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Nanoporous platinum broadband absorber has attracted interest in thermosensorics and IR photodetection due to its unique properties. In this work we report the physical mechanism underlying broadband absorption in electrochemically-grown, nanoporous Pt films by analyzing NIR-ViS-UV spectral ellipsometry data of nanoporous Pt films in dependence on the Pt film thickness (27, 35, 38, 48 nm). For the two thinner films a single layer model with a graded optical index Pt surface layer was used. For the two thicker films a two-layer optical model with a constant optical index Pt substrate layer and a graded optical index Pt surface layer was used. The graded optical index of the Pt surface layer reduces surface reflectivity and the constant optical index Pt substrate layer supports multiple reflections in the Pt film. Finally, we relate the thickness dependent optical index with the nanostructure of the nanoporous Pt film, which can be controlled in the electrochemical growth process. We observed that while in the transverse plane the multilayer exhibits graded refractive index, in the top horizontal planes the multilayer assembly exhibits discontinuous refractive index values due to the distribution of platinum crystal islands in air, which allows a metamaterial behavior of the whole system. [ABSTRACT FROM AUTHOR]

Abstract Nanoporous platinum broadband absorber has attracted interest in thermosensorics and IR photodetection due to its unique properties. In this work we report the physical mechanism underlying broadband absorption in electrochemically-grown, nanoporous Pt films by analyzing NIR-ViS-UV spectral ellipsometry data of nanoporous Pt films in dependence on the Pt film thickness (27, 35, 38, 48 nm). For the two thinner films a single layer model with a graded optical index Pt surface layer was used. For the two thicker films a two-layer optical model with a constant optical index Pt substrate layer and a graded optical index Pt surface layer was used. The graded optical index of the Pt surface layer reduces surface reflectivity and the constant optical index Pt substrate layer supports multiple reflections in the Pt film. Finally, we relate the thickness dependent optical index with the nanostructure of the nanoporous Pt film, which can be controlled in the electrochemical growth process. We observed that while in the transverse plane the multilayer exhibits graded refractive index, in the top horizontal planes the multilayer assembly exhibits discontinuous refractive index values due to the distribution of platinum crystal islands in air, which allows a metamaterial behavior of the whole system.

Porous semiconductors have garnered significant attention owing to their distinctive physical and chemical properties. In this study, optical anisotropy is presented in porous germanium (PGe) on a Si (001) substrate. Both n‐ and p‐type PGe, achieved through bipolar electrochemical etching, exhibit optical anisotropy along the Ge <001> direction, as determined by spectroscopic ellipsometry. Birefringence and depolarization factors are controllable by adjusting the etching parameters and doping concentration of the epitaxial Ge layer. The gradient porosity and pore distribution in PGe can be well captured by the optical models. The findings of optical anisotropy in PGe‐on‐Si hold promise for applications in optical elements or sensors for gas or biomolecules. [ABSTRACT FROM AUTHOR]

In order to model the propagation of light through a sand cloud, it is critical to have accurate data for the optical constants of the sand particles that comprise it. The same holds true for modeling propagation through particles of any type suspended in a medium. Few methods exist, however, to measure these quantities with high accuracy. In this paper, a characterization method based on spectroscopic ellipsometry (SE) that can be applied to a particulate material is presented. In this method, a polished disc of an adhesive compound is prepared, and its optical constants are measured. Next, a mixture of the adhesive and a sand sample is prepared and processed into a polished disc, and SE is performed. By treating the mixture as a Bruggeman effective medium, the optical constants of the particulate material are extracted. For verification of the proposed method, it is first applied to pure silica powder, demonstrating good agreement between measured optical constants and literature values. It is then applied to Arizona road dust, a standard reference material, as well as real desert sand samples. The resulting optical constant data is input into a rigorous scattering model to predict extinction coefficients for various types of sand. Modeling results are compared to spectroscopic measurements on static sand samples, demonstrating good agreement between predicted and measured spectral properties including the presence of a Christiansen feature near a wavelength of 8 µm. [ABSTRACT FROM AUTHOR]

In this work, we discuss the precision of the effective medium approximation (EMA) model in the data analysis of spectroscopic ellipsometry (SE) for solid materials with micro-rough surfaces by drawing the regime map. The SE parameters ψ (amplitude ratio) and Δ (phase difference) of the EMA model were solved by rigorous coupled-wave analysis. The electromagnetic response of the actual surfaces with micro roughness was simulated by the finite-difference time-domain method, which was validated by the experimental results. The regime maps associated with the SE parameters and optical constants n (refractive index) and k (extinction coefficient) of the EMA model were drawn by a comparison of the actual values with the model values. We find that using EMA to model micro-rough surfaces with high absorption can result in a higher precision of the amplitude ratio and extinction coefficient. The precisions of ψ, Δ, n and k increase as the relative roughness σ/λ (σ: the root mean square roughness, λ: the incident wavelength) decreases. The precision of ψ has an influence on the precision of k and the precision of Δ affects the precision of n. Changing σ alone has little effect on the regime maps of the relative errors of SE parameters and optical constants. A superior advantage of drawing the regime map is that it enables the clear determination as to whether EMA is able to model the rough surfaces or not. [ABSTRACT FROM AUTHOR]

