全文获取类型
收费全文 | 1279篇 |
免费 | 44篇 |
国内免费 | 6篇 |
专业分类
化学 | 917篇 |
晶体学 | 5篇 |
力学 | 25篇 |
数学 | 202篇 |
物理学 | 180篇 |
出版年
2023年 | 9篇 |
2022年 | 27篇 |
2021年 | 34篇 |
2020年 | 29篇 |
2019年 | 22篇 |
2018年 | 32篇 |
2017年 | 20篇 |
2016年 | 48篇 |
2015年 | 28篇 |
2014年 | 40篇 |
2013年 | 71篇 |
2012年 | 57篇 |
2011年 | 67篇 |
2010年 | 43篇 |
2009年 | 37篇 |
2008年 | 64篇 |
2007年 | 48篇 |
2006年 | 42篇 |
2005年 | 41篇 |
2004年 | 35篇 |
2003年 | 42篇 |
2002年 | 34篇 |
2001年 | 16篇 |
2000年 | 11篇 |
1999年 | 7篇 |
1998年 | 11篇 |
1997年 | 18篇 |
1996年 | 17篇 |
1995年 | 8篇 |
1994年 | 7篇 |
1993年 | 15篇 |
1992年 | 10篇 |
1991年 | 11篇 |
1990年 | 7篇 |
1987年 | 8篇 |
1985年 | 7篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1981年 | 11篇 |
1979年 | 8篇 |
1976年 | 7篇 |
1972年 | 6篇 |
1918年 | 8篇 |
1907年 | 6篇 |
1905年 | 6篇 |
1901年 | 11篇 |
1895年 | 6篇 |
1894年 | 6篇 |
1888年 | 6篇 |
1885年 | 9篇 |
排序方式: 共有1329条查询结果,搜索用时 265 毫秒
121.
Hugo M. Oliveira Marcela A. Segundo José L.F.C. Lima Manuel Miró Victor Cerdà 《Journal of chromatography. A》2010,1217(22):3575-3582
For the first time, an automatic sample pre-treatment/detection method is proposed for the multiclass determination of UV filters (namely, benzophenone-3, ethylhexylmetoxycinnamate, butylmethoxydibenzoylmethane and homosalate) in environmental samples. The new methodology comprises in-line solid-phase extraction (SPE) of the target analytes by exploiting the bead injection (BI) concept in a mesofluidic lab-on-valve (LOV) format, with subsequent determination by liquid chromatography (LC). The proposed microanalytical system, using a multisyringe burette as propulsion unit, automatically performed the overall SPE steps, including the renewal of the sorbent in each analytical cycle to prevent sample cross-contamination and the post-extraction adjustment of the eluate composition to prevent chromatographic band broadening effects. In order to expedite the LC separation, a C18 monolithic column was applied and an accelerated isocratic elution was carried out by using a cationic surfactant as mobile phase additive. The LOV-BI-LC method was proven reliable for handling and analysis of complex matrices, e.g., spiked swimming pool water and seawater, with limits of detection ranging between 0.45 and 3.2 μg L−1 for 9 mL sample volume. Linear calibration was attained up to 160 μg L−1 for homosalate and up to 35 μg L−1 for the other target analytes, with good reproducibility (RSD < 13%, for 5 different SPE columns). The hyphenated scheme is able to process a given sample simultaneously and within the same time frame than the chromatographic separation/determination of the formerly pre-treated sample, providing concentration values every 9 min. Hence, the sample throughput was enhanced up to 33 times when compared with previously reported off-line SPE methods. A drastic reduction in reagent consumption and effluent production was also attained, contributing to the development of an environment-friendly analyzer. 相似文献
122.
Fernando Godoy Alejandra Gómez A. Hugo Klahn 《Journal of organometallic chemistry》2010,695(3):346-4353
The fulvene complexes [(η6-C5Me4CH2)Re(CO)2(R)] (1a, RI; 1b, RC6F5) react at the exocyclic methylene carbon with a vinylmagnesium bromide solution to produce the anionic species [(η5-C5Me4CH2CHCH2)Re(CO)2(R)]−. Protonation with HCl at 0 °C produces the hydride complexes [trans-(η5-C5Me4CH2CHCH2)Re(CO)2(R)(H)] (2a, RI; 2b, RC6F5). Thermolysis of an hexane solution of the iodo-hydride (2a) under a CO atmosphere yields the complex [(η5-C5Me4CH2CHCH2)Re(CO)3] (3) and [Re(CO)5I] as by-product. Thermolysis of 2b produced three new products, mainly the chelated complex [(η5:η2-C5Me4CH2CHCH2)Re(CO)2] (4) and complex 3, with a non-coordinated olefin group, in moderated yield, and traces of [Re(CO)5(C6F5)]. Thermolysis of an hexane solution of 2 in presence of an excess of PMe3, afforded the phosphine derivative [(η5-C5Me4CH2CHCH2)Re(CO)2(PMe3)] (5). All the complexes were characterized by IR, 1H, 13C and 31P NMR spectroscopies and mass spectrometry. The molecular structure of 4 has also been determined. The molecule exhibits a formal three-legged piano-stool structure, with two CO groups, and the third position corresponding to the η2-coordination of the propenyl side arm of the η5-C5Me4 ring. 相似文献
123.
