Low Frequency Absorption Band Research Articles

Expanding the Low-Frequency Bandwidth of Delayed Resonators Using Quasi-Zero-Stiffness Structures

We propose a sophisticated vibration suppression technique termed the Quasi-Zero-Stiffness Delayed Resonator (QZS-DR) and delve into the associated challenges of control design, analysis, and optimization. This paper introduces the concept of a delay-independent resonant frequency, providing a theoretical explanation of the interplay between system stiffness and the resonator's resonant frequency. Utilizing the unique high-static-low-dynamic stiffness properties of quasi-zero-stiffness structures, the QZS-DR significantly reduces the dynamic stiffness of the absorber system. This reduction facilitates the expansion of the absorber's low-frequency absorption band. Our results demonstrate that the QZS-DR notably enhances the vibration absorption capabilities of the delayed absorber across low-frequency ranges, proving its substantial applicability in sectors prone to low-frequency vibrations such as automotive seating, marine vessels, and structural engineering.

Spectroscopy of a borosilicate crown glass in the wavelength range of 0.2 µm–15 cm

We report on the experimental studies of the interaction of electromagnetic radiation with borosilicate crown glass K108 in the ultrawide frequency range from 0.002 to 1500 THz. Four different types of spectrometers are used to measure the reflectivity and/or transmittance spectra. Spectral dependences of the complex dielectric permittivity, refractive index and extinction coefficient are extracted from the measured spectra. The optical properties of the borosilicate crown glass in the microwave spectral range (2–23 GHz) are investigated for the first time. There are three bands of anomalous dispersion in the spectral curve of the real part of the dielectric permittivity. Each band of the anomalous dispersion corresponds to an absorption band, which is clearly seen in the spectrum of the imaginary part of the dielectric permittivity. The wide low-frequency absorption band is attributed to the Boson peak. Two other examined absorption bands represent sharp peaks and are associated with the Si–O–Si bending and Si–O stretching vibration modes. The results of the study can be useful in the development of optics based on the borosilicate crown glass.

Design of Miniaturized Frequency-Selective Rasorber With Embedded Dual-Bow Resonators

This letter proposed a miniaturized frequency-selective rasorber (FSR). This FSR consists of a conducting sheet layer with embedded dual-bow resonators and a bandpass frequency-selective surface layer with a cross-ring slot. The structure exhibits absorption–transmission–absorption (A-T-A) characteristics that shows excellent transmission performance in 2.4 GHz band. The unit size of structure is 0.113 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda _{L}$</tex-math></inline-formula> and the thickness is 0.071 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda _{L}$</tex-math></inline-formula> , where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda _{L}$</tex-math></inline-formula> is the lowest frequency of the absorption band satisfying 80% absorption rate. The simulation results show that the center frequency of the transmission band is located at 2.42 GHz with 0.24 dB insertion loss. Under the condition of 80% absorption rate, the absorption bandwidth is 58.82% for the low-frequency absorption band, and 58.6% for the high-frequency absorption band. The concept of slope in mathematics is introduced into FSR for the first time to define steep drop characteristics. Finally, in order to verify the correctness of the analysis results, a prototype of FSR with 20×20 units is processed and measured, and the measured results are in good agreement with the simulation results.

Miniaturized frequency selective rasorber with a X‐band transmission band using cross band resonator

This article designs a miniaturized frequency selective absorber (FSR) based on X-band. The FSR consists of a resistor sheet and a bandpass frequency selective surface (FSS). It has a wide low frequency absorption band and a low insertion loss wide high frequency transmission band. The resistor element is a hexagonal metal ring with lumped resistance. Each side of the ring is inserted with three cross metal branches. The branch is equivalent to the λh/4 short-circuit branch. Three types of branches with different lengths can be used to achieve wide-band transmission. The band-pass FSS is a three-layer structure, which is composed of two layers of circular metal patches and a layer of circular aperture. The band-pass FSS must coincide with the transmission band of the resistor sheet. The resistor sheet is placed on the band-pass FSS. At low frequencies, FSR functions as an absorber. The absorption bandwidth of 10 dB is 2.2 to 6.3 GHz. At high frequencies, the transmission band is almost transparent. At this time, the 1 dB transmission bandwidth is 8.1 to 12.1 GHz. FSR has good angular stability and polarization stability. A prototype of the structure was made and measured to verify the correctness of the design.

