全文获取类型
收费全文 | 15476篇 |
免费 | 2571篇 |
国内免费 | 1911篇 |
专业分类
化学 | 11341篇 |
晶体学 | 227篇 |
力学 | 861篇 |
综合类 | 147篇 |
数学 | 1613篇 |
物理学 | 5769篇 |
出版年
2024年 | 63篇 |
2023年 | 318篇 |
2022年 | 563篇 |
2021年 | 575篇 |
2020年 | 589篇 |
2019年 | 652篇 |
2018年 | 526篇 |
2017年 | 510篇 |
2016年 | 719篇 |
2015年 | 734篇 |
2014年 | 933篇 |
2013年 | 1233篇 |
2012年 | 1431篇 |
2011年 | 1547篇 |
2010年 | 1037篇 |
2009年 | 1010篇 |
2008年 | 1015篇 |
2007年 | 990篇 |
2006年 | 800篇 |
2005年 | 683篇 |
2004年 | 580篇 |
2003年 | 443篇 |
2002年 | 414篇 |
2001年 | 350篇 |
2000年 | 328篇 |
1999年 | 252篇 |
1998年 | 218篇 |
1997年 | 170篇 |
1996年 | 185篇 |
1995年 | 148篇 |
1994年 | 157篇 |
1993年 | 118篇 |
1992年 | 115篇 |
1991年 | 94篇 |
1990年 | 87篇 |
1989年 | 77篇 |
1988年 | 44篇 |
1987年 | 41篇 |
1986年 | 42篇 |
1985年 | 40篇 |
1984年 | 29篇 |
1983年 | 16篇 |
1982年 | 6篇 |
1981年 | 10篇 |
1980年 | 6篇 |
1979年 | 11篇 |
1977年 | 6篇 |
1976年 | 8篇 |
1975年 | 6篇 |
1957年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
301.
302.
EPR study showed that the semi‐quinone radical anion of 2,3‐dicyano‐5,6‐dichloro‐1,4‐benzoquinone (DDQ) was formed in a charge transfer process between ground‐state DDQ as acceptor and each one of following ground state donors, i.e., 4‐methyl‐4′‐tridecyl‐2, 2′‐bipyridyl; 4‐methyl‐4′‐nonyl‐2, 2′‐bipyridyl; bis (2,2′‐bipyridyl) (4‐methyl‐4′‐heptadecyl‐2, 2′‐bipyridyl)ruthenium(2+) perchlorate and perylene. EPR study also showed that there are perylene cation radical and pyrene cation radical in the following experimental conditions: (a) in 98% sulfuric add. (b) 10?3 mol/L perylene (or pyrene) was dissolved in trifluoroacetic acid‐nitrobenzene (1: 1 V/V). 相似文献
303.
The condensation of 4‐amino‐3‐aryl‐5‐mercapto‐1, 2, 4‐triazoles (1a‐f) with 6‐/8‐substituted 1,4‐dihydro‐4‐oxo‐quinoline‐3‐carboxylic adds (2a‐d) in the presence of phosphorus oxychloride on refluxng or under microwave irradiation gave twenty four novel 3‐aryl‐6‐ (6‐/8‐substituted 4‐chloroquinoline‐3‐yl)‐s‐triazolo[3,4‐b]‐1, 3,4‐thiadiazoles (4a‐x), Considerable increase in the reaction rate has been observed with improved yields under microwave irradiation. The structures of the compounds synthesized were determined by elemental analyses, IR, 1H NMR and MS spectra. Their spectral properties and the reaction mechanism were also discussed. The preliminary biological test showed that some of compounds bad moderate antibacterial activities. 相似文献
304.
The catalytic properties of MCl2 (PPh3)2 (M = Fe, A; Co, B; Ni, C) in combination with ethylaluminoxane (EAO) as cocatalyst for ethylene oligomerization have been investigated. Treatment of the MCl2 (PPh3)2 complexes with EAO in toluene generated active catalysts in situ that are capable of oligomerizating ethylene to low‐carbon olefins. The catalytic activity and product distribution were affected by reaction condition, such as reaction temperature, the ratios of Al/M and the reaction time. The activity of 1.70 × 105 g oligomers/ (mol Co. h) for the catalytic system of CoCl2(PPh3)2 with EAO at 200°C was observed, with the selectivity of 91.1% to C4–10 olefins and 70.7% to C4–10 linear α‐olefins. 相似文献
305.
