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   2016篇
  免费   262篇
  国内免费   240篇
化学   1413篇
晶体学   20篇
力学   88篇
综合类   4篇
数学   330篇
物理学   663篇
  2024年   3篇
  2023年   47篇
  2022年   65篇
  2021年   81篇
  2020年   101篇
  2019年   71篇
  2018年   61篇
  2017年   66篇
  2016年   95篇
  2015年   87篇
  2014年   92篇
  2013年   139篇
  2012年   158篇
  2011年   172篇
  2010年   92篇
  2009年   113篇
  2008年   125篇
  2007年   119篇
  2006年   92篇
  2005年   94篇
  2004年   54篇
  2003年   42篇
  2002年   40篇
  2001年   44篇
  2000年   46篇
  1999年   37篇
  1998年   26篇
  1997年   27篇
  1996年   42篇
  1995年   35篇
  1994年   34篇
  1993年   29篇
  1992年   24篇
  1991年   33篇
  1990年   25篇
  1989年   14篇
  1988年   15篇
  1987年   13篇
  1986年   11篇
  1985年   10篇
  1984年   11篇
  1983年   4篇
  1982年   8篇
  1981年   5篇
  1980年   4篇
  1979年   5篇
  1977年   2篇
  1975年   2篇
  1968年   1篇
  1957年   1篇
排序方式: 共有2518条查询结果,搜索用时 31 毫秒
151.
A facile liquid‐phase exfoliation method to prepare few‐layer FeOCl nanosheets in acetonitrile by ultrasonication is reported. The detailed exfoliation mechanism and generated products were investigated by combining first‐principle calculations and experimental approaches. The similar cleavage energies of FeOCl (340 mJ m?2) and graphite (320 mJ m?2) confirm the experimental exfoliation feasibility. As a Fenton reagent, FeOCl nanosheets showed outstanding properties in the catalytic degradation of phenol in water at room temperature, under neutral pH conditions, and with sunlight irradiation. Apart from the increased surface area of the nanosheets, the surface state change of the nanosheets also plays a key role in improving the catalytic performance. The changes of charge density, density of states (DOS), and valence state of Fe atoms in the exfoliated FeOCl nanosheets versus plates illustrated that surface atomistic relationships made the few‐layer nanosheets higher activity, indicating the exfoliation process of the FeOCl nanosheets also brought about surface state changes.  相似文献   
152.
The framework of differential inclusions encompasses modern optimal control and the calculus of variations. Necessary optimality conditions in the literature identify potentially optimal paths, but do not show how to perturb paths to optimality. We first look at the corresponding discretized inclusions, estimating the subdifferential dependence of the optimal value in terms of the endpoints of the feasible paths. Our approach is to first estimate the coderivative of the reachable map. The discretized (nonsmooth) Euler–Lagrange and Transversality Conditions follow as a corollary. We obtain corresponding results for differential inclusions by passing discretized inclusions to the limit.  相似文献   
153.
The ionization spectrum of sulfur dioxide has been successfully studied by using the symmetry-adapted-cluster configuration-interaction (SAC-CI) general-R and SD-R methods and the basis set correlation-consistent polarized valence triple-zeta (cc-pVTZ). The SAC-CI general-R method reproduces the experimental spectrum well for both the main peaks and the satellite peaks of ionization spectrum of SO2. The sequence of ionic states corresponding to main peaks of SO2 has been re-determined according to the SAC-CI conclusions and it is reordered as X^2A1, A^2B2, B2A2, C^2B1,D^2A1, E^2B2 and F^2A1. Besides, the equilibrium structures and adiabatic ionization potentials (AIPs) of ionic states of main peaks of SO2 are calculated by using the SAC-CI SD-R method.  相似文献   
154.
155.
