Extended Frequency Range Research Articles

Preparation and Electromagnetic Properties of Carbon Nanofiber/Epoxy Composites

Carbon nanofibers (CNFs) with diameters ranging from 50 nm to 150 nm were prepared from polymer blends of polyacrylonitrile (PAN)/polymethyl methacrylate (PMMA) via a wet-spinning method and subsequent thermal treatment. The main factors that influence the diameter of the CNFs, including molecular weight of PAN, mass ratio of PAN/PMMA, and spinning draw ratio, are discussed. The formation of graphitization structures of the blend fibers during the carbonization process was investigated by Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction spectroscopy. The complex permittivity ϵ (ϵ′ − jϵ″), complex permeability μ (μ′ − jμ″), and microwave absorption properties of CNF/epoxy composites were studied in the X-band frequency range (8.2–12.4 GHz). ϵ′, ϵ″, μ′, and μ″’ were found to increase with increased CNF content. Measured values of these parameters were used to determine the reflection loss at various sample thicknesses, based on a model of a single-layered plane wave absorber backed by a perfect conductor. The calculated results showed that the composite with a CNF content of 8 wt% and a thickness of 2.1 mm would give a maximum reflection loss of −34 dB at 10.5 GHz with the −10 dB bandwidth over the extended frequency range of 8.8–12.4 GHz.

LOW FREQUENCY DAMPER

The low frequency damper is an autonomous equipment for damping vibrations with the 1-20Hz range. Its autonomy enables the equipment to be located in various mechanical systems, without requiring special hydraulic installations. The low frequency damper was designed for damping the low frequency oscillations occurring in the circuit controls of the upgraded IAR-99 Aircraft. The low frequency damper is a novelty in the aerospace field ,with applicability in several areas as it can be built up in an appropriate range of dimensions meeting the requirements of different beneficiaries. On this line an equipment able to damp an extended frequency range was performed for damping oscillations in the pipes of the nuclear power plants. This damper, tested in INCAS laboratories matched the requirements of the beneficiary. The low frequency damper is patented – the patent no. 114583C1/2000 is held by INCAS. Introduction Low frequency vibrations or infrasound occur in the aircraft system control. They operate permanently causing the pilot fatigue, discomfort, and in some cases induce a delay between the pilot reaction and the deflection of the control area. The analysis and synthesis of a low frequency damper came as a necessity for the IAR-99 aircraft. At longitudinal controls the pilot is subject to tiring efforts in the small and medium deflection area of the control surface. The designed equipment was studied in the laboratory through numerical simulations by the INCAS specialists which made evident the decrease and/or elimination of these vibrations. It is also useful to install in the mechanical circuits of the automatic systems of various complex installations from different activity fields a 1÷20 Hz range frequency damper. This equipment contributes substantially to the increase of the operating life of those installations. Construction The low frequency damper shown in Fig.1 consists of two modules designed independently: a hydrostatic module and a mechanical module. These modules are assembled inside a common cylinder. The hydrostatic module – located in the central area of the equipment provides the vibrations damping and the damage function. The elements of this module form a compact construction as follows: double -acting hydraulic cylinder; Supply valve; Piston hydraulic accumulator; Throttle; Relief valves to overpressure; Bleeder utilised only when loading the module with hydraulic fluid. The mechanical module – developed in cascade, from the exterior surface of the hydrostatic module to the common cylinder of the equipment, maintains the neutral position of the equipment and implicitly of the aircraft control circuit, giving an extra effort to the stick which is proportional to its stroke, thus, generating a sensibility decrease of the control in the small deflection area. The elements of this module are concentrically assembled with the hydraulic module in the following configuration: Special bush; Spring operating beakers Compression spring; The ends of the common cylinder are provided with arc limiting caps and installation elements. 64 INCAS BULLETIN No 1/ 2009 DOI: 10.13111/2066-8201.2009.1.1.12

Experimental investigation on pore size effect on the linear viscoelastic properties of acoustic foams

This paper presents linear viscoelastic measurement on a large frequency range (10(-2)-10(8) Hz) for cross-linked polymer open-cell foams of same density and different pore sizes. This large extension of frequency range is obtained by the validation of a frequency-temperature superposition principle, commonly used with polymers. At higher frequencies, the shear moduli are independent of the pore size. In acoustical insulation range (1 Hz-16 kHz), the shear moduli decreases with the foams' pore size.

