JP2012232862A - Anisotropically shaped powder and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide plate-like crystals having a relatively large particle size and having little contamination of crystals of a relatively small particle size.SOLUTION: Hydrothermal synthesis is carried out by adding oxide powder having an orthorhombic crystal structure and surface active agent to alkali hydroxide solution to synthesize seed crystals, hydrothermal synthesis is carried again by adding seed crystals, oxide powder having an orthorhombic crystal structure, and surface active agent to the seed crystals and the alkali hydroxide solution, the plate-like crystals obtained after the reaction of the second hydrothermal synthesis is washed with an organic solvent, the reaction product of the plate-like crystals after the washing is sintered at 170°C or higher and 700°C or lower.

Description

  The present invention relates to an anisotropic shaped powder composed of oriented particles in which a specific crystal plane is oriented, and a method for producing the same.

Recently, a tendency to eliminate heavy metal harmful elements such as Pb, Hg, Cd, and Cr ^{6+ has been} increased due to heightened awareness of environmental protection, and a ban on use (RoHS Directive) has been issued and enforced mainly in Europe. Lead oxide (PbO), which is a raw material that plays an important role in enhancing the functionality of electronic materials, is also a subject of concern because of environmental concerns regarding disposal issues. A wide variety of materials such as single crystals, thick films, and thin films have been developed as piezoelectric materials constituting piezoelectric devices that are widely used in fields such as electronics, mechatronics, and automobiles, with a focus on ceramics. Piezoelectric ceramics occupying most of them are Pb-based perovskite ferroelectric ceramics, and the mainstream is PbZrO ₃ —PbTiO ₃ (PZT), which contains a large amount of lead oxide as a main component. I have a problem.

  In view of this situation, research into lead-free piezoelectric materials that considers the environment is considered urgent and indispensable. Research and development of high-performance lead-free piezoelectric ceramics comparable to the performance of current PZT piezoelectric ceramics Has attracted worldwide interest.

Among them, in recent years, a composition and a manufacturing method having relatively high piezoelectric characteristics have been devised for niobate-based KNbO ₃ —NaNbO ₃ —LiNbO ₃ -based ceramics. The composition has a Curie temperature of about 250 ° C., and the piezoelectric constant d33 is 400 pm / V. Some are close to practical performance that is practical (Non-Patent Document 1).

In Patent Document 1, a plate-like powder such as NaNbO _{3 in} which a specific crystal plane is oriented and a reaction raw material are mixed, a mixture obtained by mixing is formed into a sheet, and a laminate is obtained by laminating a plurality of obtained sheets. An isotropic perovskite compound represented by the general formula: ABO ₃ by rolling, degreasing, and hydrostatic pressure (CIP) treatment of the laminate, and heating in oxygen, which is an A site It consists of a polycrystal having a main phase of a first perovskite type pentavalent metal acid alkali compound in which the main component of the element is K and / or Na and the main component of the B site element is Nb, Sb and / or Ta. And the crystal orientation ceramics in which the specific crystal plane of each crystal grain which comprises this polycrystal body has orientated, and its manufacturing method are indicated.

  Further, Patent Document 2 discloses a manufacturing method that further improves Patent Document 1, omits CIP processing, and considers mass productivity.

  On the other hand, according to Patent Document 3, as a method for obtaining a plate-like metal titanate compound, an oxide or hydroxide of titanium oxide and an A element (at least one member of the group consisting of Na, K, Rb, and Cs). Alternatively, a salt and an oxide, hydroxide, or salt of M element (at least one member of the group consisting of Li, Mg, Co, Ni, Zn, Mn (III), and Fe (III)) are used in an aqueous medium. Prepared by hydrothermal synthesis at a reaction temperature of 120 to 300 ° C., and the resulting layered titanate is reacted with an acid to convert it into plate-like titanate hydrate, and further plate-like titanate hydrate Discloses a process for reacting at least one member of the group consisting of Mg, Ca, Sr, Ba, and Pb with an oxide, hydroxide, or salt in an aqueous medium under heating.

In Non-Patent Documents 2 and 3, a hydrothermal synthesis of KOH or NaOH and Nb ₂ O ₅ and a solution containing KOH, NaOH and Nb ₂ O ₅ were used, and SDBS as a surfactant was added thereto. In addition, a technique for performing hydrothermal synthesis is disclosed.

  Patent Document 4 discloses a thin film manufacturing method in which hydrothermal synthesis is performed in two stages of crystal nucleation and crystal growth when a complex oxide thin film is formed on a substrate.

JP 2003-12373 A JP 2008-74693 A JP 2007-22857 A Japanese Patent No. 2914286

Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, and M. Nakamura, "Lead-free piezoceramics", Nature, 432, Nov. 4, 84- 87 (2004) I.C.M.S.SANTOS, etal., Studies on the hydrothermal synthesis of niobium oxides, Polyhedron, 2002, Vol.21, pp.2009-2015 Fan ZHANG, et al., Hydrothermal synthesis of (K, Na) NbO3 particles, Japanese Journal of Applied Physics, 2008, Vol.47, No.9, pp.7685-7688

  In the techniques of Patent Documents 1 and 2, a plate-like crystal oriented in a specific plane is produced via a Bi compound, and a large amount of NaCl molten salt is used for the reaction. Therefore, in order to remove NaCl and Bi after the reaction from the reaction product, a process of washing with a large amount of water and acid is included, and a long process is required to obtain plate crystals. Many manufacturing processes were complicated.

  Moreover, the technique of patent document 3 is concerned with the method of manufacturing plate-shaped powder using a hydrothermal synthesis method. However, first, a layered titanate was obtained, then reacted with an acid to convert it into a plate-like titanic acid hydrate, and further subjected to a heat reaction in barium hydroxide.

  Patent Document 4 discloses a technique of performing a hydrothermal synthesis method in two stages to form a crystalline thin film of lead zirconate titanate having a thickness of several μm on the surface of a titanium substrate.

Non-Patent Document 2 shows that NaNbO ₃ or KNbO ₃ can be synthesized under relatively mild conditions by hydrothermal synthesis of NaOH and Nb ₂ O ₅ and KOH and Nb ₂ O ₅ . Non-Patent Document 3 uses a solution containing KOH, NaOH, and Nb ₂ O ₅ , adds a surfactant, SDBS, and performs hydrothermal synthesis to synthesize cubic or spherical NaNbO ₃ . The technology to do is disclosed. Moreover, although the possibility of the template by a plate-like crystal is mentioned, it was about 100 nm in thickness and about 1.5 μm in width and was small as a template.

  As described above, the techniques described in the prior art documents are cumbersome and time consuming, and are still insufficient for industrial production. Further, it has not been considered to stably obtain plate crystals having a larger particle size.

  The present invention has been made in view of the above circumstances, and it is possible to stably obtain a plate-like crystal having a relatively large particle size and containing less crystals having a relatively small particle size. One of the purposes.

  A method for producing an anisotropically shaped powder according to one embodiment of the present invention includes hydrothermal synthesis by adding an oxide powder having an orthorhombic crystal structure to an aqueous alkali hydroxide solution and a surfactant, A step of synthesizing the crystal, the seed crystal, an oxide powder having an orthorhombic crystal structure with the seed crystal and an alkali hydroxide aqueous solution, and a surfactant are added to perform hydrothermal synthesis again. A step, a step of washing the plate crystal obtained after the reaction of the hydrothermal synthesis again with an organic solvent, a step of firing the reaction product of the plate crystal after the washing at 170 ° C. or more and 700 ° C. or less, It is supposed to have.

