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11.
12.
Foo K.S. Sudarshan T.S. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1989,17(4):588-594
Two main issues are discussed: (i) the study of arc initiation and propagation in a compressed nitrogen gas spark-gap, initiated by a low-energy (90 μJ) pulsed N2 laser (600 ps FWHM) and (ii) the modification of the discharge characteristics due to the presence of a solid insulating space made of Plexiglas (PMMA). The studies are carried out over a pressure range of 0.1 to 0.4 MPa, and for space diameters of 1, 4 and 6 cm, in a planar uniform field gap with 1-cm spacing. Coordinated measurements of discharge current and luminosity with nanosecond response times provide data on discharge current and luminosity risetimes, delay times to the onset of discharge from the instant of laser initiation, and the threshold (minimum) voltage for the onset of discharge due to laser initiation. The results point to some basic processes involving the interaction of the insular surface with : (a) the electron avalanches in the formative stage, and (b) the streamer in the discharge growth phase leading to full conduction. The results demonstrate that the insulator surface impedes the development of both the avalanche and streamer phases of the discharge development 相似文献
13.
Enantioselective Synthesis of Quaternary Carbon Stereocenters: Addition of 3‐Substituted Oxindoles to Vinyl Sulfone Catalyzed by Pentanidiums
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Dr. Lili Zong Shubo Du Kek Foo Chin Dr. Chao Wang Prof. Dr. Choon‐Hong Tan 《Angewandte Chemie (International ed. in English)》2015,54(32):9390-9393
A pentanidium‐catalyzed highly enantioselective conjugate addition of 3‐alkyloxindoles to phenyl vinyl sulfone has been demonstrated. This approach allows the construction of 3,3‐dialkyl‐substituted oxindole frameworks with high yield and excellent enantioselectivity (up to 99 %) under simple phase‐transfer conditions. A variety of oxindoles bearing all‐carbon quaternary stereogenic centers were obtained in the presence of 0.25 mol % pentanidium. Meanwhile, practicality was illustrated by a gram‐scale asymmetric synthesis of two 3,3‐dialkyl‐substituted oxindoles. The resulting adduct can be smoothly transformed to the natural product analogue in a short synthetic route. 相似文献
14.
C. Wong Po Foo E. Bini J. Huang S.Y. Lee D.L. Kaplan 《Applied Physics A: Materials Science & Processing》2006,82(2):193-203
Spider dragline silk from Nephila clavipes possesses impressive mechanical properties derived in part from repetitive primary sequence containing polyalanine regions
that self-assemble into crystalline β-sheets. In the present study, we have sought to understand more details of redox responses
related to conformational transitions of modified silk peptides and a recombinant protein containing encoded methionine triggers.
Regardless of the position of the methionine trigger relative to the polyalanine domain, chemical oxidation was rapid and
slight increases in the α-helical structure and decreases in the β-sheet and random coil content were observed by CD and FTIR
in the assembled silk-like peptides and the recombinant protein. CD results indicated that the decrease in β-sheet and random
coil conformations, coupled with the increase in helical content during oxidation, occurred during the first 30 min of the
reaction. No further conformational changes occurred after this time and the response was independent of methionine trigger
location relative to the penta-alanine domain. These results were confirmed with fluorescence studies. The design, processing
and utility of these modified redox triggered silk-like peptides and proteins suggest a range of potential utility, from biomaterials
to engineered surface coatings with chemically alterable secondary structure and, thus, properties.
PACS 87.14.Ee; 87.64.-t; 87.6.+2. 相似文献
15.
The finite strip method is applied to the vibration analysis of rectangular plates subjected to in-plane forces. Several numerical examples are presented and comparison with available solutions clearly indicates the accuracy and efficiency of the method. 相似文献
16.
Cleaning the surfaces of the as-deposited Ge2Sb2Te5 was studied by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and X-ray diffraction (XRD). The mixed native oxides on the as-deposited Ge2Sb2Te5 surface can be easily removed by dipping Ge2Sb2Te5 in de-ionized water for 1 min, while the surface morphology remains unchanged after cleaning. Native oxides only re-grow after exposure to air for more than 4 min. Although dipping in water leads to a surface layer deficient in Ge and Sb, the surface composition of Ge2Sb2Te5 can recover to its stoichiometric value after annealing at 200 °C in vacuum. The phase remains amorphous at room temperature after dipping in water, and changes to fcc and hcp after annealing at 100 and 220 °C, respectively. 相似文献
17.
V. Yamakov A. Milchev G.M. Foo R.B. Pandey D. Stauffer 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(4):659-667
A coarse-grained off-lattice bead-spring model is used to reveal the complex dynamics of a polymer chain in a quenched porous
medium in the presence of an external field B. The behavior of the mean square displacement (MSD) of the center chain bead and that of the center of mass of the chain
as a function of time is studied at different values of the barrier concentration C, the field strength B and the chain length N. In a field, important information on the way in which chains move between obstacles and overcome them is gained from the
MSD vs. time analysis in the directions parallel and perpendicular to the flow. Instead of a steady approach to uniform drift-like
motion at low C, for sufficiently strong field B we observe logarithmic oscillations in the effective exponents describing the time dependence of the MSD along and perpendicular to field. A common nature of
this phenomenon with oscillatory behavior, observed earlier for biased diffusion of tracers on random lattices, is suggested.
Received 7 August 1998 相似文献
18.
Effects of a flow field (E) on segregation and flow of polymer chains are studied in two dimensions using a hybrid (discrete‐to‐continuum) simulation. The flow rate (j) of polymer chains is found to increase monotonically with E, a linear response in the low field regime followed by a slow approach to saturation in the high field regime. The effective chain permeability (ϕc = j/E) varies nonmonotonically on increasing the field E, with a maximum (ϕcm) at a characteristic value of the field (in the range 0.2 < E < 2); ϕcm depends on the chain length. Chain aggregates exhibit an anisotropic mass distribution due to the field with a molecular bridging at high fields. The longitudinal component of the radius of gyration (Rgx) exhibits a crossover from a random walk (RW) (Rgx ˜ Lc1/2) at E = 0 to an elongated conformation (Rgx ˜Lc) at E ⪈ 0.2; the transverse component changes from Rgy ˜ Lc1/2 to Rgy ˜ Lc1/3. The width of the radial distribution function (ρ(r)) of the monomers increases while its peak varies nonmonotonically with E and is consistent with the observation of anisotropic mass distribution. 相似文献
19.
The synthesis and characterization of a series of heterocyclic liquid crystal, 4-{[(thiophen-2-yl)methylidene]amino}phenyl 4-alkoxybenzoates possessing even number of carbon atoms at the alkoxy chain (CnH2n+1O-, n = 6, 8, 10, 12, 14, 16, 18) are reported. The molecular structures of title compounds were elucidated using Fourier-transform infrared spectroscopy (FTIR) and Nuclear Magnetic Resonance (NMR) spectroscopic techniques along with mass spectrometric analysis. The phase behavior of these compounds was characterized and studied by differential scanning calorimetry and polarizing optical microscopy. All members exhibited enantiotropic nematic phase except for the highest member (n = 18) which is a non-mesogen. Influence of alkoxy chain length on the transition temperatures of crystal-to-nematic (melting point) and nematic-to-isotropic (clearing point) was studied. Nematic phase range was found to increase from n = 6 to n = 10, then it started to descend from n = 12 to n = 16 and finally the nematic phase disappeared when n changed to 18. 相似文献
20.