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The Effect of Boron Content on Wetting Kinetics in Si-B Alloy/h-BN System

The Effect of Boron Content on Wetting Kinetics in Si-B Alloy/h-BN System JMEPEG (2019) 28:3819–3825 The Author(s) https://doi.org/10.1007/s11665-018-3786-8 1059-9495/$19.00 The Effect of Boron Content on Wetting Kinetics in Si-B Alloy/h-BN System Wojciech Polkowski, Natalia Sobczak, Grzegorz Bruzda, Rafał Nowak, Donatella Giuranno, Artur Kudyba, Adelajda Polkowska, Krzysztof Pajor, Tomasz Kozieł, and Ivan Kaban (Submitted November 14, 2018; published online December 6, 2018) In this work, the effect of boron content on the high-temperature wetting behavior in the Si-B alloy/h-BN systems was experimentally examined. For this reason, hypoeutectic, eutectic and hypereutectic Si-B alloys (Si-1B, Si-3.2B and Si-5.7B wt.%, respectively) were produced by electric arc melting method and then subjected to sessile drop/contact heating experiments with polycrystalline h-BN substrates, at temperatures up to 1750 C. Similar to pure Si/h-BN system, wetting kinetics curves calculated on a basis of in situ recorded drop/substrate images point toward non-wetting behavior of all selected Si-B alloy/h-BN couples. The highest contact angle values of  150 were obtained for hypoeutectic and eutectic Si-B alloys in the whole examined temperature range. of Si-B alloys should significantly overcome energy density Keywords AMADEUS project, hexagonal boron nitride, refrac- limitations of the present molten salt-based LHTES systems tories, silicon–boron alloys, thermal energy storage, (Fig. 1). However, in order to successfully accomplish the wettability project goals, some scientific and technological challenges need to be a priori faced and solved. Besides the development of new ultra-high-temperature devices for a heat-electricity con- version [i.e., thermoionic photovoltaic converter (Ref 1)], proper ceramic materials able to withstand a long-term heating, 1. Introduction holding and cooling in contact with molten Si-B alloys need to be selected. Thus, an implementation of these materials to real Due to the high melting points and latent heat values of devices must be preceded by careful examinations of their high- silicon and boron, Si-B alloys have been assumed as excellent temperature interaction with refractories that can be used for candidates to be used as phase change materials (PCMs) for building the PCM container. applications in latent heat thermal energy storage (LHTES) and Firstly, to achieve long lifetimes and high reliability of the conversion systems working at the temperatures up to 2000 C PCM container, it must be inert toward contacting Si-B alloys (Ref 1). The main idea of a LHTES-based system is that the under operating conditions. In other words, a selection of latent heat, absorbed during melting and released upon proper refractories should be based on two main criteria: (1) a solidification, can be stored and converted into other forms of non-wettability and (2) a negligible reactivity in contact with energy, e.g., electricity. The basic idea of AMADEUS project Si-B melts. However, due to the pioneering nature of research, (Ref 1), supported by the European Commission within there is a lack of both theoretical and experimental data on Horizon 2020 Programme, is that the utilization of latent heat interaction between molten Si-B alloys and ceramics at any temperature. On the other hand, high-temperature capillarity phenomena (i.e., wetting, spreading and infiltration) have been This article is an invited submission to JMEP selected from so far widely examined for Si/ceramic systems in terms of a presentations at the 73rd World Foundry Congress and has been fabrication of photovoltaic grade silicon (Ref 2, 3). It has been expanded from the original presentation. 73WFC was held in Krakow, documented that at a temperature near melting point of Si Poland, September 23–27, 2018, and was organized by the World (T = 1414 C), almost all applied in practice ceramics, e.g., Foundry Organization and Polish Foundrymens Association. m oxides, carbides, borides or nitrides, are well wetted by molten Wojciech Polkowski, Grzegorz Bruzda, Rafał Nowak, Si, as reflected by the contact angle h < 90. This behavior Artur Kudyba, and Adelajda Polkowska, Foundry Research originates from a high chemical affinity of Si to oxygen, carbon Institute, Zakopian´ska 73 Str., 30-418 Krako´w, Poland; and nitrogen leading to an easy formation of reaction products Natalia Sobczak, Foundry Research Institute, Zakopianska 73 Str., at the interfaces of contacting materials. In fact, the only one 30-418 Krako´w, Poland; and Institute of Precision Mechanics, available ceramic showing the non-wetting behavior with Duchnicka 3 Str., 01-796 Warsaw, Poland; Donatella Giuranno, Foundry Research Institute, Zakopian´ska 73 Str., 30-418 Krako´w, molten Si is hexagonal boron nitride (h-BN). The reported Poland; and CNR - Institute of Condensed Matter Chemistry and values of the equilibrium contact angle h measured for Si/h-BN Technologies for Energy, Via E. De Marini, 6, 16149 Genoa, Italy; and couples at T < 1500 C are within the range of 95-145 (e.g., Krzysztof Pajor and Tomasz Kozieł, Faculty of Metals Engineering Ref 4, 5). In our previous work (Ref 6), it was found that the and Industrial Computer Science, AGH University of Science and non-wetting behavior of the Si/h-BN system is maintained up to Technology, Al. Mickiewicza 30, 30-059 Krako´w, Poland; and 1650 C, while a non-wetting-to-wetting transition takes place Ivan Kaban, IFW Dresden, Institute for Complex Materials, Helmholtzstraße 20, 01-069 Dresden, Germany. Contact e-mail: at higher temperatures. It has also been evidenced that the wojciech.polkowski@iod.krakow.pl. Journal of Materials Engineering and Performance