Gas diffusion, D, in partially saturated soils, constitutes a critical topic in soil sciences. However, it is a complex process and this limits its characterization and estimation. In this study, we analyzed and parameterized the soil gas diffusion using a combination of percolation theory (PT) and the effective-medium approximation (EMA). Here, we selected 126 coarse-textured soils with measurements including sand, silt, and clay content, bulk density, organic matter, porosity, soil water content measured at different pressure heads and saturation-dependent gas diffusion. First, we adopted the van Genuchten model, fit it to the soil water retention curve (SWRC), optimized its parameters, and determined the water content at the inflection point. Second, the parameters of the universal scaling law from PT and EMA were optimized by directly fitting the model to the saturation-dependent gas diffusion data. Those parameters are (1) the critical air-filled porosity, εc, (2) the crossover air-filled porosity, εx, at which the gas movement behavior changes from the percolation theory domain to the EMA domain; and (3) the average pore coordination number, z. Next, a multiple linear regression analysis (MLRA) was applied to link εc, εx and z to other soil parameters, such as soil textural and/or hydraulic properties. Uncertainties in our results were evaluated using a jack-knife resampling technique, which involved applying the MLRA more than 7000 times. Results revealed that the most accurate estimations were obtained when both soil textural and hydraulic properties were used simultaneously. However, the use of only soil textural parameters presents practical advantages, as it provides excellent estimations for εx and z, although not for εc. The latter is a critical parameter in the application of the PT and EMA to gas diffusion that requires both the soil basic properties and water saturation curve properties to be correctly estimated.

Results of numerical simulations of optical radiation focusing by a cluster of identical nanospheres close packed into a spherical volume (a "metaparticle") are presented. Parameters of the focal region (intensity and longitudinal and transverse sizes) formed by metaparticles with different internal structure are calculated. It is shown that the problem of optical radiation focusing by a spherical cluster of nanoparticles can be reduced in certain cases to the problem of light focusing by a homogeneous spherical particle with an effective refractive index derived from the effective medium theory. Moreover, certain spherical cluster topologies make it possible to improve optical focusing in the near-field region, in particular, to increase the peak intensity or enhance its spatial localization. [ABSTRACT FROM AUTHOR]

In this work, we discuss the precision of the effective medium approximation (EMA) model in the data analysis of spectroscopic ellipsometry (SE) for solid materials with micro-rough surfaces by drawing the regime map. The SE parameters ψ (amplitude ratio) and Δ (phase difference) of the EMA model were solved by rigorous coupled-wave analysis. The electromagnetic response of the actual surfaces with micro roughness was simulated by the finite-difference time-domain method, which was validated by the experimental results. The regime maps associated with the SE parameters and optical constants n (refractive index) and k (extinction coefficient) of the EMA model were drawn by a comparison of the actual values with the model values. We find that using EMA to model micro-rough surfaces with high absorption can result in a higher precision of the amplitude ratio and extinction coefficient. The precisions of ψ, Δ, n and k increase as the relative roughness σ/λ (σ: the root mean square roughness, λ: the incident wavelength) decreases. The precision of ψ has an influence on the precision of k and the precision of Δ affects the precision of n. Changing σ alone has little effect on the regime maps of the relative errors of SE parameters and optical constants. A superior advantage of drawing the regime map is that it enables the clear determination as to whether EMA is able to model the rough surfaces or not.

This work concerns the analytical study of a uniform steady flow of an incompressible, viscous and electrically conducting fluid across an array of parallel porous cylindrical fibres subjected to a transverse uniform magnetic field with the assumption that the stress jump conditions for tangential stresses at the fluid–porous interface are applied. The effective medium approximation has been used for predicting the overall bed permeability (OBP) of fibrous filtration beds. The mathematically governed equations are formulated as Stokes’ equation in the fluid region, while Brinkman’s equation is in the porous regions. These equations have been solved using the stream function method, and the corresponding flow field expressions within each region are obtained. The derivation of the drag force acting on the surface of the porous circular fibre has been presented. The effect of various physical parameters on the velocity profiles is graphically illustrated and discussed. Interestingly, the OBP is considerably increased by implementing the jump boundary conditions, and this has significant applications in filtration systems design and may have physical implications in biological systems. This model is in agreement with the published results of the existing models. [ABSTRACT FROM AUTHOR]

Microwave heating of pure alumina is studied experimentally and numerically, in a 2.45 GHz single‐mode cavity, for different density levels. Even considering a constant incident power, the results show a complex evolution of the alumina temperature: first a two‐step increase, then a maximum, and finally a cooling stage. In addition, a density dependence of the heating efficiency is observed: a more efficient heating occurs for lower densities. Using the effective medium approximation (EMA) to derive the physical data as functions of density, the numerical simulations are in contradiction with the experiments, proving that the EMA approach is not able to correctly predict the imaginary part of the permittivity. Furthermore, the simulations do not accurately describe the first moments of the heating, nor the long‐term evolution of the temperature (cooling). We then explain the origin of this discrepancy: the need to adjust the movable stub on the one hand, and to account for heat exchange between the cavity and its surroundings on the other. [ABSTRACT FROM AUTHOR]

Abstract—When creating inhomogeneous materials with required functional characteristics, theoretical methods for predicting their properties depending on the composition and structure are of great importance. This paper offers a review of methods for calculating the effective electrophysical characteristics of inhomogeneous media that can take into account such structural features of these media as the multicomponent nature, probability distributions of orientations and shapes of inclusions, and several levels of nesting. The Maxwell Garnett approximation and its generalizations for inhomogeneous matrix-type media are presented in detail: the Fricke and Bragg–Pippard formulas for the cases of ellipsoidal homogeneous inclusions, as well as my generalizations of the Maxwell Garnett approximation for the cases of ellipsoidal homogeneous anisotropic inclusions oriented according to a probabilistic law, and ellipsoidal anisotropic inclusions with anisotropic shell. Variants of the effective medium approximation for media with homogeneous inclusions are considered: the Bruggeman approximation; generalized effective medium approach of D. Stroud; generalized singular approximation. A differential approach is presented on the basis of the generalized singular approximation, with the help of which an equation with a parameter is obtained that allows one to vary the degree of differentiality of the approximation. A description of the my approach for media with inclusions in a multilayer shell is given: a generalized effective field approximation, which can be considered a generalization of the generalized singular approximation to media with several levels of nesting. Methods for taking into account the probability distributions of inclusion orientations (using the representation theory of the SO(3) group) and inclusion shapes are described. It is established that the generalized effective-field approximation for inhomogeneous media with inclusions in a multilayer shell has the greatest degree of generality. [ABSTRACT FROM AUTHOR]

