Daniel P. Riley

Density functional modeling of the local structure of kaolinite subjected to thermal dehydroxylation.

Understanding the atomic-level changes that occur as kaolinite is converted (thermally dehydroxylated) to metakaolin is critical to the optimization of this large-scale industrial process. Metakaolin is X-ray amorphous; therefore, conventional crystallographic techniques do not reveal the changes in local structure during its formation. Local structure-based experimental techniques are useful in understanding the atomic structure but do not provide the thermodynamic information which is necessary to ensure plausibility of refined structures. Here, kaolinite dehydroxylation is modeled using density functional theory, and a stepwise methodology, where several water molecules are removed from the structure, geometry optimization is carried out, and then the process is repeated. Hence, the structure remains in an energetically and thermodynamically feasible state while transitioning from kaolinite to metakaolin. The structures generated during the dehydroxylation process are validated by comparison with X-ray and neutron pair distribution function data. Thus, this study illustrates one possible route by which dehydroxylation of kaolinite can take place, revealing a chemically, energetically, and experimentally plausible structure of metakaolin. This methodology of density functional modeling of the stepwise changes in a material is not limited in application to kaolinite or other aluminosilicates and provides an accurate representation of the local structural changes occurring in materials used in industrially important processes.

What is the structure of kaolinite? Reconciling theory and experiment.

Density functional modeling of the crystalline layered aluminosilicate mineral kaolinite is conducted, first to reconcile discrepancies in the literature regarding the exact geometry of the inner and inner surface hydroxyl groups, and second to investigate the performance of selected exchange-correlation functionals in providing accurate structural information. A detailed evaluation of published experimental and computational structures is given, highlighting disagreements in space groups, hydroxyl bond lengths, and bond angles. A major aim of this paper is to resolve these discrepancies through computations. Computed structures are compared via total energy calculations and validated against experimental structures by comparing computed neutron diffractograms, and a final assessment is performed using vibrational spectra from inelastic neutron scattering. The density functional modeling is carried out at a sufficiently high level of theory to provide accurate structure predictions while keeping computational requirements low enough to enable the use of the structures in large-scale calculations. It is found that the best functional to use for efficient density functional modeling of kaolinite using the DMol3 software package is the BLYP functional. The computed structure for kaolinite at 0 K has C1 symmetry, with the inner hydroxyl group angled slightly above the a,b plane and the inner surface hydroxyls aligned close to perpendicular to that plane.

Reaction of a Ni-coated Al nanoparticle to form B2-NiAl: A molecular dynamics study

The kinetic reaction in a Ni-coated Al nanoparticle with equi-atomic fractions and diameter of approximately 4.5 nm is studied by means of molecular dynamics simulation, using a potential of the embedded atom type to model the interatomic interactions. First, the large driving force for the alloying of Ni and Al initiates solid state amorphization of the nanoparticle with the formation of Ni50Al50 amorphous alloy. Amorphization makes intermixing of the components much easier compared to the crystalline state. The average rate of penetration of Ni atoms can be estimated to be about two times higher than Al atoms, whilst the total rate of inter-penetration can be estimated to be of the order of 10−2 m/s. The heat of the intermixing with the formation of Ni50Al50 amorphous alloy can be estimated at approximately −0.34 eV/at. Next, the crystallization of the Ni50Al50 amorphous alloy into B2-NiAl ordered crystal structure is observed. The heat of the crystallization can be estimated as approximately −0.08 eV/at. Then, the B2-NiAl ordered nanoparticle melts at a temperature of approximately 1500 K. It is shown that, for the alloying reaction in the initial Ni-coated Al nanoparticle, the ignition temperature can be as low as approximately 200 K, while the adiabatic temperature for the reaction is below the melting temperature of the nanoparticle with the B2-NiAl ordered structure.

A furnace and environmental cell for the in situ investigation of molten salt electrolysis using high-energy X-ray diffraction

This paper describes the design, construction and implementation of a relatively large controlled-atmosphere cell and furnace arrangement. The purpose of this equipment is to facilitate the in situ characterization of materials used in molten salt electrowinning cells, using high-energy X-ray scattering techniques such as synchrotron-based energy-dispersive X-ray diffraction. The applicability of this equipment is demonstrated by quantitative measurements of the phase composition of a model inert anode material, which were taken during an in situ study of an operational Fray-Farthing-Chen Cambridge electrowinning cell, featuring molten CaCl(2) as the electrolyte. The feasibility of adapting the cell design to investigate materials in other high-temperature environments is also discussed.

Quantification of passivation layer growth in inert anodes for molten salt electrochemistry by in situ energy-dispersive diffraction

An in situ energy-dispersive X-ray diffraction experiment was undertaken on operational titanium electrowinning cells to observe the formation of rutile (TiO2) passivation layers on Magneli-phase (TinO2n−1; n = 4–6) anodes and thus determine the relationship between passivation layer formation and electrolysis time. Quantitative phase analysis of the energy-dispersive data was undertaken using a crystal-structure-based Rietveld refinement. Layer formation was successfully observed and it was found that the rate of increase in layer thickness decreased with time, rather than remaining constant as observed in previous studies. The limiting step in rutile formation is thought to be the rate of solid-state diffusion of oxygen within the anode structure.

