Function Of Excitation Frequency Research Articles

Non-Destructive Evaluation of Thermal Aging in EPDM Rubber Using Electromagnetic Techniques.

This study investigates the use of eddy-current technology and impedance spectroscopy in sensing the change in rubber properties after it is exposed to accelerated thermal aging. The thermal aging process, by application of temperature and pressure over time, of ethylene propylene diene monomer (EPDM) rubbers containing both carbon black (CB) and graphene are investigated. Both eddy-current sensing and electrical impedance measurement techniques were used for electromagnetic analysis. Both methods measure the in- and out-of-phase responses as a function of excitation frequency at room temperature. The measurements were performed before and after the aging process. The electrical percolation threshold was detected in the rubber samples by varying the CB content from 0 to 40 wt%. In the rubber sample containing 30 wt% CB, 0-5 wt% of the CB was replaced with graphene flakes. The substitution of graphene for CB in the EPDM rubber formulation provided an enhanced eddy-current and electrical impedance response. The findings demonstrate the feasibility of employing electromagnetic analysis techniques to investigate the extent of aging.

Extreme bursting events via pulse-shaped explosion in mixed Rayleigh-Liénard nonlinear oscillator

We study the dynamics of a parametrically and externally driven Rayleigh-Liénard hybrid model and report the emergence of extreme bursting events due to a novel pulse-shaped explosion mechanism. The system exhibits complex periodic and chaotic bursting patterns amid small oscillations as a function of excitation frequencies. In particular, the advent of rare and recurrent chaotic bursts that emerged for certain parameter regions is characterized as extreme events. We have identified that the appearance of a sharp pulse-like transition that occurred in the equilibrium points of the system is the underlying mechanism for the development of bursting events. Further, the controlling aspect of extreme events is attempted by incorporating a linear damping term, and we show that for sufficiently strong damping strength, the extreme events are eliminated from the system, and only periodic bursting is feasible.

Extraction of Pendulum Model Parameters from Steady-State Slosh Data in Diaphragm Tanks

This study proposes a new experimental approach to extract pendulum model parameters from spacecraft propellant tank slosh data. The novelty of the approach is in the utilization of steady-state triaxial force measurements at the support points of the tank under sinusoidal excitation. This approach enables repeatable and accurate determination of pendulum model parameters in contrast to the single-axis force sensing and multiple repetitions averaging techniques associated with the more common decay approach. This approach may be used for tanks with free slosh; however, this study considers a diaphragm tank with three different diaphragm configurations under translational oscillatory lateral excitation. With the aid of a parameter optimization technique prevalently used in machine learning, the proposed methodology identifies the pendulum model parameters based on an adaptive gradient descent on the error surface defined in the parameter space. The proposed approach is validated by comparing experimental measurements against model-predicted behavior, and the numerical procedure is verified against a quasi-Newton method. Moreover, the extracted pendulum model parameters enable prediction of the frequency response functions for both the apparent mass and apparent mass moment as function of excitation frequency, and the methodology enables extraction of higher-order vibration modes.

Theory of nonlinear microwave absorption in an electron-doped and gapped graphene monolayer

Abstract We present a theory of nonlinear microwave absorption in monolayer graphene grown epitaxially on a SiC substrate and electron-doped by electrostatic gating. The intensity-dependent absorption coefficient of normally incident microwave radiation is calculated by using the Boltzmann equation techniques. The Boltzmann transport equation for mobile carriers in the conduction band of the graphene layer is solved to third order in the applied microwave field, including nonlinearities due to both the nonparabolicity of the conduction band and the energy-dependent scattering rate of electrons by acoustic phonons and short-range lattice defects. The intraband absorption coefficient decreases with an increase in the intensity of incident radiation, thus suggesting that electron-doped epitaxial graphene may be exploited as a saturable absorber operating at microwave frequencies. The saturation intensity is calculated as a function of excitation frequency and electron concentration and found to be a few MW/cm 2 over the millimeter wave frequency band considered (30–300 GHz) at a typical doping level of 1012 cm−2.

Direct Imaging of High‐Frequency Multimode Spin Wave Propagation in Cobalt‐Iron Waveguides Using X‐Ray Microscopy beyond 10 GHz

Not only in fundamental wave physics but also in technical areas such as radar and communication systems, high‐frequency magnonics is increasingly attracting attention. Here, time‐resolved scanning transmission X‐ray microscopy is used to directly image high‐frequency spin wave propagation in cobalt‐iron waveguides at excitation frequencies above 10 GHz. In addition, an excitation technique is presented, which allows for versatile pump–probe experiments with radio frequency currents up to 30 GHz. With this approach, a global sinusoidal magnetic field excitation is applied to induce spin waves from the waveguide edges. Amplitude, relative phase, and k‐space information as a function of excitation frequencies and static external fields are observed, matching the theoretical predictions for confined waveguide structures. In doing so, the foundation for high‐frequency multimode spin wave excitation and propagation at the nanoscale is laid, which can be a prospective path in radar and communication systems.

