Seismic attributes, such as P- and S-wave velocity, Poisson＇s ratio, and acoustic impedances, all generally can be used for distinguishing different rock types. The nonuniqueness can be largely reduced using Lame imp...Seismic attributes, such as P- and S-wave velocity, Poisson＇s ratio, and acoustic impedances, all generally can be used for distinguishing different rock types. The nonuniqueness can be largely reduced using Lame impedances instead of acoustic impedances as additional constraints. We have followed this method to constitute a petrologic composition model of the upper crust in the Bohai Bay basin, China. We briefly review the seismic parameters used for discrimination of rock types and focus our attention on the sensitivity of different combinations of parameters to determine the composition of materials. Corrections for pressure and temperature are performed in order to compare elastic wave velocities and densities measured at room temperature and surface pressure in laboratory with those for representative rock parameters. In a second step, we find the rock classes in the tested area by contrasting known data to laboratory measurements on a variety of rock samples extracted in the area. The basic field data are P-wave velocity values collected along a seismic profile conducted in the Bozhong Depression. The different rock types belonging to a particular rock class are finally constrained by the seismic velocities, Poisson＇s ratio, density, acoustic impedance, and Lame imoedance related to the topmost 10 km of the Bohai Bav crust.展开更多
In this paper, the transmitted part of the incident wave is considered to revise Taylor＇s solution, which is used to extend its application for analytical models to predict the response of the plate with different ma...In this paper, the transmitted part of the incident wave is considered to revise Taylor＇s solution, which is used to extend its application for analytical models to predict the response of the plate with different material properties. The influence of the material properties and the boundary condition of the plate on fluid and structural dynamics is systematically investigated. The analytical results are compared with those of detailed dynamic FE simulations and the two are in good agreement. The results indicate that the analytical method is valid and suitable for the plates with different material properties subjected to underwater explosion. It is found that Taylor＇s results of the plate with small impedance are invalid, which indicates a potential application field for the analytical method.展开更多
DQ impedance-based method has been widely used to study the stability of three-phase converter systems.As the dq impedance model of each converter depends on its local dq reference frame,the dq impedance modeling of c...DQ impedance-based method has been widely used to study the stability of three-phase converter systems.As the dq impedance model of each converter depends on its local dq reference frame,the dq impedance modeling of complex converter networks gets complicated.Because the reference frames of different converters might not fully align,depending on the structure.Thus,in order to find an accurate impedance model of a complex network for stability analysis,converting the impedances of different converters into a common reference frame is required.This paper presents a comprehensive investigation on the transformation of dq impedances to a common reference frame in complex converter networks.Four different methods are introduced and analyzed in a systematic way.Moreover,a rigorous comparison among these approaches is carried out,where the method with the simplest transformation procedure is finally suggested for the modeling of complex converter networks.The performed analysis is verified by injecting two independent small-signal perturbations into the d and the q axis,and doing a point-by-point impedance measurement.展开更多
An online TL （transmission line） impedance TPIS （transmission line parameter identification system） using PMU （phasor measurement unit） was recently developed and implemented at CSG （china southern power grid c...An online TL （transmission line） impedance TPIS （transmission line parameter identification system） using PMU （phasor measurement unit） was recently developed and implemented at CSG （china southern power grid company）, Traditional approaches for TL impedance calculation only approximate the effect of conductor sags and ignore the dependence of impedances on temperature variation. Utilizing PMU measurements may improve the accuracy of TL parameters calculation. The challenge is that the parameters identified are very sensitive to noise and errors in PMU measurements, which are difficult to quantify and can be uncertain under different system operating/loading condition, TPIS provides an innovative yet practical problem formulation for TL sequence parameter estimation based on least-squares with linear constraints. A bootstrapping-based resampling technique is developed and a new metric is proposed to determine the credibility of the estimated sequence impedances. This paper discusses the proposed methodologies, challenges, as well as implementation issues identified during the development of TPIS.展开更多
The reduction of TE-scattering by a conducting cylinder with multiple surface impedance loads is investigated. Synthesis procedures are developed to find the optimal loading which result in zero scattering in the desi...The reduction of TE-scattering by a conducting cylinder with multiple surface impedance loads is investigated. Synthesis procedures are developed to find the optimal loading which result in zero scattering in the desired directions or at several frequencies. Numerical results of these procedures are presented. The theoretical predictions are confirmed with an experiment. The proposed synthesis procedure is completely general and can be applied to arbitrarily shaped conducting bodies.展开更多
Nonlinear impedances of two thermoacoustic stacks with ordered structures （plate-type and pipe-type） and one with a disordered structure （copper mesh） are studied. The linear resistances, nonlinear coefficients an...Nonlinear impedances of two thermoacoustic stacks with ordered structures （plate-type and pipe-type） and one with a disordered structure （copper mesh） are studied. The linear resistances, nonlinear coefficients and effective acoustic masses of the stacks are extracted from the experimental results based on an analogical model of nonlinear impedances of porous materials. The resistance and nonlinear coefficient of the disordered stack are found to be much larger than those of the ordered stacks, which have similar volume porosities. In the ordered stacks, the resistance is only marginally influenced by the length of the stack, while in the disordered stack, the resistance increases significantly with the length. These charac- teristics of the impedances of ordered and disordered stacks are explained with the minor loss theory and the tortuosity of a stack.展开更多
One of the factors that affect the parameters of an underground cable is earth return path impedance.Pollaczek developed a formula for the case of one-layer(homogenous) earth.But in practice the earth is composed of s...One of the factors that affect the parameters of an underground cable is earth return path impedance.Pollaczek developed a formula for the case of one-layer(homogenous) earth.But in practice the earth is composed of several layers.In this study we develop a new formula for earth return path impedance in the case of a three-layer earth.To check the accuracy of the obtained results,a comparison has been made with the finite element method(FEM).A comparison between the results of the Pollaczek formula and results of the obtained formula for a three-layer earth has been made,showing that the use of the Pollaczek formula instead of the actual formula can cause serious errors.展开更多
