How volatile is urea in a water mixture?

[Guilherme Franco]

I'm seemingly having problems of having too much evaporation of urea in the preparation of a gel. The gel is formed by drying the components in a water mixture, and it takes hours to dry at 70 ºC and dries much faster at 100 ºC (also produces a gel with better consistency), but at 100ºC it seems to be loosing too much urea, the vapour clearly smells like urea.

I couldn't find any information online about how volatile it is in a water mixture, nor could I find articles on the proper drying temperature for my gel (I've found one talking about the effects of the drying temperature on the gel structure, but not on the evaporation of urea, because it wasn't about this type of gel).

Can anybody help me on this?

Thanks!

Backstory:

I'm trying to produce a phosphor through the sol-gel combustion method, which involves creating a gel made of metal nitrates and urea, that is then heated on a furnace to the ignition temperature of urea (500 ºC).

The nitrates work as the oxidizer for the combustion, and the final solid product is a cristalline material made of the metals cations bonded to oxigen atoms in the structure.

The complete combustion should produce only water vapour, nitrogen gas and carbon dioxide as gaseous products. However, some thermal decomposition of the metal nitrates also happen, which produce NO2 as one of the products (the others are the same).

Some NO2 being produced is normal, but I'm having way too much NO2 being produced, the smoke coming from the furnace is clearly red-orange, and when I open it, even after 10 minutes at 500ºC, it's still full of NO2 inside and being produced at a fast rate in the samples.

It seems that no or almost no combustion is happening in there, the nitrates are suffering thermal decomposition and nothing else.

I'm completely sure the amount of urea I used initially was correct.

I even used this same recipe another time, but I dried the gel at 70 ºC and it worked.

 

[Borek]

I would check what is the solid urea volatility at temperatures you are interested in and start from there. While it is not the exact number you are looking for it is definitely better than nothing.

 

[Ygggdrasil]

From Wikipedia:

Urea dissolved in water is in equilibrium with the isomeric ammonium cyanate.[65] The resulting activity of the isocyanic acid ions do result in carbamylation (formation of long-chain carbamides, liberating ammonia molecule as byproduct) of proteins if proteins are present in the solution too. The carbamylation reaction may occurs at elevated temperatures even without catalysts. At room temperature, water solutions of urea are prone to same decomposition reaction in the presence of urease. The isomerization of urea in solution at room temperature without catalysts is a slow process (taking days to reach equilibrium), and freshly prepared, unheated solutions had negligible carbamylation rates.[66]

https://en.wikipedia.org/wiki/Urea#cite_note-66
https://en.wikipedia.org/wiki/Urea#Reactions

If nucleophilic species are present in your mixture, you may be reacting away your urea at high temperatures.

Furthermore, urea can react with water to form ammonia and carbon dioxide (https://en.wikipedia.org/wiki/Ammonia_volatilization_from_urea). Although the rate of this process is negligible at room temp (at least without any catalyst), I'm unsure at what rate the process occurs at elevated temperatures.

 

[Chestermiller]

https://www.researchgate.net/publication/287178967_Vapor-liquid_equilibrium_of_urea_solution_separation_system