Ferroelectrics are elements with spontaneous electric polarization (SEP). The initiators of its reversal can be applications of the electrical range E with appropriate parameters and direction vectors. This process is called repolarization. It is necessarily accompanied by hysteresis.
Common features
Ferroelectrics are components that have:
- Colossal permittivity.
- Powerful piezo module.
- Loop.
Ferroelectrics are used in many industries. Here are some examples:
- Radio engineering.
- Quantum electronics.
- Measuring technology.
- Electric acoustics.
Ferroelectrics are solids that are not metals. Their study is most effective when their state is single crystal.
Bright specifics
There are only three of these elements:
- Reversible polarization.
- Nonlinearity.
- Anomalous characteristics.
Many ferroelectrics cease to be ferroelectric when they are intemperature transition conditions. Such parameters are called TK. Substances behave abnormally. Their dielectric constant develops rapidly and reaches solid levels.
Classification
She's quite complex. Usually its key aspects are the design of the elements and the technology of formation of the SEP in contact with it during the change of phases. Here there is a division into two types:
- Having an offset. Their ions shift during phase movement.
- Order is chaos. Under similar conditions, the dipoles of the initial phase are ordered in them.
These species also have subspecies. For example, biased components fall into two categories: perovskites and pseudo-ilmenites.
The second type has a division into three classes:
- Potassium dihydrogen phosphates (KDR) and alkali metals (e.g. KH2AsO4 and KH2 PO4 ).
- Triglycine sulfates (THS): (NH2CH2COOH3)×H 2SO4.
- Liquid crystal components
Perovskites
These elements exist in two formats:
- Monocrystalline.
- Ceramic.
They contain an oxygen octahedron, which contains a Ti ion with a valence of 4-5.
When the paraelectric stage occurs, the crystals acquire a cubic structure. Ions like Ba and Cd are concentrated at the top. And their oxygen counterparts are positioned in the middle of the faces. This is how it is formedoctahedron.
When titanium ions change here, SEP is performed. Such ferroelectrics can create solid mixtures with formations of a similar structure. For example, PbTiO3-PbZrO3 . This results in ceramics with suitable characteristics for devices such as varicondas, piezo actuators, posistors, etc.
Pseudo-ilmenites
They differ in rhombohedral configuration. Their bright specificity is high Curie temperature indicators.
They are also crystals. As a rule, they are used in acoustic mechanisms on the upper large waves. The following devices are characterized by their presence:
- resonators;
- filters with stripes;
- high frequency acousto-optic modulators;
- pyro receivers.
They are also introduced into electronic and optical non-linear devices.
KDR and TGS
Ferroelectrics of the first designated class have a structure that arranges protons in hydrogen contacts. The SEP occurs when all the protons are in order.
Elements of this category are used in non-linear optical devices and in electrical optics.
In ferroelectrics of the second category, protons are ordered similarly, only dipoles are formed near glycine molecules.
The components of this group are used to a limited extent. Usually they contain pyro receivers.
Liquid crystal views
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 outer electric spectrum.
Based on these factors, the use of ferroelectrics of this type is implemented in optical sensors, monitors, banners, etc.
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 polycrystal format provided that the ceramic system is operated.
The latter easily dissolve in water and have negligible strength. They allow the formation of single crystals of solid parameters from aqueous compositions.
Domains
Most characteristics of ferroelectrics depend on domains. Thus, the switching current parameter is closely related to their behavior. They are found 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 only differences from a similar vector in neighboring sectors.
Domains separate walls that can move in the internal space of a single crystal. In this case, there is an increase in some and a decrease in other domains. When there is a repolarization, the sectors develop due to the displacement of the walls or similar processes.
Electrical properties of ferroelectrics,which are single crystals, are formed based on the symmetry of the crystal lattice.
The most profitable 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. At the same time, it is opposite in direction to the identical vector from the side of the nearest domain.
Consequently, the electrical parameters of the domains are formed on the basis of the head-tail scheme. Linear values of domains are determined. They are in the range 10-4-10-1 see
Polarization
Due to the external electric field, the vector of electric actions of domains changes. Thus, a powerful polarization of ferroelectrics arises. As a result, the dielectric constant reaches huge values.
