全文获取类型
收费全文 | 6688篇 |
免费 | 374篇 |
国内免费 | 536篇 |
专业分类
化学 | 1693篇 |
晶体学 | 13篇 |
力学 | 231篇 |
综合类 | 63篇 |
数学 | 514篇 |
物理学 | 627篇 |
综合类 | 4457篇 |
出版年
2024年 | 22篇 |
2023年 | 88篇 |
2022年 | 143篇 |
2021年 | 194篇 |
2020年 | 166篇 |
2019年 | 116篇 |
2018年 | 108篇 |
2017年 | 131篇 |
2016年 | 187篇 |
2015年 | 211篇 |
2014年 | 310篇 |
2013年 | 293篇 |
2012年 | 367篇 |
2011年 | 423篇 |
2010年 | 322篇 |
2009年 | 401篇 |
2008年 | 365篇 |
2007年 | 446篇 |
2006年 | 437篇 |
2005年 | 392篇 |
2004年 | 330篇 |
2003年 | 301篇 |
2002年 | 233篇 |
2001年 | 225篇 |
2000年 | 167篇 |
1999年 | 165篇 |
1998年 | 164篇 |
1997年 | 137篇 |
1996年 | 96篇 |
1995年 | 101篇 |
1994年 | 100篇 |
1993年 | 81篇 |
1992年 | 58篇 |
1991年 | 55篇 |
1990年 | 51篇 |
1989年 | 44篇 |
1988年 | 46篇 |
1987年 | 26篇 |
1986年 | 19篇 |
1985年 | 15篇 |
1984年 | 8篇 |
1983年 | 2篇 |
1982年 | 11篇 |
1981年 | 6篇 |
1980年 | 7篇 |
1979年 | 7篇 |
1978年 | 4篇 |
1976年 | 5篇 |
1973年 | 4篇 |
1972年 | 4篇 |
排序方式: 共有7598条查询结果,搜索用时 297 毫秒
1.
在分析非洲基础设施建设现状和特点的基础上,通过亚吉铁路建设的案例研究,探讨了中非经贸合作下非洲基础设施建设的成功效应和存在的主要问题,得出中国标准缺乏国际认可、中非双方存在文化冲突、对非“全产业链”与投融资需要强化和产业结构相似的结论。同时,对未来中非经贸合作下非洲基础设施建设的发展从消除文化差异、提高合作层次、加强投融资、改进中国标准、明确合作区发展定位、后疫情时代加快构建更紧密的中非命运共同体和加强产能合作并发挥产能优势等角度提出可行性建议。 相似文献
2.
《Arabian Journal of Chemistry》2021,14(11):103421
Processing of Carapa guianensis seeds to obtain oil on an industrial scale generates a significant amount of by-product, approximately 66% w/w, which is called cake and is a potential source of biomolecules, including simple phenolic structures. For this reason, studies were carried out on the chemical profiles of hydrolyzed extract from this agro-industrial by-product through High Performance Thin-Layer Chromatography (HPTLC) and Gas Chromatography coupled to Mass Spectrometry (GC–MS). These techniques were used to detect metabolic classes and/or groups, and to identify, for the first time, thirteen simple phenolic acids in this by-product. The sample antioxidant capacity was determined by methods of 2,2-diphenyl-1-picrylhydrazyl (DPPH)and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS+) radicals direct sequestration. The hydrolyzed fraction showed a total of 63.47% in the relative abundance of the total of compounds, standing out: p-hydroxybenzoic acid (39.19%) and protocatechuic acid (3,4-dihydroxybenzoic acid) (5.62%), both from hydroxybenzoic acids and 3-(3,4-dihydroxyphenyl)lactic acid, (7.76%) hydroxycinnamic acids derivatives. In these results, the fraction rich in simple phenolic acids was obtained, attributing the prominent behavior of this matrix antioxidant activity, expressed by (IC50: of 16.42 µg/mL and 6.52 µg/mL for DPPH and ABTS+ radicals, respectively). The research demonstrated an alternative to applicability that involves sustainability from agro-industrial. These techniques were used to detect metabolic classes and/or groups, and to identify, for the first time, thirteen simple phenolic acids in this by-product, generating a process capable of converting biomass into a bioproduct, consisting of bioactive compounds, in addition to adding value to the industrial chain. 相似文献
3.
分析了windows多重启动的过程及有关文件,据此,给出了日常维护和排除故障的策略。 相似文献
4.
5.
Parabolic Capacity and Soft Measures for Nonlinear Equations 总被引:2,自引:0,他引:2
We first introduce, using a functional approach, the notion of capacity related to the parabolic p-Laplace operator. Then we prove a decomposition theorem for measures (in space and time) that do not charge the sets of null capacity. We apply this result to prove existence and uniqueness of renormalized solutions for nonlinear parabolic initial boundary-value problems with such measures as right-hand side. 相似文献
6.
本文探讨了气囊式蓄能器的工作原理和特性.并就蓄能器在煤矿采掘机械上的应用作了研究和探讨 相似文献
7.
本文在调查研究的基础上,指出了山东省载货汽车运力发展中存在的问题,提出了相应的运力结构调整对策。这对省府有关部门制定交通政策具有参考价值。 相似文献
8.
Valentin Brimkov 《Discrete Applied Mathematics》2007,155(14):1812-1825
We consider the problem of computing the Lovász theta function for circulant graphs Cn,J of degree four with n vertices and chord length J, 2?J?n. We present an algorithm that takes O(J) operations if J is an odd number, and O(n/J) operations if J is even. On the considered class of graphs our algorithm strongly outperforms the known algorithms for theta function computation. We also provide explicit formulas for the important special cases J=2 and J=3. 相似文献
9.
A new sample business survey for agriculture, the REA survey, and a project of integration with the FADN network (RICA in
Italy) have significantly changed the production of statistical information nowadays available with reference to the agricultural
sector. On the basis of this relevant information, new economic analyses are being developed on farms’ performance, agricultural
households’ income and the Common Agricultural Policy (CAP). In this paper the authors estimate the relationship between the
levels of variables of interest and their sampling errors using models in order to improve the accessibility of the information
on estimates accuracy to the final users (agricultural analysts, policy makers).
The paper is the result of a joint research of the three authors. Sections 1–3 and 6 by Pizzoli, Sects. 4 and 5 by Rondinelli,
Sect. 7 by Filiberti, conclusions joint to the three authors. 相似文献
10.