全文获取类型
收费全文 | 82682篇 |
免费 | 6930篇 |
国内免费 | 5167篇 |
专业分类
化学 | 23933篇 |
晶体学 | 1835篇 |
力学 | 3790篇 |
综合类 | 440篇 |
数学 | 20545篇 |
物理学 | 44236篇 |
出版年
2022年 | 208篇 |
2021年 | 445篇 |
2020年 | 704篇 |
2019年 | 1003篇 |
2018年 | 948篇 |
2017年 | 739篇 |
2016年 | 599篇 |
2015年 | 529篇 |
2014年 | 1267篇 |
2013年 | 1895篇 |
2012年 | 1328篇 |
2011年 | 1917篇 |
2010年 | 2476篇 |
2009年 | 7042篇 |
2008年 | 8096篇 |
2007年 | 6623篇 |
2006年 | 6018篇 |
2005年 | 4157篇 |
2004年 | 3938篇 |
2003年 | 4152篇 |
2002年 | 4869篇 |
2001年 | 3769篇 |
2000年 | 3572篇 |
1999年 | 3410篇 |
1998年 | 2844篇 |
1997年 | 1963篇 |
1996年 | 1770篇 |
1995年 | 2239篇 |
1994年 | 2164篇 |
1993年 | 1616篇 |
1992年 | 1145篇 |
1991年 | 850篇 |
1990年 | 686篇 |
1989年 | 622篇 |
1988年 | 577篇 |
1987年 | 409篇 |
1986年 | 198篇 |
1985年 | 968篇 |
1984年 | 631篇 |
1983年 | 497篇 |
1982年 | 650篇 |
1981年 | 800篇 |
1980年 | 727篇 |
1979年 | 561篇 |
1978年 | 584篇 |
1977年 | 538篇 |
1976年 | 541篇 |
1975年 | 317篇 |
1974年 | 355篇 |
1973年 | 463篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
Johannes Wittmann 《Mathematische Nachrichten》2019,292(7):1627-1635
Let M be a closed spin manifold and let N be a closed manifold. For maps and Riemannian metrics g on M and h on N, we consider the Dirac operator of the twisted Dirac bundle . To this Dirac operator one can associate an index in . If M is 2‐dimensional, one gets a lower bound for the dimension of the kernel of out of this index. We investigate the question whether this lower bound is obtained for generic tupels . 相似文献
12.
This paper develops a framework to deal with the unconditional superclose analysis of
nonlinear parabolic equation. Taking the finite element pair $Q_{11}/Q_{01} × Q_{10}$ as an example,
a new mixed finite element method (FEM) is established and the $τ$ -independent superclose
results of the original variable $u$ in $H^1$-norm and the flux variable $\mathop{q} \limits ^{\rightarrow}= −a(u)∇u$ in $L^2$-norm are deduced ($τ$ is the temporal partition parameter). A key to our analysis is an
error splitting technique, with which the time-discrete and the spatial-discrete systems are
constructed, respectively. For the first system, the boundedness of the temporal errors is obtained. For the second system, the spatial superclose results are presented unconditionally, while the previous literature always only obtain the convergent estimates or require
certain time step conditions. Finally, some numerical results are provided to confirm the
theoretical analysis, and show the efficiency of the proposed method. 相似文献
13.
14.
15.
Diabetes mellitus (DM) is the fastest growing metabolic disorder in the world. Recently, more attention is paid to the study of natural products due to side effects of synthetic drugs. Stevia rebaudiana (Bertoni) is considered an encouraging starting point for the antidiabetic lead development. In the present study, the in vitro α-amylase inhibitory activity of the extracts of S. rebaudiana is investigated. In order to understand the molecular mechanism and future pharmacophore development, in silico study of secondary metabolites isolated from S. rebaudiana was carried out. Results indicated that water extract shows highest α-amylase inhibitory activity as compared to other extracts. Moreover, compound 20 (rebaudioside A) which has been previously reported and isolated from water extract showed the impressive binding profile with α-amylase. Therefore, our study suggests that S. rebaudiana could be used in the development of therapeutic drugs for the treatment of diabetes. 相似文献
16.
The electrochemical behavior of austenitic stainless steel (Type 304) in 3 M sulfuric acid with 3.5% recrystallized sodium chloride at specific concentrations of butan-1-ol was investigated with the aid of potentiodynamic polarization, open circuit measurement and weight loss technique. Butan-1-ol effectively inhibited the steel corrosion with a maximum inhibition efficiency of 78.7% from weight-loss analysis and 80.9% from potentiodynamic polarization test at highest concentration studied. Adsorption of the compound obeyed the Freundlich isotherm. Thermodynamic calculations reveal physiochemical interactions and spontaneous adsorption mechanism. Surface characterizations showed the absence of corrosion products and topographic modifications of the steel. Statistical analysis depicts the overwhelming influence and statistical significance of inhibitor concentration on the inhibition performance. 相似文献
17.
18.
19.
20.
Oxidative dehydrogenation (ODH) of n-octane was carried out over a vanadium–magnesium oxide catalyst in a continuous flow fixed bed reactor. The catalyst was characterized by ICP–OES, powder XRD and SEM. The catalytic tests were carried out at different gas hourly space velocities (GHSVs), viz. 4000, 6000, 8000, and 10,000 h?1. The best selectivity for octenes was obtained at the GHSV of 8000 h?1, while that for C8 aromatics was attained at the GHSV of 6000 h?1 at high temperatures (500 and 550 °C). The catalytic testing at the GHSV of 10,000 h?1 showed the lowest activity, while that at the GHSV of 4000 h?1 consistently showed the lowest ODH selectivity. Generally, the best ODH performance was obtained by the catalytic testing at the GHSVs of 6000 and 8000 h?1. No phasic changes were observed after the catalytic testing. 相似文献