全文获取类型
收费全文 | 54204篇 |
免费 | 5581篇 |
国内免费 | 3520篇 |
专业分类
化学 | 24759篇 |
晶体学 | 202篇 |
力学 | 3363篇 |
综合类 | 1346篇 |
数学 | 17013篇 |
物理学 | 16622篇 |
出版年
2023年 | 425篇 |
2022年 | 723篇 |
2021年 | 1909篇 |
2020年 | 1220篇 |
2019年 | 1269篇 |
2018年 | 1038篇 |
2017年 | 1352篇 |
2016年 | 1595篇 |
2015年 | 1472篇 |
2014年 | 2169篇 |
2013年 | 3331篇 |
2012年 | 2378篇 |
2011年 | 2750篇 |
2010年 | 2532篇 |
2009年 | 3088篇 |
2008年 | 3368篇 |
2007年 | 3549篇 |
2006年 | 2801篇 |
2005年 | 2089篇 |
2004年 | 2004篇 |
2003年 | 1896篇 |
2002年 | 4343篇 |
2001年 | 1925篇 |
2000年 | 1374篇 |
1999年 | 1210篇 |
1998年 | 1124篇 |
1997年 | 826篇 |
1996年 | 814篇 |
1995年 | 766篇 |
1994年 | 657篇 |
1993年 | 689篇 |
1992年 | 605篇 |
1991年 | 492篇 |
1990年 | 381篇 |
1989年 | 273篇 |
1988年 | 334篇 |
1987年 | 268篇 |
1986年 | 272篇 |
1985年 | 380篇 |
1984年 | 279篇 |
1983年 | 173篇 |
1982年 | 325篇 |
1981年 | 506篇 |
1980年 | 446篇 |
1979年 | 490篇 |
1978年 | 382篇 |
1977年 | 290篇 |
1976年 | 248篇 |
1974年 | 81篇 |
1973年 | 156篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
1.
2.
为提高煤与瓦斯突出矿井瓦斯抽放效果,建立了3个一级指标、14个二级指标的突出矿井瓦斯抽放限制影响因素评价指标体系,利用AHP和熵权法分别确定指标因子主、客观权重.通过实地调研分析和反馈验证了AHP-熵权法的可行性和正确性,利用加权平均法确定评价模型的综合权重.研究表明:封孔方式、钻孔半径、抽放时间、煤体裂隙发育程度和抽放负压是目前影响煤矿瓦斯抽放效果的主控因素. 相似文献
3.
4.
Dynamical analysis,circuit realization,and application in pseudorandom number generators of a fractional-order laser chaotic system 下载免费PDF全文
Chenguang Ma 《中国物理 B》2021,30(12):120504-120504
A new five-dimensional fractional-order laser chaotic system (FOLCS) is constructed by incorporating complex variables and fractional calculus into a Lorentz-Haken-type laser system. Dynamical behavior of the system, circuit realization and application in pseudorandom number generators are studied. Many types of multi-stable states are discovered in the system. Interestingly, there are two types of state transition phenomena in the system, one is the chaotic state degenerates to a periodical state, and the other is the intermittent chaotic oscillation. In addition, the complexity of the system when two parameters change simultaneously is measured by the spectral entropy algorithm. Moreover, a digital circuit is design and the chaotic oscillation behaviors of the system are verified on this circuit. Finally, a pseudo-random sequence generator is designed using the FOLCS, and the statistical characteristics of the generated pseudo-random sequence are tested with the NIST-800-22. This study enriches the research on the dynamics and applications of FOLCS. 相似文献
5.
6.
We propose a conjecture on the relative twist formula of l-adic sheaves, which can be viewed as a generalization of Kato—Saito's conjecture. We verify this conjecture under some transversal assumptions. We also define a relative cohomological characteristic class and prove that its formation is compatible with proper push-forward. A conjectural relation is also given between the relative twist formula and the relative cohomological characteristic class. 相似文献
7.
Atopic dermatitis is characterized by leukocyte migration into the skin dermis and typically driven by excessive chemokine production at the site of inflammation. Conventional topical formulations such as gels, creams, and ointments are insufficient for this treatment because of low penetration of drug molecules into the targeted skin tissues. Herein, using a simple, green, sustainable strategy, we have developed novel primary zein nanoparticles embedded in curcumin (Cur) and coated with silk sericin (ZHSCs) for the topical delivery of Cur to penetrate into the dermis and exercise anti-dermatitis effects on the lesion with minimal side-effects. Transdermal delivery experiments and porcine skin fluorescence imaging indicated that ZHSCs facilitate the penetration of Cur across the epidermis layer of skin to reach deep-seated sites. Notably, ZHSCs = 1:0.25 (zein-to-silk sericin mass ratios of 1:0.25) markedly elevated the skin permeability and cumulative turnover of Cur transferred, which were provided a greater than a 3.8-fold increase relative to free Cur. The special nanoparticles of ZHS = 1:0.25 possessed the deepest localization depth and experience a transition of the particle structure and core-shell separation after penetrating into the dermis of skin. In a cell model of dermatitis induced by tumor necrosis factor α/interferon γ co-stimulation, compared with free Cur, Cur-loaded ZHS nanoparticles down-regulated the generation of inflammatory cytokines and chemokines in keratinocytes through suppression of the nuclear translocation of NF-κBp65 and hence exerted an anti-dermatitis effect. This strategy may provide new avenues and direction for the demanding issues of valid topical delivery systems. 相似文献
8.
9.
In this work, a vanillin complex is immobilized onto MCM-41 and characterized by FT-IR, X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, thermogravimetric analysis, and BET techniques. This supported Schiff base complex was found to be an efficient and recoverable catalyst for the chemoselective oxidation of sulfides into sulfoxides and thiols into their corresponding disulfides (using hydrogen peroxide as a green oxidant) and also a suitable catalyst for the preparation of 2,3-dihydroquinazolin-4(1H)-one derivatives in water at 90°C. Using this protocol, we show that a variety of disulfides, sulfoxides, and 2,3-dihydroquinazolin-4(1H)-one derivatives can be synthesized in green conditions. The catalyst can be recovered and recycled for further reactions without appreciable loss of catalytic performance. 相似文献
10.
Thalia Tsiaka Panagiotis Zoumpoulakis Vassilia J. Sinanoglou Constantinos Makris Georgios A. Heropoulos Antony C. Calokerinos 《Analytica chimica acta》2015
High-energy assisted extraction techniques, like ultrasound assisted extraction (UAE) and microwave assisted extraction (MAE), are widely applied over the last years for the recovery of bioactive compounds such as carotenoids, antioxidants and phenols from foods, animals and herbal natural sources. Especially for the case of xanthophylls, the main carotenoid group of crustaceans, they can be extracted in a rapid and quantitative way with the use of UAE and MAE. 相似文献