The Piezoelectric Effect
Piezoelectric effect for ultrasonic cleaning
It is a phenomenon discovered in 1880 by the brothers Jacques and Pierre Curie on certain crystalline minerals, such as quartz. It takes its name from the Greek word “piezein,” which means “to press” or “to squeeze.” This effect immediately found numerous applications, but the main one is the transformation or transduction of electrical energy into mechanical vibration. In essence, by applying a sinusoidal voltage to the poles of the PZT elements, which continuously varies from positive to negative and vice versa, the elements elongate and contract at very high frequencies (33,000 Hz in our case).
In the field of ultrasonic cleaning, very high power levels are required. This has been made possible only through the development of special ceramic materials obtained by mixing fine powders of metallic oxides. generally titanium or zirconium with lead or barium (PbTiO2-PbZrO3-BaTiO3), in well-defined proportions, and then heated to mix uniformly.
The mixture is then blended with an organic binding material and placed into molds of various shapes to obtain different models: discs, rings, plates, tubes, etc. The material is heated for a specific duration, during which the powders sinter and form a highly compact structure. Finally, the cooled pieces are mechanically processed to achieve the desired precise dimensions, and electrodes (usually silver applied with vacuum metallization) are then applied to the surfaces.
At this point, the product has a simple crystalline structure with cubic symmetry and dipoles without any electrical charge at a temperature above the Curie point. On the contrary, just below the Curie point, the structure assumes a tetragonal or rhombohedral symmetry, and the dipoles acquire an electric charge.
Subsequently, the charged dipoles, although randomly oriented within the lattice structure, need to be aligned in the same direction (polarized) in order to achieve maximum piezoelectric effect. Then, the material is heated to just below the Curie point and simultaneously subjected to a strong electric field, causing the dipoles to align in the same direction. When the electric field is removed, the majority of the dipoles retain their orientation, resulting in the maximum piezoelectric effect.