全文获取类型
收费全文 | 30121篇 |
免费 | 5886篇 |
国内免费 | 9787篇 |
专业分类
化学 | 24094篇 |
晶体学 | 1217篇 |
力学 | 1789篇 |
综合类 | 1071篇 |
数学 | 4330篇 |
物理学 | 13293篇 |
出版年
2024年 | 56篇 |
2023年 | 337篇 |
2022年 | 903篇 |
2021年 | 1008篇 |
2020年 | 1041篇 |
2019年 | 1051篇 |
2018年 | 931篇 |
2017年 | 1232篇 |
2016年 | 1150篇 |
2015年 | 1542篇 |
2014年 | 1825篇 |
2013年 | 2522篇 |
2012年 | 2543篇 |
2011年 | 2764篇 |
2010年 | 2473篇 |
2009年 | 2546篇 |
2008年 | 2987篇 |
2007年 | 2639篇 |
2006年 | 2485篇 |
2005年 | 2094篇 |
2004年 | 1737篇 |
2003年 | 1323篇 |
2002年 | 1249篇 |
2001年 | 1287篇 |
2000年 | 1273篇 |
1999年 | 830篇 |
1998年 | 491篇 |
1997年 | 372篇 |
1996年 | 398篇 |
1995年 | 363篇 |
1994年 | 333篇 |
1993年 | 296篇 |
1992年 | 236篇 |
1991年 | 198篇 |
1990年 | 208篇 |
1989年 | 175篇 |
1988年 | 162篇 |
1987年 | 121篇 |
1986年 | 104篇 |
1985年 | 72篇 |
1984年 | 94篇 |
1983年 | 88篇 |
1982年 | 67篇 |
1981年 | 53篇 |
1980年 | 40篇 |
1979年 | 32篇 |
1978年 | 14篇 |
1976年 | 9篇 |
1973年 | 6篇 |
1959年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 609 毫秒
71.
Cationic polymer as a kind of flocculant is widely applied in purification treatment of waste water. Because it has positive charge group, it is able to connect strongly the suspended matters, short cellulose and other microparticles. The research on synthesis of cationic polymer and application in treatment of waste water is very universal abroad. But domestic research on those is not general. The technology of synthesis of PEM is simple, and the production cost is low. It is easy to apply in treatment of waste water.Synthesis of PEM Emulsion FlocculantSome distilled water, PVA(poly(vinyl alcohol)), EA(ethyl acrylate), and K2S2O8(potassium persulfate) were put into reaction vessel. Kept stirring up under nitrogen. When heated the solution to 40℃, dropped the water solution of MTA[(2-methacryloxylethyl)trimethyl ammonium].Maintained the temperature at 70℃, reacted about 7-8 hours. Then got the PEM emulsion. Changed the ratio of EA and MTA. Obtained a series of PEM emulsions.Stability and Convertibility of PEM EmulsionThe test results showed that when the EA/MTA was 85/15, the PEM emulsion was most stable.When the total monomer quantity was 35%, the convertibility of PEM emulsion was the highest,i.e.98.6%.The MTA Copolymerization Ratio and Morphology of PEM EmulsionWhen the monomers EA/MTA=85/15 and total monomer quantity was 35%, the MTA copolymerization ratio of PEM emulsion was 95.15%(the highest), and the PEM emulsion was some microspheres with 100-180nm of diameter.The Test Results of PEM Emulsion in Treatment of Waste Water The PEM emulsion flocculant was applied in treatment of waste water of paper mill, and measured the precipitation time(t) and transmittancy(T). The test results were showed in table 1. The optimum value of PEM which was able to make the waste water of paper mill into clear water was 0.008%. 相似文献
72.
A New Triterpene from the Orchid Pholidota yunnanensis 总被引:2,自引:0,他引:2
ZhiMingBI ZhengTaoWANG LuoShanXU 《中国化学快报》2004,15(10):1179-1181
A new triterpene, 25-methylenecyclopholidonyl p-hydroxy-trans-cinnamate, was isolated from a orchid Pholidota yunnanensis. The structure elucidation and ^1H, ^13C-NMR assignments were achieved by spectral and chemical method. 相似文献
73.
WeiMingXU LuLingWU XianHUANG 《中国化学快报》2004,15(11):1279-1280
A mild, regioselective 1, 3-dipolar cycloaddition protocol for the preparation ofphenylselenomethyl isoxazolines through substituted allyl phenyl selenides and nitrile oxides wasreported. 相似文献
74.
Qing An WU Zhen Yuan XU Mei ZHENG Dan Qian XU Jun XU Yin Chu SHENCollege of Chemical Engineering Materials Zhejiang University of Technology Hangzhou 《中国化学快报》2004,(7)
Six new 13-O-acylavermectin Bl aglycones(3-8) were synthesized from avermectin B1 aglycone and their bioactivities were evaluated against Spodoptera exigua, Spodoptera eridania, Tetranychus urticae and Aphis fabae. 相似文献
75.
WANG Bingwu XU Guangxian & CHEN Zhida State Key Laboratory of Rare Earth Materials Chemistry Applications College of Chemistry Molecular Engineering Peking University Beijing China 《中国科学B辑(英文版)》2004,47(2)
The titled inorganic fullerene-like molecule (hereafter abbreviated as IFM) was recently synthe-sized by Bai et al.[1], which attracts a lot of interests from inorganic and organometallic chemists, and questions are raised for this smart molecule: (ⅰ) Why CuCl can react with Cp*FeP5 in solvent CH3CN to form IFM? (ⅱ) What is the nature of chemical bond-ing? (ⅲ) What is the covalence of Cu in this mole-cule? In this paper we intend to answer these questions in terms of the soft-hard … 相似文献
76.
The adsorption of asymmetrical triblock copolymers from a non-selective solvent on solid surface has been studied by using Scheutjens-Fleer mean-field theory and Monte Carlo simulation method on lattice model. The main aim of this paper is to provide detailed computer simulation data, taking A8-kB20Ak as a key example, to study the influence of the structure of copolymer on adsorption behavior and make a comparison between MC and SF results. The simulated results show that the size distribution of various configurations and density-profile are dependent on molecular structure and adsorption energy. The molecular structure will lead to diversity of adsorption behavior. This discrepancy between different structures would be enlarged for the surface coverage and adsorption amount with increasing of the adsorption energy. The surface coverage and the adsorption amount as well as the bound fraction will become larger as symmetry of the molecular structure becomes gradually worse. The adsorption layer becomes thicker with increasing of symmetry of the molecule when adsorption energy is smaller but it becomes thinner when adsorption energy is higher. It is shown that SF theory can reproduce the adsorption behavior of asymmetrical triblock copolymers. However, systematic discrepancy between the theory and simulation still exists.The approximations inherited in the mean-filed theory such as random mixing and the allowance of direct back folding may be responsible for those deviations. 相似文献
77.
78.
n阶矩阵A称为完全正的,如果A有分解:A=BBT,其中B为元素非负矩阵,B的最小可能列数称为A的分解指数.本文考察低阶双非负矩阵在整数环上的完全正分解及其分解指数. 相似文献
79.
80.
The equivalent geometrical configurations of linear ordered orthogonal arrays are determined when their strengths are 3 and 4. Existence of such geometrical configurations is investigated. They are also useful in the study of (T, M, S)-nets. 相似文献