Articles

August 1, 2016 06:24 PM

Materials combining the hardness and strength of diamond with the higher thermal stability of cubic boron nitride (cBN) have broad potential value in science and engineering. Reacting nanodiamond with cBN at high pressures and high temperatures provides a pathway to such materials. Postdoctoral associate Dr. Xiaobing Liu, with CDAC partner Steve Jacobsen at Northwestern University, report the fabrication of C2-BN nanocomposite (2:1 ratio of diamond to cBN), measuring up to 10 mm in longest dimension. The nanocomposite consists of randomly-oriented, diamond and cBN domains of 50-250 nm, stitched by sp3 hybridized C-B and C-N bonds. B-C-N solid solution within the sutures leads to bulk p-type semiconductivity with an activation energy of 6.2 meV. The new study, published in the 27 July issue of Scientific Reports, shows that dislocations near the sutures accommodate lattice mismatch between diamond and cBN.

June 8, 2016 04:52 PM

Iron and oxygen are two of the most geochemically important elements on Earth. The core is rich in iron and the atmosphere is rich in oxygen, and between them is the entire range of pressures and temperatures on the planet. Iron, in its multiple oxidation states, controls the oxygen fugacity and oxygen budget. Hydrogen has a key role in the reaction of Fe and O, causing iron to rust in humid air. Recent experiments at HPCAT identified a highly stable, pyrite-structured iron oxide (FeO2) at 76 GPa and 1,800 K that holds an excessive amount of oxygen. The “rust” mineral goethite, FeOOH, decomposes under the deep lower-mantle conditions to form FeO2 and release H2. The reaction could cause accumulation of the heavy FeO2-bearing patches in the deep lower mantle, upward migration of hydrogen, and separation of the oxygen and hydrogen cycles.

May 5, 2016 06:37 PM

Applied pressure is a powerful tool for studying intermediate valent systems because their electronic states are sensitive to small changes in interatomic separation. Recent HPCAT experiments using resonant x-ray emission spectroscopy show that the pressure dependence of the f-electron occupancy in the Kondo insulator SmB6. Applied pressure reduces the f occupancy, but surprisingly, the material maintains a significant divalent character up to a pressure of at least 35 GPa. Thus, the closure of the resistive activation energy gap and onset of magnetic order are not driven by stabilization of an integer valent state. Over the entire pressure range, the material maintains a remarkably stable intermediate valence that can in principle support a nontrivial band structure.

May 4, 2016 02:34 PM

Near the Earth’s core-mantle boundary (CMB) there occur isolated regions characterized by anomalously high density and low seismic wave velocity.  The non-uniform distribution of these so-called ultralow velocity zones (ULVZs) is evidence of chemical and/or thermal heterogeneity near the CMB.  The origin of ULVZs is not well understood, as the complex relationship among elemental composition, mobility, and melt near the CMB makes it difficult to account for the various observed anomalies.  A better understanding of the origin and evolution of ULVZs would play a crucial role in understanding the evolutionary history and planetary dynamics of the Earth as a whole.  Recent experiments at HPCAT’s laser heating endstation (16-ID-B) explore the melting curve of the iron-carbon system.  Results suggest the eutectic melting curve crosses the geotherm near the CMB, and thus it is possible that regions of dense metallic melt, originating from slab material

May 3, 2016 06:54 PM

The separation of iron metal from silicate to form Earth’s core represents a fundamental physical and chemical differentiation process in our planet’s history. A longstanding problem has been determining the minor element contribution to its predominately iron-nickel alloy. Isotope fractionation will exist between phases with distinct bonding environments (e.g., Earth’s core and mantle), and separation of elements between reservoirs manifests this fractionation. Recent HPCAT experiments using nuclear forward scattering technique show the effect of pressure on iron isotopic composition, which are found to vary according to the alloy tested (FeO, FeHx, or Fe3C versus pure Fe). These results suggest that hydrogen or carbon is not the parimary light-element component in the core. The pressure dependence of iron isotopic composition provides an a new and independent constraint on core composition.

May 1, 2016 06:31 PM

Depleted uranium and its alloys are important engineering materials due to their high density. It is well known that depleted uranium can be alloyed to improve several elastic and plastic related properties, to enhance corrosion resistance, and to allow flexible heat treatability. Recent HPCAT experiments using ultrasonic interferometry have determined the elasticity, mechanical, and thermal properties of depleted uranium to 4.5 GPa. By combining X-ray radiographic, diffractive, and ultrasonic interferometric measurements of the sample, a pressure–volume curve and sound velocities have been determined. Results show general strengthening with applied load, including an overall increase in acoustic thermal conductivity. In addition to the first set of elastic/thermal results on uranium metal at high pressure, this work has also demonstrated the capacity to make such measurements on radioactive and other potentially hazardous materials.

March 14, 2016 07:21 PM

Understanding the structural response of network forming glasses to pressure is of great interest not only in condensed matter physics, geoscience, and materials science, but also in engineering and industry.  However, polyamorphism in glasses under high pressure remains poorly understood, because of experimental challenges.  A new double-stage large volume cell has been developed at HPCAT to compress GeO2 glass samples to nearly 100 GPa for in situ structure measurements. The experiments revealed new evidence of ultrahigh pressure polyamorphism in GeO2 glass with Ge-O coordination number (CN) significantly greater than 6.  The CN change is closely associated with the change in oxygen packing fraction.

February 24, 2016 04:04 PM

Under various compression rates (0.03–13.5 GPa/s), kinetics of the B1-B2 phase transition in KCl has been studied in a dynamic diamond anvil cell using time-resolved x-ray diffraction and fast imaging. The recent HPCAT experiments show that the volume fraction across the transition generally gives sigmoidal curves as a function of pressure during rapid compression. Based upon classical nucleation and growth theories (Johnson-Mehl-Avrami-Kolmogorov theories), the fit of the experimental volume fraction as a function of pressure provides information on effective activation energy and average activation volume at a given compression rate. The resulting parameters are successfully used for interpreting several experimental observables that are compression-rate dependent, such as the transition time, grain size, and over-pressurization. [Lin et al., J. Appl. Phys. 119, 045902, 2016]

February 24, 2016 04:00 PM

A novel phase of nitrogen, λ-N2, has been synthesized through low temperature compression. The new phase exhibits an exceptionally wide range of pressure stability from below 1 to 140 GPa, overlapping nine other known phases. On heating, its transformations are different to those observed in other phases, implying that the phase nitrogen adopts depends not only on P-T path, but also on the initial structural configuration, which greatly complicates its phase diagram. Although the discovery of this new phase by performing compression at low temperature reconciles differences in experiment and theory, the P-T conditions under which λ-N2 is the thermodynamic equilibrium are unknown and pose an interesting challenge for future theoretical studies.