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   5840篇
  免费   108篇
  国内免费   29篇
化学   3568篇
晶体学   15篇
力学   173篇
数学   1313篇
物理学   908篇
  2020年   58篇
  2019年   41篇
  2016年   85篇
  2015年   70篇
  2014年   73篇
  2013年   212篇
  2012年   178篇
  2011年   194篇
  2010年   134篇
  2009年   134篇
  2008年   187篇
  2007年   211篇
  2006年   174篇
  2005年   193篇
  2004年   157篇
  2003年   128篇
  2002年   137篇
  2001年   51篇
  2000年   58篇
  1999年   53篇
  1998年   45篇
  1997年   69篇
  1996年   69篇
  1995年   79篇
  1994年   84篇
  1993年   75篇
  1992年   87篇
  1991年   59篇
  1990年   64篇
  1989年   73篇
  1988年   85篇
  1987年   71篇
  1986年   84篇
  1985年   121篇
  1984年   115篇
  1983年   86篇
  1982年   120篇
  1981年   121篇
  1980年   111篇
  1979年   125篇
  1978年   106篇
  1977年   106篇
  1976年   96篇
  1975年   111篇
  1974年   81篇
  1973年   87篇
  1972年   55篇
  1971年   60篇
  1970年   63篇
  1966年   48篇
排序方式: 共有5977条查询结果,搜索用时 15 毫秒
1.
2.
Computational Optimization and Applications - It has been shown that a global minimizer of a smooth determinant of a matrix function corresponds to the largest cycle of a graph. When it exists,...  相似文献   
3.
Mononuclear coordination compounds of the type [Pd(NH2trz)4]2+ with the counterions chloride, nitrate, trifluoromethanesulfonate, and methanesulfonate were synthesized and their structures identified with single‐crystal X‐ray diffraction. In case of the synthesis with methanesulfonate as the counterion the dominant product was of the generic formula [Pd2(NH2trz)3](CH3SO3)4, and the complex [Pd(NH2trz)4](CH3SO3)2 only emerged as a byproduct. While the structure of the byproduct could be analyzed by single‐crystal X‐ray diffraction, suitable crystals of the main product [Pd2(NH2trz)3](CH3SO3)4 could not be obtained. However, stoichiometry implies a polynuclear nature with NH2trz present in the rare μ3‐η111 coordination type, i.e. with NH2trz molecules coordinating to three palladium atoms. Accordingly, identification of solids by single‐crystal analysis alone can be misleading in particular with NH2trz as a ligand due to its versatile coordination behavior. Finally, analysis by differential scanning calorimetry (DSC) revealed that the complexes were thermally stable (the onset of decomposition well above 100 °C), with [Pd2(NH2trz)3](CH3SO3)4 being the most stable compound (onset of decomposition at 204 °C).  相似文献   
4.
Four bis(dioxane) adducts of mixed‐metal trifluoroacetates, M[M′(O2CCF3)4(C4H8O2)2] (M = Na, K, Cs, M′ = In and M = Cs, M′ = Ga) were synthesized by reaction of alkali metal trifluoroacetate and group 13 element trifluoroacetate in 1,4‐dioxane and completely characterized including X‐ray structure determination. Geometric parameters, empirical bond valences and frequencies of the symmetric C=O stretching vibrations show that the moisture sensitive solids are composed of dimeric structural moieties with site symmetry 1 , containing alkali metal ions and bis(dioxane)tetrakis(trifluoroacetato)indate or ‐gallate ions. The dimeric units are further connected by weaker “intermolecular” dioxane interactions to neighboring alkali metal ions. Closer inspection of space group symmetry, unit cell parameters and atom sites allows to rationalize the compounds as members of two isotypic pairs that are further closely related due to the group‐subgroup relation of their monoclinc space groups to a common orthorhombic supergroup.  相似文献   
5.
Different colloidal particle characterization methods are examined for their suitability to determine the particle size distribution of particles extracted from steels. Microalloyed steels are dissolved to extract niobium and titanium carbonitride particles that are important for the mechanical properties of these steels. Such particles have sizes ranging from several nanometers to hundreds of nanometers depending on the precipitation stage during the thermomechanically controlled rolling process. The size distribution of the particles is analyzed by dynamic light scattering (DLS), analytical ultracentrifugation (AUC), and hollow fiber flow field-flow fractionation (HF5) and compared to data obtained for reference particles as well as data from electron microscopy, the standard sizing technique used in metallurgy today. AUC and HF5 provide high-quality size distributions, average over large particle numbers that enables statistical analysis, and yield useful insights for alloy design; however, DLS fails due to a lack of resolution. Important aspects in the conversion and comparison of size distributions obtained for broadly distributed particle systems with different measurement principles and the role of surfactants used in sample preparation are discussed.  相似文献   
6.
