A method for preparing KNN textured ceramics based on the ice template method and textured ceramics

Abstract

This invention proposes a method for preparing KNN textured ceramics based on the ice template method, including freezing slurry preparation, directional freezing, freeze drying, directional pressing, and debinding sintering. In the directional pressing step, the pressing direction is perpendicular to the lamellar structure, and the pressing pressure is 50-100 MPa, resulting in a dense ceramic body. The KNN ceramic preparation method proposed in this invention uses the ice template method combined with template grain growth technology to achieve the directional arrangement of lamellar template grains in the ceramic body, simplifying the preparation process of KNN textured ceramics and significantly reducing the amount of organic additives. The KNN ceramics prepared by this invention achieve a texture degree of 86%.

Description

Technical Field

[0001] This invention relates to the field of textured ceramics preparation technology, and in particular to a method for preparing KNN textured ceramics based on the ice template method and the textured ceramics themselves. Background Technology

[0002] The key to preparing potassium sodium niobate (KNN) textured ceramics is to achieve the directional arrangement of sheet-like template particles in the ceramic body. Currently, template directional arrangement mainly adopts casting and screen printing methods. For example, patents ZL202210754086.1, ZL202111009891.3, and ZL201910661549.8 use casting to prepare potassium sodium niobate textured ceramics, while patent ZL201010192665.9 uses screen printing to prepare potassium sodium niobate textured ceramics. However, casting and screen printing methods usually require the addition of large amounts of organic additives such as solvents, dispersants, plasticizers, and binders. Moreover, after casting or screen printing into thin films, lamination and hot pressing are necessary, making the process cumbersome and the equipment expensive. In addition, organic additives will volatilize or pyrolyze during casting, drying, hot pressing, and debinding stages, resulting in large amounts of waste gas emissions. To address this issue, it is necessary to develop template orientation technology that is simple to process and requires minimal organic additives, in order to achieve low-cost and green preparation of KNN textured ceramics. Summary of the Invention

[0003] In view of the above shortcomings, this invention proposes a method for preparing KNN textured ceramics based on the ice template method, characterized by the following steps:

[0004] S1: Preparation of cryogenic slurry: Mix KNN powder, flake KNN particles, polyvinylpyrrolidone, deionized water and gelatin, put them into a beaker and stir to obtain water-based cryogenic slurry;

[0005] S2: Directional freezing: The water-based freezing slurry is poured into a dual-temperature-field directional freezing mold and the freezing temperature is controlled at -25-40℃. The angle of the silicone rubber wedge gasket is 5-15° to allow deionized water to crystallize and solidify in a directional manner. After solidification, a frozen preform is obtained.

[0006] S3: Freeze-drying: Place the frozen green body into a freeze dryer, set the drying chamber pressure to 90-105Pa, and the freezing temperature to -5-0℃. After 40h±0.5h, the ice crystals sublimate to form layered pores, resulting in a porous green body with a layered structure.

[0007] S4: Directional pressing: The above porous blank is placed into a mold for pressing. The pressing surface of the mold is parallel to the ice crystal growth plane, and the pressing direction is perpendicular to the pressing surface.

[0008] S5: Degreasing and sintering: The dense ceramic blank is placed in a muffle furnace and heated to 350-380℃ at a heating rate of 3-5℃ / min and held for 2-2.5h. Then, the temperature is increased to 1050℃-1150℃ at a heating rate of 1-2℃ / min and held for 2-4h. After cooling, KNN textured ceramic is obtained.

[0009] The KNN ceramic preparation method proposed in this invention uses the ice template method combined with template grain growth technology to achieve the directional arrangement of plate-like template grains in the ceramic body, which simplifies the preparation process of KNN textured ceramics and greatly reduces the amount of organic additives. The KNN ceramics prepared by this invention have a texture degree of 86%. Attached Figure Description

[0010] Figure 1 This is a porous preform after freeze-drying. The black color represents the cavities or holes formed by removing ice crystals, while the gray color represents the layered structure formed by the mixture of KNN powder and flaky KNN particles.

