What’s the polarization of piezoelectric ceramics

Views: 8 Author: Site Editor Publish Time: 2018-09-13 Origin: Site

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Whether the polarization process of piezo ceramic is sufficient or not has a great influence on the material properties. Therefore, it is necessary to reasonably select the polarization conditions, the polarization electric field, the polarization temperature and the polarization time, which are referred to as the three elements of polarization.

(1)Polarized electric field

The domains can be aligned in the direction of the electric field under the action of a polarized electric field , so it is the main factor in the polarization condition. The higher is the polarization electric field, the greater is the effect of the alignment of the domains, it is more sufficient the polarization. But different formulas should be different in the height.The magnitude of the polarized electric field depends mainly on the coercive field EC of the piezoelectric ceramic. The polarization electric field must be greater than the EC to steer the domains and align in the direction of the external field. It is generally 2-3 times that of EC. The size of the EC is related to the composition and structure of the piezo ceramics. For tetragonal phase PZT-based materials, EC is increasing as the Zr/Ti ratio decreases. In the three-direction region, the change in EC with the Zr/Ti ratio is not significant. If the substitution ratio decreases, the crystal axis ratio c/a of the material, the 90o domain rotation produces a small internal stress, the rotation is easy, and the EC is lower. Soft additives reduce EC and hard additives increase EC. The practical PZT series material EC is in the range of 0.6-1.6 Kv/mm. The EC also decreases with increasing temperature. Therefore, if the polarization temperature increases, the polarization electric field can be correspondingly reduced.

The polarized electric field is also limited by the breakdown strength Eb of the piezo ceramic. Once the polarized electric field reaches the Eb size, the piezo ceramics becomes a waste after breakdown. Eb drops sharply due to the presence of pores, cracks and uneven composition. Therefore, the pre-preparation process must ensure the density and uniformity of the product. The Eb size is also related to the polarization thickness of the piezo discs and cylinders, and its relationship roughly conforms to the formula: Eb = 26.2t0.39 ,where Eb is the breakdown electric field (kV/cm) and t is the thickness (cm). Therefore, for thicker products, the polarization electric field is correspondingly reduced by increasing the polarization temperature, the polarization time is prolong to achieve a good polarization effect.

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Under the condition of polarized electric field and polarization time, when the polarization temperature of piezoelectric transducer applications is high, the orientation of the domain orientation is easier and the polarization effect is better. The main reasons are as follows: (1) The piezo crystal anisotropy decreases with the increasing temperature, and the internal stress of the domain becomes smaller, that is, the resistance is small, so polarization is easier. 2 The hysteresis loop becomes narrower with increasing temperature, that is, the coercive field becomes smaller, and actually makes the domain motion easier. 3 The space charge effect decreases with the increasing temperature. Some impurities cause a large amount of space charge in the product, resulting in a strong space charge field, shielding the externally applied polarization field, which is not conducive to polarization. When the temperature rises, the electrical conductivity of the product increases, the space charge is easy to migrate, the accumulation is reduced, and the shielding effect of the space charge field is reduced, which is favorable for polarization. The polarization temperature is related to the material composition. Some materials comprehensively reflect the piezoelectric properties of the electromechanical coupling coefficient kp value is basically not affected by the polarization temperature, which can be polarized at the lower temperatures, such as the PZT system with soft additives. Some materials require polarization at higher temperatures to have larger kp, such as PZT piezoelectric ceramic element with hard additives. In practice, when the polarization temperature is selected, the temperature is higher, because increasing the polarization temperature can shorten the polarization time and improve the polarization efficiency.However, at higher temperatures, the problem often encountered is that the resistivity of the product is too small, the leakage current is large, and the withstand voltage is low, that is, the voltage cannot be added. In addition to being related to the formulation, this is also related to poor density and low electrical resistivity. For articles that are only related to the formulation, only the polarization field is reduced and the polarization time is extended.

(3) Polarized time

Polarization time refers to the hold the pressure time required for piezo ceramic product to change from one equilibrium state to another. The time is long, the domain alignment is sufficient, and the relaxation of stress during polarization is facilitated. The polarization time is different for different materials. For the same material, the polarization time is related to the polarization electric field and the polarization temperature. When the electric field is strong and the temperature is high, the required polarization time is short; otherwise, the required polarization time is long. Considering comprehensively, the determination of polarization conditions should be based on the principle of giving full play to piezoelectric ceramics performance, improving yield and saving time. For materials with different compositions, the optimal polarization conditions should be optimized through experiments under the guidance of the polarization process . In practice, the polarization effect of piezoelectric ceramic products can be determined by measuring the piezoelectric properties (such as kp or d33), and when it no longer rises as the polarization condition increases, the polarization is considered to be sufficient. At present, in the development and production of PZT piezoelectric ceramics, the polarization conditions are generally selected as follows: Polarized electric field is 1.5-5kV/mm.Polarization temperature is about 100-180°C polarization time 10-60min.