Showing papers by "Francisco Medina published in 2016"

Recyclability Study on Inconel 718 and Ti-6Al-4V Powders for Use in Electron Beam Melting

Abstract: Powder bed-based additive manufacturing technologies offer a big advantage in terms of reusability of the powders over multiple cycles that result in cost savings. However, currently there are no standards to determine the factors that govern the powder reuse times. This work presents the results from a recyclability study conducted on Inconel 718 and Ti-6Al-4V powders. It has been found that the Inconel 718 powders are chemically stable over a large number of cycles and their reuse time is limited by physical characteristics of powders such as flowability. Ti-6Al-4V, on the other hand, finds its reuse time governed by the oxygen pick up that occurs during and in between build cycles. The detailed results have been presented.

Microstructure Development in Electron Beam-Melted Inconel 718 and Associated Tensile Properties

Abstract: During the electron beam melting (EBM) process, builds occur at temperatures in excess of 800°C for nickel-base superalloys such as Inconel 718. When coupled with the temporal differences between the start and end of a build, a top-to-bottom microstructure gradient forms. Characterized in this study is a microstructure gradient and associated tensile property gradient common to all EBM Inconel 718 builds, the extent of which is dependent on build geometry and the specifics of a build’s processing history. From the characteristic microstructure elements observed in EBM Inconel 718 material, the microstructure gradient can be classified into three distinct regions. Region 1 (top of a build) is comprised of a cored dendritic structure that includes carbides and Laves phase within the interdendritic regions. Region 2 is an intermediate transition zone characterized by a diffuse dendritic structure, dissolution of the Laves phase, and precipitation of $$\delta $$ needle networks within the interdendritic regions. The bulk structure (Region 3) is comprised of a columnar grain structure lacking dendritic characteristics with $$\delta $$ networks having precipitated within the grain interiors. Mechanically, at both 20°C and 650°C, the yield strength, ultimate tensile strength, and elongation at failure exhibit the general trend of increasing with increasing build height.

Simplified Modal Expansion to Analyze Frequency-Selective Surfaces: An Equivalent Circuit Approach

Abstract: This communication extends a previously reported analytical circuit that models the plane wave scattering by 2-D arrays of planar patches/apertures embedded in a layered media. In this extension, it is assumed that the current/electric field spatial profile in the patch/aperture region can directly be taken from the eigenfields of a waveguide whose cross section matches the scatterer boundaries. This methodology exploits all the advantages of the circuit modeling and gives place to a very efficient computational tool for many practical frequency-selective surfaces (FSSs). The circuit model is expected to work up to frequencies where the isolated scatterers have their third resonance. Thus, it allows us to provide unified equivalent circuits to deal with conical incidence and with certain symmetric cases involving two or three patches/apertures in the unit cell.

Accurate Circuit Modeling of Fishnet Structures for Negative-Index-Medium Applications

Abstract: Metallic plates with a two-dimensional (2D) periodic distribution of sub-wavelength apertures are known to exhibit extraordinary transmission of electromagnetic waves. Stacking two or more of such plates gives place to the so-called fishnet structures, which constitute a popular way of achieving an effective negative index medium at frequencies ranging from microwaves to optics. Unfortunately, a general wideband equivalent circuit has not yet been proposed to facilitate its understanding and design. This work presents this circuit model with closed-form expressions for the circuit elements, thus making it possible to obtain the electrical response for this class of structures in a very efficient way. This procedure is much faster than alternative numerical methods at the same time that it retains a high level of accuracy when compared with some other oversimplified models. The circuit model also provides a simple rationale as well as a good physical insight in order to explain the qualitative behavior of such structures, independently of the number of stacked layers.

Computationally efficient analysis of extraordinary optical transmission through infinite and truncated subwavelength hole arrays.

Abstract: The authors present a computationally efficient technique for the analysis of extraordinary transmission through both infinite and truncated periodic arrays of slots in perfect conductor screens of negligible thickness. An integral equation is obtained for the tangential electric field in the slots both in the infinite case and in the truncated case. The unknown functions are expressed as linear combinations of known basis functions, and the unknown weight coefficients are determined by means of Galerkin's method. The coefficients of Galerkin's matrix are obtained in the spatial domain in terms of double finite integrals containing the Green's functions (which, in the infinite case, is efficiently computed by means of Ewald's method) times cross-correlations between both the basis functions and their divergences. The computation in the spatial domain is an efficient alternative to the direct computation in the spectral domain since this latter approach involves the determination of either slowly convergent double infinite summations (infinite case) or slowly convergent double infinite integrals (truncated case). The results obtained are validated by means of commercial software, and it is found that the integral equation technique presented in this paper is at least two orders of magnitude faster than commercial software for a similar accuracy. It is also shown that the phenomena related to periodicity such as extraordinary transmission and Wood's anomaly start to appear in the truncated case for arrays with more than 100 (10×10) slots.

