Polycrystalline materials, crystal orientation

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. 

Besides this, the inhibiting effects are dependent on the nature of the Pt surface as has been demonstrated by investigations on single crystals [37, 38] and on polycrystalline material with preferred surface orientation [39],... 

There are pros and cons for each method of electrode preparation. The polycrystalline electrodes are cheap and also are nearest in character to those used in practical reactors inindustiy. However, a polycrystal consists ofinumerable grains (bits) of the electrode material, each having a different crystal orientation and hence a different catalytic property. One way of manufacturing an original metal may differ from another in the distribution of crystal faces of different kinds. Thus, irreproducibility of results in electrode kinetics is not only due to inadequate purification of solution,... 

We may conclude that on polycrystalline material, where various crystallographic orientations will be present at the surface, a certain degree of heterogeneity will result from the different heats of adsorption on the different crystal faces. As we see, however, from the difference in the work functions of tungsten, these differences are not extremely large. We may perhaps expect that a certain part of the observed decrease of heats of chemisorption with increasing 6 values may be ascribed to this heterogeneity, but it seems doubtful that the whole effect should be caused by it. 

Polycrystalline materials in which the crystal axes of the grains are randomly oriented all behave electrostrictively whatever the structural class of the crystallites comprising them. If the crystals belong to a piezoelectric class and their crystal axes can be suitably aligned, then a piezoelectric polycrystalline ceramic becomes possible. 

Expansion in each dimension is the same. This would only be true for isotropic materials, that is, those with a cubic crystal structure, or glass. Polycrystalline materials with non-isotropic crystalline grains would also generally demonstrate a direction independent expansion behavior, due to the averaging effect of the random orientation of their grains. 

It is also possible to produce single-crystal ingots as well as highly oriented textured polycrystalline materials (i.e., those in which the grains exhibit a preferred orientation) by directional solidification. With the Bridgman technique... 

Crystallite usually means a tiny single crystal (microcrystal). Each particle in a polycrystalline material usually consists of multiple crystallites that join together in different orientations. A small powder particle can be a single crystallite as well. 

The Ewald sphere also predicts a ring type diffraction pattern in a polycrystalline material, which is considered as an aggregate of the crystals with all possible orientations in three-dimensional space. We may consider that placing a polycrystalline solid at the center of the Ewald sphere is equivalent to a case of a single crystal by rotating the reciprocal lattice around... 

The different modifications are interconvertible by thermal treatment. For example, modification II (as obtained by crystallization from anisole) is converted to modification III by annealing at 130°C. This is a normal change in modification, and polycrystalline material of modification III (mp. 172°C) is obtained from a single crystal of modification II. However, if a single crystal of modification II is photopolymerized to less than 1% conversion before annealing, modification III is obtained after annealing, highly oriented with respect to the photopolymer and the lattice of modification II. The point is that the photopolymer of modification II serves as the site of nucleation for the formation of the other modification. 

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