阅读说明：本技术 一种具有高压电性能和高剩余极化强度铌酸钾钠陶瓷的制备方法 (Preparation method of potassium-sodium niobate ceramic with high piezoelectric property and high remanent polarization ) 是由 高大强 张曙光 廖忠新 于 2020-12-25 设计创作，主要内容包括：本发明公开了一种具有高压电性能和高剩余极化强度的铌酸钾钠陶瓷及其制备方法。所述无铅压电陶瓷化学通式为：(Na0.5K0.5)NbO3。本发明利用一种5mm和2mm氧化锆球(质量比为1∶2)球磨珠混合球磨方式,采用固相法制备的铌酸钾钠无铅压电陶瓷具有较高的压电常数d33、平面机电耦合系数Kp和微观致密度,其压电常数可达124pC/N,相对密度可达到95.5％。同时该陶瓷的剩余极化强度为23μC/cm2,高于文献里报道的利用传统固相法制备陶瓷(10～15μC/cm2)。该发明对利用传统固相法制备高性能的无铅铌酸钾钠基压电陶瓷具有重要指导意义。(The invention discloses a potassium-sodium niobate ceramic with high piezoelectric property and high remanent polarization and a preparation method thereof. The chemical general formula of the lead-free piezoelectric ceramic is as follows: (Na0.5K0.5) NbO 3. The invention utilizes a ball milling and ball milling mode of 5mm and 2mm zirconia balls (the mass ratio is 1: 2), and the potassium-sodium niobate leadless piezoelectric ceramic prepared by a solid phase method has higher piezoelectric constant d33, planar electromechanical coupling coefficient Kp and microcosmic density, the piezoelectric constant can reach 124pC/N, and the relative density can reach 95.5%. Meanwhile, the remanent polarization of the ceramic is 23 mu C/cm2, which is higher than that of the ceramic prepared by the traditional solid phase method reported in the literature (10-15 mu C/cm 2). The invention has important guiding significance for preparing the high-performance lead-free potassium sodium niobate piezoelectric ceramic by using the traditional solid phase method.)

1. A preparation method of potassium-sodium niobate ceramic with high piezoelectric property and high remanent polarization is characterized by comprising the following steps:

(1) drying materials: putting alkali metal salt K2CO3 and Na2CO3 which are easy to absorb moisture into an oven to remove moisture;

(2) preparing materials: weighing raw materials K2CO3, Na2CO3 and Nb2O5 in turn according to the stoichiometric ratio of (NaO.5KO.5) NbO3, putting the weighed raw materials into a ball milling tank, taking absolute ethyl alcohol as a ball milling medium, mixing zirconia beads with the diameter of 5mm and the diameter of 2mm as ball milling beads, and carrying out primary ball milling in a planetary ball mill to obtain wet-process slurry;

(3) and (3) briquetting and presintering: putting the obtained slurry into an oven to be dried to obtain dry powder, pressing the obtained powder into a cylindrical block by using a mold, conveying the block into a tubular furnace, and presintering in the air for 4 hours;

(4) ball milling for the second time: crushing the block after pre-sintering, transferring the obtained powder into a ball milling tank, taking absolute ethyl alcohol as a ball milling medium, mixing zirconia beads with the diameter of 5mm and the diameter of 2mm as ball milling beads, carrying out secondary ball milling in a planetary ball mill, putting slurry obtained by ball milling into a 60-90 ℃ oven for 6-9 h for drying, carrying out grinding treatment on the dried powder, and passing through a 75-mesh screen to obtain powder with fine granularity and uniform particles;

(5) molding: adding 3-4% by mass of polyvinyl alcohol solution into the powder obtained by grinding and sieving, uniformly mixing the powder with the polyvinyl alcohol solution, putting the powder into an oven at 80-90 ℃ for drying for 8-10 min, grinding the powder, sieving the powder by a 75-mesh sieve, and pressing and molding the powder obtained by sieving by using a mold to obtain a wafer type ceramic green body;

(6) and (3) sintering: placing the obtained ceramic blank into a tubular furnace, and keeping the temperature for 2 hours at 600-700 ℃ for glue removal treatment, and placing the ceramic blank obtained after glue removal into the tubular furnace, and keeping the temperature for 2 hours at 1090 ℃ to obtain a ceramic finished product;

(7) polarization: and carrying out silver coating treatment on the obtained ceramic finished product, keeping the temperature at 750-780 ℃ for 20-30 min, and polarizing the ceramic coated with the silver electrode at 120 ℃ under silicone oil.

