Polycrystalline Solid

Ty is the quantity we want the yield strength of bulk, polycrystalline solids. It is larger than the dislocation shear strength Tj (by the factor 3) but is proportional to it. So all the statements we have made about increasing apply unchanged to... 

Nucleation in solids is very similar to nucleation in liquids. Because solids usually contain high-energy defects (like dislocations, grain boundaries and surfaces) new phases usually nucleate heterogeneously homogeneous nucleation, which occurs in defect-free regions, is rare. Figure 7.5 summarises the various ways in which nucleation can take place in a typical polycrystalline solid and Problems 7.2 and 7.3 illustrate how nucleation theory can be applied to a solid-state situation. 

Sample requirements Very flexible liquids, gases, crystals, polycrystalline solids, powders, and thin films... 

In polycrystalline solids or samples consisting of various phases, each grain may sputter at a different rate producing extensive roughness in the bottom of the cra-... 

C fi3 diamond films can be deposited on a wide range of substrates (metals, semi-conductors, insulators single crystals and polycrystalline solids, glassy and amorphous solids). Substrates can be abraded to facilitate nucleation of the diamond film. 

The rise times of the elastic wave may be quite narrow in elastic single crystals, but in polycrystalline solids the times can be significant due to heterogeneities in physical and chemical composition and residual stresses. In materials such as fused quartz, negative curvature of the stress-volume relation can lead to dispersive waves with slowly rising profiles. 

In solids of cubic symmetry or in isotropic, homogeneous polycrystalline solids, the lateral component of stress is related to the longitudinal component of stress through appropriate elastic constants. A representation of these uniaxial strain, hydrostatic (isotropic) and shear stress states is depicted in Fig. 2.4. Such relationships are thought to apply to many solids, but exceptions are certainly possible as in the case of vitreous silica [88C02]. 

Downs et al. reported the synthesis of methylzinc tetrahydridoborate 164 by the two routes shown in Scheme 99.230 The compound is an extremely moisture and oxygen sensitive, colorless, polycrystalline solid, whose solid-state structure was determined by X-ray analysis. Figure 76 shows that 164 consists of helical polymers of alternating methylzinc and tetrahydridoborate ions. The zinc atom is formally pentacoordinate, making this one of the few organozinc compounds with five-coordinate zinc atom. 

One way to make the short-lived intermediates amenable to study is to increase their lifetime, usually by irradiation in the solid state and/or at very low temperatures. Then, the intermediates can be detected at the end of the irradiation by ESR or optical absorption spectroscopy. The ESR study of radicals in the solid state is done on single crystals, polycrystalline samples or frozen aqueous solution. In case of polycrystalline samples or frozen aqueous solution the identification of the radicals from the ESR spectra is difficult in many cases and, for better identification, the ESR experiment should be conducted on irradiated single crystals. Later, the method of spin trapping, developed for the liquid phase5, was extended to polycrystalline solids. In this technique the polycrystalline solids are /-irradiated and subsequently dissolved in a solution containing the spin trap. 

Figure 3.27 shows the Mossbauer spectrum that results from splitting of the 57Fe excited state, a quadrupole doublet, for a sample containing randomly oriented molecules such as found in polycrystalline solids or frozen solutions. The two doublets are separated in energy by the quadrupole splitting, A Eq, defined by the... 

Most metals and ceramics in their normal states are polycrystalline. Polycrystalline solids are composed of many interlocking small crystals, often called grains (Fig. 3.33). The surfaces of the grains that make up the solid are often similar to the external surfaces found on large crystals. 

Figure 3.33 Schematic representation of a polycrystalline solid composed of crystallites of one structure.

Grain boundaries have a significant effect upon the electrical properties of a polycrystalline solid, used to good effect in a number of devices, described below. In insulating materials, grain boundaries act so as to change the capacitance of the ceramic. This effect is often sensitive to water vapor or other gaseous components in the air because they can alter the capacitance when they are absorbed onto the ceramic. Measurement of the capacitance allows such materials to be used as a humidity or gas sensor. 

