945 research outputs found
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Extended first-principles thermochemistry for the oxidation of titanium tetrachloride
A detailed first-principles investigation of the gas-phase precursor chemistry of titanium tetrachloride (TiCl4) in an O2 environment is used to identify the thermodynamically most stable oxidation products. Candidate species are systematically proposed based on twelve manually defined base moieties in combination with possible functional groups attached to each moiety. The ground state geometry and vibrational frequencies for each candidate species are calculated using density functional theory at the B97-1/6-311+G(d,p) level of theory. A set of 2; 328 unique candidate species are found to be physically reasonable. Their thermochemical data are calculated by applying statistical thermodynamics. Standard enthalpies of formation are estimated, if unknown, by using a set of error-cancelling balanced reactions. An equilibrium composition analysis of a mixture of TiCl4/O2 (50 mol%) at 3 bar is performed to identify the thermodynamically stable products. At low temperatures, below approximately 700 K, trimer species are dominant. This is followed by a mid-temperature range of 700 to 1975 K where Ti2OCl6 is the most abundant species, before its thermodynamic stability decreases. Between 1200 and 1825 K TiCl4 is the most stable monomer. At temperatures above 1975 K TiOCl2 becomes the dominant species. This species has been measured experimentally. A structural analysis is used to suggest further potentially stable higher polymers and defines a starting point to investigate the mechanisms leading to the formation of titanium
dioxide (TiO2) particles
Method of extending hyperfine coherence times in Pr^3+:Y_2SiO_5
In this letter we present a method for increasing the coherence time of
praseodymium hyperfine ground state transitions in Pr^3+:Y_2SiO_5 by the
application of a specific external magnetic field. The magnitude and angle of
the external field is applied such that the Zeeman splitting of a hyperfine
transition is at a critical point in three dimensions, making the first order
Zeeman shift vanishingly small for the transition. This reduces the influence
of the magnetic interactions between the praseodymium ions and the spins in the
host lattice on the transition frequency. Using this method a phase memory time
of 82ms was observed, a value two orders of magnitude greater than previously
reported. It is shown that the residual dephasing is amenable quantum error
correction
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Numerical simulation and parametric sensitivity study of titanium dioxide particles synthesised in a stagnation flame
A detailed population balance model is used to simulate titanium dioxide
nanoparticles synthesised in a stagnation flame from titanium tetraisopropoxide (TTIP) precursor. A two-step simulation methodology is employed to apply the detailed particle model as a post-process to flame profiles obtained from a fully coupled simulation with detailed gas-phase chemistry, flow dynamics and a simple particle model. The detailed particle model tracks the size and coordinates of each primary in an aggregate, and is able to resolve the particle morphology, permitting direct comparison with experimental measurements through simulated TEM-style images. New sintering parameters, informed by molecular dynamics simulations in the literature, are introduced into the model to account for the sintering behaviour of sub-10 nm particles. Simulated primary and aggregate particle size distributions were in excellent
agreement with experimental measurements. A parametric sensitivity study found particle morphology to be sensitive to the sintering parameters, demonstrating the need to apply careful consideration to the sintering behaviour of nano-sized particles in modelling studies. The final particle morphology was not found to be sensitive to other model parameters
A unified evaluation of iterative projection algorithms for phase retrieval
Iterative projection algorithms are successfully being used as a substitute
of lenses to recombine, numerically rather than optically, light scattered by
illuminated objects. Images obtained computationally allow aberration-free
diffraction-limited imaging and the possibility of using radiation for which no
lenses exist. The challenge of this imaging technique is transfered from the
lenses to the algorithms. We evaluate these new computational ``instruments''
developed for the phase retrieval problem, and discuss acceleration strategies.Comment: 12 pages, 9 figures, revte
A cascade of magnetic field induced spin transitions in LaCoO3
We present magnetization and magnetostriction studies of the insulating
perovskite LaCoO3 in magnetic fields approaching 100 T. In marked contrast with
expectations from single-ion models, the data reveal two distinct first-order
spin transitions and well-defined magnetization plateaux. The magnetization at
the higher plateau is only about half the saturation value expected for spin-1
Co3+ ions. These findings strongly suggest collective behavior induced by
strong interactions between different electronic -- and therefore spin --
configurations of Co3+ ions. We propose a model of these interactions that
predicts crystalline spin textures and a cascade of four magnetic phase
transitions at high fields, of which the first two account for the experimental
data.Comment: 5 pages + supplementary materials, 5 figure
Accuracy of B(E2; 0+ -> 2+) transition rates from intermediate-energy Coulomb excitation experiments
The method of intermediate-energy Coulomb excitation has been widely used to
determine absolute B(E2; 0+ -> 2+) quadrupole excitation strengths in exotic
nuclei with even numbers of protons and neutrons. Transition rates measured
with intermediate-energy Coulomb excitation are compared to their respective
adopted values and for the example of 26Mg to the B(E2; 0+ -> 2+) values
obtained with a variety of standard methods. Intermediate-energy Coulomb
excitation is found to have an accuracy comparable to those of long-established
experimental techniques.Comment: to be published in Phys. Rev.
Elastic constants of beta-eucryptite: A density functional theory study
The five independent elastic constants of hexagonal -eucryptite have
been determined using density functional theory (DFT) total energy
calculations. The calculated values agree well, to within 15%, with the
experimental data. Using the calculated elastic constants, the linear
compressibility of -eucryptite parallel to the c-axis, , and
perpendicular to it, , have been evaluated. These values are in close
agreement to those obtained from experimentally known elastic constants, but
are in contradiction to the direct measurements based on a three-terminal
technique. The calculated compressibility parallel to the c-axis was found to
positive as opposed to the negative value obtained by direct measurements. We
have demonstrated that must be positive and discussed the implications
of a positive in the context of explaining the negative bulk thermal
expansion of -eucryptite.Comment: 3 eps figures, submitted for publicatio
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A systematic method to estimate and validate enthalpies of formation using error-cancelling balanced reactions
The Combustion Institute This paper presents an automated framework that uses overlapping subsets of reference data to systematically derive an informed estimate of the standard enthalpy of formation of chemical species and assess the consistency of the reference data. The theory of error-cancelling balanced reactions (EBRs) is used to calculate estimates of the standard enthalpy of formation. Individual EBRs are identified using linear programming. The first part of the framework recursively identifies multiple EBRs for specified target species. A distribution of estimates can then be determined for each species from which an informed estimate of the enthalpy is derived. The second part of the framework iteratively isolates inconsistent reference data and improves the prediction accuracy by excluding such data. The application of the framework is demonstrated for test cases from organic and inorganic chemistry, including transition metal complexes. Its application to a set of 920 carbon, hydrogen and oxygen containing species resulted in a rapid decrease of the mean absolute error for estimates of the enthalpy of formation of each species due to the identification and exclusion of inconsistent reference data. Its application to titanium-containing species identified that the available reference values of TiOCl and TiO(OH) 2 are inconsistent and need further attention. Revised values are calculated for both species. A comparison with popular high-level quantum chemistry methods shows that the framework is able to use affordable density functional theory (DFT) calculations to deliver highly accurate estimates of the standard enthalpy of formation, comparable to high-level quantum chemistry methods for both hydrocarbons and transition metal complexes
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