Piezoelectricity

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A piezoelectric disk generates a voltage when deformed.

Piezoelectricity (from the Greek piezo, πιέζω, "to squeeze or squeeze") is a phenomenon that occurs in certain crystals that, when subjected to to mechanical stresses, they acquire an electrical polarization in their mass, appearing a potential difference and electrical charges on their surface. Quartz crystals behave in a similar way to LC tanks, and are also known as Crystal Resonator, with the advantage of being able to generate stable and insensitive oscillation frequencies.

This phenomenon also occurs in reverse: they deform under the action of internal forces when subjected to an electric field. The piezoelectric effect is normally reversible: when the crystals are no longer subjected to an external voltage or electric field, they recover their shape.

Piezoelectric materials are natural or synthetic crystals that lack a center of symmetry. Compression or shearing causes dissociation of the centers of gravity of electrical charges, both positive and negative. As a consequence, elementary dipoles appear in the mass and, by influence, charges of opposite sign arise on the facing surfaces.

Pyroelectricity

In 1824, Sir David Brewster demonstrated piezoelectric effects using La Rochelle salt, deciding to name the effect pyroelectricity.

Groups of piezoelectric material

There are two groups of materials:

The first piezoelectric nature: quartz, turmaline, etc.
The so-called ferroelectrics: lithium tanlate, lithium nitrate, berlinite, in the form of monocrystalline materials and ceramics or polar polymers, which after being subjected to polarization acquires piezoelectric properties, as well as oriented microcrystalles.

History of piezoelectric materials

The property of piezoelectricity was first observed by Pierre and Jacques Curie in 1881 studying the compression of quartz. By subjecting it to the mechanical action of compression, the charges of matter are separated. This causes a polarization of the charge, which causes sparks to fly.

For the property of piezoelectricity to occur in matter, it must crystallize in systems that lack a center of symmetry (that have asymmetry) and, therefore, a polar axis. Of the 32 crystalline classes, in 21 the mentioned center does not exist. In 20 of these classes the piezoelectric property occurs, to a greater or lesser extent. Gases, liquids, and solids with symmetry do not possess piezoelectricity.

If pressure is exerted on the ends of the polar axis, polarization occurs: the flow of electrons is directed towards one end and generates a negative charge in it, while a positive charge is induced at the opposite end.

When narrow glass sheets with a large surface area are used, the high voltage obtained –necessary for the spark to jump– is higher. The narrow sheets are cut so that the polar axis crosses these faces perpendicularly.

The generated current is proportional to the area of the plate and to the rate of change of the pressure applied orthogonally to the surface of the plate.

Another important application of piezoelectricity results from the inverse property being fulfilled:

If the piezoelectric material plate is subjected to a variable voltage, it is compressed and relaxed, oscillating to the impulses of an electrical signal.
When this plate is in contact with a fluid it transmits its vibrations and produces ultrasounds.

The first practical application of piezoelectricity, which arises from the quality of transforming a mechanical signal (pressure) into an electrical signal (electric current), is that of sonar.

At the end of the First World War it was discovered that the sound waves produced by submarines could be detected by a piece of quartz submerged in the water, in which the generated currents were measured and made it possible to detect the direction coming from the ship. sound.

The sonar consists of a probe (piezoelectric) that is a transducer; that is: it works according to the following sequence of events:

It emits vibrations that produce ultrasonic waves in the water in the direction of the polar axis; that is, it receives its echo.
The transmitter moves so that the wave emitted "bars" the space to locate the direction in which the obstacle is found.
The echo received hits the piezoelectric crystal and produces an electric current.
Finally, the data of the distance to which is the obstacle that re-emits an echo is obtained using the calculations derived from the theory of the Doppler effect.

Crystal classes of substances containing piezoelectricity

Within the 32 crystallographic groups, there are 21 that do not have a center of symmetry. Of these, about 20 directly exhibit piezoelectricity (number 21 is cubic class 432). Ten of them are polar; that is to say: they present instantaneous polarization, because in their unit cell they contain an electric dipole, and the material exhibits pyroelectricity. Of these –when the direction of the dipole can be reversed by applying an electric field– some are also ferroelectric. The crystallographic classes are:

Crystal classes piezoelectric: 1, 2, m, 222, mm2, 4, -4, 422, 4mm, -42m, 3, 32, 3m, 6, -6, 622, 6mm, -62m, 23, -43m.
Pyroelectric crystallized classes: 1, 2, m, mm2, 4, 4mm, 3, 3m, 6, 6mm.

