Ultra-High Temperature Interaction Between h-BN-Based Composite and Molten Silicon
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Silicon has been recently proposed as a very promising phase change material for applications in latent heat thermal energy storage (LHTES) and conversion systems working at ultra-high temperatures. However, in order to successfully develop such kind of devices, suitable refractories showing low reactivity and non-wetting behavior upon the melting and storing of molten silicon at temperatures much higher than its melting point have to be selected. In our previous work, we have documented that the non-wetting behavior in Si/h-BN system is preserved at temperatures up to 1650 °C, with the absence of new reaction products formed at the interface. These findings make hexagonal boron nitride (h-BN) a reasonable first candidate for Si-based LHTES applications. Nevertheless, the rather poor mechanical strength of “pure” h-BN ought to be improved in order to enhance the reliability under thermocycling operational conditions and to increase the life period of the device. Therefore, in the present paper, we examine for the first time the interactions at ultra-high temperatures between a high strength commercial h-BN-based composite and molten Si. At temperatures up to 1750 °C, the wettability of the h-BN-based composite (h-BN + SiC + ZrO2) with molten Si is much lower if compared to the pure h-BN counterpart. Additionally, the role of reinforcements (SiC + ZrO2) and occured microstructural evolution is discussed based on the results obtained by SEM and XRD analyses.
By taking into account ecological, economical and geopolitical concerns of the energy resources and their accessibility, concentrated solar power (CSP) has been widely recognized as one of the most promising renewable energy sources. However, the lack of energy supply during sunless periods generates an urgent need to develop efficient energy storage methods to cover the gap existing between energy production and consumption. Among various available methods, latent heat thermal energy storage (LHTES) appears to be the attractive choice, mainly in terms of its large energy storage capacity.[1, 2, 3] In this method, the heat absorbed or released during a phase transformation (namely latent heat or heat of fusion) is utilized to store the energy inside containers filled with a phase change material (PCM), and then is converted into electricity. Recently, silicon has been identified as promising PCM candidate for ultra-high temperature energy storage. Indeed, Si possesses very high heat of fusion (1230 kWh/m3), at least one order of magnitude higher than the currently used salts (e.g., NaNO3—110 kWh/m3). For this reason, the use of Si as a PCM should significantly increase the energy density of LHTES devices. Nevertheless, in order to ensure a high reliability of Si-based LHTES devices, appropriate refractories for the PCM container operating at temperatures higher than the melting point of Si, need to be identified. In order to suppress degradation of the involved materials during the long-term service, one of the key requirements to select the refractory is the inertness towards the contacting PCM, i.e., reflected by a low reactivity and non-wettability by molten Si. Moreover, a reasonably high mechanical strength of the selected refractory material is crucial to provide a longer durability of the PCM container operating under thermocycling conditions. However, due to a high chemical affinity of Si to oxygen, nitrogen, and carbon, almost all the commonly used refractories are well wetted by molten Si. Among ceramic materials, hexagonal boron nitride (h-BN) is the only one reported exception exhibiting non-wetting behavior by Si at temperatures up to 1500 °C.
In our previous work, we have examined the wettability and reactivity at the Si/h-BN interface up to 1750 °C. The results of sessile drop experiments evidenced that the non-wetting behavior (reflected by the contact angle θ > 90 deg) is maintained up to 1650 °C, while for higher temperatures the contact angle was near or slightly below 90 deg, corresponding to the non-wetting-to-wetting transition. The results of microstructural characterization revealed dissolution/reprecipitation phenomena at the interface as the main interaction mechanism, without the formation of interfacial reactive products between liquid Si and h-BN. These findings provide a preliminary (since the thermal cycling behavior needs to be further examined) positive recommendation for the h-BN as the container material operating at temperatures up to 1650 °C. On the contrary, to enhance its reliability the relatively low mechanical strength of this material needs to be improved. A common way to improve mechanical performance of ceramic materials is to reinforce them with other materials, i.e., to fabricate ceramic based composites. Actually, various h-BN matrix composites are under development, while some of them have been already fully commercialized and are easily available in the form of customizable products (e.g., crucibles, nozzles, plates, tubes etc.) (e.g., Reference 10). Among various examining reinforcements, the most common are ZrO2 and SiC that are added to improve compressive strength, fracture toughness, thermal shock resistance, and oxidation resistance of the h-BN body.[11, 12, 13]
Despite a strong interest in industrial applications of h-BN-based composites, no one to the best of our knowledge has previously investigated their behavior in contact with molten Si at ultra-high temperatures. Therefore, the main purpose of the present work is to experimentally evaluate at temperatures up to 1750 °C the wettability and reactivity at the Si/(h-BN + SiC + ZrO2) interface by using the sessile drop method.
