Natural Resonance Research Articles

Meta-silencer with designable timbre

Timbre, as one of the essential elements of sound, plays an important role in determining sound properties, whereas its manipulation has been remaining challenging for passive mechanical systems due to the intrinsic dispersion nature of resonances. Here, we present a meta-silencer supporting intensive mode density as well as highly tunable intrinsic loss and offering a fresh pathway for designable timbre in broadband. Strong global coupling is induced by intensive mode density and delicately modulated with the guidance of the theoretical model, which efficiently suppresses the resonance dispersion and provides desirable frequency-selective wave-manipulation capacity for timbre tuning. As proof-of-concept demonstrations for our design concepts, we propose three meta-silencers with the designing targets of high-efficiency broadband sound attenuation, efficiency-controlled sound attenuation and designable timbre, respectively. The proposed meta-silencers all operate in a broadband frequency range from 500 to 3200 Hz and feature deep-subwavelength sizes around 50 mm. Our work opens up a fundamental avenue to manipulate the timbre with passive resonances-controlled acoustic metamaterials and may inspire the development of novel multifunctional devices in noise-control engineering, impedance engineering, and architectural acoustics.

In situ construction of complex spinel ferrimagnet in multi-elemental alloy for modulating natural resonance and highly efficient electromagnetic absorption

The contradiction between the demand for highly efficient low-frequency electromagnetic wave (EMW) absorption materials and the relatively poor permeability of existing absorbers has become increasingly prominent. However, the intrinsic natural resonance in magnetic material hinders the modulation and enhancement of permeability. Herein, (FeCoNiCu)xCr3−xO4 complex spinel ferrimagnets (CSFMs) are constructed in situ in multi-elemental alloy powders (MEAs) for the first time, via annealing treatment. The microstructure and composition of the CSFMs are regulated by adjusting the annealing temperature, aiming to modulate the natural resonance frequency and achieve highly efficient megahertz (MHz) electromagnetic absorption. Simultaneously, annealing treatment improves the crystal integrity, which is conducive to enhancing intergranular magnetic coupling to elevate permeability temperature stability and the Curie temperature. The minimum reflection loss value of the MEAs/CSFMs composites is less than −7 dB at 3 mm and the effective EMW absorption bandwidth maintains between 425 and 470 MHz at 5 mm from −50 to 150 °C. In addition, the MEAs/CSFMs composites also exhibit excellent oxidation resistance and corrosion resistance due to the oxide layer on the surface. This work offers a new and tunable strategy toward modulating the natural resonance frequency for efficient temperature-stable broadband EMW absorption at specific frequency bands.

Numerical study of the mechanisms of the longitudinal pulsed detonation in two-dimensional rotating detonation combustors

A numerical study of the longitudinal pulsed detonation (LPD) is conducted in the present paper. The occurrence mechanism of the LPD, called shock wave amplification by coherent energy release, is verified preliminarily in this study. To be specific, upstream propagating shock waves, which originate from the outlet, induce a specific gradient of reactant distribution, and then detonation waves are ignited and evolve along the gradient in close succession. It is worth noting that the occurrence of LPD does not mean that the LPD will necessarily be sustained. The low injection pressure ratio PR (i.e., the ratio of inlet pressure to outlet pressure) = 1.3 is found to be conducive to the sustenance of the LPD instability in the baseline model. A lower PR (PR ≤ 1.2) or a slightly higher PR (1.4 ≤ PR ≤ 1.8) shall lead to an unstable detonation or quenching of detonations, while a much higher PR (PR > 1.8) contributes to the formation of stable canonical rotating detonation waves. In addition, the combustion regimes of five combustors of different heights at different PR are explored. As the combustion chamber height increases, the PR of the sustainable LPD is nearly linearly increasing, and its operating frequency decreases gradually. The calculation formula between the sustainable LPD propagating frequency and the natural acoustic resonance frequency of the combustor is employed and discussed, but in consideration of its imperfection, further investigation is required.

