全文获取类型
收费全文 | 2503篇 |
免费 | 113篇 |
国内免费 | 23篇 |
专业分类
化学 | 1650篇 |
晶体学 | 27篇 |
力学 | 110篇 |
综合类 | 1篇 |
数学 | 275篇 |
物理学 | 320篇 |
无线电 | 256篇 |
出版年
2023年 | 17篇 |
2022年 | 106篇 |
2021年 | 116篇 |
2020年 | 72篇 |
2019年 | 81篇 |
2018年 | 75篇 |
2017年 | 71篇 |
2016年 | 121篇 |
2015年 | 77篇 |
2014年 | 94篇 |
2013年 | 221篇 |
2012年 | 158篇 |
2011年 | 164篇 |
2010年 | 98篇 |
2009年 | 80篇 |
2008年 | 90篇 |
2007年 | 104篇 |
2006年 | 83篇 |
2005年 | 63篇 |
2004年 | 59篇 |
2003年 | 65篇 |
2002年 | 68篇 |
2001年 | 43篇 |
2000年 | 52篇 |
1999年 | 26篇 |
1998年 | 26篇 |
1997年 | 20篇 |
1996年 | 28篇 |
1995年 | 22篇 |
1994年 | 25篇 |
1993年 | 14篇 |
1992年 | 12篇 |
1991年 | 22篇 |
1990年 | 18篇 |
1989年 | 18篇 |
1988年 | 10篇 |
1987年 | 16篇 |
1986年 | 10篇 |
1985年 | 23篇 |
1984年 | 19篇 |
1983年 | 20篇 |
1982年 | 21篇 |
1981年 | 13篇 |
1980年 | 12篇 |
1979年 | 8篇 |
1978年 | 7篇 |
1977年 | 10篇 |
1976年 | 5篇 |
1975年 | 9篇 |
1973年 | 5篇 |
排序方式: 共有2639条查询结果,搜索用时 15 毫秒
71.
O. Fatih Kilic M. Omer Sayin Ibrahim Delibalta Suleyman S. Kozat 《Signal, Image and Video Processing》2017,11(2):235-242
We introduce a new combination approach for the mixture of adaptive filters based on the set-membership filtering (SMF) framework. We perform SMF to combine the outputs of several parallel running adaptive algorithms and propose unconstrained, affinely constrained and convexly constrained combination weight configurations. Here, we achieve better trade-off in terms of the transient and steady-state convergence performance while providing significant computational reduction. Hence, through the introduced approaches, we can greatly enhance the convergence performance of the constituent filters with a slight increase in the computational load. In this sense, our approaches are suitable for big data applications where the data should be processed in streams with highly efficient algorithms. In the numerical examples, we demonstrate the superior performance of the proposed approaches over the state of the art using the well-known datasets in the machine learning literature. 相似文献
72.
Abstract A rapid and sensitive spectrophotometric method is developed for the quantitative determination of piperazine, piperazine salts and diethylcarbamazine citrate. The method is based on the formation of donor-acceptor complexes between the compound and chloranilic acid. The interaction product has a characteristic absorption maximum at about 345 nm. The system obeys Beer's law in the concentration range of 1–10 ugml?1 of piperazine hexahydrate, adipate, phosphate, citrate and diethylcarbamazine citrate. The effects of several experimantal variables have been studied and the stoichiometric composition of the complex has been determined. The developed procedure has been found to be simple, rapid and precise and its application to various dosage forms was demonstrated and yields results comparable with the standard method. 相似文献
73.
Idowu David Ibrahim Tamba Jamiru Emmanuel R. Sadiku Williams Kehinde Kupolati Stephen C. Agwuncha Gbenga Ekundayo 《Composite Interfaces》2016,23(1):15-36
There has been a growing interest in the utilization of sisal fibres as reinforcement in the production of polymeric composite materials. Natural fibres have gained recognition as reinforcements in fibre polymer–matrix composites because of their mechanical properties and environmental friendliness. The mechanical properties of sisal fibre-reinforced polymer composites have been studied by many researchers and a few of them are discussed in this article. Various fibre treatments, which are carried out in order to improve adhesion, leading to improved mechanical properties, are also discussed in this review paper. This review also focuses on the influence of fibre content and fabrication methods, which can significantly affect the mechanical properties of sisal fibre-reinforced polymer composites. 相似文献
74.
75.
