Tetragonal-phase NANO barium titanate powder, preparation method therefor, and use thereof

Abstract

The present invention relates to the technical field of dielectric ceramic powder preparation processes, and in particular, to tetragonal-phase nano barium titanate powder, a preparation method therefor, and a use thereof. The preparation method comprises the following steps: (S.1) preparing a crystalline phase-controllable mesoporous titanium dioxide precursor, adding the mesoporous titanium dioxide precursor into a barium salt solution according to a barium-titanium molar ratio of 2-4, and then adding ethanol and ammonia water to obtain a barium titanate precursor suspension, wherein the volume ratio of ethanol in the barium titanate precursor suspension is 40%-60%; (S.2) carrying out hydrothermal reaction on the barium titanate precursor suspension to obtain a barium titanate suspension; and (S.3) carrying out centrifugal washing, drying and grinding on the barium titanate suspension to obtain tetragonal-phase nano barium titanate powder. In the present application, the barium-titanium molar ratio between an anatase mesoporous titanium dioxide precursor and a barium salt in the hydrothermal reaction process and the proportion of ethanol in a hydrothermal medium in the hydrothermal reaction process are controlled at the same time, such that barium titanate powder having a higher c / a value and particle size uniformity can be obtained.

Description

A tetragonal nano-barium titanate powder and its preparation method and application Technical Field

[0001] The present invention relates to the technical field of dielectric ceramic powder preparation technology, in particular to a tetragonal nano-barium titanate powder and a preparation method and application thereof. Background Art

[0002] Barium titanate (BTO) has broad application prospects in the dielectric ceramics industry, particularly in multilayer ceramic capacitors (MLCCs), due to its excellent dielectric and ferroelectric properties. With the increasing miniaturization of electronic devices, the demand for MLCCs with thin dielectric layers and high capacitance has greatly increased. This places higher demands on the tetragonality (measured by the crystal axis ratio c / a) and particle size of the nano-BTO powder, the primary raw material for the dielectric layer.

[0003] Currently, numerous methods are available for the synthesis of barium titanate, with the hydrothermal method being widely studied due to its controllable synthesis process and simple operation. However, conventional hydrothermal methods often produce barium titanate powders in the cubic phase or tetragonal phase with a low c / a ratio. Subsequent high-temperature heat treatment is usually required to increase the c / a ratio, but this process can easily lead to agglomeration of barium titanate particles.

[0004] Therefore, the hydrothermal method to synthesize tetragonal nano-barium titanate powder with high c / a value is an important and worthy of exploration. Summary of the Invention

[0005] The present invention aims to overcome the drawback of the prior art that it is difficult to directly obtain tetragonal barium titanate with a high c / a value through a hydrothermal method. Therefore, the present invention provides a method for directly obtaining tetragonal barium titanate with a high c / a value through a one-step hydrothermal method.

[0006] To achieve the above-mentioned purpose, the present invention is implemented through the following technical solutions:

[0007] In a first aspect, the present invention provides a method for preparing tetragonal nano-barium titanate powder, comprising the following steps:

[0008] (S.1) adding a mesoporous titanium dioxide precursor with controlled crystal phase to a barium salt solution at a barium to titanium molar ratio of 2 to 4, followed by adding ethanol and aqueous ammonia to obtain a barium titanate precursor suspension;

[0009] The volume ratio of ethanol in the barium titanate precursor suspension is 40% to 60%;

[0010] (S.2) subjecting the barium titanate precursor suspension to a hydrothermal reaction to obtain a barium titanate suspension;

[0011] (S.3) The barium titanate suspension is centrifugally washed, dried, and ground to obtain tetragonal nano-barium titanate powder.

[0012] The inventors of this application have discovered that, in the process of preparing nano-barium titanate powder by a hydrothermal method, the morphology of the final nano-barium titanate powder can be regulated by controlling the reaction conditions during the hydrothermal process, thereby enabling the barium titanate to obtain a higher c / a value. In the course of further research, the inventors found that the key factors affecting the c / a value of tetragonal nano-barium titanate powder include the following three points: (1) the morphology and particle size characteristics of the mesoporous titanium dioxide precursor; (2) the barium-titanium molar ratio between the titanium dioxide precursor and the barium salt during the hydrothermal reaction; and (3) the proportion of ethanol in the hydrothermal medium during the hydrothermal reaction.

