Views: 5 Author: Site Editor Publish Time: 2018-08-10 Origin: Site
In the practical applications, the piezoelectric materials piezo ceramic transducer needs to have a high electromechanical cohesion coefficient Kt to effectively convert electrical energy and mechanical energy each other, at the same time, it is necessary to expand the dielectric coefficient as high as possible to receive the electric energy. Transmission is more efficient; and dielectric loss is (tans <0.05), mechanical loss (mechanical quality factor port is possible to ensure the sensitivity of the transducer; in addition, in order to ensure that the sound wave emitted by the transducer is at the interface of tissue .When the water is transmitted and received, the energy is better combined. The acoustic impedance needs to be as close as possible to the acoustic impedance of the human tissue, so that the sound wave interface can better perform the energy disaster when is transmitting and receiving.
The piezoelectric ceramic thickness direction electromechanical coincidence coefficient Kt is small (between 0.4 and 0.5), the dielectric constant is expanded between 100 and 2400, the dielectric loss of tans is <0.02, and the mechanical quality factor Q is between 10 to 1000, the acoustic impedance z is between 20 and 30 Mrayl, which is making it difficult to perform acoustic impedance matching. The piezo vibration sensor(polyethylene difluoride and its polymer) has a lower acoustic impedance, thus sound impedance matching is easier. These materials have a low coefficient of electromechanical engagement (kt <0.3) and high dielectric loss (a TANS 0.15), so that the piezoelectric thin film transducers made lower sensitivity.Piezoelectric composite materials have the characteristics of piezoelectric ceramics and polymers, and the electromechanical cohesion coefficient is large, which can reach 0.6-0.75, the acoustic impedance value can reach z<7.SMrayl, and a wide range of piezoelectric constants ) and low dielectric and mechanical losses make it suitable to make broadband, high sensitivity ultrasonic transducers.
In 1985, Wallace Ardensmith established a physical model of the thickness vibration mode of piezoelectric sensor working, and theoretically gave the performance parameters of the piezoelectric composites with the volume fraction of the piezoelectric ceramics. The relationship between changes is in the medical ultrasound transducer , piezoelectric materials operate in the thickness vibration mode. At this time, when is considering only the thickness vibration mode, the composite material can be approximately equivalently regarded as a piezoelectric material.Having properties can be simplified and assumed.
(1) The electric field has a component only in the thickness direction, and accordingly the two-phase material has a component only on the Z-axis.
(2) The transverse stress and strain of the two-phase material are equal .
It can be seen from the microstructure of the composite that the periodically arranged columns to reflect the propagating waves, in particular when it is propagating sound waves which resonates with the columns. At this time, the Lamb wave and the microstructure stops the composite material will form a resonance, so that the description of the composite material is as an isotropic medium is not very accurate. In order to ensure the accuracy of the WA Smith model in describing the l-type 3 piezos discs crystal, it is necessary to ensure that the l-3 type piezoelectric composite can be regarded as an isotropic medium, and the piezoelectric composite reaches the resonant frequency. When it is necessary to suppress the spatial lateral vibration mode as much as possible, only the thickness vibration mode exists. Moreover, the conditions under which the piezoelectric composite material vibrates only in the thickness direction.
It can be seen that the dielectric constant of the l-3 composite material is basically increasing with the volume fraction of the piezoelectric tube scanner.It has a linear growth trend, and the dielectric constant varies with the piezoelectric phase and is not affected by the cross-sectional shape of the piezoelectric ring. That is, in the same volume fraction, the arrangement of the piezoelectric phase is in the non-piezoelectric phase is integrated with the entire composite material.The electrical constant has no effect.