Influence of Losses, Device Size, and Mode Confinement on Integrated Micro-Ring Resonator Performance for Absorption Spectroscopy Using Evanescent Field Sensing

This work presents a detailed modeling-based analysis of integrated micro-ring resonators used for absorption spectroscopy. Generally, sensors based on micro-ring resonators detect changes in the real part of the sensing medium refractive index, at critical coupling. In absorption spectroscopy, however, micro-ring resonators are used to measure changes in the imaginary part of the index and are most sensitive away from critical coupling, with separate maxima in the under- and over-coupled regimes. In this work, we present a detailed analysis of the under-coupled regime, explaining the relationships between sensitivity, mode confinement, and losses. The analysis is based on reverse-symmetry waveguides to increase the proportion of mode power in the sensing medium and incorporates a realistic model of propagation losses based on experimental measurements of sidewall roughness. The analysis demonstrates that the resonant nature of the sensor is most effective at small radii compared to a non-resonant structure of equal size and shows a behavior of diminishing returns at larger device sizes regarding sensitivity and elevated proportions of mode power in the evanescent field.

Synthesis of Homo-Metallic Heavier Analogues of Cyclobutene and the Cyclobutadiene Dianion.

The reduction of the boryl-substituted SnII bromide {(HCDippN)2 B}Sn(IPrMe)Br with 1.5 equivalents of potassium graphite leads to the generation of the cyclic tetratin tetraboryl system K2 [Sn4 {B(NDippCH)2 }4 ], a homo-metallic heavier analogue of the cyclobutadiene dianion. This system is non-aromatic as determined by Nucleus Independent Chemical Shift Calculations (NICS(0)=-0.28, NICS(1)=-3.17), with the primary contributing resonance structures shown by Natural Resonance Theory (NRT) to involve a Sn=Sn double bond and 1,2-localized negative charges. Abstraction of the K+ cations or oxidation leads to contraction or cleavage of the Sn4 unit, respectively, while protonation generates the neutral dihydride 1,2-Sn4 {B(NDippCH)2 }4 H2 (a heavier homologue of cyclobutene) in a manner consistent with the predicted charge distribution in the [Sn4 {B(NDippCH)2 }4 ]2- dianion.

Vibration and resonance reliability analysis of non-uniform beam with randomly varying boundary conditions based on Kriging model

In order to prevent the function failure of the structure due to resonance in the working process. The vibration responses and resonance reliability of non-uniform beam (NUB) with randomly varying boundary conditions (RVBCs) are investigated in this paper. The natural frequencies calculated by Adomian decomposition method (ADM) are used to establish the sample space, based on which the Kriging model is obtained through learning. The influences of boundary support spring stiffness (BSSS) and taper ratio on natural frequencies, vibration responses, and resonance reliability are then studied. The numerical results show that within the range of studied parameters, when the BSSS of the NUB change simultaneously during vibration, the influences and trends of the BSSS on each order’s natural frequency are different and challenging to predict. Due to the geometric nonuniformity of the NUB, the influence of the spring at the left end with a smaller cross-section on the resonance failure probability is more significant than that on the other side. With the increase in taper ratio, the amplitude, maximum strain, and resonance failure probability increase first and then decrease. The method can also be used to solve the vibration characteristics and resonance reliability of NUBs with randomly varying elastic modulus, density and dimensions, which can provide a reference for the data monitoring and use maintenance of the non-uniform beam in vibration.