Structure of kaolinite and influence of stacking faults: Reconciling theory and experiment using inelastic neutron scattering analysis

The structure of kaolinite at the atomic level, including the effect of stacking faults, is investigated using inelastic neutron scattering (INS) spectroscopy and density functional theory (DFT) calculations. The vibrational dynamics of the standard crystal structure of kaolinite, calculated using DFT (VASP) with normal mode analysis, gives good agreement with the experimental INS data except for distinct discrepancies, especially for the low frequency modes (200-400 cm(-1)). By generating several types of stacking faults (shifts in the a,b plane for one kaolinite layer relative to the adjacent layer), it is seen that these low frequency modes are affected, specifically through the emergence of longer hydrogen bonds (O-H⋯O) in one of the models corresponding to a stacking fault of -0.3151a - 0.3151b. The small residual disagreement between observed and calculated INS is assigned to quantum effects (which are not taken into account in the DFT calculations), in the form of translational tunneling of the proton in the hydrogen bonds, which lead to a softening of the low frequency modes. DFT-based molecular dynamics simulations show that anharmonicity does not play an important role in the structural dynamics of kaolinite.

Characterization and dissolution of functionalized amorphous calcium phosphate biolayers using single-splat technology.

New processing routes and characterization techniques underpin further growth of biomaterials for improved performance and multifunctionality. This study investigates the characteristics and solubility of amorphous calcium phosphate (ACP) printed splats. Splats made from 20 to 60 μm molten hydroxyapatite particles were classified for shape (rounded/splashed) and cracking. Recoil of the spread droplet created a bowl-shaped splat. This has previously not been observed and could be related to the longer solidification time associated with solidification to an ACP. A central depression was created from 20 μm particles, but a bowl-shaped splat from 60 μm particles. Cracking was more prevalent for splats that solidified with an edge discontinuity. Splats immersed in pH 7.3 tris buffer displayed dissolution followed by cracking. Cracking continued over a period of 15 min as dissolution induced more cracks. Further degradation occurred by delamination of splat segments. Delamination accelerated the process of splat removal. Applied to thermal spray coatings, this highlights topography and dissolution at the splat level. The use of separate splats can potentially be used as a biolayer where splats are separate, in a line or on top of each other.

Ab initio calculations of the electronic structure and bonding characteristics of LaB{sub 6}