Volume 28(7) July 2019—3819 Fig. 1 A possible energy density coming from latent heat of different materials as a function of melting temperature compared with the energy density of other storage technologies (based on Ref 1) involved reactivity mechanism under static argon atmosphere is mostly based on the h-BN dissolution in molten Si (the dissolution rate depends on temperature), followed by a reprecipitation of h-BN platelets during cooling. Additionally, the interaction was accompanied by the formation of gaseous product (most probably N but also B O introduced from the 2 2 3 batch powders or during sintering step), which was reflected by Fig. 2 A macroview of exemplary Si-B alloys fabricated by using a lack of completely equilibrated conditions. the electric arc melting method (a). The Si-B binary phase diagram In this work, we used the sessile drop method experiments proposed by Olesinski and Abbaschian (Ref 7) with marked to examine the effect of boron content on the high-temperature composition of produced alloys interaction (wetting and spreading) during contact heating of Si-B alloys with hexagonal boron nitride (h-BN) substrates at temperatures up to 1750 C. 2. Materials and Methods The nominal compositions of Si-B alloys (Fig. 2a) were selected in accordance with the Si-B binary phase diagram proposed by Olesinski and Abbaschian (Ref 7) (Fig. 2b) as hypoeutectic (Si-1B), eutectic (Si-3.2B) and hypereutectic (Si- 5.7B) (wt.%). Additionally, ultra-highly pure Si (7N) was used as the reference material (Ref 6). The Si-B alloy samples with a mass of  0.6 g were produced from polycrystalline pure materials (Si: 99.999%; B: 99.9%—provided by Onyxmet, Poland). The main impurities in the batch materials were Al (0.008 at.%) and P (0.002 at.%) in silicon and Fe (0.023 at.%) and Al (0.012 at.%) in boron. The alloys were fabricated by the electric arc melting technique Fig. 3 Temperature profile (heating/holding/cooling scheme) of the (Buehler Arc Melter MAM-1) by using properly weight sessile drop experiment (more details are shown in Ref 6) mixtures of pure elements. The prepared Si-B mixtures were heated up in argon protective atmosphere until a complete melting and then rapidly cooled down after switching off the electric arc. In order to increase homogeneity of the Si-B alloys, polished to achieve the surface roughness of  150 nm. The each sample was twice remelted in the arc melting device. sessile drop experiments were performed with a dedicated Commercially available h-BN sinters (HeBoSint D100, experimental complex (Ref 8) working under argon atmo- Henze Boron Nitride Products AG, Lauben, Germany) were sphere, in the temperature range between 1450 and 1750 C used as the substrates in the wetting tests. Before experiments, (the applied temperature profile is shown in Fig. 3). In these contacting surfaces of h-BN substrates were gently ground and tests, in order to mimic real working conditions of a PCM 3820—Volume 28(7) July 2019 Journal of Materials Engineering and Performance device, static argon atmosphere (p = 850-900 mbar) was 3. Results and Discussion applied. More details on experimental procedure are given else- 3.1 Characterization of Si-B Alloys in As-Fabricated State where (Ref 6). During the tests, the images of Si-B/h-BN The results of microstructural characterization of the as- couples were recorded at 100 fps by using a high-speed high- fabricated Si-B alloys are shown in Fig. 4. SEM images of the resolution CDD camera. Subsequently, the acquired images cross-sectioned alloys revealed that despite using double were post-processed to compile movies and calculate wetting remelting, the microstructure of alloys is quite inhomogeneous, kinetics. After the tests, the solidified couples were removed while a degree of inhomogeneity increased with increasing from the vacuum chamber and subjected to structural evalu- boron content. A matrix of each alloy was composed of the ations. The characterization of Si-B alloy in as-fabricated state mixture of Si(B) solid solution and Si + SiB eutectic. With and Si-B/h-BN sessile drop couples was carried out by using 3 increasing boron content, the number of boron-rich precipitates Carl Zeiss Axio Observer ZM10 light microscope and FEI also increases. The results of SEM/EDS local chemical Scios field emission gun scanning electron microscope composition analyses revealed a B/Si atomic ratio of (FEGSEM) coupled with energy-dispersive x-ray spec- 3.01 ± 0.15 in the eutectic areas and B/Si = 4.11 ± 0.13 in troscopy (EDS). Fig. 4 Results of SEM/EDS evaluations of Si-B alloys in the as-fabricated state: Si-1B (a), Si-3.2B (b) and Si-5.7B (c) Journal of Materials Engineering and Performance Volume 28(7) July 2019—3821 Fig. 5 Images of Si-B/h-BN sessile drop couples in situ recorded during the high-temperature tests at T up to 1750 C dark gray particles. Additionally, black particles were found to hand, it has been proposed (Ref 13) that the triboride is a be almost Si-free and they exhibited B/C ratio of 4.07 ± 0.09. silicon-rich version of the tetraboride, so the stoichiometry of Therefore, the following structural features were recognized either compound could be expressed as SiB where x = 0 or 1. 4-x based on careful SEM/EDS examinations: Si(B) solid solution, Additionally, due to local segregations of chemical composi- Si + SiB eutectic, SiB silicon tetraboride and B C boron tion, both phases (tri- and tetraboride) might also coexist. 3 4 4 carbide. Thus, it seems that presently applied non-equilibrium It should be noted that serious discrepancies on boron-rich solidification conditions of electric arc melting processing part of the Si-B phase diagram are reported in the literature. The (i.e., rapid quenching and high solidification rates) supported most contradictory findings have been shown for the phase the formation of SiB as the most thermodynamically stability and stoichiometry of silicon borides. Although the stable phase, while the presence of B C particles should be widely accepted form of Si-B phase diagram (Ref 7) contains justified in terms of carbon impurities introduced from the only three borides, namely SiB , SiB and SiB , some earlier batch materials. 