Non-crystalline material

Waseda Y 1980 The Structure of Non-Crystalline Materials (New York McGraw-Hill)... 

N F. Motte and E. A. Davis, Electronic Processes in Non-Crystalline Materials, Clarendon Press, Oxford (1979). 

Zeolites. In heterogeneous catalysis porosity is nearly always of essential importance. In most cases porous materials are synthesized using the above de.scribed sol-gel techniques resulting in so-called amorphous catalysts. Porosity is introduced in the agglomeration process in which the sol is transformed into a gel. From X-ray Diffraction patterns it is clear that the material shows only weak broad lines, characteristic of non-crystalline materials. Silica and alumina are typical examples. Zeolites are an exception they are crystalline materials but nevertheless exhibit high (micro) porosity. Zeolites belong to the class of molecular sieves, which are porous solids with pores of molecular dimensions, i.e., typically the pore diameter ranges from 0.3 to 10 nm. Examples of molecular sieves are carbons, oxides and zeolites. 

When allowed to stand for several weeks in the sunlight in cyclohexane saturated with oxygen, [2]paracyclo[2](2,5)furanophane (41 a) generated [2.2.2]paracyclophane (119) together with some non-crystalline material. The fate of the furan component was not investigated 66b>. 

Figure 1. Cooling rate and crystallization temperatures fora variety of metallic glasses , note that modest cooling rates can result in non-crystalline materials for sizeable parts.

F. X-ray Powder Difi action Search System. Compounds that fail to crystallize may still be examined by X-ray diffraction, because non-crystalline materials, as powders, give characteristic diffraction patterns. A collection of powder diffraction patterns proves to be a very effective means by which to identify materials and indeed, one of the very earliest search systems in chemical analysis was based upon such data by Hanawalt (21) over forty years ago. The importance of these data in TSCA can be seen by examining the TSCA Inventory regulations for treatment of confidential chemicals (22). Section 710.7 of these regulations indicates that EPA intends to rely on powder diffraction data to assure the validity and seriousness of a manufacturers request for treating information on a chemical as confidential. 

Mott NF (1969) Conduction in non-crystalline materials. 3. Localized states in a pseudogap and near extremities of conduction and valence bands. Phil Mag 19 835... 

The principal breakthrough of the P-E press was the ability to compress a sufficient volume of powder (or size of single crystal) in a pressure cell and press small enough to mount in situ on a neutron beamline. P-E presses have opened high-pressiue neutron diffraction to a much wider range of users, and have become a standard experimental facility at several neutron sources. As a result, they have been used to study a wide range of materials, both crystalline and amorphous [79-83]. The presses have also been widely used at synchrotron sources for studies of non-crystalline materials and liquids, for example [84-86]. 

In situations where V(x9y9z) is not periodic, as for instance in the impurity band of doped semiconductors and in non-crystalline materials, it is still true that if N(E ) vanishes then the material is an insulator at zero temperature, but the converse is not true. This is because a finite value of N(E )9 still within the context... 

Metal-insulator transitions in both crystalline and non-crystalline materials are often associated with the existence of magnetic moments. Moments on atoms in a solid are of course an effect of correlation, that is of interaction between electrons, and their full discussion is deferred until Chapter 3. But even within the approximation of non-interacting electrons in crystalline solids, metal-insulator transitions can occur. These will now be discussed. 

The arguments are reviewed by Kaveh and Wiser (1984). They also describe observations of T2 behaviour in alkali and noble metals and—a point relevant to this book—the application to non-crystalline materials and other materials with short mean free path. They show that the time of relaxation t resulting from electron-electron scattering contains an additional term and is of the form... 

However, for the resistivity of non-crystalline materials at low temperatures, the most important effect is on Lb the inelastic diffusion length. We have seen (Chapter 1, Section 10) that thisisgivenbyLi=(DTi)1/2 and that there is a term in the conductivity proportional to //Lj, so that electron-electron scattering gives a negative term in the resistivity proportional to T with v between 1 and J. 

Fig. 5.4 Density of states in a non-crystalline material, showing the change near the Fermi energy EF predicted by Altshuler and Aronov (1978).

Mott, N. F. (1987) Conduction in Non-Crystalline Materials. Oxford University Press. Mott, N. F. (1988) Phil Mag. B58, 369. 

Mott, N.F. and E.A. Davis Electronic Processes m Non-Crystalline Materials, Oxford University Press, Inc., New York, NY, 1979. 

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