Nikolas Antolin

Fast Free Energy Calculations for Unstable High-Temperature Phases

Dept. of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210(Dated: January 10, 2012)We present a fast and accurate method to calculate vibrational properties for mechanically un-stable high temperature phases that suffer from imaginary frequencies at zero temperature. Themethod is based on standard finite-difference calculations with optimized large displacements andis significantly more efficient than other methods. We demonstrate its application for calculationof phonon dispersion relations, free energies, phase transition temperatures, and vacancy formationenergies for body-centered cubic high-temperature phases of Ti, Zr, and Hf.

Probing carbonate in bone forming minerals on the nanometre scale

To devise new strategies to treat bone disease in an ageing society, a more detailed characterisation of the process by which bone mineralises is needed. In vitro studies have suggested that carbonated mineral might be a precursor for deposition of bone apatite. Increased carbonate content in bone may also have significant implications in altering the mechanical properties, for example in diseased bone. However, information about the chemistry and coordination environment of bone mineral, and their spatial distribution within healthy and diseased tissues, is lacking. Spatially resolved analytical transmission electron microscopy is the only method available to probe this information at the length scale of the collagen fibrils in bone. In this study, scanning transmission electron microscopy combined with electron energy-loss spectroscopy (STEM-EELS) was used to differentiate between calcium-containing biominerals (hydroxyapatite, carbonated hydroxyapatite, beta-tricalcium phosphate and calcite). A carbon K-edge peak at 290 eV is a direct marker of the presence of carbonate. We found that the oxygen K-edge structure changed most significantly between minerals allowing discrimination between calcium phosphates and calcium carbonates. The presence of carbonate in carbonated HA (CHA) was confirmed by the formation of peak at 533 eV in the oxygen K-edge. These observations were confirmed by simulations using density functional theory. Finally, we show that this method can be utilised to map carbonate from the crystallites in bone. We propose that our calibration library of EELS spectra could be extended to provide spatially resolved information about the coordination environment within bioceramic implants to stimulate the development of structural biomaterials.

Ab-Initio Calculation of Spectral Absorption Coefficients in Molten Fluoride Salts with Metal Impurities

Abstract Molten salts have been proposed as coolants for numerous advanced reactor designs. It is envisioned that these reactors, both fluoride-salt–cooled high-temperature reactors and molten-salt–fueled reactors will operate at high temperatures, where the radiative heat transfer properties of the salts may be required for accurate heat transfer analysis. Experimental challenges have prevented the measurement of absorption coefficients in most salts. In an attempt to fill this gap in data, the Vienna Ab-Initio Simulation Package is used in the present research to calculate the absorption coefficient resulting from photoelectric interactions in numerous molten salts. Ab-initio molecular dynamics is used to generate the amorphous structures of a variety of salts. The pure halide salts LiF, FLiNaK, and FLiBe, are shown to be optically clear through a wide portion of the electromagnetic spectrum. Conversely, the transition metal fluoride salt KF-ZrF4 is shown to be substantially opaque. As chromium is a known impurity of concern from the corrosion of steels in reactor environments, the effect on absorption of low levels of chromium in an otherwise transparent salt is investigated and found to significantly increase absorption at relevant wavelengths.

Super-X EDS Characterization of Chemical Segregation within a Superlattice Extrinsic Stacking Fault of a Ni- based Superalloy

Superalloys are essential materials for high temperature applications in aerospace and energy production. Improving the temperature capability of disk alloys by a modest 25°C could translate into approximately a 1% increase in aircraft engine efficiency, resulting in significant cost savings as well as benefit environmental impact by reducing carbon emissions. While superalloys are prime candidates for an ICME approach to accelerated alloy development, at present this approach is severely hampered by the lack of quantitative models that connect alloying effects to deformation mechanisms. This linkage is critical for developing quantitative, physics-based deformation models, but has been limited severely by the complexity of the alloys (typically 10 components or more). One aspect, critical to a successful model is accurate determination of the degree of segregation to defects, such as faults, in order to enable the identification of the compositional configuration of such defects.