The polarization of domains is explained by their origin and development due to the shift of their boundaries.
The indicated structure of ferroelectrics causes an indirect dependence of their induction on the degree of voltage of the external field. When it is weak, the relationship between the sectors is linear. A section appears where the domain limits are shifted according to a reversible principle.
In the zone of powerful fields, such a process is irreversible. At the same time, the sectors for which the SEP vector forms the minimum angle with the field vector grow. And at a certain tension, all domains line up exactly along the field. Technical saturation is being formed.
Under such conditions, when the tension is reduced to zero, there is no similar reversal of induction. She isgets the residual Dr. If it is affected by a field with an opposite charge, it will rapidly decrease and change its vector.
The subsequent development of tension again leads to technical saturation. Thus, the dependence of the ferroelectric on polarization reversal in varying spectra is denoted. In parallel with this process, hysteresis occurs.
The intensity of the range Er, at which induction follows through the zero value, is the 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 arrangement of domains.
A hysteresis loop forms here.
Its area corresponds to the energy expended in the ferroelectric in one cycle. Due to losses, the tangent of the angle 0, 1 is formed in it.
Hysteresis loops are created at different amplitude values. Together, their peaks form the main polarization curve.
Measuring operations
The dielectric constant of ferroelectrics of almost all classes differs in solid values even at values far from TK.
Its measurement is as follows: two electrodes are applied to the crystal. Its capacity is determined in a variable range.
Aboveindicators TK 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 the Curie constant. Below transitional values, it falls rapidly.
The letter "e" in the formula means non-linearity, which is present here in a fairly narrow spectrum with a shifting voltage. Because of it and the hysteresis, the permeability and volume of the ferroelectric depend on the operating mode.
Types of permeability
Material under different operating conditions of a non-linear component changes its qualities. The following types of permeability are used to characterize them:
- Statistical (est). To calculate it, the main polarization curve is used: est =D / (e0E)=1 + P / (e 0E) » P / (e0E).
- Reverse (ep). Denotes a change in the polarization of the ferroelectric in the variable range under the parallel influence of a stable field.
- Effective (eef). Calculated from the actual current I (implies non-sinusoidal type) going in conjunction with the non-linear component. In this case, there is an active voltage U and an angular frequency w. The formula works: eef ~ Cef =I / (wU).
- Initial. It is determined in extremely weak spectra.
Two main types of pyroelectrics
These are ferroelectrics and antiferroelectrics. They havethere are BOT sectors - domains.
In the first form, one domain forms a depolarizing sphere around itself.
When a lot of domains are created, it decreases. The energy of depolarization also decreases, but the energy of the sector walls increases. The process is completed when these indicators are in the same order.
What is the behavior of the HSE when ferroelectrics are in the outer sphere, was described above.
Antiferroelectrics – assimilation of at least two sublattices placed inside each other. In each, the direction of the dipole factors is parallel. And their common dipole index is 0.
In weak spectra, antiferroelectrics are distinguished by a linear type of polarization. But as the field strength increases, they can acquire ferroelectric conditions. Field parameters develop from 0 to E1. Polarization grows linearly. On the reverse movement, she is already moving away from the field - a loop is obtained.
When the strength of the range E2 is formed, ferroelectric is converted to its antipode.
When changing the field vector E, the situation is identical. This means the curve is symmetrical.
Antiferroelectric, exceeding the Curie mark, acquires paraelectric conditions.
With the lower approach to this point, the permeability reaches a certain maximum. Above it, it varies according to the Curie-Weiss formula. However, the absolute permeability parameter at the indicated point is inferior to that of ferroelectrics.
In many cases, antiferroelectrics havecrystalline structure akin to their antipodes. In rare situations and with identical compounds, but at different temperatures, phases of both pyroelectrics appear.
The most famous antiferroelectrics are NaNbO3, NH4H2P0 4 etc. Their number is inferior to the number of common ferroelectrics.
