Leo C. Kempel

A REVIEW OF MICROWAVE-ASSISTED POLYMER CHEMISTRY (MAPC)

As a relatively new source ofprocessing energy, microwave energy offers many compelling advantages in materials processing over conventional heat sources. These advantages include greater flexibility greater speed and energy savings, improved product quality and properties, and synthesis of new materials that cannot be produced by other heating methods. Studies of microwave processing of polymeric materials in the early 1960s led to a successful industrial application in the rubber industry Since the mid-1980’s, there has been a great deal of interest in microwave processing of polymeric materials worldwide. The discipline can be categorized in two major fields: microwave-assisted polymer physics (MAPP) and microwave assisted polymer chemistry (MARC). This paper offers an overview of lie fate-of-the-art research on the field of 1, 1:1PC, including polymer processing (curing ot thermosets, processing of thermoplastics, and joining), polymer synthesis, plasma modification of polymer surfaces, plasma polymerization, polymer degradation, and production of nanomaterials. Most of these studies have focused on laboratory-scale, exploratory efforts. Challenges and possible future directions for the commercialization ofmicrowave processing technologies are discussed.

Controlled synthesis of core?shell iron?silica nanoparticles and their magneto-dielectric properties in polymer composites

Low loss core-shell iron-silica nanocomposites with improved magneto-dielectric properties at radio frequencies (1 MHz-1 GHz) were successfully fabricated. A new simple method was developed to synthesize metallic iron (Fe) nanoparticles with uniform size distribution in an aqueous environment at room temperature. Citric acid and oleic acid served as surface-capping agents to control the particle size of the synthesized Fe nanoparticles. Smaller Fe nanoparticles with narrower particle size distribution were obtained as the concentration ratio of iron ions to carboxylic acid groups decreased. The Fe nanoparticles were subsequently coated with silica (SiO(2)) layers to prevent the iron cores oxidizing. Polymer composites were prepared by incorporating Fe@SiO(2) nanoparticles with polydimethylsiloxane (PDMS) elastomers. Experimental results showed that the dielectric permittivity (ε) and magnetic permeability (μ) of the polymer composite increased with increasing amount of Fe@SiO(2) nanoparticle doping. The dielectric loss (tanδ) was near 0.020 at a frequency of 1 GHz.

An ultrawideband (UWB) switched-antenna-array radar imaging system

A low-cost ultrawideband (UWB), 1.926–4.069 GHz, phased array radar system is developed that requires only one exciter and digital receiver that is time-division-multiplexed (TDM) across 8 receive elements and 13 transmit elements, synthesizing a fully populated 2.24 m long (λ/2 element-to-element spacing) linear phased array. A 2.24 m linear phased array with a 3 GHz center frequency would require 44 antenna elements but this system requires only 21 elements and time to acquire bi-static pulses across a subset of element combinations. This radar system beamforms in the near field, where the target scene of interest is located 3–70 m down range. It utilizes digital beamforming, computed using the range migration synthetic aperture radar (SAR) algorithm. The phased array antenna is fed by transmit and receive fan-out switch matrices that are connected to a UWB LFM pulse compressed radar operating in stretch mode. The peak transmit power is 1 mW and the transmitted LFM pulses are long in time duration (2.5–10 ms), requiring the radar to transmit and receive simultaneously. It will be shown through simulation and measurement that the bi-static antenna pairs are nearly equivalent to 44 elements spaced λ/2 across a linear array. This result is due to the fact that the phase center position errors relative to a uniform λ/2 element spacing are negligible. This radar is capable of imaging free-space target scenes made up of objects as small as 15.24 cm tall rods and 3.2 cm tall metal nails at a 0.5 Hz rate. Applications for this radar system include short-range near-real-time imaging of unknown targets through a lossy dielectric slab and radar cross section (RCS) measurements.

Low-loss, high-permittivity composites made from graphene nanoribbons.

A new composite material was prepared by incorporation of graphene nanoribbons into a dielectric host matrix. The composite possesses remarkably low loss at reasonably high permittivity values. By varying the content of the conductive filler, one can tune the loss and permittivity to desirable values over a wide range. The obtained data exemplifies how nanoscopic changes in the structure of conductive filler can affect macroscopic properties of composite material.

