全文获取类型
收费全文 | 7685篇 |
免费 | 830篇 |
国内免费 | 630篇 |
专业分类
化学 | 4124篇 |
晶体学 | 66篇 |
力学 | 347篇 |
综合类 | 79篇 |
数学 | 2456篇 |
物理学 | 2073篇 |
出版年
2024年 | 13篇 |
2023年 | 59篇 |
2022年 | 82篇 |
2021年 | 108篇 |
2020年 | 190篇 |
2019年 | 195篇 |
2018年 | 185篇 |
2017年 | 251篇 |
2016年 | 212篇 |
2015年 | 235篇 |
2014年 | 361篇 |
2013年 | 1039篇 |
2012年 | 355篇 |
2011年 | 409篇 |
2010年 | 342篇 |
2009年 | 412篇 |
2008年 | 417篇 |
2007年 | 451篇 |
2006年 | 440篇 |
2005年 | 353篇 |
2004年 | 331篇 |
2003年 | 303篇 |
2002年 | 893篇 |
2001年 | 214篇 |
2000年 | 224篇 |
1999年 | 150篇 |
1998年 | 171篇 |
1997年 | 114篇 |
1996年 | 115篇 |
1995年 | 102篇 |
1994年 | 85篇 |
1993年 | 62篇 |
1992年 | 48篇 |
1991年 | 45篇 |
1990年 | 28篇 |
1989年 | 27篇 |
1988年 | 30篇 |
1987年 | 17篇 |
1986年 | 11篇 |
1985年 | 15篇 |
1984年 | 11篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 7篇 |
1980年 | 2篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1973年 | 4篇 |
1968年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有9145条查询结果,搜索用时 15 毫秒
91.
Cu/TiO2-NiO上光促表面催化CO2和H2O合成CH3OH反应规律 总被引:8,自引:0,他引:8
采用溶胶-凝胶法制备了n(电子型)-p(空穴型) 复合半导体材料0.5%Cu/TiO2-2.0%NiO (w),利用X射线衍射(XRD)、透射电镜(TEM)、红外光谱(IR)、紫外-可见光漫反射(UV-Vis)、程序升温脱附(TPD)技术对材料结构、吸光性能、化学吸附性能进行了表征,研究了该材料对CO2和H2O合成CH3OH的光促表面催化反应(PSSR)规律.结果表明,所制备材料能够明显促进目的反应,室温条件下即有CH3OH生成.在200 ℃下,由于光-表面-热的协同效应,CO2转化率得以提高,且CH3OH的选择性达到87.5% .根据实验结果,得出CO2在材料表面的卧式吸附态为CH3OH的前驱物,并对PSSR机理进行了讨论. 相似文献
92.
Tsuneo Okubo 《Colloid and polymer science》2006,284(12):1395-1401
Sedimentation and drying dissipative structural patterns formed in the course of drying colloidal silica spheres (1.2 μm in diameter) in aqueous suspension have been studied in a glass dish and a polystyrene dish. The broad ring patterns are formed within a short time in suspension state by the convection flow of water and colloidal spheres. The broad ring patterns are not formed when a dish is covered with a cap, which demonstrates the important role of the convectional flow of silica spheres and water accompanied with the evaporation of water on the air-suspension interface. The sedimentary spheres always move by the convectional flow of water, and the broad ring patterns became sharp with time. Broad ring and microscopic fine structures are formed in the solidification processes on the bases of the convectional and sedimentation patterns. Drying patterns of the colloidal suspensions containing sodium chloride are star-like ones, which strongly supports the synchronous cooperative interactions between the salt and colloidal spheres. 相似文献
93.
Acylation of 6-(1-fluorovinyl)-6-methyl- and 6-(2,2,3,3-tetrafluorocyclobutyl)-4,5-diazaspiro[2.4]hept-4-enes with acetyl chloride proceeds as electrophilic addition to the N(5) atom and is accompanied by opening of the cyclopropane ring to give 1-acetyl-3-(2-chloroethyl)-5-(1-fluorovinyl)-5-methyl- and 1-acetyl-3-(2-chloroethyl)-5-(2,2,3,3-tetrafluorocyclobutyl)-4,5-dihydropyrazoles, respectively. Under the same conditions, acylation of 6-(2,3,3-trifluorocyclobut-1-enyl)-4,5-diazaspiro[2.4]hept-4-ene is not regioselective. The (2-chloroethyl) pyrazolines obtained undergo dehydrochlorination into vinylpyrazolines in the presence of an excess of MeONa in MeOH. The reaction of 4-acetyl-6-(2,3,3-trifluorocyclobut-1-enyl)-4,5-diazaspiro[2.4]hept-5-ene with MeONa results in selective replacement of the F atom at the double bond by a methoxy group.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 419–422, February, 2005. 相似文献
94.
Xue-qiang Yin 《Tetrahedron》2005,61(7):1839-1843
A straightforward synthesis of (1S,2R,3R,4R)-4-(6-aminopurin-9-yl)-2-hydroxymethylcyclopentane-1,3-diol (2), an isomer of aristeromycin, and its 2′-deoxy derivative 3 from readily available disubstituted cyclopentenes is presented. An antiviral analysis of 2 showed it to have significant activity versus Epstein-Barr virus (IC50 0.62 μg/mL in the Elisa assay) and to be free of cytotoxicity effects against the host cells. In a much less comprehensive antiviral analysis, 3 also was active towards Epstein-Barr (IC50 7.58 μg/mL in the Elisa assay) but this was accompanied by cellular toxicity. 相似文献
95.
