Combustion synthesis is a novel type of highly exothermic, self-sustained reaction between a strong reducer and a strong oxidant. After combustion initiation, the chemical reaction propagates through the reactants as a rapidly moving combustion wave. High temperature and pressure gradients within the combustion wave result in a growth of different nanomaterials [1], i.e., we demonstrated earlier the efficient formation of silicon carbide nanowires [2]. The present study is a continuation of that research aimed at more-in-depth study of the combustion mechanism. In a typical combustion synthesis, Si powder or Si-containing powdered compound (silicides, alloys) were thoroughly mixed with poly(tetrafluoroethylene) - PTFE, in a stoichiometric ratio to obtain a homogeneous mixture. The following silicon compounds were tested: CaSi₂, Si₂Ta, Mg₂Si, NbSi₂, Cu₅Si, MoSi₂, Si₂Zr, CrSi₂, Si₂Sr, VSi₂, FeSi₂, Si₂Ti, WSi₂, Mn₁₅Si₂₆, HfSi₂, Co_0.5Ni_0.5Si₂, Ni_0.5Fe_0.5Si₂, Co_0.5Fe_0.5Si₂. Not all of them were reactive enough to instantly reduce PTFE so a small amount of a strong reductant (Mg powder) was added to commence such combustion. The reactants were transferred to a quartz crucible and then placed in a stainless steel high pressure reactor (Fig. 1) with a 350 cm³ volume. 

Fig. 1. High-pressure stainless steel reactor for combustion synthesis
After filling the reactor with either argon or carbon dioxide to a pressure of 1 MPa, the combustion process was commenced using an ohmic heating. A mechanism of the reaction can be presented in a following simplified form

MeSi + (CF₂)_n → MeF + SiC + C + SiF₄

Fig. 2 presents registered sequence of combustion stages in Si/PTFE system.
The time of reaction is about 1.2 s. The oscillation of signal intensity may be noted. Fig. 2. Registered sequence of combustion stages in Si/PTFE system The product was collected for SEM, TEM, XRD, and chemical analyses. Electron microscopy observation showed a presence of one-dimensional (1-D) silicon carbide nanocrystallites (Fig. 3) and fluoride nanoparticles along with soot agglomerates.

Fig. 3. SEM image of reaction products (starting mixture: Si/PTFE) To accelerate the combustion, sodium azide (up to 85 wt%) was also added to the starting mixture. Branched and comb-like SiC nanocrystallites were found in products (SEM images in Fig. 4) .

Fig. 4. SEM image of reaction products (starting mixture: Si/PTFE/NaN₃) After combustion initiation, the high temperature causes the pyrolysis of PTFE into C_xF_y radicals and melting of silicon. The reaction of radicals with Si generates gaseous species forming 1-D SiC nanocrystallites presumably via a well-known VLS growth.

Acknowledgement. This work was supported by NCN through grant No. UMO-2011/03/B/ST5/03256 and 2012/05/B/ST5/00709.

References:
[1] A. Huczko, M. Szala, A. Dąbrowska, “Combustion Synthesis of Nanostructured Materials”, Publ. Wydawnictwa UW, 2011, Warsaw.
[2] M. Soszyński, A. Dąbrowska, M. Bystrzejewski, A. Huczko, CrystalResearch and Technol., 45, 2010, 1241-1244.

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