全文获取类型
收费全文 | 6948篇 |
免费 | 367篇 |
国内免费 | 802篇 |
专业分类
化学 | 3466篇 |
晶体学 | 249篇 |
力学 | 671篇 |
综合类 | 102篇 |
数学 | 2094篇 |
物理学 | 1535篇 |
出版年
2024年 | 13篇 |
2023年 | 73篇 |
2022年 | 105篇 |
2021年 | 142篇 |
2020年 | 149篇 |
2019年 | 154篇 |
2018年 | 130篇 |
2017年 | 163篇 |
2016年 | 181篇 |
2015年 | 187篇 |
2014年 | 214篇 |
2013年 | 500篇 |
2012年 | 283篇 |
2011年 | 516篇 |
2010年 | 462篇 |
2009年 | 493篇 |
2008年 | 487篇 |
2007年 | 465篇 |
2006年 | 413篇 |
2005年 | 360篇 |
2004年 | 317篇 |
2003年 | 268篇 |
2002年 | 411篇 |
2001年 | 187篇 |
2000年 | 176篇 |
1999年 | 165篇 |
1998年 | 146篇 |
1997年 | 102篇 |
1996年 | 125篇 |
1995年 | 97篇 |
1994年 | 78篇 |
1993年 | 52篇 |
1992年 | 51篇 |
1991年 | 35篇 |
1990年 | 24篇 |
1989年 | 11篇 |
1988年 | 13篇 |
1985年 | 40篇 |
1984年 | 21篇 |
1983年 | 21篇 |
1982年 | 27篇 |
1981年 | 44篇 |
1980年 | 29篇 |
1979年 | 13篇 |
1978年 | 25篇 |
1977年 | 30篇 |
1976年 | 26篇 |
1975年 | 20篇 |
1974年 | 24篇 |
1973年 | 20篇 |
排序方式: 共有8117条查询结果,搜索用时 15 毫秒
91.
Recycling Chiral Organocatalyst (4S)-Phenoxy-(S)-proline for Direct Asymmetric Aldol Reaction in Ionic Liquid (bmim)PF6 总被引:1,自引:0,他引:1
Room temperature ionic liquid (bmim)PF6 was evaluated for recycling an organocatalyst (4S)-phenoxy-(S)-proline for direct asymmetric aldol reactions. The desired aldol products were obtained with good yields up to 93.2% and enantioselectivities up to 88.5%, and isolation of the products by simple extraction allowed recycling the ionic liquid containing the immobilized catalyst in subsequent reactions without significant decrease of yields and enantioselectivities. 相似文献
92.
A mixed-ligand Zn(II) complex formulated as [Zn(aldtc)2(bipy)] (aldtc=diallyldithiocarbamate; bipy=2,2′-bipyridine) was synthesized and characterized by IR, 1H and 13C NMR spectral measurements and X-ray crystallography. The crystal structure of this complex indicates that Zn has a distorted octahedral geometry. The Zn—N distances are invariant (2.168(2) Å), while those of the Zn—S are slightly different (2.5408(9) and 2.5440(9) Å). The N—Zn—N, S—Zn—S and N—Zn—S bond angles are in the range 75.35(13)–99.75(7)°, 70.48(3)–161.02(5)° and 95.26(7)–160.32(7)°, respectively. The crystal packing of the complex shows different motifs of supramolecularity resulting from both hydrophilic ((π)C—H···S) and hydrophobic ((allyl)C—H···C(π)) intermolecular interactions. These interactions result in a chain arrangement of molecules along crystallographic c axis and the chains are further connected via π···π stacking along with ((π)C—H···S along b axis leading to an overall crystal packing that can be regarded as layers of complexes along bc plane, which are held together through nonconventional hydrogen bonding and π···π stacking. 相似文献
93.
The rg structure of cyclopentene oxide has been determined by the simultaneous least squares analysis of electron diffraction and microwave spectroscopic data. The investigation has reaffirmed previous studies indicating that the molecule prefers a boat conformation. The methylene and epoxide flap angles obtained are 152.3±2.1° and 104.7±1.0° respectively. Other structural parameters determined are: rg (C-H avg.) = 1.120±0.004 Å; rg (C-C avg.) = 1.538±0.002 Å; rg (C-O) = 1.443±0.003 Å, and rg (C-C) = 1.482±0.004 Å for the carbon-carbon bond in the three membered epoxide ring. These results compare favorably with the reported structures of ethylene oxide and cyclohexene oxide. A tentative rationalization of the unusual boat conformation is also offered. 相似文献
94.