3 6 n and more recent papers, e.g., by Samsonov and Sleptsov (Ref 9) or by Tremblay and Angers (Ref 10, 11), point toward the 3.2 The Wetting Kinetics in Si-B Alloy/h-BN Systems existence of SiB instead of SiB compound, while other 4 3 The images of Si/h-BN and Si-B/h-BN sessile drop couples researchers, e.g., Aselage (Ref 12), reported that the SiB phase in situ recorded during the high-temperature tests are shown in grows from boron saturated silicon, but at the same time they Fig. 5. The wetting kinetics curves (showing a change of indicated its metastability toward SiB phase. On the other 3822—Volume 28(7) July 2019 Journal of Materials Engineering and Performance Fig. 6 Wetting kinetics curves (showing a change of contact angle h vs. testing time) calculated for Si-B alloys subjected to contact heating with h-BN substrates (data for pure Si are taken from Ref 6) contact angle h vs. testing time) calculated for pure silicon and BN substrate is slightly dissolved in initially pure Si, leading to Si-B alloys subjected to contact heating with h-BN substrates diffusion of boron into molten Si. In view of this finding, it is are shown in Fig. 6. By comparing the results obtained for Si-B reasonable to conclude that addition of boron to silicon before alloys to the behavior of pure silicon on the h-BN substrate the experiment (i.e., using Si-B alloys instead of pure Si) (described in details in Ref 6), it is concluded that the additional suppresses this phenomenon. In other words, the Si-B alloys presence of boron decreases the wettability in the system. For dissolve much less boron from the h-BN substrate, which all Si-B alloys, the contact angle values were very high (within results in a remarkable hindering of substrate dissolution and a the non-wetting regime of h > 90) in the whole examined fast achievement of a thermodynamic equilibrium. temperature range. This statement seems to be also confirmed by a strikingly However, it should be noted that the wetting kinetics curve different behavior of Si/h-BN and Si-B /h-BN couples during for the Si-5.7B hypereutectic alloy showed a noticeable cooling from 1750 C. In the former case, an increase in decrease from h = 145 at 1450 Cto h = 125 at 1750 C. contact angle h upon cooling (a so-called dewetting) was Most probably, this behavior might be attributed to an observed as the effect of drastic change in solubility of inhomogeneous initial structure of the Si-5.7B alloy, in previously diffused B, N and C atoms in liquid/solid-state particular to the presence of relatively large SiB (and B C) silicon. Consequently, a release of gaseous nitrogen and 4 4 crystals distributed in the bottom part of the alloy (Fig. 4c) precipitation of BN platelets and SiC crystals take place at directly contacting the h-BN substrate. Due to a high chemical the Si/h-BN interface during the solidification. On the other affinity of Si(B) melt to both SiB and B C phases reflected hand, Si-B alloys exhibited either the negligible alteration of 4 4 also by a very good wetting, it is believed that the existence of the h versus t curve (for Si-1B alloy) or its descending tendency this ‘‘discontinuous layer’’ in the vicinity of Si-5.7B/h-BN (for Si-3.2B and Si-5.7B alloys) during cooling. Decreasing the interface might be responsible for the observed decrease in contact angle h during cooling of eutectic and hypereutectic Si- contact angle h. This finding allows concluding that increased B alloys should be justified by the change in structure and fraction of high melting point borides in Si-B alloys having the chemistry of the interface due to the formation of wettable sil- hypereutectic composition is not beneficial in terms of the icon boride crystals at the interface area before the solidification ‘‘non-wettability requirement’’ for the selection of container of Si(B) matrix, i.e., Si-B/h-BN system was locally converted materials in LHTES device. Furthermore, what is extremely to Si-B/B C+SiB /h-BN. The results of LM and SEM/EDS 4 x important from the applications point of view, a large amount analyses of cross-sectioned solidified couples (Fig. 7) revealed of crystals having high melting points decreases a relative that: content of liquid phase providing the latent heat for the electricity generation. (1) the size and number of silicon boride crystals in the As it has been experimentally shown in earlier work (Ref 6) interface vicinity increase with increasing initial boron during the high-temperature interaction in Si/h-BN system, h- content in Si-B alloys (Fig. 7a-c); Journal of Materials Engineering and Performance Volume 28(7) July 2019—3823 Fig. 7 LM images of cross-sectioned Si-B alloys/h-BN interfaces for: Si-1B (a), Si-3.2B (b) and Si-5.7B (c) alloys. The SEM image showing a coexistence of SiB and SiB borides in the Si-5.7B alloy 3 6 (2) the EDS estimated chemical composition of the large Based on the results of SEM/EDS analyses, following gray crystals is very close to the stoichiometry of SiB structural features were recognized in the as-fabricated al- triboride. Furthermore, in the case of the Si-5.7B hyper- loys: Si(B) solid solution, Si + SiB eutectic, SiB silicon 3 4 eutectic alloy (Fig. 7d), the presence of few dark crys- tetraboride and B C boron carbide. tals having the B/Si ratio of  6.18 ± 0.15 2. The wettability tests for Si-B alloys/h-BN couples were (corresponding to the SiB hexaboride) was also noted. performed for the first time by sessile drop experiments This finding suggests that under conditions of cooling at temperatures up to 1750 C. It was established that the rates slower than that in the electric arc melting process, Si-B alloys exhibited much lower wettability with the h- the SiB and SiB phases may coexist as more BN ceramic as compared to the pure silicon counterpart. 