What is the inverse piezoelectric effect. Reverse piezoelectric effect

Used to produce ultrasound

Reverse piezoelectric effect;

magnetostriction;

electrostriction;

Piezoelectric effect - the effect of the occurrence of dielectric polarization under the action of mechanical stresses (direct piezoelectric effect). There is also an inverse piezoelectric effect - the occurrence of mechanical deformations under the action of electric field.

Reverse piezoelectric effect consists in the fact that a plate, cut in a certain way from a quartz crystal (or other anisotropic crystal), is compressed or elongated under the action of an electric field, depending on the direction of the field. If such a plate is placed between the plates of a flat capacitor, to which an alternating voltage is applied, then the plate will come into forced oscillations. The vibrations of the plate are transmitted to the particles of the environment (air or liquid), which generates an ultrasonic wave.

The phenomenon of magnetostriction is in that ferromagnetic rods (steel, iron, nickel and their alloys) change linear dimensions under the action of magnetic field directed along the axis of the rod. By placing such a rod in an alternating magnetic field (for example, inside a coil through which an alternating current flows), we will cause forced oscillations in the rod, the amplitude of which will be especially large at resonance. The oscillating end of the rod creates ultrasonic waves in the environment, the intensity of which is in direct proportion to the amplitude of the oscillations of the end.

Some materials (eg ceramics) are able to change their dimensions in an electric field. This phenomenon, called electrostriction, externally differs from the inverse piezoelectric effect in that the change in size depends only on the strength of the applied field, but does not depend on its sign. Such materials include barium titanate and lead zirconate titanate.

Transducers that use the phenomena described above are called piezoelectric, magnetostrictive, and electrostrictive, respectively.

Ultrasonic emitters.

In nature, US is found both as a component of many natural noises (in the noise of wind, waterfall, rain, in the noise of pebbles rolled by the sea surf, in the sounds accompanying lightning discharges, etc.), and among the sounds of the animal world. Some animals use ultrasonic waves to detect obstacles, orientation in space.

Ultrasound emitters can be divided into two large groups. The first includes emitters-generators; oscillations in them are excited due to the presence of obstacles in the path of a constant flow - a jet of gas or liquid. The second group of emitters is electro-acoustic transducers; they convert already given fluctuations in electrical voltage or current into mechanical fluctuations solid body, which radiates environment acoustic waves.

An electromechanical ultrasound emitter uses the phenomenon of the inverse piezoelectric effect and consists of the following elements (Fig. 1)

Plates made of a substance with piezoelectric properties;

Electrodes deposited on its surface in the form of conductive layers;

A generator that supplies an alternating voltage of the required frequency to the electrodes.

When an alternating voltage is applied to the electrodes (2) from the generator (3), the plate (1) experiences periodic stretching and compression. Forced oscillations occur, the frequency of which is equal to the frequency of the voltage change. These vibrations are transmitted to the particles of the environment, creating a mechanical wave with the appropriate frequency. The amplitude of oscillations of particles of the medium near the radiator is equal to the amplitude of oscillations of the plate.

The peculiarities of ultrasound include the possibility of obtaining waves of high intensity even at relatively small oscillation amplitudes, since at a given amplitude the energy flux density is proportional to frequency squared.

I \u003d ρ ω 2 ʋ A 2 / 2 (1)

The limiting intensity of ultrasound radiation is determined by the properties of the material of the emitters, as well as the characteristics of the conditions for their use.

The intensity range during ultrasonic generation in the UHF region is extremely wide: from 10 -14 W/cm 2 to 0.1 W/cm 2 .

For many purposes, much higher intensities are needed than those that can be obtained from the surface of the emitter. In these cases, you can use focus.

Ultrasound receivers. Electromechanical ultrasonic receivers use the phenomenon of the direct piezoelectric effect.

In this case, under the action of an ultrasonic wave, oscillations of the crystal plate (1) occur, as a result of which an alternating voltage appears on the electrodes (2), which is recorded by the recording system (3).

In most medical devices, the generator of ultrasonic waves is simultaneously used as their receiver.

Properties of ultrasound that determine its use for diagnostic and therapeutic purposes (short wavelength, directivity, refraction and reflection, absorption, half-absorption depth)

The therapeutic effect of ultrasound is due to mechanical, thermal and chemical factors. Their joint action improves the permeability of membranes, dilates blood vessels, improves metabolism, which helps to restore the equilibrium state of the body. A dosed beam of ultrasound can be used to gently massage the heart, lungs and other organs and tissues.

a) short wavelength. Orientation. The ultrasonic wavelength is much shorter than the sound wavelength. Given that the wavelength λ=υ/ν , we find: for sound with a frequency of 1 kHz, the wavelength λ sound =1500/1000=1.5 m; for ultrasound with a frequency of 1 MHz, the wavelength λ uz \u003d 1500 / 1 000 000 \u003d 1.5 mm.

Due to the short wavelength, the reflection and diffraction of ultrasound occurs on objects smaller than for audible sound. For example, a body 10 cm in size will not be an obstacle for a sound wave with λ=1.5 m, but will become an obstacle for an ultrasonic wave with λ=1.5 mm. In this case, an ultrasonic shadow appears, therefore, in some cases, the propagation of ultrasonic waves can be depicted using rays and the laws of reflection and refraction can be applied to them. That is, under certain conditions, an ultrasonic wave propagates in a directed flow, to which the laws of geometric optics apply.

b) Refraction and reflection. As with all types of waves, the phenomena of reflection and refraction are inherent in ultrasound. The laws that these phenomena obey are completely analogous to the laws of reflection and refraction of light. Therefore, in many cases, the propagation of ultrasonic waves is depicted using rays.

