The piezoelectric effect was discovered by the French scientists the Curie brothers at the end of the 19th century. At that time, it was too early to talk about the practical application of the discovered phenomenon, but at present, piezoelectric elements are widely used both in technology and in everyday life.
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The essence of the piezoelectric effect
Famous physicists have established that when some crystals (rock crystal, tourmaline, etc.) are deformed, electric charges arise on their faces. At the same time, the potential difference was small, but it was confidently fixed by the devices that existed at that time, and by connecting sections with oppositely polar charges using conductors, it was possible to obtain electricity. The phenomenon was recorded only in dynamics, at the moment of compression or stretching. Deformation in the static mode did not cause a piezoelectric effect.
Soon, the opposite effect was theoretically justified and discovered in practice - when a voltage was applied, the crystal was deformed.It turned out that both phenomena are interconnected - if a substance exhibits a direct piezoelectric effect, then the opposite is also inherent in it, and vice versa.
The phenomenon is observed in substances with an anisotropic type crystal lattice (whose physical properties are different depending on the direction) with sufficient asymmetry, as well as some polycrystalline structures.
In any solid body, the applied external forces cause deformation and mechanical stresses, and in substances with a piezoelectric effect, they also cause polarization of charges, and the polarization depends on the direction of the applied force. When changing the direction of exposure, both the direction of polarization and the polarity of the charges change. The dependence of polarization on mechanical stress is linear and is described by the expression P=dt, where t is mechanical stress, and d is a coefficient called the piezoelectric module (piezoelectric module).
A similar phenomenon occurs with the reverse piezoelectric effect. When the direction of the applied electric field changes, the direction of deformation changes. Here the dependence is also linear: r=dE, where E is the electric field strength and r is the strain. The coefficient d is the same for direct and inverse piezoelectric effects for all substances.
In fact, the above equations are only estimates. The actual dependences are much more complicated and are also determined by the direction of forces relative to the crystal axes.
Substances with a piezoelectric effect
For the first time, the piezoelectric effect was found in rock crystals (quartz). To this day, this material is very common in the production of piezoelectric elements, but not only natural materials are used in production.
Many piezoelectrics are made from substances with the ABO formula.3, e.g. BaTiO3, РbТiO3. These materials have a polycrystalline (consisting of many crystals) structure, and in order to give them the ability to exhibit a piezoelectric effect, they must be subjected to polarization using an external electric field.
There are technologies that make it possible to obtain film piezoelectrics (polyvinylidene fluoride, etc.). To give them the necessary properties, they also need to be polarized for a long time in an electric field. The advantage of such materials is a very small thickness.
Properties and characteristics of substances with a piezoelectric effect
Since polarization occurs only during elastic deformation, an important characteristic of a piezomaterial is its ability to change shape under the action of external forces. The value of this ability is determined by elastic compliance (or elastic rigidity).
Crystals with a piezoelectric effect are highly elastic - when the force (or external stress) is removed, they return to their original shape.
Piezocrystals also have their own mechanical resonant frequency. If you make the crystal vibrate at this frequency, the amplitude will be especially large.
Since the piezoelectric effect is manifested not only by whole crystals, but also by plates of them, cut under certain conditions, it is possible to obtain pieces of piezoelectric substances with resonance at different frequencies, depending on the geometric dimensions and direction of the cut.
Also, the vibrational properties of piezoelectric materials are characterized by a mechanical quality factor. It shows how many times the amplitude of oscillations at the resonant frequency increases with an equal applied force.
There is a clear dependence of the properties of a piezoelectric on temperature, which must be taken into account when using crystals. This dependence is characterized by the coefficients:
- the temperature coefficient of the resonant frequency shows how much the resonance goes away when the crystal is heated / cooled;
- the temperature expansion coefficient determines how much the linear dimensions of the piezoelectric plate change with temperature.
At a certain temperature, the piezocrystal loses its properties. This limit is called the Curie temperature. This limit is individual for each material. For example, for quartz it is +573 °C.
Practical use of the piezoelectric effect
The most famous application of piezoelectric elements is as an ignition element. The piezoelectric effect is used in pocket lighters or kitchen igniters for gas stoves. When the crystal is pressed, a potential difference arises and a spark appears in the air gap.
This area of application of piezoelectric elements is not exhausted. Crystals with a similar effect can be used as strain gauges, but this area of use is limited by the property of the piezoelectric effect to appear only in dynamics - if the changes stop, the signal stops generating.
Piezocrystals can be used as a microphone - when exposed to acoustic waves, electrical signals are formed. The reverse piezoelectric effect also allows (sometimes simultaneously) the use of such elements as sound emitters. When an electrical signal is applied to the crystal, the piezoelectric element will begin to generate acoustic waves.
Such emitters are widely used to create ultrasonic waves, in particular, in medical technology. At this the resonant properties of the plate can also be used.It can be used as an acoustic filter that selects only natural frequency waves. Another option is to use a piezoelectric element in a sound generator (siren, detector, etc.) simultaneously as a frequency-setting and sound-emitting element. In this case, the sound will always be generated at the resonant frequency, and maximum volume can be obtained with little energy consumption.
Resonance properties are used to stabilize the frequencies of generators operating in the radio frequency range. Quartz plates play the role of highly stable and high-quality oscillatory circuits in frequency-setting circuits.
There are still fantastic projects to convert the energy of elastic deformation into electrical energy on an industrial scale. You can use the deformation of the pavement under the influence of the gravity of pedestrians or cars, for example, to illuminate sections of the tracks. You can use the deformation energy of the wings of the aircraft to provide the aircraft network. Such use is constrained by the insufficient efficiency of piezoelectric elements, but pilot plants have already been created, and they have shown the promise of further improvement.
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