全文获取类型
收费全文 | 1122篇 |
免费 | 40篇 |
国内免费 | 4篇 |
专业分类
化学 | 1013篇 |
晶体学 | 4篇 |
力学 | 7篇 |
数学 | 82篇 |
物理学 | 60篇 |
出版年
2023年 | 5篇 |
2022年 | 9篇 |
2021年 | 21篇 |
2020年 | 6篇 |
2019年 | 16篇 |
2018年 | 8篇 |
2017年 | 5篇 |
2016年 | 36篇 |
2015年 | 32篇 |
2014年 | 20篇 |
2013年 | 66篇 |
2012年 | 76篇 |
2011年 | 94篇 |
2010年 | 52篇 |
2009年 | 40篇 |
2008年 | 87篇 |
2007年 | 99篇 |
2006年 | 75篇 |
2005年 | 91篇 |
2004年 | 43篇 |
2003年 | 37篇 |
2002年 | 49篇 |
2001年 | 20篇 |
2000年 | 11篇 |
1999年 | 13篇 |
1998年 | 12篇 |
1997年 | 12篇 |
1996年 | 27篇 |
1995年 | 6篇 |
1994年 | 9篇 |
1993年 | 8篇 |
1992年 | 5篇 |
1991年 | 8篇 |
1990年 | 14篇 |
1989年 | 3篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 3篇 |
1984年 | 13篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 6篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1968年 | 1篇 |
1967年 | 1篇 |
1956年 | 1篇 |
排序方式: 共有1166条查询结果,搜索用时 203 毫秒
121.
Ellis G Cano P Jadraque M Martín M López L Núñez T de la Peña E Marco C Garrido L 《Analytical and bioanalytical chemistry》2011,399(7):2379-2388
Flexible and biodegradable film substrates prepared by solvent casting from poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBHV) solutions in chloroform were microperforated by ultraviolet laser ablation and subsequently characterized
using infrared (IR) microspectroscopy and imaging techniques and scanning electron microscopy (SEM). Both transmission synchrotron
IR microspectroscopy and attenuated total reflectance microspectroscopy measurements demonstrate variations in the polymer
at the ablated pore rims, including evidence for changes in chemical structure and crystallinity. SEM results on microperforated
PHBHV substrates after cell culture demonstrated that the physical and chemical changes observed in the biomaterial did not
hinder cell migration through the pores. 相似文献
122.
New applications of cotype to the theory of absolutely summing linear operators between Banach spaces are proved in this paper. Among other consequences we extend/complement some classical results of Bennett (Duke Math J 44:603?C639, 1977) on the existence of non-absolutely summing operators between ? p spaces and of Davis and Johnson (Stud Math 51:81?C85, 1974) on the existence of compact non-absolutely summing linear operators. We also point out that some of our results are sharp. 相似文献
123.
Campillo N Viñas P Martínez-Castillo N Hernández-Córdoba M 《Journal of chromatography. A》2011,1218(14):1815-1821
Microwave-assisted extraction (MAE) and dispersive liquid-liquid microextraction (DLLME) coupled with gas chromatography-mass spectrometry (GC-MS) were evaluated for use in the extraction and preconcentration of volatile nitrosamines in meat products. Parameters affecting MAE, such as the extraction solvent used, and DLLME, including the nature and volume of the extracting and disperser solvents, extraction time, salt addition and centrifugation time, were optimized. In the MAE method, 0.25g of sample mass was extracted in 10mL NaOH (0.05M) in a closed-vessel system. For DLLME, 1.5mL of methanol (disperser solvent) containing 20μL of carbon tetrachloride (extraction solvent) was rapidly injected by syringe into 5mL of the sample extract solution (previously adjusted to pH 6), thereby forming a cloudy solution. Phase separation was performed by centrifugation, and a volume of 3μL of the sedimented phase was analyzed by GC-MS. The enrichment factors provided by DLLME varied from 220 to 342 for N-nitrosodiethylamine and N-nitrosopiperidine, respectively. The matrix effect was evaluated for different samples, and it was concluded that sample quantification can be carried out by aqueous calibration. Under the optimized conditions, detection limits ranged from 0.003 to 0.014ngmL(-1) for NPIP and NMEA, respectively (0.12-0.56ngg(-1) in the meat products). 相似文献
124.
Viñas P Martínez-Castillo N Campillo N Hernández-Córdoba M 《Journal of chromatography. A》2011,1218(5):639-646
A miniaturized liquid-phase extraction procedure based on directly suspended droplet microextraction is proposed for determining different classes of polyphenols. A derivatization reaction by means of in injection-port reaction with bis(trimethylsilyl)trifluoroacetamide is carried out to convert the polar non-volatile polyphenols into volatile derivatives. The separation and detection is carried out by coupling gas chromatography with mass spectrometry in the selected ion monitoring mode. The procedure uses undecanone, a low density organic solvent, and several factors influencing the extraction, collection efficiency and derivatization reaction are optimized. Excellent linearity was obtained for the range studied (0.05-500ngmL(-1)). The limits of detection are between 0.011 and 0.13ngmL(-1), depending on the compound, and the limits of quantification between 0.037 and 0.43ngmL(-1). The sensitivity and detection limits for polyphenols using the DSDME sample pretreatment method were very low. Enrichment factors are between 413 and 578. The recoveries obtained for spiked samples are satisfactory for all the compounds. The coupled miniaturized method is applied to the sensitive determination of both cis- and trans-resveratrol isomers, piceatannol, catechin, epicatechin, quercetin and fisetin in herbal infusions, fruits, juices and functional foods. 相似文献
125.
