758 research outputs found
Expanding the Applicability of FDM-type Technologies Through Materials Development
Currently, the most common form of additive manufacturing is material extrusion 3D
printing (ME3DP) based on fused deposition modeling (FDM®) technology which relies upon a
thermoplastic monofilament as a base material for the fabrication of three dimensional objects.
The dependence on thermoplastics as a feedstock by ME3DP platforms limits the applicability of
this additive manufacturing method. A clear-cut path towards greater applicability is the
introduction of novel materials with diverse physical properties which maintain compatibility
with 3D printing platforms based on FDM® technology. The work in this paper presents efforts
in the development of polymer matrix composites (PMC)s and polymer blends based on
acrylonitrile butadiene styrene (ABS) and polycarbonate (PC), two thermoplastic materials
commonly used by FDM®-type platforms. Mechanical testing and fractography via scanning
electron microscopy (SEM) were the two main metrics used to characterize these new material
systems. Overcoming barriers to the manufacturing of these novel 3D-printable materials
systems is also presented.Mechanical Engineerin
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Standardization, calibration and innovation: a special issue on lithic microwear method
YesThis paper introduces a special issue of the Journal of Archaeological Science that considers the current state and future directions in lithic microwear analysis. There is considerable potential for lithic microwear analysis to reconstruct past human behaviour as it can provide direct insight into past activities. Consequently, it is a technique worthy of significant additional investment and continued development. To further the cause of methodological maturation within microwear analysis and to promote standardization, calibration, and innovation, the following collection of papers present various approaches and perspectives on how greater methodological refinement and increased reliability of results can and should be achieved. Many of these papers were part of a session held at the 2011 Society for American Archaeology Meeting (SAA) in Sacramento, California, while others were selected from the 2012 International Conference on Use-Wear Analysis in Faro, Portugal. The purpose of the SAA session and this special themed issue is essentially two-fold. The first is to promote awareness of the need for methodological standardization, calibration, and continuing innovation. The second is to open a serious dialogue about how these aims could be pursued and achieved.AAE was supported by the AHRC (AH/J007935/1). HJL was supported by L’Équipe Archéometrie at the Laboratoires d’ Archéologie, a part of the Centre Interuniversitaire d’Études sur les Lettres, les Arts et les Traditions (CELAT) at Université Laval. DAM was supported by Fondation Fyssen and the University of Toronto. WJS was supported by Faculty Development Grant from Keene State College
Shubnikov-de Haas oscillations near the metal-insulator transition in a two-dimensional electron system in silicon
We have studied Shubnikov-de Haas oscillations in a two-dimensional electron
system in silicon at low electron densities. Near the metal-insulator
transition, only "spin" minima of the resistance at Landau-level filling
factors 2, 6, 10, and 14 are seen, while the "cyclotron" minima at filling
factors 4, 8, and 12 disappear. A simple explanation of the observed behavior
requires a giant enhancement of the spin splitting near the metal-insulator
transition.Comment: 4 pages, postscript figures include
Modelling the future of the Hawaiian honeycreeper : an ecological and epidemiological problem
The Hawaiian honeycreeper (Drepanididae) faces the threat of extinction; this is believed to be due primarily to predation from alien animals, endemic avian malaria (Plasmodium relictum) and climate change. A deterministic SI modelling approach is developed, incorporating these three factors and a metapopulation approach in conjunction with a quasi-equilibrium assumption to simplify the vector populations. This enables the qualitative study of the behaviour of the system. Numerical results suggest that although (partial) resistance to avian malaria may be advantageous for individual birds, allowing them to survive infection, this allows them to become carriers of infection and hence greatly increases the spread of this disease. Predation obviously reduces the life-expectancy of honeycreepers, but in turn this reduces the spread of infection from resistant carriers; therefore the population-level impact of predation is reduced. Various control strategies proposed in the literature are also considered and it is shown that predation control could either help or hinter, depending upon resistance of the honeycreeper species. Captive propagation or habitat restoration may be the best feasible solution to the loss of both heterogeneity within the population and the loss of the species as a whole
BioMiCo:A supervised Bayesian model for inference of microbial community structure
Here, we describe a novel hierarchical model for Bayesian inference of microbial communities (BioMiCo). The model takes abundance data derived from environmental DNA, and models the composition of each sample by a two-level hierarchy of mixture distributions constrained by Dirichlet priors. BioMiCo is supervised, using known features for samples and appropriate prior constraints to overcome the challenges posed by many variables, sparse data, and large numbers of rare species. The model is trained on a portion of the data, where it learns how assemblages of species are mixed to form communities and how assemblages are related to the known features of each sample. Training yields a model that can predict the features of new samples. We used BioMiCo to build models for three serially sampled datasets and tested their predictive accuracy across different time points. The first model was trained to predict both body site (hand, mouth, and gut) and individual human host. It was able to reliably distinguish these features across different time points. The second was trained on vaginal microbiomes to predict both the Nugent score and individual human host. We found that women having normal and elevated Nugent scores had distinct microbiome structures that persisted over time, with additional structure within women having elevated scores. The third was trained for the purpose of assessing seasonal transitions in a coastal bacterial community. Application of this model to a high-resolution time series permitted us to track the rate and time of community succession and accurately predict known ecosystem-level events
Perturbative spectrum of Trapped Weakly Interacting Bosons in Two Dimensions
We study a trapped Bose-Einstein condensate under rotation in the limit of
weak, translational and rotational invariant two-particle interactions. We use
the perturbation-theory approach (the large-N expansion) to calculate the
ground-state energy and the excitation spectrum in the asymptotic limit where
the total number of particles N goes to infinity while keeping the total
angular momentum L finite. Calculating the probabilities of different
configurations of angular momentum in the exact eigenstates gives us a clear
view of the physical content of excitations. We briefly discuss the case of
repulsive contact interaction.Comment: Revtex, 10 pages, 1 table, to appear in Phys. Rev.
Secondary Beam Monitors for the NuMI Facility at FNAL
The Neutrinos at the Main Injector (NuMI) facility is a conventional neutrino
beam which produces muon neutrinos by focusing a beam of mesons into a long
evacuated decay volume. We have built four arrays of ionization chambers to
monitor the position and intensity of the hadron and muon beams associated with
neutrino production at locations downstream of the decay volume. This article
describes the chambers' construction, calibration, and commissioning in the
beam.Comment: Accepted for publication in Nucl. Instr. Meth.
Quantum Transport in Two-Channel Fractional Quantum Hall Edges
We study the effect of backward scatterings in the tunneling at a point
contact between the edges of a second level hierarchical fractional quantum
Hall states. A universal scaling dimension of the tunneling conductance is
obtained only when both of the edge channels propagate in the same direction.
It is shown that the quasiparticle tunneling picture and the electron tunneling
picture give different scaling behaviors of the conductances, which indicates
the existence of a crossover between the two pictures. When the direction of
two edge-channels are opposite, e.g. in the case of MacDonald's edge
construction for the state, the phase diagram is divided into two
domains giving different temperature dependence of the conductance.Comment: 21 pages (REVTeX and 1 Postscript figure
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