Deciphering how the dielectric properties of textile materials are orchestrated by their internal components has far-reaching implications. For the development of textile-based electronics, which have gained ever-increasing attention for their uniquely combined features of electronics and traditional fabrics, both performance and form factor are critically dependent on the dielectric properties. The knowledge of the dielectric properties of textile materials is thus crucial in successful design and operation of textile-based electronics. While the dielectric properties of textile materials could be estimated to some extent from the compositional profiles, recent studies have identified various additional factors that have also substantial influence. From the viewpoint of materials characterization, such dependence of the dielectric properties of textile materials have given rise to a new possibility—information on various internal components could be, upon successful correlation, extracted by measuring the dielectric properties. In view of these considerable implications, this invited review paper summarizes various fundamental theories and principles related to the dielectric properties of textile materials. In order to provide an imperative basis for uncovering various factors that intricately influence the dielectric properties of textile materials, the foundations of the dielectrics and polarization mechanisms are first recapitulated, followed by an overview on the concept of homogenization and the dielectric mixture theory. The principal advantages, challenges and opportunities in the analytical approximations of the dielectric properties of textile materials are then discussed based on the findings from the recent literature, and finally a variety of characterization methods suitable for measuring the dielectric properties of textile materials are described. It is among the objectives of this paper to build a practical signpost for scientists and engineers in this rapidly evolving, cross-disciplinary field.

With the traditional multilayers coating, it is difficult to achieve broadband and wide-angle antireflection optical properties simultaneously. As one of the most promising candidates for omnidirectional and broadband antireflection coating, the moth-eye structure has gained considerable attention in the past several decades because of its excellent optical performance and self-cleaning function. The challenges in the use of this technology, namely, its manufacturing cost and mechanical structural stability, limit its popularization. In this paper, a moth-eye structure surface combined with a residual layer of an identical material is proposed; the proposed composite structure can be formed by a one-step process of half-etched technology. For the design and optimization of the composite moth-eye structure, composite structures were compared by performing effective medium approximation (EMA) and rigorous coupled– wave analysis (RCWA). The structural parameters of the micro–nano surface were optimized to increase the mechanical stability considerably. The results showed that the optimized composite moth-eye structure has high transmittance for incident angles from 0° to 80° in the infrared spectrum. Furthermore, the transmission diffraction at high orders was suppressed efficiently at super-wide incident angles.

Wave propagation in epsilon‐near‐zero (ENZ) media offers exciting possibilities in the field of nanophotonics. Here, a thin film self‐rolling technique to fabricate SiO2/Au based 3D multilayer structures is implemented. These cylindrical multilayer metamaterials are utilized to take advantage of the material dispersion as well as the structural dispersion to obtain self‐rolling ENZ metamaterials at the visible to near‐infrared wavelength range. The ENZ features are investigated initially by modelling the dispersion of the effective medium using circular effective medium approximation and further by the characterization of the optical response. Moreover, the potential of these structures for supporting an ENZ waveguide mode is demonstrated. This platform for dispersion engineering and obtaining ENZ metamaterials can provide opportunities for unique applications which cannot be possible with planar systems. It can be used to integrate active materials before the rolling process to actively control the layer properties, or embed single‐layer materials within different layers of ENZ metamaterials with potential applications in quantum nanophotonics. [ABSTRACT FROM AUTHOR]

The optical effect of a monolayer array of spherical silica particles, arranged in hexagonal close packing (HCP) atop ultrathin (3–15 µm) c-Si-based solar cells, as an omnidirectional light-trapping/antireflection scheme, has been investigated. The study presents an optimal light-trapping scheme (LTS) considering the effect of both the particle size and thickness of the silicon wafer simultaneously. The optical response in terms of integrated absorbance within the silicon wafer and integrated reflectance from the front were studied, both by computational and experimental means. A maximum of ~ 30% relative increment in absorbance, with respect to the bare planar silicon wafer, was computed for the optimized structure, whereas an enhancement of ~ 25% in integrated reflectance was found experimentally. The proposed LTS was applied on a 15 µm-thick a-Si(n)/p-Si(p) heterojunction solar cell and the current–voltage characterization was carried out for the various angles of incidence (0°–60°) of light. A relative increment of 18–22% in efficiency was observed for different angles of incidence. Based on the computational and experimental data, the size dependent optical phenomena and light trapping mechanisms have been analyzed and compared to obtain the most suitable light-trapping scheme for ultra-thin c-Si based solar cells. [ABSTRACT FROM AUTHOR]

Nonlinear thermal response enables flexible heat manipulation and management with artificial structures. In particular, intrinsic temperature-dependent parameters of constitutive materials guide the design of self-adaptive thermal metamaterials. However, the geometrical effect in nonlinear composites has not been adequately studied, which may limit the potential multiple functionalities and versatile control. Here, under the effective medium approximation framework, we develop a unified theory for predicting anisotropic nonlinear equivalent thermal conductivities of elliptical inclusions in homogeneous media. By means of the derived results, enhancement of value in nonlinear coefficient can be achieved in a specified direction, based on geometrically anisotropic configurations and temperature-dependent properties. Quantitative relations between directional enhancement and inclusive shape factors are given by analytical theory and verified by numerical simulation. The proposed theoretical methods can be further extended to arbitrary non-circular configurations of complex structures, and the directional nonlinearity enhancement effect will facilitate refined heat control, combined with other nonlinear mechanisms such as spatiotemporal modulation or harmonic generation.

