Method for manufacturing KNN single crystals and KNN single crystals

Abstract

To manufacture a large and single KNN single crystal using a tabular NN single crystal that is inexpensive but difficult to handle.SOLUTION: A method for manufacturing a KNN single crystal comprises: manufacturing a viscous material 4 including a tabular NN single crystal 1 and a KNN calcined powder 2 having a high AGG activity; applying shear stress by a roller 3 to the viscous material to align the orientation of the tabular NN single crystal 7; cutting a small piece 7 having a size sufficiently smaller than that of the upper surface of a compact 9 from the viscous material 4 (Fig. 2); bonding the small piece 7 to a part of the compact 9 of the KNN calcined powder having a low AGG activity (Fig. 3) to process the compact 9 by a solid phase crystal growth method; and growing a KNN crystal in an orientation corresponding to the crystal structure of the tabular NN single crystal from an area bonded by the small piece 7 of the compact 9 to obtain a single and large-sized KNN single crystal (Fig. 6 and Fig. 9 (e)) having the Miller indices (001) and (100) of the upper and lower surfaces.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a method for producing a large single-plate KNN single crystal having the same crystal orientation as a plate-like NN single crystal, and the KNN crystal thereof, by attaching a small piece of a viscous material containing KNN calcined powder and a plate-like NN single crystal to a molded body of KNN calcined powder and treating it by a solid-phase crystal growth method.

[0002] Specifically described in accordance with the description of the embodiments to be described later, the method for producing a KNN single crystal of the present invention is a solid-phase crystal growth (SSCG) method used as a growth method for a KNN single crystal, which is an alkali niobate-based material, that is, a (K 0.5 Na 0.5 ) NbO3 (KNN) - based lead-free piezoelectric material. In particular, it relates to an SSCG method in which the growth rate reaches several hundred times that of the conventional method by promoting abnormal grain growth (AGG) of crystal growth nuclei. It is characterized in that the NN single crystal used to control the crystal growth orientation is attached to the molded body of KNN calcined powder, which is the growth base material, in a special manner. By using the production method of the present invention, it is possible to control the crystal growth orientation, which has no precedent as an SSCG method for growing KNN-based single crystals at high speed. By using an NN single crystal as a substitute for an extremely expensive large-sized seed crystal that has been used in the slow conventional SSCG method, the production cost of a practical KNN single crystal can be significantly reduced.

Background Art

[0003] In KNN-based materials, which are considered environmentally friendly lead-free piezoelectric materials because they do not use lead, solid-phase crystal growth (SSCG) is known as a method for producing piezoelectric single crystals that have significantly higher performance than polycrystalline piezoelectric ceramics. This method involves compressing KNN calcined powder, which is an aggregate of single-crystal fine grains (particle size less than 1 μm), to obtain a molded body, heating it to a temperature below its melting point, and growing larger single crystals by utilizing abnormal grain growth (AGG) while the material remains solid. Because this method is carried out at a relatively low temperature without going through a melting process, it can avoid incongluent melting and elemental volatilization, which are known problems of the more common single-crystal growth method, molten growth. As a result, it is possible to suppress compositional deviations and segregation, which are particularly noticeable in KNN-based materials, caused by these phenomena. On the other hand, the SSCG method has the disadvantage that it is difficult to produce large-sized single crystals because the growth rate is extremely low. Hereafter, this solid-phase crystal growth method will be referred to as the SSCG method (A). An example of the SSCG method (A) is reported in Non-Patent Literature (1).

[0004] To overcome the shortcomings of the SSCG method (A), a high-speed solid-phase crystal growth method has been developed that significantly accelerates AGG, the driving force of SSCG, by various methods, thereby improving the growth rate by several hundred times. Hereafter, this solid-phase crystal growth method will be referred to as the SSCG method (B). Specific examples of the SSCG method (B) include the non-seed crystal solid-phase crystal growth (SFSSCG) method and the high-speed solid-phase crystal growth (RSSCG) method, which are reported in non-patent documents (2) and (3), respectively. By drastically accelerating AGG through special sintering aids, composition control, and trace element additions, the growth rate of the SSCG method (B) reaches several hundred times that of the conventional SSCG method (A).

[0005] Furthermore, as a conventional technique for producing ceramics (polycrystalline materials), a rolling orientation method, which is a type of reactive template (RTGG) method, has been reported in Non-Patent Document (4). As will be described in detail later, the method for producing KNN single crystals of the present invention utilizes the RTGG method disclosed in Non-Patent Document (4) in part during the process. However, the present invention is completely different from the invention disclosed in Non-Patent Document (4), which uses the RTGG method for producing ceramics (polycrystalline materials), in terms of the recognition of the problem, the purpose of the invention, and the effects, because it is the first to apply the RTGG method to a method for producing KNN single crystals. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] Fujii, S. Ueno, and S. Wada, Effect of sintering temperature on the growth of (K0.5 Na0.5 )NbO3 single crystals fabricated by the solid-state crystal growth method, Vol. 58, SLLD01-1- SLLD01-6 (2019)10.7567 / 1347-4065 / ab34e6 [Non-Patent Document 2] X. Yao, M. Jiang, S. Han, D. Li, Y. 10.1557 / s43578-020-00015-2 [Non-Patent Document 3] T. Morimoto, S. Shimono, Y. Yoshiichi, H. Kishimura, and K. Ishii, Conditions of large unitary (K,Na)NbO3 system single crystals for rapid solid-state crystal growth method, Japanese Journal of Applied Physics, Vol. 62, No. 3, 035501-1 035501-7 (2023)DOI 10.35848 / 1347-4065 / acbd7b [Non-Patent Document 4] K. Ishii and S. Tashiro, Orientation Control of(K,Na)NbO3 Ceramics Using NaNbO3 Particles Prepared by Single-Step Molten Salt Synthesis, Japanese Journal of Applied Physics, Vol. 52, No. 9, 09KD04-1 09KD04-6 (2013)DOI 10.7567 / JJAP.52.09KD04 [Overview of the project] [Problems that the invention aims to solve]

