Crystalline-amorphous features nature

In a discussion of these results, Bertrand et al. [596,1258] point out that S—T behaviour is not a specific feature of any restricted group of hydrates and is not determined by the nature of the residual phase, since it occurs in dehydrations which yield products that are amorphous or crystalline and anhydrous or lower hydrates. Reactions may be controlled by interface or diffusion processes. The magnitudes of S—T effects observed in different systems are not markedly different, which indicates that the controlling factor is relatively insensitive to the chemical properties of the reactant. From these observations, it is concluded that S—T behaviour is determined by heat and gas diffusion at the microdomain level, the highly localized departures from equilibrium are not, however, readily investigated experimentally. 

Ya.B. considered that the most important adsorbents—porous coal, silica gel, and the powdered manganese dioxide which he had studied experimentally—are amorphous substances, i.e., they do not have clearly articulated crystalline structure. Only thus is it possible to obtain a large developed surface—the most important feature of an adsorbent. In this case, it is natural to consider all the possible values of adsorption activity and a smooth distribution function of surface sectors according to their level of activity. 

Seldom in the study of heterogeneous catalysis does it prove possible to (I) specify precisely the concentration and nature of the active sites, (2) test whether these sites are of comparable strength and are distributed in a spatially and chemically well-defined manner, and (3) explore the structural and mechanistic features of the system using a wide range of complementary techniques, many of them in situ. Even rarer are situations in which both the access to the active sites and the shape of the reactants may be systematically and subtly varied, so that one is able to compare the performance of the active site in a crystalline environment with an essentially identical one embedded in an amorphous solid. 

Nanoparticles and the Environmenf targets naturally occurring, finely particulate minerals, many of which form at low temperature. Thus, many of the compounds of interest are those of the clay fraction . Of course, there have been decades of critical work on the structures, microstructures, and reactivity of finely crystalline or amorphous minerals, especially oxides, oxyhydroxides, hydroxides, and clays. We will not summarize what is known in general about these (for this, the reader is referred to earlier Reviews in Mineralogy volumes). Rather, our goal is to focus on the features of these materials that stem directly or indirectly from their size. 

The objective of this paper is to review the published data on ex-situ and in-situ STM of passivation of metals (Ni, Cr, Fe, Al) and alloys (Fe-Cr), with special emphasis on atomically resolved structures, and to discuss, on the basis of the reviewed data, the questions of crystalline versus amorphous character of passive films, the nature of the defects, the relation of ftie structure to the available chemical information, and the implications of the structural features in the stability and the breakdown of passive films. 

Polymers can be either amorphous or semicrystalline in structure. The structure of amorphous materials cannot be described in terms of repeating unit cells such as that of crystalline materials because of nonperiodicity, the unit cell of an amorphous material would comprise all atoms. The physics and chemistry of the amorphous state remain poorly understood in many aspects. Although numerous experiments and theoretical studies have been performed, many of the amorphous-state features remain unexplained and others are controversial. One such controversial problem is the nature of glass-liquid transition. 

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