The effect of Zn on precipitation in Al–Mg–Si alloys

Effects of addition of Zn (up to 1 wt%) on microstructure, precipitate structure and intergranular corrosion (IGC) in an Al–Mg–Si alloys were investigated. During ageing at 185?°C, the alloys showed modest increases in hardness as function of Zn content, corresponding to increased number densities of needle-shaped precipitates in the Al–Mg–Si alloy system. No precipitates of the Al–Zn–Mg alloy system were found. Using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), the Zn atoms were incorporated in the precipitate structures at different atomic sites with various atomic column occupancies. Zn atoms segregated along grain boundaries, forming continuous film. It correlates to high IGC susceptibility when Zn concentration is ~1wt% and the materials in peak-aged condition.     

Predicting buckling loads from vibration data

There are analytical methods for predicting the buckling loads of columns with the boundaries ideally fixed, i.e., simply supported or built-in, or partially fixed. Vibration-test results may furnish a practical method of measuring the fixity. In this investigation a beam, that may or may not be loaded as a column, is assumed to have a torsional spring at each end such that a zero torsional stiffness corresponds to a simply supported end and an infinite torsional stiffness corresponds to a built-in end. From a Rayleigh-Ritz analysis, the buckling load and the fundamental frequency of the beam are each computed as a function of the torsional stiffness. This procedure leads to a one-to-one nondimensional relationship between the buckling load and the natural frequency. From these calculations, it is seen that regardless of the degree of clamping of one end relative to the other end, all that is needed to predict the buckling load within a 15-percent range is a knowledge of the theoretical buckling load of the simply supported column; the theoretical fundamental frequency of the simply supported beam; and the experimental fundamental frequency. Experimental results are presented to support the theory.     

A practical two-surface plasticity model and its application to spring-back prediction

A practical two-surface plasticity model based on classical Dafalias/Popov and Krieg concepts was derived and implemented to incorporate yield anisotropy and three hardening effects for non-monotonous deformation paths: the Bauschinger effect, transient hardening and permanent softening. A simple-but-effective stress-update scheme avoiding overshooting was proposed and implemented. Constitutive parameters were fit to 5754-O aluminum alloy using uniaxial tension/compression data. Spring-back predictions using the resulting material model were compared with experiments and with single-surface material models which do not account for permanent softening. The two-surface model improved such predictions significantly as compared with single-surface models, while the differences between two-surface simulations and experiments were insignificant.     

Sharp melting in DNA-linked nanostructure systems: thermodynamic models of DNA-linked polymers

Sharp melting that has been found for DNA-linked nanostructure systems such as DNA-linked gold nanoparticles enhances the resolution of DNA sequence detection enough to distinguish between a perfect match and single base pair mismatches. One intriguing explanation of the sharp melting involves the cooperative dehybridization of DNA strands between the nanostructures. However, in the DNA-linked gold nanoparticle system, strong optical absorption by the gold nanoparticles hinders the direct observation of cooperativity. Here, with a combination of theory and experiment, we investigate a DNA-linked polymer system in which we can show that the optical profile of the system at 260 nm is directly related to the individual DNA dehybridization profile, providing a clear distinction from other possible mechanisms. We find that cooperativity plays a crucial role in determining both the value of the melting temperature and the shape of the melting profile well away from the melting temperature. Our analysis suggests that the dehybridization properties of DNA strands in confined or dense structures differ from DNA in solution.     

Fast tunable coupler for superconducting qubits

A major challenge in the field of quantum computing is the construction of scalable qubit coupling architectures. Here, we demonstrate a novel tunable coupling circuit that allows superconducting qubits to be coupled over long distances. We show that the interqubit coupling strength can be arbitrarily tuned over nanosecond time scales within a sequence that mimics actual use in an algorithm. The coupler has a measured on/off ratio of 1000. The design is self-contained and physically separate from the qubits, allowing the coupler to be used as a module to connect a variety of elements such as qubits, resonators, amplifiers, and readout circuitry over distances much larger than nearest-neighbor. Such design flexibility is likely to be useful for a scalable quantum computer.     

Effects of Processing Temperature on the Oxygen Quenching Behavior of Tris(4,7′-diphenyl-1,10′-phenanthroline) Ruthenium (II) Sequestered Within Sol-Gel-Derived Xerogel Films

Sol-gel processing methods offer novel pathways for tailoring glasses. Amongst the issues that have received the least attention are the effects of the curing temperature on the behavior and photophysics of a dopant molecule sequestered within a sol-gel-derived xerogel. Of particular interest to our group are the effects of processing variables on the ability of a dopant molecule, that is sequestered within a xerogel glass, to be accessed by an analyte and the distribution of the dopant sites within the xerogel. The thermal stability of the luminophore tris(4,7-diphenyl-1,10-phenanthroline) ruthenium (II) ([Ru(dpp)₃]²⁺) provides a convenient way to address these issues and develop an understanding of how one might best exploit curing temperature to construct improved chemical sensors. This paper focuses on quantifying how the film curing temperature affects the spectroscopy and O₂ quenching of ([Ru(dpp)₃]²⁺) sequestered within sol-gel-derived xerogel thin films. Our quenching data on films once they have been cured demonstrate that there is a dramatic increase in the sensitivity of the ([Ru(dpp)₃]²⁺) molecules to O₂ quenching when the films have been cured at elevated temperatures. This arises primarily because there are two main types of ([Ru(dpp)₃]²⁺) microenvironments within the glass and higher temperature curing leads to an increase in the bimolecular quenching rate between O₂ and ([Ru(dpp)₃]²⁺). This is accomplished as follows. Below a curing temperature of 100–150°C, 15% of the xerogel-doped ([Ru(dpp)₃]²⁺) molecules are not accessed to any detectable degree by the O₂ molecules during the ([Ru(dpp)₃]²⁺) excited-state luminescence lifetime. However, as the xerogel is cured at or above 150°C, residual silanol-bound waters (or other impurities) dissociate from the xerogel and those ([Ru(dpp)₃]²⁺) molecules that were initially inaccessible become accessible to O₂. The dissociation of these water molecules, plus other events, also causes the originally inaccessible ([Ru(dpp)₃]²⁺) population to ultimately exhibit a quenching rate that is greater than the fraction of initially accessible ([Ru(dpp)₃]²⁺) molecules that were formed under ambient curing conditions.