115 research outputs found
Phase composition, structure and properties of the spark plasma sintered ceramics obtained from the Al12Mg17-B-Si powder mixtures
In this work, composite materials were obtained by spark plasma sintering of an Al12Mg17-B-Si powder mixture. The structure, phase composition, and mechanical properties of the obtained composites were studied. It was found that various compounds based on B12 icosahedrons, such as AlB12, B4Si, and B6Si, are formed during spark plasma sintering. Based on the SEM images and results of XRD analysis of the obtained specimens, a probable scheme for the formation of the phase composition of composite materials during spark plasma sintering was proposed. An increase in the Al12Mg17-B powder content in the initial mixture from 30 to 70 wt% leads to an increase in hardness from 16.55 to 21.24 GPa and a decrease in the friction coefficient and wear rate from 0.56 to 0.32 and 13.60 to 5.60 10â5 mmâ3/(N/m), respectively
Standoff spectroscopy via remote generation of a backward-propagating laser beam
In an earlier publication we demonstrated that by using pairs of pulses of different colors (e.g., red and blue) it is possible to excite a dilute ensemble of molecules such that lasing and/or gain-swept superradiance is realized in a direction toward the observer. This approach is a conceptual step toward spectroscopic probing at a distance, also known as standoff spectroscopy. In the present paper, we propose a related but simpler approach on the basis of the backward-directed lasing in optically excited dominant constituents of plain air, N(2) and O(2). This technique relies on the remote generation of a weakly ionized plasma channel through filamentation of an ultraintense femtosecond laser pulse. Subsequent application of an energetic nanosecond pulse or series of pulses boosts the plasma density in the seed channel via avalanche ionization. Depending on the spectral and temporal content of the driving pulses, a transient population inversion is established in either nitrogen- or oxygen-ionized molecules, thus enabling a transient gain for an optical field propagating toward the observer. This technique results in the generation of a strong, coherent, counterpropagating optical probe pulse. Such a probe, combined with a wavelength-tunable laser signal(s) propagating in the forward direction, provides a tool for various remote-sensing applications. The proposed technique can be enhanced by combining it with the gain-swept excitation approach as well as with beam shaping and adaptive optics techniques
Hemilability of phosphine-thioether ligands coordinated to trinuclear Mo3S4 cluster and its effect on hydrogenation catalysis
Ligand-exchange reactions of [Mo3S4(tu)8(H2O)]Cl44H2O (tu = thiourea) with
(PhCH2CH2)2PCH2CH2SR ligands, where R = Ph (PS1), pentyl (PS2) or Pr (PS3) afford new
complexes isolated as [Mo3S4Cl3(PS1)3]PF6 ([1]PF6), [Mo3S4Cl3(PS2)3]PF6 ([2]PF6) and
[Mo3S4Cl3(PS3)3]PF6 ([3]PF6) salts in 30-50% yields as the major reaction products. The crystal
structures of [1]PF6 and [2]PF6 were determined by X-ray diffraction (XRD) analysis. Each of
the three phosphine-thioether ligands is coordinated in a bidentate chelating mode to a different
molybdenum atom of the Mo3S4 trinuclear cluster, herewith all the phosphorus atoms of the
phosphino-thioether ligand are located trans to the capping sulfur (3-S). A second product that
forms in the reaction of [Mo3S4(tu)8(H2O)]Cl44H2O with PS1 corresponds to the neutral
[Mo3S4Cl4(PS1)2(PS1*)] complex. Its XRD analysis reveals both bidentate (PS1) and
monodentate (PS1*) coordinating modes of the same ligand. In the latter mode the phosphinethioether
is coordinated to a Mo atom only via the P atom. All compounds were characterized by
1H, 31P{1H} NMR, electrospray-ionization (ESI) mass spectrometry and cyclic voltammetry
(CV). Reactions of [1]PF6, [2]PF6 and [3]PF6 with an excess of Bu4NCl in CD2Cl2 were followed
by 31P{1H} NMR. The spectra indicate equilibrium between cationic [Mo3S4Cl3(PSn)3]
+
and
neutral [Mo3S4Cl4(PSn)2(PSn*)] (n = 1, 2) species. The equilibrium constants were determined
as 2.5 ± 0.2103
, 43 ± 2 Đ
-1
and 30 ± 2 Đ
-1
(at 25°C) for [1]PF6, [2]PF6 and [3]PF6, indicating
quantitative differences in hemilabile behavior of the phosphino-thioether ligands, depending on
the substituent at sulfur. Clusters [1]PF6, [2]PF6 and [3]PF6 were tested as catalysts in reduction
of nitrobenzene to aniline with Ph2SiH2 under mild conditions. Significant differencies in the
catalytic activity were observed, which can be attributed to different hemilabile behavior of the
