(Originally published by Caltech)
April 5, 2018
Thousands of years ago, a 4.5 billion-year-old meteorite crashed into eastern Russia’s Koryak Mountains. The space rock was named the Khatyrka meteorite and was later found to contain certain properties that stunned the scientific community. Specifically, the meteorite includes exotic types of solid known as quasiperiodic crystals, or quasicrystals, that weren’t believed to exist in nature before 2009. At the nanoscale, quasicrystals display exceptional atomic patterns that fall outside the standard rules for true periodic crystals. Given their unique structure, synthetic quasicrystals can be designed to have useful properties, such as very high or very low thermal conductivity. Man-made quasicrystals were first produced in 1984 and have since found useful applications in a variety of items ranging from medical devices to cookware.
The intrigue surrounding quasicrystals, however, isn’t only about what products can be made from them. Scientists are interested in studying prehistoric extraterrestrial structures that date back millions or billions of years. Learning more about them may unlock clues as to how our asteroids, planets and solar system evolved.
Caltech professor of Geological and Planetary Sciences Paul Asimow has been studying the products created by hypervelocity impacts, which occur regularly in the asteroid belt and have affected many meteorite samples. Because the Khatyrka meteorite, too, shows evidence of having been shocked, he set out to test whether the natural quasicrystals and the shocks might be related. Recently his group members have executed experiments using a 20 mm bore, 4 m long propellant gun to recreate astronomical circumstances that might have caused the formation of the quasicrystalline structures within the Khatyrka meteorite. Their publication “Shock Synthesis of Decagonal Quasicrystals” released in the November 2017 issue of Nature’s Scientific Reports, confirms that decagonal quasicrystals – quasicrystals with 10-fold symmetrical patterns – can indeed be created in shock recovery experiments.
The experiments involved shooting a tantalum flyer into a disk comprised of stainless steel 304, aluminum alloy and Ni-rich permalloy 80 at a velocity of about 1,040 meters per second. Pieces of the resulting collision were then examined in part at the Kavli Nanoscience Institute’s cleanroom facilities using Scanning Electron Microscopy (SEM) and Focused Ion Beam (FIB) milling for Transmission Electron Microscopy (TEM) analysis to investigate their atomic structures. Postdoctoral Scholar Jinping Hu also utilized the Zeiss ORION NanoFab’s Ga beam to thin the sample to ~100 nm.
The results of the shock recovery experiments proved that decagonal quasicrystals very similar to the natural ones found in the Khatyrka meteorite can be reproduced directly by such shock synthesis. Moreover, a second publication by the Asimow group demonstrates the ability to replicate icosahedral quasicrystals (also found in the Khatyrka meteorite) via the same method.
Undergraduate researcher Julius Oppenheim writes, “the extraordinary ease with which unmixed metals mix to form quasicrystals under shock stands in stark contrast to conventional metallurgical synthesis methods, which require intimate mixing and controlled quenching of very specific bulk compositions” (“Shock Synthesis of Decagonal Quasicrystals”, p. 10). Further investigation into the enigmatic properties of quasicrystals will help us understand more about the foundation of our solar system and may also serve to improve our understanding of the materials we have here on earth.