Medium-frequency Modes Research Articles

Effect of mid-frequency pulse insertion on the microstructural and mechanical properties of AlTiN coatings prepared using superimposed HiPIMS process

Superimposed high-power impulse magnetron sputtering (HiPIMS) refers to the insertion of medium frequency (MF) pulses during the off-time of the pulse sequence to enhance the rate of thin film deposition. In this work, we investigated the effects of inserting MF pulses (0 to 42) on the deposition rate, microstructure, and mechanical properties of AlTiN films. Our results indicate that the current-voltage (I-V) characteristics discharging with an Al65Ti35 target were affected by the introduction of N2 reaction gas and the additional MF power output, due to an impedance change under given plasma conditions. All samples presented mixed phases of AlN and TiN based on the chemical composition and crystallographic analysis. Superimposition improved the power-normalized deposition rate from 0.034 to 0.149 nm/min.kW without a significant decline in the adhesion quality of the deposited films. Increasing the number of inserted MF pulses from 0 to 42 led to a decrease in the peak power density of HiPIMS from 1357 to 509 W/cm2 and MF from 115 to 60 W/cm2. This can be attributed to the fact that the configuration of the HiPIMS system in this study used only one DC power supply, which outputted HiPIMS and MF modes together. Increasing the number of inserted MF pulses decreased the hardness of the thin film from 23 to 15 GPa as well as the compressive stress, due to an increase in the average size of the columnar crystals. These findings demonstrate that the application of AlTiN films can greatly enhance the corrosion resistance of SUS304.

The role of geometry on the global instability of wakes behind streamwise rotating axisymmetric bodies

Abstract We perform direct and adjoint global stability analyses of the wake behind streamwise rotating axisymmetric bodies of hemispherical nose and cylindrical trailing edge of different length-to-diameter ratios, l , to evaluate the role of geometry on the unstable global modes. The study is limited to laminar Reynolds numbers, Re 500 , and moderate values of the rotation parameter, Ω ≤ 1 , defined as the ratio between the azimuthal velocity at the body’s surface and the freestream velocity. As the aspect ratio, l , is varied, important differences on the wake stability features are found as the rotation parameter Ω is increased. For short bodies such as hemispheres ( l = 0 ), axisymmetry breaking takes place through the destabilization of a low frequency (LF) mode, which is a modified version of the steady-state (SS) mode related to wakes without rotation, which becomes more unstable as Ω grows. This destabilizing effect of rotation vanishes for low Ω as the aspect ratio l increases (bullet-like bodies, l > 0 ), until a critical value is reached, l c 1 , from which the LF mode is stabilized as Ω grows. In addition, increasing l above two other critical thresholds, l c 2 and l c 3 , promotes, respectively, the destabilization of a high frequency (HF) mode, which is a modified version of the RSP mode at wakes without rotation, and a new medium frequency (MF) mode, whose origin remains unclear. Computation of sensitivity to base flow modifications shows that the LF mode is destabilized by rotation for short bodies (i.e. sphere or hemisphere) due to an increase in the shear along the separation line and a weakening of perturbation advection inside the recirculation bubble. Conversely, as l grows, the larger angular momentum stabilizes the LF mode by strengthening the advection of perturbations in the near wake. Destabilization of HF and MF modes are also discussed in terms of sensitivity to base flow modifications. Finally, an analysis of the structural sensitivity shows that the destabilization of the MF mode is analogous to that behind the spiral vortex breakdown, in terms of conservation of angular momentum, being it mainly driven by an inviscid mechanism whose origin lies at the rear stagnation point of the recirculation bubble.

Optical spectroscopic properties recorded for simple BOPHY dyes in condensed media: The mirror-symmetry factor.

The BOPHY structural scaffold provides opportunities for the synthesis of innumerable derivatives with linear geometries and well-controlled π-conjugation pathways. The simpler BOPHY chromophores are highly fluorescent but exhibit poor mirror symmetry between absorption and fluorescence spectra at ambient temperature. In particular, the absorption (and excitation) spectra are broad and appear as two overlapping bands of comparable intensity. In constrained media, such as low-temperature rigid glasses or stretched poly(ethylene) films, mirror symmetry is restored. Analysis of the temperature dependence recorded for simple BOPHY derivatives indicates that the vibronic envelope accompanying the electronic transitions can be well described in terms of low- and medium-frequency modes. Whereas the fluorescence spectral profile is only weakly dependent on temperature, the excitation spectrum is far more affected. The magnitude of the low-frequency mode, and the associated electron-phonon coupling, increase substantially with increasing temperature and is responsible for temperature broadening and distortion of the excitation spectrum in solution. This critical low-frequency vibronic mode is associated with out-of-plane torsional bending of the BOPHY unit. Variable temperature NMR studies failed to provide unequivocal evidence for conformational changes of one of the derivatives over the temperature range 193-353 K.

