全文获取类型
收费全文 | 9306篇 |
免费 | 505篇 |
国内免费 | 174篇 |
专业分类
化学 | 7021篇 |
晶体学 | 51篇 |
力学 | 508篇 |
数学 | 1026篇 |
物理学 | 1379篇 |
出版年
2023年 | 58篇 |
2022年 | 152篇 |
2021年 | 363篇 |
2020年 | 445篇 |
2019年 | 461篇 |
2018年 | 508篇 |
2017年 | 449篇 |
2016年 | 659篇 |
2015年 | 452篇 |
2014年 | 615篇 |
2013年 | 1134篇 |
2012年 | 832篇 |
2011年 | 730篇 |
2010年 | 501篇 |
2009年 | 457篇 |
2008年 | 473篇 |
2007年 | 386篇 |
2006年 | 256篇 |
2005年 | 224篇 |
2004年 | 172篇 |
2003年 | 144篇 |
2002年 | 100篇 |
2001年 | 37篇 |
2000年 | 25篇 |
1999年 | 30篇 |
1998年 | 23篇 |
1997年 | 21篇 |
1996年 | 27篇 |
1995年 | 17篇 |
1994年 | 15篇 |
1993年 | 14篇 |
1992年 | 19篇 |
1991年 | 13篇 |
1990年 | 11篇 |
1989年 | 11篇 |
1988年 | 7篇 |
1987年 | 15篇 |
1986年 | 6篇 |
1985年 | 17篇 |
1984年 | 16篇 |
1983年 | 6篇 |
1982年 | 16篇 |
1981年 | 8篇 |
1980年 | 12篇 |
1979年 | 7篇 |
1978年 | 8篇 |
1977年 | 7篇 |
1976年 | 5篇 |
1975年 | 5篇 |
1971年 | 3篇 |
排序方式: 共有9985条查询结果,搜索用时 31 毫秒
101.
Structural Chemistry - Density functional theory (DFT) calculations at B3LYP/6-31+G(d) level were employed to investigate the influence of the non-metal encapsulation of the second row of the... 相似文献
102.
In this work, for the first time, Solanum melongena plant extract was used for the green synthesis of Pd/MnO2 nanocomposite via reduction osf Pd(II) ions to Pd(0) and their immobilization on the surface of manganese dioxide (MnO2) nanoparticles (NPs) as an effective support. The synthesized nanocomposite were characterized by various analytical techniques such as Fourier transform infrared (FT‐IR), X‐ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), energy dispersive X‐ray spectroscopy (EDS) and UV–Vis spectroscopy. The catalytic activity of Pd/MnO2 nanocomposite was used as a heterogeneous catalyst for the one‐pot synthesis of 5‐substituted 1H‐tetrazoles from aryl halides containing various electron‐donating or electron‐withdrawing groups in the presence of K 4 [Fe (CN) 6 ] as non‐toxic cyanide source and sodium azide. The products were obtained in good yields via a simple methodology and easy work‐up. The nanocatalyst can be recycled and reused several times with no remarkable loss of activity. 相似文献
103.
Mahmoud Nasrollahzadeh Mohaddeseh Sajjadi Hartmut Komber Hossein Ali Khonakdar S. Mohammad Sajadi 《应用有机金属化学》2019,33(7)
In the present work, for the first time we have designed a novel approach for the synthesis of N‐benzyl‐N‐aryl‐5‐amino‐1H‐tetrazoles using reduced graphene oxide (rGO) decorated with Cu‐Ni bimetallic nanoparticles (NPs). In situ synthesis of Cu/Ni/rGO nanocomposite was performed by a cost efficient, surfactant‐free and environmentally benign method using Crataegus azarolus var. aronia L. leaf extract as a stabilizing and reducing agent. Phytochemicals present in the extract can be used to reduce Cu2+ and Ni2+ ions and GO to Cu NPs, Ni NPs and rGO, respectively. Analyses by means of FT‐IR, UV–Vis, EDS, TEM, FESEM, XRD and elemental mapping confirmed the Cu/Ni/rGO formation and also FT‐IR, NMR, and mass spectroscopy as well as elemental analysis were used to characterize the tetrazoles. The Cu/Ni/rGO nanocomposite showed the superior catalytic activity for the synthesis of N‐benzyl‐N‐aryl‐5‐amino‐1H‐tetrazoles within a short reaction time and high yields. Furthermore, this protocol eliminates the need to handle HN3. 相似文献
104.
Mohammad Nasir Uddin Zainul Abedin Siddique Nobuyuki Mase Monir Uzzaman Wahhida Shumi 《应用有机金属化学》2019,33(6)
A number of oxotitanium(IV) complexes of the type TiOL with bis‐unsymmetric dibasic tetradentate Schiff base (LH2) containing ONNO donor atoms have been synthesized. Mono‐Schiff base (OPD‐HNP) was prepared by the condensation of 1:3 molar ratio of 2‐hydroxy‐1‐naphthaldehyde (HNP) with o‐phenylenediamine (OPD). Dibasic unsymmetric tetradentate diamine Schiff bases were prepared by the reaction of OPD‐HNP with 2‐hydroxyacetophenone, 2‐hydroxypropeophenone, benzoylacetone, acetylacetone and ethylacetoacetate. Further, titanylacetylacetonate was reacted with these ligands to obtain their metal complexes. On the basis of analytical and physiochemical data, the formation of complexes as TiOL was suggested having square pyramidal geometry. Quantum mechanical approach also confirmed this geometry. The assessment of the synthesized ligands and their complexes showed that some behave as good inhibitors of mycelial growth against selected phytopathogic fungi but weak inhibitors against some selected bacteria. A few of them also showed antioxidant properties. 相似文献
105.