[0011] Figure 2 SEM images of the cross-section of the porous preform after pressing along the lamellar direction. a) is a cross-sectional view perpendicular to the pressing surface. b) is a cross-sectional view parallel to the pressing surface. In image a, the thin, elongated white images are lamellar KNN particles, most of which are almost parallel to the pressing surface. In image b, the white lamellar images are clearly lamellar KNN particles, most of which are almost parallel to the pressing surface. Comparing the two angles, it can be seen that under the action of the ice template, the lamellar KNN particles are pushed to the state shown in the images by the orderly oriented ice crystals.

[0012] Figure 3 This is a schematic diagram of the main steps of the present invention.

[0013] Figure 4 This is a schematic diagram of a directional freezing mold.

[0014] Figure 5 To utilize the model image formed after freezing in the freezing mold, ice crystals grow vertically upwards from the sharp corner of the mold and obliquely upwards along the wedge-shaped pad. It can be seen that the sheet-like KNN template particles are embedded inside the KNN powder matrix, exhibiting a nearly vertical, upward-aligned trend.

[0015] Figure 6 After drying, ice crystals form voids. The mixture formed by the KNN powder and the plate-like KNN particles in two adjacent layers is almost parallel, and the plate-like KNN particles in the KNN powder matrix are arranged almost parallel. The arrow direction is the direction of the pressing force, which is almost perpendicular to the layer direction, and the pressing surface is parallel to the layer.

[0016] Figure 7 The image shows the XRD pattern of KNN textured ceramics prepared by the method of this invention.

[0017] Figure 8 This is a SEM image of the KNN-textured ceramic. It can be seen that the ceramic structure is dense, with no significant pores or defects. Detailed Implementation

[0018] See Figure 1-8 The method for preparing KNN textured ceramics based on the ice template method of the present invention includes the following steps:

[0019] S1: Preparation of cryogenic slurry: Mix potassium sodium niobate powder (hereinafter referred to as KNN powder), flake potassium sodium niobate particles (hereinafter referred to as flake KNN particles), polyvinylpyrrolidone, deionized water, and gelatin in a beaker and stir for 36h±0.5h to obtain a water-based cryogenic slurry; when mixing, the amount of flake KNN particles added is 8%-12% of the mass of KNN powder, the amount of polyvinylpyrrolidone added is 0.2% of the total weight of KNN powder and flake KNN particles, the amount of deionized water added is 1.5-3.6 times the total mass of KNN powder and flake KNN particles, and the amount of gelatin added is 1%-3% of the weight of deionized water.

[0020] In this method, the amount of gelatin and polyvinylpyrrolidone organic matter used is much less than that used in existing casting and screen printing methods, and all of them are discharged after sintering, leaving almost no residue.

[0021] S2: Directional Freezing: The water-based freezing slurry is poured into a dual-temperature-field directional freezing mold, such as... Figure 4 The freezing temperature is controlled at -25-40℃, and the angle of the silicone rubber wedge gasket is 5-15°, so that the deionized water in the slurry crystallizes and solidifies in a directional manner, and a frozen blank is obtained after solidification.

[0022] In this step, the dual-temperature-field directional freezing mold has a silicone rubber wedge gasket angle of 5-15°. The water in the slurry first solidifies into ice crystals at the bottom of the freezing mold. In the dual-temperature field, the ice crystals gradually grow from the bottom in a parallel sheet-like shape. During the growth of the sheet-like ice crystals, they push against the KNN powder and sheet-like KNN particles they come into contact with. Because the sheet-like KNN particles are sheet-like, they are pushed by the ice crystals, causing them to tend to be parallel to the ice crystals. After directional pressing, this parallel tendency is further increased.

[0023] S3: Freeze-drying: The frozen preform is placed in a freeze dryer, the drying chamber pressure is set to 90-105 Pa, and the freezing temperature is -5 to 0℃. After 40 h ± 0.5 h, the ice crystals sublimate, forming layered pores, resulting in a porous preform with a layered structure. This step, which sublimates the ice crystals, aims to remove ice from the frozen preform and create pores. Figure 1 .