2 Materials and Methods
After the wetting test, the solidified couples were removed from the chamber and their microstructural characterization was performed by using the Carl Zeiss Axio Observer ZM10 light microscope (LM) and FEI Scios™ field emission gun scanning electron microscope (FEG SEM) coupled with Energy Dispersive X-ray Spectroscopy (EDS). The examinations were performed both on top-views and cross-sectioned samples. The presence of particular phases in the h-BN composite in the as-received condition and after the high temperature test was analyzed by using PANanalytical Empyrean X-ray diffractometer (XRD). In particular, the following parameters were applied upon acquiring XRD spectra: Cu Kα radiation (1.54060 Å); generator settings: 40 mA, 40 kV; 2θ range of 10 to 110 deg; a step size of 0.0020 deg.
3.1 Characterization of h-BN Composite in the As-received Condition
However, as suggested in the literature, the addition of ZrO2 to the h-BN composite may result in the formation of zirconium borides during sintering. The presence of both ZrB (rhombus, ICDD:96-900-8777, Crystal system: Cubic, Space group: Fm−3m, Space group number: 225) and ZrB2 (triangles) were confirmed in the as-received material. The coexistence of two zirconium borides might be explained by a partial evaporation of B from the ZrB2. Moreover, the high sharpness of the peaks indicates well-crystallized phases.
3.2 Wetting and Spreading Behavior
A fast-forward video of the wettability test is added as Supplementary Material 1. The applied “2 in 1” procedure allows for a real-time comparison of the pure h-BN (on the left) and the h-BN composite (on the right) substrates upon testing in contact heating with pure Si up to 1750 °C. First of all, it should be noted that the Si/(h-BN + SiC + ZrO2) composite couple exhibited more stable behavior than the Si/h-BN counterpart. In contrast to the pure h-BN sample, weak Si drop vibrations were observed on the composite substrate only for few seconds after the melting of Si. As we have concluded in our previous work, the drop vibration effect originates from the substrate dissolution followed by a releasing of overbalanced nitrogen through gas/liquid and gas/solid interfaces. Thus, it seems that highly heterogeneous structure of the composite substrate composed of h-BN matrix and at least four phases (primary SiC and ZrO2 as well as secondary ZrB and ZrB2) somehow suppresses this phenomenon. This statement is also confirmed by a more undetectable dewetting phenomenon during the cooling of Si/(h-BN + SiC + ZrO2) couple.
3.3 Reactivity in the Si/(h-BN + SiC + ZrO2) System at Temperatures up to 1750 °C—Structural Characterization
3.3.1 Examinations of the (h-BN + SiC + ZrO2) composite substrate
a not strongly altered structure of the unreacted zone as compared to the as-received material (see Figure 2), i.e., a presence of primary ZrO2 and SiC particles and a lack of secondary zirconium borides (Figures 9(d) and 10(c)).
diffraction lines from the h-BN phase were unchanged (squares);
reflections from the SiC—moissanite phase disappeared;
both ZrB (rhombus) and ZrB2 (triangles) lines were still present. The peaks from zirconium boride phases became stronger and several more reflections are visible, as compared to the material in the as-received condition[21,22];
the monoclinic ZrO2 is no longer present. Literature predicts that at high temperatures (above 1050 °C), a conversion into tetragonal ZrO2 might occur. However, the main peak of t-ZrO2 (100 deg relative intensity) at approximately 2θ = 30.0 deg is missing, indicating that in the present case such conversion did not take place. Therefore, this fact suggests that the monoclinic zirconia presented in the as-received material was completely transformed to zirconium borides during the high-temperature exposure.
