I want to increase the crystallinity of a water-insoluble phosphate by use of an autoclave.
In literature the phosphate/water dispersion was kept at 180 °C for one week. Cooling rate was not mentioned.
I wonder if it makes sense to slowly cool down the autoclave or if holding at high temperature is the important step.
I could find out by experiment, but that will take a long time, so any experience would be quite helpful.
In my experience ... it is a good idea to cool down slowly and repeat the process a number of times.
It depends too much on the system, as sometimes the crystallization occurs on cooling down a solution (cooling down makes the solution become supersaturated), but in solvothermal synthesis anything can happen and I've seen with my eyes crystals growing during heating.
If your phosphate gets dissolved at 180°C it makes sense to cool down slowly, because new nucleation has to occur and you want only few nuclei, and to grow them slowly.
But if they keep it for 1 week I suppose it stays as a suspension therefore you're trying to improve the crystals you already have, growing them bigger at the expenses of the small ones and/or more ordered.... in this case the cooling rate matters less I think.
Just for fun put 2 autoclaves in the oven, program for slow cooling, but take 1 out after the heating time...
DISCLAIMER: I have no experience with what you're trying to do, I just found the question interesting and sharing my thoughts. I might as well be wrong.
In my experience, i come across where the cooling rate does not matter in most cases, however I assume you are synthesizing AlPO/SAPO like materials which may matter. It could be stable and promote crystallinity at high temp along with order and change in crystal size, but with a change in temp you will need to consider the pH of the environment (ie, the pH environment at temp vs pH at a different temp before/after synthesis). pH will change during the reaction. I assume you cannot grab a sample at temp, in that case i would run 1) fast cooldown , 2) fast with hold, and 3) slow cool down. I would do this experiment not on your best material or your worse, but something in the middle to show its effects.
Thanks a lot for all your answers. I was hoping there might be a rule of thumb. However, I agree, the point will be whether the material will dissolve at 180 °C or not. Slow cooling only makes sense if you slowly go from above solubility to below.
One of the educts is somewhat costly, so I can't run a large amount of experiments, but maybe I'll try and do as Steven suggested.
The reason I need better crystallinity is that the XRD is very noisy and has broad reflexes making it hard to see whether any minor by-phases are present. More importantly, the luminescence indicates a material with low crystallinity as emission lines are broad and get narrower after each autoclave run. As does emission intensity. So it works, but is very time consuming and even after 4 runs with a total of about 150 h it's still not satisfactory.
I tried slow precipitation with trimethylphosphate, but that only gave a nice uniform particle size distribution (10 µm) but didn't do anything in regard to crystallinity.
We usually apply high temperature (500 deg. C and higher) annealing after hydrothermal synthesis (of metal oxides) to improve crystallinity but I do not know if your synthesised material can withstand high temperatures.
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