45915 research outputs found
Sort by
On the negative strain rate sensitivity of Hadfield steel
No full textThe room temperature strain rate sensitivity of Hadfield steel polycrystals was investigated with the aid of in situ digital image correlation. The current findings indicate that both the rapid work hardening and negative strain rate sensitivity of Hadfield steel polycrystals are influenced by dynamic strain aging
Novel approach for synthesis of magnesium borohydride, Mg(BH4)(2)
No full textMg(BH4)(2) is a complex hydride with one of the highest hydrogen contents (similar to 15%) known yet. Several synthesis routes have been reported for it, all based on the metathesis reaction of MgCl2 with NaBH4 performed in a ball-mill or in suitable solvents. In the present study a new approach for synthesis of Mg(BH4)(2) will be presented in which the more reactive MgBr2 is used instead of MgCl2. For this purpose a mixture of MgBr2 and NaBH4 (molar ratio: 1:2 and 1:2.15) was ball-milled for 6, 12 and 18 h, respectively. Mg(BH4)(2) was extracted from the reaction product (Mg(BH4)(2) + NaBr) by Soxhlet with diethylether over a day. The remaining residue after solvent evaporation was dried in vacuum at 150 degrees C for 24 h and 5h at 190 degrees C. The intermediate and final products of the reactions were analyzed using XRD, DTA/TG, Mass and Vibrational Spectroscopy. The XRD diagrams of the mixture after ball milling showed only the characteristic reflections of NaBr and the patterns obtained after solvent extraction was in all cases consistent with beta-Mg(BH4)(2). The additional weak MgBr2 reflections, decreased by increasing the ball milling time from 6 to 18 h. The DTA/TG coupled with MS revealed similar to 11% mass loss when the product was heated up to 600 degrees C. The result of MS detected that the exhaust gas is exclusively H-2. Compared to MgCl2, the use of MgBr2 has two advantages: the reaction time is considerably shorter and the excess of MgBr2 can act as additive lowering the onset temperature for hydrogen release from 290 degrees C - for pure Mg(BH4)(2) - to similar to 220 degrees C. (C) 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Canadian Hydrogen and Fuel Cell Association
State feedback control for adjusting the dynamic behavior of a piezoactuated bimorph atomic force microscopy probe
No full textWe adjust the transient dynamics of a piezoactuated bimorph atomic force microscopy (AFM) probe using a state feedback controller. This approach enables us to adjust the quality factor and the resonance frequency of the probe simultaneously. First, we first investigate the effect of feedback gains on dynamic response of the probe and then show that the time constant of the probe can be reduced by reducing its quality factor and/or increasing its resonance frequency to reduce the scan error in tapping mode AFM
A possible role of prolonged whirling episodes on structural plasticity of the cortical networks and altered vertigo perception: the cortex of sufi whirling dervishes
No full textAlthough minutes of a spinning episode may induce vertigo in the healthy human, as a result of a possible perceptional plasticity, Sufi Whirling Dervishes (SWDs) can spin continuously for an hour without a vertigo perception.This unique long term vestibular system stimulation presents a potential human model to clarify the cortical networks underlying the resistance against vertigo. This study, therefore, aimed to investigate the potential structural cortical plasticity in SWDs. Magnetic resonance imaging (MRI) of 10 SWDs and 10 controls were obtained, using a 3T scanner. Cortical thickness in the whole cortex was calculated. Results demonstrated significantly thinner cortical areas for SWD subjects compared with the control group in the hubs of the default mode network (DMN), as well as in the motion perception and discrimination areas including the right dorsolateral prefrontal cortex (DLPFC), the right lingual gyrus and the left visual area 5 (V5)/middle temporal (MT) and the left fusiform gyrus. In conclusion, this is the first report that warrants the potential relationship of the motion/body perception related cortical networks and the prolonged term of whirling ability without vertigo or dizziness
Asymptotic H-Plateau problem in H-3
No full textWe show that for any Jordan curve Gamma in S-infinity(2) (H-3) with at least one smooth point, there exists an embedded H-plane P-H in H-3 with partial derivative P-infinity(H) = Gamma for any H is an element of [0, 1)
Optofluidic waveguides written in hydrophobic silica aerogels with a femtosecond laser
No full textWe present a new method to form liquid-core optofluidic waveguides inside hydrophobic silica aerogels. Due to their unique material properties, aerogels are very attractive for a wide variety of applications; however, it is very challenging to process them with traditional methods such as milling, drilling, or cutting because of their fragile structure. Therefore, there is a need to develop alternative processes for formation of complex structures within the aerogels without damaging the material. In our study, we used focused femtosecond laser pulses for high-precision ablation of hydrophobic silica aerogels. During the ablation, we directed the laser beam with a galvo-mirror system and, subsequently, focused the beam through a scanning lens on the surface of bulk aerogel which was placed on a three-axis translation stage. We succeeded in obtaining high-quality linear microchannels inside aerogel monoliths by synchronizing the motion of the galvo-mirror scanner and the translation stage. Upon ablation, we created multimode liquid-core optical waveguides by filling the empty channels inside low-refractive index aerogel blocks with high-refractive index ethylene glycol. In order to demonstrate light guiding and measure optical attenuation of these waveguides, we coupled light into the waveguides with an optical fiber and measured the intensity of transmitted light as a function of the propagation distance inside the channel. The measured propagation losses of 9.9 dB/cm demonstrate the potential of aerogel-based waveguides for efficient routing of light in optofluidic lightwave circuits
