1,720,962 research outputs found
Feasibility of rapid debinding and sintering of additively manufactured ceramics
Full text linkIn recent years, additive manufacturing (AM) of ceramics has significantly advanced in terms of the range of equipment available, printing resolution and productivity. Among all the available AM technologies developed for ceramics, material-extrusion (MEX) based techniques outperform the others in several aspects and are widely used for rapid prototyping of ceramics. This was made possible due to the availability of low-cost open-source printers and the simplicity of the overall process. Unlike, vat photopolymerization, where differences in the refractive index between the ceramic particles and the resin increase the scattering or in SLS, where the absorption of the laser power is dependent on the optical properties of the starting powder, MEX-based AM techniques operate independently of the optical properties of the ceramic powder.
However, the bottleneck of the complete fabrication process is denoted by the slow debinding process (up to 1 °C/min) and the subsequent sintering process required to obtain a dense ceramic component, making it an energy-intensive process. Thus, there is a real need to develop feedstocks that are compatible with rapid thermal treatments and to explore energy-efficient rapid sintering approaches to accomplish this goal. In this aspect, this PhD thesis addresses two research fields that have been only marginally communicated with each other to date: non-conventional sintering and additive manufacturing.
The main goal of this thesis was to validate the feasibility of coupling rapid sintering approaches with material-extrusion (MEX)-based techniques, including direct ink writing (DIW) and fused filament fabrication (FFF). To validate this, both commercial 3 mol.% yttria stabilized zirconia (3YSZ) and laboratory-made filaments of barium titanate (BaTiO3) and alumina (Al2O3) were used. Additionally, inks with suitable viscoelastic properties for DIW were developed in-house using commercial BaTiO3 and Al2O3 powders. The printed samples with optimized processing parameters could be debinded and sintered to almost full densities without any defects in a single-step process using different rapid sintering techniques such as ultra-fast high-temperature sintering (UHS), fast firing (FF), and pressureless-spark plasma sintering (P-SPS) lasting around few seconds to minutes. The printed samples behave very differently when processed using different processing conditions. For instance, the FFF samples only work when chemical debinding is performed prior to the thermal debinding process. Additionally, the sintering process is strongly affected by the sintering atmosphere. For DIW, the samples printed with BaTiO3 could resist such rapid heating approaches but the undesired phase transformation associated when sintered in argon, makes such a process unsuitable for these compositions. Especially for UHS and P-SPS, the presence of an inert atmosphere is necessary to prevent the decomposition of the graphite felt or the die.
This thesis therefore provides a first of kind of work reporting ultra-rapid processing (debinding and sintering) of additively manufactured ceramics, thereby reducing the overall processing time by 99%. The rapid debinding/sintering approach developed in this thesis could be potentially transferred to other optimized ceramic feedstocks as well
From rapid prototyping to rapid firing: on the feasibility of high‐speed production for complex BaTiO3 components
Full text linkDirect ink writing (DIW) is an attractive additive manufacturing (AM) technology because of its simplicity, production speed, and feedstock flexibility; in addition, the use of a limited amount of binder makes the subsequent thermal debinding process easy. Nevertheless, the conventional approach to debind and sinter AMed components remains extremely slow, representing a bottleneck in the manufacturing process. In order to address such limitation, we explored different rapid sintering strategies: ultrafast high-temperature sintering (UHS), pressureless spark plasma sintering (P-SPS), and fast firing (FF), for the densification of BaTiO3 components fabricated by DIW, one of the widely used lead-free piezoceramics. All sintering technologies allow debinding and sintering of crack-free components in a few minutes instead of several hours. The final density and microstructure are strongly dependent on the sintering atmosphere (inert for UHS and P-SPS, air for FF) and a maximum relative density of only ≈72% was obtained when firing occurred in an inert environment, irrespective of the sintering technique (UHS and P-SPS). An undesired phase transition from tetragonal to hexagonal BaTiO3 was also observed upon UHS and -PSPS. On the contrary, FF in air yielded a density of about 95% in a few minutes while maintaining the desired tetragonal polymorph. The results provide proof of feasibility for rapid processing of BaTiO3 components obtained by DIW
Material-extrusion based additive manufacturing of BaTiO3 ceramics: from filament production to sintered properties
Full text linkMaterial extrusion (MEX) of thermoplastic filaments represents one of the most widely adopted additive manufacturing (AM) technologies. Unlike vat photopolymerization and powder-bed fusion methods that require high energy sources such as UV light and lasers, this fabrication method can be adapted for the fabrication of ceramics by using ceramic loaded filaments as feedstock, yet still employing relatively cheap equipment meant for polymeric materials with little adaptation of the process parameters; this potentially enables a broader diffusion of AM ceramic components. In this work, composite filaments with various weight fractions (60 – 80 wt%) of BaTiO3 were fabricated and characterized by electron microscopy, compressive mechanical testing, rheometry and thermogravimetric analysis to ensure a smooth and reliable printing process. After optimizing the printing parameters, the dense and porous printed samples were carefully debinded and sintered to obtain dense (∼ 92 %) and defect-free ceramic bodies. The sintered samples were characterized for phase development, microstructure, and pore size distribution. Careful observations reveal a particular range of pore size (0.1 – 5 μm), which originates from the binder burn out process. The dielectric and ferroelectric properties of the fabricated samples were in good agreement with those reported in previous literature. This work provides a foundation for rapid prototyping of functional electro ceramics into reliable products with desired functional properties
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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