Phase purity

Zeolites are tire product of a hydrotliennal conversion process [28]. As such tliey can be found in sedimentary deposits especially in areas tliat show signs of fonner volcanic activity. There are about 40 naturally occurring zeolite types. Types such as chabazite, clinoptilolite, mordenite and phillipsite occur witli up to 80% phase purity in quite large... 

The first analytical tool to assess tire quality of a zeolite is powder x-ray diffraction. A collection of simulated powder XRD patterns of zeolites and some disordered intergrowths togetlier witli crystallographic data is available from tlie IZA [4o]. Phase purity and x-ray crystallinity, which is arbitrarily defined as tlie ratio of tlie intensity of... 

Donnet, M., Jongen, N., Lemaitre, J., Bowen, P. and Hofmann, H., 1999. Better control of nucleation and phase purity using a new segmented flow tubular reactor Model system Precipitation of calcium oxalate. In 14th International Symposium on Industrial Crystallization. Cambridge, U.K., September 12-16, Institution of Chemical Engineers, CD ROM, pp. 1-13. 

Phase purity and crystallinity of the obtained zeolites were characterized using XRD (Siemens 5005). The morphology and crystal size were determined using SEM (Philips... 

The structure and phase purity of the synthesized materials were determined by X-ray powder diffraction (XRD). The morphology was investigated by scanning electron microscopy (SEM). 

Crystallinity/phase purity lattice expansion due to Fe incorporation Absence of amorphous matter outside the crystalline phase distribution of Fe. 

Phase purity of the sample. Crystalline impurity phases present in a powder sample (e.g. residual amounts of starting materials from a synthetic procedure) contribute additional peaks to the experimental powder XRD pattern. As a result, the pattern may look substantially different from that of a pure sample of the main phase. Clearly, careful inspection should be carried out to assess the presence of impurity phases, particularly with regard to residual amounts of unreacted starting materials. 

Aerosol processes afford materials having an optimum phase purity as contrasted to catalysts prepared by other synthetic methods. In addition, their syntheses are reproducible and metal ion concentrations are predictable based on starting... 

The typical side products in decavanadate preparations are the metavanadate and/or hexavanadate, as well as unreacted V205. These impurities are generally less soluble than the decavanadate salt and can be removed by filtration. X-Ray powder diffraction is felt to be the best criterion of phase purity unless macroscopic crystals are obtained. 

A higher temperature, Tc l 10K, phase in the Bi-Sr-Ca- Cu-O system has also been reported (78-80). This phase contains three infinite two-dimensional Cu-O layers. However, lead appears to play an important role in stabilizing this compound in phase pure form (75)(79)(80). Single crystals grown by eutectic melt growth techniques have been reported (77) but the phase purity of these crystals with respect to intergrowths containing various numbers of Cu-O planes is a concern. 

Crystal growth of these compounds is complicated by the high volatility of thallium oxides and thallium-containing compounds at elevated temperatures and the toxicity of thallium. Also, the similarity in structures leads to problems controlling phase purity and samples which appear to be single crystals based on their morphology can be shown to be complicated intergrowths by X-ray diffraction studies. 

The reference material CoFe204 was obtained by calcination of a mixture of cobalt(II) nitrate and iron(III) nitrate with the same ratio as above lCo 2Fe. The calcination was carried out at 650° C for 72 hours. Phase purity was controlled by powder X-ray diffraction (P-XRD). 

It is demonstrated that thermogravimetry data for uncalcined MCM-41 samples can be used to predict the structural quality of the calcined materials. The method is based on the comparison of weight change derivatives for a sample under study with those for a series of well-characterized samples prepared under similar conditions. Thermogravimetry data were found useful for a qualitative estimation of the overall sample quality, phase purity, degree of structural collapse and, in favorable cases, pore size of calcined MCM-41 materials. 

The current work is focused on application of thermogravimetry in qualitative characterization of structure of MCM-41 samples prepared using the hydrothermal restructuring method [22-25] and direct synthesis procedure involving hexadecyldimethylamine as an expander [25,26]. It is demonstrated that the weight-change patterns of as-synthesized samples provide qualitative information about phase purity, stability upon calcination and the resulting structure of the calcined samples. 

Determination of the phase purity of mesoporous molecular sieves (MMSs) [1,2] is important in synthesis, modification and application of these materials [3-7]. Many of the synthesis procedures reported so far involved various phase transformations [8-20] and thus the desired MMS product may be contaminated with some mesostructured impurities. One of the possible impurities is a lamellar phase, which readily forms under various synthesis conditions [1,8-25]. Because of its layered structure, the lamellar phase collapses upon calcination [1] and therefore constitutes a disordered impurity of calcined MMS samples. 

Chemical and phase purity are critical issues that drive precursor design because optimal mechanical properties are achieved only with high purity. For example, ceramics grade Nicalon fibers, with a chemical composition of ca SiCi 45O0 36 and densities 17,9... 

Chemical and phase purity are not always desirable. For example, H- and N-doped silicon carbide films behave as high temperature semiconductors, while silicon carbonitride glasses offer properties akin to glassy carbon with room temperature conductivities of 103 2 cm-118. Additional reasons for targeting materials that are not chemically or phase pure stem from the desire to control microstructural properties. 

Characterization.— The LSFTO powder was calcined at a series of temperatures (1250, 1300, and 1400°C) in air to investigate phase purity and densification behavior. X-ray diffraction (XRD) powder patterns are shown in Fig. 1. The sample is single phase after heating at 1250°C. At the higher sintering temperatures, the lines become sharper and the density increases. The density measured by the Archimedes method was 90.3% relative to theoretical value after annealing at 1400°C for 10 h. The XRD pattern sintered at 1400°C was completely indexed with a cubic unit cell with lattice parameter a = 3.898(8) A and V= 59.2(6) A3. The weak XRD peaks at 31, 43, 55, and 65° 20 are also from the perovskite phase and arise from a small amount of WL radiation in the incident beam. 

---