The absorption properties of snow and ice particles containing impurities such as black carbon (BC) and mineral dust (DS) were investigated using the geometrical optics approximation method (GOM). This study focuses on analyzing the single scattering co-albedo (SSCA) characteristics of non-spherical particles, aiming to enhance the interpretation of radiation observations in snow and ice regions. Particle sizes were varied ( r va = 50 , 200 and 500 μ m) and two types of inclusions (1 ppmw of BC and 100 ppmw DS) were considered. Significant differences in SSCA, ranging from one to two orders of magnitude were observed at λ = 0. 5 μ m between the "homogeneous model" with effective medium approximation method (EMA) and the "inhomogeneous model" with 50 BC inclusions. Similarly, differences of a factor of 2 to 4 were found for DS with 100 ppmw at λ = 0. 5 μ m and BC with 1 ppmw at λ = 0. 875 μ m. To bridge intermediate SSCA values between these models, we propose an "inhomogeneous model with EMA", introducing two parameters to account for impurity concentration and inclusion aggregation. Our findings highlight the importance of selecting appropriate single scattering calculation methods in the analysis of observation data, particularly when dealing with BC impurity with significant absorption characteristics. This research contributes to a deeper understanding of solar radiation absorption in snow and ice regions, with potential implications for the optical remote sensing of snow and ice and climate modeling studies. • The absorption of ice particles is studied in terms of the mixing state of inclusions • The "inhomogeneous model" is compared with the "homogeneous model with EMA" • An appropriate particle model is needed to analyze observations with inclusions [ABSTRACT FROM AUTHOR]

• Soil gas diffusion based on the percolation theory and effective-medium approximation. • Best estimation was obtained using both soil textural and hydraulic properties. • Soil gas diffusion parameters ε x and z were correctly estimated (R2 of 0.68 and 0.55), not for ε c. • 126 soil data featuring basic, hydraulic and gas characterization simultaneously. Gas diffusion, D , in partially saturated soils, constitutes a critical topic in soil sciences. However, it is a complex process and this limits its characterization and estimation. In this study, we analyzed and parameterized the soil gas diffusion using a combination of percolation theory (PT) and the effective-medium approximation (EMA). Here, we selected 126 coarse-textured soils with measurements including sand, silt, and clay content, bulk density, organic matter, porosity, soil water content measured at different pressure heads and saturation-dependent gas diffusion. First, we adopted the van Genuchten model, fit it to the soil water retention curve (SWRC), optimized its parameters, and determined the water content at the inflection point. Second, the parameters of the universal scaling law from PT and EMA were optimized by directly fitting the model to the saturation-dependent gas diffusion data. Those parameters are (1) the critical air-filled porosity, ε c , (2) the crossover air-filled porosity, ε x , at which the gas movement behavior changes from the percolation theory domain to the EMA domain; and (3) the average pore coordination number, z. Next, a multiple linear regression analysis (MLRA) was applied to link ε c , ε x and z to other soil parameters, such as soil textural and/or hydraulic properties. Uncertainties in our results were evaluated using a jack-knife resampling technique, which involved applying the MLRA more than 7000 times. Results revealed that the most accurate estimations were obtained when both soil textural and hydraulic properties were used simultaneously. However, the use of only soil textural parameters presents practical advantages, as it provides excellent estimations for ε x and z , although not for ε c. The latter is a critical parameter in the application of the PT and EMA to gas diffusion that requires both the soil basic properties and water saturation curve properties to be correctly estimated. [ABSTRACT FROM AUTHOR]

Ion transport through nanoporous materials is of fundamental importance for the design and development of filtration membranes, electrocatalysts, and electrochemical devices. Recent experiments have shown that ion transport across porous materials is substantially different from that in individual pores. Here, we report a new theoretical framework for ion transport in porous materials by combining molecular dynamics (MD) simulations at nanopore levels with the effective medium approximation to include pore network properties. The ion transport is enhanced with the combination of strong confinement and dominating surface properties at the nanoscale. We find that the overlap of electric double layers and ion–water interaction have significant effects on the ionic distribution, flux, and conductance of electrolytes. We further evaluate the gap between individual nanopores and complex pore networks, focusing on pore size distribution and pore connectivity. This article highlights unique mechanisms of ion transport in porous materials important for practical applications. [ABSTRACT FROM AUTHOR]

A quantitative analysis of the thermally induced degradation of various device‐relevant multi‐cation hybrid perovskite films is performed using spectroscopic ellipsometry, for temperatures between 80 and 120 °C. The studied compositions are a triple cation perovskite Cs0.05(MA0.17FA0.83)0.95Pb(Br0.17I0.83)3, a Rb‐containing variant Rb0.05Cs0.05(MA0.17FA0.83)0.90Pb(Br0.17I0.83)3, and a methylammonium‐free Rb0.05Cs0.10FA0.85PbI3 composition. A very wide combined spectral range of 200 nm to 25 μm is covered by combining the data from two separate instruments. The relative changes in organic cation concentrations are quantified from the middle infrared molecular absorption bands, leveraging the use of point‐by‐point fitting for increased sensitivity. Additionally, the formation of PbI2 and non‐perovskite δ‐CsPbI3 phases is evidenced from Bruggemann effective medium fits to the visible and ultraviolet complex refractive indices. Methylammonium is almost completely depleted from the relevant compositions within 100 to 285 min of thermal annealing. The MA‐free perovskite degrades faster at intermediate temperatures, which is attributed to phase instability due to the formation of δ‐CsPbI3 in addition to PbI2. [ABSTRACT FROM AUTHOR]

