Oak Ridge National Laboratory

About: Oak Ridge National Laboratory is a facility organization based out in Oak Ridge, Tennessee, United States. It is known for research contribution in the topics: Neutron & Ion. The organization has 31868 authors who have published 73724 publications receiving 2633689 citations. The organization is also known as: ORNL.

Size-dependent structural transformations of hematite nanoparticles. 1. Phase transition.

Abstract: Using Fourier Transform InfraRed (FTIR) spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), and Transmission Electron Microscopy (TEM), we characterize the structure and/or morphology of hematite (α-Fe2O3) particles with sizes of 7, 18, 39 and 120 nm. It is found that these nanoparticles possess maghemite (γ-Fe2O3)-like defects in the near surface regions, to which a vibrational mode at 690 cm−1, active both in FTIR and Raman spectra, is assigned. The fraction of the maghemite-like defects and the net lattice disorder are inversely related to the particle size. However, the effect is opposite for nanoparticles grown by sintering of smaller hematite precursors under conditions when the formation of a uniform hematite-like structure throughout the aggregate is restricted by kinetic issues. This means that not only particle size but also the growth kinetics determines the structure of the nanoparticles. The observed structural changes are interpreted as size-induced α-Fe2O3 ↔ γ-Fe2O3 phase transitions. We develop a general model that considers spinel defects and absorbed/adsorbed species (in our case, hydroxyls) as dominant controls on structural changes with particle size in hematite nanoparticles, including solid-state phase transitions. These changes are represented by trajectories in a phase diagram built in three phase coordinates—concentrations of spinel defects, absorbed impurities, and adsorbed species. The critical size for the onset of the α → γ phase transition depends on the particle environment, and for the dry particles used in this study is about 40 nm. The model supports the existence of intermediate phases (protohematite and hydrohematite) during dehydration of goethite. We also demonstrate that the hematite structure is significantly less defective when the nanoparticles are immersed in water or KBr matrix, which is explained by the effects of the electrochemical double layer and increased rigidity of the particle environment. Finally, we revise the problem of applicability of IR spectroscopy to the lattice vibrations of hematite nanoparticles, demonstrating that structural comparison of different samples is much more reliable if it is based on the Eu band at about 460 cm−1 and the spinel band at 690 cm−1, instead of the A2u/Eu band at about 550 cm−1 used in previous work. The new methodology is applied to analysis of the reported IR spectra of Martian hematite.

Coupled-cluster computations of atomic nuclei

Abstract: In the past decade, coupled-cluster theory has seen a renaissance in nuclear physics, with computations of neutron-rich and medium-mass nuclei. The method is efficient for nuclei with product-state references, and it describes many aspects of weakly bound and unbound nuclei. This report reviews the technical and conceptual developments of this method in nuclear physics, and the results of coupled-cluster calculations for nucleonic matter, and for exotic isotopes of helium, oxygen, calcium, and some of their neighbors.

Fine-root production dominates response of a deciduous forest to atmospheric CO2 enrichment

Abstract: Fine-root production and turnover are important regulators of the biogeochemical cycles of ecosystems and key components of their response to global change. We present a nearly continuous 6-year record of fine-root production and mortality from minirhizotron analysis of a closed-canopy, deciduous sweetgum forest in a free-air CO2 enrichment experiment. Annual production of fine roots was more than doubled in plots with 550 ppm CO2 compared with plots in ambient air. This response was the primary component of the sustained 22% increase in net primary productivity. Annual fine-root mortality matched annual production, and the mean residence time of roots was not altered by elevated CO2, but peak fine-root standing crop in midsummer was significantly higher in CO2-enriched plots, especially deeper in the soil profile. The preferential allocation of additional carbon to fine roots, which have a fast turnover rate in this species, rather than to stemwood reduces the possibility of long-term enhancement by elevated CO2 of carbon sequestration in biomass. However, sequestration of some of the fine-root carbon in soil pools is not precluded, and there may be other benefits to the tree from a seasonally larger and deeper fine-root system. Root-system dynamics can explain differences among ecosystems in their response to elevated atmospheric CO2; hence, accurate assessments of carbon flux and storage in forests in a globally changing atmosphere must account for this unseen and difficult-to-measure component.