  Here, the reaction temperature of hydrothermal synthesis for synthesizing the seed crystal is 150 ° C. or more and 250 ° C. or less, the reaction temperature of hydrothermal synthesis again is lower than the reaction temperature of hydrothermal synthesis for synthesizing the seed crystal, and It is good also as temperature exceeding 40 degreeC and less than 100 degreeC. The reaction time of hydrothermal synthesis for synthesizing the seed crystal may be 2 hours or more and 8 hours or less, and the reaction time of hydrothermal synthesis again may be longer than the reaction time of hydrothermal synthesis for synthesizing the seed crystal.

Further, the oxide powder may be Nb ₂ O ₅ having an average particle diameter of 100 nm or more and less than 2000 nm. Further, the solution concentration of an alkali hydroxide aqueous solution of potassium hydroxide (KOH) and sodium hydroxide (NaOH) used in at least hydrothermal synthesis for synthesizing seed crystals may be 4 mol / l or more and 8 mol / l or less.

  The anisotropic shaped powder according to one embodiment of the present invention is manufactured by the above-described manufacturing method, the average particle length in the major axis direction is 8 μm or more and 10 μm or less, and the average particle length in the major axis direction and the average particle length in the thickness direction are It has a pseudo cubic perovskite structure in which the ratio is 2 or more and 20 or less and the plane of the plate-like crystal is oriented in the (100) plane.

  According to the present invention, it is possible to stably obtain plate-like crystals that are suitable for industrial production, are less mixed with crystals having a relatively small particle size, and have a larger particle size and a uniform size.

It is explanatory drawing showing the example of the manufacturing method of the anisotropic shaped powder which concerns on embodiment of this invention. It is a schematic view showing the basic structure and crystal structure of the monoclinic Nb ₂ O _5. It is a schematic diagram showing the basic structure and crystal structure of orthorhombic Nb ₂ O ₅ . It is explanatory drawing showing the relationship between the addition amount of surfactant which concerns on embodiment of this invention, and the average major axis of plate-like crystal. It is explanatory drawing which shows the relationship between the heating (reaction) temperature by hydrothermal synthesis, and residual Nb ratio. It is a schematic view showing a _{K 4 Na 4 Nb 6 O 19} · 9H 2 O putative structure. Is an explanatory view showing an example of _{K 4 Na 4 Nb 6 O 19} · 9H 2 O SEM image in the case where no surfactant was used. It is a schematic view showing the putative structure of _{K 4 Na 4 Nb 6 O 19} · 9H 2 O in the case of using a surfactant. It is explanatory drawing showing the relationship between the molar concentration of KOH + NaOH solution which is alkaline aqueous solution, and the average major axis of a plate-like crystal. Example of product obtained as a result of performing hydrothermal synthesis again at 60 ° C. for 16 hours and at 40 ° C. for 16 hours in the method for producing anisotropic shaped powder according to the embodiment of the present invention. It is explanatory drawing showing. It is a diagram of the _{K 4 Na 4 Nb 6 O 19} · 9H 2 O Thermogravimetric analysis of. It is explanatory drawing showing the example of the SEM image when the plate-shaped crystal which concerns on embodiment of this invention is baked at each temperature. It is explanatory drawing showing the X-ray-diffraction pattern when the plate-shaped crystal which concerns on embodiment of this invention is baked at each temperature. SEM image of anisotropic shaped powder according to an embodiment of the present invention when hydrothermal synthesis is performed again at 60 ° C. for 10 hours and at 80 ° C. for 10 hours It is. It is explanatory drawing showing the example of an X-ray-diffraction pattern when the plate-shaped crystal which concerns on embodiment of this invention is sheet-molded and baked. It is a diagram of a Nb ₂ O ₅ powder of particle SEM images are used with X-ray diffraction pattern anisotropically shaped powder of the manufacturing method according to an embodiment of the present invention. It is the SEM image of the seed crystal produced | generated with the manufacturing method of the anisotropic shaped powder which concerns on embodiment of this invention, and the SEM image of the reaction product obtained by the again hydrothermal synthesis. It is a SEM image of anisotropically shaped powder concerning one mode of an embodiment of the invention. It is a SEM image of anisotropically shaped powder concerning a comparative example. It is explanatory drawing showing distribution of the particle size of the seed crystal which concerns on one Example of this invention. It is explanatory drawing showing distribution of the particle size of the plate-shaped crystal which concerns on one Example of this invention. It is explanatory drawing showing the example of the X-ray-diffraction pattern which concerns on a comparative example.

  A method for producing the anisotropic shaped powder according to the embodiment of the present invention will be described. The anisotropically shaped powder refers to flat particles having a major axis having a longer dimension in the longitudinal direction than the dimension in the width direction or the thickness direction (that is, the diameter of a virtual circle circumscribing the crystal), Here, they are called plate crystals.

  As illustrated in FIG. 1, the method for producing an anisotropic shaped powder according to the present embodiment includes a process of synthesizing a plate crystal serving as a seed crystal (S 1 to S 6), and a plate crystal using the seed crystal. (S7 to S12).

  In the process illustrated in FIG. 1, first, an alkali hydroxide aqueous solution is generated (S1). Here, the alkali hydroxide aqueous solution is obtained by, for example, dissolving potassium hydroxide (KOH), sodium hydroxide (NaOH), or both of these in ion-exchanged water at a predetermined ratio. Note that the above ratio when using both KOH and NaOH, the ratio of K / Na, is in the range of 0.5 to 10.

This is due to the following reason. That is, the reactivity of K and Na is different, and Na is easier to react with niobium oxide (Nb ₂ O ₅ ) added later than K. In general, K _0.5 Na _0.5 NbO ₃ is known as a lead-free piezoelectric material containing K, Na, and Nb, and composition improvements centering on this composition have been widely performed. Therefore, a plate-like crystal is produced centering on the composition system, but the composition of the reaction product is centered on NaNbO ₃ as the K / Na ratio is reduced. On the other hand, the larger the K / Na ratio, the more the composition of the reaction product is centered on KNbO ₃ . As a result of experiment, it was found that the ratio of K / Na is particularly preferably 1.5 or more and 3.5 or less in order to obtain a composition in the vicinity of K _0.5 Na _0.5 NbO ₃ .

  Further, it was found that when K / Na is less than 0.5, no plate-like crystal is formed, and it is found that plate-like crystals can be obtained even when K / Na is in the range of 0.5 to less than 1. When K / Na is in the range of 0.5 to 1.5, the melting point can be increased because the amount of Na is relatively large. When a raw material is added to a plate-like crystal to be a template later to form a sheet and fired, it is difficult to obtain a sintered body if the melting point of the template is lower than that of the raw material. The firing temperature of a normal piezoelectric body is 1200 ° C. or higher and 1300 ° C. or lower.

  Furthermore, considering the melting point of the electrode or the like, low temperature firing at about 900 ° C. or more and 1000 ° C. or less is advantageous. In this manner, the K / Na ratio is set including the viewpoint of the firing conditions corresponding to the firing temperature of each material. In the present embodiment, the ratio of K / Na is in the range of 0.5 to 10.

The solution concentration (molar concentration) of the alkali hydroxide aqueous solution (here KOH + NaOH solution) when obtaining a composition in the vicinity of K _0.5 Na _0.5 NbO ₃ is 4 mol of the KOH + NaOH solution. / L or more and 8 mol / l or less is preferable. When the solution concentration is 4 mol / l, a plate-like crystal having an average particle length in the major axis direction (longitudinal direction) of about 12 μm can be obtained. However, when the concentration of the solution is lower than 4 mol / l, for example, the efficiency of reaction with niobium oxide (Nb ₂ O ₅ ) is extremely deteriorated, and the formation of plate crystals is slowed down. It was confirmed by experiments that when the solution concentration reached 2 mol / l, the reaction did not occur and the powder was crushed.

