全文获取类型
收费全文 | 1847篇 |
免费 | 58篇 |
国内免费 | 4篇 |
专业分类
化学 | 1570篇 |
晶体学 | 18篇 |
力学 | 16篇 |
数学 | 20篇 |
物理学 | 285篇 |
出版年
2021年 | 11篇 |
2020年 | 16篇 |
2019年 | 20篇 |
2018年 | 15篇 |
2017年 | 14篇 |
2016年 | 25篇 |
2015年 | 37篇 |
2014年 | 41篇 |
2013年 | 95篇 |
2012年 | 93篇 |
2011年 | 89篇 |
2010年 | 66篇 |
2009年 | 43篇 |
2008年 | 83篇 |
2007年 | 93篇 |
2006年 | 105篇 |
2005年 | 113篇 |
2004年 | 93篇 |
2003年 | 88篇 |
2002年 | 76篇 |
2001年 | 22篇 |
2000年 | 31篇 |
1999年 | 25篇 |
1998年 | 18篇 |
1997年 | 23篇 |
1996年 | 28篇 |
1995年 | 14篇 |
1994年 | 14篇 |
1993年 | 17篇 |
1992年 | 21篇 |
1991年 | 17篇 |
1990年 | 18篇 |
1989年 | 16篇 |
1988年 | 21篇 |
1987年 | 19篇 |
1986年 | 21篇 |
1985年 | 31篇 |
1984年 | 33篇 |
1983年 | 17篇 |
1982年 | 29篇 |
1981年 | 30篇 |
1980年 | 40篇 |
1979年 | 17篇 |
1978年 | 15篇 |
1977年 | 15篇 |
1976年 | 17篇 |
1974年 | 18篇 |
1973年 | 17篇 |
1968年 | 17篇 |
1967年 | 14篇 |
排序方式: 共有1909条查询结果,搜索用时 328 毫秒
41.
Kiyoharu Tadanaga Kazuki Iwashita Tsutomu Minami Noboru Tohge 《Journal of Sol-Gel Science and Technology》1996,6(1):107-111
Nylon-6 substrates were coated with SiO2 thin films by the sol-gel method and their water permeability coefficient was evaluated. Methyltriethoxysilane (MTES) and tetraethoxysilane (TEOS) were used as starting materials. The addition of MTES to TEOS has enabled the formation of crack-free thin films on the substrates. The thin films strongly adhered to the substrates. The water permeability coefficients of nylon-6 substrates coated with these thin films decreased with the increase in the ratio of TEOS to the total alkoxides. The pretreatment of the nylon-6 substrates with -aminopropyltriethoxysilane was found to be effective to suppress the water permeability. The water permeability was suppressed by about 40% under the optimal condition. 相似文献
42.
Ishikawa T Watanabe T Tanigawa H Saito T Kotake KI Ohashi Y Ishii H 《The Journal of organic chemistry》1996,61(8):2774-2779
Nitrosation of 3-methoxyphenol and 1-naphthol were examined under both acidic (NaNO(2)-EtCO(2)H-H(2)O) and basic (i-AmNO(2)-K(2)CO(3)-DMF) conditions. Acidic nitrosations afforded ortho-directed products, whereas para-directed nitrosations were observed under basic conditions to yield p-quinone monooximes. The basic para-directed nitrosation was further examined using 15 phenols, two naphthols, and four phenolic heterocyclics. A one-pot operation of the basic nitrosation followed by methylation with dimethyl sulfate gave the corresponding methyl ethers in high yield. Two p-quinone monooximes derived from 3-methoxyphenol and 8-hydroxyquinoline showed a moderate activity against HSV-1, and the latter oxime was also effective against HSV-2. On the other hand, p-quinone monooximes derived from methyl salicylate, 1-naphthol, 7-hydroxy-2-methylbenzo[b]furan, and 8-hydroxycoumarin showed the comparable activity to that of DDI against HIV-1. 相似文献
43.
Staudinger/aza-Wittig reaction of 6,6,6-trifluoro-5-oxohexanoyl azide with PPh(3) or PBu(3) was examined. A reactive intermediate acyl imine 1 was trapped by methanol. Without nucleophile, isomerized enamide form 3 was obtained. N-Iodobenzoylation and N-haloalkylation of 3 and following radical cyclization via the 5-exo or 6-exo mode gave benzoindolizidinone, indolizidinone, and quinolizidinone derivatives 10-14, which have a trifluoromethyl group at the bridgehead position adjacent to nitrogen. Although LiAlH(4) reduction of 10 and 11 gave a mixture of saturated benzoindolizidine 15 and amino alcohol 16, reduction with BH(3)/THF selectively gave the desired 15 and indolizidine 17 from 13. 相似文献
44.
