全文获取类型
收费全文 | 4811篇 |
免费 | 800篇 |
国内免费 | 540篇 |
专业分类
化学 | 3343篇 |
晶体学 | 46篇 |
力学 | 347篇 |
综合类 | 24篇 |
数学 | 605篇 |
物理学 | 1786篇 |
出版年
2024年 | 18篇 |
2023年 | 102篇 |
2022年 | 118篇 |
2021年 | 181篇 |
2020年 | 199篇 |
2019年 | 192篇 |
2018年 | 178篇 |
2017年 | 137篇 |
2016年 | 269篇 |
2015年 | 220篇 |
2014年 | 254篇 |
2013年 | 368篇 |
2012年 | 472篇 |
2011年 | 456篇 |
2010年 | 308篇 |
2009年 | 286篇 |
2008年 | 306篇 |
2007年 | 250篇 |
2006年 | 243篇 |
2005年 | 211篇 |
2004年 | 185篇 |
2003年 | 158篇 |
2002年 | 132篇 |
2001年 | 111篇 |
2000年 | 94篇 |
1999年 | 100篇 |
1998年 | 73篇 |
1997年 | 73篇 |
1996年 | 73篇 |
1995年 | 60篇 |
1994年 | 45篇 |
1993年 | 45篇 |
1992年 | 27篇 |
1991年 | 38篇 |
1990年 | 35篇 |
1989年 | 26篇 |
1988年 | 12篇 |
1987年 | 12篇 |
1986年 | 14篇 |
1985年 | 18篇 |
1984年 | 9篇 |
1983年 | 10篇 |
1982年 | 7篇 |
1981年 | 4篇 |
1974年 | 2篇 |
1971年 | 2篇 |
1944年 | 4篇 |
1942年 | 3篇 |
1941年 | 1篇 |
1940年 | 1篇 |
排序方式: 共有6151条查询结果,搜索用时 15 毫秒
851.
Lina Ma Wenjuan Li Hui Xi Xiaohui Bai Enlu Ma Dr. Xiaoyu Yan Prof. Dr. Zhiping Li 《Angewandte Chemie (International ed. in English)》2016,55(35):10410-10413
Exploiting catalytic carbonyl–olefin metathesis is an ongoing challenge in organic synthesis. Reported herein is an FeCl3‐catalyzed ring‐closing carbonyl–olefin metathesis. The protocol allows access to a range of carbo‐/heterocyclic alkenes with good efficiency and excellent trans diastereoselectivity. The methodology presents one of the rare examples of catalytic ring‐closing carbonyl–olefin metathesis. This process is proposed to take place by FeCl3‐catalyzed oxetane formation followed by retro‐ring‐opening to deliver metathesis products. 相似文献
852.
Identification of HcgC as a SAM‐Dependent Pyridinol Methyltransferase in [Fe]‐Hydrogenase Cofactor Biosynthesis
下载免费PDF全文
![点击此处可从《Angewandte Chemie (International ed. in English)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Takashi Fujishiro Liping Bai Dr. Tao Xu Dr. Xiulan Xie Dr. Michael Schick Jörg Kahnt Prof. Dr. Michael Rother Prof. Dr. Xile Hu Dr. Ulrich Ermler Dr. Seigo Shima 《Angewandte Chemie (International ed. in English)》2016,55(33):9648-9651
Previous retrosynthetic and isotope‐labeling studies have indicated that biosynthesis of the iron guanylylpyridinol (FeGP) cofactor of [Fe]‐hydrogenase requires a methyltransferase. This hypothetical enzyme covalently attaches the methyl group at the 3‐position of the pyridinol ring. We describe the identification of HcgC, a gene product of the hcgA‐G cluster responsible for FeGP cofactor biosynthesis. It acts as an S‐adenosylmethionine (SAM)‐dependent methyltransferase, based on the crystal structures of HcgC and the HcgC/SAM and HcgC/S‐adenosylhomocysteine (SAH) complexes. The pyridinol substrate, 6‐carboxymethyl‐5‐methyl‐4‐hydroxy‐2‐pyridinol, was predicted based on properties of the conserved binding pocket and substrate docking simulations. For verification, the assumed substrate was synthesized and used in a kinetic assay. Mass spectrometry and NMR analysis revealed 6‐carboxymethyl‐3,5‐dimethyl‐4‐hydroxy‐2‐pyridinol as the reaction product, which confirmed the function of HcgC. 相似文献
853.
In this article, we prove the existence of a random attractor for stochastic nonclassical diffusion equations on unbounded domains, and the asymptotic compactness of the random dynamical system is established by a tail-estimates method. 相似文献
854.
Fractional‐order switching type control law design for adaptive sliding mode technique of 3D fractional‐order nonlinear systems
下载免费PDF全文
![点击此处可从《Complexity》网站下载免费的PDF全文](/ch/ext_images/free.gif)
In this article, an adaptive sliding mode technique based on a fractional‐order (FO) switching type control law is designed to guarantee robust stability for a class of uncertain three‐dimensional FO nonlinear systems with external disturbance. A novel FO switching type control law is proposed to ensure the existence of the sliding motion in finite time. Appropriate adaptive laws are shown to tackle the uncertainty and external disturbance. The calculation formula of the reaching time is analyzed and computed. The reachability analysis is visualized to show how to obtain a shorter reaching time. A stability criteria of the FO sliding mode dynamics is derived based on indirect approach to Lyapunov stability. Effectiveness of the proposed control scheme is illustrated through numerical simulations. © 2015 Wiley Periodicals, Inc. Complexity 21: 363–373, 2016 相似文献
855.
