全文获取类型
收费全文 | 36917篇 |
免费 | 7184篇 |
国内免费 | 6372篇 |
专业分类
化学 | 27711篇 |
晶体学 | 636篇 |
力学 | 2359篇 |
综合类 | 492篇 |
数学 | 4256篇 |
物理学 | 15019篇 |
出版年
2024年 | 76篇 |
2023年 | 632篇 |
2022年 | 1097篇 |
2021年 | 1306篇 |
2020年 | 1470篇 |
2019年 | 1535篇 |
2018年 | 1311篇 |
2017年 | 1381篇 |
2016年 | 1770篇 |
2015年 | 1963篇 |
2014年 | 2402篇 |
2013年 | 3037篇 |
2012年 | 3523篇 |
2011年 | 3670篇 |
2010年 | 2765篇 |
2009年 | 2877篇 |
2008年 | 3060篇 |
2007年 | 2547篇 |
2006年 | 2377篇 |
2005年 | 1941篇 |
2004年 | 1462篇 |
2003年 | 1202篇 |
2002年 | 1229篇 |
2001年 | 1030篇 |
2000年 | 826篇 |
1999年 | 660篇 |
1998年 | 463篇 |
1997年 | 422篇 |
1996年 | 367篇 |
1995年 | 300篇 |
1994年 | 313篇 |
1993年 | 239篇 |
1992年 | 194篇 |
1991年 | 157篇 |
1990年 | 152篇 |
1989年 | 101篇 |
1988年 | 104篇 |
1987年 | 78篇 |
1986年 | 82篇 |
1985年 | 53篇 |
1984年 | 33篇 |
1983年 | 36篇 |
1982年 | 17篇 |
1981年 | 15篇 |
1980年 | 18篇 |
1979年 | 27篇 |
1978年 | 9篇 |
1977年 | 12篇 |
1974年 | 9篇 |
1925年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
草酰胺桥联大环二羰四胺铜(Ⅱ)-钴(Ⅱ)异双核配合物的合成、结构、表征和电化学性质 总被引:2,自引:0,他引:2
合成了一种具有草酰胺桥联大环二羰四胺结构的 Cu( ) -Co( )异双核配合物 ,用红外光谱、电子光谱、摩尔电导、热重分析、室温磁矩、循环伏安等对其进行了表征 .初步推定 Cu( ) -Co( )异双核配合物具有草酰胺桥联大环二羰四胺结构 .室温磁矩测定表明 ,通过草酰胺桥联配体金属离子间有一定的反铁磁性自旋偶合作用 .循环伏安法测定了配合物的半波电位 ,实验表明 ,此类配合物能够稳定高价态 Cu( ) ,外延桥基配位对大环内腔 Cu( )离子的氧化还原过程无影响 .单核配合物 X射线晶体衍射研究表明 ,其为单斜晶系 ,空间群 P2 1 /c,a=0 .73 861 (1 5 ) nm,b=2 .1 2 1 1 (4 ) nm,c=0 .95 2 5 0 (1 9) nm,β=94.70 (3 )°,R1 =0 .0 5 1 3 ,w R2 =0 .1 1 77,Z=4.Cu( )处于大环四胺平面正方中心上方 0 .0 3 nm,并具有外延草酰胺桥 相似文献
993.
Yuan Yuan Wang Wei Li Li Yi Dai 《中国化学快报》2007,18(10):1187-1190
Cationic ring-opening polymerization of 3,3-bis(chloromethyl)oxacyclobutane catalyzed by BF_3·OEt_2 was carried out in ionic liquids [bmim]BF_4 and [bmim]PF_6.The influences of BCMO concentration and molar ratio of BCMO/BF_3·OEt_2 on the molecular weights and yield of PBCMO were investigated.The polymerization in ionic liquids proceed to high conversions,although molecular weights are limited,similar to polymerization in organic solvent such as CH_2Cl_2.Follow a viewpoint of green chemistry, we feel ionic liquid [bmim]BF_4 is superior to [bmim]PF_6.Extracting [bmim]PF_6 from the product using organic solvent as extractant limits its advantage as a green reaction media. 相似文献
994.
