631 research outputs found
The density factor in the synthesis of carbon nanotube forest by injection chemical vapor deposition
Beneath the seeming straight-forwardness of growing carbon nanotube(CNT) forests by the injection chemical vapor deposition(CVD) method, control of the forest morphology on various substrates is yet to be achieved. Using ferrocene dissolved in xylene as the precursor, we demonstrate that the concentration of ferrocene and the injection rate of the precursor dictate the CNT density of these forests. However, CNT density will also be affected by the substrates and the growth temperature which determine the diffusion of the catalyst adatoms. The CNT growth rate is controlled by the temperature and chemical composition of the gases in the CVD reactor. We show that the final height of the forest is diffusion limited, at least in the conditions of our experiments. Because of the proximity and entanglement of the CNTs in a forest, the growing CNTs can lift-up the inactive CNTs resulting in reduced density toward the base of the forest unless the nucleation rate of the new catalyst particles is sufficiently high to replenish the inactive catalyst particles. Significant loss of CNT attachment by the lift-up effect reduces the adhesion of the forest to the substrate. Optimizing the ferrocene concentration in the precursor, precursor injection rate, gas mixture, substrate, and temperature is necessary to achieve desired forest morphology for specific applications
Families of Graphs With Chromatic Zeros Lying on Circles
We define an infinite set of families of graphs, which we call -wheels and
denote , that generalize the wheel () and biwheel ()
graphs. The chromatic polynomial for is calculated, and
remarkably simple properties of the chromatic zeros are found: (i) the real
zeros occur at for even and for odd;
and (ii) the complex zeros all lie, equally spaced, on the unit circle
in the complex plane. In the limit, the zeros
on this circle merge to form a boundary curve separating two regions where the
limiting function is analytic, viz., the exterior and
interior of the above circle. Connections with statistical mechanics are noted.Comment: 8 pages, Late
The VISTA Science Archive
We describe the VISTA Science Archive (VSA) and its first public release of
data from five of the six VISTA Public Surveys. The VSA exists to support the
VISTA Surveys through their lifecycle: the VISTA Public Survey consortia can
use it during their quality control assessment of survey data products before
submission to the ESO Science Archive Facility (ESO SAF); it supports their
exploitation of survey data prior to its publication through the ESO SAF; and,
subsequently, it provides the wider community with survey science exploitation
tools that complement the data product repository functionality of the ESO SAF.
This paper has been written in conjunction with the first public release of
public survey data through the VSA and is designed to help its users understand
the data products available and how the functionality of the VSA supports their
varied science goals. We describe the design of the database and outline the
database-driven curation processes that take data from nightly
pipeline-processed and calibrated FITS files to create science-ready survey
datasets. Much of this design, and the codebase implementing it, derives from
our earlier WFCAM Science Archive (WSA), so this paper concentrates on the
VISTA-specific aspects and on improvements made to the system in the light of
experience gained in operating the WSA.Comment: 22 pages, 16 figures. Minor edits to fonts and typos after
sub-editting. Published in A&
Nitrogen cost minimization is promoted by structural changes in the transcriptome of N-deprived Prochlorococcus cells
Prochlorococcus is a globally abundant marine cyanobacterium with many adaptations that reduce cellular nutrient requirements, facilitating growth in its nutrient-poor environment. One such genomic adaptation is the preferential utilization of amino acids containing fewer N-atoms, which minimizes cellular nitrogen requirements. We predicted that transcriptional regulation might further reduce cellular N budgets during transient N limitation. To explore this, we compared transcription start sites (TSSs) in Prochlorococcus MED4 under N-deprived and N-replete conditions. Of 64 genes with primary and internal TSSs in both conditions, N-deprived cells initiated transcription downstream of primary TSSs more frequently than N-replete cells. Additionally, 117 genes with only an internal TSS demonstrated increased internal transcription under N-deprivation. These shortened transcripts encode predicted proteins with an average of 21% less N content compared to full-length transcripts. We hypothesized that low translation rates, which afford greater control over protein abundances, would be beneficial to relatively slow-growing organisms like Prochlorococcus. Consistent with this idea, we found that Prochlorococcus exhibits greater usage of glycine-glycine motifs, which causes translational pausing, when compared to faster growing microbes. Our findings indicate that structural changes occur within the Prochlorococcus MED4 transcriptome during N-deprivation, potentially altering the size and structure of proteins expressed under nutrient limitation.Gordon and Betty Moore Foundation (Grant GBMF495)Simons Foundation (Award 329108)National Science Foundation (U.S.) (Grant DBI-0424599
Reproductive Failure in UK Harbour Porpoises Phocoena phocoena : Legacy of Pollutant Exposure?