The threat and global concern of energy crises have significantly increased over the last two decades. Because solar light and water are abundant on earth, photocatalytic hydrogen evolution through water splitting has been considered as a promising route to produce green energy. Therefore, semiconductor photocatalysts play a key role in transforming sunlight and water to hydrogen energy. To date, various photocatalysts have been studied. Among them, TiO2 has been extensively investigated because of its non-toxicity, high chemical stability, controllable morphology, and high photocatalytic activity. In particular, 1D TiO2 nanofibers (NFs) have attracted increasing attention as effective photocatalysts because of their unique 1D electron transfer pathway, high adsorption capacity, and high photoinduced electron–hole pair transfer capability. However, TiO2 NFs are considered as an inefficient photocatalyst for the hydrogen evolution reaction (HER) because of their disadvantages such as a large band gap (~3.2 eV) and fast recombination of photoinduced electron–hole pairs. Therefore, the development of a high-performance TiO2 NF photocatalyst is required for efficient solar light conversion. In recent years, several strategies have been explored to improve the photocatalytic activity of TiO2 NFs, including coupling with narrow-bandgap semiconductors (such as ZnIn2S4). Recently, microwave (MW)-assisted synthesis has been considered as an important strategy for the preparation of photocatalyst semiconductors because of its low cost, environment-friendliness, simplicity, and high reaction rate. Herein, to overcome the above-mentioned limiting properties of TiO2 NFs, we report a 2D/1D ZnIn2S4/TiO2 S-scheme heterojunction synthesized through a microwave (MW)-assisted process. Herein, the 2D/1D ZnIn2S4/TiO2 S-scheme heterojunction was constructed rapidly by using in situ 2D ZnIn2S4nanosheets decorated on 1D TiO2 NFs. The loading of ZnIn2S4 nanoplates on the TiO2 NFs could be easily controlled by adjusting the molar ratios of ZnIn2S4 precursors to TiO2 NFs. The photocatalytic activity of the as-prepared samples for water splitting under simulated solar light irradiation was assessed. The experimental results showed that the photocatalytic performance of the ZnIn2S4/TiO2 composites was significantly improved, and the obtained ZnIn2S4/TiO2 composites showed increased optical absorption. Under optimal conditions, the highest HER rate of the ZT-0.5 (molar ratio of ZnIn2S4/TiO2= 0.5) sample was 8774 μmol·g-1·h-1, which is considerably higher than those of pure TiO2 NFs (3312 μmol·g-1·h-1) and ZnIn2S4nanoplates (3114 μmol·g-1·h-1) by factors of 2.7 and 2.8, respectively. Based on the experimental data and Mott-Schottky analysis, a possible mechanism for the formation of the S-scheme heterojunction between ZnIn2S4 and TiO2 was proposed to interpret the enhanced HER activity of the ZnIn2S4/TiO2heterojunctionphotocatalysts.
相似文献
95.
Eun-Ku Lee Kwang-Deog Jung Oh-Shim Joo Yong-Gun Shul 《Reaction Kinetics and Catalysis Letters》2004,83(1):25-30
The room temperature wet catalytic oxidation was conducted in a batch reactor with V/MgO catalyst. The XRD study of the catalyst
used indicated that V/MgO could not only oxidize H2S to sulfur selectively, but also prevent the sulfidation of metal oxide
effectively at the room temperature. The XPS study indicated that the H2S oxidation with V/MgO could proceed by a redox mechanism
(V5+↔ V4+) and that V3+ formation (V4+→ V3+), was responsible for the deactivation of V/MgO.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
96.
A thin-layer chromatographic procedure is reported for the separation of serotonin from epinephrine and norepinephrine. The method involves the use of ethylenediaminetetraacetic acid-impregnated silica gel G plates and n-butanol-ethanol-acetic acid-water (8:2:1:3) as developer. Serotonin is well separated from both epinephrine and norepinephrine. Epinephrine and norepinephrine overlapped slightly but were both detectable. The procedure is thus applicable to the separation of all three biogenic amines in spite of the absence of an absolutely clear separation of epinephrine from norepinephrine. 相似文献
97.
Ren Yanliang Cheng Lin Wan Jian Li Yongjian Liu Junjun Yang Guangfu Zhang Lihua Yang Song 《中国科学B辑(英文版)》2006,49(1):88-96
The electronic singlet vertical excited states of photosynthetic reaction center (PSRC) in Rhodopseudomonas (Rps.) viridis were investigated by ZINDO and INDO/S methods. The effects of the interactions of pigment-pigment and pigment-protein on
the electronic excitations were examined. The calculation results showed that the interactions of pigment-pigment and pigment-protein
play an important role in reasonably assigning the experimental absorption and circular dichroism (CD) spectra of PSRC in
Rps. virids. By comparing the theoretically computed excited states with the experimental absorption and CD spectra, satisfactory assignments
of the experimental spectroscopic peaks were achieved. 相似文献
98.
碳氟链与碳氢链表面活性剂在固液界面上的吸附 总被引:1,自引:0,他引:1
全氟辛酸及其钠盐和十二烷基硫酸钠在R972上的吸附等温线均为S型或LS型,指示固液界面吸附过程中有表面疏水缔合物生成.碳氟表面活性剂的饱和吸附量显著高于碳氢表面活性剂的饱和吸附量.加电解质于液相使各体系吸附量上升.对于碳氟表面活性剂,甚至引起吸附等温线类型变化.例如,不加电解质时全氟辛酸在R972上的吸附等温线为S型,而加入HCl(c=0.05mol·dm-3)使吸附等混线变成LS型.全氟辛酸比全氟辛酸钠在R972上的吸附更强.几种表面活性剂在R972上的吸附均随温度升高而减少。应用两阶段吸附模型及通用吸附等温线公式可以很好地解释所得实验结果. 相似文献
99.
100.
1 INTRODUCTION In 1984, thiolate ligand was successfully introduced into molybdenum carbonyl compound by the reaction of Mo(CO)6 with [Et4N]SR( R = C6H5, But) in a moderate condition[1]. From then on, a series of dinuclear molybdenum(0) carbonyl thiolate compounds have been synthesized and characterized by using a variety of thiolate ligands in our research group. It was found that a planar Mo2S2 unit is in the compound [Et4N]2[Mo2(CO)8(SC6H4-CH_3-p)2][2] (2) and a "butterfly" t… 相似文献