Grey condensate
EPMA (Exp. R4c
B. Si-SiO 2 condensate characterization
Visual inspection and SEM analysis
A brown, glassy material glues the SiC substrates, as it happened to the sample from Experiment R4a shown in Figure 111. The brown condensate covers particles which are already covered in white condensate. The substrates stick together as if the brown compound was molten during the experiment. Different shades of brown appear at different temperatures. In particular, the color shifts from dark brown to grey while going downwards in the crucible, i.e. towards high temperatures. The brown condensate sticks to the graphite parts.
When removed, the brown condensate is still attached to the white condensate removed from the graphite parts.
Figure 111: Brown condensate layer generated between SiC particles, white condensate and white scale.
Sample from Exp. R4a., T = 1665 - 1800°C.
(a) (b)
129 As far as microstructure is concerned, SE-SEM pictures reveals a matrix, which contains spherical inclusions (Figure 112). The spheres are interconnected by treads. The spheres diameter ranges in the order of magnitude of 0.5-1 µm. The sample would charge under SEM analysis, implying that the compound should contain a considerable amount of an oxide.
Figure 112: Left: Spheres and wires from experiment R1b, T = 1340 - 1570°C; Right: Top view of brown condensate embedding spheres inside the matrix. Sample from experiment R4a, T = 1665 - 1800°C.
EPMA
The EPMA point analysis gave almost equivalent atomic percentages of Si and O, but also a carbon signal up to 10 at. % (out of 14 points analyzed in a sample from experiment R4a, with an analysis radius of 10 µm). Hence, if carbon was neglected, the brown condensate composition can be approximated to a mixture of Si spheres in a SiO2 matrix, in a molar ratio 1:1 (Table 22). This is the theoretical composition for the Si-SiO2 condensate, which neglects the contribution of carbon. However, the empirical error caused by the carbon layer (previously discussed for EPMA analysis in the SiC-SiOx mixture) is also present for the Si-SiO2 mixture.
Table 22: Characterization of brown Si-SiO2 mixture (at. %).
Element Theoretical composition (Si:SiO2 1:1 molar)
EPMA (Exp. 4a, 14 Points)
Si 50.0% (43.1 ± 2.6)%
O 50.0% (49.4 ± 0.4)%
C 0 % (7.5 ± 2.6)%
Element mapping showed that the spheres contain more than 90 wt. % silicon, whereas the matrix composition corresponds to SiO2. The EPMA was also performed as an area mapping over the same sample, which analyses the distribution of Si, C and O over an area. Figure 113 shows elemental mapping of brown condensate from Experiment R4a, for the elements Si, C and O. Carbon signal is also present where the spheres are located. Carbon concentration in the spheres can be up to 8 wt. %. However, C measurement are semi-quantitative, and a carbon coating is applied during sample preparation, therefore it is believed that the carbon content is lower.
Figure 113: EPMA mapping of brown condensate, Experiment R4a, wt. % on the colored scales.
TEM characterization
The elemental mapping in Figure 113 shows numerous spots giving a carbon signal, especially close to the silicon spheres. However, the SEM resolution was not high enough to identify if this carbon signal was caused by a carbon-containing phase, or by empirical errors. Therefore, the Si-SiO2 condensate is characterized with a technique at a higher resolution. Since brown condensate is a thick and dense material, a thin slab could be extracted through FIB preparation, and used for TEM analysis.
The FIB sample was collected from Experiment R1b. Figure 114 shows how the FIB sample looked like before undergoing TEM characterization. The borders of the spheres are not perfectly smooth. The light glance at the particle borders is caused by the charging effect of the silica surface (Figure 114 and Figure 115). Appendix B shows the process used for preparing the sample by FIB. The final sample size is about 30 x 30 x 0.1 µm.
131 Figure 114: Final aspect of the FIB-TEM sample of brown condensate. Left: Frontal view; Right: Cross section thickness. FIB sample.
Figure 115: Details of spheres borders. FIB sample.
