Method for producing barium titanate nanocrystals

Abstract

To provide a method that enhances the assemblability of barium titanate nanocrystals.SOLUTION: A method for producing barium titanate nanocrystals comprises a preparation step of obtaining a raw material mixture containing a water-soluble barium salt, a titanium-containing raw material, an alkali component, an organic carboxylic acid and water, and a heating step of heating the raw material mixture to obtain the barium titanate nanocrystals. The titanium-containing raw material contains a mixture of a water-soluble titanium complex or at least one kind selected from the group consisting of titanium tetrachloride and titanium hydroxide and a hydroxy acid. In the raw material mixture, the molar ratio of the organic carboxylic acid to the barium ions is 0.5 or more and 2 or less, and the molar ratio of the hydroxide ions to the barium ions is 1 or more and 2 or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for producing barium titanate nanocrystals. [Background technology]

[0002] Barium titanate nanocubes are nanocrystals with a hexahedral shape. Barium titanate nanocrystals can be used as aggregates in which multiple crystal particles are regularly arranged, and it is believed that the density of the aggregates can be increased up to theoretical values. The high density of the aggregates and localized interfacial distortion brought about by the unique particle shape result in high dielectric properties. Taking advantage of the self-organization of nanocrystals and the resulting high dielectric properties of the aggregates, it is expected that innovative devices such as high-capacity ferroelectric memories, ultra-high-performance dielectric elastomers, and high-performance optical films will be realized (e.g., Non-Patent Document 1). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Kazumi Kato, "Development of high-performance small devices using single-crystal nanocubes," ALCA New Technology Briefing (February 24, 2015), presentation material, p. 7 Summary of the Invention [Problem to be solved by the invention]

[0004] As devices become more powerful and functional, there is a need to improve the aggregation of barium titanate nanocrystals in order to enhance the dielectric properties of the aggregates of barium titanate nanocrystals. However, this is still insufficient. [Means for solving the problem]

[0005] In view of the above, one aspect of the present invention relates to a method for producing barium titanate nanocrystals, comprising: a preparation step of obtaining a raw material mixture containing a water-soluble barium salt, a titanium-containing raw material, a water-soluble titanium complex, an alkali component, an organic carboxylic acid, and water; and a heating step of heating the raw material mixture to obtain barium titanate nanocrystals, wherein the titanium-containing raw material comprises a water-soluble titanium complex or a mixture of at least one selected from the group consisting of titanium tetrachloride and titanium hydroxide and a hydroxy acid, and the molar ratio of the organic carboxylic acid to barium ions in the raw material mixture is 0.5 or more and 2 or less, and the molar ratio of hydroxide ions to barium ions is 1 or more and 2 or less. [Effects of the Invention]

[0006] According to the present invention, the aggregation of barium titanate nanocrystals can be improved. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram schematically showing one side of a barium titanate nanocrystal. [Figure 2] 1 is a TEM image of an aggregate of BT nanocrystals obtained by the manufacturing method of Example 2. [Figure 3] 1 is a TEM image of an aggregate of BT nanocrystals obtained by the manufacturing method of Example 3. [Figure 4] 1 is a TEM image of an aggregate of BT nanocrystals obtained by the manufacturing method of Comparative Example 1. [Figure 5] 1 is a TEM image of an aggregate of BT nanocrystals obtained by the manufacturing method of Comparative Example 2. [Figure 6] 1 is a TEM image of an aggregate of BT nanocrystals obtained by the manufacturing method of Comparative Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0008] [Barium titanate nanocrystals] The BT nanocrystals obtained by the manufacturing method of barium titanate nanocrystals (hereinafter also referred to as BT nanocrystals) according to an embodiment of the present invention have a square angle of 0.8 or more. The square angle is determined by the absolute maximum length L nm, diagonal width W nm, and actual area SO nm of the crystal, which are determined from an image of the crystal by a transmission electron microscope. 2 Using the above, it is obtained by (L × W) / (2 × S0). Also, the above actual area S0nm 2 The length Ls of one side of a square corresponding to the size is greater than 20 nm and is not more than 100 nm. The coefficient of variation of the length Ls is not more than 20%.

[0009] When the square angle, length Ls, and coefficient of variation of length Ls are within the above ranges, BT nanocrystals have excellent aggregation properties and can form high-density aggregates. For example, the porosity of the aggregate can be reduced to 20% or less, and even to approximately 16% or less. The term "BT nanocrystals forming an aggregate" means that multiple BT nanocrystals are regularly arranged based on their self-organizing properties.

[0010] If the length Ls is 20 nm or less, the crystal aggregation decreases. If the length Ls is 20 nm or less, the crystal size becomes small, and the proportion of voids between the crystals in the aggregate (porosity) increases. The voids can be formed by the influence of additives such as organic carboxylic acids used during crystal synthesis that adhere to the crystal surface. Note that the voids referred to here refer to the areas of the aggregate other than those occupied by the crystals, and the portions of the crystal surface coated with organic carboxylic acids are considered voids. The smaller the crystal size, the greater the amount of organic carboxylic acid adhered per unit mass of the crystal, and the greater the porosity. Note that the organic carboxylic acids mentioned above are derived from materials used in the production process of BT nanocrystals.

[0011] If the length Ls exceeds 100 nm, the BT nanocrystals will have poor dispersibility in the dispersion medium (increased sedimentation) and poor self-alignment, which is also disadvantageous in terms of miniaturizing devices.

[0012] If the square angle is less than 0.8, the corners of the BT nanocrystals become more rounded, the gaps between the crystals become larger, and the aggregation of the crystals decreases. If the coefficient of variation of the length Ls exceeds 20%, the variation in crystal size increases, and the aggregation of the crystals decreases.

