Dependence Of Resonance Frequency Research Articles

Dynamics of Discotic Fluoroalkylated Triphenylene Molecules Studied by Proton NMR Relaxometry

The Larmor frequency and temperature dependence of the proton nuclear magnetic resonance (NMR) spin-lattice relaxation time was measured in the isotropic and columnar phases of both chain-end fluorinated triphenylene disk-like and fully hydrogenated molecules. In the columnar phase, the results are interpreted in terms of the collective motions, due to the deformations of the columns, and individual molecular translational self-diffusion displacements and rotations/reorientacions. In the isotropic phase, local molecular motions and order fluctuations as a pretransitional effect were considered. The activation energies of the molecular motions of the partially fluorinated molecule were found to be higher than those corresponding to the hydrocarbon homologue. Our findings show a clear difference in the relaxation dispersion between the two liquid crystals homologues. In particular it is observed that the columnar undulations have a much stronger contribution to the relaxation rate in the low frequency regime in the case of the fully hydrogenated triphenylene. The effect of fluorination of the pheripheral chain enhances the columnar mesophase's stability.

Standing second sound wave in many-layer systems

The variation of thermodynamic parameters of superfluid helium in standing second sound wave is studied. Such double-layer systems as heater–helium and helium–detector are considered. Heat transfer in the heater and the detector was described with usual thermal conductivity equation. Temperature and heat transfer in superfluid 4He was described with the system of equations for superfluid with taking into accounts both dissipative thermal conductivity mode and second sound mode. Resonance frequency and amplitude dependence on dissipation at the heater and the detector was studied. The unusual resonance was found in the double-layer system helium–detector.

Fabrication and Testing of Polymer Cantilevers for VOC Sensor

Abstract The mode dependences of resonance frequencies and quality factors in a PMMA (polymethyl methacrylate) cantilever and a PC (Polycarbonate) cantilever at atmospheric pressure are investigated to obtain fundamental data for polymer-based VOC (volatile organic compounds) sensors. The PMMA cantilever and the PC cantilever are fabricated using hot embossing, bonding and polishing techniques. The resonance frequencies and the quality factors are found to vary largely depending on the vibration mode. The measured resonance frequencies correlate well with the theory of undamped resonance frequency of a flexural-vibrating cantilever. The measured quality factors varied from less than 20 to more than 100 depending on the vibration mode. A higher mode tended to exhibit a higher quality factor. The highest quality factor of 100 was obtained on the 4th flexural mode in both the PMMA and PC cantilever.

BioLC-Oscillator: A Self-Powered Wireless Glucose-Sensing System with the Glucose Dependent Resonance Frequency

BioLC-Oscillator: A Self-Powered Wireless Glucose-Sensing System with the Glucose Dependent Resonance Frequency

Energetics of stochastic resonance

In this paper, we discuss the motion of a Brownian particle in a double-well potential driven by a periodic force in terms of energies delivered by the periodic and the noise forces and energy dissipated into the viscous environment. It is shown that, while the power delivered by the periodic force to the Brownian particle is controlled by the strength of the noise, the power delivered by the noise itself is independent of the amplitude and frequency of the periodic force. The implications of this result for the mechanism of stochastic resonance in an equilibrium system is that it is not energy from the noise force which enhances a small periodic force, but rather an increase of energy delivered by the periodic force, regulated by the strength of the noise. We further re-evaluate the frequency dependence of stochastic resonance in terms of energetic terms including efficiency.

Ultrasensitive Magnetometers Based on Carbon-Nanotube Mechanical Resonators

We describe herein how a nanoelectromechanical system based on a carbon nanotube used as a force sensor can enable one to assess the magnetic properties of a single and very small nano-object grafted onto the nanotube. Numerical simulations performed within the framework of the Euler-Bernoulli theory of beams predict that the magnetic field dependence of the nanotube mechanical resonance frequency is a direct probe for the nano-object magnetic properties and that a sensitivity around a few (few hundreds) Bohr magnetic moments at low temperature (room temperature) can be expected.

Preparation and characterization of Ba2Co2Fe12O22 ferrite via glucose sol–gel method

Ba2Co2Fe12O22 (Co2Y) was synthesized by sol–gel method using glucose as chelating agent. X-Ray diffraction studies indicate that sintering temperature as low as 950 °C is sufficient to produce Co2Y ferrites. Co2Y ferrites calcined at 1,000 °C exhibit good magnetic prosperities in high frequency, with the resonance frequency up to 11 GHz and intrinsic permeability about 5 even at 6 GHz. The heat-treated temperature dependence of coercivity, initial permeability and resonance frequency is close related to the particle shape and size.

Numerical Analysis for Acoustic Resonance of One-Dimensional Nonlinear Elastic Bar

The free vibration acoustic resonance of a one-dimensional nonlinear elastic bar has been studied within a framework of the calculus of variation. The one-dimensional deformation function is expanded by a complex Fourier series, and the coefficients as well as the resonance frequency are determined from a direct analysis based on the Ritz method. This numerical analysis revealed that the elastic bar shows prominent nonlinear features such as the amplitude dependence of resonance frequency and the small oscillation of pseudo-nodal points. From a spectral analysis for the first resonant vibration mode, we confirmed nonlinear excitations of high-frequency and/or high-wave-number vibration modes. The mechanism of the mode selection is discussed.