Although previous studies have demonstrated that transcranial focused ultrasound stimulation protects the ischemic brain,clear criteria for the stimulation time window and intensity are lacking.Electrical impedance to...Although previous studies have demonstrated that transcranial focused ultrasound stimulation protects the ischemic brain,clear criteria for the stimulation time window and intensity are lacking.Electrical impedance tomography enables real-time monitoring of changes in cerebral blood perfusion within the ischemic brain,but investigating the feasibility of using this method to assess post-stroke rehabilitation in vivo remains critical.In this study,ischemic stroke was induced in rats through middle cerebral artery occlusion surgery.Transcranial focused ultrasound stimulation was used to treat the rat model of ischemia,and electrical impedance tomography was used to measure impedance during both the acute stage of ischemia and the rehabilitation stage following the stimulation.Electrical impedance tomography results indicated that cerebral impedance increased after the onset of ischemia and decreased following transcranial focused ultrasound stimulation.Furthermore,the stimulation promoted motor function recovery,reduced cerebral infarction volume in the rat model of ischemic stroke,and induced the expression of brain-derived neurotrophic factor in the ischemic brain.Our results also revealed a significant correlation between the impedance of the ischemic brain post-intervention and improvements in behavioral scores and infarct volume.This study shows that daily administration of transcranial focused ultrasound stimulation for 20 minutes to the ischemic hemisphere 24 hours after cerebral ischemia enhanced motor recovery in a rat model of ischemia.Additionally,our findings indicate that electrical impedance tomography can serve as a valuable tool for quantitatively evaluating rehabilitation after ischemic stroke in vivo.These findings suggest the feasibility of using impedance data collected via electrical impedance tomography to clinically assess the effects of rehabilitatory interventions for patients with ischemic stroke.展开更多
In a rapid cycling synchrotron(RCS),the magnetic field is synchronized with the beam energy,creating a highly dynamic magnetic environment.A ceramic chamber with a shielding layer(RF shield),composed of a series of co...In a rapid cycling synchrotron(RCS),the magnetic field is synchronized with the beam energy,creating a highly dynamic magnetic environment.A ceramic chamber with a shielding layer(RF shield),composed of a series of copper strips connected to a capacitor at either end,is typically employed as a vacuum chamber to mitigate eddy current effects and beam coupling impedance.Consequently,the ceramic chamber exhibits a thin-walled multilayered complex structure.Previous theoretical studies have suggested that the impedance of such a structure has a negligible impact on the beam.However,recent impedance measurements of the ceramic chamber in the China Spallation Neutron Source(CSNS)RCS revealed a resonance in the low-frequency range,which was confirmed by further theoretical analysis as a source of beam instability in the RCS.Currently,the magnitude of this impedance cannot be accurately assessed using theoretical calculations.In this study,we used the CST Microwave Studio to confirm the impedance of the ceramic chamber.Further simulations covering six different types of ceramic chambers were conducted to develop an impedance model in the RCS.Additionally,this study investigates the resonant characteristics of the ceramic chamber impedance,finding that the resonant frequency is closely related to the capacitance of the capacitors.This finding provides clear directions for further impedance optimization and is crucial for achieving a beam power of 500 kW for the CSNS Phase-Ⅱ project(CSNS-Ⅱ).However,careful attention must be paid to the voltage across the capacitors.展开更多
A digital measuring method for power system harmonic impedances is presented.Based on the idea of injecting current to the power system and measuring harmonic impedances directly,a digital receiving set is designed to...A digital measuring method for power system harmonic impedances is presented.Based on the idea of injecting current to the power system and measuring harmonic impedances directly,a digital receiving set is designed to receive harmonic signals from the power system.To obtain the harmonic impedances,the signals are handled by digital filters.Simulation results show that this measuring set fits the requirement of industrial measurement.展开更多
Developing effective strategies to regulate graphene’s conduction loss and polarization has become a key to expanding its application in the electromagnetic wave absorption(EMWA)field.Based on the unique energy band ...Developing effective strategies to regulate graphene’s conduction loss and polarization has become a key to expanding its application in the electromagnetic wave absorption(EMWA)field.Based on the unique energy band structure of graphene,regulating its bandgap and electrical properties by introducing heteroatoms is considered a feasible solution.Herein,metal-nitrogen doping reduced graphene oxide(M–N-RGO)was prepared by embedding a series of single metal atoms M–N_(4) sites(M=Mn,Fe,Co,Ni,Cu,Zn,Nb,Cd,and Sn)in RGO using an N-coordination atom-assisted strategy.These composites had adjustable conductivity and polarization to optimize dielectric loss and impedance matching for efficient EMWA performance.The results showed that the minimum reflection loss(RL_(min))of Fe–N-RGO reaches−74.05 dB(2.0 mm)and the maximum effective absorption bandwidth(EAB_(max))is 7.05 GHz(1.89 mm)even with a low filler loading of only 1 wt%.Combined with X-ray absorption spectra(XAFS),atomic force microscopy,and density functional theory calculation analysis,the Fe–N_(4) can be used as the polarization center to increase dipole polarization,interface polarization and defect-induced polarization due to d-p orbital hybridization and structural distortion.Moreover,electron migration within the Fe further leads to conduction loss,thereby synergistically promoting energy attenuation.This study demonstrates the effectiveness of metal-nitrogen doping in regulating the graphene′s dielectric properties,which provides an important basis for further investigation of the loss mechanism.展开更多
As the penetration rate of distributed energy increases,the transient power angle stability problem of the virtual synchronous generator(VSG)has gradually become prominent.In view of the situation that the grid impeda...As the penetration rate of distributed energy increases,the transient power angle stability problem of the virtual synchronous generator(VSG)has gradually become prominent.In view of the situation that the grid impedance ratio(R/X)is high and affects the transient power angle stability of VSG,this paper proposes a VSG transient power angle stability control strategy based on the combination of frequency difference feedback and virtual impedance.To improve the transient power angle stability of the VSG,a virtual impedance is adopted in the voltage loop to adjust the impedance ratio R/X;and the PI control feedback of the VSG frequency difference is introduced in the reactive powervoltage link of theVSGto enhance the damping effect.Thesecond-orderVSGdynamic nonlinearmodel considering the reactive power-voltage loop is established and the influence of different proportional integral(PI)control parameters on the system balance stability is analyzed.Moreover,the impact of the impedance ratio R/X on the transient power angle stability is presented using the equal area criterion.In the simulations,during the voltage dips with the reduction of R/X from 1.6 to 0.8,Δδ_(1)is reduced from 0.194 rad to 0.072 rad,Δf_(1)is reduced from 0.170 to 0.093 Hz,which shows better transient power angle stability.Simulation results verify that compared with traditional VSG,the proposedmethod can effectively improve the transient power angle stability of the system.展开更多
In this study,a phosphate-based conversion coating(PCC)was applied as a precursor before forming silicate-fluoride(SiF)and silicate-phosphate-fluoride(SiPF)based flash-plasma electrolytic oxidation(Flash-PEO)coatings ...In this study,a phosphate-based conversion coating(PCC)was applied as a precursor before forming silicate-fluoride(SiF)and silicate-phosphate-fluoride(SiPF)based flash-plasma electrolytic oxidation(Flash-PEO)coatings on AZ31B magnesium alloy.The main novelty is the successful incorporation of calcium,zinc,manganese and phosphate species into the Flash-PEO coatings via a precursor layer rather than using the electrolyte.The precursor also led to longer lasting and more intense discharges during the PEO process,resulting in increased pore size.Corrosion studies revealed similar short-term performance for all coatings,with impedance modulus at low frequencies above 10^(7)Ωcm^(2),and slightly better performance for the SiPF-based coating.Nonetheless,the enlarged pores in the PEO coatings functionalized with the PCC precursor compromised the effectiveness of self-healing mechanisms by creating diffusion pathways for corrosive species,leading to earlier failure.These phenomena were effectively monitored by recording the open circuit potential during immersion in 0.5 wt.%NaCl solution.In summary,this study demonstrates that conversion coatings are a viable option for the functionalization of PEO coatings on magnesium alloys,as they allow for the incorporation of cationic and other species.However,it is crucial to maintain a small pore size to facilitate effective blockage through self-healing mechanisms.展开更多
