Why we can't have large area of ceramic (Flat ceramic) as Flat glass. I know there is a problem of moving the ceramic over a bath of molten tin
Sintering of ceramic is a surface force driven mechanism; to get an even surface you must apply large pressure and even then you wil not receive a totally dense body.
Glass is an amorphous material (no crystals inside) and its viscosity drops with increasing temperature, hence it can flow at high enough temperature.
Ceramics on the other hand are crystalline and have melting points that are extremely high (generally > 2000 K). Casting a ceramic on a liquid metal is therefore not a popular method, but it would be nice to try (using a ceramic with lower melting point). There will be issues like diffusion and chemical reactions at the material interface at such high temperature. Upon solidification (crystallization) the interface between metal and ceramic will not remain flat. This process in avoided by using an amorphous material, but apart from glass-types, are there any amorphous ceramics?
You have raised an interesting point. In principle one could do float ceramics, but it would require melt crystallizing on metal bath. However, there are couple constrains in such process:
1) Glass will be viscous at temperatures above Tg (including above liquidus temperature) while ceramics will freeze below liquidus temperature. Therefore, with glass you have much wider temperature window.
2) For the reasons from point 1), you will have less time unless the bath is really hot (its temperature is around liquidus temperature of ceramics).
3) Because of the high bath temperature you would be faced with evaporation from bath and sublimation from ceramics.
4) Energy required for keeping the bath hot enough would be much higher because of the higher temperatures. Also the energy loss on bath walls are higher.
5) At such temperatures most of the heating solutions are made that they provide heath from the top.
6) Bath could infiltrate pores between grains.
7) You would need to avoid bath oxidation. Or you would end up with ceramics being fused to metal from bath.
8) As already mentioned by Eric, interface would be rough.
As you can see most of the constrains result in lesser technical and economical feasibility.
@ eric: Ceramics by its definition is polycrystalline. The only ceramics that would come close to glass category is the one with nano-sized grains or maybe glass-ceramics.
How flat do you want it to have? With techniques like tape casting or paper derived casting methods you can go down to 4 µm flat ceramic substrates of various materials. We do that in our group so ;).
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