[0013] The length Ls may be 21 nm or more and 100 nm or less, 22 nm or more and 55 nm or less, or 30 nm or more and 55 nm or less. The square angle may be 0.85 or more.

[0014] From the viewpoint of further improving the aggregation of the crystals, the coefficient of variation of the length Ls may be less than 15%, may be 14% or less, or may be 12% or less.

[0015] For example, in the preparation process of the raw material mixture described below, by appropriately adjusting the molar ratio of (organic carboxylic acid / Ba ion) and the molar ratio of (hydroxide ion / Ba ion), it is possible to control the square angle, length Ls, and coefficient of variation of length Ls within the above ranges.

[0016] The squareness is an index related to the shape of BT nanocrystals, and refers to the degree of squareness on the side of the hexahedral BT nanocrystals. The squareness is a maximum of 1, and the more rounded the corners of the BT nanocrystals, the smaller the squareness. A squareness of 1 means that the corners of the BT nanocrystals are not rounded and the BT nanocrystals are neat hexahedrons.

[0017] The length Ls is an index relating to the size of the BT nanocrystals, and the hexahedral structure of the BT nanocrystals If the side of the crystal is assumed to be square, this corresponds to the length of one side of the square.

[0018] (Measurement of the square angle, length Ls, and coefficient of variation of length Ls of BT nanocrystals) The square angle, length Ls, and coefficient of variation of length Ls of the BT nanocrystals are determined by the following method.

[0019] (i) Preparation of BT nanocrystal aggregates 0.1 g of BT nanocrystals is placed in 10 mL of a nonpolar dispersion medium (e.g., toluene) and ultrasonicated for 1 minute using an ultrasonic cleaner (SND, US-106) to obtain a dispersion of BT nanocrystals. The dispersion of BT nanocrystals is then dropped onto the support film of a TEM grid and air-dried to obtain an aggregate of BT nanocrystals (an integrated film).

[0020] (ii) Imaging the aggregates using a transmission electron microscope (TEM) An image of the assembly (integrated film) is obtained using a TEM (JEM-2100F, manufactured by JEOL Ltd.) The magnification of the TEM image is adjusted appropriately so that it contains approximately 300 to 600 crystal particles.

[0021] (iii) Image processing The TEM image of the aggregate is processed using image analysis software (Mitani Corporation, WinROOF2015). Specifically, the contrast of the TEM image is adjusted appropriately, and binarization processing is performed to distinguish between areas occupied by crystals and areas other than crystals. The contrast value is set to, for example, 20 or more. In the binarization processing, processing is performed by specifying the brightness range based on the brightness histogram. The lower limit of the threshold is set to 0, and the upper limit of the threshold can be set appropriately within a range greater than 0 and below the peak top of the histogram.

[0022] (iv) Measurement of the four angles (arithmetic mean) of BT nanocrystals Using the TEM image after processing in (iii) above, the "actual area S0," "absolute maximum length L," and "diagonal width W" necessary for calculating the four angles are extracted from the "shape feature" function. Here, the actual area S0 (nm 2) is the area of ​​one side surface 1 of the BT nanocrystal shown in Figure 1. The absolute maximum length L (nm) is the maximum distance between any two points on the contour of side surface 1. In Figure 1, the absolute maximum length L is the distance between two points A1 and A2. The distance between two line segments P1 and P2 that are parallel to the length direction of the absolute maximum length L and sandwich side surface 1 is the diagonal width W (nm). The direction of the diagonal width W is perpendicular to the direction of the absolute maximum length L. A hypothetical rectangle 2 (a dashed-dotted rectangle) is drawn on one side surface 1 of the BT nanocrystal shown in Figure 1. Hypothetical rectangle 2 is obtained by drawing four line segments connecting the two points A1 and A2 and the two points B1 and B2 where the two line segments P1 and P2 touch side surface 1, as shown in Figure 1. The corners of hypothetical rectangle 2 touch the rounded corners of side surface 1 from the inside. Using the absolute maximum length L and diagonal width W, assume (L × W) / 2 (assumed quadrangle 2) area S1 (nm 2 ) If L=W, then hypothetical quadrilateral 2 is a square.

[0023] Using the actual area S0 and the assumed area S1, calculate the four angles using the formula below. Squareness=S1 / S0=(L×W) / (2×S0) The four angles are determined for approximately 300 to 600 BT nanocrystals using the above method, and the arithmetic mean of these angles is calculated.

[0024] (v) Measurement of the length Ls (arithmetic mean) of BT nanocrystals and its coefficient of variation The length Ls (arithmetic mean) of the BT nanocrystals and its coefficient of variation are calculated by the following method. The actual area S0 (nm ) of approximately 300 to 600 BT nanocrystals obtained in the process of calculating the four angles is calculated. 2 ) square root: (S0) 1 / 2 The length Ls (nm) is calculated for each BT nanocrystal, and the arithmetic mean and coefficient of variation are calculated. The coefficient of variation is the relative standard deviation, which is the value obtained by dividing the standard deviation by the arithmetic mean.

[0025] (Measurement of porosity of aggregates) The porosity of the aggregate of BT nanocrystals is determined by the following method. A TEM image of the BT nanocrystal aggregate was obtained using the same method as for determining the square angle. A region where approximately 100 to 300 BT nanocrystals were regularly arranged was randomly selected, and the TEM image was subjected to the image processing (iii) described above using the same method as for determining the square angle. After image processing, the area enclosed by the outline of the aggregate region, Sa, and the area occupied by voids within the aggregate region, Sv, were determined, and the porosity (%) was calculated as (Sv / Sa) × 100. Note that all areas within the aggregate region other than those occupied by BT nanocrystals were considered voids. Sv can be calculated by subtracting the total area of ​​the BT nanocrystals within the aggregate region from Sa.

