Heterogeneous crystalline phase

When a DP of approximately 10 is reached, the oligomeric polyoxymethylene glycols are no more soluble, precipitation occurs and further polymerization proceeds in the heterogeneous, crystalline phase. The heterogeneous equilibria are... 

Alloys are metallic materials prepared by mixing two or more molten metals. They are used for many purposes, such as construction, and are central to the transportation and electronics industries. Some common alloys are listed in Table 5.5. In homogeneous alloys, atoms of the different elements are distributed uniformly. Examples include brass, bronze, and the coinage alloys. Heterogeneous alloys, such as tin-lead solder and the mercury amalgam sometimes used to fill teeth, consist of a mixture of crystalline phases with different compositions. 

The properties of alloys are affected by their composition and structure. Not only is the crystalline structure important, but the size and texture of the individual grains also contribute to the properties of an alloy. Some metal alloys are one-phase homogeneous solutions. Examples are brass, bronze, and the gold coinage alloys. Other alloys are heterogeneous mixtures of different crystalline phases, such as tin-lead solder and the mercury-silver amalgams used to fill teeth. 

It was found (616) that the course of the heterogeneous reaction of 1-hydroxy-5,5- dimethyl-2,4- diphenyl-3-imidazoline-3-oxide with PhLi depends on the crystalline phase of the starting compound, which can be obtained, predominantly, in cyclic or open chain tautomeric forms. 

Although the role of crystalline phases in the leachability of HT materials is unclear and must be examined from case to case, the identified silicates and oxides are overall more resistant to corrosion than silicate glass and residues of incineration (Scholze 1991). Thus, a clear assessment of the durability of HT materials as a function of crystalline components must take into account the combined effects of their enrichment or depletion in trace metals, their individual leachability, the increase (but sometimes decrease) in overall reactivity due to local heterogeneities and increased Sspec (Jacquet-Francillon et al. 1982 Bickford Jantzen 1984 Jantzen Plodinec 1984 Scholze 1991 Adams 1992 Sproull et al. 1994 Sterpenich 1998). 

Fig. 4. Typical X-ray diffractograms of 100-125 pm ground HT materials, showing purely vitreous character (P8) or vitrocrystalline character (P6, PI 6 ). For most vitrocrystalline samples, mineral phases are distributed throughout the glass matrix (e.g., P6", homogeneous vitrocrystalline), but some samples (e.g., PI6, heterogeneous vitrocrystalline) exhibit patches of concentrated crystalline phases visibly separated from the bulk vitreous matrix.

Whatever the method of control may be it seems evident that it is by no means completely efficient for the solid polymer is always a rather small fraction of the total. Price believes, presumably because any optical activity tends to be concentrated in the crystalline phase, that the amorphous and crystalline polymers are products of two different reactions, one in solution and the other heterogeneous (27). While this view is not impossible, it could be argued that solution reaction might be expected to lead to either inversion or retention of configuration and, hence, optically active polymer. Furthermore, some reports suggest rather strongly that the distinction between the two types of polymer is a rather arbitrary one based in part on polymer symmetry and in part on molecular weight. 

Although nano-scale metal particles have been of leading importance in heterogeneous catalysis for decades, these particles have generally been prepared by high temperatures reduction methods. (1,2,3) Under such conditions approach to the most thermodynamically stable state has often moved further than desirable. That is, metastable mono-metallic phases cannot be prepared because they convert to the most stable crystalline phase, and metastable bimetallic particles would phase separate according to thermodynamic tendencies. 

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