Sub-terahertz/terahertz electron resonances in hard ferrimagnets

Intensive development of ultrafast electronics requires materials with high-frequency spin dynamics. In this light, the insulators that possess the magnetization precession phenomenon due to magnetic anisotropy are dark horses. On the one hand, modern hard magnetic materials reveal relatively moderate resonance frequencies of the ferromagnetic mode (generally, dozens of GHz), which are lower than the frequencies of the antiferromagnetic resonances; on the other hand, the research in this area is quite scanty, which implies a room for a breakthrough. Here, an example of a hard ferrimagnetic insulator (cobalt ferrite CoFe2O4) was obtained in the form of nanoparticles and bulk ceramics via high-temperature methods. Due to high magnetic anisotropy fields, the samples in a single domain state show broad hysteresis loops. The materials also possess intensive resonance absorption at frequencies higher than 0.20 THz in zero external magnetic fields. For the first time, natural ferromagnetic resonance (NFMR) frequencies higher than 0.30 THz were registered. The ceramic sample demonstrates the highest-known NFMR frequency of 0.35 THz. The model based on the Landau-Lifshitz equation was developed to explain the demonstrated magnetodynamic properties and shed light on those of hard ferrimagnets in general. The practical application of the electron resonances in hard magnetic insulators, including cobalt ferrite, Al-doped M-type hexaferrite, and epsilon iron oxide, is discussed. Our findings reveal that these materials should provide several orders of magnitude more powerful spin pumping at sub-terahertz/terahertz frequencies compared to insulating antiferromagnets, even under unpolarized irradiation and even in the absence of external magnetic fields. This opens new horizons for the development of practical ultrafast electronics.

Broadband electromagnetic dissipation superiority of hierarchical ZnO flakes co-decorated with CoFe/CoFe2O4 and rGO

Currently, in order to solve the increasing issues caused by the electromagnetic radiation pollution and broadened radar detection bandwidth, electromagnetic wave dissipating materials with high performance of broadband absorption are desired urgently. Herein, the CoFe/CoFe2O4/ZnO/rGO composites composed of hierarchical ZnO flakes with anchored CoFe/CoFe2O4 and rGO-doping were synthesized via a facile aging approach and subsequent calcination process under vacuum atmosphere. The constituents and electromagnetic parameters of absorbers could be efficiently modulated through tuning the additive amount of raw GO. Notably, the composite with GO additive amount of 0.025 g exhibits a prominent broadband electromagnetic wave absorbing capacity. In detail, the widest effective absorption bandwidth reaches 7.66 GHz with thin thickness of just 2.5 mm. This superior broadband microwave absorption property is owing to the synergistic effect of hierarchical nanostructure and well-matched impedance, as well as enhanced interfacial polarization, conduction loss, eddy current, natural and exchange magnetic resonances. The investigation demonstrates the specific CoFe/CoFe2O4/ZnO/rGO composite is a potential candidate to be applied for broadband military radar stealth and serious electromagnetic radiation contamination.

Investigation on Gold-Ligand Interaction for Complexes from Gold Leaching: A DFT Study.

Gold leaching is an important process to extract gold from ore. Conventional alkaline cyanide process and alternative nontoxic lixiviants including thiosulfate, thiourea, thiocyanate, and halogen have been widely investigated. However, density functional theory (DFT) study on the gold complexes Au(CN)2-, Au(S2O3)23-, Au[SC(NH2)2]2+, Au(SCN)2-, and AuCl2- required for discovering and designing new highly efficient and environmentally friendly gold leaching reagents is lacking, which is expected to support constructive information for the discovery and designation of new high-efficiency and environmentally friendly gold leaching reagents. In this study, the structure information, electron-transferring properties, orbital interaction, and chemical bond composition for complexes Au(CN)2-, Au(S2O3)23-, Au[SC(NH2)2]2+, Au(SCN)2-, and AuCl2- depending on charge decomposition analysis (CDA), natural bond orbital (NBO), natural resonance theory (NRT), electron localization function (ELF), and energy decomposition analysis (EDA) were performed based on DFT calculation. The results indicate that there is not only σ-donation from ligand to Au+, but also electron backdonation from Au+ to ligands, which strengthens the coordinate bond between them. Compared with Cl-, ligands CN-, S2O32-, SC(NH2)2, and SCN- have very large covalent contribution to the coordinate bond with Au+, which explains the special stability of Au-CN and Au-S bonds. The degree of covalency and bond energy in Au-ligand bonding decreases from Au(CN)2-, Au(S2O3)23-, Au[SC(NH2)2]2+, Au(SCN)2-, to AuCl2-, which interprets the stability of the five complexes: Au(CN)2- > Au(S2O3)23- > Au[SC(NH2)2]2+ > Au(SCN)2- > AuCl2-.

