A low temperature phase transition method of a tetragonal barium titanate

Abstract

The application provides a low-temperature phase transition method of tetragonal barium titanate, and belongs to the technical field of inorganic functional materials. The application uses barium chloride and / or barium fluoride as a key component of a barium-containing molten salt, and the barium-containing molten salt provides high-concentration Ba 2+ A local "barium source" environment is formed, and the grain growth is synergistically inhibited through the following multiple mechanisms: the high-activity Ba 2+ The surface barium volatilization can be effectively inhibited, the vacancies can be filled, the surface stoichiometric ratio and high surface energy can be maintained, and the driving force of grain growth can be weakened; the Ba 2+ The Ba in the barium-containing molten salt can be adsorbed on the surface of barium titanate particles and at the grain boundaries, a "pinning" effect is generated, and the grain boundary migration is hindered; the high-concentration Ba 2+ The balance is moved to the left, the dissolution of the barium titanate particles is inhibited, and the grain coarsening path caused by dissolution-precipitation is blocked.

Description

Technical Field

[0001] This invention relates to the field of inorganic functional materials technology, and in particular to a low-temperature phase transformation method for tetragonal barium titanate. Background Technology

[0002] Barium titanate (BaTiO3) is an important electronic ceramic material. Its tetragonal phase exhibits ferroelectricity at room temperature, making it crucial for manufacturing components such as multilayer ceramic capacitors (MLCCs) and positive temperature coefficient thermistors (PTCs). Currently, the preparation of tetragonal barium titanate powder mainly employs high-temperature solid-state methods (>1200℃), but this method is energy-intensive and prone to grain coarsening. While hydrothermal and sol-gel methods can prepare nanoparticles, they often yield products with low cubic or tetragonal phase content, and the processes are complex and costly.

[0003] Molten salt synthesis has attracted attention due to its ability to lower reaction temperatures and promote grain development. Current molten salt methods for synthesizing barium titanate mostly involve the direct reaction of barium and titanium sources in molten salt. However, this method presents significant challenges in controlling the morphology and phase structure of the product, and requires relatively high temperatures to achieve a high tetragonal phase content, leading to easy grain growth. In particular, for synthesized cubic barium titanate powders with specific morphologies and sizes, there is a lack of efficient, low-temperature solutions for inducing a tetragonal phase transformation during subsequent processing while maximizing the preservation of their original fine-grained characteristics. While conventional heat treatment can induce phase transformation, it inevitably results in severe grain sintering and growth, thus negating the advantages of nanoparticles. Summary of the Invention

[0004] The purpose of this invention is to provide a low-temperature phase transformation method for tetragonal barium titanate. The low-temperature phase transformation method provided by this invention can realize the phase transformation from cubic barium titanate to tetragonal barium titanate at a relatively low temperature and can effectively suppress grain growth.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a low-temperature phase transformation method for tetragonal barium titanate, comprising: wet mixing and drying cubic barium titanate, barium-containing molten salt and water to obtain a mixture, and then calcining the mixture to obtain tetragonal barium titanate; The barium-containing molten salt includes barium chloride and / or barium fluoride.

[0006] Preferably, the barium-containing molten salt further includes alkali metal halides and / or alkaline earth metal halides.

[0007] Preferably, when the barium-containing molten salt further includes alkali metal halides and / or alkaline earth metal halides, the total mass percentage of barium chloride and / or barium fluoride in the barium-containing molten salt is ≥20%.

[0008] Preferably, the mass ratio of the barium-containing molten salt to the cubic phase barium titanate is (1~50):1.

[0009] Preferably, the mass ratio of the barium-containing molten salt to the cubic phase barium titanate is (3~10):1.

[0010] Preferably, the calcination temperature is 800~1100℃ and the calcination time is 0.5~10h.

[0011] Preferably, the calcination temperature is 850~1000℃ and the calcination time is 2~8h.

[0012] Preferably, the heating rate to the calcination temperature is 4~6℃ / min, and the calcination atmosphere is air or a protective atmosphere.

[0013] Preferably, the protective atmosphere is nitrogen or argon.

[0014] Preferably, the cubic phase barium titanate has a particle size of 20~500nm.

