Ferroelectric polycrystalline

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. 

Single crystal and bulk BaTiOs exhibits a sharp paraelectric-to-ferroelectric transition at 393K. In the presence of submicron grains, the transition becomes diffuse and can be absent for polycrystalline BaTiOs. Twin boundaries along the four crystallographically equivalent 11 planes constitute the main lattice defects. Junctions between such twin boundaries can be frequently observed within a grain. The local atomic arrangement of the core of twin intersections was studied by focal-series reconstruction (Jia etal. 1999). 

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. 

Figure 1.6 Ferroelectric hysteresis of single domain single crystal (dashed line) and polycrystalline sample (full line)...

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... 

Polycrystalline ferroelectric PTO thin films on platinized silicon substrates... 

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.)...

It should be noted that a poling process is often necessary with single-crystal ferroelectric bodies because they contain a multiplicity of randomly oriented domains. There is therefore a sequence of states of increasing orderliness polycrystalline ferroelectric ceramics, poled ferroelectric ceramics, single-crystal ferroelectrics and single-domain single crystals. 

Tenne DA, Soukiassian A, Xi XX, Taylor TR, Hansen PJ, York RA, Speck JS (2004) Effect of thermal strain on the ferroelectric phase transition in polycrystalline Bao.5Srt).5Ti03 thin films studied by Raman spectroscopy. Appl Phys Lett 85 4124... 

The first illustration is provided by ferroelectrics belonging to the family of pyridinium salts. Complex interplay between the contributions of van der Waals, Coulomb, dipolar and hydrogen-bonding interactions are expected because of the hybrid nature of the compound. The majority of reported NMR experiments are proton second-moment and relaxation studies on polycrystalline samples. The most sophisticated NMR methods with regard to resolution, symmetry and time-scale interpretations applied to the historical problem of assigning a pure order disorder or displacive mechanism to a ferroelectric phase transition will provide the second example with the study of squaric acids and perovskites compounds like BaTi03. 

Figure 15.14 (a) Ferroelectric hysteresis loop for a single-crystal, (b) Polycrystalline sample. 

How can a ferroelectric solid be made from a polycrystalline aggregate ... 

The process by which polycrystalline sohds can be made ferroelectric is ... 

For ferroelectrics that undergo a first-order transition [e.g., BaTiOj, (Ba, Sr)Ti03, PbTiOj, and KNbOj] Tew < 0c. For example, experimental measurements on polycrystalline BaTiOs have shown that can be more than 10°C less than 0c. A first-order transition involves a discontinuous change in P with T. 

An important point now emerges the requirement that the piezoelectric, pyroelectric and ferroelectric effects are restricted to non-centrosymmetric crystals implies that these physical phenomena should not be observed in a polycrystalline solid. This is because the individual grains of a polycrystalline body will polarise in random directions that will cancel overall. This was changed by the discovery, in 1945, of a way to endow polycrystalline ceramic articles with ferroelectric properties. 

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