Silicon is one of the most abundant elements in nature. Among its other traits, it's also a good stand-in to study how materials behave in planetary interiors. Its behavior when shock compressed was a subject of vigorous debate. Using a novel experimental geometry, scientists revealed how silicon acts. They found that many of the assumptions made about silicon were incorrect.
When scientists want to know more about what happens inside planets, they often try to imitate the conditions in the lab. They need to subject samples to intense temperatures and pressures at the same time. This study demonstrates a novel experimental configuration that creates such conditions. The configuration is sensitive to transformations. It also offers exact data, something other techniques couldn't.
In addition to being a key material in the semi-conductor industry, and one of the most studied materials at high pressure, silicon as a ceramic-like material is an ideal model system to investigate the behavior of more complex ceramics enstatite (MgSiO3) found inside planets and exoplanets.
Using laser-driven shock compression techniques at the Matter in Extreme Conditions endstation, combined with the extremely bright, time-resolved and collimated X-rays produced by the Linac Coherent Light Source, the world's first hard X-ray free electron laser, scientists revealed the nature of the shear-relieving response of silicon for the first time. Rather than plastic deformation, the nature of the shear-relieving mechanism at the elastic limit was a solid-solid phase transition to the metallic b-tin phase. The team observed the pressure of this phase transition boundary was significantly lower than its hydrostatic counterpart, contrary to the previous belief that phase transitions would move to higher pressures under shock compression due to being kinetically hindered.
Finally, the novel experimental configuration was more sensitive to the onset of both solid-solid and solid-liquid phase transformations, and the onset of the liquid phase boundary was in excellent agreement with the hydrostatically determined melting line. This suggests that in ceramic materials, while solid-solid phase boundaries may be affected by uniaxial shock compression, solid-liquid boundaries are not. Such considerations are very important when using such methods to explore materials at planetary core conditions.
Emma Elizabeth McBride
SLAC National Accelerator Laboratory
The Volkswagen Foundation (E.E.M, A.S.), Engineering and Physical Sciences Research Council (United Kingdom) (J.S.W), and the French Agence Nationale de la Recherche funded this work. Use of the Linac Coherent Light Source, SLAC National Accelerator Laboratory, is supported by the Department of Energy, Office of Science, Office of Basic Energy Sciences. The Matter in Extreme Conditions instrument is supported by the Department of Energy, Office of Science, Office of Fusion Energy Sciences.
E.E. McBride, A. Krygier, A. Ehnes, et al., "Phase transition lowering in dynamically compressed silicon." Nature Physics (2018). [DOI: 10.1038/s41567-018-0290-x]
Deutsches Elektronen-Synchrotron press release: Silicon in a state of shock