For quantitative characteristics of the process, the concept of the reflection coefficient R=I ref /I o is introduced, where I ref is the intensity of the reflected ultrasonic wave; I about - the intensity of the incident. This is a dimensionless quantity that varies from zero (no reflection) to one (total reflection).

The more the wave resistances (ρυ) of the media differ, the more share reflected energy and the fraction of energy passing through the interface is smaller.

The wave resistance of biological media is approximately 3000 times greater than the wave resistance of air (R=1/3000), so the reflection at the boundary air-skin is 99.99%. If the emitter is applied directly to the human skin, then the ultrasound will not penetrate inside, but will be reflected from a thin layer of air between the emitter and the skin. To eliminate the air layer, the surface of the skin is covered with a layer of an appropriate lubricant (water jelly), which acts as a transition medium that reduces reflection.

The lubricant must meet the relevant requirements: have an acoustic resistance close to the acoustic resistance of the skin, have a low ultrasonic absorption coefficient, have a significant viscosity, wet the skin well, and be non-toxic (vaseline oil, glycerin, etc.).

c) Absorption, depth of half-absorption. Next important property ultrasound is its absorption in media: the energy of mechanical vibrations of the particles of the medium is converted into the energy of their thermal motion. The energy of the mechanical wave absorbed by the medium in this case causes heating of the medium. This effect is described by the formula:

I \u003d I o. e-cl (3)

where I is the intensity of the ultrasonic wave that has traveled the distance l in the medium; I o - initial intensity; k is the absorption coefficient of ultrasound in the medium; e is the base of natural logarithms (e = 2.71).

Along with the absorption coefficient, the half-absorption depth is also used as a characteristic of ultrasonic absorption.

The half-absorption depth is the depth at which the intensity of the ultrasound wave is halved.

The depth of half-absorption for different tissues has a different meaning. Therefore, for medical purposes, ultrasound waves of various intensities are used: small - 1.5 W / m 2, medium - (1.5-3) W / m 2 and large - (3-10) W / m 2.

Absorption in a liquid medium is much less than in soft tissues and even more so in bone tissue.

8. Interaction of ultrasound with matter: acoustic flows and cavitation, release of heat and chemical reactions, sound reflection, sound vision).

a) Acoustic flows and cavitation. Ultrasonic waves of high intensity are accompanied by a number of specific effects. Thus, the propagation of ultrasonic waves in gases and liquids is accompanied by the movement of the medium, acoustic flows (sonic wind) arise, the speed of which reaches 10 m/s. At frequencies in the UHF range (0.1-10) MHz in an ultrasonic field with an intensity of several W / cm 2, a liquid can be spouted and sprayed with the formation of a very fine mist. This feature of ultrasound propagation is used in ultrasonic inhalers.

Among the important phenomena that arise during the propagation of intense ultrasound in liquids is acoustic cavitation-growth in the ultrasonic field of bubbles from existing submicroscopic nuclei of gas or vapor in liquids up to fractions of a mm in size, which begin to pulsate with the frequency of ultrasound and collapse in the positive phase of pressure. When gas bubbles collapse, large local pressures of the order thousand atmospheres, spherical shock waves are formed. Such an intense mechanical action on particles can lead to various effects, including destructive ones, even without the influence of the thermal action of ultrasound. Mechanical effects are especially significant under the action of focused ultrasound.

Another consequence of the collapse of cavitation bubbles is a strong heating of their contents (up to a temperature of about 10,000 0 C), accompanied by ionization and dissociation of molecules.

The phenomenon of cavitation is accompanied by erosion of the working surfaces of the emitters, cell damage, etc. However, this phenomenon also leads to a number of beneficial effects. So, for example, in the area of cavitation, enhanced mixing of the substance occurs, which is used to prepare emulsions.

b) Heat release and chemical reactions. The absorption of ultrasound by a substance is accompanied by the transfer of mechanical energy into the internal energy of the substance, which leads to its heating. The most intense heating occurs in the regions adjacent to the interface between media, when the reflection coefficient is close to unity (100%). This is due to the fact that, as a result of reflection, the intensity of the wave near the boundary increases and, accordingly, the amount of absorbed energy increases. This can be verified experimentally. It is necessary to attach an ultrasound emitter to a wet hand. Soon, a sensation (similar to pain from a burn) occurs on the opposite side of the palm, caused by ultrasound reflected from the skin-air interface.

Tissues with a complex structure (lungs) are more sensitive to ultrasound heating than homogeneous tissues (liver). Relatively much heat is released at the border of soft tissues and bone.

Local heating of tissues by fractions of degrees contributes to vital activity biological objects, increases the intensity of metabolic processes. However, prolonged exposure may cause overheating.

In some cases, focused ultrasound is used for local effects on individual body structures. This effect allows you to achieve controlled hyperthermia, i.e. heating up to 41-44 0 C without overheating of neighboring tissues.

An increase in temperature and pressure drops that accompany the passage of ultrasound can lead to the formation of ions and radicals that can interact with molecules. In this case, such chemical reactions can occur that are not feasible under normal conditions. The chemical action of ultrasound is manifested, in particular, by the splitting of a water molecule into H + and OH - radicals, followed by the formation of hydrogen peroxide H 2 O 2 .

c) Reflection of sound. Sound vision. Based on the reflection of ultrasonic waves from inhomogeneities sound vision, used in medical ultrasound. In this case, the ultrasound reflected from the inhomogeneities is converted into electrical vibrations, and the latter into light vibrations, which makes it possible to see certain objects on the screen in a medium opaque to light.