[0013] Among them, regarding the morphology and particle size characteristics of the mesoporous titanium dioxide precursor, the anatase mesoporous titanium dioxide prepared by the present invention provides the dual advantages of anatase and mesoporous structure. The mesoporous structure further increases the specific surface area of ​​the anatase structure in the hydrothermal medium, provides more reaction sites and higher reaction activity, and promotes the subsequent hydrothermal method to fully carry out the barium titanate reaction, which is beneficial to improve the c / a value of the barium titanate powder and obtain a barium titanate powder with high product purity and low impurity content. Moreover, the mesoporous structure is composed of a large number of small-particle nano-titanium dioxide particles. The uniform and fine titanium dioxide nanoparticle precursor increases the solubility of titanium dioxide in the hydrothermal reaction, which is not only more conducive to a full hydrothermal reaction to obtain a tetragonal phase structure, but also reduces the heterogeneity of the nucleation and growth of barium titanate caused by the solubility difference, thereby ensuring the uniformity and dispersibility of the subsequent hydrothermal synthesis of barium titanate nanoparticles.

[0014] Regarding the barium-titanium molar ratio, the inventors found that when the barium-titanium molar ratio is in the range of 2 to 4, the c / a value of the nano-barium titanate powder can be higher than 1.008, which is sufficient to meet the needs of practical applications. Further research on the principle behind this ratio found that in theory, the formation of nano-barium titanate particles only needs to ensure that the barium-titanium molar ratio is 1:1. In the present application, the inventors found that when the barium-titanium molar ratio is in the range of 2 to 4, it can ensure that there are sufficient barium ions around the mesoporous titanium dioxide dissolved in the hydrothermal medium to react quickly with it, thereby increasing the probability of forming barium titanate crystal nuclei, thereby effectively obtaining small and uniform tetragonal barium titanate nanoparticles. When the barium-titanium molar ratio is less than 2, the probability of forming barium titanate crystal nuclei is greatly reduced, resulting in a significant decrease in the yield of barium titanate. At the same time, the reduction of barium titanate crystal nuclei also leads to a significant increase in the particle size of the generated barium titanate, which is not conducive to final utilization. When the molar ratio of barium to titanium was further increased, the inventors found that due to the excessive concentration of barium ions in the reaction system, the synthesized barium titanate contained barium carbonate impurities, which led to a significant decrease in its performance and was not suitable for practical applications.

[0015] Regarding the proportion of ethanol in the hydrothermal medium during the hydrothermal reaction, the inventors discovered that the ethanol content significantly impacts the formation of tetragonal nano-barium titanate powder with a high c / a ratio. This is due to the adsorption of hydroxyl groups on titanium sites or the catalytic oxidation of ethanol during the hydrothermal reaction. These side reactions exacerbate the hydroxylation process, thereby reducing the c / a ratio of the barium titanate. After in-depth research on the ethanol content in the hydrothermal medium, the inventors discovered that when the ethanol content is between 40% and 60%, competition between these side reactions suppresses the hydroxylation process during the hydrothermal reaction, thereby maintaining the c / a ratio of the resulting tetragonal nano-barium titanate powder. Furthermore, these conditions reduce the polarity of the hydrothermal solvent, resulting in a tetragonal nano-barium titanate powder with good particle size uniformity.

[0016] Therefore, in this application, by using anatase mesoporous titanium dioxide precursor, and simultaneously controlling the barium-titanium molar ratio between the titanium dioxide precursor and the barium salt during the hydrothermal reaction and the proportion of ethanol in the hydrothermal medium during the hydrothermal reaction, it is possible to obtain barium titanate powder with a higher c / a value and particle size uniformity. The three have a synergistic effect and none of them can be missing.

[0017] Preferably, the method for preparing the mesoporous titanium dioxide precursor in step (S.1) comprises the following steps:

[0018] (1) Adding the titanium source and pH regulator simultaneously dropwise into an ethanol aqueous solution containing a template, a pH stabilizer, and a dispersant, and performing a hydrothermal reaction after uniform dispersion to obtain a reaction product;

[0019] (2) washing and drying the obtained reaction product and then calcining it to remove residual organic matter;

[0020] (3) Grinding and homogenizing the calcined product to obtain anatase or rutile mesoporous titanium dioxide precursor with controllable powder crystal phase;

[0021] The pH regulator in step (1) is concentrated hydrochloric acid or ammonia water, and the pH value of the system is controlled to be 0.5-10 after adding the pH regulator.

