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Making Nanopowders – Solution Combustion Synthesis

Theory:

The Science of Nanomaterials is a topic of particular interest in scientific community and is easily one of the most promising fields for technological development in the 21st century [1]. The blatant and principal characteristic of nanomaterials is their particle size (usually  1 – 100nm), which results in enormous surface area. They have a wide range of uses for advanced applications, from catalysts, to fuel cells and biotechnology [2]. Iron nano-powder is, for example used for reduction of hexavalent chromium in heavily polluted wastewaters as a mean of removing the dangerous chromate ion CrO42- from the environment. We will publish an article about that soon. Solution combustion synthesis or SCS is an effective method for producing nano-sized materials. To this date, over 1000 complex oxide powders have been synthesized using this method [2]. The classic example of SCS is a self-sustained reaction between metal nitrates and a fuel, for which glycine or urea is ussually used.  Once the reaction is initiated (at about 200°C), a rapid high temperature wave (1000-3000°C) propagates through the mixture at speed about 0.1 to 10cm/s. In our experiment we used ferric nitrate Fe(NO3)3 (prepared by the reaction of elemental iron and nitric acid) and glycine. The reaction itself  doesn’t need oxygen to proceed, but if present, it will enter the reaction in quite a complex way[2]:

Fe(NO3)3 + 5/3*ΦH2NCH2COOH + 15/4(1-Φ)*O2 → 1/2Fe2O3 + 10/3*ΦCO2 + (5/3*Φ+3)/2N2 + 25/6*ΦH2O

Where Φ=1 implies oxygen-free conditions, the lower the value of Φ, the more oxygen enters the combustion reaction. Evolution of gases as sideproducts is absolutely crucial and the amount of gases produced directly influences the particle size of the product. The gases break large clusters and create pores between particles [3]. The oxidant/fuel ratio thus affects not only the particle size but the morphology as well. When large excess of fuel is used, typically only agglomerates up to tens of micrometers are formed (e. g. ErFeO3 synthesis – 100% over-stoichiometry of glycine).

EFO-NG-900-3

SEM photograph of erbium-iron mixed oxide.

Chemicals:

  • Nitric acid (3.17 mol/L, 60 mL)
  • Iron (4.4 g, 0.078 mol)
  • Glycine (12 g, 0.16 mol)

Procedure:

Nitric acid is poured into a beaker. Iron powder is added and the mixture is carefully heated. A lot of nitrogen dioxide is produced, so the experiment must be performed outside or in the fumehood. Then, glycine powder is added. The solution is heated again and water is boiled off. After a few minutes, a gel-like mixture is formed. It is important not to stir the mixture,  as it would cool down the mixtureunder critical temperature, and the combustion may not occur. Suddenly, nitrogen gas starts evolving and bright-red hot Fe2O3 dentrites start rising from the mixture. This product is very fragile, and breaks into micro-particles very easily.

You can also check out the whole procedure and reaction in this video:

 

Cluster of Fe3O4 dust produced by the reaction.

Cluster of Fe2O3 dust produced by the reaction.

References:

[1] Alves, K., Annelise, Bergmann, Carlos P., Berutti, Felipe Amorim (2013). Novel Synthesis and Characterization of Nanostructured Materials. Berlin, Heidelberg: Springer Berlin Heidelberg, p.11.

[2] Alexander S. Mukasyan,Paul Epstein,Peter Dinka (2007) Solution combustion synthesis of nanomaterials

[3] Alves, K., Annelise, Bergmann, Carlos P., Berutti, Felipe Amorim (2013). Novel Synthesis and Characterization of Nanostructured Materials. Berlin, Heidelberg: Springer Berlin Heidelberg, p.16-17.

 

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