[0026] [Method of manufacturing barium titanate nanocrystals] A method for producing barium titanate nanocrystals according to one embodiment of the present invention includes a preparation step of preparing a raw material mixture and a heating step of heating the raw material mixture to obtain barium titanate nanocrystals. The raw material mixture includes a water-soluble barium salt, a titanium-containing raw material, an alkali component, an organic carboxylic acid, and water. The titanium-containing raw material includes a water-soluble titanium complex or a mixture of at least one selected from the group consisting of titanium tetrachloride and titanium hydroxide with a hydroxy acid. In the raw material mixture, the molar ratio of the organic carboxylic acid to barium ions is 0.5 or more and 2 or less, and the molar ratio of hydroxide ions to barium ions is 1 or more and 2 or less.

[0027] The molar ratio of organic carboxylic acid to barium ions is the ratio of the molar amount of organic carboxylic acid to the molar amount of Ba ions, and is hereinafter also referred to as the molar ratio (organic carboxylic acid / Ba ions). The molar ratio of hydroxide ions to barium ions is the ratio of the molar amount of hydroxide ions to the molar amount of Ba ions, and is hereinafter also referred to as the molar ratio (hydroxide ions / Ba ions). The molar amount of Ba ions refers to the molar amount of Ba ions when all of the Ba derived from the water-soluble barium salt exists as ions in the raw material mixture. The molar amount of Ba ions when all of the Ba derived from the water-soluble barium salt exists as ions can also be said to be the molar amount of Ba derived from the water-soluble barium salt.

[0028] The molar amount of the organic carboxylic acid refers to the molar amount of the organic carboxylic acid added to the raw material mixture, and the organic carboxylic acid may be ionized or not. However, when the organic carboxylic acid contains two or more carboxy groups in one molecule (for example, when the organic carboxylic acid is a dicarboxylic acid or a carboxylic anhydride (a hydrolyzate thereof in the raw material mixture)), the molar amount of the organic carboxylic acid is converted into the molar amount of the carboxy groups contained in the organic carboxylic acid.

[0029] The above-mentioned molar amount of hydroxide ions refers to the molar amount of hydroxide ions present in the raw material mixture. The hydroxide ions present in the raw material mixture refer to the hydroxide ions remaining in the raw material mixture after the neutralization reaction of the alkaline and acid components contained in the raw materials. The molar amount of hydroxide ions can be calculated from the amounts of raw materials charged.

[0030] When the molar ratios of (organic carboxylic acid / Ba ion) and (hydroxide ion / Ba ion) are within the above ranges, BT nanocrystals with excellent aggregation properties can be efficiently obtained, and high-density aggregates can be formed.

[0031] When the (hydroxide ion / Ba ion) molar ratio is within the above range, the amount of seeds produced (the number of crystal grains) decreases, which in turn accelerates crystal growth, resulting in an exponential increase in crystal size (particle diameter). On the other hand, the crystal size variation may increase and the crystal recovery rate may drop sharply. This phenomenon was previously unknown and was newly discovered by the present inventors. Based on this finding, the present inventors further pursued intensive research. As a result, they newly discovered that when the (hydroxide ion / Ba ion) molar ratio is within the above range and the (organic carboxylic acid / Ba ion) molar ratio is within the above range, seed production and crystal growth can be arbitrarily controlled, and crystals with a length Ls of more than 20 nm can be stably obtained while reducing the variation in crystal size (while keeping the coefficient of variation of the length Ls at 20% or less). The present invention is based on this new finding.

[0032] If the molar ratio of (organic carboxylic acid / Ba ion) is less than 0.5, the square angle of the BT nanocrystals will be low and they will not be able to be regularly arranged.If the molar ratio of (organic carboxylic acid / Ba ion) is more than 2, the viscosity of the raw material mixture will increase and the shape of the resulting BT nanocrystals will become non-uniform.

[0033] The smaller the molar ratio of (hydroxide ion / Ba ion), the larger the crystal size (length Ls) tends to be. When the molar ratio of (hydroxide ion / Ba ion) exceeds 2, the length Ls may be 20 nm or less. In addition, the coefficient of variation of the length Ls may exceed 20%, and the square angle may be less than 0.8.

[0034] If the molar ratio (hydroxide ion / Ba ion) is less than 1, the reaction rate decreases, and the BT nanocrystals are not sufficiently obtained, and the aggregates are not sufficiently formed. Also, the length Ls may exceed 100 nm.

[0035] To further improve the aggregation of BT nanocrystals, the (hydroxide ion / Ba ion) molar ratio may be 1 or more and 1.8 or less, preferably 1 or more and less than 1.75, and more preferably 1.1 or more and 1.72 or less. In this case, crystal growth is promoted while the variation in crystal shape is reduced, making it easier to obtain high-quality BT nanocrystals that can form high-density aggregates. The (hydroxide ion / Ba ion) molar ratio is more preferably 1.4 or more and 1.72 or less. When the (hydroxide ion / Ba ion) molar ratio is within the above range, it is possible to efficiently obtain a sufficient amount of BT nanocrystals to form aggregates while reducing the variation in crystal shape and improving the aggregation of BT nanocrystals. The (hydroxide ion / Ba ion) molar ratio is particularly preferably 1.64 or more and 1.72 or less. When the (hydroxide ion / Ba ion) molar ratio is within the above range, it is possible to further increase the square angle of the BT nanocrystals and improve the aggregation.

