Sintered magnesia

Magnesia magma. See Magnesium hydroxide Magnesia oxide fume Magnesia, sintered Magnesia usta. See Magnesium oxide... 

Magnesia, sintered Magnesia usta Magnesite, caustic-calcined Magnesium oxide fume Periclase... 

Table 1 Typical Properties of Several Selected Magnesia Sinters...

Tabie 2 T) ical Properties of Several Selected Synthetic Magnesia Sinters (S) and Fused Magnesia Grains (F)... 

Some materials have characteristic microstructures that can be used for confirmation of their presence. For instance, fused magnesia, sintered magnesia, and seawater magnesia are structurally different. Similarly, calcined alumina, tabular alumina, and white fused and brown fused alumina all have very different characteristics. 

Sol—Gel Sintered Aluminum Oxide. A new and much more versatile sintered alumina abrasive is now produced from aluminum monohydrate, with or without small additions of modifiers such as magnesia, by the sol—gel process (see Sol-gel technology). The first modified sol—gel abrasive on the market, Cubitron, was patented (27) and produced by the 3M Corporation for products such as coated belts and disks. The success of this material promoted intensive research into sol—gel abrasives. 

The technical problem in die high teiiiperamre application of Si3N4 is that unlike the pure material, which can be prepared in small quantities by CVD for example, die commercial material is made by sintering the nitride with additives, such as MgO. The presence of the additive increases the rate of oxidation, when compared with the pure material, by an order of magnitude, probably due to the formation of liquid magnesia-silica solutions, which provide short-circuits for oxygen diffusion. These solutions are also known to reduce the mechanical strength at these temperatures. 

Figure 9.11. Microstructures of porous sintered alumina prepared undoped (right) and when doped with magnesia (left). Optical micrographs, originally 250x (after Burke 1996).

Calcination temperature is very important in the production process and dictates the particle size, purity and reactivity of the product. A dead-burned, sintered dense microcrystaUine product is obtained at calcination temperature of 1,400 to 1,700°C. A caustic-burned product is obtained when magnesium carbonate or hydroxide is calcined at 600 to 700°C. A hght grade (specific gravity 2.9) highly reactive caustic-hurned magnesia that contains some moisture and carbon dioxide is obtained at about 600°C. A denser form from... 

Commercial sintered magnesia-spinel refractory materials are divided into three categories magnesia rich, stoichiometric, and alumina rich.1 Typical properties of magnesia rich spinel bricks are given in Table 14.1.2... 

Properties Magnesia-spinel brick with alumina Magnesia-spinel brick with sintered spinel... 

MgO-spinel clinker is obtained by the same procedure, but with a substantial proportion of free MgO (75-85%). MgO-spinel is produced by sintering at temperatures of 1600-1800°C and a dense product is obtained. MgO-spinel refractory has better high temperature mechanical properties in arc furnace roof application compared to magnesia-chrome refractories, but it has lower strength than a high-alumina refractory. 

Grain size and boundaries, impurities, additives, particle size, porosity, sintering temperature and time, heating and cooling rate, and shaping practice play an important role in controlling many physical, mechanical and chemical properties of magnesia-based bricks.29... 

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