3.3.2 Examinations of the Si drop and Si/(h-BN + SiC + ZrO2) interface
Nevertheless, as it comes from the Reaction  and Reactions [2a, 2b], the transformation of ZrO2 into zirconium borides requires a pre-existence of free C in the system. Kumar et al.[24,25] have documented that one of the main features of the non-wetting interaction of (h-BN + SiC + ZrO2) composite with molten Si-killed steel at temperatures up to 1600 °C is a thermal decomposition followed by a dissolution and/or removal of SiC, accompanied by formation of gaseous CO product, and thus leading to an existence of a porous zone in the affected volume of the composite. Analogously, since it is well established that SiC is also completely unstable in contact with molten Si, we may assume a similar high temperature structural evolution mechanism for the Si/(h-BN + SiC + ZrO2) system involving the decomposition process of SiC phase and giving free carbon needed for the Reactions  and/or [2a, 2b].
Furthermore, it is reasonable to assume that the composite also contained some amount of boron oxide (B2O3). It is commonly introduced either as a sintering aid for rapid densification, or due to the oxidation of BN powder during sintering step.
Furthermore, as it was discussed by Chen et al. the impurity B2O3 contained in BN facilitates the formation of ZrB phases in nominal (h-BN + SiC + ZrO2) material through the reactions with zirconia and silicon carbide also assisted by formed gases.
Therefore, reactions between composite constituents assisted by formed gaseous reaction products; as well as existence of phases (e.g., liquid B2O3) that evaporate rapidly under low-oxygen partial-pressure atmosphere, might be indicated as a possible explanation for the observed inhomogeneity of the composite (especially by the presence of a near-surface porous zone).
A high probability for the occurrence of this mechanism during the high temperature test is also justified by the following evidences observed in the outer part of the tested substrate: (I) a lack of SiC particles and (II) a coexistence of zirconium borides and pores (Figures 8(b) and 9(b)). Accordingly, it is suggested that the porosity might be formed in accordance to aforementioned reactions including (I) the SiC decomposition (that can be alternatively assisted by the production of B2O2 gas—Reaction ) and (II) the transformation of ZrO2 accompanied by of the production of gaseous CO and N2, respectively.
Furthermore, the in situ formation of porosity (and especially open pores) in the outer part of the composite alters its initial surface morphology, and thus might also affect the wetting behavior and contact angle values recorded during the wetting test with molten Si. As we previously stated, the vibration of Si drop on the h-BN ceramic at high temperature is caused by the action of gaseous nitrogen formed by dissolution of the substrate and then is released through gas/liquid interface. In the present case, it seems that the absence of Si drop vibration on the (h-BN + SiC + ZrO2) composite substrate could be justified by the existence of open porosity in the near surface area, enabling a fast gas removing (N2, CO) from the interaction zone. This situation is very similar to that reported by Kanai et al. for the Si/SiO2 system. They documented that the presence of grooves or dimples on the SiO2 substrates facilitates the “leakage” of SiO gaseous product; and as a result, the Si droplet did not vibrate, as opposite to the counterpart tested on the silica substrate having flat surfaces. Consequently, since conditions for perfectly flat and smooth surface are not fulfilled in such case, the measured contact angle values, probably overestimated in some extent, should be treated as apparent.
The h-BN matrix is slightly dissolved in the molten Si, while the dissolution rate and degree increase with rising temperature.
The substrate dissolution process is assisted by a diffusion of boron, nitrogen and trace impurities leading to a gradual change of chemistry of molten Si towards Si-B-X alloys.
SiC particles embedded in the near surface area of h-BN matrix are decomposed in contact with molten Si (according to Si + SiC → Si + C mechanism) while reactively formed C is dissolved in liquid Si and then rebuilt during cooling in a form of SiC particles at the interface on the drop side (dissolution-reprecipitation mechanism),
ZrO2 is unstable at such high testing temperatures, and under the presence of free C, it is transformed to more stable zirconium borides in accordance to reactions (1) or (2).