Characterization and modeling of temperature effect on the shear mode properties of magnetorheological elastomers

The temperature dependency of the viscoelastic properties of magnetorheological elastomers (MREs) operating in the shear mode was experimentally investigated under broad ranges of strain amplitude, excitation frequency, and applied magnetic field. Experiments were performed with isotropic MRE samples with volume fraction of carbonyl iron particles dispersed in the silicone rubber matrix under controlled temperature, ranging from to together with different levels of magnetic field density ( to ). The results revealed significant effect of temperature on the linear and nonlinear viscoelastic properties of MREs, in addition to the effects of strain amplitude, rate, and applied magnetic flux density. The temperature dependency of mechanical properties of an unfilled rubber matrix was also measured, which served as a reference. The shear properties of the MRE revealed reductions in both the storage and loss moduli with increasing temperature. The temperature dependency of the MRE, however, diminished under shear strain in excess of the critical strain. The results also revealed more pronounced temperature effect at higher temperatures and excitation frequencies, while it was relatively lesser with increasing magnetic field. Increase in temperature resulted in enhanced magnetorheological effect, while a clear trend with regard to the excitation frequency could not be established. Finally, a phenomenological model has been developed to predict the storage and loss moduli of the MRE as a function of excitation frequency, applied magnetic flux density, and temperature. The superb performance of the proposed model in predicting viscoelastic moduli under various operating and environmental conditions has been demonstrated through comparison of the experimental and simulation results.

Imaging of caustic-like spin wave beams using optical heterodyne detection

We investigated caustic-like spin wave beams radiated from a narrow waveguide to a continuous film using a Permalloy. For this purpose, we developed a spatially resolved optical heterodyne detection of propagating spin waves by using the polar Kerr effect, which allows the measurement of the intensity and the phase information of the spin waves. We characterized magnetostatic surface spin wave modes in one-dimensional (1D) propagation and caustic-like spin wave beams in two-dimensional (2D) propagation as a function of excitation frequency. The fast Fourier transformations of the real space images reconstruct 1D and 2D spin wave dispersion, which agree with the theoretical calculations.

Carrier mobility of silicon by sub-bandgap time-resolved terahertz spectroscopy.

Low density charge mobility from below bandgap, two-photon photoexcitation of bulk silicon (Si) is interrogated using time-resolved terahertz spectroscopy (TRTS). Total charge mobility is measured as a function of excitation frequency and fluence (charge carrier density), cut angle, and innate doping levels. Frequency dependent complex photoconductivities are extracted using the Drude model to obtain average and DC-limit mobility and carrier scattering times. These dynamic parameters are compared to values from contact-based Hall, above bandgap photoexcitation, and comparable gallium arsenide (GaAs) measurements. Mobilities are shown to increase beyond Hall values at low carrier densities and are modestly higher with increasing dopant density. The former occurs in part from below bandgap photoexcitation exhibiting abnormally small (faster) scattering times, while both reflect unique conduction characteristics at lowest (> 2x1012 cm-3) carrier densities achieved through photodoping.

Spectral Quantification of Nonlinear Elasticity Using Acoustoelasticity and Shear-Wave Dispersion

Tissue biomechanical properties are known to be sensitive to pathological changes. Accordingly, various techniques have been developed to estimate tissue mechanical properties. Shear-wave elastography (SWE) measures shear-wave speed (SWS) in tissues, which can be related to shear modulus. Although viscosity or stress-strain nonlinearity may act as confounder of SWE, their explicit characterization may also provide additional information about tissue composition as a contrast modality. Viscosity can be related to frequency dispersion of SWS, which can be characterized using multi-frequency measurements, herein called spectral SWE (SSWE). Additionally, nonlinear shear modulus can be quantified and parameterized based on SWS changes with respect to applied stress, a phenomenon called acoustoelasticity (AE). In this work, we characterize the nonlinear parameters of tissue as a function of excitation frequency by utilizing both AE and SSWE together. For this, we apply incremental amounts of quasi-static stress on a medium, while imaging and quantifying SWS dispersion via SSWE. Results from phantom and ex vivo porcine liver experiments demonstrate the feasibility of measuring frequency-dependent nonlinear parameters using the proposed method. SWS propagation in porcine liver tissue was observed to change from 1.8 m/s at 100 Hz to 3.3 m/s at 700 Hz, while increasing by approximately 25% from a strain of 0% to 12% across these frequencies.