2. The preparation method of the potassium-sodium niobate ceramic with high piezoelectric property and high remanent polarization according to claim 1, wherein the temperature of the oven in the step (1) is 200-250 ℃, and the drying time is 2-5 h.

3. The method for preparing the potassium-sodium niobate ceramic with the high piezoelectric property and the high remanent polarization according to claims 1-2, wherein the mass ratio of the zirconia beads with the diameter of 5mm and the zirconia beads with the diameter of 2mm in the step (2) is 1: 2, and the mass ratio of the raw materials, the ball milling beads and the absolute ethyl alcohol is 1: 8: 4.

4. The preparation method of the potassium-sodium niobate ceramic with high piezoelectric property and high remanent polarization according to claims 1 to 3, wherein the rotation speed of the planetary ball mill in the step (2) is 400rpm, and the ball milling time is 10 hours.

5. The preparation method of the potassium-sodium niobate ceramic with high piezoelectric property and high remanent polarization according to claims 1 to 4, wherein the baking temperature of the slurry in the step (3) is 60 to 90 ℃, and the baking time is 6 to 9 hours.

6. The method for preparing the potassium-sodium niobate ceramic with the high piezoelectric property and the high remanent polarization according to claims 1 to 5, wherein the pre-sintering temperature in the tube furnace in the step (3) is 850 ℃.

7. The method for preparing potassium-sodium niobate ceramic with high piezoelectric property and high remanent polarization according to claims 1 to 6, wherein the mass ratio of 5mm diameter zirconia beads to 2mm diameter zirconia beads in step (4) is 1: 2, and the mass ratio of raw materials, ball milling beads and absolute ethyl alcohol is 1: 8: 4.

8. The preparation method of the potassium-sodium niobate ceramic with high piezoelectric property and high remanent polarization according to claims 1 to 7, wherein the rotation speed of the planetary ball mill in the step (4) is 400rpm, and the ball milling time is 10 hours.

9. The method for preparing the potassium-sodium niobate ceramic with the high piezoelectric property and the high remanent polarization according to claims 1 to 8, wherein the mass ratio of the powder in the step (5) to the polyvinyl alcohol solution is 10: 3.

10. The method for preparing potassium-sodium niobate ceramic having high piezoelectric properties and high remanent polarization according to claims 1 to 9, wherein the ceramic green body formed by pressing in the step (5) with a mold has a diameter of about 15mm and a thickness of about 1 mm.

11. The method for preparing potassium-sodium niobate ceramic having high piezoelectric properties and high remanent polarization according to claims 1 to 10, wherein the polarization electric field in the step (7) is 3kV/mm, and the polarization time is 30 min.

Technical Field

The invention belongs to the technical field of perovskite type lead-free piezoelectric ceramics, and particularly relates to a preparation method of potassium-sodium niobate ceramics with high piezoelectric performance and high remanent polarization.

