Search results for " Transition"
showing 10 items of 2751 documents
Die Kristallisation von Mehrkomponentensystemen aus hochpolymeren Stoffen
1977
Es wird untersucht, welche Anderungen im Kristallisationsverhalten hochpolymerer Stoffe auftreten, wenn man von einem Einkomponentensystem auf ein Mehrkomponentensystem ubergeht. Beispiele fur Mehrkomponentensysteme sind Homopolymere mit einer breiten Molekulargewichtsverteilung, Mischungen aus verschiedenen Homopolymeren, gequollene Polymere, Blockcopolymere und statistische Copolymere. Das Auftreten einer Molekulargewichtsverteilung ausert sich vor allem bei einer Kristallisation mit gestreckten Ketten, und zwar in der Weise, das eine Fraktionierung nach Molekullangen auftritt. Des weiteren bewirkt sie eine Verbreiterung des Schmelzbereichs. Die Anwesenheit eines zweiten nichtkristallisie…
Infrared study of the MoO3 doping efficiency in 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP)
2013
AbstractElectrochemical doping produces clear changes in the vibrational spectra of organic semiconductors as we show here for the system molybdenum oxide (MoO3) doped into the charge transport material 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP). Based on density-functional theory (DFT) calculations of vibrational spectra, the new spectral features can be attributed to the CBP cation that forms as a result of electron transfer from CBP to MoO3. The intensity of the new vibrational lines is a direct measure for the probability of charge transfer. MoO3 agglomerating within the CBP matrix limits the active interface area between the two species. The appearance of a broad electronic transition …
High-pressure study of substrate material ScAlMgO4
2011
We report on the structural properties of ScAlMgO4 studied under quasi-hydrostatic pressure using synchrotron high-pressure x-ray diffraction up to 40 GPa. We also report on single-crystal studies of ScAlMgO4 performed at 300 K and 100 K. We found that the low-pressure phase remains stable up to 24 GPa. At 28 GPa, we detected a reversible phase transformation. The high-pressure phase is assigned to a monoclinic distortion of the low-pressure phase. No additional phase transition is observed up to 40 GPa. In addition, the equation of state, compressibility tensor, and thermal expansion coefficients of ScAlMgO4 are determined. The bulk modulus of ScAlMgO4 is found to be 143(8) GPa, with a str…
Nucleation mechanism for the direct graphite-to-diamond phase transition
2011
Graphite and diamond have comparable free energies, yet forming diamond from graphite is far from easy. In the absence of a catalyst, pressures that are significantly higher than the equilibrium coexistence pressures are required to induce the graphite-to-diamond transition. Furthermore, the formation of the metastable hexagonal polymorph of diamond instead of the more stable cubic diamond is favored at lower temperatures. The concerted mechanism suggested in previous theoretical studies cannot explain these phenomena. Using an ab initio quality neural-network potential we performed a large-scale study of the graphite-to-diamond transition assuming that it occurs via nucleation. The nucleat…
Pressure-induced phase transitions in AgClO4
2011
11 pags, 9 figs, 4 tabs. -- PACS number(s): 62.50.−p, 64.70.K−, 61 .50.Ks, 64.30.−t
New Pressure-Induced Polymorphic Transitions of Anhydrous Magnesium Sulfate
2017
The effects of pressure on the crystal structure of the three known polymorphs of magnesium sulfate have been theoretically study by means of DFT calculations up to 45 GPa. We determined that at ambient conditions gamma MgSO4 is an unstable polymorph, which decompose into MgO and SO3, and that the response of the other two polymorphs to hydrostatic pressure is non isotropic. Additionally we found that at all pressures beta MgSO4 has a largest enthalpy than alpha MgSO4. This indicates that beta MgSO4 is thermodynamically unstable versus alpha MgSO4 and predicts the occurrence of a beta alpha phase transition under moderate compression. Our calculations also predict the existence under pressu…
Salt-induced microheterogeneities in binary liquid mixtures
2017
The salt-induced microheterogeneity (MH) formation in binary liquid mixtures is studied by small-angle x-ray scattering (SAXS) and liquid state theory. Previous experiments have shown that this phenomenon occurs for antagonistic salts, whose cations and anions prefer different components of the solvent mixture. However, so far the precise mechanism leading to the characteristic length scale of MHs has remained unclear. Here, it is shown that MHs can be generated by the competition of short-ranged interactions and long-ranged monopole-dipole interactions. The experimental SAXS patterns can be reproduced quantitatively by fitting to the derived correlation functions without assuming any speci…
The synthesis of zeolites from dry powders
1995
Summary ZSM-5 was synthesized by different methods with a gradually decreasing water content ending in a reaction mixture with absolutely dry reagents in form of a powder in the complete absence of a solution phase. Amorphous precursors obtained by drying SiO 2 * Al 2 O 3 gels at 650°C were transformed into zeolites in the presence of dried NH 4 F and TPABr. The reaction products were characterized by XRD, REM, TG/DTA, MAS NMR and Electron Micropobe. Pure ZSM-5 or Silicalite-1 was obtained in all cases. Some water is probably formed as a reaction product, but the maximum water pressure is appreciably below the saturation pressure at the given reaction conditions. To explain the formation of…
The Scope of Zinc Oxide Bulk Growth
2005
The techniques classically used for the growth of bulk ZnO crystals, melt-growth, vapour growth, solution-growth and hydrothermal growth, are reviewed. In the case of vapor growth, numerical simulations suggest that such species as H2 + H2O, CH4, Zn, C or Fe could act as sublimation activators. The electrical and structural properties of crystals obtained by these various techniques are compared.
Studies of structures and properties of polymeric systems containing bis-(hydroxy-arylidene)alkanones as NLO-active chromophores
2002
Abstract NLO-properties of polymer systems containing bis-(hydroxy-arylidene)alkanone chromophores were studied experimentally and analyzed using ab initio quantum chemical calculations. A monoclinic crystal structure (space group P2111) of the polyester containing fragments of such chromophores in the backbone was simulated and a reasonable agreement between the experimental and simulated X-ray powder diffraction patterns was achieved. Ab initio quantum-mechanical estimations of the SHG-observable macroscopic second-order non-linearity tensor coefficients, obtained for the polymer crystal structure at the HF SCF level, led to the major dXZZ-coefficient of 1.9 pm/V. Films of ionic complexes…