[0022] In order to better control the synthesis cost and the tetragonality of barium titanate, the inventors found that the choice of titanium source is also crucial. Titanium dioxide has attracted much attention due to its advantages of being easily available, having no strict storage requirements, and being easy to scale up, but under mild hydrothermal conditions, its complex crystal structure and solubility should be further considered to affect the properties of the final synthesized barium titanate powder. In this application, the inventors unexpectedly discovered that the crystal structure of titanium dioxide can be regulated by controlling the type of pH regulator and pH conditions in the hydrothermal reaction. The applicant found that different pH values ​​are the key to obtaining mesoporous titanium dioxide of different crystal forms. The higher the pH value, the more conducive it is to the synthesis of anatase mesoporous titanium dioxide, while the lower the pH value, the more conducive it is to the synthesis of rutile mesoporous titanium dioxide.

[0023] In the present application, a template and a dispersant are added during the hydrothermal preparation of a mesoporous titanium dioxide precursor, thereby obtaining a mesoporous titanium dioxide precursor composed of a large number of small-particle nano-titanium dioxide particles. Under the same conditions, a more uniform and fine titanium dioxide nanoparticle precursor increases the solubility of the titanium dioxide precursor in the subsequent hydrothermal reaction to generate barium titanate. At the same time, the mesoporous structure further increases the specific surface area of ​​the anatase structure in the hydrothermal medium, provides more reaction sites and higher reaction activity, promotes the subsequent hydrothermal preparation of barium titanate reaction to proceed fully, is more conducive to improving the c / a value, and obtains a barium titanate powder with high product purity and low impurity content. In the subsequent hydrothermal generation of barium titanate, the previously synthesized mesoporous titanium dioxide first dissolves in the hydrothermal medium to form species, and then reacts with barium ions to form barium titanate crystal nuclei. The increase in solubility is not only more conducive to a sufficient hydrothermal reaction to obtain a tetragonal phase structure, but also reduces the uneven nucleation and growth of barium titanate caused by the solubility difference, thereby ensuring the uniformity and dispersibility of the subsequent hydrothermal synthesis of barium titanate nanoparticles.

[0024] Preferably, the mesoporous titanium dioxide precursor in the step (S.1) is an anatase-type mesoporous titanium dioxide precursor.

[0025] Compared with rutile titanium dioxide, which has a more thermodynamically stable crystal structure, anatase titanium dioxide has a higher solubility in the hydrothermal process, and the formation of mesopores further increases the specific surface area in contact with the hydrothermal medium, thereby easily forming more barium titanate crystal nuclei to obtain tetragonal barium titanate nanoparticles with fine and uniform particle size. Therefore, anatase mesoporous titanium dioxide is preferred as a precursor.

[0026] Preferably, the titanium source in step (1) is any one of tetrabutyl titanate, titanium tetrachloride, and titanium isopropoxide.

[0027] Preferably, the pH stabilizer is any one of urea, acetylacetone, acetic acid, and thioglycolic acid;

[0028] The template agent is any one of glucose, carbonaceous polysaccharide microspheres, polyethylene glycol, organic amine, and soluble starch;

[0029] The dispersant is any one of cetyltrimethylammonium bromide, polyvinylpyrrolidone and o-xylene.

[0030] Preferably, the hydrothermal condition in step (1) is to heat to 160-190°C using a heating jacket, heat-conducting oil or molten salt for a hydrothermal reaction of 2-6 hours;

[0031] In step (3), the calcination temperature is 400-700°C and the calcination time is 2-4 hours.

[0032] Preferably, the barium source in step (S.1) is any one of barium hydroxide, barium chloride and barium acetate.

[0033] Preferably, the pH of the barium titanate precursor suspension in step (S.1) is ≥13;

[0034] Preferably, in the step (S.2), the hydrothermal temperature is 220°C to 260°C, and the hydrothermal time is 12 to 40 hours.

[0035] In a second aspect, the present invention further provides a tetragonal nano-barium titanate powder, which is prepared by the above method;

[0036] The c / a value of the tetragonal nano-barium titanate powder is greater than 1.008;

[0037] The average particle size of the tetragonal nano-barium titanate powder is less than 200 nm.

[0038] In a third aspect, the present invention also provides the use of the above-mentioned tetragonal nano-barium titanate powder in the dielectric ceramic industry.