Ultrafast magnetization dynamics in magnetic tunneling junctions

To understand the dynamic magnetic properties and its relationship to ultrafast field sensing, we performed micromagnetic simulations on actual MgO-based magnetic tunneling junction (MTJ) structures. The results indicate that an MTJ with a larger aspect ratio yields a smaller response time under a sudden change of an external magnetic field. Such an effect is mainly due to the coherent rotation from the central cores inside the junction element. Damping coefficient is shown to play an important role in ultrafast sensing and the optimal value need to be a few times larger than typical values found in common ferromagnetically soft materials. Our calculations further show that the response time can be reduced by increasing the free layer thickness and/or its saturation magnetization. Finally, we have obtained the dependence of intrinsic resonance frequency on the sensor size while keeping the same aspect ratio.

Determination of microdroplet contact angles using electrically driven droplet oscillations

Oscillatory deformations of micrometer-sized NaCl-water droplets by an AC electric field are used for contact angle measurements on superhydrophobic surfaces. Contact angles are determined from the dependence of the lowest-order resonant frequency of the electrically driven droplet oscillations on the droplet size. The resonant frequency and size of a droplet are found using whispering gallery mode spectroscopy. Measurements are compared with those performed with direct mechanical driving of the droplets using a piezoelectric transducer, and a good agreement is found. The demonstrated contact angle measurement method can be readily integrated into the planar architecture of microfluidic chips.

Temperature-Dependent Response of Radially Polarized Piezoceramic Cylinders to Harmonic Loadings

Modeling the behavior of a radially polarized, hollow piezoelectric cylinder working at different temperature conditions is the topic of this article. The temperature dependence is taken into account through temperature-dependent material properties, pyroelectricity, and thermal dimension changes, simultaneously. The cylinder is electrically excited at its outer surface by a harmonic voltage. Two different structural boundary conditions being clamped-free and traction-free cases are considered. Symmetry of the structure, boundary conditions, and applied harmonic loads lead to an axisymmetric problem which is solved analytically. Temperature dependence of the first resonance frequency, radial stress, and displacement of the cylinder for different structural boundary conditions are then investigated. It was shown that the resonance frequency of the device is almost temperature independent in the traction-free case while it changes in the clamped-free case. The derived equations are verified by other published data and the finite-element software of Comsol Multiphysics.

Effects of air susceptibility on proton resonance frequency MR thermometry

The temperature dependence of the proton resonance frequency (PRF) is often used in MR thermometry. However, this method is prone to even very small changes in local magnetic field strength. Here, we report on the effects of susceptibility changes of surrounding air on the magnetic field inside an object and their inferred effect on the measured MR temperature. MR phase thermometry was performed on spherical agar phantoms enclosed in cylindrical containers at 7 T. The air susceptibility inside the cylindrical container was changed by both heating the air and changing the gas composition. Changing the temperature of surrounding air from 23 to 69°C caused an apparent MR temperature error of 2 K. When ambient air was displaced by 100% oxygen, the MR temperature error increased to 40 K. The magnetic field shift and therefore error in inferred MR temperature scales linearly with volume susceptibility change and has a strong and nontrivial dependence on the experimental configuration. Air susceptibility changes associated with oxygen concentration changes greatly affect PRF MR thermometry measurements. Air temperature changes can also affect these measurements, but to a smaller degree. For uncalibrated MR thermometry, air susceptibility changes may be a significant source of error.

The use of pressure dependent subharmonic resonance to increase the signal to noise ratio of ultrasound contrast agent imaging.

Conventional methods of subharmonic (SH) imaging of ultrasound contrast agents (UCAs) employ a frequency which is approximately twice the resonance frequency of the UCA (the SH resonance frequency). However, this approach may be limited by a low signal to noise ratio. To address this problem, the Hoff model was solved for several UCAs and the SH resonance frequencies of the bubble were calculated for different applied pressures (PAs). By increasing the PA above the pressure threshold for SH generation, the SH resonance frequency shifts toward lower values. Calculation of the scattered pressure in the regime of stable oscillations show that, if the bubble is sonicated with its pressure dependent SH resonance frequencies, the backscattered signal can be significantly stronger than when sonicating with the conventional SH resonance frequency (although the initiation of the SH occurs at a higher PA). The enhancement of the SH and ultraharmonic amplitudes can be up to 7 and 14 dB, respectively. This enhancement may lead to better UCA SH imaging, especially for molecular based imaging protocols.

Subharmonic response from ultrasound contrast microbubbles for noninvasive blood pressure estimation.