  On the other hand, the higher the solution concentration is in the above range, the better the reaction proceeds and the crystal grain size tends to decrease. Further, when the solution concentration is increased, the excess alkali component to be cleaned increases, and a large amount of organic solvent is required for cleaning, which leads to an increase in subsequent work load and cost. For this reason, the molar concentration range of the KOH + NaOH solution is more preferably 4 mol / l or more and 6 mol / l or less. It was found that the reaction proceeded too much when the solution concentration exceeded 8 mol / l, and the formation of plate crystals was dull, and the crushed state was reached at 10 mol / l.

The reason why the crystals are in a crushed state when the solution concentration is outside the above range is considered to be related to the crystal structure of the oxide powder as shown below. That is, when the solution concentration is too low, reaction with Nb ₂ O ₅ does not occur, and bonding between adjacent crystals does not occur, so that the plate does not have a plate shape. On the other hand, if the solution concentration is too high, the reaction proceeds so much that the bonded portion between the crystals is roughly cut, resulting in a crushed reaction product. From the above, the upper limit of the solution concentration is 8 mol / l, the preferred upper limit is 6 mol / l, and the molar concentration of the KOH + NaOH solution is 4 mol / l or more and 8 mol / l or less (more preferably 6 mol / l or less). ). This solution concentration is not related to the ratio of K / Na.

Next, the oxide powder is put into the aqueous alkali hydroxide solution (here, KOH + NaOH solution) generated as described above, and the mixture is stirred and mixed (S2). Here, oxide powder having an orthorhombic crystal structure is used. For example, Nb ₂ O ₅ , TiO ₂ , Ta ₂ O ₅ or the like having an average particle diameter (arithmetic average value of the diameter of the circumscribed circle of the crystal) of 100 nm or more and less than 2000 nm (0.1 μm or more and less than 2 μm) is used. The smaller the particle size of this powder, the better the reactivity. However, if the particle size is less than 100 nm, it tends to agglomerate and it becomes difficult to adjust the particle size, so that there is a problem that a commercially available grinder with mass productivity cannot be used. Further, when the thickness is 2000 nm or more, there is a problem that it is difficult to form a plate-like crystal.

Here, the crystal structure of the oxide powder such as Nb ₂ O ₅ is preferably orthorhombic. This is because an orthorhombic system having no oxygen defects is more likely to form a plate-like crystal than a monoclinic system having oxygen defects. This is considered to be due to the following reasons. For example, two types of crystal systems, known as monoclinic and orthorhombic, are known for Nb ₂ O ₅ as described above. Monoclinic Nb ₂ O ₅ is shown in FIG. In the monoclinic system, an NbO ₆ basic structure in which oxygen is arranged at the apex of the regular octahedron and Nb is arranged at the center thereof is connected in such a manner that the apexes are shared to form the entire crystal structure. Therefore the actual chemical formula _Nb 2 _O in ₅ without Nb ₂ O _5-[delta] and oxygen is insufficient little. When this monoclinic Nb ₂ O _5-δ reacts with an alkali component to produce the crystal structure of K ₄ Na ₄ Nb ₆ O ₁₉ · 9H ₂ O described below, It is considered that the bond is broken and the reaction proceeds. As a result, compared to the orthorhombic system, the formed particles have no shape anisotropy.

On the other hand, the orthorhombic Nb ₂ O ₅ shown in FIG. 3 has two types of structures: the octahedral NbO ₆ basic structure and the icosahedral NbO ₇ basic structure in which oxygen is arranged at the apex and Nb is arranged at the center. A crystal structure is formed in combination. Then, as can be seen from FIG. 3, this structure has not only vertices but also a part that shares the side in the plane, and the part that shares the side has a stronger coupling strength than the share of only the vertex. it is conceivable that. Therefore, it is considered that the bond is easily cut between the layers sharing only the apex, and this is considered to cause the reaction product to easily form an anisotropic shape, that is, a plate shape. Such an effect is the same even when orthorhombic TiO ₂ or Ta ₂ O ₅ is used.

For example, when the K / Na ratio is 1.5, the amount of oxide powder (Nb ₂ O ₅ ) used relative to the alkali hydroxide aqueous solution (here, KOH + NaOH solution) is about 1 wt% or more and 15 wt% or less. Is preferred. This is because the production efficiency of the reaction product is poor when the amount is less than 1 wt%, and when the amount exceeds 15 wt%, the reaction cannot be completed and unreacted components remain, which is not efficient. From the viewpoint of production efficiency, more preferably, the amount of the oxide powder (Nb ₂ O ₅ here) used relative to the aqueous alkali hydroxide solution (here, KOH + NaOH solution), for example, when the ratio of K / Na is 1.5, 2 wt% or more and 10 wt% or less. In addition, since it is a hydroxide ion that determines this reaction, regardless of the ratio of K / Na, for example, even when the ratio of K / Na is 3.5, it is set to 2 wt% or more and 10 wt% or less.

Further, a surfactant such as SDBS (sodium dodecylbenzenesulfonate) is added here (S3). When SDBS is used as the surfactant and Nb ₂ O ₅ is used as the oxide powder, the amount of SDBS added is, as illustrated in FIG. 4, the particle size of the reaction product (diameter of circumscribed circle of the product) Is known to affect. Fig. 4 shows the amount of SDBS added and the reaction products shown in Shan Bai, et. Al., "Influence of Hydrothermal Conditions on size of template particles for textured ceramics", JJAP, Vol. 50 (2011), 01BE17. It is explanatory drawing showing the relationship between a particle size and thickness.

In the present embodiment, a suitable ratio of the particle diameter to the thickness is found with reference to FIG. 4, and in order to obtain the suitable ratio, a range of 0.1 wt% to 5 wt% with respect to Nb ₂ O ₅ To do. If it is less than 0.1 wt%, the particle size of the reaction product after reaction (diameter of the circumscribed circle of the product) becomes large and it is difficult to form a plate crystal. On the other hand, if it exceeds 5 wt%, the plate crystals become too fine. According to the experiment, the more preferable addition amount of SDBS was 0.5 wt% or more and 3 wt% or less with respect to Nb ₂ O ₅ . At the time of implementation, for example, it may be 1.5 wt%.

  The mixed solution obtained by adding the surfactant is placed in an autoclave container lined with Teflon (registered trademark) and sealed, and heated at a preset temperature for a preset time to perform hydrothermal synthesis (S4). . The heating temperature is between 150 ° C. and 250 ° C. Below 150 ° C, the reaction is incomplete. The limit of 250 ° C. is in consideration of the heat resistance of the Teflon-lined autoclave. Desirably, it is 150 degreeC or more and 200 degrees C or less. The heating time in the process S4 is about 2 hours to 8 hours. If it is shorter than 2 hours, the reaction becomes insufficient. In addition, the progress of the reaction changes logarithmically, and the reaction is remarkable in the initial stage. However, the reaction settles down as time passes, and even if the time is extended, the change is small, so the heating time is lower than 2 hours. The upper limit is not particularly set and may be set in consideration of productivity. For example, the heating temperature is 150 ° C. and the heating time is 4 hours.