The impedance flow sensors have usually been considered as the competitive components in the multi-flow measurement, primarily due to their advantages of fast response, high dynamic range and sensitivity, non-invasive and/or non-intrusive measurements. In this review, the impedance sensors have been categorized into two categories, i.e., the probes for local/average parameter measurement and sensor arrays for distributed parameter measurement. Maxwell’s equations and boundary conditions have been referred as a starting point to understand the measuring principle of impedance sensors, from which the sensitivity distribution can be attained. The sensitivity distribution can be utilized to characterize the sensor and employed as an objective function in optimizing the sensor geometry. The EMA models usually offer a way to relate the impedance measurements to individual phase fraction, which have been critically reviewed. In addition, the application range, spatial and temporal resolutions, advantages and disadvantages of the sensor arrays, i.e., the wire mesh sensor, field focusing sensor, electrical resistance and capacitance tomography sensors have also been provided. Finally, the future trends of impedance sensor development have been briefly discussed. [ABSTRACT FROM AUTHOR]

Liefferink et al. (GM 22: 57, 2020) reported the effect of liquid volume fraction on the penetration hardness/shear modulus and the friction coefficient of several types of wet sand. However, no analytical equation can simultaneously explain these two observations. In this paper, we formulate the effective elastic modulus of wet granular material as a function of the liquid volume fraction using the effective medium approximation. We also propose the equation of the friction coefficient between the surface of an object and the surface of the wet granular materials. The equations for the modulus of elasticity and coefficient of friction as a function of the liquid volume fraction can well explain the observed data by using a few adjusted parameters. [ABSTRACT FROM AUTHOR]

The design of multiphase optical nanocomposites necessitates the understanding of coexisting phases' morphology and chemistry which have deterministic impacts on light–matter interaction. A prominent example is gradient refractive index (GRIN) materials proposed to realize an arbitrarily shaped, single optical component with minimal chromatic aberration. Ge‐As‐Pb‐Se materials are promising for GRIN due to their ability to exhibit spatially varying volume fraction of high‐index Pb‐rich phases in low index matrices. These materials are characterized to date, exclusively using transmission electron microscopy to reveal their phase separation and induced crystalline phase(s). It is found in the study that the intrinsic 2D perspective of the technique has hindered the identification of true morphology. To clarify this ambiguity, atom probe tomography (APT) is utilized to gain the first‐ever observation of the nanocomposites' microstructure and its evolution upon heat treatment in a 3D space. The APT‐quantified geometry and chemistry of coexisting phases are considered to predict the effective media's optical behaviors which closely match experimental data, enabling the establishment of the material's predictive and accurate process–structure–property relationship. Findings in the study demonstrate the robustness and advantage of the APT‐assisted characterization in the design and realization of GRIN materials. [ABSTRACT FROM AUTHOR]

• A unified model was developed for unfrozen and frozen soil thermal conductivity. • New model accurately reproduces the increase at low moisture and during freezing. • High predicative scores were obtained using a large compiled dataset. Soil thermal conductivity (STC) is a crucial parameter in modeling land surface processes. However, the current STC models are developed separately for unfrozen and frozen soils, leading to inconsistent understanding. In this study, we propose a unified model based on the work of Ghanbarian and Daigle (2016) originally developed for unfrozen soils. The unified model comprises three parameters: critical volume fraction (ϕ c), scaling exponent (t), and a compensating factor (α), and considers dry soil as the low-conductivity component (weighted by air volume fraction) and saturated soil as the high-conductivity component (weighted by volumetric liquid content for unfrozen state and total water content for frozen state). Specifically, ϕ c represents a critical point where high-conductivity component begins to govern the behavior of effective STC, characterized by t. α allows for accurate calibration of saturated STC. Using a dataset of 90 unfrozen samples (693 measurements) and 74 frozen samples (255 measurements), pedotransfer functions (PTFs) for the three parameters were trained. The unified model successfully reproduces the sharp rise in STC at low moisture conditions during wetting and the increase during freezing. Compared to an empirical model (Côté and Konrad, 2005a) and a theoretical model (Tian et al., 2016), the unified model demonstrates higher predictive skill for unfrozen and frozen soils, achieving Nash-Sutcliffe efficiency coefficients of 0.96 and 0.90, respectively. This work contributes to a more consistent and comprehensive understanding of STC in cold environments and has the potential to be integrated into land surface models. [ABSTRACT FROM AUTHOR]

There is growing requirement to use recycled aggregate in concrete to demote industrial wastes produced by construction sectors. Thus, to develop simple but meaningful formulations to assess the mechanical properties of concrete, when it is mixed with both natural and recycled aggregates, becomes essential. In the present study an analytical approach to calculate the elastic modulus of concrete containing both natural and recycled concrete aggregates is proposed. The approach is developed on the concept of effective medium approximation but considering the effect of Poisson's ratio. Two analytical formulas are derived. One is for the two-phase concrete (mortar and aggregate) that is mixed with 100% natural aggregate or 100% recycled concrete aggregate. The other is for the three-phase concrete (mortar, natural aggregate, and recycled concrete aggregate) that is mixed with both natural and recycled aggregates. The analytical formulations are validated using the results obtained from both the numerical simulations and experimental tests. The present analytical model not only provides the simple and meaningful formulations to calculate the effective elastic modulus but also can be used to directly examine the influence of mortar, natural aggregate, recycled concrete aggregate and its replacement ratio to natural aggregate on the elastic modulus of the mixed concrete. [ABSTRACT FROM AUTHOR]

The emerging fifth-generation (5G) wireless and mobile communications demand high-performance phase shifters to enable the multi-channel transmit/receive systems with phase control ability. In this work, a size-miniaturized wideband phase shifter based on slow-wave half-mode substrate integrated waveguide (SW-HMSIW) is presented. The proposed SW-HMSIW is consisting of conventional HMSIW section and periodically-patterned non-uniform microtrip polyline unit cells. Patterning polyline unit cells in HMSIW can effectively enhance the product of the equivalent permittivity and permeability of SW-HMSIW, so that the slow-wave effect can be exhibited and its associated size reduction will be further achieved. Moreover, lengths and widths of segments of the proposed polyline unit cell can influence the equivalent permittivity and permeability of the SW-HMSIW, and consequently change its phase constant and cutoff frequency. Hence, for a SW-HMSIW with a fixed physical length, an electrical length variation, i.e, phase shift, can be achieved as segments of the polyline employ various lengths or widths. Based on this principle, a wideband equal-width equal-length SW-HMSIW phase shifter is implemented. Measured results are in good agreement with simulated ones, with a phase shift of 90.5 ± 3 degree and a fractional bandwidth of 43.0% achieved. Compared with some similar reported works, the proposed one shows a size reduction more than 80% as well as good magnitude and phase deviations performance. Moreover, its miniaturized size and equal-width equal-length structure make it suitable for the complex multi-channel modules and networks of the 5G applications.