Background

The piezoelectric ceramic can realize the function of interconversion between mechanical energy and electric energy, is convenient for manufacturing various functional components with low loss, high reliability, miniaturization and the like, and has very wide application in various fields of national daily production and life, such as transducers, sensors, drivers and the like. The mainstream material of the piezoelectric ceramic device in the market at present is lead-based lead zirconate titanate (PZT) material, heavy metal element lead contained in the lead-based material has great harm to many organisms including human beings and the environment, and a plurality of countries have introduced various related laws aiming at limiting the use of the lead-based piezoelectric ceramic material. Therefore, it is an extremely important task for researchers to develop a lead-free piezoelectric ceramic material that can replace lead zirconate titanate (PZT). Three perovskite type lead-free piezoelectric ceramic materials (including barium titanate series, sodium bismuth titanate series and potassium sodium niobate series) are expected to be substituted for lead-based piezoelectric ceramic materials at present because the perovskite type lead-free piezoelectric ceramic materials do not contain harmful elements such as lead and the like. Although the piezoelectric property of barium titanate ceramic is high, the temperature stability is not high due to the lower Curie temperature (120 ℃), and the application range of the barium titanate ceramic as a lead-based piezoelectric ceramic substitute material is greatly limited. Although sodium bismuth titanate ceramics have the advantages of high Curie temperature, high remanent polarization strength and the like, the density is not high, the coercive field is high, the polarization difficulty is high and the piezoelectric performance is low due to the volatilization problem of the Bi element. The potassium-sodium niobate ceramic has high Curie temperature (420 ℃) and low piezoelectric performance (d33 is 80pC/N), researchers regulate and control phase boundaries around room temperature by doping, constructing binary or ternary systems and other methods, and construct a trigonal-orthogonal phase boundary, an orthogonal-tetragonal phase boundary, a trigonal-tetragonal phase boundary or a trigonal-orthogonal-tetragonal phase boundary and the like around the room temperature. As the most basic composition component in potassium sodium niobate-based ceramics, pure potassium sodium niobate ceramics prepared by using a traditional solid phase method often have low density (relative density is lower than 95%) after sintering due to the volatilization problem of alkali metal elements of potassium and sodium, so that residual polarization strength and piezoelectric property are often not high. The density of the pure sodium potassium niobate ceramic material can be improved by using a special preparation method such as hot-press sintering, a sol-gel method and the like, so that the residual polarization strength and the piezoelectric property of the ceramic material are improved, but the special preparation method has the defects of complex process, high cost and the like. The patent CN 111517788A provides a preparation method for preparing potassium-sodium niobate ceramic with high remanent polarization strength by hot pressing sintering under oxygen atmosphere, 60mL/min oxygen gas flow is introduced into a hot pressing furnace, sintering pressure of 30-60 MPa is applied to a sample after the sintering temperature is reached, the temperature is kept for 20-100 min, oxygen annealing treatment is carried out on the hot pressing sintered sample, the obtained (Na0.5K0.5) NbO3 ceramic has high remanent polarization strength which can reach 24-27 mu C/cm2, but the cost is higher, and meanwhile, the piezoelectric property of the potassium-sodium niobate ceramic by hot pressing sintering under the oxygen atmosphere is not researched. Therefore, how to prepare the high-density potassium-sodium niobate ceramic by using the traditional solid phase method which is low in cost and easy for industrial mass production so as to improve the residual polarization strength and the piezoelectric property becomes a research hotspot in the field of the potassium-sodium niobate ceramic.

Disclosure of Invention

The invention aims to solve the technical problems in the prior art, and provides a traditional solid-phase preparation method of a potassium-sodium niobate (Na0.5K0.5) NbO3 ceramic material with high piezoelectric property and high remanent polarization, which solves the problems of low density, poor piezoelectric property and small remanent polarization of the potassium-sodium niobate ceramic prepared by the traditional solid-phase method.

In order to solve the technical problem of the invention, the following technical scheme is adopted.

A preparation method of potassium-sodium niobate ceramic with high piezoelectric property and high remanent polarization strength comprises the following steps:

(1) drying materials: placing alkali metal salt K2CO3 and Na2CO3 which are easy to absorb moisture into an oven, and baking for 2-5 hours at 200-250 ℃ to remove moisture;

(2) preparing materials: weighing raw materials K2CO3, Na2CO3 and Nb2O5 in turn according to the stoichiometric ratio of (Na0.5K0.5) NbO3, putting the weighed raw materials into a ball milling tank, taking absolute ethyl alcohol as a ball milling medium, and mixing the raw materials in a mass ratio of 1: 2, mixing zirconia beads with the diameter of 5mm and the diameter of 2mm as ball milling beads, and mixing the following raw materials: ball milling: the mass ratio of the absolute ethyl alcohol is 1: 8: 4, and the mixture is subjected to primary ball milling for 10 hours in a planetary ball mill at the rotating speed of 400rpm to obtain wet slurry;

(3) and (3) briquetting and presintering: putting the obtained slurry into a drying oven, baking for 6-9 h at 60-90 ℃ to obtain dry powder, pressing the obtained powder into cylindrical blocks by using a mould, feeding the blocks into a tubular furnace, and pre-sintering for 4h at 850 ℃ in the air;

