Polycrystalline physical properties

As can be seen from this figure, the heat-resistance was remarkably improved by the drastic changes in the microstructure from amorphous to polycrystalline structure. Another type of SiC-based fiber, SA fiber (2), has a sintered SiC polycrystalline structure and includes very small amounts of aluminum. This fiber exhibits outstanding high temperature strength, coupled with much improved thermal conductivity and thermal stability compared with the Nicalon and Hi-Nicalon fibers. The fabrication cost of the SA fiber is also reduced to near half of that of the Hi-Nicalon Type S [ 17]. The SA fiber makes SiC/SiC composites even more attractive to the many applications [18]. In the next section, the production process, microstructure and physical properties of the SA fiber are explained in detail. 

The single crystals thus far prepared are too small for many physical property measurements. Also, the two techniques described for preparation of powders result in fine grained (and not sintered) materials which are also not appropriate for many measurements. A different technique, however, was recently developed which produces near theoretical density polycrystalline pellets (56). Stoichiometric mixtures of BaO, K02 and Bi2Os are mixed and melted in N2 gas and quickly quenched onto a copper block under the N2 atmosphere. The process must be performed... 

Soon after the discovery of the quaternary borocarbide superconductors in 1994 a remarkable progress in the investigation of their physical properties could be asserted (Muller and Narozhnyi, 2001b). One reason for this rapid progress is the favorable synthesis properties of this class of materials, which resulted in high-quality polycrystalline samples over a wide range of compositions, as well as thin... 

Of the thirty-two crystal classes, twenty-two lack an inversion center and are therefore known as non-centrosymmetric, or acentric. Crystalline and polycrystalline bulk materials that belong to acentric crystal classes can exhibit a variety of technologically important physical properties, including optical activity, pyroelectricity, piezoelectricity, and second-harmonic generation (SHG, or frequency doubling). The relationships between acentric crystal classes and physical properties of bulk materials are summarized in Table 9.1.1. 

We conclude that the microscopic etch mechanism must be the same for single crystals and sputter deposited, polycrystalline ZnO Al. For the latter, the tendency for crater formation is masked by inhomogeneous chemical or physical properties like porosity, composition or, in case of dynamic deposition, multilayered ZnO Al films. This multilayer structure results from the fact that structural properties of ZnO Al deposited by a sputter process varies depending on the position of the film relative to the race track of the sputter target [131,132]. This dependence is important for the etch rate of in-line sputter deposited films [133]. 

In the absence of good quality single crystal samples, the physical properties of indium nitride have been measured on non-ideal thin films, typically ordered polycrystalline material with crystallites in the 50 nm to 500 nm range. Structural, mechanical and thermal properties have only been reported for epitaxial films on non-lattice-matched substrates. 

Often, it is not possible to obtain single crystals that are large enough to be worked with in a convenient manner. In those cases, physical properties must be measured on poly crystalline samples. There is always discrepancy, or disagreement, between measured physical properties of single crystals and polycrystals due to microstructural effects. Hence, physical properties measured from polycrystalline samples are sometimes considered less reliable from a reproducibility standpoint. 

The synthesis of macroscopic amounts of C o and C70 (fullerenes) has stimuiated a variety of studies on their chemical and physical properties. We recently demonstrated that C o and C70 become conductive when doped with alkali metals. Here we describe iow-temperature studies of potassium-doped both as films and bulk samples, and demonstrate that this material becomes superconducting, Superconductivity is demonstrated by microwave, resistivity and Melssner-effect measurements. Both polycrystalline powders and thin-flim samples were studied. A thin film showed a resistance transition with an onset temperature of 16 K and essentially zero resistance near 5 K. Bulk samples showed a well-defined Meissner effect and magnetic-field-dependent microwave absorption beginning at 18 K. The onset of superconductivity at 18 K is the highest yet observed for a molecular superconductor. 

Ultrasonic interferometry, in which the travel time of high-frequency elastic waves through a sample is measured, also yields elastic moduli. Because it is a physical property measurement, rather than an optical spectroscopy, it can be used equally well on poly-crystalline samples as single-crystals, although polycrystalline measurements only yield the bulk elastic properties, bulk modulus and shear modulus, G. High-pressure ultrasonic interferometry techniques were initially developed in the piston cylinder... 

By far the dominant technique in solid state/materials chemistry is powder diffraction since single crystals are often difficult to synthesize and not representative of the bulk, which can show non-stoichiometry and disorder. However, for definitive characterization of physical properties such as magnetic ordering or electron transport, single crystals are often used, as the directional properties of these processes are lost in a polycrystalline sample. 

G.Harbeke, Polycrystalline Semiconductors Physical Properties and Applications (Heidelberg, Springer, 1985), pp. 223-262. 

The model suggests that charge transfer is a primary mechanism of Schottky barrier formation, and a good agreement with the experimental results is found for polycrystalline inorganic semiconductors. It should be emphasized that the model is based on the thermod)mamic equilibrium of electrons across the interface between the metal and the semiconductor, and is facilitated by the interface bonds. Therefore, it does not depend on the details of the interface reactions, so long as the physical properties of the semiconductor, such as IP and Eg, remain intact at the interface. The model does not apply to interfaces where strong chemical reactions result in the domination of the interface by new reacted species. 

The precision with which this relationship between the imposed impulse and the physical property needs to be expressed depends on a number of factors. Of these, the nature of the sample is of considerable importance. Gases, liquids, amorphous solids and glasses, and polycrystalline arrays are isotropic. That is, the physical property is the same in all directions. In these cases there is little to be gained by specifying the imposed impulse and response as vectors, and scalars give aU the information about the physical property that is needed. In many other cases the directional nature of the processes becomes aU important, and the material is regarded as anisotropic. This happens when measuring molecular properties, properties of objects such as nanotubes, and most crystals. In this case it is necessary to specify the direction of... 

An alloy is a mixture of two or more materials, at least one of which is a metal. Alloys can have a microstructure consisting of solid solutions, where secondary atoms are introduced as substitutionals or interstitials (discussed further in the next chapter and Module 5, Plant Materials) in a crystal lattice. An alloy might also be a crystal with a metallic compound at each lattice point. In addition, alloys may be composed of secondary crystals imbedded in a primary polycrystalline matrix. This type of alloy is called a composite (although the term "composite" does not necessarily imply that the component materials are metals). Module 2, Properties of Metals, discusses how different elements change the physical properties of a metal. 

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