[0007] In piezoelectric single crystals, high piezoelectricity is only exhibited in specific directions determined by the orientation of the crystal structure. Therefore, controlling the crystal growth orientation so that the shape of the compacted molded body (the growth matrix for the single crystal) and the orientation of the crystal structure match the intended use is essential for practical growth methods of piezoelectric single crystals.

[0008] In the conventional SSCG method (A) disclosed in Non-Patent Literature (1), a KTaO3 single crystal having a specific plane orientation and shape is used as a seed crystal, and the seed crystal is embedded in a calcined powder molded body so that the crystal orientation faces a specific direction. From the surface of the seed crystal, which has incorporated atoms of the surrounding KNN calcined powder, the KNN single crystal grows in the same orientation as the seed crystal, resulting in control of the growth orientation. Because the seed crystal is directly grasped and embedded in the KNN calcined powder molded body so that the crystal orientation faces a specific direction, a relatively large seed crystal (in the example of Non-Patent Literature 1, a plate-like shape with top and bottom surfaces of 2-3 mm square and a thickness of about 0.5 mm) is required for ease of handling. On the other hand, in this SSCG method (A), the crystal growth rate is extremely small, and growth saturates after prolonged heating, so only KNN single crystals with sides of a few mm square at most could be obtained. The heating time is long, up to a week, and despite using expensive KTaO3 seed crystals, only small KNN single crystals, about 10 times the volume of the seed crystal, can be grown. Because the cost ratio of the seed crystal is high, the cost of single crystal production per volume is extremely high.

[0009] On the other hand, even with the high-speed SSCG method (B) disclosed in non-patent documents (2) and (3), effective control of the growth orientation using KTaO3 single crystals as seed crystals has not been successful. This is because the remarkably high activity of AGG, which contributes to a significant improvement in crystal growth rate, inhibits the selective growth of single crystals from the seed crystal surface alone. In other words, as a side effect of the remarkably active AGG, multiple single crystals grow disorderly from crystal nuclei other than the seed crystal present in the molded body of KNN calcined powder. As a result, multiple single crystals with various growth orientations grow simultaneously, making it impossible to obtain only large single crystals that grow in the same orientation as the seed crystal. It has been virtually impossible to control the growth orientation using a single seed crystal so that only large single crystals whose final external shape and crystal structure growth orientation match the intended use can be efficiently obtained. While embedding multiple KTaO3 seed crystals in various parts of the molded body increases the proportion of single crystals grown from the seed crystals, the cost increases with the number of seed crystals required. Furthermore, because the single crystals growing within the molded body compete with each other, the individual single crystals become smaller, resulting in an extremely high manufacturing cost per unit volume.

[0010] As explained above, conventional technologies have the problem that the SSCG method (A) currently has extremely high manufacturing costs, and that the high-speed SSCG method (B), which was expected to reduce manufacturing costs, makes it virtually impossible to control the growth orientation of the crystal structure using seed crystals, making it impossible to obtain large single crystals with a growth orientation that matches the intended use.

[0011] This invention has been made in light of the prior art, challenges, and problems, and aims to provide a manufacturing method for obtaining a large, single single crystal by controlling the crystal growth orientation in a solid-phase crystal growth method. This is achieved by arranging inexpensive, plate-shaped NN single crystals, which are difficult to handle and therefore difficult to use as seed crystals, onto a molded body of KNN calcined powder with good operability so that the crystal orientation is in a specific direction that matches the intended use. [Means for solving the problem]

[0012] To achieve the aforementioned objective, the method for manufacturing a KNN single crystal described in claim 1 is: A viscous material is produced containing plate-like NN single crystals and a first KNN calcined powder with relatively high AGG activity. Applying shear stress to the viscous material aligns the orientation of the plate-like NN single crystals to a specific direction. something Cut into small pieces, The small pieces are attached to a molded body made of a second KNN calcined powder having a relatively lower AGG activity than the first KNN calcined powder. The molded body is treated by a solid-phase crystal growth method, and KNN crystals are grown in the region of the molded body to which the small pieces are attached, in an orientation corresponding to the crystal structure of the plate-like NN single crystal.

[0013] The method for manufacturing a KNN single crystal described in claim 2 is the method for manufacturing a KNN single crystal described in claim 1, The present invention is characterized by subjecting the plate-shaped NN single crystal to a molten salt treatment using Na2CO3 before manufacturing the viscous material.