Impact of Lanthanum-Doping on the Physical and Electrical Properties of Cobalt Ferrites

Cobalt ferrites have attracted extraordinary attention due to their high coercivity, chemical stability, and mechanical hardness. Lanthanum doped-cobalt ferrites having chemical formula CoLaxFe2-xO4 with composition x = (0.00, 0.015, 0.045, 0.060) were synthesized by chemical co-precipitation method. The prepared samples were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and current-voltage (I-V) technique. The structure of the crystal was analyzed by X-ray diffraction. The crystallite size of nanoparticles was examined in the range of 21–25 nm, and a fluctuating trend was found with the inclusion of La3+ cations. The X-ray diffraction patterns verify the contraction of lattice constant and unit cell volume with the substitution of La3+ cations except for the concentration of x = 0.060. Lattice constant was in the range of 8.34 A–8.41 A while unit volume cell was in the range of 580 A3–596 A3. The resistivity of all samples was calculated by the application of two probes I-V technique. The maximum resistivity of the order of 81.129 × 105 Ω cm was found for the concentration of x = 0.060 at 723 K which makes it useful for high-frequency gimmicks applications. The resistivity and drift mobility were found inversely related to each other. The inverse relation low-frequency absorption band and high-frequency absorption bands were analyzed by Fourier-transform infrared spectroscopy technique.

Ultra-broadband terahertz absorption using bi-metasurfaces based multiplexed resonances.

In this paper, we demonstrate an ultra-broadband terahertz (THz) bi-metasurfaces absorber composed of two stacking metasurfaces backed by a metallic ground plane. The bottom metasurface consists of four multiplexed cross resonators with different geometries on a thin parylene layer, achieving a bandwidth of 3.80 THz with the absorption higher than 50% at high frequency. Meanwhile, the top metasurface, including two multiplexed cross resonators with different sizes on a relatively thicker parylene layer, provides a low frequency absorption band with an additional Salisbury screen absorption peak that connects the two absorption bands of the two metasurfaces, therefore enabling an ultra-broadband absorption. The experimental absorption spectrum of the bi-metasurfaces shows a bandwidth of 4.46 THz while the absorption exceeding 50% and a full width at half maxima (FWHM) of 97.7%. The ultra-broadband absorber will be a promising candidate for THz broadband detection.

Effect of temperature on structural, morphological and magnetic properties of Cd0.7Co0.3Fe2O4 nanoparticles

Cadmium-substituted cobalt ferrite (Cd0.7Co0.3Fe2O4) nanoparticles were synthesized using a chemical synthesis method and synthesized particles were calcinated at 300°C and 600°C respectively. The samples were characterized in order to understand the temperature effect on structural, morphological, thermal, and magnetic properties. X-ray diffraction data confirm the formation of single-phase cubic structure and the average grain sizes were evaluated. The microstructural features were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and compositional analysis carried out by energy dispersive spectroscopy (EDS). A vibrating sample magnetometer (VSM) was used to investigate the magnetic properties. The hysteresis curves of Cd0.7Co0.3Fe2O4 nanoparticles show enhancement of the coercivity with the increasing calcinated temperature. This enhancement is attributed to the transition from a multi-domain to a single-domain nature. The high and low frequency absorption bands of Cd0.7Co0.3Fe2O4 were investigated using FT-IR analysis.

Elastic properties of germanium substituted lithium ferrite

Polycrystalline ferrites with general formula Li0.5+0.5xGexFe2.5−1.5xO4 (where x=0.0, 0.1, 0.2, 0.3 and 0.4) were prepared by the standard ceramic technique. The analysis of XRD patterns revealed the formation of single-phase cubic spinel structure. The lattice constant was found to decrease with the increase in germanium content. Infrared spectra were performed at room temperature in the wave number range of 200–1000cm−1. Two prominent bands were observed; high frequency band ‘υ1’ around 583cm−1 is assigned to the tetrahedral complexes and low frequency band ‘υ2’ around 392cm−1 is assigned to the octahedral complexes. Low frequency absorption band ‘υ3’ appeared at 240cm−1 is assigned to lattice vibrations. Force constant for the tetrahedral and octahedral sites was calculated by using IR data. Elastic moduli such as Young’s modulus (E), bulk modulus (B), modulus of rigidity (G), Poisson ratio (σ) and Debye temperature (θD) were determined. In the present work the elastic moduli increase with germanium content whereas Poisson’s ratio almost remains constant. A linear relationship between Debye temperature and average sound velocity was observed.