Pilar Cabildo Concepción López Marta Pérez-Torralba M. Rosario Torres José Elguero 《Journal of Molecular Structure》2011,985(1):75-81
3-Bromo-1-methyl-7-nitro-1H-indazole (1), 3-bromo-2-methyl-7-nitro-2H-indazole (2) and 3,7-dinitro-1(2)H-indazole (3) have been synthesized and characterized by X-ray diffraction, 13C and 15N NMR spectroscopy in solution and in solid-state. The dihedral angles obtained in the crystal structures are in good agreement with the molecular parameters calculated using DFT B3LYP calculations employing the 6-311++G(d,p) basis set. Compounds 1 and 2 present intermolecular halogen bonds between the bromine and the oxygen atoms of the nitro group and in compound 3 inter- and intramolecular hydrogen bonding exists. 相似文献
126.
Reviriego F Navarro P Arán VJ Jimeno ML García-España E Latorre J Yunta MJ 《The Journal of organic chemistry》2011,76(20):8223-8231
By using an improved synthetic method reported earlier, the cyclic stannoxanes obtained from RN-diethanolamine (R = Me, Bu) and dibutyltin oxide have been reacted with 1H-pyrazole-3,5-dicarbonyl dichloride to afford 26-membered diaza tetraester crowns (1, R = Me; 3, R = Bu) and 39-membered triaza hexaester crowns (2, R = Me; 4, R = Bu). The new structures were identified from their analytical and spectroscopic ((1)H and (13)C NMR, FAB-MS, and/or ESI-MS) data. Both diaza tetraester crowns (1 and 3), containing two 1H-pyrazole units, self-assemble into dimeric species through the formation of four hydrogen bonds involving the two NH pyrazole groups and the two tertiary amine groups of both crowns, as proved by X-ray crystallography and NMR analysis. Preliminary NMR, ESI-MS, MALDI-TOF-MS, and molecular modeling studies suggest that, in CDCl(3) solution, 1 interacts with ethyleneurea (ETU), affording 1:1, 2:1, and 2:2 1-ETU complexes. 相似文献
127.
Phenolic compounds in Brassica vegetables 总被引:2,自引:0,他引:2
Phenolic compounds are a large group of phytochemicals widespread in the plant kingdom. Depending on their structure they can be classified into simple phenols, phenolic acids, hydroxycinnamic acid derivatives and flavonoids. Phenolic compounds have received considerable attention for being potentially protective factors against cancer and heart diseases, in part because of their potent antioxidative properties and their ubiquity in a wide range of commonly consumed foods of plant origin. The Brassicaceae family includes a wide range of horticultural crops, some of them with economic significance and extensively used in the diet throughout the world. The phenolic composition of Brassica vegetables has been recently investigated and, nowadays, the profile of different Brassica species is well established. Here, we review the significance of phenolic compounds as a source of beneficial compounds for human health and the influence of environmental conditions and processing mechanisms on the phenolic composition of Brassica vegetables. 相似文献
128.
Ignatyev I Montejo M Ortega PG González JJ 《Physical chemistry chemical physics : PCCP》2011,13(41):18507-18515
Activation barrier heights for the dehydration reaction of geminal carbinols and silanediols R'R″X(OH)(2) (X = C, Si) were estimated at the B3LYP and MP2 levels of theory employing Dunning's correlation-consistent triple-zeta basis sets. It was shown that the barrier height for carbon derivatives steadily decreases upon substitution by R groups, usually termed as electron-donating, such as alkyl and amino groups. Substitution by electron-withdrawing groups leads, however, only to small changes in barrier heights compared to that of methanediol. A similar tendency was also found for silicon derivatives, but high activation barriers of this reaction remain even for amino group substituted silanediols. Introduction of additional water molecules into the reactive space of carbinol dehydration drastically reduces barrier heights and brings the transition state energy for methanediol close to the experimental value. The difference between dehydration barrier heights for both methanediol and carbinols with electron-rich substituents becomes well-defined for dimeric species. The higher acidity of the hydroxyl group protons in molecules containing halogens and C==O groups brings about a noticeable growth in the dehydration barrier heights of these compounds. This difference in barrier heights for oligomeric species may be the reason for the stability of carbinols with electron-rich substituents. 相似文献
129.
Gupta R Baldock SJ Carreras P Fielden PR Goddard NJ Mohr S Razavi BS Brown BJ 《Lab on a chip》2011,11(23):4052-4056
The primary requirement for a mixing operation in droplet-based microfluidic devices is an accurate pairing of droplets of reaction fluids over an extended period of time. In this paper, a novel device for self-synchronous production of droplets has been demonstrated. The device uses a change in impedance across a pair of electrodes introduced due to the passage of a pre-formed droplet to generate a second droplet at a second pair of electrodes. The device was characterised using image analysis. Droplets with a volume of ~23.5 ± 3.1 nl (i.e.~93% of the volume of pre-formed droplets) were produced on applying a voltage of 500 V. The synchronisation efficiency of the device was 83%. As the device enables self-synchronised production of droplets, it has a potential to increase the reliability and robustness of mixing operations in droplet-based microfluidic devices. 相似文献
130.
Pilar E. López Albert Isidro-Llobet Luis J. Cruz Andrés Parra Fernando Albericio 《Tetrahedron letters》2005,46(45):7737-7741
p-Nitrobenzyloxycarbonyl (pNZ) is used for the permanent protection of ornithine in the synthesis of derivatives of the anti-tumor cyclodepsipeptide Kahalalide F that contain acid labile residues. 相似文献