Global stability analysis of the axisymmetric wake past a spinning bullet-shaped body

AbstractWe analyze the global linear stability of the axisymmetric flow around a spinning bullet-shaped body of length-to-diameter ratio $L/D=2$, as a function of the Reynolds number, $Re=\rho w_{\infty } D /\mu $, and of the rotation parameter $\varOmega =\omega D/(2 w_{\infty })$, in the ranges $Re<450$ and $0\leq \varOmega \leq 1$. Here, $w_{\infty }$ and $\omega $ are the free-stream and the body rotation velocities respectively, and $\rho $ and $\mu $ are the fluid density and viscosity. The two-dimensional eigenvalue problem (EVP) is solved numerically to find the spectrum of complex eigenvalues and their associated eigenfunctions, allowing us to explain the different bifurcations from the axisymmetric state observed in previous numerical studies. Our results reveal that, for the parameter ranges investigated herein, three global eigenmodes, denoted low-frequency (LF), medium-frequency (MF) and high-frequency (HF) modes, become unstable in different regions of the $(Re,\varOmega )$-parameter plane. We provide precise computations of the corresponding neutral curves, that divide the $(Re,\varOmega )$-plane into four different regions: the stable axisymmetric flow prevails for small enough values of $Re$ and $\varOmega $, while three different frozen states, where the wake structures co-rotate with the body at different angular velocities, take place as a consequence of the destabilization of the LF, MF and HF modes. Several direct numerical simulations (DNS) of the nonlinear state associated with the MF mode, identified here for the first time, are also reported to complement the linear stability results. Finally, we point out the important fact that, since the axisymmetric base flow is $SO(2)$-symmetric, the theory of equivariant bifurcations implies that the weakly nonlinear regimes that emerge close to criticality must necessarily take the form of rotating-wave states. These states, previously referred to as frozen wakes in the literature, are thus shown to result from the base-flow symmetry.

Mode-specific vibrational relaxation of photoexcited guanosine 5′-monophosphate and its acid form: a femtosecond broadband mid-IR transient absorption and theoretical study

UV-pump/broadband-mid-IR-probe transient absorption (TA) experiments and ab initio quantum mechanical (QM) calculations were used to investigate the photophysics in heavy water of the neutral and acid forms of guanosine 5'-monophosphate (GMP and GMPD(+), respectively). Excited GMP undergoes ultrafast internal conversion (IC) and returns to the electronic ground state in less than one picosecond with a large amount of excess vibrational energy. The spectroscopic signals are dominated by vibrational cooling - a process in which the solute dissipates vibrational energy to the solvent. For neutral GMP, cooling proceeds with a time constant of 3 ps. Following IC, at least some medium-frequency modes such as the carbonyl stretch and an in-plane ring vibration are excited, suggesting that the vibrational energy distribution is non-statistical. This is consistent with predicted structural changes upon passage through the S1/S0 conical intersection. GMPD(+) differs from GMP by a single deuteron at the N7 position, but has a dramatically longer lifetime of 200 ps. Vibrational cooling of the S1 state of GMPD(+) was monitored via several medium-frequency modes that were assigned using QM calculations. These medium-frequency modes are also vibrationally excited in a non-statistical fashion. Excitation of these modes is in line with the change in geometry at the S1 minimum of GMPD(+) predicted by QM calculations. Furthermore, these modes relax at different rates, fully consistent with QM calculations, which predict that excited vibrational states of the carbonyl stretch couple strongly to the D2O solvent and thus deactivate via intermolecular energy transfer (IET). In contrast, the ring stretch couples strongly to other ring modes of the guanine chromophore and appears to decay via intramolecular vibrational energy redistribution (IVR).

Stabilization of Medium-Frequency Modes in Isolated Microgrids Supplying Direct Online Induction Motor Loads

Driven by the economical and environmental concerns, the importance of distributed generation-based microgrids is increasing. However, the presence of induction motor (IM) loads in converter-based microgrids might induce source-load admittance mismatch between the IM load and microgrid converters leading to medium-frequency instabilities, particularly in isolated microgrids. To overcome this issue, this paper presents detailed small-signal admittance models, analysis, and stabilization of a voltage-source inverter (VSI)-based microgrid with direct online IM load. The resultant source (VSI) and load (IM) admittances form a multi-input-multi-output (MIMO) system. Using the generalized Nyquist stability criterion, it is shown that the interactions between the lightly-damped dynamics of IMs and isolated VSI-based microgrid have significant destabilizing effect. Active compensation techniques are proposed to maintain the system stability. Time-domain simulations and experimental results are presented to validate the theoretical analysis.

