A secondary hydrothermal crystallization method for preparing core-shell structured nano-modified barium titanate-based formula powder

Abstract

The present invention discloses a secondary hydrothermal crystallization preparation method for a core-shell structured nano-modified barium titanate-based powder. The method uses a secondary hydrothermal reaction crystallization control technology to achieve nano-scaling and homogenization of the base material. A proton capture agent is added to reduce the proportion of mismatched OH-ions, minimizing the cubic metastable phase structure present at the phase transition point temperature. Bulk ion doping is used to suppress and broaden the Curie peak, thereby increasing the dielectric constant, reducing dielectric loss, and optimizing temperature characteristics. The method improves sintering characteristics and enhances dielectric properties by forming a core-shell structured nano-modified barium titanate-based powder. The method achieves the preparation of a core-shell structured nano-modified barium titanate-based powder, recycles excess barium chloride, sodium hydroxide, and hydrochloric acid, and is suitable for the efficient preparation of barium titanate ceramic powder for high-performance MLCCs.

Description

Technical Field

[0001] The invention belongs to the technical field of hydrothermal crystallization preparation technology of nano functional materials, and particularly relates to a secondary hydrothermal crystallization preparation method of a core-shell structure nano modified barium titanate-based formula powder. Background Art

[0002] MLCC (Multi-layer Ceramic Capacitors) is short for chip-type multilayer ceramic capacitors. They consist of ceramic dielectric diaphragms with internal electrodes stacked in an offset manner, sintered at high temperature to form a ceramic chip. Metal external electrodes are then sealed at both ends of the chip, forming a structure similar to a monolith. MLCCs are a key component of ceramic capacitors, offering advantages such as a wide capacitance range, excellent frequency characteristics, a wide operating voltage and temperature range, an ultra-small size, and non-polarity. They are essential components for noise bypassing, power filtering, energy storage, differentiation, integration, and oscillation circuits. They are widely used in military and industrial fields such as aerospace, aviation, shipbuilding, weapons, medical equipment, rail transportation, and automotive electronics.

[0003] The explosive growth of smart consumer electronics has driven demand for related MLCC products. The diverse range of functional applications is also driving increasing demands for MLCC products. The development of high-capacity MLCCs is driven by a continuous pursuit of thinner dielectric thicknesses and higher layer counts. For example, the world's latest technology currently produces dielectrics with a thickness of just 1μm and designs with over 1,000 layers. "Smaller, thinner, and higher specific capacitance" is the future development direction for MLCC products, placing higher demands on materials, equipment, and process technology. Key requirements include nano-sizing of base materials, high dielectric constants, low dielectric loss, and excellent temperature characteristics.

[0004] Barium titanate-based ceramics are a new type of environmentally friendly electronic ceramic material with a very high dielectric constant (room temperature dielectric constant of ~1600, and a maximum dielectric constant of 15000 at the Curie temperature) and low dielectric loss. They are widely used in electronic components such as multilayer ceramic capacitors, supercapacitors, thermistors, ferroelectric and piezoelectric devices.

[0005] However, pure BaTiO3 ceramics are prone to abnormal grain growth during sintering, resulting in low densification, a relatively low room-temperature dielectric constant, and significant temperature variation, all of which fail to meet practical application requirements. To obtain high-quality BaTiO3 ceramics with a high dielectric constant ε in the operating temperature range, minimal temperature variation of ε, and both dielectric loss and breakdown strength that meet the required standards, BaTiO3 ceramics must be modified. This involves doping and forming a core-shell structure to improve sintering characteristics and shift and broaden the Curie peak.

[0006] Barium titanate belongs to the typical ABO3 perovskite structure and is also a ferroelectric with a density of 6.08g / cm 3 , melting point 1625 ℃, with five different crystal structures, namely rhombohedral, tetragonal, cubic and hexagonal. Among them, cubic and tetragonal are the two most widely used phases. Above 120 ℃, the cubic phase structure exists stably. Its unit cell structure and octahedral structure are as follows Figure 1 and 2 shown.

