Ultrasonic transducer responsible for the reception and reflection of ultrasound
Ultrasonic probe, also known as ultrasonic transducer, is a key device for transmitting and receiving ultrasonic waves. It is divided into transmitting transducer and receiving transducer. The transmitting transducer converts other forms of energy into ultrasonic waves, and the receiving transducer converts the received ultrasonic waves into other energy that is easy to measure. There are many forms of energy, so the form of the transducer is also different. In ultrasonic measurement, piezoelectric transducers are commonly used, followed by magnetostrictive transducers.
(1) Piezoelectric transducer
The receiving and reflection of the piezoelectric transducer is based on the piezoelectric effect and the inverse piezoelectric effect. The emission of ultrasonic waves uses the inverse piezoelectric effect of piezoelectric materials to apply an alternating voltage to the electrodes on the end face of the piezoelectric material. Along the thickness direction of the crystal, mechanical vibrations with the same frequency as the applied alternating voltage will be generated and emitted outward Sound waves realize the conversion of electrical energy and sound energy (mechanical energy). Ultrasonic reception uses the piezoelectric effect of piezoelectric materials. When piezoelectric materials are subjected to the sound pressure of sound waves, both ends of the material will generate charges that are synchronized with the changes in sound pressure, thereby converting sound energy (mechanical energy) into electrical energy.
Piezoelectric materials can be quartz crystals, piezoelectric ceramics, piezoelectric semiconductors, polymer piezoelectric materials, and the like. According to different needs, there are many vibration modes of the piezoelectric sheet, such as the thickness vibration of the sheet, the length vibration of the longitudinal sheet, the radial vibration of the disc, the bending vibration and so on. Among them, sheet thickness vibration is the most used.
Because the piezoelectric crystal itself is relatively brittle, and due to various requirements for insulation, sealing, corrosion resistance, impedance matching, and poor protection of the environment, piezoelectric elements are often installed in a shell to form a probe. Among them, the spherical sound film plays a role in improving impedance matching and increasing radiation power. Its vibration frequency is above several hundred kilohertz, and a piezoelectric sheet with thickness vibration is used.
(2) Magnetostrictive transducer
The receiving and transmitting of the magnetostrictive transducer is based on the magnetostrictive effect and the inverse magnetostrictive effect. The emission of ultrasonic waves utilizes the magnetostrictive effect of the magnetostrictive material. The magnetostrictive material is placed in an alternating magnetic field, which causes it to produce alternating changes in mechanical dimensions along the direction of the magnetic field, that is, mechanical vibration, thereby generating ultrasound. Ultrasonic reception uses the inverse magnetostrictive effect of magnetostrictive materials. When the magnetostrictive materials are subjected to sound pressure, they will cause internal deformation and stress, which will cause the boundaries between the magnetic domains to move. The magnetization vector rotates, so that the magnetization and permeability of the material change accordingly. Adding a coil to the magnetostrictive material can convert the magnetic change of the material into a change in the coil current, so it can be used to receive ultrasonic waves.
Magnetostrictive materials can be certain ferromagnetic materials and their alloys, such as nickel, nickel-iron alloys, aluminum-iron alloys, iron-cobalt-vanadium alloys, etc., and ferrites containing zinc and nickel. Different magnetostrictive materials have different magnetostrictive effects when an alternating magnetic field is applied, and nickel has the largest magnetostrictive effect.
Commonly used magnetostrictive transducers are made of nickel sheets with a thickness of 0.1～0.4mm. Insulation measures are taken between the sheets to reduce the loss of eddy current. Its shapes are rectangular, window-shaped and so on.