Ferroelectricity / ferroelectric polarization

Because of very high dielectric constants k > 20, 000), lead-based relaxor ferroelectrics, Pb(B, B2)02, where B is typically a low valence cation and B2 is a high valence cation, have been iavestigated for multilayer capacitor appHcations. Relaxor ferroelectrics are dielectric materials that display frequency dependent dielectric constant versus temperature behavior near the Curie transition. Dielectric properties result from the compositional disorder ia the B and B2 cation distribution and the associated dipolar and ferroelectric polarization mechanisms. Close control of the processiag conditions is requited for property optimization. Capacitor compositions are often based on lead magnesium niobate (PMN), Pb(Mg2 3Nb2 3)02, and lead ziac niobate (PZN), Pb(Zn 3Nb2 3)03. 

Above a specific temperature, the Curie temperature, a ferroelectric substance becomes paraelectric since the thermal vibrations counteract the orientation of the dipoles. The coordinated orientation of the dipoles taking place during the ferroelectric polarization is a cooperative phenomenon. This behavior is similar to that of ferromagnetic substances, which is the reason for its name the effect has to do nothing with iron (it is also called seignette or rochelle electricity). 

In sodium nitrite the ferroelectric polarization only occurs in one direction. In BaTiOs it is not restricted to one direction. BaTiOs has the structure of a distorted perovskite between 5 and 120 °C. Due to the size of the Ba2+ ions, which form a closest packing of spheres together with the oxygen atoms, the octahedral interstices are rather too large for... 

Figure 3. The ferroelectric polarization versus temperature for W82,W7 (O), 2% PPDA polymer ( ) and 2% HDDA polymer (A) in W82,W7.

Figure 4. Ferroelectric polarization as a function of polymerization temperature for 2% PPDA (O) and 2% HDDA ( ) in W82,W7. LC phases in which polymerizations were initiated are also denoted.

The introduction of a polymer network into an FLC dramatically changes phase and electro-optic behavior. Upon addition of monomer to the FLC, the phase transitions decrease and after polymerization return to values close to that observed in the neat FLC. The phase behavior is similar for the amorphous monomers, HDD A and PPDA. The electro-optic properties, on the other hand, are highly dependent on the monomer used to form the polymer/FLC composite. The ferroelectric polarization decreases for both HDDA and PPDA/FLC systems, but the values for each show extremely different temperature dependence. Further evidence illustrating the different effects of each of the two polymers is found upon examining the polarization as both the temperature and LC phase of polymerization are changed. In PPDA systems the polarization remains fairly independent of the polymerization temperature. On the other hand, the polarization increases steadily as the polymerization temperature of HDDA systems is increased in the ordered LC phases. 

It may be noted that simple MOPAC AMI calculations suggest that the dipole moment of NOBOW is oriented antiparallel to the molecular arrow. As indicated in Figure 8.25, this means that for an up field, the molecular arrows are pointing down. Given the definition of the sign of P in FLCs, this also means that domains of the ShiCaPa phase with positive chirality have negative ferroelectric polarization, and vice versa. 

The isotropic spherical matrix, on the other hand, seems to correspond to dynamic nanoclusters which fluctuate fast on the NMR timescale. However, the anisotropic ferroelectric polar nanoclusters are frozen on the NMR timescale. 

Electrooptic Properties, The electrooptic properties of the PLZT materials are intimately related to their ferroelectric properties. Consequently, varying the ferroelectric polarization with an electric field such as in a hysteresis loop, produces a change in the optical properties of the ceramic. In addition, the magnitude of the observed electrooptic effect is dependent on both the strength and direction of the electric field,... 

The ferroelectric hysteresis originates from the existence of irreversible polarization processes by polarization reversals of a single ferroelectric lattice cell (see Section 1.4.1). However, the exact interplay between this fundamental process, domain walls, defects and the overall appearance of the ferroelectric hysteresis is still not precisely known. The separation of the total polarization into reversible and irreversible contributions might facilitate the understanding of ferroelectric polarization mechanisms. Especially, the irreversible processes would be important for ferroelectric memory devices, since the reversible processes cannot be used to store information. 

A spherical model was used in Ref. [15] in order to obtain the shape of the domains, reversed under the fdb conditions. This model was widely applied for studies of different processes that take place in the field of afm tip (see Ref. [65]), including ferroelectric polarization reversal [66-69], In this model the field of the tip apex is supposed to coincide with a field of a metallic sphere, the radius of which is equal to the radius of curvature of the tip apex. Using a simple approximation it may be supposed that the tip charge is concentrated in the center of the sphere [15,64-69], We will take into account a more general model and check the accuracy of the simple spherical model application to the ferroelectric domain breakdown condition. 

In conclusion, we reported the investigation of inner and outer interfaces in pzt in order to quantity both the amount of effective ferroelectric polarization and change in dielectric properties. With pfm and kpfm we find a transition layer occurring at the Pt/PZT interface within... 

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. 

The most widely known case of phonon-induced charge transfer may be that in ferroelectric oxides, such as BaTi03. In the classic picture of ferroelectricity polarization is produced by positive and negative ions displaced in opposite directions. The polarization is given by uZ, where u is the atomic displacement and Z is the ionic charge. In reality, however, the actual polarization is much larger because of charge transfer ... 

A number of factors can influence the behavior of ferroelectric thin films and multilayer stmctures with layer thickness at nanometer scale. One of the major factors is strain in epitaxial structures [15]. Recent demonstrations of huge strain effect on ferroelectric properties include changes in the phase diagram [16-22], dramatic enhancement of ferroelectric polarization, and increase of the ferroelectric phase transition temperature [23-27], induced ferroelectricity in non-ferroelectric materials like SrTi03 or KTa03 [28-33], or even simple rocksalt binary oxides like BaO ([34], theoretically predicted). 

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