全文获取类型
收费全文 | 2729篇 |
免费 | 226篇 |
国内免费 | 335篇 |
专业分类
化学 | 520篇 |
晶体学 | 5篇 |
力学 | 75篇 |
综合类 | 114篇 |
数学 | 2153篇 |
物理学 | 423篇 |
出版年
2024年 | 4篇 |
2023年 | 15篇 |
2022年 | 52篇 |
2021年 | 60篇 |
2020年 | 67篇 |
2019年 | 57篇 |
2018年 | 66篇 |
2017年 | 68篇 |
2016年 | 74篇 |
2015年 | 52篇 |
2014年 | 122篇 |
2013年 | 241篇 |
2012年 | 121篇 |
2011年 | 172篇 |
2010年 | 145篇 |
2009年 | 189篇 |
2008年 | 196篇 |
2007年 | 195篇 |
2006年 | 161篇 |
2005年 | 146篇 |
2004年 | 154篇 |
2003年 | 144篇 |
2002年 | 135篇 |
2001年 | 96篇 |
2000年 | 70篇 |
1999年 | 93篇 |
1998年 | 79篇 |
1997年 | 72篇 |
1996年 | 43篇 |
1995年 | 33篇 |
1994年 | 30篇 |
1993年 | 16篇 |
1992年 | 18篇 |
1991年 | 11篇 |
1990年 | 17篇 |
1989年 | 14篇 |
1988年 | 12篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 6篇 |
1984年 | 7篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1979年 | 2篇 |
1977年 | 2篇 |
1974年 | 2篇 |
1970年 | 1篇 |
1959年 | 1篇 |
1936年 | 2篇 |
排序方式: 共有3290条查询结果,搜索用时 156 毫秒
211.
Richard F. Patterson Jeff Connor Jeannette Kline 《Central European Journal of Mathematics》2008,6(3):488-496
The notion of oscillation for ordinary sequences was presented by Hurwitz in 1930. Using this notion Agnew and Hurwitz presented
regular matrix characterization of the resulting sequence space. The primary goal of this article is to extend this definition
to double sequences, which grants us the following definition: the double oscillation of a double sequence of real or complex
number is given P-lim sup(m,n)→∞;(α,β)→∞|S
m,n
-S
α,β
|. Using this concept a matrix characterization of double oscillation sequence space is presented. Other implication and variation
shall also be presented.
相似文献
212.
感知矩阵的构造是压缩感知从理论走向工程应用的关键技术之一.由于托普利兹感知矩阵能够支持快速算法且与离散卷积运算相对应,因此具有重要的研究意义.然而常用的随机托普利兹感知矩阵因其元素的不确定性,使得它在实际应用中受到了诸多约束,例如内存消耗较高和不易于硬件加载.基于此,本文结合双极性混沌序列的内在确定性和托普利兹矩阵的优点,提出了基于双极性混沌序列的托普利兹块状感知矩阵.具体地,首先介绍了双极性混沌序列的产生并分析了它的统计特性.其次,构造了双极性托普利兹块状混沌感知矩阵,从相关性方面证明了新建的感知矩阵具有近乎最优的理论保证,并同时证实了它满足约束等距条件.最后,研究了该感知矩阵针对一维信号和图像的压缩测量效果,并与典型感知矩阵进行了对比.结果表明,提出的感知矩阵对这些测试信号具有更好的测量效果,而且它在内存开销、计算复杂度和硬件实现等方面均具有明显的优势.特别地,该感知矩阵非常适用于多输入-单输出线性时不变系统的压缩感知测量问题. 相似文献
213.