[0014] The method for manufacturing the KNN single crystal according to claim 3 is the method for manufacturing the KNN single crystal according to claim 2, wherein the molten salt treatment is performed using a salt obtained by diluting Na2CO3 with NaCl.

[0015] The method for manufacturing the KNN single crystal according to claim 4 is the method for manufacturing the KNN single crystal according to claim 2 or 3, wherein the molten salt treatment is performed for a treatment time such that the atomic ratio of Bi to Nb contained in the plate-like NN single crystal is less than 3%.

[0016] The method for manufacturing the KNN single crystal according to claim 5 is the method for manufacturing the KNN single crystal according to claim 1, wherein shearing stress is applied to the viscous material by a rolling process using a roller.

[0017] The method for manufacturing the KNN single crystal according to claim 6 is the method for manufacturing the KNN single crystal according to claim 5, wherein the size of the small pieces is 1 mm to 2 mm square.

[0018] The method for manufacturing the KNN single crystal according to claim 7 is the method for manufacturing the KNN single crystal according to claim 6, wherein the number of the plate-like NN single crystals contained in the small pieces is 10 to 30.

[0019] The KNN single crystal according to claim 8 is Surrounded by four black, arc-shaped KNN polycrystalline bodies, a single KNN single crystal having Miller indices of (100) and (001) for the upper and lower surfaces. Rectangle

[0021] Specific features of the present invention that solve the "problems to be solved by the invention" described above will be described in accordance with the specific configuration and effects of the embodiments of the invention while comparing with conventional examples, and are as follows 1 to 3.

[0022] 1. This invention utilizes plate-shaped NaNbO3(NN) single crystals, which were conventionally used in the fabrication of piezoelectric oriented ceramics, as an extremely inexpensive seed crystal instead of expensive KTaO3 single crystals. The first feature of this invention is a method that significantly reduces the manufacturing cost per unit volume of KNN single crystals by using small-sized (20 μm to 30 μm square) plate-shaped NN single crystals synthesized by a one-step molten salt method to control the crystal growth orientation.

[0023] 2. These plate-like NN single crystals are mechanically brittle and easily broken, and their small size (less than 30 μm) makes them difficult to handle for embedding (or attaching / bonding) in a specific orientation into a molded body of KNN calcined powder as seed crystals. As a solution, the present invention utilizes the reactive template (RTGG) method used in the production of oriented ceramics. In this RTGG method, the above-mentioned NN single crystals are mixed into a turbid solution of an organic binder dissolved in a solvent and KNN calcined powder, and shear stress is applied to the turbid solution to align the internal plate-like NN single crystals to a specific orientation. The second feature of the present invention is that it utilizes the rolling orientation method, which is a type of this RTGG method.

[0024] The rolling orientation method, which involves partially drying the turbid liquid and kneading it into a clay-like substance to produce a clay-like green material, and then applying shear stress by roller rolling, offers significantly better operability than the conventional RTGG method. In this invention, aligned plate-shaped NN single crystals are indirectly joined to the powder compact by attaching the clay-like green material, cut into squares of approximately 1 mm to 2 mm on each side, to the center of the upper surface of the powder compact.

[0025] Furthermore, while the aforementioned Non-Patent Document (4) reports on the synthesis of plate-like NN single crystals by a one-step molten salt method without surface treatment in molten salt, and the production of oriented ceramics using the rolling orientation method, this Non-Patent Document (4) is solely concerned with the production of oriented ceramics and does not describe the use of the rolling orientation method for growing single crystals, which are completely different from ceramics, which are aggregates of large single crystal grains. Nor does it contain any content that would suggest this.

[0026] 3. In the SSCG method (B), due to the significant AGG activity that contributes to rapid crystal growth, simply joining the as-synthesized plate-like NN single crystals does not guarantee that large single crystals will grow using these seed crystals as growth nuclei. Therefore, in this invention, to improve the probability of rapid growth of KNN single crystals using the plate-like NN single crystals as growth nuclei, the plate-like NN single crystals after synthesis are heat-treated in molten salt to remove Bi, which suppresses AGG, i.e., a Bi-de-treatment is performed. In addition, a third feature of this invention is that the AGG activity of the KNN calcined powder mixed with the plate-like NN single crystals when producing the clay-like green material is made slightly higher than that of the KNN calcined powder used in the compacted molded body, thereby selectively facilitating crystal growth only in the vicinity of the compressed clay-like green material, i.e., in the vicinity of the plate-like NN single crystals contained therein. [Effects of the Invention]

[0027] According to the present invention, by employing small-sized plate-shaped NN single crystals as seed crystals and utilizing a joining method using the rolling orientation method for the production of KNN single crystals by the SSCG method, it has become possible to control the crystal growth orientation in the high-speed SSCG method (B), which was previously impossible. Furthermore, when attempting to produce single crystals using the conventional SSCG method (A), it was necessary to use expensive KTaO3 single crystals (as of April 2024, the unit price of a 1cm square, 0.5mm thick crystal plate was 200,000 yen). However, according to the present invention, the equipment required to produce the minute plate-shaped NN single crystals used as material is only a relatively inexpensive muffle furnace with an upper heating temperature of about 1250°C and a platinum dish with a capacity of 150mL to 200mL that can be reused. Using inexpensive starting materials (total cost less than 1000 yen as of April 2024) and this equipment, 10 9 Since more than one sheet can be obtained at once, the manufacturing cost of the minute plate-shaped NN single crystals used in this invention is extremely low.

