Shock waves are rapid compression waves created in the air that travel faster than the normal speed of sound. It occurs when gunpowder explodes or an object flies at supersonic speeds. A current that reaches its maximum value and disappears in a short period of time, from 1/1000 to 1/1 millionth of a second. It refers to the electric current that occurs when lightning strikes.
In other words, it refers to a wave generated by a strong impact, and is a compression wave with large amplitude accompanied by large changes in pressure, temperature, and density.
Extracorporeal shock wave therapy treats chronic diseases within the body by artificially generating shock waves outside the body and applying mechanical stimulation to the body.
Extracorporeal shock waves are used to treat a variety of diseases, and are also used to treat various joint diseases such as tennis elbow and golf elbow, plantar fasciitis, Achilles tendinitis, muscle pain, and calcific tendinitis that occur in the feet, as well as kidney or ureteral stones.
Piezoelectric effect
When ultrasonic waves are emitted into the human body, human tissues vibrate slightly, causing various medical phenomena. Vibration is used to activate or destroy human cells, and image diagnosis is possible using reflection characteristics between tissues. Additionally, it is possible to diagnose the heart, fetus, etc. using the Doppler effect.
Electromagnetic induction phenomenon
The electromagnetic induction phenomenon uses the force (Lorenz force) received by electrons in a magnetic field in the same way as a dynamic speaker. It has low efficiency in high frequency areas and is not used often these days.
Magnetostriction phenomenon
When a coil is wound around a ferrite structure and current flows, a natural frequency determined by the physical properties or resonance of the structure is generated, generating sound waves. This phenomenon is called magnetostriction.
Piezoelectric effect
The piezoelectric effect can be divided into the positive effect (Figure 1 below), in which an electric charge is generated when external pressure is applied to the piezoelectric element, and the reverse effect (Figure 2 below), in which deformation occurs when voltage is applied.
Piezoelectric elements include natural materials such as crystal and Rossell’s salt, barium titanate (BaTiO3), or lead titanate (PbZrTiO3, PZT), but recently, PZT, which has the best characteristics, is mainly used.
piezoelectric effect( picture 1 )
Inverse piezoelectric effect ( picture 2 )
Characteristic of piezoelectric
Almost all solid substances have a crystal structure. Among the 32 point groups, 20 asymmetric groups with electrically insulating properties have piezoelectric properties. Among these, 10 groups are pyroelectric crystals that have spontaneous polarization (a property in which polarization occurs when the temperature changes), and the direction of polarization that changes due to an electric field is called ferroelectricity. The piezoelectric phenomenon exists in ferroelectric crystals. The representative crystal structure of a ferroelectric is shown in the figure below.
It has a structure where Ti/Zr ions are at the center of the hexahedron, Pb is at the vertices, and oxygen is at the center of each of the six sides. (Perovskite structure) This material is called PZT and is the most representative piezoelectric material. At room temperature, as shown on the right side of the picture, the Ti/Zr ions are not located exactly in the center but slightly above. As a result, a dipole exists, and when an electric field is applied, the dipole expands and contracts, resulting in piezoelectric properties. Because of these characteristics, there may be positive and negative effects of piezoelectricity. Above the Curie temperature (approximately 330°C for PZT), it is located in the exact center as shown in the picture on the left and loses its piezoelectric properties.
To quantitatively formalize the piezoelectric phenomenon, the piezoelectric equation, which can be expressed through electro-mechanical conversion factors, is used, but since this becomes a bit complicated, it will be omitted here.
Polarization
Ferroelectrics such as PZT are lumped together to form crystals through a solid-state reaction through sintering, and have a dipole arrangement as shown in the left picture below. When a strong DC electric field is applied here, the dipoles are aligned in the direction of the electric field, resulting in a state as shown in the center picture below. Even after the external electric field is removed, residual polarization exists as shown in the picture on the right below, so this ferroelectric material finally acquires piezoelectric properties. This process is called polarization.
