Thermal expansion of SiC at high pressure‐temperature and implications for thermal convection in the deep interiors of carbide exoplanets

Abstract

Recent astrophysical observations have shown that some stars have sufficiently high carbon‐to‐oxygen ratios and may host planets composed mainly of carbides instead of silicates and oxides. From the low thermal expansion of SiC at 1 bar, it can be inferred that the buoyancy force of thermal anomalies is much lower in the carbide planets than in the silicate planets. However, numerous studies have shown that high pressure in planetary interiors can fundamentally change the physical properties of materials. We have measured the pressure‐volume‐temperature relations of two SiC polymorphs (3C and 6H) at pressures and temperatures up to 80 GPa and 1900 K and 65 GPa and 1920 K, respectively, in the laser‐heated diamond anvil cell combined with synchrotron X‐ray diffraction. We found no evidence of dissociations of these phases up to our maximum pressure condition, supporting the stability of SiC to 1900 km depth in Earth‐size Si‐rich carbide planets. Following the Mie‐Grüneisen approach, we fit our data to the Birch‐Murnaghan or the Vinet equations of state combined with the Debye approach. We found that the pressure‐induced change in the thermal expansion parameter of SiC is much smaller than that of Mg silicate perovskite (bridgmanite). Our new measurements suggest that the thermal buoyancy force may be stronger in the deep interiors of Si‐rich carbide exoplanets than in the “Earth‐like” silicate planets.

High-Pressure, High-Temperature Behavior of Silicon Carbide: A Review

“…Several interesting findings are presented, including a higher thermal expansion for SiC than previous measurements, although this is likely due to the high temperatures of their study. The thermal expansion found in [23] is on the order of 1 × 10 −5 1/K at 2500 K, nearly an order of magnitude higher than previous studies at room temperature. They also find that the thermal expansion changes very little with pressure.…”

“…Impurities may also play a role in the expressed polytypes as has been observed in ambient pressure experiments [13]. In reported static experiments at high-pressures the starting polytype structure remains throughout pressure loading and unloading [22,23], at least up to the conditions of the transition to the rocksalt structure [20,24,25]. See Figure 2 for a compilation of experimental data on the stability of common polytypes at pressure.…”

“…DAC studies find that 15R remains under compression to at least 35 GPa at 300 K [22], 3C remains stable to 105 GPa at 300 K and 6H remains stable to 95 GPa at 300 K before transition to a dense, high-pressure rocksalt structure [25]. LHDAC experiments find that both 6H and 3C remain in experiments covering conditions of~10-80 GPa and~3000 K (shown in blue) before transition to the rocksalt structure [18,23,24,27,28]. Shock experiments up to 25 GPa and~2000 K find an increase in the proportion of rhombohedral and cubic phases at high P-T and a decrease in hexagonal phases [16].…”

“…A recent study [23] used the LHDAC coupled with in situ X-ray diffraction to measure the thermal expansion of both the 3C-and 6H-SiC polytypes [23]. Their measurements spanned a range of conditions up to 80 GPa and 1900 K for 3C-SiC, and up to 65 GPa and 1920 K for 6H-SiC.…”

X-Ray Diffraction under Extreme Conditions at the Advanced Light Source

“…The potential for creating new material with interesting physical properties using combined high pressure and high temperatures as achieved in a LHDAC has been demonstrated by a series of 12.2.2 experiments, which created, among others, novel transition metal nitrides, borides, and carbides (e.g., Kaner et al [24], Chung et al [25], Mohammadi et al [26], Friedrich et al [27,28], and Santamaria-Perez et al [29]). Equally interesting is the possibility of exploring P-T phase diagrams and thermo-elastic properties of (Earth-) materials at very high pressures and temperatures (e.g., Armentrout and Kavner [30], Nisr et al [31]). (1) 1090 nm fiber laser, (2) IR path, (3) sample position, (4) 80 mm f/2.8 objective lens (5) motorized IR mirror, transparent for vis (6) image and pyrometry beam path, (7) monochromatic 16-bit camera with 700 nm notch filter, (8) 8-bit color GigE camera, (9) motorized remotely controlled mirror, (10) pyrometry signal fed into OceanOptics Jaz spectrometer (OceanOptics, Largo, FL, USA) via optical fiber, (11) light source for sample illumination.…”