A palladium–fibroin complex (Pd/Fib.) was prepared by the addition of sonicated fibroin fiber in water to palladium acetate solution. Pd (OAc)2 was absorbed by fibroin and reduced with NaBH4 at room temperature to the Pd(0) nanoparticles. Powder‐X‐ray diffraction, scanning electron microscopy–energy‐dispersive X‐ray spectroscopy, Fourier transform‐infrared, CHN elemental analysis and inductively coupled plasma‐atomic emission spectroscopy were carried out to characterize the Pd/Fib. catalyst. Catalytic activity of this finely dispersed palladium was examined in the Heck coupling reaction. The catalytic coupling of aryl halides (‐Cl, ‐Br, ‐I) and olefins led to the formation of the corresponding coupled products in moderate to high yields under air atmosphere. A variety of substrates, including electron‐rich and electron‐poor aryl halides, were converted smoothly to the targeted products in simple procedure. Heterogeneous supported Pd catalyst can be recycled and reused several times. 相似文献
106.
The magnetic core of manganese ferrite (MnFe2O4) nanoparticles has a significant stability in comparison with ferrite (Fe3O4) nanoparticles. The unique supramolecular properties of β‐cyclodextrin (β‐CD), such as hydrophobic cavity, hydrophilic exterior and ‐OH functional groups, make it a good candidate for functionalization and catalytic application. So, a surface‐modified magnetic solid support with the Cu (II)‐β‐CD complex was prepared. The structure of nanoparticles was characterized by Fourier transform‐infrared spectroscopy, X‐ray powder diffraction, thermogravimetric analysis, vibrating‐sample magnetometry, inductively coupled plasma‐optical emission spectrometry and scanning electron microscope analyses. The catalytic activity of these nanoparticles was investigated in the synthesis of spiropyrans and high yields of desired products obtained under green media. Some advantages of this novel catalyst for this reaction are high yields, short reaction times, green solvent and conditions, easy workup procedure, negligible copper leaching, reusability without a significant diminish in catalytic efficiency, and simple separation of nanocatalyst by using an external magnet alongside the environmental compatibility and sustainability. 相似文献
107.
Taghavizadeh Yazdi Mohammad Ehsan Modarres Masoomeh Amiri Mohammad Sadegh Darroudi Majid 《Research on Chemical Intermediates》2019,45(3):1105-1116
Research on Chemical Intermediates - We have synthesized silver nanoparticles (Ag-NPs) via a simple and eco-friendly method through the utilization of aqueous aerial parts of Salvia leriifolia... 相似文献
108.
In this paper, we investigate the complete moment convergence for dependent linear processes with random coefficients to form 相似文献
109.
Bahareh Heidari Majid M. Heravi Mohammad Reza Nabid Roya Sedghi Seyyed Emad Hooshmand 《应用有机金属化学》2019,33(1)
A novel nanocatalyst was designed and prepared. Initially, the surface of magnetic graphene oxide (M‐GO) was modified using thionyl chloride, tris(hydroxymethyl)aminomethane and acryloyl chloride as linkers which provide reactive C═C bonds for the polymerization of vinylic monomers. Separately, β‐cyclodextrin (β‐CD) was treated with acryloyl chloride to provide a modified β‐CD. Then, in the presence methylenebisacrylamide as a cross‐linker, monomers of modified β‐CD and acrylamide were polymerized on the surface of the pre‐prepared M‐GO. Finally, palladium acetate and sodium borohydride were added to this composite to afford supported palladium nanoparticles. This fabricated nanocomposite was fully characterized using various techniques. The efficiency of this easily separable and reusable heterogeneous catalyst was successfully examined in Suzuki–Miyaura cross‐coupling reactions of aryl halides and boronic acid as well as in modified Suzuki–Miyaura cross‐coupling reactions of N‐acylsuccinimides and boronic acid in green media. The results showed that the nanocatalyst was efficient in coupling reactions for direct formation of the corresponding biphenyl as well as benzophenone derivatives in green media based on bio‐based solvents. In addition, the nanocatalyst was easily separable, using an external magnet, and could be reused several times without significant loss of activity under the optimum reaction conditions. 相似文献
110.
In this work, a new nanocatalyst, Fe2W18Fe4@NiO@CTS, was synthesized by the reaction of sandwich‐type polyoxometalate (Fe2W18Fe4), nickel oxide (NiO), and chitosan (CTS) via sol–gel method. The assembled nanocatalyst was systematically characterized by FT‐IR, UV–vis, XRD, SEM, and EDX analysis. The catalytic activity of Fe2W18Fe4@NiO@CTS was tested on oxidative desulfurization (ODS) of real gasoline and model fuels. The experimental results revealed that the levels of sulfur content and mercaptan compounds of gasoline were lowered with 97% efficiency. Also, the Fe2W18Fe4@NiO@CTS nanocatalyst demonstrated an outstanding catalytic performance for the oxidation of dibenzothiophene (DBT) in the model fuel. The major factors that influence the desulfurization efficiency and the kinetic study of the ODS reactions were fully detailed and discussed. The probable ODS pathway was proposed via the electrophilic mechanism on the basis of the electrophilic characteristic of the metal‐oxo‐peroxo intermediates. The prepared nanocatalyst could be reused for 5 successive runs without any appreciable loss in its catalytic activity. As a result, the current study suggested the potential application of the Fe2W18Fe4@NiO@CTS hybrid nanocatalyst as an ideal candidate for removal of sulfur compounds from fuel. 相似文献