[0039] Therefore, the present invention has the following beneficial effects:

[0040] The hydrothermal method used in the present invention has simple preparation conditions, is easy to operate, has low cost, and can be mass-produced. The barium titanate powder synthesized from the anatase mesoporous titanium dioxide precursor prepared by this method has a c / a value of up to 1.0095, a narrow particle size distribution, and an average particle size of approximately 95 nm. This is because the anatase structure has greater solubility during the hydrothermal process, and the mesopores further increase the specific surface area, making the hydrothermal process react faster and more fully, resulting in a narrow particle size distribution of the tetragonal barium titanate obtained. When ethanol in the hydrothermal medium is present in an appropriate proportion of 40% to 60%, the hydroxylation process is inhibited to obtain a high c / a value. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG1 is an XRD spectrum of the mesoporous titanium dioxide powder with controllable crystal phase prepared by the hydrothermal method in Examples 1-4 of the present invention.

[0042] FIG2 is an XRD spectrum of tetragonal barium titanate powder prepared by hydrothermal method in Examples 1-4 of the present invention.

[0043] FIG3 is a SEM image of anatase mesoporous titanium dioxide powder prepared in Example 1 of the present invention.

[0044] FIG4 is a TEM image of anatase mesoporous titanium dioxide powder prepared in Example 1 of the present invention.

[0045] FIG5 is a SEM image of the tetragonal barium titanate powder prepared in Example 1 of the present invention.

[0046] FIG6 is a partially enlarged XRD pattern of the tetragonal barium titanate powder prepared in Example 1 of the present invention.

[0047] FIG. 7 is a SEM image of the tetragonal barium titanate powder prepared in Example 5 of the present invention.

[0048] FIG8 is a SEM image of the tetragonal barium titanate powder prepared in Example 6 of the present invention. Implementation Method

[0049] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only a portion of the embodiments of the present invention, rather than all of the embodiments. Therefore, all other embodiments derived by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention. Example

[0050] (1) Barium precursor: 18.93 g of barium hydroxide octahydrate was added to 20 mL of deionized water, heated in an 80 °C water bath with stirring until dissolved to obtain a barium salt solution.

[0051] (2) Titanium precursor: 0.6M glucose and 0.6M urea were added to 30 mL of a mixed solution of deionized water and ethanol in a ratio of 1:2, and magnetic stirring was applied until all the solids were dissolved. Then, 3.64 g of hexadecyltrimethylammonium bromide was added, and 4 mL of tetrabutyl titanate was added dropwise at a constant rate using a fixed double dropping funnel under ice-water bath and vigorous stirring. At the same time, ammonia water was added dropwise to adjust the pH of the above solution to 10 to obtain anatase structure. After stirring evenly, the mixture was heated to 190 °C using a heating jacket or thermal oil for hydrothermal reaction for 3 h. The reaction product was washed several times with anhydrous ethanol and deionized water, dried in an 80 °C oven for more than 12 h, and calcined at 400 °C for 3 h to remove residual organic matter. The calcined and ground powder was dispersed in ethanol, refined and homogenized using an ultrasonic cell disruptor to obtain a well-dispersed suspension, which was then placed in a vacuum oven for drying. Finally, a mesoporous titanium dioxide precursor with controllable crystal phase was obtained by grinding. The XRD spectrum of the obtained mesoporous titanium dioxide powder is shown in Figure 1, its SEM image is shown in Figure 3, and its TEM image is shown in Figure 4.

[0052] (3) Mixing: Weigh a certain amount of mesoporous titanium dioxide precursor with controlled crystal phase according to a molar ratio of barium to titanium of 4 and transfer it to the barium salt solution. Stir evenly, then add ethanol (the proportion of ethanol in the hydrothermal medium is 50%) and 15 mL of ammonia water. Control the solution pH ≥ 13 to obtain a barium titanate precursor suspension.

[0053] (4) Synthesis: Add the barium titanate precursor suspension into a 50 mL hydrothermal reactor and heat it to 260 °C using a heating jacket, heat-conducting oil, or molten salt for 30 h. Cool it to room temperature and open the reactor to obtain the barium titanate suspension.

[0054] (5) Washing: The hydrothermally synthesized barium titanate suspension was centrifuged and washed multiple times with acetic acid, deionized water, and ethanol. The supernatant was discarded, leaving the white precipitate at the bottom.