An ultrasonic microscope has been created at frequencies in the UHF range - a device similar to an ordinary microscope, the advantage of which over an optical one is that biological studies do not require preliminary staining of the object. With an increase in the frequency of the ultrasonic wave, the resolution increases (smaller inhomogeneities can be detected), but their penetrating power decreases, i.e. the depth at which structures of interest can be explored decreases. Therefore, the ultrasound frequency is chosen so as to combine sufficient resolution with the required depth of investigation. So, for ultrasound examination of the thyroid gland located directly under the skin, 7.5 MHz waves are used, and for the examination of the abdominal organs, a frequency of 3.5 - 5.5 MHz is used. In addition, the thickness of the fat layer is also taken into account: for thin children, a frequency of 5.5 MHz is used, and for overweight children and adults, a frequency of 3.5 MHz.

9. Biophysical effect of ultrasound: mechanical, thermal, physical and chemical.

Under the action of ultrasound on biological objects in irradiated organs and tissues at distances equal to half the wavelength, pressure differences from units to tens of atmospheres can occur. Such intense impacts lead to a variety of biological effects, the physical nature of which is determined by the joint action mechanical, thermal and physico-chemical phenomena accompanying the propagation of ultrasound in the medium.

mechanical action is determined by variable acoustic pressure and consists in vibrational micromassage of tissues at the cellular and subcellular levels, an increase in the permeability of cellular, intracellular and tissue membranes due to the depolymerizing effect of ultrasound on hyaluronic acid and chondroitin sulfate, which entails an increase in hydration of the dermal layer.

thermal effect is associated with the transformation of mechanical energy into thermal energy, while heat is generated unevenly in the tissues of the body. Especially a lot of heat accumulates at the boundaries of the media due to the difference in the acoustic resistance of tissues, as well as in tissues that absorb ultrasonic energy in greater quantities (nervous, bone tissue), and in places that are poorly supplied with blood.

Physical and chemical action due to the fact that chemical energy causes mechanical resonance in the tissues of the body. Under the influence of the latter, the movement of molecules is accelerated and their decomposition into ions intensifies, the isoelectric state changes. New electric fields are formed, electrical changes occur in the cells. The structure of water and the state of hydration shells change, radicals and various products of sonolysis of biological solvents appear. As a result, stimulation of physicochemical and biochemical processes in tissues, activation of metabolism occurs.

1. Piezoelectric effect.

In some crystals, polarization can occur even without an external field if the crystal is subjected to mechanical deformations. This phenomenon, discovered in 1880 by Pierre and Jacques Curie, was called the piezoelectric effect.

To detect piezoelectric charges, metal plates are applied to the edges of the crystal plate. When the plates are open, a potential difference appears between them during deformation. When the plates are closed, induced charges are formed on them, equal in magnitude to the polarization charges, but opposite in sign, and in the circuit connecting the plates, a current appears in the process of deformation. Consider the main features of the piezoelectric effect on the example of quartz. SiO2 quartz crystals exist in various crystallographic modifications. The crystals of interest to us (a-quartz) belong to the so-called trigonal crystallographic system and usually have the shape shown in Fig.1. They resemble a hexagonal prism bounded by two pyramids, but have a number of additional faces. Such crystals are characterized by four crystal axes that define important directions within the crystal.

One of these axes - Z connects the tops of the pyramids. Three other X1, X2, X3 are perpendicular to the Z axis and connect the opposite edges of the hexagonal prism. The direction defined by the Z-axis is piezoelectrically inactive: when compressed or stretched along this direction, no polarization occurs. On the contrary, when compressed or stretched in any direction perpendicular to the Z axis, electric polarization occurs. The Z axis is called the optical axis of the crystal, and the X1, X2, X3 axes are called the electrical or piezoelectric axes.

Consider a quartz plate cut perpendicular to one of the piezoelectric axes X. The axis perpendicular to Z and X will be denoted by Y (Fig. 2). Then it turns out that when the plate is stretched along the X axis, opposite polarization charges appear on the faces ABCD and EFGH perpendicular to it. This piezoelectric effect is called longitudinal. If we change the sign of the deformation, i.e., go from tension to compression, then the signs of the polarization charges will also change to the opposite ones.

Rice. 1. Quartz crystal.

The appearance of polarization charges of certain signs for a given type of deformation (tension or, respectively, compression) shows that the ends of the X axes are unequal, and certain directions can be attributed to the X axes (which is indicated in Fig. 1 by arrows). This means that for a given deformation, the sign of the charge depends on whether the X axis is directed along the outer normal to the face or along the inner one. Such axes with unequal ends are called polar axes. Unlike the polar axes X1, X2, X3, the ends of the Z axis are completely equal and it is a non-polar axis.

Rice. 2. Quartz plate cut perpendicular to the piezoelectric axis.

The unevenness of the ends of the polar axis manifests itself, of course, not only in the piezoelectric effect, but also in other phenomena. For example, the rate of chemical etching of faces located at different ends of the polar axis turns out to be different, and the resulting etching patterns differ from each other.

Along with the longitudinal piezoelectric effect, there is also a transverse piezoelectric effect. It consists in the fact that during compression or tension along the Y axis, polarization occurs along the X axis, and polarization charges appear on the same faces ABCD and EFGH. It turns out that the signs of the charges on each face in compression along Y (in the transverse effect) are the same as in tension along X (in the longitudinal effect).

The piezoelectric effect is explained as follows In ionic crystals, due to the mismatch between the centers of positive and negative ions, there is an electric moment even in the absence of an external electric field. However, this polarization usually does not manifest itself, since it is compensated by charges on the surface. When the crystal is deformed, the positive and negative ions of the lattice are displaced relative to each other, and therefore, generally speaking, the electric moment of the crystal changes. This change in the electric moment is manifested in the piezoelectric effect.