[0007] Currently, most domestic barium titanate manufacturers use a solid-phase synthesis method. However, this method produces large and unevenly distributed powders, making them primarily used in the production of mid- and low-end electronic components. High-temperature, high-pressure hydrothermal processing is the core technology used internationally to mass-produce high-purity, nanoscale barium titanate powders.

[0008] The hydrothermal method prepares the barium titanate ultrafine particles. At room temperature, some grains still retain the stable cubic phase structure at high temperature, which is not conducive to the dielectric properties of the material. From the perspective of crystallization chemistry, this is because the two adjacent Ti-O6 octahedrons are connected by matching the vertex angles. If there is a mismatch, an OH- is suspended at one vertex of the mismatched Ti-O6 octahedron, and it is located in the BaTiO3 structure. 2+ The position of BaTiO3 makes the pores along the a, b or c axis in the BaTiO3 structure blocked, so when the temperature drops below the phase transition point, BaTiO3 2+ The pores cannot be displaced along the a, b or c axis, and the Ti-O6 octahedron cannot be deformed. Therefore, at room temperature, the BaTiO3 ultrafine grains can still retain the cubic metastable phase structure, which affects the dielectric constant properties of the ceramic powder. The results of thermal analysis show that after high-temperature sintering (temperature is about 1346℃), the oxygen decomposed by OH- can be replenished at the octahedron corners, and the pores are opened. When the temperature drops to the phase transition point, BaTiO3 2+ It can move along the c-axis, the Ti-O6 octahedron is deformed, and the crystal transforms from cubic phase to tetragonal phase.

[0009] However, high-temperature sintering can lead to severe grain growth in ceramic powders, hindering the production of high-end MLCC electronic components. Therefore, the present invention utilizes a secondary hydrothermal reaction crystallization control technique to achieve nano-scaling and homogenization of the base material. A proton scavenger is added to reduce the proportion of mismatched OH-ions, minimizing the presence of cubic metastable phase structures at the phase transition point. Bulk ion doping suppresses and broadens the Curie peak, increasing the dielectric constant, reducing dielectric loss, and optimizing temperature characteristics. Finally, a nano-modified barium titanate-based powder with a core-shell structure is formed to improve sintering characteristics and enhance dielectric properties. This results in the production of smaller, thinner, and higher-capacity barium titanate ceramic powders for high-end MLCCs. Summary of the Invention

[0010] The present invention provides a secondary hydrothermal crystallization preparation method for a core-shell structured nano-modified barium titanate-based formula powder, which realizes the preparation of the core-shell structured nano-modified barium titanate-based formula powder and the recycling of excess barium chloride, sodium hydroxide and hydrochloric acid. The method is suitable for the efficient preparation of barium titanate ceramic powder for high-performance MLCC.

[0011] The present invention discloses a method for preparing a core-shell structured nano-modified barium titanate-based powder by secondary hydrothermal crystallization, which comprises the following steps:

[0012] Step S01, under nitrogen protection, dissolving barium chloride dihydrate crystals, bulk doping additives, proton capture agents and deoxygenated deionized water in an ultrasonic dissolution tank, so that the mass ratio of barium chloride in the solution reaches 5-25%, and the dissolution temperature is 20-40° C.;

[0013] Step S02: Under nitrogen protection, 30-32% ion-exchange membrane alkali solution, titanium tetrachloride solution, and the barium chloride solution dissolved in step S01 are mixed in a high-efficiency mixer to generate a white slurry to prepare a first-level hydrothermal precursor slurry, maintaining a molar ratio of Ba:Ti:NaOH of 1.5-3.0:1.0:10.0-20.0;

[0014] Step S03, transporting the precursor slurry liquid of the first-level hydrothermal reaction in step S02 to the first-level hydrothermal reaction crystallizer, sealing it and replacing the atmosphere in the crystallizer;

[0015] Step S04: using induction heating in a first-stage hydrothermal reaction crystallizer, setting a heating rate of 0.5-20°C / min, a reaction temperature of 160-300°C, and a constant temperature time of 0.5-72h;