Modeling and analysis of the effect of substrate on the flexible piezoelectric films for kinetic energy harvesting from textiles

In the last few years, a lot of research focused on increasing of smart textiles products such as woven and knitted structures, which are able to show significant change in their mechanical properties (such as shape and stiffness), in a practical way in response to the stimuli. In this paper, we investigate the potential of a flexible piezoelectric film stuck onto three woven textile matrices: cotton, polyester/cotton, and Kermel, for harvesting mechanical energy from the textile and converting it into electrical energy. At first, a brief introduction of energy harvesting using the piezoelectric material and smart textile is presented. Furthermore, a basic model showing the operation of polyvinylidene fluoride with 33 mode is established. The second part is focused on standard approach model of energy harvesting based on resistive load and freestanding piezo-polymer for the examination of the performance of 33-mode polyvinylidene fluoride energy harvester and the prediction of harvested energy quantity. A power analytical model generated by a smart structure type polyvinylidene fluoride that can be stuck onto fabrics and flexible substrates is investigated. On the other hand, the effects of various substrates and the sticking of these substrates on the piezoelectric material are reported. Additionally, the output power density of this theoretical model of woven textile matrices could reach a value that was seven times higher than freestanding piezo-polymer. Three types of the substrates have been compared as function of excitation frequency and the compressive applied force.

A New Structural Dynamic Model for Pump Mechanical Seals Vibration Analysis Incorporating Squeeze Motion of O-Ring Seals and General Dynamic Motion of the Pump Housing and the Pump Shaft

New models are developed for flexibly mounted stator (FMS) and flexibly mounted rotor (FMR) mechanical seals that incorporate the radial reaction force components produced by supporting O-rings due to relative squeezing motion across the O-rings. Supporting data come from tests done in relation to O-ring supports for ball bearing races. The reaction-force model is linear but a nonlinear function of excitation frequency. The model accounts for the axial displacement doz of the O-ring from the mass center of the seal stator (FMS configuration) or seal rotor (FMR configuration), which couples the radial and pitch–yaw motion of the model's stiffness and damping matrices. Greens' coned-face-seal model is used to define the reaction moment arising across the seal faces via stiffness and damping matrices. The damping matrix does not coincide with Green's. His is constant; the matrix developed here contains terms that are harmonic at twice theprecession frequency. When averaged over one precession cycle, the new average damping matrix coincides with Green's result. When the averaged damping matrix is used, the resultant model is linear. However, because of the viscoelastic reaction-force and reaction-moment models used for the O-ring coefficients, most of the stiffness and damping matrices are strong functions of the assumed precession frequency. The new FMR model contains a skew-symmetric stiffness matrix due to the O-ring damping terms. In rotordynamics, skew symmetric stiffness matrices due to internal damping in the rotor can lead to rotordynamic instabilities.

Quasi-static and dynamic motions of the columellar footplate in ostrich (Struthio camelus) measured ex vivo

The nature of the movement of the columellar footplate (CFP) in birds is still a matter of ongoing debate. Some sources claim that rocking motion is dominant, while others propose a largely piston-like motion. In this study, motions of the CFP are experimentally investigated in the ostrich using a post-mortem approach. For quasi-static loads, micro-CT scans of ostrich heads were made under positive and negative middle-ear pressures of 1 kPa. For dynamic loads, laser Doppler vibrometry was used to measure the velocity on multiple locations of the CFP as a function of excitation frequency from 0.125 to 4 kHz, and digital stroboscopic holography was used to assess the 1D full-field out-of-plane displacement of the CFP at different excitation frequencies. To expose the CFP in the experiments, measurements were made from the medial side of the CFP after opening and draining the inner ear. To determine the influence of the inner-ear load on CFP motions, a finite element model was created of the intact ostrich middle ear with inner-ear load included. For quasi-static loads, the CFP performed largely piston-like motions under positive ME pressure, while under negative ME pressure the difference between piston and rocking motion was smaller. For dynamic loads, the CFP motion was almost completely piston-like for frequencies below 1 kHz. For higher frequencies, the motions became more complicated with an increase of the rocking components, although they never exceeded the piston component. When including the inner-ear load to the model, the rocking components started to increase relative to the piston component when compared to the result of the model with unloaded CFP, but only at high frequencies above 1 kHz. In this frequency range, the motion could no longer be identified as purely piston-like or rocking. As a conclusion, the current results suggest that CFP motion is predominantly piston-like below 1 kHz, while at higher frequencies the motion becomes too complicated to be described as purely piston-like or rocking.

