In this first blog post of the piezoelectrics series, a brief overview is provided on the fundamentals of the phenomenon. Piezoelectric materials allow conversion of energy from the mechanical domain to the electrical domain and vice versa. They can be used to create various sensors or actuators: applied periodic electrical signal can result in the generation of ultrasonic waves for imaging purposes; stresses, such as observed for a cantilever suspending a mass in an accelerometer can be translated to electrical signals.
The piezoelectric materials are usually grouped into three categories: (i) naturally occurring (single) crystal substrates, (ii) ceramics with perovskite structure (iii) polymer films. In all cases, an electric dipole moment is required, which can either arise from asymmetric charge distribution in their unit cell or artificially introduced via poling. These two cases are discussed in the following.
First, refer to the following figure that showcases a non-piezoelectric material: the overall charge center of positive and negative ions in the unit cell coincide, and even with applied deformation, these cancel out, and no overall polarisation appears. Note even if we consider elongation in the horizontal direction due to the compression, the charges still cancel out.
However, in crystalline piezoelectric materials, the unique distribution of charges gives rise to a dipole moment when the material is deformed. Consider the example 2D lattice as shown below. A unit cell is shown outlined with dashed lines. Without any external stress, the centroid of positive and negative charges coincide and marked by a black dot. When the material is compressed (right figure), the distance between the atoms remains the same, which is only possible by expanding the material horizontally. This in turn moves the positive and negative charges denoted by a star (*) apart, and their centroid no longer coincide, but are shown by blue and red dots, creating an electric dipole.
A side note here: the electric field always points from positive charges to negative charges, however, moving the charges close to each other (to form a dipole), to have a continuous arrow for the electric field, the polarisation vector is seemingly reversed and always points from the negative to the positive charge (illustrated on the below).
The second group of materials are the engineered materials, usually having the structure shown in the following figure. The corner ions are bivalent metal ions, the face centered ions bivalent oxygen ions and a tetravalent metal ion is used inside the cell to achieve the dipole moment. Important to emphasize there is zero overall charge of the material. By using a strong external electrical field, the tetravalent metal ion can be displaced and a remanent dipole (red arrow) is formed.Poling creates a charge difference between surfaces, that can be neutralized due to airborne free charges, non-zero material conductivity or loading electronics. Therefore, usually the voltage difference between the electrodes of an unstressed piezoelectric (even a poled one) is zero.
For the inverse piezoelectric effect (electrical domain to mechanical) assume there is an external electric field applied horizontally on the crystalline or vertically on the poled material. The field would move the positive charges in the direction of the field and the negative charges against the field, resulting in an opposite movement of charges. Considering the distances between atoms fixed, the material must deform to accommodate the moving charges (in the poled case, mainly the tetravalent metal ion would move).
The direct piezoelectric process involves creation of new surface charges when the material is placed under stress. For crystals, this has been discussed in detail above already, for poled materials, applying stress moves the tetravalent metal ion closer to the centroid of negative charges, or further away. In both cases, a change in polarisation (magnitude and/or sign) occurs, which results in surface charge change. Note that the material is assumed to have an initial zero voltage due to the airborne charges as discussed above, so the sign of the deformation corresponds to the sign of the output voltage in all cases.
Important to see the piezoelectric phenomenon as a dynamic process: even if the material is kept compressed, it cannot be used as a ‘battery’, the removed charges will not regenerate. New surface charges appear either when further compressing or expanding the material.
In the next blog post the various modes will be discussed and how these materials can be used.