Dielectric resonances at optical frequencies using metal nanoshells

A first-principles systematic investigation is reported to identify the dielectric (quasistatic) resonances properties of layered metal–dielectric particles. Specifically, we present remarkable results of finite-element simulations that model the effective permittivity of two-dimensional (or cross sections of infinite three-dimensional parallel, infinitely long, identical, circular cylinders, where the properties and characteristics are invariant along the cylinder axis) three-phase heterostructures made of coated discs embedded in a dielectric host. The derivation assumes that the quasistatic approximation holds valid even at relatively high angular frequencies of the harmonic external field. The coating (shell) material is taken as a conventional Drude noble metal and the core and shell phases are assumed to be dielectric, non-dispersive and lossless. One of the main results of this study is that the real part of the effective permittivity of an isotropic host matrix containing metallic nanoshells is tunable from negative through zero to positive values. The simulations reveal that a Drude–Lorentz-like dielectric function reproduces the overall shape of the effective complex permittivity of these nanostructures over an extended range of frequencies. The data nearly make a complete circle when all the (ε′, ε″) pairs for the various frequencies are included over the bandwidth about each resonance. For a given resonant frequency, the resonant features are determined by the complex permittivity of the metal (shell) phase, but also depend on the dielectric environment. Kramers–Kronig relations provide a consistency test for data of the complex effective permittivity. Numerical simulations are also used to investigate the sensitivity of field enhancement effects against frequency. The position and width of the resonance have the ability to be tailored by the selection of constituent metal phases and their nominal thicknesses.

Interpretation of Broadband Ocean-Bottom Seismometer Horizontal Data Seismic Background Noise

The seismic background noise observed on the OBS (ocean-bottom seismometer) recordings is well known to be strong. With the recent appearance of broadband OBS, the background noise in the extended frequency range is even more significant. By taking into account the various degrees of freedom of the seismic sensors, it has been shown that the seismometers are not only translational sensors but also rotational ones with a sensitivity that depends on the installation. The strong similarity of the background noise recorded on the two horizontal traces of the broadband Hippocampe OBS data deployed during the Rosmarin program is interpreted as a rotational motion around the vertical axis. This torsion motion is so important that it dominates the other noise seismic contributions like translations and tilt (rotational motion around a horizontal axis) and represents more than 90% of the background noise signal at the periods between 5 and 50 sec. Use of the difference between the two horizontal traces makes it possible to reduce the background noise made up primarily of this torsion motion and thus to enhance the displacement signal but also strongly modify it. An effort should be made to better control the installation of the sensors on the seafloor. A better solution would be to simultaneously record rotations at the same position as that of the seismometer. Such data will permit the correction of the seismic traces of the OBS data and strongly reduce the background noise.

A Hybrid Winding Model of Disc-Type Power Transformers for Frequency Response Analysis

The paper presents a hybrid model of disc-type power transformer winding for frequency response analysis (FRA) based on traveling wave and multiconductor transmission line (MTL) theories. Each disc of a winding is described by traveling wave equations, which are connected to each other in a form of MTL matrix model. This significantly reduces the order of the model with respect to previously established MTL models of transformer winding. The model is applied to frequency response simulation of two single-phase transformers. The simulations are compared with the experimental data and calculated results using lumped parameter and MTL models reported in other publications. It is shown that the model can be used for FRA result interpretation in an extended range of frequencies up to several mega Hertz and resonance analysis under very fast transient overvoltages (VFTOs).

Synthesis, Characterization and Electromagnetic Wave Absorption Properties of Z-Type Hexaferrites Prepared by Molten Salt Method

The Co2Z hexaferrites powders, Ba3Co2Fe24O41, were synthesized by the molten salt method. The results indicated that the ferrite powders possess hexagonal shape and are well separated from one another. The composite with 40vol% Co2Z content has shown a minimum reflection loss of -24dB at 9.5GHz with a -20dB bandwidth over the extended frequency range from 8.3 to 10.2GHz in a quite thin sample with a thickness of 3.0mm. The molten salt method could be a potential way to enhance the properties of the composites filled with hexagonal ferrite powders for electromagnetic applications.