[0016] Step S05: After cooling the slurry obtained in step S04, the pH is adjusted to 6.0-7.0 using 1.0-2.0 mol / L dilute hydrochloric acid, and continuous solid-liquid separation and washing are performed using a ceramic membrane separation device. The conductivity of the mother liquor at the end of the washing is 300-1000 μS / cm;

[0017] Step S06, efficiently and uniformly dispersing the cleaned bulk-doped barium titanate wet crystals in step S05, the additive constituting the shell material, the pH adjuster, and deoxygenated water into a slurry having a solid content of 5.0-20%, wherein the mass ratio of the bulk-doped barium titanate wet crystals to the additive constituting the shell material is 9-19;

[0018] Step S07: The slurry evenly dispersed in step S06 is transported to a secondary hydrothermal reaction crystallizer, which is sealed and the atmosphere in the crystallizer is replaced;

[0019] Step S08, using induction heating in a secondary hydrothermal reaction crystallizer, setting the heating rate to 0.5-10°C / min, the reaction temperature to 120-280°C, and the constant temperature time to 0.5-48h;

[0020] Step S09: After cooling the slurry obtained in step S08, the pH is adjusted to 6.0-7.0 using 1.0-2.0 mol / L dilute hydrochloric acid, and continuous solid-liquid separation and washing are performed using a high-speed horizontal spiral sedimentation centrifuge or a rotary ceramic membrane separator. The conductivity of the mother liquor at the end of the washing is 100-1000 μS / cm;

[0021] Step S10: The clean core-shell structured modified barium titanate-based powder slurry washed in step S09 is spray-dried or flash-dried to obtain spherical modified barium titanate-based powder secondary particles, with a residual moisture content of <1%, an air inlet temperature of 200-300° C., and an air outlet temperature of 105-120° C.;

[0022] Step S11: The spherical modified barium titanate-based formula powder secondary particles obtained by drying in step S10 are deagglomerated by airflow milling and packaged as finished products to obtain a final grain main particle size D50 ranging from 60 to 100 nm and D97 less than 200 nm;

[0023] Step S12: The mother liquor separated in step S05 is passed through a multi-stage nanofiltration membrane device to obtain a concentrated aqueous solution of barium chloride and a clear aqueous solution of barium chloride, respectively. The concentrated aqueous solution of barium chloride is configured with matching barium chloride dihydrate crystals and deoxygenated deionized water to form a barium chloride solution as a precursor of a first-stage hydrothermal reaction crystallization system, with a barium chloride content of 5-25%, thereby realizing the recycling of excess barium chloride;

[0024] Step S13, the clear aqueous solution of barium chloride obtained in step S12, wherein the barium chloride concentration is about 500-1000 ppm, is subjected to deep removal of residual barium ions by using an ion exchange membrane or barium sulfate precipitation;

[0025] Step S14, using a bipolar membrane electrodialysis device to prepare a 1.0-2.0 mol / L sodium hydroxide solution and a 1.0-2.0 mol / L hydrochloric acid solution, respectively;

[0026] Step S15, using a vacuum flash evaporation process to concentrate the sodium hydroxide solution in step S14, with a sodium hydroxide concentration of 30-32% and a vacuum flash evaporation temperature of 50-90°C.

[0027] Preferably, in the secondary hydrothermal crystallization preparation method of the core-shell structured nano-modified barium titanate-based formula powder, the bulk doping additive in step S01 includes one or more of MnCl2·4H2O, MgCl2·6H2O, NdCl3·6H2O and GdCl3·6H2O.

[0028] Preferably, in the secondary hydrothermal crystallization preparation method of the core-shell structured nano-modified barium titanate-based formula powder, the proton capture agent in step S01 is one or more of pyridine, propylene oxide and organic amine.

[0029] Preferably, in the secondary hydrothermal crystallization method for preparing the core-shell structured nano-modified barium titanate-based formula powder, the main particle size of the bulk-doped barium titanate wet crystals washed clean in step S05 is 50-100 nm.

[0030] Preferably, in the secondary hydrothermal crystallization preparation method of the core-shell structured nano-modified barium titanate-based formula powder, the additive constituting the shell material in step S06 is NbCl5·6H2O, and the pH regulator is NaOH.