We have studied [Co{sub 2}MnGe/V]{sub N} multilayers with a thickness of the V layers t{sub V} between 1.5 and 10 nm and a fixed thickness of the Heusler layer t{sub Co{sub 2}}{sub MnGe}=3 nm by x-ray scattering, neutron reflectivity, and magnetization measurements. In the thickness range t{sub V}{ =}4 nm the multilayers undergo a cluster glass transition at T{sub f}{approx_equal}150 K. At high temperatures above T{sub N} or T{sub f} the mutilayers are superparamagnetic with a huge cluster magnetic moment {mu}{sub c}{>=}10{sup 5}{mu}{sub B}.The halides CaCl{sub 2},CaBr{sub 2}, and CrCl{sub 2} all adopt, at room temperature, the same distorted rutile structure, in orthorhombic space group Pnnm, known as the calcium chloride structure. Upon heating, CaCl{sub 2} and CaBr{sub 2} each undergoes a continuous transformation to the true tetragonal rutile structure, in space group P4{sub 2}/mnm, the transition temperatures being 235 and 553 deg. C, respectively. By contrast, the structural change in CrCl{sub 2} upon heating is just further elongation of octahedra already lengthened by Jahn-Teller effects, and no phase transition occurs. The orthorhombic structure is maintained by a strong and temperature-dependent geometrical coupling of the orthorhombic strain to the order parameter, represented by the tilt angle of the CrCl{sub 6} octahedron.The author has studied the influence of fermion-boson conversion on Mott states near Feshbach resonances. It is demonstrated that Mott states are unstable with respect to fermion-boson conversion. A branch of collective modes in superfluids has been found, which involve antisymmetric phase oscillations in fermionic and bosonic channels and are always gapped. The low-energy quantum dynamics of a Fermi-Bose superfluid can be fully characterized by either an effective coupled U(1)xU(1) quantum rotor Hamiltonian or a coupled XXZxXXZ spin model.Anisotropic Pr-Fe-B thin films with perpendicular texture have been prepared by magnetron sputtering and subsequent heat treatment. After crystallization at 873 K, the films deposited at 773 K show a perpendicular anisotropic texture, due to the existence of a weak anisotropy in the as-deposited films. The deposition rate has been proved to be an important parameter for the control of microstructure, morphology, and coercivity. The coercivity mechanism of the anisotropic Pr-Fe-B films has been studied by analyzing the temperature dependence of coercivity from 5 to 300 K, based on the micromagnetic model. Nucleation of reversed domains, taking place preferentially at the grain surface where the magnetic anisotropy is reduced and the local demagnetization field is the highest, is determined to be the leading mechanism in controlling the magnetization reversal processes of the anisotropic Pr-Fe-B films. Though the grains in the films are strongly magnetically coupled, the magnetization reversal processes in the Pr-Fe-B thin films are not realized by uniform rotations of the magnetic moments.Electron spin resonance was applied on samples of Gd{sub 5.09}Ge{sub 2.03}Si{sub 1.88}. The results are discussed under the scope of magnetization measurements, optical metallography, and wavelength dispersive spectroscopy. Polycrystalline arc-melted samples submitted to different heat treatments were investigated. The correlation of the electron spin resonance and magnetization results permitted a characterization of the present phases and their transitions. Two coexisting phases in the temperature range between two phase transitions have been identified and associated to distinct crystallographic phases. Additionally, the magnetic moment at high temperatures has been estimated from the measured effective g factor. A peak value of 21.5 J/kg K for the magnetocaloric effect was obtained for a sample heat treated at 1500 deg. C for 16 h.Geometrical models of neutral single vacancy-arsenic complexes are determined from first principles and used for atomistic simulation of Rutherford backscattering channeling (RBS-C) spectra in heavily As-doped crystalline silicon, both with and without compensating Si self-interstitials. The goal is to investigate whether the relaxation patterns of complexes containing different numbers (from 1 to 4) of As atoms can be used as a fingerprint in structural analysis by conventional RBS-C. Simulation of RBS-C spectra in million-atoms supercells containing a population of As{sub m}V, show the off-lattice displacement of the Si atoms neighboring the vacancy, due to Jahn-Teller effect. On the other side, As displacement is found to be similar in all clusters investigated. The present results suggest that in the case of samples equilibrated at high temperature, the lack of any significant disorder of Si atoms is consistent with the hypothesis of electrically inactive As being in the form of either As{sub 3}V or As{sub 4}V complexes.The ternary germanide La{sub 3}Pd{sub 4}Ge{sub 4} has been prepared by arc melting. This compound takes a body-centered lattice with an orthorhombic unit cell with the lattice parameters of a=4.2200(3) A, b=4.3850(3) A, and c=25.003(2) A. The crystal structure of La{sub 3}Pd{sub 4}Ge{sub 4} is U{sub 3}Ni{sub 4}Si{sub 4}-type with the space group of Immm, consisting of the combination of structural units of AlB{sub 2}-type and BaAl{sub 4}-type layers. This compound is a type-II superconductor with a critical temperature (T{sub c}) of 2.75 K. The lower critical field H{sub c1}(0) is estimated to be 54 Oe. The upper critical field H{sub c2}(0) estimated by linear extrapolation of the H{sub c2}(T) curves is about 4.0 kOe, whereas the Werthamer-Hefland-Hohemberg theory gives H{sub c2}(0){sup WHH}=3.0 kOe. This is an interesting observation of superconductivity in the compounds with U{sub 3}Ni{sub 4}Si{sub 4}-type structure. The coherence length {xi}(0) of 330 A and the penetration depth {lambda}(0) of 2480 A are derived.

Measuring helium bubble diameter distributions in tungsten with grazing incidence small angle x-ray scattering (GISAXS)

Grazing incidence small angle x-ray scattering was performed on tungsten samples exposed to helium plasma in the MAGPIE and Pisces-A linear plasma devices to measure the size distributions of resulting helium nano-bubbles. Nano-bubbles were fitted assuming spheroidal particles and an exponential diameter distribution. These particles had mean diameters between 0.36 and 0.62 nm. Pisces-A exposed samples showed more complex patterns, which may suggest the formation of faceted nano-bubbles or nano-scale surface structures.

Observation of a helium ion energy threshold for retention in tungsten exposed to hydrogen/helium mixture plasma

Helium retention is measured in tungsten samples exposed to mixed H/He plasma in the Magnum-PSI linear plasma device. It is observed that there is very little He retention below helium ion impact energies of 9.0 +- 1.4 eV, indicating the existence of a potential barrier which must be overcome for implantation to occur. The helium retention in samples exposed to plasma at temperatures  >1000 K is strongly correlated with nano-bubble formation measured using grazing incidence small-angle x-ray scattering. The diameters of nano-bubbles were not found to increase with increasing helium concentration, indicating that additional helium must be accommodated by increasing the bubble concentration or an increase in bubble pressure. For some samples pre-irradiation with heavy ions of 2.0 MeV energy is investigated to simulate the effects of neutron damage. It is observed that nano-bubble sizes are comparable between samples pre-irradiated with heavy-ions, and those without heavy-ion pre-irradiation.