An image-based effective medium approximation (EMA) is developed so as to permit very fast transport properties evaluations of 3D porous media. From an image-based porous network (IBPN) built upon digital image processing of 3D binary images, we focus on throat's local geometrical properties at the pore scale, for being the most sensible structural units which build up the local pressure. This approach is a 3D image–based extension of the critical point approach proposed in 2D fractures. We show, from analyzing various core rock samples available in the literature, that the asymptotic assumptions associated with the preeminence of critical points in throats are indeed geometrically relevant. We then describe how the image-based EMA evaluated from the conductances computed from the discrete IBPN can be reliably evaluated. The proposed method is evaluated upon the estimation of core sample permeability from binarized image obtained using X-ray tomography. Since it combines digital image treatments with statistical data post-processing without the need of computational fluid dynamics (CFD) computation, it is extremely cost efficient. The results are compared with a micro-scale Stokes flow computation in various rock samples. The sensitivity to the pore discretization also is discussed and illustrated. [ABSTRACT FROM AUTHOR]

Giant permittivity (GP) materials are of interest as possible capacitors for energy storage. Ferroelectric and related homogeneous materials suffer from strongly temperature dependent dielectric properties. Therefore, within the last two decades, the interest has been focused to dielectric studies of various inhomogeneous GP materials, where the essential role was shown to be due to weak but inhomogeneous conductivity. Here we study, within the known models based on effective medium approximation, the broadband dielectric spectra of conducting core – less conducting shell structures, which are expected to be the most advantageous for revealing the GP effects. Examples of core–shell structures, in which the dielectric properties of both components are independent of frequency, are studied analytically and numerically within the generalized brick model. For the simple case of coated-spheres model the dielectric dispersion is of a single Debye-relaxation type and the optimal GP properties are for highly conducting cores and thinnest possible nonconducting and high-permittivity shells. For non-spherical core–shell particles the conditions for macroscopically isotropic composite are briefly discussed. [ABSTRACT FROM AUTHOR]

The presented experiments and studies are intended for photovoltaic applications of crystalline silicon. This work deals with chemical treatment of the surface of n-type silicon wafers with different resistivity to reduce their reflectivity. Chemical surface treatment of silicon is an alternative method to using the antireflection layer. Optical losses caused by the reflection of light from the surface of the solar cells significantly reduce their efficiency. The investigated samples were prepared by the electrochemical etching method in the solution based on hydrofluoric acid and ethanol. The analysis of the prepared samples is divided into two parts, namely experimental measurements, and theoretical modeling. Experimental measurements are performed using UV-VIS spectroscopy, spectroscopic ellipsometry and SEM microscopy. Theoretical modeling is based on the construction and optimization of theoretical model of optical response (reflectivity and ellipsometric parameters) to determine the effective refractive index and thickness of formed structure. Effective refractive index of studied samples in theoretical model of optical response is based on Looyenga effective medium approximation and Tauc-Lorentz dispersion model. [ABSTRACT FROM AUTHOR]

The influence of the internal structure of inhomogeneous particles on their radiative properties is an open issue repeatedly questioned in many fields of science and technology. The importance of a refined description of the particle composition and structure, going beyond mean-field approximations, is generally recognized. Here, we focus on describing internal inhomogeneities from a statistical point of view. We introduce an analytical description based on the two-point density-density correlation function, or the corresponding static structure factor, to calculate the extinction cross sections. The model agrees with numerical predictions and is validated experimentally with colloidal aggregates in the 0.3–6 μm size range, which serve as an inhomogeneous model system that can be characterized enough to work without any free parameters. The model can be tightly compared to measurements with single particle extinction and scattering and spectrophotometry and suggests a simple behavior for 90° scattering from fractal aggregates as a function of extinction, which is also confirmed experimentally and numerically. We also discuss the case of absorbing particles and report the experimental results for water suspensions of black carbon for both the forward and 90° scattering properties. In this case, the total scattering and the extinction cross sections determine the single scattering albedo, which agrees with numerical simulations. The three parameters necessary to feed radiative transfer models, namely, extinction, asymmetry parameter, and single scattering albedo, can all be set by the analytical model, with explicit dependence on a few parameters. Results are applicable to radiative transfer problems in climate, paleoclimate, star and planetary formation, and nanoparticle optical characterization for science and industry, including the intercomparison of different optical methods such as those adopted by ISO standards. [ABSTRACT FROM AUTHOR]

This paper expands our previous numerical studies predicting the optical properties of highly ordered mesoporous thin films from two-dimensional (2D) nanostructures with cylindrical pores to three-dimensional (3D) structures with spherical pores. Simple, face centered, and body centered cubic lattices of spherical pores and hexagonal lattice of cylindrical pores were considered along with various pore diameters and porosities. The transmittance and reflectance were numerically computed by solving 3D Maxwell's equations for transverse electric and transverse magnetic polarized waves normally incident on the mesoporous thin films. The effective optical properties of the films were determined by an inverse method. Reflectance of 3D cubic mesoporous thin films was found to be independent of polarization, pore diameter, and film morphology and depended only on film thickness and porosity. By contrast, reflectance of 2D hexagonal mesoporous films with cylindrical pores depended on pore shape and polarization. The unpolarized reflectance of 2D hexagonal mesoporous films with cylindrical pores was identical to that of 3D cubic mesoporous films with the same porosity and thickness. The effective refractive and absorption indices of 3D films show good agreement with predictions by the 3D Maxwell–Garnett and nonsymmetric Bruggeman effective medium approximations, respectively.

