Macroscopic polarization, ferroelectrics

M ribbons, chromonics 996 Mach-Zehnder interferometer 132 macrocyeles, hydrocarbon cores 719 f macrocyclic complexes, metallomesogens 926 macroscopic flow, chiral nematics 379 macroscopic polarization, ferroelectrics 541 ff macroscopic properties... 

Fig. 17a-c. Sketches of the molecular arrangements for the smectic structure with alternating layer-to-layer tilt a conventional and chevron smectic C layering in low molecular mass mesogens b ferroelectric hilayer chevron structures for achiral side-chain polymers c antiferroelectric hilayer chevron structures for achiral side-chain polymers. Arrows indicate the macroscopic polarization in the direction of the molecular tilt... 

In their ferroelectric state, the electro-optically useful PLZT compositions have an almost cubic structure, with the polar c axis being typically only about 1% longer than the a axes. Consequently the optical properties are almost isotropic and this, in part, is why high transparency can be achieved in the ceramic form. When an electric field is applied to the ceramic, domain alignment, or a field-enforced transition to the ferroelectric state, leads to the development of macroscopic polarization and so to uniaxial optical properties, i.e. the optic axis... 

The ferroelectric polarisation has been calculated for the tetragonal phase of KNbOs [58] and for HCl [45] in the case of KNbOs, the macroscopic polar-... 

Ferroelectric liquid crystals (FLC) are of great interest due to their fast electro-optical response which is about 1,000 times faster than conventional twisted nematic cells [131]. The geometry used is called a surface stabilized FLC cell which utilizes a very thin gap (=2 pm) to unwind the FLC supramolecular pitch (=1-2 pm) since the bulk FLC materials do not show macroscopic polarization. This very thin gap, however, leads to difficulties in manufacturing large panels and very poor shock resistance. Researchers have proposed the concept of microphase stabilized FLC [79,109, 130] using FLC-coil diblock copolymers for electro-optical applications as shown in Fig. 15. This concept takes advantage of ferroelectric liquid crystallinity and block copolymer microphase separation since the block... 

The polyphilic quality leads to a further ordering of the molecules in their meso-phase Fig. 34 gives a simple schematic drawing of a directed lamellar phase. The molecules have a polar orientation within the layers and long range correlations between them. This special mesophase structure causes macroscopically polar properties. With regard to this kind of molecular arrangement some polyphilic compounds have been successfully studied in respect of ferroelectric properties [161-165]. 

Pyro- and Piezoelectric Properties The electric field application on a ferroelectric nanoceramic/polymer composite creates a macroscopic polarization in the sample, responsible for the piezo- and pyroelectricity of the composite. It is possible to induce ferroelectric behavior in an inert matrix [Huang et al., 2004] or to improve the piezo-and pyroelectricity of polymers. Lam and Chan [2005] studied the influence of lead magnesium niobate-lead titanate (PMN-PT) particles on the ferroelectric properties of a PVDF-TrFE matrix. The piezoelectric and pyroelectric coefficients were measured in the electrical field direction. The Curie point of PVDF-TrFE and PMN-PT is around 105 and 120°C, respectively. Different polarization procedures are possible. As the signs of piezoelectric coefficients of ceramic and copolymer are opposite, the poling conditions modify the piezoelectric properties of the sample. In all cases, the increase in the longitudinal piezoelectric strain coefficient, 33, with ceramic phase poled) at < / = 0.4, the piezoelectric coefficient increases up to 15 pC/N. The decrease in da for parallel polarization is due primarily to the increase in piezoelectric activity of the ceramic phase with the volume fraction of PMN-PT. The maximum piezoelectric coefficient was obtained for antiparallel polarization, and at < / = 0.4 of PMN-PT, it reached 30pC/N. 

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. 

In the field-off state the macroscopic polarization of the antiferroelectric phase is zero. With increasing field, the induced polarization, at first, increases linearly with field and then, at a certain threshold, the antiferroelectric (AF) structure with alternating molecular tilt transforms in the ferroelectric one (F) with a uniform tilt, see Fig. 13.24a. Correspondingly, the macroscopic polarization jumps from a low value to the level of the local polarization Po [34]. The process is quite similar to that observed in crystalline antiferroelectrics. With a certain precaution we can speak about a field-induced AF-F non-equilibrium phase transition . The magnitude of the switched polarization in some antiferroelectric materials can be quite... 

Dantlgraber, G. Eremin, A. Diele, S. Hauser, A. Kresse, H. Pelzl, G. Tschierske, C. Chirality and macroscopic polar order in a ferroelectric smectic liquid-crystalline phase formed by achiral polyphilic bent-core molecules. Angew. Chem. Int. Ed. 2002, 41, 2408-2412. 

Liquid crystals can be composed both of polar and apolar molecules. An important fact in connection with polar substances is that in uniaxial phases there is no polar ordering of the molecules. In average the dipole moments aligned in a given direction are compensated by those aligned in the opposite direction. As a consequence no spontaneous macroscopic polarization develops. More generally one can state that rotation of the director by n does not affect the physical state of the liquid crystal. In biaxial phases built of chiral molecules, such as the chiral smectic C phase, the situation is different. In these systems the compensation of the dipole moments is not perfect, a macroscopic polarization appears in the direction perpendicular both to the layer normal and the director. These phases are therefore ferroelectric. Ferroelectric liquid crystals are currently perhaps the... 

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. 

The storage effects could also be realized in polymer liquid crystals. On cooling, ferroelectric liquid crystal polymers with the electric field applied, the macroscopic polarization is frozen in the glassy state [74]. Thus, the polymer film becomes a pyroelectric and a piezoelectric. Unfortunately, the glassy state is too viscous to allow the field-induced reorientation of the polarization and the film cannot be considered to be a ferroelectric. 

Lagerwall ST (2007) The necessary conditions for macroscopic polarization. In Ferroelectric and antiferroelectric liquid crystals. Wiley-VCH Verlag GmbH, Weinheim, pp 57-91. doi 10.1002/9783527613588.ch3... 

This area of research has seen an explosion of interest in recent years and has been well reviewed by others [39 1]. This extra element in the assembly of these mesogens is responsible for a number of interesting polar properties in this class of materials [42], As shown in Figure 4(b), the oxo-metal compounds provide a dipole moment to many of these mesogens. This allows them to transfer their oxygen atoms between members of the stack to switch the macroscopic polarity [42], Some have shown ferroelectric behavior [42] that is possibly due to the inability to pack opposing dipoles in a hexagonal lattice [43]. 

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