3 6 stable than the silicon tetraboride. For both Si-1B hypoeutectic and Si-3.2B eutectic alloys, very high contact angle h values of  150 were re- In addition, boron has been already recognized as the corded in the whole examined temperature range. The Si- surface active element in many metal-boron systems (Ref 14), 5.7B hypereutectic alloy shows slightly lower contact an- which means that B atoms preferentially segregate at the gle h values, most probably due to its inhomogeneous liquid–vapor interface (Ref 15). Therefore, the observed initial structure including the presence of primary high decrease in contact angle during cooling of Si-B alloys might melting point borides in the vicinity of the surface con- be also related to a probably negative effect of increased boron tacting the h-BN substrate. content on the melt surface tension. 3. Since wetting phenomenon in Si/h-BN system at ultra- high temperature is dominated by the dissolution of h- BN in molten Si followed by reprecipitation during cool- ing, the lack of wetting in Si-B alloy/h-BN system under 4. Conclusions the same conditions as for pure Si/h-BN is caused by a suppression of this mechanism. The following conclusions are drawn from the obtained 4. Regarding the predicted application of Si-B/h-BN sys- experimental results and supported by appropriate data from the tems in the ultra-high-temperature LHTES devices, it is literature: suggested to use hypoeutectic or near eutectic composi- tions of Si-B alloys. This choice is justified not only by 1. Silicon–boron alloys having various boron contents were presently documented negligible interaction with the h- successfully fabricated by electric arc melting process. 3824—Volume 28(7) July 2019 Journal of Materials Engineering and Performance 3. Z. Yuan, W.I. Huang, and K. Mukai, Wettability and Reactivity of BN ceramic, but also by theoretically highest available Molten Silicon with Various Substrates, Appl. Phys. A Mater., 2004, 78, latent heat for these alloys (Ref 1). p 617–622 4. J.A. Champion, B.J. Keene, and S. Allen, Wetting of Refractory Materials by Molten Metallides, J. Mater. Sci., 1973, 8, p 423–426 Acknowledgments 5. B. Drevet, R. Voytovych, R. Israel, and N. Eustathopoulos, Wetting and Adhesion of Si on Si N and BN Substrates, J. Eur. Ceram. Soc., 3 4 The project AMADEUS has received funds from the European 2009, 29, p 2363–2367 Unions Horizon 2020 research and innovation program, FET- 6. W. Polkowski, N. Sobczak, R. Nowak, A. Kudyba, G. Bruzda, A. OPEN action, under Grant Agreement 737054. The sole respon- Polkowska, M. Homa, P. Turalska, M. Tangstad, J. Safarian, E. sibility for the content of this publication lies with the authors. It Moosavi-Khoonsari, and A. Datas, Wetting Behavior and Reactivity of Molten Silicon with h-BN Substrate at Ultrahigh Temperatures up to does not necessarily reflect the opinion of the European Union. 1750 C, J. Mater. Eng. Perform., 2018, 27, p 5040–5053 Neither the REA nor the European Commission is responsible for 7. R.W. Olesinski and G.J. Abbaschian, The B-Si (Boron-Silicon) any use that may be made of the information contained therein. System, Bull. Alloys Phase Diagr., 1984, 5, p 479–484 The authors wish to express their thanks to S. Donath for the help 8. N. Sobczak, R. Nowak, W. Radziwill, J. Budzioch, and A. Glenz, with sample preparation. Experimental Complex for Investigations of High Temperature Cap- illarity Phenomena, Mater. Sci. Eng. A, 2008, 495, p 43–49 9. G.V. Samsonov and V.M. Sleptsov, Preparation of Boron-Silicon Alloys, Powder Metall. Met. Ceram., 1964, 3, p 488–496 Open Access 10. R. Tremblay and R. Angers, Preparation of High Purity SiB by Solid This article is distributed under the terms of the Creative Commons State Reaction Between Si And B, Ceram. Int., 1989, 15, p 73–78 Attribution 4.0 International License (http://creativecommons.org/ 11. R. Tremblay and R. Angers, Mechanical Characterization of Dense Silicon Tetraboride (SiB4), Ceram. Int., 1992, 18, p 113–117 licenses/by/4.0/), which permits unrestricted use, distribution, and 12. T.L. Aselage, The Coexistence of Silicon Borides with Boron- reproduction in any medium, provided you give appropriate credit Saturated Silicon: Metastability of SiB , J. Mater. Res., 1998, 13,p to the original author(s) and the source, provide a link to the 1786–1794 Creative Commons license, and indicate if changes were made. 13. C. Brosset and B. Magnusson, The Silicon-Boron System, Nature, 1960, 187, p 54–55 14. A.F. Vishkarev, Y.V. Kryakovskii, S.A. Bliznukov, V.I. Yavoiski, Influence of Rare-Earth Elements on the Surface Tension of Liquid References Iron, in Surface Phenomena in Metallurgical Processes: Proceedings 1. A. Datas, A. Ramos, A. Marti, C. del Canizo, and A. Luque, Ultra High of an Interinstitute Conference, ed. by A.I. Belyaev (Springer, US, Temperature Latent Heat Energy Storage and Thermophotovoltaic 1965), pp. 166–171 Energy Conversion, Energy, 2016, 107, p 542–549 15. A. Passerone, M.L. Muolo, F. Valenza, and R. Novakovic, Thermo- 2. B. Drevet and N. Eustathopoulos, Wetting of Ceramics by Molten Silicon dynamics and Surface Properties of Liquid Cu-B Alloys, Surf. Sci., and Silicon Alloys: A Review, J. Mater. Sci., 2012, 47, p 8247–8260 2009, 603, p 2725–2733 Journal of Materials Engineering and Performance Volume 28(7) July 2019—3825 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Materials Engineering and Performance Springer Journals

The Effect of Boron Content on Wetting Kinetics in Si-B Alloy/h-BN System

