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Vacuum sintering process of SiO2 soot body

Quartz glass is a special industrial technical glass composed of SiO2 single oxide. It has excellent physical and chemical properties. It is an irreplaceable basic raw material in the development of emerging industries and pillar industries. It is widely used in aerospace, nuclear technology, astronomy, optical communication, semiconductors , laser, electric light source and chemical industry and other high-tech fields. Usually, quartz glass preparation methods mainly include electrofusion, gas refining, chemical vapor deposition (CVD), plasma chemical vapor deposition (PCVD) and indirect methods. The first four methods mentioned above are to directly prepare quartz glass from quartz raw materials at a high temperature above 1800 °C, so they are collectively referred to as direct methods.

Compared with the direct method, the indirect method is to indirectly obtain quartz glass through the two steps of SiO2 soot deposition and sintering. In this method, a low-density SiO2 soot body is prepared by chemical vapor deposition from a silicon-containing compound raw material at a temperature not higher than 1200 ℃, and then the soot body is sintered under vacuum or atmosphere conditions to obtain quartz glass. Compared with the direct method, the indirect method has many unique advantages. SiO2 soot, as an intermediate for the indirect preparation of quartz glass, is a block composed of amorphous and nano-sized SiO2 particles with certain strength and pores. Its porosity and average density are 66.67%~87.50% and 0.28~0.73 g, respectively. /cm3, while the density of quartz glass is 2.2 g/cm3. Therefore, the process of sintering densification and transparency of the soot is the key to obtain high-performance quartz glass, and the evolution and elimination of pores are the core of optimizing the sintering process.

In the past ten years, the indirect method of preparing quartz glass has become one of the international research hotspots. At the same time, the sintering of loose bodies mainly adopts helium gas atmosphere zone melting and sintering for transparency. As a non-renewable resource, helium gas is increasingly consumed, and its price remains high, resulting in the low economical efficiency of glass preparation. In this work, vacuum sintering is used instead of helium atmosphere zone fusion sintering, which can greatly shorten the sintering time and reduce the cost, and the negative pressure effect is beneficial to improve the properties of the glass endoplasm.

1) The SiO2 loose body is composed of SiO2 solid spherical particles with high sphericity, high dispersion and diameter of 20-180 nm. There are many pores among the particles, and the pore size distribution is wide, mainly macropores, and the porosity is 67.52. %, the specific surface area is 63.07 m2/g.

2) The SiO2 soot body is sintered under vacuum conditions. When the sintering temperature is 1000 ℃, the soot body begins to shrink, and the macro-scale shrinkage is completed at 1200 ℃, and the shrinkage ratio is 30%. This sintering stage conforms to the traditional theory of powder sintering, that is, the center distance The double-sphere model is shortened, and the driving force for sintering is the difference in vacancy concentration between the neck and the center of the interface, which is dominated by diffusion and mass transfer; when the vacuum sintering temperature is higher than 1200°C to 1250°C, the small SiO2 particles in the loose body begin to melt into a transparent glass body. There is an obvious transition boundary between the solid-phase bulk body and the liquid-phase vitreous body. The pores of the loose body gradually change from random connected pores to spherical isolated closed pores. It decreased to 30.76 nm, and the average specific surface area decreased from 63.07 m2/g to 9.60 m2/g. This stage was dominated by liquid phase sintering, which was viscous flow and mass transfer. After shrinking and disappearing, the loose body transforms into transparent silica glass without bubbles. The mass transfer mechanism at this stage is still the viscous flow mass transfer and the viscous diffusion effect of the gas.

3) The phase composition of the SiO2 soot during sintering densification and transparency under vacuum conditions is always the same as that before sintering, all of which are amorphous and do not appear crystallization.

Selection of vacuum sintering equipment: The RVS vacuum sintering furnace provided by SIMUWU is a high-quality product for this kind of process. It has the characteristics of good temperature uniformity, high temperature control accuracy, and efficient glue removal system. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing.