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We demonstrate a new method for realizing modified Luneburg lens antennas with nearly continuously graded permittivity profiles in three‐dimensions. The method used a quasi‐conformal transformation optics (QCTO) approach to modify the geometry and permittivity of a spherical Luneburg lens to have a flat surface for convenient integration of antenna feeds. The modified lens was then fabricated using Fused Deposition Modeling (FDM) printing with an effective media approach that employs space‐filling curves. The method was validated by designing and fabricating a modified Luneburg lens antenna designed to operate in the Ka‐band. The antenna performance of the sample was measured experimentally and shown to compare well to predicted results using full wave simulations. The device was able to achieve a reasonably high degree of beam steering (ie, −55° to +55°) over the entire Ka‐band. We believe this new approach provides a cost‐effective and scalable means of realizing practical passive beam steering lenses that operate over a broad range of frequencies. [ABSTRACT FROM AUTHOR]

Monolithic hierarchical fractal assemblies of silicananoparticles are referred to as aerogels, and despite an impressivecollection of attractive macroscopic properties, fragility has beenthe primary drawback to applications. In that regard, polymer-cross-linkedsilica aerogels have emerged as strong lightweight nanostructuredalternatives rendering new applications unrelated to aerogels before,as in ballistic protection, possible. In polymer-cross-linked aerogelsskeletal nanoparticles are connected covalently with a polymer. However,the exact location of the polymer on the elementary structure of silicaand, therefore, critical issues, such as how much is enough, haveremained ambiguous. To address those issues, the internal nanoporoussurfaces of silica wet-gels were modified with norbornene (NB) bycogelation of tetramethyl orthosilicate (TMOS) with a newly synthesizedderivative of nadic acid (Si-NAD: N-(3-triethoxysilylpropyl)-5-norbornene-2,3-dicarboximide).As inferred by both rheological and liquid 29Si NMR data, Si-NADreacts more slowly than TMOS, yielding a TMOS-derivedskeletal silica network surface-derivatized with NB via monomer-clusteraggregation. Then, ring-opening metathesis polymerization (ROMP) offree NB in the nanopores engages surface-bound NB moieties and bridgesskeletal nanoparticles either through cross-metathesis or a newlydescribed stitching mechanism. After solvent exchange and drying withsupercritical fluid CO2into aerogels (bulk densities inthe range 0.27–0.63 g cm–3, versus 0.20 gcm–3of the native network), the bridging natureof the polymer is inferred by a >10-fold increase in mechanicalstrength and a 4-fold increase in the energy absorption capabilityrelative to the native samples. The cross-linking polymer was freedfrom silica by treatment with HF, and it was found by GPC that itconsists of a long and a short component, with around 400 and 10 monomerunits, respectively. No evidence (by SAXS) was found for the polymercoiling up into particles, consistent with the microscopic similarity(by SEM) of both native and cross-linked samples. Most importantly,the polymer does not need to spill over higher aggregates for greatlyimproved mechanical strength; mechanical properties begin improvingafter the polymer coats primary particles. Extremely robust materialsare obtained when the polymer fills most of the fractal space withinsecondary particles. [ABSTRACT FROM AUTHOR]

Monolithic polyimide aerogels (PI-ISOs) have been prepared by drying wet-gels synthesized viaa rather underutilized room-temperature reaction of pyromellitic dianhydride (PMDA) with 4,4′-methylene diphenyl diisocyanate (MDI). The reaction is followed up to the gelation point by liquid 13C-NMR in DMSO-d6and it proceeds through a seven-member ring intermediate that collapses to the imide by expelling CO2. PI-ISOs are characterized comparatively with aerogels referred to as PI-AMNs, obtained viathe classic reaction of PMDA and 4,4′-methylenedianiline (MDA). The two materials are chemically identical, they show similar degrees of crystallinity (30–45%, by XRD) and they both consist of similarly sized primary particles (6.1–7.5 nm, by SANS). By N2-sorption porosimetry they contain both meso- and macroporosity and they have similar BET surface areas (300–400 m2g−1). Their major difference, however, is that PI-AMNs are particulate while PI-ISOs are fibrous. The different morphology has been attributed to the rigidity of the seven-member ring intermediate of PI-ISOs. PI-AMNs shrink significantly during processing (up to 40% in linear dimensions), but mechanically are much stronger materials than PI-ISOs of the same density. Upon pyrolysis at 800 °C both PI-ISO and PI-AMN are converted to porous carbons; PI-AMNs loose their nanomorphology and more than 2/3 of their surface area, as opposed to PI-ISOs, which retain both. Etching with CO2at 1000 °C increases the BET surface area of both PI-AMN (to 417 m2g−1) and PI-ISO (to 1010 m2g−1), and improves the electrical conductivity of the latter by a factor of 70. [ABSTRACT FROM AUTHOR]

We describe the additive manufacturing results pertaining to a multi-function antenna aperture. The antenna consists of customized high dielectric and low loss feedstocks as the enabling technology. The 3D printed prototype shows agreement with simulation while providing excellent performance. [ABSTRACT FROM AUTHOR]

In this paper we present the development of feedstock materials, specifically tailored permittivity materials used in fused filament fabrication (FFF), for microwave applications. One of the major bottlenecks in utilizing additive manufacturing for useful radiofrequency (RF) applications is the lack of suitable materials. Flexible, high-dielectric constant and low dielectric loss tangent composite materials, consisting of a mixture of polymer and ceramic nanoparticles in controlled volume fractions, are manufactured. Effective medium approximations (EMA) to predict composite properties, such as Bruggeman approximations, are validated with measurements between 18 – 40 GHz and are presented. [ABSTRACT FROM AUTHOR]