41,452 research outputs found
The librational dynamics of satellites
Librational dynamics of satellite
Microstructure and mechanical properties of bulk yttria-partially-stabilized zirconia
A commercially available bulk 4.5 mole percent yttria-(Y2O3) partially stabilized zirconia (PSZ) was studied by light microscopy, X-ray analysis, microhardness measurement, and fracture toughness testing. The growth of the precipitates and the phase transformations were studied as a function of aging in air at 1500 C. Aging curves were constructed for both the as received and the solution annealed and quenched materials; the curves showed hardness peaks at 1397 and 1517 Kg/sq mm respectively. The rectangular plate shaped tetragonal precipitates were found to have a 110 habit plane. A total of twelve different types of tetragonal precipitates were found. Grinding of the Y2O3 PSZ into powder did not cause a significant amount of metastable tetragonal precipitates to transform into the monoclinc phase, thus indicating that transformation toughening is not a significant mechanism for the material
Radiation-Driven Warping: The Origin of Warps and Precession in Accretion Disks
A geometrically thin, optically thick, warped accretion disk with a central
source of luminosity is subject to non-axisymmetric forces due to radiation
pressure; the resulting torque acts to modify the warp. In a recent paper,
\cite{pri96} used a local analysis to show that initially planar accretion
disks are unstable to warping driven by radiation torque. Here we extend this
work with a global analysis of the stable and unstable modes. We confirm
Pringle's conclusion that thin centrally-illuminated accretion disks are
generically unstable to warping via this mechanism; we discuss the
time-evolution and likely steady-state of such systems and show specifically
that this mechanism can explain the warping of the disk of water masers in NGC
4258 and the 164-day precession period of the accretion disk in SS 433.
Radiation-driven warping and precession provides a robust mechanism for
producing warped, precessing accretion disks in active galactic nuclei and
X-ray binary systems.Comment: 16 pages, latex, 3 figure
Orbital correlations in the pseudo-cubic \emph{O} and rhombohedral -phases of LaMnO
The local and intermediate structure of stoichiometric LaMnO has been
studied in the pseudocubic and rhombohedral phases at high temperatures (300 to
1150 K). Neutron powder diffraction data were collected and a combined Rietveld
and high real space resolution atomic pair distribution function analysis
carried out. The nature of the Jahn-Teller (JT) transition around 750 K is
confirmed to be orbital order to disorder. In the high temperature orthorhombic
() and rhombohedral () phases the MnO octahedra are still fully
distorted locally. The data suggest the presence of local orbitally ordered
clusters of diameter \AA (four MnO octahedra) implying
strong nearest neighbor JT anti-ferrodistortive coupling.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Le
Acoustic characterization of crack damage evolution in sandstone deformed under conventional and true triaxial loading
We thank the Associate Editor, Michelle Cooke, and the reviewers, Ze'ev Reches and Yves Guéguen, for useful comments which helped to improve the manuscript. We thank J.G. Van Munster for providing access to the true triaxial apparatus at KSEPL and for technical support during the experimental program. We thank R. Pricci for assistance with technical drawings of the apparatus. This work was partly funded by NERC award NE/N002938/1 and by a NERC Doctoral Studentship, which we gratefully acknowledge. Supporting data are included in a supporting information file; any additional data may be obtained from J.B. (e-mail: [email protected]).Peer reviewedPublisher PD
Growing Pains or Opportunities? A Customer Survey of Three Farmers\u27 Markets in One Rural Community
The continued growth of farmers\u27 markets is presenting new challenges to Extension. As the number of markets expands, how can Extension help those in the same community work together for mutual benefit? The study reported here examined similarities and differences among customers attending three different farmers\u27 markets within a single locality in Gettysburg, Pennsylvania. Based on 370 customer surveys, study results underscore the diversity of markets operating within the same community and provide insights into ways Extension might assist markets to work together to expand their shared customer base, increase revenues, and better serve local residents
Emerging applications of integrated optical microcombs for analogue RF and microwave photonic signal processing
We review new applications of integrated microcombs in RF and microwave
photonic systems. We demonstrate a wide range of powerful functions including a
photonic intensity high order and fractional differentiators, optical true time
delays, advanced filters, RF channelizer and other functions, based on a Kerr
optical comb generated by a compact integrated microring resonator, or
microcomb. The microcomb is CMOS compatible and contains a large number of comb
lines, which can serve as a high performance multiwavelength source for the
transversal filter, thus greatly reduce the cost, size, and complexity of the
system. The operation principle of these functions is theoretically analyzed,
and experimental demonstrations are presented.Comment: 16 pages, 8 figures, 136 References. Photonics West 2018 invited
paper, expanded version. arXiv admin note: substantial text overlap with
arXiv:1710.00678, arXiv:1710.0861
High performance photonic microwave filters based on a 50GHz optical soliton crystal Kerr micro-comb
We demonstrate a photonic radio frequency (RF) transversal filter based on an
integrated optical micro-comb source featuring a record low free spectral range
of 49 GHz yielding 80 micro-comb lines across the C-band. This record-high
number of taps, or wavelengths for the transversal filter results in
significantly increased performance including a QRF factor more than four times
higher than previous results. Further, by employing both positive and negative
taps, an improved out-of-band rejection of up to 48.9 dB is demonstrated using
Gaussian apodization, together with a tunable centre frequency covering the RF
spectra range, with a widely tunable 3-dB bandwidth and versatile dynamically
adjustable filter shapes. Our experimental results match well with theory,
showing that our transversal filter is a competitive solution to implement
advanced adaptive RF filters with broad operational bandwidths, high frequency
selectivity, high reconfigurability, and potentially reduced cost and
footprint. This approach is promising for applications in modern radar and
communications systems.Comment: 19 pages, 12 figures, 107 reference
First principles based atomistic modeling of phase stability in PMN-xPT
We have performed molecular dynamics simulations using a shell model
potential developed by fitting first principles results to describe the
behavior of the relaxor-ferroelectric (1-x)PbMg1/3Nb2/3O3-xPbTiO3 (PMN-xPT) as
function of concentration and temperature, using site occupancies within the
random site model. In our simulations, PMN is cubic at all temperatures and
behaves as a polar glass. As a small amount of Ti is added, a weak polar state
develops, but structural disorder dominates, and the symmetry is rhombohedral.
As more Ti is added the ground state is clearly polar and the system is
ferroelectric, but with easy rotation of the polarization direction. In the
high Ti content region, the solid solution adopts ferroelectric behavior
similar to PT, with tetragonal symmetry. The ground state sequence with
increasing Ti content is R-MB-O-MC-T. The high temperature phase is cubic at
all compositions. Our simulations give the slope of the morphotropic phase
boundaries, crucial for high temperature applications. We find that the phase
diagram PMN-xPT can be understood within the random site model.Comment: 27 pages, 9 figure
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