[0024] S4: Directional Pressing: The porous green body is placed in a mold for pressing. The pressing surface of the mold is parallel to the plane where the ice crystals grow, and the pressing direction is perpendicular to the pressing surface. Here, "parallel" can be understood as approximately parallel. If the plate-like ice crystals grow vertically, the voids formed after drying are also nearly parallel in the vertical direction. After directional pressing, this tendency towards near-parallelism is further increased, and the plate-like KNN particles in the dense ceramic green body become even closer to parallel. Figure 2 , 5 The pressing pressure is 50-100 MPa, resulting in a dense ceramic body;

[0025] S5: Degreasing and Sintering: The dense ceramic green body is placed in a muffle furnace and heated to 350-380℃ at a rate of 3-5℃ / min, held for 2-2.5 hours, then heated to 1050℃-1150℃ at a rate of 1-2℃ / min, held for 2-4 hours, and cooled to obtain KNN textured ceramic. In this step, organic additives such as gelatin and polyvinylpyrrolidone are thermally desorbed. With continued heating, small-particle-size KNN powder undergoes epitaxial growth on the surface of large-particle-size lamellar KNN particles, i.e., the KNN grains have a certain orientation, ultimately forming textured ceramic. This step utilizes template crystal growth technology, with lamellar KNN particles as templates, and KNN powder grows directionally along the surface of the lamellar KNN particles into grains, which gradually grow larger. Figure 3 .

[0026] In step S2 of this solution, as follows: Figure 2 , 5 This method uses KNN template particles in sheet form. During the directional freezing step, the sheet-like material is easily pushed, compressed, and moved by the ice crystals under the driving force of ice crystal crystal formation, turning towards a direction parallel to the ice crystals, thus forming an arrangement approximately parallel to the ice crystals. After the directional pressing step, the sheet-like material tends to be even more parallel, making it easier to form layers parallel to the pores. This method differs from methods where the shape of the KNN in the raw material is not limited. For example, existing technologies use equiaxed or irregularly shaped KNNs. Even when applied to the ice template method, because the material shape has no special characteristics, it can only be separated and isolated by the pushing force of ice crystals. However, during sintering, the growth or fusion of the raw material does not have a grain growth guide; it is simply fusion. The grain growth cannot control the directionality. The crystallographic direction is in all directions, without directionality, and textured ceramics cannot be formed.

[0027] Combining the characteristics of texture structure: texture is essentially oriented grains, which have a certain directionality, meaning that the crystallographic orientation of most grains is close or consistent, thus resulting in better performance in this direction. In this scheme, a small number of plate-like KNN particles are uniformly distributed within KNN powder, forming a template. The surrounding KNN powder acts as a matrix, growing directionally along the surface of the plate-like KNN particles. Guided by the plate-like KNN particles, the crystallographic orientation of most KNN powder grains is close or consistent. In this invention, to form texture, the plate-like KNN particles act as an inducing condition when the KNN powder grows into grains. Because the plate-like powder is large, smaller KNN powder particles will grow on its surface, and the growth direction is along the direction provided by the plate-like KNN particles. Since all the plate-like KNN particles are almost parallel, the final grown KNN grains have a certain directionality. The piezoelectric properties of this invention are excellent, with a piezoelectric coefficient d33 = 150-160 pC / N.

[0028] Further, in step S1, the sheet-like KNN powder has a length of 20±1 μm and a thickness of approximately 2±0.2 μm. The KNN is (K 0.5 Na 0.5 )NbO3.

[0029] The KNN textured ceramics prepared by this method have a texture degree of 86%.

[0030] Example 1

[0031] The following materials were added: (K,Na)NbO3 (KNN powder) prepared by molten salt synthesis (average particle size 2 μm); plate-like (K,Na)NbO3 (plate-like KNN particles) template particles prepared by molten salt synthesis (added at 10% of the mass of KNN powder); the plate-like KNN particles had a length of 20±1 μm and a thickness of approximately 2±0.2 μm; polyvinylpyrrolidone (added at 0.2% of the total weight of KNN powder and plate-like KNN particle template); and deionized water (added at a certain amount). Three times the total mass of powder and flake-shaped KNN particle template, and gelatin (added at 2% of the weight of deionized water) were weighed and placed in a beaker, and magnetically stirred for 36 hours; the freezing temperature was -35℃; the wedge angle was 10°; the drying chamber pressure was 100Pa, and the freezing temperature was -5℃; the directional pressing pressure was 100MPa; the temperature was increased to 350℃ at a heating rate of 5℃ / min and held for 2 hours, then increased to 1150℃ at a heating rate of 2℃ / min and held for 4 hours. After cooling, the KNN textured ceramic was obtained; the XRD pattern is shown below. Figure 7 As shown, the ceramic microstructure is as follows Figure 8 As shown. The KNN ceramic prepared in this embodiment has a texture of 86% and a piezoelectric coefficient d33 = 160 pC / N.

