521 research outputs found
Modulating active sites in MOFs for improved Lewis acid or base catalysis
International audienc
Foot-and-mouth disease in Tanzania from 2001 to 2006.
Foot-and-mouth disease (FMD) is endemic in Tanzania, with outbreaks occurring almost each year in different parts of the country. There is now a strong political desire to control animal diseases as part of national poverty alleviation strategies. However, FMD control requires improving the current knowledge on the disease dynamics and factors related to FMD occurrence so control measures can be implemented more efficiently. The objectives of this study were to describe the FMD dynamics in Tanzania from 2001 to 2006 and investigate the spatiotemporal patterns of transmission. Extraction maps, the space-time K-function and space-time permutation models based on scan statistics were calculated for each year to evaluate the spatial distribution, the spatiotemporal interaction and the spatiotemporal clustering of FMD-affected villages. From 2001 to 2006, 878 FMD outbreaks were reported in 605 different villages of 5815 populated places included in the database. The spatial distribution of FMD outbreaks was concentrated along the Tanzania-Kenya, Tanzania-Zambia borders, and the Kagera basin bordering Uganda, Rwanda and Tanzania. The spatiotemporal interaction among FMD-affected villages was statistically significant (P≤0.01) and 12 local spatiotemporal clusters were detected; however, the extent and intensity varied across the study period. Dividing the country in zones according to their epidemiological status will allow improving the control of FMD and delimiting potential FMD-free areas
Analytical evaluation of the X-ray scattering contribution to imaging degradation in grazing-incidence X-ray telescopes
The focusing performance of X-ray optics (conveniently expressed in terms of
HEW, Half Energy Width) strongly depend on both mirrors deformations and photon
scattering caused by the microroughness of reflecting surfaces. In particular,
the contribution of X-ray Scattering (XRS) to the HEW of the optic is usually
an increasing function H(E) of the photon energy E. Therefore, in future hard
X-ray imaging telescopes of the future (SIMBOL-X, NeXT, Constellation-X, XEUS),
the X-ray scattering could be the dominant problem since they will operate also
in the hard X-ray band (i.e. beyond 10 keV). [...]
Several methods were proposed in the past years to estimate the scattering
contribution to the HEW, dealing with the surface microroughness expressed in
terms of its Power Spectral Density (PSD), on the basis of the well-established
theory of X-ray scattering from rough surfaces. We faced that problem on the
basis on the same theory, but we tried a new approach: the direct, analytical
translation of a given surface roughness PSD into a H(E) trend, and - vice
versa - the direct translation of a H(E) requirement into a surface PSD. This
PSD represents the maximum tolerable microroughness level in order to meet the
H(E) requirement in the energy band of a given X-ray telescope.
We have thereby found a new, analytical and widely applicable formalism to
compute the XRS contribution to the HEW from the surface PSD, provided that the
PSD had been measured in a wide range of spatial frequencies. The inverse
problem was also solved, allowing the immediate evaluation of the mirror
surface PSD from a measured function H(E). The same formalism allows
establishing the maximum allowed PSD of the mirror in order to fulfill a given
H(E) requirement. [...]Comment: 10 pages, 6 figures, published in Astronomy & Astrophysics, sect.
"Astronomical Instrumentation". In this version, a typo in two equations has
been corrected. After the correction, the other results, formulae and
conclusions in the paper remain unchange
Nine Principles of Semantic Harmonization
Medical data is routinely collected, stored and recorded across different institutions and in a range of different formats. Semantic harmonization is the process of collating this data into a singular consistent logical view, with many approaches to harmonizing both possible and valid. The broad scope of possibilities for undertaking semantic harmonization do lead however to the development of bespoke and ad-hoc systems; this is particularly the case when it comes to cohort data, the format of which is often specific to a cohort's area of focus. Guided by work we have undertaken in developing the 'EMIF Knowledge Object Library', a semantic harmonization framework underpinning the collation of pan-European Alzheimer's cohort data, we have developed a set of nine generic guiding principles for developing semantic harmonization frameworks, the application of which will establish a solid base for constructing similar frameworks
Determination of the nature of the Cu coordination complexes formed in the presence of NO and NH3 within SSZ-13
Ammonia-selective catalytic reduction (NH3-SCR) using Cu zeolites is a well-established strategy for the abatement of NOx gases. Recent studies have demonstrated that Cu is particularly active when exchanged into the SSZ-13 zeolite, and its location in either the 6r or 8r renders it an excellent model system for fundamental studies. In this work, we examine the interaction of NH3-SCR relevant gases (NO and NH3) with the Cu2+ centers within the SSZ-13 structure, coupling powder diffraction (PD), X-ray absorption spectroscopy (XAFS), and density functional theory (DFT). This combined approach revealed that, upon calcination, cooling and gas exposure Cu ions tend to locate in the 8r window. After NO introduction, Cu-ions are seen to coordinate to two framework oxygens and one NO molecule, resulting in a bent Cu-nitrosyl complex with a Cu-N-O bond angle of similar to 150 degrees. Whilst Cu seems to be partially reduced/changed in coordination state, NO is partially oxidized. On exposure to NH3 while the PD data suggest the Cu2+ ion occupies a similar position, simulation and XAFS pointed toward the formation of a Jahn-Teller distorted hexaamine complex [Cu(NH3)(6)](2+) in the center of the cha cage. These results have important implications in terms of uptake and storage of these reactive gases and potentially for the mechanisms involved in the NH3-SCR process
Error estimates for solid-state density-functional theory predictions: an overview by means of the ground-state elemental crystals
Predictions of observable properties by density-functional theory
calculations (DFT) are used increasingly often in experimental condensed-matter
physics and materials engineering as data. These predictions are used to
analyze recent measurements, or to plan future experiments. Increasingly more
experimental scientists in these fields therefore face the natural question:
what is the expected error for such an ab initio prediction? Information and
experience about this question is scattered over two decades of literature. The
present review aims to summarize and quantify this implicit knowledge. This
leads to a practical protocol that allows any scientist - experimental or
theoretical - to determine justifiable error estimates for many basic property
predictions, without having to perform additional DFT calculations. A central
role is played by a large and diverse test set of crystalline solids,
containing all ground-state elemental crystals (except most lanthanides). For
several properties of each crystal, the difference between DFT results and
experimental values is assessed. We discuss trends in these deviations and
review explanations suggested in the literature. A prerequisite for such an
error analysis is that different implementations of the same first-principles
formalism provide the same predictions. Therefore, the reproducibility of
predictions across several mainstream methods and codes is discussed too. A
quality factor Delta expresses the spread in predictions from two distinct DFT
implementations by a single number. To compare the PAW method to the highly
accurate APW+lo approach, a code assessment of VASP and GPAW with respect to
WIEN2k yields Delta values of 1.9 and 3.3 meV/atom, respectively. These
differences are an order of magnitude smaller than the typical difference with
experiment, and therefore predictions by APW+lo and PAW are for practical
purposes identical.Comment: 27 pages, 20 figures, supplementary material available (v5 contains
updated supplementary material
- …