Rice. 3 qualitatively explains the occurrence of the piezoelectric effect in quartz. Here, projections of positive Si ions (dashed circles) and negative O ions (open circles) are shown schematically in a plane perpendicular to the optical Z axis. This figure does not correspond to the actual configuration of ions in a quartz unit cell, in which the ions do not lie in the same plane, their number is greater than shown. He, however, correctly conveys the symmetry relative position ions, which is already sufficient for a qualitative explanation.

Rice. 3a) corresponds to an undeformed crystal. On face A, perpendicular to the X1 axis, there are protruding positive charges, and on face B, parallel to it, there are protruding negative charges. Under compression along the X1 axis (Fig. 3b), the unit cell is deformed. In this case, positive ion 1 and negative ion 2 are “pressed” into the cell, which causes the protruding charges (positive on plane A and negative on plane B) to decrease, which is equivalent to the appearance of a negative charge on plane A and a positive charge on plane B. When stretched along the axis X1, the opposite occurs (Fig. 3c): ions 1 and 2 are “pushed out” of the cell. Therefore, an additional positive charge arises on face A, and a negative charge appears on face B.

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Rice. 3. To an explanation of the piezoelectric effect.

Calculations in solid state theory, in agreement with experience, show that the piezoelectric effect can exist only in crystals in which the unit cell does not have a center of symmetry. For example, the unit cell of CsCl crystals (Fig. 4) has a center of symmetry, and these crystals do not exhibit piezoelectric properties. The arrangement of ions in a quartz cell is such that there is no center of symmetry in it, and therefore a piezoelectric effect is possible in it.

Rice. 4. Unit cell of a cesium chloride CsCl crystal.

The value of the polarization vector Р (and the surface density of piezoelectric charges proportional to it o ") in a certain range of changes is proportional to the magnitude of mechanical deformations. Let us denote the deformation of one-sided stretching along the X axis by and:

where d is the thickness of the plate, and Dd is its change during deformation. Then, for example, for the longitudinal effect we have:

The value b is called the piezoelectric modulus. The b sign can be either positive or negative. Since it is a dimensionless quantity, b is measured in the same units as P, i.e. in C/m2. Value surface density piezoelectric charges on the faces perpendicular to the X axis is s "=Px

Due to the occurrence of piezoelectric polarization, the electric displacement D inside the crystal also changes during deformation. In this case, in the general definition of displacement, P should be understood as the sum of Pe + Pu, where Pe is due to the electric field, and Pu is the deformation. In the general case, the directions of E, Pe, and Pu do not coincide, and the expression for D turns out to be complicated. However, for some directions coinciding with the axes of high symmetry, the directions of these vectors turn out to be the same. Then for the offset value we can write:

where E is the electric field strength inside the crystal, and e is the permittivity at constant strain. The relation is valid, for example, for one-sided tension (compression) deformation along one of the electric axes X. It is one of the two main relations in the theory of piezoelectricity (the second relation is given).

The piezoelectric effect occurs not only in the case of unilateral tensile deformation, but also in shear deformations.

Piezoelectric properties are observed, in addition to quartz, in a large number of other crystals. Much stronger than in quartz, they are expressed in Rochelle salt. Strong piezoelectrics are crystals of compounds of elements of the 2nd and 6th groups periodic system(CdS, ZnS), as well as many other chemical compounds.

2. Reverse piezoelectric effect.

Along with the piezoelectric effect, there is also an opposite phenomenon: in piezoelectric crystals, the occurrence of polarization is accompanied by mechanical deformations. Therefore, if an electric voltage is applied to the metal plates fixed on the crystal, then the crystal is polarized and deformed under the action of the field.

It is easy to see that the need for the existence of an inverse piezoelectric effect follows from the law of conservation of energy and the existence of a direct effect. Consider a piezoelectric plate (Fig. 5) and assume that we compress it with external forces F. If there were no piezoelectric effect, then the work of external forces would be equal to the potential energy of an elastically deformed plate. In the presence of a piezoelectric effect, charges appear on the plate and an electric field arises, which contains additional energy. According to the law of conservation of energy, it follows that when the piezoelectric plate is compressed, a lot of work is done, which means that additional forces F1 appear in it, which counteract the compression. This is the force of the inverse piezoelectric effect. From the above reasoning, a connection between the signs of both effects follows. If in both cases the signs of the charges on the faces are the same, then the signs of the deformations are different. If, when the plate is compressed, the charges indicated in Fig. 5, then when the same polarization is created by an external field, the plate will stretch.

Fig.5. Relationship between direct and reverse piezoelectric effects.

The inverse piezoelectric effect has an outward resemblance to electrostriction. However, these two phenomena are different. The piezoelectric effect depends on the direction of the field and when the direction of the field changes to the opposite, it changes sign. Electrostriction does not depend on the direction of the field. The piezoelectric effect is observed only in some crystals that do not have a center of symmetry. Electrostriction occurs in all dielectrics, both solid and liquid.

If the plate is fixed and cannot be deformed, then when an electric field is created, an additional mechanical stress will appear in it. Its value s is proportional to the electric field strength inside the crystal:

where b is the same piezoelectric modulus as in the case of the direct piezoelectric effect. The minus in this formula reflects the ratio of the signs of the direct and inverse piezoelectric effects indicated above.

