Ferroelectric ceramics become polarized when cooled below the Curie point, and the polarity can be reversed by an electric field. They have a perovskite crystal structure and are used in FRAM, capacitors, ultrasound imaging, and optical displays.
Ferroelectric ceramics are a class of crystalline pyroelectric materials, i.e. materials that become electrically polarized when cooled below a particular temperature. The critical temperature in this sense is the Curie point, which is perhaps best known as the temperature above which ferromagnetic materials such as iron lose their magnetism. The term ferroelectric, however, has no direct connection to iron. In materials exhibiting the ferroelectric effect, the polarity can be reversed under the influence of an electric field of appropriate orientation. Many ceramic materials with this property can be produced by heating the powdered ingredients to the required temperature and allowing them to crystallize as the material cools.
Materials exhibiting this property typically have a perovskite crystal structure, a term derived from the mineral perovskite (CaTiO3) or calcium titanate. These compounds have the general formula ABX3, where A is a large cation, B is a much smaller cation, and X is an anion, usually oxygen. The crystalline structure of these materials is such that the “A” cations form a cubic lattice with, within each cube, a “B” cation surrounded by six “X” anions. Perovskite structures do not have a center of symmetry, as the ‘B’ cation tends to drift away from the center – this is essential for the ferroelectric effect. Examples of ferroelectric ceramics with this type of crystalline structure are barium titanate (BaTiO3), lead titanate (PbTiO3) and potassium niobate (KNbO3).
When an electric field is applied, the “B” cations change positions within the crystal lattice according to the orientation of the field and remain in these positions when the field is turned off. This causes the material to become electrically polarized. However, the positions of the “B” cations can be altered by applying an electric field with a different orientation. In this way, the ferroelectric ceramic can record information and can then be used for computer memory.
One of the most important applications of ferroelectricity is ferroelectric random access memory (FRAM). This offers very fast data archiving and retrieval, with the advantage that archived data is retained in the absence of power. Ferroelectric ceramics are also very suitable for use in capacitors. Multilayer capacitors made of hundreds of thin sheets of barium titanate with molded electrodes are produced in large quantities and have a wide range of uses, such as in ultrasound imaging and high-sensitivity infrared cameras. Other applications involve thin-film ferroelectric ceramics, which can be used in optical waveguides and optical memory displays.
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