Method of moments solution for a printed patch/slot antenna on a thin finite dielectric substrate using the volume Integral equation

In this paper, a volume integral equation (VIE)-based modeling method suitable for a patch or slot antenna on a thin finite dielectric substrate is developed and tested. Two new key features of the method are the use of proper dielectric basis functions and proper VIE conditioning, close to the metal surface, where the surface boundary condition of the zero tangential E -component must be extended into adjacent tetrahedra. The extended boundary condition is the exact result for the piecewise-constant dielectric basis functions. The latter operation allows one to achieve a good accuracy with one layer of tetrahedra for a thin dielectric substrate and thereby greatly reduces computational cost. The use of low-order basis functions also implies the use of low-order integration schemes and faster filling of the impedance matrix. For some common patch/slot antennas, the VIE-based modeling approach is found to give an error of about 1% or less in the resonant frequency for one-layer tetrahedral meshes with a relatively small number of unknowns. This error is obtained by comparison with fine finite-element method (FEM) simulations, or with measurements, or with the analytical mode matching approach. Hence it is competitive with both the method of moments surface integral equation approach and with the FEM approach for the printed antennas on thin dielectric substrates.

E-pulse diagnostics of simple layered materials

The feasibility of using the E-pulse technique to diagnose changes in the properties of simple layered materials is undertaken. Numerical results for a conductor-backed lossy slab and a Salisbury screen show that changes in permittivity, conductivity and thickness can be diagnosed in the presence of white Gaussian noise.

COMPARISON OF UWB SHORT-PULSE AND STEPPED-FREQUENCY RADAR SYSTEMS FOR IMAGING THROUGH BARRIERS

A canonical problem is used to investigate the efiects of various radar parameters on the performance of both stepped- frequency and short-pulse through-barrier radar imaging systems. For simplicity, a two-dimensional problem is considered, consisting of a perfectly conducting strip located behind a lossy dielectric slab of inflnite extent illuminated by line sources. To assess the impact of the parameters on system performance, radar received images of the target are created using the re∞ected fleld computed at several positions in front of the barrier and adjacent to the sources. Speciflc radar parameters considered include sample rate, A/D bit length, pulse width, and target SNR for a time-domain system. For a stepped- frequency system, A/D bit length, bandwidth, and target SNR are considered.

Scattering by cavity-backed antennas on a circular cylinder

Conformal arrays are popular antennas for aircraft and missile platforms due to their inherent low weight and drag properties. However, to date there has been a dearth of rigorous analytical or numerical solutions to aid the designer. In fact, it has been common practice to use limited measurements and planar approximations in designing such non-planar antennas. The finite element-boundary integral method is extended to scattering and radiation by cavity-backed structures in an infinite, metallic cylinder. In particular, the formulation specifics such as weight functions, dyadic Greens function, implementation details, and particular difficulties inherent to cylindrical structures are discussed. Special care is taken to ensure that the resulting computer program has low memory demand and minimal computational requirements. Both scattering and radiation parameters are computed and validated as much as possible.

Electromagnetic Material Characterization using a Partially-Filled Rectangular Waveguide

Electromagnetic material characterization measurements are an important aspect of modern design. Rectangular waveguides are often used in the material characterization process to facilitate measurement since they are readily available and because rectangular samples can be easily machined and placed in the cross-sectional plane of the waveguide for reflection and transmission testing and subsequently analyzed using a simple theory [1, 2]. However, this technique can produce unacceptably large errors for high loss or highly reflecting samples due to a poor signal-to-noise ratio (SNR) for the transmission measurement.In this paper, a partially-filled waveguide method is presented that enhances transmission quality and accuracy in electromagnetic material characterization measurements. The partially-filled waveguide geometry improves transmission but leads to the excitation of higher-order modes. A mode-matching technique is developed here to accommodate the resulting waveguide discontinuity and a Newton root search method is utilized to subsequently extract the electromagnetic properties of the test sample. Experimental measurements for several materials will be presented at X-band frequencies (8–12 GHz) to verify the theoretical analysis.

Permittivity of dielectric composite materials comprising graphene nanoribbons. The effect of nanostructure.

New lightweight, flexible dielectric composite materials were fabricated by the incorporation of several new carbon nanostructures into a dielectric host matrix. Both the permittivity and loss tangent values of the resulting composites were widely altered by varying the type and content of the conductive filler. The dielectric constant was tuned from moderate to very high values, while the corresponding loss tangent changed from ultralow to extremely high. The data exemplify that nanoscale changes in the structure of the conductive filler result in dramatic changes in the dielectric properties of composites. A microcapacitor model most explains the behavior of the dielectric composites.