5,049 research outputs found
On characteristic classes of singular hypersurfaces and involutive symmetries of the Chow group
For any algebraic scheme and every we define an associated involution of its Chow group , and
show that certain characteristic classes of (possibly singular) hypersurfaces
in a smooth variety are interchanged via these involutions. For
we show that such involutions are induced by involutive
correspondences
Valley-selective optical Stark effect in monolayer WS2
Breaking space-time symmetries in two-dimensional crystals (2D) can
dramatically influence their macroscopic electronic properties. Monolayer
transition-metal dichalcogenides (TMDs) are prime examples where the
intrinsically broken crystal inversion symmetry permits the generation of
valley-selective electron populations, even though the two valleys are
energetically degenerate, locked by time-reversal symmetry. Lifting the valley
degeneracy in these materials is of great interest because it would allow for
valley-specific band engineering and offer additional control in valleytronic
applications. While applying a magnetic field should in principle accomplish
this task, experiments to date have observed no valley-selective energy level
shifts in fields accessible in the laboratory. Here we show the first direct
evidence of lifted valley degeneracy in the monolayer TMD WS2. By applying
intense circularly polarized light, which breaks time-reversal symmetry, we
demonstrate that the exciton level in each valley can be selectively tuned by
as much as 18 meV via the optical Stark effect. These results offer a novel way
to control valley degree of freedom, and may provide a means to realize new
valley-selective Floquet topological phases in 2D TMDs
A non-contact vision-based system for multi-point displacement monitoring in a cable-stayed footbridge
This is the author accepted manuscript. The final version is available from Wiley via the DOI in this record.Vision-based monitoring receives increased attention for measuring displacements of civil
infrastructure such as towers and bridges. Currently, most field applications rely on artificial targets for
video processing convenience, leading to high installation effort and focus on only single-point
displacement measurement e.g. at mid-span of a bridge. This study proposes a low-cost and non-contact
vision-based system for multi-point displacement measurement based on a consumer-grade camera for
video acquisition and a custom-developed package for video processing. The system has been validated
on a cable-stayed footbridge for deck deformation and cable vibration measurement under pedestrian
loading. The analysis results indicate that the system provides valuable information about bridge
deformation of the order of a few cm induced, in this application, by pedestrian passing. The measured
data enables accurate estimation of modal frequencies of either the bridge deck or the bridge cables and
could be used to investigate variations of modal frequencies under varying pedestrian loads
Kinematic Analysis and Dimensional Synthesis of a Meso-Gripper
Abstract
In recent years, applications in industrial assemblies within a size range from 0.5 mm to 100 mm are increasing due to the large demands for new products, especially those associated with digital multimedia. Research on grippers or robotic hands within the mesoscopic scale of this range has not been explored in any great detail. This paper outlines the development of a gripper to bridge the gap between microgrippers and macrogrippers by extending the gripping range to the mesoscopic scale, particularly without the need to switch grippers during industrial assembly. The mesoscopic scale gripper (meso-gripper) researched in this work has two modes of operation: passive adjusting mode and angled gripping mode, adapting its configuration to the shape of object automatically. This form of gripping and the associated mechanism are both novel in their implementation and operation. First, the concept of mesoscopic scale in robotic gripping is presented and contextualized around the background of inefficient hand switching processes and applications for assembly. The passive adjusting and angled gripping modes are then analyzed and a dual functional mechanism design proposed. A geometric constraint method is then demonstrated which facilitates task-based dimensional synthesis after which the kinematics of synthesized mechanism is investigated. The modified synthesized mechanism gripper is then investigated according to stiffness and layout. Finally, a 3D printed prototype is successfully tested, and the two integrated gripping modes for universal gripping verified.</jats:p
Hyperbaric laser chemical vapor deposition of high-strength aluminum- silicon-carbide nanocomposite fibers for aerospace and transportation applications
For over 25 years, hyperbaric pressure laser chemical vapor deposition (HP-LCVD) has been studied by various authors as a mean for growing three-dimensional structures and fibers [1-2]. Novel normally-immiscible materials (NIMs) [3], amorphous/glassy ceramics [4], and high-strength fibers have been grown [5]. However, the highest experimental pressures to date have only reached beyond the critical point of certain alkanes (\u3c60 bar) [6]. Our group has found it useful to synthesize materials from high pressure fluids, where the ensuing cooling rates after deposition can exceed 106 K/s. This has enabled the growth of (metastable) amorphous and nanostructured materials, including diamond-like carbon and boron carbides [7-8]. For this work, freestanding nanocomposite fibers were grown from mixtures of Bis(trimethylsilyl)methane and various organometallic and halide aluminum precursors. A chopped, cw fiber laser at 1064nm and diode lasers at 808nm were used for this work. The 1/e2 laser beam waists were approximately 10-15 microns across. The resulting Al-Si-C fibers could be grown continuously—and were nanostructured due to the precursor pressures and laser powers employed. A variety of phases were found to be present, including aluminum carbide, silicon carbide, carbon, and silicon-rich phases. Scanning electron microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) were used to characterize the composition and structure of the resulting materials. A map of the ternary phase diagram under these non-equilibrium conditions will be provided and discussed in detail. These fibers will find utility in reinforcements for ceramic- and metal-matrix composites for aerospace and transportation applications.
References:
[1]F. T. Wallenberger, P. C. Nordine, M. Boman, Composites Science and Technology, 1994, 51, 192.
[2]J. L. Maxwell, US Patent #5,786,023, 1996.
[3]J. L. Maxwell, M. R. Black, C. A. Chavez, K. R. Maskaly, M. Espinoza, M. Boman, Applied Physics A-Materials Science and Engineering, 2008, 91, 507.
[4]F. T. Wallenberger, P. C. Nordine, Journal of Materials Research, 1994, 9, 527.
[5]M. Boman, D. Bauerle, Journal of the Chinese Chemical Society, 1995, 42, 405.
[6] J. Maxwell, Unpublished Results.
[7]J. Maxwell, M. Boman, W. Springer, J. Narayan, S. Gnanavelu, Journal of the American Chemical Society, 2006, 128, 4405.
[8]J. Maxwell, C. Chavez, W. Springer, K. Maskaly, D. Goodin, Diamond and Related Materials, 2007, 16, 1557
Room-temperature atmospheric argon plasma jet sustained with submicrosecond high-voltage pulses
In this letter, an experimental study is presented to characterize a room-temperature plasma jet in
atmospheric argon generated with submicrosecond voltage pulses at 4 kHz. Distinct from
sinusoidally produced argon discharges that are prone to thermal runaway instabilities, the pulsed
atmospheric argon plasma jet is stable and cold with an electron density 3.9 times greater than that
in a comparable sinusoidal jet. Its optical emission is also much stronger. Electrical measurement
suggests that the discharge event is preceded with a prebreakdown phase and its plasma stability
is facilitated by the short voltage pulses
Sharp bursts of high-flux reactive species in submicrosecond atmospheric pressure glow discharges
In this letter, the authors present an experimental study of the temporal characteristics of
submicrosecond pulsed atmospheric glow discharges. Using electrical measurements and
nanosecond-resolved optical emission spectroscopy, they show that a long initial period of each
voltage pulse is spent building up space charges and is then followed by a large current pulse in the
voltage-falling phase. Reactive plasma species such as oxygen atoms and OH radicals are produced
in a train of sharp and independent pulses of 50–100 ns wide. Finally, their production is shown to
increase significantly as the voltage pulse width reduces or the repetition frequency increases
- …