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We studied the effect of a cross-conjugated bridging group (χC) on charge-transfer in a push-pull chromophore system. The hyperpolarizability of such molecules was found to be comparable to that of a fully π-conjugated molecule (πC) with the same donor and acceptor. The cross-conjugated moiety was then applied as a pendant to a fully π-conjugated chromophore containing a tricyanopyrroline acceptor (TCP). The addition of a χC moiety did not alter the intrinsic hyperpolarizability and provides an avenue for extending and aiding πC systems. The molecules were examined by X-ray diffraction (XRD), hyper-Raleigh scattering (HRS) and UV-visible (UV-vis) spectroscopy. Experimental results were compared with the predictions of density functional theory (DFT). Cross-conjugated molecules have comparable β values, relative to πC molecules, due to reduced spatial overlap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Thus, the χC architecture could facilitate independent modification of donor and acceptor strengths while minimizing unfavorable effects on electronic transitions and dipole moments.
Monolithic Nb5Si3 films and microlaminates consisting of alternating, equally thick layers of Nb and Nb5Si3 were synthesized by magnetron sputtering. Thick monolithic Nb5Si3 films (25,000 nm) were deposited on a sapphire substrate to set process parameters and evaluate the microstructure and mechanical properties of as-deposited crystalline films. Nb5Si3/Nb micro-laminates with modulation wavelengths (i.e., bilayer thickness) of 40 and 200 nm were deposited on Nb substrates. Mechanical properties (elastic modulus, microhardness, compressive yield strength) of the films and microlaminates were studied using the nanoindentation method and Vickers microhardness. Mechanical property test results are presented.
Several sandwich panels with Ti-6AI-4V facesheets and syntactic metal foam (i.e., alumina microballoons (mb) in aluminum matrix) core were produced by a single step casting press. Optical microscopy and scanning acoustic microscopy techniques were used to examine the integrity of Ti/Al and mb/Al matrix interfaces. Mechanical property tests including compression, tension, flexure, and impact tests were performed to determine the characteristics of as-cast Ti//(alumina)mb/Al//Ti laminates. Presented in this paper are the specific details of design, processing, and mechanical behavior of the as-cast laminates.
Internal friction behavior in cast 8-ply [0°1 P55Gr/Mg-0.6%Zr alloy and P55Gr/Mg-1%Mn composites as a function of vibratory strain amplitude was measured at 80 kHz using a Marxtype piezoelectric composite oscillator. Both the matrix and composite exhibited strain amplitude independent internal friction below ε ≈ 10−6, while significant strain amplitude dependence was noted at higher strain levels. A maxima in damping was observed for most of the specimens tested. Heat treatment to enlarge grain size was found to increase both the strain amplitude independent and dependent internal friction of the composite. Strain amplitude dependence of the internal friction, including the existence of the maxima, was explained by the Granato-Lucke (G-L) dislocation internal friction model. Dislocation densities obtained from various TEM images from the fiber-matrix interface were compared to values predicted by G-L theory.
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