In this study, a 38 mL monolith with homogeneous porous structure was produced by a single polymerization from glycidyl methacrylate (GMA) and ethylene dimethacrylate (EDMA) in the presence of porogens and an initiator. The uniform temperature distribution within the reaction system was achieved by adding reactant mixture continuously and enhancing the heat transfer ability of the polymerization system. Homogeneous porous structure in the monolith was proved by SEM and the pore size distribution profiles measured by mercury intrusion porosimetry. Experimental results from proteins separation indicated that the dynamic capacity and resolution of radial flow monolithic column were independent of flow rates. Furthermore, the pressure drop on the column was linearly dependent on the flow rate and did not exceed 1.7 MPa even at a flow rate of 50 mL/min, which proved that the prepared monolith could be used in the quick separation and preparation of biopolymers. 相似文献
995.
Using modern NMR techniques, including DQF-COSY, NOESY, HETCOR and HMBC, the full assignments of all 1H and 13C chemical shifts were made for stigmastane-3,6-dione and stigmast-4-ene-3,6-dione, isolated from the roots of Piper nigrum (Piperaceae). Their stereochemistry was resolved by a combination of 2D NOESY data and analysis of coupling constants. The two compounds were isolated from the genus Piper for the first time. 相似文献
996.
One linear template 13 and one cyclophane template 15, both incorporating two electron rich 1,4‐dialkoxybenzene units and one diamide unit, have been synthesized. By utilizing donor‐acceptor interaction and/or intermolecular hydrogen bonding assembling principles, one novel hetero[3]rotazane 22·4Cl, possessing one neutral and one tetracationic ring components, has been synthesized from 13, through neutral [2]rotaxane 21 as intermediate. With 15 as template, tetracationic [2]catenane 23·4PF6 was assembled by using donor‐acceptor interaction, but no neutral [2]rotaxane could be obtained under the typical conditions of hydrogen bonding assembling principle. The interlocked supramolecular compounds have been characterized and their spectral properties are investigated. 相似文献
997.
Wei Y Guo X Shuang S Dong C Sun X Wong M 《Journal of photochemistry and photobiology. B, Biology》2005,81(3):190-194
Novel dimeric capsules are generated from the noncovalent assembly of 5, 10, 15, 20-tetrakis(4-N-ethylpyridiniurmyl)prophyrin (TEPyP) and tetracarboxyl-phenyl calix[4]arene. The self-assembly system was investigated based on UV-Vis absorption and fluorescent spectra. The factors affecting the interaction process including pH and concentration were examined in detail. The association constants between TEPyP and calix[4]arene were determined by the nonlinear least squares fit. The results showed that the basic medium is favourable to the interaction and electrostatic interaction was determinate in the processes of self-assembly process. The related mechanism was discussed. 相似文献
998.
A new class of proton exchange composite membranes made by incorporating phosphosilicate gels into SPPO matrix was prepared and characterized. The thermal stability was evaluated by TGA and DSC, and the amorphous structure information was provided from XRD. The experimental results showed that the composite membranes have good stability to oxidation by Fenton's reagent test, and the membrane dimension is hardly changed, even at high temperature. The hydration number values of the persulfonic acid group of composite membranes were lower than that of Nafion 112 at room temperature, but the water uptake of composite membranes at 80°C was higher than that of Nafion 112. With increasing relative humidity and doping amount, the conductivity of the composite membranes increased. Moreover, the conductivities of water-equilibrated composite membranes were higher than that of Nafion 112 (0.0871 S/cm) at room temperature, and the highest conductivity for the composite membrane was 0.216 S/cm. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
999.
1000.
Microdroplets display distinctive interfacial chemistry, manifested as accelerated reactions relative to those observed for the same reagents in bulk. Carbon dioxide undergoes C–N bond formation reactions with amines at the interface of droplets to form carbamic acids. Electrospray ionization mass spectrometry displays the reaction products in the form of the protonated and deprotonated carbamic acid. Electrosonic spray ionization (ESSI) utilizing carbon dioxide as nebulization gas, confines reaction to the gas–liquid interface where it proceeds much faster than in the bulk. Intriguingly, trace amounts of water accelerate the reaction, presumably by formation of superacid or superbase at the water interface. The suggested mechanism of protonation of CO2 followed by nucleophilic attack by the amine is analogous to that previously advanced for imidazole formation from carboxylic acids and diamines.Microdroplets display distinctive interfacial chemistry, manifested as accelerated reactions relative to those observed for the same reagents in bulk. 相似文献