Microrheology of complex fluids using optical tweezers: a comparison with macrorheological measurements

The increasing interest in the mechanical properties of complex systems at mesoscopic scale has recently fueled the development of new experimental techniques, collectively indicated as microrheology. Unlike bulk-based approaches (macrorheology), these new techniques make use of micrometric probes (usually microspheres) which explore the mechanical properties of the surrounding medium. In this paper we discuss the basic idea of microrheology and we will focus on one specific technique based on optical tweezers (OT). The discussion starts from Newtonian fluids to tackle the more general case of complex fluids, also showing results of these techniques on solutions of a relevant biomolecule: hyaluronic acid (HA). In particular, we study the viscoelastic properties of low molecular weight HA (155?kDa) at low ionic strength over an extended frequency range (0.1?1000?Hz) and in a wide range of concentrations (0.01?20?mg?ml?1), which include both the dilute and semidilute regime. In the concentration range here explored and within the test frequencies covered by our techniques, samples prevalently exhibit a viscous behavior, the elastic contribution becoming significant at the highest concentrations. By comparing OT outcomes to those obtained by a traditional rheometer, we found that they were in good agreement in the overlapping frequency range of the two techniques, thus confirming the reliability of the microrheological approach.

Effects of frequency allocation on lexical tone identification by Mandarin-speaking children with a cochlear implant

Conclusion. Frequency allocation with extended frequency ranges yielded significantly higher accuracy in pediatric CI recipients’ lexical tone identification. These findings suggest that frequency allocation with extended frequency ranges may be useful in improving lexical tone recognition for at least some pediatric CI recipients. Objectives. To assess the effects of frequency allocation on lexical tone identification by Mandarin-speaking children with a cochlear implant (CI). Subjects and methods. In a prospective study, 15 prelingually deafened children between 7.17 and 16.17 years of age served as participants. Using Med-el CI devices, each participant's accuracy in lexical tone identification was compared in two conditions: first, the experimental condition, i.e. use of the extended frequency range from 233 to 8501 Hz; second, the control condition, i.e. use of the participant's clinically assigned frequency range from 300 to 8404 Hz. Results. The group mean of pediatric CI users’ accuracy in lexical tone identification was 88.02% (SD = 6.31%) in the experimental condition and 83.82% (SD = 9.84%) in the control condition. The group mean was 4.20% (SD = 5.48%) higher in the experimental condition than that in the control condition; this difference was statistically significant (t(14) = 2.97, p=0.010).

Broadband Viscoelastic Spectroscopy: A New Technique for Characterizing Rheological Behavior of Solid Foods

The broadband viscoelastic spectroscopy (BVS) is a new technique for characterizing rheological behavior of solid foods. Conventional methods to obtain rheological behavior of materials over a broad range of frequencies are time-temperature superposition (TTS) technique and direct method using specific instruments. TTS technique works only for materials that are thermorheologically simple and over a limited range of about three decades of time or frequency. Materials for which the measured curves do not overlap by making a horizontal shift on the frequency axis are thermorheologically complex. Food systems are inherently complex with multiple constituents rendering them thermorheologically complex. Thus, it is necessary to acquire data directly over a wide frequency range in a single test to truly characterize such complex materials. The problems associated with TTS can be circumvented by using the BVS, which is capable of directly measuring viscoelastic properties over an extended frequency range. Applications of the BVS are introduced for characterizing viscoelastic behavior of food materials. Some advantages of the BVS technique over the TTS and other available instruments are also discussed.

Dielectric properties of differently flexible polyions: a scaling approach

The dielectric relaxations associated with counterion polarization along some typical polyion lengths have been measured in an extended frequency range (from 10 kHz to 2 GHz) for four different polyelectrolyte solutions, differing for the polyion molecular weight and the backbone stiffness. Here, we deal with the so-called intermediate dielectric relaxation, falling between the polarization process concerning the whole polyion chain and the polarization process associated with the field-induced re-orientation of the water molecule dipoles. These observed intermediate relaxations have been characterized by means of two parameters, i.e., the dielectric strength Deltaepsilon and the relaxation frequency nu0, and their dependence on the polyion concentration has been described according to the scaling model of a polyelectrolyte solution. These dependencies follow the expected exponents of the scaling laws, both for the dilute and semidilute (unentangled and entangled) regimes. The different concentration regimes evidenced from dielectric relaxation measurements are in very good agreement with the ones determined by means of the zero-shear viscosity measurements. Our results confirm that the intermediate dielectric relaxation in polyelectrolyte solutions should be attributed to counterion fluctuations along some segments (e.g. the subunits of the Mandel model) of the polyion chain, independently of its overall stiffness. This counterion polarization effect is rather confined to the local structure of the polyion chain instead of the whole chain and it is largely independent of the polyion conformation.