[0028] Therefore, according to the present invention, it is possible to manufacture KNN single crystals with controlled growth orientation at a significantly reduced manufacturing cost per unit volume compared to conventional methods. [Brief explanation of the drawing]

[0029] [Figure 1]This graph shows the relationship between the immersion time (horizontal axis) and the Bi / Nb atomic ratio (vertical axis) during the molten salt treatment of a plate-shaped NN single crystal in the embodiment. [Figure 2] This embodiment is a conceptual diagram of a rolling orientation method used when producing a viscous material containing plate-like NN single crystals and KNN calcined powder. [Figure 3] This figure shows the process of attaching small pieces of viscous material to a molded body of KNN calcined powder in one embodiment. [Figure 4] This is a perspective view of a die and punch used in the manufacture of a molded article in this embodiment. [Figure 5] This is a manufacturing process diagram of the molded body according to the embodiment. [Figure 6] In this embodiment, the image below shows a photograph of a KNN single crystal that has grown significantly with its growth orientation controlled, and the image above shows the edges that have appeared on the single crystal in the same photograph, clearly indicated by dashed lines. [Figure 7] Figure 6 is a graph of X-ray diffraction measurement results showing that the single crystal grew in the (100)(001) plane orientation. [Figure 8] In this embodiment, the left figure shows an external view of a rectangular plate-shaped KNN single crystal grown inside a molded body, and the right figure shows a state in which the ceramic surrounding the KNN single crystal has been cut in a crescent shape. [Figure 9] This diagram compares conventional technology with the present invention, where Figure (a) shows conventional ceramic firing, Figure (b) shows conventional solid-phase crystal growth (SSCG(A), technology described in Non-Patent Document 1), Figure (c) shows high-speed solid-phase crystal growth (SSCG(B), SFSSCG method, technology described in Non-Patent Document 2), Figure (d) shows high-speed solid-phase crystal growth (SSCG(B), RSSCG method, technology described in Non-Patent Document 3), and Figure (e) shows the present invention. [Figure 10] The image below shows a photograph of a KNN single crystal grown by the high-speed solid-phase crystal growth method (SSCG(B), RSCG method) described in Non-Patent Literature 3, and the image above shows the edges appearing on the single crystal in the same photograph clearly indicated by dashed lines. [Modes for carrying out the invention]

[0030] Embodiments for carrying out the present invention will be described with reference to the drawings. This invention relates to a method for manufacturing KNN single crystals by the SSCG method (B) using small-sized plate-shaped NN single crystals as seed crystals. In particular, it has various features in the technique of providing seed crystals to a molded body made of KNN calcined powder in order to produce a single, larger KNN single crystal than conventional methods. Specifically, the method involves surface treatment to remove elements that hinder crystal growth from the surface of the plate-shaped NN single crystal, producing a green material (hereinafter also referred to as a viscous material), which is a clay-like material containing the treated plate-shaped NN single crystal and KNN calcined powder, rolling and cutting the viscous material to produce small pieces, and attaching the small pieces to a molded body made of KNN calcined powder, thereby aligning the orientation of the plate-shaped NN single crystals in a specific direction and providing them to the molded body. By firing this molded body using the SSCG method (B), the plate-shaped NN single crystals rapidly enlarge, and a large single-plate KNN single crystal is obtained in which surfaces with Miller indices of (100)(011) appear on the upper and lower surfaces of the molded body. The following description and referenced drawings are merely examples of embodiments for carrying out the invention and illustrate configurations that fall within the scope of the present invention.

[0031] [1] The synthesis of small-sized plate-like NN single crystals by a one-step molten salt method disclosed in Non-Patent Literature (4) utilizes a reaction with a Bi layered ferroelectric crystal as a precursor. As a result, a large amount of unreacted Bi atoms remain inside the synthesized plate-like NN single crystal, and it is thought that these Bi atoms suppress the AGG of KNN. The inventors of the present invention considered that AGG is unlikely to occur from the surface of NN crystals containing many Bi atoms, and that it would be difficult to effectively utilize plate-like NN single crystals as seed crystals in this state. After further investigation, they sought a method to reduce the concentration of Bi atoms on the surface of plate-like NN single crystals, and as a result, they arrived at the idea of ​​performing a molten salt treatment with Na2CO3 before using the plate-like NN single crystal as a seed crystal. Furthermore, considering that molten salt treatment with high-purity Na2CO3 would cause a decomposition reaction in the plate-like NN single crystals, we concluded that, in order to avoid adverse effects on the plate-like NN single crystals, it is a practically reasonable method to use a salt obtained by diluting Na2CO3 several hundred times with NaCl for molten salt treatment, rather than performing the treatment with Na2CO3 alone.