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Materials Science; Characterization and Evaluation of Materials; Tribology, Corrosion and Coatings; Quality Control, Reliability, Safety and Risk; Engineering Design
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JMEPEG (2019) 28:3819–3825 The Author(s) https://doi.org/10.1007/s11665-018-3786-8 1059-9495/$19.00 The Effect of Boron Content on Wetting Kinetics in Si-B Alloy/h-BN System Wojciech Polkowski, Natalia Sobczak, Grzegorz Bruzda, Rafał Nowak, Donatella Giuranno, Artur Kudyba, Adelajda Polkowska, Krzysztof Pajor, Tomasz Kozieł, and Ivan Kaban (Submitted November 14, 2018; published online December 6, 2018) In this work, the effect of boron content on the high-temperature wetting behavior in the Si-B alloy/h-BN systems was experimentally examined. For this reason, hypoeutectic, eutectic and hypereutectic Si-B alloys (Si-1B, Si-3.2B and Si-5.7B wt.%, respectively) were produced by electric arc melting method and then subjected to sessile drop/contact heating experiments with polycrystalline h-BN substrates, at temperatures up to 1750 C. Similar to pure Si/h-BN system, wetting kinetics curves calculated on a basis of in situ recorded drop/substrate images point toward non-wetting behavior of all selected Si-B alloy/h-BN couples. The highest contact angle values of  150 were obtained for hypoeutectic and eutectic Si-B alloys in the whole examined temperature range. of Si-B alloys should significantly overcome energy density Keywords AMADEUS project, hexagonal boron nitride, refrac- limitations of the present molten salt-based LHTES systems tories, silicon–boron alloys, thermal energy storage, (Fig. 1). However, in order to successfully accomplish the wettability project goals, some scientific and technological challenges need to be a priori faced and solved. Besides the development of new ultra-high-temperature devices for a heat-electricity con- version [i.e., thermoionic photovoltaic converter (Ref 1)], proper ceramic materials able to withstand a long-term heating, 1. Introduction holding and cooling in contact with molten Si-B alloys need to be selected. Thus, an implementation of these materials to real Due to the high melting points and latent heat values of devices must be preceded by careful examinations of their high- silicon and boron, Si-B alloys have been assumed as excellent temperature interaction with refractories that can be used for candidates to be used as phase change materials (PCMs) for building the PCM container. applications in latent heat thermal energy storage (LHTES) and Firstly, to achieve long lifetimes and high reliability of the conversion systems working at the temperatures up to 2000 C PCM container, it must be inert toward contacting Si-B alloys (Ref 1). The main idea of a LHTES-based system is that the under operating conditions. In other words, a selection of latent heat, absorbed during melting and released upon proper refractories should be based on two main criteria: (1) a solidification, can be stored and converted into other forms of non-wettability and (2) a negligible reactivity in contact with energy, e.g., electricity. The basic idea of AMADEUS project Si-B melts. However, due to the pioneering nature of research, (Ref 1), supported by the European Commission within there is a lack of both theoretical and experimental data on Horizon 2020 Programme, is that the utilization of latent heat interaction between molten Si-B alloys and ceramics at any temperature. On the other hand, high-temperature capillarity phenomena (i.e., wetting, spreading and infiltration) have been This article is an invited submission to JMEP selected from so far widely examined for Si/ceramic systems in terms of a presentations at the 73rd World Foundry Congress and has been fabrication of photovoltaic grade silicon (Ref 2, 3). It has been expanded from the original presentation. 73WFC was held in Krakow, documented that at a temperature near melting point of Si Poland, September 23–27, 2018, and was organized by the World (T = 1414 C), almost all applied in practice ceramics, e.g., Foundry Organization and Polish Foundrymens Association. m oxides, carbides, borides or nitrides, are well wetted by molten Wojciech Polkowski, Grzegorz Bruzda, Rafał Nowak, Si, as reflected by the contact angle h < 90. This behavior Artur Kudyba, and Adelajda Polkowska, Foundry Research originates from a high chemical affinity of Si to oxygen, carbon Institute, Zakopian´ska 73 Str., 30-418 Krako´w, Poland; and nitrogen leading to an easy formation of reaction products Natalia Sobczak, Foundry Research Institute, Zakopianska 73 Str., at the interfaces of contacting materials. In fact, the only one 30-418 Krako´w, Poland; and Institute of Precision Mechanics, available ceramic showing the non-wetting behavior with Duchnicka 3 Str., 01-796 Warsaw, Poland; Donatella Giuranno, Foundry Research Institute, Zakopian´ska 73 Str., 30-418 Krako´w, molten Si is hexagonal boron nitride (h-BN). The reported Poland; and CNR - Institute of Condensed Matter Chemistry and values of the equilibrium contact angle h measured for Si/h-BN Technologies for Energy, Via E. De Marini, 6, 16149 Genoa, Italy; and couples at T < 1500 C are within the range of 95-145 (e.g., Krzysztof Pajor and Tomasz Kozieł, Faculty of Metals Engineering Ref 4, 5). In our previous work (Ref 6), it was found that the and Industrial Computer Science, AGH University of Science and non-wetting behavior of the Si/h-BN system is maintained up to Technology, Al. Mickiewicza 30, 30-059 Krako´w, Poland; and 1650 C, while a non-wetting-to-wetting transition takes place Ivan Kaban, IFW Dresden, Institute for Complex Materials, Helmholtzstraße 20, 01-069 Dresden, Germany. Contact e-mail: at higher temperatures. It has also been