Shiding Miao Dr. Hugo Leeman Steven De Feyter Prof. Robert A. Schoonheydt Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(8):2461-2469
The Langmuir–Blodgett (L–B) technique has been employed for the construction of hybrid films consisting of three components: surfactant, clay, and lysozyme (Lys). The surfactants are octadecylammonium chloride (ODAH) and octadecyl ester of rhodamine B (RhB18). The clays include saponite and laponite. Surface pressure versus area isotherms indicate that lysozyme is adsorbed by the surfactant–clay L–B film at the air–water interface without phase transition. The UV‐visible spectra of the hybrid film ODAH–saponite–Lys show that the amount of immobilized lysozyme in the hybrid film is (1.3±0.2) ng mm?2. The average surface area (Ω) per molecule of lysozyme is approximately 18.2 nm2 in the saponite layer. For the multilayer film (ODAH–saponite–Lys)n, the average amount of lysozyme per layer is (1.0±0.1) ng mm?2. The amount of lysozyme found in the hybrid films of ODAH–laponite–Lys is at the detection limit of about 0.4 ng mm?2. Attenuated total reflectance (ATR) FTIR spectra give evidence for clay layers, ODAH, lysozyme, and water in the hybrid film. The octadecylammonium cations are partially oxidized to the corresponding carbamate. A weak 1620 cm?1 band of lysozyme in the hybrid films is reminiscent of the presence of lysozyme aggregates. AFM reveals evidence of randomly oriented saponite layers of various sizes and shapes. Individual lysozyme molecules are not resolved, but aggregates of about 20 nm in diameter are clearly seen. Some aggregates are in contact with the clay mineral layers, others are not. These aggregates are aligned in films deposited at a surface pressure of 20 mN m?1. 相似文献
124.
The association and photobehavior of Rose Bengal (RB) in the presence of dipalmitoylphosphatidyl choline (DPPC) small unilamellar liposomes is determined by the temperature. At temperatures above the main phase transition of the bilayer, the incorporation of the dye is ca. 2.5 times more efficient than that taking place when the bilayer is in the gel state. In both temperature ranges, adsorption isotherms show a noticeable anti-cooperativity that can be related to electrostatic repulsion between bound molecules. The photophysics and the photochemistry of the bound dye molecules also depend on the bilayer status. In particular, in the liquid crystalline state the surrounding of the dye is more polar and production of singlet oxygen is less efficient (Φ∼0.1). This reduced singlet oxygen production is partially due to a low triplet yield (ΦT=0.35) and triplet self-quenching due to a high local RB concentration. In spite of these, tryptophan is efficiently photobleached when RB is associated to liposomes in the liquid crystalline state, probably due to a Type I mechanism favored by its high local concentration in the sensitized surroundings. 相似文献
125.
The phase inversion that undergoes an emulsion while being sheared is a sudden phenomenon that is still puzzling. In this Letter, we report an experimental investigation on propagative coalescence by using a microfluidic device where a calibrated two-dimensional emulsion is created and destabilized. The velocity of propagation and the probability of the coalescence are reported as a function of the size and the spatial distribution of the drops, respectively. We then discuss the efficiency of this novel scenario of phase inversion and suggest that inversion can be favored by the existence of a drop size distribution. 相似文献
126.
We give sufficient conditions on a real number β and on a closed set F in a general space of homogeneous type (X,d,μ) in such a way that μ(B(x,d(x,F)))β becomes a Muckenhoupt weight. In order to prove our result, we modify the underlying space so that it becomes 1-Ahlfors regular. 相似文献
127.
Alexandre Plastino Hugo Barbalho Luis Filipe M. Santos Richard Fuchshuber Simone L. Martins 《Journal of Heuristics》2014,20(1):39-74
Metaheuristics represent an important class of techniques to solve, approximately, hard combinatorial optimization problems for which the use of exact methods is impractical. Some researches have been combining machine learning techniques with metaheuristics to adaptively guide and improve the search for near optimal solutions. An example of such development is the DM-GRASP, a hybrid version of the Greedy Randomized Adaptative Search Procedures (GRASP) metaheuristic which incorporates a data mining process. In this hybrid proposal, after executing half of the total number of iterations, the data mining process extracts patterns from an elite set of sub-optimal solutions for the optimization problem. These patterns present characteristics of near optimal solutions and can be used to guide the following half GRASP iterations in the search through the solution space. In this work, we explore new versions of the DM-GRASP metaheuristic to experiment, not a single activation, but multiple and adaptive executions of the data mining process during the metaheuristic execution. We also applied the data mining technique into a reactive GRASP to show that a more sophisticated and not memoryless GRASP approach can also benefit from the use of this technique. In order to evaluate these new proposals, we adopted the server replication for reliable multicast problem since the best known results for this problem were obtained by GRASP and DM-GRASP implementations. The computational experiments, comparing traditional GRASP, DM-GRASP, and the new proposals, showed that multiple and adaptive executions of the data mining process can improve the results obtained by the DM-GRASP hybrid metaheuristic—the new proposals were able to find better results in less computational time for the reliable multicast problem. 相似文献
128.
Claudete J. Valduga Denise B. Santis Hugo S. Braibante Mara E. F. Braibante 《Journal of heterocyclic chemistry》1999,36(2):505-508
The condensation of 4-phenyl substituted β-enamino ketones 1a-d and β-enamino esters 5a-d with hydroxylamine hydrochloride using K-10 as the solid support under sonication was studied to evaluate the formation of isoxazole and 5-isoxazolone rings from β-enamino compounds with a substituted aromatic ring. Isoxazoles 2a-c, 3c-d and 5-isoxazolones 6a-c and 7a-d were obtained. The use of K-10/ultrasound in this reaction furnished novel results in some cases. 相似文献
129.
Ohne Zusammenfassung 相似文献
130.
Ohne Zusammenfassung 相似文献