The total mechanical stress inside the crystal is the sum of the stress caused by the deformation and the stress caused by the electric field. It is equal to:

Here C is the modulus of elasticity under unilateral tension deformation (Young's modulus) at a constant electric field. Formulas (51.2) and (52.2) are the main relations in the theory of piezoelectricity.

When writing formulas, we chose u and E as independent variables and considered D and s to be their functions. This, of course, is not necessary, and we could consider as independent variables another pair of quantities, one of which is mechanical and the other electrical. Then we would also get two linear relations between u, s, E and D, but with different coefficients. Depending on the type of problems under consideration, various forms of writing the main piezoelectric relations are convenient.

Since all piezoelectric crystals are anisotropic, the constants e, C, and b depend on the orientation of the plate faces relative to the crystal axes. In addition, they depend on whether the side faces of the plate are fixed or free (they depend on the boundary conditions during deformation). To give an idea of the order of magnitude of these constants, we present their values for quartz in the case when the plate is cut perpendicular to the X axis and its side faces are free:

e=4, 5; C=7.8 1010 N/m2; b=0.18 C/m2.

Let us now consider an example of applying the basic relations (4) and (5) Let us assume that a quartz plate, cut out as indicated above, is stretched along the X axis, and the plates touching the faces are open. Since the charge of the plates before deformation was equal to zero, and quartz is a dielectric, then after deformation the plates will be uncharged. According to the definition of electrical displacement, this means that D=0. Then from relation (4) it follows that, during deformation, an electric field with strength will appear inside the plate:

Substituting this expression into formula (5), we find for the mechanical stress in the plate:

s=Cu-b(-(b/e0e)u)=C(1+(b2/e0eC))u (7)

The stress, as in the absence of the piezoelectric effect, is proportional to the strain. However, the elastic properties of the plate are now characterized by the effective modulus of elasticity

C" == C (1 + b2/e0eC). (8)

which is greater than C. The increase in elastic rigidity is caused by the appearance of an additional stress during the inverse piezoelectric effect, which prevents deformation. The influence of the piezoelectric properties of the crystal on its mechanical properties is characterized by the value: K2=b2/e0eC (9)

The square root of this value (K) is called the electromechanical coupling constant. Using the above values of e, C and b, we find that for quartz K2 ~ 0.01 For all other known piezoelectric crystals, K2 is also small compared to unity and does not exceed 0.1 .

Let us now estimate the magnitude of the piezoelectric field. Let us assume that a mechanical stress of 1 1055 N/m2 is applied to the faces of a quartz plate perpendicular to the X axis. Then, according to (7), the deformation will be equal to u=1, 3 10-6. Substituting this value into formula (6), we obtain |E|==5900 V/m=59 V/cm. With a plate thickness of, say, d==0.5 cm, the voltage between the plates will be equal to U=Ed~30 V. We see that piezoelectric fields and voltages can be quite significant. By using stronger piezoelectrics instead of quartz and using properly chosen types of deformation, piezoelectric voltages measuring many thousands of volts can be obtained.

The piezoelectric effect (direct and reverse) is widely used for the construction of various electromechanical transducers. For this, composite piezoelements are sometimes used, designed to carry out deformations of various types.

Figure 6 shows a double piezoelectric element (composed of two plates) working in compression. The plates are cut from the crystal in such a way that they are either compressed or stretched at the same time. If, on the contrary, such a piezoelectric element is compressed or stretched by external forces, then tension appears between its plates. The connection of the plates in this piezoelectric element corresponds to the parallel connection of capacitors.

Fig.6. Double piezo element working in compression.

3. Use of the piezoelectric effect in science and technology.

The main part of any equipment for sounding an acoustic musical instrument is a piezo transducer (Transducer). This detail transforms mechanical vibrations strings and soundboards into an electrical signal.

A similar function in an electric guitar is performed by a magnetic pickup: single or humbucker. But the physics of the operation of an electric guitar pickup is different - it converts the changes in the magnetic field introduced by steel strings. Piezo pickup for acoustics works with any strings, including synthetic ones. The piezo pickup is placed under the bone of the guitar (the plate on which the strings rest). This is a UST sensor

There is another way to place the piezo sensor - it is glued to the guitar deck (from the inside, closer to the stand). The signal from such a sensor will be weaker, because the strings do not press it, it receives only soundboard vibrations. However, he has more information about the properties of the body of the guitar. This sensor is called AST (1470).

The combination of signals from UST and AST gives a very complex and interesting picture and allows you to realistically sound the instruments of the highest class. However, it is not always necessary to use two sensors.

Piezoelectric transducers:

Piezoelectrics are reversible electromechanical transducers, that is, they are capable of converting mechanical energy into electrical energy and, conversely, electrical energy into mechanical energy. Converters based on the use of the direct piezoelectric effect are called generator converters; they have a mechanical input and an electrical output. Converters based on the use of the reverse piezoelectric effect are called motor converters; they have electrical input and mechanical outputs. There are many piezoelectric devices based on the use of both direct and reverse effects. The direct effect is used, for example, in microphones, sound pickups, sensors of mechanical forces, displacements and accelerations, household gas lighters, etc. The reverse effect served as the basis for the creation of telephones, loudspeakers, ultrasonic emitters, relays, motors, etc.

known and found practical use piezoelectric transducers - piezoelectric transformers (abbreviated as piezotransformers). Schematically, the device of the piezotransformer is shown in the figure explaining that it is a piezoelectric transducer in the form of a four-terminal circuit with only electrical input and output.

