Polar materials, ferroelectrics

In the broad range of ceramic materials that are used for electrical and electronic apphcations, each category of material exhibits unique property characteristics which directiy reflect composition, processing, and microstmcture. Detailed treatment is given primarily to those property characteristics relating to insulation behavior and electrical conduction processes. Further details concerning the more specialized electrical behavior in ceramic materials, eg, polarization, dielectric, ferroelectric, piezoelectric, electrooptic, and magnetic phenomena, are covered in References 1—9. 

Recently, ferroelectric materials, especially in thin film form, have attracted the attention of many researchers. Their large dielectric constants make them suitable as dielectric layers of microcapacitors in microelectronics. They are also investigated for application in nonvolatile memory using the switchable dielectric polarization of ferroelectric material. To characterize such ferroelectric materials, a high-resolution tool is required for observing the microscopic distribution of remanent (or spontaneous) polarization of ferroelectric materials. 

With this background, we have proposed and developed a new purely electrical method for imaging the state of the polarizations in ferroelectric and piezoelectric material and their crystal anisotropy. It involves the measurement of point-to-point variations of the nonlinear dielectric constant of a specimen and is termed scanning nonlinear dielectric microscopy (sndm) [1-7]. This is the first successful purely electrical method for observing the ferroelectric polarization distribution without the influence of the screening effect from free charges. To date, the resolution of this microscope has been improved down to the subnanometer order. 

Eleven acentric crystal classes are chiral, i.e., they exist in enantiomorphic forms, whereas ten are polar, i.e., they exhibit a dipole moment. Only five (1,2, 3, 4, and 6) have both chiral and polar symmetry. All acentric crystal classes except 432 possess the same symmetry requirements for materials to display piezoelectric and SHG properties. Both ferroelectricity and pyroelectricity are related to polarity a ferroelectric material crystallizes in one of ten polar crystal classes (1, 2, 3,4, 6, m, mm2, 3m, 4mm, and 6mm) and possesses a permanent dipole moment that can be reversed by an applied voltage, but the spontaneous polarization (as a function of temperature) of a pyroelectric material is not. Thus all ferroelectric materials are pyroelectric, but the converse is not true. 

Eq. (2.87) also suggests the possibility of spontaneous polarization , i.e. lattice polarization in the absence of an applied field. Considering Eq. (2.87), xe 00 as No. y -> 1, implying that under certain conditions lattice polarization produces a local field which tends to further enhance the polarization - a feedback mechanism. Such spontaneously polarized materials do exist and, as mentioned in Section 2.3, ferroelectrics constitute an important class among them. 

Pyroelectric crystals are ones that are spontaneously polarizable (see below) and in which a change in temperature produces a change in that spontaneous polarization. A limited number of pyroelectric crystals have the additional property that the direction of spontaneous polarization can be reversed by application of an electric field, in which case they are known as ferroelectrics. Thus a ferroelectric is a spontaneously polarized material with reversible polarization. Before proceeding much further it is important to appreciate that not all crystal classes can exhibit polar effect. 

The vast majority of nematogens are polar compounds but the absence of ferroelectricity in the nematic phase shows that there is equal probability of the dipoles pointing in either direction. Because of this it is generally assumed that the permanent dipolar contribution to the orientational order is negligibly small. However, a simple calculation shows that the interaction between neighbouring dipoles is by no means trivial compared with dispersion forces, particularly in strongly polar materials. We shall now consider a model which takes into account the influence of permanent dipoles and is at the same time consistent with the non-polar character of the medium. ... 

L. Dinescu, K.E. Maly, R.P. Lemieux, Design of photonic liquid crystal materials synthesis and evaluation of new chiral thioindigo dopants designed to photomodulate the spontaneous polarization of ferroelectric liquid crystals. J. Mater. Chem. 9, 1679-1686 (1999)... 

Further, given the need to fabricate thin-film devices with features measured in nanometers in conjunction with micro- and nanofluidics, an understanding of the nature of polarization and ferroelectric domain growth in materials compatible with fabrication at these scales is vital. The applications for this technology span many disciplines nonvolatile memory for computer technology, sensors for medical and biological diagnostics, and actuators and microwave devices for consumer applications [14]. 

Polarization that exists within a material in the absence of the application of an external field. It requires nonenantiomorphic polar symmetry. It is usually used in the context of electric fields and electrical polarization. Materials that exhibit spontaneous polarization and are piezoelectric are able to retain an ionic polarizarimi and therefore said to be ferroelectric. 

