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The Piezoelectric Effect

Piezoelectricity

The Piezo Effect

The piezo effect was discovered in the year 1880 by the brothers Jacques and Pierre Curie.

During experiments with Tourmaline crystals they found that electrical charges appeared on the surface when the crystal was mechanically deformed. The quantity of the electrical charge was exactly proportional to the load applied. 

When a piezoelectric material is mechanically deformed the electric charges contained within the elementary cells get displaced and form an electrical field over the entire body. The so produced charge can be collected on the respective surfaces of the piezoelectric body. This is called the direct piezo effect.

The piezo effect is also invertible. If we apply a voltage across the  same surfaces, the piezoelectric body will deform itself in a similar way. This phenomenon is called the inverse (or converse) piezo effect.

We retain:

  • Piezoelectric materials are extremely sensitive to mechanical deformation.

  • The relation between charge output and input force is strictly linear (the amount of charge follows exactly the force).

  • The charge is produced due to the deformation of the piezo however this deformation is extremely small.

  • Most of the piezo materials are quite rigid, in many cases comparable to a material like aluminum.
     

 

Piezoelectric Crystals

If we talk about piezoelectric crystals we normally mean a single crystal. I.e. a body made from only one continuous crystal.

Probably the most famous piezoelectric crystal is the quartz. Quartz can be found in the nature but for technical applications it is normally man-made.

Natural quartz

Chemically speaking the quartz is made of silicon (Si) and oxygen (O).

The arrangement of silicon and oxygen is in the form of a so called tetrahedron like shown in the picture.

The oxygen atoms are forming a tetrahedron around each silicon atom meaning that each silicon is surrounded by four oxygens.

Oxygen  O

Silicon  Si

Silicon - oxygen tetrahedron

(the size of the atoms is not to scale)

The structure of a quartz crystal is highly complex.

The picture shows an impression of how it would look inside a quartz when we could visualize the Si-O tetrahedrons.

However we don't need to be afraid of this complexity. Instead we are going back to the basic and simple tetrahedron.

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Structure of quartz built up with silicon - oxygen tetrahedrons

The silicon and oxygens carry an electric charge. The oxygens are negatively and the silicon is positively charged. In the illustration the respective charges are shown in blue (-) and red (+).

When such a tetrahedron element is deformed mechanically the positive charge of the silicon is shifted downwards and so the tetrahedron will become positively at the bottom and more negatively at the top.

(      hover with the mouse pointer over the picture to apply a vertical load.)

The positive charge is shifted down when a load is applied.

(The deformation is exaggerated to show the effect)

This picture shows a simplified model of the quartz structure. Every dot repesents a tetrahedron which are  arranged in hexagons. In reality, as we have seen, the situation is far more complex.

Although the real orientation of the tetrahedrons is somewhat different all of them will be affected with their central Si atom pushed downwards when a vertical load is applied. The Si-O units are producing an electric charge in the same direction which means that as a result a net charge appears at the top and bottom surface of the body.

Simplified model of a quarz structure showing the charge distribution. Blue (-) red (+).
Note that the pink colour is due to blue and red overlapping

 

Piezoelectric Ceramics

Another group of piezoelectric materials are the piezo ceramics.

All the piezo ceramics are man-made.

One of the world’s most widely used piezoelectric ceramic material is lead zirconate titanate or PZT.

PZT is a mixture of lead zirconate and lead titanate.

The material is not a single crystal but is a conglomerate of little crystals or crystallits. The base unit of such a crystallite is a cube built with lead (Pb) atoms in the corners and oxygen (O) atoms in the center of each face. Inside this structure we find a smaller atom either titanium (Ti) or zirconium (Zr).

Over the temperature range this structure can take two slightly different states. Above a certain temperature, called Curie-temperature, the crystal structure is a simple cube. It is completely symmetric and is not piezoelectric.

Pb ²⁺

Ti ⁴⁺  or   Zr ⁴⁺

O ²⁻

Above the Curie point:

Cubic  structure with symmetric arrangement of  positive and negative charges. Not piezo electric.

However, when the crystal is cooled down below the Curie-temperature the crystal cube is stretched a bit in one direction and the Ti or Zr atom is squeezed out of the center. This happens by itself and is called spontaneous polarization. Now we recognize a similar structure as we had on the quartz. In fact the Ti or Zr atom has a strong positive charge while the oxygens are negatively charged and we find the same mechanism that makes the material piezo electric.

The polarization can happen in the direction of any face of the cube so there are 6 possible directions.

polarization

Below the Curie point:

Polarized structure with the central positive atom shifted up, creating an asymmetric distribution of the positive and negative charges.

Shows piezo electricity.

Poling of Piezo Ceramic

A piezo ceramic is a conglomerate of crystallites which stick together due to a kind of baking process called sintering. During the cooling the spontaneous polarization occurs and adjoining elements align themselves forming domains with parallel orientation. The alignment provides an individual but uniform polarization to each domain. The direction of polarization in the different domains is completely random so the ceramic element has no overall polarization. However there is a possibility to align the domains in a ceramic element.

By heating it close to the Curie temperature and exposing it to a strong electric field the polarization of the domains are aligned and “freeze up” when the electric field is removed.

To create an electric field we need to place electrodes on opposite surfaces and apply a high voltage. The negative voltage or electric charge on the top will attract the positively chreged Zr and Ti atoms and pull them upwards which aligns the individual domains.

This treatment called poling.

Random orientation of different Weiss domains

Polarization in a strong DC electric field

When the electric field is removed most of the polarizations are locked into a configuration of near alignment. However the alignment will not be perfectly straight because each domain has distinctly allowed directions. The element now has a permanent or remnant polarization and is piezoelectric.

We can now understand that heating of a piezo element above the Curie temperature will destroy the alignment of the polarization and thereby the integral piezo effect of the element.

Remnant polarization after removal of the electric field