Formation of SiO and Related Si-Based Materials Through Carbothermic Reduction of Silica-Containing Slag

Abstract

Silicon based nanoparticulates of various composition and morphology have been produced by smelting reduction method which includes a carbothermic reduction of SiO 2 -Al 2 O 3 -CaO starting materials to SiO vapor at 2073 K and transfer of the vapor with a carrier-gas to cooler surfaces inside the experimental reactor where the nanoparticulates were deposited. The chemical composition of the starting materials was matched to the basic composition of silica-rich coal ash which is considered as a potential source of Si in this study. Special emphasis was placed upon examining the degree of SiO 2 reduction from starting materials, purity and morphology of the as-obtained nanomaterials. It is shown that up to 20% of Si can be converted from the silica-based melt into rounded nanoparticles, nanoparticle chains and nanowires containing Si,O and C in variable proportions depending on deposition temperature and gas flow conditions. The diameter of nanoparticulates was estimated to be in the range of 20$100 nm. The nanoparticles and chains were found to be deposited at lower temperature locations (293$1320 K) while the nanowires were obtained at higher temperatures (1320$1570 K). There was a tendency for an increase in Si concentration in order of nanowires, nanoparticles chains and nanoparticles. The carbon concentration, on the contrary, was much higher in nanowires as compared to that in nanoparticles. Although the degree of SiO 2 reduction from silica-containing melt to SiO vapor is limited because a part of SiO 2 reacts with C producing SiC, its good controllability, high productivity and possibility for processing cheap starting materials make the smelting reduction a very attractive technique for production of silicon based nanostructured materials.

Characterization of Si-Based Nanoparticulates Produced by Carbothermic Reduction of Silica-Containing Slag

“…It is notable that XPS measurements showed larger oxygen concentration in the silicon suboxide as compared to that obtained by WDX measurements presented in our previous paper. 1) This inconsistency can be explained by the fact that XPS analysis is very sensitive to the presence of oxygen on the sample surface while EPMA indications show bulk concentration. Obviously, such a surface oxidation could occur during the nanoparticle formation.…”

“…Thus, during annealing, amorphous SiO decomposes into Si nanocrystals and amorphous SiO 2 with the decomposition degree increasing with annealing time. This phenomenon is well known from the relevant literatures (for example 1) ) as reaction of SiO disproportionation which can be expressed by eq. (1).…”

“…This finding coincides qualitatively with the SEM observations discussed earlier. 1) For example, it was found that in the temperature range of 1490-1580 K, the asobtained deposit is composed of nanowires the diameter of which was estimated as 40-60 nm. It is worthy of note, however, that analyzing crystallite size by means of SEM view is very hard since both the Si and SiC crystallites may be covered with a shell of amorphous SiO x .…”

“…6(a) in the previous report by the present authors. 1) Checking for crystallinity of these nanoparticles indicated the presence of amorphous phase only. Following EDX examination revealed the average atomic ratio of Si to O to be 0.88 indicating that the nanoparticles are largely composed of silicon monoxide.…”

“…In the previous study, 1) we proposed a process for production of silicon based nanoparticulates by smelting reduction method which includes a high temperature carbothermic reduction of silica-rich SiO 2 -Al 2 O 3 -CaO melt to SiO vapor and transfer of the vapor with a carrier-gas to lower temperature zones inside the experimental reactor where the vapor was deposited on the walls of graphite tube and filter. Both temperature and gas flow conditions were found to be of prime importance in controlling the process yield, purity and morphology of the obtained products.…”