Sensitivity of Cell Cultures on Time-Varying Low-Frequency Magnetic Field Changes

This article is focused on the in vitro experimental verification of the basic necessary conditions for valid interpretation of an ion parametric resonance prediction model of biological reaction on an externally applied, extremely low-frequency magnetic field. Experiments are performed on model organism Saccharomyces cerevisiae strain BY4741, with the intention to target calcium ions, which already produced interesting preliminary results within authors’ previous works, confirming specific magnetic field settings being bioactive. To study the nature and origin of biological reactions more exhaustingly, the research is aimed at two chosen physical aspects of ion parametric resonance theory. In the first part, experiments are set up to verify the resonance point validity for calcium ions and to perform a sensitivity analysis of the biological system response around this point. In the second part, the main attention is paid to the verification of the validity of maximum biological effect at specific BAC/BDC ratios, reflecting the ion parametric resonance theory as well as the current state of the art and knowledge. Both series of experiments have shown interesting results, confirming 29.89 Hz as a bioactive frequency for a BAC/BDC ratio of 1.8. These observations are in close correlation with the assumptions of the ion parametric resonance model for targeting calcium ions. The performed sensitivity analysis around the resonance point looks even more interesting, as conducted preliminary experiments resulted in two more pronounced bioactive frequencies of 24.89 and 34.89 Hz, in terms of biological response significance, than the resonance frequency assumed by an investigated prediction model. The narrow width of the peak, which is typical for physical phenomena of resonant nature, was not observed within this series of experiments, which would be addressed more precisely in future work of authors.

Quantifying Vertical Resonance Energy in Aromatic Systems with Natural Bond Orbitals

AbstractNatural bond orbitals (NBOs) provide the familiar Lewis type (2c–2e−) localized description of a molecule. Interactions between nearly filled (2e− π or σ) orbitals and empty (π* or σ*) anti‐bonding orbitals represent delocalization in the system and creates a framework to study stereoelectronic interactions. Here we show that deleting the interactions (NBODel) between π and π* orbitals in aromatic systems and acquiring the energy with the NBO program provides a highly intuitive and quantitative picture of π‐aromaticity that correlates with the well‐established nucleus‐independent chemical shift (NICS) method. This natural bond orbital resonance energy (NBO‐RE) measures the vertical resonance energy (VRE) of aromatic systems without the use of an external reference structure. The NBO‐RE method is applicable to the study of local aromaticity in polycyclic aromatic hydrocarbons (PAHs) and other non‐planar systems.

Feasibility analysis of a wave powered autonomous underwater vehicle

An autonomous underwater vehicle (AUV) powered by renewable energy would enable it to extend endurance and forward presence with no logistics tail for fuel. A wave powered concept AUV is proposed and analyzed for this purpose. The AUV is in a torpedo shape and employs a fully hull-encapsulated mass-spring-damper system as its power take-off (PTO) unit. While in the recharging mode, the AUV surfaces and converts wave energy for electricity. Considering the AUV of a common size, our analysis indicates that resonance of the AUV and/or its PTO, to be excited by ocean waves, becomes impossible in typical ocean conditions. Then by making the AUV almost fully submerged to eliminate its intrinsic resonant nature and by applying some realistic constraints, the AUV’s power generation capability without resonance is characterized. Our simulation results show that the AUV is still capable of generating power at a decent level in a wide range of the wave frequency upon optimization. Some guiding principles have been established for AUV design of this type.