5 Summary and Conclusions
In this work, the high temperature interaction of commercially available (h-BN + SiC + ZrO2) composite with molten Si was experimentally examined by sessile drop experiments carried out at temperatures up to 1750 °C. The obtained results show that the examined h-BN composite exhibits a lack of wettability with molten Si over the whole examined temperature range. The recorded contact angle of θ1420 = 133 ± 1 to θ1750 = 113 ± 1 deg is distinctly higher than those measured for the Si/pure h-BN system under the same operating conditions (θ1420 = 129 ± 1 to θ1750 = 90 ± 5 deg, respectively). Based on the results of LM, SEM/EDS and XRD analyses, it is proposed that the involved ultra-high temperature interaction mechanism in the Si/(h-BN + SiC + ZrO2) system is mostly dominated by two phenomena: (i) a dissolution of the h-BN substrate leading to a change of chemistry of molten Si toward a Si-B-X alloy and (ii) a formation of discontinuous interfacial SiC layer during the cooling step.
In the view of non-wettable surface requirement, it is reasonable to conclude that the investigated (h-BN + SiC + ZrO2) composite as container material for molten PCM will allow for increasing the operating temperature of the LHTES system, in respect to the pure h-BN.
On the other hand, the results of structural examinations point towards a lack of thermodynamic equilibrium in the as-received material under applied testing conditions. An occurrence of chemical reactions between composite components leading to significant changes of its microstructure and phase composition due to the ultra-high temperature exposure was observed.
The structural evolution of the examined h-BN-based composite is dominated by the thermal destabilization of primary SiC phase upon high temperature exposition favoring reactions between the other composite components (h-BN and ZrO2). Consequently, the structure becomes strongly inhomogeneous with easily distinguished reacted and unreacted zones separated by the transition area. The reacted zone is characterized by transformation of zirconia to zirconium borides. Moreover, the reaction is accompanied by gaseous products leading to the formation of porosity in the outer zone of the composite substrate.
A thermal cycling behavior of the Si/(h-BN + SiC + ZrO2) system (i.e., during consecutive melting/solidification steps) ought to be further examined in order to test the thermal stability of reactively formed products. Our findings might provide a first indication in the selection of refractories for the PCM casing in Si-based LHTES device.
6 Supplementary Information
A fast-forward movie recorded during the wettability test is attached as the Supplementary Material 1.
The project AMADEUS has received funds from the European Union’s Horizon2020 research and innovation program, FET-OPEN action, under Grant Agreement 737054. The sole responsibility for the content of this publication lies with the authors. It does not necessarily reflect the opinion of the European Union. Neither the REA nor the European Commission are responsible for any use that may be made of the information contained therein. The authors acknowledge Mr M. Guaragno for his technical support concerning the XRD measurements.
Conflict of interest
The authors declare that there is no conflict of interests.
Supplementary material 1 (AVI 5780 kb)
- 7.W. Polkowski, N. Sobczak, R. Nowak, A. Kudyba, G. Bruzda, A. Polkowska, M. Homa, P. Turalska, J. Safarian, E. Moosavi-Khoonsari, A. Datas: J. Mater. Eng. Perform., 2017, 36: 45, https://doi.org/10.1007/s11665-017-3114-8Google Scholar
- 8.S.T. Mileiko: in Metal and ceramic based composites, Composite Materials Series, S.T. Mileiko, ed., 1997, vol. 12, pp. 233–305. https://doi.org/10.1016/s0927-0108(97)80023-9
- 10.Henze HeBoSint Boron Nitride Products Overview: http://www.henze-bnp.com/PDF/HeBoSint_PI_GB.pdf?m=1502355729 accessed on 27/04/2017
- 20.J. Eichler, Ch. Lesniak: J. Eur. Ceram. Soc., 2008, vol. 28, pp. 1105-9. https://doi.org/10.1016/j.jeurceramsoc.2007.09.005 CrossRefGoogle Scholar
- 27.A.W. Weimer: in: Carbide, Nitride and Boride Materials Synthesis and Processing, A.W. Weimer, ed., Springer, Netherlands, 1997, pp. 79–113. https://doi.org/10.1007/978-94-009-0071-4
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