全文获取类型
收费全文 | 65798篇 |
免费 | 2073篇 |
国内免费 | 217篇 |
专业分类
化学 | 40900篇 |
晶体学 | 390篇 |
力学 | 1121篇 |
综合类 | 5篇 |
数学 | 12148篇 |
物理学 | 13524篇 |
出版年
2023年 | 412篇 |
2022年 | 546篇 |
2021年 | 968篇 |
2020年 | 976篇 |
2019年 | 1008篇 |
2018年 | 1231篇 |
2017年 | 1158篇 |
2016年 | 2225篇 |
2015年 | 1882篇 |
2014年 | 1924篇 |
2013年 | 4084篇 |
2012年 | 3960篇 |
2011年 | 4051篇 |
2010年 | 2709篇 |
2009年 | 2391篇 |
2008年 | 3565篇 |
2007年 | 3349篇 |
2006年 | 3059篇 |
2005年 | 2935篇 |
2004年 | 2489篇 |
2003年 | 2091篇 |
2002年 | 1815篇 |
2001年 | 1337篇 |
2000年 | 1282篇 |
1999年 | 926篇 |
1998年 | 745篇 |
1997年 | 688篇 |
1996年 | 848篇 |
1995年 | 611篇 |
1994年 | 685篇 |
1993年 | 637篇 |
1992年 | 655篇 |
1991年 | 552篇 |
1990年 | 582篇 |
1989年 | 472篇 |
1988年 | 464篇 |
1987年 | 450篇 |
1986年 | 428篇 |
1985年 | 569篇 |
1984年 | 555篇 |
1983年 | 415篇 |
1982年 | 425篇 |
1981年 | 436篇 |
1980年 | 380篇 |
1979年 | 346篇 |
1978年 | 331篇 |
1977年 | 325篇 |
1976年 | 329篇 |
1974年 | 287篇 |
1973年 | 314篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
A simple and sensitive electroanalytical method was developed for microvolume trace elements determination. Commercially available
interdigitated microelectrodes (IDA) from ALS Japan Inc. based on a thin carbon film technology were used. Anodic stripping
voltammetry with collection at IDA was applied. Carbon IDA microbands were coated with a pre-deposited mercury film for better
performance. The method with this sensor enables analysis of some heavy metals in total sample volumes as small as 0.05 cm3. The method was applied to Pb in blood samples determination by a standard addition method. 相似文献
932.
ágnes Zsigmond Ramóna Madácsi Ferenc Notheisz Erzsébet Mernyák and Ferenc Joó 《Reaction Kinetics and Catalysis Letters》2006,87(2):297-304
Summary [RuCl2(PPh3)3], [{RuCl2(TPPMS)2}2] and their heterogenized analogs were applied in the selective hydrogenation of 17-keto- and α,β-unsaturated ketosteroids.
In basic conditions these complexes selectively hydrogenated the C=O bonds, similarly to the results obtained in the case
of α,β-unsaturated aldehydes. A new method was developed for the synthesis of an expensive steroid alcohol, which can be prepared
traditionally in a more complicated way. 相似文献
933.
Fernández G del Mar Graciani M Rodríguez A Múñoz M Moyá ML 《Journal of colloid and interface science》2000,225(1):47-53
The ligand substitution reactions Fe(CN)(5)(4-(t)bupy)(3-) + 4-CNpy and Fe(CN)(5)(4-(t)bupy)(3-) + pzCO(2)(-) (4-(t)Bupy = 4-tert-butylpyridine; 4-CNpy = 4-cyanopyridine; pzCO(2)(-) = pyrazinecarboxylate) were studied in several aqueous salt and micellar solutions. Kinetic data in aqueous solutions showed that the two processes follow a dissociative mechanism, D, and the dependence of the first-order rate constants on [salt] on electrolyte aqueous solutions allow the estimation of the activation volumes corresponding to both reactions. Under true first-order conditions no kinetic micellar effects were found in anionic (SDS) and nonionic (Triton X-100) aqueous micellar solutions. In cationic micellar solutions (CTAB, CTAC, and TTAB) small kinetic micellar effects were found. These were related to the different ionic concentrations and the different polarity and structure of the Stern layer surrounding the cationic micellar aggregates, where the reactions take place, with respect to pure water. Copyright 2000 Academic Press. 相似文献
934.
L. C. Balbás J. A. Alonso G. Borstel 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1987,6(3):219-226
The non local weighted-density approximation (WDA) to the exchange-correlation potentialV xc (r) is used to compute electron densities, ionization potentials and electron affinities of several atoms and ions. Especial care is taken in calculating and discussing 〈r 2〉 in noble gas atoms. Good results are obtained for all those quantities, but it is critical that the WDA is applied in such a way that it preserves the correct larger behaviour ofV xc (r). A comparison of the calculated electron density with that of the Hartree-Fock method shows the effects of coulomb correlation. The density rearrangements upon the introduction of correlation agree qualitatively with the results of Configuration-Interaction calculations 相似文献
935.