MEMS scanners for display, imaging, and spectroscopy and their dynamic characterization
No full textMoving micro-mechanical structures combined with laser light sources and micro-optics enable a number of powerful applications in display, imaging, and spectroscopy. Examples of systems developed in our laboratory are: rotational scanners developed for micro-projectors, dynamic diffraction gratings with large out-of-plane motion developed for Fourier Transform spectrometers, and 2 degree-of-freedom MEMS stages that carry micro-lens arrays for laser beam steering and imaging applications. Precise control of motion is critical in all those applications. We developed a number of optical characterization tools for point-based and full-field dynamic characterization of micro and nano mechanical structures. In this paper, we first briefly discuss the applications and then describe the details of the optical characterization tools. First setup is a stroboscopic interferometry for dynamic deformation analysis. Second setup is a simple technique for simultaneous in-plane and out-of-plane measurement with nanometric precision. The setup is constructed using one photo detector and a Mirau-type interference objective. For out-of-plane measurements, interference fringes are used to compute the the deflection amount. For in-plane measurements, knife edge technique is used to modulate the reflected beam intensity using a sharp edge in the object. Third setup is a simple optical angle sensor for rotational mirrors, which uses only one bi-cell photo detector. The setup is able to measure amplitude, phase, and quality factor of torsional devices
Focused bulk ultrasonic waves generated by ring-shaped laser illumination and application to flaw detection
No full textFocused bulk ultrasonic waves have been generated in aluminum plates by surface irradiation with ring-shaped laser light. The waves are detected by a piezoelectric transducer. Compression and shear peak amplitudes drop quickly when the detector is moved away from the epicenter, This shows that strong focusing exists at the epicenter as the result of constructive interference of the waves generated by different parts of the ring, The focusing persists when the radius of the laser light is scanned over a large range, indicating that the elastic disturbance concentrates in depth along the ring's central axis. Numerical simulations are presented for comparison. The ''pencil-like'' acoustic wave structure is used to observe a sample plate with an artificial flaw. Strong new features including compress-shear mode conversion at the site of the flaw are observed. These features are used to locate the flaw within the sample
Statistical thermodynamics of residue fluctuations in native proteins
No full textStatistical thermodynamics of residue fluctuations of native proteins in a temperature, pressure, and force reservoir is formulated. The general theory is discussed in terms of harmonic and anharmonic fluctuations of residues. The two elastic network models based on the harmonic approximation, the anisotropic network and the Gaussian network models are discussed as the limiting cases of the general theory. The heat capacity and the correlations between the energy fluctuations and residue fluctuations are obtained for the harmonic approximation. The formulation is extended to large fluctuations of residues in order to account for effects of anharmonicity. The fluctuation probability function is constructed for this purpose as a tensorial Hermite series expansion with higher order moments of fluctuations as coefficients. Evaluation of the higher order moments using the proposed statistical thermodynamics model is explained. The formulation is applied to a hexapeptide and the fluctuations of residues obtained by molecular dynamics simulations are characterized in the framework of the model developed. In particular, coupling of two different modes in the nonlinear model is discussed in detail
Quantum fluid dynamics in the Lagrangian representation and applications to photodissociation problems
No full textThis paper considers the practical utility of quantum fluid dynamics (QFD) whereby the time-dependent Schrodinger's equation is transformed to observing the dynamics of an equivalent "gas continuum." The density and velocity of this equivalent gas continuum are respectively the probability density and the gradient of the phase of the wave function. The numerical implementation of the QFD equations is carried out within the Lagrangian approach, which transforms the solution of Schrodinger's equation into following the trajectories of a set of mass points, i.e., subparticles, obtained by discretization of the continuum equations. The quantum dynamics of the subparticles which arise in the present formalism through numerical discretization are coupled by the density and the quantum potential. Numerical illustrations are performed for photodissociation of nocl and NO2 treated as two-dimensional models. The dissociation cross sections sigma(omega) are evaluated in the dramatically short CPU times of 33 s for nocl and 40 s for NO2 on a Pentium-200 mhz PC machine. The computational efficiency comes from a combination of (a) the QFD representation dealing with the near monotonic amplitude and phase as dependent variables, (b) the Lagrangian description concentrating the computation effort at all times into regions of highest probability as an optimal adaptive grid, and (c) the use of an explicit time integrator whereby the computational effort grows only linearly with the number of discrete points