was subsequently sintered.

10. The preparation method according to claim 9, characterized in that, The amount of the binder added is 4-8 wt% of the mixture; preferably, the binder is a polyvinyl alcohol aqueous solution with a mass fraction of 6-8%; the descaling conditions are to heat to 700-800°C at a heating rate not exceeding 2°C / min and hold at that temperature for no more than 3 hours.

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