Dear Sam Choppala,

Regarding your first question about methyl cellulose. This water-soluble binder works well in a basic solution at temperatures below 15°C (cold water). SiO2 makes the solution acidic and this allows the methyl cellulose to polymerize. For acidic solutions there is another water-soluble binder - polyvinyl alcohol (PVA), which you can successfully use here.

The second question (sintering of SiO2 ceramic) is much more complex than you think. Pure SiO2 ceramic does not exist and cannot be produced by sintering. The problem with this is the polymorphic transformations of SiO2, which are associated with large changes in volume (when heated, alfa quartz first transforms into beta quartz and then tridymite and cristobalite form). In order to avoid this problem with polymorphic transformations, SiO2 (quartz) must be sintered with additions of CaO and FeO. The resulting refractory material (silica brick) is not dense technical ceramics, but porous coarse-grained refractory stone used in furnace construction.

Hello Vadim Verlotski ,

Thank you for your answer!

Regarding the methylcellulose question, I've tried using PVA; however, I am faced with a similar issue in that the SiO2 does not mix well. This is most likely not due to the polymerization issue that you mentioned for methylcellulose. Please see the attached image for a visual description. It seems to be due to nonhomogeneous mixing, but I have tried experimenting with premixing PVA with water to avoid particle clumping as well as modifying different speeds at which the slurry is mixed. Additionally, this mixture dries very quickly when exposed to air. My goal is to achieve a "toothpaste-like" viscosity. Could you please advise on how to overcome the mixing issue?

Regarding the sintering question, thank you for the feedback. Could you please point me to research studying the strengths for each polymorph. Ideally from the crystal structure of each polymorph, the hexagonal structure would have the highest strength, and having lower grain sizes would increase strength as well. Does this mean sintering to a tridymite phase would result in a stronger part than sintering to a cristobalite phase?

I appreciate your help and feedback!

Thanks & Regards,

Sam Choppala

Dear Sam Choppala,

You are correct in that the problem with the high viscosity of the silica slip is not related to the polymerization of polyvinyl alcohol. The problem is in the silicon dioxide itself, which in aqueous suspensions is prone to the formation of viscous and strong precipitates even without any binder. In this property, silica is very different from other mineral powders, which form much less stable precipitates. The smaller the silica particles, the more serious the problem. If you want to get loose silica sediment (toothpaste consistency), then you will have to forego water and use a non-polar organic liquid, such as kerosene.

As for the sintering of silicon dioxide, I'm afraid you misunderstood me. I meant that silicon dioxide in its pure form does not sinter at all! Polymorphic transformations destroy the formed sintered body and no component can be obtained. It doesn't matter if you use quartz, cristobalite or tridymite. The only exception is amorphous SIO2 (quartz glass), which can be sintered at temperatures up to 1100°C. In this case, no polymorphic transformations occur.