Barium titanate ceramic powder

Barium titanate (BaTiO3) is the main raw material of titanate-based electronic ceramics, and as a ferroelectric material, it is widely used in multilayer ceramic capacitors, sonar, infrared radiation detection, grain boundary ceramic capacitors, positive temperature coefficient thermal ceramics, etc. It has a broad application prospect and is known as the pillar of electronic ceramics. With the development of miniaturization, light weight, reliability and thinness of electronic equipment and its components, the demand for high purity ultra-fine barium titanate powder is becoming more and more urgent.
I. What is barium carbonate
Barium titanate is a consistent molten compound with a melting point of 1618℃. It has five crystalline variants: square crystalline, cubic crystalline, tetragonal crystalline, trigonal crystalline and hexagonal crystalline; it is stable in square crystalline form at room temperature. 
1、Ferroelectricity of barium titanate
When BaTiO3 is subjected to a strong current electric field, a continuous polarization effect will be produced below 120℃. Polarized barium titanate has two important properties: ferroelectricity and piezoelectricity. There are many small regions with different spontaneous polarization directions in the barium titanate ferroelectric crystal. Each region is composed of many cells with the same spontaneous polarization direction, called "electric domains". Crystals with electric domain structure are called ferroelectric crystals or ferroelectrics. Figure 3 shows the changes in the shape and geometry of the domains of BaTiO3-based ferroelectric ceramics under the action of an applied electric field.
2. Curie temperature of barium titanate
BaTiO3 Curie temperature Tc refers to the phase transition temperature between tetragonal and cubic phases, i.e. the lowest temperature at which ferroelectric crystals lose spontaneous polarization (loss of electric domain structure). the BaTiO3 Curie temperature is about 120°C.
Second, the preparation method of barium carbonate powder
The preparation methods of barium titanate powder can be divided into solid-phase method, liquid-phase method and gas-phase method. Among them, the solid phase method is the most traditional method and an important method for the industrial production of barium titanate such as barium titanate at home and abroad; the liquid phase method has obvious advantages and can prepare high purity ultra-fine barium titanate powder. In foreign countries, oxalic acid co-precipitation method and hydrothermal method have been applied to industrial production, and the development of gas phase method is slow and immature.
1、Liquid phase method
Liquid phase method, also known as wet chemical method, is a method of preparing ultrafine powder by atoms and ions through two stages of nucleation and growth, which is characterized by easier nucleation, uniform components, and can produce high purity powder, and also facilitate the addition of trace elements for modification. In general, the liquid phase method is better than the solid phase method for the preparation of high quality barium titanate powder. It can be subdivided into hydrothermal synthesis method, precipitation method, sol-gel method, etc.
2、Phase solid method
The solid phase method refers to mixing and grinding the oxides of chromium metal elements (TIO2/BaO) or their acidic salts (TIO2, Ba(CO3)2) that make up barium titanate, and then calcining them for a long time at 1100℃ to form the desired powder through solid phase reaction.
The reaction equation is.
BaCO3+TiO2→BaTiO3+CO2
The advantages are: simple and mature process, reliable equipment, and cheap raw material price.
The disadvantages are: the chemical composition of the prepared powder is not uniform, easy to agglomerate, coarse particles; the powder purity is low, and the reaction is carried out at high temperature, and the energy consumption is relatively large.
3、Precipitation method
Precipitation method is mainly divided into direct precipitation method, co-precipitation method.
(1) direct precipitation method
Direct precipitation method is to add the appropriate precipitant to the metal solution, and control the appropriate conditions so that the precipitant reacts with the metal ions to produce ceramic powder precipitates. Ba(OC3H7) and Ti(OC5H11)4 are dissolved in propanol and decomposed with water to obtain precipitated BaTiO3. the process flow is shown below.
The advantages are: the process is simple, it is carried out under atmospheric pressure, no high temperature is required, and the reaction conditions are mild.
The disadvantages are: easy to introduce TiO3, BaCO3 and other magazines, and the particle distribution is wide, and certain post-treatment is needed.
4、Hydrothermal synthesis method
Hydrothermal synthesis method of BaTiO3 powder refers to the precursors containing barium and titanium, generally barium hydroxide and hydrated barium oxide slurry, placed in a certain temperature and pressure vessel, under hydrothermal conditions for chemical reaction, and after a period of time, BaTiO3 powder is directly generated in the hydrothermal medium.
At present, Shanghai Silicate Research Institute of Chinese Academy of Sciences obtained tetragonal phase barium titanate nanopowders by hydrothermal synthesis of barium chloride and titanium tetroxide as raw materials at 240°C for 12 hours. In the case of raw material barium-titanium ratio of 1:6, barium chloride is firstly dissolved with distilled water to make a certain concentration solution, then mixed with a certain amount of titanium tetrachloride and added with excess sodium hydroxide, mixed evenly and placed in a high-pressure stainless pot reaction kettle lined with tetrafluoroethylene, and dried all the precipitates at 80℃ after a period of closed and elevated temperature reaction to produce barium titanate powder.
Advantages.
(1) The results show that the prepared powder has complete grain development, uniform particle distribution and small inter-particle agglomeration.
(2) The ideal stoichiometric ratio of the material can be obtained and the particle size can be controlled.
(3) The raw material is cheap and the production cost is low.
(4) The powder can be directly used for processing and molding without calcination, avoiding the disadvantages of particle agglomeration and impurity mixing in the calcination process.
Disadvantages are.
(1) The cost is too high due to poor reaction conditions.
(2) Lack of thermodynamic data for the solid-liquid reaction and the problem of chloride salt corrosion.
In summary, if the hydrothermal method can solve the above problems, the application of hydrothermal method will be very promising.
(2) Co-precipitation method
Co-precipitation method is to add a mixed solution of TiC2 to oxalic acid solution with surfactant at room temperature. After continuous stirring, the precipitation reaction generates barium titanate oxalate precipitate and barium titanate precursor. precursor of BaTiO3, and barium titanate powder is prepared after filtration, washing, drying and calcination. The process flow diagram is shown below
The advantages are: the resulting powder has low impurity content and is easy to dope.
The disadvantage is: the powder agglomeration is more serious, and the barium and titanium are more difficult to control.
5、Sol-gel method
Sol-gel method is a method of hydrolysis and condensation of metal alcohol salts or inorganic salts to form soles, and then sol-gel, drying and heat treatment to obtain powder. The purity of raw materials is ensured by using the sol-gel method, such as distillation or recrystallization. Process engineering does not introduce impurity particles, high purity of powder, uniform composition, small particle size, and strong chemical activity.
Currently. Domestic researchers Cao Yong depression method, that is, the reaction of barium stearate and titanate butanol to prepare barium titanate powder. The process is to dissolve barium stearate into stearic acid, and then add equimolar titanium butoxide to prepare the gel, and calcine the barium titanate powder with a particle size of 20mm at 800℃.
Due to the high cost of raw materials, high toxicity of organic solvents, rapid agglomeration of powders after high temperature treatment, strict operating conditions of sol-gel method and difficult control, it is still out of laboratory research stage.
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