Dielectric plug-loaded two-port transmission line measurementtechnique for dielectric property characterization of granular andliquid materials

There are numerous dielectric property characterization techniques available in the microwave regime each with its own uniqueness, advantages and disadvantages. The two-port completely-filled waveguide (transmission line) technique is a robust measurement approach which is well suited for solid dielectric materials. In this case, the dielectric material can be relatively easily machined to fit inside the waveguide and the subsequent measurement of the scattering parameters of this two-port device renders the dielectric properties of the material filling the waveguide. However, this technique is not well suited for measuring the dielectric properties of granular and liquid materials. These materials are used in the production of various composites which are increasingly replacing the use of metals in many environments. If this technique is directly applied to these types of materials, several approximations either in the measurement apparatus or the formulation must be made. To overcome this problem, this paper describes a modification to this measurement technique utilizing two dielectric plugs which are used to house the granular or the liquid dielectric material. In this approach no approximation to the measurement apparatus is made while the presence of the plugs are fully accounted for in the derivations. Using this technique, the dielectric properties of cement powder, corn oil, antifreeze solution and tap water, constituting low- and high-loss dielectric materials (granular and liquid) were measured. In addition, the important issue of measurement uncertainty associated with this technique is also fully addressed. The issue of optimal choice of various measurement parameters is also discussed as it relates to the measurement uncertainty     

Comments on real-space Green's function of an isolated point-chargein an unbounded anisotropic medium

The electric potential of an isolated point-charge in an unbounded anisotropic dielectric can be derived by an approach which merely involves a coordinate transformation and the knowledge of the solution to the corresponding isotropic problem     

Extinction and scattering of a guided beam in a hollow dielectric waveguide.

We present a theoretical approach to the problem of mode scattering by a spherical object that is placed inside a circular dielectric waveguide. This approach is based on the separation-of-variables method for each subsystem, namely, the spherical inclusion and the circular dielectric cylinder, and on the concept of the generalized recursive T-matrix algorithm for multilayered structures. We apply the technique to the backward and the forward scattering of a quasi-optical beam in the form of the fundamental HE11 mode by a sphere inside a circular hollow dielectric waveguide. The results calculated for the perfectly conducting spherical objects inside the circular hollow dielectric waveguide are compared with corresponding measured data of the backward-and the forward-scattering characteristics at the 4-mm wave band.     

New results on electromagnetic imaging based on the inversion of synthetic and measured scattered-field data

A new approach for the inversion of synthetic and measured scattered data is proposed in this paper. The approach is based on an iterative technique in which the nonlinear equations of the inverse-scattering problem are solved within the pth-order Born approximation. A regularization scheme based on an inexact-Newton method is applied. Several numerical simulations and experimental results are reported. Multiple separated dielectric cylinders are localized and reconstructed in a noisy environment.     

Minimax design of optically transparent and reflective coatings

We consider the design problem of creating coatings that are either highly reflective or highly transparent. The goal is to create an optical element, consisting of planar dielectric layers, that reflects (or transmits) energy over a given range of wavelengths and angles of incidence. The approach that we take is to formulate the problem as a minimax optimization problem. We demonstrate that the approach can be effective in producing coatings of a few layers with desirable properties.     

Towards a fracture mechanics for brittle piezoelectric and dielectric materials

A theoretical fracture mechanics for brittle piezoelectric and dielectric materials is developed consistent with standard features of elasticity and dielectricity. The influence of electric field and mechanical loading is considered in this approach and a Griffith style energy balance is used to establish the relevant energy release rates. Results are given for a finite crack in an infinite isotropic dielectric and for steady state cracking in a piezoelectric strip. In the latter problem, the effect of charge separation in the material and discharge in the crack are considered. Observations of crack behavior in piezoelectrics under combined mechanical and electrical load are discussed to assess which features of the theory are useful.     

Short-Range Image-Based Method for the Inspection of Strong Scatterers Using Microwaves

In this paper, a new inverse-scattering-based reconstruction method is presented. The aim of the approach is to "invert" wavefleld samples, which are collected by experimental tomographic systems working at radio frequencies and microwaves. Imaging systems operating in this band require the solution of a nonlinear and highly ill-posed inverse problem. The need to regularize the inverse problem is addressed here by considering an efficient inexact Newton method that is able to inspect strong scatterers. In this paper, the mathematical formulation of the approach is detailed and discussed. Moreover, the results of several numerical simulations concerning the reconstructions of dielectric structures in noisy environments and in several applicative scenarios are reported.     