evidenced that the wojciech.polkowski@iod.krakow.pl. Journal of Materials Engineering and Performance Volume 28(7) July 2019—3819 Fig. 1 A possible energy density coming from latent heat of different materials as a function of melting temperature compared with the energy density of other storage technologies (based on Ref 1) involved reactivity mechanism under static argon atmosphere is mostly based on the h-BN dissolution in molten Si (the dissolution rate depends on temperature), followed by a reprecipitation of h-BN platelets during cooling. Additionally, the interaction was accompanied by the formation of gaseous product (most probably N but also B O introduced from the 2 2 3 batch powders or during sintering step), which was reflected by Fig. 2 A macroview of exemplary Si-B alloys fabricated by using a lack of completely equilibrated conditions. the electric arc melting method (a). The Si-B binary phase diagram In this work, we used the sessile drop method experiments proposed by Olesinski and Abbaschian (Ref 7) with marked to examine the effect of boron content on the high-temperature composition of produced alloys interaction (wetting and spreading) during contact heating of Si-B alloys with hexagonal boron nitride (h-BN) substrates at temperatures up to 1750 C. 2. Materials and Methods The nominal compositions of Si-B alloys (Fig. 2a) were selected in accordance with the Si-B binary phase diagram proposed by Olesinski and Abbaschian (Ref 7) (Fig. 2b) as hypoeutectic (Si-1B), eutectic (Si-3.2B) and hypereutectic (Si- 5.7B) (wt.%). Additionally, ultra-highly pure Si (7N) was used as the reference material (Ref 6). The Si-B alloy samples with a mass of  0.6 g were produced from polycrystalline pure materials (Si: 99.999%; B: 99.9%—provided by Onyxmet, Poland). The main impurities in the batch materials were Al (0.008 at.%) and P (0.002 at.%) in silicon and Fe (0.023 at.%) and Al (0.012 at.%) in boron. The alloys were fabricated by the electric arc melting technique Fig. 3 Temperature profile (heating/holding/cooling scheme) of the (Buehler Arc Melter MAM-1) by using properly weight sessile drop experiment (more details are shown in Ref 6) mixtures of pure elements. The prepared Si-B mixtures were heated up in argon protective atmosphere until a complete melting and then rapidly cooled down after switching off the electric arc. In order to increase homogeneity of the Si-B alloys, polished to achieve the surface roughness of  150 nm. The each sample was twice remelted in the arc melting device. sessile drop experiments were performed with a dedicated Commercially available h-BN sinters (HeBoSint D100, experimental complex (Ref 8) working under argon atmo- Henze Boron Nitride Products AG, Lauben, Germany) were sphere, in the temperature range between 1450 and 1750 C used as the substrates in the wetting tests. Before experiments, (the applied temperature profile is shown in Fig. 3). In these contacting surfaces of h-BN substrates were gently ground and tests, in order to mimic real working conditions of a PCM 3820—Volume 28(7) July 2019 Journal of Materials Engineering and Performance device, static argon atmosphere (p = 850-900 mbar) was 3. Results and Discussion applied. More details on experimental procedure are given else- 3.1 Characterization of Si-B Alloys in As-Fabricated State where (Ref 6). During the tests, the images of Si-B/h-BN The results of microstructural characterization of the as- couples were recorded at 100 fps by using a high-speed high- fabricated Si-B alloys are shown in Fig. 4. SEM images of the resolution CDD camera. Subsequently, the acquired images cross-sectioned alloys revealed that despite using double were post-processed to compile movies and calculate wetting remelting, the microstructure of alloys is quite inhomogeneous, kinetics. After the tests, the solidified couples were removed while a degree of inhomogeneity increased with increasing from the vacuum chamber and subjected to structural evalu- boron content. A matrix of each alloy was composed of the ations. The characterization of Si-B alloy in as-fabricated state mixture of Si(B) solid solution and Si + SiB eutectic. With and Si-B/h-BN sessile drop couples was carried out by using 3 increasing boron content, the number of boron-rich precipitates Carl Zeiss Axio Observer ZM10 light microscope and FEI also increases. The results of SEM/EDS local chemical Scios field emission gun scanning electron microscope composition analyses revealed a B/Si atomic ratio of (FEGSEM) coupled with energy-dispersive x-ray spec- 3.01 ± 0.15 in the eutectic areas and B/Si = 4.11 ± 0.13 in troscopy (EDS). Fig. 4 Results of SEM/EDS evaluations of Si-B alloys in the as-fabricated state: Si-1B (a), Si-3.2B (b) and Si-5.7B (c) Journal of Materials Engineering and Performance Volume 28(7) July 2019—3821 Fig. 5 Images of Si-B/h-BN sessile drop couples in situ recorded during the high-temperature tests at T up to 1750 C dark gray particles. Additionally, black particles were found to hand, it has been proposed (Ref 13) that the triboride is a be almost Si-free and they exhibited B/C ratio of 4.07 ± 0.09. silicon-rich version of the tetraboride, so the stoichiometry of Therefore, the following structural features were recognized either compound could be expressed as SiB where x = 0 or 1. 4-x based on careful SEM/EDS examinations: Si(B) solid solution, Additionally, due to local segregations of chemical composi- Si + SiB eutectic, SiB silicon tetraboride and B C boron tion, both phases (tri- and tetraboride) might also coexist. 