[0015] This invention provides a low-temperature phase transformation method for tetragonal barium titanate, comprising: wet mixing and drying cubic barium titanate, a barium-containing molten salt, and water to obtain a mixture; and then calcining the mixture to obtain tetragonal barium titanate; wherein the barium-containing molten salt comprises barium chloride and / or barium fluoride. This invention uses barium chloride and / or barium fluoride as key components of the barium-containing molten salt, which provides a high concentration of Ba. 2+ A local "barium source" environment is formed, which inhibits grain growth through the following multiple mechanisms: (1) Thermodynamic inhibition of barium volatilization: the high activity of Ba in barium-containing molten salts 2+ It can effectively suppress surface barium volatilization and fill vacancies, maintain surface stoichiometry and high surface energy, and weaken the driving force for grain growth; (2) Kinetic pinning of grain boundaries: Ba in barium molten salt 2+ It can be adsorbed on the surface and grain boundaries of barium titanate particles, producing a "pinning" effect and hindering grain boundary migration; (3) Equilibrium shift inhibits dissolution: the high concentration of Ba provided by barium molten salt 2+ To achieve balance (BaTiO3⇌Ba 2+ +TiO3 2- The particle size shifts to the left, inhibiting the dissolution of barium titanate particles and thus blocking the grain coarsening path caused by dissolution-reprecipitation. Results from the examples show that the tetragonal barium titanate grain size D prepared using cubic barium titanate in this invention is [not specified]. 50 The grain size D of the cubic barium titanate raw material 50 The ratio is less than twice that of the standard, and the (002) and (200) diffraction peaks in the X-ray diffraction pattern are clearly split, and the lattice constant ratio c / a>1.008. Attached Figure Description

[0016] Figure 1 SEM image of the cubic phase barium titanate used in Example 1; Figure 2 The XRD pattern of the cubic phase barium titanate used in Example 1; Figure 3 SEM image of the tetragonal barium titanate prepared in Example 1; Figure 4 The image shows the XRD pattern of the tetragonal barium titanate prepared in Example 1. Figure 5 The XRD pattern of the tetragonal barium titanate prepared in Example 4; Figure 6 SEM image of barium titanate prepared in Comparative Example 1; Figure 7 The XRD pattern of barium titanate prepared in Comparative Example 1; Figure 8 SEM image of barium titanate prepared in Comparative Example 2; Figure 9 SEM image of barium titanate prepared in Comparative Example 3. Detailed Implementation

[0017] This invention provides a low-temperature phase transformation method for tetragonal barium titanate, comprising: wet mixing and drying cubic barium titanate, barium-containing molten salt and water to obtain a mixture, and then calcining the mixture to obtain tetragonal barium titanate; The barium-containing molten salt includes barium chloride and / or barium fluoride.

[0018] In this invention, the particle size of the cubic barium titanate is preferably 20-500 nm, more preferably 100-400 nm, and even more preferably 200-300 nm. This invention does not impose any specific limitation on the source of the cubic barium titanate; it can be obtained from commercially available cubic barium titanate known to those skilled in the art or from conventional methods such as hydrothermal methods or sol-gel methods.

[0019] In this invention, the barium-containing molten salt comprises barium chloride and / or barium fluoride. Preferably, the barium-containing molten salt also comprises alkali metal halides and / or alkaline earth metal halides. When the barium-containing molten salt further comprises alkali metal halides and / or alkaline earth metal halides, the total mass percentage of barium chloride and / or barium fluoride in the barium-containing molten salt is preferably ≥20%. In embodiments of this invention, the total mass percentage of barium chloride and / or barium fluoride in the barium-containing molten salt can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 100%. By controlling the total mass percentage of barium chloride and / or barium fluoride in the barium-containing molten salt, this invention ensures a sufficient barium ion concentration. Using barium chloride and / or barium fluoride as key components of the barium-containing molten salt, this invention's effect far exceeds that of a simple physical isolation medium. At high temperatures, the barium-containing molten salt forms a liquid phase, promoting the formation of Ba... 2+ and O 2- Plasma diffusion reduces the activation energy for the transition from cubic to tetragonal phase.

[0020] In this invention, the preferred mass ratio of the barium-containing molten salt to the cubic barium titanate is (1~50):1. In embodiments of this invention, the mass ratio of the cubic barium titanate to the barium-containing molten salt can be 1:1, 2:1, 3:1, 4:1, 5:1, 10:1, 15:1, 20:1, 25:1, 30:1, 35:1, 40:1, 45:1, or 50:1.

