Motivation:
Silicon carbide is one of the main components in cicumstellar envelopes. It has a strong feature in the infrared region peaked around 11.3 microns. It was believed that the parameters of the synthesis, for example the temperature, in the gas phase and in the circumstellar shells are nearly the same. Material scientists are interested in SiC, especially in nanoparticles, too. SiC plays an important role for producing ceramics and changing the attributes of them. It has been shown that the laser induced pyrolysis of a gazeous phase is a good method to produce nanoparticles with different attributes. The produced particles are small in diameter, have a small distribution of size, are mainly spherical, are extremly pure and have a low agglomeration level. Therefor these particles are a good precursor for surface treatments of ceramics.
Advantages of this method:
extremly pure products (no contact with surface of the chamber)
extremly fine powders (d < 1 Micron)
small distribution of size
continous synthesis
well defined reaction zone
variable reaction conditions (temp., pressure, ...)
homogenous nucleation of molecules
control of growing rate and residence time in reaction zone
The disadvantage of this method is that you partly have to use corrosiv, flammable, explosiv or toxic precursors.
But you have a wide field of different possibilities of synthesis. You can for example synthesize C, SiC, Si, Si3N4, Si/C/N, SiO2, SiCO, Fullerene, and so on.
The principle of the laser induced pyrolysis of a gazeous phase is that infrared-emitting laser induces a chemical reaction (pyrolysis). The requirement for this is that the emission line of the laser overlaps with an absorption line of one or more precursors. In my case of the SiC synthesis this requirement is met by silane (SiH4). The 10P(20) emission line of the CO2-IR-laser peaks at 10.591 Microns and is the strongest emission line and overlaps with the absorption line of the silane. If the choosen precursors do not meet the requirements you have to use additional substances, so called photocatalysts like sulphur hexafluoride (SF6). The photocatalyst will absorb the energy and transmit it to the precursors by shocks.
Influences by different parameters on the reaction:
choosen precursors
composition of the reaction mixture
pressure in the reaction chamber
laserpower
flow rate of the gazeous precursors
All these parameters will take influence on the reaction temperature and this again will be responsible for the reaction progress. The temperature increases if the flow rate decreases, the laser power increases or the pressure in the reaction chamber increases. The composition, the diameter and the particlegrowth are directly dependent of the temperature. The average diameter of the particles decreases if the pressure decreases, the power decreases or if silane is choosen instead of methylsilane as precursor.
In my experiments silane and acetylene are choosen as precursors since they have the largest negativ reactionentalphy for the reaction to SiC. Therefore they will react easier than other precursor. The reaction for getting stochiometric SiC is theoretically:
2(SiH4) + 1(C2H2) --> 2(SiC) + 5(H2)
The requirements were that the reaction is complete and that there exists no other reaction besides this one. Therefore it is useful to vary the composition of the precursors silane:acetylene in the region between 2.5 and 1. Ratios larger than 2.5 will lead to the synthesis of pure Si, ratios smaller than 1 will lead to the synthesis of polymeric substances of acetylene. The average diameter of the particles have to be smaller than 50 nm. After the synthesis of SiC the next step is to produce Si/C/N-substances. For that purpose I will take ammonia as an additive to get the expected ratio of nitrogene.
Dominik Clément, 28. August 1998