5:15 PM - 7:15 PM
[PPS07-P11] Evaporation experiments of silicon carbide in low-pressure H2-H2O gas mixture: Implications for the pre-accretional thermal history of dust in the protosolar disk
Keywords:presolar grain, silicon carbide (SiC), evaporation, kinetics, protosolar disk
Presolar silicon carbide (SiC) grains can be an excellent indicator of the pre-accretional history of dust in the protosolar disk because of their thermodynamically unstable nature in oxidative protosolar disk gas (Larimer & Bartholomay, 1979). Evaporation kinetics of SiC under various temperature-oxygen fugacity conditions at 1 atm has been investigated in detail by Menydybaev et al. (2002). However, kinetics obtained at 1 atm may not be applicable to the low-pressure disk conditions. Here, we carried out evaporation experiments of SiC in the low-pressure gas of an H2–H2O gas mixture to determine the reaction mechanism and kinetics of evaporation of SiC in the disk.
Evaporation experiments were conducted at 1779–1523 K and total pressures (Ptot) of 0.5 and 2.5 Pa of an H2-H2O gas mixture (Ptot) in a high-temperature vacuum furnace with a gas flow system. The starting material was a chip (~4 × (4–5) × 0.6 mm) cut from a plate of polycrystalline b (3C)-SiC. Three different flow gas conditions were employed in this study; (1) Ptot = 0.5 Pa and PH2O of 2.2 × 10–3 Pa, (2) Ptot= 2.5 Pa and PH2O of 6.1 × 10–3 Pa, and (3) Ptot = 2.5 Pa and PH2O of 1.5 × 10–2 Pa. The water vapor pressures in these experiments were determined by the rate of consumption of liquid water in the gas flow system and the total gas flow rate. The simulated redox conditions are similar to or slightly more oxidative than that in the canonical protosolar disk (e.g., Lodders, 2003). The sample weight before and after the experiments was measured with an ultra-micron balance. FIB lift-out thin sections of the heated samples prepared with a FIB-SEM (Hitachi NX2000; Thermo Fisher Scientific Versa 3D) were analyzed with a STEM-EDS (JEOL JEM-2800). Micro-Raman spectroscopic observations were also made for the phase characterization.
STEM-EDS analysis of the heated samples showed that there was no oxide layer on the sample surface, but porous carbon-rich layers. These results are consistent with those obtained by Raman spectroscopy. The estimated evaporation flux J depends weakly on temperatures typically higher than ~1610–1670 K, while larger temperature dependences of J were observed at lower temperatures. The PH2O-dependence of J in the weak temperature-dependent regime suggests that the evaporation reaction rate is controlled by the water vapor supply to the SiC surface and the associated surface dissociation of the adsorbed water molecules under low-pressure conditions. The kinetic parameters in the temperature-dependent regime of this study and of previous investigations exhibit correlations with the redox conditions of the surrounding gas.
The survivability of presolar SiC grains was then compared with that of presolar amorphous silicate grains whose presolar signature would be erased by oxygen isotope exchange with the protosolar disk (Yamamoto et al., 2018, 2020). The present results indicated that, in the steady-state accreting protosolar disk (Ishizaki et al., 2023), presolar SiC grains would survive when they are kept at temperatures lower than ~1200–1500 K, which are much higher than the temperatures required to preserve oxygen isotopic signatures of presolar amorphous silicate grains (<~600–800 K). The abundance ratios of presolar silicate to SiC grains normalized to its initial ratio increased with increasing the heliocentric distance from the Sun (r), reaching the values of ~0.7–0.9 at r >~4–5 au. The ratios of ~0.7–0.9 are consistent with the primitive interplanetary dust particle-normalized ratios of Queen Alexandra Range (QUE) 99177 and Dominion Range (DOM) 08006 meteorites, suggesting that the parent bodies of these meteorites accreted from materials mainly originated from the regions of r > ~ 4–5 au. The present results suggest that the abundance ratios of presolar silicate to SiC grains can provide crucial information on the pre-accretional thermal history of materials in the protosolar disk.
Evaporation experiments were conducted at 1779–1523 K and total pressures (Ptot) of 0.5 and 2.5 Pa of an H2-H2O gas mixture (Ptot) in a high-temperature vacuum furnace with a gas flow system. The starting material was a chip (~4 × (4–5) × 0.6 mm) cut from a plate of polycrystalline b (3C)-SiC. Three different flow gas conditions were employed in this study; (1) Ptot = 0.5 Pa and PH2O of 2.2 × 10–3 Pa, (2) Ptot= 2.5 Pa and PH2O of 6.1 × 10–3 Pa, and (3) Ptot = 2.5 Pa and PH2O of 1.5 × 10–2 Pa. The water vapor pressures in these experiments were determined by the rate of consumption of liquid water in the gas flow system and the total gas flow rate. The simulated redox conditions are similar to or slightly more oxidative than that in the canonical protosolar disk (e.g., Lodders, 2003). The sample weight before and after the experiments was measured with an ultra-micron balance. FIB lift-out thin sections of the heated samples prepared with a FIB-SEM (Hitachi NX2000; Thermo Fisher Scientific Versa 3D) were analyzed with a STEM-EDS (JEOL JEM-2800). Micro-Raman spectroscopic observations were also made for the phase characterization.
STEM-EDS analysis of the heated samples showed that there was no oxide layer on the sample surface, but porous carbon-rich layers. These results are consistent with those obtained by Raman spectroscopy. The estimated evaporation flux J depends weakly on temperatures typically higher than ~1610–1670 K, while larger temperature dependences of J were observed at lower temperatures. The PH2O-dependence of J in the weak temperature-dependent regime suggests that the evaporation reaction rate is controlled by the water vapor supply to the SiC surface and the associated surface dissociation of the adsorbed water molecules under low-pressure conditions. The kinetic parameters in the temperature-dependent regime of this study and of previous investigations exhibit correlations with the redox conditions of the surrounding gas.
The survivability of presolar SiC grains was then compared with that of presolar amorphous silicate grains whose presolar signature would be erased by oxygen isotope exchange with the protosolar disk (Yamamoto et al., 2018, 2020). The present results indicated that, in the steady-state accreting protosolar disk (Ishizaki et al., 2023), presolar SiC grains would survive when they are kept at temperatures lower than ~1200–1500 K, which are much higher than the temperatures required to preserve oxygen isotopic signatures of presolar amorphous silicate grains (<~600–800 K). The abundance ratios of presolar silicate to SiC grains normalized to its initial ratio increased with increasing the heliocentric distance from the Sun (r), reaching the values of ~0.7–0.9 at r >~4–5 au. The ratios of ~0.7–0.9 are consistent with the primitive interplanetary dust particle-normalized ratios of Queen Alexandra Range (QUE) 99177 and Dominion Range (DOM) 08006 meteorites, suggesting that the parent bodies of these meteorites accreted from materials mainly originated from the regions of r > ~ 4–5 au. The present results suggest that the abundance ratios of presolar silicate to SiC grains can provide crucial information on the pre-accretional thermal history of materials in the protosolar disk.