  Here, the temperature during hydrothermal synthesis will be described. FIG. 5 is an explanatory diagram showing the relationship between the heating (reaction) temperature by hydrothermal synthesis and the residual Nb ratio. The residual Nb ratio on the vertical axis is about 10 ° (f1), about 11 ° (f2) and about 15 ° (f3) with respect to the diffraction intensity by X-ray diffraction at each heating temperature, with 2θ being about 28 ° (f4). ) Ratio f4 / (f1 + f2 + f3). As can be understood from the experimental example shown in FIG. 5, the amount of residual Nb can be reduced by raising the heating temperature, can be reduced from 150 ° C. to 4%, and is minimal at around 180 ° C. Residual Nb indicates an unreacted content with K and Na. If this unreacted content is too large, K and Na are added to the sheet raw material in the subsequent sheet forming stage, and the unreacted Nb at the time of firing. May need to be eliminated by reacting. Thus, since the residual Nb becomes an extra component after all, it is desirable to reduce it at the stage of hydrothermal synthesis, and the adjustment of the component at the time of sheet forming can be facilitated by the reduction.

If no surfactant is used at this stage, fine particles of K ₄ Na ₄ Nb ₆ O ₁₉ · 9H ₂ O (hereinafter referred to as “446”) are first generated. The structure of the 446 crystal is a structure in which both sides of the two oxygen octahedron layers 12 are sandwiched by one crystal water layer 11 as shown in FIG. Due to this crystal structure, the 446 crystal powder has a slightly slower growth rate in the crystal water plane direction than the direction parallel to the crystal water plane, and is a flat spherical (meteorite) powder as shown in the SEM photograph shown in FIG. To grow.

However, in this flat spherical powder, the ratio of diameter to thickness is not a value necessary for producing oriented ceramics. When the synthesis temperature is higher than 200 ° C. or the synthesis time is longer than 8 hours, the high-potential 446 powder gradually disappears and the synthesis reaction further proceeds, and the (K, Na) NbO _{3 having the} most stable perovskite structure. It changes to fine particles (hereinafter referred to as “112”). This 112 crystal is a perovskite ferroelectric, and has spontaneous polarization, so that it is charged and has a property of being easily aggregated. When 112 particles agglomerate and grow to a crystal of several hundred nanometers or more, the difference in surface energy of the crystal plane is large, so that eventually it becomes a cube and also becomes a large particle due to aggregation. This is an unsuitable particle shape.

  On the other hand, when a surfactant is used, first, 446 crystals are formed. However, as shown in FIG. 8, surfactants (including anionic, cationic, nonionic, and amphoteric) 13 are attached to crystal water layer 11 or oxygen octahedron layer 12 of 446 crystal particles. Since the surfaces are covered, the surfactant 13 inhibits contact of components necessary for crystal growth on these surfaces. Therefore, the growth rate of the 446 crystal in the direction of the crystal water surface 11 is suppressed more strongly than when no surfactant is used, and the formation of a thinner plate-like crystal is promoted as compared with the case where no surfactant is used. The surfactant also plays a role of dispersing 446 crystals.

  Furthermore, as illustrated schematically in FIG. 8, the surfactant 13 is attached to the crystallization water layer 11 or the oxygen octahedron layer 12 of 446 particles, so that the potential of the 446 crystal is lowered and the synthesis temperature is set to 200 ° C. Even if it is made higher or the synthesis time is increased to 8 hours or more, only 446 particles grow large, and the perovskite structure does not change. For this reason, the ratio of the thickness to the size and diameter of the formed 446 particles can also be controlled by adjusting the type, amount used, synthesis temperature, and synthesis time of the surfactant, for example, potassium sodium niobate type A plate-like crystal which is a plate-like template suitable for producing high-performance lead-free piezoelectric oriented ceramics can be produced. For example, a plate-like crystal having a diameter of about 4 μm and a thickness of about 250 nm (diameter / thickness ratio of 16: 1) is produced at 150 ° C. for 4 hours.

  When SEM images of the reaction products obtained at the respective molar concentrations were observed, the relationship between the molar concentration of the KOH + NaOH solution, which is an alkaline aqueous solution, and the average major axis of the plate crystals was as shown in FIG. That is, it was found that plate crystals having an average major axis of 3 μm or more and 12 μm or less can be obtained by setting the molar concentration of the KOH + NaOH solution to 4 mol / l or more and 8 mol / l or less.

  As the surfactant, SDBS is used in the above example, but it is a surfactant that adheres to the crystal water layer or the oxygen octahedron layer and has an effect of inhibiting the contact between the crystal surface and the reaction product. Instead of this, as an anionic surfactant, in addition to SH (sodium hexametaphosphate), SDS (sodium dodecyl sulfate), LIDS (lithium dodecyl sulfonate), HDBS (dodecylbenzene sulfonic acid), etc. It may be used. Further, as a cationic surfactant, CTAC (hexadecyltrimethylammonium chloride), DTAC (dodecyltrimethylammonium chloride), DDAC (didodecyldimethylammonium chloride), DODAC (dioctadecyldimethylammonium chloride) or the like may be used. . In addition to PEG (polyethylene glycol), PVA (polyvinyl alcohol), PA (polyacrylamide), AGE (alkyl monoglyceryl ether), and the like may be used as the nonionic surfactant. In addition, LAB (lauryl dimethylaminoacetic acid betaine), ADAO (alkyl dimethylamine oxide), ACB (alkyl carboxy betaine), or the like may be used as the amphoteric surfactant.

  Thereafter, the system is cooled, and after cooling, the container is opened, and the seed crystal as a reaction product is taken out (S5). This seed crystal includes plate crystals of various sizes. Thereafter, this seed crystal is filtered and washed with an organic solvent such as ethanol or methanol until neutrality is removed from the seed crystal as a reaction product (S6). At this time, it was found that in the case of washing with pure water, a part of the reaction product was dissolved in water to form a suspension. Therefore, in the present embodiment, a cleaning solution that can clean the alkali component and does not dissolve a part of the reaction product is used. In view of ease of handling and availability, an organic solvent such as ethanol or methanol is preferable, and methanol is preferable in consideration of cost. Note that step S6 is not necessarily performed.

Subsequently, using the seed crystal obtained here, the seed crystal is grown almost uniformly to generate a relatively large plate crystal (S7 to S12). Also here, first, an alkali hydroxide aqueous solution is generated (S7). Here, the aqueous alkali hydroxide solution is the same as that in step S1. Then, the oxide powder such as Nb ₂ O ₅ and the seed crystal generated up to step S6 are added to the alkali hydroxide aqueous solution (here, KOH + NaOH solution) generated in step S7, and stirred and mixed (S8). ). Here again, the oxide powder is the same as that used in step S2 and has an orthorhombic crystal structure.

  The seed crystal powder to be used is 50 wt% or less, particularly 20 wt% or more and 40 wt% or less with respect to the total powder.

Further, as in step S3, a surfactant such as SDBS (sodium dodecylbenzenesulfonate) is added here (S9). When SDBS is used as the surfactant and Nb ₂ O ₅ is used as the oxide powder, the amount of SDBS added is 0.1 wt% or more and 5 wt% or less with respect to Nb ₂ O ₅ with reference to FIG. Range.

  The mixed solution obtained by adding the surfactant is placed in an autoclave container lined with Teflon (registered trademark) and sealed, and heated at a preset temperature for a preset time to perform hydrothermal synthesis (S10). . This is the same as step S4 in which the seed crystal is generated, but the heating temperature is different. Here, even a synthesis at a heating temperature lower than that in the narrowly defined hydrothermal synthesis is referred to as a hydrothermal synthesis. That is, the heating temperature in this step S10 is set lower than the temperature in step S4 for generating the seed crystal and higher than 40 ° C.