Piezoelectricity Equations

The constitutive equations of piezoelectric materials combine stresses, strains, and electrical behavior:

D=epsilon E ;

D is the density of electric flow, $epsilon ;$ is the permitivity and E is the electric field:

S=s T ;

S is strain and T is stress.

These equations can be combined into a single equation where the relationship between load and strain is considered:

{S} = left [s^E right ]{T}+[d^T]{E}

{D} = [d]{T}+left [ epsilon^T right ] {E}

d represents the piezoelectric constants of the material, and the superscript E indicates that the magnitude is measured under constant or zero electric field, and the superscript T signals that this is a transposed form of matrix.

This can be rewritten in matrix form like this:

begin{bmatrix} S_1 \ S_2 \ S_3 \ S_4 \ S_5 \ S_6 end{bmatrix} = begin{bmatrix} s_{11}^E & s_{12}^E & s_{13}^E & 0 & 0 & 0 \ s_{21}^E & s_{22}^E & s_{23}^E & 0 & 0 & 0 \ s_{31}^E & s_{32}^E & s_{33}^E & 0 & 0 & 0 \ 0 & 0 & 0 & s_{44}^E & 0 & 0 \ 0 & 0 & 0 & 0 & s_{55}^E & 0 \ 0 & 0 & 0 & 0 & 0 & s_{66}^E=2left(s_{11}^E-s_{12}^Eright) end{bmatrix} begin{bmatrix} T_1 \ T_2 \ T_3 \ T_4 \ T_5 \ T_6 end{bmatrix} + begin{bmatrix} 0 & 0 & d_{31} \ 0 & 0 & d_{32} \ 0 & 0 & d_{33} \ 0 & d_{24} & 0 \ d_{15} & 0 & 0 \ 0 & 0 & 0 end{bmatrix} begin{bmatrix} E_1 \ E_2 \ E_3 end{bmatrix}

begin{bmatrix} D_1 \ D_2 \ D_3 end{bmatrix} = begin{bmatrix} 0 & 0 & 0 & 0 & d_{15} & 0 \ 0 & 0 & 0 & d_{24} & 0 & 0 \ d_{31} & d_{32} & d_{33} & 0 & 0 & 0 end{bmatrix} begin{bmatrix} T_1 \ T_2 \ T_3 \ T_4 \ T_5 \ T_6 end{bmatrix} + begin{bmatrix} epsilon {}_{11} & 0 & 0 \ 0 & epsilon {}_{22} & 0 \ 0 & 0 & epsilon {}_{33} end{bmatrix} begin{bmatrix} E_1 \ E_2 \ E_3 end{bmatrix}

Uses

One of the most widespread uses of this type of crystal is in electric lighters. Inside they have a piezoelectric crystal which the ignition mechanism hits sharply. This dry blow causes a high concentration of electrical charge, capable of creating a voltaic arc or spark, which ignites the lighter.

Another important application of a piezoelectric crystal is its use as a vibration sensor. Each of the pressure variations produced by the vibration causes a current pulse proportional to the force exerted.

It has easily turned a mechanical vibration into an electrical signal ready to be amplified. Simply connect an electrical cable to each of the crystal faces and send this signal to an amplifier. For example, in piezoelectric guitar pickups.

A very important additional application of piezoelectricity, but in this case in reverse, occurs in the fuel injectors of internal combustion engines. By applying a potential difference to a piezoelectric material, it is possible to open the injector, which allows the fuel, at very high pressure, to enter the cylinder. The use of piezoelectric injectors makes it possible to control, with great precision, injection times and the amount of fuel that is introduced into the engine. This results in improvements in consumption, features and performance of different engines.

Materials

Materials used in electronics:

Quartz
Blonde
Sal de Seignette
Ceramics
Piezoelectric ceramic
Technical ceramics

Applications

Acute speakers (tweeters: small speakers).
Capsuula (Capula)pick-up) of playdy.
Electric lighters
Electronic hots and gas stoves.
Crystal Oscillator
Sensors
Ultrasonic transducers (such as the heads of ecographers).
Transducer piezoelectric
piezoelectric transformers.
Ultrasound dentists, for the removal of the interdental tatar or "sarro".

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