Sintering and reaction

Sintering can be brought about by a variety of reactions. Of these, material transport by viscous flow is important in glasses but less so in metals or ceramics. Evaporation and condensation is important in rather volatile compounds such halides and some oxides. Diffusion - bulk, grain boundary and surface diffusion - is important for refractory materials and, for these materials, the presence of traces 

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Sintering, activated sintering and reaction sintering are used to produce maximum density in sintered bodies and to make special parts with controlled porosity. 

Impermeable silicon carbides of both types, sintered and reaction bonded, perform generally better than the permeable refractories as shown in Table 19-3 Both reaction bonded and sintered products can be exposed to higher temperature for longer periods of time with lower weight loss than the oxide, Si3N4 or Si20N2 bonded refractories. This is due to the lower surface area available for reaction and to the greater relative inertness of their bond phases. 

Specific surface area. The specific surface area is an important criterion for the sintering activity (solid state sintering), dissolution processes (liquid phase sintering), and reaction with gaseous or solid substances during carburization. Commonly, it is in the range of 0.01 m /g (coarse powders) up to 12m /g (very fine powders). 

The addition of sawdust increases the porosity of the charge and facilitates gas circulation. Chlorine is added to reduce impurities.I Alpha SiC is produced above 2100°C and pSiC at 1500-1600°C. Shapes are produced by standard ceramic forming technologies, pressureless sintering, and reaction bonding coatings are produced by CVD.t H ... 

Figure 11.57 Sintering and reaction schedule for reaction sintering of zircon-AI2O3 powder mixtures. At A, only zircon and AI2O3 are detected by x-ray analysis at B, the reaction to produce mulUte and ZrC>2 is completed. (From Ref. 94.)...

The initial laboratory tests on the application of SiC began from oxide-bonded SiC. Lab tests showed that N-SiC is as corrosive-resistant as O-SiC. There was a hope that SiA10N -bonded SiC would be more resistant [165], but the experiments proved opposite. In experiments, also AIN (sintered and reaction-sintered), SiC with Ti02 addition, aluminium titanate, Al-Mg, and Al-Ni spinels were tested [166-168]. 

Stanciu, L., Groza, J., Kodash, V., Crijan, M., and Zaharescu, M. (2001) Electrical field effects in sintering and reaction to... 

Hydrogenation of the oxides of carbon to methane according to the above reactions is sometimes referred to as the Sabatier reactions. Because of the high exothermicity of the methanization reactions, adequate and precise cooling is necessary in order to avoid catalyst deactivation, sintering, and carbon deposition by thermal cracking. 

Sihcon nitride is one of the few nonmetaUic nitrides that is able to form alloys with other refractory compounds. Numerous soHd solutions of P-Si N and AI2O2 have gained technical interest. Many companies have begun to mass produce reaction-sintered and hot-pressed Si N parts. 

The heat released from the CO—H2 reaction must be removed from the system to prevent excessive temperatures, catalyst deactivation by sintering, and carbon deposition. Several reactor configurations have been developed to achieve this (47). 

In this chapter shock modification of powders (their specific area, x-ray diffraction lines, and point defects) measurements via analytical electron microscopy, magnetization and Mossbauer spectroscopy shock activation of catalysis, solution, solid-state chemical reactions, sintering, and structural transformations enhanced solid-state reactivity. 

If the phases present can be unambiguously identified, microscopy can be used to determine the geometry of interface initiation and advance, and to provide information about particle sizes of components of mixed reactants in a powder. Problems of interpretation arise where materials are poorly crystallized and where crystallites are small, opaque, porous or form solid solutions. With the hot-stage microscope, the progress of reactions can be followed in some instances and the occurrence of sintering and/or melting detected. 

Fluid bed electrodes consist of a bed of particles supported by a structure such as a coarse sinter and fluidized by an upward stream of electrolyte and two different configurations have been described where the current path is parallel or perpendicular to the direction of fluidization (Backhurst et al., 1969). Such electrodes have been used for electrosynthetic reactions and, in particular, a pilot plant for the reduction of... 

A pellet is pressed of an intimate mixture of finely divided reactants and reaction induced either by arc melting and high-T annealing or by solid-state sintering in an electrical or high-frequency furnace. Isolating the borides from reactive container components can be a problem. The use of boron nitride liners has proved effective. In some cases the protective liner is made of sintered boride containing the same elements as the boride in preparation. 

The stability of catalyst is one of the most important criteria to evaluate its quality. The influence of time on stream on the conversion of n-heptane at SSO C is shown in Fig. 5. The conversion of n-heptane decreases faster on HYl than on FIYs with time, so the question is Could the formation of coke on the catalyst inhibit diffusion of reactant into the caves and pores of zeolite and decrease the conversion According to Hollander [8], coke was mainly formed at the beginning of the reaction, and the reaction time did not affect the yield of coke. Hence, this decrease might be caused by some impurities introduced during the catalyst synthesis. These impurities could be sintered and cover active sites to make the conversion of n-heptane on HYl decrease faster. 

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