Emad A. Jaffar Al-Mulla Nor Azowa Bt Ibrahim Kamyar Shameli Mansor Bin Ahmad Wan Md. Zin Wan Yunus 《Research on Chemical Intermediates》2013,39(3):1313-1321
In this study, fatty amides (FAs) synthesized from palm olein were used to extract and separate Mo(VI) from acidic media. Effects of various parameters upon the separation of Mo(VI) from Co(II), Ni(II), Al(III) and Mn(II), including extractant concentration, metal ion concentration, contact time, diluent, and acidity, were investigated. It was found that Mo(VI) was successfully separated from the above commonly associated metal ions by stripping from the loaded organic phase. Different acidic and alkaline solutions were used. Ammonium hydroxide solution was an optimal. Extraction of Mo(VI) into the organic phase involved the formation of 1:3 complexes. This work presents the development of a low-cost and environmentally friendly extractant to recycle and recover molybdenum. 相似文献
76.
Let G be a locally compact abelian group with a fixed Haar measure and ω be a weight on G. For 1 < p < ∞, we study uniqueness of uniform and C*-norm properties of the invariant weighted algebra L p (G, ω). 相似文献
77.
78.
Youssef Ibrahim Moharram El-Hallag Ibrahim Shibl Selim Sameh Mahmoud 《Russian Journal of Electrochemistry》2021,57(6):644-653
Russian Journal of Electrochemistry - Generic products are not identical to their branded equivalents. They are typically off-patent. Therefore, this paper suggests a selective, reliable, and... 相似文献
79.
This paper studies the fabrication and characterization of 80 nm zinc oxide anti-reflective coating (ARC) on flexible 1.3 μm thin film microcrystalline silicon (μc-Si) solar cell. High resolution X-ray diffraction (HR-XRD) shows a c-axis oriented ZnO (0 0 2) peak (hexagonal crystal structure) at 34.3° with full width at half maximum (FWHM) of 0.3936°. Atomic force microscope (AFM) measures high surface roughness root-mean-square (RMS) of the layer (50.76 nm) which suggests scattering of the incident light at the front surface of the solar cell. UV–vis spectrophotometer illustrates that ZnO ARC has optical transmittance of more than 80% in the visible and infra-red (IR) regions and corresponds to band gap (Eg) of 3.3 eV as derived from Tauc equation. Inclusion of ZnO ARC successfully suppresses surface reflectance from the cell to 2% (at 600 nm) due to refractive index grading between the Si and the ZnO besides quarter-wavelength (λ/4) destructive interference effect. The reduced reflectance and effective scattering effect of the incident light at the front side of the cell are believed to be the reasons why short-circuit current (Isc) and efficiency (η) of the cell improve. 相似文献
80.
Oxygen bridged Homobinuclear Mn(II) compounds with Anthranilic acid: Theoretical calculations,oxidation and catalase activity 下载免费PDF全文
Two new homobinuclear manganese compounds with mixed ligands, [Mn2(μ1,1–2‐NH2C6H4COO)2(phen)4](ClO4)2(CH3OH) ( 1 ), and [Mn2(μ1,3–2‐NH2C6H4COO)2(bipy)4](ClO4)2 ( 2 ) (NH2C6H4COOH = anthranilic acid, bipy = 2,2′‐bipyridine, phen = 1,10‐ phenanthroline) were synthesized and thoroughly characterized by elemental analysis, IR, UV and single crystal X‐ray crystallography. X‐ray structure analysis shows that in the mono‐ and bidentate carboxylate bridged compounds, Mn–Mn distances of 1 and 2 are 3,461 Å, and 4,639 Å, respectively. The energy of the compounds was determined with a DFT (Density Functional Theory) calculation on B3LYP/6‐31G(d,p) optimized geometry by using the B3LYP/6‐31G(d,p) basis set. These compounds acts as biomimetic catalyst and show catalase‐like activity for the hydrogen peroxide dismutation at room temperature in different solvents with remarkable activity (TOF, Turnover frequency = mol of subst./(mol of cat. × time)) up to 12640 h?1 with 1 , and 17910 h?1 with 2 in Tris–HCl buffer). Moreover, the catalytic activity of 1 and 2 has been studied for oxidation of alcohols (cinnamyl alcohol, benzyl alcohol, cyclohexanol, 1‐octanol and 1‐heptanol) and alkenes (cyclohexene, styrene, ethyl benzene, 1‐octene and 1‐hexene) in a homogeneous catalytic system consisting t‐butylhydroperoxide (TBHP) as an oxidant in acetonitrile. Both compounds exhibited very high activity in the oxidation of cyclohexene to cyclohexanone (~80% selectivity, ~99% conversion in 1 h, TOF = 243 h?1 and 226 h?1) and cinnamyl alcohol to cinnamaldehyde (~64% selectivity) as the main product with very high TOF value (9180 h?1 and 13040 h?1 in the first minute of reaction) (~100% conversion in 0.5 h) with TBHP at 70 °C in acetonitrile, for 1 and 2 , respectively. 相似文献