全文获取类型
收费全文 | 92204篇 |
免费 | 7563篇 |
国内免费 | 5932篇 |
专业分类
化学 | 27452篇 |
晶体学 | 1833篇 |
力学 | 4193篇 |
综合类 | 486篇 |
数学 | 26116篇 |
物理学 | 45619篇 |
出版年
2023年 | 394篇 |
2022年 | 349篇 |
2021年 | 662篇 |
2020年 | 1072篇 |
2019年 | 1342篇 |
2018年 | 1314篇 |
2017年 | 1053篇 |
2016年 | 864篇 |
2015年 | 764篇 |
2014年 | 2002篇 |
2013年 | 3313篇 |
2012年 | 1909篇 |
2011年 | 2620篇 |
2010年 | 3031篇 |
2009年 | 7860篇 |
2008年 | 8817篇 |
2007年 | 7084篇 |
2006年 | 6407篇 |
2005年 | 4452篇 |
2004年 | 4185篇 |
2003年 | 4274篇 |
2002年 | 5488篇 |
2001年 | 3824篇 |
2000年 | 3662篇 |
1999年 | 3491篇 |
1998年 | 2877篇 |
1997年 | 2056篇 |
1996年 | 1832篇 |
1995年 | 2278篇 |
1994年 | 2209篇 |
1993年 | 1640篇 |
1992年 | 1143篇 |
1991年 | 851篇 |
1990年 | 697篇 |
1989年 | 622篇 |
1988年 | 583篇 |
1987年 | 421篇 |
1985年 | 990篇 |
1984年 | 668篇 |
1983年 | 512篇 |
1982年 | 672篇 |
1981年 | 825篇 |
1980年 | 750篇 |
1979年 | 583篇 |
1978年 | 605篇 |
1977年 | 553篇 |
1976年 | 551篇 |
1975年 | 321篇 |
1974年 | 361篇 |
1973年 | 467篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
Presenting the devices invented by Toepler, Holtz, Wimshurst, and Wommelsdorf, the history of the influence machines, the progress made, and the remaining deficits are discussed. The theoretical considerations published in the literature lack generalizable statements on the efficiency. Based on R. W. Pohl's model (1927), an elementary analysis is made, with the result that the upper limit of the theoretically achievable efficiency of influence machines is only 50%. 相似文献
3.
A finite support product of clones of Jensen's minimal singleton forcing is used to define a model in which any non-empty analytically definable set of reals contains an analytically definable real (the full basis theorem), but there is no analytically definable wellordering of the reals. 相似文献
4.
5.
Marta Wojciechowska Piotr Stepnowski Marek Gołębiowski 《Biomedical chromatography : BMC》2022,36(5):e5346
This paper presents the effect of insecticides on the composition of the surface compounds of one of the most harmful insects, Tenebrio molitor, by analysis using GC–MS. As a result of the use of insecticides, the composition of the chemical compounds on the surface of insects changes, depending on the insecticides used. The most numerous groups of the marked compounds were fatty acids, alkanes, esters and sterols. The content of the identified compounds in the larvae increased at both 24 and 48 h after the application of insecticides, in comparison with the control samples. The content of identified compounds in the samples taken from the females increased 24, 48 and 72 h after the application of insecticides in comparison with the control samples. By contrast, in samples prepared from males, the content of identified compounds decreased 24 h after the application of insecticides, compared with the control samples. The highest content of chemical compounds was for fatty acids and alkanes after the use of insecticides. The content of fatty acids after the application of the insecticide with deltamethrin was 62.1 ± 3.3–466.9 ± 5.9 μg/g, and after the application of the insecticide with cyfluthrin was 49.9 ± 1.9–458.3 ± 4.2 μg/g. However, the content of alkanes after the use of deltamethrin was 115.6 ± 4.2–4672.0 ± 32.1 μg/g, and after the use of cyfluthrin was 189.4 ± 3.8–3975.0 ± 10.2 μg/g. 相似文献
6.
《Arabian Journal of Chemistry》2022,15(11):104297
This study aimed to investigate the chemical composition of Tribulus terrestris L. fruit (TT) extract named TT15 and its protective effect against ischemic stroke (IS) as well as corresponding mechanisms. The chemical composition of TT15 was analyzed by liquid chromatography-mass spectrometry (LC-MS), and the compound identification was conducted via searching the in-house database. The LC-MS-based multi-omics approach was applied to search the differential metabolites and differential proteins in rat brain tissue and to explore the biomarker and molecular mechanism of TT15 against middle cerebral artery occlusion (MCAO). A total of 20 compounds were identified from TT15, mainly including alkaloids, flavonoids, phenols, quinones, and esters. These 20 compounds significantly affected the metabolism of 44 metabolites and the expression of 51 proteins. Joint pathway analysis showed that these metabolites and proteins were mainly involved in the response to elevated platelet cytosolic Ca2+ and platelet activation, which inferred that TT15 may exert a protective effect against cerebral ischemic injury via regulating platelet function. This study provides useful information for further exploration of the mechanisms of TT extract against IS. 相似文献
7.
Combustion processes in porous media have been used by the petroleum engineering industry to extract heavy oil from reservoirs. This study focuses on a one-dimensional nonlinear hybrid system consisting of reaction–diffusion–convection equations coupled with ordinary differential equations, which models a combustion front moving through a porous medium with parallel layers. The state variables are the temperature and fuel concentration in each layer. Coupling occurs in both the reaction function and differential operator coefficients. We prove the existence of a classical solution, first locally and then globally over time, to an initial and boundary value problem for the corresponding system. The proof uses a new approach for combustion problems in porous media. The local solution is obtained by defining an operator in a set of Hölder continuous functions and using Schauder’s fixed-point theorem to find a fixed point as the desired solution. Using Zorn’s lemma, we extend the local solution to a global solution, proving that the first-order spatial derivative of the temperature in each layer is a bounded function. 相似文献
8.
Qinhai ZHANG 《Frontiers of Mathematics in China》2022,17(1):1
We survey some unsolvable conjectures in finite p-groups and their research progress. 相似文献
9.
10.
Hasan Kara Muhammad Aamir Ali Hüseyin Budak 《Mathematical Methods in the Applied Sciences》2021,44(1):104-123
In this paper, we define interval‐valued left‐sided and right‐sided generalized fractional double integrals. We establish inequalities of Hermite‐Hadamard like for coordinated interval‐valued convex functions by applying our newly defined integrals. 相似文献