As for SEM analysis, elemental mapping can be performed also at TEM resolution. EDX area mapping visualizes the qualitative concentration of Si, O and C in a sample. EDX was carried out on a portion of the FIB sample, which contained both large and small silicon spheres. A brighter color corresponds to a stronger concentration of the detected element. The results are summarized in Figure 116.
The first thing noticed was that the Si signal is stronger inside the spheres and a lower outside. This confirms the previous results from the area mapping in the SEM characterization. However, it can be added that the wires interconnecting the spheres are also made of elemental silicon. The oxygen mapping shows, as expected, a low signal from the spheres and a stronger outside them. Again, it can be stated that the Si-SiO2 condensate consists of Si spheres embedded in a SiO2 matrix.
The second result is that both Si and C are present around the spheres. Therefore, it is believed that there are SiC crystals located around the spheres. By looking at the C elemental mapping (Figure 116c), it can noticed that the C signal on the spheres borders is more intense than inside the spheres. At the same time, in the Si mapping, the signal at the spheres borders is represented by an orange zone, i.e. the concentration of Si is lower than inside the spheres, but higher than in the matrix. The size of the SiC protuberances ranges between 50-150 nm.
Only the largest spheres show relevant amounts of carbide on their boundaries.
The very low C solubility of the two phases is also noticed in the elemental mapping. The SiO2 matrix can dissolve low amounts of carbon (in the order of magnitude of ppb). In fact, the matrix appears dark blue in the C mapping.
The Si spheres are dark blue in both the C and O mappings and is only dissolving O and C on a ppm level.
(a) (b)
(a) (b)
Many of the large spheres show SiC nanocrystals, whereas the smallest ones do not. The small spheres might not be large enough to react with CO(g) to a sufficient extent.
Figure 116: Original TEM bright field picture (a). Mapping of Si (b), C (c) and O (d).
Finally, it was seen that crystallographic defects are widely spread both in the spheres and wires. The most common defects are twin grain boundaries. The structure is not monocrystalline. Figure 117a displays the twin grain boundaries inside a sphere of 2 µm diameter. There are differences in the diffraction pattern between the bulk and the twinning pattern (Figure 117b,c), but the overall pattern is similar. Figure 118 illustrates a close view of the twinning defect. The bulk diffraction pattern of the sphere coincides with a reference diffraction pattern of pure Si [90]. The pattern at the twinning is typical of a ∑3 {111} grain boundary.
As is happened for the SiC-SiOx condensates, it is not possible to comment the crystallographic structure of the SiO2 matrix, since it may change to amorphous under X-Ray radiation [90].
Figure 117: Left: Silicon sphere from Experiment 1b used for diffraction pattern. Center: Diffraction pattern with twinning: Right: Diffraction pattern without twining, indexed.
(a)
(b) (c)
133 Figure 118: Twin grain boundary in silicon.
XPS
Samples from experiment R4a were used for XPS analysis (Figure 119a). The survey spectrum from Figure 120 shows that the amount of carbon is significant, due to surface contamination of adventitious carbon. Some of the C signal comes also from SiC in the brown condensate. The Si:O ratio is close to 1, as the overall composition of brown Si-SiO2 condensate.
The position of the peaks, their areas and FWHM are collected in Table 23. F, N and Na are coming from organic impurities. Again, the positions of the peaks in the survey spectrum correspond to those from references [86], [91], as well as to those found in the SiC-SiOx condensates (see Table 17).
Table 23: XPS analysis of brown condensate, survey spectrum
Name Position At. %
O 1s 532.77 37
C 1s 284.77 28
Si 2p 103.77 34
N 1s 399.77 0.5
F 1s 689.27 <0.5
Na 1s 1071.27 <0.5
Figure 119: Left: brown condensate XPS sample; Right: SE picture from the area chosen for the analysis.
Figure 120: Survey spectrum of brown condensate from experiment R4a.
As far as the localized spectra are concerned, the Si-2p core level spectrum gave also two peaks at 101 and 103.5 eV (Figure 121a). The latter is related to Si-O bonds. Elemental silicon should give its own peak at ≈100 eV, but the Si-Si peak might be overlapping with Si-C (≈101 eV). The Si-Si peak is lower than the Si-O peak. The shape of the Si-Si peak is distorted because of the overlapping with the Si-C peak, as it happened for the SiC-SiOx
condensate.