[0036] The hydroxide ion concentration in the raw material mixture may be, for example, 0.3 mol / L or more and 0.8 mol / L or less, 0.3 mol / L or more and 0.6 mol / L or less, or 0.4 mol / L or more and 0.6 mol / L or less. The hydroxide ion concentration in the raw material mixture refers to the value calculated by dividing the molar amount of hydroxide ions present in the raw material mixture by the total amount (liters) of water contained in each raw material (including water) used to prepare the raw material mixture.

[0037] [Raw material mixture preparation process] In this step, a raw material mixture containing barium and titanium is obtained. In preparing the raw material mixture, the water-soluble barium salt serving as the barium source and the titanium-containing raw material serving as the titanium source may be added so that the molar ratio of barium to titanium (Ba / Ti) is within the range of 0.95 or more and 1.5 or less (preferably 0.97 or more and 1.2 or less). In the preparation step, the water-soluble barium salt, the titanium-containing raw material, and water may be added, followed by the addition of an organic carboxylic acid and an alkali component. The water-soluble barium salt and the water-soluble titanium complex may each be used as an aqueous solution. Furthermore, a mixture of at least one of titanium tetrachloride and titanium hydroxide with a hydroxy acid may be used by mixing with water.

[0038] (Water-soluble barium salts) The water-soluble barium salt may be one that dissolves in water during the preparation process, or one that is poorly soluble in water during the preparation process but dissolves when heated during the heating process (e.g., barium salts of higher fatty acids such as barium oleate). Examples of water-soluble barium salts include barium chloride, barium hydroxide, barium salts of fatty acids, and barium nitrate. Among these, barium hydroxide is preferred. Barium hydroxide contains no elements other than barium titanate and water. Therefore, when barium hydroxide is used as the barium source, the incorporation of impurities derived from other elements during the synthesis of BT nanocrystals is suppressed, making it easier to consistently obtain BT nanocrystals with excellent aggregation properties. The water-soluble barium salt may be used alone or in combination of two or more.

[0039] The concentration of Ba ions in the raw material mixture may be 0.2 mol / L or more and 2 mol / L or less, or 0.2 mol / L or more and 1 mol / L or less. In this case, it is easy to efficiently obtain high-quality barium titanate nanocrystals. The concentration of Ba ions in the raw material mixture refers to the value obtained by dividing the molar amount of Ba ions when all of the Ba derived from the water-soluble barium salt is present as ions in the raw material mixture by the total amount of water (in liters) contained in each raw material (including water) used to prepare the raw material mixture.

[0040] (Titanium-containing raw materials) The titanium-containing raw material may be a water-soluble titanium complex. The ligand of the water-soluble titanium complex preferably contains a hydroxy acid (salt). The hydroxy acid may be an α-hydroxy acid. Examples of the α-hydroxy acid include glycolic acid, citric acid, malic acid, tartaric acid, and lactic acid. The salt is, for example, an ammonium salt. From the viewpoint of easy availability, the water-soluble titanium complex is preferably a titanium complex containing an ammonium salt of lactic acid as a ligand, and more preferably titanium bis(ammonium lactate) dihydroxide (TALH). One type of water-soluble titanium complex may be used alone, or two or more types may be used in combination.

[0041] The titanium-containing raw material may be a mixture of at least one selected from the group consisting of titanium tetrachloride and titanium hydroxide with a hydroxy acid. In the mixture, a titanium complex may or may not be formed. As the hydroxy acid, those exemplified above can be used, such as lactic acid. Titanium hydroxide is desirable in terms of cost and ease of handling.

[0042] The Ti in the raw material mixture is poorly soluble in water and may exist as a precipitate containing Ti. Such precipitates may include, for example, Ti(OH)4, TiO(OH)2, and TiO2. In this specification, the Ti concentration in the raw material mixture refers to the value obtained by dividing the molar amount of Ti derived from the titanium-containing raw material by the total amount (liters) of water contained in each raw material (including water) used to prepare the raw material mixture.

[0043] (organic carboxylic acid) The organic carboxylic acid plays a role in controlling the crystal shape. The organic carboxylic acid coordinates to the (100) plane of the barium titanate crystal. This suppresses crystal growth on the (100) plane and promotes crystal growth on the (111) plane, making it easier to control the crystal shape to a hexahedron.

[0044] The organic carboxylic acid preferably includes a fatty acid. The number of carbon atoms in the main chain of the fatty acid is preferably 6 or more, more preferably 10 or more, and particularly preferably 15 or more. The fatty acid may be a saturated fatty acid or an unsaturated fatty acid. Saturated fatty acids having 15 or more carbon atoms in the main chain include, for example, palmitic acid and stearic acid. Unsaturated fatty acids having 15 or more carbon atoms in the main chain include, for example, oleic acid, linoleic acid, linolenic acid, eleostearic acid, and arachidonic acid. Among these, oleic acid is preferred from the viewpoint of ease of obtaining hexahedral nanocrystals. One type of organic carboxylic acid may be used alone, or two or more types may be used in combination. Note that the hydroxy acid contained in the titanium-containing raw material is not included in the organic carboxylic acid added in the raw material mixture preparation process.

[0045] In the raw material mixture, the molar ratio of (organic carboxylic acid / Ba ion) may be 0.7 or more and 1.65 or less, or 0.7 or more and 1.5 or less. In this case, the (100) and (111) faces of the barium titanate crystals tend to grow in a balanced manner, making it easier to control the crystal shape to a hexahedron, thereby reducing the variation in the crystal shape and advantageously reducing the porosity of the aggregate.

