全文获取类型
收费全文 | 25807篇 |
免费 | 2541篇 |
国内免费 | 5272篇 |
专业分类
化学 | 27293篇 |
晶体学 | 149篇 |
力学 | 549篇 |
综合类 | 181篇 |
数学 | 2220篇 |
物理学 | 3228篇 |
出版年
2024年 | 30篇 |
2023年 | 360篇 |
2022年 | 536篇 |
2021年 | 878篇 |
2020年 | 1259篇 |
2019年 | 1069篇 |
2018年 | 1100篇 |
2017年 | 1026篇 |
2016年 | 1132篇 |
2015年 | 1003篇 |
2014年 | 1579篇 |
2013年 | 2454篇 |
2012年 | 1453篇 |
2011年 | 1733篇 |
2010年 | 1397篇 |
2009年 | 1504篇 |
2008年 | 1587篇 |
2007年 | 1761篇 |
2006年 | 1592篇 |
2005年 | 1520篇 |
2004年 | 1441篇 |
2003年 | 1154篇 |
2002年 | 709篇 |
2001年 | 604篇 |
2000年 | 607篇 |
1999年 | 512篇 |
1998年 | 449篇 |
1997年 | 401篇 |
1996年 | 345篇 |
1995年 | 336篇 |
1994年 | 329篇 |
1993年 | 243篇 |
1992年 | 243篇 |
1991年 | 175篇 |
1990年 | 143篇 |
1989年 | 131篇 |
1988年 | 110篇 |
1987年 | 78篇 |
1986年 | 75篇 |
1985年 | 81篇 |
1984年 | 66篇 |
1983年 | 33篇 |
1982年 | 55篇 |
1981年 | 41篇 |
1980年 | 34篇 |
1979年 | 29篇 |
1978年 | 33篇 |
1977年 | 34篇 |
1976年 | 43篇 |
1974年 | 39篇 |
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
101.
采用聚丙烯胺溶胶-凝胶法制备了类钙钛矿La1-xNDxSrCoO4(x=0.1~0.9)复合氧化物催化剂,考察了其对CO和C3H8的催化氧化活性,并运用XRD、IR和TPR等手段对催化剂进行了表征.结果表明,所制备的样品均具有K2NiF4型结构,适量Nd2O3的加入增加了LaSrCoO4的催化活性,使LaSrCoO4催化剂粒度变小、晶格畸变率变大及与氧的结合能力减弱,从而有利于CO和C3H8氧化活性的提高. 相似文献
102.
Hiroshi Onishi 《Catalysis Surveys from Japan》2002,6(1-2):1-8
The present account describes first time examples of scanning tunneling microscope (STM) visualization of reaction intermediates on a metal oxide surface. The topographic response of reactant-adsorbed TiO2(110) surfaces to a temperature increment or to a pressure increment was monitored as a sequence of STM images. Acetates thermally decomposing to ketene were resolved in a temperature-jump STM observation. The kinetics of the acetate consumption was determined on the number of acetates resolved in the microscope images and agreed with the macroscopic rate law of ketene production. A pressure-jump study revealed how a chemisorbed carboxylate (RCOO-) is exchanged by another carboxylate (R'COO-) supplied from the ambient vapor phase. An impinging R'COOH was immobilized on the top of a RCOO- and then squeezed itself into the monolayer of the RCOO-. One of the carboxylates in the squeezed state returned to the vapor phase via the immobilized state. 相似文献
103.
104.
L. T. Ashchepkov D. V. Davydov 《Computational Mathematics and Mathematical Physics》2006,46(11):1910-1917
Noncooperative games of a finite number of persons with interval-valued payoff functions are considered. The concept of an equilibrium situation is introduced. A reduction of such games to deterministic noncooperative games is proposed. Properties of the reduced games are discussed. Interval antagonistic and bimatrix games are examined, and illustrative examples are considered. 相似文献
105.
《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(1):130-137
We have shown previously that iodosylbenzene–iron(III ) porphyrin intermediates ( 2 ) are generated in the reactions of oxoiron(IV ) porphyrin π‐cation radicals ( 1 ) and iodobenzene (PhI), that 1 and 2 are at equilibrium in the presence of PhI, and that the epoxidation of olefins by 2 affords high yields of epoxide products. In the present work, we report detailed mechanistic studies on the nature of the equilibrium between 1 and 2 in the presence of iodoarenes (ArI), the determination of reactive species responsible for olefin epoxidation when two intermediates (i.e., 1 and 2 ) are present in a reaction solution, and the fast oxygen exchange between 1 and H218O in the presence of ArI. In the first part, we have provided strong evidence that 1 and 2 are indeed at equilibrium and that the equilibrium is controlled by factors such as the electronic nature of iron porphyrins, the electron richness of ArI, and the concentration of ArI. Secondly, we have demonstrated that 1 is the sole active oxidant in olefin epoxidation when 1 and 2 are present concurrently in a reaction solution. Finally, we have shown that the presence of ArI in a reaction solution containing 1 and H218O facilitates the oxygen exchange between the oxo group of 1 and H218O and that the oxygen exchange is markedly influenced by factors such as ArI incubation time, the amounts of ArI and H218O used, and the electronic nature of ArI. The latter results are rationalized by the formation of an undetectable amount of 2 from the reaction of 1 and ArI through equilibrium that leads to a fast oxygen exchange between 2 and H218O. 相似文献
106.
In this paper, the authors deal with bifunctions defined on complete metric spaces and with values in locally convex spaces ordered by closed convex cones. The aim is to provide a vector version of Ekeland’s theorem related to equilibrium problems. To prove this principle, a weak notion of continuity of a vector-valued function is considered, and some of its properties are presented. Via the vector Ekeland’s principle, existence results for vector equilibria are proved in both compact and noncompact domains. 相似文献
107.
The kinetics of reactions occurring at the gas-exposed surfaces of charged mixed ionic electronic conductors (MIECs) are examined
from theoretical first principles. Analysis based on the classical electrochemical potential-transition state theory model
reveals that the nature of the reactions is electrochemical in general. However, the influence of the surface potential on
the reaction rate is opposite for adsorption and incorporation reactions. Two-dimensional finite volume models of an MIEC
as working electrode in a half-cell configuration are presented. The results for a simple, two-step reduction process show
that the effect of the surface potential on the rate of reactions is minimal for incorporation-limited reactions but more
influential for adsorption-limited reactions.
An erratum to this article is available at . 相似文献
108.
A highly stereoselective synthesis of the side Chain enantiomers of brassinolide and dolicholide through an aldol reaction of 2-(4-methoxy benzyloxyl) proplonylaldehyde (5) with the anion of 1- ( tert -butyldimethylsiloxy )-3-methyl-2-butanone ( 6 ) is described. 相似文献
109.
The N-carboethoxy precursor to (±)-tecomanine has been prepared in 11 steps from 2-methyl-1-buten-3-yne. The key step, Pauson-Khand cyclization of a methylated 5-aza-6-nonen-1-yne succeeds, but only in low yield, a consequence of the dialkyl substitution about the azaenyne framework. Nevertheless, the overall sequence to that point is one of the more efficient to be described. 相似文献
110.
本文研究了催化反应非线性奇摄动边值问题.利用微分不等式理论和方法,得到了问题的解的任意次近似渐近估计. 相似文献