Polycrystalline ferroelectric materials

Kato, K. (2011) Tailored liquid alkoxides for the chemical solution processing of Pb-free ferroelectric thin films, in Multifunctional Polycrystalline Ferroelectric Materials. Processing and Properties, Springer Series in Materials Science (eds L. Pardo and J. Ricote), Canopus Academic Publishing Limited, London. 

This kind of microstructure also influences other kinds of conductors, especially those with positive (PTC) or negative (NTC) temperature coefficients of resistivity. For instance, PTC materials (Kulwicki 1981) have to be impurity-doped polycrystalline ferroelectrics, usually barium titanate (single crystals do not work) and depend on a ferroelectric-to-paraelectric transition in the dopant-rich grain boundaries, which lead to enormous increases in resistivity. Such a ceramic can be used to prevent temperature excursions (surges) in electronic devices. 

In many ferroelectric materials, the net piezoelectric effect is a result of both intrinsic and extrinsic responses. Here, intrinsic refers to the response that would result from an appropriately oriented single crystal (or ensemble thereof, in a polycrystalline sample). The extrinsic response is typically the result of motion of non-180° domain walls. The principle of these... 

Single-crystal materials, however, are very expensive and in some cases impossible to fabricate. In response to this challenge, polycrystalline ferroelectrics with controlled crystallographic textures attempt to match, and in some cases overcome the limitations of their single-crystal counterparts. Figure 3 summarizes the results for two chemistries, BaTiOs and PZN-PT, as reported in [82]. Here,... 

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. 

Piezoelectricity appears in natural crystals such as quartz, tourmaline, rochelle salt as well as in artificially produced ceramics and polymers such as e. g. nylon or copolymers of vinylidenefluoride (VDF) with trifluoroethylene (TrFE) or with tetrafluorethylene (TeFE). Most of the piezoelectric materials used for commercial sensor applications are synthetically produced polycrystalline ferroelectric ceramics such as e.g. lead-zirconate-titanate (PZT). 

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. 

Usually, the value of the spontaneous polarization P, depends on the temperature. Figure 22-4 shows the temperature dependence of the spontaneous polarization in barium titanate (BaTiOs) crystal. As temperature changes, a variation of the charge density can be observed on those surfaces of the sample which are perpendicular to the unique polar axis in a crystal without twins, or in a poled polycrystalline solid (ceramic) with oriented grains. The phenomenon, where the spontaneous polarization changes with temperature, is called pyroelectric effect . All ferroelectric materials exhibit pyroelectric effect. 

In this chapter we have introduced some general concepts of the ferroelectric and piezoelectric properties, as well as some typical methods for measurement of the ferroelectric materials fabricated by the sol-gel technique, hi general, sol-gel derived ferroelectric materials can be formed to various shapes, such as bar, disk, fiber, and film, in fact, the most successful product is ferroelectric thin film. In principle, the measurement of ferroelectric thin film derived by the sol-gel processing is similar to the measurement of a bulk material. In general, the properties of carefully prepared ferroelectric films are comparable to those of bulk polycrystalline samples. However, there is a little difference between the resulted... 

Polymer composites are multiphase materials containing, usually, inorganic fillers or reinforcing materials embedded in an anurphous or polycrystalline matrix. The dielectric properties of the inclusions arc, usually, very different from those of the nutrix. For ferroelectric appUcatioos, inorganic ferroelectric materials (e.g.. ceramics) are often used u fillers. 

One of the important characteristics of ferroelectrics is that the dielectric constant obeys the Curie- Weiss law (equation 6.48), similar to the equation relating magnetic susceptibility with temperature in ferromagnetic materials. In Fig. 6.55 the temperature variation of dielectric constant of a single crystal of BaTiOj is shown to illustrate the behaviour. Above 393 K, BaTiOj becomes paraelectric (dipoles are randomized). Polycrystalline samples show less-marked changes at the transition temperature. 

The first section details the purely intrinsic response of ferroelectrics and discusses their anisotropic properties, the useful application of which can be controlled by the correct orientation of a sin e crystal or by texturing a polycrystalline material. Attention is then focused on one of the most significant extrinsic contributions to the polarization response of ferroelectrics, namely the motion of domain walls. The effect of the domain wall contribution can be controlled by the hardening-softening of... 

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. 

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