全文获取类型
收费全文 | 25126篇 |
免费 | 462篇 |
国内免费 | 104篇 |
专业分类
化学 | 14009篇 |
晶体学 | 163篇 |
力学 | 763篇 |
综合类 | 9篇 |
数学 | 3471篇 |
物理学 | 7277篇 |
出版年
2023年 | 115篇 |
2022年 | 318篇 |
2021年 | 397篇 |
2020年 | 360篇 |
2019年 | 319篇 |
2018年 | 409篇 |
2017年 | 364篇 |
2016年 | 625篇 |
2015年 | 525篇 |
2014年 | 610篇 |
2013年 | 976篇 |
2012年 | 1257篇 |
2011年 | 1440篇 |
2010年 | 804篇 |
2009年 | 762篇 |
2008年 | 1211篇 |
2007年 | 1185篇 |
2006年 | 1066篇 |
2005年 | 1502篇 |
2004年 | 1535篇 |
2003年 | 1082篇 |
2002年 | 723篇 |
2001年 | 609篇 |
2000年 | 558篇 |
1999年 | 354篇 |
1998年 | 291篇 |
1997年 | 292篇 |
1996年 | 358篇 |
1995年 | 316篇 |
1994年 | 307篇 |
1993年 | 328篇 |
1992年 | 324篇 |
1991年 | 264篇 |
1990年 | 217篇 |
1989年 | 205篇 |
1988年 | 179篇 |
1987年 | 175篇 |
1986年 | 174篇 |
1985年 | 224篇 |
1984年 | 212篇 |
1983年 | 184篇 |
1982年 | 187篇 |
1981年 | 184篇 |
1980年 | 168篇 |
1979年 | 150篇 |
1978年 | 176篇 |
1977年 | 138篇 |
1976年 | 134篇 |
1974年 | 132篇 |
1973年 | 122篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
911.
I. M. J. J. van de Ven-Lucassen M. F. Kemmere P. J. A. M. Kerkhof 《Journal of solution chemistry》1997,26(12):1145-1167
The Taylor dispersion technique is used to measure the ternary mutual diffusion coefficients of aqueous nonelectrolyte solutions at 25°C. The dispersion of the injected solutes is recorded by a differential refractometer and an ultraviolet-visible detector. The diffusion coefficients are calculated directly by fitting the theoretical dispersion equations to about six experimental curves simultaneously. If the ternary diffusion effects in the measured dispersion profiles are not confused by the inaccuracy of the experimental method or an unfavorable relative detector sensitivity, the diffusion coefficients are precise. For the system methanol + acetone + water, it is shown that the Taylor dispersion method is unsuitable for the determination of all the diffusion coefficients if the methanol mole fraction is less than 0.45 or the acetone mole fraction if more than 0.001. 相似文献
912.
The reaction of boron heterocycles 1 and 2 with n-butyl lithium and alkyl halides led to (N→B) phenyl[N-alky-N-(2-alkyl)aminodiacetate-O,O′,N]boranes 3–6(a–b), 7(b) and 9(b), where alkyl can be in exo and/or endo position, and phenyl[N-alkyl-N-(2-alkyl)aminodiacetate-O,O′,N]boranes 7(c) and 8(c) isomers, which do not display the intramolecular N→B coordination bond. The existence of steric interactions between N-benzyl and the alkyl group at 2 position was indicated by 1H and 13C NMR, while, the δ(11B) values confirm the tetrahedral and trigonal environment of the 11B nucleus in these compounds. Moreover, the compounds were characterized by COSY, HETCOR and homonuclear proton decoupling experiment. The study of the intramolecular N→B coordination by dynamic NMR afforded a ΔG‡ value of 81.09 kJ/mol for compound 6(b). 相似文献
913.
It has been developed an automatic stop-flow procedure for sequential photometric determination of anionic and cationic surfactants in a same sample of water. The flow system was based on multicommutation process that was designed employing two solenoid micro-pumps and six solenoid pinch valves, which under microcomputer control carry out fluid propelling and reagent solutions handling. A homemade photometer using a photodiode as detector and two light emitting diodes (LEDs) with emission at 470 nm (blue) and 650 nm (red) as radiation sources, which was tailored to allow the determination of anionic and cationic surfactants in waters. The procedure for anionic surfactant determination was based on the substitution reaction of methyl orange (MO) by the anionic surfactant sodium dodecylbenzene sulfonate (DBS) to form an ion-pair with the cetyl pyridine chloride (CPC). Features such as a linear response ranging from 0.35 to 10.5 mg L−1 DBS (R = 0.999), a detection limit of 0.06 mg L−1 DBS and a relative standard deviation of 0.6% (n = 11) were achieved. For cationic surfactant determination, the procedure was based on the ternary complex formation between cationic surfactant, Fe(III) and chromazurol S (CAS) using CPC as reference standard solution. A linear response range between 0.34 and 10.2 mg L−1 CPC (R = 0.999), a detection limit of 0.05 mg L−1 CPC and a relative standard deviation of 0.5% (n = 11) were obtained. In both cases, the sampling throughput was 60 determinations per hour. Reagents consumption of 7.8 μg MO, 8.2 μg CPC, 37.2 μg CAS and 21.6 μg Fe(III) per determination were achieved. Analyzing river water samples and applying t-test between the results found and those obtained using reference procedures for both surfactant types provide no significant differences at 95% confidence level. 相似文献
914.