Dynamical Equivalent Circuit for 1-D Periodic Compound Gratings

Abstract: Metallic compound gratings are studied in this work by means of an analytical equivalent circuit approach in order to obtain its transmission and reflection properties when illuminated by a TM-polarized plane wave. A compound grating consists of the periodic repetition of a finite number of slits carved out of a thick metal slab (reflection grating) or connecting two separated open regions through groups of slits in the metal slab (transmission grating). The equivalent circuit is rigorously obtained starting from a simplified version of the integral equation for the electric field at the slits apertures. That equivalent circuit involves transmission-line sections that account for the fundamental and lowest order diffracted modes (which does give the “dynamical” nature to the present equivalent circuit), and lumped components to model the effect of all the higher order diffracted modes. All the relevant and complex features of the spectra can be satisfactorily explained in terms of the topology and characteristics of the equivalent circuit. In contrast with some previously reported circuit models, all the dynamical and quasi-static circuit elements are analytically and explicitly obtained in terms of the geometric and electrical parameters of the grating. The accuracy of the approximate circuit model is very good over a very wide band, as it is demonstrated by comparison with full-wave data computed with commercial electromagnetic solvers.

Wideband analytical equivalent circuit for one-dimensional periodic stacked arrays.

Abstract: A wideband equivalent circuit is proposed for the accurate analysis of scattering from a set of stacked slit gratings illuminated by a plane wave with transverse magnetic or electric polarization that impinges normally or obliquely along one of the principal planes of the structure. The slit gratings are printed on dielectric slabs of arbitrary thickness, including the case of closely spaced gratings that interact by higher-order modes. A Π-circuit topology is obtained for a pair of coupled arrays, with fully analytical expressions for all the circuit elements. This equivalent Π circuit is employed as the basis to derive the equivalent circuit of finite stacks with any given number of gratings. Analytical expressions for the Brillouin diagram and the Bloch impedance are also obtained for infinite periodic stacks.

Balanced bandpass filter based on magnetically coupled coplanar waveguide folded-stepped impedance resonators

Abstract: A novel balanced differential bandpass filter implemented in coplanar waveguide (CPW) technology is presented. The filter is based on the use of two magnetically coupled CPW folded-stepped impedance resonators. The use of magnetic coupling, instead of the more common electric coupling, provides inherent common-mode rejection. Measured and simulated results demonstrate the benefits of the proposed balanced filter.

Making metals transparent: A circuit model approach

Abstract: Solid metal films are well known to be opaque to electromagnetic waves over a wide frequency range, from low frequency to optics. High values of the conductivity at relatively low frequencies or negative values of the permittivity at the optical regime provide the macroscopic explanation for such opacity. In the microwave range, even extremely thin metal layers (much smaller than the skin depth at the operation frequency) reflect most of the impinging electromagnetic energy, thus precluding significant transmission. However, a drastic resonant narrow-band enhancement of the transparency has recently been reported. The quasi-transparent window is opened by placing the metal film between two symmetrically arranged and closely spaced copper strip gratings. This letter proposes an analytical circuit model that yields a simple explanation to this unexpected phenomenon. The proposed approach avoids the use of lengthy numerical calculations and suggests how the transmissivity can be controlled and enhanced by manipulating the values of the electrical parameters of the associated circuit model.

Pseudo-analytical circuits for dual-polarized FSS

Abstract: A circuit model approach is here presented to characterize bi-dimensional arrangements of dual-polarized elements. This contribution is an extension of the physical insightful approach previously proposed by some of the authors. The main novelty here resides in the fact that the scatterer may support two different current profiles. This feature becomes crucial for the accurate characterization of multiple current practical scenarios, such as the analysis and design of dual-polarized frequency selective surfaces. In particular, we illustrate its applicability when, due to oblique incidence, both an even and an odd current profiles are excited in a crossed-slot element.

Design of a dual-band metamaterial absorber in WLAN bands with high stability over incidence angle and polarization

Abstract: A dual-band metamaterial absorber operating at two wireless local area network bands, 2.45 GHz and 5.61 GHz, is designed based on a systematic methodology that involves equivalent circuit model calculations and full-wave electromagnetic simulations. The unit cell of the absorber consists of two kinds of Jerusalem cross resonators, one of which is rotated 45 degrees with respect to the other, on a metal backed lossy dielectric. Interdigitated capacitors are introduced as the capacitive components of one of the Jerusalem cross resonators to reduce the absorption resonance frequency and electrical size. The proposed dual-band metamaterial absorber shows absorption rates of 99.83 % and 98.98% for the lower and upper absorption bands, respectively. Furthermore, it presents a robust performance under oblique incidence angles for both transverse-electric and transverse-magnetic polarizations.

