全文获取类型
收费全文 | 2975篇 |
免费 | 216篇 |
国内免费 | 395篇 |
专业分类
化学 | 3382篇 |
晶体学 | 13篇 |
力学 | 4篇 |
综合类 | 17篇 |
物理学 | 170篇 |
出版年
2024年 | 7篇 |
2023年 | 24篇 |
2022年 | 95篇 |
2021年 | 88篇 |
2020年 | 144篇 |
2019年 | 113篇 |
2018年 | 96篇 |
2017年 | 109篇 |
2016年 | 107篇 |
2015年 | 130篇 |
2014年 | 139篇 |
2013年 | 276篇 |
2012年 | 174篇 |
2011年 | 146篇 |
2010年 | 134篇 |
2009年 | 152篇 |
2008年 | 152篇 |
2007年 | 139篇 |
2006年 | 133篇 |
2005年 | 132篇 |
2004年 | 141篇 |
2003年 | 134篇 |
2002年 | 77篇 |
2001年 | 65篇 |
2000年 | 65篇 |
1999年 | 69篇 |
1998年 | 77篇 |
1997年 | 44篇 |
1996年 | 65篇 |
1995年 | 49篇 |
1994年 | 48篇 |
1993年 | 53篇 |
1992年 | 46篇 |
1991年 | 28篇 |
1990年 | 19篇 |
1989年 | 15篇 |
1988年 | 13篇 |
1987年 | 9篇 |
1986年 | 15篇 |
1985年 | 10篇 |
1984年 | 15篇 |
1983年 | 7篇 |
1982年 | 9篇 |
1981年 | 6篇 |
1980年 | 6篇 |
1979年 | 5篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有3586条查询结果,搜索用时 0 毫秒
991.
三维全扫描荧光法在油田单井评价中的应用研究 总被引:3,自引:0,他引:3
采用信息量大的三维全扫描荧光法,以发射、激发波长和荧光强度三维空间荧光图和荧光强度等高线图描述样品中的芳烃含量。以总荧光强度、特征荧光强度及R值来研究芳烃含量随井深的变化情况和油气运移机制。 相似文献
992.
Aaryn Jones 《Analytica chimica acta》2003,485(1):51-55
Pure subcritical water has been found to be an efficient mobile phase for reversed-phase separations of both polar and moderately polar compounds. However, subcritical water must be modified with organic solvent in order to elute nonpolar analytes in an efficient manner. In this study, the separation of polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and benzene, toluene, and p-xylene (BTX) was performed by using heated methanol-water mixtures as the mobile phase. Temperatures employed in this study ranged from 21 to 140 °C, while the percentage of methanol in the mobile phase ranged from 52 to 90%. The retention times of analytes were matched under different mobile phase conditions by increasing the temperature and decreasing the percentage of methanol in the mobile phase. 相似文献
993.
994.
Kenji Kamada Dr. Liudmil Antonov Prof. Satoru Yamada Dr. Koji Ohta Dr. Takashi Yoshimura Kazukuni Tahara Prof. Akiko Inaba Motohiro Sonoda Prof. Yoshito Tobe Prof. 《Chemphyschem》2007,8(18):2671-2677
Two‐photon absorption (TPA) properties of two trefoil‐shaped compounds with different edge linkages—tris(hexadehydrotribenzo[12]annulene) and tris(tetradehydrotribenzo[12]annulene)—and three asterisk‐shaped compounds having no edge‐linkage—hexakis(phenylethynyl)benzenes—are investigated experimentally by the open‐aperture Z‐scan and TPA‐induced fluorescence methods with wavelength tuneable femtosecond pulses. The compound with ethynylene edge‐linkage exhibits the most intense TPA (the maximal TPA cross section is 1300±170 GM at 572 nm where 1 GM=10?50 cm4 s molecule?1 photon?1). The TPA activity of the compounds is primarily explained in terms of the planarity of the molecules in relation with the type of edge‐linkage. 相似文献
995.
