Among ceramic capacitors, high dielectric constant series capacitors, now mainly use BaTiO3 (barium titanate) as the main component of the dielectric.
BaTiO3 has a perovskite crystal structure as shown in the figure below. Above the Curie temperature, it is a cubic crystal (cubic), with Ba2+ ions at the apex, O2- ions at the center of the surface, and Ti4+ ions at the center of the cube. position.
The above figure shows the cubic crystal structure when the temperature is above the Curie temperature (about 125°C). In the normal temperature range below this temperature, one axis (C axis) is elongated and the other axis is slightly shortened. tetragonal) crystal structure.
At this time, polarization occurs as a result of Ti4+ ions shifting in the extension direction of the crystal unit. However, this polarization occurs even in the absence of an external electric field or voltage, so it is called spontaneous polarization. (Spontaneous polarization).
In this way, it has the characteristics of spontaneous polarization and can change the direction of spontaneous polarization according to the external electric field, so it is called a strong dielectric type.
(Sometimes the rhombohedral crystal system is called the trigonal crystal system, and the orthorhombic crystal system is called the monoclinic crystal system.)
In addition, when BaTiO3 is heated above the Curie temperature, the crystal structure will undergo a phase transition from a square crystal to a cubic crystal. With this change, the spontaneous polarization will disappear, and the domain will not exist.
When it is cooled below the Curie temperature, the phase transition from cubic crystal to square crystal occurs near the Curie temperature, and the C axis direction will be extended by about 1%, the other axes will be slightly shortened, spontaneous polarization and domains will be generated . At the same time, the grains will be subjected to pressure due to deformation.
At this time, a large number of tiny domains are generated in the crystal grains, and the spontaneous polarization of each domain is in a state where the phase transition easily occurs even in the case of a low electric field.
If it is placed below the Curie temperature and placed in an unloaded state, as time goes by, the domains generated in a random direction will have a larger size and gradually progress toward a more stable energy form (Figure 90°domain) Arrange again to release the pressure caused by the deformation of the crystal.
In addition, the space charge (slow moving ions and void points, etc.) of the grain boundary layer will move and generate polarization of the space charge. The polarization of the space charge will have an effect on the spontaneous polarization and hinder the phase transition of the spontaneous polarization.
Therefore, the spontaneous polarization will gradually re-arrange to a state where the spontaneous polarization tends to be stable from the beginning of its generation. At the same time, space charge polarization is generated in the grain boundary layer, and the phase transition of the spontaneous polarization Obstructed.
In this state, in order to cause the spontaneous polarization of each domain to undergo a phase transition, a stronger electric field is necessary.
The same as the phase transition of spontaneous polarization per unit volume is the permittivity. Therefore, if the domains that undergo phase transition under a weak electric field are reduced, the electrostatic capacity will decrease.
The above content is generally regarded as the principle of aging characteristics.