[0031] Preferably, in the secondary hydrothermal crystallization preparation method of the core-shell structured nano-modified barium titanate-based formula powder, the pH regulator of the separated mother liquor in step S12 is HCl.

[0032] Compared with the existing technology, the present invention realizes the preparation of nano-scale modified barium titanate-based formula powder with core-shell structure and the recycling of excess barium chloride, sodium hydroxide and hydrochloric acid, which is suitable for the development direction of "smaller, thinner and higher specific volume" of barium titanate ceramic powder for high-performance MLCC. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments recorded in this application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0034] Figure 1 Shown is a unit cell structure diagram in the background art;

[0035] Figure 2 Shown is a diagram of an octahedral structure in the background art;

[0036] Figure 3 The figure shows a flow chart of a method for preparing a core-shell structure nano-modified barium titanate-based formula powder through secondary hydrothermal crystallization in a specific embodiment of the present invention;

[0037] Figure 4 Shown is a photograph of the first-stage hydrothermal crystallized barium titanate product prepared in Example 1;

[0038] Figure 5 Shown is the XRD pattern of the first-stage hydrothermal crystallized barium titanate product prepared in Example 2;

[0039] Figure 6 Shown is the Rietlveld refinement image of the primary hydrothermal crystallized barium titanate product prepared in Example 2. DETAILED DESCRIPTION

[0040] The following is a detailed description of the technical solutions in the embodiments of the present invention, with reference to the accompanying drawings. Obviously, the described embodiments are only a portion of the embodiments of the present invention, not all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0041] Example 1

[0042] like Figure 1 The preparation method flow shown is as follows: the preparation system includes: a dissolution and high-efficiency mixing and batching system; a primary hydrothermal reaction crystallization system; a solid-liquid separation and washing system for doped barium titanate slurry; a core-shell structure precursor batching and secondary hydrothermal reaction crystallization system; a core-shell structure nano-modified barium titanate-based formula powder slurry solid-liquid separation and washing system; a core-shell structure nano-modified barium titanate-based formula powder wet crystal dispersion, drying, gas powder and packaging system; a primary hydrothermal crystallization separation mother liquor excess barium chloride, sodium hydroxide and hydrochloric acid separation and recycling system.

[0043] (1) Dissolution and efficient mixing system: 356.5 kg of barium chloride dihydrate crystals, 3.5 kg of neodymium chloride hexahydrate, 1.0 kg of manganese chloride tetrahydrate, 1.0 kg of magnesium chloride hexahydrate, 50 L of propylene oxide and 2000 kg of deoxygenated deionized water are taken. Under nitrogen protection, the above ingredients are dissolved in an ultrasonic dissolution tank with deoxygenated deionized water, and the mixture is stirred and dissolved for 30 minutes at a dissolution temperature of 35°C. The dissolved liquid is filtered through a ceramic precision filter to remove trace insoluble impurities in the liquid. Under nitrogen protection, 1693.0 kg of 32% ion membrane liquid alkali solution, 100 L of titanium tetrachloride solution and the dissolved barium chloride liquid are mixed in an SK type tubular static mixer to generate white slurry, and a first-level hydrothermal precursor slurry is prepared, maintaining Ba:Ti:NaOH=1.6:1.0:14.88 (molar ratio);

[0044] (2) First-stage hydrothermal reaction crystallization system: The precursor slurry is transported to the first-stage hydrothermal reaction crystallizer, which is sealed and the atmosphere inside the crystallizer is replaced. The hydrothermal reaction crystallization process is carried out by induction heating. The heating rate is set to 3°C / min, the reaction temperature is ~240°C, and the constant temperature time is 16h.