  This is because the seed crystal does not grow at 40 ° C. as shown in FIG. 10 in which heating is performed at 60 ° C. for 16 hours (Example 4) and heating at 40 ° C. for 16 hours (Comparative Example 2). It is. Further, at a temperature higher than 100 ° C., the reaction is not a seed crystal but a reaction between the oxide powders newly added in step S8, and as a result, the growth in the major axis direction stops and the temperature changes to the growth in the thickness direction. it is conceivable that. The heating time grows as the time is longer than the time in the step S4 of generating the seed crystal, but even if it is 16 hours or more, it does not reach about 12 μm or more. Considering production efficiency, it should be 24 hours or less. Specifically, the heating temperature is 50 ° C. or more and 100 ° C. or less, the heating time is 1 hour or more and 6 hours or less, more preferably 60 ° C. or more and 80 ° C. or less and 2 hours or more and 4 hours or less. Experimentally, this temperature range is more suitable for seed crystal growth.

  After cooling, the container is opened after cooling, and the reaction product is filtered out and washed with an organic solvent such as ethanol or methanol until neutrality is removed from the reaction product (S11). . At this time, in the case of washing with pure water, a part of the reaction product may be dissolved in water to form a suspension, so that the alkaline component can be washed and a part of the reaction product is dissolved. Use non-cleaning solution. In view of ease of handling and availability, an organic solvent such as ethanol or methanol is preferable, and methanol is preferable in consideration of cost.

  Next, after the washing is completed, the reaction product is dried at a temperature of 70 ° C. to 200 ° C. or naturally dried at room temperature (S12), and further the reaction product is baked at 170 ° C. to 700 ° C. (S13). ). Note that step S12 may be omitted as it also serves as a drying step in step S13. By calcination in this step S13, the crystal water present in the crystals is dehydrated to obtain plate crystals. Although the firing atmosphere is not particularly limited, it may be performed in the air in combination with the previous drying step. FIG. 11 shows the results of thermogravimetric analysis of the plate crystals obtained in step S11. In FIG. 11, the upper part shows thermogravimetric analysis, the vertical axis represents the mass ratio, the horizontal axis represents the temperature, and the change in the mass ratio due to heating is shown. Here, when heated to a temperature of 170 ° C., a change in weight is observed, which is considered to be due to elimination of crystal water from the crystal at this temperature. From this result, it can be said that there is no effect of dehydrating crystal water unless the firing temperature is 170 ° C. or higher. Moreover, the lower part of FIG. 11 shows a differential thermal analysis result. From the graph in the lower part of FIG. 11, an endothermic effect is observed at 170 ° C., and it is considered that there is a change in crystal structure at 300 ° C. to 350 ° C.

  FIG. 12 shows an SEM image after the plate crystals obtained in step S13 were baked at 250 ° C., 450 ° C., and 600 ° C. for 1.5 hours and dehydrated. It can be seen that the plate-like shape is maintained at any temperature. The X-ray diffraction pattern of the plate crystal of this example is shown in FIG. According to FIG. 13, when the firing temperatures are 450 ° C. and 600 ° C., 2θ shows peaks at around 22 ° and around 32 °, and it can be seen that a perovskite structure is formed. It has been confirmed that when the firing temperature exceeds 700 ° C., cracks occur in the plate-like crystals, and the particles may sinter and lose their plate-like shape. Therefore, the firing temperature is preferably 350 ° C. or higher and 700 ° C. or lower, and more preferably 450 ° C. or higher and 600 ° C. or lower.

  In FIG. 14, when hydrothermal synthesis is performed again at 60 ° C. for 10 hours (Example 1, (a)) and when performed at 80 ° C. for 10 hours (Example 2, (b) The SEM image of)) is shown. As shown in FIG. 14, the crystal has a hexagonal shape, and the major axis (longest diagonal distance) does not exceed about 10 μm. According to experiments, it was found that the growth of crystals stopped when the major axis exceeded 10 μm, and all of them had a major axis of about 10 μm. In the present embodiment, when the crystals are arranged in substantially the same size as described above, a material suitable for sheet molding can be obtained.

  Next, the obtained plate-like crystal is formed into a sheet, and 10 sheets of this sheet are stacked to form a laminate, then formed into CIP (Cold Isostatic Pressing), and then baked at 950 ° C. The diffraction pattern is shown in FIG. As shown in FIG. 15, it was observed that this laminate had a pseudo cubic perovskite structure and a strong peak intensity on the (100) plane and the (200) plane that was the secondary reflection.

  Thus, in the present embodiment, the average particle length in the major axis direction is at least 8 μm or more, preferably 10 μm or more, and the average particle length in the major axis direction (average particle length in the major axis direction, where the average is an arithmetic average) The average particle length in the thickness direction (average particle length in the thickness direction) is 2 or more and 20 or less, and is composed of a plate-like crystal having a pseudo cubic perovskite structure. An anisotropically shaped powder (plate crystal) having a plane (front and back surfaces having a relatively large area, hereinafter referred to as a crystal plane) oriented in the (100) plane can be obtained.

  Further, after adjusting the composition obtained by mixing the plate-like crystal oriented in a specific crystal plane obtained by the manufacturing method of the present embodiment with a reaction raw material such as potassium niobate, barium titanate, strontium titanate, etc. In the same manner as above, the sheet is formed into a sheet, and the sheets are laminated to form a laminate, and the laminate is fired. The ceramic sintered body obtained by this firing has crystal orientation and becomes a piezoelectric ceramic having high piezoelectric constant characteristics at a high Curie temperature. Sintering at this time may be performed at a temperature of 900 ° C. to 1300 ° C. for only 2 hours to 10 hours.

  Next, an example in the case of producing an anisotropic shaped powder by the method for producing an anisotropic shaped powder of the present embodiment and a comparative example for the same will be described. The conditions are summarized as shown in the following [Table 1] to [Table 4]. Among these, [Table 1] and [Table 2] are conditions relating to the seed crystal synthesis process (one table is divided because it is long), and [Table 3] and [Table 4] are plate crystal production processes. (This is also divided because one table is long).

Example 1
An example in the case of producing an anisotropically shaped powder by the method for producing an anisotropically shaped powder of the present embodiment will be described. First, KOH and NaOH are mixed at a ratio of K / Na = 1.5, weighed so that KOH + NaOH = 4 mol / l, and then dissolved in ion-exchanged water.

Next, 1.87 g of Nb ₂ O ₅ powder having an average particle diameter of 200 nm and an orthorhombic structure is weighed and added to 15 ml of the previously prepared KOH + NaOH solution. Further, 1.5 wt% surfactant SDBS is added to the Nb ₂ O ₅ powder and stirred. Here, Nb ₂ O ₅ powder of particle SEM images using FIG 16 (acceleration voltage 5 kV, 15000 times) and X-ray diffraction pattern (a), showing the (b).

  The mixed solution thus obtained is sealed in a Teflon-lined autoclave container. Heat at 150 ° C. for only 4 hours. Then, it cooled, the container was opened after cooling, and when the reaction product was taken out, the quantity of the reaction product obtained here was 2.24g. The reaction product was washed several times with ethanol. After the washing was completed, the reaction product was dried in air at 80 ° C., and a powder that became seed crystals was taken out.

  Next, an alkaline solution is prepared in order to perform hydrothermal synthesis again. KOH and NaOH are mixed at a ratio of K / Na = 1.5, weighed so that KOH + NaOH = 4 mol / l, and then dissolved in ion-exchanged water.

Then, 0.94 g (67 wt%) of Nb ₂ O ₅ powder having an average particle diameter of 200 nm and an orthorhombic structure and 0.46 g (33 wt%) of the reaction product as a seed crystal obtained by the previous hydrothermal synthesis. Is weighed and added into 15 ml of the previously prepared KOH + NaOH solution. Further, 1.5 wt% surfactant SDBS is added to the Nb ₂ O ₅ powder and stirred.

