The advantages of hydrothermal method are:
(1) The grains of the prepared powder are well developed, the particle size distribution is uniform, and there is less agglomeration among the particles.
(2) The ideal stoichiometric composition of the material can be obtained, the particle size is controllable.
(3) Cheaper raw materials, low production cost.
(4) The subsequent treatment of powder does not need calcination, and can be directly used for processing and molding, to avoid the disadvantages of agglomeration growth of grains and easy mixing of impurities in the process of calcination.
The disadvantage is that:
(1) Harsh reaction environmental conditions, resulting in high costs.
(2) The lack of thermodynamic data for solid-liquid reaction and the existence of corrosion caused by chloride salts.
To sum up, if the hydrothermal method can solve the above problems, the application prospect of hydrothermal method will be very broad.
2.2 precipitation
Precipitation method is mainly divided into direct precipitation method, co-precipitation method.
(1) Direct precipitation method
Direct precipitation method refers to adding appropriate precipitator to metal salt solution and controlling appropriate conditions to make the precipitator react with metal ions to produce ceramic powder precipitate. Ba (OC3H7) 2 and Ti (OC5H11) 4 were dissolved in isopropyl alcohol and decomposed by adding water to obtain the precipitated BaTiO3. The process flow is shown in the figure below.
Figure 5. The process of preparing barium titanate powder by direct precipitation method
The advantages of direct precipitation method are: simple process, under the condition of atmospheric pressure, do not need high temperature, reaction conditions are mild.
Disadvantages: it is easy to introduce TiO2, BaCO3 and other impurities, and the particle size distribution is wide, which requires certain post-treatment.
(2) Co-precipitation method
The co-precipitation method is to add the mixed solution of TiCl4 and BaCl2 to the oxalic acid solution at room temperature, and add surfactants, constantly stirring, precipitation reaction to generate barium titanate oxalate precipitation, the precursor of BaTiO3, after filtration, washing, drying, calcination, to prepare BaTiO3 powder. The process flow is shown in the figure below.
FIG. 6 Technological process of preparing barium titanate powder by co-precipitation method
The advantages of co-precipitation method are that the obtained powder has low impurity content and is easy to be doped. The disadvantages are: but the agglomeration of powder is more serious, the ratio of barium to titanium is difficult to control.
2.3 Sol-gel method
Sol-gel method is based on metal alcohols or inorganic salts as raw materials, through hydrolysis, condensation, the solution to form sol, and then sol gelation, drying and heat treatment to get powder a method. Sol-gel method uses distillation or recrystallization and other technologies to ensure the purity of raw materials, process engineering does not introduce impurity particles, the powder prepared by high purity, uniform composition, small particle size, strong chemical activity.
At present, domestic researchers use SAG method, namely barium stearate and butyl titanate reaction to prepare BaTiO3 powder. The process is to dissolve barium stearate in stearate acid, then add equimolar butyl titanate to react to get gel, calcined at 800℃ to get BaTiO3 powder with particle size of 20nm.
Sol-gel method because of the high cost of raw materials, organic solvent toxicity and high temperature treatment will make the powder quickly agglomerate, strict operating conditions and difficult to control, so it is difficult to industrialization, is still in the laboratory research stage.