全文获取类型
收费全文 | 285178篇 |
免费 | 2597篇 |
国内免费 | 638篇 |
专业分类
化学 | 153407篇 |
晶体学 | 4273篇 |
力学 | 12221篇 |
综合类 | 9篇 |
数学 | 32716篇 |
物理学 | 85787篇 |
出版年
2020年 | 2319篇 |
2019年 | 2591篇 |
2018年 | 3487篇 |
2017年 | 3436篇 |
2016年 | 5153篇 |
2015年 | 3176篇 |
2014年 | 4819篇 |
2013年 | 11823篇 |
2012年 | 9154篇 |
2011年 | 11006篇 |
2010年 | 7739篇 |
2009年 | 7508篇 |
2008年 | 10314篇 |
2007年 | 10512篇 |
2006年 | 9730篇 |
2005年 | 8876篇 |
2004年 | 8151篇 |
2003年 | 7312篇 |
2002年 | 7142篇 |
2001年 | 7668篇 |
2000年 | 5992篇 |
1999年 | 4624篇 |
1998年 | 4033篇 |
1997年 | 4010篇 |
1996年 | 3824篇 |
1995年 | 3483篇 |
1994年 | 3683篇 |
1993年 | 3350篇 |
1992年 | 3810篇 |
1991年 | 3807篇 |
1990年 | 3732篇 |
1989年 | 3577篇 |
1988年 | 3668篇 |
1987年 | 3572篇 |
1986年 | 3397篇 |
1985年 | 4507篇 |
1984年 | 4772篇 |
1983年 | 3974篇 |
1982年 | 4102篇 |
1981年 | 4039篇 |
1980年 | 4027篇 |
1979年 | 3957篇 |
1978年 | 4267篇 |
1977年 | 4185篇 |
1976年 | 4232篇 |
1975年 | 3917篇 |
1974年 | 3931篇 |
1973年 | 4102篇 |
1972年 | 2875篇 |
1971年 | 2409篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
L. N. Dianova T. G. Koksharova N. V. Volkova 《Chemistry of Heterocyclic Compounds》1992,28(3):321-323
Nucleophilic substitution of 5-methylthio- and 5-cyanomethylthio-7-amino-s-triazolo[1,5-c]pyrimidines has been carried out using sodium hydroxide, ammonia, hydrazine hydrate, and four amines. The cyanomethyl group is particularly reactive under these conditions and 5-cyanomethylthiotriazolopyrimidine can be used to functionalize this heterocycle.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 382–385, March, 1992. 相似文献
993.
994.
995.
L. D. Smirnov S. V. Nikitin A. I. Chernyshev A. A. Sorokin V. P. Lezina V. G. Zabrodnyaya M. M. Kaganskii 《Chemistry of Heterocyclic Compounds》1992,28(12):1425-1431
It has been shown that the alkylation of 2-methyl-9-hydroxy-4H-pyrido[1,2-a]pyrimidin-4-one takes place at the oxygen atom, but electrophilic substitution takes place mainly at position 8 of the molecule (the ortho position relative to the hydroxy group).Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1660–1666, December, 1992. 相似文献
996.
L Jirovetz G Buchbauer W J?ger A Woidich A Nikiforov 《Biomedical chromatography : BMC》1992,6(3):133-134
After inhalation experiments with sandalwood oil and the pure fragrance compounds coumarin and alpha-terpineol, substances were detected and measured in the blood samples of test animals (mice) using gas chromatography/mass spectrometry (GC/MS) (MID) in connection with GC/FTIR (SWC), GC/AES (carbon and oxygen trace) and flame ionization detection/gas chromatography. Using tiglinic acid benzyl ester as the internal standard the following concentrations in serum could be found: alpha-santalol 6.1 ng/mL, beta-santalol 5.3 ng/mL and alpha-santalene 0.5 ng/mL. In separate inhalation experiments with coumarin and with alpha-terpineol the corresponding concentrations were 7.7 ng/mL and 6.9 ng/mL, respectively. 相似文献
997.
Middle-ear phenomenology: the view from the three windows. 总被引:1,自引:0,他引:1
998.
999.
We study the effect of arrival model uncertainties on the optimal routing in a system of parallel queues. For exponential service time distributions and Bernoulli routing, the optimal mean system delay generally depends on the interarrival time distribution. Any error in modeling the arriving process will cause a model-based optimal routing algorithm to produce a mean system delay higher than the true optimum. In this paper, we present an asymptotic analysis of the behavior of this error under heavy traffic conditions for a general renewal arrival process. An asymptotic analysis of the error in optimal mean delay due to uncertainties in the service time distribution for Poisson arrivals was reported in Ref. 6, where it was shown that, when the first moment of the service time distribution is known, this error in performance vanishes asymptotically as the traffic load approaches the system capacity. In contrast, this paper establishes the somewhat surprising result that, when only the first moment of the arrival distribution is known, the error in optimal mean delay due to uncertainties in the arrival model is unbounded as the traffic approaches the system capacity. However, when both first and second moments are known, the error vanishes asymptotically. Numerical examples corroborating the theoretical results are also presented.This work was supported by the National Science Foundation under Grants ECS-88-01912 and EID-92-12122 and by NASA under Contract NAG 2-595.The authors wish to thank an anonymous referee for pointing out Ref. 20, thus avoiding the need for an explicit proof of convexity of the cost function considered in the paper. 相似文献
1000.
Brush S.G. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1992,20(6):577-589
Hannes Alfven's research program in solar system physics provides a valuable case study for the historians and philosophers of science. The response of the scientific community to his remarkable contributions is a severe test of widely held views about how science is or should be done. Seven of those contributions are examined, in the light of two doctrines of scientific methodology, those of Karl Popper (1959) and Imre Lakatos (1978). The seven are: application of the magnetic braking concept to the origin of the solar system; magnetohydrodynamic waves; field-aligned (`Birkeland') currents; critical ionization velocity and the existence of planetary rings; jet streams; electrostatic double layers; and partial corotation 相似文献