Liquid-phase sintering solution-precipitation

O.-H. Kwon and G.L. Messing, Kinetic analysis of solution-precipitation during liquid-phase sintering of alumina, J. Am. Ceram. Soc. 73(2), 275-281 (1990). 

Silicon nitride powder particles are contaminated by layers of silica on their surfaces. Oxide additives MxOy mixed with silicon nitride react with this silica and some of the nitride itself at sintering temperatures to form an oxynitride liquid which promotes liquid phase sintering and densification by solution-precipitation [1, 6, 10]. The a-Si3N4 dissolves in the liquid and is... 

The stages of liquid phase sintering are summarized in Fig. 10.3. The extent to which each stage influences densification is dependent on the volume fraction of liquid so there are many variants in this conceptual picture. When the volume fraction of liquid is high, complete densification can be achieved by the rearrangement process alone. On the other hand, at the low liquid contents common for many systems, the solid skeleton inhibits densification, so that solution-precipitation and final stage sintering are required to achieve further densification. 

Derive Eq. (10.38) for the shrinkage during the solution-precipitation stage of liquid-phase sintering when the phase boundary reaction is the rate controlling mechanism. 

PuOj from direct calcination of Pu(N03 )4. The precipitation steps of the above processes can be avoided by the direct calcination of the plutonium nitrate solution to PuOa. Calcination has been carried out at 350 C in a liquid-phase screw calciner. Half a mole of ammonium sulfate per mole of plutonium is added to the feed solution to increase the production of reactive PuOi. The calcination time and temperature must be low enough to minimize sintering, which would otherwise reduce the chemical reactivity of the oxide particles for subsequent conversion to a halide. 

Chemical precipitation is a popular method for synthesizing solid materials from solution in which a liquid-phase reaction is utilized to prepare insoluble compounds. The precipitates are composed of crystalline or amorphous fine particles. Usually, rare earth oxides are prepared by calcinations of the hydroxide or oxalate gel precipitated from a reaction of an aqueous or alcohol solution of inorganic salt (nitrate, chloride, sulfate, and ammonium nitrate, etc.) with an alkali solution (NaOH, NH4OH, and (NH2)2 H20) or an oxalic acid solution [15-21]. However, it is very difficult to obtain ultrafine particles because of growth and sintering of the particles during the calcinations. 

The a P transformation in silicon nitride requires a lattice reconstruction. This type of process occurs usually only when the transforming material is in contact with a solvent. The greater solubility of the more unstable a form drives it into solution after which it precipitates as the less soluble, more stable P form. The predominantly a silicon nitride powder used to produce dense silicon nitride ceramics is observed to transform to the P modification during the sintering process at temperatures in excess of 140QOC when the original a phase is in contact with a M-Si-O-N (M = Mg, Y, etc) oxynitride liquid phase. 

The nature of bonding in the carbides is known to be a mixture of covalent and metallic with little ionic tendency. If solid solutions were formed, Ta from TaSi substimted the transition metal atoms in the carbide lattice. This may occur either by cations diffusion or by solntion-precipitation. Given the low diffusion coefficient of this class of materials, it is presumed that lattice diffusion can occur only at very high temperature. Indeed, solution re-precipitation seems to be the dominant mechanism, in light of the sintering behaviour characterized by a relatively low T, 1400-1600°C (Table 2). The well-defined boundary between core and shell and the morphology of the interface between them also put forward a re-precipitation from liquid phase over a diffusion process. 

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