John W. Schultz

Impedance response and modeling of composites containing aligned semiconductor whiskers: Effects of dc-bias partitioning and percolated-cluster length, topology, and filler interfaces

Impedance spectroscopy and modeling were used to investigate the partitioning of 0-40 V dc bias in composites containing alumina and different volume fractions of silicon carbide whiskers (SiCw), which formed low-connectivity percolated clusters. Differences in response between long (∼25 cm) composite rods and thin (∼1.7 mm) slices thereof were interpreted in terms of the relative contributions to the impedance from the electrodes and SiCw-percolated clusters of the composite samples. Bias had minimal effect on the impedance of rods, because its distribution across the long percolated clusters within translated to low electric fields at the SiCw-SiCw interfaces. The impedance of thin slices was more sensitive to bias and was mainly due to such interfaces. The associated dc resistance and effective capacitance decreased significantly with increasing dc bias. A model for symmetrical Schottky energy barriers at interfaces fit the capacitance trend and outputted a parameter Φi/κ¯i. Different models for the no...

Novel liquid crystal resins for stereolithography – processing parameters and mechanical analysis

The build characteristics of two liquid crystal (LC) reactive monomers were studied using a table‐top stereolithography apparatus (TTSLA). LC materials contain stiff, rod‐like mesogenic segments in their molecules, which can be aligned causing an anisotropy in properties. When cured in the aligned state the anisotropic structure is “locked in” resulting in materials with anisotropic physical and mechanical properties. By varying the alignment of layers, properties such as thermal expansion coefficient can be optimized. High heat distortion (or glass transition) temperatures are possible depending on the monomer chemical structure. Working curves for the LC resins were developed under various conditions. A permanent magnet placed outside the TTSLA vat was used to uniformly align the monomer in the nematic state. Photo‐initiator type and content; alignment of the nematic phase; and operating conditions affected the working curve parameters. Glass transition temperatures of post‐cured parts ranged from 75 to 1488C depending on the resin and processing conditions. Mechanical analysis data revealed a factor of two difference between glassy moduli measured in the molecular alignment versus the transverse alignment directions. Based on these initial studies, more advanced resins with higher glass transitions are being developed at the University of Dayton.

Thermal-expansion and fracture toughness properties of parts made from liquid crystal stereolithography resins

Abstract Stereolithography is a layered manufacturing technique that uses a laser to selectively polymerize a thermosetting resin to form complex three-dimensional parts. This rapid prototyping technique has evolved over the years from a model maker to a rapid fabrication technique for soft-tooling and parts for in-service testing. With this evolution in applications has come increased demand for high-performance resins. Liquid crystal (LC) resins can be processed by stereolithography methods to provide parts with superior and in some cases unique properties compared with conventional resins. LC resins consist of rod-like molecules that can be aligned prior to cure resulting in anisotropic mechanical and physical properties. A magnet outside the resin vat can be used to control the molecular alignment. Alignment can be varied within a layer or from layer-to-layer to achieve desired properties much as is done with fiber reinforced plastics. Two properties that are important for many applications are toughness and thermal dimensional-stability. Initial studies showed that fracture toughness values of LC coupons were about twice that of isotropic coupons manufactured from the same resin. Also, we have demonstrated that the in-plane thermal expansion of LC parts can be minimized over a wide temperature range by rotating the molecular alignment 90° from one layer to the next.

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.

Near-field probe measurements of microwave scattering from discontinuities in planar surfaces

In microwave scattering, nonradiating fields may contribute to radiating fields by local perturbations such as geometric discontinuities or variations in impedance or electromagnetic properties. Near-field measurements of scattering bodies provide insight into these scattering mechanisms by measuring both radiated and nonradiated fields. In this research, an H-field probe measured scattering from simple discontinuities in planar bodies at frequencies between 2 and 10 GHz. Illumination of the test-body was furnished by a focused lens system with a Gaussian-like tapered beam that locally illuminated inhomogeneities on the body. Measured data and model calculations are presented for scattered H-fields near canonical discontinuities (e.g. gaps and edges in conducting planes). Calculations of the plane wave spectrum of the measured and modeled data were used to distinguish specular reflected components from surface modes. A focused beam was simulated in a finite-difference time-domain (FDTD) model with a weighted sum of plane waves. FDTD results agreed with the measured near-field data.