1,720,956 research outputs found
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
Investigating The Re-Usability Characteristics And Limits Of Polypropylene Powder In Laser Sinter Additive Manufacturing
Get PDFDissertationThe aim of this research was to investigate the re-usability characteristics and limits of polypropylene (PP) powder in laser sinter additive manufacturing (AM). Initially, the objective was to analyse Laser PP CP 60 from Diamond Plastics GmbH, and preliminary work was carried out on it, but the focus eventually turned to Laser PP CP 75 after processing difficulties were experienced with the former material. Polymer laser sintering (LS) subjects feed powders to high temperatures, leading to degradation of their thermal, rheological, and physical properties, impeding their recyclability. Therefore, it is imperative to examine the degree of deterioration or aging of polymers before re-using the materials. The recyclability of polymers is measured based on powder particle size, morphology, density, rheological properties, and thermal characteristics. Attempts were made to determine suitable processing parameters of Laser PP CP 60 polypropylene powder grade from Diamond Plastics in the first part of the analysis. This was followed by experiments to characterize the powder after a single print cycle to quantify degradation at this stage using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and melt flow index testing (MFI). The focus was then turned to the recyclability of Laser PP CP 75 per print cycle by printing a set of test coupons at pre-determined positions in the build volume of an EOSINT P 380 LS machine. The powder remaining in the machine and the cake powder surrounding the coupons after each print cycle was thoroughly mixed using a concrete mixer for about 30 minutes. A sample of the mixed powder was re-introduced into the P 380 machine to print another set of test specimens. In this study, the re-usability of the PP powder was determined by characterizing and comparing the used powder after each cycle with powder from previous cycles and fresh material.
It was established from the preliminary testing of Laser PP CP 60 that slightly different process parameter settings were required to those provided by the supplier to achieve the highest ultimate tensile strength, smoothest surface, and best dimensional accuracy of printed parts. The three characterized batches of Laser PP CP 60 powder (virgin, aged, virgin-aged mixture) exhibited poor (not 100% spherical) but acceptable morphology and particle size distribution. It was also found that the sintering window of virgin Laser PP CP 60 increased significantly (by 28%) after a single cycle of printing, from a value of 21.04 ℃ for the virgin powder to 26.95 ℃ for the recycled powder. The degree of crystallinity of virgin Laser PP CP 60 decreased from 13.92% to 12.12% after a single printing cycle and then increased to 12.48% after the addition of 50% virgin material. All the three batches, namely virgin powder, used powder, and a 50% virgin:50% used mixture, showed high degradation temperatures of 457.30 ℃, 456.05 ℃, and 455.95 ℃, respectively. Lastly, the three batches of powder showed low MFI values of 6.1, 6.5, and 6.4 g/10 min in the same order.
Turning to Laser PP CP 75, it was concluded that the powder could be re-used for four re-use cycles without having to mix with virgin material because it does not form an “orange peel,” as is the case with polyamide-12 (PA 12) (used here as reference material). Besides, the MFI trend indicates that the viscosity of the material is not significantly degraded, which promotes recyclability. The DSC assessment established that the sintering window of Laser PP CP 75 increased with each re-use cycle. The results indicate that the shrinkage rate and curling of the material might reduce with the number of re-use cycles because a wide and sufficient sintering window prevents crystallization of the polymers during processing. From the TGA test, the breakdown temperature of Laser PP CP 75 was seen to increase slightly with the number of re-use cycles, from 455.53 ℃ (virgin material) to 457.53 ℃ after the 4th re-use cycle. Thus, the material does not break down at the temperatures prevailing during printing, making it suitable for LS processing and re-use. The SEM analysis revealed that the average size of Laser PP CP 75 powder particles is not significantly influenced by re-use cycles, making the powder recyclable. Furthermore, the powder did not exhibit signs of agglomeration for the 1st, 2nd, and 3rd re-use cycles. However, this phenomenon was observed in the 4th re-use cycle of printing. Lastly, tensile testing revealed that the material had the highest ultimate tensile strength after the 3rd printing cycle (7.4 MPa), after which the value decreased with recycling. Overall, it can be concluded that PP powder has superior recyclability properties to those of PA 12, which requires mixing with virgin material after every print cycle. Besides, the MFI, particle size and thermal properties are insignificantly altered, which does not hamper the re-use of the material for subsequent printing cycles. Parts printed with virgin Laser PP CP 75 had an average dimensional error of 3.02% and 4.06% after the 4th re-use cycle. Therefore, Laser PP CP 75 might not be commercially suitable because of dimensional accuracy and observed processing difficulties
Mixing And Reuse Of Polymer Laser Sintering Powders To Ensure Homogeneity - A Review
Get PDFArticleSelective laser sintering is one of the most common additive manufacturing technologies for polymeric materials. However, the technique is limited due to the high cost of the few available feedstock materials. Besides, laser sintering is a high-temperature process, which subjects polymeric materials to thermal stresses, resulting in the degeneration of powder. Besides, only a small portion of the powder fed into the built chamber is used in the printing of components. Hence, the need to recycle the used polymeric powder. This is achieved by mixing used and virgin powder in the ratios stipulated by the manufacturers of the powders. This review investigates the types of powder, types of mixing mechanism, parameters that affect efficient mixing, the homogeneity of mixed powder, mechanisms of segregation, and selection of the most appropriate mixers in the context of selective laser sintering of polymeric materials
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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
Get PDFWe 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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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