[0055] (6) Drying: The washed product was placed in an oven at 80 °C and dried for not less than 12 h. The obtained product was ground to obtain barium titanate powder. The XRD spectrum of the prepared tetragonal barium titanate powder is shown in Figure 2. The locally enlarged XRD spectrum shows obvious splitting peaks of the tetragonal phase structure as shown in Figure 6. Its SEM image is shown in Figure 5. Example

[0056] A mesoporous titanium dioxide precursor with controllable crystal phase was prepared by a hydrothermal method using the method of Example 1. The difference is that in step (2), 6 mL of the initial titanium source was added dropwise at a uniform rate, and the mixture was heated to 160°C by a heating mantle, heat-conducting oil, or molten salt for a hydrothermal reaction for 6 h. The XRD spectrum of the prepared mesoporous titanium dioxide powder is shown in Figure 1, and the XRD spectrum of the finally prepared tetragonal barium titanate powder is shown in Figure 2. Example

[0057] A mesoporous titanium dioxide precursor with controllable crystal phase was prepared by a hydrothermal method using the method of Example 1, except that in step (2), concentrated hydrochloric acid was added dropwise to adjust the pH of the solution to 4 to obtain a mixed phase structure of anatase and rutile. The XRD spectrum of the obtained mesoporous titanium dioxide powder is shown in FIG1 , and the XRD spectrum of the finally prepared tetragonal barium titanate powder is shown in FIG2 . Example

[0058] A mesoporous titanium dioxide precursor with controllable crystal phase was prepared by a hydrothermal method using the method of Example 1. The difference is that in step (2), concentrated hydrochloric acid was added dropwise to adjust the pH of the above solution to 0.5 to obtain a rutile structure. The dried product was calcined at 700°C for 3 h to remove residual organic matter. The XRD spectrum of the obtained mesoporous titanium dioxide powder is shown in Figure 1, and the XRD spectrum of the finally prepared tetragonal barium titanate powder is shown in Figure 2. Example

[0059] Tetragonal nano-barium titanate powder was prepared by a hydrothermal method using the method of Example 1. The difference is that in steps (1), (2), and (3), the barium source is barium chloride, the titanium source is titanium tetrachloride, and the Ba / Ti ratio used is 3. The SEM image of the finally obtained barium titanate powder is shown in FIG7 . Example

[0060] Tetragonal nano-barium titanate powder was prepared by a hydrothermal method using the method of Example 1. The difference is that in steps (1), (2), and (3), the barium source is barium acetate, the titanium source is titanium isopropoxide, and the Ba / Ti ratio used is 2. The SEM image of the finally obtained barium titanate powder is shown in Figure 8. Example

[0061] Tetragonal nano-barium titanate powder was prepared by a hydrothermal method using the method of Example 1, except that in step (3), the proportion of ethanol in the hydrothermal medium was 40%. Example

[0062] Tetragonal nano-barium titanate powder was prepared by a hydrothermal method using the method of Example 1, except that in step (3), the proportion of ethanol in the hydrothermal medium was 60%. Example

[0063] Tetragonal nano-barium titanate powder was prepared by hydrothermal method using the method of Example 1, except that in step (4), heating was performed at 220°C by means of a heating jacket, heat-conducting oil or molten salt for 40 h.

[0064] Example 10:

[0065] Tetragonal nano-barium titanate powder was prepared by hydrothermal method using the method of Example 1, except that in step (4), the mixture was heated to 250° C. for hydrothermal reaction for 12 h using a heating jacket, heat-conducting oil or molten salt.

[0066] Comparative Example 1:

[0067] The method of Example 1 was used to prepare a mesoporous titanium dioxide precursor with controllable crystal phase by a hydrothermal method, except that in step (2), 2 mL of the initial titanium source was added dropwise at a uniform rate.

[0068] Comparative Example 2:

[0069] The method of Example 1 was used to prepare a mesoporous titanium dioxide precursor with controllable crystal phase by a hydrothermal method, except that in step (2), 8 mL of the initial titanium source was added dropwise at a uniform rate, and the mixture was heated to 160 °C by a heating jacket, heat-conducting oil or molten salt for a hydrothermal reaction for 12 h.

[0070] Comparative Example 3:

[0071] The method of Example 1 was used to prepare a mesoporous titanium dioxide precursor with controllable crystal phase by a hydrothermal method, except that in step (2), a mixed solution of deionized water and ethanol in a ratio of 1:1 was used.

[0072] Comparative Example 4:

[0073] The method of Example 1 was used to prepare a mesoporous titanium dioxide precursor with controllable crystal phase by a hydrothermal method, except that in step (2), a mixed solution of deionized water and ethanol in a ratio of 2:1 was used.

[0074] Comparative Example 5:

[0075] Tetragonal nano-barium titanate powder was prepared by a hydrothermal method using the method of Example 1, except that in steps (1), (2), and (3), the barium source was barium chloride, the titanium source was titanium tetrachloride, and the Ba / Ti ratio used was 1.