This research was supported by a Marie Curie International Outgoing Fellowship within the Seventh European Community Framework Programme (Project Cetacean-stressors, PIOF-GA-2010-276145 to PDJ and SM). Additional funding was provided through the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS) (Grants SSFA/2008 and SSFA / ASCOBANS / 2010 / 5 to SM). Analysis of Scottish reproductive and teeth samples was funded by the EC-funded BIOCET project (BIOaccumulation of persistent organic pollutants in small CETaceans in European waters: transport pathways and impact on reproduction, grant EVK3-2000-00027 to GJP), and Marine Scotland (GJP). Samples examined in this research were collected under the collaborative Cetacean Strandings Investigation Programme (http://ukstrandings.org/), which is funded by the Department for Environment, Food and Rural Affairs (Defra) and the UK’s Devolved Administrations in Scotland and Wales (http://sciencesearch.defra.gov.uk/Default.aspx?Menu=Menu&Module=More&Location=None&Completed=0&ProjectID=15331) (grants to PDJ, RD). UK Defra also funded the chemical analysis under a service-level agreement with the Centre for Environment, Fisheries and Aquaculture Science (grants to RJL, JB). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Peer reviewedPublisher PD
The DeRisk database: Extreme Design Waves for Offshore Wind Turbines
The estimation of extreme loads from waves is an essential part of the design
of an offshore wind turbine. Standard design codes suggest to either use
simplified methods based on regular waves, or to perform fully nonlinear
computations. The former might not provide an accurate representation of the
extreme waves, while the latter is computationally too intensive for design
iterations. We address these limitations by using the fully nonlinear solver
OceanWave3D to establish the DeRisk database, a large dataset of extreme waves
kinematics in a two-dimensional domain. From the database, which is open and
freely available, a designer can extract fully-nonlinear wave kinematics for a
wave condition and water depth of interest by identifying a suitable
computation in the database and, if needed, by Froude-scaling the kinematics.
The nonlinear solver is validated against the DeRisk model experiments at two
different water depths, and , and an excellent agreement
is found for the analyzed cases. The experiments are used to calibrate
OceanWave3D's numerical breaking filter constant, and the best agreement is
found for . We compare the experimental static force with
predictions by the DeRisk database and the Rainey force model, and with
state-of-the-art industrial practices. For milder storms, we find a good
agreement in the predicted extreme force between the present methodology and
the standard methodologies. At the deep location and for stronger storms, the
largest loads are given by slamming loads due to breaking waves. In this
condition, the database methodology is less accurate than the embedded stream
function method and more accurate than the WiFi JIP methodology, providing
generally nonconservative estimates. For strong storms at the shallower
location, where wave breaking is less dominating, the database methodology is
the most accurate overall.Comment: Submitted to Marine Structures (Elsevier), 46 pages, 16 figures, 6
tables. The database associated with the publication is available at
https://data.dtu.dk/articles/dataset/The_DeRisk_Database/1032203
Pharmacokinetics of β-Lactam Antibiotics:Clues from the Past to Help Discover Long-Acting Oral Drugs in the Future
β-Lactams
represent perhaps the most important class of antibiotics
yet discovered. However, despite many years of active research, none
of the currently approved drugs in this class combine oral activity
with long duration of action. Recent developments suggest that new
β-lactam antibiotics with such a profile would have utility
in the treatment of tuberculosis. Consequently, the historical β-lactam
pharmacokinetic data have been compiled and analyzed to identify possible
directions and drug discovery strategies aimed toward new β-lactam
antibiotics with this profile
Ground State Entropy of Potts Antiferromagnets: Bounds, Series, and Monte Carlo Measurements
We report several results concerning , the
exponent of the ground state entropy of the Potts antiferromagnet on a lattice
. First, we improve our previous rigorous lower bound on for
the honeycomb (hc) lattice and find that it is extremely accurate; it agrees to
the first eleven terms with the large- series for . Second, we
investigate the heteropolygonal Archimedean lattice, derive a
rigorous lower bound, on , and calculate the large- series
for this function to where . Remarkably, these agree
exactly to all thirteen terms calculated. We also report Monte Carlo
measurements, and find that these are very close to our lower bound and series.
Third, we study the effect of non-nearest-neighbor couplings, focusing on the
square lattice with next-nearest-neighbor bonds.Comment: 13 pages, Latex, to appear in Phys. Rev.
The Structure of Chromatic Polynomials of Planar Triangulation Graphs and Implications for Chromatic Zeros and Asymptotic Limiting Quantities
We present an analysis of the structure and properties of chromatic
polynomials of one-parameter and multi-parameter families
of planar triangulation graphs , where is a vector of integer parameters. We use these to study the
ratio of to the Tutte upper bound ,
where and is the number of vertices in . In particular, we calculate limiting values of this ratio as for various families of planar triangulations. We also use our
calculations to study zeros of these chromatic polynomials. We study a large
class of families with and and show that these have
a structure of the form for , where
, , and , and for . We derive properties of the
coefficients and show that has a
real chromatic zero that approaches as one or more of
the . The generalization to is given. Further, we
present a one-parameter family of planar triangulations with real zeros that
approach 3 from below as . Implications for the ground-state
entropy of the Potts antiferromagnet are discussed.Comment: 57 pages, latex, 15 figure
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