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Factors affecting the Qm value of piezoelectric ceramic mechanical quality factor

Views:4     Author:Site Editor     Publish Time: 2019-10-08      Origin:Site

1 domain motion
In the Pb (Zr0. 52Ti0. 48)O3 PZT material piezoelectric ceramics composition, the addition of oxides such as La2O3, Nb2O5, ThO2 and WO3 reduces the Qm value . To explain the Qm reduction, it proposed the following mechanism. When is replacing Pb2 + , Ti4 + , Zr4 + in PZT ceramics with a metal ion is having a higher valence than Pb2 + or Ti4 + or Zr4 + such as La3 + , Nb5 + , W6 + , etc., in order to maintain the lattice power properties, Pb vacancies are generated in piezo ceramics. These Pb vacancies reduce the internal stress that hinders the rotation of the domain, so the domain become easy to move, the internal friction increases, the dielectric loss increases, and the Qm value decreases. It is believed that the motion of the domain wall causes The Qm value is reduced. In addition, the formula deduced by Yamauchi and Takahashi knows that the Q-1m value is proportional to the dielectric loss. One of the causes of dielectric loss is the energy consumption caused by the domain wall vibration hysteresis under the action of the alternating electric field. Therefore, the hysteresis of the domain wall vibration causes the loss of the medium, which causes the Qm value to decrease. Thus, in the low Qm doped modified piezoelectric ceramic material, the difficulty of domain wall motion directly corresponds to the Qm value.

2 Space Charge

When Pb2 + , Ti4 + , Zr4 + in PZT piezo ceramics are replaced by metal ions having a lower valence than Pb2 + or Ti4 + and Zr4 + such as K+, Na + , Cr3 + , Fe3 + , the Qm value increases remarkably. This can be explained by the effect of space charge on the domain. Pure PZT piezoelectric ceramics exhibit hole-type conduction due to lead vacancies, while positive-low-valent metal ion additives act as acceptors in the body. The addition of a low cost metal ion greatly increases the negative center and carrier holes in the piezoceramic body, that is, generates a large amount of space charge. In order to eliminate the electric field generated by the discontinuous change of the domain wall, the negatively charge space charge is concentrated in the domain. The positive terminal and the positively charged space charge is concentrated at the negative end of the domain wall, forming a space charge field (Eq) that is the same as the polarization direction of the domain. When an external electric field is applied to turn the domain, not only overcoming the spontaneous polarization of the original domain, and also overcoming the space charge field Eq. That is, the effect of space charge suppresses the motion of the domain, thereby reducing the internal friction and increasing the Qm value. To estimate the presence in the ceramic .The amount of space charge introduced a can observe The definition is the parameter of the amount of space charge ( Ps - Pi) / Ps to characterize the amount of space charge. The value of ( Ps - Pi ) / Ps varies with the type and quantity of impurities. For low-cost metal additives (hard additive), as the amount of doping increases, the amount of space charge increases, while the amount of space charge for high-priced additives (soft additives) is small and almost impossible to measure. Therefore, the pressure of high Qm value In electric ceramics, the change of Qm value is related to space charge. When the temperature rises, the space charge migrates in the piezo ceramic crystals, which reduces the accumulation of space charge, promotes the movement of the domain wall, and reduces the Qm value. For the addition of soft materials, due to ceramics, The amount of space charge generated in the body is small, and the change of temperature causes the migration of space charge to be small. Therefore, the temperature stability of Qm is generally higher than that of hard additive.

3 Body resistivity For soft additive-modified materials

the volume resistivity is 1 to 2 orders of magnitude higher than that of the hard additive-modified material because a small amount of dopant provides excess electrons. The original hole recombination reduces the concentration of electron holes in the piezo ceramic body, thereby increasing the volume resistivity. The high volume resistivity is beneficial to increase the polarization electric field strength of the piezoelectric ceramic, and the domain orientation movement is more sufficient, resulting in The dielectric loss increases and the Qm value decreases. For hard additive doped materials, the increase in space charge and bulk resistance is accompanied by an increase Qm. The lower the temperature, the greater the bulk resistivity of the material, It is difficult to migrate the space charge, the domain motion is suppressed, and the Qm value is decreased. When the relationship between the volume resistivity and the Qm value is quantitatively characterized, as described above, the Qm value has been expressed as a function of the space charge amount and the volume resistivity.

4 Grain size Large grain size during the polarization process,

 the clamping stress generated at the grain boundary is small, the material is easy to be polarized, the internal friction is reduced, and the Qm value is increased. However, if the grain size is too large, the grain gap will be caused. Increased, compactness, affecting piezoelectric performance; grain is too small, the grain boundary has strong clamping effect on the domain, making the domain steering movement difficult, reducing the piezoelectric performance. Therefore, the grain size must be moderate. The study of CeO2 doped Pb (Mn1/ 3Sb2/ 3)2PbTiZrO3 ternary piezoelectric ceramics shows that, after adding CeO2, the ceramic is dense and the grains are moderate, which improves the microstructure of the piezo ceramic. A standing wave ultrasonic motor material with high voltage and high Qm value was obtained.


5 Lattice constant study
The phase diagram of PZT-based piezoelectric disc transducer materials can be found that adjusting the Zr/Ti ratio and the third or fourth component will cause changes in lattice parameters; in addition, doping ions enter the lattice, crystal The lattice parameter also changes. If the crystal axis ratio increases c/ a, at the time of polarization, the spontaneous polarization Ps of some structures is difficult to rotate, resulting in a decrease in piezoelectric performance; and the decrease in piezo crystal axis ratio c/a makes Ps. The reversal is easy, resulting in an increase in internal friction and a decrease in Qm. Therefore, for binary or multi-element lead-based piezoelectric ceramic materials, the lattice parameters also affect the Qm value.
6 Vibration mode
 The Qm value reflects the degree of mechanical loss caused by the internal friction of the piezoelectric vibrator during resonance. Obviously, the Qm value is related to the vibration mode. It use the PMN2PZT piezoelectric ceramic material to fabricate the piezoelectric vibrator. When the Qm value and temperature of the gyroscope quadruple vibrator and the cylindrical vibrator are related to the temperature, it is found that the Qm curve of the cylindrical vibrator is approximately 30 °C lower than the tetragonal vibrator, which means the tetragonal vibration. The Qm value of about 0°C corresponds to the Qm value of the cylindrical vibrator of -30 to 80 °C. Therefore, adjusting the Qm value can be studied not only by the choice of material composition, but also by changing the vibration mode.

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