Dehydration of Castor Oil over NaHSO4/MCM‐41 Catalyst Modified by n‐Dodecyltriethoxysilane
下载免费PDF全文
![点击此处可从《无机化学与普通化学杂志》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Cui‐Xia Yan Jian‐Fei Ding Tian‐Lin Ma Rong Shao Wei Xu Jie He Peng‐Fei Wang 《无机化学与普通化学杂志》2017,643(12):772-779
n‐Dodecyltriethoxysilane (DTEOS) modified NaHSO4/MCM‐41 catalysts (silanized catalysts) were synthesized by different impregnation sequences and evaluated in the liquid‐phase dehydration of castor oil. The samples were evaluated by X‐ray diffraction, nitrogen adsorption‐desorption, SEM, TEM, FT‐IR spectroscopy, XPS, 29Si MAS NMR spectroscopy, contact angle measurements, NH3‐TPD, and pyridine‐FT‐IR spectroscopy. The analyses demonstrated that silanization enhanced the hydrophobicity of the catalysts, and the impregnation sequence of silanized catalysts had a significant effect on the NaHSO4 dispersion, surface area, acid distribution, and hydrophobicity of the silanized catalysts. The catalytic activity of the silanized catalysts was much higher than that of NaHSO4/MCM‐41. Among the silanized catalysts, the catalyst prepared by simultaneous impregnation with DTEOS and NaHSO4 showed the highest iodine value of 141.8 [g(I2) per 100 g] and lowest hydroxyl value of 11.3 [mg(KOH) · g–1]. 相似文献
214.
A major concern affecting the efficient use of composite laminates is the effect of low velocity impact damage on the structural integrity [1–3]. The aim of this study is to characterize and assess the effect of laminate thickness, ply-stacking sequence and scaling technique on the damage resistance of CFRP laminates subjected to low velocity impact. Drop-weight impact tests are carried out to determine impact response. Ultrasonic C-scanning and cross-sectional micrographs are examined to assess failure mechanisms of the different configurations.It is observed that damage resistance decreases as impact energy increases. In addition, thicker laminates show lower absorbed energy but, conversely, a more extensive delamination due to higher bending stiffness. Thinner laminates show higher failure depth. Furthermore, quasi-isotropic laminates show better performance in terms of damage resistance. Finally, the results obtained demonstrate that introducing ply clustering had a negative effect on the damage resistance and on the delamination area. 相似文献
215.
P.M. Ligrani R. Van Den Braembussche M. Roustan 《International Journal of Heat and Fluid Flow》1983,4(2):103-106
Results from an experimental study of flow behaviour at the inlet of a vaneless diffuser of a centrifugal compressor are presented. Measurements from a crossed hot-wire probe are given for operating points having inlet flow coefficients ranging from 0.006 to 0.019 at different Reynolds numbers. Instantaneous, time-averaged, and phase-averaged absolute velocity and flow angle at the diffuser inlet are deduced from the hot-wire signals after correction for mean density variations. These results show how flow behaviour varies in stable, rotating stall and surge regimes of compressor operation 相似文献
216.
Xianjie Fang 《Tetrahedron letters》2010,51(2):317-8716
A domino sequence of cyclization of 1,6-enyne with 2-bromoarylaldehyde followed by the intramolecular Aldol-condensation has been realized in the presence of 5 mol % of palladium catalyst. This sequence provides a convenient protocol to access polycyclic ring systems with readily available starting materials under simple Pd(0)-catalyzed conditions. The intramolecular Aldol-condensation has been proposed as the key step for the catalytic cycle. 相似文献
217.
218.
Johanna Andersson Dr. Minna Li Prof. Per Lincoln 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(36):11037-11046
The binuclear ruthenium complex [μ‐bidppz(phen)4Ru2]4+ has been extensively studied since the discovery of its unusual threading intercalation interaction with DNA, a binding mode with extremely slow binding and dissociation kinetics. The complex has been shown to be selective towards long stretches of alternating AT base pairs, which makes it interesting, for example, as a model compound for anti‐malaria drugs due to the high AT content of the genome of the malaria parasite P. falciparum. We have investigated the effect of bridging ligand structure on threading intercalation ability and found that length and rigidity as well as the size of the intercalated ring system are all factors that affect the rate and selectivity of the threading intercalation. In particular, we discovered a new DNA‐threading compound, [μ‐dppzip(phen)4Ru2]4+, which appears to be just at the border of being capable of threading intercalation and displays even greater selectivity for AT‐DNA than the parent compound, [μ‐bidppz(phen)4Ru2]4+. 相似文献
219.
220.
研究了极大代数上线性系统的3维最小实现问题,给出了特征方程为λ<'3> c<,0>λ<'0>=c<,2>λ<'2> c<,1>λ,当c<,1>=c<'2><,2>,c<,0>=C<,1>C<,2>时,无穷序列{g<,2>}<,0><'∞>存在3维最小实现的充要条件. 相似文献