When it comes to the C-1s core level spectrum (Figure 121b), the Si-C signal is lower in this sample, compared to the SiC-SiOx condensates. The carbide signal (282.88 eV) is low, but not negligible. XPS is a surface technique, therefore some of the carbide signal comes from the spheres exposed at the surface shown in Figure 119b.
Contamination of the substrate is negligible, as the sample was detached from the SiC substrate.
The experimental errors discussed for the SiC-SiOx condensates (i.e. SiC contribution to the signal, single peak fitting for Si-2p electrons and adventitious carbon) are also valid for the Si-SiO2 condensate analysis.
(a) (b)
135 Figure 121: XPS spectra for C-1s (left) and Si-2p (right) electrons in brown condensate from experiment R4a.
XRD polytypes analysis
For the SiO2 polytypes distribution, a sample from experiment IF7a was extracted. The sample comes from the temperature interval 1580 – 1750°C. The experiment used Si-SiO2 pellets for gas production (pSiO = 1.0), exposed at 1890°C for 2 hours. The amount of polytypes was inquired by TOPAS by peak fitting with the internal standard method. The results are shown in Table 24 and Table 25. The main results are three:
1. The Si/SiO2 ratio is higher than expected. This value is still comparable to the theoretical Si:SiO2 ratio expected from Reaction 2 (1:1 molar, 32:68 weight). The silicon content could be higher than expected, as the condensate layer was scratched for the outer portion of the sample. The model was also not very precise in quantifying the amount of silica polytypes, as described in Appendix H. Some of the amorphous phase might come also from the crystallographic defects in SiC and Si. Other causes for contamination are those who reduce the SiO2 content (e.g. presence of SiC from the substrate).
2. The percentage of SiC is higher than expected (≈20%), due to contamination from the scratched SiC substrates. β-SiC may come from SiC crystals embedded in the Si spheres, as shown during TEM analysis. The amount of β-SiC (14%) is in the order of magnitude of the estimated amount by TEM (5-10%). The contamination from of α-SiC from the substrate is small (≈5%).
3. Most of the silica is amorphous (64 wt.%). The cristobalite phase makes 26 wt.% of the sample, and quartz is present at 10 wt.%. There is less amorphous phase compared to the SiC-SiOx condensate. Therefore, the sample has partially undergone a recrystallization process under cooling.
Table 24: Relative amount of each polytype for silica and SiC, calculated by TOPAS v5 for brown condensate. Quartz Cristobalite Amorph. α-SiC β-SiC
10% 26% 64% 23% 77% 48.83 32.83 0.67 0.47
The size distribution of silicon spheres changes within sub-millimetric distances in a sample. A metallographic analysis was carried out on 9 brown condensate samples, to understand how the silicon phase changes its microstructure through a brown condensate deposited layer. Table 26 lists the samples considered for the study.
The temperature interval corresponds to the position in the condensation chamber where the sample was extracted from. All the samples were analyzed by BSE-SEM. Sample 1-3 showed the clearest contrast between the spheres and the matrix and were analyzed with Image J®. Samples 4-7 do not show good contrast between Si and SiO2, but a qualitative analysis by visual inspection shows similar results collected for samples 1-3. Samples 8-9 contain representative overview pictures of microstructural configurations at smaller magnification (x100).
Table 26: Samples used for Si spheres analysis.
Sample Experiment name T interval (°C) Number of pictures
1 P40 R10a – sample a 1420-1590°C 23
Figure 122 shows how the sphere size increases while getting closer to the outer surface of the sample. The white rectangles show the dimension of each picture taken during line scanning. At the point closest to the SiC substrate (x = 0 µm), the average silicon sphere size is 256 nm. At x = 300 µm, the size has already increased to 475 nm. The growth continues again towards the edge, where an average size of 555 nm was calculated at x = 660 µm. While the average size increases, the counted particles in every picture decrease, from 415 (x = 0 µm) to 104 (x =660 µm). The counted spheres and the average sphere diameter are plotted in Figure 124.