Nanocomposites and nanomaterials

Chromium nanosilicides: a thermodynamic justification for synthesis methods
Gab A.I., Shakhin D.B., Malyshev V.V., Panchenko Yu.V.
University”Ukraine”, 23 Lvivska St., Kyiv-03115, Ukraine. 
E-mail: viktor.malyshev.igic@gmail.com

This work aim is the theoretical substantiation of the possibility of high-temperature electrochemical synthesis (HTES) and metallothermic synthesis (MS) of chromium silicides.
For 1000 K electrolysis temperature, silicon containing compounds can be arranged in the silicon equilibrium deposition potential descending order as follows: SiF4 > Na2SiO3 > Li2SiO3 > Mg2SiO4 > Na2Si2O5 > Li2Si2O5 > MgSiO3 > SiO2. 
From the decomposition voltage values, it follows that the most energetically profitable process is the direct silicon deposition. However, it will depend on the kinetic characteristics of the process which will take place. 
Decomposition potentials for chromium (-0.7  -1.0 V) and for silicon (-1.6  -1.9 V) differ substantially. Therefore, HTES of chromium silicides is possible only in kinetic mode. To optimize the chromium silicides yield, HTES was performed in KCl-KF-K2SiF6-K2CrO4 system. Depending on the electrolyte composition and parameters of electrolysis, both individual Cr2O3, Cr3Si, and CrSi2 phases and mixtures thereof were obtained. 
Thermodynamic calculations for MS were carried out of the free Gibbs energy change values for the reactions of interaction between CrCl3, SiF4, and metal-reductant (M – Na, K; Me – Mg, Ca). Calculations results allow to arrange chromium silicides of different composition and metals-reductants in the following series by the free Gibbs energy decrease: CrSi2 > CrSi > Cr5Si3 > Cr3Si; Mg > Na > K > Ca. So, the most thermodynamically advantageous process is the formation of silicide Cr3S with the use of Ca as metal-reductant in this case.
The thermodynamic calculations results obtained allow finding optimum parameters for chromium silicides MS realization. Depending on the chromium and silicon containing components ratio, silicides of different composition can be obtained. In addition, depending on the use of M and Me, fluorides of sodium, potassium, magnesium, or calcium can be formed. 
The chromium silicides MS possibility was proven experimentally. The results of structural studies indicate that synthesized powders consist of nanoparticles with average particle size 35-60 nm.