[0046] The concentration of the organic carboxylic acid (e.g., oleic acid) in the raw material mixture may be, for example, 0.15 mol / L or more and 0.5 mol / L or less. The concentration of the organic carboxylic acid in the raw material mixture refers to the value calculated by dividing the molar amount of the organic carboxylic acid added to the raw material mixture by the total amount (liters) of water contained in each raw material (including water) used to prepare the raw material mixture.

[0047] (Alkaline component) By including an alkali component in the raw material mixture, it is possible to promote crystal growth and reduce variations in crystal shape. From the viewpoint of cost reduction and environmental load reduction, the alkali component preferably includes an alkali metal hydroxide. Examples of alkali metal hydroxides include sodium hydroxide and potassium hydroxide. Of these, sodium hydroxide is preferred as the alkali metal hydroxide.

[0048] The concentration of sodium hydroxide in the raw material mixture may be adjusted as appropriate depending on the amounts of the other raw materials added, and is, for example, 0.5 mol / L or more and 1.5 mol / L or less. The concentration of sodium hydroxide in the raw material mixture refers to the value calculated by dividing the molar amount of sodium hydroxide added to the raw material mixture by the total amount of water (in liters) contained in the raw materials (including water) used to prepare the raw material mixture.

[0049] The alkaline component may contain an amine compound for the purpose of reducing the variation in crystal shape. Examples of the amine compound include primary amine compounds such as tert-butylamine, secondary amine compounds such as dimethylamine, and tertiary amine compounds such as trimethylamine. Because amine compounds are highly ecotoxic and expensive, it is preferable that the alkaline component does not contain an amine compound. By adjusting the hydroxide ion concentration using an alkali metal hydroxide without using an amine compound, the variation in crystal shape can be reduced.

[0050] [Heating process of raw material mixture] In this process, the raw material mixture is heated to synthesize barium titanate. That is, BT nanocrystals are obtained. Barium titanate can be obtained using a hydrothermal reaction, and the raw material mixture is heated while being stirred. From the viewpoint of efficiently obtaining high-quality BT nanocrystals, the heating temperature is preferably 150°C or higher and 250°C or lower. When the heating temperature is 150°C or higher, the barium titanate synthesis reaction tends to proceed smoothly. When the heating temperature is 250°C or lower, decomposition of the organic carboxylic acid is suppressed, and the effect of controlling the crystal shape by the organic carboxylic acid is more reliably obtained. From the viewpoint of sufficiently progressing the reaction and increasing productivity, the heating time is, for example, 1 hour or higher and 80 hours or lower, or may be 12 hours or higher and 48 hours or lower.

[0051] [BT nanocrystal cleaning process] Furthermore, after the heating step, a washing step may be performed in which the BT nanocrystals are dispersed in alcohol and washed. Examples of alcohol that can be used include ethanol, methanol, and 2-propanol. In the washing step, the BT nanocrystals obtained in the heating step are separated from water (including residual components such as organic carboxylic acids dissolved in the water).

[0052] The washing step includes, for example, step (a) of adding alcohol to the reaction solution (water containing BT nanocrystals) after the heating step and then centrifuging to obtain a precipitate, and step (b) of dispersing the precipitate in alcohol and then centrifuging to obtain a precipitate. Step (b) may be repeated multiple times. In step (a), it is preferable to thoroughly mix the reaction solution with the alcohol before centrifugation.

[0053] [BT nanocrystal classification process] Furthermore, after the heating step, a classification step may be performed in which the BT nanocrystals are dispersed in a non-polar dispersion medium and classified by centrifugation. The classification step is preferably performed after the washing step. Examples of dispersion methods include ultrasonic treatment and stirring with a stirring blade. Non-polar dispersion media include low-polarity organic dispersion media. Examples of the dispersion media that can be used include non-polar (low-polarity) aromatic hydrocarbon dispersion media (e.g., toluene, benzene) and aliphatic hydrocarbon dispersion media (e.g., hexane).

[0054] [Example] Examples of the present invention will be specifically described below, but the present invention is not limited to the following examples.

[0055] Example 1 (Raw material mixture preparation process) A 300 mL polytetrafluoroethylene beaker was charged with 10.1 g of titanium bis(ammonium lactate) dihydroxide (TALH) aqueous solution (a titanium source), 56.1 g of ion-exchanged water, and 5.6 g of barium hydroxide octahydrate (a barium source) and stirred for 3 minutes. A mixture containing TALH (a titanium source) and barium hydroxide (a barium source) was obtained. The TALH aqueous solution was manufactured by Sigma-Aldrich (product number 388165, concentration 50 wt%). The barium hydroxide octahydrate was manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. (product number 020-00242, special grade).

[0056] While stirring the mixture containing the titanium source and the barium source, 6.5 mL of 7.5 M aqueous sodium hydroxide solution and 4.8 g of oleic acid were added to the mixture, in that order, and stirred for 5 minutes. The sodium hydroxide used was Fujifilm Wako Pure Chemical Industries, Ltd., product code 198-13765 (special grade reagent). The oleic acid used was Miyoshi Oil & Fats Co., Ltd., product name PM500. In this way, a raw material mixture was prepared.

[0057] The Ba ion concentration and Ti concentration in the raw material mixture were each 0.25 mol / L. The sodium hydroxide concentration in the raw material mixture was 0.73 mol / L. The oleic acid concentration in the raw material mixture was 0.20 mol / L. The hydroxide ion concentration in the raw material mixture was 0.47 mol / L. In the raw material mixture, the molar ratio (oleic acid / Ba ion) was 0.8. In the raw material mixture, the molar ratio (hydroxide ion / Ba ion) was 1.88.