Analytical modeling of non-symmetric and non-uniform compound gratings

Abstract: Analytical models based on suitable equivalent circuits are proposed to analyze transmission compound gratings made of non-symmetric and/or non-uniform slits. These structures exhibit transmission dips related with the so-called phase resonances, which are known to appear, in the case of highly symmetrical structures and under normal incidence, when the number of air-filled slits per period is three o more. However, if non-uniform identical slits are properly grouped, simpler structures may exhibit such resonances. This work reports on the analytical modeling of this kind of structures, which have been studied before making use of experimental or numerical approaches. The circuit model provides a relatively simple explanation to the observed transmission spectra and drastically reduces the computation time.

Excitation of Discrete and Continuous Spectrum in Subdiffraction Wire-Medium Type Lenses

Abstract: Subwavelength imaging of the near field of a magnetic line-source excitation is studied for several wire-medium (WM) lens topologies using complex-plane analysis of the radiation integral. Nonlocal homogenization is used for the wire medium, resulting in an analytical expression for the transfer function of the lens. It is shown that by evaluating the Sommerfeld integral of the transmitted field in terms of the discrete and continuous spectra provides a general framework for better understanding of electromagnetic phenomena involved with subwavelength imaging. Results are obtained for a WM slab, and for a wire medium loaded with graphene monolayers and periodic arrays of graphene patches, demonstrating the interplay of the discrete and continuous spectral components in different operating regimes of the lenses. The imaging with a stack of silver slabs is also considered for comparison purposes.

Discrete and continuous spectrum analysis: An alternative perspective on subwavelength imaging

Abstract: In this paper, we establish a general framework for understanding subwavelength imaging by means of complex-plane analysis of the Sommerfeld integral for a magnetic line source over a subwavelength imaging lens. The Sommerfeld integral of the transmitted field is calculated in terms of discrete and continuous spectra. This spectral decomposition makes it possible to achieve physical insight regarding the imaging mechanisms. The results are obtained for a stack of silver slabs, an isolated wire-medium (WM) slab, and for a WM loaded with graphene sheets (GSs) and graphene patches (GPs).

Accurate circuit models for the analysis of stacked metal gratings

Abstract: Stacked periodic planar structures have been a continuous subject of research interest due to their ability to control the polarization, transmission, reflection, and absorption of electromagnetic waves. The scattering of plane waves by one-dimensional (1-D) and two-dimensional (2-D) arrays of metal patches (or apertures) can be accurately modeled using suitable equivalent circuits. The structures obtained by stacking periodic systems of this type can easily be modeled using transmission line sections to account for the dielectric region between the periodic surfaces. However, this task is not straightforward if the separation between the periodically structure surfaces is electrically small. This contribution will describe a methodology to obtain valid and accurate circuit models for tightly spaced periodic arrays of apertures. The proposed theory allows for the modeling of devices that have attracted a lot of attention in recent years, such as the fishnet structures employed to obtain an effective negative index of refraction.

Equivalent admittance approach for the scattering of patch/slot-based frequency selective surfaces

Abstract: The equivalent admittance of a 2-D array of printed metallic patches/apertures is obtained here by extending a precedent equivalent circuit approach reported by some of the authors. The goal of the present extension is to cover more complex situations like multi-resonant scatterers, several scatterers per unit cell, and conical incidence. After studying the limits of validity of the equivalent circuit approach, it is found that this quasi closed-form approach is a very efficient numerical tool which also provides a very convenient physical insight into the involved electromagnetic phenomena.

Discrete and continuous spectrum in subwavelength imaging with wire-medium type lenses

Abstract: In this paper, subwavelength imaging of the near field with the magnetic line-source excitation is studied for several wire-medium topologies by means of the complex-plane analysis of the radiation integral. The Sommerfeld integral of the transmitted field is calculated in terms of the discrete and continuous spectra which describes a general framework for understanding the subwavelength imaging. Results are presented for an isolated wire-medium (WM) slab, and for a wire medium loaded with graphene sheets (GSs) and graphene patches (GPs). This study demonstrates the effects of the discrete and continuous spectral components in different operating regimes of the lenses.

Analytical and highly efficient numerical modeling of electromagnetic periodic structures

Abstract: Periodic printed metallodielectric structures have been used for a long time to control the transmission, absorption and polarization of electromagnetic waves from the RF / microwave domains up to the optical range. During the last two decades, the modeling of these structures has become a popular research topic due several incentives. Such incentives are both of technical (frequency selective surfaces, reflectarrays, planar lenses, etc.) and scientific (extraordinary transmission, exotic diffraction effects, etc.) nature. In this contribution we present some recent advances carried out by our research group along two directions: (i) approximate analytical circuit-like models, and (ii) highly efficient computational methods. The goal is to develop fast and accurate tools to be used in conjunction with optimizers to design antennas, filters and other devices. These techniques also provide simple models to explain the exotic behavior of some periodic structures.