Lei Zhang Zhi‐Liang Liu Lin‐Hong Weng Dai‐Zheng Liao Zong‐Hui Jiang Shi‐Ping Yan Pan‐Wen Shen Martin Baumgarten 《Helvetica chimica acta》2001,84(4):834-841
The structure of a novel oxido‐aminoxyl (=`nitronyl nitroxide') biradical, 2,2′‐(1‐oxidopyridine‐2,6‐diyl)bis[4,5‐dihydro‐4,4,5,5‐tetramethyl‐3‐oxido‐1H‐imidazol‐1‐oxyl] hydrate ( 1 ⋅H2O) was established by X‐ray analysis in the solid state: monoclinic, space group P21/c, Z=4 with a=12.621(4), b=15.704(5), and c=13.001(4) Å, and β=115.202(6)°. Variable‐temperature magnetic susceptibilities show a weak antiferromagnetic interaction between the two oxido‐substituted aminoxyl moieties of 1 , indicative of a singlet ground state. AM1 Calculations located minima for the possible structure based on the X‐ray crystal structure. A hybride density‐functional‐theory calculation with the UB3LYP method from the X‐ray crystal structure establishes the same spin sign in the two aminoxyl moieties and shows that a small spin density is localized at the C‐atoms of the pyridine moiety. These theoretic results are in good agreement with the determined weak antiferromagnetic interaction of 1 . 相似文献
306.
本文用X射线衍射及差热分析等方法, 研究了BaO-B_2O_3-GeO_2(BaO≤50 mol%)三元系的室温截面相图。发现了一个新的三元化合物Ba_3B_6Ge_2O_(16)(3BaO·3B_2O_3·2GeO_2)。并且研究了BaB_2O_4-Ba_3B_6Ge_2O_(16), BaGeO_3-Ba_3B_6Ge_2O_(16), BaB_2O_4-BaGeO_3三个二元系相图。它们都是共晶体系, 共晶温度分别为937±3 ℃、879±3 ℃875±3 ℃; 共晶组分分别为(BaO)_(0.42)(B_2O_3)_(0.42)(GeO_2)_(0.16), (BaO)_(0.42)(B_2O_3)_(0.24)(GeO_2)_(0.34), (BaO)_(0.50)(B_2O_3)_(0.27)(GeO_2)_(0.23)。这三个二元系组成一个三元共晶体系, 其三元共晶温度为870±3 ℃, 共晶组分为(BaO)_(0.46)(B_2O_3)_(0.27)(GeO_2)_(0.27)。 相似文献
307.
Rong-Chang Liang Arnost Reiser 《Journal of polymer science. Part A, Polymer chemistry》1987,25(2):451-465
A group of block copolymers containing diacetylenes as chain extenders in their hard segments was prepared, based on urethanes, esters, ureas, and amides as hard segments and polybutadienes, polyethers, polyesters, and polysiloxanes as soft segments. Almost all block copolymers were photoreactive, but there was a wide range of sensitivities. The photoreactivity of the copolymers was found to depend on the reactivity of the monomer unit, on the width of the diacetylene stacks in the hard segments, and on the degree of phase separation in the solid films. To explore the range of monomer reactivities we prepared 15 crystalline monomers. Urethanes were in general the most reactive, and this was attributed in part to the specific effect of hydrogen bonding which brings about a shortening of the C1 to C4 distance between diacetylenes tend to reduce the photoreactivity. The behavior of identical diacetylene units in the monomer crystal, in the homopolymer, and in the block copolymer is discussed in this paper. 相似文献
308.
309.
Jie Ju Yu Jin Li Jian Rong Gao Jian Hong Jia Liang Han Wei Jian Sheng Yi Xia Jia 《中国化学快报》2011,22(4):382-384
An aqueous solution of hydrogen peroxide and hydrogen bromide illuminated by a 60 W incandescent light bulb serves as a source of bromine radicals.Various substituted toluenes(NO2,Cl,Br,H,CH3) were high selectively brominated at the benzyl position for monobromination in CH2C12 at ice water with catalyst free.This simple but effective bromination of toluene derivatives with an aqueous H2O2-HBr system is characterized with the use of inexpensive reagents and a lower impact on the environment, which make it a good alternative to the existing bromination methods. 相似文献
310.
Block copolymer-supported Ag Nps (nanoparticles) have either a "cherry"-like or "raspberry"-like morphology [Antonietti, et al., Adv. Mater. 7 (1995) 1000-1005] depending on the amount of silver nitrate loading and the external conditions. Sonication favors silver nitrate and polyethyleneimine diffusion; the nucleation sites are well distributed in the micellar cores, so it is easy to form the cherry-like Ag NP colloids. However, when the amount of silver nitrate is decreased, it is heating that induces the formation of raspberry-like Ag NP colloids. The Ag NP colloids were investigated by transmission electron microscopy to demonstrate the nanosize dimensions and the location of the Ag NPs in the micelles. X-ray diffraction was employed to determine the crystal structure of the Ag NPs. UV-vis spectroscopy was employed for further qualitative characterization of the optical properties of Ag NPs. 相似文献