Electrostatic tractions at crack faces and their influence on the fracture mechanics of piezoelectrics

The influence of electrostatic tractions acting upon crack faces on the fracture mechanical quantities in piezoelectric materials under electromechanical loading is investigated. The physical background are the mechanical and dielectric equilibria at an interface between two dielectric domains and related mechanical stresses. The model is applied to a crack problem, where a dielectric interface exists between the solid material and the insulating crack medium. The analytical solution for a crack in an infinite piezoelectric body accounting for intrinsic charges and electrostatic stresses on the crack faces gives insight into the influence of crack boundary conditions on the field intensity factors. Varying loading conditions and the dielectric permittivity of the flaw yields a parameter range in which induced crack surface tractions are relevant. Then, the calculation of the J-integral for thermodynamically consistent crack boundary conditions is discussed. The line integral along the crack faces is replaced by a simple jump term. This approach comes out to be exact only for a simplified model of the electrostatic tractions.     

A new ellipsometric approach for determining dielectric function of graphene in the infrared spectral region

Simple ellipsometric method for determining dielectric constants of absorbing two-dimensional materials on dielectric substrates is developed. The method is based on the analytical formulas obtained in the framework of a long-wave limit. An important feature of this approach lies in the fact that for data handling the problematic numerical calculation methods are not in use. The inversion problem is resolved analytically. The developed method has no need for the initial guesses of the desired parameters that is very useful from the practical point of view, for example, in the light of in-line control.     

Germanium‐Assisted Direct Growth of Graphene on Arbitrary Dielectric Substrates for Heating Devices

Direct growth of graphene on dielectric substrates is a prerequisite to the development of graphene‐based electronic and optoelectronic devices. However, the current graphene synthesis methods on dielectric substrates always involve a metal contamination problem, and the direct production of graphene patterns still remains unattainable and challenging. Herein, a semiconducting, germanium (Ge)‐assisted, chemical vapor deposition approach is proposed to produce monolayer graphene directly on arbitrary dielectric substrates. By the prepatterning of a catalytic Ge layer, the graphene with desired pattern can be achieved conveniently and readily. Due to the catalysis of Ge, monolayer graphene is able to form on Ge‐covered dielectric substrates including SiO₂/Si, quartz glass, and sapphire substrates. Optimization of the process parameters leads to complete sublimation of the catalytic Ge layer during or immediately after formation of the monolayer graphene, enabling direct deposition of large‐area and continuous graphene on dielectric substrates. The large‐area, highly conductive graphene synthesized on a transparent dielectric substrate using the proposed approach has exhibited a wide range of applications, including in both defogger and thermochromic displays, as already successfully demonstrated here.     

Analysis of inductive multiport microwave devices employing a novel double parallel plate approach

The authors present a novel surface integral equation approach to analyse inductive microwave devices with several parallel input-output ports containing inductive metallic and dielectric obstacles. The technique decomposes the main problem into three subproblems, employing different kinds of Green's functions. One of the subproblems uses the classical Green's functions of an infinite unbounded medium with the constitutive parameters of the dielectric obstacles. A novel point of the technique is the formulation of the two other subproblems with two different, 90deg rotated parallel plate regions. In these regions the parallel plate waveguide Green's functions are used to simplify the modelling of the excitation problem. The second novel aspect of the work is in the treatment of these Green's functions, to smooth their behaviour, and to improve their convergence. Several numerical results are presented to demonstrate the usefulness of the algorithms proposed. Also, several results for inductive microwave devices are presented to show the practical value of the approach.     

Broadband measurement method using the admittance change betweentwo standards: interpretations of measurements and applications

A new approach to the open-circuit method in broadband measurement is described. The special case of a transition from coaxial to circular waveguide, applied in heterogeneous dielectric solid material characterization by time domain spectroscopy (TDS), is discussed. This method is absolute in the sense that it does not require a relative reference in solid material measurements for system calibration. Some applications are given to verify the validation and the limits of the approach to this problem     

Dielectric Absorption in Memory Capacitors

Experience from experiments with a sample and hold circuit showed that dielectric absorption constitutes a fundamental limitation of the accuracy in analog memories. To gain more insight into the problem several materials commonly used as dielectrics in capacitors have been investigated. The theory of dielectric absorption is discussed. Results from measurements on capacitors with paper, cellulose acetate, parylene, polyester, polycarbonate, and polystyrene dielectrics are reported. The components in Dow's model, which describes the dielectric absorption, are calculated for the polycarbonate dielectric. A compensation circuit is suggested, which considerably reduces the effect of dielectric absorption in analog memories and electronic integrators.     

Hybrid finite element-boundary integral algorithm to solve the problem of scattering from a finite array of cavities with multilayer stratified dielectric coating

This work presents a hybrid finite element-boundary integral algorithm to solve the problem of scattering from a finite array of two-dimensional cavities engraved in a perfectly electric conducting screen covered with multilayer stratified dielectric coating. The solution region is divided into interior regions containing the cavities and the region exterior to the cavities. The finite element formulation is applied only inside the interior regions to derive a linear system of equations associated with unknown field values. Using a two-boundary formulation, the surface integral equation employing a closed-form multilayer Green's function in the spatial domain is applied at the opening of the cavities as a boundary constraint to truncate the solution region. The closed-form Green's function in the spatial domain for multilayer planar coating is expressed in terms of complex images using the generalized pencil-of-function method in conjunction with a two-level sampling approach. Placing the truncation boundary at the opening of the cavities and inside the dielectric coating results in a highly efficient solution in terms of computational resources, which makes the algorithm well suited for optimization problems involving scattering from grating surfaces. The near fields are generated for array of cavities with different dimensions and inhomogeneous fillings covered with dielectric layers.     