[0021] This invention does not impose any particular limitations on the specific operation of wet mixing of the cubic barium titanate, barium-containing molten salt, and water; any conventional mixing method that ensures uniform mixing is acceptable. In embodiments of this invention, the wet mixing method may be magnetic stirring; the wet mixing time may be 1-5 hours, or even 2-3 hours.

[0022] The present invention does not impose any special limitations on the specific drying operation. Based on the technical common sense of those skilled in the art, it is sufficient to ensure that the mixture is completely dried. In the embodiments of the present invention, the drying temperature can be 80~120℃, or 90℃, 100℃ or 110℃; the drying time can be 6~24h, or 9h, 12h, 15h, 18h or 21h.

[0023] In this invention, the calcination temperature is preferably 800~1100℃; the calcination time is preferably 0.5~10h; the heating rate to the calcination temperature is preferably 4~6℃ / min, more preferably 5℃ / min; the calcination atmosphere is preferably air or a protective atmosphere; the protective atmosphere is preferably nitrogen or argon. In embodiments of this invention, the calcination temperature can be 800℃, 850℃, 900℃, 950℃, 1000℃, 1050℃, or 1100℃; the calcination time can be 0.5h, 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, or 10h.

[0024] After calcination, the present invention preferably further includes washing, filtering and drying the calcined product in sequence.

[0025] In this invention, the detergent used for washing is preferably deionized water or a dilute acid solution; the temperature of the deionized water is preferably 70-90°C, more preferably 80°C; the dilute acid solution is preferably a dilute nitric acid solution; and the mass concentration of the dilute acid solution is preferably 0.01-1%. This invention does not specifically limit the number of washes; based on the technical knowledge of those skilled in the art, the number of washes can be increased until AgNO3 solution (for Cl-) is used. - (forming AgCl precipitate) or CaCl2 solution (for F) - (The formation of CaF2 precipitate) indicates that no halide ions are detected. This invention removes residual impurities through washing.

[0026] In this invention, the detergent after washing is preferably collected and then the molten salt is recovered through evaporation crystallization or fractional crystallization. This invention does not impose any particular limitations on the specific operations of evaporation crystallization and fractional crystallization; any evaporation crystallization and fractional crystallization processes well known to those skilled in the art can be used.

[0027] The present invention does not have any special limitations on the specific operation of the filtration. Any filtration method known to those skilled in the art can be used to remove most of the water.

[0028] The present invention does not impose any particular limitation on the specific drying operation. Based on the technical knowledge of those skilled in the art, the operation should be able to remove residual moisture and other impurities. In embodiments of the present invention, the drying temperature can be 80~120℃, or it can be 90℃, 100℃, or 110℃.

[0029] This invention uses barium chloride and / or barium fluoride as key components of barium-containing molten salts, which provide a high concentration of Ba. 2+ This creates a localized "barium source" environment, which synergistically inhibits grain growth through the following multiple mechanisms: (1) Thermodynamic inhibition of barium volatilization: At high temperatures, barium on the surface of barium titanate is prone to volatilization, forming vacancies, reducing surface energy, and driving grain growth (Ostwald ripening); while the high-activity Ba in barium-containing molten salts 2+ It can effectively suppress surface barium volatilization, fill vacancies, maintain surface stoichiometry and high surface energy, and weaken the driving force for grain growth.

[0030] (2) Kinetic pinning of grain boundaries: Ba in barium molten salt 2+ It can be adsorbed on the surface and grain boundaries of barium titanate particles, producing a "pinning" effect that hinders grain boundary migration.

[0031] (3) Equilibrium shift inhibits dissolution: The high concentration of Ba provided by barium molten salt 2+ To achieve balance (BaTiO3⇌Ba 2+ +TiO3 2- The particle moves to the left, inhibiting the dissolution of barium titanate particles and thus blocking the grain coarsening path caused by dissolution-reprecipitation.

[0032] Compared with the prior art, the present invention has the following beneficial effects: (1) Low-temperature and efficient phase transition: The low-temperature phase transition method provided by this invention can achieve a complete transformation from cubic phase to tetragonal phase at 800~1100℃, and the phase transition temperature is significantly lower than that of the traditional solid-state method (>1100℃). The products prepared by the low-temperature phase transition method provided by this invention have a high tetragonal phase content, obvious splitting of the (002) / (200) peaks in the XRD pattern, and the c / a ratio is usually greater than 1.008.

[0033] (2) Excellent grain size control: The present invention can effectively suppress grain growth during heat treatment by using the unique chemical environment of barium molten salt. Compared with direct calcination without molten salt at the same temperature, the low-temperature phase transformation method provided by the present invention can control the grain growth multiple to within 2 times, while direct calcination will lead to severe sintering and grain growth of several times.