Nonlinear Dynamics - The cellular microstructure of periodic architected materials can be enriched by local intracellular mechanisms providing innovative distributed functionalities. Specifically,...  相似文献   
7.
Combined quantum mechanics/molecular mechanics (QM/MM) calculations were used to investigate the reaction mechanism of taxadiene synthase (TXS). TXS catalyzes the cyclization of geranylgeranyl diphosphate (GGPP) to taxadiene (T) and four minor cyclic products. All these products originate from the deprotonation of carbocation intermediates. The reaction profiles for the conversion of GGPP to T as well as to minor products were calculated for different configurations of relevant TXS carbocation complexes. The QM region was treated at the M06-2X/TZVP level, while the CHARMM27 force field was used to describe the MM region. The QM/MM calculations suggest a reaction pathway for the conversion of GGPP to T, which slightly differs from previous proposals regarding the number of reaction steps and the conformation of the carbocations. The QM/MM results also indicate that the formation of minor products via water-assisted deprotonation of the carbocations is highly exothermic, by about −7 to −23 kcal/mol. Curiously, however, the computed barriers and reaction energies indicate that the formation of some of the minor products is more facile than the formation of T. Thus, the present QM/MM calculations provide detailed insights into possible reaction pathways and into the origin of the promiscuity of TXS, but they do not reproduce the product distribution observed experimentally. © 2019 Wiley Periodicals, Inc.  相似文献   
8.
Thermal storage and transfer fluids have important applications in industrial, transportation, and domestic settings. Current thermal fluids have relatively low specific heats, often significantly below that of water. However, by introducing a thermochemical reaction to a base fluid, it is possible to enhance the fluid's thermal properties. In this work, density functional theory (DFT) is used to screen Diels–Alder reactions for use in aqueous thermal fluids. From an initial set of 52 reactions, four are identified with moderate aqueous solubility and predicted turning temperature near the liquid region of water. These reactions are selectively modified through 60 total functional group substitutions to produce novel reactions with improved solubility and thermal properties. Among the reactions generated by functional group substitution, seven have promising predicted thermal properties, significantly improving specific heat (by as much as 30.5%) and energy storage density (by as much as 4.9%) compared to pure water.  相似文献   
9.
Evaluation of ligand-binding affinity using the atomic coordinates of a protein-ligand complex is a challenge from the computational point of view. The availability of crystallographic structures of complexes with binding affinity data opens the possibility to create machine-learning models targeted to a specific protein system. Here, we describe a new methodology that combines a mass-spring system approach with supervised machine-learning techniques to predict the binding affinity of protein-ligand complexes. The combination of these techniques allows exploring the scoring function space, generating a model targeted to a protein system of interest. The new model shows superior predictive performance when compared with classical scoring functions implemented in the programs Molegro Virtual Docker, AutoDock4, and AutoDock Vina. We implemented this methodology in a new program named Taba. Taba is implemented in Python and available to download under the GNU license at https://github.com/azevedolab/taba . © 2019 Wiley Periodicals, Inc.  相似文献   
10.
DNA-based aptamers are commonly used recognition elements in biosensors for a range of target molecules. Here, the development of a wavelength-shifting optical module for a DNA-based adenosine-binding aptamer is described. It applies the combination of two photostable cyanine-styryl dyes as covalent modifications. This energy-transfer pair is postsynthetically attached to oligonucleotides via a copper(I)-catalyzed azide–alkyne cycloaddition by two structurally different approaches: 1) as nucleotide modifications at the 2′-position of uridines and 2) as nucleotide substitutions using (S)-amino-1,2-propanediol as acyclic linker between the phosphodiester bridges. Both dyes exhibit a remarkable photostability. A library of DNA aptamers consisting of different combinations of the two dyes in diagonal orientations were evaluated by their emission color contrast as readout. Further optimization led to aptasensors with improved fluorescent readout as compared with previously reported aptasensors. This approach described is synthetically facile using simple propargylated phosphoramidites as DNA building blocks. As such, this approach could be applied for other dyes and other chemical/biological applications.  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