Zur Thermodynamik des Systems Si–C–Cl–H und Darstellung von phasenreinem SiC

A method is described for the calculation of the preparation conditions of binary and more complicated compounds by vapour phase deposition basing upon their thermodynamics. With that the deposition conditions of the pure SiC-Phase in the system Si–C–Cl–H and the codeposition conditions of the coexisting phases were determined. The experimental results are in a good agreement with the considerable large preparation regions found by calculation, expecially above 1700 K.     

A parametric study of the synthesis and purification of single-walled carbon nanotubes using the high-pressure carbon monoxide process

Single-walled carbon nanotubes (SWCNTs) were synthesized using the high-pressure carbon monoxide disproportionation process. The SWCNT diameter, diameter distribution and yield can be varied depending on the process parameters. Important parameters are the temperature, the pressure, the CO gas flow rate and the nozzle injection velocity and geometry for the injection of reactant gas into the reaction zone. Carbon nanotubes as small as 1.0 nm in diameter have been produced. The purity and yield of the deposited material were increased with increasing CO gas flow by means of rapid heating of the gas mixture and using an optimum injection profile. Highly pure SWCNTs were produced at 1250 K, pressures between 5 and 10 bar and gas in the turbulent flow regime in the cold line of 2000–2500 sccm CO. The raw materials were purified by oxidation in high vacuum at 523 K in wet Ar/20 vol. % O₂ to remove SWCNT carbon-like impurities and to oxidize the iron catalyst nanoparticles. The iron oxides were removed by chemical treatment in concentrated HCl/C₂H₅OH mixture solution. The SWCNTs were analyzed by scanning electron microscopy, high-resolution transmission electron microscopy, atomic absorption spectroscopy and optical absorption spectroscopy to determine the purity, the diameter and diameter distribution, the chemical composition and the catalyst morphology, as well as the optical properties of deposited SWCNTs in dependence on the synthesis parameters. PACS 29.30.-h     

Thermotropic liquid crystalline compounds with lateral long-chain substituents (IV). Physical properties of 1,4-bis-[4-n-octyloxybenzoyloxy]-2-n-alkylbenzenes

In 15 Substances of general formula transition temperatures, transition enthalpies and entropies, densities and phase transition volumina, order parameters of a dissolved dye and optical refractive indices were measured. Additional miscibility and X-ray investigations were performed. The results are discussed on the basis of molecular statistical theories of the nematic state and point to a conformation of the long lateral chains which is nearly parallel to the molecular long axes. The results further show the dominating importance of the steric repulsive forces in stabilizing the nematic state.     

Strongly driven exciton resonances in quantum wells: light-induced dressing versus coulomb scattering

The nonequilibrium dynamics of a two-dimensional electron-hole gas is studied in the regime of strong and resonant pumping of the exciton resonance. The Coulomb collision rates are consistently determined by taking into account the light-induced coherence of the two-band system that leads to a dressing of the carrier spectral functions. The light dressing dramatically reduces the Coulomb scattering efficiency. Results are presented for Rabi oscillations in the time domain and dynamical Stark splitting in the pump-probe absorption spectra.     

Generation of frustrated liquid crystal phases by mixing an achiral nematic-smectic-C mesogen with an antiferroelectric chiral smectic liquid crystal

By mixing the achiral liquid crystal HOAB, exhibiting a nematic (N)-smectic-C (SmC) mesophase sequence, with the chiral antiferroelectric liquid crystal (AFLC) (S,S)-M7BBM7, forming the antiferroelectric SmC(a)(*) phase, at least seven different mesophases have been induced which neither component forms on its own: a twist-grain-boundary (TGB(*)) phase, two or three blue phases, the untilted SmA(*) phase, as well as all three chiral smectic-C-type "subphases," SmC(alpha)(*), SmC(beta)(*), and SmC(gamma)(*). The nature of the induced phases and the transitions between them were determined by means of optical and electro-optical investigations, dielectric spectroscopy, and differential scanning calorimetry. The induced phases can to a large extent be understood as a result of frustration, TGB(*) at the border between nematic and smectic, the subphases between syn and anticlinic tilted smectic organization. X ray scattering experiments reveal that the smectic layer spacing as well as the degree of smectic order is relatively constant in the whole mixture composition range in which AFLC behavior prevails, whereas both these parameters rapidly decrease as the amount of HOAB is increased to such an extent that no other smectic-C-type phase than SmC/SmC(*) exists. By tailoring the composition we are able to produce liquid crystal mixtures exhibiting unusual phase sequences, e.g., with a direct isotropic-SmC(a)(*) transition or a temperature range of the SmC(beta)(*) subphase of about 50 K.     

Thermotropic liquid crystalline compounds with lateral long-chain substituents (III). Molar volumina,viscosities, optical refractive indices and order parameters of 1,4-Bis-[4-n-octyloxybenzoyloxy]-2-n-alkylbenzenes

In the substance class of the general formula densities, thermal expansion coefficients, viscosities, optical refractive indices and the order parameters of a dissolved dye were measured in the isotropic and nematic states. The results are discussed briefly.     

Influence of lateral branches on the properties of liquid-crystalline 1,4-Bis-[4-n-hexylbenzoyloxy]-2-subst.-benzenes (I).: measurement of density,viscosity and optical anisotropy

8 compounds of the substance class of the 1,4-bis-[4-n-hexylbenzoyloxy]-2-subset.-benzenes and additionally 1,4-bis-[4-n-hexyleyclohexanoyloxy]-2-ethylbenzene were sythesized and density, viscosity and optical refractive indices were measured in the nematic and isotropic state. The lateral branching causes decrease of the melting and clearing temperatures, diminishing of the optical anisotropy and order parameter, but increase of the viscosity.