Low-cost, high-density Mn–Co spinel coatings for stainless steel interconnect via efficient microwave heating

The high preparation cost and poor density of spinel coatings for stainless steel interconnect have always been puzzling conundrums for the solid oxide fuel cell stack developers. Herein, the microwave heating technology for fabricating Mn–Co spinel coating is developed to address these issues. We designed a device spontaneously providing reducing atmospheres and realized the low-cost, high-efficiency reduction of Mn–Co spinel via microwave heating. Dense coatings (MnO + Co) are formed on the steel surface at ∼1100 °C. The excellent density is related to the melting of Co particles induced by microwave selective heating. At initial, microwave heating is dominated by dipolar polarization, interfacial polarization, and natural resonance. After forming metallic Co, conductive loss and the lens effect further accelerate the reduction process. High-quality spinel coatings are formed during the subsequent oxidation at 800 °C, exhibiting an extremely low area-specific resistance increase rate.

MnFe2O4/ rGO/Diatomite composites with excellent wideband electromagnetic microwave absorption

Electromagnetic interference has become one urgent problem in the current society. Efficient preparation of materials with excellent electromagnetic wave absorption is challenging but urgently needed. In this work, MnFe2O4/rGO/Diatomite (M/rGO/DT) cladding structure composites were prepared using a direct, one-step hydrothermal technique. The ternary composite has many microwave absorption mechanisms, including interfacial dipole polarization, conductive loss, natural resonance, flux loss, huge kudos to the three-dimensional cladding cross-linked structure and close-packed MnFe2O4 particles. As a result, compared with unitary or binary composites this ternary composite offers superior capabilities in low and medium frequency microwave absorption. The composite's minimum reflection loss (RLmin) is − 76.56 dB at 7.9 GHz, its thickness is 2.485 mm, and its effective absorption bandwidth (EAB) is up to 3.61 GHz. The remarkable electromagnetic wave attenuation ability of the generated ternary composite shows that the work in this research may provide an innovative and simple technique for the preparation of superior microwave absorbing materials (MAMs).

Synthesis of RGO/Cu@ FeAl2O4 Composites and Its Applications in Electromagnetic Microwave Absorption Coatings.

In order to satisfy the requirements of wide frequency bands, the lightweight and strong absorption for the electromagnetic wave absorbing materials, a uniform mixture of FeAl2O4 with RGO/Cu (reduction graphene oxide, RGO) was obtained by the mechanical mixing method, and composite coating was obtained by plasma spraying. The addition of RGO/Cu into FeAl2O4 is conducive to improve the dielectric properties and the impedance matching performance of spinel. When the RGO/Cu composite powders are doped by 10 wt.%, the reflection loss at 15 GHz is -16 dB and the absorption bandwidth is 2 GHz, indicating that the composite material has potential application value in the field of high-frequency wave absorption. The research on the electromagnetic wave absorption mechanism shows that its superior wave absorption performance is determined by the synergistic effect of multiple loss mechanisms such as interfacial polarization, dipole relaxation, natural resonance, exchange resonance, and eddy current loss.

Electromagnetic wave absorption properties of core double-shell structured α-Fe(Si)@Fe3O4@SiO2 composites

Core double-shell hierarchical structured α-Fe(Si)@Fe3O4@SiO2 (FFS) microwave absorbing composites have been fabricated by high-temperature mechano-chemical method. Heterogeneous interfaces of Fe/Si, α-Fe(Si)/Fe3O4, Fe3O4/SiO2 and high-density defects were formed by coupling the controlled temperature field and mechanical force. A dramatic reduction in the average particle size of the composite powder was caused by increasing ball-to-powder ratio. Electromagnetic wave absorption (EMWA) performance of the FFS composites was investigated in the 2–18 GHz frequency range. A balance of impedance matching and strong attenuation characteristics was achieved due to the asymmetric charge distribution at the heterogeneous interfaces, high-density defects, natural resonance, and exchange resonance of the FFS composites. When the ball-to-powder ratio is 40:1, the minimum reflection loss (RLmin) of the FFS composite is −47.04 GHz at a frequency of 5.28 GHz with an absorption layer thickness of 3.1 mm. A broad effective absorbing bandwidth (EAB, RL < −10 dB) of 5.6 GHz (12.4–18 GHz) with a thickness of 1.3 mm covers almost the entire Ku-band. After annealing at 1173 K in an argon atmosphere, the FFS composite with 30:1 ball-to-powder ratio shows excellent EMWA performance in the S-band, where the RLmin reaches −50.18 dB at 2.88 GHz with a thickness of 5.0 mm.