  The mixed solution thus obtained is sealed in a Teflon-lined autoclave container. Heated at 60 ° C. for only 10 hours. Then, it cooled, the container was opened after cooling, and when the reaction product was taken out, the quantity of the reaction product obtained here was 1.7g. The reaction product was washed several times with ethanol and dried, and the powder was taken out.

  FIG. 17A shows a scanning electron microscope (SEM) image (SEM: manufactured by Hitachi, accelerating voltage: 8 kV, magnification) of a reaction product (seed crystal) obtained by hydrothermal synthesis for generating a seed crystal. : 3000 times). From this SEM image, the obtained powder had a major axis (length in the longest direction in the plate-like plane) of 1 to 6 μm, a thickness of 0.1 to 0.8 μm, an average of 5 μm of major axis, and a thickness of 0 It was observed to be 5 μm (500 nm). FIG. 20 shows the particle size distribution of this seed crystal. When the ratio of the volume of the crystal of the particle diameter to the total volume is the volume frequency for each particle diameter, the seed crystal has a particle diameter of 3% or more in the volume frequency in the range of 2 μm to 8 μm, It can be seen that the peak (particle size with the largest volume for each particle size) is 5 μm.

  FIG. 17B shows a scanning electron microscope (SEM) image (SEM: manufactured by Hitachi, accelerating voltage: 20 kV, magnification: 3500 times) of the reaction product obtained by the second hydrothermal synthesis. is there. From the SEM image, it was observed that the obtained powder had a major axis of 5 to 12 μm, a thickness of 0.5 to 1 μm, an average major axis of 10 μm, and an average thickness of 0.7 μm (700 nm). In addition, the powder did not collapse small and grew uniformly, and the shape was mostly hexagonal and the width was close to the major axis. FIG. 21 shows the particle size distribution of the plate crystals. According to this, if the ratio of the volume of the crystal of the particle size to the total volume for each particle size is the volume frequency, the particle size at 3% or more by volume frequency after the second hydrothermal synthesis is It is in the range of 5 μm to 11 μm, and the peak (particle size with the largest volume for each particle size) is 10 μm.

  Further, the obtained plate-like crystal was baked at 450 ° C. and dehydrated, and then formed into a sheet and laminated into 10 layers. Further, CIP molding was performed and firing was performed at 950 ° C. The X-ray diffraction pattern at this time is as shown in FIG. 13, and it was confirmed that the peak intensity of (100) and its secondary reflection (200) is strong in the pseudo cubic perovskite structure. From the above, it was found that an anisotropically shaped powder in which the (100) crystal plane was oriented was obtained.

(Example 2)
Next, Example 2 under the same conditions as in Example 1 except that the reaction temperature in the second hydrothermal synthesis is 80 ° C. will be described. Also in this example, KOH and NaOH are mixed at a ratio of K / Na = 1.5, weighed so that KOH + NaOH = 4 mol / l, and then dissolved in ion-exchanged water.

Next, 1.87 g of Nb ₂ O ₅ powder having an average particle diameter of 200 nm and an orthorhombic structure is weighed and added to 15 ml of the previously prepared KOH + NaOH solution. Further, 1.5 wt% surfactant SDBS is added to the Nb ₂ O ₅ powder and stirred.

  The mixed solution thus obtained is sealed in a Teflon-lined autoclave container. Heat at 150 ° C. for only 4 hours. After cooling, the container is opened and the reaction product is taken out. The amount of the reaction product obtained here was 2.24 g. After washing the reaction product several times with ethanol, the reaction product is dried in air at 80 ° C. And the powder which is a seed crystal is taken out.

Next, an alkaline solution is prepared in order to perform hydrothermal synthesis again. KOH and NaOH are mixed at a ratio of K / Na = 1.5, weighed so that KOH + NaOH = 4 mol / l, and then dissolved in ion-exchanged water. Next, 0.94 g (67 wt%) of Nb ₂ O ₅ powder having an average particle diameter of 200 nm and an orthorhombic structure and a reaction product 0 as a seed crystal obtained from hydrothermal synthesis to produce a seed crystal .46 g (33 wt%) is weighed and added into 15 ml of the previously prepared KOH + NaOH solution. Further, 1.5 wt% surfactant SDBS (sodium dodecylbenzenesulfonate) is added to the Nb ₂ O ₅ powder and stirred.

  The mixed solution thus obtained is sealed in a Teflon-lined autoclave container. Heat at 80 ° C. for only 10 hours. After cooling, the container is opened and the reaction product is taken out. The amount of the reaction product obtained here was 1.7 g. The reaction product is washed several times with ethanol, dried and the powder is removed.

  Next, after the washing was completed, the reaction product was dried in air at 80 ° C., and then the reaction product was calcined at 450 ° C.

  FIG. 18 shows a scanning electron microscope (SEM) image (SEM: manufactured by Hitachi, accelerating voltage: 8 kV, magnification: 3000 times) of the reaction product obtained as a result. From the SEM image, it was observed that the obtained powder had a major axis of 5 to 10 μm, a thickness of 0.5 to 1 μm, an average major axis of 8 μm, and an average thickness of 0.7 μm (700 nm).

(Comparative Example 1)
Next, a comparative example in which the reaction temperature in the second hydrothermal synthesis is set to 100 ° C. using the seed crystal of Example 1 will be described.

  Also in this example, KOH and NaOH are mixed at a ratio of K / Na = 1.5, weighed so that KOH + NaOH = 4 mol / l, and then dissolved in ion-exchanged water.

Next, 1.87 g of Nb ₂ O ₅ powder having an average particle diameter of 200 nm and an orthorhombic structure is weighed and added to 15 ml of the previously prepared KOH + NaOH solution. Further, 1.5 wt% surfactant SDBS is added to the Nb ₂ O ₅ powder and stirred.

  The mixed solution thus obtained is sealed in a Teflon-lined autoclave container. Heat at 150 ° C. for only 4 hours. After cooling, the container is opened and the reaction product is taken out. The amount of the reaction product obtained here was 2.24 g. After washing the reaction product with ethanol several times, the reaction product is dried in air at 80 ° C., and the powder as a seed crystal is taken out.

Next, an alkaline solution is prepared in order to perform hydrothermal synthesis again. KOH and NaOH are mixed at a ratio of K / Na = 1.5, weighed so that KOH + NaOH = 4 mol / l, and then dissolved in ion-exchanged water. Then, 0.94 g (67 wt%) of Nb ₂ O ₅ powder having an average particle diameter of 200 nm and an orthorhombic structure and 0.46 g of a reaction product, which is a seed crystal obtained from hydrothermal synthesis to produce a seed crystal ( 33 wt%) is weighed and added into 15 ml of the previously prepared KOH + NaOH solution. Further, 1.5 wt% surfactant SDBS is added to the Nb ₂ O ₅ powder and stirred.

  The mixed solution thus obtained is sealed in a Teflon-lined autoclave container. Heat at 100 ° C. for 4 hours or more. After cooling, the container is opened and the reaction product is taken out. The amount of the reaction product obtained here was 1.7 g. The reaction product is washed several times with ethanol, dried and the powder is removed.

  FIG. 19 shows a scanning electron microscope (SEM) image (SEM: manufactured by Hitachi, accelerating voltage: 7 kV, magnification: 5000 times) of the obtained reaction product. From this SEM image, the obtained powder had a major axis of 1 to 5 μm, a thickness of 1 to 3 μm, an average major axis of 4 μm, and an average thickness of 2 μm. As a result, it was found that when the reaction temperature reached 100 ° C., the growth in the longitudinal direction stopped, and even when the reaction time was increased, the major axis did not grow and grew only in the thickness direction. As a result, it was observed that the comparative example was thicker than each of the examples, was no longer plate-like, and was not suitable for sheet molding.

