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   2123篇
  免费   42篇
  国内免费   14篇
化学   1439篇
晶体学   24篇
力学   35篇
数学   305篇
物理学   376篇
  2023年   12篇
  2022年   22篇
  2021年   41篇
  2020年   33篇
  2019年   33篇
  2018年   23篇
  2017年   29篇
  2016年   55篇
  2015年   33篇
  2014年   52篇
  2013年   136篇
  2012年   119篇
  2011年   158篇
  2010年   73篇
  2009年   76篇
  2008年   149篇
  2007年   159篇
  2006年   144篇
  2005年   132篇
  2004年   99篇
  2003年   72篇
  2002年   71篇
  2001年   44篇
  2000年   46篇
  1999年   36篇
  1998年   32篇
  1997年   26篇
  1996年   29篇
  1995年   24篇
  1994年   13篇
  1993年   18篇
  1992年   11篇
  1991年   12篇
  1990年   15篇
  1989年   14篇
  1988年   19篇
  1987年   11篇
  1986年   11篇
  1985年   10篇
  1984年   10篇
  1983年   5篇
  1982年   6篇
  1981年   6篇
  1980年   4篇
  1977年   4篇
  1976年   8篇
  1975年   5篇
  1974年   4篇
  1973年   5篇
  1972年   8篇
排序方式: 共有2179条查询结果,搜索用时 15 毫秒
71.
Phosphorylation of phenols with diethyl cyanophosphonate in methylene chloride solution at 0°C is an easy, rapid and good yielding reaction.  相似文献   
72.
73.
A series of twelve novel 1-acetamide-5-methoxy-2-oxindoles were prepared in moderate to good yields (30–88%) by an Ugi-SN2 one pot process with a xanthate mediated free radical cyclization. Additionally, Density Functional Theory (DFT) based calculations were performed at UB3LYP/6-311G(d) level in order to rationalize the energy profile of the xanthate mediated free radical cyclization because the benzo-fused γ-lactams (ortho products) were obtained as sole regioisomers over the spiro β-lactams (ipso products).  相似文献   
74.
POCIS (polar organic chemical integrative samplers) are increasingly used for sampling polar compounds. Although very efficient for a wide range of pollutants, the classic configuration of the device has a number of limitations, in particular for the sampling of highly polar analytes and hydrophobic compounds. This study presents a new version of the POCIS passive sampler which uses a highly porous Nylon membrane of 30 μm pore size, enabling the sampling of hydrophobic pollutants and improving the accumulation rate of other pollutants. This newly designed tool and the classic POCIS were both tested during a laboratory experiment to evaluate the accumulation kinetics of a selection of pesticides and pharmaceuticals. The observed results show unexpected accumulation kinetics for the new version of POCIS. To explain the data, the use of an intraparticulate diffusion model was required, which also enabled us to propose another explanation of the burst effect observed with the classic POCIS, primarily related to the potential wetting of the device as the first step in the accumulation of compounds.
Figure
Picture of classical and nylon version of POCIS  相似文献   
75.
A systematic study was conducted on current efficiency (CE), corrosion and structural changes in SnO2-based inert anodes (made of 96wt%SnO2+2wt%Sb2O3+2wt%CuO) on a laboratory Hall-Heroult aluminium cell. The influence of operating parameters and electrolyte composition on the CE and corrosion process were evaluated. The CE was found to be more than 90% and catastrophic corrosion took place at low percent of Al2O3, high percent of LiF, low cryolite ratio and high current densities. From all the structural changes that took place in the SnO2-based inert anodes, we assumed that the most important contribution was due to the migration of CuO towards the outer limits of the constituent grains of SnO2 based ceramic. The complex process occurred during the formation of various phases and their sintering ability both directly depended on Cu/Sb molar ratio.  相似文献   
76.
A Knill-Laflamme-Milburn (KLM) type quantum computation with bosonic neutral atoms or bosonic ions is suggested. Crucially, as opposite to other quantum computation schemes involving atoms (ions), no controlled interactions between atoms (ions) involving their internal levels are required. Versus photonic KLM computation, this scheme has the advantage that single-atom (ion) sources are more natural than single-photon sources, and single-atom (ion) detectors are far more efficient than single-photon ones.  相似文献   
77.
We address the problem of "nonlocal computation," in which separated parties must compute a function without any individual learning anything about the inputs. Surprisingly, entanglement provides no benefit over local classical strategies for such tasks, yet stronger nonlocal correlations allow perfect success. This provides intriguing insights into the limits of quantum information processing, the nature of quantum nonlocality, and the differences between quantum and stronger-than-quantum nonlocal correlations.  相似文献   
78.
The x-ray structure analysis of the oxygen-surfactant-mediated growth of Ni on Cu(001) identifies up to 0.15 monolayers of oxygen in subsurface octahedral sites. This questions the validity of the general view that surfactant oxygen floats on top of the growing Ni film. Rather, the surfactant action is ascribed to an oxygen-enriched zone extending over the two topmost layers. Surface stress measurements support this finding. Our results have important implications for the microscopic understanding of surfactant-mediated growth and the change of the magnetic anisotropy of the Ni films.  相似文献   
79.

Against the practical context of thermal straightening, hair samples were obtained with a chemical (bleaching) as well as a cumulative thermal history (0–800 s, 200 °C). On these samples, tensile testing and DSC analysis, both in the wet state, were conducted to obtain the elastic moduli Ew as well as denaturation temperatures TD and enthalpies ΔHD. 3D plots show overall linearity for the relationships between the parameters for natural hair. For bleached hair, pronounced nonlinearities develop beyond 300 s of thermal treatment. At this stage, TD as well as Ew approaches limiting values, consistent with the state of a highly cross-linked, thermoset polymer. 2D projections are used to investigate the correlations between pairs of parameters. The results show that bleaching imparts a specific sensitivity for thermal damage, namely, to the matrix proteins, which more readily than the intermediate filaments (IF) turn into a thermoset. Overall, correlations between parameters hold well prior to the thermoset range. It is thus suggested that tensile testing to determine the elastic modulus and DSC come to consistent and equivalent results, at least, for the current experimental context. However, while Ew combines contributions of IFs and matrix, DSC differentiates the specific property changes of these components.

  相似文献   
80.
In this paper, we find new refinements to the Massey inequality, which relates the Shannon and guessing entropies, introducing a new concept: the Massey gap. By shrinking the Massey gap, we improve all previous work without introducing any new parameters, providing closed-form strict refinements, as well as a numerical procedure improving them even further.  相似文献   
[首页] « 上一页 [3] [4] [5] [6] [7] 8 [9] [10] [11] [12] [13] 下一页 » 末  页»
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