Ferroelectric ceramics are a class of crystalline pyroelectric materials — that is, materials that become electrically polarized when cooled below a particular temperature. The critical temperature in this respect is the Curie point, which is perhaps better known as the temperature above which ferromagnetic materials such as iron lose their magnetism. The term ferroelectric, however, has no direct connection with iron. In materials which exhibit the ferroelectric effect, the polarity can be reversed under the influence of an electric field of the appropriate orientation. Many ceramic materials with this property can be manufactured by heating powdered ingredients to the required temperature and allowing crystallization as the material cools.
Materials that exhibit this property typically have a perovskite crystal structure, a term which comes 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 crystal structure of these materials is such that the “A” cations form a cubic lattice with, inside each cube, a “B” cation surrounded by six “X” anions. Perovskite structures do not have a center of symmetry, in that the “B” cation tends to be displaced away from the center — this is essential for the ferroelectric effect. Examples of ferroelectric ceramics with this type of crystal structure are barium titanate (BaTiO3), lead titanate (PbTiO3) and potassium niobate (KNbO3).
When an electric field is applied, the “B” cations change position within the crystal lattice according to the orientation of the field, and remain in these positions when the field is switched off. This results in the material becoming electrically polarized. The positions of the “B” cations can, however, be altered by applying an electric field with a different orientation. In this way, ferroelectric ceramics can record information and can therefore be used for computer memory.
One of the most important applications of ferroelectricity is ferroelectric random access memory (FRAM). This offers very fast storage and retrieval of data, with the advantage that the stored data is preserved when there is no power supply. Ferroelectric ceramics are also very suitable for use in capacitors. Multi-layer capacitors consisting of hundreds of thin sheets of barium titanate with printed electrodes are manufactured in large quantities and have a wide range of uses, for example 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.
