Effective Medium Analysis Research Articles

Principles of progressive slow-sound and critical coupling condition in broadband sonic black hole absorber

Recent advances in sonic black hole (SBH) provide new opportunities for controlling sound waves and designing wave manipulation devices. SBH is a device that consists of partitions with gradually decreasing inner radii inserted into an acoustic duct. Several studies have reported that SBH can achieve a broadband sound absorption coefficient close to 1, avoiding the issue of alternating high and low absorption coefficients observed in traditional sound absorbers. However, the fundamental mechanisms and principles behind this behavior are not yet fully understood. This study aims to investigate the detailed sound absorption mechanisms of SBH, including the progressive slow-sound effect and the critical coupling condition that leads to broadband sound absorption. To achieve this goal, an analytical model based on the effective medium approach is developed to investigate the layer-by-layer retardation in sound propagation. The sound absorption coefficient is then determined based on the surface impedance calculation. The effective medium analysis reveals that SBH enables a unique condition to progressively decelerate wave propagation across its layers. As a result, the critical coupling condition becomes more easily established with smoothly increasing SBH partitions and more discretised layers, as elucidated by the complex frequency analysis results. The physical insights gained from this study reveal the distinctive features of SBH compared to classical sound absorbers, paving the way for its engineering applications.

Probabilistic Analysis of Composite Materials with Hyper-Elastic Components.

This work is a comprehensive literature overview in the area of probabilistic methods related to composite materials with components exhibiting hyper-elastic constitutive behavior. A practical area of potential applications is seen to be rubber, rubber-like, or even rubber-based heterogeneous media, which have a huge importance in civil, mechanical, environmental, and aerospace engineering. The overview proposed and related discussion starts with some general introductory remarks and a general overview of the theories and methods of hyper-elastic material with a special emphasis on the recent progress. Further, a detailed review of the current trends in probabilistic methods is provided, which is followed by a literature perspective on the theoretical, experimental, and numerical treatments of interphase composites. The most important part of this work is a discussion of the up-to-date methods and works that used the homogenization method and effective medium analysis. There is a specific focus on random composites with and without any interface defects, but the approaches recalled here may also serve as well in sensitivity analysis and optimization studies. This discussion may be especially helpful in all engineering analyses and models related to the reliability of elastomers, whose applicability range, which includes energy absorbers, automotive details, sportswear, and the elements of water supply networks, is still increasing, as well as areas where a stochastic response is the basis of some limit functions that are fundamental for such composites in structural health monitoring.

An Empirical Loss Model for an Additively Manufactured Luneburg Lens Antenna

This research applies Effective Medium Theory and 3D Finite Element Analysis to model the transmissive loss through a waveguide fed additively manufactured Luneburg lens. New results are presented that provide rational function approximations for accurately modeling the aperture, beam, and radiation loss factors of the antenna. It introduces a normalized loss tangent and shows that the loss factors are dependent on the product of this parameter and the lens radius. Applying the constraint that the main beam of the radiation pattern contains 50% of accepted power, a maximum useful radius is tabulated for common polymers used in additive manufacturing.

How can green building development promote carbon emission reduction efficiency of the construction industry?--Based on the dual perspective of industry and space.

Green building development(GBD) plays an important role in improving carbon emission reduction efficiency of the construction industry(CEEOCI), and accelerating the construction industry to achieve carbon neutrality goals. In this paper, for the first time, the topic of "green building development promotes carbon neutrality in the construction industry" is included in the crossover research category of industrial economics and space economics. A framework for the analysis of the impact mechanism of GBD and CEEOCI under the dual perspective of "industry-space" is proposed. Based on China's provincial panel data from 2008 to 2019, using the intermediary adjustment model and the spatial Dubin model to analyze and test the industrial mechanism, spatial mechanism, and spatial effect attenuation boundary of the GBD effect on CEEOCI. The results show GBD is found to promote CEEOCI, and both the GBD and CEEOCI have a clear "center-periphery" spatial pattern. Meanwhile, the impact of GBD on CEEOCI has a "industry-space" two-dimensional mechanism. From an industrial perspective, GBD can change the supply-demand relationship of the construction market by promoting the greening of supporting industries in the construction industry and stimulating green consumption in the market, and ultimately achieve the overall improvement of CEEOCI. From a spatial perspective, GBD can form a new sectoral growth pole and provide elemental support for the improvement of CEEOCI through the polarization effect, but the polarization effect is higher in the range of 1000 km, while attenuation occurs outside the range of 1000 km. This paper provides not only a scientific and effective medium and macro analysis framework for green building development emission reduction performance evaluation but also a new theoretical basis and policy direction for the construction industry to improve carbon reduction efficiency and achieve industrial carbon neutrality.

