Piezoelectric Effect

Piezoelectric Effect

The old version of this article was advertised years ago (2009) and you can still find this article here.

Experimental

Piezoelectricity is the ability of certain materials to produce a voltage when subjected to mechanical stress. Piezoelectric materials also show the opposite effect, called converse piezoelectricity, where application of an electrical field creates mechanical stress (size modification) in the crystal.

Figure 1 Piezoelectric effect

The effect known as piezoelectricity was discovered by brothers Pierre and Jacques Curie; they showed that crystals of tourmaline, quartz, topaz, cane sugar, and Rochelle salt (sodium potassium tartrate tetra hydrate) generate electric charge from mechanical stress. Quartz and Rochelle salt exhibited the most piezoelectricity.

The class of piezoelectric materials was enlarged, when was observed that many other materials exhibit the effect, like berlinite (AlPO4) and gallium orthophosphate (GaPO4), ceramics with perovskite or tungsten-bronze structures (BaTiO3, KNbO3, LiNbO3, LiTaO3, BiFeO3, NaxWO3, Ba2NaNb5O5, Pb2KNb5O15). Later was observed that polymer materials like rubber, wool, hair, wood fiber, and silk exhibit piezoelectricity to some extent. The polymer polyvinlidene fluoride, (-CH2-CF2-)n, exhibits piezoelectricity several times larger than quartz

Background and actual explanation

In a piezoelectric material, the positive and negative electrical charges are separated, but symmetrically distributed, so that the crystal overall is electrically neutral. When a stress is applied, this symmetry is destroyed, and the charge asymmetry generates a voltage.

In reverse effect, when an external voltage is applied, on such crystal, because the charges inside the crystal are separated, the applied voltage affects different points within the crystal differently, resulting in the distortion and size modifications.

The effect was studied by Curie brothers before 1900, using a quadrant electrometer and a piezoelectric crystal subjected to an external force.

It was considered a curiosity when was discovered, but in time, the effect gained a lot of applications.

Except of an ulterior classification of piezoelectric substances depending on the type or crystal symmetry, no significant advancements were registered in a quantum treatment for this subject. The quantum mechanic theoreticians are able to apply the quantum idea to a lot of cosmic phenomena, but none was able to apply it to piezoelectric effect. Famous books of physics or physical chemistry omit the topic completely. Maybe instead of cutting leaves to the dogs, some theoreticians will give a complete quantum treatment of this simple effect. The subsequent text will present some ideas important for actual theoreticians and for a future quantum theoretical treatment.

Why the actual explanation is absurd

In actual orthodox explanation, it is not clear what does it mean a charge separation in a crystal, and where this charge is generated.

In any material (piezoelectric or not), electrons are bound on nucleus with strong electric forces. In order to remove an electron from an atom, it is necessary to give at a specific atom an energy greater then ionization energy. Considering a quartz crystal, which is simply a variety of silicon dioxide, in order to produce a charge separation it is necessary to give a ionization energy greater then 13,6 eV for an oxygen atom or greater then 8,15 eV for a silicon atom.

An easy to follow math (entire demonstration in the book), will show that external pressure exerted on faces of quartz crystal does not produce the ionization of quartz material.

The actual physicists are not able to explain how is possible to furnish a smaller energy like ionization energy to a quartz crystal, and to obtain a charge separation. Maybe in the meantime, the ionization process is produced as result of quantum tunneling effect?

Let’s analyze in detail this possible charge apparition and its movement. Considering a cube made from a piezoelectric material, two equal forces act on the x dimension as in fig. 2:

Figure 2 Charge generation in piezoelectric crystal

In the piezoelectric material a small number of compensated (positive and negative) charges are figured. As result of external force, actual orthodox physics suppose a charge separation, but…what kind of charge and where are they appearing?

Do the charges appear only at the surface of compressed material or they appear in entire volume?

Because the atoms are neutral before the mechanical stress, it must be supposed that under force action an equal number of positive charge and negative charges appears. In the same time, it is a common sense concept to admit the immobility of nucleus, and the mobility of electrons.

Let’s suppose for the first case, the charges are appearing only to the surface of piezoelectric material as in fig.3. In this case, on both side of material an equal number of negative charges are available for conduction. A voltmeter connected between these faces should register a null difference of potential. This is because there is no electric charge moving into external circuit. If a ,,charge” is produced at the surface of crystal, the potential of both surfaces modify simultaneously and no potential difference appear. The result is in contradiction with experiment.

Figure 3 Case of surface charge generation

Maybe actual theoreticians are able to demonstrate that positive nuclei are traveling around circuit?

A second possibility regards generation of charges in the entire volume of piezoelectric material as in fig. 4. This is a more realistic idea as far crystals shrinks under external forces and due to the distances modifications between atoms, it is normal to suppose that charges are generated in entire crystal volume.

Figure 4. Case of volume charge generation

With such distribution, an electron generated somewhere inside piezoelectric material, with an energy greater then ionization energy, will leave its nucleus and …. will be soon attracted by another nucleus. There is an equal probability for electrons to arrive on a face of crystal; again no potential differential should appear on crystal surface.

Of course, there is a possibility for generated electrons to group together and to travel to one of crystal surface, but in this case actual theoreticians should provide a mechanism able to produce an electron grouping in a region of piezoelectric material. For a common sense mind, it is completely absurd to believe that such charge distribution permit a grouping of positive charge into a spatial region and negative charge in another region.

In proposed theory, there is no reason for a charge separation on a crystal surface and in fact, in reality, there are no such phenomena as result of a mechanical force action.

If by absurd, a separation of charge would take place, the most commen effect should be the generation of an emmision spectra. A charge displacement is less probable like a charge extinction due to recombination of newly generated cations and electrons. In this case an emmision spectra  fig. 5., should be observed each time as result of piezoelectricity. 

 Fig. 5. Emmision spectra due to charge extinction 

A further problem is related to:  How the piezoelectric effect fits with quantum hypothesis?

Similar to another well known physic effect (photoelectric effect), if the quantum hypothesis is valid, the potential difference should appear only when the pressure acting on crystal overpasses a certain limit. Does this thing happen in reality? It is well known from the time of Curie’s experiments that generated potential is related to applied pressure, without a threshold pressure for generated potential. In the book, the experiments are repeated and described in details, and no threshold pressure for piezoelectric potential apparition was observed. Therefore, a ,,common mind” explanation of piezoelectric effect will ruled out either quantum hypothesis and an ionization process in quartz crystal. There is no electron removed from silicon or oxygen atom.

The pyroelectricity is closed related to piezoelectricty, and express the ability of certain materials to generate ,,electrical charges” when heated. The same discussion made for piezoelectric effect is valid for pyroelectricity. It is absurd to believe that a temperature of 200 C is able to produce ionization or a charge displacement in a material. The entire discussion is presented in the book.