Developing sensorless techniques for estimating battery expansion is essential for effective mechanical state monitoring,improving the accuracy of digital twin simulation and abnormality detection.Therefore,this paper...Developing sensorless techniques for estimating battery expansion is essential for effective mechanical state monitoring,improving the accuracy of digital twin simulation and abnormality detection.Therefore,this paper presents a data-driven approach to expansion estimation using electromechanical coupled models with machine learning.The proposed method integrates reduced-order impedance models with data-driven mechanical models,coupling the electrochemical and mechanical states through the state of charge(SOC)and mechanical pressure within a state estimation framework.The coupling relationship was established through experimental insights into pressure-related impedance parameters and the nonlinear mechanical behavior with SOC and pressure.The data-driven model was interpreted by introducing a novel swelling coefficient defined by component stiffnesses to capture the nonlinear mechanical behavior across various mechanical constraints.Sensitivity analysis of the impedance model shows that updating model parameters with pressure can reduce the mean absolute error of simulated voltage by 20 mV and SOC estimation error by 2%.The results demonstrate the model's estimation capabilities,achieving a root mean square error of less than 1 kPa when the maximum expansion force is from 30 kPa to 120 kPa,outperforming calibrated stiffness models and other machine learning techniques.The model's robustness and generalizability are further supported by its effective handling of SOC estimation and pressure measurement errors.This work highlights the importance of the proposed framework in enhancing state estimation and fault diagnosis for lithium-ion batteries.展开更多
The active sound absorption technique excels in mitigating low-frequency sound waves,yet it falls short when dealing with medium and high-frequency sound waves.To enhance the sound-absorbing effect of medium and high-...The active sound absorption technique excels in mitigating low-frequency sound waves,yet it falls short when dealing with medium and high-frequency sound waves.To enhance the sound-absorbing effect of medium and high-frequency sound waves,a novel semi-active sound absorption method has been introduced.This method modulates the surface impedance of a loudspeaker positioned behind the sound-absorbing material,thereby altering the sound absorption coefficient.The theoretical sound absorption coefficient is calculated using MATLAB and compared with the experimental one.Results show that the method can effectively modulates the absorption coefficient in response to varying incident sound wave frequencies,ensuring that it remains at its peak value.展开更多
All-solid-state lithium batteries(ASSLBs)are strongly considered as the next-generation energy storage devices for their high energy density and intrinsic safety.The solid-solid contact between lithium metal and solid...All-solid-state lithium batteries(ASSLBs)are strongly considered as the next-generation energy storage devices for their high energy density and intrinsic safety.The solid-solid contact between lithium metal and solid electrolyte plays a vital role in the performance of working ASSLBs,which is challenging to investigate quantitatively by experimental approach.This work proposed a quantitative model based on the finite element method for electrochemical impedance spectroscopy simulation of different solid-solid contact states in ASSLBs.With the assistance of an equivalent circuit model and distribution of relaxation times,it is discovered that as the number of voids and the sharpness of cracks increase,the contact resistance Rcgrows and ultimately dominates the battery impedance.Through accurate fitting,inverse proportional relations between contact resistance Rcand(1-porosity)as well as crack angle was disclosed.This contribution affords a fresh insight into clarifying solid-solid contact states in ASSLBs.展开更多
Carbon fibre,steel fibre and graphite were used as conductive fillers to prepare cementitious materials with excellent electrothermal properties.The electrically conductive cementitious materials with different volume...Carbon fibre,steel fibre and graphite were used as conductive fillers to prepare cementitious materials with excellent electrothermal properties.The electrically conductive cementitious materials with different volume dosages were analysed through compressive and flexural strength,electrochemical impedance spectroscopy and temperature rise tests.An equivalent circuit model was established to study the electrically conductive heat generation mechanism in the electrically conductive cementitious composites.The results indicate that the mechanical properties of cementitious composite materials with a ternary conductive phase are better than those of pristine cementitious materials because the fibrous filler improves their mechanical properties.However,the incorporation of graphite in the material reduces its strength.Introducing fibrous and point-like conductive phase materials into the cementitious material enhances the overall conductive pathway and considerably reduces the electrical resistance of the cementitious material,enhancing its conductive properties.The volume ratios of carbon fibre,steel fibre and graphite that achieve an optimal complex doping in the cementitious material were 0.35%,0.6%and 6%,respectively.This was determined using the mutation point of each circuit element parameter as the percolation threshold.In addition,at a certain safety voltage,there is a uniform change between the internal and surface temperatures of the conductive cementitious material,and the heating effect in this materialis is considerably better than that in the pristine cementitious material.展开更多
As a novel 2D material,Ti_(3)C_(2)T_(x)-MXene has become a major area of interest in the field of microwave absorption(MA).However,the MA effect of common Ti_(3)C_(2)T_(x)-MXene is not prominent and often requires com...As a novel 2D material,Ti_(3)C_(2)T_(x)-MXene has become a major area of interest in the field of microwave absorption(MA).However,the MA effect of common Ti_(3)C_(2)T_(x)-MXene is not prominent and often requires complex processes or combinations of other ma-terials to achieve enhanced performance.In this context,a kind of gradient woodpile structure using common Ti_(3)C_(2)T_(x)-MXene as MA ma-terial was designed and manufactured through direct ink writing(DIW)3D printing.The minimum reflection loss(RL_(min))of the Ti_(3)C_(2)T_(x)-MXene-based gradient woodpile structures with a thickness of less than 3 mm can reach-70 dB,showing considerable improve-ment compared with that of a completely filled structure.In addition,the effective absorption bandwidth(EAB)reaches 7.73 GHz.This study demonstrates that a Ti_(3)C_(2)T_(x)-MXene material with excellent MA performance and tunable frequency band can be successfully fab-ricated with a macroscopic structural design and through DIW 3D printing without complex material hybridization and modification,of-fering broad application prospects by reducing electromagnetic wave radiation and interference.展开更多
[Background and purposes]In recent years,there has been growing attention in academia and industry on the development of high-performance electromagnetic wave(EMW)absorbing materials.However,creating lightweight broad...[Background and purposes]In recent years,there has been growing attention in academia and industry on the development of high-performance electromagnetic wave(EMW)absorbing materials.However,creating lightweight broadband absorbers remains a challenge in terms of practical applications.EMW absorbing materials primarily rely on the magnetic loss of magnetic materials and/or the dielectric loss of dielectric materials to convert EMW energy into thermal energy for dissipation.Among various magnetic materials,Fe_(3)O_(4) plays an irreplaceable role in EMW absorption due to its high saturation magnetization,low cost and compatible dielectric loss in the gigahertz frequency range.Nevertheless,the high density,large matching thickness and narrow absorption bandwidth of Fe_(3)O_(4) pose significant challenges for practical applications.In contrast,one-dimensional(1D)structures not only retain the characteristic properties of lightweight,chemical stability and high dielectric loss,but also exhibit anisotropic structures and large aspect ratios.Additionally,researchers have found that the minimum reflection loss(RL)of hollow carbon materials with mesopores is nearly four times that of non-porous hollow carbon materials and nine times that of dense carbon materials.According to Maxwell's EMW theory,composites consisting of Fe_(3)O_(4) and one-dimensional(1D)mesoporous carbon materials can leverage their respective advantages by optimizing the composition and structure of the composites to balance u,and Er,thereby enhancing EMW absorption performance.Additionally,numerous studies have demonstrated that composites composed of multi-component heterostructures significantly enhance the EAB.This enhancement is primarily ascribed to the numerous interface polarization losses generated by the additional heterostructure interfaces,which also improve the overall impedance matching of the composites.In this study,we leverage the advantages of magnetic/carbon composites,one-dimensional(1D)mesoporous carbon and multi-component heterostructures to prepare a composite of 1D mesoporous carbon-coated manganese oxide(Mn_(3)O_(4) and MnO,denoted as Mn_(x)O_(y))embedded with Fe_(3)0_(4) nanoparticles(Mn_(x)O_(y)/C@Fe_(3)O_(4)).This composite was synthesized and its formation mechanism and microstructure were analyzed in detail.At the same time,the influence of this Mn_(x)O_(y)/C@Fe_(3)O_(4) structure on EMW properties and absorbing performance was further discussed.