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   79672篇
  免费   389篇
  国内免费   379篇
化学   25227篇
晶体学   792篇
力学   6766篇
数学   32317篇
物理学   15338篇
  2022年   53篇
  2021年   60篇
  2020年   55篇
  2019年   63篇
  2018年   10448篇
  2017年   10271篇
  2016年   6134篇
  2015年   905篇
  2014年   357篇
  2013年   410篇
  2012年   3924篇
  2011年   10609篇
  2010年   5691篇
  2009年   6115篇
  2008年   6684篇
  2007年   8847篇
  2006年   312篇
  2005年   1376篇
  2004年   1609篇
  2003年   2027篇
  2002年   1082篇
  2001年   269篇
  2000年   304篇
  1999年   166篇
  1998年   209篇
  1997年   155篇
  1996年   216篇
  1995年   128篇
  1994年   95篇
  1993年   109篇
  1992年   62篇
  1991年   73篇
  1990年   58篇
  1989年   72篇
  1988年   68篇
  1987年   64篇
  1986年   61篇
  1985年   55篇
  1984年   55篇
  1983年   44篇
  1982年   47篇
  1981年   41篇
  1980年   49篇
  1979年   48篇
  1978年   38篇
  1914年   45篇
  1913年   40篇
  1912年   40篇
  1909年   41篇
  1908年   40篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
191.
Thymidylate synthase (TS), one of folate-dependent enzymes, is a key and well-recognized target for anticancer agents. In this study, a series of 6-aryl-5-cyano thiouracil derivatives were designed and synthesized in accordance with essential pharmacophoric features of known TS inhibitors. Nineteen compounds were screened in vitro for their anti-proliferative activities toward HePG-2, MCF-7, HCT-116, and PC-3 cell lines. Compounds \(\mathbf{21}_{\mathbf{c}}\), \(\mathbf{21}_{\mathbf{d}}\), and 24 exhibited high anti-proliferative activity, comparable to that of 5-fluorouracil. Additionally, ten compounds with potent anti-proliferative activities were further evaluated for their ability to inhibit TS enzyme. Six compounds (\(\mathbf{21}_{\mathbf{b}}\), \(\mathbf{21}_{\mathbf{c}}\), \(\mathbf{21}_{\mathbf{d}}\), 22, 23 and 24) demonstrated potent dose-related TS inhibition with \(\hbox {IC}_{50}\) values ranging from 1.57 to \(3.89\,\upmu \hbox {M}\). The in vitro TS activity results were consistent with those of the cytotoxicity assay where the most potent anti-proliferative compounds of the series showed good TS inhibitory activity comparable to that of 5-fluorouracil. Furthermore, molecular docking studies were carried out to investigate the binding pattern of the designed compounds with the prospective target, TS (PDB-code: 1JU6).  相似文献   
192.
An approach for photonic generation of an arbitrary chirped microwave waveform with an increased time-bandwidth product (TBWP) is proposed and experimentally demonstrated. In the proposed model, light from the mode locked laser is splitted into two parts by using 1 × 2 power splitter: one is sent to linearly chirped fiber Bragg grating (LCFBG) through circulator and the other is time delayed by fiber delay line. The optical pulse in upper arm is time stretched by the LCFBG. Meanwhile, the optical pulse in lower arm experiences a time delay and then stretched by the dispersive single mode fiber. Temporal interference pattern is generated with an increasing or decreasing free spectral range by combination of two time-stretched optical pulses. Finally, the temporal interference pattern which is obtained at the output of optical coupler is transformed into an arbitrary chirped microwave waveform by using a photo-detector. The main advantage of this proposed model is high TBWP in the range of 750–1000 which ultimately results in an increased range resolution of radio detection and ranging.  相似文献   
193.
A transfer-reaction experiment of 9Be(9Be, 10Be)8Be was performed at a beam energy of 45 MeV. Excited states in 10Be up to 18.80 MeV are produced using missing mass and invariant mass methods. Most of the observed high-lying resonant states, reconstructed from the α + 6He and t + 7Li decay channels, agree with the previously reported results. In addition, two new resonances at 15.6 and 18.8 MeV are identified from the present measurement. The 18.55 MeV state is found to decay into both the t + 7Lig.s. and t + 7Li* (0.478 MeV) channels, with a relative branching ratio of 0.93 ± 0.33. Further theoretical investigations are encouraged to interpret this new information on cluster structure in neutron-rich light nuclei.  相似文献   
194.
Misfit-layered calcium cobaltites (Ca3Co4O9, Ca3Co3.9Fe0.1O9, and Ca3Co3.9Mn0.1O9), as anode materials for lithium-ion batteries, were synthesized by a simple hydro-decomposition method. All synthesized samples do not show any impurity phase. They exhibited plate-like particle with the particle size of 1–2 μm. The specific capacities of doped samples showed higher electrochemical performance compared to the undoped sample. After charge/discharge of 50 cycles, the specific capacities of Ca3Co4O9, Ca3Co3.9Fe0.1O9, and Ca3Co3.9Mn0.1O9 were 343, 562, and 581 mAh g?1, respectively. The doped samples showed an increase of over 60% compared to the undoped sample. The cyclic voltammetry profile of the doped samples showed the enhanced reactivity corresponding to their improved electrochemical performance. The capacity improvement of doped samples resulted from the metal oxide/Li conversion reactions, volume change, and high reactivity.  相似文献   
195.
In this paper, the synchronization problem of a chaotic supply chain management system is studied. A novel adaptive modified projective synchronization method is introduced to control the behaviour of the leader supply chain system by a follower chaotic system and to adjust the leader system parameters until the measurable errors of the system parameters converge to zero. The stability evaluation and convergence analysis are carried out by the Lyapanov stability theorem. The proposed synchronization and antisynchronization techniques are studied for identical supply chain chaotic systems. Finally, some numerical simulations are presented to verify the effectiveness of the theoretical discussions.  相似文献   
196.
The current-voltage relationships of deposited structures are measured for cluster structures consisting of nanoparticles of lead telluride. Variation in the value of the tunnel current is shown. Optimum conditions for the possible emergence of quantum-hopping conductivity due to carrier tunneling (the characteristic sizes of the nanoclusters and the distances between them) are determined.  相似文献   
197.
198.
199.
200.
We propose a total measure of multi-particle quantum correlation in a system of N two-level atoms (N qubits). We construct a parameter that encompasses all possible quantum correlations among N two-level atoms in arbitrary symmetric pure states and define its numerical value to be the total measure of the net atom-atom correlations. We use that parameter to quantify the total quantum correlations in atomic Schrödinger cat states, which are generated by the dispersive interaction in a cavity. We study the variation of the net amount of quantum correlation as we vary the number of atoms from N=2 to N=100 and obtain some interesting results. We also study the variation of the net correlation, for fixed interaction time, as we increase the number of atoms in the excited state of the initial system, and notice some interesting features. We also observe the behaviour of the net quantum correlation as we continuously increase the interaction time, for the general state of N two-level atoms in a dispersive cavity.  相似文献   
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