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851.
An easy solvent-free method is described for the conversion of ketones into β-keto sulfones in high yields that involves the in situ generation of α-tosyloxyketones, followed by nucleophilic substitution with sodium arene sulfinate in the presence of tetra-butylammonium bromide at room temperature. The salient features of this one-pot protocol are short reaction times, cleaner reaction profiles, and simple work-up that precludes the use of toxic solvents. 相似文献
852.
H. Liu O. Vasquez V. R. Santiago L. Diaz F. E. Fernandez 《Journal of Electronic Materials》2004,33(10):1171-1175
VO2 thin films deposited on MgO and fused silica glass substrates were prepared by the pulsed laser deposition (PLD) technique,
which shows phase transition (PT) from the monoclinic semiconductor phase to a metallic tetragonal rutile structure at temperatures
over 68°C. The observed PT is reversible, showing a typical hysteresis. The PT can also be induced through optical pumping
by laser excitation. In this case, it was found that the optically induced PT is ultrafast and passive, but not thermally
initiated. In order to understand the PT mechanism, a study of transient holography using degenerate-four-wavemixing (DFWM)
measurement was conducted. A Nd:YAG pulsed laser with pulse duration of 30 psec operating at 532 nm was employed as the coherent
light source. This showed that the observed transient holography in VO2 thin film is associated with the excited state dynamical process, which essentially causes the structural change, or so-called
optically induced PT. The observed extremely large polarizability is believed to relate to the large offset in the potential
well minimum between the ground state and excited state. Through an unidentified intermediate state, the transient lattice
distortion triggered the structural change. 相似文献
853.
本文介绍一种跳频频率合成器 ,并对它进行了较详细的分析。该频率合成器将国外新出现的直接数字频率合成器技术和传统的锁相环技术有机地结合起来 ,具有结构简单、频率分辨率高和频率转换速度快等特点。 相似文献
854.
This letter presents a new one-dimensional chaotic map with infinite collapses. Theoretical analyses show that the map has complicated dynamical behavior and ideal distribution. The map can be applied in chaotic spreading spectrum communication and chaotic cipher. 相似文献
855.
讨论了锁相式频率合成器的基本原理,设计了一种通用可编程锁相式频率合成器,介绍了其编程置型格式,提出了一种可提高程控分频器工作频率的电路设计方法,并给出了其模拟波形。该电路的最高合成频率为100MHz最小频率间隔为100Hz,在工程上具有广泛的应用前景。 相似文献
856.
Dar-Jong Lin 《European Polymer Journal》2006,42(7):1581-1594
The phase equilibrium boundaries of the membrane forming system, water/triethyl phosphate (TEP)/PVDF, at 25 °C were determined experimentally using cloud-point and equilibrium absorption methods. Based on the phase diagram, appropriate dope and bath compositions were selected to prepare microporous membranes by means of the isothermal immersion-precipitation technique. As a metastable casting dope with respect to crystallization was adopted, the formed membranes exhibited a uniform cross-section composed of interlocked crystal elements coexisting with the network of continuous pores, as was revealed by high resolution FESEM imaging. Morphologies of the membranes’ top surfaces were found to depend heavily on the bath strength, which was controlled by the TEP content. By changing the bath gradually from pure water to 70% TEP, the top surface evolved from a dense skin (asymmetric membrane) to a totally porous morphology (symmetric membrane). Wide angle X-ray diffraction analysis indicated that PVDF crystallized into α-type structure for all of the synthesized membranes. The crystallinity as determined from diffraction peak deconvolution was ≈65%, which value was confirmed by Differential Scanning Calorimetry (DSC). The obtained thermograms also showed a similar melting peak temperature (Tm ≈ 169 °C) for all membranes. Furthermore, water fluxes and tensile strengths of the membranes were measured. The results were found to correlate with the morphologies of the membranes. 相似文献
857.
A laser-heated sample in a diamond anvil cell and synchrotron X-ray radiation was used to carry out structural characterization of the phase transformation of Fe2O3 at high pressures (30-96 GPa) and high temperature. The Rh2O3(II) (or orthorhombic perovskite) structure transforms to a new phase, which exhibits X-ray diffraction data that are indicative of a CaIrO3-type structure. The CaIrO3-type structure exhibited an orthorhombic symmetry (space group: Cmcm) that was stable at temperatures of 1200-2800 K and pressure of 96 GPa (the highest pressure used). Unambiguous assignment of such a structure requires experimental evidence for the presence of two Fe species. Based on the equation of state of gold, the phase boundary of the CaIrO3-type phase transformation was P (GPa)=59+0.0022×(T−1200) (K). 相似文献
858.
边界元法在带可动边界的瞬时热传导问题中的应用 总被引:1,自引:0,他引:1
该文利用边界无法对二维瞬时传热和相变问题进行求解。采用空间二次曲线单元对边界进行离散化,并采用时间相关基本解正确处理了相应边界积分。计算表明该文数值格式稳定,结果与已有文献相比较吻合一致,且该文模式时段可放大,从而提高了计算效能和稳定性。 相似文献
859.
860.
Highly oriented (100) thin films of LaVO3 and La1−xSrxVO3 have been fabricated by pulsed laser deposition in a reducing atmosphere. The films show a transition from insulating to metallic behaviour in the composition region of x, 0.175<x<0.200. In the single crystals of the antiferromagnetic insulating phase, a first-order structural phase transition is observed few degrees below the magnetic transition, which manifests itself as a kink in the temperature dependence of resistivity. In the highly oriented thin films of LaVO3 and La1−xSrxVO3 fabricated on lattice matched substrates in this study, the structural phase transformation in the insulating phase has been suppressed. The electrical conduction is found to take place via hopping through localized states at low temperatures. The metallic compositions show a non-linear (T1.5) behaviour in the temperature dependence of resistivity. V (2p) core level spectra of these films show a gradual change in the relative intensities of V3+ and V4+ ions as the value of x increases. 相似文献