151,617 research outputs found
Long-term in situ observations on typhoon-triggered turbidity currents in the deep sea
This work is supported by the National Science Foundation of China (grants 91528304, 41576005, and 41530964). We thank J. Li, X. Lyu, P. Li, K. Duan, J. Ronan, Y. Wang, P. Ma, and Y. Li for cruise assistance; G. de Lange and J. Hinojosa for editing an early version of manuscript; and E. Pope and two anonymous reviewers for their reviews.Peer reviewedPublisher PD
Implementing liquid lubrication in micro-electro-mechanical systems
Even though there are many types of micro-electro-mechanical devices (MEMS), few commercialized MEMS devices can achieve functions requiring rotation. This is a result of a lack of methods providing efficient lubrication for the micro sliding contacts. Liquid hydrodynamic bearings which are commonly used in the macro sliding contacts could still be efficient in micro sliding contacts if appropriate bearing design and lubricants are applied. This thesis reports on the research focused on miniaturizing liquid hydrodynamic bearings thereby improving the performance of the micro sliding contacts in MEMS devices.
A finite volume model considering mass-conservation and cavitation has been developed to investigate the performance of the micro pocketed parallel thrust bearings. Simulation results show that the micro thrust bearing performance is largely affected by cavitation. It also shows that micro pocketed parallel thrust bearings with appropriate pocket shapes can generate sufficient load support and operate in the low friction coefficient full-film lubrication regime when low viscosity lubricant is supplied (e.g. hexadecane).
Micro fabrication process has been developed to manufacture the micro pocketed parallel thrust bearings. The manufactured micro bearings have been examined both on a MEMS tribometer and on a prototype micro energy harvesting turbine. Test results show that low friction coefficient, torque and power loss can be achieved in a wide rotational speed range from the micro bearings with proper design and lubricated by low viscosity and low surface tension liquid (e.g. hexadecane).
Effects of friction modifiers and partial surface textures on the micro liquid hydrodynamic bearings performance have also been investigated. Results indicate that both methods are still efficient in friction reduction.
As a conclusion, miniaturized liquid hydrodynamic bearings with appropriate bearing design and lubricants are efficient to achieve low friction coefficient and wear rate for the micro sliding contacts in a wide rotational speed range.Open Acces
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Observation of the and the Y(2175) in
The cross sections for and e^+ e^- \to \phi
\fzero are measured from threshold to using
initial state radiation. The analysis is based on a data sample of 673
fb collected on and below the resonance with the Belle
detector at the KEKB asymmetric-energy collider. First measurements
are reported for the resonance parameters of the in the
mode: MeV/ and MeV/. A structure at ,
corresponding to the so called Y(2175), is observed; its mass and width are
determined to be MeV/ and , respectively.Comment: 11 pages, 6 figures. Add one plot. Accepted by Phys.Rev.D(RC
Fermi Large Area Telescope observations of the Vela-X Pulsar Wind Nebula
We report on gamma-ray observations in the off-pulse window of the Vela
pulsar PSR B0833-45, using 11 months of survey data from the Fermi Large Area
Telescope (LAT). This pulsar is located in the 8 degree diameter Vela supernova
remnant, which contains several regions of non-thermal emission detected in the
radio, X-ray and gamma-ray bands. The gamma-ray emission detected by the LAT
lies within one of these regions, the 2*3 degrees area south of the pulsar
known as Vela-X. The LAT flux is signicantly spatially extended with a best-fit
radius of 0.88 +/- 0.12 degrees for an assumed radially symmetric uniform disk.