[0076] Comparative Example 6:

[0077] Tetragonal nano-barium titanate powder was prepared by a hydrothermal method using the method of Example 1, except that in steps (1), (2), and (3), the barium source was barium chloride, the titanium source was titanium tetrachloride, and the Ba / Ti ratio used was 5.

[0078] Comparative Example 7:

[0079] Tetragonal nano-barium titanate powder was prepared by a hydrothermal method using the method of Example 1, except that in step (3), the proportion of ethanol in the hydrothermal medium was 20%.

[0080] Comparative Example 8:

[0081] Tetragonal nano-barium titanate powder was prepared by a hydrothermal method using the method of Example 1, except that in step (3), the proportion of ethanol in the hydrothermal medium was 80%.

[0082] Comparative Example 9:

[0083] Tetragonal nano-barium titanate powder was prepared by hydrothermal method using the method of Example 1, except that in step (4), the hydrothermal reaction was heated to 200 °C for 60 h using a heating jacket, heat-conducting oil or molten salt.

[0084] Comparative Example 10:

[0085] Tetragonal nano-barium titanate powder was prepared by hydrothermal method using the method of Example 1, except that in step (4), the hydrothermal reaction was heated to 270° C. for 10 h using a heating jacket, heat-conducting oil or molten salt.

[0086] Comparative Example 11:

[0087] Tetragonal nano-barium titanate powder was prepared by a hydrothermal method using the method of Example 1, except that in step (2), the titanium dioxide precursor used was a non-mesoporous commercial anatase titanium dioxide precursor.

[0088]

Performance testing and result analysis

[0089] (I) Effect of pH value on the hydrothermal preparation of mesoporous titanium dioxide precursor with controllable crystal phase, see Table 1.

[0090] Table 1 Effect of pH value on the phase-controlled mesoporous titanium dioxide precursor prepared by hydrothermal method

[0091] .

[0092] As can be seen from Table 1, different pH values ​​are the key to obtaining mesoporous titanium dioxide with different crystal forms. The higher the pH value, the more conducive it is to the synthesis of anatase mesoporous titanium dioxide. Compared with rutile titanium dioxide, which has a more thermodynamically stable crystal structure, anatase titanium dioxide has a higher solubility in the hydrothermal process, and the formation of mesopores further increases the specific surface area in contact with the hydrothermal medium, making it easier to form more barium titanate nuclei, thereby obtaining small and uniform tetragonal barium titanate nanoparticles. Therefore, anatase mesoporous titanium dioxide is preferred as a precursor.

[0093] (2) Effect of initial titanium source dosage on the phase-controlled mesoporous titanium dioxide precursor prepared by hydrothermal method, see Table 2.

[0094] Table 2 Effect of initial titanium source dosage on the phase-controlled mesoporous titanium dioxide precursor prepared by hydrothermal method

[0095] .

[0096] As can be seen from Table 2, the crystalline phase of mesoporous titanium dioxide obtained with different initial titanium source dosages is all anatase. However, the particle size of mesoporous titanium dioxide synthesized in the range of 4-6 mL of the initial titanium source is small and uniform. This is because as the concentration of the reactants increases, the nucleation sites of titanium dioxide nanoparticles increase, resulting in finer mesoporous titanium dioxide. When the initial titanium source dosage is 2 mL, the initial nucleation sites are small, and titanium dioxide easily grows on the formed nucleation sites, resulting in a larger anatase titanium dioxide particle size. When the initial titanium source dosage is increased to 8 mL, the particle size of the titanium dioxide obtained is not significantly different from that of 6 mL. Therefore, 4-6 mL is preferred as the optimal dosage of the initial titanium source.

[0097] (III) Effect of the ratio of deionized water to ethanol on the phase-controlled mesoporous titanium dioxide precursor prepared by the hydrothermal method, see Table 3.

[0098] Table 3 Effect of the ratio of deionized water to ethanol on the phase-controlled mesoporous titanium dioxide precursor prepared by hydrothermal method

[0099] .

[0100] As can be seen from Table 3, when the deionized water to ethanol ratio is 1:2, the initial particle size of the titanium dioxide precursor obtained under this condition is finer than that of the other cases in the comparative example, further increasing the solubility of the titanium dioxide precursor in the hydrothermal process, which is beneficial for obtaining small and uniform tetragonal barium titanate nanoparticles. In addition, since glucose is slightly soluble in ethanol, the ethanol content should not be too high. When the deionized water to ethanol ratio is higher, the dissolution of glucose becomes difficult and requires longer stirring time. Therefore, a deionized water to ethanol ratio of 1:2 is preferred.

