2,336 research outputs found
Mechanical properties of polycrystalline graphene based on a realistic atomistic model
Graphene can at present be grown at large quantities only by the chemical
vapor deposition method, which produces polycrystalline samples. Here, we
describe a method for constructing realistic polycrystalline graphene samples
for atomistic simulations, and apply it for studying their mechanical
properties. We show that cracks initiate at points where grain boundaries meet
and then propagate through grains predominantly in zigzag or armchair
directions, in agreement with recent experimental work. Contrary to earlier
theoretical predictions, we observe normally distributed intrinsic strength (~
50% of that of the mono-crystalline graphene) and failure strain which do not
depend on the misorientation angles between the grains. Extrapolating for grain
sizes above 15 nm results in a failure strain of ~ 0.09 and a Young's modulus
of ~ 600 GPa. The decreased strength can be adequately explained with a
conventional continuum model when the grain boundary meeting points are
identified as Griffith cracks.Comment: Accepted for Physical Review B; 5 pages, 4 figure
On the use of non-canonical quantum statistics
We develop a method using a coarse graining of the energy fluctuations of an
equilibrium quantum system which produces simple parameterizations for the
behaviour of the system. As an application, we use these methods to gain more
understanding on the standard Boltzmann-Gibbs statistics and on the recently
developed Tsallis statistics. We conclude on a discussion of the role of
entropy and the maximum entropy principle in thermodynamics.Comment: 29 pages, uses iopart.cls, major revisions of text for better
readability, added a discussion about essentially microcanonical ensemble
Uranium leaching from contaminated soil utilizing rhamnolipid, EDTA, and citric acid
Biosurfactants have recently gained attention as "green" agents that can be used to enhance the remediation of heavy metals and some organic matter in contaminated soils. The overall objective of this paper was to investigate rhamnolipid, a microbial produced biosurfactant, and its ability to leach uranium present in contaminated soil from an abandoned mine site. Soil samples were collected from two locations in northern Arizona: Cameron (site of open pit mining) and Leupp (control-no mining). The approach taken was to first determine the total uranium content in each soil using a hydrofluoric acid digestion, then comparing the amount of metal removed by rhamnolipid to other chelating agents EDTA and citric acid, and finally determining the amount of soluble metal in the soil matrix using a sequential extraction. Results suggested a complex system for metal removal from soil utilizing rhamnolipid. It was determined that rhamnolipid at a concentration of 150 mu M was as effective as EDTA but not as effective as citric acid for the removal of soluble uranium. However, the rhamnolipid was only slightly better at removing uranium from the mining soil compared to a purified water control. Overall, this study demonstrated that rhamnolipid ability to remove uranium from contaminated soil is comparable to EDTA and to a lesser extent citric acid, but, for the soils investigated, it is not significantly better than a simple water wash
Traditional Ecological Knowledge Policy Considerations for Abandoned Uranium Mines on Navajo Nation
Environmental justice is a prominent issue for Native American Nations within the United States. One example is the abandoned uranium mines on the Navajo Nation that were left unremediated since the Cold War. Often, environmental policy is developed for issues facing Native American Nations that do not include input from those Nations. Instead, Native American Nations should have the opportunity to address environmental issues using their traditional ecological knowledge (TEK). TEK has ties to natural laws long respected by tribal communities; these laws provide the foundation for addressing the complex relationship between nature and humans. Often, policy development addressing environmental concerns is determined by non-Native American stakeholders, which can have negative effects on the Native American communities. These policies harm Native Americans rather than ultimately helping them. The focus of this discussion is how TEK can play a role in environmental policy development for the Navajo Nation surrounding abandoned uranium mines
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