[0034] (3) Strong applicability of raw materials and processes: The low-temperature phase change method provided by the present invention can synthesize cubic barium titanate raw materials with different particle sizes (20~500nm) by different methods. It has good applicability of raw materials, wide process window and good repeatability.

[0035] (4) Green and environmentally friendly, low cost: The present invention uses barium molten salt as raw material. After preparation, the barium molten salt can be recycled and reused through simple water washing, evaporation and crystallization, which reduces the cost of raw materials and waste salt discharge. The whole process is simple and easy to scale up production.

[0036] The method provided in this invention produces tetragonal barium titanate with a grain size D of [missing information]. 50 The grain size D of the cubic barium titanate raw material 50The ratio is less than twice that of the standard, and the (002) and (200) diffraction peaks in the X-ray diffraction pattern are clearly split, and the lattice constant ratio c / a>1.008.

[0037] The tetragonal barium titanate prepared by the method provided in this invention can be applied to various electronic components such as multilayer ceramic capacitors, positive temperature coefficient thermistors, piezoelectric ceramics, electro-optic devices, or ferroelectric memories.

[0038] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0039] Example 1 A low-temperature phase transition method for tetragonal barium titanate is as follows: 10g of cubic barium titanate, 50g of barium-containing molten salt, and 150mL of deionized water are mixed and magnetically stirred for 2h, then dried in an oven at 100℃ for 12h to obtain a mixture. The mixture is then placed in a muffle furnace and heated to 1000℃ at a heating rate of 5℃ / min for 3h. After cooling to room temperature in the furnace, it is repeatedly washed and filtered with deionized water at 80℃ until no halide ions can be detected with 0.1mol AgNO3 solution. The washing filtrate is collected, and NaCl and BaCl2 are recovered sequentially by fractional crystallization. The filter cake after washing and filtration is finally dried at 100℃ for 12h to obtain tetragonal barium titanate. The particle size of the cubic phase barium titanate is 50 nm; The barium-containing molten salt is composed of barium chloride and sodium chloride, with a mass ratio of barium chloride to sodium chloride of 2:1.

[0040] The SEM image of the cubic phase barium titanate used in Example 1 is shown below. Figure 1 As shown; the XRD pattern of the cubic phase barium titanate used in Example 1 is shown below. Figure 2 As shown; SEM image of the tetragonal barium titanate prepared in Example 1 is shown. Figure 3 As shown; the XRD pattern of the tetragonal barium titanate prepared in Example 1 is shown. Figure 4 As shown. By Figures 1-4 It can be seen that the XRD pattern of the tetragonal barium titanate shows that the (002) and (200) peaks near 45° are clearly split, indicating that it is a tetragonal phase, and the calculated c / a is greater than 1.008. The SEM pattern of the tetragonal barium titanate shows that the product particles are uniform and the average particle size is less than 80 nm. Compared with the raw material, the growth is very small, indicating that a low-temperature phase transformation was successfully achieved in the barium-containing molten salt formed by BaCl2-NaCl and the grain growth was effectively suppressed.

[0041] Example 2 A low-temperature phase transition method for tetragonal barium titanate is as follows: 10g of cubic barium titanate, 60g of barium-containing molten salt, and 200mL of deionized water are mixed and magnetically stirred for 2h, and then dried in an oven at 100℃ for 12h to obtain a mixture. The mixture is then placed in a muffle furnace and heated to 1100℃ at a heating rate of 5℃ / min for 3h. After cooling to room temperature in the furnace, it is repeatedly washed and filtered with deionized water at 80℃ until no halide ions can be detected by a 1% calcium chloride solution. The washing filtrate is collected, and the barium-containing molten salt is recovered by fractional crystallization. The filter cake after washing and filtration is finally dried at 100℃ for 12h to obtain tetragonal barium titanate. The particle size of the cubic phase barium titanate is 60~80nm; The barium-containing molten salt is composed of barium fluoride and sodium chloride, with a mass ratio of barium fluoride to sodium chloride of 3:1.

[0042] The product prepared in Example 2 mainly consists of tetragonal barium titanate, with a c / a ratio greater than 1.008 and an average particle size of approximately 100 nm. This demonstrates that barium-containing molten salts containing barium fluoride also exhibit excellent performance.

