Barium titanate (BaTiO3) is the main raw material department of titanate electronic ceramics, as a kind of ferroelectric materials, with its excellent dielectric properties, are widely used in multilayer ceramic capacitors, sonar and infrared radiation detection, grain boundary ceramic capacitor, positive temperature coefficient of thermal ceramics, etc., has a broad application prospect, is known as the backbone of electronic ceramics. With the development of small, light, reliable and thin electronic equipment and its components, the demand for high purity ultra-fine barium titanate powder is more and more urgent.
Overview of barium titanate
Barium titanate is a consistent melting compound with a melting point of 1618℃. There are five crystal variants: hexagonal, cubic, tetragonal, tetragonal and trigonal. Stable as tetragonal crystal at room temperature.
1. Ferroelectric properties of barium titanate
When BaTiO3 is subjected to high current electric field, the continuous polarization effect will be generated below 120℃ at Curie point. Polarized barium titanate has two important properties: ferroelectric and piezoelectric.
The BaTiO3 ferroelectric has many small regions of spontaneous polarization in different directions. Each region is composed of many cells of spontaneous polarization in the same direction. These small regions are called domains. Crystals with domains are called ferroelectrics or ferroelectrics. FIG. 3 shows the change of the geometrical dimension of BaTiO3 based ferroelectric ceramics under the action of external electric field.
2. Curie temperature of barium titanate
BaTiO3 Curie temperature Tc refers to the phase transition temperature between tedral and cubic phases, namely the lowest temperature at which the ferroelectric crystal loses its spontaneous polarization (the disappearance of the domain structure). BaTiO3 Curie temperature is about 120℃
Preparation method of barium titanate 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, which is also an important method for the industrial production of barium titanate and other titanates at home and abroad. The liquid phase method has obvious advantages, which can prepare high purity and ultrafine barium titanate powder. In foreign countries, oxalate coprecipitate method and hydrothermal method have been applied in industrial production. The development of gas phase method is slow and immature.
1. Solid phase method
Solid phase method is to mix and grind the oxides (TiO2, BaO) or their acidic salts (TiO2, Ba (CO3) 2) of the metal elements of barium titanate, and then calcination at 1100℃ for a long time to form the required powder through solid phase reaction. The reaction equation is:
2. Liquid phase method
Liquid phase method, also known as wet chemical method, is a method to prepare ultrafine powder by nucleation and growth of atoms and ions. It is characterized by easy nucleation, uniform components, and can produce high-purity powder. It is also convenient to add trace elements for modification. In general, the liquid phase method is better than the solid phase method in preparing high quality barium titanate powder. It can be subdivided into hydrothermal synthesis method, precipitation method, sol-gel method, etc.
2.1 Hydrothermal synthesis
The hydrothermal synthesis method of BaTiO3 powder means that the precursor containing barium and titanium, usually barium hydroxide and hydrated titanium oxide water slurry, is placed in a container with a certain temperature and pressure, and the chemical reaction is carried out under hydrothermal conditions. After a certain time, the BaTiO3 powder is directly generated in hydrothermal medium.
At present, using barium chloride and titanium tetracloride as raw materials in Shanghai Institute of Ceramics, Chinese Academy of Sciences, tetra phase barium titanate nano powder was synthesized by hydrothermal synthesis at 240℃ for 12 hours. In the raw material barium titanium ratio of 1: 6’s case, the barium chloride dissolved in distilled water with a certain concentration of solution, then mixed with a certain amount of titanium tetrachloride, and join the excess sodium hydroxide, after mixing in the lining of tetrafluoroethylene high-pressure reaction kettle, stainless steel airtight heating reaction after a period of time, will the sediment filtering, washing, until no Cl – check out, After drying at 80℃, barium titanate powder was prepared.
BaCO3+ TiO2 → BaTiO3+ CO2
Solid phase method has the advantages of simple and mature process, reliable equipment and cheap raw materials.
Disadvantages are: the chemical composition of the prepared powder is not uniform, easy to agglomerate, particle size is coarse; Powder purity is low, and the reaction is carried out at high temperature, energy consumption is also large.