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Introduction
Cristobalite is a low density SiO2 homomorphous variant, and its thermodynamic stability range is 1470 ℃~1728 ℃ (under normal pressure). β Cristobalite is its high-temperature phase, but it can be stored in metastable form to a very low temperature until a shift type phase transformation occurs at about 250 ℃ α Cristobalite. Although cristobalite can be crystallized from SiO2 melt in its thermodynamic stability zone, most cristobalite in nature is formed under metastable conditions. For example, diatomite transforms into cristobalite chert or microcrystalline opal (opal CT, opal C) during diagenesis, and their main mineral phases are α Cristobalite), whose transition temperature is in the stable zone of quartz; Under the condition of granulite facies metamorphism, cristobalite precipitated from the rich Na Al Si melt, existed in garnet as an inclusion and coexisted with albite, forming a temperature and pressure condition of 800 ℃, 01GPa, also in the stable zone of quartz. In addition, metastable cristobalite is also formed in many non-metallic mineral materials during heat treatment, and the formation temperature is located in the thermodynamic stability zone of tridymite.
Formative mechanism
Diatomite transforms into cristobalite at 900 ℃~1300 ℃; Opal transforms into cristobalite at 1200 ℃; Quartz is also formed in kaolinite at 1260 ℃; The synthetic MCM-41 mesoporous SiO2 molecular sieve was transformed into cristobalite at 1000 ℃. Metastable cristobalite is also formed in other processes such as ceramic sintering and mullite preparation. For the explanation of the metastable formation mechanism of cristobalite, it is agreed that it is a non-equilibrium thermodynamic process, mainly controlled by the reaction kinetics mechanism. According to the metastable formation mode of cristobalite mentioned above, it is almost unanimously believed that cristobalite is transformed from amorphous SiO2, even in the process of kaolinite heat treatment, mullite preparation and ceramic sintering, cristobalite is also transformed from amorphous SiO2.
Purpose
Since the industrial production in the 1940s, white carbon black products have been widely used as reinforcing agents in rubber products. In addition, they can also be used in pharmaceutical industry, pesticide, ink, paint, paint, toothpaste, paper, food, feed, cosmetics, batteries and other industries.
The chemical formula of white carbon black in the production method is SiO2nH2O. Because its use is similar to that of carbon black and is white, it is named white carbon black. According to different production methods, white carbon black can be divided into precipitated white carbon black (precipitated hydrated silica) and fumed white carbon black (fumed silica). The two products have different production methods, properties and uses. The gas phase method mainly uses silicon tetrachloride and silicon dioxide obtained by air combustion. The particles are fine, and the median particle size can be less than 5 microns. Precipitation method is to precipitate silica by adding sulfuric acid to sodium silicate. The median particle size is about 7-12 microns. The fumed silica is expensive and not easy to absorb moisture, so it is often used as a matting agent in coatings.
The water glass solution of nitric acid method reacts with nitric acid to generate silicon dioxide, which is then prepared into electronic grade silicon dioxide through rinsing, pickling, deionized water rinsing and dehydration.