Phases, reaction between crystalline

More detailed studies demonstrate that crystalline Al2(W04)3 is produced from the direct solid-state reaction between crystalline WO3 and the alumina support and occurs by the migration of alumina to the WO3 particles at these elevated temperatures. The initial transformation of the surface tungsten-oxide phase to crystalline WO3 particles at elevated temperatures, when the alumina surface area decreases, reveals that the tungsten-oxide species are not miscible in the alumina support and are actually diffusing away from the alumina support. Thus, it appears that the surface tungsten oxide-alumina mixed oxide forms an "immiscible oxide system" because of the inability of the alumina support to accommodate the large charge and size of the cation. More recent Raman studies... 

Multiblock polyethylene-polydimethylsiloxane copolymers were obtained by the reaction of silane terminated PDMS and hydroxyl terminated polyethylene oligomers in the presence of stannous octoate as the catalyst 254). The reactions were conducted in refluxing xylene for 24 hours. PDMS block size was kept constant at 3,200 g/mole, whereas polyethylene segment molecular weights were varied between 1,200 and 6,500 g/mole. Thermal analysis and dynamic mechanical studies of the copolymers showed the formation of two-phase structures with crystalline polyethylene segments. 

As an example, consider the following. Suppose we have a crucible half-filled with a powder. We now fill the crucible with another powder of different composition and then heat the filled crucible. Any solid state reaction which does occur can only do so at the boundary of the two layers of powders. If the reaction is A -t- B = AB, then we find that the reaction product, which is also a solid, forms as a phase boundary between the two layers. The same condition exists in a solid state reaction between two crystalline particles having differing compositions. That is, they can only react at the interface of each particle. This is illustrated in the following diagram, which is a model of how the components react through a phase boundary ... 

Early workers, and some later ones, ignored the fact that aluminium is always found in the orthophosphoric acid liquid of the practical cement its presence profoundly affects the course of the cement-forming reaction. It affects crystallinity and phase composition, and renders deductions based on phase diagrams inappropriate. Nevertheless we first describe the simple reaction between zinc oxide and pure orthophosphoric acid solution, which was the system studied by the earliest workers. 

Crisp et al. (1978) were able to follow the course of the cement-forming reaction using infrared spectroscopy and to confirm previous observations. They found that the technique could be used to distinguish between crystalline and amorphous phases of the cement. Hopeite shows a number of bands between 1105 and 1000 cm this multiplicity has been explained by postulating a distortion of the tetrahedral orthophosphate anion. (Two-thirds of the zinc ions are tetrahedrally coordinated to four phosphate ions, and the remainder are octahedrally coordinated to two phosphate and four water ligands.)... 

Oxysalt bonded cements are formed by acid-base reactions between a metal oxide in powdered solid form and aqueous solutions of metal chloride or sulphate. These reactions typically give rise to non-homo-geneous materials containing a number of phases, some of which are crystalline and have been well-characterized by the technique of X-ray diffraction. The structures of the components of these cements and the phase relationships which exist between them are complex. However, as will be described in the succeeding parts of this chapter, in many cases there is enough knowledge about these cements to enable their properties and limitations to be generally understood. 

Melting of two crystalline phases or reactions between them... 

The conversion rates of n-hexane are shown as a function of the crystallinity parameter Qai for different temperatures. We found that the catalytical activity increases simultaneously with the increased crystallinity of the composites, the crystallization products. According this linear correlation it can be concluded that the catalytical active sites, the acidic centers in the zeolitic framework, are always, independent of the crystal content of the composite material, accessible for educt of the test reaction, the n-hexane molecules. This leads to the assumption that the crystallization must start on the interface (at the phase border) between the solution (contains the alkalinity and the template) and the solid (porous glass) surface and has to carry on to the volume phase of the glass resulting finally in complete transformed granules. 

We have not determined the number of Vlv species. We assume that they come from two distinct sources. Some of them may already be present in the V2CL samples (21). Most of them come from a reaction between and Na+ of NaY zeolite crystals giving vanadium bronze phases (27-29) among which two crystalline phases were identified NaVv5VIV015 and Na5VvnVlv032. 

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