Articles

March 20, 2015 06:56 PM

Compressing single-crystal coesite ​SiO2 under hydrostatic pressures of 26–53 GPa at room temperature, HPCAT experiments show a new polymorphic phase transition featured by the formation of multiple previously unknown triclinic phases of ​SiO2 on the transition pathway as structural intermediates. These intermediate phases are accompanied by extensive splitting of the original coesite X-ray diffraction peaks, appearing as dramatic peak broadening and weakening, resembling an amorphous material. This work provides new insights into the densification mechanism of tetrahedrally bonded structures common in nature. (Q.Y. Hu, et al, Nature Communications, 6, doi 10.1038, 2015)

 

March 3, 2015 04:57 PM

As iron is heated, the arrangement of the atoms in the solid changes several times before the iron finally melts.  This unusual behavior is one reason why steel is so strong. The atomic-level details of how and why iron takes on so many different forms during heating remains a mystery, however. Recent work by Caltech CDAC Partner Brent Fultz, current and former Caltech CDAC students Lisa Mauger, Matthew Lucas, and Jorge Muñoz; and HPCAT Beamline Scientists Yuming Xiao and Paul Chow provides evidence for how iron's magnetism plays a role in the melting behavior of iron, and it is this detailed understanding that could help metallurgists develop better and stronger steel.

February 11, 2015 12:16 PM

Energetic radiation can cause dramatic changes in the physical and chemical properties of actinide materials, degrading their performance in fission-based energy systems. The radiation tolerance, referring to the ability to retain atomic structures and properties during irradiation, is of primary concern for the design of nuclear fuels with long operating lifetimes, adequate performance in reactor accident scenarios, and management of wasteforms. While the effect of displacive radiation producing damage through atomic displacement caused by elastic collisions of nuclei has been relatively well studied, the effect of highly ionizing radiation is poorly understood.

February 9, 2015 05:15 PM

Carbon materials are known to possess strong chemical bonds with sp, sp2, and sp3 hybridizations, displaying an impressively rich variety of atomic arrangements, such as graphene, fullerenes, nanotubes, in addition to the well-known crystalline forms of graphite and diamond. These carbon allotropes display distinct mechanical properties, originating from the difference in atomic structures. Searching for new carbon allotropes with unusual physical properties has long been a subject of extensive studies.  Type-II glass-like ​carbon is a widely used material with a unique combination of properties including low density, high strength, extreme impermeability to gas and liquid and resistance to chemical corrosion. It can be considered as a ​carbon-based nanoarchitectured material, consisting of a disordered multilayer graphene matrix encasing numerous randomly distributed nanosized fullerene-like spheroids.

December 4, 2014 06:09 PM

One-dimensional sp3 carbon nanomaterials is formed by high pressure solid state reaction of benzene. They form in close packed bundles of nano-carbon threads capped with hydrogen. These nano threads promise extraordinary properties such as strength and stiffness higher than that of sp2 carbon nanotubes and polymers. The core of the nanothreads is a long, thin strand of carbon atoms arranged just like the fundamental unit of a diamond's structure -- zig-zag "cyclohexane" rings of six carbon atoms bound together, in which each carbon is surrounded by others in the strong triangular-pyramid shape of a tetrahedron. (T.C. Fitzgibbons, et al, Nature Materials, 21 SEPTEMBER 2014, DOI: 10.1038/NMAT4088)

November 21, 2014 03:32 PM

Extensive experimental studies of nitrogen have revealed a rich and varied phase diagram, with several molecular solids at modest pressures and temperatures, and several molecular, amorphous, and polymeric solids and liquids at ultrahigh pressures and temperatures. Many more phases have been predicted from computational work. Recent attempts to synthesize a novel phase of nitrogen at pressures and temperatures exceeding 125 GPa and 3000 K (well above the synthesis conditions of the polymeric cubic gauche phase) have yielded a single-bonded, layered polymeric nitrogen (LP-N) phase with a remarkable structure and similarly remarkable properties. The new structure is characterized by 3D (cg-N) to 2D transition to the theoretically predicted Pba2 structure, consisting of seven-membered N-N rings.

November 18, 2014 08:35 PM

In order for silicon to be more attractive for use in new technology, its indirect band gap needs to be altered. A new form of silicon has been synthesized with a quasi-direct band gap that falls within the desired range for solar absorption, something that has never before been achieved. The silicon is an open-framework allotrope in the same way that diamonds and graphite are both forms of carbon. Unlike the conventional diamond structure, this new allotrope consists of an interesting zeolite-type structure, which is comprised of channels with five-, six-and eight-membered silicon rings.  The new compound was obtained using a novel high-pressure precursor process. First, a compound of silicon and sodium, Na4Si24, was formed under high-pressure conditions. Next, this compound was recovered to ambient pressure, and the sodium was completely removed by heating under vacuum.

October 20, 2014 11:31 AM

A pressure-volume isotherm in cerium metal at 1100 K was measured in a large volume press of the Paris-Edinburgh type up to 6 GPa. The volume was determined by imaging a rectangular shape of the sample via white X-ray radiography. Energy dispersive x-ray diffraction spectra were recorded to ensure that the highly reactive cerium in the cell assembly remained pure at this temperature. Even at 1100 K the p-V equation of state of liquid cerium shows a pronounced decrease of the bulk modulus above the gamma-phase region similar to the 775 K isotherm in the solid that also shows an inflection point between gamma and alpha-type cerium. The inflection point in the 1100 K isotherm indicating the minimum in the bulk modulus separating the gamma from the alpha-type liquid is located at approximately 3.5 GPa. (M.J. Lipp, et al., J. Phys., Conference Series, 500, 032011, 2014)

October 20, 2014 12:42 PM

The material 2,4,6-trinitrotoluene (TNT) is a molecular explosive that exhibits chemical stability in the molten phase at ambient pressure. The experiments partially performed at HPCAT show that the chemical stability of molten TNT is limited, existing in a small domain of pressure-temperature conditions below 2 GPa. Decomposition dominates the phase diagram at high temperatures beyond 6 GPa. From the calculated bulk temperature rise, it is unlikely that TNT melts on its principal Hugoniot. The work is published in D.M. Dattelbaum, et al, Appl. Phys. Lett., 104, 021911, (2014)