[0058] (Process for heating raw material mixture) The raw material mixture prepared above was transferred to a 100 mL PTFE container (HUT-100, manufactured by San-Ai Scientific Co., Ltd.) and sealed in a hot stirrer reaction decomposition apparatus (RDV-TMS-100 aluminum block and HHE-19G-U hot stirrer, manufactured by San-Ai Scientific Co., Ltd.) where it was heated with stirring. The stirring speed was 536 rpm, the heating temperature was 230°C, and the heating time was 24 hours. In this way, barium titanate was synthesized in water. In other words, BT nanocrystals were obtained.

[0059] (BT nanocrystal cleaning process) The BT nanocrystals obtained in the heating process were dispersed in ethanol and washed. Specifically, 20 mL of ethanol was added to the reaction solution (water containing BT nanocrystals) after the hydrothermal reaction, and this was transferred to a centrifuge tube. The centrifuge tube was then heated by standing in a temperature-controlled hot bath at 50-70°C for 5 minutes, and then the tube was shaken to mix and disperse the reaction solution in the ethanol. The centrifuge tube was placed in a tabletop laboratory centrifuge (Sigma, 3-16L) and centrifuged at a centrifugal acceleration of 2500G for 10 seconds. The supernatant was then completely removed, and a precipitate was obtained (step (a)).

[0060] Next, 40 mL of ethanol was added to the precipitate in the centrifuge tube, and the mixture was heated and stirred as described above to disperse the precipitate in the ethanol. The mixture was then centrifuged as described above, and the supernatant was completely removed to obtain the precipitate (step (b)). Step (b) was repeated twice. The precipitate was transferred to a magnetic dish and dried in a constant temperature bath at 90°C for 6 hours. BT nanocrystals were thus obtained.

[0061] The BT nanocrystals obtained above were evaluated as follows. [Evaluation 1: Measurement of square angle (arithmetic mean), length Ls (arithmetic mean), and coefficient of variation of length Ls] The square angle (arithmetic mean), length Ls (arithmetic mean), and coefficient of variation of length Ls of the BT nanocrystals were determined by the method described above (steps (i) to (v) above).

[0062] For the image processing (iii) above, the TEM images of the aggregates were processed using image analysis software (Mitani Shoji Co., Ltd., WinROOF2015) according to the following procedure. First, the scale was calibrated. Specifically, the "Manual Calibration" function was used to measure the length of the scale bar listed in the analysis conditions section of the TEM image and align it with the actual size. Next, a rectangular ROI was used to select the entire area excluding the analysis conditions section displayed below the TEM image, and the selected area was then clipped to extract the image region related to the crystal particles (aggregates). Next, to enhance the contrast of the TEM image, the "Brightness / Contrast" function was used, setting the contrast value to 60. Next, median processing was performed to average the brightness. This median processing was performed using the "Filter" function with a filter size of 9 × 9 pixels. Next, sharpening was performed to emphasize the boundary between the crystal particles and the background in the TEM image of the aggregate. This sharpening was performed using the "Edge" function with a filter size of 7 × 7 pixels. The image after the above processing was then binarized by specifying the brightness range based on the brightness histogram. The lower threshold was set to 0, and the upper threshold was set to the peak top of the histogram. After setting the threshold, areas where the binarization was insufficient were slightly corrected using the pen tool.

[0063] [Evaluation 2: Porosity measurement] The porosity of the aggregate of BT nanocrystals was determined by the method described above.

[0064] [Evaluation 3: Measurement of BT reaction rate] Five grams of the BT nanocrystals obtained above were transferred to an alumina crucible and heated in a constant temperature bath at 130°C for 30 minutes to completely dry. The mass of the dried solid, W1 (g), was measured. The dried solid was then calcined at 800°C for 2 hours, and the mass of the calcined solid, W2 (g), was measured. The ratio (percentage) of mass W2 to mass W1, i.e., (W2 / W1) × 100, was calculated as the BT conversion rate (%).

[0065] The barium source becomes barium oleate in the raw material mixture, which decomposes during the heating process and reacts with the titanium source. Unreacted barium source remains as barium oleate, and during the firing process, the oleic acid derived from the unreacted residual component (barium oleate) decomposes and volatilizes. When there are few unreacted residual components, the amount of volatilization is small, resulting in a high BT conversion reaction rate.

[0066] Examples 2 to 5 In the preparation process of the raw material mixture, the amounts of sodium hydroxide and oleic acid added were changed to set the concentrations of sodium hydroxide and oleic acid in the raw material mixture to the values ​​shown in Table 1. The hydroxide ion concentration in the raw material mixture and the molar ratios of (oleic acid / Ba ion) and (hydroxide ion / Ba ion) were the values ​​shown in Table 1. BT nanocrystals were produced and evaluated in the same manner as in Example 1, except for the above.

[0067] Example 6 In the preparation process of the raw material mixture, the amounts of the barium source and titanium source added were changed to obtain the Ba ion concentration and Ti concentration in the raw material mixture, respectively, as shown in Table 1. The amounts of sodium hydroxide and oleic acid added were also changed to obtain the sodium hydroxide and oleic acid concentrations in the raw material mixture, respectively, as shown in Table 1. The hydroxide ion concentration and the molar ratios of (oleic acid / Ba ions) and (hydroxide ions / Ba ions) in the raw material mixture were the values ​​shown in Table 1. Except for the above, BT nanocrystals were produced and evaluated in the same manner as in Example 1.

