U.S. Air Force Research Laboratory Materials and Manufacturing Directorate

Films subsumed with topological defects are transformed into complex, topographical surface features with light irradiation of azobenzene-functionalized liquid crystal polymer networks (azo-LCNs). Using a specially designed optical setup and photoalignment materials, azo-LCN films containing either singular or multiple defects with strengths ranging from |½| to as much as |10| are examined. The local order of an azo-LCN material for a given defect strength dictates a complex, mechanical response observed as topographical surface features.

Sulfur is a very promising cathode material for rechargeable energy storage devices. However, sulfur cathodes undergo a noticeable volume variation upon cycling, which induces mechanical stress. In spite of intensive investigation of the electrochemical behavior of the lithiated sulfur compounds, their mechanical properties are not very well understood. In order to fill this gap, we developed a ReaxFF interatomic potential to describe Li-S interactions and performed molecular dynamics (MD) simulations to study the structural, mechanical, and kinetic behavior of the amorphous lithiated sulfur (a-LixS) compounds. We examined the effect of lithiation on material properties such as ultimate strength, yield strength, and Young's modulus. Our results suggest that with increasing lithium content, the strength of lithiated sulfur compounds improves, although this increment is not linear with lithiation. The diffusion coefficients of both lithium and sulfur were computed for the a-LixS system at various stages of Li-loading. A grand canonical Monte Carlo (GCMC) scheme was used to calculate the open circuit voltage profile during cell discharge. The Li-S binary phase diagram was constructed using genetic algorithm based tools. Overall, these simulation results provide insight into the behavior of sulfur based cathode materials that are needed for developing lithium-sulfur batteries.

Abstract Phototuning of more than 2000 nm is demonstrated in an azobenzene‐based cholesteric liquid crystal (azo‐CLC) consisting of a high‐helical‐twisting‐power, axially chiral bis(azo) molecule (QL76). Phototuning range and rate are compared as a function of chiral dopant concentration, light intensity, and thickness. CLCs composed of QL76 maintain the CLC phase regardless of intensity or duration of exposure. The time necessary for the complete restoration of the original spectral properties (position, bandwidth, baseline transmission, and reflectivity) of QL76‐based CLC is dramatically reduced from days to a few minutes by polymer stabilization of the CLC helix.

Abstract The facile synthesis of well‐aligned, main‐chain liquid crystalline elastomers that retain the cholesteric phase (CLCEs) is reported. The selective reflection inherent to this phase is thermally tuned more than 200 nm in these solid films, across the visible spectrum. The optical response is directly correlated to thermomechanical expansion of the CLCE film thickness. The bandwidth of the selective reflection of the CLCEs is increased to more than 200 nm by the incorporation of photosensitive chiral dopants that introduce heterogeneity in the pitch distribution. The mirror‐like reflection of this CLCE film is also thermochromic, shifting from the visible to infrared. Reminiscent of cephalopods, when combined with the mechanical deformation of voxelated nematic liquid crystal elastomer, the thermochromic response of the CLCE produces solid‐state elements with concurrent variation of specular and diffuse reflectance. These results demonstrate distinctive potential opportunities for liquid crystal elastomers to control light enabling new application in textiles, optics, and architecture.

Microsphere‐assisted imaging has emerged as an extraordinary simple technique of obtaining optical super‐resolution. This work addresses two central problems in developing this technology: i) methodology of the resolution measurements and ii) limited field‐of‐view provided by each sphere. It is suggested that a standard method of resolution analysis in far‐field microscopy based on convolution with the point‐spread function can be extended into the super‐resolution area. This allows developing a unified approach to resolution measurements, which can be used for comparing results obtained by different techniques. To develop the surface scanning functionality, the high‐index ( n ∼ 2) barium titanate glass microspheres were embedded in polydimethylsiloxane (PDMS) thin‐films. It is shown that such films adhere to the surface of nanoplasmonic structures so that the tips of embedded spheres experience the objects’ optical near‐fields. Based on rigorous criteria, the resolution ∼ λ /6‐ λ /7 (where λ is the illumination wavelength) is demonstrated for arrays of Au dimers and bowties. Such films can be translated along the surface of investigated samples after liquid lubrication. It is shown that just after lubrication the resolution is diffraction limited, however the super‐resolution gradually recovers as the lubricant evaporates. image

Marine-based deoxyribonucleic acid (DNA), purified from waste products of the Japanese fishing industry, has recently become a material of interest in photonics applications. Using highly purified DNA, unique processing techniques developed specifically to transform the purified DNA into a biopolymer suitable for optical device fabrication are reported.

