Wetting Behavior During Cycling Si Melting/Solidification Processes
The fast-forward video recorded during the test is available as the Electronic Supplementary Material 1. What should be noted is that the silicon showed a high cyclic stability in terms of onset and offset temperatures of melting and solidification processes (Fig. 2a). In fact, the melting offset temperatures were close to the theoretical melting point of Si (Tm = 1414°C), and only a slight decrease was observed during the consecutive cycles. As was also expected, a significant undercooling effect was noted. It was documented that in each cycle the solidification was finished at a temperature around 1345°C, thus giving ∆T ~ 70°C. The change of contact angle after melting and holding the Si drop in consecutive cycles is presented in Fig. 2b, while a set of corresponding Si drop/h-BN substrate images taken after 5 min holding at 1450°C in each cycle, is presented in Fig. 3. It was found that, during the whole experiment, the contact angle values were within the non-wettability regime (i.e., the θav was higher than 90°). However, it was noted that (1) in the 10th cycle, a substantial increase of contact angle from θav ~ 100° to θav ~ 125–130° took place (Fig. 2b) and (2) the Si drop was spontaneously moved along the substrate surface during subsequent cycles (Fig. 3b).
Structural Characterization of the Si/h-BN Couple After Melting/Solidification Cycles
A macroscopic view of the Si/h-BN couple after 15 cycles of melting/solidification is shown in Fig. 4a. The presence of some modification of the h-BN surface related to a spontaneous in situ detachment and movement of the Si drop can be distinguished. The results of SEM examinations of this area (Fig. 4b, c) revealed the presence of fine h-BN platelets having different sizes. A difference in the size of the platelets reflects the spontaneous movement path of the Si drop (see Fig. 3b); the smallest were located in the outer zone, while their size increased towards the central part of the h-BN substrate area initially covered by the Si drop. The results of the EDS analysis (Fig. 4d) confirmed the existence of small Si particles (so-called “daughter droplets”) that were left behind during movement of the main (“mother”) Si drop.
What is important is that there is no evidence for the formation of any interfacial continuous product layers (e.g., silicon nitride Si3N4, as was suggested in Ref. 8. After that, the cross-section of Si/h-BN couple was prepared by using a precise metallographic cutter, while cold-mounting in epoxy resin was first applied to protect the sample (Fig. 5).
The results of SEM inspections carried out on the cross-sectioned sample documented (1) the formation of a platelets zone (with a thickness of ~ 150 µm) in the h-BN substrate’s subsurface area (Fig. 6a); (2) a slight change of the chemistry of initially ultra-high-purity Si towards a Si-B alloy reflected by the presence of a very few Si + SiB3 eutectic features between the Si grains (Fig. 6b); and (3) a lack of any new products formed at the interface, except for some single needle-like particles observed on the drop side (Fig. 6c). Based on the measured increased content of B and N, they were recognized as h-BN platelets that were detached from the substrate during the thermocycling experiment.
Finally, based on the results of the conducted high-temperature thermocycling test and the structural characterizations of the obtained materials, the following interaction mechanism in the Si/h-BN system during consecutive melting/solidification processes of Si might be proposed. As in the case of our previous experiment,6 the interaction is dominated by a slight dissolution of the h-BN substrate in the molten Si, followed by the reprecipitation (a re-growth) of the h-BN platelets during solidification. Consequently, boron from the h-BN substrate easily diffuses into the molten Si, while nitrogen is released through the gas/liquid interface as a gaseous product due to its very low solubility in Si. By taking into account an extremely high latent heat of boron,2 an introduction of small amounts of this element into the PCM candidate should be considered as beneficial in terms of the performance of LHTES devices. Interestingly, in the present work, it has also been documented that the in situ modification of the h-BN surface morphology affects the wetting characteristics in the Si/h-BN system. Most probably, the observed increase of contact angle in the 10th cycle and the movement of Si drop in subsequent cycles is an effect of its uplifting and mechanical pinning on regrown h-BN platelets. Thus, based on the obtained results, the involved interaction mechanism is described as follows:
A Si piece is placed on a h-BN substrate having a smooth and flat surface. During the first melting at the temperature of 1414°C, initially Si does not wet the h-BN substrate. However, when the testing temperature is increased to 1450°C, the h-BN substrate is slightly dissolved in the molten Si and the contact angle decreases (Fig. 7a).
Upon cooling from 1450°C to 1300°C, the h-BN crystals having a platelet-like morphology reprecipitate in the dissolved substrate area. Since the reprecipitation process takes place at a temperature higher than that needed for the full solidification of the Si drop, a new solid/solid interface is formed in situ. What is important is that a needle-like morphology of grown interfacial h-BN crystals combined with a lack of wettability with molten Si gives a discontinuous contact with the Si drop (Fig. 7b). Furthermore, due to the fact that the contacting substrate surface loses its flatness and smoothness, the recorded contact angles are far from the applicability of Young’s equation, and thus they should be treated as apparent in nature. Similar findings on recorded contact angles were noted in our recent work,9 in which open (and near-surface) porosity was reactively formed in the h-BN-based composite subjected to contact heating with molten Si at an ultra-high temperature.
During consecutive melting/solidification cycles the Si/h-BN interface undergoes further changes. The pre-existing h-BN platelets are cyclically dissolved in contact with molten Si, and the new ones are reprecipitated upon cooling to 1300°C. This behavior results in a prominent differentiation of h-BN platelet size: the largest were observed in areas subjected to the longest direct contact with molten Si. As a consequence of the highly developed inhomogeneous surface topography, the Si drop can undergo a spontaneous “self-detachment” movement (Fig. 7c).
This behavior is analogous to that described by Liang et al.10 showing a superhydrophobicity of highly crystallized h-BN microplatelets formed in accordance with a “surface microarchitecture” approach. They have documented that the presence of such morphological features significantly increases the contact angle value of a water drop on a h-BN substrate, i.e., it gives a so-called “lotus leaf” effect. What is important is that in such a case a lack of perfectly flat and rigid surface conditions makes the recorded contact angle values far from equilibrium, i.e., they are apparent in nature. Although in the present work the wetting characteristic was similar, it should be noted that the orientation of the platelets reprecipitated in consecutive cycles is rather random, so that it is quite hard to exactly predict the behavior of the Si/h-BN system during cycling melting/solidification processes. Nevertheless, in the view of the non-wettability requirement for the selection of refractories for the PCM container, the in situ formation of fine h-BN crystals having a platelet-like morphology on the contacting ceramic surface is clearly favorable.