[0101] (3) Effect of Ba / Ti ratio on tetragonal nano-barium titanate powder prepared by hydrothermal method, see Table 4.

[0102] Table 4 Effect of Ba / Ti ratio on tetragonal nano-barium titanate powder prepared by hydrothermal method

[0103] .

[0104] As shown in Table 4, when the Ba / Ti ratio is between 2 and 4, the c / a values ​​of hydrothermally prepared nanobarium titanate powders are all above 1.008, which is sufficient for practical applications. A higher Ba / Ti ratio increases the probability of barium titanate nucleation, which is beneficial for obtaining small and uniform tetragonal barium titanate nanoparticles. However, when the Ba / Ti ratio is further increased to 5, the barium ion concentration is too high, and the synthesized barium titanate contains barium carbonate impurities. Therefore, a Ba / Ti ratio of 2 to 4 is preferred.

[0105] (IV) The proportion of ethanol in step (3) for tetragonal nano-barium titanate powder prepared by hydrothermal method is shown in Table 5.

[0106] Table 5 Effect of ethanol ratio on tetragonal nano-barium titanate powder prepared by hydrothermal method

[0107] .

[0108] As shown in Table 5, when the ethanol ratio is high or low, the c / a value of the barium titanate obtained in the comparative example is low, resulting in adsorption of hydroxyl groups on the titanium sites or catalytic oxidation of ethanol, which intensifies the hydroxylation process. Only when ethanol is present in the hydrothermal medium at an appropriate ratio of 40% to 60% is the hydroxylation process suppressed due to competition between side reactions. Therefore, the preferred ethanol ratio is 40% to 60%.

[0109] (V) Effects of hydrothermal temperature and time on tetragonal nano-barium titanate powder prepared by hydrothermal method, see Table 6.

[0110] Table 6 Effects of hydrothermal temperature and time on tetragonal nano-barium titanate powder prepared by hydrothermal method

[0111] .

[0112] As can be seen in Table 6, when the hydrothermal temperature is low (200°C), extending the hydrothermal time still fails to achieve the ideal c / a value, but the impact of dissolution differences is magnified, resulting in poor particle size uniformity of the obtained barium titanate. Furthermore, prolonged hydrothermal treatment is time-consuming and energy-consuming. Furthermore, considering the operating temperature requirements of the hydrothermal reactor, excessively high temperatures increase reactor wear. To provide sufficient driving force for the formation of the tetragonal phase within a relatively short period of time, a more suitable temperature and time were selected in conjunction with the experimental investigations of the present invention. Therefore, the preferred hydrothermal temperature and time are 220°C to 260°C for 12 to 40 hours.

[0113] (6) The effects of mesoporous and non-mesoporous titanium dioxide precursors on tetragonal nano-barium titanate powder prepared by hydrothermal method, see Table 7.

[0114] Table 7 Effects of mesoporous and non-mesoporous titanium dioxide precursors on tetragonal nano-barium titanate powders prepared by hydrothermal method

[0115] .

[0116] As can be seen from Table 7, under the same experimental conditions as Example 1, the barium titanate powder obtained using a non-mesoporous commercial anatase titanium dioxide precursor exhibited a low c / a value and large particle size. This is because the specific surface area of ​​non-mesoporous titanium dioxide in contact with the hydrothermal medium is not as large as that of mesoporous titanium dioxide, resulting in a reduced dissolution rate and reactivity of titanium dioxide, insufficient driving force for crystal structure change, and thus a failure to achieve the desired c / a value. Furthermore, the rate of barium titanate nucleation is slowed, resulting in a larger particle size. Furthermore, the tetragonal nanobarium titanate particles provided in the hydrothermal synthesis of barium titanate using a titanium dioxide precursor, disclosed in Patent Publication No. CN111762810A, entitled "A Method for Preparing Tetragonal Nanobarium Titanate," ranged from 60 to 200 nm and had a c / a value of 1.0082, similarly lower than the performance achieved with the mesoporous titanium dioxide precursor in the present invention.