(Example 3)
Next, an example in which the reaction time in the second hydrothermal synthesis is increased will be described.

  First, KOH and NaOH are mixed at a ratio of K / Na = 1.5, weighed so that KOH + NaOH = 6 mol / l, and then dissolved in ion-exchanged water.

Next, 0.8 g of Nb ₂ O ₅ powder having an average particle diameter of 200 nm and an orthorhombic structure is weighed and added to 15 ml of the previously prepared KOH + NaOH solution. Further, 1.5 wt% surfactant SDBS is added to the Nb ₂ O ₅ powder and stirred.

  The mixed solution thus obtained is sealed in a Teflon-lined autoclave container. Heat at 150 ° C. for only 4 hours. Then, it cooled, the container was opened after cooling, and when the reaction product was taken out, the quantity of the reaction product obtained here was 0.96g. The reaction product was washed several times with ethanol, and after the washing was completed, the reaction product was dried in air at 80 ° C., and a powder serving as a seed crystal was taken out.

  When the SEM image of the obtained seed crystal was observed in the same manner as described above, the obtained powder had a major axis of 1 to 6 μm, a thickness of 0.2 to 0.5 μm, an average of a major axis of 4 μm, and a thickness of 0.2 μm. (200 nm) was observed.

  Next, an alkaline solution is prepared in order to perform hydrothermal synthesis again. KOH and NaOH are mixed at a ratio of K / Na = 1.5, weighed so that KOH + NaOH = 4 mol / l, and then dissolved in ion-exchanged water.

Then, 0.53 g (70 wt%) of Nb ₂ O ₅ powder having an average particle diameter of 200 nm and an orthorhombic structure and 0.16 g (30 wt%) of a reaction product as a seed crystal obtained by the previous hydrothermal synthesis Is weighed and added into 15 ml of the previously prepared KOH + NaOH solution. Further, 1.5 wt% surfactant SDBS is added to the Nb ₂ O ₅ powder and stirred.

  The mixed solution thus obtained is sealed in a Teflon-lined autoclave container. Heated at 80 ° C. for only 16 hours. Then, it cooled, the container was opened after cooling, and when the reaction product was taken out, the quantity of the reaction product obtained here was 0.8g. The reaction product was washed several times with ethanol and dried, and the powder was taken out.

  Next, after the washing was completed, the reaction product was dried in air at 80 ° C., and then the reaction product was calcined at 450 ° C.

  When the SEM image of the obtained reaction product was observed, the obtained powder had a major axis of 5 to 10 μm, a thickness of 0.3 to 0.6 μm, an average major axis of 8 μm, and an average thickness of 0.5 μm (500 nm). It was observed that

Example 4
Next, an example in which hydrothermal synthesis is performed again under different conditions using a seed crystal synthesized by the same method as in Example 3 will be described.

  An alkaline solution in which KOH and NaOH are mixed at a ratio of K / Na = 1.5 is used and weighed so that KOH + NaOH = 4 mol / l, and then dissolved in ion-exchanged water.

Then, 0.53 g (70 wt%) of Nb ₂ O ₅ powder having an average particle diameter of 200 nm and an orthorhombic structure and 0.16 g (30 wt%) of a reaction product as a seed crystal obtained by the previous hydrothermal synthesis Is weighed and added into 15 ml of the previously prepared KOH + NaOH solution. Further, 1.5 wt% surfactant SDBS is added to the Nb ₂ O ₅ powder and stirred.

  The mixed solution thus obtained is sealed in a Teflon-lined autoclave container. Heated at 60 ° C. for only 16 hours. Then, it cooled, the container was opened after cooling, and when the reaction product was taken out, the quantity of the reaction product obtained here was 0.8g. The reaction product was washed several times with ethanol and dried, and the powder was taken out.

  Next, after the washing was completed, the reaction product was dried in air at 80 ° C., and then the reaction product was calcined at 450 ° C.

  When the SEM image was observed about the reaction product obtained by hydrothermal synthesis again, the obtained powder had a major axis of 5 to 10 μm, a thickness of 0.3 to 0.5 μm, an average major axis of 8 μm, and an average thickness. It was observed to be 0.4 μm (400 nm).

(Comparative Example 2)
Next, an example in which hydrothermal synthesis is performed again under another condition using a seed crystal synthesized by the same method as in Example 3 will be described.

  As an alkaline solution for performing hydrothermal synthesis again, KOH and NaOH are mixed at a ratio of K / Na = 1.5, weighed so that KOH + NaOH = 4 mol / l, and then dissolved in ion-exchanged water.

Then, 0.53 g (70 wt%) of Nb ₂ O ₅ powder having an average particle diameter of 200 nm and an orthorhombic structure and 0.16 g (30 wt%) of a reaction product as a seed crystal obtained by the previous hydrothermal synthesis Is weighed and added into 15 ml of the previously prepared KOH + NaOH solution. Further, 1.5 wt% surfactant SDBS is added to the Nb ₂ O ₅ powder and stirred.

  The mixed solution thus obtained is sealed in a Teflon-lined autoclave container. Heated at 40 ° C. for only 16 hours. Then, it cooled, the container was opened after cooling, and when the reaction product was taken out, the quantity of the reaction product obtained here was 0.8g. The reaction product was washed several times with ethanol and dried, and the powder was taken out.

  When an SEM image of the reaction product obtained by the second hydrothermal synthesis was observed, the obtained powder was in a pulverized state and could not be measured. In other words, plate crystals could not be obtained.

(Comparative Example 3)
Next, an example in which hydrothermal synthesis is performed again under different conditions using a seed crystal synthesized by the same method as in Example 3 will be described.

  As an alkaline solution for performing hydrothermal synthesis again, KOH and NaOH are mixed at a ratio of K / Na = 1.5, weighed so that KOH + NaOH = 4 mol / l, and then dissolved in ion-exchanged water.

Then, 0.53 g (70 wt%) of Nb ₂ O ₅ powder having an average particle diameter of 2000 nm and a monoclinic structure and 0.16 g (30 wt%) of a reaction product as a seed crystal obtained by the previous hydrothermal synthesis Is weighed and added into 15 ml of the previously prepared KOH + NaOH solution. Further, 1.5 wt% surfactant SDBS is added to the Nb ₂ O ₅ powder and stirred.

  The mixed solution thus obtained is sealed in a Teflon-lined autoclave container. Heated at 80 ° C. for only 16 hours. Then, it cooled, the container was opened after cooling, and when the reaction product was taken out, the quantity of the reaction product obtained here was 0.8g. The reaction product was washed several times with ethanol and dried, and the powder was taken out.

  When an SEM image of the reaction product obtained by hydrothermal synthesis again was observed, the obtained powder was not grown and a plate-like crystal could not be obtained.

The Nb ₂ O ₅ powder was pulverized with a ball mill for 96 hours to obtain a powder having an average particle diameter of 500 nm. When the X-ray diffraction pattern of the powder particles was taken, the crystal structure remained monoclinic. Using this pulverized Nb ₂ O ₅ powder, a reaction product was obtained in the same production process as described above. When an SEM image of this reaction product was observed, plate crystals were not formed.

  From the above examples, it was found that a plate-like crystal can be obtained by using an oxide powder having an orthorhombic crystal structure.

(Example 5)
Next, an example in which the molar concentration of the KOH + NaOH solution is changed in the process of manufacturing the seed crystal will be described.

  First, KOH and NaOH are mixed at a ratio of K / Na = 1.5, and weighed so that the molar concentrations of the KOH + NaOH solution are 2 mol / l, 4 mol / l, 6 mol / l, 8 mol / l and 10 mol / l, respectively. Then, an example of dissolving in ion-exchanged water is prepared.