[0045] (3) Solid-liquid separation and washing system of doped barium titanate slurry: After the first-stage hydrothermal reaction crystallization is completed, the slurry is cooled to a temperature of <60°C through a winding high-efficiency heat exchanger. Under nitrogen protection, it is transported to an SK-type tubular static reactor by a delivery pump and mixed with 1.0 mol / L dilute hydrochloric acid. The pH of the outlet slurry is 6.0. Rotating ceramic membrane separation equipment is used for continuous solid-liquid separation and washing. The conductivity of the mother liquor at the end of washing is ~600μS / cm. Samples are taken for TEM morphology and XRD phase structure analysis, and Rietlveld refinement is performed on them. The results are as follows: Figure 4-6 As shown;

[0046] (4) Core-shell structure precursor ingredients and secondary hydrothermal reaction crystallization system: The cleaned bulk-doped barium titanate crystals, the additive NbCl5·6H2O constituting the shell material, ~32% ion membrane liquid alkali and deoxygenated deionized water are efficiently and evenly dispersed in an ultrasonic stirrer to form a slurry with a solid content of ~15%. The mass ratio of bulk-doped barium titanate to the additive constituting the shell material is 0.94:0.06. The evenly dispersed slurry is transported to a secondary hydrothermal reaction crystallizer, sealed and the atmosphere in the crystallizer is replaced. The secondary hydrothermal reaction crystallizer is heated by induction, the heating rate is set to 0.5-10℃ / min, the reaction temperature is ~160℃, and the constant temperature time is ~6h. The core is nano-doped barium titanate and the shell is the chemical reaction product of barium titanate and Nb2O5;

[0047] (5) Core-shell structure nano-modified barium titanate-based powder slurry solid-liquid separation and washing system: After the secondary hydrothermal reaction crystallization is completed, it is cooled to a slurry liquid temperature of <60°C through a wound high-efficiency heat exchanger. Under nitrogen protection, it is transported by a delivery pump to an SK-type tubular static reactor and mixed with 1.0 mol / L dilute hydrochloric acid. The outlet slurry pH is 7.0. Rotating ceramic membrane separation equipment is used for continuous solid-liquid separation and washing. The conductivity of the mother liquor at the washing end point is ~300μS / cm;

[0048] (6) Wet crystal dispersion, drying, air powder and packaging system of core-shell structure nano-modified barium titanate-based powder: The cleaned ~35% core-shell structure modified barium titanate-based powder slurry is subjected to high-speed centrifugal spray drying to obtain secondary particles of core-shell structure modified barium titanate-based powder, with a residual moisture content of <1%, an inlet air temperature of ~250°C, and an outlet air temperature of ~106°C; deagglomeration is carried out using a flat ceramic airflow pulverizing device and the finished product is packaged to obtain the final particle size of D50~80nm and D97~150nm;

[0049] (7) Separation and recycling of excess barium chloride, sodium hydroxide and hydrochloric acid in the first hydrothermal crystallization separation mother liquor: The first hydrothermal reaction crystallization separation mother liquor (2.2% barium chloride and 13% sodium chloride) is passed through a multi-stage nanofiltration membrane device to obtain a concentrated aqueous solution of barium chloride and a clear aqueous solution of barium chloride, respectively. The barium chloride concentrated aqueous solution is composed of 13.0% barium chloride and 13% sodium chloride. The barium chloride concentrated aqueous solution is configured with matching barium chloride dihydrate crystals and deoxygenated deionized water to form a barium chloride solution (barium chloride content ~13.0%) as a precursor of the first hydrothermal reaction crystallization system, thereby realizing the recycling of excess barium chloride. The clear aqueous solution of barium chloride (barium chloride concentration is about 700ppm, sodium chloride 13.0%) is deeply removed of residual barium ions (<100ppm) by ion exchange membrane. The pure sodium chloride solution obtained after deep purification is used in a bipolar membrane electrodialysis device to produce a 1.0 mol / L sodium hydroxide solution and a 1.0 mol / L hydrochloric acid solution. The dilute hydrochloric acid is recycled as a pH regulator. The 1.0 mol / L sodium hydroxide solution is concentrated using a vacuum flash evaporation process to a sodium hydroxide concentration of ~32% at a vacuum flash evaporation temperature of ~80°C. The solution is then returned to the primary hydrothermal reaction crystallization batching system for recycling, with a circulation rate of 98%.

