Dear Ashfaq , it depends on particle specification. no idea what material is in your interest, but I can give you a general idea why you need two step. at calcification step you keep it for some duration at relatively low ToC, low T is not enough for crystal formation, but high enough for source material decomposition. (there can be other processes as well, such as melting, gas generation, surfactant evaporation, solvent removal, and so on...) note that time is important at this point. once decomposition step is over, the T is increased to improve particle crystalline. " In synthesizing manganite nano particles or any other nanoparticles, is it necessary to perform calcination step " can you provide some reference work so I can take a look, maybe I can give better explanation.

Best

Artavazd

thanks Dear Artavazd. I have attached a paper. The question is why dont we directly go for annealing instead of calcination?

Dear Ashfaq,

Thanks for reference. I just checked the experimental part and it says nearly what I mentioned above. here is the experimental part from the work:

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Strontium doped lanthanum manganite nanoparticles ( and 0.27) were prepared using the citrate gel technique. Lanthanum nitrate, strontium nitrate and manganese acetate were used as raw materials; citric acid was

used as a fuel, and ethylene glycol was used as the gel forming agent. The precursor salts were mixed in distilled water. Citricn acid and ethylene glycol were added to the solution which was heated at 120 for 1 h under constant stirring. After gel formation, heating was continued at 160 until a grey powder was obtained. The powders were then dried at 400 for 3.5

hours. The samples were annealed in air at 800 in a box furnace for 3 hours to remove defects and improve crystallinity.

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1. it solt-solution-organic based method that results Metal oxide, you first remove the solvent, (there is some not correct part as well, ethylene glycol is mot only get former, but also fuel ass well.) simply you get homonegiously distributed metal ions over organics (citrate, ethylene glycol)

2. heating at 160 leads slight decomposition of solts

3. 400oC is enough to degrade the all organics, there will be a lot of gas generated that also keeps metal ions in distance from each other - key factor for small (nm) size particle synthesis. ,

4. note that it 3.5h calcination is required for complete decomposing organics and nitrates.

5. if you stop the calcination process shorter than 3.5 h and check the samples, then you can find a dozens of inorganic solts (carbonates, nitrates, nitrites, ets.)

NOTE: here is important the fact that several concurrent and parallel processes occure, and limitang the time at intermedit T may change the process direction resulting different type and size of NPs. you can investigate by TEM and SEM a few samples to get more realistic picture.

6. they also mention that "The samples were annealed in air at 800 in a box furnace for 3 hours to remove defects and improve crystallinity."

Hopefully I could give you some idea going under the hood. check "solution combustion synthesis process related research works, here is a very good work from Prof. Mukasyan for reference.

Article Combustion Synthesis of Nanoscale Oxide Powders: Mechanism, ...

Good luck

Best

Artavazd

Thanks Dr.Artavazd.

'' 400oC is enough to degrade the all organics, there will be a lot of gas generated that also keeps metal ions in distance from each other - key factor for small (nm) size particle synthesis''. I would like to have some more detail or paper regarding the effect on particle size.

consisting of a decarbonation, moreover during annealing it goes through carbonation and from 450 ° C all organic matter will eliminate.

Dear Ashfag,

about gas generation - 400oC or around that (as Dr. Bouazza Tbib suggested) is the T where most of organics undergo decomposition, additionally nitrates decompose, + carbonates, also nitrate + organic reaction starts as self-ingnation combustion process.

about ions staing apart - this formulation is kind of simplified. here is what happening: organics and metal salts are first dissolved to form homogeneous solution and then dried. Typically it forms homogeneous mixture, although inhomogeneity may take place as well depending solubility of salt (if the salt has low solubility, then it will crystallize during solvent evaporation, but as nitrates have good solubility, than no problem) so in homogeneous mixture the metal ions (or small groups of ions i form of small particles of salt i.e. microscopically non-homogeneous mixture) are apart ( separated by organics), otherwise there will be no homogeneous mixture. latter, when the T is being increased, the organics decompose and/or react to nitrates and generates gas. gas bubbles prevents these metals ions (or small group of metal atoms*) aggregating into large group i.e. big particles. formed small particles have nm size but not good crystalline.

* metal type also must be concidered. typibally in the case of noble metals ions end up forming reduced metal, organic species (radicals etc.) reduce the metal ions to metal atoms. in the case of transition metals (sometimes noble as well), the thermodynamically stable state is oxide. thus, metal ions end up forming oxide particles by either nitraate decomposition of carbonate decomposition

check these reference.

1. see previous reference

2. https://www.researchgate.net/publication/317413677_Solution_Combustion_Synthesis_of_Copper_Nanopowders_The_Fuel_Effect

3. chapter 3 of Article Combustion synthesis of zero-, one-, two- and three-dimensio...

4. Article Solution Combustion Synthesis of Nanoscale Materials

thanks a lot.

It is very important to consider the TGA curve and its differentiation to identify the calcination temperature. This curve shows the stages the changes of the material with change of temperature Through this curve you can determined calcination temperature and initiation of crystallization and crystallization temperature as well.

I have found this very helpful in understanding small molecule sol-gel processes:

Van Bael, M. K., Hardy, A., & Mullens, J. (2013). Aqueous Precursor Systems. In Chemical solution deposition Of functional oxide thin films (pp. 93-140). Springer Vienna.

https://link.springer.com/content/pdf/10.1007/978-3-211-99311-8_5.pdf

I agree with Essam that TGA curves are probably your first step into understanding the decomposition of your amorphous precursor. There are a number of factors which can change the decomposition temperature (for my system this temperature is 550 °C). Trying to understand the decomposition pathways is very complex so the blackbox TGA curves normally suffice. TGA will give you the time and temperature you need to calcine in order to fully decompose the organics.

The small molecule solgel techniques take a lot of process refining in order to fine a suitable process for your system. There is no standard way to do it like when doing solid state synthesis. You have to consider heterogeneity forming in the gel, during the decomposition, before your desired phase crystallizes, and the favorability of your phase to form. In-situ non-ambient XRD is a great way to monitor the process and crystallization.

I have always struggled with the two step process of first calcining the organics followed by another heat treatment. The organics are what hold the homogeneity. Another heat cycle without those organics to limit bonding and diffusion should lead to agglomeration of like species and thus a loss of homogeneity before the crystallization temperature of the desired phase. from the TGA curves the decomposition seems to be pretty quick I would imagine that if you are going to higher temperatures anyway that a single step process with a brief hold (determine by TGA) at that calcination temperature should suffice. That said my experience is limited and experimentation with this is limited. Can anyone else comment on their experiences with a two step vs single step process?

I think the answer of the Ashfaq question should be restricted to the material prepared and to the method of preparation used. For magnetite, crystal formation would be achieved at 400 C and further heating increases the crystal growth (be careful about hematite formation). Heating at 160 C should not be considered as calcination; it is just for water evaporation. Decomposition of organics start with further heating as an "autocombustion" process.