Estimation of local organ‐level blood pressure can help in diagnosing and monitoring heart and vascular diseases. Subharmonic signals from ultrasound contrast microbubbles have been proposed as a noninvasive alternative to the current practice of using a manometer‐tipped catheter. Approximately 10 dB linear decrease in subharmonic component with 25 kPa pressure increase (typical blood pressure variation) has been reported for several contrast microbubbles. A theoretical investigation of the underlying phenomenon will be reported here. First the well‐established model of a free microbubble is studied to show that reduction in subharmonic with ambient pressure increase occurs only below a certain excitation frequency. Above another critical frequency, subharmonic signal increases with ambient pressure. In between the variation is nonmonotonic. Furthermore, where it decreases with ambient pressure, the relationship is linear only above a certain excitation pressure. The behavior is explained by analyzing the dependence of resonance frequency of a bubble on the ambient pressure. The dependence of the critical frequencies on bubble radius and possibly bubble size distribution will be discussed. Behaviors for several models for encapsulated contrast microbubbles will also be reported. [Work Supported by the NSF and the NIH.]

Resonance and Q Performance of Ellipsoidal ENG Subwavelength Radiators

The resonance performance and Q-factor of electrically small ellipsoidal radiators made of epsilon negative (ENG) material is investigated. It is demonstrated that the material polarization can successfully provide resonance radiation at the negative material constitutive parameters. In principle, arbitrary low resonant frequencies for a fixed antenna dimension can be achieved. The dependence of resonant frequency on the shape of the structure is determined. Special attention is devoted to the sphere, thin disk, and long rod, and physical insights into the radiation characteristics and Q (or bandwidth) are highlighted.

Lumped parameter model for resonant frequency estimation of a thermoacoustic engine with gas-liquid coupling oscillation

Gas-liquid coupling oscillation is a novel approach to reducing the resonant frequency and to elevating the pressure amplitude of a thermoacoustic engine. If a thermoacoustic engine is used to drive low-frequency pulse tube refrigerators, the frequency matching between the thermoacoustic engine and the refrigerator plays an important role. Based on an acoustic-electric analogy, a lumped parameter model is proposed to estimate the resonant frequency of a standing-wave thermoacoustic engine with gas-liquid coupling oscillation. Furthermore, a simplified lumped parameter model is also developed to reduce the computation complexity. The resonant frequency dependence on the mean pressure, the gas space volume, and the water column length is computed and analyzed. The impact of different working gases on the resonant frequency is also discussed. The effectiveness of the models is validated by comparing the computed results with the experimental data of the gas-liquid coupling oscillation system. An increase in the mean working pressure can lead to a rise in the resonant frequency, and a lower resonant frequency can be achieved by elongating the liquid column. In comparison with nitrogen and argon, carbon dioxide can realize a lower frequency due to a smaller specific heat ratio.

Resonant frequencies of electroelastic vibrations of piezoceramic plates

The harmonic electroelastic vibrations of a thin ring plate are considered. The effect of boundary conditions on the natural frequencies is studied. The asymptotic properties of the frequency spectrum are determined. The dependence of the resonant and antiresonant frequencies and the dynamic electromechanical-coupling coefficient on the relative size of plate is analyzed

Investigation of the dielectric properties of BaTiO3 single crystals of different qualities by the thermal noise method

Comparative dielectric investigations of bulk barium titanate crystals grown using different techniques have been performed by the thermal noise and traditional methods. The temperature dependences of the permittivity have been used to determine the phase transition temperatures and the Curie-Weiss constants. The investigation performed using the thermal noise method has revealed a number of phenomena that complicate the separation of the noise associated with the change in the impedance of the sample. Possible approaches to the elimination or minimization of contributions from different effects to the total noise voltage have been described. The piezoelectric resonances that also contribute to the total noise signal have been found for the first time and investigated by the thermal noise method. The possible nature of the obtained temperature dependences of the piezoelectric resonance frequencies has been discussed.

Analytical model of the temperature dependent properties of microresonators immersed in a gas

A comprehensive theoretical model of microresonators immersed in a viscous gas of varying temperature is presented and verified by experiments. Analytical expressions for both the temperature dependent resonant frequency and quality factor of the first flexural eigenmode were derived extending Sader’s theory of viscous damping to small temperature variations. The model provides useful implications for the thermal stabilization of microresonators immersed in a gas as well as for the reduction in the influence of the temperature dependent gas properties on the resonant frequency. Finally, an analytical expression is deduced for the mass detection capability of a microresonator that undergoes temperature variations.

Non-monotonic pressure dependence of resonant frequencies of microelectromechanical systems supported on squeeze films

The resonant frequencies of released microcantilevers, microbeams, and microplates are among the most important response characteristics for microelectromechanical systems such as resonators, sensors, and radio frequency (RF) switches. It is generally believed that the resonance frequencies of such structures decrease monotonically as the surrounding gas pressure is increased from vacuum conditions. However, we find that for microbeams supported on gas films the natural frequencies of the device can first increase and then decrease with increasing gas pressure from vacuum, with the extent of non-monotonicity depending on device geometry. This anomalous property of a wide class of microelectromechanical systems is explained in terms of the competing inertial and compressive effects of the supporting squeeze film.