The 200 MeV to 20 GeV LAT spectrum of this source is well described by a
power-law with a spectral index of 2.41 +/- 0.09 +/- 0.15 and integral flux
above 100 MeV of (4.73 +/- 0.63 +/- 1.32) * 10^{-7} cm^{-2} s^{-1}. The first
errors represent the statistical error on the fit parameters, while the second
ones are the systematic uncertainties. Detailed morphological and spectral
analyses give strong constraints on the energetics and magnetic field of the
pulsar wind nebula (PWN) system and favor a scenario with two distinct electron
populations.Comment: 21 pages, 5 figures, accepted for publication in Astrophysical
Journa
Measurement of the B0-anti-B0-Oscillation Frequency with Inclusive Dilepton Events
The - oscillation frequency has been measured with a sample of
23 million \B\bar B pairs collected with the BABAR detector at the PEP-II
asymmetric B Factory at SLAC. In this sample, we select events in which both B
mesons decay semileptonically and use the charge of the leptons to identify the
flavor of each B meson. A simultaneous fit to the decay time difference
distributions for opposite- and same-sign dilepton events gives ps.Comment: 7 pages, 1 figure, submitted to Physical Review Letter
Discovery of very high energy gamma rays from PKS 1424+240 and multiwavelength constraints on its redshift
We report the first detection of very-high-energy (VHE) gamma-ray emission
above 140 GeV from PKS 1424+240, a BL Lac object with an unknown redshift. The
photon spectrum above 140 GeV measured by VERITAS is well described by a power
law with a photon index of 3.8 +- 0.5_stat +- 0.3_syst and a flux normalization
at 200 GeV of (5.1 +- 0.9_stat +- 0.5_syst) x 10^{-11} TeV^-1 cm^-2 s^-1, where
stat and syst denote the statistical and systematical uncertainty,
respectively. The VHE flux is steady over the observation period between MJD
54881 and 55003 (2009 February 19 to June 21). Flux variability is also not
observed in contemporaneous high energy observations with the Fermi Large Area
Telescope (LAT). Contemporaneous X-ray and optical data were also obtained from
the Swift XRT and MDM observatory, respectively. The broadband spectral energy
distribution (SED) is well described by a one-zone synchrotron self-Compton
(SSC) model favoring a redshift of less than 0.1. Using the photon index
measured with Fermi in combination with recent extragalactic background light
(EBL) absorption models it can be concluded from the VERITAS data that the
redshift of PKS 1424+240 is less than 0.66.Comment: accepted for publication, Ap
Fermi Large Area Telescope Observations of the Crab Pulsar and Nebula
We report on gamma-ray observations of the Crab Pulsar and Nebula using 8
months of survey data with the Fermi Large Area Telescope (LAT). The high
quality light curve obtained using the ephemeris provided by the Nancay and
Jodrell Bank radio telescopes shows two main peaks stable in phase with energy.
The first gamma-ray peak leads the radio main pulse by (281 \pm 12 \pm 21) mus,
giving new constraints on the production site of non-thermal emission in pulsar
magnetospheres. The improved sensitivity and the unprecedented statistics
afforded by the LAT enable precise measurement of the Crab Pulsar spectral
parameters: cut-off energy at E_c = (5.8 \pm 0.5 \pm 1.2) GeV, spectral index
of Gamma = (1.97 \pm 0.02 \pm 0.06) and integral photon flux above 100 MeV of
(2.09 \pm 0.03 \pm 0.18) x 10^{-6} cm^{-2} s^{-1}. The first errors represent
the statistical error on the fit parameters, while the second ones are the
systematic uncertainties. Pulsed gamma-ray photons are observed up to ~ 20 GeV
which precludes emission near the stellar surface, below altitudes of around 4
to 5 stellar radii in phase intervals encompassing the two main peaks. The
spectrum of the nebula in the energy range 100 MeV - 300 GeV is well described
by the sum of two power-laws of indices Gamma_{sync} = (3.99 \pm 0.12 \pm 0.08)
and Gamma_{IC} = (1.64 \pm 0.05 \pm 0.07), corresponding to the falling edge of
the synchrotron and the rising edge of the inverse Compton components,
respectively. This latter, which links up naturally with the spectral data
points of Cherenkov experiments, is well reproduced via inverse Compton
scattering from standard Magnetohydrodynamics (MHD) nebula models, and does not
require any additional radiation mechanism.Comment: 17 pages, 9 figures, Accepted for publications in Astrophysical
Journa
Search for chargino-neutralino production with mass splittings near the electroweak scale in three-lepton final states in √s=13 TeV pp collisions with the ATLAS detector
A search for supersymmetry through the pair production of electroweakinos with mass splittings near the electroweak scale and decaying via on-shell W and Z bosons is presented for a three-lepton final state. The analyzed proton-proton collision data taken at a center-of-mass energy of √s=13 TeV were collected between 2015 and 2018 by the ATLAS experiment at the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb−1. A search, emulating the recursive jigsaw reconstruction technique with easily reproducible laboratory-frame variables, is performed. The two excesses observed in the 2015–2016 data recursive jigsaw analysis in the low-mass three-lepton phase space are reproduced. Results with the full data set are in agreement with the Standard Model expectations. They are interpreted to set exclusion limits at the 95% confidence level on simplified models of chargino-neutralino pair production for masses up to 345 GeV
Search for direct stau production in events with two hadronic tau-leptons in root s=13 TeV pp collisions with the ATLAS detector
A search for the direct production of the supersymmetric partners ofτ-leptons (staus) in final stateswith two hadronically decayingτ-leptons is presented. The analysis uses a dataset of pp collisions corresponding to an integrated luminosity of139fb−1, recorded with the ATLAS detector at the LargeHadron Collider at a center-of-mass energy of 13 TeV. No significant deviation from the expected StandardModel background is observed. Limits are derived in scenarios of direct production of stau pairs with eachstau decaying into the stable lightest neutralino and oneτ-lepton in simplified models where the two staumass eigenstates are degenerate. Stau masses from 120 GeV to 390 GeV are excluded at 95% confidencelevel for a massless lightest neutralino
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