[Methods]Firstly,MnO_(2) nanowires were synthesized by using a simple hydrothermal method.Then,the MnO_(2) nanowires served as templates for the synthesis of MnO_(2)/PDA@Fe^(3+)composites through the in-situ polymerization of dopamine and Fe^(3+)adsorption.Finally,1D mesoporous carbon-coated manganese oxide composite embedded with Fe_(3)O_(4) nanoparticles(Mn_(x)O_(y)/C@Fe_(3)O_(4))composites were obtained after heat treatment at 550℃ in N_(2).The crystal structure of the samples was analyzed using X-ray diffractometer with Cu Ka irradiation.Scanning electron microscopy(SEM)and high-resolution transmission electron microscopy(TEM)were used to observe microstructure and morphology of the samples.Nitrogen sorption measurements were obtained at 77 K on a Quantachrome surface area and pore size analyzer to measure the specific surface area and pore size distribution.XPS analysis was performed on X-ray photoelectron spectrometer with monochromatic Al Ka radiation.Magnetization curves of the samples were recorded with a Quantum Design physical property measurement system(PPMS-9)at room temperature.The electromagnetic parameters of the Mn_(x)O_(y)/C@Fe_(3)O_(4) composites were measured using an Agilent N5230C network analyzer in the frequency range of 2-18 GHz.For electromagentic testing,the Mn,Oy/C@Fe34 composites and paraffin wax were mixed at 50°C according to the mass ratio of 15 wt.%,20 wt.%and 25 wt.%,and pressed in a special mold to make coaxial rings(inner diameter=3.04 mm,outer diameter-7 mm),which were denoted as S-1,S-2 and S-3,respectively.[Results]SEM images illustrate the preparation process of iD mesoporous carbon-coated manganese oxide embedded with Fe3O4 nanoparticles composites(Mn_(x)O_(y)/C@Fe_(3)O_(4)).Most of the manganese oxide(Mn,Oy)was reduced to granular after heat treatment,while the outer carbon layer remains its 1D morphology and the carbon layer is interspersed with Fe_(3)O_(4) nanoparticles.The diffraction peaks of MnO_(2) nanowires align well with the body-centered tetragonal a-MnO2.For the Mn_(x)O_(y)/C@Fe_(3)O_(4) composites,the signals of α-MnO_(2) disappears,followed by the emergence of Mn_(3)O_(4) and three prominent diffraction peaks for the cubic MnO.In addition,four weak diffraction peaks correspond to the magnetite Fe_(3)O_(4),consistent with the HRTEM results.The corresponding nitrogen adsorption-desorption isotherm and pore size distribution curve are presented to further analyze the mesoporous structure of composite.The surface composition and element valence states of the Mn_(x)O_(y)/C@Fe_(3)O_(4) composite were investigated by using XPS.The polarization relaxation processes were analyzed according to the Debye theory which describes the relationship between e'and e".Besides the polarization loss,the contribution of the conduction loss plays an important role for the overall dielectric loss.The magnetization curve of Mn_(x)O_(y)/C@Fe_(3)O_(4) exhibits typical ferromagnetic behavior.The permittivity parameter(Co),defined as Co=u"(u)^(-2)f^(-1) determine the contribution of eddy current effect to magnetic loss.The tand values are all larger than those of tand,for the three samples,indicating that the loss capacity of Mn_(x)O_(y)/C@Fe_(3)O_(4) composites is mainly derived from the dielectric loss.Although tand,is smaller,it plays an important role in improving the impedance matching of Mn_(x)O_(y)/C@Fe_(3)O_(4) composites.When the filler loading is 15 wt.%,the RL of sample S-1 is about-10.0 dB at the thickness of 1.5 mm with narrow EAB.As the filler loading increased to 20 wt.%,the RL of sample S-2 reached-62.0 dB at a thickness of 2.2 mm and the EAB was 6.4 GHz at a small thickness of 1.7 mm.When the filler loading is further increased to 25 wt.%,the microwave absorption performance of sample S3 decreased significantly with a little region of RL<-10.0 dB at the thickness of 5.0 mm.The values of[Zin/Zol of the three samples at thicknesses of 1.5-5.0 mm were calculated.Due to good impedance matching of S-2,the incident EMW can enter the material and then can be dissipated through dipole polarization loss,interface polarization loss,conduction loss,eddy current loss and natural ferromagnetic resonance loss.[Conclusions]1D Mn_(x)O_(y)/C@Fe_(3)O_(4) was synthesized via a process involving the coating of polydopamine,adsorption of Fe(ll)salts and heat treatment,using MnO_(2) nanowires as templates.The multi-component heterostructure of the Mn_(x)O_(y)/C@Fe_(3)O_(4) composite(Mn_(3)O_(4),MnO,Fe_(3)O_(4),and C)enhances the interfacial interactions between the different phases,providing increased interface polarization loss under the action of an alternating electromagnetic field.The numerous defects and terminal groups in the mesoporous carbon provide abundant dipole polarization centers.Additionally,the presence of mesopores reduces the weight of the material while increasing the multiple scattering losses of the electromagnetic waves within the material.The ID carbon structure in the matrix forms a conductive network between adjacent fibers,facilitating electron migration and transition,thereby enhancing conductive loss.The incorporation of magnetic Fe_(3)O_(4) nanoparticles introduces eddy current loss and natural ferromagnetic resonance loss,thus increasing magnetic loss.Moreover,the synergistic effect between dielectric and magnetic losses improves the impedance matching of the material,leading to excellent EMW absorption performance.展开更多
Based on simplified calculations of one-dimensional wave systems,loading pressure platform curves of Al-Cu gradient materials(GMs)impactor were designed.The Al-Cu GMs were prepared using tape-pressing sintering,and th...Based on simplified calculations of one-dimensional wave systems,loading pressure platform curves of Al-Cu gradient materials(GMs)impactor were designed.The Al-Cu GMs were prepared using tape-pressing sintering,and their acoustic properties were characterized to match the design path.The parallelism of the Al-Cu GM was confirmed using a three-dimensional surface profilometry machine.A one-stage light-gas gun was used to launch the Al-Cu GM,impacting an Al-LiF target at a velocity of 400 m/s.The results of the experimental strain rate demonstrate that the Al-Cu GMs can realize the precise control of the strain rate within the range of 10^(4)‒10^(5)/s in the high-speed impact experiments.展开更多
基金supported by the National Natural Science Foundation of China (Grant 20930140)the Chinese Academy of Sciences and the sponsors of the Centre for Reservoir Geophysics of the Imperial College
文摘Seismic attributes, such as P- and S-wave velocity, Poisson＇s ratio, and acoustic impedances, all generally can be used for distinguishing different rock types. The nonuniqueness can be largely reduced using Lame impedances instead of acoustic impedances as additional constraints. We have followed this method to constitute a petrologic composition model of the upper crust in the Bohai Bay basin, China. We briefly review the seismic parameters used for discrimination of rock types and focus our attention on the sensitivity of different combinations of parameters to determine the composition of materials. Corrections for pressure and temperature are performed in order to compare elastic wave velocities and densities measured at room temperature and surface pressure in laboratory with those for representative rock parameters. In a second step, we find the rock classes in the tested area by contrasting known data to laboratory measurements on a variety of rock samples extracted in the area. The basic field data are P-wave velocity values collected along a seismic profile conducted in the Bozhong Depression. The different rock types belonging to a particular rock class are finally constrained by the seismic velocities, Poisson＇s ratio, density, acoustic impedance, and Lame imoedance related to the topmost 10 km of the Bohai Bav crust.
