Rick L. Moore

Effective Medium Calculations of the Electromagnetic Behavior of Single Walled Carbon Nanotube Composites

Dielectric properties of single walled carbon nanotube assemblies were calculated with an effective medium approximation at frequencies from 200 MHz to 200 GHz. The model treats the carbon nanotubes as layered cylinders, each with a core, a graphene layer and an outer layer, to investigate the dielectric properties of coated and filled nanotubes. The graphene and metal layer properties were modeled with a Drude approximation based on literature data. A generalized Bruggeman model was then used to determine the macroscopic behavior of the modified carbon nanotubes in a composite structure as a function of volume fraction, frequency, and aspect ratio. The depolarization factors in this model were scaled by the normalized effective permittivity to better account for percolation behavior. The model showed a wide variety of frequency dependent dielectric properties. Uncoated tubes were calculated to form highly conductive materials at volume fractions of just a few percent and metal-coated tubes enhanced the conductivity by an order of magnitude. Calculations of nanotubes with insulating coatings showed that high dielectric constants with moderate to low dielectric loss were possible.

High power ultra wideband radar exotic material response

This paper is the first of two that present analytical and numerical analysis of high power and ultra wide band radars interacting with EMI and radar suppression materials. Magnetic materials are often used in isolators, thin film EMI materials and/or appliques. Such magnetic composites may be exposed to wide band and/or high power signals in operational radar systems. We describe exotic interactions where material responses derive from wideband and/or high power non linearity. The responses are similar to recent publications that find changes in the magnetization spectra (proportional to material permeability) that may occur when a device and/or material is exposed to high power pulsed or pulsed-CW RF (radio frequency) sources. Numerical simulations are used to study response of micro and nano scale magnetic particles within composites. Synthesis and testing of devices and materials containing nanoscale (10s of nanometer size) magnetic particulates is a major thrust in device improvements. Nonlinear response is studied as a function of material intrinsic magnetic properties, radar ERP, power spectrum of the incident RF field, particle geometry (shape and size) of the magnetic material and uniformity in shape and size.

Comparison of Techniques for Microwave Characterization of Percolating Dielectric -Metallic Media and Resolution of Discrepancies in Measured Data

The measured microwave effective dielectric properties of metal-dielectric composites show discrepancies when data from free space, resonant cavity or transmission line measurements are compared. Discrepancies are especially evident for materials where the metallic concentration is near the percolation threshold. This paper presents theory and measured data which highlight and resolve these discrepancies. Electrical correlation length is the relevant parameter which must be considered in choice of measurement technique. Agreement between effective medium models and measurement are best when focussed beam measurement techniques are utilized so as to produce planar wave-fronts whose extent is 3–4 freespace wavelengths when incident on the sample.

Millimeter wave dielectric measurements at elevated temperatures

Interest in the constitutive parameters of materials at elevated temperatures continues to increase as new materials are developed for high-temperature applications. A simple, relatively nondestructive method for testing materials at millimeter wave frequencies is presented here. Measurements have been performed between 30 and 100 GHz at temperatures up to 1500 degrees C in a commercially available furnace. Measurements to date have been performed on dielectric materials with dielectric constants ranging from 1.2 to 10. The measured dielectric constant and loss tangent of a low-loss foam material are shown.<<ETX>>

Permittivity of fiber polymer composites environmental effects: comparison of measurement and theory

The authors present the results of a study to determine the relative changes in fiber/organic resin composite permittivity after exposure to moisture, SO/sub 2/ and ultraviolet (UV) light. Effective medium theories are applied in an attempt to predict the permittivities of the composites given the permittivity of the constituent fiber and polymer. It is found that environmental changes due to moisture have severe effects on polyester composite permitivities. Changes near 400% in loss tangent and 20% in dielectric constant are observed. Effective medium theories are found useful for predicting permitivity in composite panels with fiber reinforcement. Moisture effects on the permittivity of these composites may also be predicted. The observed changes due to other environmental exposures are dominated by moisture pickup. Thus, no direct effects of UV and SO/sub 2/ can be determined from these measurements.<<ETX>>