Spectral evidence for multi-pathway contribution to the upconversion pathway in NaYF4:Yb3+,Er3+ phosphors.

Although upconversion phosphors have been widely used in nanomedicine, laser engineering, bioimaging, and solar cell technology, the upconversion luminescence mechanism of the phosphors has been fiercely debated. A comprehensive understanding of upconversion photophysics has been significantly impeded because the number of photons incorporated in the process in different competitive pathways could not be resolved. Few convincing results to estimate the contribution of each of the two-, three-, and four-photon channels of near-infrared (NIR) energy have been reported in yielding upconverted visible luminescence. In this study, we present the energy upconversion process occurring in NaYF4:Yb3+,Er3+ phosphors as a function of excitation frequency and power density. We investigated the upconversion mechanism of lanthanide phosphors by comparing UV/VIS one-photon excitation spectra and NIR multi-photon spectra. A detailed analysis of minor transitions in one-photon spectra and luminescence decay enables us to assign electronic origins of individual bands in multi-photon upconversion luminescence and provides characteristic transitions representing the corresponding upconversion channel. Furthermore, we estimated the quantitative contribution of multiple channels with respect to irradiation power and excitation energy.

Imaging via complete cantilever dynamic detection: general dynamic mode imaging and spectroscopy in scanning probe microscopy

We develop and implement a multifrequency spectroscopy and spectroscopic imaging mode, referred to as general dynamic mode (GDM), that captures the complete spatially- and stimulus dependent information on nonlinear cantilever dynamics in scanning probe microscopy (SPM). GDM acquires the cantilever response including harmonics and mode mixing products across the entire broadband cantilever spectrum as a function of excitation frequency. GDM spectra substitute the classical measurements in SPM, e.g. amplitude and phase in lock-in detection. Here, GDM is used to investigate the response of a purely capacitively driven cantilever. We use information theory techniques to mine the data and verify the findings with governing equations and classical lock-in based approaches. We explore the dependence of the cantilever dynamics on the tip–sample distance, AC and DC driving bias. This approach can be applied to investigate the dynamic behavior of other systems within and beyond dynamic SPM. GDM is expected to be useful for separating the contribution of different physical phenomena in the cantilever response and understanding the role of cantilever dynamics in dynamic AFM techniques.

Frequency-domain measurement of the spin-imbalance lifetime in superconductors

We have measured the lifetime of spin imbalances in the quasiparticle population of a superconductor ($\tau_s$) in the frequency domain. A time-dependent spin imbalance is created by injecting spin-polarised electrons at finite excitation frequencies into a thin-film mesoscopic superconductor (Al) in an in-plane magnetic field (in the Pauli limit). The time-averaged value of the spin imbalance signal as a function of excitation frequency, $f_{RF}$ shows a cut-off at $f_{RF} \approx 1/(2\pi\tau_s)$. The spin imbalance lifetime is relatively constant in the accessible ranges of temperatures, with perhaps a slight increase with increasing magnetic field. Taking into account sample thickness effects, $\tau_s$ is consistent with previous measurements and of the order of the electron-electron scattering time $\tau_{ee}$. Our data are qualitatively well-described by a theoretical model taking into account all quasiparticle tunnelling processes from a normal metal into a superconductor.

Internal resonance of an axially moving unidirectional plate partially immersed in fluid under foundation displacement excitation

This paper studies characteristics of 1:3 internal resonances and its bifurcations for an axially moving unidirectional plate partially immersed in a fluid under foundation displacement excitation. The fluid is assumed to be inviscid and incompressible. Based on Von Kármán large deflection equations of thin plates, and consider the influences of axial movement of the plate, axial tension, fluid‐structure interaction and foundation displacement, nonlinear vibration equations of an axially moving plate are established. By applying the Galerkin technique, the vibration equations are discretized, and thus achieve nonlinear differential equations on mode coordinates. Due to the influence of foundation displacement, there are more than two excitation items in the mode equations. Moreover, excitation amplitude is not a constant value, but a function of excitation frequency. Adopting numerical method and approximate analysis method, the equations are solved, moreover frequency‐response curves and time histories under internal resonance system are also obtained, the stability of periodic solution is also discussed. Global bifurcation phenomenon of averaging equations is studied for 1:3 internal resonance system. Changing course of periodic solution of the system and its complex nonlinear dynamic characteristics are revealed through bifurcation diagrams and phase trajectory diagrams.

