Characterization of Si-B Alloys in As-Fabricated State
The results of microstructural characterization of the as-fabricated Si-B alloys are shown in Fig. 4. SEM images of the cross-sectioned alloys revealed that despite using double remelting, the microstructure of alloys is quite inhomogeneous, while a degree of inhomogeneity increased with increasing boron content. A matrix of each alloy was composed of the mixture of Si(B) solid solution and Si + SiB3 eutectic. With increasing boron content, the number of boron-rich precipitates also increases. The results of SEM/EDS local chemical composition analyses revealed a B/Si atomic ratio of 3.01 ± 0.15 in the eutectic areas and B/Si = 4.11 ± 0.13 in dark gray particles. Additionally, black particles were found to be almost Si-free and they exhibited B/C ratio of 4.07 ± 0.09. Therefore, the following structural features were recognized based on careful SEM/EDS examinations: Si(B) solid solution, Si + SiB3 eutectic, SiB4 silicon tetraboride and B4C boron carbide.
It should be noted that serious discrepancies on boron-rich part of the Si-B phase diagram are reported in the literature. The most contradictory findings have been shown for the phase stability and stoichiometry of silicon borides. Although the widely accepted form of Si-B phase diagram (Ref 7) contains only three borides, namely SiB3, SiB6 and SiBn, some earlier and more recent papers, e.g., by Samsonov and Sleptsov (Ref 9) or by Tremblay and Angers (Ref 10, 11), point toward the existence of SiB4 instead of SiB3 compound, while other researchers, e.g., Aselage (Ref 12), reported that the SiB3 phase grows from boron saturated silicon, but at the same time they indicated its metastability toward SiB6 phase. On the other hand, it has been proposed (Ref 13) that the triboride is a silicon-rich version of the tetraboride, so the stoichiometry of either compound could be expressed as SiB4-x where x = 0 or 1. Additionally, due to local segregations of chemical composition, both phases (tri- and tetraboride) might also coexist.
Thus, it seems that presently applied non-equilibrium solidification conditions of electric arc melting processing (i.e., rapid quenching and high solidification rates) supported the formation of SiB4 as the most thermodynamically stable phase, while the presence of B4C particles should be justified in terms of carbon impurities introduced from the batch materials.
The Wetting Kinetics in Si-B Alloy/h-BN Systems
The images of Si/h-BN and Si-B/h-BN sessile drop couples in situ recorded during the high-temperature tests are shown in Fig. 5. The wetting kinetics curves (showing a change of contact angle θ vs. testing time) calculated for pure silicon and Si-B alloys subjected to contact heating with h-BN substrates are shown in Fig. 6. By comparing the results obtained for Si-B alloys to the behavior of pure silicon on the h-BN substrate (described in details in Ref 6), it is concluded that the additional presence of boron decreases the wettability in the system. For all Si-B alloys, the contact angle values were very high (within the non-wetting regime of θ > 90°) in the whole examined temperature range.
However, it should be noted that the wetting kinetics curve for the Si-5.7B hypereutectic alloy showed a noticeable decrease from θ = 145° at 1450 °C to θ = 125° at 1750 °C. Most probably, this behavior might be attributed to an inhomogeneous initial structure of the Si-5.7B alloy, in particular to the presence of relatively large SiB4 (and B4C) crystals distributed in the bottom part of the alloy (Fig. 4c) directly contacting the h-BN substrate. Due to a high chemical affinity of Si(B) melt to both SiB4 and B4C phases reflected also by a very good wetting, it is believed that the existence of this “discontinuous layer” in the vicinity of Si-5.7B/h-BN interface might be responsible for the observed decrease in contact angle θ. This finding allows concluding that increased fraction of high melting point borides in Si-B alloys having the hypereutectic composition is not beneficial in terms of the “non-wettability requirement” for the selection of container materials in LHTES device. Furthermore, what is extremely important from the application’s point of view, a large amount of crystals having high melting points decreases a relative content of liquid phase providing the latent heat for the electricity generation.
As it has been experimentally shown in earlier work (Ref 6) during the high-temperature interaction in Si/h-BN system, h-BN substrate is slightly dissolved in initially pure Si, leading to diffusion of boron into molten Si. In view of this finding, it is reasonable to conclude that addition of boron to silicon before the experiment (i.e., using Si-B alloys instead of pure Si) suppresses this phenomenon. In other words, the Si-B alloys dissolve much less boron from the h-BN substrate, which results in a remarkable hindering of substrate dissolution and a fast achievement of a thermodynamic equilibrium.
This statement seems to be also confirmed by a strikingly different behavior of Si/h-BN and Si-B /h-BN couples during cooling from 1750 °C. In the former case, an increase in contact angle θ upon cooling (a so-called dewetting) was observed as the effect of drastic change in solubility of previously diffused B, N and C atoms in liquid/solid-state silicon. Consequently, a release of gaseous nitrogen and precipitation of BN platelets and SiC crystals take place at the Si/h-BN interface during the solidification. On the other hand, Si-B alloys exhibited either the negligible alteration of the θ versus t curve (for Si-1B alloy) or its descending tendency (for Si-3.2B and Si-5.7B alloys) during cooling. Decreasing the contact angle θ during cooling of eutectic and hypereutectic Si-B alloys should be justified by the change in structure and chemistry of the interface due to the formation of wettable silicon boride crystals at the interface area before the solidification of Si(B) matrix, i.e., Si-B/h-BN system was locally converted to Si-B/B4C+SiBx /h-BN. The results of LM and SEM/EDS analyses of cross-sectioned solidified couples (Fig. 7) revealed that:
the size and number of silicon boride crystals in the interface vicinity increase with increasing initial boron content in Si-B alloys (Fig. 7a-c);
the EDS estimated chemical composition of the large gray crystals is very close to the stoichiometry of SiB3 triboride. Furthermore, in the case of the Si-5.7B hypereutectic alloy (Fig. 7d), the presence of few dark crystals having the B/Si ratio of ~ 6.18 ± 0.15 (corresponding to the SiB6 hexaboride) was also noted. This finding suggests that under conditions of cooling rates slower than that in the electric arc melting process, the SiB3 and SiB6 phases may coexist as more stable than the silicon tetraboride.
In addition, boron has been already recognized as the surface active element in many metal-boron systems (Ref 14), which means that B atoms preferentially segregate at the liquid–vapor interface (Ref 15). Therefore, the observed decrease in contact angle during cooling of Si-B alloys might be also related to a probably negative effect of increased boron content on the melt surface tension.