Infrared reflectance analysis is facilitated via the comparison of spectra recorded in situ to a databank of actual or synthetic infrared reflectance spectra. It has recently been shown that reference spectra corresponding to the many different morphological forms of the same chemical can be generated synthetically using the imaginary, k, and real, n, components of the complex refractive index, n=n+ik. One method to obtain the n and k vectors is infrared ellipsometry, which measures the changes in amplitude, tan Ψ, and phase, Δ, of polarized light reflected from the sample both as a function of wavenumber and angle of incidence. The method requires specularly reflected light, so best results are usually obtained with polished planar samples of large surface area. Due to the difficulties of obtaining such samples, however, we investigate the possibility of pressing powders of neat materials and obtaining the corresponding optical constants from the pellets. In this paper, variability in the sample pellet and preparation method is investigated, as is variability in the fitting procedure for the derived optical constants. The n/k vectors are derived from the measured ellipsometric parameters, tan ψ and Δ, as they are fit by an oscillator model which yield n(ν) and k(ν) vectors as a function of wavenumber, ν. Construction of the oscillator model is not automatic and depends on significant input from the analyst as well as the sample's physical characteristics. For pellet pressing, the experimental variability was found to be minimized for size-selected powdered samples as gauged by the minimal variance in ψ and Δ for three different pellets; similarly, the analytical precision for multiple measurements of the same pellet was also quite good, suggesting that a pressed pellet is a viable sample preparation method. Experimental variabilities were comparatively small; the greatest variability came in the analytic fitting procedure with differences in the k-peak values up to 10% for only the sharpest bands arising from four different fits to the same data set. The final ellipsometric n/k data are compared to literature values obtained from crystalline ammonium sulfate ((NH4)2SO4) samples as well as single-angle reflectance measurements that also used pressed pellets. Comparison with the previous literature values shows generally good agreement, although larger k-values are observed for the independent sets of data derived from pressed pellets. These data are suggested as an improved set of optical constants for (NH4)2SO4. [ABSTRACT FROM AUTHOR]

The rheology of crystallizing polymers is critical to polymer processing, but our current understanding of how crystallinity affects rheology during crystallization is lacking. The challenge is twofold: first, we must measure rheology simultaneously with crystallinity, and then we must develop models that can describe those measurements as well as prior phenomenological observations. Here, we further develop a generalized effective medium model to describe the frequency-dependent shear modulus of a crystallizing polymer. Through a simple model system, we show that the percolation transition in the effective medium model recovers the relaxation dynamics of a critical gel with a relaxation spectrum that can be approximated as the power mean of the initial melt and final semicrystalline material. We demonstrate the success of this model on the isothermal crystallization of polycaprolactones. From the generalized effective medium model, we can calculate the percolation fraction and power-law relaxation exponent at the critical point, even when the measurement frequency range is dominated by shear thinning of the melt phase. [ABSTRACT FROM AUTHOR]

We investigated a diffusion-like equation with a bounded speed of signal propagation (the so called telegrapher's equation) in a random media. We discuss some properties of the mean-value solution in a well-defined perturbation theory. The frequency-dependent effective-velocity of propagation is studied in the long and short time regime. We show that due to the wave-like character of telegrapher's equation the effective-velocity is a complex dispersive function in time. Exact results and asymptotic perturbative long-time behaviors (for an exponential space-correlated binary disorder) are presented, showing their agreement and corroborating the goodness of the effective medium approximation in continuous system. [ABSTRACT FROM AUTHOR]

Porosity, one of the important quality attributes of pharmaceutical tablets, directly affects the mechanical properties, the mass transport and hence tablet disintegration, dissolution and ultimately the bioavailability of an orally administered drug. The ability to accurately and quickly monitor the porosity of tablets during manufacture or during the manufacturing process will enable a greater assurance of product quality. This tutorial systematically outlines the steps involved in the terahertz-based measurement method that can be used to quantify the porosity of a tablet within seconds in a non-destructive and non-invasive manner. The terahertz-based porosity measurement can be performed using one of the three main methods, which are (i) the zero-porosity approximation (ZPA); (ii) the traditional Bruggeman effective medium approximation (TB-EMA); and (iii) the anisotropic Bruggeman effective medium approximation (AB-EMA). By using a set of batches of flat-faced and biconvex tablets as a case study, the three main methods are compared and contrasted. Overall, frequency-domain signal processing coupled with the AB-EMA method was found to be most suitable approach in terms of accuracy and robustness when predicting the porosity of tablets over a range of complexities and geometries. This tutorial aims to concisely outline all the necessary steps, precautions and unique advantages associated with the terahertz-based porosity measurement method. [ABSTRACT FROM AUTHOR]

Networks that are organized as a hierarchy of modules have been the subject of much research, mainly focusing on algorithms that can extract this community structure from data. The question of why modular hierarchical (MH) organizations are so ubiquitous in nature, however, has received less attention. One hypothesis is that MH topologies may provide an optimal structure for certain dynamical processes. We revisit a MH network model that interpolates, using a single parameter, between two known network topologies: from strong hierarchical modularity to an Erdős–Rényi random connectivity structure. We show that this model displays a similar small-world effect as the Kleinberg model, where the connection probability between nodes decays algebraically with distance. We find that there is an optimal structure, in both models, for which the pair-averaged first passage time (FPT) and mean cover time of a discrete-time random walk are minimal, and provide a heuristic explanation for this effect. Finally, we show that analytic predictions for the pair-averaged FPT based on an effective medium approximation fail to reproduce these minima, which implies that their presence is due to a network structure effect. [ABSTRACT FROM AUTHOR]

