What Are Ferroelectric Materials?

Ferroelectric materials refer to a class of materials with ferroelectric effects, which is a branch of pyroelectric materials. Research on ferroelectric materials and its applications has become one of the most popular research topics in the field of condensed matter physics and solid electronics. Crystals are due to their excellent performance. Many electro-optic crystals and piezoelectric materials are ferroelectric crystals. Ferroelectric crystals are of great significance both in technology and theory.

Piezoelectric materials: matter

As far back as ancient times, it has been known that certain materials have temperature-dependent self-generated dipole distances because they have the ability to attract other light and small objects when heated. In 1824 Brewster observed that many ores had pyroelectric properties ^[1]

All ferroelectric materials have both ferroelectricity and piezoelectricity. Ferroelectricity refers to the spontaneous polarization of materials in a certain temperature range. Because the positive and negative charge centers in the ferroelectric lattice do not coincide, an electric dipole moment can be generated even without an external electric field, and its spontaneous polarization can change direction under the action of an external electric field ^.

Many ferroelectric crystals were found, but they can be divided into two categories:

At present, according to the mechanism that generates sensing and driving functions, ferroelectric ceramics can be divided into 3 types

Layered ferroelectric ceramics

There are many studies, and lead zirconate titanate with perovskite structure is used to prepare ferroelectric ceramic materials.

Referred to as PZT series. The outstanding advantages of this series are that Pr with larger remanent polarization is about 10 ~ 35 C / cm ² and low heat treatment temperature (about 600 ° C). However, with the deepening of research, it has been found that the performance of the PZT series deteriorates after cumulative polarization inversion, which is mainly manifested in the occurrence of high leakage current and severe fatigue problems. In addition, the volatilization of lead is also harmful to the human body. Therefore, it is of great practical significance to research and develop lead-free ferroelectric ceramics with excellent performance. The bismuth-based layered perovskite structural material belongs to the ferroelectric material class, has good performance, and does not contain lead, so it has attracted widespread attention. The general formula of the material is (Bi ₂ O ₂ ) ²⁺ A _n-1 B _n O _{3n + 1} ) ^2- where A is +1, +2 or +3 valence and B is + 3, + 4 or + 5 valence Ions, n is the number of oxygen octahedral BO ₆ layers in the perovskite-like layer, in which the perovskite-like layer (A _n-1 B _n O _{3n + 1} ) ^2- and the bismuth oxygen layer (Bi ₂ O ₂ ) ²⁺ are alternately arranged . SrBi ₄ Ti ₄ O _15, abbreviated as SBTi, n = 4, n = 5 or n = 7, the ceramic is a bismuth-based layered perovskite structure ferroelectric ceramic material. The study found that the single crystal with larger remanent polarization has a polarization direction along the a or b axis 2Pr = 58C / cm2

[1] The thermal stability is also good, and the Curie temperature is 520 ° C.

[2] In addition, SBTi ceramic is a non-lead series material, which is a more promising ferroelectric ceramic material. However, since Bi is volatile, it is easy to form bismuth vacancies during the preparation and use of materials, thereby forming oxygen vacancies, which affects the fatigue resistance and ferroelectric properties of the material. In order to meet the needs of practical applications, the ferroelectric performance of this series of materials needs to be improved and improved. Therefore, researchers at home and abroad have done a lot of research in changing the preparation methods, preparation methods, and adjusting the composition of materials.

Relaxation type ferroelectric ceramics

Relaxation ferroelectrics (RF). Is the paraelectric-ferroelectric transition a class of ferroelectric materials that are dispersed phase transitions? It has both ferroelectric and relaxation phenomena. Compared with typical ferroelectrics, a typical characteristic of relaxed ferroelectrics is the complex dielectric constant, * () = '() "(), is the real part of the angular frequency, '() shows a relatively broad and gentle peak with temperature change, and the temperature Tm corresponding to its maximum ' () value moves to high temperature as increases. This feature is related to the transformation of structure glass The characteristics of spin glass transitions are very similar. Therefore, the relaxation type ferroelectric is also called polar glass, and the corresponding relaxation ferroelectric phase transition is also called polar glass transition. So far, although a lot of experimental measurements and theoretical explorations have been carried out on relaxed ferroelectric phase transitions, there is still no generally accepted model of relaxed ferroelectric phase transitions. So the research on the mechanism of relaxed ferroelectric phase transitions has been One of the hot issues in this field. In addition, some existing relax ferroelectrics have excellent ferroelectric, piezoelectric, and pyroelectric properties, so they have a wide range of important applications.

Therefore, optimizing the performance of existing relaxed ferroelectrics and the synthesis of new types of relaxed ferroelectrics will have important potential application value, and it is also another hot issue in this field. SrTiO3 is a non-polluting functional ceramic material, so new materials based on SrTiO3 have industrial advantages. It was found that the introduction of Bi ions into SrTiO3 produced a typical ferroelectric relaxation behavior, and the dielectric spectrum was measured, but the minimum measurement frequency was 100 Hz. Generally speaking, the characteristic time of glass transition is 50 ~ 102s, so the measurement of the dielectric spectrum of polar vitreous body in the lower frequency range is undoubtedly valuable for understanding its glass transition mechanism.

Antiferroelectric ceramics

In the late 1980s, research work on PZST antiferroelectric ceramics with large electrostrain and large electromechanical conversion capacity as active materials for transducers or large displacement actuators gradually appeared. The Institute of Materials Research at the University of Pennsylvania in the United States has carried out a feasibility study on the application of PZST antiferroelectric ceramics as active materials for large displacement actuators. Optimization, reduce the phase transition field strength, increase the longitudinal strain variable, the maximum longitudinal strain variable reaches 0.85%, the phase transition field strength is 48 kV / cm, and the hysteresis width is 20 kV / cm. It is pointed out that the anti-ferroelectric ceramics of the "square-width" type hysteresis loop show serious hysteresis loss under the alternating electric field, so they are not suitable for application in the alternating state.

Ferroelectricity: NVFRAM \ FFET Dielectricity: Large capacity capacitor \ Tunable microwave device \ PTC

High-performance ferroelectric materials are a class of functional materials with wide application prospects. From the current research status, research and development and application of ferroelectric materials with high performance are still in the development stage. Researchers have chosen different ferroelectric materials for research, and continue to explore the preparation process, but so far the research on some properties of ferroelectric materials has not reached a satisfactory level. For example, the types of ceramic powders and polymers used to prepare ferroelectric composite materials are still very single. Theoretical research on their composite interfaces has just begun, and research on the fatigue resistance characteristics of ferroelectric memory devices has yet to be developed. In short, ferroelectric materials are a class of important functional materials with broad development prospects. The research and application of their characteristics requires our continuous research and exploration, and we should pay sufficient attention to them.