A Switchable Ultra-Wideband Metamaterial Absorber with Polarization-Insensitivity and Wide-incident Angle at THz Band

In this paper, we report a switchable ultra-wideband metamaterial absorber with polarization-insensitivity and wide-incident angle at THz band which is composed of VO2 disk, polyimide dielectric substrate, and gold ground plane. The results show that the absorption is greater than 90% from 3.5–8 THz for a temperature of 300 K and this absorption band disappears when the temperature rises to 350 K. The absorption property of our proposed metamaterial absorber is insensitive to polarization states and angles and it can withhold high absorption of more than 80% for wide-incident angles, up to 60° for TE mode and TM mode. The wideband absorption mechanism is elucidated using an effective medium and surface current analysis.

Graphene-integrated hybridized metamaterials for wide-angle tunable THz absorbers

Metamaterial absorbers with excellent absorption properties and real-time tunability are desirable for practical applications. In this paper, we design and numerically investigate a polarization insensitive wide-angle metamaterial absorber with tunable absorption strength formed by finite-length symmetric metallic plates and graphene sheet. This structure achieves a near perfect absorption, and remains highly absorptive with wide incident-angle, higher than 80% at 60o for TE mode and around 90% at 70o for TM mode. Absorption strength can be varied between 97% and 52% by controlling the Fermi energy of graphene while the operating frequency remains relatively stable. Also without the presence of a ground plane as in other conventional absorbers, waves outside of the absorption band are allowed to travel through. The absorbing principle and absorption tunability mechanism are elucidated using effective medium analysis and finite integration simulations.

Structural parameters of hyperbolic metamaterials controlling high-k mode resonant wavelengths

Layered semiconductor hyperbolic metamaterials (HMMs) are composite materials composed of alternating subwavelength-doped (metal) and undoped (dielectric) semiconductor layers. These materials support the propagation of light with large wave vectors through modes called volume plasmon polaritons (VPPs). In this paper, we use finite-element modeling and effective medium analysis (EMA) to investigate how the number of periods, the period thickness, and the overall HMM thickness affect the VPP mode-resonant wavelengths. We show that the overall HMM thickness has a larger impact on shifting the resonant wavelengths of the VPP modes than the subwavelength structure. We also show that the main limitation of EMA for this application is an inability to account for the boundary conditions at the substrate.

A Modified Maxwell Garnett Model: Hysteresis in phase change materials

The dielectric properties of materials undergoing phase transitions are key to a number of modern and developing technologies, particularly when hysteresis is demonstrated in the response. This paper presents a modified Maxwell Garnett model for analysing electromagnetic hysteresis, using an asymmetric effective medium approximation to describe intermediate states in the phase change, establishing a link between effective medium and hysteresis analysis. The model has few input parameters and provides a phenomenological approach to describing electromagnetic hysteresis in various phase change materials.

Controlling the absorption strength in bidirectional terahertz metamaterial absorbers with patterned graphene

THz metamaterials that exhibit excellent absorption properties with real-time tunability have been of great interest for various applications. In this paper, we combine symmetric metamaterials and patterned graphene to achieve an electrically-switchable bidirectional absorption window operating in terahertz frequencies. The proposed metamaterial can homogeneously absorb the incident waves while allowing significant transmission outside the absorption band. Our simulations show that the tunable electrical conductivity of the patterned graphene can result in a desired control of the absorptivity between 41% and 91%. Finite integration simulations, standard retrieval procedure, equivalent circuit model, and effective medium analysis are employed to elaborate our idea.

Effective medium and equivalent circuit analysis of extraordinary transmission through metallic grating in the infrared range

Extraordinary transmission (EOT) through one- and two-dimensional deep metallic grating in the infrared range is explored by numerical electromagnetic (EM) field analysis, such as the finite difference time domain method. The transmittance at normal incidence is greater than the porosity that is defined as a proportion of the gap area to the total cross-section when the gap is much smaller than the wavelength of incident infrared light. The EOT mechanism is investigated using two approaches—the equivalent electrical circuit (EEC) model and the effective medium approximation (EMA). The transmittance and reflectance profiles calculated using the EEC model agree with those obtained by the numerical EM analysis. EMA is applied on the basis of the idea that the deep metallic grating can be regard as hyperbolic metamaterials. Then, the effective refractive index of the deep metallic grating is real with a negligible imaginary part, in infrared range. This means that the metallic grating behaves as a dielectric medium, resulting in the great transmission and existence of a Brewster angle.

