全文获取类型
收费全文 | 3852篇 |
免费 | 396篇 |
国内免费 | 262篇 |
专业分类
化学 | 1523篇 |
晶体学 | 6篇 |
力学 | 107篇 |
综合类 | 71篇 |
数学 | 588篇 |
物理学 | 817篇 |
无线电 | 1398篇 |
出版年
2024年 | 8篇 |
2023年 | 38篇 |
2022年 | 121篇 |
2021年 | 97篇 |
2020年 | 111篇 |
2019年 | 116篇 |
2018年 | 126篇 |
2017年 | 155篇 |
2016年 | 197篇 |
2015年 | 199篇 |
2014年 | 239篇 |
2013年 | 285篇 |
2012年 | 276篇 |
2011年 | 264篇 |
2010年 | 202篇 |
2009年 | 230篇 |
2008年 | 253篇 |
2007年 | 252篇 |
2006年 | 219篇 |
2005年 | 179篇 |
2004年 | 173篇 |
2003年 | 145篇 |
2002年 | 141篇 |
2001年 | 63篇 |
2000年 | 86篇 |
1999年 | 56篇 |
1998年 | 42篇 |
1997年 | 34篇 |
1996年 | 35篇 |
1995年 | 30篇 |
1994年 | 16篇 |
1993年 | 18篇 |
1992年 | 22篇 |
1991年 | 14篇 |
1990年 | 9篇 |
1989年 | 7篇 |
1988年 | 8篇 |
1987年 | 3篇 |
1986年 | 12篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1959年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有4510条查询结果,搜索用时 15 毫秒
961.
《Journal of heterocyclic chemistry》2017,54(2):1598-1603
Hexamethylenetetramine catalyzes synthesis of new polyfunctionalized 4H‐pyrans by the reaction of aromatic aldehyde, malononitrile, and β‐keto esters via one‐pot three‐component procedure in water medium. Addition of reactants was performed by two methods led to achieve similar results. Using hexamethylenetetramine in catalytic amount not only represents the economic face of the reaction but also due to the use of water, a green and safe reaction condition is organized. Thus, the current strategy provides the benefits of high productivity, convenient operation, and environmental friendliness. The structure of all products were proved by elemental analysis, IR, 1H NMR, and 13C NMR spectroscopy. 相似文献
962.
《应用有机金属化学》2017,31(11)
Superparamagnetic nanoparticles of modified thioglycolic acid (γ‐Fe2O3@SiO2‐SCH2CO2H) represent a new, efficient and green catalyst for the one‐pot synthesis of novel spiro[benzo[a ]benzo[6,7]chromeno[2,3‐c ]phenazine] derivatives via domino Knoevenagel–Michael–cyclization reaction of 2‐hydroxynaphthalene‐1,4‐dione, benzene‐1,2‐diamines, ninhydrin and isatin. This novel magnetic organocatalyst was easily isolated from the reaction mixture by magnetic decantation using an external magnet and reused at least six times without significant loss in its activity. The catalyst was fully characterized using various techniques. This procedure was also applied successfully for the synthesis of benzo[a ]benzo[6,7]chromeno[2,3‐c ]phenazines. 相似文献
963.
In order to adaptively estimate the generalized principal component from input signals,a novel generalized principal component analysis algorithm was proposed based on the Hebbian linear neuron model.Since the autocorrelation matrices of the signals were estimated directly from the sampled data at the current time,the proposed algorithm had low computation complexity.Trough analyzing all of the equilibrium points by Lyapunov method,it is proven that if and only if the weight vector in the neuron had the same direction with the generalized principal component,the proposed algorithm attains the convergence status.Simulation results shows that compared with some same type algorithms,the proposed algorithm has faster convergence speed. 相似文献
964.
Zhitao Jiang Xue Wang Jianchun Wang Chundi Liu Jinhuo Pan 《Biomedical chromatography : BMC》2019,33(9)
Sedum sarmentosum Bunge (SSB) is a traditional Chinese herbal medicine containing multiple components that has been extensively used clinically to treat chronic viral hepatitis and some inflammatory diseases. Total flavonoids are major pharmacologically active components of SSB. To gain a deeper understanding of SSB resources, we analyzed eight chemical constituents in 33 batches of SSB from 11 regions in China. An accurate, precise and sensitive ultra‐high‐performance liquid chromatography coupled with triple quadrupole electrospray tandem mass spectrometry method was developed for the determination of eight flavonoids in SSB. Under the optimized chromatographic conditions, good separation for the eight target components was obtained on an Agilent Zobax SB C18 (50 × 2.1 mm, 5 μm) column within 4 min. The established methods were validated with good linearity (r ≥ 0.9988), precision (RSD ≤ 2.68%), stability (1.43–3.28%) and repeatability (1.14–2.89%). Moreover, the average recoveries were 95.91–100.68%, and the RSDs were 1.50–3.80%. In addition, the analytical conditions of UPLC–ESI–MS/MS provided better sensitivity with a shorter analysis time when compared with the HPLC–DAD method. Hierarchical clustering analysis and principal component analysis were performed to estimate and classify these samples based on the contents of the eight chemical constituents. This study provided the theoretical basis and scientific evidence for the development and utilization of SSB resources. 相似文献
965.
