Physico-Chemical nanomaterials science

Heat Resistance of Nanocrystalline Powders, Single Crystals, and Coatings of CrSi2 Obtained from Molten Salts
Sapsay O.I.1, Shakhnin D.B.1, Kublanovsky V.S.2, Malyshev V.V.1,2
1 University “Ukraine”. 23 Lvivska St., Kyiv-03115, Ukraine. 
E-mail: sapsay@email.ua
2 Vernadsky Institute of General and Inorganic Chemistry of the NAS of Ukraine. 32/34 Palladina Ave., Kyiv-03142, Ukraine.

The purpose of this work is the synthesis of heat resistant chromium disilicide nanocrystalline powders, single crystals, and coatings.
Chromium disilicide CrSi2 was synthesized as nanocrystalline powder by co-reducing of anhydrous CrCl3 and Na2SiF6 with Na metal according to the following reaction:
CrCl3 + 2Na2SiF6 + 11Na → CrSi2 + 12NaF + 3NaCl
Since the reaction between reduced Cr and Si is carried out in molten salt system, it probably leads to nanocrystalline particles formation.
CrSi2 single crystals were grown from elements in the molten Sn-Zn (1:10 weight) system. Cr and Si powders of 99.9% purity (taken in molar ratio 1:2) were added to Sn-Zn melt (1:10 by weight).
CrSi2 coatings were obtained by electroless deposition of Si onto pure Cr substrate surface from molten KCl-NaCl-NaF-Na2SiF6-Si system.
The heat resistance of the nanocrystalline powders, single crystals and coatings to oxidation in air at elevated temperatures was studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) methods.
As TGA curves show, no CrSi2 samples change their mass up to a 600° C. Within the range 600-1000 °C, samples weight gradually increased by 6.8% for nanopowders, 4.6% for crystals, and up to 5.3% (minor surface oxidation) for coatings. With further temperature increase, samples surface oxidation is observed. In the DSC curve for the nanoparticles, there is a major exothermic peak in the temperature range close to 800 °C. For crystals and coatings, exothermic peak in DSC curve is observed at a temperature about 700° C. The correlation of TGA and DSC curves data obtained in these cases suggests that, up to the temperature 600 °C, chromium silicide is not oxidized. The intense oxidation process is starting at a temperature 810-820 °C for nanosized powders and 700 °C for CrSi2 crystals and coatings. Then, a protective SiO2 layer is formed on the chromium silicide surface.