[0032] [2] Figure 1 shows experimental results by the inventors of the present invention, and is a graph showing the relationship between the atomic ratio of Bi to Nb contained in the surface of a plate-like NN single crystal (Bi / Nb ratio (%), vertical axis), measured using X-ray photoelectron spectroscopy, and the soaking time (h), horizontal axis, during which the plate-like NN single crystal is immersed in molten salt. The immersion temperature in the molten salt treatment is 810°C. According to these experimental results, it is thought that after the Na2CO3 in the molten salt is consumed, the Bi component that is discharged from the surface of the plate-like NN single crystal into the molten salt as Bi2O3 is reabsorbed into the surface of the plate-like NN single crystal by prolonged heating. Therefore, the inventors of the present invention considered that the Bi removal effect would be reduced if the immersion time was too short or too long, and conducted repeated experiments in the time range of 3 hours 30 minutes to 4 hours 30 minutes, during which the Bi content was less than 3%, to investigate the success probability of long-orientation control. As a result, it was found that within this time range, the success probability of growth orientation control using the treated plate-like NN single crystal exceeds 50%. The following description of the embodiment and the contents shown in Figures 6 to 8 described later were obtained from experiments using plate-shaped NN single crystals treated for 4 hours, which is considered to be the most effective immersion time.

[0033] [3] Figure 2 is a conceptual diagram of the rolling orientation method used in the embodiment. Since this method is a type of reactive template (RTGG), a binder and solvent are added to small-sized (20 μm to 30 μm square) plate-shaped NN single crystals 1 and KNN calcined powder 2 that have been treated with molten salt to be used as seed crystals, and a turbidity is prepared in the same way as in the RTGG method. In the rolling orientation method, this turbidity is kneaded after being brought to a semi-dried state to produce a clay-like green material 4 as a viscous material. The prepared clay-like green material 4 is sandwiched between release sheets 6 and rolled to a thickness of 0.3 mm with rollers 3. Due to the shear stress applied by rolling, an oriented clay-like green material 5 is obtained in which the upper and lower surfaces ((100)(001) planes in pseudotetragonal representation) of the plate-shaped NN single crystals 1 inside the clay-like green material 4 are aligned parallel to the release sheet 6.

[0034] [4] After rolling, the upper release sheet 6 is peeled off, and the oriented clay-like green material 5 on the lower release sheet 6 is cut into numerous square pieces 7 measuring 1 mm to 2 mm on each side. As shown in the left diagram of Figure 3, one of the pieces 7 is placed on the center of the upper surface of a disc-shaped molded body 9 made of KNN calcined powder, and pressure is applied to adhere (compress) it as indicated by the arrow. As shown in the right diagram of Figure 3, the piece 7 is crushed to form a circular oriented clay-like green material 10, and the plate-like NN single crystals 1 contained in this oriented clay-like green material 10 are in a position parallel to the upper surface of the molded body 9 made of KNN calcined powder. The compression of the pieces 7 to the molded body 9 is performed with a predetermined pressure using a press machine or the like used in the manufacture of the molded body 9, which will be described later. In this example, the pressure is such that the 1 mm square, 0.3 mm thick oriented clay-like green material 5 piece 7 is crushed to form a circular oriented clay-like green material 10 with a diameter of 2 mm on the molded body 9.

[0035] The disc-shaped molded body 9 is produced using a mold with the structure shown in Figure 4, namely die D and two types of punches A and B. Die D is cylindrical and has a through-circular hole 20. Punch A has a rod portion 21 with an outer diameter equal to the inner diameter of the hole 20 of die D and a length equal to the length of the hole 20 of die D, and a disc portion 22 with the same diameter as die D is attached to one end of the rod portion 21. Punch B has a rod portion 23 with an outer diameter equal to the inner diameter of the hole 20 of die D and a length shorter than the rod portion 21 of punch A, and a disc portion 22 with the same dimensions as punch A is attached to one end of the rod portion 23.

[0036] The disc-shaped molded body 9 is manufactured using the process shown in Figure 5. As shown in Figure 5(a), the rod portion 21 of punch A is inserted through the lower opening of the hole 20 of die D, and a spacer 30 is placed between the upper surface of the disc portion 22 of punch A and the lower surface of die D. The thickness of the spacer 30 is approximately three times the finished thickness of the molded body 9. The upper part of the hole 20 of die D becomes a cylindrical cavity with a height corresponding to the thickness of the spacer 30 and opens to the upper surface of die D.

[0037] As shown in Figure 5(b), the KNN calcined powder 40 is filled into the cavity of the die D's hole 20 so as to completely fill it without any gaps and so as not to leak out of the cavity. The KNN calcined powder 40 is mixed with an adhesive (organic binder) as a binder to prevent the molded body from swelling, falling apart, or cracking after press molding.

[0038] As shown in Figure 5(c), die D is moved relatively upward relative to punch A, and spacer 31 is added. The cavity of the hole in die D expands, and another cavity is created above the filled KNN calcined powder 40. The rod portion 23 of punch B is inserted into this cavity.

[0039] As shown in Figure 5(d), spacers 30 and 31 are removed, and opposing forces are applied to punches A and B with a press machine as indicated by the arrows, thereby applying pressure to the KNN calcined powder 40 and solidifying it.

[0040] As shown in Figure 5(e), after pressing, the punch B is removed and the die D is pushed down as indicated by the arrow, allowing the molded body 9 to be removed from the die D.