A universal approach to develop 3D printable, free-standing, and current collector-embedded electrode inks has been established.

Interactions between two layered silicate sheets, as found in various nanoscale materials, are investigated as a function of sheet separation using molecular dynamics simulation. The model systems are periodic in the xy plane, open in the z direction, and subjected to stepwise separation of the two silicate sheets starting at equilibrium. Computed cleavage energies are 383 mJ /m(2) for K-mica, 133 mJ /m(2) for K-montmorillonite (cation exchange capacity=91), 45 mJ /m(2) for octadecylammonium (C(18))-mica, and 40 mJ /m(2) for C(18)-montmorillonite. These values are in quantitative agreement with experimental data and aid in the molecular-level interpretation. When alkali ions are present at the interface between the silicate sheets, partitioning of the cations between the surfaces is observed at 0.25 nm separation (mica) and 0.30 nm separation (montmorillonite). Originally strong electrostatic attraction between the two silicate sheets is then reduced to 5% (mica) and 15% (montmorillonite). Weaker van der Waals interactions decay within 1.0 nm separation. The total interaction energy between sheets of alkali clay is less than 1 mJ /m(2) after 1.5 nm separation. When C(18) surfactants are present on the surfaces, the organic layer (>0.8 nm) acts as a spacer between the silicate sheets so that positively charged ammonium head groups remain essentially in the same position on the surfaces of the two sheets at any separation. As a result, electrostatic interactions are efficiently shielded and dispersive interactions account for the interfacial energy. The flexibility of the hydrocarbon chains leads to stretching, disorder, and occasional rearrangements of ammonium head groups to neighbor cavities on the silicate surface at medium separation (1.0-2.0 nm). The total interaction energy amounts to less than 1 mJ /m(2) after 3 nm separation.

Experimental data on the oxidation kinetics of SiC ‐containing diborides of Zr and Hf in the temperature regime of 1473–2273 K are interpreted using a mechanistic model. The model encompasses counter‐current gas diffusion in the internal SiC depleted zone, oxygen permeation through borosilicate glass channels in the oxide scale, and boundary layer evaporation at the surface. The model uses available viscosity, thermodynamic and kinetic data for boria, silica, and borosilicate glasses, and a logarithmic mean approximation for compositional variations. The internal depletion region of SiC is modeled with CO/CO 2 counter diffusion as the oxygen transport mechanism. Data reported for pure SiC in air/oxygen, for ZrB 2 containing varying volume fractions of SiC , and for SiC–HfB 2 ultra‐high temperature ceramics ( UHTC s) by different investigations were compared with quantitative predictions of the model. The model is found to provide good correspondence with laboratory‐furnace‐based experimental data for weight gain, scale thicknesses, and depletion layer thicknesses. Experimental data obtained from arc‐jet tests at high enthalpies are found to fall well outside the model predictions, whereas lower enthalpy data were closer to model predictions, suggesting a transition in mechanism in the arc‐jet environment.

Liquid crystal elastomers with tunable actuation strain are synthesized with simple techniques that enable complexly patterned actuation.

Polar polyimides with added CN dipoles exhibit higher discharged energy density than nonpolar polyimides such as Ultem and Kapton.

Artificial hair sensors consisting of a piezoresistive carbon-nanotube-coated glass fiber embedded in a microcapillary are assembled and characterized. Individual sensors resemble a hair plug that may be integrated in a wide range of host materials. The sensors demonstrate an air-flow detection threshold of less than 1 m/s with a piezoresistive sensitivity of 1.3% per m/s air-flow change.

Cu-rich surface defects on MOF crystallites lead to enhanced polysulfide uptake and improved capacity retention in lithium sulfur batteries.

Research on fatigue crack formation from a corroded 7075-T651 surface provides insight into the governing mechanical driving forces at microstructure-scale lengths that are intermediate between safe life and damage tolerant feature sizes. Crack surface marker-bands accurately quantify cycles (Ni) to form a 10–20 μm fatigue crack emanating from both an isolated pit perimeter and EXCO corroded surface. The Ni decreases with increasing-applied stress. Fatigue crack formation involves a complex interaction of elastic stress concentration due to three-dimensional pit macro-topography coupled with local micro-topographic plastic strain concentration, further enhanced by microstructure (particularly sub-surface constituents). These driving force interactions lead to high variability in cycles to form a fatigue crack, but from an engineering perspective, a broadly corroded surface should contain an extreme group of features that are likely to drive the portion of life to form a crack to near 0. At low-applied stresses, crack formation can constitute a significant portion of life, which is predicted by coupling macro-pit and micro-feature elastic–plastic stress/strain concentrations from finite element analysis with empirical low-cycle fatigue life models. The presented experimental results provide a foundation to validate next-generation crack formation models and prognosis methods.