We propose a new approach for high-throughput continuous titration based on a flow ratiometry. The method was applied to potentiometric titrations of acids and bases. A base solution, the flow rate FB of which was varied in response to controller output voltage Vc, was merged with an acid solution under constant total (acid + base) flow rate. Downstream, the pH of the mixed solution was measured with a flow-through glass electrode. Initially, Vc, and thus FB was increased linearly. At the instant the equivalence point (EP) was sensed, the ramp direction of Vc was reversed from upward to downward. The direction was reversed to upward when EP was sensed again. Such the feedback-based operation gives a triangular waveform of Vc, because there is a delay corresponding mainly to the transit time of merged solutions to reach the sensor. The value of Vc that gives EP composition, VE, was estimated by averaging the most recent maximum and minimum values of Vc. Next, fixed triangular waves of Vc was used to control FB. The amplitude and the scan rate of the waves were fixed narrower and faster, respectively, than those in the feedback-based operation in order to improve the throughput rate. The EP can be located as long as the scan range covers VE. These automated processes limited the titration to just the narrow range around EP, and thus realized extremely high throughput rate of maximally 17.6 titrations per minute (=3.4 s per titration) at R.S.D. = 0.35%. 相似文献
45.
Four possible diastereomers of pentenocin B were synthesized in a stereocontrolled manner, and the first total synthesis of a natural enantiomer of (+)-pentenocin B unequivocally established the absolute stereochemistry to be 4S,5R,6R. 相似文献
46.
The vertical ionization potentials of difluoramine are calculated by perturbation corrections to Koopmans' theorem. The calculation shows that difluoraimine has three overlapping bands between 15 and 16 eV. The calculated results compare well with the experimental values. The photoelectron spectrum of difluoramine is compared with that of OF2 and CH2F2. 相似文献
47.
Neil C. Concibido Tetsuji Okuda Wataru Nishijima Mitsumasa Okada 《Reaction Kinetics and Catalysis Letters》2006,89(2):369-376
Summary The hydrodechlorination of tetrachloroethylene in methanol (MeOH) over palladium catalysts on activated carbon (Pd/C) at 20°C
and atmospheric pressure was rendered safer by using ethanol (EtOH), isopropanol (IPA) or water-alcohol mixtures as solvents.
Adding water to MeOH was better than using EtOH or IPA since it resulted in a faster reaction and better stability of Pd/C,
which could also be completely reactivated. 相似文献
48.
Tsutomu Kagiya Masatsugu Izu Masakazu Hatta Takehisa Matsuda Kenichi Fukui 《Journal of polymer science. Part A, Polymer chemistry》1967,5(5):1129-1135
N,N′-Dipropionylethylenediamine was synthesized by the ring-opening addition reaction of 2-ethyl-2-imidazoline with propionic acid at 220°C. By applying this reaction to polymerization, polyamides were synthesized by the ring-opening polyaddition reaction at 220°C. of 1,4-bis(imidazoline-2-yl)butane with adipic acid, succinic acid, sebacic acid, and terephthalic acid. The reaction product of 1,4-bis(imidazoline-2-yl)butane with adipic acid, which was proposed to be nylon 26, was compared with an authentic sample of nylon 26 and shown to possess a very similar infrared spectrum and melting point. 相似文献
49.
Masatomo Iwao Toshiro TakeuchiNaotaka Fujikawa Tsutomu FukudaFumito Ishibashi 《Tetrahedron letters》2003,44(24):4443-4446
A highly efficient route to 3,4-diarylpyrrole marine alkaloids has been developed using Hinsberg-type pyrrole synthesis and palladium-catalyzed Suzuki cross-coupling of the 3,4-dihydroxypyrrole bis-triflate derivatives as key reactions. Based on this approach, formal syntheses of permethyl storniamide A and ningalin B, and a total synthesis of lamellarin G trimethyl ether have been achieved. 相似文献
50.
Two new phenolic compounds, glicoricone (3) and licofuranone (4), were isolated from a species of licorice brought from the northwestern region of China, and their structures were assigned. Among the twelve licorice constituents examined for the inhibition of monoamine oxidase (MAO), six compounds, 3, 4, genistein (6), licopyranocoumarin (7), licocoumarone (14) and glycyrrhisoflavone (15), inhibited the enzyme with the IC50 (concentration required for 50% inhibition of the enzyme activity) values of 6.0 x 10(-5)-1.4 x 10(-4) M. Glycyrrhizin (1) also inhibited MAO with the IC50 value of 1.6 x 10(-4) M. 相似文献