[0050] Example 2

[0051] like Figure 1 The preparation method flow shown is as follows: the preparation system includes: a dissolution and high-efficiency mixing and batching system; a primary hydrothermal reaction crystallization system; a solid-liquid separation and washing system for doped barium titanate slurry; a core-shell structure precursor batching and secondary hydrothermal reaction crystallization system; a core-shell structure nano-modified barium titanate-based formula powder slurry solid-liquid separation and washing system; a core-shell structure nano-modified barium titanate-based formula powder wet crystal dispersion, drying, gas powder and packaging system; a primary hydrothermal crystallization separation mother liquor excess barium chloride, sodium hydroxide and hydrochloric acid separation and recycling system.

[0052] (1) Dissolution and efficient mixing system: 445.6 kg of barium chloride dihydrate crystals, 4.0 kg of neodymium chloride hexahydrate, 0.9 kg of manganese chloride tetrahydrate, 0.9 kg of magnesium chloride hexahydrate, 60 L of propylene oxide and 2000 kg of deoxygenated deionized water are taken. Under nitrogen protection, the above ingredients are dissolved in an ultrasonic dissolution tank with deoxygenated deionized water, stirred and dissolved for 30 minutes, and the dissolution temperature is ~35°C. The dissolved liquid is filtered through a ceramic precision filter to remove trace insoluble impurities in the liquid; under nitrogen protection, 2116.5 kg of 32% ion membrane liquid alkali solution, 100 L of titanium tetrachloride solution and the dissolved barium chloride liquid are mixed in an SK type tubular static mixer to generate white crystal slurry, and a first-level hydrothermal precursor slurry is prepared, maintaining Ba:Ti:NaOH=2.0:1.0:14.9 (molar ratio);

[0053] (2) First-stage hydrothermal reaction crystallization system: The precursor slurry is transported to the first-stage hydrothermal reaction crystallizer, which is sealed and the atmosphere inside the crystallizer is replaced. The hydrothermal reaction crystallization process is carried out by induction heating. The heating rate is set to 5°C / min, the reaction temperature is ~250°C, and the constant temperature time is 14h.

[0054] (3) Doped barium titanate slurry solid-liquid separation and washing system: After the first-stage hydrothermal reaction crystallization is completed, the slurry is cooled to a slurry temperature of <60°C through a wound high-efficiency heat exchanger. Under nitrogen protection, it is transported by a delivery pump to an SV-type tubular static reactor and mixed with 2.0 mol / L dilute hydrochloric acid. The pH of the outlet slurry is 6.2. Rotating ceramic membrane separation equipment is used for continuous solid-liquid separation and washing. The conductivity of the mother liquor at the washing end point is ~700μS / cm;

[0055] (4) Core-shell structure precursor ingredients and secondary hydrothermal reaction crystallization system: The cleaned bulk-doped barium titanate crystals, the additive NbCl5·6H2O constituting the shell material, ~32% ion membrane liquid alkali and deoxygenated deionized water are efficiently and evenly dispersed in an ultrasonic stirrer to form a slurry with a solid content of ~10%. The mass ratio of bulk-doped barium titanate to the additive constituting the shell material is 0.95:0.05. The evenly dispersed slurry is transported to the secondary hydrothermal reaction crystallizer, sealed and the atmosphere in the crystallizer is replaced. The secondary hydrothermal reaction crystallizer is heated by induction, the heating rate is set to ~3℃ / min, the reaction temperature is set to ~180℃, and the constant temperature time is ~5h. The core is nano-doped barium titanate and the shell is the chemical reactant of barium titanate and Nb2O5;

[0056] (5) Core-shell structure nano-modified barium titanate-based powder slurry solid-liquid separation and washing system: After the secondary hydrothermal reaction crystallization is completed, it is cooled to a slurry liquid temperature of <60°C through a winding high-efficiency heat exchanger. Under nitrogen protection, it is transported by a delivery pump to an SV-type tubular static reactor and mixed with 2.0 mol / L dilute hydrochloric acid. The outlet slurry pH is 7.0. Rotating ceramic membrane separation equipment is used for continuous solid-liquid separation and washing. The conductivity of the mother liquor at the washing end point is ~350μS / cm;

