Ferroelectric domain switching

The normal state of a ferroelectric crystal is one in which the microstructure consists of a set of coherent domains, each of which has the internal electric dipoles parallel to each other, but not aligned with those of neighbouring domains. Domain walls are not atomically smooth but have a thickness of between 0.5 and 1 nm. Polarisation builds up at domain boundaries and a single domain can have surface charges of the order of 1.5 x lO electrons cm which can generate an internal electric field of 300MV or more. 

---

Ferroelectric Domain Switching is recognized, different from that in micro-nano type ceramic composites. 

W., Lu, D.N., Fang and K.C., Hwang, Micromechanics of ferroelectric domain switching behavior Part I Coupled electromechanical field of domain inclusions, Theor. App. Fract. Meehan., 37, 29 38 (2001)... 

Figure 8.35 Illustration of bistable ferroelectric EO switching observed for blue focal conic domains of MHOBOW. SmCsPF structure can be assigned for these domains. In this phase director structure in all layers is oriented identically. This structure and corresponding extinction bmsh orientation for cylindrical focal conic are illustrated at bottom of figure for two bistable ferroelectric states. Note that for material with 30° tilt angle, such as MHOBOW, two ferroelectric states look very similar in still photos. In fact, two states result from large rotation of extinction brushes through 60°, as can be easily seen when observing switching in motion.

Analogous C(V) curves were recorded on pzt bulk ceramics with compositions around the morphotropic phase boundary (mpb). Figure 1.25 displays the relative permittivity as a function of DC-bias for a tetragonal (x = 0.48), a morphotropic (x = 0.52) and a rhombohedral (.x = 0.58) sample. In contrast to thin films additional humps observed in the e E) curves. This could be explained by different coercive fields for 180° and non-180° domains [31]. Their absence in ferroelectric thin films could be taken as evidence for suppressed non-180° domain switching in thin films [30],... 

Two types of contributions to dielectric and piezoelectric properties of ferroelectric ceramics are usually distinguished [6], [9-12], One type is called an intrinsic contribution, and it is due to the distortion of the crystal lattice under an applied electric field or a mechanical stress. The second type is called an extrinsic contribution, and it results from the motion of domain walls or domain switching [8], To provide an understanding of material properties of pzt, several methods to separate the intrinsic and extrinsic contributions were proposed. These methods are indirect, and are based on measurements of the dielectric and piezoelectric properties of ferroelectric ceramics [8], [10-12], In the experiments reported in this paper a different approach is adopted, which is based on measurements of high-resolution synchrotron X-ray powder diffraction. The shift in the positions of the diffraction peaks under applied electric field gives the intrinsic lattice deformation, whereas the domain switching can be calculated from the change in peak intensities [13,14],... 

Fig. 2.44 Schematic illustrating the changes accompanying the application of electrical and mechanical stresses to a polycrystalline ferroelectric ceramic (a) stress-free - each grain is non-polar because of the cancellation of both 180° and 90° domains (b) with applied electric field - 180° domains switch producing net overall polarity but no dimensional change (c) with increase in electric field 90° domains switch accompanied by small ( 1%) elongation (d) domains disorientated by application of mechanical stress. (Note the blank grains in (a) and (b) would contain similar domain structures.)...

As mentioned above, domain switching in ferroelectrics is accompanied by domain nucleation, moving domain walls and restructuring of dipoles and charges. A characteristic feature of this irreversible process is the appearance of a hysteresis loop in the dependence of dielectric displacement... 

In addition to resulting in very large k at Tq, spontaneous polarization will result in hysteresis loops, as shown in Fig. 15.14. At low applied fields, the polarization is reversible and almost linear with the applied field. At higher field strengths, the polarization increases considerably due to switching of the ferroelectric domains. Further increases in the electric field continue to increase the polarization as a result of further distortions of the TiOf, octahedra. ... 

Ren, X. (2004) Large electric-field-induced strain in ferroelectric crystals by point-defect-mediated reversible domain switching. Nat. Mater., 3, 91—94. 

Sun, C-T. and Achuthan, A. 2004. Domain-switching criteria for ferroelectric materials subjected to electrical and mechanical loads. Journal of the American Ceramic Society, 87 [3] 395-400. 

Shaikh, M.G., Phanish, S., and Sivakumar, S.M. 2006. Domain switching criteria for ferroelectrics. Computational Materials Science 37, pp. 178-186. 

Li, F.X, and Rajapakse, R.K.N.D. 2007. A constrained domain-switching model for polycrystalline ferroelectric ceramics. Part I Model formulation and application to tetragonal materials. ActaMaterialia 55, 6472-6480. 

Zhang, W. and Bhattacharya, K. 2005a. A computational model of ferroelectric domains. Part I model formulation and domain switching Acta Materialia 53, 185-198. 

Another distinguishing feature of ferroelectric behavior is the polarization versus electric field P—B) hysteresis loop. The hysteresis loop results from the domain reorientation which occurs as the electric field direction is varied. The size and shape of the loop is determined by the magnitude of the dipole moment of the unit cell and the domain-switching characteristics of the material. Hysteresis loop behavior is measured using either a Sawyer—Tower circuit or a Diamant—Pepinsky bridge. Details of the construction and operation of a Sawyer—Tower circuit are given in Reference 24. Thin film properties have also been measmed with these two devices, and in addition, a commercially available measurement system has been widely used. ... 

Figure 3.10a shows the polarization arrangement in a ceramic system (note domains were not explicitly drawn in here, the arrows only represent the net polarization in each grain). The macroscopic piezoelectric effect is zero due to the cancellation of oppositely polarized grains. If the ceramic material is ferroelectric, it can be made piezoelectric by aligning the polarization of different grains using an external electric field through the domain switching process. A net polarization may be produced along the field direction as illustrated in Fig. 3.10b. As the electromechanical characteristic of piezoelectric effect is reversible, piezoelectric materials can be used for both sensing and actuation functions (see Chaps. 6 and 7).

The structural unit of the copolymer -(-CH2-CF2)n-(-CF2-CHF-)m- contains n and m corresponding monomer links. The ferroelectric properties are attributed to transverse dipole moments, formed by positive hydrogen and negative fluorine atoms. Below the temperature of the ferroelectric phase transition (about 80-100°C), the main chain of the polymer is in all-trans form and the dipole moments are parallel, at least, within ferroelectric domains separated fi-om each other by domain walls. The ferroelectric switching is due to an electric field induced, collective flip-flop of the dipoles around the backbone of the polymer. Several recent studies were devoted to a local ferroelectric switching of the domains in cast P(VDF-TrFE) films [6-8]. To this effect, a powerful technique, called Electrostatic Force Microscopy (EFM) [9] was used which was developed for studies of domains in thin ferroelectric films, see papers [10, 11] and references therein. 

In case of the inverse contrast, the field oriented ferroelectric domains are looking as areas charged by electric current from the tip and only a comparison with piezoelectric measurements may finally show whether a ferroelectric has been switched or has not. Below we present the results of such a comparison (note, for all images below the tip is grounded, i.e. has a zero potential). 

Zeng, X., and Rajapakse, R.K.N.D., Domain switching induced fracture toughness variation in ferroelectrics. Smart Mat. Struc., 2001, 10, 203 211. 

---