Crystal activity

Semiconducting Properties. Sihcon carbide is a semiconductor it has a conductivity between that of metals and insulators or dielectrics (4,13,46,47). Because of the thermal stabiUty of its electronic stmcture, sihcon carbide has been studied for uses at high (>500° C) temperature. The Hall mobihty in sihcon carbide is a function of polytype (48,49), temperature (41,42,45—50), impurity, and concentration (49). In n-ty e crystals, activation energy for ioniza tion of nitrogen impurity varies with polytype (50,51). 

A crystal activated with Ti + ions presents an absorption that peaks at 514 nm and the corresponding emission spectrum peaks at 600 nm. A sample of this crystal, which has an optical density of 0.6 at the absorption peak, is illuminated with an Ar laser emitting at 514 nm with a power of 2 mW. (a) Determine the laser power of the beam after it passes through the crystal, (b) If the quantum efficiency isrj = 0.6, determine the intensity (in photons per second) emitted as luminescence and the power dissipated as heat in the crystal. 

Niviere, V., Hatchikian, C., Cambillau, C. and Frey, M. (1987) Crystallization, preliminary X-ray study and crystal activity of the hydrogenase from Desulfovibrio gigas. J. Mol. Biol., 195, 969-71. 

Kaiser and Garret i D first reported two-photon absorptions in CaFj crystals activated with Eu ". ... 

All unite developed up to now are based on use of an active oscillator, as shown schematically in Fig, 6.5. This circuit keeps the crystal actively in resonance so that any type of oscillation duration or frequency measurement can be carried out. In this type of circuit the oscillation is maintained as long as sufficient energy is provided by the amplifier to compensate for losses in the crystal oscillation circuit and the crystal can effect the necessary phase shift. The basic stability of the crystal oscillator is created through the sudden phase change that takes place near the series resonance point even with a small change in crystal frequency, see Fig. 6.6. 

The structure is thoroughly analyzed to identify putative binding sites, e.g., by the known location of co-crystallized active compounds, or by applying in silico methods to identify such sites... 

It was possible for two of the systems chosen that the nucleation and crystallization activation energies could be determined separately by distinguishing the induction period and crystal growth period in the overall crystallization process. Of the two hypotheses proposed for zeolite crystallization, in the gel phase or from the solution phase, the data support the latter hypothesis for crystal growth with the crystal-liquid surface enhancing the nucleation process in seeded systems. The precise mechanism of nucleation in unseeded systems remains to be determined. 

CARBORANE. A cry stalline compound composed of boron, carbon, and hydrogen. It can be synthesized in various ways, chiefly by the reaction of a borane (penta-or deca-) with acetylene, either at high temperature in the gas phase or in the presence of a Lewis base. Alkylated derivatives have been prepared. Carborancs have different structural and chemical characteristics and should not be conTused with hydrocarbon derivatives or boron hydrides. The predominant structures arc the cage type, the nest type, and the web type, these terms being descriptive of the arrangement of atoms in the crystals. Active research on cargorane chemistry has been conducted under sponsorship of the U.S. Office of Naval Research, http //www.onr.navy.mil/... 

The topochemical differences in the physical environment of dissolved and crystallized active centers explain easily the difference in reactivity ratios between the two phases. The gain in free energy arising from immediate crystallization of growing chain ends enhances the incorporation of trioxane into the crystalline copolymer. Simultaneous crystallization is considered an important driving force in copolymerization as well as in the homopolymerization of trioxane. On the other hand, dioxolane units do not fit the crystal lattice of polyoxymethylene and reduce the crystallinity of the polymer. This impedes the incorporation of dioxolane units into the crystalline copolymer. 

Taking standard states of crystal and melt phase to be pure NaAlSi20g crystal and liquid, respectively, at the pressure and temperature of interest, Blundy et al. then converted crystal activities to composition using the observation of Holland... 

S. R. Rotman, E. Luria, N. Yitzhaki, and A. Eyal, Proc. SPIE-Int. Soc. Opt. Eng., 1996, 2716 (Spectroscopy of Crystals Activated by Rare-Earth and Transition Metal Ions), 20. 

The substance generally used to detect x-rays is a sodium iodide crystal activated with a small amount of thallium. It emits violet light under x-ray bombardment. (The details of this emission are roughly as follows. Absorbed x-rays ionize some atoms, i.e., raise some electrons from the valence to the... 

The decrease in crystal activity at r , coupled with strongly hindered diffusion in solution in the SECM configuration, results in a rapid depletion in the local concentration of electroactive material in the tip-substrate gap and crucially at the crystal/solution interface. Eventually a value for the interfacial undersaturation is reached that exceeds the threshold required to spontaneously nucleate fresh dissolution sites from the perfect surface and dissolution is initiated again at rcrit. This is reflected in the sudden surge of current. To model this simply, the entire surface was allowed to become active once the undersaturation at any point on the surface exceeded a critical value (50). Thus, the following condition applied, in conjunction with Eqs. (31) and (33) ... 

The glucamylase from Rhizopus delemar has been purified free from a-amylase and, later, crystallized. Activity was optimal at pH 4.5 and was diminished by calcium ions (compare, a-amylases). With amylose, amylopectin, glycogen, and soluble starch, the conversion into D-glucose exceeded 90% " and, since residual dextrins were not formed, amyloly-sis was not impeded by the (1 6)-interchain linkages. Enzymes showing similar activity are also produced by black-koji mold (Aspergillus awamori (Kawachi))" and the bacterium Clostridium acetohutylicum ... 

Solid components not pure crystals, activities non-zero... 

Of all the scintillators existing in the market, the Nal crystal activated with thallium, NaI(Tl), is the most widely used for the detection of y-rays. Nal(Tl) scintillation counters are used when the energy resolution is not the most important factor of the measurement. They have the following advantages over Ge(Li) and Si(Li) detectors ... 

A further development of the displayed approach to the electron-phonon interaction and relaxation processes in crystals activated by lanthanide ions came from the high-resolution study of Pr CsCdBr3 [7]. These crystals have recently attracted a considerable interest being a promising material for up-conversion lasers due to the property of their quasi-one-dimensional lattice to incorporate Ln ions in pairs. 

Morozov A, Morozova L, Trefilov A, Feofilov P (1970) Spectral and Inminescent characteristics of flnorapatite single crystals activated by rate earth ions. Opt Spectros 29 590-596 Mnrray JR, Oreskes N (1997) Uses and hmitations of cathodolnminescence in the stndy of apatite paragenesis. Econ Geol 92 368-376... 

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