全文获取类型
收费全文 | 2461篇 |
免费 | 444篇 |
国内免费 | 314篇 |
专业分类
化学 | 1854篇 |
晶体学 | 30篇 |
力学 | 223篇 |
综合类 | 13篇 |
数学 | 318篇 |
物理学 | 781篇 |
出版年
2024年 | 11篇 |
2023年 | 72篇 |
2022年 | 96篇 |
2021年 | 111篇 |
2020年 | 150篇 |
2019年 | 106篇 |
2018年 | 119篇 |
2017年 | 103篇 |
2016年 | 150篇 |
2015年 | 158篇 |
2014年 | 157篇 |
2013年 | 203篇 |
2012年 | 199篇 |
2011年 | 204篇 |
2010年 | 142篇 |
2009年 | 121篇 |
2008年 | 129篇 |
2007年 | 120篇 |
2006年 | 108篇 |
2005年 | 90篇 |
2004年 | 66篇 |
2003年 | 55篇 |
2002年 | 79篇 |
2001年 | 69篇 |
2000年 | 52篇 |
1999年 | 45篇 |
1998年 | 49篇 |
1997年 | 37篇 |
1996年 | 41篇 |
1995年 | 24篇 |
1994年 | 20篇 |
1993年 | 26篇 |
1992年 | 21篇 |
1991年 | 18篇 |
1990年 | 15篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 5篇 |
1986年 | 7篇 |
1985年 | 6篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1975年 | 1篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1964年 | 1篇 |
1957年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有3219条查询结果,搜索用时 46 毫秒
71.
In this paper, a novel multidimensional scaling (MDS) based on information measures method is proposed to analyze financial stock markets. In order to examine the effectiveness of this method, we applied it to the classification of two types of artificial series, the logistic map model and the cubic map model, as well as stock time series. Moreover, the traditional MDS using Euclidean dissimilarity is also provided as a reference for comparisons. The results show that the MDS based on information measures can give us more detailed, exact and clearer information on the classification of simulation series and stock time series than the MDS using Euclidean dissimilarity. In addition, the proposed graphical method may also assist in the construction of multivariate econometric models. 相似文献
72.
In this paper, a model is built to explore how the parameters (amplified spontaneous emission (ASE), temperature and round-trip loss) influence the output power in a thin-disk laser. It is found that optical efficiency of the disk laser is reduced with the increase of ASE, temperature or round-trip loss. The parameters are optimized to maximize the output power based on our model. We find that it is necessary to balance the need to lower the temperature with the need to control ASE during the optimization process. But the balance becomes more difficult to achieve with the increase of round-trip loss. We conclude that output power of more than 2.6 MW with a single disk can be achieved, but the necessary disk size (more than 0.5 m) is far beyond the actual technical limits. But it is possible to achieve output power of over a hundred kilowatts using a 10 cm disk in the near future. 相似文献
73.
Sien Liu Bangyue He Hongyi Li Xiaofeng Zhang Yaping Shang Prof. Weiping Su 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(63):15628-15633
The Rh-catalyzed ortho-C(sp2)−H functionalization of 8-aminoquinoline-derived benzamides with aliphatic acyl fluorides generated in situ from the corresponding acids has been developed. This reaction initiated with 8-aminoquinoline-directed ortho-C(sp2)−H acylation, which was accompanied by subsequent intramolecular nucleophilic acyl substitution of amide group to produce alkylidene phthalides This approach exhibits high stereo-selectivity for Z-isomer products, and tolerates a variety of functional groups as well as aliphatic carboxylic acids with diverse structural scaffolds. 相似文献
74.
75.
The thermal rearrangement of [3]polynorbornane bis-imide rack-mounted 7-oxabenzonorbornadienes has been conducted using flash vacuum pyrolysis (FVP) at 520 °C and is compared with the FVP of similar 7-oxabenzonorbornadienes off the rack. The isomerisation is considered to involve (a) C–O bond cleavage to a vinylogous 1,5-dipole, (b) formation of a benzene epoxide by nucleophilic ring-closure and (c) valence-isomerisation of the benzene epoxide to the oxepine. Competing fragmentation to the isobenzofuran by ejection of acetylene and other rearrangements become prominent pathways off the rack, whereas isomerisation to the oxepine is highly favored on the rack. 相似文献
76.
77.
78.
Le Fang Wei-Tao Gong Manivannan Kalavathi Dhinakaran Li Shang Gui-Ling Ning 《Chemical Papers》2016,70(5):663-666
A novel Hg2+ ion induced reversible ring contraction was achieved employing the intramolecular reaction of isobutylene with an aromatic hydroxyl group of cyclophane; reversibility of the reaction was facilitated by excess addition of NaBH4 which also resulted in complexation. The ring contraction and expansion was monitored by UV-VIS absorption, and by fluorescence and 1H NMR spectra. Switchable fluorescence behavior (on—off—on) was observed when the ring-size was tuned from a 19-membered ring to an 18-membered and vice versa. This fine tuning has the potential to be applied in the construction of new supramolecular devices. 相似文献
79.
Henry reaction is one of the most classical reactions to construct synthetically useful product nitro alcohol, which as a privileged skeleton is widely distributed in various pharmaceuticals. This review summarizes the recent progress of copper-catalyzed asymmetric Henry reaction from 2011 to 2016. The significant progress that has been made in this area will be highlighted and some of challenges that the author believes may be hindering further progress will be revealed. 相似文献
80.
Xiao Hu An Zeng Ming-Sheng Shang 《The European Physical Journal B - Condensed Matter and Complex Systems》2016,89(2):46
Recommender system is an effective tool to find the most relevant information for onlineusers. By analyzing the historical selection records of users, recommender system predictsthe most likely future links in the user-item network and accordingly constructs apersonalized recommendation list for each user. So far, the recommendation process ismostly investigated in static user-item networks. In this paper, we propose a model whichallows us to examine the performance of the state-of-the-art recommendation algorithms inevolving networks. We find that the recommendation accuracy in general decreases with timeif the evolution of the online network fully depends on the recommendation. Interestingly,some randomness in users’ choice can significantly improve the long-term accuracy of therecommendation algorithm. When a hybrid recommendation algorithm is applied, we find thatthe optimal parameter gradually shifts towards the diversity-favoring recommendationalgorithm, indicating that recommendation diversity is essential to keep a high long-termrecommendation accuracy. Finally, we confirm our conclusions by studying therecommendation on networks with the real evolution data. 相似文献