MRI measurements of water diffusion and blood perfusion are increasingly used for the evaluation of organ functionality and tissue viability (e.g., in tumors). While diffusion-weighted imaging is performed without contrast agents, measurement of blood perfusion is normally performed based on the administration of paramagnetic substances such as gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA). Simultaneous measurements of these two parameters are often preferred. However, it may be argued that Gd-DTPA causes constriction of small blood vessels or alters hemodynamic parameters such as blood viscosity, thereby corrupting subsequent measurements of the apparent diffusion constant (ADC). The objective of the current study was to investigate the possible changes in the ADC in tumors following intravenous administration of 0.2 and 0.4 mmol/kg of Gd-DTPA in mice. The study was conducted with C3H mouse mammary carcinomas inoculated in the right foot of the animal subjects. The results were compared with findings in a sham group, demonstrating that Gd-DTPA had no significant impact on the ADC as measured in a 7-T animal system.  相似文献   
156.
Through the use of monodisperse core/shell quantum dots (QDs) as photosensitizers for the first time, a novel strategy for the fabrication of QD-photosensitized nano-TiO2 films was demonstrated. Core/shell QDs were self-assembled on nano-TiO2 films through carboxyls as anchoring groups to metal oxides. Atomic force microscopy and some other experiments showed the fabrication strategy is successful. Reactive oxygen species detection experiments indicated that such films have photosensitization ability. The results of bactericidal and DNA damage experiments demonstrate that such films have excellent photoactivity.  相似文献   
157.
Seven 271Ds decay chains were identified in the bombardment of 208Pb targets with 311.5 and 314.3 MeV 64Ni projectiles using the Berkeley Gas-filled Separator. These data, combined with previous results, provide an excitation function for this reaction. From these results, an optimum energy of 321 MeV was estimated for the production of (272)111 in the new reaction 208Pb(65Cu,n). One decay chain was observed, resulting in a cross section of 1.7(+3.9)(-1.4) pb. This experiment confirms the discovery of element 111 by the Darmstadt Group who used the 209Bi(64Ni,n)(272)111 reaction.  相似文献   
158.
The output properties of electrons accelerated by the vacuum laser acceleration scheme CAS (capture and acceleration scenario) are addressed. The transport process of the electron bunch, the fraction of the CAS electrons of the incident electrons, the correlation of electron energy with position and scattering angle, the energy spectrum and angular distributions as well as the emittance of the outgoing electrons are studied at a laser intensity of a0=10. In addition, the effects of the laser intensity, beam width, and pulse duration on the properties of the output electrons are also examined. Physical explanations of those output characteristics are presented based on the mechanism behind the CAS scheme. The feasibility of CAS to become a realistic laser accelerator scheme is explored. PACS 41.75.Jv; 42.60.Jf; 41.85.Ja  相似文献   
159.
Nanoparticles and nanorods of CuSb2O6 are prepared by hydrothermal method and its high temperature α-phase is stabilized at room temperature. The average size of the nanoparticles is ca. 13.7 nm. The nanorods, with a width of ca. 20 nm and an aspect ratio of ca. 5, are the agglomerates composing of smaller nanoparticles with an average size of ca. 8.3 nm. Compared with the high temperature α-phase of bulk sample at 400 K, the lattice of nanophases elongated in ab plane and compressed along c direction. The CuSb2O6 nanoparticles exhibit predominant paramagnetic phenomenon. The difference in magnetic properties of the nanoparticles and nanorods indicates the interfacial interaction of agglomerated nanoparticles.  相似文献   
160.
Spinel CoFe2O4 coating on the surface of hollow glass microspheres of low density was synthesized by co-precipitation method. The phase structures, morphologies, particle size, shell thickness, chemical compositions of the composites have been characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and energy dispersive X-ray spectroscopy (EDS). The results show that CoFe2O4 coating on hollow glass microspheres can be achieved, and the coating layers are constituted by CoFe2O4 nanoparticles of mean size ca. 10 nm. The as-synthesized powder materials were uniformly dispersed into the phenolic cement, then the mixture was pasted on metal plate with the area of 200 mm×200 mm as the test plate. The test of microwave absorption was carried out by the radar-absorbing materials (RAM) reflectivity far field radar cross-section (RCS) method. The results indicate that the coated CoFe2O4/hollow glass microspheres composites can be applied in lightweight and strong absorption microwave absorbers.  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