(4) ball milling for the second time: crushing the block after the pre-sintering, transferring the obtained powder into a ball milling tank, taking absolute ethyl alcohol as a ball milling medium, mixing zirconia beads with the mass ratio of 1: 2, the diameter of 5mm and the diameter of 2mm as ball milling beads, and carrying out secondary ball milling on the mixture, wherein the raw materials are as follows: ball milling: the mass ratio of the absolute ethyl alcohol is 1: 8: 4, performing ball milling for 10 hours at the rotation speed of 400rpm of the planetary ball mill, putting slurry obtained by ball milling into a 60-90 ℃ oven for 6-9 hours for drying, performing grinding treatment on the dried powder, and sieving the powder through a 75-mesh sieve to obtain powder with fine granularity and uniform particles;

(5) molding: adding 3-4% by mass of polyvinyl alcohol solution into the powder obtained by grinding and sieving treatment, wherein the mass ratio of the powder to the polyvinyl alcohol solution is 10: 3, uniformly mixing the powder and the polyvinyl alcohol solution, putting the mixture into an oven at 80-90 ℃ for drying for 8-10 min to dry the water, grinding the mixture, sieving the mixture by using a 75-mesh sieve, and pressing and forming the powder obtained by sieving treatment by using a mould to obtain a disc-shaped ceramic blank, wherein the diameter of the ceramic blank is about 15mm, and the thickness of the ceramic blank is about 1 mm;

(6) and (3) sintering: placing the obtained ceramic blank into a tubular furnace, and keeping the temperature of 600-700 ℃ for 2h for carrying out glue discharging treatment, and placing the ceramic blank obtained after glue discharging into the tubular furnace, and keeping the temperature of 1090 ℃ for 2h to obtain a ceramic finished product;

(7) polarization: and carrying out silver coating treatment on the obtained ceramic finished product, keeping the temperature at 750-780 ℃ for 20-30 min, and polarizing the ceramic coated with the silver electrode for 30min at 120 ℃ under silicone oil, wherein the polarizing electric field is 3 kV/mm.

According to the invention, by changing the ball milling bead proportion of (Na0.5K0.5) NbO3 ceramic in the primary and secondary ball milling processes in the traditional solid phase preparation process, different ball milling bead proportions are found to influence the uniformity of components, so that the compactness and microstructure of the ceramic are influenced, and further the parameters of Curie temperature, orthogonal tetragonal phase transition temperature, relative free dielectric constant, remanent polarization strength, piezoelectric constant d33, plane electromechanical coupling coefficient Kp and the like of the ceramic are influenced. The proportion of the ball milling beads in the primary ball milling process and the secondary ball milling process of the comparison sample is respectively (1) agate beads with the diameter of 5mm are used for primary ball milling, and agate beads with the diameter of 2mm are used for secondary ball milling. (2) The primary and secondary ball milling were mixed using 10mm diameter and 5mm diameter agate beads in a mass ratio of 1: 2. The pure-phase perovskite potassium-sodium niobate ceramic is prepared by a traditional solid phase method, is in an orthogonal phase at room temperature, has a relative density of 93.8-95.5%, a piezoelectric constant d33 of 115-124 pC/N, a planar electromechanical coupling coefficient of 0.35-0.38, a remanent polarization of 20.88-23.83 mu C/cm2, a dielectric loss tan delta of 0.017-0.028 and a relative free dielectric constant epsilon r of 383-422. The ball milling proportion can ball mill the powder raw material more uniformly, reduce the agglomeration phenomenon among raw material particles and reduce the piezoelectric property deterioration caused by nonuniform components. The potassium-sodium niobate ceramic obtained by the ball milling bead proportioning has the highest density, the highest remanent polarization, the highest relative free dielectric constant and the highest piezoelectric performance.