Ines Baer Beatriz de la Calle Philip Taylor 《Analytical and bioanalytical chemistry》2010,396(1):443-456
This review gives an overview of current knowledge about 3-monochloropropane-1,2-diol (3-MCPD) formation and detection. Although
3-MCPD is often mentioned with regard to soy sauce and acid-hydrolysed vegetable protein (HVP), and much research has been
done in that area, the emphasis here is placed on other foods. This contaminant can be found in a great variety of foodstuffs
and is difficult to avoid in our daily nutrition. Despite its low concentration in most foods, its carcinogenic properties
are of general concern. Its formation is a multivariate problem influenced by factors such as heat, moisture and sugar/lipid
content, depending on the type of food and respective processing employed. Understanding the formation of this contaminant
in food is fundamental to not only preventing or reducing it, but also developing efficient analytical methods of detecting
it. Considering the differences between 3-MCPD-containing foods, and the need to test for the contaminant at different levels
of food processing, one would expect a variety of analytical approaches. In this review, an attempt is made to provide an
up-to-date list of available analytical methods and to highlight the differences among these techniques. Finally, the emergence
of 3-MCPD esters and analytical techniques for them are also discussed here, although they are not the main focus of this
review. 相似文献
915.
Gabriela HuelgasLynne K. LaRochelle Lucrecia RivasYekaterina Luchinina Rubén A. ToscanoPatrick J. Carroll Patrick J. WalshCecilia Anaya de Parrodi 《Tetrahedron》2011,67(24):4467-4474
The catalytic asymmetric addition of alkyl groups to ketones has received considerable attention. Outlined herein is the synthesis of two new ligands based on the C2-symmetric 11,12-diamino-9,10-dihydro-9,10-ethanoanthracene. The scope of the new ligands has been evaluated in the catalytic asymmetric addition of diethylzinc to a variety of ketones. Enantioselectivities as high as 99% have been achieved. The structures of two of these ligands have been determined by X-ray crystallography and are compared with related structures. Additionally, the structure of a titanium complex bound to a bis(sulfonamide) diol ligand is reported. 相似文献
916.
de Vargas-Sansalvador IM Fay C Phelan T Fernández-Ramos MD Capitán-Vallvey LF Diamond D Benito-Lopez F 《Analytica chimica acta》2011,(2):216-222
A new system for CO2 measurement (0–100%) based on a paired emitter–detector diode arrangement as a colorimetric detection system is described. Two different configurations were tested: configuration 1 (an opposite side configuration) where a secondary inner-filter effect accounts for CO2 sensitivity. This configuration involves the absorption of the phosphorescence emitted from a CO2-insensitive luminophore by an acid–base indicator and configuration 2 wherein the membrane containing the luminophore is removed, simplifying the sensing membrane that now only contains the acid–base indicator. In addition, two different instrumental configurations have been studied, using a paired emitter–detector diode system, consisting of two LEDs wherein one is used as the light source (emitter) and the other is used in reverse bias mode as the light detector. The first configuration uses a green LED as emitter and a red LED as detector, whereas in the second case two identical red LEDs are used as emitter and detector. The system was characterised in terms of sensitivity, dynamic response, reproducibility, stability and temperature influence. We found that configuration 2 presented a better CO2 response in terms of sensitivity. 相似文献
917.
918.
Mariana R. Lopes Carlos J. A. de Souza Marina Q. R. B. Rodrigues Daniela A. Costa Ancély F. dos Santos Leandro L. de Oliveira Humberto J. O. Ramos Valéria M. Guimarães Wendel B. Silveira Flávia M. L. Passos Luciano G. Fietto 《Applied biochemistry and biotechnology》2014,172(5):2412-2424
An extracellular β-glucanase secreted by Kluyveromyces marxianus was identified for the first time. The optimal conditions for the production of this enzyme were evaluated by response surface methodology. The optimal conditions to produce β-glucanase were a glucose concentration of 4 % (w/v), a pH of 5.5, and an incubation temperature of 35 °C. Response surface methodology was also used to determine the pH and temperature required for the optimal enzymatic activity. The highest enzyme activity was obtained at a pH of 5.5 and a temperature of 55 °C. Furthermore, the enzyme was partially purified and sequenced, and its specificity for different substrates was evaluated. The results suggest that the enzyme is an endo-β-1,3(4)-glucanase. After optimizing the conditions for β-glucanase production, the culture supernatant was found to be effective in digesting the cell wall of the yeast Saccharomyces cerevisiae, showing the great potential of β-glucanase in the biotechnological production of soluble β-glucan. 相似文献
919.
Solid-phase micro extraction (SPME) and headspace derivatization of clenbuterol followed by GC–FID and GC–SIMMS quantification 总被引:1,自引:0,他引:1
Solid-phase micro extraction (SPME) and on-fiber derivatization followed by Gas Chromatography coupled with Flame Ionization Detection (GC-FID) or Selected Ion Monitoring Mass Spectrometry (GC-SIMMS) allows for simple yet sensitive quantification for the hexamethyldisilazane derivative of the beta-agonist clenbuterol. Using an 85- micro m polyacrylate fiber, the analysis method is optimized with respect to extraction time, derivatization time and temperature, and solution pH. In addition, the use of a rapid temperature ramping injection port allows for optimization of fiber desorption conditions. Under optimal conditions, the limits of detection for the hexamethyldisilazane derivative of clenbuterol are 1.1 ppb by FID and 0.20 ppb by SIMMS. 相似文献
920.