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Devitrification Of Quartz Glass
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Devitrification is a process that changes a semi-stable glass into a crystallized stable cristobalite. Here ‘Vitri’ means glass and ‘devitrification’ means the process of becoming un-glasslike. It mainly occurs when quartz glass is heated with impurities attached to it or fired at high temperatures for longer duration. Practically, this process never occurs at temperatures below 1100ºC as long as no impurity is attached to it.
This process mainly takes place when the temperature rises above 1200ºC and is accelerated as temperature goes higher. If some impurities are present on the surface of quartz glass, devitrification can easily occur even at temperature below 1100ºC. Alkalis, such as K, Na and Ca, or metal compounds of alkaline can produce devitrification tens or hundreds of times the norm.
A glass by definition is a liquid at all temperatures. A normal glass consists of the glass formers (B2O3 and SiO2), fluxes (alkaline metals), and the network modifying alkaline earths (ZnO, BaO, CaO). Few oxides are able to form glass networks include P2O5, GeO2, As2O3, Sb2O3, As2O5, and to a limited degree ZrO2, Bi2O3 and V2O5. The oxides of Al, Zn, Ti, Pb and Be can be included in different concentrations, but will not on their own, yield a glass. The glass matrix includes non-bridging oxygen ions and bridging oxygen ions. Precisely, glass is a metal oxide polymer with Silicon dioxide being the major co-polymer of quartz glasses.
In three dimensions, quartz glass polymer can be represented as a random 'web' of silicon ions each bonded to either three or four oxygen atoms. Moving ahead along any branch of the polymer chain we observe Si-O-Si-O-Si-O-Si-O with an irregular matrix modifier ion (as lead, Pb) substituting for a Si. Different Branches of Si will form another similar chain of O-Si-O-Si-etc. Sometimes, a chain ends at non-bridging oxygen, which has a strong negative charge. The non-bridging oxygen bulges into a microscopic cavity in the glass matrix within which resides a counter balancing, positively charged alkaline metal flux ion (as Na+).
A pure Silicon dioxide glass is known as fused quartz. The process of machining quartz can be done by using quartz crystals. There should be no matrix or flux modifiers attached to these crystals. Because of the constant strength of the Si-O bonds, it produces extremely high temperature softening glass. Frequent interruptions of Si-O matrix with modifying ions (with weaker metal oxide bonds) will lower the viscosity of the soften glass. The more alkaline oxides added to the glass, the more the viscosity outline is depressed. When glass devitrifies, it doesn't essentially returns to its former crystalline and solid constituents. In many cases, a devitrified quartz glass will not be chemically modified, but relatively it will crystallize into its ceramic analogue.
Stress-induced devitrification is indirectly induced by applied force and actually has little to do with either deformation (strain) or force (stress). The small surface geometric irregularities which are created by applying force (crazing in regions of tension and wrinkles in the area of compression - neither of which affect the large structure of the glass) increase the surface to volume ratios of the outer microns of the glass. It allows the devitrified micro-structures to become heated by the fire, allowing the higher vapor pressure fluxes to volatilize, thus producing a hard quartz-like glass.
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