[0043] Example 3 A low-temperature phase transition method for tetragonal barium titanate is as follows: 10g of cubic barium titanate, 80g of barium-containing molten salt, and 200mL of deionized water are mixed and magnetically stirred for 2h, then dried in an oven at 100℃ for 12h to obtain a mixture. The mixture is then placed in a muffle furnace and calcined at 850℃ for 5h at a heating rate of 5℃ / min. After cooling to room temperature in the furnace, it is repeatedly washed and filtered with deionized water at 80℃ until no halide ions can be detected with 0.1mol AgNO3 solution. The washing filtrate is collected, and the barium-containing molten salt is recovered by fractional crystallization. The filter cake after washing and filtration is finally dried at 100℃ for 12h to obtain tetragonal barium titanate. The cubic phase barium titanate has a particle size of 200 nm; The barium-containing molten salt is composed of barium chloride, sodium chloride, and potassium chloride, with a mass ratio of 3:1:1.

[0044] The product prepared in Example 3 mainly consists of tetragonal barium titanate, with a c / a ratio greater than 1.008 and an average particle size of approximately 250 nm. The tetragonal barium titanate exhibits good dispersibility. This demonstrates that barium-containing molten salts formed from barium chloride, sodium chloride, and potassium chloride also possess excellent performance.

[0045] Example 4 A low-temperature phase transition method for tetragonal barium titanate is as follows: 10g of cubic barium titanate, 30g of barium-containing molten salt, and 100mL of deionized water are mixed and magnetically stirred for 2h, then dried in an oven at 100℃ for 12h to obtain a mixture. The mixture is then placed in a muffle furnace and calcined at 950℃ for 2h at a heating rate of 5℃ / min. After cooling to room temperature in the furnace, it is repeatedly washed and filtered with deionized water at 80℃ until no halide ions can be detected with 0.1mol AgNO3 solution. The washing filtrate is collected, and the barium-containing molten salt is recovered by fractional crystallization. The filter cake after washing and filtration is finally dried at 100℃ for 12h to obtain tetragonal barium titanate. The cubic phase barium titanate has a particle size of 100 nm; The barium-containing molten salt is barium chloride.

[0046] The XRD pattern of the tetragonal barium titanate prepared in Example 4 is shown below. Figure 5 As shown. By Figure 5 It can be seen that the XRD pattern shows that the (002) and (200) peaks near 45° are clearly split, and the calculated c / a is greater than 1.008.

[0047] Particle size analysis showed that the average particle size of the tetragonal barium titanate prepared in Example 4 was about 120 nm, with no significant increase.

[0048] Comparative Example 1 A method for preparing barium titanate is as follows: 10g of cubic phase barium titanate is placed in a muffle furnace and heated to 1000℃ at a heating rate of 5℃ / min for 3h, and then cooled to room temperature with the furnace to obtain barium titanate. The cubic phase barium titanate has a particle size of 50 nm.

[0049] SEM image of barium titanate prepared in Comparative Example 1 is shown below. Figure 6 As shown; the XRD pattern of barium titanate prepared in Comparative Example 1 is shown. Figure 7 As shown. By Figure 6 and Figure 7 It can be seen that the barium titanate prepared in Comparative Example 1 underwent severe hard agglomeration and sintering, with particles adhering to each other and grain size increasing to 200-800 nm, showing a significant increase in particle size. XRD patterns show that although barium titanate has certain tetragonal phase characteristics, the grains are severely coarsened.

[0050] Comparative Example 2 A method for preparing barium titanate is as follows: 10g of cubic barium titanate, 30g of sodium chloride and 150mL of deionized water are mixed and magnetically stirred for 2h, and then dried in an oven at 100℃ for 12h to obtain a mixture. The mixture is then placed in a muffle furnace and heated to 1100℃ at a heating rate of 5℃ / min for 3h. After cooling to room temperature in the furnace, it is repeatedly washed and filtered with deionized water at 80℃ until no halide ions can be detected by 0.1mol AgNO3 solution. The filter cake after washing and filtration is finally dried at 100℃ for 12h to obtain barium titanate. The cubic phase barium titanate has a particle size of 50 nm.

[0051] SEM image of barium titanate prepared in Comparative Example 2 is shown below. Figure 8 As shown. By Figure 8 It can be seen that the particle size of barium titanate is significantly increased and the size is uneven.