Self-organized 3D photonic superstructures loaded with plasmonic hybrid nanorods were found to undergo structural transformation from body-centered cubic to simple cubic upon NIR-light irradiation resulting from the "photothermal effect" of gold nanorods. Furthermore, dynamic NIR light-directed red, green and blue reflections of the nanocomposites were demonstrated.

Three-dimensional electron backscatter diffraction data, obtained by serial sectioning a nickel–base superalloy, has been analysed to measure the geometric arrangement of grain boundary planes at triple junctions. This information has been used to calculate the grain boundary character distribution (GBCD) and the grain boundary energy distribution (GBED). The twin content from the three-dimensional GBCD calculation compares favourably with the twin content estimated by stereology. Important factors in the analysis are the alignment of the parallel layers, the ratio of the out-of-plane to in-plane spacing of the discrete orientation data and the discretisation of the domain of grain boundary types. The results show that grain boundaries comprised of (111) planes occur most frequently and that these grain boundaries have a relatively low energy. The GBCD and GBED are inversely correlated.

Sharp leading edge (LE) samples of UHTC (20 vol% SiC – HfB 2 ) and SiC were exposed to simulated hypersonic flight conditions using a direct‐connect scramjet rig and their thermal and oxidation responses measured. The measured back‐wall temperatures and scale thicknesses were significantly smaller than might be expected from stagnation temperatures at the LE. Furthermore, the scale that formed around the LE was more uniform than expected from the steep drop in cold wall heat flux with distance from the tip. These results were interpreted and rationalized using physics‐based models. An aerothermal model in combination with an oxidation model accounted for the observed scale thicknesses at the tip and their slight variation with distance. The scale thicknesses were similar to values reported for exposures in furnaces at temperatures calculated for the tip, but less than those reported in arc jet tests. The formation of hafnon (HfSiO 4 ) and the absence of external glassy layer and of silica in the outer portions of the oxide region are unique to scramjet tested samples, presumably due to the high fluid flow (high shear and evaporation) rates.

Optically reconfigurable monodisperse chiral microspheres of self-organized helical superstructures with dynamic chirality were fabricated via a capillary-based microfluidic technique. Light-driven handedness-invertible transformations between different configurations of microspheres were vividly observed and optically tunable RGB photonic cross-communications among the microspheres were demonstrated.

Laser acceleration of ions to ≳MeV energies has been achieved on a variety of Petawatt laser systems, raising the prospect of ion beam applications using compact ultra-intense laser technology. However, translation from proof-of-concept laser experiment into real-world application requires MeV-scale ion energies and an appreciable repetition rate (>Hz). We demonstrate, for the first time, proton acceleration up to 2 MeV energies at a kHz repetition rate using a milli-joule-class short-pulse laser system. In these experiments, 5 mJ of ultrashort-pulse laser energy is delivered at an intensity near onto a thin-sheet, liquid-density target. Key to this effort is a flowing liquid ethylene glycol target formed in vacuum with thicknesses down to 400 nm and full recovery at 70 μ s, suggesting its potential use at ≫kHz rate. Novel detectors and experimental methods tailored to high-repetition-rate ion acceleration by lasers were essential to this study and are described. In addition, particle-in-cell simulations of the laser–plasma interaction show good agreement with experimental observations.

Abstract A novel type of all‐natural, biocompatible, and very robust nanoscale free‐standing biohybrids are reported. They are obtained by integrating a silk fibroin matrix with functional inorganic nanoplatelets using a spin‐assisted layer‐by‐layer assembly. The organized assembly of the silk fibroin with clay (montmorillonite) nanosheets results in highly transparent nanoscale films with significantly enhanced mechanical properties, including strength, toughness, and elastic modulus, as compared to those for the pristine silk nanomaterials. Moreover, replacing clay nanoplatelets with a highly reflective Langmuir monolayer of densely packed silver nanoplates causes a similar enhancement of the mechanical properties, but in contrast to the materials above, highly reflective, mirror‐like, nanoscale flexible films are created. This strategy offers a new perspective for the fabrication of robust all‐natural flexible nanocomposites with exceptional mechanical properties important for biomedical applications, such as reinforced tissue engineering. On the other hand, the ability to convert silk‐based nanoscale films into mirror‐like biocompatible flexible films can be intriguing for prospective photonics and optical exploitation of these nanobiohybrids.