Structural and magnetic studies on Zr doped ZnO diluted magnetic semiconductor

In this study, Zirconium doped Zn1−xZrxO (with x = 0.00–0.10) samples have been prepared by formal solid-state reaction technique. The Zr doped ZnO samples annealed at 1100 °C and characterized by different characterization techniques, such as X-ray diffraction (XRD), Scanning electron microscope (SEM), Vibrating sample magnetometer (VSM) and Fourier transform infrared spectroscopy (FTIR). The X-ray diffraction (XRD) used to study the structural properties. XRD pattern showed that lattice parameters, “a”, “c”, unit cell volume and Zn–O bond length increase with doping content (x ≤ 0.04) where as these decrease with x > 0.04. On the other hand, reverse trend observed with lattice distortion. The crystallite size decreases with increasing doping content of Zr. FTIR employed to investigate functional chemical bonding properties of different elements and compounds present in materials. The low, medium and high frequency absorption bands observed at 630, 1500 and 3435 cm−1, which were the common features of Zn–O, H–O–H and O–H bond respectively. SEM used to study surface morphology and measured grain size of specimen. The surface becomes dense and grain size decreases with increasing degree of Zr contents. The SEM micrograph also shows the presence of spherical micro size particles and formation of pores in samples. Magnetic properties were obtained using VSM. The samples exhibit room temperature ferromagnetism. The magnetic hysteresis loops show variation in the value of magnetic parameter. The saturation magnetization (Ms) and coercivity (Hc) decrease, while remanence magnetization (Mr) shows gradually increasing trend with Zr content. VSM measurement reveals that sample Zn0.96Zr0.4O show better result as compared to x = 0.06–0.10.

Microwave assisted combustion synthesis of CdFe2O4: Magnetic and electrical properties

CdFe2O4 particles were synthesized by the microwave assisted combustion method using two different fuels—glycine and urea. Microwave heating provides higher chemical yield within a minute. The synthesized particles were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), ac impedance spectroscopy, vibrating sample magnetometry (VSM) and electron spin resonance (ESR) methods. XRD analysis shows the cubic structure of CdFe2O4. The high and low frequency absorption bands of CdFe2O4 were found using FTIR analysis. Spherical morphology was revealed from the SEM images. ESR and VSM measurements reveal the antiferromagnetic behavior of CdFe2O4. The electrical conductivities of CdFe2O4 synthesized using glycine and urea are 6.5×10−7Scm−1 and 4.7×10−8Scm−1 respectively at 240oC. At elevated temperatures an occurrence of increase in conductivity was observed, which indicates the semiconducting behavior of CdFe2O4. The dielectric spectral analysis reveals that dielectric constant of CdFe2O4 decreases with frequency and increases with temperature.

Structural and magnetic phase evolution study on needle-shaped nanoparticles of magnesium ferrite

Nanoparticles of magnesium ferrite (MgFe 2O 4) have been synthesized by chemical co-precipitation route. The microstructure, infrared spectral and magnetic properties have been studied by means of X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, infrared spectroscopy, high field magnetization, low field ac susceptibility and Mossbauer spectroscopic measurements. Transmission electron microscopy (TEM) observation showed that the nanocrystals present a needle-like shape with an aspect ratio of around 3.5 and long axis of 37 nm. The kinetics of needle-shaped nanocrystals formation has been discussed. The Mg 2+ seems to play key role in governing the rate of growth of growing planes of nanocrystals. The magnetic behaviour is explained by invoking the concept of super-paramagnetism assuming core–shell structure of the nanoparticles. Mossbauer spectral and susceptibility studies showed that the as-prepared nano-sized ferrite is super paramagnetic at room temperature and magnetic ordering evolves on baking and annealing the nanopowdered sample due to increase in the crystalline size. The high temperature annealing transforms the nanostructured ferrite to ordered magnetic structure of ceramic ferrite having long range ferrimagnetic ordering. Infrared (IR) spectral analysis reveals the weak high frequency shoulder due to disparity in the masses of cations present at tetrahedral sites, while splitting of low and high frequency absorption bands has been explained on the basis of Jahn–Teller effect in Fe 2+-ions which lead to a non-cubic component of the crystal field potential.

