Microcrystalline phases

Cubic Phase of Boron Nitride c-BN. The cubic phase of boron nitride (c-BN) is one of the hardest materials, second only to diamond and with similar crystal structure. It is the first example of a new material theoretically predicted and then synthesized in laboratory. From automated synthesis a microcrystalline phase of cubic boron nitride is recovered at ambient conditions in a metastable state, providing the basic material for a wide range of cutting and grinding applications. Synthetic polycrystalline diamonds and nitrides are principally used as abrasives but in spite of the greater hardness of diamond, its employment as a superabrasive is limited by a relatively low chemical and thermal stability. Cubic boron nitride, on the contrary, has only half the hardness of diamond but an extremely high thermal stability and inertness. 

Microcrystals exhibit properties distinctly different from those of bulk solids. The fractional change in lattice spacing has been found to increase with decreasing particle size in FejOj. Magnetic hyperfine fields in a-FejOj and FejO are lower in the microcrystalline phase compared to those of the bulk crystalline phases. The tetra-gonality (i.e. the departure of the axial ratio from unity) of ferroelectric BaTiOj decreases with decrease in particle size in PZT, the low-frequency dielectric constant decreases and the Curie temperature increases with decreasing particle size. The small particle size in microcrystals cannot apparently sustain low-frequency lattice vibrations. 

Currently silicon is still one of the most important semiconductors as it is the basis of any computer chip. It exhibits an indirect band gap of 1.1 eV at room temperature in the microcrystalline phase. Similar to Ge, silicon nanoparticles show a size-dependent photoluminescence. It was reported by Katayama el al. that a thin Si layer can be electrodeposited in l-ethyl-3-methylimidazolium hexafluorosilicate at 90 °C [44], However, upon exposure to air the deposit reacted completely to SiC>2, which makes it difficult to decide whether the deposit was semiconducting or not. Recently, we showed for the first time that silicon can be well electrodeposited from SiCU in the air and water stable ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([BMPJTfiN) [45, 46]. This ionic liquid can be... 

However, the syndiotactic segments of extended chains (613 cm"1) are obviously situated in a different local environment. These more rigid segments form into microcrystalline phases which orient as units to much higher degrees than the amorphous isotactic sequences. The C-Cl peak at 613 cm"1 arises mainly from seqeunces of four or more trans conformations in syndiotactic repeat units. With syndiotactic sequences of four or more repeat units, crystallinity can form (17,18). This ease of crystallization has been ascribed to the strong dipole-dipole interaction between the C-Cl bonds. 

Electron Electron diffraction, including LEED, RHEED 1 nm Identification of microcrystalline phases Li-U... 

In addition, it is important to note that the isotropic forms of pyrolytic carbon produce at least a two-phase microstructure during formation, consisting of the previously-described turbostratic microcrystalline phase along with an amorphous carbon phase presumably interspersed between the crystalline regions (12). 

The presence of a non-crystalline or a microcrystalline phase in a sample produces a broad hump (s.c. amorphous halo) in the diffraction pattern. The area under the halo is proportional to the content of the amorphous material. The complex ash samples often produce several partially overlapping halos in the diffraction pattern. This, and the presence of diffraction peaks due to crystalline phases have presented a challenge for the integration. The matrix effects need also be taken into account in the calibration. In this work we report results from a frilUscale FB test. Selected samples were analysed with XRD and results were compared to the SEM-EDS data of the same samples. It was found that the content of amorphous material in the bed increased as the test proceeded. 

Fig. 6.4.2. chemical shift powder pattern spectra of Boc-Gly-Gly-[ N]Gly-OBz obtained by cross-polarization and H decoupling. (A) Crystalline phase (monoclinic). (B) Microcrystalline phase (triclinic). Dotted lines represent spectral simulations. Asymmetry, n, for the mono-clinic sample is 0.064 while 17 = 0.44 for the triclinic sample. Chemical shift reference is arbitrary. (Reprinted with permission from Hiyama et al. [10].)... 

For a crystalline/crystalline blend, Yoshie et al. [151] studied blends of PVA and poly(3-hydroxybutyrate) (PHB). They found that PVA/PHB is compatible only when the blend contains a larger amount of PVA, and Model C was found with amorphous and crystalline PHB. Kwak et al. [94] studied poly(ether-ester)/PVC to find a common Ti, but double-exponential Tip decays. Model B was proposed with a mixed amorphous phase and two microcrystalline phases for component polymers. Note that Guo [95] reexamined this blend and pointed out that these assignments have to be reconsidered. 

Instead, the presence of vanadium in all samples has been detected by UV-VIS spectroscopy. The sample Nb/V=9/1 even shows the presence of a microcrystalline phase (UV band at about 420 nm) as reported in figure 1 ... 

In the case of using a-SiC H as window material in p-i-n amorphous solar cells this will not only reduce the interface reflections and surface absorptions of incident light, but also greatly improve the transmissivity of the p layer in the solar cells. By introducing a microcrystalline phase into a-SiC to obtain the higher doping efficiency, the efficiency of the solar cells can be increased further [17]. 

Germanium is an important elemental semiconductor which exhibits an indirect band gap of 0.67 eV at room temperature in the microcrystalline phase. In contrast to the microcrystalline element, nanocrystalline germanium is a direct semiconductor and a promising material in the optoelectronic industry. The... 

Morphology. In the development of the diffusion sorption, and permeation Eqs. 11.2 through 11.13, it is assumed that the polymer phase is a homogeneous and isotropic phase that is, an isotropic amorphous polymer. The presence of a crystalline microphase complicates this assumption considerably and makes the diffusion process in semicrystaUine polymers a complex phenomenon. SemicrystaUine polymers consist of a microcrystalline phase dispersed in an amorphous phase. The dispersed crystalhne phase decreases the... 

The catalysis science of supported metal oxide catalysts, especially supported vanadia catalysts, has lagged behind their industrial development. In the 1970s, two models were proposed for the active metal oxide component a three-dimensional microcrystalline phase (e.g., small metal oxide crystallites) or a two-dimensional surface metal oxide overlayer (e.g., surface metal oxide monolayer). In the 1980s, many studies demonstrated that the active metal oxide components were primarily present as two-dimensional surface metal oxide overlayers, below monolayer coverage, and that the surface metal oxide overlayers control the catalytic properties of supported metal oxide catalysts. The synergistic interaction between the surface vanadia overlayer and the underlying oxide support prompted Ceilings to state. . that neither the problem of the structure of suppored vanadium oxide nor that of the special role of TiOa as a support have definitely been solved. Further work on these and related topics is certainly necessary. In more recent years, many fundamental studies have focused on the molecular structural determination of the surface vanadia phase and to a lesser extent the molecular structure-reactivity relationships of supported vanadia catalysts. " ... 

Figure 10.3 Typical DSC thermograms exhibiting multiple thermal transitions in TPU. (1) Multiple thermal transitions in an unannealed specimen, in which endotherm I represents the disordering of short-range order, endotherm II represents the disordering of long-range order, and endotherm III represents the fusion of microcrystalline phase. (2) Thermal transition in a specimen that was annealed at 130 °C. (3) Thermal transition in a specimen that was annealed at 150 °C for a long period. (Reprinted from Seymour and Cooper, Journal of Polymer Science, Polymer Letter Edition 9 689.

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