Response to Acoustic Forcing of Laminar Coflow Jet Diffusion Flames

Toward the goal of understanding and controlling instability in combustion systems, we present a fundamental characterization of the interaction of the buoyancy-induced instability in flickering flames with forced excitation of fuel supply. Laminar coflow diffusion flames were acoustically forced, whose frequency responses were recorded as a function of excitation frequency and amplitude. The evolving structure of such flames was also examined through the use of video analysis and particle imaging velocimetry (PIV). For specific combinations of excitation frequency and amplitude, the frequency response of the flames was found to couple to that of the forcing, where the contribution of natural puffing frequency disappears. Such instances of coupling exhibited many harmonics of the excitation frequency, related indirectly to the natural puffing frequency. We showed how such harmonics form, through application of PIV, and furthermore unveiled insight into the physics of how the flame couples to the forcing under certain conditions. Our frequency response characterization provides quantitative results, which are of utility for both modeling studies and active-control strategies.

Measurement and Simulation of the Induced Current on a Wire Using S-Parameter Method

This study investigates the measurement of current on a wire located inside a cavity with an aperture as a function of excitation frequency with the use of a simple S parameter model. Validation is provided by comparing simulations and measurements. Using this approach, it will be possible to make an accurate prediction of the current distribution on the wire, and thereby to describe potential interference effects to equipment placed along the wire bundle at frequencies of interest.

Numerical investigation of ultrasonic attenuation through 2D trabecular bone structures reconstructed from CT scans and random realizations

In this paper, we compare ultrasound interrogations of actual CT-scanned images of trabecular bone with artificial randomly constructed bone. Even though it is known that actual bone does not have randomly distributed trabeculae, we find that the ultrasound attenuations are close enough to cast doubt on any microstructural information, such as trabeculae width and distance between trabeculae, being gleaned from such experiments. More precisely, we perform numerical simulations of ultrasound interrogation on cancellous bone to investigate the phenomenon of ultrasound attenuation as a function of excitation frequency and bone porosity. The theoretical model is based on acoustic propagation equations for a composite fluid-solid material and is solved by a staggered-grid finite-difference scheme in the time domain. Numerical experiments are performed on two-dimensional bone samples reconstructed from CT-scanned images of real human calcaneus and from random distributions of fluid-solid particles generated via the turning bands method. A detailed comparison is performed on various parameters such as the attenuation rate and speed of sound through the bone samples as well as the normalized broadband ultrasound attenuation coefficient. Comparing results from these two types of bone samples allows us to assess the role of bone microstructure in ultrasound attenuation. It is found that the random model provides suitable bone samples for ultrasound interrogation in the transverse direction of the trabecular network.

A piezoelectric micropump with an integrated sensor based on space-division multiplexing

This paper presents a new concept of a self-sensing piezoelectric micropump based on space-division multiplexing (SDM). By means of dividing the electrode of the piezoelectric diaphragm into two parts of driving unit and sensing unit, a sensor is integrated onto the piezoelectric micropump. A prototype micropump is fabricated from a finished segmented-electrode piezoelectric diaphragm in our laboratory. Through basic experiments using tap water as a working fluid, frequency and voltage characteristics of the flowrate, backpressure and sensing voltage are investigated. The frequency ranging 0–50Hz can be determined by the sensing voltage. When the maximal flowrate of 26.55ml/min is recorded at a driving frequency of 30Hz, the sensing voltage also achieves the maximum of 3.02Vp–p at a fixed excitation voltage of 150Vp–p. Experimental results indicate there is pretty good agreement between the sensing voltage and the flowrate as a function of excitation frequency or voltage. Moreover, it indicates that the piezoelectric micropump itself can accurately determine the optimal frequency through monitoring the sensing voltage with the method of SDM. In the backpressure-voltage experiment, along with the backpressure changing from 0 to the maximum of 7.1kPa at an excitation voltage of 210Vp–p, the sensing voltage also changes from 0 to the maximal voltage of 16.6Vp–p correspondingly at a fixed frequency of 30Hz. Experimental result also shows there is a linear relationship between the backpressure and the sensing voltage. Therefore, both theoretical analysis and experiments prove that the self-sensing method based on SDM could well reflect the variation law of the flowrate and backpressure for piezoelectric micropumps. The micropump design shows the feasibility of our current effort to realize the measurement of the output flow and backpressure without the additional testing instruments.