全文获取类型
收费全文 | 6091篇 |
免费 | 780篇 |
国内免费 | 1447篇 |
专业分类
化学 | 5757篇 |
晶体学 | 41篇 |
力学 | 452篇 |
综合类 | 67篇 |
数学 | 988篇 |
物理学 | 1013篇 |
出版年
2024年 | 24篇 |
2023年 | 132篇 |
2022年 | 229篇 |
2021年 | 349篇 |
2020年 | 515篇 |
2019年 | 317篇 |
2018年 | 282篇 |
2017年 | 258篇 |
2016年 | 354篇 |
2015年 | 279篇 |
2014年 | 413篇 |
2013年 | 563篇 |
2012年 | 354篇 |
2011年 | 360篇 |
2010年 | 288篇 |
2009年 | 329篇 |
2008年 | 341篇 |
2007年 | 331篇 |
2006年 | 315篇 |
2005年 | 276篇 |
2004年 | 253篇 |
2003年 | 304篇 |
2002年 | 299篇 |
2001年 | 182篇 |
2000年 | 141篇 |
1999年 | 115篇 |
1998年 | 106篇 |
1997年 | 74篇 |
1996年 | 93篇 |
1995年 | 67篇 |
1994年 | 64篇 |
1993年 | 44篇 |
1992年 | 57篇 |
1991年 | 22篇 |
1990年 | 22篇 |
1989年 | 18篇 |
1988年 | 23篇 |
1987年 | 16篇 |
1986年 | 21篇 |
1985年 | 29篇 |
1984年 | 16篇 |
1983年 | 6篇 |
1982年 | 17篇 |
1981年 | 7篇 |
1980年 | 4篇 |
1979年 | 5篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1971年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有8318条查询结果,搜索用时 15 毫秒
81.
Radical anions of nitrobenzothiazoles: EPR study of conjugative properties of benzothiazolyl systems
Francesco Ciminale 《Tetrahedron letters》2004,45(30):5849-5852
The electrochemical reduction in DMSO of the five isomers of nitrobenzothiazole (NBTZ) gave quite persistent radical anions that could be easily characterised by EPR spectroscopy. By contrast, the chemical reduction in alkaline solution, that is by t-BuOK in DMSO or by glucose and MeOK in MeOH, presented some problems with 6- and 4-NBTZ, and in the case of 2-NBTZ did not provide any detectable paramagnetic species. The internal consistency of coupling constants of the nitrobenzothiazole radical anions is in good agreement with the conjugative properties of the various benzothiazolyl systems and allows rectifying a recent EPR characterisation of 6-NBTZ radical anion. 相似文献
82.
CuO/活性炭和Fe2O3/活性炭催化还原NO 总被引:4,自引:0,他引:4
CuO/活性炭和Fe_2O_3/活性炭催化还原NO高志明,赵震,杨向光,吴越(中国科学院长春应用化学研究所长春130022)关键词活性炭,还原,NO,氧化铜,氧化铁目前,对固定源的NO处理是采用V2O5/TiO2作催化剂,NH3作还原剂的选择催化还原方... 相似文献
83.
L. Poul S. Ammar N. Jouini F. Fievet F. Villain 《Journal of Sol-Gel Science and Technology》2003,26(1-3):261-265
A new route for the preparation of powdery metal, oxide and hydroxide materials is presented as a chimie douce alternative to the sol-gel method. It consists in the reduction or the hydrolysis of a metal salt dissolved and heated in a polyol medium. It appears through zinc and cobalt examples that the use of acetate precursors contrarily to chloride or sulfate ones leads to the precipitation of a solid (metal, oxide, hydroxide) whose nature depends on two main factors: the hydrolysis ratio, defined by the water to metal molar ratio, and the reaction temperature. As in the sol-gel method, acetate leads to the formation of intermediate alkoxyacetate complex. The absence of water favors metal formation while its presence favors oxide or hydroxyacetate formation. 相似文献
84.
The reactions of substituted 3-cyanopyridine-2(1H)-thiones and 3-cyano-2-(methylthio)pyridines with lithium aluminum hydride in anhydrous diethyl ether afforded the corresponding 3-aminomethyl derivatives, which were used in the synthesis of the corresponding amides. 相似文献
85.