A neural network approach to microwave imaging

We present a neural network approach to microwave imaging for medical diagnosis. The problem is to reconstruct the complex permittivity of the biological tissues illuminated by the transverse magnetic (TM) incident waves. In order to avoid the inherent ill‐posedness of the inverse scattering problem, we introduce a stochastic process based on Markov random field and a priori knowledge. A coupled gradient neural network is proposed to deal with the mixed‐variable problem because the reconstructed dielectric permittivities are continuous complex variables and the line processes, which can preserve the edges of the reconstructed image, are binary variables. We report the numerical results of a simple human forearm model. We also point out the advantages and the limitations of this method. © 2001 John Wiley & Sons, Inc. Int J Imaging Syst Technol 11, 159–163, 2000     

Electromagnetic wave propagation in a plane dielectric layer under critical conditions

An exact solution to the problem of a plane wave propagation in a plane-stratified dielectric layer of arbitrary thickness is studied. An approach based on self-consistent finite differences is suggested, replacing the layer by a stack of homogeneous sublayers of variable thickness matched to the local behavior of the permittivity profile. For the region around zero permittivity, it is predicted that small perturbations of the physical parameters of the problem cause qualitative and quantitative changes in the solution. The conditions for the excitation of a surface wave are formulated. Computed data for the wave are presented.     

Approximation of the solution of inverse problem of scattering of electromagnetic waves on plane dielectrics

We propose a method for the determination of the material parameters of dielectric coatings according to the measured values of scattered electromagnetic fields which enables us to introduce an efficient procedure of processing of the data of measurements in the methods of nondestructive testing. As a specific feature of the proposed method for the solution of the formulated inverse problem, we can mention the possibility of reconstruction of piecewise continuous profiles of dielectric permittivity for laminated materials. The procedure of reconstruction is based on the method of integral equations. The solution of the problem is obtained approximately. The measured values of the coefficient of reflection of plane electromagnetic waves are extrapolated to the high-frequency region, which enables us to guarantee higher accuracy of reconstruction of the functions of dielectric permittivity for the analyzed structures.     

Uncertainty in multiple penetration depth fringing electric field sensor measurements

Measurement of multilayer material properties with fringing electric field dielectrometry sensors requires the processing of terminal admittance data from multiple electrode pairs. The choice of the algorithmic approach for data analysis is critical because of a high sensitivity to small measurement uncertainties. While individual channel uncertainties are usually within a reasonable instrumentation range, measurement of properties of multiple material layers fails unless special techniques are used to prevent numerical instabilities. This paper demonstrates the mechanism of uncertainty in a successive stair-step property estimation in which the estimates of one layer's properties are used to estimate the subsequent layer's properties. Three-wavelength measurements of dielectric permittivity and conductivity illustrate the problem in two-layer experimental setups. Limitations of the stair-step approach and possible future improvements are discussed.     

Dispersive surface waves in anisotropic linear electromagnetoelasticity

The occurrence of surface waves on a dispersive piezoelectric half-space is considered on the basis of a fully electromagnetic approach. A linear constitutive model is adopted which accounts for both dielectric and magnetic time-dispersion and a Coulomb-type gauge condition is introduced to derive a Stroh eigenvalue problem in the frequency domain. Compatibility conditions are obtained for surface modes in the ranges of quasi-acoustic and quasi-electromagnetic speeds. The propagation properties are derived having recourse to an asymptotic analysis for the transformed constitutive tensors and it is found that at most three surface waves can propagate in each range of speed. Dispersive solutions are also discussed in the quasi-static limit.     

Kerr effect enhancement in ferromagnetic films

The enhancement of the Kerr magneto-optic effect by overcoating a ferromagnetic film with a dielectric film is studied. It is treated as an impedance-matching problem for the light wave. Closed form solutions are obtained for the polar Kerr effect. The same analysis, with modifications, can be applied to the longitudinal Kerr effect. The effect of the dielectric thickness on the enhancement is continuously monitored with a laser beam (λ = 6328Å) during the dielectric film deposition. The amplification of the longitudinal Kerr signal with SiO films on iron film is of the order of five, and the enhancement varies periodically as a function of dielectric thickness. The experimentally observed period, 1600Å, agrees well with the calculated value. The sense ofB-Hloops, which also varies periodically as a function of the dielectric thickness, is explainable in terms of the phase factor which appears in the difference between the reflection coefficients.