Ferroelectrics are elements with spontaneous electrical polarization (BEP). The initiators of its circulation can be applications of the electric range E with suitable parameters and direction vectors. Such a process is called repolarization. Hysteresis necessarily accompanies it.
Common features
Ferroelectrics are components that are inherent in:
- Colossal dielectric constant.
- Powerful piezo module.
- The loop.
The use of ferroelectrics is carried out in many industries. Here are some examples:
- Radio engineering.
- Quantum Electronics.
- Measuring technique.
- Electric acoustics.
Ferroelectrics are non-metal solids. Their study is most effective when their state is single-crystal.
Bright specifics
These elements have only three:
- Reversible polarization.
- Nonlinearity.
- Abnormal characteristics.
Many ferroelectrics cease to be such when they are in temperature transition conditions. Such parameters are called T K. Substances behave abnormally. Their dielectric constant is rapidly developing and reaches solid indicators.
Classification
She's pretty complicated. Usually its key aspects are the design of the elements and the technology of the formation of the SES in contact with it when changing phases. There is a subdivision into two types:
- Having an offset. In phase motion, ions are shifted in them.
- Order is randomness. Under similar conditions, the dipoles of the initial phase are ordered in them.
These species also have subspecies. So, components with displacement are divided into two categories: perovskites and pseudo-ilmenites.
The second type has a division into three classes:
- Dihydrogen phosphates of potassium (KDR) and alkali metals (e.g. KH 2 AsO 4 and KH 2 PO 4 ).
- Triglycine sulfates (TGS): (NH 2 CH 2 COOH 3 ) × H 2 SO 4.
- Liquid crystal components
Perovskites
Such elements exist in two formats:
- Monocrystalline.
- Ceramic.
They contain an oxygen octahedron, which contains a Ti ion with a valency of 4-5.
When the paraelectric stage occurs, the crystals acquire a cubic structure. At the apex, ions of the Ba and Cd type are concentrated. And their oxygen counterparts are positioned in the middle of the faces. So the octahedron is formed.
When titanium ions change here, an SEP is carried out. Such ferroelectrics can create solid mixtures with structures of a similar structure. For example, PbTiO 3 -PbZrO 3 . Thanks to this, ceramics with suitable characteristics are obtained for such devices as variconds, piezo drives, posistors, etc.
Pseudo-menial
They differ in rhombohedral configuration. Their striking specificity is the high temperature Curie.
They are also crystals. As a rule, they are involved in acoustic mechanisms at the upper large waves. The following devices are characterized by their presence:
- resonators;
- filters with stripes;
- acousto-optical modulators of high frequencies;
- pyroeceivers.
They are also embedded in electronic and optical nonlinear devices.
KDR and TGS
Ferroelectrics of the first designated class have a structure that protons are ordered in hydrogen contacts. BOT occurs when all protons are arranged in order.
Elements of this category are used in nonlinear optical devices and in electrical optics.
In ferroelectrics of the second category, protons are ordered similarly, only a dipole is formed near glycine molecules.
The components of this group are used to a limited extent. Usually they contain pyro receivers.
Liquid crystal species
They are characterized by the presence of polar molecules, arranged in order. Here, the main specifics of ferroelectrics are clearly manifested.
Their optical qualities are affected by temperature and the vector of the external electrical spectrum.
Based on these factors, the use of ferroelectrics of this type is implemented in optical sensors, monitors, banners, etc.
The differences between the two classes
Ferroelectrics are formations with ions or dipoles. They have significant differences in their properties. So, the first components do not dissolve in water at all, but they have powerful mechanical strength. They are easily formed in the format of polycrystals, subject to the action of the ceramic system.
The latter are easily soluble in water and have negligible strength. They allow the formation of single crystals of solid parameters from aqueous compositions.
Domains
Most characteristics of ferroelectrics are domain dependent. So, the switching current parameter is closely related to their behavior. They are both in single crystals and in ceramics.
The domain structure of ferroelectrics is a sector of macroscopic dimensions. In it, the vector of arbitrary polarization has no discrepancies. And there are differences only from a similar vector in neighboring sectors.
Domains separate walls that are capable of shifting in the inner space of a single crystal. In this case, there is an increase in some and a decrease in other domains. When repolarization occurs, sectors develop due to wall displacement or similar processes.
The electrical properties of ferroelectrics, which are single crystals, are formed on the basis of the lattice symmetry of the crystals.
The most favorable energy structure is characterized by the fact that the domain boundaries in it are electrically neutral. Thus, the polarization vector is projected onto the boundary of a particular domain and is equal to its length. Moreover, it is opposite in direction to an identical vector from the side of the nearest domain.