Moving target emulators for ultra wide band signals: Electrically modulated fragmented surfaces

MTI radar systems and signal processing algorithms may be made more efficient if a wide range of perfect and background distorted Doppler signatures could be made available to test system hardware and algorithms. This paper addresses emulators of moving vehicle Doppler signatures. Measured Doppler spectra of tracked and wheeled vehicles are used to guide the development of moving target emulators made from ldquoswitchablerdquo reflectivity panels\. The geometry of the panels are electrically percolating surfaces The percolating patterns provide 10:1 bandwidths and thus satisfy UWB criteria for frequency response and they use minimal modulating elements. A potential problem is the number of modulating elements on a panel that are required to synthesize the complex Doppler waveform measured on a ground vehicle.

Ultra Wide Band -High Power Radar-Material Responses II (Waveform and Material Density)

This paper presents numerical calculations for high power ultra wide band radars interacting with EMI and radar suppression magnetic materials and components, e.g. isolators, EMI, coatings and/or various appliqus. Exposure may occur within the radar system or on a radiated target. New dynamic micromagnetic simulators are applied to predict non-linear permeability time and frequency response of micro and nano scale magnetic particles which may be used in composites. The paper predicts non-linear material response as a function of incident pulse shape, frequency spectra, various incident fields strengths for hard and soft magnetic material; calculates magnetic particle permeability and then applies effective media theory to predict pulse reflection. Results suggest exotic interactions may significantly alter the input impedance of devices and surfaces containing magnetic materials; changing reflection and absorption.

Measured and FDTD Calculated Ultra Wide Band (UWB) RCS for Treated Test Fixtures

This paper presents comparisons of measured, and finite difference time domain (FDTD) calculations of ultra wide Band scattering for dimensionally scaled, shaped and treated fixtures and measurements on a ground plane fed by vertically polarized TEM horn antenna operating from 50 MHz to 18.05 GHz. method of moment (MoM) calculations are used to cross-check measurement and FDTD. UWB exotic material effects should be more easily observed at low scattering levels. Our findings indicate that for multiple target characteristics and over UWB bandwidths, FDTD code furnish a sufficient level of predictive accuracy (i.e. -30 to -40 dBsm) such that it might be applied in RCS predictions of treated targets where exotic RF - material interactions resulting from high power UWB waveforms might be used. RCS predictions could then be applied to improve signal processing algorithms. The paper discussed measurement technique, choice of fixtures, FDTD model, the measurement-model level of agreement and outlines a path to verification.

Calculation of electromagnetic constitutive parameters of insulating magnetic materials with conducting inclusions

A Method of Moments (MoM) electromagnetic model of percolating conducting films was applied to calculate the effective parameters of the composite formed by conducting inclusions placed within a dispersive magnetic but nondispersive dielectric matrix. The MoM calculations demonstrate a coupling between the magnetic properties of the matrix and the effective composite permittivity and frequency dispersion of the composite. The coupling of permittivity and permeability is observed near the percolation threshold of the composite and for high conductivity inclusions. The prediction agrees with physical expectations since near percolation the conduction correlation length dominates the effective permittivity of the composite and this correlation length is determined by both the permittivity and permeability of the composite.

Luneberg lens design: optimization using effective medium theories

A procedure for minimizing the weight of a Luneberg lens composed of a composite made from a low dielectric foam and a spherical pigment is presented. The spherical pigment is distributed radially to obtain the Luneberg distribution of the permittivity. The lens considered here is greater than 2 ft in diameter, and therefore minimization of weight is a priority. Effective medium theories are used with measured permittivities and densities of pigments and foams to derive a function which minimizes weight while maximizing the Luneberg electromagnetic design. A Maxwell-Garnet (1904) mixture theory is used to obtain closed-form solutions.<<ETX>>