[0057] (6) Wet crystal dispersion, drying, air powder and packaging system of core-shell structure nano-modified barium titanate-based powder: The cleaned ~35% core-shell structure modified barium titanate-based powder slurry is subjected to high-speed centrifugal spray drying to obtain secondary particles of core-shell structure modified barium titanate-based powder, with a residual moisture content of <1%, an inlet air temperature of ~250°C, and an outlet air temperature of ~106°C; deagglomeration is carried out using a flat ceramic airflow pulverizing device and the finished product is packaged to obtain the final particle size of D50~85nm and D97~160nm;

[0058] (7) Separation and recycling of excess barium chloride, sodium hydroxide and hydrochloric acid in the first hydrothermal crystallization separation mother liquor: The first hydrothermal reaction crystallization separation mother liquor (2.75% barium chloride and 16.3% sodium chloride) is passed through a multi-stage nanofiltration membrane device to obtain a concentrated aqueous solution of barium chloride and a clear aqueous solution of barium chloride, respectively. The barium chloride concentrated aqueous solution is composed of 18.0% barium chloride and 16.3% sodium chloride. The barium chloride concentrated aqueous solution is configured with matching barium chloride dihydrate crystals and deoxygenated deionized water to form a barium chloride solution (barium chloride content ~18.0%) as a precursor of the first hydrothermal reaction crystallization system, thereby realizing the recycling of excess barium chloride. The clear aqueous solution of barium chloride (barium chloride concentration is about 600ppm, sodium chloride 16.3%) is deeply removed of residual barium ions (<100ppm) by ion exchange membrane. The pure sodium chloride solution obtained after deep purification is used in a bipolar membrane electrodialysis device to produce a 2.0 mol / L sodium hydroxide solution and a 2.0 mol / L hydrochloric acid solution. The dilute hydrochloric acid is recycled as a pH regulator. The 2.0 mol / L sodium hydroxide solution is concentrated using a vacuum flash evaporation process to a sodium hydroxide concentration of ~32% at a vacuum flash evaporation temperature of ~85°C. The solution is then returned to the primary hydrothermal reaction crystallization batching system for recycling, with a circulation rate of 98.5%.

[0059] This embodiment is only an illustrative description of this patent and does not limit its scope of protection. People skilled in the art may also make partial changes to it. As long as it does not exceed the spirit of this patent, it will be regarded as an equivalent replacement of this patent and will be within the scope of protection of this patent.