基金financially supported by the National Natural Science Foundation of China(Grant Nos.51279038,51509228,and 51479041)
文摘In this paper, the transmitted part of the incident wave is considered to revise Taylor＇s solution, which is used to extend its application for analytical models to predict the response of the plate with different material properties. The influence of the material properties and the boundary condition of the plate on fluid and structural dynamics is systematically investigated. The analytical results are compared with those of detailed dynamic FE simulations and the two are in good agreement. The results indicate that the analytical method is valid and suitable for the plates with different material properties subjected to underwater explosion. It is found that Taylor＇s results of the plate with small impedance are invalid, which indicates a potential application field for the analytical method.
基金The support of the first and fourth authors is given by National Key R&D Program of China,2018YFB0905200.The support for the second and third authors is coming from BIRD171227/17 project of the University of Padova.
文摘DQ impedance-based method has been widely used to study the stability of three-phase converter systems.As the dq impedance model of each converter depends on its local dq reference frame,the dq impedance modeling of complex converter networks gets complicated.Because the reference frames of different converters might not fully align,depending on the structure.Thus,in order to find an accurate impedance model of a complex network for stability analysis,converting the impedances of different converters into a common reference frame is required.This paper presents a comprehensive investigation on the transformation of dq impedances to a common reference frame in complex converter networks.Four different methods are introduced and analyzed in a systematic way.Moreover,a rigorous comparison among these approaches is carried out,where the method with the simplest transformation procedure is finally suggested for the modeling of complex converter networks.The performed analysis is verified by injecting two independent small-signal perturbations into the d and the q axis,and doing a point-by-point impedance measurement.
文摘An online TL （transmission line） impedance TPIS （transmission line parameter identification system） using PMU （phasor measurement unit） was recently developed and implemented at CSG （china southern power grid company）, Traditional approaches for TL impedance calculation only approximate the effect of conductor sags and ignore the dependence of impedances on temperature variation. Utilizing PMU measurements may improve the accuracy of TL parameters calculation. The challenge is that the parameters identified are very sensitive to noise and errors in PMU measurements, which are difficult to quantify and can be uncertain under different system operating/loading condition, TPIS provides an innovative yet practical problem formulation for TL sequence parameter estimation based on least-squares with linear constraints. A bootstrapping-based resampling technique is developed and a new metric is proposed to determine the credibility of the estimated sequence impedances. This paper discusses the proposed methodologies, challenges, as well as implementation issues identified during the development of TPIS.
文摘The reduction of TE-scattering by a conducting cylinder with multiple surface impedance loads is investigated. Synthesis procedures are developed to find the optimal loading which result in zero scattering in the desired directions or at several frequencies. Numerical results of these procedures are presented. The theoretical predictions are confirmed with an experiment. The proposed synthesis procedure is completely general and can be applied to arbitrarily shaped conducting bodies.
基金supported by the National Basic Research Program of China(Grant No.2012CB921504)the National Natural Science Foundation of China(GrantNos.11374154,10904067,and 11174142)+2 种基金the Ph.D.Programs Foundation of Ministry of Education of China(Grant No.20090091120050)the PriorityAcademic Program Development of Jiangsu Higher Education Institutions,Chinathe Fundamental Research Funds for the Central Universities of Ministryof Education of China(Grant No.1101020402)
文摘Nonlinear impedances of two thermoacoustic stacks with ordered structures （plate-type and pipe-type） and one with a disordered structure （copper mesh） are studied. The linear resistances, nonlinear coefficients and effective acoustic masses of the stacks are extracted from the experimental results based on an analogical model of nonlinear impedances of porous materials. The resistance and nonlinear coefficient of the disordered stack are found to be much larger than those of the ordered stacks, which have similar volume porosities. In the ordered stacks, the resistance is only marginally influenced by the length of the stack, while in the disordered stack, the resistance increases significantly with the length. These charac- teristics of the impedances of ordered and disordered stacks are explained with the minor loss theory and the tortuosity of a stack.
文摘One of the factors that affect the parameters of an underground cable is earth return path impedance.Pollaczek developed a formula for the case of one-layer(homogenous) earth.But in practice the earth is composed of several layers.In this study we develop a new formula for earth return path impedance in the case of a three-layer earth.To check the accuracy of the obtained results,a comparison has been made with the finite element method(FEM).A comparison between the results of the Pollaczek formula and results of the obtained formula for a three-layer earth has been made,showing that the use of the Pollaczek formula instead of the actual formula can cause serious errors.
基金supported by the Fundamental Research Funds for the Central Universities,Nos.G2021KY05107,G2021KY05101the National Natural Science Foundation of China,Nos.32071316,32211530049+1 种基金the Natural Science Foundation of Shaanxi Province,No.2022-JM482the Education and Teaching Reform Funds for the Central Universities,No.23GZ230102(all to LL and HH).
文摘Although previous studies have demonstrated that transcranial focused ultrasound stimulation protects the ischemic brain,clear criteria for the stimulation time window and intensity are lacking.Electrical impedance tomography enables real-time monitoring of changes in cerebral blood perfusion within the ischemic brain,but investigating the feasibility of using this method to assess post-stroke rehabilitation in vivo remains critical.In this study,ischemic stroke was induced in rats through middle cerebral artery occlusion surgery.Transcranial focused ultrasound stimulation was used to treat the rat model of ischemia,and electrical impedance tomography was used to measure impedance during both the acute stage of ischemia and the rehabilitation stage following the stimulation.Electrical impedance tomography results indicated that cerebral impedance increased after the onset of ischemia and decreased following transcranial focused ultrasound stimulation.Furthermore,the stimulation promoted motor function recovery,reduced cerebral infarction volume in the rat model of ischemic stroke,and induced the expression of brain-derived neurotrophic factor in the ischemic brain.Our results also revealed a significant correlation between the impedance of the ischemic brain post-intervention and improvements in behavioral scores and infarct volume.This study shows that daily administration of transcranial focused ultrasound stimulation for 20 minutes to the ischemic hemisphere 24 hours after cerebral ischemia enhanced motor recovery in a rat model of ischemia.Additionally,our findings indicate that electrical impedance tomography can serve as a valuable tool for quantitatively evaluating rehabilitation after ischemic stroke in vivo.These findings suggest the feasibility of using impedance data collected via electrical impedance tomography to clinically assess the effects of rehabilitatory interventions for patients with ischemic stroke.
