Sintering segregation

An important attribute of these materials comes from the two distinct cationic sites (Oh and Td), and the feasibility of cations migration among them due to the redox nature of metalions, while keeping the spinel structure intact. This particular aspect helps to avoid possible segregation/sintering of metal-ions and remain stable for longer period, compared to a mixed metal oxide catalyst. This unique property of spinels makes them an attractive candidate for number of catalytic reactions. 

Creation of non-segregating blends of powder ingredients, as in sintering of fines for steel or agricultural chemical granules. 

Other potential applications are ceramic powders coated with their sintering aids, zirconia coated withyttria stabilizer, tungsten carbide coated with cobalt, or nickel, alumina abrasive powders coated with a relatively brittle second phase such as MgAl204 and plasma spray powders without the segregation of alloying elements. 

Discuss the role of the surface free-energy in phenomena such as alloy segregation, surface reconstruction, faceting and sintering of small particles. How does a gas environment (air, vacuum) affect the surface composition of alloys ... 

Another procedure is to rapidly dry the solution at a rate that does not allow the elements to segregate. As an example, aerosols can be sprayed through a furnace heated at the reaction temperature of 900°C. This yields spherical submicronic particles that are most suitable to sinterable compacts. Another way is to spray the solution onto a substrate. This is a simple method of thick film deposition. Very ionic species such as nitrates dissolved in water can be sprayed onto a hot substrate. Long-chain carboxylates, soluble in volatile organic solvents, can be deposited as well, even at room temperature. 

However, copper alkoxides with longer chains appear to be more soluble in their parent alcohol. S. Shibata et al. (20) have used the n-butoxides of Y, Ba and Cu dissolved in n-butanol and hydrolyzed with water. They obtain a precipitate of oxides that is composed of a very fine submicron powder that readily sinters starting above 250°C. However, the different reaction rates for the hydrolysis and the precipitation of the three different cations lead to cationic segregation. 

The mechanical properties of ceramics maybe increased by minimising impurity segregation, decreasing grain size and increasing sintered density. 

Segregation of Pt and Ir occured during oxygen treatment. Pt and Pt-Ir catalysts sintered in similar ways upon hydrogen treatment.96 ... 

Creation of non-segregating blends of particulates, as in the sintering of fines in the steel industry. 

Catalytic molecular surface species may undergo drastic changes in their structure in the presence of reactants. For example, polymeric clusters may transform into highly distorted monomeric species. A crystalline phase may become mobile at its Tammann temperature, as shown by Raman spectroscopy, and it may spread over oxide supports driven by the reduction of the overall surface free energy. Reactive environments trigger many structural transformations, exemplified by particle sintering, dispersion of bulk phases, segregation of surface species into bulk phases, and solid-state reactions between supported oxides and supports. 

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