Toshiaki Sunazuka Masaki Handa Tatsuya Shirahata Kazuhiko Otoguro Satoshi ōmura 《Tetrahedron》2004,60(36):7845-7859
In the current studies, we used the Kakisawa-Kashman modification of the Mosher NMR method to determine the complete absolute stereochemistry of arisugacins. We also report the convergent total synthesis of (+)-arisugacins A and B by a sequence including (i) ruthenium complex-catalyzed asymmetric reduction of the cyclohexenone derivative; (ii) stereoselective construction of the arisugacin skeleton by a Knoevenagel-type reaction of an α,β-unsaturated aldehyde derivative with production of a 4-hydroxy-2-pyrone derivative as a key reaction; and (iii) stereoselective dihydroxylation to give the diol derivative, followed by deoxygenation. Accordingly, we defined the absolute structures of arisugacins A and B as 4a-(R),6a-(R),12a-(R), and 12b-(S). Finally, we characterized the bioactivities of the synthetic intermediates to understand the structure-activity relationships of the arisugacins. 相似文献
96.
Summary The solution conformations of 1,4-diaryl-tetrahydro-1H,3H-furo[3,4-c]furans were determined by means of the LIS technique using a modified LIS-simulation program specially designed for the requirements of the aromatic 1,2,3-trimethoxy arrangement as a bidentate coordination site for the NMR shift reagent. The resulting geometries were confirmed by force field calculations (MM2-87) and compared with X-ray derived structures. Reliable data for the conformations in solution allowed a quantitative estimation of the circular dichroism based on the coupled oscillator model.
Lanthanideninduzierte Verschiebungen von aromatischen 1,2,3-Trimethoxy-Verbindungen: Konformationsanalyse von Tetrahydrofurofuran-Lignanen in Lösung
Zusammenfassung Es wurden mittels der LIS-Technik die Konformationen von 1,4-Diaryl-tetrahydro-1H,3H-furo[3,4-c]furanen in Lösung bestimmt. Dazu mußte ein modifiziertes LIS-Simulationsprogramm entworfen werden, das auf die besonderen Verhältnisse der aromatischen 1,2,3-Trimethoxy-Gruppierung eingeht, welche in den untersuchten Verbindungen als zweizähnige Koordinationsstelle für das NMR-Verschiebungsreagens fungiert. Die resultierenden Geometrien wurden mittels Kraftfeldrechnungen (MM2-87) überprüft und soweit vorhanden auch mit Röntgenstrukturdaten verglichen. Für die Lösungskonformationen wurde eine quantitative Abschätzung des Circulardichroismus auf der Basis des Modells für gekoppelte Oszillatoren durchgeführt.相似文献
97.
Two series of nano-sized N-containing MOx–ZnO (M: Fe, W) composite powders were synthesized by spray pyrolysis. The nitrogen content was controlled from 500 to 2100 ppm by changing the powder ingredients and spray pyrolysis temperature. Ultraviolet-visible (UV-Vis) spectra indicated that the N-containing MOx–ZnO powders absorbed not only ultraviolet light (λ<387 nm) like pure ZnO powder, but also part of visible light (λ<650 nm). Acetaldehyde decomposition was used as a probe reaction to evaluate the photocatalysis of these composite powders. The photocatalytic activity of the N-containing ZnO powder was pronouncedly enhanced by the WO3 addition under both UV and visible-light irradiation. However, that of the N-containing ZnO powder was suppressed by the Fe2O3 addition. A model of the semiconductor energy-band structure is proposed to explain the enhancement and suppression of photocatalytic activity. 相似文献
98.
Concurrent solvent evaporation with a loop-type interface was used for on-line HPLC-CGC in the analysis of methylated dibenzothiophene (DBT) isomers in oil samples. The chromatographic behavior of 20 methyl DBT's was studied by HPLC on an aminopropylsilane DBTA phase and by GC on a selective methyl-phenylsilicone phase. That provided a method for analyzing by GC-flame photometric detection, the individual components of the DBT family, previously picked out of the crude oil matrix by HPLC. The GC oven temperature was shown to be critical during HPLC eluent introduction into the GC pre-column. Too high a temperature induced a severe broadening of early eluted peaks whereas a temperature too close to the boiling point of the liquid at the inlet pressure induced double peaks. Optimized conditions were retained on this basis and may be used for the analysis of other families of polyaromatic hydrocarbons. 相似文献
99.
100.
The alicyclic ring contraction of perfluoro-1-phenyltetralin in reaction with antimony pentafluoride
Perfluoro-1-phenyltetralin (1) heated with antimony pentafluoride at 130 °C, then treated with water, gave a mixture of perfluorinated 3-methyl-2-phenylindenone (3), 3-methyl-2-phenylindene (4), 3-hydroxy-1-methyl-3-phenylindan (5), 1-methyl-3-phenylindan (6), 9-methyl-1,2,3,4,5,6,7,8-octahydroanthracene (7), and 1,9-dimethyl-5,6,7,8-tetrahydro-β-naphthindan (8). When heated with SbF5 in the presence of HF, then treated with water, compound 1 is transformed to a mixture of products 3-6. The reaction at 170 and 200 °C forms compounds 3-6 together with perfluoro-2-(cyclohexen-1-yl)-3-methylindene (10). 相似文献