[0032] The texture degree is calculated as follows:

[0033]

[0034] Where: f is the texture degree in the <00l> direction, calculated within the diffraction peaks of 2θ = 20°-70°; ∑I (00l) and ∑I 0(00l) ∑I represents the sum of the intensities of the (00l) peak in the XRD diffraction patterns of textured and untextured ceramics, respectively; (hkl) and ∑I 0(hkl) These represent the sum of the intensities of the (hkl) peak in the XRD diffraction patterns of textured and untextured ceramics, respectively.

[0035] Example 2

[0036] The raw material used is the same KNN powder as in Example 1, but without the addition of flaky KNN particles. The proportions of the ingredients and the process conditions remain unchanged. The KNN ceramic prepared under the conditions of this example has no texture and a piezoelectric coefficient d33 = 71 pC / N.

[0037] As can be seen from the comparison of the two embodiments above, the ceramic prepared by the present invention using KNN powder and plate-like KNN particles, combined with ice template method and template grain growth technology, has a high texture and excellent piezoelectric properties. Without plate-like KNN particles, the prepared ceramic structure lacks texture, and the piezoelectric properties are far lower than those of this solution.

Claims

1. A method for preparing KNN textured ceramics based on ice templating method, characterized in that Includes the following steps: S1: Preparation of cryogenic slurry: Mix KNN powder, flake KNN particles, polyvinylpyrrolidone, deionized water and gelatin, put them into a beaker and stir to obtain water-based cryogenic slurry; S2: Directional freezing: The water-based freezing slurry is poured into a dual-temperature-field directional freezing mold and the freezing temperature is controlled at -25-40℃. The angle of the silicone rubber wedge gasket is 5-15° to allow deionized water to crystallize and solidify in a directional manner. After solidification, a frozen preform is obtained. S3: Freeze-drying: Place the frozen green body into a freeze dryer, set the drying chamber pressure to 90-105Pa, and the freezing temperature to -5-0℃. After 40h±0.5h, the ice crystals sublimate to form layered pores, resulting in a porous green body with a layered structure. S4: Directional pressing: The above porous blank is placed into a mold for pressing. The pressing surface of the mold is parallel to the ice crystal growth plane, and the pressing direction is perpendicular to the pressing surface. S5: Degreasing and sintering: The dense ceramic blank is placed in a muffle furnace and heated to 350-380℃ at a heating rate of 3-5℃ / min and held for 2-2.5h. Then, the temperature is increased to 1050℃-1150℃ at a heating rate of 1-2℃ / min and held for 2-4h. After cooling, KNN textured ceramic is obtained. In step S1, during mixing, the amount of flake KNN added is 8%-12% of the mass of KNN powder, the amount of polyvinylpyrrolidone added is 0.2% of the total weight of KNN powder and flake KNN particles, the amount of deionized water added is 1.5-3.6 times the total mass of KNN powder and flake KNN, and the amount of gelatin added is 1%-3% of the weight of deionized water.

2. The method for preparing KNN textured ceramics based on ice-templating according to claim 1, characterized in that In the S4 directional pressing step, the pressing pressure is 50-100 MPa.

3. The method for preparing KNN textured ceramics based on the ice template method as described in claim 1, characterized in that: In step S1, the chemical composition of KNN is (K0.5,Na0.5)NbO3.

4. A KNN textured ceramic prepared by the method of preparing KNN textured ceramic based on the ice template method according to any one of claims 1-3 has a texture degree of 86%.

5. The piezoelectric coefficient d33 of the KNN textured ceramic as described in claim 4 is 150-160 pC / N.

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