The synthesis and catalytic application of a novel MgO containing periodic mesoporous organosilica with ionic liquid framework (MgO@PMO‐IL) is described. The prepared MgO@PMO‐IL was characterized by Fourier transform‐infrared spectroscopy, N2 adsorption/desorption, transmission electron microscopy, field emission‐scanning electron microscopy, thermogravimetric and inductively coupled plasma analyses. This nanocatalyst was successfully applied as a highly efficient and recoverable catalyst for the synthesis of novel spirooxindole‐furan derivatives via the three‐component reaction of 1,3‐dicarbonyl compounds, N‐phenacyl pyridinium salts and isatin derivatives. The products were achieved in high to excellent yields with a simple work‐up procedure and short reaction times, and the catalyst could be recovered through a simple filtration process and successfully reused seven times without any significant decrease in its efficiency. 相似文献
966.
F. I. Onuska S. Davies 《International journal of environmental analytical chemistry》2013,93(2-3):137-150
Abstract The objectives of this study were to identify, classify and categorize polychlorinated biphenyl (PCB) residues in samples collected from the Hamilton and Wheatley Harbour environmental compartments. The study is built around the use of a principal components analysis method, namely the soft independent modelling of class analogy (SIMCA) technique. This multivariate method is widely used for evaluating differences and observing similarities among multiple objects. The results obtained from this work confirm that the gas chromatographic data sets obtained with the samples provide a good approximation of the pattern derived from a determination of the composition of commercial Aroclors in water, sediment and biota samples. The data analysis technique provides insight into the origin of PCB contamination in environmental samples and indicates pathways for the environmental degradation or bioaccumulation of PCBs. This investigation contributes some evidence that multivariate reduction techniques are suitable for the investigation of complex data sets in environmental studies. 相似文献
967.
《Analytical letters》2012,45(15):3131-3141
ABSTRACT Principal Component Analysis (PCA) was applied to a set of physico-chemical variables obtained from 41 samples of summer orange juice, in order to reduce the number of variables. Working with the covariance matrix, three components (which explained 98.27% of the variance) were taken. With the correlation matrix, four components which explained: 85.65% of the variance were taken. With the scores corresponding to both matrixes a principal component regression (PCR) was carried out against the dependent variable of Brix grades, so as to obtain two statistical models that would allow the detection of adulterations in pure orange juice, based on dilution and later masking by the addition of sugar. The models were tested with simulated dilutions of 41 samples of juice, to assess the effectiveness of each for the detection of adulterations. Both models turned out to be equally effective, detecting adulterations starting from about 15 % of dilution. 相似文献
968.
Polypyrrole/Fe3O4/CNT as a recyclable and highly efficient catalyst for one‐pot three‐component synthesis of pyran derivatives
下载免费PDF全文
![点击此处可从《应用有机金属化学》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Polypyrrole (PPY)/Fe3O4/CNT has been synthesized and characterized by FT‐IR, TEM and SEM techniques and its catalytic activity has been evaluated in the synthesis of several series of pyran derivatives. Tetrahydrobenzo[b]pyranes, 4H‐pyran‐3‐carboxylates, 4H,5H‐pyrano[3,2‐c]chromenes and dihydropyrano[2,3‐c]pyrazoles have been successfully prepared from one‐pot three‐component condensation of aldehyde, malononitrile and active methylene‐containing compounds (dimedone /ethyl acetoacetate/4‐hydroxycoumarin/3‐methyl‐2‐pyrazoline‐5‐one) using PPY/Fe3O4/CNT as a new and reusable heterogeneous catalyst. The present method offer several advantages such as; high yields of products, short reaction times, easy work‐up procedure and easy separation of the catalyst from the reaction mixture due to its magnetic character. Furthermore, chemoselective synthesis of bis‐benzo[b]pyran from terephthalaldehyde can be achieved by this method. 相似文献
969.
《Journal of computational chemistry》2018,39(17):1051-1058
Generalizing our recent work on relativistic generalizations of the quantum theory of atoms in molecules, we present the general setting under which the principle of stationary action for a region leads to open quantum subsystems. The approach presented here is general and works for any Hamiltonian, and when a reasonable Lagrangian is selected, it often leads to the integral of the Laplacian of the electron density on the region vanishing as a necessary condition for the zero‐flux surface. Alternatively, with this method, one can design a Lagrangian that leads to a surface of interest (though this Lagrangian may not be, and indeed probably will not be, “reasonable”). For any reasonable Lagrangian for the electronic wave function and any two‐component method (related by integration by parts to the Hamiltonian) considered, the Bader definition of an atom is recaptured. © 2018 Wiley Periodicals, Inc. 相似文献
970.
Alexey P. Ilyin Vladimir V. Kobak Irina G. Dmitrieva Yulia N. Peregudova Veronika A. Kustova Yulia S. Mishunina 《合成通讯》2013,43(7):903-910
We describe an efficient synthesis of novel 7,8‐dihydrothieno [3′,2′:4,5]pyrrolo[1,2‐a]pyrazin‐5(6H)‐one heterocyclic structures. The target compounds were prepared by a novel modification of four‐component Ugi reaction of heterocyclic bifunctional keto acids, isonitriles, and amines. We demonstrate the usefulness and versatility of the developed approach, which is amenable to production of combinatorial libraries. 相似文献