Material characteristics recrystallization

In one of the most significant observations, small amounts of recrystallized material were observed in rutile at shock pressure of 16 GPa and 500 °C. Earlier studies in which shock-modified rutile were annealed showed that recovery was preferred to recrystallization. Such recrystallization is characteristic of heavily deformed ceramics. There has been speculation that, as the dislocation density increases, amorphous materials would be produced by shock deformation. Apparently, the behavior actually observed is that of recrystallization there is no evidence in any of the work for the formation of amorphous materials due to shock modification. Similar recrystallization behavior has also been observed in shock-modified zirconia. 

Hill et al. [117] extended the lower end of the temperature range studied (383—503 K) to investigate, in detail, the kinetic characteristics of the acceleratory period, which did not accurately obey eqn. (9). Behaviour varied with sample preparation. For recrystallized material, most of the acceleratory period showed an exponential increase of reaction rate with time (E = 155 kJ mole-1). Values of E for reaction at an interface and for nucleation within the crystal were 130 and 210 kJ mole-1, respectively. It was concluded that potential nuclei are not randomly distributed but are separated by a characteristic minimum distance, related to the Burgers vector of the dislocations present. Below 423 K, nucleation within crystals is very slow compared with decomposition at surfaces. Rate measurements are discussed with reference to absolute reaction rate theory. 

The o-dichlorobenzene extracts were combined and analyzed by GLC. Four peaks were observed under standard GLC conditions in the 10 to 15 min retention time range which is characteristic of hexachloro-dibenzo-p-dioxins (sample 1 in Table IV). The mixture was fractionally sublimed (120° to 175°G/1 mm). The major crop was harvested at 175 °G and recrystallized from anisole. Analysis of this material by GLG indicated that two isomeric hexachlorodibenzo-p-dioxins were present (sample 2). Overall yield (1.5 grams) of the product was 1-3% at 99+% purity, as determined by GLG and mass spectrometry. 

The characteristics of starting materials and obtained after recrystallization are given in... 

Figure 3.2. Differential calorimetric curves for the molecular glasses (a) Spiro-sexiphenyl (second heating curve) and (b) Spiro-PBD (first and second heating curve). The glass transition is indicated by a characteristic step, the melting point by an endothermic peak. In (a) recrystallization occurs above Tg, which can be seen by an exothermic peak. The material in (b) forms a stable amorphous glass without recrystallization. The melting point from the first heating curve of a crystalline sample (dotted line) disappears in the second heating cycle (solid line). Only the glass transition is visible.

A recent result on Al-implanted 4H-SiC, which is in sharp contrast to the common opinion that amorphization should be avoided, has been reported by E. Kalinina from the Ioffe Institute in St. Petersburg [124]. She showed that very good activation could be achieved by RT implantation of Al at doses in excess of 10 cm . This completely amorphized material is shown to be fairly well recrystallized by an RTA process that also causes some of the Al to diffuse into the low-doped n-type epilayer, forming the p n junction at a larger depth than the highly defective implanted area. Nearly ideal forward IV-characteristics were also shown for current densities of several kA cm . Even if the stability of such heavily implanted devices may be questioned this result shows that there is still a long way to go before a fully optimized implantation technology is at hand. 

An initial run on a 76 ug sample of Nylon 6, 6 yarn (seen in fig. 14) shows a processing effect at about 210°C, the same temperature at which this material displayed a break in the TMA extension curve. After shock cooling, the processing mark has been erased and the cold recrystallization peak appears--an indication that the glass transition is about 25°C lower. Figure 15 shows the effect of program-cooling and the characteristic nylon double peak in the subsequent melt (9). 

Zhang and co-workers reported partial conversion of a mesoporous starting material (SBA-15) into a mesoporous aluminosilicate with zeolitic characteristics in a so-called vapour phase transport method.[82] In this process, Al is firstly introduced onto the mesoporous surface, followed by a filling of the mesopores with a carbonaceous species, and finally a partial recrystallization of aluminosilicate in the vapour of the SDA is conducted. The advantage of this method, compared with the hydrothermal recrystallization method of Kloetstra et al., lies in the fact that the mesopore structure collapses to a lesser extent as the crystallization is limited to the surface of the mesoporous precursor. 

In this section, methods of separation and purification which depend on the partitioning of material between two phases will be considered. If one phase is a solid, the method is called adsorption, and if the process is carried out in such a way as to multiply the efficiency of a single adsorption, it is commonly called chroviatography. If the material is distributed between two liquid phases or a gas and a liquid phase, the process is called extraction. These methods are particularly valuable for separating mixtures, since they discriminate between compounds on the basis of physical properties different from those which were important for the methods previously described. Thus, two solid compounds which cannot be separated conveniently by recrystallization because of their very similar solubility characteristics may often be separated by chromatography. 

In the decompositions of some particularly stable crystalline materials, the reactant stracture does not undergo recrystallization or disintegration, although there may be modification of lattice parameters following the loss of a small stable molecule, such as HjO or NHj, from the reactant phase. Such molecules diffuse outwards between structural components that are sufficiently stable to survive unmodified. Reaction rates are controlled by Fick s laws, ease of movement being determined by the dimensions of the intracrystalline channels. The participation of diffusion control is often recognized by the appearance of the characteristic A term in the rate equation estabhshed. Theoretical aspects of diffusion control have been discussed by Okhotnikov el uf/. [53-57]. 

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