[0041] The diameter of the molded body 9 is determined by the inner diameter of the hole 20 in the die D of the mold. The thickness of the molded body 9 can be arbitrarily set by appropriately adjusting the thickness of the spacers 30 and 31. In this embodiment, multiple types of molds were manufactured with the inner diameter of the hole 20 in the die D and the outer diameters of the rod portions 21 and 23 of punches A and B ranging from 15 mm to 35 mm. The press pressure was set to approximately 100 MPa regardless of the dimensions of the mold. Specifically, molded bodies 9 of multiple dimensions were manufactured, ranging from a thickness of 3 mm and a diameter of 15 mm (press pressure of 2 tons) to a thickness of 5 mm and a diameter of 35 mm (press pressure of 10 tons), and were used to manufacture KNN single crystals, all of which yielded good results. The molding was performed by pressing only; no heating was used.

[0042] [5] The small pieces 7 cut into 1mm to 2mm squares can be easily picked up with tweezers, and are easy to handle. Furthermore, because the plate-shaped NN single crystals 1 are covered with a soft clay-like material, even inside the circular oriented clay-like green material 10 that is joined to the molded body 9 by compression, the plate-shaped NN single crystals 1 are hardly crushed and maintain their original shape and aligned orientation. In this way, by using the clay-like green material, fragile plate-shaped NN single crystals, which are small in size (20μm to 30μm square) and difficult to handle on their own, can be joined to a molded body with an aligned orientation through simple operations.

[0043] [6] For the preparation of the clay-like green material 4, KNN calcined powder 2, which has a higher AGG activity than the KNN calcined powder used in the molded body 9, is used. The AGG activity of the KNN calcined powder is adjusted by changing the amount of Bi2O3 that is added to the KNN calcined powder after it has been prepared. If the amount of Bi2O3 added decreases, the AGG activity increases, and if it increases, the AGG activity decreases. By making only the area near the plate-like NN single crystal 1 have a higher AGG activity, and by cutting small pieces of the oriented clay-like green material 5 into 1mm to 2mm square pieces 7 and joining them to a very limited part of the molded body 9, it is possible to improve the possibility of selective growth of KNN single crystals using the plate-like NN single crystal 1 as a growth nucleus, while keeping low the possibility of KNN single crystals growing larger in other parts of the molded body 9 where the AGG activity is relatively low.

[0044] [7] The proportion of plate-shaped NN single crystals 1 mixed into the clay-like green material 4 should be very small, less than 1 / 10000 of the KNN calcined powder 2. If the amount of plate-shaped NN single crystals 1 mixed in is too small, the probability of KNN single crystals growing using the plate-shaped NN single crystals 1 as seed crystals will fall below a certain level. If the amount mixed in is too large, KNN single crystals will grow simultaneously from multiple plate-shaped NN single crystals 1 contained in the oriented clay-like green material 10 joined to the molded body 9 and compete, making it difficult to obtain large KNN single crystals. The amount of plate-shaped NN single crystals 1 mixed in should be appropriately adjusted so that a large KNN single crystal grows as a single, enlarged plate-like crystal, with only one single crystal. Specifically, since the weights and ratios of the plate-like NN single crystals 1, KNN calcined powder 2, and green components (binder and solvent) contained in the initially prepared clay-like green material 4 are known, the number of plate-like NN single crystals 1 contained in one small piece 7 can be determined from the weight of one small piece 7 produced from this material and the specific gravity of the plate-like NN single crystals 1. The ratio of each component included when initially preparing the clay-like green material 4 should be determined so that this number matches the value mentioned above.

[0045] [8] When the molded body 9 shown in the right-hand figure of Figure 3, that is, the disc-shaped molded body 9 in which an oriented clay-like green material 10 containing an appropriate number of plate-like NN single crystals 1 is provided in the center of the upper surface, is processed by the high-speed SSCG method (B), a KNN single crystal with a size of 15 mm square or larger and the same thickness as the molded body 9 is grown. The lower part of Figure 6 is a photograph of a plate-like crystal produced by the high-speed SSCG method (B) while controlling the growth orientation in this embodiment, and the upper part of Figure 6 is a diagram in which the edges that appeared on the single crystal in the same photograph are indicated by dashed lines. Inside the disc-shaped molded body 9, a large, somewhat light-colored rectangular KNN single crystal is growing, surrounded by four black, arc-shaped KNN polycrystalline bodies (ceramics). In the center of the upper surface of the KNN single crystal, an edge 11 with a shape close to a square is visible, suggesting that the same (100)(001) plane as the plate-like NN single crystal has grown.

[0046] Figure 7 shows the measurement results obtained by powder X-ray diffraction analysis of the crystal surface shown in Figure 6. Sharp diffraction peaks 12 were observed only near 22° and 45° of 2θ, confirming that the upper and lower surfaces of this single crystal are parallel to the (100)(001) plane. In large plate-shaped single crystals, it is practically easy to apply driving voltage and stress to the upper and lower surfaces, which are parallel and have a large area, and the induced strain and charge can be easily extracted from these upper and lower surfaces. Figures 6 and 7 show that the crystal growth orientation was correctly controlled so that these upper and lower surfaces of the plate-shaped KNN single crystal coincide with the (100)(001) plane, which exhibits high piezoelectricity in KNN-based materials.

