Sintering silica

These opaque minerals are common in Pb-Zn vein-type deposits in Japan Opaque minerals identified from silica sinter containing gold and mercury are krennerite, col-oradoite and metacinnabar (Osorezan, Japan Waiotapu, New Zealand). These minerals are not found in low sulfidation-type Au-Ag deposits in Japan but are reported from Kobechizawa, and Date in south Hokkaido which are massive and disseminated types and similar to hot spring type deposits. 

Cyclodextrin derivatives have been successfully used also for enantioseparations in packed capillary CEC. In this technique CyDs are used covalently immobilized to silica gel [69], to particulate silica sintered as a monolith [70], or to a monolithic organic matrix [71, 72]. In addition, CyDs have been used as dynamic modifiers of the stationary phase [73], as well as chiral mobile phase additives in combination with an achiral stationary phase [74, 75]. 

Filter on a dry tared silica sintered crucible, wash the precipitate with water then with a few millilitres of ethanol, and dry under a vacuum. Dry in an oven at 105°C for 30 min. 

Silica sinters by a viscous flow sintering mechanism while titania sinters by grain boundary diffusion. Kingery et al. (28) and Kobata et al. (29) have derived the... 

Silica sintering (dashed curves) and crystallization (solid curves) under dry and wet" conditions upper and lower sintering bounds correspond to assumed pore diameters of SOO and 5 nm, respectively. The lower viscosity observed by Sacks and Tseng [48] shifts all curves to lower times at any temperature. Their data for sol-gel derived glasses are also included X marks treatments that resulted in crystallization with little sintering and S marks treatments that produced sintered glasses free of crystallinity. From Uhlmann et al. [122], pp. 173-183 in Science of Ceramic Chemical Processing, eds. L.L. Hench and D.R. Ulrich (Wiley, New York, 1986). 

Into a 500 ml. round-bottomed flask, fitted with a reflux condenser, place 42 g. of potassium hydroxide pellets and 120 g. (152 ml.) of absolute ethyl alcohol. Heat under reflux for 1 hour. Allow to cool and decant the liquid from the residual solid into another dry 500 ml. flask add 57 g. (45 ml.) of A.R. carbon dtsulphide slowly and with constant shaking. Filter the resulting almost solid mass, after cooling in ice, on a sintered glass funnel at the pump, and wash it with two 25 ml. portions of ether (sp. gr. 0-720), followed by 25 ml. of anhydrous ether. Dry the potassium ethyl xanthate in a vacuum desiccator over silica gel. The yield is 74 g. If desired, it ma be recrystallised from absolute ethyl alcohol, but this is usually unneceasary. 

Sodamide Soda-silica glass Soda-sinter process Sodatol... 

Fig. 4. Alumina—silica—chromia fiber after 120 h at 1426°C showing crystallization and sintering at contact points. Magnified x5000.

Fig. 6. Dependence of stmctural density of alkoxide-derived porous silica gels as a function of sintering temperature held for 12 h for three different...

Rigid Porous Media These are available in sheets or plates and tubes. Materials used include sintered stainless steel and other metals, graphite, aluminum oxide, silica, porcelain, and some plastics—a gamut that allows a wide range of chemical and temperature resistance. Most applications are for clarification. 

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. 

In the sintering of such materials as silicon nindde, a silica-rich liquid phase is formed which remains in the sintered body as an intra-granular glass, but this phase, while leading to consolidation, can also lead to a deterioration in the high-temperature mechanical properties. 

Zorbax PSM particles are made from small (80-2000 A), extremely uniform colloidal silica sol beads. In a patented polymerization process, these beads are agglutinated to form spherical particles. The size of the Zorbax PSM particles is controlled by the polymerization process, and the pore size is determined by the size of the silica sol beads. After polymerization, the silica is heated to remove the organic polymer and sinter the particles. The result is a spherical, porous, mechanically stable, pure silica particle that provides excellent chromatographic performance (Pig. 3.1). 

Filter crucibles with a porous filter base are available in porcelain (porosity 4), in silica (porosities 1,2,3,4), and in alumina (coarse, medium and fine porosities) these have the advantage as compared with sintered crucibles, of being capable of being heated to much higher temperatures. Nevertheless, the heating must be gradual otherwise the crucible may crack at the join between porous base and glazed side. 

This operation is the separation of the precipitate from the mother liquor, the object being to get the precipitate and the filtering medium quantitatively free from the solution. The media employed for filtration are (1) filter paper (2) porous fritted plates of resistance glass, e.g. Pyrex (sintered-glass filtering crucibles), of silica (Vitreosil filtering crucibles), or of porcelain (porcelain filtering crucibles) see Section 3.24. 

Care must be taken with both sintered glass and porous base crucibles to avoid attempting the filtration of materials that may clog the filter plate. A new crucible should be washed with concentrated hydrochloric acid and then with distilled water. The crucibles are chemically inert and are resistant to all solutions which do not attack silica they are attacked by hydrofluoric acid, fluorides, and strongly alkaline solutions. 

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