Rice. 7 Piezoelectric transformer

The action of the piezotransformer is based on the use of both direct and reverse piezoelectric effects. The electrical voltage applied to the input electrodes of the piezotransformer, as a result of the inverse piezoelectric effect, causes deformation of the entire volume of the piezoelectric material and an electrical (secondary) voltage appears on the output electrodes as a result of the direct piezoelectric effect. In a piezotransformer, there is, as it were, a double conversion of energy - electrical into mechanical, and then mechanical into electrical. Like an electromagnetic transformer, a piezotransformer is used to convert electrical voltage. By selecting the size of the electrodes and their location, you can get different values of the transformation ratio. Piezotransformers are usually used in the resonant mode, in which large values of the transformation ratio (on the order of several hundred) are achieved. Piezotransformers are used in high-voltage secondary power supplies.

Let us consider in general terms the phenomena occurring in a piezoelectric for two cases of piezoelectric energy conversion.

Piezoelectric element (PE) - a piezoelectric body of a certain size, geometric shape and orientation relative to the main crystallographic axes (or the direction of polarization in the case of piezoceramics, having conductive plates (electrodes).

Rice. 8 Piezoelectric element: 1 - piezoelectric plate; 2 - electrodes made of conductive material, superimposed on the edges of the plate

Thus, the piezoelectric element is an electrical capacitor with a solid (crystalline or ceramic) dielectric. A feature of such a capacitor is the presence of piezoelectric properties in the dielectric that fills the space between the electrodes. Below it will be shown how important the presence of the piezoelectric effect is and how it affects the electrical and mechanical characteristics of the piezoelectric element. If the piezoelectric element is used as an electromechanical transducer, then its orientation is chosen based on the requirements for achieving the greatest effect. External forces (both mechanical and electrical) acting on a piezoelectric element can be either distributed or concentrated. Distributed forces allow a more efficient conversion to be achieved. Therefore, for more efficient polarization of the piezoelectric volume, electrodes are used. covering the entire area of the faces of the piezoelectric element, and to create a uniformly distributed mechanical stress - linings of an elastic material that fit well to the edges of the piezoelectric element and convert external concentrated forces into distributed ones.

An external force causes deformation of the piezoelectric element, its polarization and the appearance of opposite electric charges on the electrodes. The magnitude of the electric charge or the resulting voltage can be measured by the corresponding measuring device attached to the electrodes of the piezoelectric element. The external force imparts energy to the piezoelectric element in the form of elastic deformation, which can be calculated if the magnitude of the acting force and the stiffness of the piezoelectric element are known. Simultaneously with the deformation of the piezoelectric element, an electrical voltage appears on its electrodes. Consequently, part of the energy imparted to the piezoelectric element by an external force turns out to be electrical and its value can be calculated if the electrical voltage on the electrodes and the capacitance of the piezoelectric element are known.

The external mechanical force acting on the piezoelectric element informs the latter of the energy W0 in the form of the energy of elastic deformation and the energy of the charge of the capacitance of the piezoelectric element. If we denote the energy of elastic deformation of the piezoelectric element as Wm, and the electric energy of the charge of its capacitance as We, then total energy W0 reported to the piezoelectric element will be equal to their sum. As in any reversible transducer, in this case, a reverse action (piezoelectric reaction) occurs, which consists in the fact that the electrical voltage that has arisen as a result of the direct piezoelectric effect creates (already as a result of the inverse piezoelectric effect) mechanical stresses and deformations that counteract external forces. This is manifested in an increase in the rigidity of the piezoelectric element. If the electrical voltage arising due to the piezoelectric effect is eliminated, for example, by shorting the electrodes of the piezoelectric element, then the reverse piezoelectric action will not be observed, therefore, a decrease in the rigidity of the piezoelectric element should occur.

Similar reasoning can be done for the case of the inverse piezoelectric effect, i.e., the impact on the piezoelectric element of an external electrical force. In this case, an external source of electrical energy imparts energy to the piezoelectric element in the form of the energy of the charge of the capacitance of the piezoelectric element and the mechanical energy of its elastic deformation. Here, too, the reverse action takes place. If deformation is prevented by rigid clamping of the piezoelectric element, then a change in its capacitance can be detected. This fact is easily observed in strong piezoelectrics, while for weak ones, such as quartz, the change in capacitance is small (about 1%). It is easy to come to this conclusion, taking into account thermodynamic considerations. It is known from the theory of piezoelectricity that the elastic coefficients of piezoelectrics depend on electrical conditions, just as their permittivity coefficients depend on mechanical conditions. This is natural, since piezoelectricity, by definition, implies a relationship between elastic and dielectric properties. Therefore, the description of the piezoelectric properties of the material is impossible without the use of elastic and dielectric coefficients, indicating the boundary mechanical and electrical conditions.

The piezoelectric effect is more fully characterized by the energy coefficient u, called the electromechanical coupling coefficient (EMC) and defined by the ratio k = We / W0 = Wm / W0, where W0 is the entire energy applied to the piezoelectric element, and We and Wm are the converted (electrical and mechanical) energy. The EMC coefficient is very useful for comparing piezoelectric materials whose piezoelectric, elastic and dielectric coefficients can differ significantly. This coefficient is different for static and dynamic conversion modes, in the latter case it also depends on the type and mode of oscillation. The EMC coefficient, like piezoelectric modules, depends on the direction of the acting forces relative to the crystallographic axes of the crystal. It determines such an essential characteristic of the resonator as the relative width of the resonance curve. The larger the EMC coefficient, the larger the relative width of the resonant curve. The energy conversion of a piezoelectric element cannot be complete, so the EMC coefficient cannot be greater than 1. For the so-called weak piezoelectrics, to which quartz belongs, the EMC coefficient does not exceed a few percent, for strong piezoelectrics, such as Rochelle's salt or piezoceramics, it can reach 50 ...90%.