[0117] In summary, the adjustment of multiple parameters in the present invention has a synergistic effect. For example, the mesoporous structure of anatase mesoporous titanium dioxide, which is composed of fine and uniform particles, increases the specific surface area and reaction activity, further improving the solubility of the titanium dioxide precursor in the hydrothermal reaction. In the subsequent reaction, a larger Ba / Ti ratio is adopted to further provide sufficient barium ions to react with the dissolved titanium species in a timely manner, increasing the nucleation probability of barium titanate, obtaining a high c / a value, and being conducive to obtaining a fine and uniform barium titanate powder. In addition, the synergistic effect between the higher reaction temperature range (220~260 ℃) and the proportion of ethanol in the hydrothermal medium (40%~60%) is more conducive to controlling the hydroxyl defects formed in the hydrothermal reaction, thereby ensuring the acquisition of a high c / a value of the barium titanate powder.

[0118] Although the above content (in conjunction with the accompanying drawings) describes the preferred embodiments of the present invention, the present invention is not limited to the above specific implementation methods. The above specific implementation methods are merely illustrative and not restrictive. Under the guidance of the present invention, ordinary technicians in this field can also make many forms of specific changes without departing from the scope of protection of the present invention and the claims. These all fall within the scope of protection of the present invention.

Claims

1. A method for preparing tetragonal nano-barium titanate powder, characterized in that: The following steps are involved: (S.1) adding a mesoporous titanium dioxide precursor with controllable crystal phase to a barium salt solution at a barium-titanium molar ratio of 2 to 4, and then adding ethanol and ammonia water to obtain a barium titanate precursor suspension; The volume ratio of ethanol in the barium titanate precursor suspension is 40% to 60%; (S.2) subjecting the barium titanate precursor suspension to a hydrothermal reaction to obtain a barium titanate suspension; (S.3) The barium titanate suspension is centrifugally washed, dried, and ground to obtain tetragonal nano-barium titanate powder.

2. The method for preparing tetragonal nano-barium titanate powder according to claim 1, characterized in that: The method for preparing the mesoporous titanium dioxide precursor in step (S.1) comprises the following steps: (1) adding a titanium source and a pH adjuster simultaneously dropwise into an ethanol aqueous solution containing a template, a pH stabilizer and a dispersant, and performing a hydrothermal reaction after uniform dispersion to obtain a reaction product; (2) washing and drying the obtained reaction product and then calcining it to remove residual organic matter; (3) Grinding and homogenizing the calcined product to obtain anatase or rutile mesoporous titanium dioxide precursor with controllable powder crystal phase; The pH regulator in step (1) is concentrated hydrochloric acid or aqueous ammonia, and after adding the pH regulator, the pH value of the system is controlled to be 0.5-10.

3. The method for preparing tetragonal nano-barium titanate powder according to claim 1 or 2, characterized in that: The mesoporous titanium dioxide precursor in the step (S.1) is an anatase-type mesoporous titanium dioxide precursor.

4. The method for preparing tetragonal nano-barium titanate powder according to claim 2, characterized in that: In the step (1), the titanium source is any one of tetrabutyl titanate, titanium tetrachloride, and titanium isopropoxide.

5. The method for preparing tetragonal nano-barium titanate powder according to claim 2, characterized in that: The template agent is any one of glucose, carbonaceous polysaccharide microspheres, polyethylene glycol, organic amine, and soluble starch; The pH stabilizer is any one of urea, acetylacetone, acetic acid, and thioglycolic acid; The dispersant is any one of hexadecyltrimethylammonium bromide, polyvinylpyrrolidone and o-xylene.

6. The method for preparing a mesoporous titanium dioxide precursor with controllable crystal phase according to claim 2, 4 or 5, characterized in that: The hydrothermal conditions in step (1) are to heat to 160-190°C by means of a heating jacket, heat-conducting oil or molten salt for a hydrothermal reaction of 2-6 hours; In the step (3), the calcination temperature is 400-700°C and the calcination time is 2-4 hours.

7. The method for preparing tetragonal nano-barium titanate powder according to claim 1, characterized in that: In the step (S.1), the barium source is any one of barium hydroxide, barium chloride and barium acetate.

8. The method for preparing tetragonal nano-barium titanate powder according to claim 1 or 7, characterized in that: In the step (S.1), the pH of the barium titanate precursor suspension is ≥ 13; In the step (S.2), the hydrothermal temperature is 220°C to 260°C, and the hydrothermal time is 12 to 40 h.

9. A tetragonal nano-barium titanate powder, characterized in that: It is prepared by the method described in any one of claims 1 to 8; The c / a value of the tetragonal nano-barium titanate powder is greater than 1.008; The average particle size of the tetragonal nano-barium titanate powder is less than 200 nm.

10. Application of the tetragonal nano-barium titanate powder as claimed in claim 9 in the dielectric ceramic industry.

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