Physical properties mullite

Selected composite materials and their basic physical properties are summarized in Table 2. The GE GEN-IV all-oxide fiber composites have a relatively low volume fraction (30%) of Nextel 610 fiber and a matrix composed of alumina and silica with approximately 35% porosity [12, 30, 37]. COI produces a number of aluminosilicate matrix materials with a selection of different Nextel oxide fibers, but with higher fiber loading and matrix porosity than the GE materials [140, 141]. Two examples of nominally silica-free porous composites (Table 2) with improved thermal stabilities are the mullite-alumina formulation developed at UCSB [4,44,142,143] and the all-alumina COI N720/A-1 [75, 79, 141, 144],... 

Mullite brick contains very little free AI2O3 therefore its chemical resistance is superior to that of the high alumina brick. Typical physical properties are as... 

Also known as bauxitland cement. Kurlbaum Method. A method for the determination of flame temperature by means of an optical pyrometer (F. Kurlbaum, Phys. Z., 3,187,1902). Kyanite. A mineral having the same composition (Al2SiOs) as sillimanite and andalusite, but with different physical properties. The chief sources are Virginia and S Carolina (USA), and India. When fired, kyanite breaks down at 1300°C into mullite and cristobalite with a volume expansion of about 10% it is therefore calcined before use. Calcined kyanite is used in making aluminous refractories. 

Sillimanite. A mineral having the same composition (Al2Si05) as kyanite and andalusite but with different physical properties. The chief sources are S. Africa and India. Sillimanite changes into a mixture of mullite and cristobalite when fired at a high temperature (1550°C) this change occurs without any significant alteration in volume (cf. kyanite). The mineral is used as a refractory. 

Consists of up to 95% mullite crystals. Made from pure kaolins. Chemical and physical properties are those of mullite. (See MULLITE, KYANITE, CLAY.)... 

Physical properties such as the compressive strength of clays and zirconia have been enhanced by the application of pressure during a chemical reaction ( ). Chemical reactions can occur at lower temperatures under an applied pressure than under vacuum. Carruthers and Wheat ( ) found mullite formation at 650°C in a china clay hot pressed during the dehydroxylation reaction with 30,000 psi. The process of dissolution can also be affected by an applied pressure. 

The most common physical properly of the ash zeolites is its specific surface area, which is dependent on the extent of dissolution of fly ash particles in alkaline solvents [19, 22, 31]. In line with this, another important physical property of zeolites is their void volume which can directly be correlated with the CEC of the synthesized product (e.g., fly ash zeolites, Na-Pl) and which in turn depends upon the specific area as depicted in Fig. 2.3a, b [1, 22]. Moreover, both CEC and the surface area of the ash zeolites are found to undergo significant variations with increase in molarity and the reaction time, as depicted in Fig. 2.3c, d [22, 29]. From the trends depicted in Fig. 2.3c, it can be observed that the CEC increases, marginally, with an increase in concentration, however, the same is noticed to be fluctuating, randomly, with an increase in reaction time. This can be attributed to the variations in the pore size and volume, as depicted in Fig. 2.3a. On the contrary, the surface area maintains an increasing trend with increase in concentration and the reaction time, as depicted in Fig. 2.3d, which can be attributed to increase in dissolution of fly ash ingredients (viz., glass. Quartz and Mullite). 

Table 5.2 Physical and mechanical properties of graded mullite-ZTA...

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