Pulse-train control of branching processes: Elimination of background and intruder state population

The authors introduce and describe pulse train control (PTC) of population branching in strongly coupled processes as a novel control tool for the separation of competing multiphoton processes. Control strategies are presented based on the different responses of processes with different photonicities and/or different frequency detunings to the pulse-to-pulse time delay and the pulse-to-pulse phase shift in pulse trains. The control efficiency is further enhanced by the property of pulse trains that complete population transfer can be obtained over an extended frequency range that replaces the resonance frequency of simple pulses. The possibility to freely tune the frequency assists the separation of the competing processes and reduces the number of subpulses required for full control. As a sample application, PTC of leaking multiphoton resonances is demonstrated by numerical simulations. In model systems exhibiting sizable background (intruder) state population if excited with single pulses, PTC leading to complete accumulation of population in the target state and elimination of background population is readily achieved. The analysis of the results reveals different mechanisms of control and provides clues on the mechanisms of the leaking process itself. In an alternative setup, pulse trains can be used as a phase-sensitive tool for level switching. By changing only the pulse-to-pulse phase shift of a train with otherwise unchanged parameters, population can be transferred to any of two different target states in a near-quantitative manner.

Square-lattice large-pitch hollow-core photonic crystal fiber

We report on the design, fabrication and characterization of silica square-lattice hollow core photonic crystal fibers optimized for low loss guidance over an extended frequency range in the mid-IR region of the optical spectrum. The fiber's linear optical properties include an ultra-low group velocity dispersion and a polarization cross-coupling as low as -13.4 dB over 10 m of fiber.

Preparation and microwave–absorbing properties of NiFe2O4-polystyrene composites

Abstract NiFe 2 O 4 nanoparticles were synthesized by the polyacrylamide gel method with acrylamide as the monomer and N,N ′-methylenediacrylamide as lattice agent. The average crystallite sizes of the nickel ferrites annealed at 500, 600 and 800 °C are about 10, 30 and 50 nm, respectively. Ferrite–polystyrene composites were made by hot pressing, and microwave-absorbing properties of the composites with different contents of 35, 45, 55 and 65 wt% ferrite were investigated by testing complex permeability and complex permittivity in the X-band (8.2–12.4 GHz) frequency range. All the parameters, e ′, e ″, μ ′ and μ ″, increase with increasing ferrite content. The reflection losses were calculated based on a model of a single-layered plane wave absorber backed by a perfect conductor. The composite with 65 wt% ferrite content shows a minimum reflection loss of −13 dB at 11.5 GHz with a −10 dB bandwidth over the extended frequency range of 10.3–13 GHz for an absorber thickness of 2 mm.

Dynamic train–turnout interaction in an extended frequency range using a detailed model of track dynamics

A time domain solution method for general three-dimensional dynamic interaction of train and turnout (switch and crossing) that accounts for excitation in an extended frequency range (up to several hundred Hz) is proposed. Based on a finite element (FE) model of a standard turnout design, a complex-valued modal superposition of track dynamics is applied using the first 500 eigenmodes of the turnout model. The three-dimensional model includes the distribution of structural flexibility along the turnout, such as bending and torsion of rails and sleepers, and the variations in rail cross-section and sleeper length. Convergence of simulation results is studied while using an increasing number of eigenmodes. It is shown that modes with eigenfrequencies up to at least 200 Hz have a significant influence on the magnitudes of the wheel–rail contact forces. Results from using a simplified track model with a commercial computer program for low-frequency vehicle dynamics are compared with the results from using the detailed FE model in conjunction with the proposed method.

