Synthèse par plasma et frittage de poudres ultrafines de carbure de silicium
This study explores a new route for the synthesis of SiC Ultra Fine Powder (UFP) directly from solid silicon particles and methane, using thermal plasma technology. Both d.c. plasma and r.f. induction plasma have been employed to realize the SiC synthesis. The steps involved are the vaporization of the Si powder, followed by the vapor phase carburization and condensation of SiC UFP. On the basis of the experimental results of this reaction, in-situ boron doped SiC UFP was subsequently synthesized using the r.f. induction plasma process. The sintering properties of the various plasma-synthesized SiC UFP's were evaluated and compared to the behaviour of commercial powders. In performing the d.c. plasma synthesis, an inclined plasma torch, a rotary reactor and the cross trajectory injection of reactants were used to enhance the plasma heating efficiency. SiC UFP was synthesized by introducing Si powders to a rotating graphite crucible where it was heated by the plasma jet and the Si vapor so produced was subsequently carburized by CH[subscript 4] injected at the evaporator crucible exit. The SiC powder so obtained was of very fine particle size (20-40 nm). The sintering properties of the obtained SiC UFP was not satisfactory due to the high free Si and C content, originating from the unexpected very low and variable evaporation rate of Si in the plasma and thus poor controllability of the synthesis process. R.F. induction plasma technology possesses the unique characteristics of relatively large plasma volume, long residence time and availability of axial reactants injection, application of which ensured the Si vaporization and facilitated product powder quality control. The product powders, obtained from co-injected Si and CH[subscript 4], were characterized by XRD, ED, TGA, SEM, TEM, EPMA, IR, and BET. The combined results indicated that the purest SiC powder was able to be collected from the microfine filter, and was composed of both [alpha] and [beta]-phases of SiC, with only small amounts of free Si and C content. The particle size of the SiC powder was typically in the range of 40 to 60 nm, corresponding to a specific surface area of 30 to 50 m[superscript 2]/g. The initial silicon evaporation rate was found to depend strongly on the particle size of the starting Si powder. A parametric study of the synthesis showed that the quality of the powder obtained varied with the plasma plate power ant the position of the injection probe. The plasma gas composition employed was found to separately influence the proportions of [alpha] and [beta]-SiC in the synthesized SiC powder. With an Ar/N[subscript 2] mixture as the plasma gas, the ratio of the [alpha] to [beta] phases was less than 1.0, whereas the ratio was greater than 1.5 when using a mixture of Ar/H[subscript 2] as plasma gas. The Si powder feed rate and the input C/Si molar ratio in the injected reactants significantly affected both the formation of the SiC and the free Si and free C content in the synthesized powder. The use of a hot wall reactor through the lining of the cynlindrical plasma reactor wall with graphite, improved the conversion of Si to SiC. By appropriate selection of experimental conditions, SiC UFP of high purity (98 wt% SiC, 0.3 wt% free Si, 1.0 wt% free C. and other metallic elements) was obtained. The experimental results support the view that the formation mechanism for SiC UFP is dominated by the reaction of Si vapor with the thermal decomposition products of CH[subscript 4]. The plasma-synthesized SiC UFP was subjected to pressureledd sintering in an induction fumace in the presence of various sintering aids. With the addition of B[subscript 4]C (2.0 wt% B) by mechanical mixing, the powders could be only partially densified, with the highest value, 84.5% of theoretical density achieved at 2170°C for 30 min. Through the use of “in-flight” (in-situ) boron-doping of the powder during the plasma synthesis step (1.65 wt% B), the UFP obtained could be densified to above 90% of its TD at 2050°C for 30 min. The addition of oxide sintering aids (3.0 wt% Y[subscript 2]O[subscript 3] + 7.0 wt% Al[subscript 2]O[subscript 3]) by mechanical mixing produce sintered pellet of 95% of TD at 2000°C fot 75 min. The Vickers microhardness of the sintered pellets produced in this case was as high as 31.2 GPa, values that are superior to those for commercial [alpha]-SiC powder sintered under the same conditions. In order to improve our understanding of the basic phenomena involved, extensive microstructural (SEM, TEM), phase transformation (XRD, ED), physical (shrinkage, weight loss, porosity, hardness), as well as chemical analysis (PGNAA, EDS, XPS, TGA) were carried out. This helped establish a relationship between the properties of the as-synthesized powder and their sintering properties. The influences of sintering temperature, sintering time, additive concentration, and the powder purity on the densification behavior of the plasma-synthesized powders was investigated. The results were compared with data obtained using commercial powders.
- Génie – Thèses