Publications by authors named "Larry K Aagesen"

8 Publications

  • Page 1 of 1

Phase-field model of oxidation: Kinetics.

Phys Rev E 2020 Feb;101(2-1):022802

Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA.

The kinetics of oxidation is examined using a phase-field model of electrochemistry when the oxide film is smaller than the Debye length. As a test of the model, the phase-field approach recovers the results of classical Wagner diffusion-controlled oxide growth when the interfacial mobility of the oxide-metal interface is large and the films are much thicker than the Debye length. However, for small interfacial mobilities, where the growth is reaction controlled, we find that the film increases in thickness linearly in time, and that the phase-field model naturally leads to an electrostatic overpotential at the interface that affects the prefactor of the linear growth law. Since the interface velocity decreases with the distance from the oxide vapor, for a fixed interfacial mobility, the film will transition from reaction- to diffusion-controlled growth at a characteristic thickness. For thin films, we find that in the limit of high interfacial mobility we recover a Wagner-type parabolic growth law in the limit of a composition-independent mobility. A composition-dependent mobility leads to a nonparabolic kinetics at small thickness, but for the materials parameters chosen, the deviation from parabolic kinetics is small. Unlike classical oxidation models, we show that the phase-field model can be used to examine the dynamics of nonplanar oxide interfaces that are routinely observed in experiment. As an illustration, we examine the evolution of nonplanar interfaces when the oxide is growing only by anion diffusion and find that it is morphologically stable.
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http://dx.doi.org/10.1103/PhysRevE.101.022802DOI Listing
February 2020

Grand-potential-based phase-field model for multiple phases, grains, and chemical components.

Phys Rev E 2018 Aug;98(2-1):023309

Department of Nuclear Engineering, Texas A&M University, AI Engineering Building, College Station, Texas 77843, USA.

Grand-potential-based phase-field model for multiple phases, grains, and chemical components is derived from a grand-potential functional. Due to the grand-potential formulation, the chemical energy does not contribute to the interfacial energy between phases, simplifying parametrization and decoupling interface thickness from interfacial energy, which can potentially allow increased interface thicknesses and therefore improved computational efficiency. Two-phase interfaces are stable with respect to the formation of additional phases, simplifying implementation and allowing the variational form of the evolution equations to be used. Additionally, we show that grand-potential-based phase-field models are capable of simulating phase separation, and we derive conditions under which this is possible.
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http://dx.doi.org/10.1103/PhysRevE.98.023309DOI Listing
August 2018

High-Operating-Temperature Direct Ink Writing of Mesoscale Eutectic Architectures.

Adv Mater 2017 Feb 15;29(7). Epub 2016 Dec 15.

John A. Paulson School of Engineering and Applied Sciences, Wyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge, MA, 02138, USA.

High-operating-temperature direct ink writing (HOT-DIW) of mesoscale architectures that are composed of eutectic silver chloride-potassium chloride. The molten ink undergoes directional solidification upon printing on a cold substrate. The lamellar spacing of the printed features can be varied between approximately 100 nm and 2 ┬Ám, enabling the manipulation of light in the visible and infrared range.
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http://dx.doi.org/10.1002/adma.201604778DOI Listing
February 2017

Template-Directed Directionally Solidified 3D Mesostructured AgCl-KCl Eutectic Photonic Crystals.

Adv Mater 2015 Aug 14;27(31):4551-9. Epub 2015 Jul 14.

Department of Materials Science and Engineering, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA.

3D mesostructured AgCl-KCl photonic crystals emerge from colloidal templating of eutectic solidification. Solvent removal of the KCl phase results in a mesostructured AgCl inverse opal. The 3D-template-induced confinement leads to the emergence of a complex microstructure. The 3D mesostructured eutectic photonic crystals have a large stop band ranging from the near-infrared to the visible tuned by the processing.
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http://dx.doi.org/10.1002/adma.201502265DOI Listing
August 2015

Resonance amplification of defect emission in ZnO-inverted opal.

Opt Lett 2009 May;34(10):1519-21

Tyndall National Institute, University College Cork, Lee Maltings, Cork, Ireland.

Transformation of broadband emission of oxygen defects in the carcass of ZnO-inverted opal into a multiple-mode amplified spontaneous emission band has been observed in the spectral interval of a photonic bandgap upon increasing excitation intensity. The mode structure has been assigned to amplification of emission coupled to resonance modes of the self-selected distributed Bragg resonator. The surprisingly low 2 W/cm(2) onset of amplification has been explained by the long radiative decay time of defect states populated according to the three-level excitation scheme. The decrease of emission intensity between amplified peaks has been associated with the saturation of the ZnO defect emission.
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http://dx.doi.org/10.1364/ol.34.001519DOI Listing
May 2009

Growth of highly ordered colloidal photonic crystals using a modeling approach.

Nanotechnology 2008 Oct 22;19(43):435204. Epub 2008 Sep 22.

Materials Science and Engineering Department, Materials Research Institute, Northwestern University, Evanston, IL 60208, USA.

Colloidal photonic crystals were grown using a vertical deposition method. The effect of colloidal concentration and deposition rate on crystalline quality and domain size was studied by means of response surface design. The crystalline quality was assessed using the optical reflectance. The results show that a strong negative correlation exists between the reflectance and the full width at half-maximum (FWHM), as illustrated by the close optimal region between maximizing reflectance and minimizing FWHM. For reflectance and FWHM, the quadratic effect of colloidal concentration is highly significant. For domain size, the colloidal concentration-lifting speed interaction effect was found to be significant. The observed colloidal concentration-lifting speed interaction effect explains the controversy that a low deposition rate is desirable at low colloidal concentrations, whereas a high deposition rate is favorable at high colloidal concentrations. Predictive models relating the important factors to the reflectance, FWHM, and domain size are proposed in the paper. The resulting optimal recipe shows a well-ordered structure with good optical reflectance, consistent with the prediction from modeling.
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http://dx.doi.org/10.1088/0957-4484/19/43/435204DOI Listing
October 2008

Chemosorption-related shift of a photonic bandgap in photoconductive ZnO inverse opal.

Opt Lett 2008 Mar;33(5):461-3

Tyndall National Institute, University College Cork, Lee Maltings, Cork, Ireland.

A change of up to 40% of the relative transmission at the photonic bandgap edge has been observed in photoconductive inverted ZnO opals under ultraviolet laser irradiation. This effect has been related to the irradiation-stimulated change of the refraction index of the photonic crystal. The desorption (chemosorption) of oxygen molecules on the surface of the ZnO backbone leading to destruction (formation) of a depletion layer at the ZnO surface has been suggested as the mechanism responsible for the slow variation of polarizability of the inverted ZnO opal.
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http://dx.doi.org/10.1364/ol.33.000461DOI Listing
March 2008
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