Drawings

FIG. 1 is a scanning electron microscope image of a potassium-sodium niobate ceramic powder raw material obtained by a second ball milling after pre-sintering in an embodiment of the present invention;

FIG. 2 is an X-ray diffraction pattern of a potassium-sodium niobate ceramic powder raw material obtained by a second ball milling after pre-sintering in an embodiment of the present invention;

FIG. 3 is an X-ray diffraction pattern of a potassium-sodium niobate ceramic wafer sintered according to an embodiment of the present invention;

FIG. 4 is a scanning electron microscope image of the surface of a potassium-sodium niobate ceramic wafer sintered and prepared by an embodiment of the invention;

FIG. 5 is a dielectric temperature spectrum of a potassium-sodium niobate ceramic wafer prepared by sintering according to an embodiment of the present invention;

FIG. 6 is the hysteresis loop of the sintered potassium-sodium niobate ceramic wafer in accordance with the present invention.

Detailed Description

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown.

Example 1

A preparation method of potassium-sodium niobate ceramic with high piezoelectric property and high remanent polarization is characterized by comprising the following steps:

(1) drying materials: putting alkali metal salt K2CO3 and Na2CO3 which are easy to absorb moisture into an oven 230 ℃ for 3h to dry and remove moisture;

(2) preparing materials: weighing raw materials K2CO3, Na2CO3 and Nb2O5 in turn according to the stoichiometric ratio of (Na0.5K0.5) NbO3, putting the weighed raw materials into a ball milling tank, taking absolute ethyl alcohol as a ball milling medium, mixing zirconia beads with the diameter of 5mm and the diameter of 2mm as ball milling beads for the first time, wherein the mass ratio of the zirconia beads with the diameter of 5mm to the zirconia beads with the diameter of 2mm is 1: 2, and the raw materials: ball milling: the mass ratio of the absolute ethyl alcohol is 1: 8: 4, and the mixture is subjected to primary ball milling for 10 hours in a planetary ball mill at the rotating speed of 400rpm to obtain wet slurry;

(3) and (3) briquetting and presintering: putting the obtained slurry into an oven at 80 ℃ for 7h for drying to obtain dry powder, pressing the obtained powder into a cylindrical block by using a mold, conveying the block into a tubular furnace, and presintering for 4h at 850 ℃;

(4) ball milling for the second time: crushing the block after pre-sintering, transferring the obtained powder into a ball milling tank, taking absolute ethyl alcohol as a ball milling medium, mixing zirconia beads with the diameter of 5mm and the diameter of 2mm as ball milling beads for the second ball milling, wherein the mass ratio of the zirconia beads with the diameter of 5mm to the zirconia beads with the diameter of 2mm is 1: 2, the mass ratio of the powder to the ball milling beads to the absolute ethyl alcohol is 1: 8: 4, carrying out the second ball milling for 10h in a planetary ball mill at the rotating speed of 400rpm, putting the slurry obtained by the ball milling into an oven with the temperature of 80 ℃ for 7h for drying, grinding the dried powder, and passing through a screen with the size of 75 meshes to obtain powder with fine and uniform particles;

(5) molding: adding 3% by mass of polyvinyl alcohol solution into the powder obtained after grinding and sieving, wherein the mass ratio of the powder to the polyvinyl alcohol solution is 10: 3, uniformly mixing the powder with the polyvinyl alcohol solution, putting the mixture into an oven at 80 ℃ for drying for 10min to dry the water, grinding the mixture, sieving the dried mixture with a 75-mesh sieve, and performing compression molding on the powder obtained after sieving by using a mold to obtain a disc-type ceramic green blank, wherein the diameter of the ceramic green blank is about 15mm, and the thickness of the ceramic green blank is about 1 mm;

(6) and (3) sintering: placing the obtained ceramic green body into a tubular furnace, preserving heat for 2h at 700 ℃ for glue discharging treatment, and placing the ceramic green body obtained after glue discharging into the tubular furnace, and sintering at 1090 ℃ for 2h to obtain a ceramic finished product;

(7) polarization: and (3) carrying out silver coating treatment on the obtained ceramic finished product, keeping the temperature at 750 ℃ for 30min, and polarizing the ceramic coated with the silver electrode at 120 ℃ under silicone oil, wherein the polarizing electric field is 3kV/mm, and the polarizing time is 30 min.

