3,447 research outputs found
Trajectory Optimization for Spacecraft Collision Avoidance
The last several decades have resulted in an unfortunate byproduct of space exploration and development: orbital debris. Satellites in Low Earth Orbit have been required to make an ever increasing number of course corrections in order to avoid collisions. Despite efforts to the contrary, collisions continue to occur, each time creating additional debris and increasing the requirement for the remaining satellites to maneuver. Every required maneuver decreases a satellite\u27s service life. The purpose of this study is to develop a minimum thrust profile to maneuver an orbiting satellite out of its projected error ellipse before a collision occurs. For comparison, both the impulsive and continuous thrust cases were considered as well as in-plane versus out-of plane maneuvering. This study made use of the Radau Pseudospectral Method to develop this minimum thrust profile. This method was run in MATLAB® using General Pseudospectral Optimal Control Software (GPOPS-II). Once the optimal solution was obtained, Systems Tool Kit® was used to simulate the resulting calculated trajectories and confirm avoidance of the error ellipse
Magnetic excitations in weakly coupled spin dimers and chains material Cu2Fe2Ge4O13
Magnetic excitations in a weakly coupled spin dimers and chains compound
Cu2Fe2Ge4O13 are measured by inelastic neutron scattering. Both structure
factors and dispersion of low energy excitations up to 10 meV energy transfer
are well described by a semiclassical spin wave theory involving interacting
Fe () chains. Additional dispersionless excitations are
observed at higher energies, at meV, and associated with
singlet-triplet transitions within Cu-dimers. Both types of excitations
can be understood by treating weak interactions between the Cu and
Fe subsystems at the level of the Mean Field/ Random Phase
Approximation. However, this simple model fails to account for the measured
temperature dependence of the 24 meV mode.Comment: 9 pages, 11 figure
Superconductivity in the New Platinum Germanides MPt4Ge12 (M = Rare-earth and Alkaline-earth Metals) with Filled Skutterudite Structure
New germanium-platinum compounds with the filled-skutterudite crystal
structure were synthesized. The structure and composition were investigated by
X-ray diffraction and microprobe analysis. Magnetic susceptibility, specific
heat, and electrical resistivity measurements evidence superconductivity in
LaPt4Ge12 and PrPt4Ge12 below 8.3K. The parameters of the normal and
superconducting states were established. Strong coupling and a crystal electric
field singlet groundstate is found for the Pr compound. Electronic structure
calculations show a large density of states at the Fermi level. Similar
behavior with lower T_c was observed for SrPt4Ge12 and BaPt4Ge12.Comment: RevTeX, 4 figures, submitted to Physical Review Letters July 12, 200
Determination of polyphenols in wines by reaction with 4-aminoantipyrine and photometric flow-injection analysis
A new flow-injection analytical procedure is
proposed for the determination of the total amount of
polyphenols in wines; the method is based on the formation
of a colored complex between 4-aminoantipyrine and
phenols, in the presence of an oxidizing reagent. The oxidizing
agents hexacyanoferrate(III), peroxodisulfate, and
tetroxoiodate(VII) were tested.
Batch trials were first performed to select appropriate
oxidizing agents, pH, and concentration ratios of reagents,
on the basis of their effect on the stability of the colored
complex. Conditions selected as a result of these trials
were implemented in a flow-injection analytical system in
which the influence of injection volume, flow rate, and reaction-
coil length, was evaluated. Under the optimum conditions
the total amount of polyphenols, expressed as gallic
acid, could be determined within a concentration range of
36 to 544 mg L–1, and with a sensitivity of 344 L mol–1 cm–1
and an RSD <1.1%. The reproducibility of analytical
readings was indicative of standard deviations <2%. Interference
from sugars, tartaric acid, ascorbic acid, methanol,
ammonium sulfate, and potassium chloride was negligible.
The proposed system was applied to the determination
of total polyphenols in red wines, and enabled analysis of
approximately 55 samples h–1. Results were usually precise
and accurate; the RSD was <3.9% and relative errors,
by the Folin–Ciocalteu method, <5.1%
Redefining Technology in Development Work: A Need for Learning Outcomes in ICT Projects
Redefining Technology in Development Work:
A Need for Learning Outcomes in ICT Projects
Jayson W. Richardson
Gregory C. Sales
Technology has redefined, and will likely continue to redefine, educational processes and procedures around the world. In many development efforts, however, the focus is on the hardware, software, or professional development. Rare is the international development project focused on teaching and learning. In this special issue of the Forum for International Research in Education (FIRE), we propose a reframing of ICT by shifting the paradigm to focus on Instructional quality, Curriculum development, and Teaching pedagogy. Thinking of ICTs in this I-C-T context moves the discussion away from the technology itself, and spotlights changing and improving teaching and learning. In this special issue the Information and Communication Technology for Development Special Interest Group (ICT4D SIG) of the Comparative and International Education Society (CIES), solicited articles that focus on improving and measuring teaching and learning outcomes in technology-focused development projects
Low-lying optical phonon modes in the filled skutterudite CeRu4Sb12
The phonon dynamics of filled skutterudite CeRu4Sb12 have been studied at
room temperature by inelastic neutron scattering. Optical phonons associated
with a large vibration of Ce atoms are observed at a relatively low energy of E
= 6 meV, and show anticrossing behavior with acoustic phonons. We propose that
the origin of the low lattice thermal conductivity in filled skutterudites can
be attributed to intensive Umklapp scattering originating from low-lying
optical phonons. By an analysis based on a Born-von Karman force model, the
longitudinal force constants of the nearest Ce-Sb and Ce-Ru pairs are estimated
to be 0.025 mdyn/A, while that of the nearest Ru-Sb pair is estimated to be 1.4
mdyn/A, indicating that the Ce atoms are bound very weakly to the surrounding
rigid RuSb6-octahedron cages.Comment: 4pages, 5 figures, J. Phys. Soc. Jpn. (2006) in pres
Charge Order Superstructure with Integer Iron Valence in Fe2OBO3
Solution-grown single crystals of Fe2OBO3 were characterized by specific
heat, Mossbauer spectroscopy, and x-ray diffraction. A peak in the specific
heat at 340 K indicates the onset of charge order. Evidence for a doubling of
the unit cell at low temperature is presented. Combining structural refinement
of diffraction data and Mossbauer spectra, domains with diagonal charge order
are established. Bond-valence-sum analysis indicates integer valence states of
the Fe ions in the charge ordered phase, suggesting Fe2OBO3 is the clearest
example of ionic charge order so far.Comment: 4 pages, 5 figures. Fig. 3 is available in higher resolution from the
authors. PRL in prin
Incommensurate Charge Order Phase in Fe2OBO3 due to Geometrical Frustration
The temperature dependence of charge order in Fe2OBO3 was investigated by
resistivity and differential scanning calorimetry measurements, Mossbauer
spectroscopy, and synchrotron x-ray scattering, revealing an intermediate phase
between room temperature and 340 K, characterized by coexisting mobile and
immobile carriers, and by incommensurate superstructure modulations with
temperature-dependent propagation vector (1/2,0,tau). The incommensurate
modulations arise from specific anti-phase boundaries with low energy cost due
to geometrical charge frustration.Comment: 4 p., 5 fig.; v2: slightly expanded introduction + minor changes. PRL
in prin
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