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What is the Piezoelectric Effect of an Ultrasound Transducer?

The diasonograph generates incident ultrasound waves (transmitted waves) and receives reflected ultrasound waves (echoes) through probes. It is an important part of diagnostic equipment. The high-frequency electric energy excites the crystal in the probe to produce mechanical vibration, and the mechanical vibration of the reflected ultrasound wave can be converted into an electric pulse by the probe. That is to say, the probe can convert electric energy into sound energy, and it can also convert sound energy into electric energy, so the probe is also called an ultrasound transducer. The principle comes from the piezoelectric effect of the crystal.

1. Piezoelectric crystal of the ultrasound transducer

Crystals with piezoelectric effect properties are called piezoelectric crystals. At present, the crystal sheets commonly used in portable usg probe include artificial or natural crystals such as lead zirconate, barium titanate, quartz, and lithium sulfate. Barium titanate and lead zirconate are polycrystalline ceramic bodies sintered at high temperatures. After the blank is sintered into a ceramic body, it is properly ground and trimmed to obtain the required geometric dimensions. After polarization by a high-voltage direct current electric field, it has piezoelectric properties and becomes ultrasound transducer parts.

2. The piezoelectric effect of the ultrasound transducer

Positive piezoelectric effect

In a certain direction of the crystal or ceramic, mechanical force is applied to make it deform, and the two force-bearing surfaces of the crystal or ceramic produce charges with opposite signs; the direction of deformation is opposite, the polarity of the charge changes accordingly, and the charge density is proportional to the applied mechanical force. This effect of stimulating the surface charge due to the mechanical force is called the positive piezoelectric effect.

Inverse piezoelectric effect

Applying a voltage along the direction of the electric field on the surface of the crystal or ceramic will cause the geometric shape of the crystal or ceramic to be strained under the action of the electric field. The direction of the voltage changes and the direction of the strain also changes. The deformation is proportional to the electric field voltage. The deformation effect induced by the electric field is called the inverse piezoelectric effect.

In general, the piezoelectric effect is linear. However, when the electric field is too strong or the pressure is high, a nonlinear relationship will appear.

Crystals and ceramics have different natural frequencies of mechanical vibration due to the different cutting positions and geometric dimensions. When the frequency of the applied alternating voltage is consistent with the natural frequency, the mechanical vibration generated is the strongest; when the frequency of the applied mechanical force is consistent with the natural frequency, the generated charge is also the most. The frequency of the excitation pulse in the ultrasound diagnostic apparatus must be the same as the natural frequency of the ultrasound transducer.

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