I'm finding a powder that can improve the formation of cristobalite at low temperature when I start from pure silica powders.
Can you help me?
Thank for any answers gentlemen..
Classical catalysts for the crystallization of cristobalite from crystalline quartz are CaO and FeO. These are used in the sintering of silica refractory bricks. However, alfa quartz and tridymit changes into cristobalite at high temperatures (1350-1470°C). Crystallization of the cristobalite from quartz glass is observed at significantly lower temperatures even without a catalyst (from about 1100°C). When the surface of the quartz glass comes into contact with various minerals, cristobalite crystallizes as well at more lower temperatures (> 800°C). Besides CaO and FeO, it could also be different melts of metals (Al, Zn and others) or light fusible salts (chlorides, sulphates, vanadates and others). For example, aluminum melt causes a slow decomposition of a quartz glass vessel at 800°C. Quartz glass vessel crumbles due to partial crystallisatin of cristobalite.
Yes, but it depends on your task. Do you want to make a cristobalite powder from powder of quartz glass (fused silica)? If cristobalite powder should be pure, then CaO and FeO are not the best choice because they are difficult to remove. In this case, a better catalyst would be a water-soluble salt melt, for example, sodium sulfate (melting temperature of Na2SO4 is 884°C).
PS. Cristobalite powder is a cheap commercial product that can be purchased in tons. It is made by high temperature heat treatment of simple sand (heating > 1470°C and after that quench into the water). http://www.sibelco.com/materials/cristobalite/
In this case we can't add cristobalite, but is better if it's generate during sintering. We can't add composite with solfur...It's hard I know.. Can you suggest some works? You are very kind in any case. I'm grateful for helping me.
As a catalyst for the formation of cristobalite, is working also cristobalite by itself. If you add a bit of cristobalite powder to quartz glass powder, crystallization of the glass will proceed faster.
I can't add cristobalite directly for safety reasons, but I can generate in other ways. Can you suggest other catalizer besides the already mentioned CaO and FeO? Thank you very much mr. Vadim Verlotski
The easiest way to moisturize the amorphous SiO2 powder a small amount of dilute sodium salt solution (NaOH, NaCl). In the particles should be thin film solution. When it dries sodium cations concentrate at the point in which there are seeds of the cristobalite.
Thank you very much Dr. Alexey Vasilievitsh Belyakov, if you can share some papers (even the titles) with me I will be very grateful.
You can suggest the starting concentrations of NaOH or NaCl and what is the activation temperature of crystallization due to the same additives?
The content of NaCl (through an aqueous solution) into SiO2 should be about 0.1 wt.%. Additives KCl (instead of NaCl) is sufficient 0.01 wt.%. The heat treatment temperature of 1050 °C, the holding time of about 5 hours. Note that cristobalite immediately begins to move into tridymite. Therefore, the exposure time should be selected by changing the exposure time and temperature. It is necessary to take into account the size of the particles. The smaller the particles, the faster formation of cristobalite.
Dr. Alexey Vasilievitsh Belyakov, Can you suggest some papers with this kind of crystallization example?
Thank you very much for all the aswers.
Belyakov, A. V. Easily removed ceramic rods for casting gas-turbine engine blades using molding patterns / A. V. Belyakov, I. V. Razumnova, I. M. Demonis and Yu. I. Folomeikin // Glass and Ceramics Volume 69, Numbers 3-4 (2012), 125-129, DOI: 10.1007/s10717-012-9429-x
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