[0047] From the molded body 9 on which the KNN single crystal has grown, as shown in the lower part of Figure 6 and the left part of Figure 8, four black, arc-shaped KNN polycrystalline bodies (ceramics) can be cut and removed as shown in the right part of Figure 8 to obtain a rectangular plate-shaped KNN single crystal. Then, this KNN single crystal can be cut into sizes and shapes suitable for the intended use as a piezoelectric material, etc., using a cross section perpendicular to the (100)(001) plane (a cross section perpendicular to the plane of paper in Figures 6 and 8).

[0048] Furthermore, in the above manufacturing process, if the SSCG method (B) is continued, it is possible to make the entire disc-shaped molded body 9 a KNN single crystal. However, in that case, cracks and warping often occur in the finished KNN single crystal. For this reason, in the manufacturing method of the embodiment, heating is stopped at an appropriate time so that the four corners of the grown rectangular (square) KNN single crystal do not reach the outer surface of the circular molded body 9. As a result, as mentioned above, a large, approximately square-shaped KNN single crystal that is free from cracks and warping and easy to process can be obtained in a subsequent process of simply cutting and removing the four black, arc-shaped KNN polycrystalline bodies (ceramics).

[0049] [9] The raw materials required for the production of small-sized plate-like NN single crystals using the one-step molten salt method are several tens of grams each of Bi2O3, Nb2O5, and NaCl reagents of general quality grade, and the only necessary equipment is a platinum dish with a capacity of 150 mL to 200 mL as a reaction vessel and a relatively inexpensive muffle furnace with an upper heating temperature of about 1250 °C. Using these, 10 can be synthesized in one step. 9 More than 10 plate-like NN single crystals can be produced. In this embodiment, the number is reduced to 50% or less by the molten salt treatment after synthesis, but still 10 8 More than 10 plate-like NN single crystals can be obtained at once. This number is 3 × 10 when using the method of the embodiment, compared to the conventional SSCG method (A) and the high-speed SSCG method (B). 6 This amount is sufficient to produce more than [number] KNN single crystals. Therefore, according to this embodiment, the cost ratio of the seed crystal used is significantly reduced, making it possible to drastically reduce the manufacturing cost of practical alkali niobium single crystals.

[0050] Here, referring to the comparison diagram in Figure 9 and the photographs shown in Figures 6 and 10, we will explain again the differences between the method for manufacturing KNN single crystals according to this embodiment and the conventional crystal growth methods described earlier in the sections on "Background Art" and "Problems to be Solved by the Invention."

[0051] Figure 9(a) shows a typical ceramic firing process. In this method, by heating a molded body of KNN calcined powder, the aggregates of small single crystal grains (polycrystalline) that make up the molded body grow uniformly into aggregates of slightly larger single crystal grains (also polycrystalline), and the ceramic is sintered.

[0052] Figure 9(b) shows the normal solid-phase crystal growth (SSCG(A)) described in Non-Patent Literature 1. In this method, a very expensive, large KTaO3 crystal embedded in the center of the KNN calcined powder molded body undergoes AGG (anomalous grain growth), absorbing the surrounding calcined powder and growing into a single crystal. However, a ceramic region is formed around the single crystal, and the crystal grains of this ceramic region are not easily incorporated into the central single crystal, so the single crystal remains at a certain size. In other words, the growth rate of the single crystal is low, and the dimensions of the final single crystal are also small.

[0053] Figure 9(c) shows the high-speed solid-phase crystal growth (SSCG(B), SFSSCG method) described in Non-Patent Literature 2. According to this method, by manipulating the chemical composition of the KNN calcined powder constituting the molded body to cause the activity of AGG (anomalous grain growth) to run wild, some of the calcined powder in the molded body becomes a growth nucleus without the use of seed crystals, and multiple single crystals grow extremely rapidly and disorderly. The enlarged single crystals compete with each other and the growth saturates. The shape and growth orientation of the enlarged single crystals become disordered.

[0054] Figure 9(d) shows the high-speed solid-phase crystal growth (SSCG(B), RSSCG method) described in Non-Patent Literature 3, and Figure 10 shows a photograph of the appearance of a KNN single crystal obtained by this method (bottom figure) and a diagram (top figure) in which the edges appearing on the single crystal in the same photograph are indicated by dashed lines. According to this method, by adjusting the AGG activity of the calcined powder constituting the molded body to a moderately high level without causing it to run wild, a single crystal can be rapidly enlarged from only a single growth nucleus. However, the process window in which single growth can be achieved is narrow (i.e., the fabrication conditions are strict), and therefore, even if a large single-plate-like single crystal is obtained, the growth orientation is undetermined and cannot be controlled. As shown in Figure 10, the plane that appears to have Miller indices of (100)(001) is a KNN single crystal that grew nonparallel to the upper and lower surfaces of the molded body, and in particular, as can be seen from the dashed lines shown in the upper figure (photograph) of Figure 10, the shape of the edges in the central part is greatly distorted, and the crystal orientation is unknown. This state is schematically represented in the right-hand diagram of Figure 9(d). To utilize such a KNN single crystal, it is necessary to determine its crystal orientation and cut it along the appropriate direction, which would result in significant costs for commercialization.

