Multi phase ceramics

Oxynitride glasses may be heat treated to form glass-ceramics, effectively multi-phase composites. The process involves heat treatment at two different temperatures, firstly to induce nucleation, then to allow crystal growth of the nuclei. The crystalline phases formed depend on both the composition of the parent glass and the temperatures used for heat treatment. The extent of their formation and growth, the relative amounts and distributions of different phases (including residual glass) and their characteristics will determine the overall properties of the particular composite. The formation of these types of materials and their properties is outlined below. 

Crystallisation of oxynitride glasses to form multi-phase glass-ceramic composites... 

Phase diagrams are extremely useful in determining the reactions that occur when alkali oxides react with many common ceramics. Most scientists and engineers are easily able to evaluate binary phase diagrams that correspond to an alkali reaction with single oxide ceramics however, when multi-oxide ceramics such as mullite are involved, multi-component phase diagrams are not fully used and extremely time-consuming experimentation is unnecessarily conducted. 

Porous membrane modules were therefore effectively used in bioreactors as an alternative to direct two-liquid contact systems, as long as phase breakthrough was avoided. This required a careful control of the transmembrane pressure, particularly if surface-active material was produced during bioconversions [126,184, 187]. Fouling problems also developed in membrane-assisted multi-phase separation systems. This was observed by Conrad and Lee in the recovery of an aqueous bioconversion product from a broth containing 20% soybean oil by using ceramic membranes fouling was caused mainly by soluble proteins and surfactants [188]. 

As is known, composite materials are two- or multi-phase with well-defined interphase border. Such materials contain the reinforcing elements immersed into a polymeric, ceramic or metal matrix. Mechanical properties of composites depend on structure and properties of the interphase border. Phases of usual composite materials have micron and submicron sizes. 

Typical characterization of the thermal conversion process for a given molecular precursor involves the use of thermogravimetric analysis (TGA) to obtain ceramic yields, and solution NMR spectroscopy to identify soluble decomposition products. Analyses of the volatile species given off during solid phase decompositions have also been employed. The thermal conversions of complexes containing M - 0Si(0 Bu)3 and M - 02P(0 Bu)2 moieties invariably proceed via ehmination of isobutylene and the formation of M - O - Si - OH and M - O - P - OH linkages that immediately imdergo condensation processes (via ehmination of H2O), with subsequent formation of insoluble multi-component oxide materials. For example, thermolysis of Zr[OSi(O Bu)3]4 in toluene at 413 K results in ehmination of 12 equiv of isobutylene and formation of a transparent gel [67,68]. 

The growth of such structures is possible only from a gas phase and probably occurs as a result of dehydropolymerisation (polycondensation) [4,11 ]. Under more harsh reaction conditions multi-walled nanotubes grow as a loop on ceramic reactor walls (Fig. 3.4). We suggest that the benzene molecule could be the main fragment in the graphene network formation. At temperatures >600°C benzene rapidly undergoes dehydrogenation followed by diphenyl formation that can be considered... 

In this chapter, we present the results of computational studies on the above mentioned novel inorganic systems namely AlPOs, carbon nanotubes and supercritical fluid extraction from the adsorbed phase over ceramics. Multi-technique computational methods such as Computer Graphics (CG), molecular mechanics (MM), quantum chemistry (QC) and molecular dynamics (MD) were applied. The attempts made to design synthetic sorbents at molecular level are reviewed. 

Sol-gel process is proven to be an attractive fabrication method of multi-component oxide ceramics. In addition to the achieved homogeneity and purity of the products, the sol-gel method also enables a lower phase-formation and sintering temperature in comparison to the conventional sintering of powder With good size scaling possibility, colloidal sol-gel materials are suitable for depositing layers on macroporous substrates to serve as support of polymeric sol-gel derived layers preventing infiltration of the sol. 

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