130,444 research outputs found
Real-time observations of materials under dynamic loading conditions at the micron scale
No full textThesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2016.Cataloged from PDF version of thesis. Page 171 blank.Includes bibliographical references (pages [151]-170).New methods are introduced for direct real-time observations of materials under dynamic loading. Traditionally, converging shock waves, which are of great interest for studying fundamental aspects of behavior of materials under extreme conditions, have been studied on the spatial scale ~1 m using facilities such as implosion chambers. In this work, direct real-time visualization of converging shock waves in a thin liquid layer was demonstrated at the micron scale in an all-optical experiment. Shock waves were generated in a 10 [mu]m-thick layer of water by sub-nanosecond laser pulses focused into a ring of 100 [mu]m radius. Time-resolved interferometry imaging with a femtosecond probe pulse was used to obtain full-field images at variable delays tracing the converging shock wave as it collapsed at the focal point leading to the formation of a cavitation bubble. Pressure values calculated from velocity measurements agreed with those obtained from quantitative analysis of interferometric images. Other samples were investigated using the same experimental setup. Focusing surface waves leading to material damage were observed on a glass substrate and conversion from highly-ordered pyrolytic graphite to nano-crystalline carbon was demonstrated under shock conditions. The developed technique will enable shock-induced chemical reactions to be investigated with direct access to the shocked reacting material for spectroscopic tools in a small-scale all-optical experiment. A novel approach for hypervelocity impact testing of materials using microprojectiles and a table-top laser system was also developed in this work. Microparticles were placed on a transparent substrate coated with a laser-absorbing polymer layer. Ablation of the polymer by a short laser pulse accelerated the particles to high velocities (~1 km/s). The impact events of high-speed particles with target material surfaces were monitored using an ultrafast multi-frame camera. In particular, the impact response of poly(urethane urea) (PUU) elastomers was investigated in order to further the understanding of the molecular influence on the dynamical behavior of PUUs. The results will provide an impetus for modeling microscale impact responses and high-strain rate deformation for a large spectrum of materials ranging from viscoelastic polymers and gels to elastic-plastic metals.by David Veysset.Ph. D
Laser-induced versus shock wave induced transformation of highly ordered pyrolytic graphite
No full textWe demonstrate that in-plane 2D propagation and focusing of a laser-induced shock wave result in enhanced nano-crystallization of highly ordered pyrolytic graphite. Throughout the 2D shock focusing technique, which enables to clearly distinguish between the laser-induced and the shock-induced transformation/transition, our findings establish the role of the shock wave during the transformation/transition process. This configuration could open the way to an alternative path for laser shock fabrication of graphitic compounds and would give access to real time investigation of shock waves mediated phase transitions.United States. Army Research Office (W911NF-13-D-0001
MeSH term explosion and author rank improve expert recommendations
No full textInformation overload is an often-cited phenomenon that reduces the productivity, efficiency and efficacy of scientists. One challenge for scientists is to find appropriate collaborators in their research. The literature describes various solutions to the problem of expertise location, but most current approaches do not appear to be very suitable for expert recommendations in biomedical research. In this study, we present the development and initial evaluation of a vector space model-based algorithm to calculate researcher similarity using four inputs: 1) MeSH terms of publications; 2) MeSH terms and author rank; 3) exploded MeSH terms; and 4) exploded MeSH terms and author rank. We developed and evaluated the algorithm using a data set of 17,525 authors and their 22,542 papers. On average, our algorithms correctly predicted 2.5 of the top 5/10 coauthors of individual scientists. Exploded MeSH and author rank outperformed all other algorithms in accuracy, followed closely by MeSH and author rank. Our results show that the accuracy of MeSH term-based matching can be enhanced with other metadata such as author rank
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
No full textBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
Dispelling the Myths Behind First-author Citation Counts
No full textWe 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
Scholarly Communication and Publishing Lunch and Learn Talk #11: The ULS Open Access Author Fee Fund
No full textAt the May 2014 talk, you will learn about the ULS Open Access Author Fee Fund--what it is, why we do it, how it works, and how the program is going so far
The R&D Tax Incentives
No full textThis article sets out some background information and reflections of the author on the R&D tax incentive schemes included in the Common Corporate Tax Base (CCTB) Proposal. In particular the author analyzes the stimulus to private R&D through ad hoc tax incentives included in the CCTB Proposal and dives into the actual provisions included in the Proposal highlighting the most relevant issues connected with their design and interpretation. Moreover, the author explores the interaction between the CCTB Proposal and the granting by Member States of domestic R&D tax incentives
- …
