全文获取类型
收费全文 | 178005篇 |
免费 | 2818篇 |
国内免费 | 1641篇 |
专业分类
化学 | 94069篇 |
晶体学 | 3157篇 |
力学 | 7453篇 |
综合类 | 94篇 |
数学 | 20285篇 |
物理学 | 57406篇 |
出版年
2021年 | 1419篇 |
2020年 | 1637篇 |
2019年 | 1798篇 |
2018年 | 2183篇 |
2017年 | 2302篇 |
2016年 | 3520篇 |
2015年 | 2253篇 |
2014年 | 3529篇 |
2013年 | 8157篇 |
2012年 | 6348篇 |
2011年 | 7541篇 |
2010年 | 5509篇 |
2009年 | 5441篇 |
2008年 | 7070篇 |
2007年 | 6878篇 |
2006年 | 6502篇 |
2005年 | 5887篇 |
2004年 | 5399篇 |
2003年 | 4877篇 |
2002年 | 4773篇 |
2001年 | 5453篇 |
2000年 | 4038篇 |
1999年 | 3130篇 |
1998年 | 2587篇 |
1997年 | 2468篇 |
1996年 | 2290篇 |
1995年 | 2166篇 |
1994年 | 2127篇 |
1993年 | 2035篇 |
1992年 | 2340篇 |
1991年 | 2283篇 |
1990年 | 2219篇 |
1989年 | 2137篇 |
1988年 | 2145篇 |
1987年 | 2140篇 |
1986年 | 2038篇 |
1985年 | 2645篇 |
1984年 | 2689篇 |
1983年 | 2388篇 |
1982年 | 2500篇 |
1981年 | 2311篇 |
1980年 | 2288篇 |
1979年 | 2429篇 |
1978年 | 2466篇 |
1977年 | 2389篇 |
1976年 | 2437篇 |
1975年 | 2364篇 |
1974年 | 2328篇 |
1973年 | 2505篇 |
1972年 | 1623篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
92.
93.
94.
Levoglucosenone reacts with α-aminoazoles to yield azolo[1,5-a]pyrimidine systems fused with a carbohydrate fragment. The reaction oocurs much more smoothly than in the case of other α,β-unsaturated
ketones. The reactions of levoglucosenone with β-dicarbonyl compounds (dimedone, barbituric acid) in the presence of a base
results in the pyran ring closure, which has never been observed earlier in reactions of β-dicarbonyl compounds with α,β-unsaturated
ketones under the conditions of basic catalysis. The structures of products were established by IR and NMR spectroscopy.
For Part 3, see Ref. 1.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 553–558, March, 1997. 相似文献
95.
Ya. V. Fattakhov M. F. Galyautdinov T. N. L’vova I. B. Khaibullin 《Technical Physics》1997,42(12):1457-1459
The first results regarding the formation of a two-dimensional periodic structure of local melting regions on a silicon surface
upon pulsed light irradiation are presented. The conditions are established, and the mechanism of the formation of such structures
is discussed.
Zh. Tekh. Fiz. 67, 97–99 (December 1997) 相似文献
96.
A converging perturbation series that can be summed analytically has been obtained for intersubband transitions of electrons
coherently tunneling through the middle of a dimensionally quantized level in an asymmetric double-barrier structure in a
high-frequency terahertz electric field. The possibility of a substantial increase in tunneling current accompanied by either
absorption or emission of a photon has been demonstrated. The quantum efficiency of radiative transitions between dimensionally
quantized levels can be up to 66%.
Zh. éksp. Teor. Fiz. 112, 237–245 (July 1997) 相似文献
97.
Summary Consider the solution of one-dimensional linear initial-boundary value problems by a finite element method of lines using a piecewiseP
th
-degree polynomial basis. A posteriori estimates of the discretization error are obtained as the solutions of either local parabolic or local elliptic finite element problems using piecewise polynomial corrections of degreep+1 that vanish at element ends. Error estimates computed in this manner are shown to converge in energy under mesh refinement to the exact finite element discretization error. Computational results indicate that the error estimates are robust over a wide range of mesh spacings and polynomial degrees and are, furthermore, applicable in situations that are not supported by the analysis.This research was partially supported by the U.S. Air Force Office of Scientific Research, Air Force Systems Command, USAF, under Grant Number AFOSR 90-0194; by the U.S. Army Research Office under Contract Number DAAL03-91-G-0215; and by the National Science Foundation under Institutional Infrastructure Grant Number CDA-8805910 相似文献
98.
V. M. Shopa B. N. Polevoi I. I. Popadyuk V. I. Tusevich 《Journal of Mathematical Sciences》1993,63(3):398-401
We propose a method of determining the contact pressures between the shells in a packet under the influence of nonlinear internal and constant external pressure. Using the equations of the general moment theory of shells we determine the stress-deformed state of a packet of finite cylindrical shells taking account of frictional forces on the contacting surfaces. One table. Bibliography: 10 titles.Translated fromMatematicheskie Metody i Fiziko-Mekhanicheskie Polya, Issue 30, 1989, pp. 94–98. 相似文献
99.
V. A. Samsonov L. B. Volodarskii V. L. Korolev G. Kh. Khisamutdinov 《Chemistry of Heterocyclic Compounds》1994,30(10):1243-1249
The reaction of 4-aminobenzofurazan with aryldiazonium salts leads to the formation of 4-amino-5-aryl-azobenzofurazans and 5-amino-2-aryl-4-nitroso-2H-benzotriazoles, products of the rearrangement of the initially formed 4-amino-7-(arylazo)benzofurazans. Oxidation of the benzofurazan as well as of the triazole derivatives gives 7-aryl-1,2,3-triazol[4,5-e]benzofurazans. The chemical properties of some of the compounds obtained have been investigated.Novosibirsk Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk 630090. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1432–1438, October, 1994. Original article submitted July 25, 1994. 相似文献
100.
M. A. Mikhaleva B. M. Bolotin T. A. Kizner E. S. Serebrykova 《Chemistry of Heterocyclic Compounds》1994,30(1):79-81
We have synthesized 5-alkyl(alkoxy)-2-(o-lzydroxy-p-alkoxyphenyl)pyrimidines and considered the effect of the OH group on the inesomorphism. We have found that in binuclear o-hydroxyarylpyrimidines, an ortho OH group forming an intramolecular hydrogen bond increases Tmp, inhibits the appearance of nematic properties, and promotes the appearance of a smectic A.Translated from Khimiya Geterotsiklicheskikh Soedinenii, Vol. 30, No. 1, pp. 90–92, January, 1994. 相似文献