Wetting Behavior and Reactivity of Molten Silicon with h-BN Substrate at Ultrahigh Temperatures up to 1750 °C

Abstract

For a successful implementation of newly proposed silicon-based latent heat thermal energy storage systems, proper ceramic materials that could withstand a contact heating with molten silicon at temperatures much higher than its melting point need to be developed. In this regard, a non-wetting behavior and low reactivity are the main criteria determining the applicability of ceramic as a potential crucible material for long-term ultrahigh temperature contact with molten silicon. In this work, the wetting of hexagonal boron nitride (h-BN) by molten silicon was examined for the first time at temperatures up to 1750°C. For this purpose, the sessile drop technique combined with contact heating procedure under static argon was used. The reactivity in Si/h-BN system under proposed conditions was evaluated by SEM/EDS examinations of the solidified couple. It was demonstrated that increase in temperature improves wetting, and consequently, non-wetting-to-wetting transition takes place at around 1650°C. The contact angle of 90°± 5°is maintained at temperatures up to 1750°C. The results of structural characterization supported by a thermodynamic modeling indicate that the wetting behavior of the Si/h-BN couple during heating to and cooling from ultrahigh temperature of 1750°C is mainly controlled by the substrate dissolution/reprecipitation mechanism.

Ultra-High Temperature Interaction Between h-BN-Based Composite and Molten Silicon

“…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, [7] 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 ZrO 2 as well as secondary ZrB and ZrB 2 ) somehow suppresses this phenomenon.…”

“…High purity Si and commercial sintered h-BN composite having the nominal phase composition of (h-BN + SiC + ZrO 2 ) were used for the sessile drop wetting test. As in our previous work, [7] the selected Si material was amorphous polysilicon from the commercial Siemens process, with a purity of 7N. [14] The commercially available (h-BN + SiC + ZrO 2 ) hot isostatically pressed composite HeBoSint Ò O120 (Henze, Germany), was used as substrate.…”

“…[7] The Si/h-BN composite couple was subjected to the wettability test by the sessile drop method combined with a contact heating procedure, by using the experimental apparatus described in detail elsewhere. [15] For the sake of comparison, the test was performed in ''2 in 1'' mode as shown in Figure 1(a) (i.e., two couples were simultaneously examined under the same experimental environment), by using as the counterpart the ''pure'' h-BN (HeBoSint Ò D100 material produced by Henze, Germany) examined in our previous work [7] ). The applied testing procedure of the heating/cooling scheme including five intervals (i.e., 1450°C/5 min; 1550°C/5 min; 1650°C/5 min; 1700°C/5 min and 1750°C/10 min) is shown in Figure 1(b).…”

“…[5] 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. [6] In our previous work, [7] 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 h > 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 Effect of Boron Content on Wetting Kinetics in Si-B Alloy/h-BN System

“…The reported values of the equilibrium contact angle h measured for Si/h-BN couples at T < 1500°C are within the range of 95°-145°(e.g., Ref 4,5). In our previous work (Ref 6), it was found that the non-wetting behavior of the Si/h-BN system is maintained up to 1650°C, while a non-wetting-to-wetting transition takes place at higher temperatures. It has also been evidenced that the 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.…”