Tailoring electronic and thermal transport properties of CaO(CaMnO3)m-based (m=1 and m=∞) composites for thermoelectric power generation

Oxide thermoelectric (TE) materials are promising for waste heat recovery at high temperatures thanks to their good chemical stability at elevated temperatures and low cost. We study Nb-doped n-type TE oxides of the CaO(CaMnO3)m-series. The CaMnO3 (m = ∞) and Ca2MnO4 (m = 1) derivatives feature extremely opposite transport coefficients, where the m = ∞ structure exhibits high electrical and thermal conductivity, and the m = 1 derivative exhibits the opposite combination. We synthesize composite materials based on these two phases of different ratios to draw correlations between the TE properties, microstructure evolution, and composition of the material. We determine the optimum sintering temperature to be 1373 K, and measure both thermal and electronic transport coefficients, then perform a thorough general effective medium (GEM) analysis. Interestingly, we find that most ratios obey to the GEM behavior, where deviations are elucidated in terms of interfacial effects. This study provides us with tools for identifying the significance of bulk vs. interfacial effects in design of composite materials with controllable transport properties.

Modified Maxwell Garnett model for hysteresis in phase change materials

Analytical modelling of hysteresis is used to provide crucial prediction and insight for phase transitions in materials. Here we present a modified Maxwell Garnett model for analysing electromagnetic hysteresis. The model uses an asymmetric effective medium approximation to describe intermediate states in the phase change, establishing a link between effective medium and hysteresis analysis. Numerical calculation was performed on an example material, vanadium dioxide, for quantitative demonstration and future experimental verification. The model is easy to use, requires very few input parameters, and provides a phenomenological approach to describing electromagnetic hysteresis in various phase change materials.

Enhanced thermal stability of organic solar cells comprising ternary D-D-A bulk-heterojunctions

Ternary absorber blends have recently been identified as promising concepts to spectrally broaden the absorption of organic bulk-heterojunction solar cells and hence to improve their power conversion efficiencies. In this work, we demonstrate that D-D-A ternary blends comprising two donor polymers and the acceptor PC61BM can also significantly enhance the thermal stability of the solar cell. Upon harsh thermal stress at 120 °C for 2 h, the ternary solar cells show only a minor relative deterioration of 10%. Whereas the polymer/fullerene blend PTB7-Th:PC61BM is rather unstable under these conditions, its degradation was efficiently suppressed by incorporating the near infrared-absorbing polymer PDTP–DFBT. Spectroscopic ellipsometry investigations and an effective medium analysis of the ternary absorber blend revealed that the domain conformation in presence of PDTP–DFBT remains stable whereas the domain conformation changes in its absence. The ternary PTB7-Th:PDTP–DFBT:PC61BM solar cells yield thermally stable power conversion efficiencies of up to 6%.

Water based fluidic radio frequency metamaterials

Electromagnetic metamaterials offer great flexibility for wave manipulation and enable exceptional functionality design, ranging from negative refraction, anomalous reflection, super-resolution imaging, transformation optics to cloaking, etc. However, demonstration of metamaterials with unprecedented functionalities is still challenging and costly due to the structural complexity or special material properties. Here, we demonstrate for the first time the versatile fluidic radio frequency metamaterials with negative refraction using a water-embedded and metal-coated 3D architecture. Effective medium analysis confirms that metallic frames create an evanescent environment while simultaneously water cylinders produce negative permeability under Mie resonance. The water-metal coupled 3D architectures and the accessory devices for measurement are fabricated by 3D printing with post electroless deposition. Our study also reveals the great potential of fluidic metamaterials and versatility of the 3D printing process in rapid prototyping of customized metamaterials.

Effective medium analysis of absorption enhancement in short-pitch metal grating incorporated organic solar cells.

The effective medium theory is applied to analyze the absorption enhancement in organic solar cells with a short-pitch metal grating. A 37% improvement in the absorption of the active layer is achieved with respect to that of a planar control cell. It is inspired that the propagating surface plasmon modes are excited at the interface between the effective medium layer and the flat metal plate, resulting in a reduction of light reflection. In real structure, the electric field redistributes with its intensity in the region with active materials infiltrated in the grooves increases significantly, exhibiting like hot spots to assist in achieving broadband absorption enhancement. Moreover, the localized surface plasmon resonances excited at the top of the metal ridges also contribute to the absorption enhancement in the cells.