Various applications:

US Patent N3239283. American inventors J. Broz and W. Lauberdorfer developed a bearing design in which friction is destroyed by vibration, but no special mechanisms are required to create it. Bearing bushings are made of piezoelectric material. The current causes the piezoelectric material to contract and expand, creating a vibration that destroys friction.

The installation of piezoelectric transducers on jet aircraft saves almost a third of the fuel that was used to generate electricity, therefore, it allows to increase the flight range. Here, vibrations and vibrations of the fuselage and wings are directly converted into electricity.

Philips is successfully developing the idea of a piezoelectric drive for low power mechanisms. In particular, she created a traffic light, the batteries of which are charged by the noise of cars at the intersection.

There is talk of creating soundproof partitions for apartment buildings from piezoelectric materials. Here, the double effect is both noise absorption and electricity generation, for example, for heating apartments.

Piezoelectric inkjet printing. Piezoelectric inkjet heads for printers were developed in the seventies. In most of these printers, the ink chamber is pressurized by a piezoelectric disc that changes shape (bent) when an electrical voltage is applied to it. Curving, the disk, which serves as one of the walls of the ink chamber, reduces its volume. Under the action of excess pressure, liquid ink is emitted from the nozzle in the form of a drop.

The piezoelectric microphone, designed by Soviet scientists S. N. Rzhevkin and A. I. Yakovlev in 1925, has a plate of a substance with piezoelectric properties as a sound pressure sensor. Sound waves fall on the microphone's piezocrystal and compress it. With the help of a piezocrystal, the energy of sound waves is converted into weak electricity. This small current is then fed to the amplifier, which makes it strong enough to provide normal work loudspeaker. Working as a pressure sensor allowed the creation of the first hydrophones and the recording of ultra-low frequency sounds characteristic of marine life.

Lighter household piezoelectric ZP-1 "Tolne". The lighter is designed to ignite gas in the burners of household gas appliances. The spark source is a piezoelectric element. By pressing the key, the compression force is transmitted to the piezoelectric elements, as a result of which a spark occurs between the contacts located inside the metal nozzle, worn on the elongated end of the piezo lighter. The spark that ignites the gas is generated both when the key is pressed and when it is released.

Piezoelectric emitters are used to generate ultrasound with frequencies up to 50 MHz. The main element of the piezoelectric emitter is a piezoelectric plate, which, due to the inverse piezoelectric effect, performs forced mechanical oscillations in an alternating electric field.

Bibliography

“Electricity” S.G. Kalashnikov, Moscow, 1977

“Electrotechnical materials” Yu.V. Koritsky, Moscow, 1968

“Radio transmitting devices” G.A. Zeitlenka, Moscow, 1969

http://www.terralab.ru/299680/?r1=rss&r2=remote;

http://www.b-band.ru/pieza.html;

The piezoelectric effect (Greek piezo - pressure and electricity) is a phenomenon that characterizes the occurrence of electric polarization (induction) under the action of mechanical stresses or the occurrence of deformation under the influence of an electric field in certain substances (piezocrystals). If a piezoelectric plate, cut in a certain way, is subjected to mechanical stresses (compression, tension, shear), then electric charges appear on its surface due to polarization - this is the so-called direct piezoelectric effect; when such a plate is placed in an electric field, its deformation arises, which depends linearly on the electric field strength - the inverse piezoelectric effect.
The mechanism of the direct piezoelectric effect is explained by the appearance or change in the dipole moment of the unit cell of the crystal lattice as a result of the displacement of charges under the action of mechanical stresses. Under the action of an electric field on elementary charges in a cell, their displacement occurs and, as a result, a change in the average distances between them, i.e. deformation (reverse piezoelectric effect).
The piezoelectric effect was discovered in 1880 by the brothers P. and J. Curie, who observed it in quartz and some other crystals.
A necessary condition for the existence of the piezoelectric effect is the absence of a center of symmetry in the crystal. Only in this case, the application of voltages can lead to the appearance of an uncompensated electric charge, i.e. to the occurrence of polarization. Piezoelectrics are quartz, tourmaline, senget salt, barium titanate, potassium dihydrogen phosphate, antimony sulfoiodide, potassium sulfide, etc. It is also inherent in human bones.
The principle of the direct piezoelectric effect is used in the manufacture of ultrasonic vibration receivers. The reverse piezoelectric effect is used to produce ultrasound, and all therapeutic ultrasound devices are based on this effect. The essence of obtaining ultrasound is as follows. If an alternating electrical voltage is applied to the end surfaces of a piezocrystal plate cut in a certain way using electrodes, then its thickness will alternately decrease in accordance with the frequency of the alternating current. With a decrease in the thickness of the plate in the surrounding layers of the environment, a rarefaction is formed, and with its increase, a thickening of the particles of the medium. As a result of a periodic change in the thickness of the plate, called a piezoelectric transducer, an ultrasonic wave arises in the medium, propagating in a direction perpendicular to the surface of the plate. The change in the thickness of the plates of piezoelectric crystals is very small, it is proportional to the applied electrical voltage: AS = L U, where AS is the change in the dimensions of the plate: L is the piezoelectric module: U is the applied voltage.
In order to increase the intensity of ultrasonic vibrations, the phenomenon of resonance is used, which requires taking into account the frequency of natural vibrations of the substance. If the frequency of the alternating voltage applied to the piezocrystal coincides with its own (resonant) frequency, then the amplitude of the oscillation of the plate will be the largest. Accordingly, the intensity of ultrasonic waves propagating into the environment will also be maximum. In turn, the resonant frequency of the plate depends on its size: the thinner the plate, the greater its resonant frequency. For example, for a quartz plate with a thickness of 1 mm, the resonant frequency corresponds to 2.88 MHz, and for a thickness of 0.5 mm - 5.76 MHz.
Previously, quartz plates were used as a piezoelectric element in ultrasonic therapeutic devices. Today it is being replaced by barium titanate ceramics, which have many times the piezoelectric effect.