PS1 and PS2/PS3 ligands
Continuous Symmetries of Difference Equations
Lie group theory was originally created more than 100 years ago as a tool for
solving ordinary and partial differential equations. In this article we review
the results of a much more recent program: the use of Lie groups to study
difference equations. We show that the mismatch between continuous symmetries
and discrete equations can be resolved in at least two manners. One is to use
generalized symmetries acting on solutions of difference equations, but leaving
the lattice invariant. The other is to restrict to point symmetries, but to
allow them to also transform the lattice.Comment: Review articl
Search for charginos in e+e- interactions at sqrt(s) = 189 GeV
An update of the searches for charginos and gravitinos is presented, based on
a data sample corresponding to the 158 pb^{-1} recorded by the DELPHI detector
in 1998, at a centre-of-mass energy of 189 GeV. No evidence for a signal was
found. The lower mass limits are 4-5 GeV/c^2 higher than those obtained at a
centre-of-mass energy of 183 GeV. The (\mu,M_2) MSSM domain excluded by
combining the chargino searches with neutralino searches at the Z resonance
implies a limit on the mass of the lightest neutralino which, for a heavy
sneutrino, is constrained to be above 31.0 GeV/c^2 for tan(beta) \geq 1.Comment: 22 pages, 8 figure
Measurement of the gluon fragmentation function and a comparison of the scaling violation in gluon and quarks jets
The fragmentation functions of quarks and gluons are measured in various three-jet topologies in Z decays from the full data set collected with the Delphi detector at the Z resonance between 1992 and 1995. The results at different values of transverse momentum-like scales are compared. A parameterization of the quark and gluon fragmentation functions at a fixed reference scale is given. The quark and gluon fragmentation functions show the predicted pattern of scaling violations. The scaling violation for quark jets as a function of a transverse momentum-like scale is in a good agreement with that observed in lower energy e+eâ annihilation experiments. For gluon jets it appears to be significantly stronger. The scale dependences of the gluon and quark fragmentation functions agree with the prediction of the DGLAP evolution equations from which the colour factor ratio CA/CF is measured to be: CACF=2.26±0.09stat.±0.06sys.±0.12clus.,scale
The Physics of the B Factories
This work is on the Physics of the B Factories. Part A of this book contains a brief description of the SLAC and KEK B Factories as well as their detectors, BaBar and Belle, and data taking related issues. Part B discusses tools and methods used by the experiments in order to obtain results. The results themselves can be found in Part C
Study of B0_s anti-B0_s oscillations and B0_s lifetimes using hadronic decays of B0_s mesons
Oscillations of B0s mesons have been studied in samples selected from about
3.5 million hadronic Z decays detected by DELPHI between 1992 and 1995. One
analysis uses events in the exclusive decay channels: B0s -> Ds- pi+ or Ds- a1+
and B0s -> anti-D0 K- pi+ or anti-D0 K- a1+, where the D decays are completely
reconstructed. In addition, B0s anti-B0s oscillations have been studied in
events with an exclusively reconstructed Ds accompanied in the same hemisphere
by a high momentum hadron of opposite charge. Combining the two analyses, a
limit on the mass difference between the physical B0s states has been obtained:
Delta(m_B0s) > 4.0 ps^{-1} at the 95% C.L. with a sensitivity of Delta(m_B0s)
= 3.2 ps^{-1}. Using the latter sample of events, the B0s lifetime has been
measured and an upper limit on the decay width difference between the two
physical B0s states has been obtained:
tau(B0s) = 1.53^{+0.16}_{-0.15}(stat.) +/- {0.07}(syst.) ps
\Delta\Gamma(B0s)/\Gamma(B0s) < 0.69 at the 95% C.L.
The combination of these results with those obtained using Ds+- lepton-+
sample gives:
Delta(m_B0s) > 4.9 ps^{-1} at the 95% C.L.
with a sensitivity of Delta(m_B0s) = 8.7 ps^{-1}.
tau(B0s) = 1.46 +/- 0.11 ps and \Delta\Gamma(B0s)/\Gamma(B0s) < 0.45 at the
95% C.L.Comment: 42 pages, 13 figure
Biological Earth observation with animal sensors
Space-based tracking technology using low-cost miniature tags is now delivering data on fine-scale animal movement at near-global scale. Linked with remotely sensed environmental data, this offers a biological lens on habitat integrity and connectivity for conservation and human health; a global network of animal sentinels of environmen-tal change
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