Example 2

A preparation method of potassium-sodium niobate ceramic with high piezoelectric property and high remanent polarization is characterized by comprising the following steps:

(1) drying materials: putting alkali metal salt K2CO3 and Na2CO3 which are easy to absorb moisture into an oven at 200 ℃ for 5 hours to dry and remove moisture;

(2) preparing materials: weighing raw materials K2CO3, Na2CO3 and Nb2O5 in turn according to the stoichiometric ratio of (Na0.5K0.5) NbO3, putting the weighed raw materials into a ball milling tank, taking absolute ethyl alcohol as a ball milling medium, taking zirconia beads with the diameter of 5mm as ball milling beads for primary ball milling, wherein the mass ratio of the raw materials to the ball milling beads to the absolute ethyl alcohol is 1: 8: 4, and performing primary ball milling for 10 hours in a planetary ball mill at the rotating speed of 400rpm to obtain wet-process slurry;

(3) and (3) briquetting and presintering: putting the obtained slurry into a 60 ℃ oven for 9h to dry to obtain dry powder, pressing the obtained powder into a cylindrical block by using a mold, conveying the block into a tubular furnace, and presintering for 4h at 850 ℃;

(4) ball milling for the second time: crushing the block after pre-sintering, transferring the obtained powder into a ball milling tank, taking absolute ethyl alcohol as a ball milling medium, taking zirconia beads with the diameter of 2mm as ball milling beads for secondary ball milling, wherein the mass ratio of the powder to the ball milling beads to the absolute ethyl alcohol is 1: 8: 4, carrying out secondary ball milling for 10h in a planetary ball mill at the rotating speed of 400rpm, putting slurry obtained by ball milling into a 60 ℃ drying oven for 9h for drying, grinding the dried powder, and passing through a 75-mesh screen to obtain powder with fine granularity and uniform particles;

(5) molding: adding a polyvinyl alcohol solution with the mass fraction of 4% into the powder obtained after grinding and sieving, wherein the mass ratio of the powder to the polyvinyl alcohol solution is 10: 3, uniformly mixing the powder with the polyvinyl alcohol solution, putting the powder into a 90 ℃ oven to be dried for 8min, carrying out grinding treatment, sieving the powder with a 75-mesh sieve, and carrying out compression molding on the powder obtained after sieving by using a mold to obtain a disc-type ceramic green blank, wherein the diameter of the ceramic green blank is about 15mm, and the thickness of the ceramic green blank is about 1 mm;

(6) and (3) sintering: putting the obtained ceramic green body into a tubular furnace, preserving heat for 2h at 600 ℃ for carrying out binder removal treatment, putting the ceramic green body obtained after binder removal into the tubular furnace, and sintering for 2h at 1090 ℃ to obtain a ceramic finished product;

(7) polarization: and (3) carrying out silver coating treatment on the obtained ceramic finished product, keeping the temperature at 780 ℃ for 20min, and polarizing the ceramic coated with the silver electrode at 120 ℃ under silicone oil, wherein the polarizing electric field is 3kV/mm, and the polarizing time is 30 min.

Example 3

A preparation method of potassium-sodium niobate ceramic with high piezoelectric property and high remanent polarization is characterized by comprising the following steps:

(1) drying materials: putting alkali metal salt K2CO3 and Na2CO3 which are easy to absorb moisture into an oven at 250 ℃ for 2h to dry and remove moisture;

(2) preparing materials: sequentially weighing raw materials K2CO3, Na2CO3 and Nb2O5 according to the stoichiometric ratio of (Na0.5K0.5) NbO3, putting the weighed raw materials into a ball milling tank, taking absolute ethyl alcohol as a ball milling medium, mixing agate beads with the diameter of 10mm and the diameter of 5mm to serve as ball milling beads for first ball milling, wherein the mass ratio of the agate beads with the diameter of 10mm to the agate beads with the diameter of 5mm is 1: 2, and the raw materials: ball milling: the mass ratio of the absolute ethyl alcohol is 1: 8: 4, and the mixture is subjected to primary ball milling for 10 hours in a planetary ball mill at the rotating speed of 400rpm to obtain wet slurry;

(3) and (3) briquetting and presintering: putting the obtained slurry into a 90 ℃ oven for 6h to obtain dry powder, pressing the obtained powder into a cylindrical block by using a mold, conveying the block into a tubular furnace, and presintering for 4h at 850 ℃;

