Larry K. Aagesen

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

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.

High‐Operating‐Temperature Direct Ink Writing of Mesoscale Eutectic Architectures

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 µm, enabling the manipulation of light in the visible and infrared range.

Phase-field simulations of GaN growth by selective area epitaxy from complex mask geometries

Three-dimensional phase-field simulations of GaN growth by selective area epitaxy were performed. The model includes a crystallographic-orientation-dependent deposition rate and arbitrarily complex mask geometries. The orientation-dependent deposition rate can be determined from experimental measurements of the relative growth rates of low-index crystallographic facets. Growth on various complex mask geometries was simulated on both c-plane and a-plane template layers. Agreement was observed between simulations and experiment, including complex phenomena occurring at the intersections between facets. The sources of the discrepancies between simulated and experimental morphologies were also investigated. The model provides a route to optimize masks and processing conditions during materials synthesis for solar cells, light-emitting diodes, and other electronic and opto-electronic applications.

Advancing quantitative description of porosity in autogenous laser-welds of 304L stainless steel

Porosity in linear autogenous laser welds of 304L stainless steel has been investigated using micro-computed tomography to reveal defect content in fifty-four welds made with varying delivered power, travel speed and focal lens. Trends associated with porosity size and frequencies are shown and interfacial measures are employed to provide quantitative descriptors of pore shape, directionality, interspacing and solid linear fraction. Lastly, the coefficient of variation associated with equivalent pore radii is reported toward a discussion of microstructural variability and the influence of process-parameters on such variability.

Coupling 3D Quantitative Interrogation of Weld Microstructure with 3D Models of Mechanical Response

Porosity resulting from linear autogenous laser-welds of 304L stainless steel are non-destructively examined and digitally reproduced by means of micro-computed tomography. These digitized microstructures are then imported into a finite element framework in which the pores are surrounded by an idealized, homogenized geometry, and exposed to a plastic strain-inducing failure load. Variations in equivalent plastic strain, strain at peak load and load-to-failure were all found to bear some correlation with the digitized microstructure’s local and global porosity content in simulation. Furthermore, experimental results show agreements in deformation trends predicted by simulation but reveal simulations underestimate both peak load and strain-to-failure.

Template-Directed Directionally Solidified Three-Dimensionally Mesostructured AgCl-KCl Eutectic Photonic Crystals.

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Porosity in millimeter-scale welds of stainless steel : three-dimensional characterization.

A variety of edge joints utilizing a continuous wave Nd:YAG laser have been produced and examined in a 304-L stainless steel to advance fundamental understanding of the linkage between processing and resultant microstructure in high-rate solidification events. Acquisition of three-dimensional reconstructions via micro-computed tomography combined with traditional metallography has allowed for qualitative and quantitative characterization of weld joints in a material system of wide use and broad applicability. The presence, variability and distribution of porosity, has been examined for average values, spatial distributions and morphology and then related back to fundamental processing parameters such as weld speed, weld power and laser focal length.

Influence of surface nano-patterning on the placement of InAs quantum dots

We have examined the influence of spontaneous nano-patterning on the placement of InAs quantum dots (QDs) on (Al)GaAs surfaces using an experimental-computational approach. Both atomically flat and mounded surfaces, generated via a surface instability induced by the Ehrlich-Schwoebel barrier, are employed as templates for the subsequent deposition of InAs QDs. Using height profiles from atomic-force micrographs, we simulate QD deposition with a 2D phase field model, which describes the time evolution of the InAs layer driven by a chemical potential gradient. For flat surfaces, phase-field simulations result in QD densities comparable to experimental observations. For mounded surfaces, the simulations reveal QDs preferentially positioned in regions of positive curvature (substrate valleys), e.g., at the edge of surface mounds, consistent with the anisotropic QD placement observed experimentally. We discuss the role of curvature-driven diffusion in the spontaneous ordering of QDs, demonstrating the applicability of this mechanism to AlGaAs mounds.We have examined the influence of spontaneous nano-patterning on the placement of InAs quantum dots (QDs) on (Al)GaAs surfaces using an experimental-computational approach. Both atomically flat and mounded surfaces, generated via a surface instability induced by the Ehrlich-Schwoebel barrier, are employed as templates for the subsequent deposition of InAs QDs. Using height profiles from atomic-force micrographs, we simulate QD deposition with a 2D phase field model, which describes the time evolution of the InAs layer driven by a chemical potential gradient. For flat surfaces, phase-field simulations result in QD densities comparable to experimental observations. For mounded surfaces, the simulations reveal QDs preferentially positioned in regions of positive curvature (substrate valleys), e.g., at the edge of surface mounds, consistent with the anisotropic QD placement observed experimentally. We discuss the role of curvature-driven diffusion in the spontaneous ordering of QDs, demonstrating the applicab...