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 , i.e., we demonstrated earlier the efficient formation of silicon carbide nanowires . 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, Co0.5Ni0.5Si2, Ni0.5Fe0.5Si2, Co0.5Fe0.5Si2. 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 CxFy 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.
 A. Huczko, M. Szala, A. Dąbrowska, “Combustion Synthesis of Nanostructured Materials”, Publ. Wydawnictwa UW, 2011, Warsaw.
 M. Soszyński, A. Dąbrowska, M. Bystrzejewski, A. Huczko, CrystalResearch and Technol., 45, 2010, 1241-1244.