Structure of ultrasonic single crystal probe

I. Piezoelectric Chip
　　Piezoelectric chips are mostly round and square. As shown in the figure:
　　The thickness of the piezoelectric chip is inversely proportional to the ultrasonic frequency. For example, the frequency-thickness constant of lead zirconate titanate (PZT-5) is 1890 kHz/mm. When the chip thickness is 1 mm, the natural frequency is 1.89 MHz; when the thickness is 0.7 mm, the natural frequency is approximately 2.5 MHz.
　　The diameter of the piezoelectric chip is inversely proportional to the diffusion angle.
　　Both sides of the piezoelectric chip are coated with a silver layer as a conductive electrode. The bottom of the chip is connected to a ground wire, and the top of the chip is connected to a wire leading to the circuit.

　　II. Protective Film
　　In order to prevent the chip from being worn away by direct contact with the workpiece, a protective film is bonded under the chip in the flaw detector's straight probe.
　　There are two types of protective films: soft and hard. Soft ones can use thin plastic films (about 0.3 mm thick), which are better in contact with workpieces with rough surfaces. Hard ones can use stainless steel sheets or ceramic sheets.
　　When the thickness of the protective film is an integer multiple of half the wavelength (the wavelength in the protective film), the acoustic wave transmission is maximized. When the thickness is an odd multiple of a quarter wavelength, the transmission is minimized.
　　The material properties of the protective film should pay attention to the impedance matching. Let the acoustic impedance of the protective film be Z, the acoustic impedance of the chip be Z1, and the acoustic impedance of the workpiece to be measured be Z2. The condition is:

　　After the chip and protective film are bonded, the resonant frequency of the probe will decrease.
　　When bonding the protective film and the chip, the bonding layer should be as thin as possible, and no air should be allowed to enter.
　　The formula for the adhesive is:
　　618 epoxy resin: diethylenetriamine: dibutyl phthalate = 100:8:10
　　After bonding, apply a certain pressure and leave it for 24 hours, then bake it at 60℃~80℃ for 4 hours.
　　III. Damping
　　The damping block, also known as the absorption block, reduces the mechanical quality factor Qm of the piezoelectric chip and absorbs acoustic energy. Without a damping block, when the electrical oscillation pulse stops, the piezoelectric chip continues to vibrate due to inertia, increasing the ultrasonic pulse width, increasing the blind zone, and reducing the resolution. As shown in the figure below.

Oscillation of piezoelectric ceramic片

　When the acoustic impedance of the block is equal to the acoustic impedance of the chip, the effect is best. The commonly used absorption block formula is as follows:

　　Tungsten powder: epoxy resin: diethylenetriamine (hardener): dibutyl phthalate (plasticizer) = 35g:10g:0.5g:1g
　　To ensure good bonding between the chip and the damping block, clean the surface of the chip and chip seat with acetone before pouring, and heat it to 60-80℃ before pouring. The epoxy resin and tungsten powder should be fully mixed. After pouring, tilt the probe so that the upper surface of the damping block is tilted by about 20°. This can eliminate the reflection of acoustic waves on the absorption block and reduce the clutter on the fluorescent screen. As shown in the figure above.

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