Nucleation, preferred orientations

X-ray difl raaion (structure grain size preferred orientation stress) Scanning laser microscopy Optical microscopy Oocnl thickness topography nucleation general morphology internal oxidation) l.R. spectroscopy (specialised analysis and applications)... 

Both columnar and equiaxed microstructures are obtained from polycrystal growth. Columnar micro structure is composed of columnar grains of preferred orientation that grow from the first nucleated location on a substrate surface. The microstructure is attributed to high supersaturation and low temperature and hence more limited diffusion. A columnar microstructure is suitable for high-temperature structural applications, such as thermal barrier coatings. Figure 6.6 shows the typical columnar CVD SiC microstructure in C/SiC composites. 

Specimens that were simply pressed in a mold and not extruded showed no preferred orientation there were rings in the X-ray diffraction patterns. Blends between c. 40% and c. 80% PC containing predominantly PC crystallites which were caused by the presence of the COP phase. They were not found in the pure PC. Good mechanical mixing turned out to be a precondition for effective nucleation. Bad mechanical mixture... 

After nucleation in the apatite-leucite glass-ceramic, however, the apatite grows anisotropicaily in a preferred orientation into needlelike apatite (Holand et al., 1995). Previously, these crystals had only been produced under hydrothermal conditions (Newesely 1972 Jaha et al., 1997). Their morphology corresponds to that of the hydroxyapatite in natural teeth (enamel). 

A calcium metaphosphate glass rod composed of Ca0 P205 = 0.95 (molar ratio) was produced. One area of this rod was heated to the softening point in a special furnace at 600°-650°C. In the process, nucleation and controlled ciystallization of P-Ca(P03)2 needlelike crystals occurred. It was interesting to note that crystals formed along the glass rod in one direction only, from the hotter to the colder areas. As the glass rod was pushed farther into the furnace, crystallization continued in the hot areas until unidirectional Ca-metaphosphate was produced as the main crystal phase. That is, the crystals were precipitated anisotropically and with preferred orientation. 

Here, the diffusing single molecules can be pinned at these defects, if low deposition rates are used. The molecules then form nucleation centers at which crystallites are formed. There is no evidence that the molecules in these crystallites have a preferred orientation with respect to the substrate surface. If the deposition rate is enhanced, aggregation is kinetically favored if compared to diffusion. Therefore closed layers are formed like on amorphous substrates in which the molecules... 

Huang et al. [2001] conducted an extensive study of EO crystal orientation as a function of crystallization temperature within the cylinders of an EO/S diblock (EO/S 9/9 blended with S homopolymer to produce EO cylinders). When crystallized at —50°C, where the nucleation density is quite high and the resulting crystals quite small, no preferred orientation was present. At Tc = 10°C and above, the crystals were oriented so that the fast-growth direction for EO (the [120] direction) was aligned with the cylinder axis, as shown in Figure 6.2. When crystallized between —30°C and 0°C, a preferred tilt of the fast growth direction to the cylinder axis was observed, as Quiram et al. [1998]... 

Figure 5. Top, preferred orientation exhibited by a) benzoic acid crystals and b) 4-nitroaniline crystals grown from the gas phase onto (OTO)sa- Nucleation is initiated at the [10T]sa ledges. Bottom, schematic representation of the orientations of benzoic acid and 4-nitroaniline on (OTO)sa. The lattice parameters and directions for succinic acid, benzoic acid and 4-nitroaniline are also depicted. (Adapted from ref. 15.)...

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