3 4 4 carbide. Thus, it seems that presently applied non-equilibrium It should be noted that serious discrepancies on boron-rich solidification conditions of electric arc melting processing part of the Si-B phase diagram are reported in the literature. The (i.e., rapid quenching and high solidification rates) supported most contradictory findings have been shown for the phase the formation of SiB as the most thermodynamically stability and stoichiometry of silicon borides. Although the stable phase, while the presence of B C particles should be widely accepted form of Si-B phase diagram (Ref 7) contains justified in terms of carbon impurities introduced from the only three borides, namely SiB , SiB and SiB , some earlier batch materials. 3 6 n and more recent papers, e.g., by Samsonov and Sleptsov (Ref 9) or by Tremblay and Angers (Ref 10, 11), point toward the 3.2 The Wetting Kinetics in Si-B Alloy/h-BN Systems existence of SiB instead of SiB compound, while other 4 3 The images of Si/h-BN and Si-B/h-BN sessile drop couples researchers, e.g., Aselage (Ref 12), reported that the SiB phase in situ recorded during the high-temperature tests are shown in grows from boron saturated silicon, but at the same time they Fig. 5. The wetting kinetics curves (showing a change of indicated its metastability toward SiB phase. On the other 3822—Volume 28(7) July 2019 Journal of Materials Engineering and Performance Fig. 6 Wetting kinetics curves (showing a change of contact angle h vs. testing time) calculated for Si-B alloys subjected to contact heating with h-BN substrates (data for pure Si are taken from Ref 6) contact angle h vs. testing time) calculated for pure silicon and BN substrate is slightly dissolved in initially pure Si, leading to Si-B alloys subjected to contact heating with h-BN substrates diffusion of boron into molten Si. In view of this finding, it is are shown in Fig. 6. By comparing the results obtained for Si-B reasonable to conclude that addition of boron to silicon before alloys to the behavior of pure silicon on the h-BN substrate the experiment (i.e., using Si-B alloys instead of pure Si) (described in details in Ref 6), it is concluded that the additional suppresses this phenomenon. In other words, the Si-B alloys presence of boron decreases the wettability in the system. For dissolve much less boron from the h-BN substrate, which all Si-B alloys, the contact angle values were very high (within results in a remarkable hindering of substrate dissolution and a the non-wetting regime of h > 90) in the whole examined fast achievement of a thermodynamic equilibrium. temperature range. This statement seems to be also confirmed by a strikingly However, it should be noted that the wetting kinetics curve different behavior of Si/h-BN and Si-B /h-BN couples during for the Si-5.7B hypereutectic alloy showed a noticeable cooling from 1750 C. In the former case, an increase in decrease from h = 145 at 1450 Cto h = 125 at 1750 C. contact angle h upon cooling (a so-called dewetting) was Most probably, this behavior might be attributed to an observed as the effect of drastic change in solubility of inhomogeneous initial structure of the Si-5.7B alloy, in previously diffused B, N and C atoms in liquid/solid-state particular to the presence of relatively large SiB (and B C) silicon. Consequently, a release of gaseous nitrogen and 4 4 crystals distributed in the bottom part of the alloy (Fig. 4c) precipitation of BN platelets and SiC crystals take place at directly contacting the h-BN substrate. Due to a high chemical the Si/h-BN interface during the solidification. On the other affinity of Si(B) melt to both SiB and B C phases reflected hand, Si-B alloys exhibited either the negligible alteration of 4 4 also by a very good wetting, it is believed that the existence of the h versus t curve (for Si-1B alloy) or its descending tendency this ‘‘discontinuous layer’’ in the vicinity of Si-5.7B/h-BN (for Si-3.2B and Si-5.7B alloys) during cooling. Decreasing the interface might be responsible for the observed decrease in contact angle h during cooling of eutectic and hypereutectic Si- contact angle h. This finding allows concluding that increased B alloys should be justified by the change in structure and fraction of high melting point borides in Si-B alloys having the chemistry of the interface due to the formation of wettable sil- hypereutectic composition is not beneficial in terms of the icon boride crystals at the interface area before the solidification ‘‘non-wettability requirement’’ for the selection of container of Si(B) matrix, i.e., Si-B/h-BN system was locally converted materials in LHTES device. Furthermore, what is extremely to Si-B/B C+SiB /h-BN. The results of LM and SEM/EDS 4 x important from the applications point of view, a large amount analyses of cross-sectioned solidified couples (Fig. 7) revealed of crystals having high melting points decreases a relative that: content of liquid phase providing the latent heat for the electricity generation. (1) the size and number of silicon boride crystals in the As it has been experimentally shown in earlier work (Ref 6) interface vicinity increase with increasing initial boron during the high-temperature interaction in Si/h-BN system, h- content in Si-B alloys (Fig. 7a-c); Journal of Materials Engineering and Performance Volume 28(7) July 2019—3823 Fig. 7 LM images of cross-sectioned Si-B alloys/h-BN interfaces for: Si-1B (a), Si-3.2B (b) and Si-5.7B (c) alloys. The SEM image showing a coexistence of SiB and SiB borides in the Si-5.7B alloy 3 6 (2) the EDS estimated chemical composition of the large Based on the results of SEM/EDS analyses, following gray crystals is very close to the stoichiometry of SiB structural features were recognized in the as-fabricated al- triboride. Furthermore, in the case of the Si-5.7B hyper- loys: Si(B) solid solution, Si + SiB eutectic, SiB silicon 3 4 eutectic alloy (Fig. 7d), the presence of few dark crys- tetraboride and B C boron carbide. tals having the B/Si ratio of  6.18 ± 0.15 2. The wettability tests for Si-B alloys/h-BN couples were (corresponding to the SiB hexaboride) was also noted. performed for the first time by sessile drop experiments This finding suggests that under conditions of cooling at temperatures up to 1750 C. It was established that the rates slower than that in the electric arc melting process, Si-B alloys exhibited much lower wettability with the h- the SiB and SiB phases may coexist as more BN ceramic as compared to the pure silicon counterpart. 