Molecular miscible blend of poly(2-cyano-1,4-phenyleneterephthalamide) and polyvinylpyrrolidone characterized by two-dimensional correlation FTIR and solid state 13C NMR spectroscopy

Miscibility of blends of poly(2-cyano-1,4-phenyleneterephthalamide/polyvinylpyrrolidone) (CN-PPTA/PVP) was investigated by dilute solution viscometry, two-dimensional (2D) correlation Fourier transformed infrared (FTIR) spectroscopy and solid state 13C NMR spectroscopy. It was shown that a large proportion of the PVP, the water-soluble component, could not be removed from CN-PPTA by extraction with water, and even with boiling water for blend films, suggesting that the flexible aliphatic PVP chain forms a blend with the rigid aromatic CN-PPTA chain through strong intermolecular interaction making it too difficult to dissolve even in boiling water. Viscometry on a polymer mixture of dilute solution showed that [ η] exp exhibited larger value than [ η] theo in all mixtures used in this experiment, suggesting occurrence of a strong attractive interaction between the two polymers. 2D correlation FTIR spectroscopy revealed that the carbonyl absorption band of PVP at 1675 cm −1 shifted to a new low frequency absorption band at 1640 cm −1 with a change of 35 cm −1, suggesting strong hydrogen bonding with NH (amide II) proton of CN-PPTA. Another new absorption band at 1685 cm −1 was due to the carbonyl absorption band of CN-PPTA shifting to a higher frequency than that at 1662 cm −1, indicating that some of the carbonyl groups in the CN-PPTA components of the blends were in a free state or in a non-hydrogen bonded state as a consequence of the participation of NH proton of CN-PPTA in hydrogen bonding, resulting in the absorption bands of NH bend deformation of CN-PPTA at 1542 and 1313 cm −1 being shifted to higher wavenumber of 1556 and 1324 cm −1, respectively. Solid state 13C NMR spectroscopy revealed a chemical shift for C O of the PVP component in the blend fiber changing down-field (shift to left) at 177.346 ppm with a difference of 1.812 ppm; this was due to a lower electron density around the carbon atom of C O of lactam via hydrogen bonding with NH proton of amide in the CN-PPTA component, suggesting that a homogeneous blend of the CN-PPTA and PVP was produced on a molecular scale via hydrogen bonding.

Determination of Planck Mean Absorption Coefficients for Hydrocarbon Fuels

Infrared transmission of propane, n-heptane and propylene samples was measured using Fourier transform infrared spectroscopy (FTIR) for temperatures ≤1000 K to facilitate calculation of absorption coefficients of fuel molecules at temperatures representative of non-premixed flame interiors. Spectrally resolved fits of the absorption coefficient data using a semi-empirical quantum-based expression provide a basis for calculating infrared spectra at any temperature. Experimentally-derived Planck mean absorption coefficients (κp) of these fuels are compared with that of methane calculated from the HITRAN database since methane absorption has been used to model fuel absorption in fires. κP's for propane and heptane have similar characteristics over the entire temperature range. Methane and propylene with their higher proportion of absorption in low frequency bands have peak κP at lower temperatures where blackbody radiation intensity peaks near the spectral range of these bands. Propylene with low frequency absorption bands associated with the C=C bond has the highest κP at temperatures <600 K. N-heptane has the largest κP at temperatures ≥800 K where blackbody emission intensity peaks near or above the spectral range of the C–H stretching bands. Implications of these results on fuel absorption of radiative heat transfer in flames are discussed.