EfficientO(N2)approach to solve the Bethe-Salpeter equation for excitonic bound states

Excitonic effects in optical spectra and electron-hole pair excitations are described by solutions of the Bethe-Salpeter equation (BSE) that accounts for the Coulomb interaction of excited electron-hole pairs. Although for the computation of excitonic optical spectra in an extended frequency range efficient methods are available, the determination and analysis of individual exciton states still requires the diagonalization of the electron-hole Hamiltonian $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{H}$. We present a numerically efficient approach for the calculation of exciton states with quadratically scaling complexity, which significantly diminishes the computational costs compared to the commonly used cubically scaling direct-diagonalization schemes. The accuracy and performance of this approach is demonstrated by solving the BSE numerically for the Wannier-Mott two-band model in $\mathbf{k}$ space and the semiconductors MgO and InN. For the convergence with respect to the $\mathbf{k}$-point sampling, a general trend is identified, which can be used to extrapolate converged results for the binding energies of the lowest bound states.

Establishing a link to given radiated emission limits during extending the frequency range above 1 GHz

Abstract. Up to now most limits for radiated emission are given as values for the electrical field strength. Battermann, 2007 has shown that the frequency range extension for radiated emission measurements above 1 GHz generates a lot of problems while performing the test on a classical test site as depicted in Fig. 1. This paper will give a motivation to use an other measurand namely the total-radiated-power than electrical field strength by using alternative test sites like reverberation chambers, TEM-waveguides, Fully Anechoic Rooms (FAR) etc. Nevertheless most of the existing standards still specify electrical field strength limits. This paper will show how to set the parameters in the given algorithm to establish a link between measured total radiated power and equivalent electrical field values.

A new setup for measuring the mechanical properties of porous materials

A new device for determining complex Young's modulus of porous materials in a extended frequency range is proposed. Classical methods are based on quasitatic or dynamic response of porous material. These methods generaly neglect the coupling between the surrounding fluid and the porous frame so that they are restricted to low frequency range (<100 Hz) or specific sample shape. Dynamic methods provide relevant information only at the resonance frequencies of the frame. The proposed method extends the quasistatic method towards high frequencies: 1. the porous sample is setup in a cavity in order to avoid the coupling with the external fluid, 2. a specific electrodynamic transducer has been developped to get the mechanical impedance of the sample from the measurement of the electrical impedance, 3. mechanical properties of the frame are derived by inverse method using Biot theory so that the frequency range is not restricted to the quasistatic domain. First results obtained with a prototype validate the method in comparison with two classical methods.

Microwave absorption properties of Mn- and Ti-doped strontium hexaferrite

Abstract M-type strontium hexaferrite SrFe 9 Mn 1.5 Ti 1.5 O 19 was synthesized by solid-state method. X-ray diffraction (XRD), scanning electron microscopic (SEM), energy dispersive spectrometer (EDS) and susceptometer were used for characterization of obtained powders. The ferrite–polymer composites with different ferrite ratios of 50%, 60%, 70% and 80% in polyvinylchloride matrix were prepared. The microwave absorption properties of these composites were investigated in 12–20 GHz frequency range. The results showed that the particle size had a wide range of distribution and the magnetoplumbite structure was formed in the sample. The composite with 70% ferrite content has shown a maximum reflection loss of −26 dB at 18.84 GHz with the −15 dB bandwidth over the extended frequency range of 16.4–19.4 GHz for an absorber thickness of 1.8 mm. It was concluded that the prepared composites could be good candidates for electromagnetic compatibility and other practical applications at high frequency.

Dielectric-Parameter Measurements of SiC at Millimeter and Submillimeter Wavelengths

High-precision continuous spectra of the absorption coefficient, refractive index, complex dielectric permittivity, and loss tangent for several silicon carbide (SiC) specimens are reported in this paper over a broad millimeter- and submillimeter-wave range for the first time. Measurements have been successfully performed using three different types of specially constructed spectrometer systems: a dispersive Fourier transform spectroscopy, an automated 60-GHz open resonator, and a free-space quasi-optical millimeter-wave spectrometer equipped with high-power backward-wave oscillator sources and their associated state-of-the-art dielectric-measurement techniques. Data are presented as continuous functions of frequency from 30 to 600 GHz. The employment of various measurement systems and techniques ensured the measurement of polycrystalline SiC specimens with various degrees of absorption and dispersion characteristics over an extended frequency range with high precision. Results presented here provide comprehensive information of SiC on its optical and dielectric behavior as a function of frequency and purity for its potential application in semiconductor and radio-frequency devices and circuits. An error analysis of measured dielectric-parameter results is also provided.