首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   178篇
  免费   2篇
化学   124篇
晶体学   1篇
力学   5篇
数学   10篇
物理学   40篇
  2021年   3篇
  2020年   3篇
  2019年   2篇
  2018年   2篇
  2016年   2篇
  2015年   2篇
  2014年   4篇
  2013年   10篇
  2012年   6篇
  2011年   11篇
  2010年   4篇
  2009年   3篇
  2008年   7篇
  2007年   3篇
  2006年   6篇
  2005年   5篇
  2004年   5篇
  2003年   7篇
  2002年   4篇
  2001年   6篇
  1999年   3篇
  1998年   5篇
  1995年   5篇
  1994年   8篇
  1993年   2篇
  1991年   2篇
  1990年   1篇
  1989年   1篇
  1988年   2篇
  1987年   3篇
  1986年   1篇
  1985年   5篇
  1984年   2篇
  1983年   1篇
  1982年   6篇
  1981年   6篇
  1980年   3篇
  1979年   3篇
  1978年   1篇
  1977年   3篇
  1976年   2篇
  1975年   6篇
  1974年   3篇
  1973年   2篇
  1972年   1篇
  1971年   1篇
  1970年   1篇
  1966年   1篇
  1936年   1篇
  1924年   2篇
排序方式: 共有180条查询结果,搜索用时 17 毫秒
21.
In this paper, we study a discretized version of the (generalized) Korteweg-de Vries equation, t u + x 3 u + u 4 x u = 0. After a number of estimates, we utilize the Contraction Mapping Principle to prove the global well-posedness of this equation in a certain discrete Banach space. Our results are analogous to those of Kenig, Ponce, and Vega in the continuous setting. However, due to the nature of the Fourier multipliers, the proofs of several of these estimates in the discrete setting require new techniques. Our results yield a numerical procedure for computing the solution. We present a numerical algorithm which is based on successive iterations to obtain a fixed point guaranteed by the Contraction Mapping Principle. This fixed point is the desired solution to the discrete equation.  相似文献   
22.
We experimentally investigate the phase dynamics of laser networks with homogenous time-delayed mutual coupling and establish the fundamental rules that govern their state of synchronization. We identified a specific substructure that imposes its synchronization state on the entire network and show that for any coupling configuration the network forms at most two synchronized clusters. Our results indicate that the synchronization state of the network is a nonlocal phenomenon and cannot be deduced by decomposing the network into smaller substructures, each with its individual synchronization state.  相似文献   
23.
Dark soliton formation in mode-locked lasers is investigated by means of a power-energy saturation model that incorporates gain and filtering saturated with energy, and loss saturated with power. It is found that general initial conditions evolve (mode-lock) into dark solitons under appropriate requirements also met in experimental observations. The resulting pulses are essentially dark solitons of the unperturbed nonlinear Schr?dinger equation. Notably, the same framework also describes bright pulses in anomalous and normally dispersive lasers.  相似文献   
24.
The potential energy surfaces of both neutral and dianionic SnC2P2R2 (R=H, tBu) ring systems have been explored at the B3PW91/LANL2DZ (Sn) and 6‐311+G* (other atoms) level. In the neutral isomers the global minimum is a nido structure in which a 1,2‐diphosphocyclobutadiene ring (1,2‐DPCB) is capped by the Sn. Interestingly, the structure established by X‐ray diffraction analysis, for R=tBu, is a 1,3‐DPCB ring capped by Sn and it is 2.4 kcal mol?1 higher in energy than the 1,2‐DPCB ring isomer. This is possibly related to the kinetic stability of the 1,3‐DPCB ring, which might originate from the synthetic precursor ZrCp2tBu2C2P2. In the case of the dianionic isomers we observe only a 6π‐electron aromatic structure as the global minimum, similarly to the cases of our previously reported results with other types of heterodiphospholes. 1 , 4 , 19 The existence of large numbers of cluster‐type isomers in neutral and 6π‐planar structures in the dianions SnC2P2R22? (R=H, tBu) is due to 3D aromaticity in neutral clusters and to 2D π aromaticity of the dianionic rings. Relative energies of positional isomers mainly depend on: 1) the valency and coordination number of the Sn centre, 2) individual bond strengths, and 3) the steric effect of tBu groups. A comparison of neutral stannadiphospholes with other structurally related C5H5+ analogues indicates that Sn might be a better isolobal analogue to P+ than to BH or CH+. The variation in global minima in these C5H5+ analogues is due to characteristic features such as 1) the different valencies of C, B, P and Sn, 2) the electron deficiency of B, 3) weaker pπ–pπ bonding by P and Sn atoms, and 4) the tendency of electropositive elements to donate electrons to nido clusters. Unlike the C5H5+ systems, all C5H5? analogues have 6π‐planar aromatic structures as global minima. The differences in the relative ordering of the positional isomers and ligating properties are significant and depend on 1) the nature of the π orbitals involved, and 2) effective overlap of orbitals.  相似文献   
25.
By measurement of the proton relaxation times T 1 and T 2 of CH3OD containing Co++ ions in dilute solution and the dependence of the observed decay time (T 2 ?) on t ep the separation between pulses in a Carr-Purcell sequence the coordination number is shown to be six. Since this result has previously been obtained by an independent method, the use of the dependence of T 2 ? on t ep to determine coordination numbers is justified for solutions of paramagnetic ions. The validity of analytical expressions derived previously has been checked by numerical methods, and the analytical expression is shown to be valid even in the presence of H 0 and H 1 inhomogeneity and inaccurate pulses of finite duration, provided that the Meiboom-Gill method is used. In an Appendix insight derived from the results of the numerical solutions is used to give a brief derivation of the dependence of T 2 ? on H 0, exchange rate and t ep.  相似文献   
26.
The metastable zone width for the solution of organic nonlinear material: indole-3-aldehyde (IA) has been determined for the various solvents of acetone, methanol and ethyl acetate and the corresponding IA crystals have been grown by temperature lowering technique. The surface analysis on different planes of the grown crystals has been carried out. The range of transmission and powder second harmonic generation efficiency of IA have been reported.  相似文献   
27.
28.
Summary Polymers can be difficult to fully characterize by conventional chromatographic methods [i.e., gas chromatography (GC), high performance liquid chromatography (HPLC) and size exclusion chromatography (SEC)] due to temperature instability, detector incompatibility, nonvolatility, molecular weight considerations or insufficient chromatographic efficiency. Capillary supercritical fluid chromatography (SFC) offers several advantages for polymer analysis. Complex distributions can be resolved, low temperatures can be used and individual oligomers can be detected, allowing differences in molecular weight distributions to be quantitatively and qualitatively measured. In this paper, results will be shown for the analysis of various types of polymers demonstrating how changes in chromatographic parameters affect the analysis
Industrielle Anwendungen der Chromatographie mit überkritischen fluiden Phasen: Polymeranalyse
  相似文献   
29.
The molecule 1-phenylphosphaethyne, C6H5CP, was detected by microwave spectroscopy when Ph(Me3Si)CPCl was pyrolyzed in the gas phase at 750°C. Asymmetric rotor transitions between 26.5 and 40 GHz with J = 16 to 22 were analyzed to yield A0 = 5669.044(72), B0 = 933.79209(71), and C0 = 801.59279(75). These data indicate that the bond length r(CC) between the ring and CP group carbon atom is 1.467 Å.  相似文献   
30.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号