Next, 0.8 g of Nb ₂ O ₅ powder having an average particle diameter of 200 nm and an orthorhombic structure is weighed and added to 15 ml of each KOH + NaOH solution prepared earlier. Further, 1.5 wt% surfactant SDBS is added to the Nb ₂ O ₅ powder and stirred.

  The mixed solution thus obtained is sealed in a Teflon-lined autoclave container. Heat at 170 ° C. for 4 hours. Then, it is cooled, the container is opened after cooling, and the reaction product is taken out. The amount of the reaction product obtained here was 0.95 g, respectively. The reaction product was washed several times with ethanol, and after the washing was completed, the reaction product was dried in air at 80 ° C., and a powder serving as a seed crystal was taken out.

  When SEM images of the reaction products obtained at the respective molar concentrations were observed, the relationship between the molar concentration of the KOH + NaOH solution, which is an alkaline aqueous solution, and the average major axis of the plate crystals was as shown in FIG. That is, it was found that plate crystals having an average major axis of 3 μm to 12 μm can be obtained by setting the molar concentration of the KOH + NaOH solution to 4 mol / l or more and 8 mol / l or less. Further, it was found that there is a critical point at 4 mol / l and 8 mol / l, and that it is difficult to obtain a plate-like crystal if it is less than 4 mol / l or exceeds 8 mol / l. In the range where the molar concentration of the KOH + NaOH solution was less than 4 mol / l or more than 8 mol / l, plate crystals could hardly be observed, so the average major axis could not be calculated and could not be represented in this graph.

  From the above results, it was found that the solution concentration of the aqueous alkali hydroxide solution is preferably 4 mol / l or more and 8 mol / l or less in at least hydrothermal synthesis for synthesizing seed crystals.

  Next, the second hydrothermal synthesis shown below was performed using the seed crystal in the case where the molar concentration was 6 mol / l.

In the alkaline solution, KOH and NaOH are mixed at a ratio of K / Na = 1.5, weighed so that KOH + NaOH = 6 mol / l, and then dissolved in ion-exchanged water. Next, a reaction product 0 as a seed crystal obtained from hydrothermal synthesis in which 0.53 g (70 wt%) of Nb ₂ O ₅ powder having an average particle diameter of 200 nm and an orthorhombic structure was formed as a seed crystal. .16 g (30 wt%) is weighed and added into 15 ml of the previously prepared KOH + NaOH solution. Further, 1.5 wt% surfactant SDBS is added to the Nb ₂ O ₅ powder and stirred.

  The mixed solution thus obtained is sealed in a Teflon-lined autoclave container. Heat at 90 ° C. for 16 hours. After cooling, the container is opened and the reaction product is taken out. The amount of the reaction product obtained here was 0.7 g. The reaction product is washed several times with ethanol, dried and the powder is removed.

  Next, after the washing was completed, the reaction product was dried in air at 80 ° C., and then the reaction product was calcined at 450 ° C.

  When the SEM image of the obtained reaction product was observed, the obtained powder had a major axis of 5 to 12 μm, a thickness of 0.5 to 0.8 μm, an average major axis of 10 μm, and an average thickness of 0.6 μm (600 nm). It was observed that

(Comparative Example 4)
An example in which the powder is produced under the same conditions as in Example 3 but no surfactant SDBS is added will be described as Comparative Example 4.

  That is, KOH and NaOH are mixed at a ratio of K / Na = 1.5, weighed so that KOH + NaOH = 6 mol / l, and then dissolved in ion-exchanged water.

Next, 0.8 g of Nb ₂ O ₅ powder having an average particle diameter of 200 nm and an orthorhombic structure is weighed and added to 15 ml of the previously prepared KOH + NaOH solution and stirred.

  The mixed solution thus obtained is sealed in a Teflon-lined autoclave container. Heat at about 150 ° C. for a predetermined time. Then, it cooled, the container was opened after cooling, and when the reaction product was taken out, the quantity of the reaction product obtained here was 0.96g. The reaction product was washed several times with ethanol, and after the washing was completed, the reaction product was dried in air at 80 ° C., and a powder serving as a seed crystal was taken out.

FIG. 22 (a) shows X-ray diffraction patterns of powders obtained by setting the heating time in hydrothermal synthesis for synthesizing the seed crystal to 2 hours, 4 hours, 16 hours, and 24 hours, respectively. According to FIG. 22 (a), the X-ray diffraction pattern of the powder obtained at the time of heating for 2 hours and at the time of heating for 4 hours is K ₄ Na ₄ Nb ₆ O ₁₉ · shown as the reference pattern in FIG. 22 (b). There are many portions that match the X-ray diffraction pattern of 9H ₂ O, and the main component is assumed to be K ₄ Na ₄ Nb ₆ O ₁₉ · 9H ₂ O. However, when it exceeds 16 hours, the X-ray diffraction pattern is This is largely different from the example of 22 (b), and it is presumed that the main component of the produced powder was different from that of K ₄ Na ₄ Nb ₆ O ₁₉ · 9H ₂ O.

Furthermore, referring to those SEM images, the powder obtained by heating for 16 hours has a perovskite structure, and no plate-like crystals are observed. Moreover, although the SEM image of the powder obtained by heating for 4 hours is shown in FIG. 7, it is rounder than the crystal illustrated in FIG. It has a structure.
From the above results, it can be seen that plate crystals are easily obtained by adding a surfactant.

Claims (6)

Adding an oxide powder having an orthorhombic crystal structure to an aqueous alkali hydroxide solution and a surfactant to perform hydrothermal synthesis to synthesize a seed crystal;
Adding the seed crystal, the oxide powder having an orthorhombic crystal structure to the aqueous alkali hydroxide solution, and a surfactant, and performing hydrothermal synthesis again;
Washing the plate crystal obtained after the reaction of the hydrothermal synthesis again with an organic solvent;
Baking the reaction product of the plate crystal after the washing at 170 ° C. or higher and 700 ° C. or lower;
A method for producing an anisotropically shaped powder characterized by comprising:   2. The production method according to claim 1, wherein a reaction temperature of hydrothermal synthesis for synthesizing a seed crystal is 150 ° C. or more and 250 ° C. or less, and a reaction temperature of hydrothermal synthesis again is hydrothermal synthesis for synthesizing a seed crystal. The method for producing an anisotropically shaped powder, characterized in that the temperature is lower than the reaction temperature of, and exceeds 40 ° C and less than 100 ° C.   3. The production method according to claim 1, wherein the reaction time of hydrothermal synthesis for synthesizing the seed crystal is 2 hours or more and 8 hours or less, and the reaction time of hydrothermal synthesis again is hydrothermal for synthesizing the seed crystal. A method for producing an anisotropically shaped powder characterized in that the reaction time is longer than the synthesis reaction time. 4. The method according to claim 1, wherein the oxide powder is Nb ₂ O ₅ having an average particle diameter of 100 nm or more and less than 2000 nm. Manufacturing method.   5. The production method according to claim 1, wherein a solution of an alkali hydroxide aqueous solution of potassium hydroxide (KOH) and sodium hydroxide (NaOH) used in hydrothermal synthesis for synthesizing at least a seed crystal. A method for producing an anisotropically shaped powder, characterized in that the concentration is 4 mol / l or more and 8 mol / l or less. An anisotropic shaped powder produced using the production method according to any one of claims 1 to 5,
The average particle length in the major axis direction is 8 μm or more and 10 μm or less, and the ratio of the average particle length in the major axis direction to the average particle length in the thickness direction is 2 or more and 20 or less, and the plane of the plate-like crystal is oriented in the (100) plane An anisotropically shaped powder having a pseudo cubic perovskite structure.

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