主题：【原创】铁电材料介绍

History
The term ferroelectricity is used in analogy to ferromagnetism, in which a material exhibits a permanent magnetic moment. Ferromagnetism was already known when ferroelectricity was discovered in the late 1800s. Thus, the prefix "ferro", meaning iron, was used to describe the property despite that that most ferroelectric materials do not have iron in their lattice.


[edit] Applications
Placing a ferroelectric material between two conductive plates creates a ferroelectric capacitor. Ferroelectric capacitors exhibit nonlinear properties and usually have very high dielectric constants. The fact that the internal electric dipoles can be forced to change their direction by the application of an external voltage gives rise to hysteresis in the "polarization vs voltage" property of the capacitor. In this case, polarization is defined as the total charge stored on the plates of the capacitor divided by the area of the plates. Hysteresis means memory and ferroelectric capacitors are used to make ferroelectric RAM for computers and RFID cards.

The combined properties of memory, piezoelectricity, and pyroelectricity make ferroelectric capacitors some of the most useful technological devices in modern society. Ferroelectric capacitors are at the heart of medical ultrasound machines (the capacitors generate and then listen for the ultrasound "ping" used to image the internal organs of a body), high quality infrared cameras (the infrared image is projected onto a two dimensional array of ferroelectric capacitors capable of detecting temperature differences as small as millionths of a degree Celsius), fire sensors, sonar, vibration sensors, and even fuel injectors on diesel engines. Engineers use the high dielectric constants of ferroelectric materials to concentrate large values of capacitance into small volumes, resulting in the very tiny surface mount capacitor. Without the space savings allowed by surface mount capacitors, compact laptop computers and cell phones simply would not be possible. As well, the electro-optic modulators that form the backbone of the Internet are made with ferroelectric materials.

A ferroelectric tunnel junction (FTJ) is a contact made up by nanometer-thick ferroelectric film placed between metal electrodes. The thickness of the ferroelectric layer is thin enough to allow tunneling of electrons. The piezoelectric and interface effects as well as the depolarization field may lead to a giant electroresistance (GER) switching effect.


[edit] Materials
The internal electric dipoles of a ferroelectric material are physically tied to the material lattice so anything that changes the physical lattice will change the strength of the dipoles and cause a current to flow into or out of the capacitor even without the presence of an external voltage across the capacitor. Two stimuli that will change the lattice dimensions of a material are force and temperature. The generation of a current in response to the application of a force to a capacitor is called piezoelectricity. The generation of current in response to a change in temperature is called pyroelectricity. There are two main types of ferroelectrics: displacive and order-disorder. The effect in barium titanate, a typical ferroelectric of the displacive type, is due to a polarization catastrophe, in which, if an ion is displaced from equilibrium slightly, the force from the local electric fields due to the ions in the crystal increase faster than the elastic-restoring forces. This leads to an asymmetrical shift in the equilibrium ion positions and hence to a permanent dipole moment. In an order-disorder ferroelectric, there is a dipole moment in each unit cell, but at high temperatures they are pointing in random directions. Upon lowering the temperature and going through the phase transition, the dipoles order, all pointing in the same direction within a domain.

Another important ferroelectric material is lead zirconate titanate.

Ferroelectric crystals often show several transition temperatures and domain structure hysteresis, much as do ferromagnetic crystals. The nature of the phase transition in some ferroelectric crystals is still not well understood.

The ferroelectric effect also finds use in liquid crystal physics by incorporation of a chiral dopant into an achiral smectic C matrix. These liquid crystals exhibit the Clark-Lagerwall effect[1] which effects a change in one bistable state to another upon switching of electric field direction.