Vangskåsen and Høgsand [15] found a green compound on the surface of the brown condensate at 1570-1580°C.
It consists of SiC particles dispersed in a SiO2 matrix (Figure 63). It was thought that Si and SiO2 may separate at high temperatures, or that Si can react with CO and form SiC. Pultz and Hertl [72] found a green-grey condensate containing SiO2, SiC and Si. The green condensate color is coming from the dispersion of SiC into the SiO2 matrix [1], [11]. Mølnås [11] expects that the green condensates also follow the ICM-model [47].
Figure 63: Green condensate [11].
The black condensate composition was not analyzed [54], so it remains unknown for now. It can come as continuous spheres with 15 µm diameter, but it can also be tube-shaped (Figure 64). A spherical type structure is a sign of the presence of a vapor-solid reaction. The compound would be noticed together with brown condensate if a SiO2+Si+SiC charge was heated at 2000°C for 40 minutes. It is thought that this could be carbon black.
Figure 64: a): Spherical black condensate microstructure; b): Tube-shaped black condensate [54].
The grey condensate shows a needle-like structure, with length of the order of magnitude of 1 µm [54] (Figure 65). Its composition is unknown. As for the black condensate, it can form at holding a SiO2+Si+SiC charge at 2000°C for at least 40 minutes.
Figure 65: Grey condensate [54].
Summary
Table 5 resumes all the features of the condensates reviewed in this report.
81 Table 5: Condensates properties, revised after [12], [15], [53].
Color Microstructure Temperature Substrate Appearance Compounds
Brown
condensate Colored layer SiC, SiO2
Black
with L=1µm 1760-1790°C White and brown condensate
Together with white, brown and
black
Unknown
D. The oxide assisted growth of nanowires
Introduction
In the previous section, it was seen that SiC-SiOx core-shell nanowires are one of the products of the interaction between SiO(g) and CO(g). Numerous studies have tried to explain the reaction mechanism in the gas phase.
Zhang et al. [77] proposed the Oxide Assisted Growth (OAG) as a mechanism of reaction. The mechanism was widely revised by other authors [35], [36], [78], [79]. Understanding the mechanism of formation of nanowires can help in controlling the SiO(g)-CO(g) interaction.
The OAG model can explain the generation of core-shell nanowires from a gas phase, without using a catalyst.
The starting materials to produce nanowires are the gas species. They are generalized in the model as HXO(g), O(g) and YO(g). In our case, HXO stands for “High temperature oxide”, i.e. there is no hydrogen in this compound.
In the white condensates nanowires, the HXO compound will correspond to SiO(g). O(g) is monoatomic oxygen, and YO(g) is CO(g).
OAG-grown nanowires consist of a XnYm species in the core, and an oxide phase at the shell. The XnYm core phase will be SiC. Either n or m can be equal to zero. In fact, both Si-SiO2 and SiC-SiO2 nanowires can be associated to this mechanism [37]. The outer shell phase is made of SiOx. The oxide is a combination of (SiO)x nanoclusters, and consequent segregation into Si and SiO2 domains. The suboxide phase is molten unless the tip size is large than 10-100 nm. Such small dimensions will favor size-dependent melting point depression. The theory behind this phenomenon is explained at the end of this section.
Mechanism
Hu et al. [35], [36] carry out a broad discussion about the mechanism of formation of SiC-SiOx nanowires. SiO(g) and CO(g) are produced from a mixture of Si, SiO2 and C powders. Si and C react with a low concentration of O2(g) and generate SiO(g) and CO(g). The minimum concentration needed to trigger these reactions is very low [36]. Si powders adsorb oxygen at room temperature during milling. The poor tightness of the experimental setup chosen favors also air leakages in the setup.
It was assumed that the gas mixture consists of mainly SiO(g) and CO(g), with traces of CO2(g) and O2(g). Carbon dioxide is believed to form as an intermediate, confirming the theoretical discussion carried out by Schei [1] for SiC formation. Boudouard reaction is also considered responsible for CO2 formation by Hu et al. [35], [36].
Once the gas mixture is produced, the nanowires generation occurs in three stages (Figure 66).
Stage I: Incubation (Production of nanoclusters). SiO(g) and CO(g) will deposit on the surface of a substrate, to produce SiC and SiOx nanoclusters. SiC is solid and does not react easily with the oxide, unless the temperature is increased to favor the gas producing reaction (Reaction -1). SiOx is liquid, and it will deposit as a separate phase, thanks to its low miscibility with SiC [15].
Stage II: Nucleation (Lateral growth). The oxides deposition at the sides will define the lateral growth of the core phase, and the final diameter of the nanowire. The external phase is made of suboxides that bind to the core phase, thanks to their dangling bonds. The dangling bonds facing the vapor phase will entrap more suboxides, thus favoring diffusion of silicon to the nanowire core. Eventual oxygen dissolved in the core will diffuse to the edge, thus balancing the stoichiometric composition of the external layer. Once the shell is completely formed and surrounds the core phase, the lateral growth of the wire stops.
The final product of this step is called seed. It consists of a SiC core, surrounded by a SiOx shell, and a droplet at the top. The droplet is rich in liquid SiOx. The formation of SiC and SiOx on the top of the seed is always spontaneous, as well as exothermic.
83 Stage III: Growth and termination. The droplet is the point where SiC and SiOx are further produced from this moment. Being the material at the droplet in molten state, the adsorption of gases at the surface and their diffusion to the core are favored. During the vertical growth, the suboxide phase (SiOx) will also collect more molecules from the vapor phase, thanks to its dangling bonds. Silicon carbide nanoclusters can precipitate and further act as new available nuclei for nanowires.
SiC-SiOx nanowires grow along a fixed axis during Stage III. The vertical growth will continue along a preferential direction. The amount of impurities in the core is reduced but stacking faults and other crystallographic defects are inevitable. This explains the irregular orientation of the nanowires, according to Hu et al. [35], [36].
Growth continues until the nanowires stop receiving SiO(g) and CO(g) at the droplet. At this point, termination occurs. The exothermic reaction producing SiC and SiOx stops. The temperature decreases, and the droplet solidifies.
Figure 66: Sketch of oxide assisted growth mechanism for SiC-SiOx nanowires [35]