8 research outputs found
Thermodynamics of Alon II : phase relations
No full textThe dependence of the lattice parameter of g-aluminum oxynitride (Alon) on its compn. was investigated. The width of the homogeneity of Alon varies with temp.: at 1850 Deg Alon is the stable phase between 66 mol% Al2O3 (lattice parameter 0.7953 nm) and 81 mol% Al2O3 (lattice parameter 0.7932 nm). This region becomes smaller at lower temps. A
Thermodynamics of Alon I : stability at lower temperatures
No full textThe formation and stability of aluminum oxynitride (Alon) under various circumstances are described using Ellingham and activity diagrams. With the help of these diagrams, the connection between seemingly uncorrelated exptl. observations on the stability of Alon from the literature can be understood. Thermodn. data sets for Alon are crit. reviewed. Alon is stable only within a small region of oxygen and nitrogen pressures, and it is not stable a
Thermal expansion of cubic Si3N4 with the spinel structure
No full textThe thermal expansion coeff. of cubic Si3N4 with the spinel structure was detd. with high temp. x-ray diffraction. The exptl. value agrees well with the lattice parameter predicted by first principles methods, and is significantly larger than the value for b-Si3N4. This difference is discussed in terms of the chem. bonding in these 2 modifications of Si3N4
Effect of high-temperature treatment on Fe/ZSM-5 prepared by chemical vapor deposition of FeCl3. I. Physicochemical characterization
No full textThe effect of severe (hydrothermal) treatment on Fe/ZSM-5 prepared by sublimation of FeCl3 is studied by a combination of high-resolution TEM, EXAFS, 57Fe Mössbauer spectroscopy, IR, UV–vis, nitrogen adsorption, 27Al NMR spectroscopy, and low-temperature nitrous oxide decomposition. The heterogeneous nature of Fe/ZSM-5 is stressed with a preponderance of iron oxide particles on the external zeolite surface. Additionally, neutral iron oxide nanoparticles and charge-compensating iron complexes are located in the micropores. Severe calcination at 973 K induces the growth and ordering of the iron oxide aggregates. Moreover, some of the occluded neutral iron oxide nanoparticles are transformed into charge-compensating iron complexes upon a protolysis reaction with the Brønsted protons. These effects are more pronounced in the case of steaming at 973 K, additionally resulting in the removal of Al from framework positions. Despite the low dispersion of iron oxide in Fe/ZSM-5, relatively low Fe---Fe coordination numbers were derived from the EXAFS data for Fe/ZSM-5; high-temperature treatments increased this number. This low value points to the disordered nature of the iron oxide aggregates rather than to the presence of an abundant fraction of binuclear iron clusters. Titration of sites active in nitrous oxide decomposition shows that their amount increases upon increasing severity of treatment of Fe/ZSM-5. Their number, however, remains very small (a few percent of the total iron) and appears to correlate to the amount of Fe2+ present after room temperature exposure to vacuum conditions. A comparison to a commercial HZSM-5 zeolite with a very low iron content is made. The catalytic performance of these materials is discussed in a companion paper (J. Catal. (2003))
Nanoscale fibrillar crystals of PET from dilute quiescent solution
No full textThe crystallization behavior of poly(ethylene terephthalate) (PET, IV 2 dL/g) from solution in biphenyl¿diphenyl ether mixed solvent is examined. Reversible gelation of the polymer solution is observed during cooling of the solutions. Light scattering and DSC analysis are used to follow the heating and cooling processes, thus determining the crystallization temperature and the melting point, which are found to be nearly independent of the polymer concentration (0.25¿5%). High degree of crystallization (>50%) is observed in the PET crystallized from the solution at 170 ¿C. Morphological characteristics of the crystals obtained after solvent removal are determined by WAXD, FTIR, SEM and TEM examination. The crystallization of PET into unique high aspect ratio fibrillar morphology during cooling of the solutions explains their gelation even at low PET concentration. Thin films made from the thus obtained PET could be drawn five times at 250 ¿C, resulting in only moderate values of modulus and strength