Figure 5.3 Schematic representation of the penetration profile for bulk, grain boundary, and dislocation diffusion in a polycrystalline solid. The initial part of the curve is bell shaped, and the part due to short-circuit diffusion is made up of linear segments. The insets show the distribution of the tracer in the sample.

Besides the multiplicity of defects that can be envisaged, there is also a wide range of solid phases within which such defects can reside. The differences between an alloy, a metallic sulfide, a crystalline fluoride, a silicate glass, or an amorphous polymer are significant. Moreover, developments in crystal growth and the production of nanoparticles have changed the perspective of earlier studies, which were usually made on polycrystalline solids, sometimes with uncertain degrees of impurity present. 

Figure 6.11 (a) Definition of the dihedral angle, , at a junction of three grain boundaries in a polycrystalline solid, (b) Schematic illustration of the shape of an inclusion phase for different dihedral angles. 

The chemistry of metal oxides can be understood only when their crystal structure is understood. Knowledge of the geometric structure is thus a prerequisite to understanding the properties of metal oxides. The bulk structure of polycrystalline solids can usually be determined by x-ray... 

The energy associated with grain boundaries in polycrystalline solids, i.e., yx erg/cm2, has been mentioned in Section III. 1. It is reviewed here even more briefly than the surface energy (ys or y ) with which this paper is primarily concerned. 

The usefulness of quadrupolar effects on the nuclear magnetic resonance c I 7 yi nuclei in the defect solid state arises from the fact that point defects, dislocations, etc., give rise to electric field gradients, which in cubic ciystals produce a large effect on the nuclear resonance line. In noncubic crystals defects of course produce an effect, but it may be masked by the already present quadrupole interaction. Considerable experimental data have been obtained by Reif (96,97) on the NMR of nuclei in doped, cubic, polycrystalline solids. The effect of defect-producing impurities is quite... 

Although anisotropy of the chemical shift is expected for nuclei in non-cubic symmetry, the average value obtained for a polycrystalline solid should be comparable to these shifts for liquids, which are also average values. The chemical shift for concentrations of fluorine in the intermediate and on the high part of the concentration range of the samples examined was found to be (—40 40 ppm) relative to F (aqueous). No large shift for the fluoride ion occurs in the solids examined compared to the heavier halides in the solid state 118), indicative of the ionicity of the A1—F bond involved. 

Deep pink polycrystalline solid Rup4 has been prepared by treatment of AsFs with [Ru(F)g] in anhydrous HF solution." A combination of X-ray synchrotron and neutron powder diffraction data reveal that each Ru-atom has six F ligands with an octahedral framework, four in the same plane, each shared with another Ru-atom, to form a puckered-sheet array (Ru—F (bridge) = 2.00 A and 2.00 A, Ru—F—Ru 133°). ... 

There have been three important theories in recent years on the sputtering of amorphous and polycrystalline solids. They are attributed to Sigmund , Thompson , and Brandt and Laubert . The predictions about various aspects of sputter-... 

Compounds of the I—VII group in the periodic table are known to exhibit good ionic conductivity and have attracted much attention as possible candidates for solid electrolytes. A typical family of compounds is Lil, CuCl, CuBr, and Agl. Historically, polycrystalline solid electrolytes were noticed to show significantly higher ionic conductivity than bulk crystals, since a half century ago. Furthermore, a large increase in conductivity was reported for the system of the mixture of a solid electrolyte such as CuCl (1) and Agl (2) with submicrometer particles of several sorts of insulating materials. In this case, the size of the metal halide itself was on the order of a micrometer or larger. It was also reported that the enhanced conductivity was approximately proportional to the inverse of the size of the electrolyte substances (2). Hence it is natural to make an effort to obtain fine particles of metal halides in order to get better conductivity. 

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