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Extrusion freeforming of millimeter-wave electromagnetic bandgap(EBG)photonic crystals
No full textExtrusion freeforming can be used for the rapid prototyping of millimeter-wave electromagnetic bandgap (EBG) structures. In this work, an alumina-polymer paste with a relatively high volatility solvent (propanol) was used and the characteristics of the ceramic paste, particularly the rheological features are described. The advantage of high volatility solvent is that the viscosity and elastic modulus of the paste are increased sharply as the solvent evaporates. Thus, the rigidity of the extruded filament is quickly increased as a small amount of solvent evaporates. Finally, by employing this procedure, different EBG structures such as 2-D, 3-D woodpile and aperiodic structures were fabricated and their bandgaps were measured. The experimental results show that extrusion freeforming is a relatively simple and easy method to fabricate these woodpile structures with a bandgap in the 90-110 GHz regio
Fabrication and evaluation of solid freeformed electromagnetic bandgap structures
No full textMicrowave electromagnetic bandgap (EBG) structures have potential applications in improving the radiating features of antennas and the transmission properties of waveguides. Extrusion freeforming is a rapid prototyping technique for assembling ceramic dielectric lattice structures directly from a computer design file. It is without heating, cooling or polymerization processes to contend with at the construction stage. Various limitations on overall build thickness prompted us to explore lamination by welding to produce larger three-dimensional quasi-crystals. Microwave transmission through normal and side incidence showed that the bandgap frequency was in the 90–110 GHz region in all directions, matching the design that was informed by computer modelling. In order to image the EBG internal crystal structure, micro-computed tomography was used for scanning and reconstruction. Finally, the effect of structural defects including roundness and sagging of filaments and deviation of inter-filament spacing caused by fabrication errors on bandgap frequency was investigated.<br/
Fabrication of electromagnetic crystals by extrusion freeforming
No full textElectromagnetic crystals have potential applications from microwave to optical frequencies. At present, several methods can be applied to fabricate these structures, for example, stereolithography and wafer etching. In this work, extrusion freeforming, one of the linear solid freeforming methods, was used to fabricate electromagnetic crystals and the forming, drying, debinding and sintering stages are described. This method allows structures to be designed on a computer file and rendered directly as physical objects so that prototypes can be created quickly. A wide range of ceramic powders can be used so that dielectric constant and loss factor can be tailored. Structures with deliberate defects such as missing filaments or circular structures can be made
Solvent-based paste extrusion solid freeforming
No full textSolvent-based extrusion freeforming is capable of building complex ceramic 3D structures and can be used in the fabrication of hard and soft tissue scaffolds, photonic crystals for millimetre wave, ordered ceramic preforms for metal matrix composites, precise molten metal filters and potentially for terahertz bandgap metamaterials. This is a powder-based rapid prototyping process, the principle of which is the realization of liquid to solid transition through solvent evaporation in the presence of a binder. We describe the characteristics of pastes prepared from different powders, notably the rheological properties at different solvent fractions and illustrate some of the structures fabricated by this techniqu
Fine lattice structures fabricated by extrusion freeforming: process variables
No full textThis paper describes the main factors affecting the rapid prototyping of fine lattices by extrusion freeforming of powder, notably equipment accuracy, paste preparation, extrusion and post-processing and their effects on filament deposition and the defects that might be caused. Effective methods were devised in order to reduce the incidence of these defects. The results provide guidance for fabrication of very fine lattices from powder extrusion (comprising <100 ?m diameter filaments) and improvement of sample qualit
Fabrication of millimeter-wave electromagnetic bandgap crystals using microwave dielectric powders
No full textElectromagnetic bandgap (EBG) structures active in the 90–110 GHz region widely used by security imaging radar were created using different ceramics by a rapid prototyping method informed by finite difference time domain modeling. This solid free-forming method uses a high volatility solvent-based ceramic paste extruded through fine nozzles allowing ceramic powders to be assembled on a multiaxis building platform avoiding machining, etching, or the alignment of loose rods and created to designs downloaded directly from a computer file. Lattices were made from two high dielectric constant ceramics: La(Mg0.5, Ti0.5)O3 and (Zr0.8, Sn0.2)TiO4 and compared with those of Al2O3 demonstrating three EBG structures with different dimensions and dielectric constants but with the same bandgap. The effects of manufacturing tolerances on bandgap frequency are investigated by simulatio
Rapid prototyping of ceramic millimeterwave metamaterials: Simulations and experiments
No full textRapid prototyping by an extrusion freeforming technique, of ceramic metamaterials based on a woodpile structure at millimeterwave frequencies has been performed. The finite difference time domain technique is applied for the design and characterization of the proposed metamaterials. The transmittance of the millimeterwave metamaterials is measured in the range of 75–110 GHz. Both measurement and simulation results are in good agreement
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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