Claims

1. A method for preparing a core-shell structured nano-modified barium titanate-based powder by secondary hydrothermal crystallization, characterized in that: The following steps are included in sequence: Step S01, under nitrogen protection, dissolving barium chloride dihydrate crystals, bulk doping additives, proton capture agents and deoxygenated deionized water in an ultrasonic dissolution tank, so that the mass ratio of barium chloride in the solution reaches 5-25%, and the dissolution temperature is 20-40°C; Step S02: Under nitrogen protection, 30-32% ion-exchange membrane alkali solution, titanium tetrachloride solution, and the barium chloride solution dissolved in step S01 are mixed in a high-efficiency mixer to generate a white slurry to prepare a first-level hydrothermal precursor slurry, maintaining a molar ratio of Ba:Ti:NaOH of 1.5-3.0:1.0:10.0-20.0; Step S03, transporting the precursor slurry liquid of the first-level hydrothermal reaction in step S02 to the first-level hydrothermal reaction crystallizer, sealing it and replacing the atmosphere in the crystallizer; Step S04: using induction heating in a first-stage hydrothermal reaction crystallizer, setting the heating rate to 0.5-20°C / min, the reaction temperature to 160-300°C, and the constant temperature time to 0.5-72h; Step S05: After cooling the slurry obtained in step S04, the pH is adjusted to 6.0-7.0 using 1.0-2.0 mol / L dilute hydrochloric acid, and continuous solid-liquid separation and washing are performed using a ceramic membrane separation device. The conductivity of the mother liquor at the end of the washing is 300-1000 μS / cm; Step S06, efficiently and uniformly dispersing the cleaned bulk-doped barium titanate wet crystals in step S05, the additive constituting the shell material, a pH adjuster, and deoxygenated water into a slurry having a solid content of 5.0-20%, wherein the mass ratio of the bulk-doped barium titanate wet crystals to the additive constituting the shell material is 9-19; Step S07: The slurry evenly dispersed in step S06 is transported to a secondary hydrothermal reaction crystallizer, which is sealed and the atmosphere in the crystallizer is replaced; Step S08: using an induction heating secondary hydrothermal reaction crystallizer, setting the heating rate to 0.5-10°C / min, the reaction temperature to 120-280°C, and the constant temperature time to 0.5-48h; Step S09: After cooling the slurry obtained in step S08, the pH is adjusted to 6.0-7.0 using 1.0-2.0 mol / L dilute hydrochloric acid, and continuous solid-liquid separation and washing are performed using a high-speed horizontal spiral sedimentation centrifuge or a rotary ceramic membrane separator. The conductivity of the mother liquor at the end of the washing is 100-1000 μS / cm; Step S10: The clean core-shell structured modified barium titanate-based powder slurry washed in step S09 is spray-dried or flash-dried to obtain spherical modified barium titanate-based powder secondary particles, with a residual moisture content of <1%, an air inlet temperature of 200-300° C., and an air outlet temperature of 105-120° C.; Step S11: The spherical modified barium titanate-based formula powder secondary particles obtained by drying in step S10 are deagglomerated by airflow milling and packaged as finished products to obtain a final grain main particle size D50 ranging from 60 to 100 nm and D97 less than 200 nm; Step S12: The mother liquor separated in step S05 is passed through a multi-stage nanofiltration membrane device to obtain a concentrated aqueous solution of barium chloride and a clear aqueous solution of barium chloride, respectively. The concentrated aqueous solution of barium chloride is mixed with matching barium chloride dihydrate crystals and deoxygenated deionized water to form a barium chloride solution as a precursor of a first-stage hydrothermal reaction crystallization system, with a barium chloride content of 5-25%, thereby realizing the recycling of excess barium chloride; Step S13, the clear aqueous solution of barium chloride obtained in step S12, wherein the barium chloride concentration is 500-1000 ppm, is subjected to deep removal of residual barium ions by using an ion exchange membrane or barium sulfate precipitation; Step S14, using the purified sodium chloride solution obtained in step S13 to prepare a 1.0-2.0 mol / L sodium hydroxide solution and a 1.0-2.0 mol / L hydrochloric acid solution respectively using a bipolar membrane electrodialysis device; In step S15, the sodium hydroxide solution in step S14 is concentrated by a vacuum flash evaporation process, wherein the sodium hydroxide concentration is 30-32% and the vacuum flash evaporation temperature is 50-90°C.

2. The method for preparing the core-shell structure nano-modified barium titanate-based formula powder by secondary hydrothermal crystallization according to claim 1, characterized in that: In step S01 , the bulk doping additive includes one or more of MnCl 2 ·4H 2 O, MgCl 2 ·6H 2 O, NdCl 3 ·6H 2 O, and GdCl 3 ·6H 2 O.

3. The method for preparing the core-shell structure nano-modified barium titanate-based formula powder by secondary hydrothermal crystallization according to claim 1, characterized in that: In step S01 , the proton capture agent is one or more of pyridine, propylene oxide and organic amine.

4. The method for preparing the core-shell structure nano-modified barium titanate-based formula powder by secondary hydrothermal crystallization according to claim 1, characterized in that: The main particle size of the cleaned bulk-doped barium titanate wet crystals in step S05 is 50-100 nm.

5. The method for preparing the core-shell structure nano-modified barium titanate-based formula powder by secondary hydrothermal crystallization according to claim 1, characterized in that: In step S06 , the additive constituting the shell material is NbCl 5 ·6H 2 O, and the pH adjuster is NaOH.

6. The method for preparing the core-shell structure nano-modified barium titanate-based formula powder by secondary hydrothermal crystallization according to claim 1, characterized in that: The pH regulator of the separated mother liquor in step S12 is HCl.

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