对近几年来高迁移率有机薄膜晶体管材料研究的主要发展作了简要介绍和评述,讨论了高迁移率有机半导体材料存在的问题和发展方向. 相似文献
996.
Graham Sandford Andrzej Tadeusiak Judith A.K. Howard 《Journal of fluorine chemistry》2007,128(10):1216-1220
A short series of bromo- and nitro-polyfluorobenzene derivatives gave the corresponding tetrahydroquinoxaline systems upon reaction with N,N′-dimethylethylene diamine. This annelation strategy gives useful quantities of functionalised tetrahydroquinoxaline systems which may be interesting scaffolds for the drug discovery arena. 相似文献
997.
Summary Elution strengths of 11 common HPLC solvents on a polymeric C18 phase were compared using a marker set of polycyclic aromatic hydrocarbons. Naphthalene, pyrene, benzo[ghi]perylene, and three larger naphthologues of 8, 10, and 12 rings (constituting a naphthalene zigzag series) were chosen because they span the solvent strength range up to and including the strongest solvents, tetrahydrofuran (THF) and chlorobenzene. Four pairs of similarly shaped isomers were used to probe solvent selectivity. With the exception of THF, HPLC solvent strength correlated with observed red shifts of fluorescence band maxima in each solvent. For THF, the pure solvent and blended mixtures behaved quite differently. 相似文献
998.
Valve based/flow modulated comprehensive two-dimensional gas chromatography-flame ionization detection (GC x GC-FID) was used for quantification of C6 through C12 aromatic hydrocarbons by carbon number in gasolines. A 0.53 mm i.d. non-polar first dimension column was coupled to a 0.53 mm i.d. polar second dimension column through a double loop eight port valve modulator. Depending on the sample type, normalized percent and internal standard (I.S.) quantification was performed. For normalized percent quantification, a one-point calibration performed with one aromatic compound per carbon number/class provided an average % accuracy of 2.1% and a short-term n--1 relative standard deviation of 1.0%. For total aromatic compounds good agreement with the more complex conventional multidimensional GC technique was obtained. However, GC x GC has certain advantages over most other methods, mainly increased selectivity for total and carbon number aromatic content. The identification of the aromatic hydrocarbons was confirmed by GC x GC-MS. 相似文献
999.
Paul G. Apen Paul G. Rasmussen 《Journal of polymer science. Part A, Polymer chemistry》1992,30(2):203-210
A novel betaine internal salt ( 3 ) has been synthesized by the reaction of pyridine with 1-methyl-2-bromo-4,5-dicyanoimidazole ( 1 ). The reaction of pyridine with 1 serves as a model for the reaction of poly(4-vinylpyridine) with 1 . The reaction of poly(4-vinylpyridine) with 1 gives betaine salt substituted polymers which were characterized by IR, NMR, UV-visible spectroscopy, and viscometry. The substituted polymers were compared to the model compound ( 3 ) and to unsubstituted poly(4-vinylpyridine) in order to determine polymer structure and the degree of substitution. One of the substituted polymers shows polyelectrolyte behavior. Thermal characterization of the substituted polymers shows two exothermic transitions at 260 and 340°C attributed to chemical reactions of the pendant groups. 相似文献
1000.
Cristau P Temal-Laïb T Bois-Choussy M Martin MT Vors JP Zhu J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(9):2668-2679
A unified strategy for the synthesis of mauritines A (5), B (6), C (7), and F (10) has been developed based on a key intramolecular nucleophilic aromatic substitution reaction (S(N)Ar) for the formation of the strained 14-membered paracyclophane. It was demonstrated that the outcome of the cycloetherification is independent of the stereochemistry of the peptide backbone and that both (1R)-16 and (1S)-16 cyclized smoothly to provide the corresponding macrocycle. On the other hand, dehydration of the secondary benzylic alcohol, via the phenylselenide intermediate, is configuration dependent. (1R)-25 underwent the two-step syn-elimination much more easily than (1S)-22. A modified reductive deamination procedure via the diazonium intermediate was developed. A complete assignment of proton and carbon NMR spectroscopy signals for these natural products is reported for the first time. 相似文献