Finite-temperature infrared spectroscopy of polycyclic aromatic hydrocarbon molecules. II. Principal mode analysis and self-consistent phonons

Following previous work [F. Calvo et al. J. Chem. Phys. 132, 124308 (2010)], infrared spectra of several polycyclic aromatic hydrocarbon molecules are simulated with classical and quantum molecular dynamics trajectories. The interactions are modeled using a tight-binding potential energy surface and quantum delocalization is accounted for using the partially adiabatic centroid and ring-polymer molecular dynamics frameworks, both built upon the path-integral representation. The spectra obtained directly by Fourier transformation of the dipole moment autocorrelation function are here compared with several quasiharmonic approximations that provide additional information about the vibrational modes. A principal mode analysis (PMA) is carried out from the covariance matrix of atomic displacements in classical and quantum trajectories. The method systematically overestimates the line shifts due to anharmonicities, except in the power spectra of atomic displacements, and is not robust in predicting IR intensities for such large molecules. Alternatively, effective normal modes have also been determined by adapting the self-consistent phonon (SCP) theory of condensed matter physics to the present tight-binding model, in both classical and quantum mechanical descriptions. The SCP approximation turns out as semiquantitative in estimating the redshift of tight stretching modes, and performs better for classical systems. More problematic, it predicts that many low- or medium-frequency modes should be blueshifted, in contradiction with the molecular dynamics results. The sets of anharmonic normal modes extracted from the PMA and SCP approaches reveal important mixings within the tightest C-H and C-C stretching modes, which are also manifested on the corresponding power spectra.

End-Cap Effects on Vibrational Structures of Finite-Length Carbon Nanotubes

Vibrational structures of C60-related finite-length nanotubes, C(40+20n) and C(42+18n) (1 < or = n < or = 4), in which n is, respectively, the number of cyclic cis- and trans-polyene chains inserted between fullerene hemispheres, are analyzed from density functional theory (DFT) calculations. To illuminate the end-cap effects on their vibrational structures, the corresponding tubes terminated by H atoms C(20n)H20 and C(18n)H18 (1 < or = n < or = 5) are also investigated. DFT calculations show a broad range of vibrational frequencies for the finite-size nanotubes: high-frequency modes (1100-1600 cm(-1)) containing oscillations along tangential directions (tangential modes), medium-frequency modes (700-850 cm(-1)) whose oscillations are located on the edges or end caps, and low-frequency modes (300-600 cm(-1)) involving oscillations along the radial directions (radial modes). Broadening of the calculated frequencies is due to the number of nodes in the standing waves of normal modes in the finite-size tubes. In the capped tubes, calculated vibrational frequencies are insensitive to the number of chains (n), whereas in the uncapped tubes, most vibrational frequencies change significantly with an increase in tube length. The discrepancy in the size dependency is reasonably understood by their C-C bonding networks; the capped tubes have similar bond-length alternation patterns within the polyene chains irrespective of n, whereas the uncapped tubes have various bond-deformation patterns. Thus, DFT calculations illuminate that the edge effects have strong impacts on the vibrational frequencies in the finite-size nanotubes.

Properties of low and medium frequency modes in two-fluid plasma

Based on a two-fluid plasma model where the electron mass and the displacement current are neglected, the eigenmode properties such as the frequency and the compressibility are studied. It is found that these properties strongly depend on the two-fluid parameter kℓi, where k is the wave number of a mode and ℓi is the ion skin depth. Especially it is found that as the two-fluid parameter kℓi increases beyond unity, the Alfvén wave, which is an incompressible mode in the magnetohydrodynamics (MHD) limit, becomes compressible and its phase velocity approaches to the acoustic speed. The slow magnetosonic wave, which is compressible in the MHD limit, becomes incompressible as kℓi increases. Implications of these results are also discussed.

The numerical evaluations of the density of states weighted Franck–Condon factor in large molecules. Effects of frequency changes

The numerical integration method (NIM) is applied to calculations of the density of states weighted Franck–Condon (DWFC) factor which plays an important role in the theory of nonradiative transitions. The 200 point Legendre–Gauss quadrature formula is used to save computing time. Values of the integrand are given with the quadruple precision to avoid accumulated errors occuring in the numerical integration. The illustrative calculations are carried out for the T1→S0 intersystem crossing in benzene. If frequency changes of all vibrational modes are taken into account, the energy gap law applies very well, but it fails without frequency changes. This fact suggests that the ordinary saddle point method based on the harmonic displaced potential surface model is inaccurate. The DWFC factor is sensitive to frequency changes of a high frequency mode which plays an important role in nonradiative transition. When low and medium frequency modes are added, as a group, to high frequency ones, they give a significant effect upon the DWFC factor. If a frequency change of a low frequency mode is hypothetically reduced to an extremely low value, the DWFC factor rapidly increases. This relates to the proximity effect by Wassam and Lim in the nonradiative transition in N-heterocyclic aromatic hydrocarbons. Finally, Metz’s method is discussed.