全文获取类型
收费全文 | 586827篇 |
免费 | 6230篇 |
国内免费 | 1906篇 |
专业分类
化学 | 307293篇 |
晶体学 | 8842篇 |
力学 | 26271篇 |
综合类 | 14篇 |
数学 | 71502篇 |
物理学 | 181041篇 |
出版年
2021年 | 4568篇 |
2020年 | 4996篇 |
2019年 | 5452篇 |
2018年 | 6847篇 |
2017年 | 6924篇 |
2016年 | 10545篇 |
2015年 | 6811篇 |
2014年 | 10555篇 |
2013年 | 27387篇 |
2012年 | 20848篇 |
2011年 | 25339篇 |
2010年 | 17529篇 |
2009年 | 17340篇 |
2008年 | 22861篇 |
2007年 | 23132篇 |
2006年 | 21704篇 |
2005年 | 19236篇 |
2004年 | 17755篇 |
2003年 | 15703篇 |
2002年 | 15554篇 |
2001年 | 17681篇 |
2000年 | 13442篇 |
1999年 | 10710篇 |
1998年 | 8840篇 |
1997年 | 8599篇 |
1996年 | 8376篇 |
1995年 | 7449篇 |
1994年 | 7196篇 |
1993年 | 6986篇 |
1992年 | 7712篇 |
1991年 | 7814篇 |
1990年 | 7413篇 |
1989年 | 7206篇 |
1988年 | 7232篇 |
1987年 | 7061篇 |
1986年 | 6758篇 |
1985年 | 9168篇 |
1984年 | 9299篇 |
1983年 | 7643篇 |
1982年 | 8050篇 |
1981年 | 7838篇 |
1980年 | 7587篇 |
1979年 | 7938篇 |
1978年 | 8023篇 |
1977年 | 7942篇 |
1976年 | 7914篇 |
1975年 | 7636篇 |
1974年 | 7394篇 |
1973年 | 7611篇 |
1972年 | 4922篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
The corresponding 2,3-diphenyl[3,2-a]pyrimidinium salts were obtained by the condensation of 4,5-diphenyl-2-aminothiazole perchlorate with -diketones, -chlorovinyl ketones, -chlorovinyl aldehydes, and 1,1,3,3-tetraethoxypropane. The PMR spectra were used to prove the structures of the compounds obtained, particularly to select the structures of the isomeric salts obtained in the condensation with unsymmetrical -diketones, -chlorovinyl ketones, and -chlorovinyl aldehydes. As a rule, the latter give one isomer — the -unsubstituted (relative to the bridge nitrogen atom) derivative.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 629–631, May, 1972. 相似文献
993.
Acid activated metakaolins (AAMKs) have been prepared by calcination of the natural clay at 600 degrees C to provide a metakolin which was then leached at 80 degrees C for 3 h using 1M, 2M, 3M, and 6M HCl. These materials were characterized and their ability to transform the off gases from HDPE decomposition into useful aromatic species was evaluated. The amount of adsorbed water and the number of acid sites increased with the severity of acid treatment. Variable temperature DRIFTS spectroscopy of pyridine treated samples revealed that both Br?nsted and Lewis acid centers were present until 425 degrees C. Pyridine bonded to the Lewis acid centers was more thermally stable. The AAMKs were all selective to the production of toluene with respectable, but lesser, amounts of xylenes and trimethylbenzenes. This selectivity contrasts with that of acid leached and pillared smectites which are selective toward trimethylbenzene. 相似文献
994.
G. A. Gailyunas G. V. Nurtdinova V. P. Yur'ev G. A. Tolstikov S. R. Rafikov 《Russian Chemical Bulletin》1982,31(4):806-812
Conclusions The high catalytic activity of (CH2=CHSiR3)·Fe(CO)4 complexes was demonstrated in the reaction of vinylsilanes with polyhalomethanes and hydrosilanes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 914–920, April, 1982. 相似文献
995.
When the cations bound to purple membrane are removed it turns blue, and when this blue membrane is irradiated its color changes to pink. Irradiation of pink membrane leads to the reformation of blue membrane. We have determined that the quantum efficiency for the formation of pink membrane from deionized blue membrane is 1.6 ± 0.6 ± 10 4 at 0o C, pH 5.0. We also found that the quantum efficiency for the back photoconversion, i.e. the formation of blue membrane from pink membrane, is 8.8 ± 1.6 ± 10-3 at 0o C, 55 times greater than that of the forward photoconversion reaction. The extinction coefficients of the pink membrane and blue membrane were determined to be 44 500 ± 670 cm-1 M-1 at 491 nm and 54 760 ± 830 cm-1 M -1 at 603 nm, respectively, assuming light-adapted purple membrane is 63 000 cm-1 M -1 at 568 nm. The quantum efficiency for forming pink membrane from blue membrane is much lower than that for forming the photointermediate of the blue membrane's photocycle. Their relationship is similar to that of light-adaptation and photocycle of the dark-adapted purple membrane. 相似文献
996.
997.
Abstract From April 1982 to April 1986 we used photodynamic therapy to treat 192 patients with various malignancies. In all, 172 injections of hematoporphyrin derivative and 135 injections of dihematoporphyrin ether were made intravenously to sensitize the tumors; 684 photodynamic treatments were given using a tunable dye argon laser system for the light source. The technical problems of the light source and delivery systems are presented and the efficacy of therapy is analyzed. 相似文献
998.
999.
The use of pH gradients in borate buffer with a polyhydroxy compound for separation of proteins in a free-flow electrophoretic apparatus is outlined. The composition of the pH gradients, the free-flow apparatus design and its operation, and the main causes of failures in the protein separation are considered. 相似文献
1000.