Bing Bai Byungjoon Yoo Xiuquan Deng Iljoo Kim Dehua Gao 《Computational & Mathematical Organization Theory》2016,22(2):184-211
IT capability as an important source of competitive advantage has been strongly emphasized in the strategic management literature, yet the formation and evolution of IT capability in the ever-changing business environment are not well explained. To fill this gap, this paper takes routine as the unit of analysis, depicts a micro-interpretation of the formation and evolution of IT capabilities from the routine-based and dynamic perspective, and uses the agent-based simulation methodology to simulate the evolutionary process of IT capability as well as to identify the underlying principles. In order to provide a better presentation of the evolutionary process, a routine-based view of the enterprise explicitly recognizes relationships of IT resources and capabilities. The simulation results show that the evolution of IT capability is a dynamic adaption and learning process. From the routine-based view, the evolution of IT capability is indeed the process of variation, selection, and retention for IT routines. 相似文献
856.
Nickel-cobalt binary oxide/reduced graphene oxide (G-NCO) composite with high capacitance is synthesized via a mild method for electrochemical capacitors. G-NCO takes advantages of reduced graphene oxide (RGO) and nickel-cobalt binary oxide. As an appropriate matrix, RGO is beneficial to form homogeneous structure and improve the electron transport ability. The binary oxide owns more active sites than those of nickel oxide and cobalt oxide to promote the redox reaction. Attributed to the well crystallinity, homogeneous structure, increased active sites, and improved charge transfer property, the G-NCO composite exhibits highly enhanced electrochemical performance compared with G-NiO and G-Co3O4 composites. The specific capacitance of the G-NCO composite is about 1750 F g?1 at 1 A g?1 together with capacitance retention of 79 % (900/1138 F g?1) over 10,000 cycles at 4 A g?1. To research its practical application, an asymmetric supercapacitor with G-NCO as positive electrode and activated carbon as negative electrode was fabricated. The asymmetric device exhibits a prominent energy density of 37.7 Wh kg?1 at a power density of 800 W kg?1. The modified G-NCO composite shows great potential for high-capacity energy storage. 相似文献
857.
We introduce the so-called naive tests and give a brief review of the new developments. Naive testing methods are easy to understand and perform robustly, especially when the dimension is large. We focus mainly on reviewing some naive testing methods for the mean vectors and covariance matrices of high-dimensional populations, and we believe that this naive testing approach can be used widely in many other testing problems. 相似文献
858.
Superconvergence and recovery type a posteriori error estimators are analyzed for Pian and Sumihara's 4-node hybrid stress quadrilateral finite element method for linear elasticity problems. Superconvergence of order O(h~(1+min){α,1}) is established for both the displacement approximation in H~1-norm and the stress approximation in L~2-norm under a mesh assumption, where α 0 is a parameter characterizing the distortion of meshes from parallelograms to quadrilaterals. Recovery type approximations for the displacement gradients and the stress tensor are constructed, and a posteriori error estimators based on the recovered quantities are shown to be asymptotically exact. Numerical experiments confirm the theoretical results. 相似文献
859.
Ya-Ting Wang Hou Chen Dong-Ju Wang Liang-Jiu Bai Hui Xu Wen-Xiang Wang 《Chemical Papers》2016,70(9):1171-1184
Corn stalk-based adsorbents modified from corn stalk were prepared by Cu(0)-mediated reversible-deactivation radical polymerization (Cu(0)-mediated RDRP). They were applied to remove metal ions and they exhibited good adsorption capacity, especially for Hg(II). Adsorption properties of corn stalk can be enhanced by introducing cyano, amino, amidoxime, and carboxyl groups onto its surface, which results in efficient adsorbents for different metal ions. TGA, SEM, EA, and FTIR analyses were employed to characterize the structures of corn stalk-graft-polyacrylonitrile (CS-g-PAN), corn stalk-graft-polyacrylamide (CS-g-PAM), amidoxime corn stalk-graft-polyacrylonitrile (AO CS-g-PAN) and carboxyl corn stalk-graft-poly(methyl acrylate) (CO CS-g-PMA). The maximum adsorption capacity for Hg(II) was 8.06 mmol g?1 of AO CS-g-PAN. Kinetics of the Hg(II) adsorption on AO CS-g-PAN was found to follow the pseudo-second-order model and the adsorption isotherms were well fitted with the Freundlich isotherm model. 相似文献
860.
A facile approach was exploited for fast preparation of polymer-based monoliths in UV-transparent fused-silica capillaries via “one-pot” photo-initiated thiol-acrylate polymerization reaction of dipentaerythritolpenta-/hexaacrylate (DPEPA) and 1-octadecanethiol (ODT) in the presence of porogenic solvents (1-butanol and ethylene glycol). Due to relative insensitivity of oxygen inhibition in thiol-ene free-radical polymerization, the polymerization could be performed within 5 min. The effects of composition of prepolymerization solution on the morphology and permeability of poly(ODT-co-DPEPA) monoliths were investigated in detail by adjusting the content of monomer and binary porogen ratio. The physical properties of poly(ODT-co-DPEPA) monoliths were characterized by Fourier transform infrared spectroscopy (FT-IR), mercury intrusion porosimetry (MIP) and nitrogen adsorption/desorption measurement. The evaluation of chromatographic performance was carried out by capillary liquid chromatography (cLC). The results indicated that the poly(ODT-co-DPEPA) monolith was homogeneous and permeable, and also possessed a typical reversed-phase retention mechanism in cLC with high efficiency (∼75,000 N m−1) for separation of alkylbenzenes. Eventually, the further separation of tryptic digest of proteins by cLC tandem mass spectrometry (cLC-MS/MS) demonstrated its potential in the analysis of biological samples. 相似文献