The state of metals in the Pt/Al2O3 and (Pt-Cu)/Al2O3 catalysts as indicated by IR spectroscopy with isotope dilution of 12C16O with 13C18O molecules

Adsorption of 13C18O+12C16O mixtures on the Pt(2.9%)/γ-Al2O3, (Pt(2.6%)+Cu(2.7%))/γ-Al2O3, and (Pt(2.6%)+Cu(5.1%))/γ-Al2O3 catalysts was studied by FTIR spectroscopy. On the metallic Pt surface at coverages close to saturation, CO is adsorbed both strongly and weakly to form linear species for which the vibrational frequencies of the isolated 13C18O molecules adsorbed on Pt are ∼1940 and ∼1970 cm−1, respectively. The redistribution of intensities of the high-and low-frequency absorption bands in the spectra of adsorbed 13C18O indicates that these linear forms are present on the adjacent metal sites. The weak adsorption is responsible for the fast isotope exchange between the gaseous CO and CO molecules adsorbed on metal. The Pt-Cu alloys, in which the electronic state of the surface Pt atoms characteristic of monometallic Pt remains unchanged, are formed on the surface of the reduced Pt-Cu bimetallic catalysts. The decrease in the vibrational frequencies of the isolated C=O bonds in the isolated Pt-CO complexes suggests that the CO molecules adsorbed on the Cu atoms affect the electronic properties of Pt.

Fundamental and second-order phonon processes in CdTe and ZnTe

Fundamental and higher-order photon-phonon interactions dominate the far infrared absorption and dispersion spectrum of most semiconductors and dielectrics. We present a detailed investigation of the temperature dependence of the dielectric function of the important II-VI semiconductors CdTe and ZnTe in the frequency range below 3 THz, between 10 and 300 K. From the dielectric function we determine the temperature dependence of the fundamental transverse-optical (TO) frequency in CdTe and ZnTe as well as the TO phonon damping rate in CdTe, and the dynamic ionic charge of both crystals is inferred from the measurements. Furthermore, our experimental data enable unambiguous assignment of low-frequency absorption bands to sum and difference combinations of fundamental phonon modes at critical points away from the Brillouin zone center.

Intermolecular interactions in the liquid crystal-fullerene C70 system

The formation of complexes in the fullerene C70-toluene-liquid crystal system is investigated. The precision study of spectral manifestations of the intermolecular interaction in this system is performed for two low-frequency absorption bands of C70. These manifestations are found to be specific for each of the bands analyzed.

Photoinduced Infrared bands in short polyacetylene segments

We present the theoretical analysis of the photoinduced absorption infrared spectra of Durham type polyacetylene and diblock copolymers. Good agreement with experimental data is found, taking into account the contribution of short segments, in all these samples, expecially in the case of Durham type (CH) x exposed to air. In particular we have well reproduced the different structures of the low frequency absorption band in the photoinduced spectra.

Low-frequency conductivity of the one-dimensional strong coupling Hubbard model

Abstract The real part of the conductivity (absorption) of the Hubbard model at strong coupling is studied using exact diagonalization. The low-frequency absorption band which appears in the doped system is examined in both the standard t - J model and in the strong coupling limit of the Hubbard model which includes three-site spin-dependent hopping terms. The conductivity is shown to differ markedly in the two models, with the three-site terms being essential to obtain reasonable agreement with the Hubbard model.

Influence of interstitially soluted iron on structural, optical and electrical properties of β-rhombohedral boron

A systematical investigation of a series of interstitially doped B:Fe solid solutions of the β-rhombohedral boron structure with compositions up to FeB 29.5, which is close to the maximum solubility, is presented. At an iron concentration of about 2.5% the conductivity character changes from the p-type behaviour of pure β-rhombohedral boron to n-type. The IR and Raman active phonons change systematically. The damping ‖ c is much stronger than ⊥ c. In the case of the stretching mode of the central boron atom in the unit cell, occupied and unoccupied unit cells can be distinguished. An additional splitting of this vibration coincides with the formation of magnetic clusters and the sign reversal of the Seebeck coefficient. Some low frequency absorption bands are attributed to atoms in the definite voids of the structure. The question of whether they are local vibrational modes or electronic transitions remains unsolved. The iron atoms lead to an electronic level at 0.133 eV above the Jahn-Teller induced split-off valence band of β-rhombohedral boron.

Anisotropic IR absorptivity of single domain YBa 2Cu 3O 7

Far infrared measurements on detwinned single crystals at 2 K give an anisotropic absorptivity for the a and b polarizations. A ≊ 0.5% for the axis from ≊ 100cm −1 up to the optical edge at 400cm −1. The a axis conductivity shows low frequency absorption bands of electronic strength.