Crystallites phase

Still another model for the spreading of an active oxide (V2O5) under oxidizing conditions onto the surface of a support (anatase) was proposed by Haber et al. [40]. In this model, the active phase crystallite spreads spontaneously at the beginning of the thermal treatment (hot plate effect) which leads to amorphiza-tion of the active phase. Thereafter the spreading is... 

The former phenomenon is usual referred to as particle-size effect and is pronounced for structure-sensitive reactions [1,2], i.e., catalytic reactions where the rate and/or selectivity is significantly different from one crystallographic plane to another. Structure-sensitive reactions (e.g., isomerizations) frequently occur on catalytic sites consisting of an ensemble of surface atoms with specific geometry. It is thus reasonable to expect that as the active-phase crystallite size decreases, there will be a different distribution of crystallographic planes on the catalyst surface, with the possible disappearance of ensemble sites, so that both the catalyst activity and... 

In this contribution we demonstrate that, using an easy wet impregnation method that is suitable for deposition of active phase on powder supports, the morphology of the active phase crystallites of silica-supported M0O3 catalysts can be efficiently adjusted by simply varying the nature and the amount of the precursor used in the synthesis. 

Sample Washcoat procedure Active phase (before firing) XRD phases (crystallite size /nm)... 

The a-phase crystallites are stable up to maximally 120°C. The a-form crystallinity changes into S-form crystallinity at higher temperatures. This a/S crystal transition is indicated as the Tm -value of PK copolymer. The fusion effect at about 258°C which is important for the processing of the polymer, is thus fusion of S-form material only. 

Many thermoplastic materials are semi-crystalline and the precise nature of this crystallinity plays a major role in determining their properties. In addition to the level of crystallinity, factors such as crystal phase, crystallite numbers, orientation, size and shape are all important, but the exact way in which they affect properties is far from understood. 

The size of crystallites for both phases present in the Ni-Mo-O alloy powder deposits was determined by the analysis of certain peaks on the diffractograms shown in Figs. 5.55, 5.57, and 5.59a. Results are presented in Table 5.5. As expected, the size of M0O3 and NiMo04 phase crystallites increases with increasing the annealing temperature of powders [122]. 

The first experiments with electric fields showed that an increasing field initially lengthens the blue phase lattice parameter and causes birefringence. A theory for weak fields was given by Lubin and Hornreich [102]. With larger fields, the blue phases may transform between themselves, to the helical phase, and ultimately to the nematic phase [23], [103], [104], [105], [106], [107], [108], [109]. Fields also affect the orientation and facetting of blue phase crystallites [81], [82], [110]. 

Polyvinylidene fluoride is a semicrystalline fluoropolymer with at least three distinct crystaUine forms. The most common type is nonpolar and centrosym-metric, called the a phase, and forms when the polymer is cooled from its melt. Deformation of the atype crystallites such as stretching the extruded film at 80°C, results in packing of the unit cells in parallel planes into the polar P phase. The third crystallite is the y phase, which is intermediate between a and phases. Orientation of the /phase also generates phase crystallites. 

---