[0068] Example 7 Lactic acid was added to 5.3 g of an aqueous titanium tetrachloride solution (concentration 16% by mass) and stirred to obtain 8.8 g of a mixture of titanium tetrachloride and lactic acid. Lactic acid was added so that the molar ratio of lactic acid to titanium derived from titanium tetrachloride in the mixture was 2.

[0069] In the preparation process of the raw material mixture, a mixture of titanium tetrachloride and lactic acid was used as the titanium source instead of the TALH aqueous solution. The amounts of ion-exchanged water and the amounts of sodium hydroxide aqueous solution and oleic acid added to the mixture containing the titanium source and barium source were changed to obtain the concentrations of sodium hydroxide and oleic acid in the raw material mixture shown in Table 1. The hydroxide ion concentration and the molar ratios of (oleic acid / Ba ion) and (hydroxide ion / Ba ion) in the raw material mixture were the values ​​shown in Table 1. Except for the above, BT nanocrystals were prepared and evaluated using the same method as in Example 1. The titanium tetrachloride aqueous solution used was Fujifilm Wako Pure Chemical Industries, Ltd., product code 203-08955. The lactic acid used was Fujifilm Wako Pure Chemical Industries, Ltd., product code 128-00056.

[0070] Example 8 Aqueous sodium hydroxide solution was slowly added to 5.3 g of an aqueous titanium tetrachloride solution (concentration 16% by mass) until the pH reached 7, yielding a precipitate. The precipitate was filtered and washed with water to yield a titanium hydroxide cake. Ion-exchanged water was added to the titanium hydroxide cake until the total mass reached 5.3 g, and the mixture was repulped to prepare a titanium hydroxide slurry. Lactic acid was added to the titanium hydroxide slurry to obtain 8.8 g of a mixture of titanium hydroxide and lactic acid. Lactic acid was added so that the molar ratio of lactic acid to titanium derived from titanium hydroxide in the mixture was 2.

[0071] In the preparation process of the raw material mixture, a mixed solution of titanium hydroxide and lactic acid was used as the titanium source instead of the aqueous TALH solution. The amounts of ion-exchanged water and the amounts of aqueous sodium hydroxide and oleic acid added to the mixture containing the titanium source and barium source were changed to obtain the concentrations of sodium hydroxide and oleic acid in the raw material mixture shown in Table 1. The hydroxide ion concentrations and the molar ratios of (oleic acid / Ba ions) and (hydroxide ions / Ba ions) in the raw material mixture were the values ​​shown in Table 1. Except for the above, BT nanocrystals were produced and evaluated in the same manner as in Example 1. The same titanium tetrachloride aqueous solution and lactic acid as in Example 7 were used.

[0072] [Table 1]

[0073] Comparative Examples 1, 3, and 6 In the preparation of the raw material mixture, barium chloride was used as the barium source. The amounts of sodium hydroxide and oleic acid added were varied to obtain the concentrations of sodium hydroxide and oleic acid in the raw material mixture shown in Table 1.

[0074] To the aqueous solution containing the titanium source and the barium source, tert-butylamine was further added as an amine compound. The amount of tert-butylamine added was adjusted so that the concentration of tert-butylamine in the raw material mixture was 0.48 mol / L. The concentration of the amine compound in the raw material mixture was calculated by dividing the molar amount of the amine compound added to the raw material mixture by the total amount (liters) of water contained in each raw material (including water) used in the raw material mixture.

[0075] The hydroxide ion concentration, (oleic acid / Ba ion) and (hydroxide ion / Ba ion) molar ratios in the raw material mixture were the values ​​shown in Table 1. Except for the above, BT nanocrystals were produced and evaluated in the same manner as in Example 1.

[0076] Comparative Examples 2 and 5 In the preparation process of the raw material mixture, the amounts of sodium hydroxide and oleic acid added were changed to set the concentrations of sodium hydroxide and oleic acid in the raw material mixture to the values ​​shown in Table 1. The hydroxide ion concentration in the raw material mixture and the molar ratios of (oleic acid / Ba ion) and (hydroxide ion / Ba ion) were the values ​​shown in Table 1. BT nanocrystals were produced and evaluated in the same manner as in Example 1, except for the above.

[0077] Comparative Example 4 In the preparation process of the raw material mixture, barium chloride was used as the barium source. The amounts of the barium source and titanium source added were changed to obtain the barium and titanium concentrations in the raw material mixture shown in Table 1. The amounts of sodium hydroxide and oleic acid added were changed to obtain the sodium hydroxide and oleic acid concentrations in the raw material mixture shown in Table 1.

[0078] To the aqueous solution containing the titanium source and the barium source, tert-butylamine was further added as an amine compound. The amount of tert-butylamine added was adjusted so that the concentration of tert-butylamine in the raw material mixture was 0.48 mol / L. The concentration of the amine compound in the raw material mixture was calculated by dividing the molar amount of the amine compound added to the raw material mixture by the total amount (liters) of water contained in each raw material (including water) used in the raw material mixture.

[0079] The hydroxide ion concentration in the raw material mixture and the molar ratios of (oleic acid / Ba ion) and (hydroxide ion / Ba ion) were the values ​​shown in Table 1. Except for the above, BT nanocrystals were produced and evaluated in the same manner as in Example 1.

[0080] Table 2 shows the evaluation results of the BT nanocrystals obtained by the manufacturing methods of Examples 1 to 8 and Comparative Examples 1 to 6.

