Feststoffbildung durch Kristallisation und Fällung

Formation of solids by crystallization and precipitation. Formation of solids from solutions can take place by crystallization or precipitation. The principal factor is the relationship between solubility and supersaturation. In crystallization, the solubility of the crystallising substance is so high that the formation of solids occurs largely in the metastable zone in the immediate vicinity of the solubility limits. Crystal growth and nucleation are functions of supersaturation. If a high supersaturation is required at low substance-specific growth rates for an adequate crystal growth, high nucleation rates and hence small crystals will result. On this basis, the authors present information for the specific design of crystallization processes. In contrast, the solubility of the precipitated product must be very low for precipitation. Direct crystal formation is possible only for substance systems of high solubility. As a rule, however, the precipitated substance is so insoluble that solids are formed via amorphous intermediates. The results of a large number of experiments show the influence of various parameters of the precipitation process on the filtrability of the precipitated product.     

Photoconductivity of polymeric dibenz [b,i] 1,4,8,11-tetraaza [14] annulenato (μ-pyrazin)iron(II) [Fetaa(pyz)]n

Summary Results of experiments on dark- and photoconductivity of polymeric dibenz[b,i] 1,4,8,11-tetraaza[14]annulenato(-pyrazin) iron(II) [Fetaa(pyz)]_n are described. By studying the dependence of photoconductivity on electric field, intensity and wavelength it is shown that photogeneration of charge carriers occurred at short wavelengths by an Onsager mechanism and in the near-infrared region by a photoinjection process from the electrodes.Dedicated to Prof. Dr. G. Manecke on the occasion of his 70th birthdayThe authors would like to thank the Volkswagen Foundation for financial support.     

Thermal and dynamic mechanical properties of hydroxypropyl cellulose films

Differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) were used to characterize the morphology of solvent cast hydroxypropyl cellulose (HPC) films. DSC results were indicative of a semicrystalline material with a melt at 220°C and a glass transition at 19°C (T₁), although an additional event was suggested by a baseline inflection at about 80°C (T₂). Corresponding relaxations were found by DMTA. A secondary relaxation at ?55°C was attributed to the interaction between hydroxyl groups of the polymer and residual diluent. The tan δ peak at T₂ was found to arise from an organized phase, presumably from a liquid-crystal mesophase formed while in solution. Crosslinking with a diisocyanate increased the peak temperature of the two primary relaxations, and resulted in a more clearly defined peak for the T₂ transition. From this behavior it was concluded that both T₁ and T₂ are similar to glass transitions (T_g's) associated with an amorphous component and a more highly ordered phase (due to a residual liquid crystal superstructure) in the HPC bulk.     

Molgewichtsverteilung und Langkettenverzweigung bei der anionischen Polymerisation von Butadien

The copolymerization of butadien with small amounts of divinyl benzene (DVB) has been investigated. The polymerization has been carried out with a lithiumamide as initiator in n-hexane as solvent under addition of tetrahydrofuran (THF) to govern the content of vinyl groups. By gel permeation chromatography coupled with a viscometer the molecular weight distribution (MWD) and long chain branching as a function of the conversion and of the molecular weight have been determined. Using a differential refractometer and a UV-photometer as detectors the distribution of the branching agent DVB vs. molecular weight has been studied. With our example long chain branching decreases with increasing conversion. The insertion of DVB occurs mainly in the first phase of the reaction. With increasing conversion the MWD becomes broader. At very high degrees of conversion the molecular weight increases considerably by coupling reactions.     

Spectroscopic Evidence for a Nitrite Intermediate in the Catalytic Reduction of NOx with Ammonia on Fe/MFI

The mechanism of selective catalytic reduction (SCR) of NO_x with NH₃ over Fe/MFI was studied using in situ FTIR spectroscopy. Exposing Fe/MFI first to NH₃ then to flowing NO + O₂ or using the reversed sequence, invariably leads to the formation of ammonium nitrite, NH₄NO₂. In situ FTIR results in flowing NO + NH₃ + O₂ at different temperatures show that NH₃ is strongly adsorbed and reacts with impinging NO_x. The intensity of the NH₄NO₂ bands initially increases with temperature, but passes through a maximum at 120 °C because the nitrite decomposes to N₂ + H₂O. The mechanistic model rationalizes that the consumption ratio of NO and NH₃ is close to unity and that the effect of water vapor depends on the reaction temperature. At high temperature H_2O enhances the rate because it is needed to form NH₄NO₂. At low temperature, when adsorbed H₂O is abundant it lowers the rate because it competes with NO_x for adsorption sites.     

Formation of internal cracks in steel billets during soft reduction

To investigate the formation of internal cracks in steel billets during soft reduction, fully coupled thermo-mechanical finite element models were developed using the commercial software ABAQUS, also casting and soft reduction tests were carried out in a laboratory strand casting machine. With the finite element models, the temperature distribution, the stress and strain states in the bil-let were calculated. The relation between internal cracks and equivalent plastic strain, as well as maximal principal stress was ana-lyzed. The results indicate that tensile stresses can develop in the mushy zone during soft reduction and the equivalent strain nearby the zero ductility temperature (ZDT) increases with decreasing solid fraction. Internal cracks can be initiated when the accumulated strain exceeds the critical strain or the applied tensile stress exceeds the critical fracture stress during solidification.     

Stereological determination of curvature distributions of spatial fibre systems

Under the assumption of homogeneity and isotropy, this paper solves the problem of determining spatial curvature distributions of fibre processes from curvatures of projection curves from thin sections. As an example, a sample of dislocation lines in a hot-rolled Fe-Ni alloy is discussed.