Glass phases controlling chemical properties

Chemical and phase purity are not always desirable. For example, H- and N-doped silicon carbide films behave as high temperature semiconductors, while silicon carbonitride glasses offer properties akin to glassy carbon with room temperature conductivities of 103 2 cm-118. Additional reasons for targeting materials that are not chemically or phase pure stem from the desire to control microstructural properties. 

The batches for rapidly fired porcelains have to be adapted to the short firing times to obtain product properties comparable with those of conventionally fired porcelains, despite the diffusion-controlled slow chemical reactions. Suitable mixtures contain lower concentrations of quartz and fluxes and contain prefired raw materials such as calcined kaolin (fireclay). Satisfactory tran.sparency can be obtained by using very Fine particulate quartz sorts, which dissolve sufficiently rapidly in the glass phase. 

Chemical properties, ranging from resorbability to chemical stability, can be controlled according to the nature of the crystal, the glass phase or the... 

The majority of ceramics are multiphased materials that comprise both ciystallized and vitreous phases. Porcelain thus consists of silicate glass reinforced by acicular crystals of crystallized muUite, but we can also observe millimetric ciystal agglomerates with a very porous microstracture (iron and steel refiactoiy materials), or fine grained polycrystals (< 10 pm) without vitreous phases and with very low porosity (hip prosthesis in alumina or zirconia). It should be reiterated that, in addition to the chemical nature of the compound(s) in question, it is the microstracture of the material (size and shape of the grains, rate and type of porosity, distribution of the phases) that controls the properties. 

The phase structure of glasses has a significant effect on their physical properties, which is discussed below with reference to chemical durability. The magnitude of the phase separation can be altered by heat treatment, and enhanced or reduced by the addition of various oxides to the melt. In particular, the addition of alumina to commercial soda-lime-silica glasses reduces the tendency to phase separation, improving chemical resistance (Doremus, 1973). A detailed study of the microstructure of soda-lime-silica glasses has been published by Burnett and Douglas (1970). The control of phase separation in the melt is now commercially important for processes such as the... 

Soft segments One of the two phases which make up polyurethane. The soft segment is composed of long-chain polyether or polyester polyols. The soft segment controls many of the polyurethane properties such as tensile and tear strength, hydrolysis and chemical resistance, glass transition temperature, and flexibility. 

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