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981.
Thongpanchang T Paruch K Katz TJ Rheingold AL Lam KC Liable-Sands L 《The Journal of organic chemistry》2000,65(6):1850-1856
The questions considered in this paper are why, as agents for resolving helicenols, camphanate esters are particularly effective, and why, in all 19 examples studied, when the (1S)-camphanates of (P)- and (M)-helicen-1-ols are chromatographed on silica gel, the former has the lower R(f)(). Models are proposed for the favored conformations of the esters, and to support the models, evidence is provided from five X-ray diffraction analyses and four ROESY analyses supplemented by molecular mechanics calculations. The essential discovery is that, presumably to avoid a steric interaction between a methyl on the camphanate's bridge and the helicene skeleton, the O=CCO conformation is anti-periplanar in (M)-helicenol camphanates and syn-periplanar in (P)-helicenol camphanates. In the former, the lactone carbonyl points toward the helicene ring system, and in the latter, it points away. 相似文献
982.
Y. J. Wang C.-H. Lam X. Zhang O. K.C. Tsui 《The European physical journal. Special topics》2007,141(1):181-187
According to classical theory of phase transition,
fluctuations in systems with low dimensions are so violent that the
phase boundary between unstable and metastable states would be
smeared. In this experiment, we measure the growth of surface
fluctuations on an unstable polymer film with a thickness, h0 =
5.1 nm, which is much less than the spinodal thickness, h
sp (= 243 nm) thereby the film is in the very deep unstable
region. We find the film to show rupturing behavior markedly
different from that of an unstable film. Specifically, nucleation of
holes – a characteristic rupturing feature of metastable films –
is prominent, which is surprising for a film in the very deep
unstable region even provision is given to thin films being
two-dimensional and hence are susceptible to broadening of the phase
boundary by fluctuations. Monte Carlo simulation shows that the
nucleated holes can be caused by stochastic thermal fluctuations.
Our result thus confirms the broadening of the phase boundary in
thin films by fluctuations to be extremely large. As a consequence,
the phase behavior of thin films cannot be predicted by the
mean-field calculated phase boundary, which however has been the
general practice so far. 相似文献
983.
984.
Supported in part by NSERC of Canada 相似文献
985.
The assumption of jets with bounded pT leads to the following consequences: (a) In e+e? annihilation the coefficient of the cos2θ term (a) of single-particle inclusive distributions] exhibits a unique dependence on the observed particle momentum. This dependence provides a measure of 〈pT2〉 in a jet. (b) In two-particle inclusive annihilation the average of the squared relative pT is determined by the same parameter and by the momenta of the two particles. (c) The observation that in the current fragmentation region of leptoproduction the pT distribution of fast hadrons is broader than that of show ones is explained. 相似文献
986.
Photoneutron mean energies of 38 elements were measured as a function of peak bremsstrahlung energy for elements with 23 ≦ Z ≦ 83. Results are compared with neutron mean energies calculated from statistical theory, using for nuclear level densities modified Fermi gas formulae with and without pairing corrections and a constant temperature formula. Except near closed shells the Fermi gas formula with pairing corrections gives reasonable to good correlation between experimental and theoretical data. Derived values of the nuclear level density parameter a-except near Z = 82-are in quantitative agreement with those from recent neutron resonance data. 相似文献
987.
Bismuth sodium barium titanate/poly(vinylidene fluoride-trifluoroethylene) 70/30 [(Bi0.5Na0.5)0.94Ba0.06TiO3-P(VDF-TrFE)] 0-3 composites were prepared by a hot-press method for different volume fractions of (Bi0.5Na0.5)0.94Ba0.06TiO3 ceramic powder in a P(VDF-TrFE) 70/30 copolymer matrix. The relative permittivity and dielectric loss of the composites increased with increase in the volume fraction of the ceramics, which well follows the Bruggeman model. The polarization responses of the composites were strongly dependent on the ceramic volume fraction. The composites with a higher ceramic volume fraction showed an increase in remanent polarization. At room temperature, a 0.3(Bi0.5Na0.5)0.94Ba0.06TiO3-0.7P(VDF-TrFE) composite showed a relative permittivity εr=30, remanent polarization and coercive field 相似文献
988.
McKenzie K Grosse N Bowen WP Whitcomb SE Gray MB McClelland DE Lam PK 《Physical review letters》2004,93(16):161105
We demonstrate the generation of broadband continuous-wave optical squeezing from 280 Hz-100 kHz using a below-threshold optical parametric oscillator (OPO). The squeezed state phase was controlled using a noise locking technique. We show that low frequency noise sources, such as seed noise, pump noise, and detuning fluctuations, present in optical parametric amplifiers, have negligible effect on squeezing produced by a below-threshold OPO. This low frequency squeezing is ideal for improving the sensitivity of audio frequency measuring devices such as gravitational-wave detectors. 相似文献
989.
H.M.?Lam M.H.?HongEmail author S.?Yuan T.C.?Chong 《Applied Physics A: Materials Science & Processing》2004,79(8):2099-2102
This work investigates pulsed laser ablation for Ga2O3 nanoparticles. Nanoparticles with diameters of 10 to 500 nm were deposited on silicon substrates in large quantities, by KrF excimer laser ablation of a GaN (99.99% purity) target in high purity nitrogen (99.9995%) background gas at room temperature, without a catalyst. The particle size and phase structure of the as-deposited nanoparticles are examined by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), and selected-area electron diffraction (SAD). FE-SEM images show that the nanoparticles aggregate to form micron-size nanoclusters at chamber pressures of 1 and 5 Torr. On the other hand, nanoparticles aggregate with chain-like nanostructures, are synthesized at high chamber pressures (10 Torr). TEM images further reveal that chain-like nanostructures are formed by the aggregation of individual spherical and ellipsoidal nanoparticles. Photoluminescence measurement shows stable and broad blue emission at 445 nm. PACS 81.20.-N; 81.15.Fg; 75.50.Vv 相似文献
990.
Do Van Lam Van-Toan Nguyen Euijin Roh Quang-Tung Ngo Wanuk Choi Jae-Hyun Kim Hyunuk Kim Ho-Suk Choi Seung-Mo Lee 《Particle & Particle Systems Characterization》2021,38(9):2100119
Metal nanoparticles encapsulated in graphitic carbon can show high catalytic efficiency and stability, yet the production method remains improved. Herein, it is demonstrated that a Ni-based metal–organic framework [EG-MOF-74(Ni)] can be rapidly transformed into ultrasmall Ni-nanoparticles with different sizes (4–11 nm) encapsulated in graphitic carbon via the laser-scribing method. The synthesized sample shows the best electrocatalytic performances with excellent stability in alkaline electrolyte for oxygen/hydrogen evolution reactions with overpotentials of 0.35/0.18 V at a current density of 10 mA cm−2 when the Ni particle size is ≈6 nm. This is because of its well-developed micro/mesoporous structure, high electronic transport, and large electrochemical active surface area. An electrolyzer with Ni-nanoparticles encapsulated in the graphitic carbon shows a current density of 10 mA cm−2 at a voltage of 1.6 V, which is comparable to the Pt/C and RuO2 counterparts. The laser-based synthesis can serve as a powerful tool for the size-controlled synthesis of various catalysts out of MOFs. 相似文献