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   13033篇
  免费   433篇
  国内免费   45篇
化学   10410篇
晶体学   73篇
力学   210篇
数学   1666篇
物理学   1152篇
  2023年   88篇
  2022年   218篇
  2021年   502篇
  2020年   282篇
  2019年   330篇
  2018年   252篇
  2017年   212篇
  2016年   433篇
  2015年   441篇
  2014年   415篇
  2013年   829篇
  2012年   857篇
  2011年   968篇
  2010年   586篇
  2009年   576篇
  2008年   814篇
  2007年   795篇
  2006年   679篇
  2005年   618篇
  2004年   524篇
  2003年   440篇
  2002年   431篇
  2001年   163篇
  2000年   132篇
  1999年   110篇
  1998年   113篇
  1997年   98篇
  1996年   127篇
  1995年   86篇
  1994年   84篇
  1993年   77篇
  1992年   60篇
  1991年   75篇
  1990年   54篇
  1989年   72篇
  1988年   62篇
  1987年   49篇
  1986年   52篇
  1985年   63篇
  1984年   82篇
  1983年   43篇
  1982年   66篇
  1981年   58篇
  1980年   55篇
  1979年   46篇
  1978年   54篇
  1977年   38篇
  1976年   32篇
  1975年   24篇
  1963年   20篇
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
991.
992.
993.
994.
995.
The purpose of this study was to investigate the reliability of functional magnetic resonance imaging (fMRI) by using either a projection screen or video goggles as stimulus modality. A sequence of visual stimuli were presented to the same subject at different occasions. The sequence was optimized with a genetic algorithm. In five sessions the stimuli were presented using a projection screen viewed through a mirror in the head coil and in five sessions using video goggles. Failure to detect visual activation in the medial left hemisphere was observed in sessions using the projection screen as stimulus modality. Decreased thresholds for P values and cluster size resulted in activation outside the occipital lobe and did not significantly increase activated areas in this region. Results in this study indicate that presentation of fMRI tasks with visual routes is more reliable with direct input through video goggles than with the conventional use of projection screens. Failure to detect crucial visual areas has severe consequences for tumor surgery in the visual cortex. Inferior visual impression might also have negative consequences for cognitive tests with high demand on attention and perception.  相似文献   
996.
The mechanism of NO interaction with nanosized Ru(Pd,Pt)-doped SnO(2) was studied by electron paramagnetic resonance, M?ssbauer, and electric resistance measurements. Three steps were proposed for the reaction between the semiconductor oxide and the gaseous component: (i) the formation of bielectronic oxygen vacancies (V(o)) in SnO(2); (ii) their single-ionization (V(o)(*)) with injection of electrons into the SnO(2) conduction band; (iii) the subsequent transfer of electrons from V(o)(*) to [Ru(Pd,Pt)](4+). The last process induces the formation of further oxygen vacancies which reduce the transition metal centers to lower oxidation states; the redox processes is enhanced and the electrical resistance in transition metal-doped SnO(2) is stronger modified with respect to the undoped material.  相似文献   
997.
We generalize the linearized path integral approach to evaluate quantum time correlation functions for systems best described by a set of nuclear and electronic degrees of freedom, restricting ourselves to the adiabatic approximation. If the operators in the correlation function are nondiagonal in the electronic states, then this adiabatic linearized path integral approximation for the thermal averaged quantum dynamics presents interesting and distinctive features, which we derive and explore in this paper. The capability of these approximations to accurately reproduce the behavior of physical systems is demonstrated by calculating the diffusion constant for an excess electron in a metal-molten salt solution.  相似文献   
998.
Two flavonoid glycosides containing acetylated allose, and two iridoid glucosides were obtained from a EtOH extract of Stachys corsica, an endemic plant of Sardinia and Corsica. All the compounds have previously been isolated from other species of Stachys.  相似文献   
999.
N,N-Dimethyl-pyrrolidinium iodide, and the effect of doping with LiI, has been investigated using DSC, NMR, and impedance spectroscopy. It was found that the addition of a small amount of LiI enhances the ionic conductivity by up to 3 orders of magnitude for this ionic solid. Furthermore, a slight decrease in phase transition onset temperatures, as well as the appearance of a superimposed narrow line in the (1)H NMR spectra with dopant, suggest that the LiI facilitates the mobility of the matrix material, possibly by the introduction of vacancies within the lattice. (7)Li NMR line width measurements reveal a narrow Li line width, decreasing in width and increasing in intensity with temperature, indicating mobile Li ions.  相似文献   
1000.
AOT/water/decane microemulsions have been used to entrap the water-soluble 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS4). Quasi-elastic light scattering technique has confirmed the confinement of the porphyrin and its various aggregates into the inner water pool. Various species have been detected as function of the size of the microemulsions, concentration of the porphyrin, pH, and aging of the solutions by using a combination of UV-vis absorption, steady fluorescence emission, fluorescence lifetime measurements, and time-resolved fluorescence anisotropy. Under neutral pH conditions, the porphyrin is present as the free base monomer (S414) in the inner water compartment, and it is free to rotate when the size of the droplet is large enough and the porphyrin concentration is low. On increasing the concentration and/or decreasing the microemulsion size, a H-dimer of the free base (S406) is prevalently formed. Aging both the S414 and S406 species leads to the formation of a new species (S424), which has been postulated as a H-type dimer of the diacid porphyrin. On decreasing the pH, the species S414 and S406 almost instantaneously convert into the diacid porphyrin, which is monomeric (S434). This latter is an intermediate in the eventual formation of J-aggregated TPPS4 (S490). A marked stability has been observed for the S424 species, which do not interconvert on changing the pH of the bulk aqueous phase.  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