Hard ferrite magnetic insulators revealing giant coercivity and sub-terahertz natural ferromagnetic resonance at 5-300 K.

The temperature behavior of the magnetic properties is crucial for the application of magnetic materials. Recently, giant room temperature coercivities (20-36 kOe) and sub-terahertz natural ferromagnetic resonance (NFMR) frequencies (160-250 GHz) were observed for single-domain M-type hexaferrites with high aluminum substitution. Herein, the temperature dependences of the magnetic properties and natural ferromagnetic resonance are studied at 5-300 K for single-domain Sr1-x/12Cax/12Fe12-xAlxO19 (x = 1.5-5.5) particles. It is shown that the samples maintain their magnetic hardness over the whole temperature range. The coercivity and NFMR frequencies have a maximum shifting to the low-temperature region with a rise in aluminum concentration. The highest coercivity of 42 kOe and the maximum NFMR frequency of 297 GHz are observed for x = 5.5 at 180 K.

Digital camera's parameter identification for vibration-based structural health monitoring under different dynamic responses

Vibration-based structural health monitoring (SHM) is critical in assessing structures' integrity. However, traditional methods of contact vibration sensors, such as accelerometers, face inherent limitations regarding accessibility, coverage, and their impact on structural dynamics. In recent years, digitalisation has introduced novel possibilities for overcoming these challenges through camera-based measurements. Nonetheless, a knowledge gap exists concerning the interdependencies among camera speed, resolution, and the accuracy of extracted dynamic features from captured results. This paper aims to address this gap by examining the influence of camera settings on the accuracy and reliability of visual-based vibration measurements. The study measured the natural frequencies and resonance amplitudes of the cracked cantilever beams exhibiting diverse dynamics. During the tests, the camera records the vibration behaviour with different speed and resolution settings. Through analysis of the acquired data, this study seeks to establish trade-offs between camera parameters and measurement accuracy, ultimately enhancing the reliability and effectiveness of crack detection within visual-based vibration monitoring.

Electromagnetic properties of composite filaments with ferromagnetic fillers for 3D printing

The paper presents the results of a study of composite materials based on the polymer acrylonitrile styrene acrylate (ACA) and filler M-type hexaferrite (BaFe12O19). The results of X-ray diffractometry of hexaferrite powder are presented. The porosity of the obtained composite filament samples was studied. After 9 extrusion cycles, the porosity is reduced to less than 1%. In the EHF range, the transmission coefficient through a sample of a composite material was measured. A sharp decrease in the transmission coefficient is observed in the frequency range from 42 to 55 GHz, which is due to the natural ferromagnetic resonance in BaFe12O19 hexaferrite.

Nanoceramics of metastable ε-Fe2O3: effect of sintering on the magnetic properties and sub-terahertz electron resonance.

In this study, we demonstrate the sintering of metastable ε-Fe2O3 nanoparticles into nanoceramics containing 98 wt% of the epsilon iron oxide phase and with a specific density of 60%. At room temperature, the ceramics retain a giant coercivity of 20 kOe and a sub-terahertz absorption at 190 GHz inherent in the initial nanoparticles. The sintering leads to an increase in the frequencies of the natural ferromagnetic resonance at 200-300 K and larger coercivities at temperatures below 150 K. We propose a simple but working explanation of the low-temperature dynamics of the macroscopic magnetic parameters of the ε-Fe2O3 materials via the transition of the smallest nanoparticles into a superparamagnetic state. The results are confirmed by the temperature dependence of the magnetocrystalline anisotropy constant and micromagnetic modeling. In addition, based on the Landau-Lifshitz formalism, we discuss features of the spin dynamics in ε-Fe2O3 and the possibility of using nanoceramics as sub-terahertz spin-pumping media. Our observations will expand the applicability of ε-Fe2O3 materials and promote their integration into telecommunication devices of the next generation.