Crystalline-amorphous transition

The major area of application for solids and liquids is chemical fingerprinting and the identification of unknown compounds. For solids, Raman is also used for phase identification, following amorphous/crystalline transitions, measurement of stress and strain, and, in the microscope mode, the detection and analysis of defects, including particles during wafer processing. 

Remark In the field of structural investigations one must mention the work of Kuzmenko et al. (1972, 1976, 1980, 1984a,b, 1986a,b) and Kuzmenko and Melnikov (1982, 1988) who have studied the amorphous crystalline transition in Yb films. It is observed to occur under particular experimental conditions application of a perpendicular magnetic field, a flow of helium gas, an oblique deposition and a short current pulse. The most important parameter is the thickness of the sample. The transition is explained in terms of avalanche (shock) crystallization. However no XRD or EDP data are exhibited and the crystalline phases of the metal (fee or hep) are not defined. One must notice that Haussler and Bauman (1980) using a parallel (to the surface) magnetic field do not observe this transition at 4,2 K. 

To erase information by the transition amorphous — crystalline, the amorphous phase of the selected area must be crystallized by annealing. This is effected by illumination with a low power laser beam (6—15 mW, compared to 15—50 mW for writing/melting), thus crystallizing the area. This crystallization temperature is above the glass-transition point, but below the melting point of the material concerned (Eig. 15, Erase). 

Partially Crystalline Transition Metal Sulphide Catalysts. Chiannelli and coworkers (6, 7, 8) have shown how, by precipitation of metal thio-molybdates from solution and subsequent mild heat-treatment many selective and active hydrodesulphurization catalysts may be produced. We have shown (18) recently that molybdenum sulphide formed in this way is both structurally and compositionally heterogeneous. XRES, which yields directly the variation in Mo/S ratio shows up the compositional nonuniformity of typical preparations and HREM images coupled to SAED (see Figure 2) exhibit considerable spatial variation, there being amorphous regions at one extreme and highly crystalline (18, 19) MoS at the other. 

At a given (low) temperature and pressure a crystalline phase of some substance is thermodynamically stable vis a vis the corresponding amorphous solid. Furthermore, because of its inherent metastability, the properties of the amorphous solid depend, to some extent, on the method by which it is prepared. Just as in the cases of other substances, H20(as) is prepared by deposition of vapor on a cold substrate. In general, the temperature of the substrate must be far below the ordinary freezing point and below any possible amorphous crystal transition point. In addition, conditions for deposition must be such that the heat of condensation is removed rapidly enough that local crystallization of the deposited material is prevented. Under practical conditions this means that, since the thermal conductivity of an amorphous solid is small at low temperature, the rate of deposition must be small. 

A polymer may be amorphous, crystalline, or a combination of both. Many polymers actually have both crystalline and amorphous regions, i.e., a semicrystalline polymer. The Tg is a transition related to the motion in the amorphous regions of the polymer [3,8,9], Below the Tg, an amorphous polymer can be said... 

Figure 1.26 summarizes the property behavior of amorphous, crystalline, and semicrystalline materials using schematic diagrams of material properties plotted as functions of temperature. Again, pressures affect the transition temperatures as schematically depicted in Fig. 1.27 for a semi-crystalline polymer. 

Another interesting example is a crystalline to amorphous phase transition in Eu(OH)3 (Chen et al., 1994a) at room temperature. The initial structure is the UCI3 -type structure (space group P63 /m ) which was confirmed by X-ray diffraction. Figure 11 shows that broad bands appear under pressure and completely replace the former sharp lines at around 5.5 GPa. The... 

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