Optical properties and surface damage studies of crystalline silicon caused by swift iron ions

p-Type silicon samples irradiated with 70MeV 56Fe5+ ions for various fluences varying between 5×1012 and 4×1014ionscm−2 have been studied using spectroscopic ellipsometry and Fourier transform infrared spectroscopy. The microstructure of the irradiated samples was modeled from ellipsometric data, using a multilayer optical model and Bruggeman effective medium approximation. The values of pseudodielectric function, absorption coefficient and Penn gap energy were determined with respect to ion fluence. The effective medium analysis suggests that the superficial silicon layer can be explained as a mixture of crystalline and damaged silicon. The thickness of the damaged layer and percentage of voids present in the layer were found to increase with increase in the ion fluence. The effect of disorder on the interband optical spectra, especially on the critical point E1 at 3.4eV was found to vary with ion fluence. A red shift in the critical point E1 with increasing ion fluence was observed. FTIR study showed of silicon samples irradiated with 70MeV 56Fe5+ ions produced the oscillations in the spectral region 1000–400cm−1. As irradiated sample showed more pronounced fringes, while contrast of the fringes and amplitude both were found to decrease with increase in depth.

Dimensionality effects of carbon-based thermal additives for microporous adsorbents

Abstract We present a systematic study of carbon nanomaterials with different geometries and thermal properties, including few-layer graphene (FLG), graphene oxide (GO), and functionalized carbon nanotubes (fCNT) as additives to enhance the thermal conductivity of microporous adsorbent materials. The dimensionality and intrinsic thermal conductivity of the additives were found to be critical for both maximizing the thermal conductivity enhancement, and minimizing the reduction in the adsorption capacity of the active materials. We demonstrated that two-dimensional (2D) FLG was the most effective thermal additive for zeolite (ZT) adsorbents due to its high thermal conductivity and preferential 2D geometry. Meanwhile, negligible enhancement was observed from one-dimensional (1D) additives such as fCNTs, which is consistent with the predictions from a modified effective medium analysis (EMA). Our work provides insights for the development of additives to enhance the thermal performance of porous materials in applications such as adsorption heat pumps, gas storage, and separation processes.

Effective medium analysis of thermally evaporated Ag nanoparticle films for plasmonic enhancement in organic solar cell

Films of silver nanoparticles have optical properties that are useful for applications such as plasmonic light trapping in solar cells. We report on the simple fabrication of Ag nanoparticle films via thermal evaporation, with and without subsequent annealing. These films result in a random array of particles of various shapes and sizes. The modeling of such a vast collection of particles is still beyond reach of the modern computers. We show that it is possible to represent the silver island films by the Bergman effective mediums with the same optical properties. The effective medium method provides us with deep insight about the shape, the size and the distribution of nanoparticles. The far field simulations of solar cells, in which the silver island film is replaced with an effective medium layer, show a reduction in the absorption of active layer. Besides, the near field simulations based on finite-difference time-domain technique demonstrate that the near field effects on active layer absorption are negligible and this method highlights the importance of nanoparticles shapes. The PCPDTBT:PCBM solar cells with embedded silver island films are fabricated, and it is found that their performances show the similar trend. This insight can be used for the optical analysis of thermally evaporated Ag nanoparticle films for the improvement of organic solar cells.

Dimensional effects on the tunneling conductivity of gold-implanted nanocomposite films

We study the dependence of the electrical conductivity on the gold concentration of Au-implanted polymethylmethacrylate (PMMA) and alumina nanocomposite thin films. For Au contents larger than a critical concentration, the conductivity of Au-PMMA and Au-alumina is well described by percolation in two dimensions, indicating that the critical correlation length for percolation is larger than the thickness of the films. Below the critical loading, the conductivity is dominated by tunneling processes between isolated Au particles dispersed in PMMA or alumina continuous matrices. Using an effective medium analysis of the tunneling conductivity, we show that Au-PMMA behaves as a tunneling system in two dimensions, as the film thickness is comparable to the mean Au particle size. On the contrary, the conductivity of Au-alumina films is best described by tunneling in three dimensions, although the film thickness is only a few times larger than the particle size. We interpret the enhancement of the effective dimensionality of Au-alumina films in the tunneling regime as due to the larger film thickness as compared to the mean interparticle distances.

Broadband negative permeability using hybridized metamaterials: Characterization, multiple hybridization, and terahertz response

There is an increased interest to create artificial magnetic metamaterials that show a negative permeability over a wide frequency range. In this paper, we experimentally and numerically demonstrate a broadband negative permeability using symmetric cut-wire-pair metamaterial structures. This finding is based on the second-order hybridization, which is activated by manipulating the correlation between the coupling within a single cut-wire pair and the coupling between neighboring cut-wire pairs. An effective medium analysis is performed to identify the role of the internal and external interactions in the hybridized metamaterials. An extended second-order hybridization scheme is proposed, which describes the electromagnetic response of more complex systems that exhibit an extremely wide band of negative permeability. In addition, the terahertz response of the cut-wire-pair dimer is further explored by scaling down the dimensions of the structures.