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   511427篇
  免费   8269篇
  国内免费   4487篇
化学   257114篇
晶体学   7452篇
力学   25349篇
综合类   248篇
数学   71467篇
物理学   162553篇
  2021年   3523篇
  2020年   4090篇
  2019年   4405篇
  2018年   12568篇
  2017年   13250篇
  2016年   11204篇
  2015年   6115篇
  2014年   8071篇
  2013年   19137篇
  2012年   19077篇
  2011年   28981篇
  2010年   19035篇
  2009年   18951篇
  2008年   25692篇
  2007年   28485篇
  2006年   16334篇
  2005年   19826篇
  2004年   15841篇
  2003年   14407篇
  2002年   12651篇
  2001年   12824篇
  2000年   10051篇
  1999年   7604篇
  1998年   6462篇
  1997年   6334篇
  1996年   6374篇
  1995年   5599篇
  1994年   5614篇
  1993年   5451篇
  1992年   5755篇
  1991年   5908篇
  1990年   5491篇
  1989年   5338篇
  1988年   5309篇
  1987年   5139篇
  1986年   4969篇
  1985年   6654篇
  1984年   6839篇
  1983年   5701篇
  1982年   5997篇
  1981年   5680篇
  1980年   5321篇
  1979年   5670篇
  1978年   5939篇
  1977年   5822篇
  1976年   5919篇
  1975年   5400篇
  1974年   5562篇
  1973年   5668篇
  1972年   4164篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
A temperature control unit was implemented to vary the temperature of samples studied on a commercial Mobile Universal Surface Explorer nuclear magnetic resonance (MOUSE-NMR) apparatus. The device was miniaturized to fit the maximum MOUSE sampling depth (25 mm). It was constituted by a sample holder sandwiched between two heat exchangers placed below and above the sample. Air was chosen as the fluid to control the temperature at the bottom of the sample, at the interface between the NMR probe and the sample holder, in order to gain space. The upper surface of the sample was regulated by the circulation of water inside a second heat exchanger placed above the sample holder. The feasibility of using such a device was demonstrated first on pure water and then on several samples of bread dough with different water contents. For this, T1 relaxation times were measured at various temperatures and depths and were then compared with those acquired with a conventional compact closed-magnet spectrometer. Discussion of results was based on biochemical transformations in bread dough (starch gelatinization and gluten heat denaturation). It was demonstrated that, within a certain water level range, and because of the low magnetic field strength of the MOUSE, a linear relationship could be established between T1 relaxation times and the local temperature in the dough sample.  相似文献   
42.
Crystallography Reports - The orientation of grains and the special boundaries formed by them in multicrystalline silicon has been studied by electron backscattered diffraction. It is found that...  相似文献   
43.
Crystallography Reports - Some questions related to the establishment, development, and prospects of physical studies using inelastic thermal-neutron scattering are briefly reviewed. The directions...  相似文献   
44.
Theoretical and Mathematical Physics - The fractional derivative method is used to take wave dispersion into account in the wave equation when describing the propagation of gravitational soliton...  相似文献   
45.
A facile biosynthesis route was followed to prepare zinc oxide nanoparticles (ZnO NPs) using Euphorbia milii (E. milii) leaf constituents. The SEM images exhibited presence of spherical ZnO NPs and the corresponding TEM images disclosed monodisperse nature of the ZnO NPs with diameter ranges between 12 and 20 nm. The Brunauer–Emmett–Teller (BET) analysis revealed that the ZnO NPs have specific surface area of 20.46 m2/g with pore diameter of 2 nm–10 nm and pore volume of 0.908 cm3/g. The EDAX spectrum exemplified the existence of Zn and O elements and non-appearance of impurities that confirmed pristine nature of the ZnO NPs. The XRD pattern indicated crystalline peaks corresponding to hexagonal wurtzite structured ZnO with an average crystallite size of 16.11 nm. The FTIR spectrum displayed strong absorption bands at 512 and 534 cm?1 related to ZnO. The photocatalytic action of ZnO NPs exhibited noteworthy degradation of methylene blue dye under natural sunlight illumination. The maximum degradation efficiency achieved was 98.17% at an illumination period of 50 min. The reusability study proved considerable photostability of the ZnO NPs during photocatalytic experiments. These findings suggest that the E. milii leaf constituents can be utilized as suitable biological source to synthesis ZnO NPs for photocatalytic applications.  相似文献   
46.
Crystallography Reports - Methods of coherent X-ray diffraction imaging of the spatial structure of noncrystalline objects and nanocrystals (nanostructures) are considered. Particular attention is...  相似文献   
47.
In a 2:2 reaction with silver(I) chloride or bromide, 1,5-bis(1-phospholano)pentane ( 1a ) afforded frame-like macrocyclic structures, with intra- ( 2 , Cl) or intermolecular ( 3 , Br) halido bridges. In contrast, 1,7-bis(1-phospholano)heptane ( 1b ) formed coordination polymers 4a (Cl) and 4b (Br) with bridging bis-phospholane and halido ligands. A unique paddle wheel-type metallacryptand structure 5 was obtained from 1a and silver(I) bromide in a 2:3 reaction (M:L). All complexes were fully characterized by NMR, IR spectroscopy, mass spectrometry, and X-ray crystallography.  相似文献   
48.
To achieve efficient proton pumping in the light-driven proton pump bacteriorhodopsin (bR), the protein must be tightly coupled to the retinal to rapidly convert retinal isomerization into protein structural rearrangements. Methyl group dynamics of bR embedded in lipid nanodiscs were determined in the dark-adapted state, and were found to be mostly well ordered at the cytosolic side. Methyl groups in the M145A mutant of bR, which displays only 10 % residual proton pumping activity, are less well ordered, suggesting a link between side-chain dynamics on the cytosolic side of the bR cavity and proton pumping activity. In addition, slow conformational exchange, attributed to low frequency motions of aromatic rings, was indirectly observed for residues on the extracellular side of the bR cavity. This may be related to reorganization of the water network. These observations provide a detailed picture of previously undescribed equilibrium dynamics on different time scales for ground-state bR.  相似文献   
49.
Triene 6π electrocyclization, wherein a conjugated triene undergoes a concerted stereospecific cycloisomerization to a cyclohexadiene, is a reaction of great historical and practical significance. In order to circumvent limitations imposed by the normally harsh reaction conditions, chemists have long sought to develop catalytic variants based upon the activating power of metal–alkene coordination. Herein, we demonstrate the first successful implementation of such a strategy by utilizing [(C5H5)Ru(NCMe)3]PF6 as a precatalyst for the disrotatory 6π electrocyclization of highly substituted trienes that are resistant to thermal cyclization. Mechanistic and computational studies implicate hexahapto transition-metal coordination as responsible for lowering the energetic barrier to ring closure. This work establishes a foundation for the development of new catalysts for stereoselective electrocyclizations.  相似文献   
50.
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