Vadim?B.?Fetisov Alexander?N.?Ermakov Galina?M.?Belysheva Andrey?V.?Fetisov Valentin?M.?Kamyshov Khiena?Z.?BraininaEmail author 《Journal of Solid State Electrochemistry》2004,8(8):565-571
The present paper deals with the electrochemical behavior of magnetite microcrystals in an acid medium. A voltammetric method employing a carbon-paste electroactive electrode (CPEE) with an organic binder was used. It was found that the cathodic voltammograms, which were recorded at different scan rates, formed a set bounded in the space of i–E parameters by a generalizing voltammetric curve corresponding to the effective potential scan rate eff. In other words, all curves are situated under one enveloping curve, just as the smaller dolls sit in the largest doll of a Russian doll. Reverse currents (a cathodic current in the anodic direction of the potential scan) were observed on the cyclic voltammogram. Forward and reverse currents obey the same laws and have one and the same generalizing curve, which could be taken as the magnetite characteristic. 相似文献
86.
The reductions of several substituted acetophenones using supercritical 2-propanol were carried out to estimate the Hammett's reaction constant (ρ=0.33). Also, the reduction of acetophenone using supercritical deuteriated 2-propanol was carried out to determine the rate-determining step. The kinetic isotope effects were observed in the reduction using 2-deuterio-2-propanol (kH/kD=1.6) and O-deuterio-2-propanol (kH/kD=2.0). These findings suggest that the reaction proceeds via a cyclic transition state between acetophenone and 2-propanol similar to that of the Meerwein-Ponndorf-Verley reduction. 相似文献
87.
88.
We describe the first example of a catalytic enantioselective intermolecular reductive aldol reaction. Three types of reactions were studied: (1) reactions between acetophenone and methyl acrylate; (2) reactions between symmetric ketones and β-substituted α,β-unsaturated esters; and (3) reactions between acetophenone derivatives and an allenic ester. Although only moderate enantioselectivity was obtained in the first reaction type, high to excellent enantioselectivity was realized in the enantio-induction at the α-position in the second reaction type and at the δ-position in the third reaction type. Specifically, the third reaction type afforded the corresponding tertiary alcohols with up to 99% ee. Pre-activation of the nucleophile by silyl enolate formation is not necessary in these one-pot catalytic enantioselective reductive aldol reactions. 相似文献
89.
《中国化学快报》2020,31(5):1207-1212
Developing high efficiency and low cost electrocatalysts is critical for the enhancement of oxygen reduction reaction (ORR), which is the fundamental for the development and commercialization of renewable energy conversion technology. Herein, zinc-nitrogen-carbon (Zn-N-C) was prepared by using biomass resource chitosan via a facile carbon bath method. The obtained Zn-N-C delivered a high specific surface area (794.7 cm2/g) together with pore volume (0.49 cm3/g). During the electrochemical evaluation of oxygen reduction reaction (ORR), Zn-N-C displayed high activity for ORR with an onset potential E0 = 0.96 VRHE and a half wave potential E1/2 = 0.86 VRHE, which were more positive than those of the commercial 20 wt% Pt/C benchmark catalyst (E0 = 0.96 VRHE and E1/2 = 0.81 VRHE). In addition, the Zn-N-C catalyst also had a better stability and methanol tolerance than those of the Pt/C catalyst. 相似文献
90.
An efficient clean-up procedure coupled with a high performance liquid chromatography (HPLC) with on-line electrochemical (EC) reduction and fluorescence detection (FLD) was developed to quantify nitrated polycyclic aromatic hydrocarbons (NPAHs) in the airborne particulate. In this process, NPAHs were extracted ultrasonically followed by analysis by using a reversed phase column with an aqueous eluent containing 70% aqueous acetonitrile and sodium monocholoroacetate as a buffer solution. The extraction efficiencies were above 83% for 1-nitropyrene and 1,3-dinitropyrene (1,3-DNP) 1,6-DNP, and 1,8-DNP, and calibration graphs were linear with very good correlation coefficients (r>0.999) and the detection limits were in the range of 1.0-2.2 pg for dinitropyrenes and nitropyrene. The proposed method provides a relatively simple and convenient procedure for determining the NPAHs samples in airborne particulate. 相似文献