In 1756, the Russian academician F. Epinus discovered that when a tourmaline crystal is heated, electrostatic charges appear on its faces. Subsequently, the atom phenomenon was given the name of the pyroelectric effect. F. Aepinus assumed that the reason for the electrical phenomena observed when the temperature changes is the uneven heating of two surfaces, leading to the appearance of mechanical stresses in the crystal. At the same time, he pointed out that the constancy in the distribution of poles at certain ends of the crystal depends on its structure and composition, thus F. Aepinus came close to the discovery of the piezoelectric effect.

The piezoelectric effect in crystals was discovered in 1880 by the brothers P. and J. Curie, who observed the appearance on the surface of plates cut in a certain orientation from a quartz crystal, electrostatic charges under the action of mechanical stresses. These charges are proportional to the mechanical stress, change sign with it, and disappear when it is removed. The formation of electrostatic charges on the surface of a dielectric and the occurrence of electric polarization inside it as a result of mechanical stress is called the direct piezoelectric effect.

Along with the direct one, there is an inverse piezoelectric effect, which consists in the fact that in a plate cut from a piezoelectric crystal, mechanical deformation occurs under the action of an electric field applied to it; moreover, the magnitude of the mechanical deformation is proportional to the electric field strength. The inverse piezoelectric effect should not be confused with the phenomenon of electrostriction, i.e., with the deformation of a dielectric under the action of an electric field. With electrostriction, there is a quadratic dependence between the deformation and the field, and with the piezoelectric effect, it is linear.

In addition, electrostriction occurs in a dielectric of any structure and occurs even in liquids and gases, while the piezoelectric effect is observed only in solid dielectrics, mainly crystalline ones.

Piezoelectricity appears only in those cases when the elastic deformation of the crystal is accompanied by a shift in the centers of gravity of the positive and negative charges of the unit cell of the crystal, i.e., when it causes an induced dipole moment, which is necessary for the occurrence of electric polarization of the dielectric under the action of mechanical stress. In structures with a center of symmetry, no uniform deformation can disturb the internal equilibrium of the crystal lattice and, therefore, only 20 classes that do not have a center of symmetry are piezoelectric. The absence of a center of symmetry is a necessary but not sufficient condition for the existence of the piezoelectric effect, and therefore not all acentric crystals have it.

The piezoelectric effect cannot be observed in solid amorphous and cryptocrystalline dielectrics, since this contradicts their spherical symmetry. The exception is when they become anisotropic under the influence of external forces and thus partially acquire the properties of single crystals. The piezoelectric effect is also possible in some types of crystal textures.

Until now, the piezoelectric effect has not found a satisfactory quantitative description within the framework of modern atomic theory of the crystal lattice. Even for structures of the simplest type, it is impossible to even approximately calculate the order of the piezoelectric constants.

Each piezoelectric is an electromechanical transducer, therefore its important characteristic is the electromechanical coupling coefficient k. The square of this coefficient is the ratio of the energy manifested in mechanical form for a given type of deformation to the total electrical energy received at the input from the power source.

In many cases of piezoelectrics, their elastic properties are essential, which are described by elastic moduli c (Young's moduli Eyu) or reciprocals - elastic constants s.

When using piezoelectric elements as resonators, in some cases, a frequency coefficient is introduced, which is the product of the resonant frequency of the piezoelectric element and the geometric size that determines the type of vibration. This value is proportional to the speed of sound in the direction of propagation of elastic waves in the piezoelectric element. Currently, many substances (more than 500) are known to have exhibited piezoelectric activity. However, only a few find practical application.

2. Reverse piezoelectric effect.

Fig.5. Relationship between direct and reverse piezoelectric effects.

The total mechanical stress inside the crystal is the sum of the stress caused by the deformation and the stress caused by the electric field. It is equal to:

e=4, 5; C=7.8 1010 N/m2; b=0.18 C/m2.

Substituting this expression into formula (5), we find for the mechanical stress in the plate:

s=Cu-b(-(b/e0e)u)=C(1+(b2/e0eC))u (7)

The stress, as in the absence of the piezoelectric effect, is proportional to the strain. However, the elastic properties of the plate are now characterized by the effective modulus of elasticity

C" == C (1 + b2/e0eC). (8)

Fig.6. Double piezo element working in compression.

And also for metrological purposes. 3. The main criteria for evaluating non-contact vibration transducers To compare non-contact methods for measuring vibration parameters and vibration transducers based on them, it is advisable to use, in addition to the listed parameters, the following evaluation criteria: the nature of physical fields or radiation interacting during measurements; ...

Those. to protect the source from information leakage, it is required to violate the energy and time conditions for the existence of a leakage channel by using means of protection that differ in physical principles. Technical characteristics of the acousto-converting channel Acousto-electric transducer is a device that converts electromagnetic energy into the energy of elastic waves in the medium and vice versa. AT...

Raw mixture and reduces the stability of their crystal lattices and, therefore, accelerates the process of formation of the material. The study of the influence of nickel and copper additives on the density of piezoceramic blanks is presented in Fig. 1. 2. Density measurement results show that alloyed ceramics have higher density at all firing temperatures. So for ceramics with the addition of copper, the density is already at ...