[0081] [Table 2]

[0082] In Examples 1 to 8, the aggregation of BT nanocrystals was improved, and the porosity was reduced to 20% or less in all cases. In particular, Examples 2 to 6, in which the molar ratio (hydroxide ion / Ba ion) was 1.1 or more and 1.72 or less, achieved even lower porosities. Low porosities were also achieved in Example 7, which used a mixed solution of titanium tetrachloride and lactic acid, and Example 8, which used a mixed solution of titanium hydroxide and lactic acid. The high-density aggregates of BT nanocrystals obtained by the manufacturing methods of Examples 1 to 8 can exhibit excellent dielectric properties when used in ferroelectric memories or dielectric elastomers. As an example, TEM images of the aggregates of BT nanocrystals obtained by the manufacturing methods of Examples 2 and 3 are shown in Figures 2 and 3.

[0083] In Comparative Examples 1 to 3, the molar ratio (hydroxide ion / Ba ion) was greater than 2, resulting in an increase in porosity of 24% or more. In Comparative Example 1, the coefficient of variation of the length Ls was 20% or less, but the length Ls was less than 20 nm, resulting in increased voids between the crystals and a decrease in the degree of crystal aggregation. In Comparative Example 2, the coefficient of variation of the length Ls was greater than 20%, resulting in increased variation in crystal size and a decrease in the degree of crystal aggregation compared to Comparative Example 1. In Comparative Example 3, the square angle was less than 0.8, resulting in increased rounding of the corners of the hexahedral crystals and an increase in voids between the crystals, resulting in a decrease in the degree of crystal aggregation compared to Comparative Example 1. TEM images of the BT nanocrystal aggregates obtained by the manufacturing methods of Comparative Examples 1 to 3 are shown in Figures 4 to 6.

[0084] In Comparative Example 4, where the molar ratio of (hydroxide ion / Ba ion) was less than 1, and Comparative Example 6, where the molar ratio of (oleic acid / Ba ion) was more than 2, the BT conversion reaction rate was low, and sufficient crystals were not obtained, so aggregates could not be formed. In Comparative Example 5, where the molar ratio of (oleic acid / Ba ion) was less than 0.5, the morphology control ability of oleic acid was not exerted, the crystals had low square angles, were not regularly arranged, and aggregates could not be formed. [Industrial Applicability]

[0085] The BT nanocrystals obtained by the method for producing BT nanocrystals according to the present invention are useful for electronic devices such as multilayer capacitors. [Explanation of symbols]

[0086] 1: One side of a BT nanocrystal, 2: Hypothetical rectangle, L: Absolute maximum length, W: Diagonal width, A1, A2: Two points where the distance on the contour line of the crystal side is the greatest, P1, P2: Two line segments that are parallel to the length direction of the absolute maximum length L and sandwich the crystal side, B1, B2: Points where line segments P1, P2 contact the crystal side

Claims

1. A water-soluble barium salt, a titanium-containing raw material, an alkali component, an organic carboxylic acid, and water. a preparation step of obtaining a raw material mixture comprising: a heating step of heating the raw material mixture to obtain barium titanate nanocrystals; Including, the titanium-containing raw material comprises a water-soluble titanium complex or a mixture of at least one selected from the group consisting of titanium tetrachloride and titanium hydroxide with a hydroxy acid; In the raw material mixture, the molar ratio of the organic carboxylic acid to barium ions is 0.5 or more and 2 or less, and the molar ratio of hydroxide ions to barium ions is greater than or equal to 1 and less than 1.75; Method for producing barium titanate nanocrystals.

2. A method for producing a titanium-containing composition comprising: a water-soluble barium salt; a titanium-containing raw material; an alkaline component; an organic carboxylic acid; and water. a preparation step of obtaining a raw material mixture comprising: a heating step of heating the raw material mixture to obtain barium titanate nanocrystals; Including, the titanium-containing raw material comprises a water-soluble titanium complex or a mixture of at least one selected from the group consisting of titanium tetrachloride and titanium hydroxide with a hydroxy acid; In the raw material mixture, the molar ratio of the organic carboxylic acid to barium ions is 0.5 or more and 2 or less, and the molar ratio of hydroxide ions to barium ions is equal to or greater than 1 and equal to or less than 2; A method for producing barium titanate nanocrystals, wherein the ligand of the water-soluble titanium complex contains a hydroxy acid.

3. A method for producing a titanium-containing composition comprising: a water-soluble barium salt; a titanium-containing raw material; an alkaline component; an organic carboxylic acid; and water. a preparation step of obtaining a raw material mixture comprising: a heating step of heating the raw material mixture to obtain barium titanate nanocrystals; Including, the titanium-containing raw material comprises a water-soluble titanium complex or a mixture of at least one selected from the group consisting of titanium tetrachloride and titanium hydroxide with a hydroxy acid; In the raw material mixture, the molar ratio of the organic carboxylic acid to barium ions is 0.5 or more and 2 or less, and the molar ratio of hydroxide ions to barium ions is equal to or greater than 1 and equal to or less than 2; The water-soluble titanium complex comprises titanium bis(ammonium lactate) dihydroxide.

4. A method for producing barium titanate nanocrystals as described in claim 2 or 3, wherein the molar ratio of hydroxide ions to barium ions is 1 or more and less than 1.

75.

5. The method for producing barium titanate nanocrystals according to any one of claims 1 to 4, wherein the organic carboxylic acid includes oleic acid.

6. The method for producing barium titanate nanocrystals according to any one of claims 1 to 5, wherein the water-soluble barium salt comprises at least one selected from the group consisting of barium hydroxide and barium chloride.

7. The method for producing barium titanate nanocrystals according to any one of claims 1 to 6, further comprising, after the heating step, a washing step of dispersing the crystals in alcohol and washing them.

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