Until the late sixties the only known ferroelectrics, piezoelectrics, and pyroelectrics were certain inorganic monocrystals, or polycrystalline ceramics like lead titanate zirconate perovskites. Other known materials with macroscopic polarization were electrets, (for example mixmres of beeswax and rosin) in which the polarization was produced by application of the electric field in the melted state and then by cooling and the solidification of the polarized material. 

A polar material, whose electric dipoles can reverse direction as a consequence of an external electric field, is by definition a ferroelectric material. These materials form a subgroup of pyroelectric materials. 

Some pyroelectric materials exhibit special physical properties - so calledferroelectric properties - and build a special subgroup of pyroelectric materials, so called ferroelectrics. The spontaneous polarization could be switched (reversed or rotated) from its stable orientation to the other orientation for such materials. Ferroelectric ciystal could be also treated as pyroelectric crystals with switchable polarization, in that sense. 

Pb(ZrxTii x)03 (PZT) ferroelectric thin films are studied for their high dielectric constant and ferroelectric properties (e.g. high remanent polarization). Material is an excellent material in bulk ceramic applications. The dielectric constant lOSSeo at 1 MHz frequency was reported for poly crystalline PZT (x=0.50) thin film, remanent polarizationPr = 21.5 x 10 Ccm and coercive field of 3.9 kVmm ... 

It is important to recognize that a useful piezoelectric effect is defined macroscopically. Each unit cell has to contribute constructively in order for the macroscopic effect to occur. It is the global symmetry that determines the macroscopic piezoelectric effect. For example, a piezoelectric ceramic containing randomly oriented crystal grains has no piezoelectric effect even though the symmetry of each unit cell allows piezoelectricity. A net polarization in the material is a sufficient but not a necessary condition for the presence of piezoelectricity for example, quartz is one of the popular piezocrystals without polarization. The existence of a polarization, however, does make the piezoelectric effect much more pronounced. In fact, the best piezoelectric materials are all ferroelectric materials. Most importantly, the hydrostatic piezoelectric effect belongs uniquely to polar materials. 

Within single crystals and ceramic crystallites, respectively, the dipole moments of neighbouring domains are either perpendicular or anti-parallel to each other. For polycrystalline materials the orientation of the crystallites and thus of the domains is randomly distributed. In the original state these materials do not exhibit a macroscopic polarization and thus no piezoelectric effect. However, the latter can be induced by applying a static electric field below the Curie temperature where the domains of uniform dipole moments arrange towards the polarization field (paraelectric polarization). The field strength applied should be between the saturation and the breakdown range. Due to this polarization the ferroelectric material becomes piezoelectric. 

There are two common ways to categorize dielectric materials polar or nonpolar and paraelectric or ferroelectric. Polar materials include those that are primarily molecular in nature, such as water, and nonpolar materials include both electronically and ionically polarized materials. Paraelectric materials are polarized only in the presence of an applied electric field and lose their polarization when the field is removed. Ferroelectric materials retain a degree of polarization after the field is removed. Materials used as ceramic substrates are usually nonpolar and paraelectric in nature. An exception is silicon carbide, which has a degree of molecular polarization. 

Both, piezoelectric and pyroelectric behavior is possible only in ferroelectric ceramics, or in otherwise polar materials that are deposited as textured thin films. 

Ferroelectric liquid crystals are a novel state of matter, a very recent addition to the science of ferroelectrics which, in itself, is of relatively recent date. The phenomenon which was later called ferroelectricity was discovered in the solid state (on Rochelle salt) in 1920 by Joseph Valasek, then a PhD student at the University of Minnesota. His first paper on the subject [1] had the title Piezo-Electric and Allied Phenomena in Rochelle Salt. This was at the time when solid state physics was not a fashionable subject and it took several decades until the importance of the discovery was recognized. Valasek had then left the field. Later, however, the development of this branch of physics contributed considerably to our understanding of the electrical properties of matter, of polar materials in particular and of phase transitions and solid state physics in general. In fact, the science of ferroelectrics is today an intensely active field of research. Even though its technical and commercial importance is substantial, many breakthrough applications may still lie ahead of us. The relative importance of liquid crystals within this broader area is also constantly growing. This is illustrated in Fig. 1,... 

When dealing with ferroelectric liquid crystals, we use the same conceptual framework already developed for solid polar materials. An important part of this is the Landau formalism describing phase transitions (still not incorporated in any textbook on thermodynamics), based on symmetry considerations. It is important to gain some familiarity with the peculiarities of this formalism before applying it to ferroelectric liquid crystals. In this way it will be possible to recognize cause and effect more easily than if both subject matters were introduced simultaneously. 

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