coal - 2-4-2008 at 10:37
Does anyone have any cleaver ideas on how to make ammonium silicate which do NOT involve the obvious ammonium sulphate + sodium silicate
I ask because it seems to me that it would be great for makeing high temperature ceramics; when heated the ammonia would break down leaving pure
silica intimately bound and mixed with all the other particles. Sodium contamination would flux everything.
Sorry if this is a retarded question 
-coal
DJF90 - 2-4-2008 at 13:53
Good idea, although I suspect that you'd be left with silica in clumps/powder and not a solid structure. In order to form a solid structure you'd need
to fuse the silica, a process that requires a temperature of about 2000C. However these are just my thoughts and other members may have an entirely
different opinion.
12AX7 - 2-4-2008 at 14:21
Doesn't work. Silicate pKa1 = 9.3, pKa2 = 12, ammonium pKa = 9.
To do what you describe, ethyl silicate ((CH3CH2)4SiO4) is often used, hydrolyzing to amorphous silica gel.
Tim
Zinc - 2-4-2008 at 14:33
How to make ethyl silicate?
garage chemist - 2-4-2008 at 15:50
The synthesis of ethyl silicate is described in Brauer, which you can download in the library.
coal - 3-4-2008 at 03:25
The Brauer for download in the libabry doesn't appear to mention it. It does cover silanes and oxysilanes. Nothing organic with an SiO4 though.
I suspect the synth would be decidedly non-trivial however, probably not doable in my shead 
coal
garage chemist - 3-4-2008 at 03:30
Wrong, the synthesis of ethyl silicate is described in the Brauer from the library! It is called tetraethoxysilane there.
The necessary SiCl4 can be prepared from the elements in a heated tube.
The bigger problem would be to get the pure and completely anhydrous ethanol- you would have to purchase that from the pharmacy and do the
absolutation yourself.
not_important - 3-4-2008 at 06:13
While hydrolysis of solutions of esters of inorganic oxides gives the best mixing, you can do quite well with co-precipitation from aqueous solution.
For example, you can produce a decent mullite by making two somewhat dilute stock solutions, on of ammonium sulfate and aluminium sulfate, the other
of sodium silicate and sodium carbonate. Mix quickly, the allow to sit undisturbed. Wash well with dilute ammonia + ammonium (nitrate, carbonate, or
sulfate) solution, which means a number of washes or by flowing the wash through the gel, moving to increasingly dilute aqueous ammonia; the ammonia
and ammonium salts helps displace sodium ions. You need both sodium silicate and carbonate to fully react with the sulfates and get the proper Al:Si
ratio.
coal - 3-4-2008 at 08:22
garage chemist: tetraethoxysilane is Si(OC2H5)4 whereas the formula 12AX7 specified was (CH3CH2)4SiO4, there don't appear to be enough oxygen atoms in
the tetraethoxysilane for it to actually be a full silicate. mind you it probably wouldn't matter too much as it appears to hydrolyze to silica
anyway
coal
garage chemist - 3-4-2008 at 08:52
Tetraethoxysilane and the compound 12AX7 mentioned are the same.
Ethyl silicate, tetraethyl silicate, tetraethyl orthosilicate (TEOS) are synonyms for the same compound.
Nicodem - 3-4-2008 at 09:08
The confusion results just from writing down somewhat ambiguous but still corect rational formulas. It is otherwise pretty clear that Si(OC2H5)4 and
(CH3CH2)4SiO4 can only represent the same one structure.
The correct nomenclature for these compounds is ortosilicates, hence the above mentioned Si(OEt)4 is actually called tetraethyl orthosilicate (often abbreviated as TEOS by the materials people) and not ethyl silicate which one would tend to assign to (EtO)2SiO
instead. Another possibility is to use additive nomenclature based on silanes, which would result in tetraethoxysilane.
coal - 5-4-2008 at 04:39
ah i see now, my mistake 
SiCl4 then the tetraethyl orthosilicate still seems a bit of a hassle to go to to make stuff stick together though think i'll have to stick with phosphate bonding
coal
garage chemist - 5-4-2008 at 06:05
You can make colloidal silica (H4SiO4 solution) by running dilute sodium silicate solution through a cation exchanger.
That would be even better than ammonium silicate, and if it has to be ammonium silicate you can always add ammonia to it and get what you need.
not_important - 5-4-2008 at 06:26
Phosphates and high temperature ceramics don't seem to go together well. They tend to form multiple phases, and often have thermal expansion
mismatches as well as phase transformations as they heat up and cool down. A few percent in the glassy phases does work as a stiffener, the multiple
-O- bonds greatly reducing sag.
coal - 5-4-2008 at 12:37
hmm silica gel might have potential. Wonder if it could be formed into a useful shape, a wet process for making silica crucibles would be handy!
coal
Nicodem - 5-4-2008 at 14:15
If you just want some ionic water soluble silicate with an organic cation that would decompose during the sintering, you can try preparing
tetramethylammonium silicate or (Me4N)2SiO3. However, I don't know how one would go to prepare this except maybe by hydrolizing Si(OEt)4 in the
commercial solution of tetramethylammonium hydroxide (which is relatively expensive). Well, at least Si(OEt)4 is quite cheap.
PS: Be careful with these ortosilicate esters since, even though their MSDS sheets don't say anything about it, rumors among the materials researchers
say that they are quite possibly carcinogenic (I guess they base their fears on fine silica causing cancer - and guess what these esters hydrolyze
into?).