[0052] Comparative Example 3 A method for preparing barium titanate is as follows: 10g of cubic barium titanate, 30g of composite molten salt and 150mL of deionized water are mixed and magnetically stirred for 2h, and then dried in an oven at 100℃ for 12h to obtain a mixture. The mixture is then placed in a muffle furnace and heated to 1100℃ at a heating rate of 5℃ / min for 3h. After cooling to room temperature in the furnace, it is repeatedly washed and filtered with deionized water at 80℃ until no halide ions can be detected by 0.1mol AgNO3 solution. The filter cake after washing and filtration is finally dried at 100℃ for 12h to obtain barium titanate. The particle size of the cubic phase barium titanate is 50 nm; The composite molten salt is composed of sodium chloride and potassium chloride, with a mass ratio of sodium chloride to potassium chloride of 1:1.

[0053] SEM image of barium titanate prepared in Comparative Example 3 is shown below. Figure 9 As shown. By Figure 9 It can be seen that the barium titanate exhibits significant grain growth and uneven grain size.

[0054] Meanwhile, the (002) / (200) peak splitting of barium titanate prepared in Comparative Example 3 was far less obvious than that in Example 1, indicating that the phase transition was incomplete.

[0055] The comparison of the examples and comparative examples shows that when molten salt (whether single or composite) lacking a "barium source" environment is used for calcination, the molten salt can only provide limited physical isolation and cannot effectively inhibit grain growth and promote phase transformation in depth chemically.

[0056] In summary, the present invention uses cubic barium titanate as raw material and a barium-containing molten salt composed of barium chloride and / or barium fluoride, along with other salts, as the reaction medium. The molten salt is uniformly coated onto the surface of the barium titanate particles through wet mixing, followed by drying and calcination at 800-1100°C. During calcination, the barium-containing molten salt not only provides a liquid environment to promote ion migration, but more importantly, it provides a high concentration of barium ions (Ba). 2+ The environmental conditions, thermodynamically and kinetically, synergistically inhibited barium volatilization and grain boundary migration of barium titanate at high temperatures, thereby significantly reducing the phase transition temperature and effectively suppressing grain growth. The calcined product, after washing and drying, yielded barium titanate powder with high tetragonal phase content and controllable grain size. The washing filtrate can be recycled to contain barium molten salt.

[0057] The process provided by this invention is simple, energy-efficient, environmentally friendly, and the barium-containing molten salt is recyclable, environmentally friendly, and easy to scale up. The resulting tetragonal barium titanate has high phase purity and uniform and controllable grain size, and can be directly used to prepare high-performance multilayer ceramic capacitors (MLCCs), positive temperature coefficient thermistors (PTCs), piezoelectric ceramics, and ferroelectric memories, etc., and has broad market prospects.

[0058] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A low-temperature phase transition method for tetragonal barium titanate, comprising: The cubic phase barium titanate, barium-containing molten salt, and water are wet-mixed and dried to obtain a mixture. The mixture is then calcined to obtain the tetragonal phase barium titanate. The barium-containing molten salt includes barium chloride and / or barium fluoride.

2. The low-temperature phase transition method according to claim 1, characterized in that, The barium-containing molten salt also includes alkali metal halides and / or alkaline earth metal halides.

3. The low-temperature phase transition method according to claim 2, characterized in that, When the barium-containing molten salt also includes alkali metal halides and / or alkaline earth metal halides, the total mass percentage of barium chloride and / or barium fluoride in the barium-containing molten salt is ≥20%.

4. The low-temperature phase transition method according to claim 1, characterized in that, The mass ratio of the barium-containing molten salt to the cubic phase barium titanate is (1~50):

1.

5. The low-temperature phase transition method according to claim 4, characterized in that, The mass ratio of the barium-containing molten salt to the cubic phase barium titanate is (3~10):

1.

6. The low-temperature phase transition method according to claim 1, characterized in that, The calcination temperature is 800~1100℃, and the calcination time is 0.5~10h.

7. The low-temperature phase transition method according to claim 6, characterized in that, The calcination temperature is 850~1000℃, and the calcination time is 2~8h.

8. The low-temperature phase transition method according to claim 6, characterized in that, The heating rate to the calcination temperature is 4~6℃ / min, and the calcination atmosphere is air or a protective atmosphere.

9. The low-temperature phase transition method according to claim 8, characterized in that, The protective atmosphere is nitrogen or argon.

10. The low-temperature phase transition method according to claim 1, characterized in that, The particle size of the cubic phase barium titanate is 20~500nm.

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