基金supported by the Guangdong Basic and Applied Basic Research Foundation,China(No.2021B1515140007).
文摘In a rapid cycling synchrotron(RCS),the magnetic field is synchronized with the beam energy,creating a highly dynamic magnetic environment.A ceramic chamber with a shielding layer(RF shield),composed of a series of copper strips connected to a capacitor at either end,is typically employed as a vacuum chamber to mitigate eddy current effects and beam coupling impedance.Consequently,the ceramic chamber exhibits a thin-walled multilayered complex structure.Previous theoretical studies have suggested that the impedance of such a structure has a negligible impact on the beam.However,recent impedance measurements of the ceramic chamber in the China Spallation Neutron Source(CSNS)RCS revealed a resonance in the low-frequency range,which was confirmed by further theoretical analysis as a source of beam instability in the RCS.Currently,the magnitude of this impedance cannot be accurately assessed using theoretical calculations.In this study,we used the CST Microwave Studio to confirm the impedance of the ceramic chamber.Further simulations covering six different types of ceramic chambers were conducted to develop an impedance model in the RCS.Additionally,this study investigates the resonant characteristics of the ceramic chamber impedance,finding that the resonant frequency is closely related to the capacitance of the capacitors.This finding provides clear directions for further impedance optimization and is crucial for achieving a beam power of 500 kW for the CSNS Phase-Ⅱ project(CSNS-Ⅱ).However,careful attention must be paid to the voltage across the capacitors.
文摘A digital measuring method for power system harmonic impedances is presented.Based on the idea of injecting current to the power system and measuring harmonic impedances directly,a digital receiving set is designed to receive harmonic signals from the power system.To obtain the harmonic impedances,the signals are handled by digital filters.Simulation results show that this measuring set fits the requirement of industrial measurement.
基金supported by National Natural Science Foundation of China(NSFC 52432002,52372041,52302087)Heilongjiang Touyan Team Program,the Fundamental Research Funds for the Central Universities(Grant No.HIT.OCEF.2021003)the Shanghai Aerospace Science and Technology Innovation Fund(SAST2022-60).
文摘Developing effective strategies to regulate graphene’s conduction loss and polarization has become a key to expanding its application in the electromagnetic wave absorption(EMWA)field.Based on the unique energy band structure of graphene,regulating its bandgap and electrical properties by introducing heteroatoms is considered a feasible solution.Herein,metal-nitrogen doping reduced graphene oxide(M–N-RGO)was prepared by embedding a series of single metal atoms M–N_(4) sites(M=Mn,Fe,Co,Ni,Cu,Zn,Nb,Cd,and Sn)in RGO using an N-coordination atom-assisted strategy.These composites had adjustable conductivity and polarization to optimize dielectric loss and impedance matching for efficient EMWA performance.The results showed that the minimum reflection loss(RL_(min))of Fe–N-RGO reaches−74.05 dB(2.0 mm)and the maximum effective absorption bandwidth(EAB_(max))is 7.05 GHz(1.89 mm)even with a low filler loading of only 1 wt%.Combined with X-ray absorption spectra(XAFS),atomic force microscopy,and density functional theory calculation analysis,the Fe–N_(4) can be used as the polarization center to increase dipole polarization,interface polarization and defect-induced polarization due to d-p orbital hybridization and structural distortion.Moreover,electron migration within the Fe further leads to conduction loss,thereby synergistically promoting energy attenuation.This study demonstrates the effectiveness of metal-nitrogen doping in regulating the graphene′s dielectric properties,which provides an important basis for further investigation of the loss mechanism.
基金supported by theMajor Science and Technology Projects of China Southern Power Grid(Grant number CGYKJXM20210328).
文摘As the penetration rate of distributed energy increases,the transient power angle stability problem of the virtual synchronous generator(VSG)has gradually become prominent.In view of the situation that the grid impedance ratio(R/X)is high and affects the transient power angle stability of VSG,this paper proposes a VSG transient power angle stability control strategy based on the combination of frequency difference feedback and virtual impedance.To improve the transient power angle stability of the VSG,a virtual impedance is adopted in the voltage loop to adjust the impedance ratio R/X;and the PI control feedback of the VSG frequency difference is introduced in the reactive powervoltage link of theVSGto enhance the damping effect.Thesecond-orderVSGdynamic nonlinearmodel considering the reactive power-voltage loop is established and the influence of different proportional integral(PI)control parameters on the system balance stability is analyzed.Moreover,the impact of the impedance ratio R/X on the transient power angle stability is presented using the equal area criterion.In the simulations,during the voltage dips with the reduction of R/X from 1.6 to 0.8,Δδ_(1)is reduced from 0.194 rad to 0.072 rad,Δf_(1)is reduced from 0.170 to 0.093 Hz,which shows better transient power angle stability.Simulation results verify that compared with traditional VSG,the proposedmethod can effectively improve the transient power angle stability of the system.
基金support of the PID2021-124341OB-C22/AEI/10.13039/501100011033/FEDER,UE(MICIU)J.M.Vega also acknowledges the Grant RYC2021-034384-I funded by MICIU/AEI/10.13039/501100011033 and by“European Union Next Generation EU/PRTR”.
文摘In this study,a phosphate-based conversion coating(PCC)was applied as a precursor before forming silicate-fluoride(SiF)and silicate-phosphate-fluoride(SiPF)based flash-plasma electrolytic oxidation(Flash-PEO)coatings on AZ31B magnesium alloy.The main novelty is the successful incorporation of calcium,zinc,manganese and phosphate species into the Flash-PEO coatings via a precursor layer rather than using the electrolyte.The precursor also led to longer lasting and more intense discharges during the PEO process,resulting in increased pore size.Corrosion studies revealed similar short-term performance for all coatings,with impedance modulus at low frequencies above 10^(7)Ωcm^(2),and slightly better performance for the SiPF-based coating.Nonetheless,the enlarged pores in the PEO coatings functionalized with the PCC precursor compromised the effectiveness of self-healing mechanisms by creating diffusion pathways for corrosive species,leading to earlier failure.These phenomena were effectively monitored by recording the open circuit potential during immersion in 0.5 wt.%NaCl solution.In summary,this study demonstrates that conversion coatings are a viable option for the functionalization of PEO coatings on magnesium alloys,as they allow for the incorporation of cationic and other species.However,it is crucial to maintain a small pore size to facilitate effective blockage through self-healing mechanisms.
