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   10312篇
  免费   1063篇
  国内免费   201篇
化学   8758篇
晶体学   12篇
力学   28篇
综合类   5篇
数学   27篇
物理学   2746篇
  2023年   71篇
  2022年   119篇
  2021年   202篇
  2020年   337篇
  2019年   263篇
  2018年   236篇
  2017年   178篇
  2016年   361篇
  2015年   344篇
  2014年   440篇
  2013年   677篇
  2012年   411篇
  2011年   488篇
  2010年   385篇
  2009年   455篇
  2008年   435篇
  2007年   493篇
  2006年   436篇
  2005年   371篇
  2004年   326篇
  2003年   342篇
  2002年   1213篇
  2001年   199篇
  2000年   150篇
  1999年   164篇
  1998年   160篇
  1997年   93篇
  1996年   103篇
  1995年   116篇
  1994年   56篇
  1993年   54篇
  1992年   42篇
  1991年   41篇
  1990年   29篇
  1988年   24篇
  1987年   24篇
  1985年   92篇
  1984年   117篇
  1983年   106篇
  1982年   149篇
  1981年   113篇
  1980年   97篇
  1979年   100篇
  1978年   106篇
  1977年   144篇
  1976年   118篇
  1975年   129篇
  1974年   165篇
  1973年   133篇
  1972年   82篇
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
991.
Yb(OTf)3 catalyzed efficient Hantzsch reaction via four-component coupling reactions of aldehydes, dimedone, ethyl acetoacetate and ammonium acetate at ambient temperature was described as the preparation of polyhydroquinoline derivatives. The process presented here is operationally simple, environmentally benign and has excellent yield. Furthermore, the catalyst can be recovered conveniently and reused efficiently.  相似文献   
992.
Precise control and knowledge of surface structures are essential inorder to meet the requirements of today's and future materials. One possiblegrowth technique capable of meeting the requirements is atomic layer epitaxy(ALE). ALE is based on sequentially applied saturated gas-solid reactions,which provide the means for adsorption controlled material deposition atatomic layer level. In this paper the potentiality of the use of porousmaterials in a detailed study of adsorption controlled growth is discussed.At the same time the study promotes the application of adsorption controlledmaterials processing for advanced catalysts manufacturing.  相似文献   
993.
The AM1 method was used to analyze the factors that correlate with regioselectivity in the addition of radicals to 1,2-disubstituted unsaturated compounds. The rate constants of the addition of.CCl3 radicals to RCH=CHC(O)X (R = Ph, Me; X = N-pyrrolidyl) were determined by ESR. The analysis of the spin density distribution in mono- and 1,2-disubstituted alkenes and the experimental values for the rate constants of the addition of.CCl3 radicals to these alkenes allowed the authors to conclude that the efficiency of the addition of.CC13 to unsaturated compounds depends only on steric effects.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 452–455, March, 1995.  相似文献   
994.
Features of the ignition kinetics of an H2/air mixture in the supersonic flow behind an inclined shock front are analyzed when asymmetric vibrations of a small amount (<1%) of O3 molecules specially introduced into the initial mixture are excited by 9.7 μm wavelength radiation. It is shown that this radiation leads to intensification of the chain reactions and makes it possible to organize combustion at small distances from the front (of the order of 1 m) of even relatively weak shocks at small values of the laser radiation energies absorbed by the gas. This method of initiating combustion in a supersonic flow is 10–100 times more efficient than the thermal method.__________Translated from Izvestiya Rossiiskoi Academii Nauk, Mekhanika Zhidkosti i Gaza, No. 2, 2005, pp. 157–167.Original Russian Text Copyright © 2005 by Lukhovitskii, Starik, and Titova.  相似文献   
995.
996.
The reactivity of the heteronuclear oxide cluster [Ga2Mg2O5].+, bearing an unpaired electron at a bridging oxygen atom (Ob.?), towards methane and ethane has been studied using Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR‐MS). Hydrogen‐atom transfer (HAT) from both methane and ethane to the cluster ion is identified experimentally. The reaction mechanisms of these reactions are elucidated by state‐of‐the‐art quantum chemical calculations. The roles of spin density and charge distributions in HAT processes, as revealed by theory, not only deepen our mechanistic understanding of C? H bond activation but also provide important guidance for the rational design of catalysts by pointing to the particular role of doping effects.  相似文献   
997.
We demonstrate the fabrication of graphene liquid marbles as photothermal miniature reactors with precise temperature control for reaction kinetics modulation. Graphene liquid marbles show rapid and highly reproducible photothermal behavior while maintaining their excellent mechanical robustness. By tuning the applied laser power, swift regulation of graphene liquid marble’s surface temperature between 21–135 °C and its encapsulated water temperature between 21–74 °C are demonstrated. The temperature regulation modulates the reaction kinetics in our graphene liquid marble, achieving a 12‐fold superior reaction rate constant for methylene blue degradation than at room temperature.  相似文献   
998.
In an endeavor to provide an efficient route to natural product hybrids, described herein is an efficient, highly stereoselective, one‐pot process comprising an organocatalytic conjugate addition of 1,3‐dicarbonyls to α,β‐unsaturated aldehydes followed by an intramolecular isocyanide‐based multicomponent reaction. This approach enables the rapid assembly of complex natural product hybrids including up to four different molecular fragments, such as hydroquinolinone, chromene, piperidine, peptide, lipid, and glycoside moieties. The strategy combines the stereocontrol of organocatalysis with the diversity‐generating character of multicomponent reactions, thus leading to structurally unique peptidomimetics integrating heterocyclic, lipidic, and sugar moieties.  相似文献   
999.
Mechanochemistry is an effective method for the preparation of multicomponent crystal systems. In the present work, we propose an alternative to the established liquid‐assisted grinding (LAG) approach. Polymer‐assisted grinding (POLAG) is demonstrated to provide a new class of catalysts for improving reaction rate and increasing product diversity during mechanochemical cocrystallization reactions. We demonstrate that POLAG provides advantages comparable to the conventional liquid‐assisted process, whilst eliminating the risk of unwanted solvate formation as well as enabling control of resulting particle size. It represents a new approach for the development of functional materials through mechanochemistry, and possibly opens new routes toward the understanding of the mechanisms and pathways of mechanochemical cocrystal formation.  相似文献   
1000.
The non‐aqueous Li–air (O2) battery has attracted intensive interest because it can potentially store far more energy than today′s batteries. Presently Li–O2 batteries suffer from parasitic reactions owing to impurities, found in almost all non‐aqueous electrolytes. Impurities include residual protons and protic compounds that can react with oxygen species, such as the superoxide (O2?), a reactive, one‐electron reduction product of oxygen. To avoid the parasitic reactions, it is crucial to have a fundamental understanding of the conditions under which reactive oxygen species are generated in non‐aqueous electrolytes. Herein we report an in situ spectroscopic study of oxygen reduction on gold in a dimethyl sulfoxide electrolyte containing phenol as a proton source. It is shown directly that O2?, not HO2, is the first stable intermediate during the oxygen reduction process to hydrogen peroxide. The unusual stability of O2? is explained using density functional theory (DFT) calculations.  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