Researchers at Lawrence Livermore National Laboratory, Livermore, Calif., report that they have compressed iron up to 5.6 million atmospheres, a record pressure for solid iron. The Lab’s Yuan Ping used the Omega laser at the Laboratory for Laser Energetics at the University of Rochester, N.Y., to achieve the record pressure by multi-shock compression. A series of shocks (rather than a single shock) keeps the entropy low while compressing the material, which is key to keeping the temperature lower than the melting point, so the iron will remain solid.
Diagnosing the material properties under extreme conditions is as important as the creation of high-pressure states. The team used an X-ray technique called EXAFS (extended X-ray absorption fine structure). EXAFS is a powerful tool widely used in materials science, but its application in materials under extreme conditions is still in its infancy. This work presents the first EXAFS data in high-energy-density (HED) matter.
The EXAFS data show that the close-packed structure of iron is stable in the regime explored, confirming simulation predictions and other experimental studies by X-ray diffraction up to 3 million atmospheres. Unexpectedly, the team found that the temperature at peak compression is significantly higher than that from pure compressive work. Extra heat is generated by inelastic distortion of the lattice, termed plastic work. It is found that upon fast compression in a few billionths of a second, the strength of iron is enhanced, leading to more plastic work and the elevated temperature.