基金Fund supported this work for Excellent Youth Scholars of China(Grant No.52222708)the National Natural Science Foundation of China(Grant No.51977007)+1 种基金Part of this work is supported by the research project“SPEED”(03XP0585)at RWTH Aachen Universityfunded by the German Federal Ministry of Education and Research(BMBF)。
文摘Developing sensorless techniques for estimating battery expansion is essential for effective mechanical state monitoring,improving the accuracy of digital twin simulation and abnormality detection.Therefore,this paper presents a data-driven approach to expansion estimation using electromechanical coupled models with machine learning.The proposed method integrates reduced-order impedance models with data-driven mechanical models,coupling the electrochemical and mechanical states through the state of charge(SOC)and mechanical pressure within a state estimation framework.The coupling relationship was established through experimental insights into pressure-related impedance parameters and the nonlinear mechanical behavior with SOC and pressure.The data-driven model was interpreted by introducing a novel swelling coefficient defined by component stiffnesses to capture the nonlinear mechanical behavior across various mechanical constraints.Sensitivity analysis of the impedance model shows that updating model parameters with pressure can reduce the mean absolute error of simulated voltage by 20 mV and SOC estimation error by 2%.The results demonstrate the model's estimation capabilities,achieving a root mean square error of less than 1 kPa when the maximum expansion force is from 30 kPa to 120 kPa,outperforming calibrated stiffness models and other machine learning techniques.The model's robustness and generalizability are further supported by its effective handling of SOC estimation and pressure measurement errors.This work highlights the importance of the proposed framework in enhancing state estimation and fault diagnosis for lithium-ion batteries.
基金National Natural Science Foundation of China(No.51705545)。
文摘The active sound absorption technique excels in mitigating low-frequency sound waves,yet it falls short when dealing with medium and high-frequency sound waves.To enhance the sound-absorbing effect of medium and high-frequency sound waves,a novel semi-active sound absorption method has been introduced.This method modulates the surface impedance of a loudspeaker positioned behind the sound-absorbing material,thereby altering the sound absorption coefficient.The theoretical sound absorption coefficient is calculated using MATLAB and compared with the experimental one.Results show that the method can effectively modulates the absorption coefficient in response to varying incident sound wave frequencies,ensuring that it remains at its peak value.
基金supported by the Beijing Natural Science Foundation(Z200011,L233004)the National Key Research and Development Program(2021YFB2500300)+3 种基金the National Natural Science Foundation of China(52394170,52394171,22109011,22393900,and 22108151)the Tsinghua-Jiangyin Innovation Special Fund(TJISF)(2022JYTH0101)the S&T Program of Hebei(22344402D)the Tsinghua University Initiative Scientific Research Program.
文摘All-solid-state lithium batteries(ASSLBs)are strongly considered as the next-generation energy storage devices for their high energy density and intrinsic safety.The solid-solid contact between lithium metal and solid electrolyte plays a vital role in the performance of working ASSLBs,which is challenging to investigate quantitatively by experimental approach.This work proposed a quantitative model based on the finite element method for electrochemical impedance spectroscopy simulation of different solid-solid contact states in ASSLBs.With the assistance of an equivalent circuit model and distribution of relaxation times,it is discovered that as the number of voids and the sharpness of cracks increase,the contact resistance Rcgrows and ultimately dominates the battery impedance.Through accurate fitting,inverse proportional relations between contact resistance Rcand(1-porosity)as well as crack angle was disclosed.This contribution affords a fresh insight into clarifying solid-solid contact states in ASSLBs.
基金Funded by the Key R&D Projects of Shanxi Province(No.201903D321113)the Shanxi Provincial Natural Science Foundation(No.202303021212359)the General Research Projects of Taiyuan University(No.23TYYB07)。
文摘Carbon fibre,steel fibre and graphite were used as conductive fillers to prepare cementitious materials with excellent electrothermal properties.The electrically conductive cementitious materials with different volume dosages were analysed through compressive and flexural strength,electrochemical impedance spectroscopy and temperature rise tests.An equivalent circuit model was established to study the electrically conductive heat generation mechanism in the electrically conductive cementitious composites.The results indicate that the mechanical properties of cementitious composite materials with a ternary conductive phase are better than those of pristine cementitious materials because the fibrous filler improves their mechanical properties.However,the incorporation of graphite in the material reduces its strength.Introducing fibrous and point-like conductive phase materials into the cementitious material enhances the overall conductive pathway and considerably reduces the electrical resistance of the cementitious material,enhancing its conductive properties.The volume ratios of carbon fibre,steel fibre and graphite that achieve an optimal complex doping in the cementitious material were 0.35%,0.6%and 6%,respectively.This was determined using the mutation point of each circuit element parameter as the percolation threshold.In addition,at a certain safety voltage,there is a uniform change between the internal and surface temperatures of the conductive cementitious material,and the heating effect in this materialis is considerably better than that in the pristine cementitious material.
基金support from the National Key Research and Development Program of China(No.2021YFB3701503)the Key Research and Development Program of Ningbo,China(No.2023Z107).
文摘As a novel 2D material,Ti_(3)C_(2)T_(x)-MXene has become a major area of interest in the field of microwave absorption(MA).However,the MA effect of common Ti_(3)C_(2)T_(x)-MXene is not prominent and often requires complex processes or combinations of other ma-terials to achieve enhanced performance.In this context,a kind of gradient woodpile structure using common Ti_(3)C_(2)T_(x)-MXene as MA ma-terial was designed and manufactured through direct ink writing(DIW)3D printing.The minimum reflection loss(RL_(min))of the Ti_(3)C_(2)T_(x)-MXene-based gradient woodpile structures with a thickness of less than 3 mm can reach-70 dB,showing considerable improve-ment compared with that of a completely filled structure.In addition,the effective absorption bandwidth(EAB)reaches 7.73 GHz.This study demonstrates that a Ti_(3)C_(2)T_(x)-MXene material with excellent MA performance and tunable frequency band can be successfully fab-ricated with a macroscopic structural design and through DIW 3D printing without complex material hybridization and modification,of-fering broad application prospects by reducing electromagnetic wave radiation and interference.
