Polycrystalline materials, properties

The properties of siHcon carbide (4—6) depend on purity, polytype, and method of formation. The measurements made on commercial, polycrystalline products should not be interpreted as being representative of single-crystal siHcon carbide. The pressureless-sintered siHcon carbides, being essentially single-phase, fine-grained, and polycrystalline, have properties distinct from both single crystals and direct-bonded siHcon carbide refractories. Table 1 Hsts the properties of the hiUy compacted, high purity material. 

Dielectric, piezoelectric and pyroelectric properties of LiTa03 derived ceramics containing additives of LiF and MgF2 were investigated and reported on in [407]. The materials were prepared at 900°C by means of two methods Reaction sintering, yielding powdered polycrystalline material ... 

Diamond is obtained as a polycrystalline material by CVD with properties similar to these of natural diamond. Efforts to produce single crystal thin films have so far been largely unsuccessful. 

Recent developments of materials and devices with structures in nanometer length scales have created new opportunities and challenges in the science of thermal transport. Interfaces play a particularly important role in the properties of nanoscale structures and nanostructured materials [97-98], This is why a renewed interest for contact resistance arose in recent years with studies of nanocomposite, semicrystalline and polycrystalline materials where contact resistances has a controlling role to determine the bulk thermal conductivity of the material [99-100],... 

The behavior of polycrystalline materials is often dominated by the boundaries between the crystallites, called grain boundaries. In metals, grain boundaries prevent dislocation motion and reduce the ductility, leading to hard and brittle mechanical properties. Grain boundaries are invariably weaker than the crystal matrix, and... 

The mechanical properties of materials are of enormous influence on the way these can be processed. After a molten metal has cooled to a solid, the solid metal is mechanically moulded into the desired form. The way of cooling and the final shape to a large extent determine the properties of the product. By means of a heat treatment it is possible to alter the properties of some metals. In this process the polycrystalline material is converted to a less polycrystalline form and eventually a state is reached which comes close the the ideal state, i.e. the metal is nearlv monocrvstalline (figure 10.10). 

The preparation of well-defined single-crystal surfaces of copper oxides in UHV or of well-sintered powders, starting from polycrystalline materials, is difficult. Consequently, structure-catalytic property correlations remain to be established. 

This process is obviously a natural scattering process in polycrystalline materials, since polycrystalline films exhibit a high concentration of crystallographic defects, especially dislocations [133,134]. However, this process is rarely used to explain experimental data of carrier transport in polycrystalline semiconductors and especially transparent conducting oxides [88], which is mainly due to the fact that in most works on transport properties of polycrystalline films the density of defects was not determined. Podor [135] investigated bended n-type Ge crystals with a dislocation density around 107 cm 2... 

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