Let us provide you with a very simple definition first to get things clear. Certain materials tend to accumulate electric charges when a mechanical stress is applied to it. The piezoelectric effect is an effect that simply describes the fact that a pressure applied to a piezoelectric material will generate a voltage.
Now, how does it work? And where does it come from?
Piezoelectricity and the piezoelectric effect
The word piezoelectricity comes from the Greek word piezein, which means squeeze or press and electron, which means “amber” and is an ancient source of electric charge.
The French physicists Jacques and Pierre Curie discovered in 1880 that electric charges could accumulate in certain solid materials in response to an applied mechanical stress.
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.
The piezoelectric materials are usually grouped into three categories:
- naturally occurring (single) crystal substrates,
- ceramics with perovskite structure
- polymer films.
For example, some materials which show a more pronounced piezoelectric effect are:
- Crystals (Quartz, Potassium Nibonate …)
- Certain Ceramics (Lead Zirconate Titanate or PZT, Barium Titanate, …)
- Biological material (Bone,…)
- DNA and various proteins
What is interesting is that the piezoelectric effect is mostly linear and reversible. For example, take one of the most used piezoelectric materials, the lead zirconate titanate (or PZT) crystals will generate measurable piezoelectricity when their static structure is deformed by about 0.1% of the original dimension. Conversely, those same crystals will change about 0.1% of their static dimension when an external electric field is applied to the material.
The inverse piezoelectric effect is very useful because it is implemented in many transducers to produce ultrasonic sound waves.
Now let’s have a closer look at where it comes from.
What is the difference between a non-piezoelectric material and a piezoelectric material?
First, let’s look at 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 that even if we consider elongation in the horizontal direction due to the compression, the charges still cancel out.
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 coincides but are shown by blue and red dots, creating an electric dipole.
Let’s see a bit more visually how this all work in the next part.
How does piezoelectricity appear under pressure in ceramic or crystal materials?
Materials (like everything in the world) are composed of molecules which are arranged in a certain way.
When the material is in a free state (without any pressure), those molecules will be arranged in a certain way which corresponds to an equilibrium of the matter in which the charges of the molecules cancels itself if we look at the whole.
When a pressure is applied however, those molecules change position and align into a dipolar state in which the global charge distribution isn’t null anymore and 2 sides of the materials become polarized.
But why charge is changing for piezoelectric materials and not for any other material?
Like we mentioned in the previous part, it is because of the special arrangement of the piezoelectric material crystals in a hexagonal configuration.
If you look at the atoms that compose a Quartz Material (commonly used in watches as resonators) crystal for example, you will notice that they are arranged like this:
What happens when the piezoelectric crystal is compressed?
To understand how this work, we must look at the center of the positive and negative charges.
When you compress the crystal, the 2 positive charges on the top move horizontally and not vertically, which causes the center of positive charge to change position upward.
Same for the negative charges.
The average of the 3 negative charges moves downward.
In an uncompressed crystal, the positive charges and negative charges just cancel each other and the resultant charge distribution is null.
When you compress the crystal in a certain orientation, you are slightly shifting the average position of the positive charges in one direction and the average of the negative charges in the other direction.
This creates an accumulation of positive charges on one face and an accumulation of negative charges on the other face.
If you then wire up those faces, the positively charges face will start to pull electrons negatively charged towards it through the wire and the negatively charged face will repel electrons.
That’s how a voltage is generated from the piezoelectric effect!
Note: it’s 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.
Now, It’s cool to know how it works… but is there a way to “ experiment” a bit with all of that to gain a practical understand of how the piezoelectric effect work??
Sure! Simulation can do that.
How to simulate the Piezoelectric effect?
With OnScale, you can easily create a model and simulate the piezoelectric effect as well as the inverse piezoelectric effect.
OnScale calculates Electric Voltage, Mechanical deformation and stresses and acoustic pressure in one and unique model and in a fully coupled way.
Check out our step by step tutorial to learn how to how to set up a simple 2D model of a PZT disc residing in a water tank using the designer mode basic geometry shapes.