(4) ball milling for the second time: crushing the block after pre-sintering, transferring the obtained powder into a ball milling tank, taking absolute ethyl alcohol as a ball milling medium, mixing agate beads with the diameter of 10mm and agate beads with the diameter of 5mm as ball milling beads for the second ball milling, wherein the mass ratio of the agate beads with the diameter of 10mm to agate beads with the diameter of 5mm is 1: 2, the mass ratio of the powder to the ball milling beads to the absolute ethyl alcohol is 1: 8: 4, carrying out the second ball milling for 10h in a planetary ball mill at the rotating speed of 400rpm, drying the slurry obtained by ball milling in a 90 ℃ oven for 6h, grinding the dried powder, and passing through a 75-mesh screen to obtain powder with fine granularity and uniform particles;

(5) molding: adding 3% by mass of polyvinyl alcohol solution into the powder obtained after grinding and sieving, wherein the mass ratio of the powder to the polyvinyl alcohol solution is 10: 3, uniformly mixing the powder with the polyvinyl alcohol solution, putting the powder into a 90 ℃ oven to be dried for 8min, grinding the powder, sieving the powder with a 75-mesh sieve, and performing compression molding on the powder obtained after sieving by using a mold to obtain a disc-type ceramic green body, wherein the diameter of the ceramic green body is about 15mm, and the thickness of the ceramic green body is about 1 mm;

(6) and (3) sintering: putting the obtained ceramic green body into a tubular furnace, preserving heat at 650 ℃ for 2h, carrying out binder removal treatment, putting the ceramic green body obtained after binder removal into the tubular furnace, and sintering at 1090 ℃ for 2h to obtain a ceramic finished product;

(7) polarization: and (3) carrying out silver coating treatment on the obtained ceramic finished product, keeping the temperature at 780 ℃ for 20min, and polarizing the ceramic coated with the silver electrode at 120 ℃ under silicone oil, wherein the polarizing electric field is 3kV/mm, and the polarizing time is 30 min.

The potassium-sodium niobate leadless piezoelectric ceramics prepared in the examples 1 to 3 were analyzed in structure and performance as follows:

(I) structural analysis

The scanning electron microscope results in fig. 1 show that the diameter of the raw material particles obtained by the second ball milling in three different ball milling manners of examples 1 to 3 is between 0.2 and 1 micron. In the embodiment 2, the agglomeration phenomenon of the raw material particles is obvious, and the diameter of the particle cluster can reach 2 microns; example 3 also had agglomerated clusters of particles 2 microns in diameter. Although example 1 also showed particle agglomeration, the particle cluster diameters were all less than 1 micron. It can be seen that the ball-milling proportion in example 1 is better than that in examples 2 and 3, and the agglomeration phenomenon of particles can be reduced, so that the powder is more uniformly mixed with the polyvinyl alcohol solution in the subsequent granulation step, and thus, the distribution of pores caused by removing the polyvinyl alcohol molecules mixed in the ceramic green body at high temperature in the degumming step is more uniform, the discharge of pores in the crystal grain forming and growing process in the sintering step is more sufficient, the density of the ceramic finished product obtained after sintering is higher, and the ceramic performance in example 1 is also more excellent.

FIG. 2 shows the X-ray diffraction (XRD) results of examples 1-3 after pre-firing at 850 ℃ and secondary ball milling. It can be seen from the figure that all the calcined powders have a pure phase perovskite structure. In addition, structural analysis was performed on the ceramics obtained by sintering at 1090 ℃ in three different ball milling manners of examples 1 to 3, and the results shown in fig. 3 show that the ceramics obtained in examples 1 to 3 are all in a pure phase perovskite structure and are in an orthogonal phase at room temperature. The strong ratio of the left peak to the right peak of the diffraction peak splitting peak of example 3 at about 22 degrees and 45 degrees is obviously higher than that of examples 1 and 2, which shows that different ball milling modes can influence the microstructure of the final ceramic.

Fig. 4 shows that the ceramic material of example 3 has the smallest average crystal grain and obvious pores, which results in low density, when the surface of the ceramic material sintered in examples 1 to 3 is observed by using a scanning electron microscope. The average grain size of the ceramic material of the example 1 is the largest, the packing among the grains is very dense, no obvious pores are formed, the compactness is the highest, and the performance is the most excellent, and the average grain size and the compactness of the ceramic material of the example 2 are between the example 1 and the example 3.