Therefore, the electrical parameters of the domains are formed on the basis of the head-tail scheme. The linear values of the domains are determined. They are in the range of 10 -4 -10 -1 cm.
Polarization
Due to the external electric field, the vector of electric actions of the domains changes. Thus, a powerful polarization of ferroelectrics arises. As a result, the dielectric constant reaches enormous values.
The polarization of domains is explained by their origin and development due to a shift in their boundaries.
The indicated structure of ferroelectrics becomes the cause of the indirect dependence of their induction on the degree of voltage of the external field. When it is weak, the type of connection between sectors is linear. There is a plot on which domain limits are shifted according to a reversible principle.
In the zone of powerful fields, such a process is irreversible. At the same time, sectors grow in which the SES vector forms a minimum angle with the field vector. And with a certain tension, all domains are built exactly on the field. Formed technical saturation.
Under such conditions, with a decrease in tension to zero, there is no similar inversion of induction. She receives the residual indicator D r . If a field with an opposite charge acts on it, it will rapidly decrease and change its vector.
The subsequent development of tension again leads to technical saturation. Thus, the dependence of a ferroelectric on repolarization in varying spectra is indicated. In parallel with this process is hysteresis.
The tension range E p, at which induction follows through a zero value, is a coercive force.
Hysteresis process
With it, the domain boundaries are irreversibly shifted under the influence of the field. It means the presence of dielectric losses due to energy costs for the location of the domains.
A hysteresis loop is formed here.
Its area corresponds to the energy expended in ferroelectric per cycle. Due to losses, the tangent of an angle of 0.1 is formed in it.
At different amplitude indicators, hysteresis loops are created. Their vertices together form the main polarization curve.
Measuring operations
The dielectric constant of ferroelectrics of almost all classes differs in respectable values even at values remote from T K.
Its measurement is as follows: two electrodes are applied to the crystal. Its capacity in a variable range is determined.
Above T K, permeability has a certain thermal dependence. This can be calculated based on the Curie-Weiss law. The following formula works here:
e = 4pC / (T-Tc).
In it, C is a Curie constant. Below transitional values, it is rapidly falling.
The letter “e” in the formula means non-linearity, which is present here in a sufficiently narrow spectrum with a shifting voltage. Because of it and hysteresis, the permeability and volume of a ferroelectric depend on the operating mode.
Types of permeability
Material under different operating conditions of a nonlinear component changes its qualities. For their characteristics, the following types of permeability are used:
- Statistical (e article). For its calculation, the main polarization curve is used: e article = D / (e 0 E) = 1 + P / (e 0 E) »P / (e 0 E).
- Reversible (e p). Indicates a change in the polarization of a ferroelectric in an alternating range with the parallel influence of a stable field.
- Effective (e eff). It is calculated by the current current index I (the non-sinusoidal type is implied), which is coupled with a nonlinear component. In this case, there is an active voltage U and an angular frequency w. The formula works: e eff ~ C eff = I / (wU).
- Initial. It is determined in extremely weak spectra.
Two main types of pyroelectrics
Ferroelectrics and antiferroelectrics are considered as such. They have SES sectors - domains.
In the first form, one domain forms a depolarizing sphere around itself.
When many domains are created, it decreases. The energy of depolarization is also reduced, but the energy of the sector walls is increased. The completion of the process occurs when these indicators become in one order.
What is the behavior of the SES when the ferroelectrics are in the outer sphere, was described above.
Antiferroelectrics - the assimilation of at least two sublattices placed in each other. In each direction, the dipole factors are parallel. And their total dipole indicator is 0.
In weak spectra, antiferroelectrics differ in the linear type of polarization. But as the field strength increases, they can acquire the conditions of ferroelectrics. Field parameters range from 0 to exponent E 1. Polarization grows linearly. On the reverse movement, it is already moving away from the field - a loop is obtained.
When the tension of the range E 2 is formed, the ferroelectric is converted into its antipode.
When changing the field vector E, the situation is identical. This means the symmetry of the curve.
Antiferroelectric, exceeding the Curie mark, acquires paraelectric conditions.
With a lower approach to this point, permeability reaches a certain maximum. Above it, it varies according to the Curie-Weiss formula. However, the absolute parameter of permeability at the indicated point is inferior to that of ferroelectrics.
In many cases, antiferroelectrics have a crystalline structure related to their antipodes. In rare situations and with identical compounds, but at different temperatures, phases of both pyroelectrics appear.
The most famous antiferroelectrics are NaNbO 3, NH 4 H 2 P0 4 , etc. Their number is inferior to the number of common ferroelectrics.