全文获取类型
收费全文 | 101814篇 |
免费 | 28409篇 |
国内免费 | 24573篇 |
专业分类
化学 | 53276篇 |
晶体学 | 1004篇 |
力学 | 5388篇 |
综合类 | 20篇 |
数学 | 11862篇 |
物理学 | 54024篇 |
无线电 | 29222篇 |
出版年
2024年 | 788篇 |
2023年 | 1396篇 |
2022年 | 1503篇 |
2021年 | 1489篇 |
2020年 | 1789篇 |
2019年 | 2970篇 |
2018年 | 2837篇 |
2017年 | 3576篇 |
2016年 | 4325篇 |
2015年 | 4948篇 |
2014年 | 4686篇 |
2013年 | 6742篇 |
2012年 | 7099篇 |
2011年 | 7856篇 |
2010年 | 10556篇 |
2009年 | 10418篇 |
2008年 | 4516篇 |
2007年 | 3967篇 |
2006年 | 3790篇 |
2005年 | 3727篇 |
2004年 | 4779篇 |
2003年 | 3844篇 |
2002年 | 3666篇 |
2001年 | 3579篇 |
2000年 | 2793篇 |
1999年 | 2786篇 |
1998年 | 2466篇 |
1997年 | 2293篇 |
1996年 | 2660篇 |
1995年 | 2995篇 |
1994年 | 3066篇 |
1993年 | 3188篇 |
1992年 | 2769篇 |
1991年 | 2407篇 |
1990年 | 1987篇 |
1989年 | 2102篇 |
1988年 | 1993篇 |
1987年 | 1249篇 |
1986年 | 1276篇 |
1985年 | 980篇 |
1984年 | 1107篇 |
1982年 | 1047篇 |
1981年 | 843篇 |
1980年 | 906篇 |
1979年 | 636篇 |
1978年 | 637篇 |
1977年 | 728篇 |
1976年 | 1139篇 |
1973年 | 521篇 |
1972年 | 595篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Within the framework of the effective-mass approximation and the dipole approximation, considering the three-dimensional confinement of the electron and hole and the strong built-in electric field(BEF) in strained wurtzite Zn O/Mg0:25Zn0:75O quantum dots(QDs), the optical properties of ionized donor-bound excitons(D+, X)are investigated theoretically using a variational method. The computations are performed in the case of finite band offset. Numerical results indicate that the optical properties of(D+, X) complexes sensitively depend on the donor position, the QD size and the BEF. The binding energy of(D+, X) complexes is larger when the donor is located in the vicinity of the left interface of the QDs, and it decreases with increasing QD size. The oscillator strength reduces with an increase in the dot height and increases with an increase in the dot radius. Furthermore, when the QD size decreases, the absorption peak intensity shows a marked increment, and the absorption coefficient peak has a blueshift. The strong BEF causes a redshift of the absorption coefficient peak and causes the absorption peak intensity to decrease remarkably. The physical reasons for these relationships have been analyzed in depth. 相似文献
2.
Dr. Antonio M. Rodríguez Dr. Pilar Prieto Prof. Antonio de la Hoz Prof. Ángel Díaz-Ortiz D. Raúl Martín Prof. José I. García 《ChemistryOpen》2015,4(3):308-317
The aim of this work was to determine the parameters that have decisive roles in microwave-assisted reactions and to develop a model, using computational chemistry, to predict a priori the type of reactions that can be improved under microwaves. For this purpose, a computational study was carried out on a variety of reactions, which have been reported to be improved under microwave irradiation. This comprises six types of reactions. The outcomes obtained in this study indicate that the most influential parameters are activation energy, enthalpy, and the polarity of all the species that participate. In addition to this, in most cases, slower reacting systems observe a much greater improvement under microwave irradiation. Furthermore, for these reactions, the presence of a polar component in the reaction (solvent, reagent, susceptor, etc.) is necessary for strong coupling with the electromagnetic radiation. We also quantified that an activation energy of 20–30 kcal mol−1 and a polarity (μ) between 7–20 D of the species involved in the process is required to obtain significant improvements under microwave irradiation. 相似文献
3.
Antonio R. Montoro Bustos Marta Garcia-Cortes Hector González-Iglesias Jorge Ruiz Encinar José M. Costa-Fernández Miguel Coca-Prados Alfredo Sanz-Medel 《Analytica chimica acta》2015
A generic strategy based on the use of CdSe/ZnS Quantum Dots (QDs) as elemental labels for protein quantification, using immunoassays with elemental mass spectrometry (ICP-MS), detection is presented. In this strategy, streptavidin modified QDs (QDs-SA) are bioconjugated to a biotinylated secondary antibody (b-Ab2). After a multi-technique characterization of the synthesized generic platform (QDs-SA-b-Ab2) it was applied to the sequential quantification of five proteins (transferrin, complement C3, apolipoprotein A1, transthyretin and apolipoprotein A4) at different concentration levels in human serum samples. It is shown how this generic strategy does only require the appropriate unlabeled primary antibody for each protein to be detected. Therefore, it introduces a way out to the need for the cumbersome and specific bioconjugation of the QDs to the corresponding specific recognition antibody for every target analyte (protein). Results obtained were validated with those obtained using UV–vis spectrophotometry and commercial ELISA Kits. 相似文献
4.
We investigate the effect of dopant random fluctuation on threshold voltage and drain current variation in a two-gate nanoscale transistor. We used a quantum-corrected technology computer aided design simulation to run the simulation (10000 randomizations). With this simulation, we could study the effects of varying the dimensions (length and width), and thicknesses of oxide and dopant factors of a transistor on the threshold voltage and drain current in subthreshold region (off) and overthreshold (on). It was found that in the subthreshold region the variability of the drain current and threshold voltage is relatively fixed while in the overthreshold region the variability of the threshold voltage and drain current decreases remarkably, despite the slight reduction of gate voltage diffusion (compared with that of the subthreshold). These results have been interpreted by using previously reported models for threshold current variability, load displacement, and simple analytical calculations. Scaling analysis shows that the variability of the characteristics of this semiconductor increases as the effects of the short channel increases. Therefore, with a slight increase of length and a reduction of width, oxide thickness, and dopant factor, we could correct the effect of the short channel. 相似文献
5.
6.
7.
8.
9.
10.
Susana Estopiñá-Durán Liam J. Donnelly Euan B. Mclean Bryony M. Hockin Prof. Alexandra M. Z. Slawin Dr. James E. Taylor 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(15):3950-3956
A combination of pentafluorophenylboronic acid and oxalic acid catalyses the dehydrative substitution of benzylic alcohols with a second alcohol to form new C−O bonds. This method has been applied to the intermolecular substitution of benzylic alcohols to form symmetrical ethers, intramolecular cyclisations of diols to form aryl-substituted tetrahydrofuran and tetrahydropyran derivatives, and intermolecular crossed-etherification reactions between two different alcohols. Mechanistic control experiments have identified a potential catalytic intermediate formed between the aryl boronic acid and oxalic acid. 相似文献