基金National Natural Science Foundation of China (52371171, 52222106, 51971008, 52121001)Fund of National Key Laboratory of Scattering and Radiation (Beijing Institute of Environmental Features)。
文摘[Background and purposes]In recent years,there has been growing attention in academia and industry on the development of high-performance electromagnetic wave(EMW)absorbing materials.However,creating lightweight broadband absorbers remains a challenge in terms of practical applications.EMW absorbing materials primarily rely on the magnetic loss of magnetic materials and/or the dielectric loss of dielectric materials to convert EMW energy into thermal energy for dissipation.Among various magnetic materials,Fe_(3)O_(4) plays an irreplaceable role in EMW absorption due to its high saturation magnetization,low cost and compatible dielectric loss in the gigahertz frequency range.Nevertheless,the high density,large matching thickness and narrow absorption bandwidth of Fe_(3)O_(4) pose significant challenges for practical applications.In contrast,one-dimensional(1D)structures not only retain the characteristic properties of lightweight,chemical stability and high dielectric loss,but also exhibit anisotropic structures and large aspect ratios.Additionally,researchers have found that the minimum reflection loss(RL)of hollow carbon materials with mesopores is nearly four times that of non-porous hollow carbon materials and nine times that of dense carbon materials.According to Maxwell's EMW theory,composites consisting of Fe_(3)O_(4) and one-dimensional(1D)mesoporous carbon materials can leverage their respective advantages by optimizing the composition and structure of the composites to balance u,and Er,thereby enhancing EMW absorption performance.Additionally,numerous studies have demonstrated that composites composed of multi-component heterostructures significantly enhance the EAB.This enhancement is primarily ascribed to the numerous interface polarization losses generated by the additional heterostructure interfaces,which also improve the overall impedance matching of the composites.In this study,we leverage the advantages of magnetic/carbon composites,one-dimensional(1D)mesoporous carbon and multi-component heterostructures to prepare a composite of 1D mesoporous carbon-coated manganese oxide(Mn_(3)O_(4) and MnO,denoted as Mn_(x)O_(y))embedded with Fe_(3)0_(4) nanoparticles(Mn_(x)O_(y)/C@Fe_(3)O_(4)).This composite was synthesized and its formation mechanism and microstructure were analyzed in detail.At the same time,the influence of this Mn_(x)O_(y)/C@Fe_(3)O_(4) structure on EMW properties and absorbing performance was further discussed.[Methods]Firstly,MnO_(2) nanowires were synthesized by using a simple hydrothermal method.Then,the MnO_(2) nanowires served as templates for the synthesis of MnO_(2)/PDA@Fe^(3+)composites through the in-situ polymerization of dopamine and Fe^(3+)adsorption.Finally,1D mesoporous carbon-coated manganese oxide composite embedded with Fe_(3)O_(4) nanoparticles(Mn_(x)O_(y)/C@Fe_(3)O_(4))composites were obtained after heat treatment at 550℃ in N_(2).The crystal structure of the samples was analyzed using X-ray diffractometer with Cu Ka irradiation.Scanning electron microscopy(SEM)and high-resolution transmission electron microscopy(TEM)were used to observe microstructure and morphology of the samples.Nitrogen sorption measurements were obtained at 77 K on a Quantachrome surface area and pore size analyzer to measure the specific surface area and pore size distribution.XPS analysis was performed on X-ray photoelectron spectrometer with monochromatic Al Ka radiation.Magnetization curves of the samples were recorded with a Quantum Design physical property measurement system(PPMS-9)at room temperature.The electromagnetic parameters of the Mn_(x)O_(y)/C@Fe_(3)O_(4) composites were measured using an Agilent N5230C network analyzer in the frequency range of 2-18 GHz.For electromagentic testing,the Mn,Oy/C@Fe34 composites and paraffin wax were mixed at 50°C according to the mass ratio of 15 wt.%,20 wt.%and 25 wt.%,and pressed in a special mold to make coaxial rings(inner diameter=3.04 mm,outer diameter-7 mm),which were denoted as S-1,S-2 and S-3,respectively.[Results]SEM images illustrate the preparation process of iD mesoporous carbon-coated manganese oxide embedded with Fe3O4 nanoparticles composites(Mn_(x)O_(y)/C@Fe_(3)O_(4)).Most of the manganese oxide(Mn,Oy)was reduced to granular after heat treatment,while the outer carbon layer remains its 1D morphology and the carbon layer is interspersed with Fe_(3)O_(4) nanoparticles.The diffraction peaks of MnO_(2) nanowires align well with the body-centered tetragonal a-MnO2.For the Mn_(x)O_(y)/C@Fe_(3)O_(4) composites,the signals of α-MnO_(2) disappears,followed by the emergence of Mn_(3)O_(4) and three prominent diffraction peaks for the cubic MnO.In addition,four weak diffraction peaks correspond to the magnetite Fe_(3)O_(4),consistent with the HRTEM results.The corresponding nitrogen adsorption-desorption isotherm and pore size distribution curve are presented to further analyze the mesoporous structure of composite.The surface composition and element valence states of the Mn_(x)O_(y)/C@Fe_(3)O_(4) composite were investigated by using XPS.The polarization relaxation processes were analyzed according to the Debye theory which describes the relationship between e'and e".Besides the polarization loss,the contribution of the conduction loss plays an important role for the overall dielectric loss.The magnetization curve of Mn_(x)O_(y)/C@Fe_(3)O_(4) exhibits typical ferromagnetic behavior.The permittivity parameter(Co),defined as Co=u"(u)^(-2)f^(-1) determine the contribution of eddy current effect to magnetic loss.The tand values are all larger than those of tand,for the three samples,indicating that the loss capacity of Mn_(x)O_(y)/C@Fe_(3)O_(4) composites is mainly derived from the dielectric loss.Although tand,is smaller,it plays an important role in improving the impedance matching of Mn_(x)O_(y)/C@Fe_(3)O_(4) composites.When the filler loading is 15 wt.%,the RL of sample S-1 is about-10.0 dB at the thickness of 1.5 mm with narrow EAB.As the filler loading increased to 20 wt.%,the RL of sample S-2 reached-62.0 dB at a thickness of 2.2 mm and the EAB was 6.4 GHz at a small thickness of 1.7 mm.When the filler loading is further increased to 25 wt.%,the microwave absorption performance of sample S3 decreased significantly with a little region of RL<-10.0 dB at the thickness of 5.0 mm.The values of[Zin/Zol of the three samples at thicknesses of 1.5-5.0 mm were calculated.Due to good impedance matching of S-2,the incident EMW can enter the material and then can be dissipated through dipole polarization loss,interface polarization loss,conduction loss,eddy current loss and natural ferromagnetic resonance loss.[Conclusions]1D Mn_(x)O_(y)/C@Fe_(3)O_(4) was synthesized via a process involving the coating of polydopamine,adsorption of Fe(ll)salts and heat treatment,using MnO_(2) nanowires as templates.The multi-component heterostructure of the Mn_(x)O_(y)/C@Fe_(3)O_(4) composite(Mn_(3)O_(4),MnO,Fe_(3)O_(4),and C)enhances the interfacial interactions between the different phases,providing increased interface polarization loss under the action of an alternating electromagnetic field.The numerous defects and terminal groups in the mesoporous carbon provide abundant dipole polarization centers.Additionally,the presence of mesopores reduces the weight of the material while increasing the multiple scattering losses of the electromagnetic waves within the material.The ID carbon structure in the matrix forms a conductive network between adjacent fibers,facilitating electron migration and transition,thereby enhancing conductive loss.The incorporation of magnetic Fe_(3)O_(4) nanoparticles introduces eddy current loss and natural ferromagnetic resonance loss,thus increasing magnetic loss.Moreover,the synergistic effect between dielectric and magnetic losses improves the impedance matching of the material,leading to excellent EMW absorption performance.
基金Natural Science Foundation of Hubei Province(2024AFB432)National Natural Science Foundation of China(52171045,12302436,52302095)Research Fund of Jianghan University(2023JCYJ05)。
文摘Based on simplified calculations of one-dimensional wave systems,loading pressure platform curves of Al-Cu gradient materials(GMs)impactor were designed.The Al-Cu GMs were prepared using tape-pressing sintering,and their acoustic properties were characterized to match the design path.The parallelism of the Al-Cu GM was confirmed using a three-dimensional surface profilometry machine.A one-stage light-gas gun was used to launch the Al-Cu GM,impacting an Al-LiF target at a velocity of 400 m/s.The results of the experimental strain rate demonstrate that the Al-Cu GMs can realize the precise control of the strain rate within the range of 10^(4)‒10^(5)/s in the high-speed impact experiments.