(II) analysis of Properties

The results of measurement of the dielectric temperature spectra of the ceramic samples of examples 1 to 3 are shown in FIG. 5. The dielectric temperature spectra of the ceramic materials of examples 1-3 have relatively small differences, and Table 1 shows the phase transition temperature To-t and Curie temperature Tc of the orthorhombic phase and the tetragonal phase of the ceramic materials of examples 1-3. The result shows that the phase transition temperature To-t of the orthorhombic phase and the tetragonal phase of the ceramic material in the embodiment 1 is 7-8 ℃ higher than that in the embodiments 2 and 3, and the Curie temperature is slightly higher than that in the embodiments 2 and 3. The lower Curie temperature of example 2, which is only 415 ℃, shows that the difference of the ball milling bead proportions can cause slight difference of the microstructure of the final ceramic product, so that some difference of the phase transition temperature occurs.

TABLE 1 phase transition temperatures To-t and Curie temperatures Tc of the orthorhombic and tetragonal phases of examples 1 To 3

FIG. 6 shows the results of the hysteresis loop of the ceramic materials of examples 1 to 3. It can be seen that the ceramic material of example 1 has the highest saturation polarization and the ceramic material of example 3 has the lowest saturation polarization. Table 2 shows the remanent polarization values of the ceramic materials of examples 1-3, and it can be seen that the remanent polarization of the ceramic material of example 1 is 23.83 μ C/cm2, and the remanent polarization of the ceramic material of example 3 is the lowest, 20.88 μ C/cm 2. These values are obviously improved compared with the residual polarization strength of 10-15 μ C/cm2 of potassium-sodium niobate ceramic prepared by the traditional solid phase method reported in the literature, which indicates that the ball milling modes of examples 1-3 have obviously different effects, the ball milling mode of example 1 has the best effect and the highest residual polarization, and the reason that the piezoelectric constant d33 is very high can be partially explained.

TABLE 2 remanent polarization of examples 1-3

Table 3 shows parameters such as the piezoelectric coefficient d33, the planar electromechanical coupling coefficient Kp, the dielectric loss tan. delta., the relative free dielectric constant ε r, the density ρ, and the relative density ρ' of the ceramic materials of examples 1 to 3. The ceramic material in example 1 has the highest density of 4.31g/cm3, the relative density of 95.5% (the theoretical density of potassium-sodium niobate ceramic is 4.51g/cm3), and is higher than the relative density (92% -94%) of potassium-sodium niobate ceramic prepared by the traditional solid phase method reported in the literature, so the ceramic materials in examples 1 and 2 have higher piezoelectric performance, the piezoelectric coefficients d33 of the ceramic materials in examples 1 and 2 can respectively reach 124 and 120pC/N, and the Kp of the ceramic materials is respectively 0.38 and 0.37. The ceramic material of example 3 has a relatively low density and a lower piezoelectric performance than those of examples 1 and 2, and has a d33 of 115pC/N and a Kp of 0.35.

The above results show that the ceramic material of example 1 has the best effect by using the mixed ball milling of zirconia beads with the diameter of 5mm and zirconia beads with the diameter of 2mm, the obtained ceramic has the highest compactness and the best performance, the ceramic material of example 2 has the second effect by using 5mm zirconia beads for the first ball milling and 2mm zirconia beads for the second ball milling, and the ceramic material of example 3 has the second effect by using the mixed ball milling of 10mm agate balls and 5mm agate balls. In the embodiment 1, the ceramic material is subjected to mixed ball milling by using zirconia beads with the diameter of 5mm and the diameter of 2mm, so that powder can be milled more uniformly, the agglomeration phenomenon of particles is reduced, the processes of ceramic preparation such as pre-sintering, granulation, binder removal and sintering can be smoothly and fully carried out, and the phenomenon that the piezoelectric and ferroelectric properties of the ceramic material are deteriorated due to non-uniform components caused by non-uniform ball milling is avoided.

Table 3: the results of the piezoelectric coefficient d33, the planar electromechanical coupling coefficient Kp, the dielectric loss tan delta, the relative free dielectric constant ε r, the density ρ, and the relative density ρ' of the ceramic materials of examples 1 to 3