3 6 stable than the silicon tetraboride. For both Si-1B hypoeutectic and Si-3.2B eutectic alloys, very high contact angle h values of  150 were re- In addition, boron has been already recognized as the corded in the whole examined temperature range. The Si- surface active element in many metal-boron systems (Ref 14), 5.7B hypereutectic alloy shows slightly lower contact an- which means that B atoms preferentially segregate at the gle h values, most probably due to its inhomogeneous liquid–vapor interface (Ref 15). Therefore, the observed initial structure including the presence of primary high decrease in contact angle during cooling of Si-B alloys might melting point borides in the vicinity of the surface con- be also related to a probably negative effect of increased boron tacting the h-BN substrate. content on the melt surface tension. 3. Since wetting phenomenon in Si/h-BN system at ultra- high temperature is dominated by the dissolution of h- BN in molten Si followed by reprecipitation during cool- ing, the lack of wetting in Si-B alloy/h-BN system under 4. Conclusions the same conditions as for pure Si/h-BN is caused by a suppression of this mechanism. The following conclusions are drawn from the obtained 4. Regarding the predicted application of Si-B/h-BN sys- experimental results and supported by appropriate data from the tems in the ultra-high-temperature LHTES devices, it is literature: suggested to use hypoeutectic or near eutectic composi- tions of Si-B alloys. This choice is justified not only by 1. Silicon–boron alloys having various boron contents were presently documented negligible interaction with the h- successfully fabricated by electric arc melting process. 3824—Volume 28(7) July 2019 Journal of Materials Engineering and Performance 3. Z. Yuan, W.I. Huang, and K. Mukai, Wettability and Reactivity of BN ceramic, but also by theoretically highest available Molten Silicon with Various Substrates, Appl. Phys. A Mater., 2004, 78, latent heat for these alloys (Ref 1). p 617–622 4. J.A. Champion, B.J. Keene, and S. Allen, Wetting of Refractory Materials by Molten Metallides, J. Mater. Sci., 1973, 8, p 423–426 Acknowledgments 5. B. Drevet, R. Voytovych, R. Israel, and N. Eustathopoulos, Wetting and Adhesion of Si on Si N and BN Substrates, J. Eur. Ceram. Soc., 3 4 The project AMADEUS has received funds from the European 2009, 29, p 2363–2367 Unions Horizon 2020 research and innovation program, FET- 6. W. Polkowski, N. Sobczak, R. Nowak, A. Kudyba, G. Bruzda, A. OPEN action, under Grant Agreement 737054. The sole respon- Polkowska, M. Homa, P. Turalska, M. Tangstad, J. Safarian, E. sibility for the content of this publication lies with the authors. It Moosavi-Khoonsari, and A. Datas, Wetting Behavior and Reactivity of Molten Silicon with h-BN Substrate at Ultrahigh Temperatures up to does not necessarily reflect the opinion of the European Union. 1750 C, J. Mater. Eng. Perform., 2018, 27, p 5040–5053 Neither the REA nor the European Commission is responsible for 7. R.W. Olesinski and G.J. Abbaschian, The B-Si (Boron-Silicon) any use that may be made of the information contained therein. System, Bull. Alloys Phase Diagr., 1984, 5, p 479–484 The authors wish to express their thanks to S. Donath for the help 8. N. Sobczak, R. Nowak, W. Radziwill, J. Budzioch, and A. Glenz, with sample preparation. Experimental Complex for Investigations of High Temperature Cap- illarity Phenomena, Mater. Sci. Eng. A, 2008, 495, p 43–49 9. G.V. Samsonov and V.M. Sleptsov, Preparation of Boron-Silicon Alloys, Powder Metall. Met. Ceram., 1964, 3, p 488–496 Open Access 10. R. Tremblay and R. Angers, Preparation of High Purity SiB by Solid This article is distributed under the terms of the Creative Commons State Reaction Between Si And B, Ceram. Int., 1989, 15, p 73–78 Attribution 4.0 International License (http://creativecommons.org/ 11. R. Tremblay and R. Angers, Mechanical Characterization of Dense Silicon Tetraboride (SiB4), Ceram. Int., 1992, 18, p 113–117 licenses/by/4.0/), which permits unrestricted use, distribution, and 12. T.L. Aselage, The Coexistence of Silicon Borides with Boron- reproduction in any medium, provided you give appropriate credit Saturated Silicon: Metastability of SiB , J. Mater. Res., 1998, 13,p to the original author(s) and the source, provide a link to the 1786–1794 Creative Commons license, and indicate if changes were made. 13. C. Brosset and B. Magnusson, The Silicon-Boron System, Nature, 1960, 187, p 54–55 14. A.F. Vishkarev, Y.V. Kryakovskii, S.A. Bliznukov, V.I. Yavoiski, Influence of Rare-Earth Elements on the Surface Tension of Liquid References Iron, in Surface Phenomena in Metallurgical Processes: Proceedings 1. A. Datas, A. Ramos, A. Marti, C. del Canizo, and A. Luque, Ultra High of an Interinstitute Conference, ed. by A.I. Belyaev (Springer, US, Temperature Latent Heat Energy Storage and Thermophotovoltaic 1965), pp. 166–171 Energy Conversion, Energy, 2016, 107, p 542–549 15. A. Passerone, M.L. Muolo, F. Valenza, and R. Novakovic, Thermo- 2. B. Drevet and N. Eustathopoulos, Wetting of Ceramics by Molten Silicon dynamics and Surface Properties of Liquid Cu-B Alloys, Surf. Sci., and Silicon Alloys: A Review, J. Mater. Sci., 2012, 47, p 8247–8260 2009, 603, p 2725–2733 Journal of Materials Engineering and Performance Volume 28(7) July 2019—3825

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