Ordered nanoporous anodic alumina (NAA) samples are fabricated by the two-step anodization process. The surface of NAA is modified by means of the spray pyrolysis of titanium dioxide (TiO 2). The morphology and optical properties of the samples are investigated. UV–Vis-NIR reflectance of NAA and NAA coated with TiO 2 has been both measured and simulated. The influence of pore diameter and thickness of both NAA and TiO 2 in reflectance is analyzed. Pore diameters between 30 nm and 80 nm and thicknesses from 3 to 14 μm are evaluated for the NAA. TiO 2 coating thicknesses from 87 nm to 438 nm are determined by the number of sprayed TiO 2 layers. Samples up to 40 nm TiO 2 coating exhibit an increase in the intensity of the oscillations produced by NAA. Whereas, thicker TiO 2 coatings generate a modulated oscillating behavior in the UV–Vis-NIR region. Numerical simulation based on the Transfer Matrix Method has also been carried out to analyze the measured reflectance spectra. The porosity of films has been modeled using an effective medium approximation. It is found the importance of including some absorption in the UV–Vis zone of the NAA layers to obtain accurate predictions of the optical behavior of NAA. Unlabelled Image • Thin titanium dioxide coatings modulate the anodic alumina pore diameter. • Thin titanium dioxide coatings enhance the intensity of the interference spectrum. • Thick titanium dioxide coatings generate reflective hybrid layers with a modulated oscillatory behavior. • UV absorption should be included in the optical models of oxalic acid anodic alumina. [ABSTRACT FROM AUTHOR]

In this work we study the optical properties of one-dimensional photonic crystals in which layers of silica nanoparticles are alternated with layers of indium tin oxide nanoparticle (ITO)/titania nanoparticle mixture, using the transfer matrix method. The dielectric function of the mixed ITO/TiO2 nanoparticle layer is carefully accounted for with a generalized Rayleigh equation for the ternary mixture ITO:TiO2:air. We studied the light transmission of the multilayer photonic crystal as a function of the ITO/TiO2 ratio. We observe that, by increasing the ITO content and its carrier density in the three-phase mixture, the intensity of the plasmon resonance in the near infrared (NIR) increases and the intensity of the photonic band gap (visible) decreases. Thus, our study is of major importance for the realization of electrochromic smart windows, in which separate and independent NIR and visible light control is required. [ABSTRACT FROM AUTHOR]

An all-dielectric polarization-dependent metamaterial consisting of polymer and graphite is presented based on an anisotropic reflectionless TM waveguide designed using impedance-tunable transformation-optics. The simple structure for the metamaterial is given by discretizing it into several pieces of meshed panels in which a series of special cells are configured by effective medium approximations. The simulated results show that the metamaterial exhibits different transmittance for TE and TM polarization at 0° and 45° incident angle from 2.8 to 5.2 GHz. A metamaterial sample is fabricated via a two-step approach involving the 3D printing of polylactic acid panels and a novel graphite tape mask filling technique of graphite powder. The measured transmittance of the sample is −0.5 dB for TM polarization from 2.8 to 5.2 GHz. For TE polarization it is, on average, −4 dB and −3 dB at 0° and 45° incident angle respectively. [ABSTRACT FROM AUTHOR]

Abstract The acoustic performance of a locally resonant phononic crystal comprising steel cylindrical scatterers arranged periodically in a viscoelastic medium is analytically and numerically studied. Analytical expressions for the damping and dipole resonance frequency of the phononic array of solid cylinders embedded in a viscoelastic medium are derived. Based on effective medium approximation, effective geometric and material properties of the composite layer with embedded cylindrical scatterers are also derived. Results from the analytical model are compared with those from a numerical model based on the finite element method. The phononic crystal is immersed in water and the effects of water backing and steel-air backing on its acoustic performance are investigated. Dipole resonance of the cylinders as well as constructive interference between waves scattered by the cylinders and reflected from the steel backing plate are shown to lead to high sound absorption, attributed to conversion of longitudinal to shear waves which are subsequently dissipated in the rubber medium. Different geometric and material parameters are examined to provide further insight into the physical mechanisms associated with high sound absorption of the phononic crystal. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights: • A simple antireflective design over 3.7–4.8 µm MWIR range is reported for Si substrate. • Ge-MgF 2 bilayer prepared by e-beam evaporation gives high performances (T av = 98.56%). • The refractive index of the Ge layer is tuned by Oblique Angle Deposition (OAD). • Key insights about nanostructure, pollution and optical properties are provided. • New routes are proposed to keep on improving the performances. Abstract This work reports the development of a perfect antireflective coating designed to operate over the 3.7–4.8 µm MWIR region, in order to improve the optics used in thermal detection systems. A simulated stack with 99.97% average transmittance is designed for a silicon support using an effective refractive index approach. This structure made of two layers is deposited by electron beam evaporation to achieve an optical transmittance as high as 99.21% and a 98.56% average. These performances are similar to that reported for the best MWIR interferential antireflective systems, but rely on a much simpler design. The first layer is fabricated by oblique angle deposition to obtain Ge nanocolumns that are then coated with a dense MgF 2 capping film. Because environmental pollution can cause light absorption in the infrared region, different techniques are combined to provide key insights about the nanostructure and the related optical properties: ellipsometry, spectrophotometry and scanning-transmission electron microscopy combined with energy-dispersive X-ray spectrometry. With this approach, essential chemical aspects like oxidation, water adsorption in the Ge layer and intermixing at the Ge/MgF 2 interface are evidenced. Advanced models based on effective medium approximation, considering this set of information, are addressed here to gain a better description of the optical response of the surface. In a context of performance optimization of this almost perfect antireflective bilayer, new routes and perspectives to limit or reduce these compositional issues are proposed. [ABSTRACT FROM AUTHOR]