[0055] Figure 9(e) shows an embodiment of the present invention, and Figure 6, as described above, is a photograph of the appearance of a KNN single crystal obtained by the embodiment (bottom figure) and a diagram showing the edges appearing on the single crystal in the same photograph with dashed lines (top figure). In the embodiment, an appropriate amount of inexpensive plate-shaped NN single crystals 1 with aligned orientations are mixed with a clay-like green material 4 of calcined powder with relatively high AGG activity, attached to a molded body made of calcined powder with relatively low AGG activity, and the RSSCG method is performed. The plate-shaped NN single crystals rapidly enlarge by consuming the surrounding calcined powder with high activity, and large single-plate KNN single crystals are formed, and a large single-plate KNN single crystal is obtained in which the growth orientation is the same as that of the plate-shaped NN single crystal and Miller index (100)(001) planes appear on the top and bottom surfaces.

[0056] As described above, in the method for manufacturing KNN single crystals of this embodiment, the KNN calcined powder 2 mixed into the clay-like green material 4 has a higher AGG activity than the KNN calcined powder constituting the molded body 9. Therefore, when small pieces 7 of the clay-like green material 4 are attached to the molded body 9 and solid-phase crystal growth is performed, it is possible to make the area near the plate-like NN single crystal 1 contained in the small piece 7 have a high AGG activity.

[0057] Furthermore, in the method for manufacturing KNN single crystals according to the embodiment, the size of the small piece 7 attached to the molded body 9 is made sufficiently smaller than the size of the upper surface of the molded body 9 to which the small piece 7 is attached. As a result, the small piece 7 is joined to a limited part of the molded body 9, reducing the possibility of KNN single crystals growing from other parts of the molded body 9 where the AGG activity is low.

[0058] According to the experimental results, when the size of the small piece 7 is approximately 1-2 mm square, and the diameter of the molded body is within the range of 15 mm to 35 mm, the KNN single crystal, which has grown using the plate-like NN single crystal as a seed crystal, rapidly increases in size and spreads throughout the molded body, allowing for the production of large, single-plate KNN single crystals without any problems. Therefore, there is no particular numerical limit on the area ratio between the small piece and the upper surface of the molded body, but the smaller the dimensions of the small piece, the better in order to avoid the single crystal growth starting from different locations on the upper surface of the molded body. However, if it is too small, it becomes difficult to pick up with tweezers and hinders handling, so in this embodiment, it was set to 1-2 mm.

[0059] Furthermore, according to the KNN single crystal manufacturing method of this embodiment, by appropriately setting the ratio of plate-shaped NN single crystals 1 mixed into the clay-like green material 4 to the KNN calcined powder 2, it is not the case that there are too few plate-shaped NN single crystals 1, which would reduce the possibility of KNN single crystals growing from the plate-shaped NN single crystals 1 as seed crystals to below a certain level, nor is it the case that there are too many plate-shaped NN single crystals 1, which would cause KNN single crystals to grow simultaneously from multiple plate-shaped NN single crystals 1 and compete with each other. As a result, a single, large KNN single crystal can be obtained through appropriate single crystal growth. [Explanation of symbols]

[0060] 1. Plate-like NN single crystal 2 KNN calcined flour 3 rollers 4. Clay-like KNN calcined powder green material containing plate-like NN single crystals. 5. Oriented KNN calcined powder green material in which the internal plate-like NN single crystals are aligned by roller rolling. 6. Release sheet 7 Small pieces of oriented clay-like green material 5 8. Pressing operation using a press machine 9. Molded body made from KNN calcined powder 10. Oriented clay-like green material that has been compressed into a circular shape and joined to a molded body. 11. Square-shaped edges of KNN single crystals 12 X-ray diffraction peaks

Claims

1. A viscous material is produced containing plate-like NN single crystals and a first KNN calcined powder with relatively high AGG activity. Shear stress is applied to the viscous material to align the orientation of the plate-like NN single crystals in a specific direction, and the resulting material is cut into small pieces. The small pieces are attached to a molded body made of a second KNN calcined powder having a relatively lower AGG activity than the first KNN calcined powder. A method for producing a KNN single crystal, characterized by processing the molded body by a solid-phase crystal growth method and growing a KNN crystal in an orientation corresponding to the crystal structure of the plate-like NN single crystal from the region of the molded body to which the small piece is attached.

2. The method for producing a KNN single crystal according to claim 1, characterized in that the plate-shaped NN single crystal is subjected to a molten salt treatment using Na2CO3 before the production of the viscous material.

3. The method for producing a KNN single crystal according to claim 2, characterized in that the molten salt treatment is performed using a salt obtained by diluting Na2CO3 with NaCl.

4. The method for producing a KNN single crystal according to claim 2 or 3, characterized in that the molten salt treatment is performed for a processing time such that the atomic ratio of Bi to Nb contained in the plate-like NN single crystal becomes less than 3%.

5. A method for producing a KNN single crystal according to claim 1, characterized in that shear stress is applied to the viscous material by a rolling process using rollers.

6. The method for manufacturing a KNN single crystal according to claim 5, characterized in that the dimensions of the small piece are 1 mm to 2 mm square.

7. The method for producing a KNN single crystal according to claim 6, characterized in that the number of plate-like NN single crystals contained in the small piece is 10 to 30.

8. A single rectangular KNN single crystal surrounded by four black, arc-shaped KNN polycrystalline bodies, wherein the Miller indices of its upper and lower surfaces are (100)(001).

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