The photometric and fluorometric characteristics of the complex formed by 1,4-dihydroxyanthraquinone with Y(III) in 20–80% water-ethanol solution are described by the study of several variables. The stoichiometry and stability constant of the complex in the solution are 1:1 and log K = 4.57, respectively. The 1,4-dihydroxyanthraquinone-Y(III) solid complex has been prepared and studied by infrared spectroscopy and thermal analysis. The thermal behavior of this compound has been studied using thermogravimetry and differential scanning calorimetry techniques and the residue verified by X-ray diffraction. 相似文献
936.
The crystal structure of 1-[N-(2-aminoethyl)aminopropyl]silatrane has been determined by x-ray diffraction at room temperature. The Si←N bond distance (2.165(2) Å) is in the range observed for other 1-X-propylsilatranes (X = CN, OH, SH, Cl and SCN). The structure is partially disordered: the silatrane moiety displays a disorder that is typical for silatranes and the aminoethyl group terminating the planar chain linked to silicon is rotationally disordered. 相似文献
937.
A. Dávid G. Horváth Z. Mészáros T. Meisel Z. Halmos 《Journal of Thermal Analysis and Calorimetry》1974,6(1-2):59-66
The thermal behaviour of tetrahydroperparine (THP) and its derivatives with various acids has been investigated. The acid is liberated from the formate derivative basically in two steps, while the other aliphatic acids are released quantitatively before the thermal decomposition of the THP molecule. The thermoanalytical curves, electrical conductivity data measured in the molten phase, and infrared spectra prove that while part of the formic acid is hydrogen-bonded the other part is bound ionically in the molecule. It may be assumed that the marked biological activity of the compound can be explained by this difference in the nature of the bonds. 相似文献
938.
M. Salmerón Sánchez Y. Touzé A. Saiter J. M. Saiter J. L. Gómez Ribelles 《Colloid and polymer science》2005,283(7):711-720
The enthalpy relaxation of poly(hydroxyethyl methacrylate) (PHEMA), poly(ethyl methacrylate) (PEMA) and poly(ethyl acrylate) (PEA) networks, obtained by DSC, are compared. The temperature interval of the glass transition broadens in the sequence PEA-PEMA-PHEMA. The plots of the enthalpy loss during the annealing for 200 min at different temperatures below Tg show that the structural relaxation process also takes place in PHEMA in a broader temperature interval than in PEA or PEMA. The modelling of the structural relaxation process using a phenomenological model allows determining the temperature dependence of the relaxation times concluding that the fragility in PHEMA is significantly lower than in PEMA. Both features are ascribed to the connectivity of the polymer chains in PHEMA via hydrogen bonding. The role of the presence of the methyl group bonded to the main chain is analysed by comparing the results obtained in PEA and PEMA. 相似文献
939.
A pressure-assisted capillary chromatography-mass spectrometry method based on the use of a monolithic octadecylsilica (ODS) capillary is proposed for the determination of aliphatic amines. A 25 mM citric acid buffer containing 10% methanol is used as running electrolyte. Separation is achieved by simultaneously applying a capillary electrophoresis (CE) voltage of 13 kV and an overimposed pressure of 8 bar. The use of pressure is required to ensure stable electrospray conditions. Analysis times are reduced by using a capillary column consisting of a 30 cm long monolithic silica capillary column bound with ODS and a fused-silica capillary column also 30 cm long. The proposed method was successfully applied to the determination of low-molecular-weight aliphatic amines in tap and river water. The analysis of real samples requires cleanup and preconcentration, which can be performed automatically by inserting a minicolumn in the replenishment system of the commercial instrument. 相似文献
940.
Infrared laser ablation study of pressed soil pellets with inductively coupled plasma atomic emission spectrometry 总被引:1,自引:0,他引:1
Mikolás J Musil P Stuchliková V Novotný K Otruba V Kanický V 《Analytical and bioanalytical chemistry》2002,374(2):244-250
Potential of infrared laser ablation (LA) coupled with ICP-AES as a technique suitable for the determination of trace elements (Zn, Cu, Ni, Cr, and V) in agricultural soils was studied. Operating parameters such as laser beam energy, laser beam focusing with respect to the sample surface, and velocity of the sample translation in the plane perpendicular to the laser beam were optimized. Soil samples were mixed with powdered Ag as a binder, and an internal standard (GeO(2)), and pressed into pellets. Calibration samples were prepared by adding known amounts of oxides of elements of interest into soils of known elemental composition and then processed in the same way as the analyzed samples. Calibration curves were found to be linear at least up to several hundreds of mg kg(-1) for the elements of interest. The elemental contents obtained by using LA-ICP-AES were compared with those obtained by analysis using wet chemistry followed by ICP-AES with pneumatic nebulization (PN). The results were in good agreement. Accuracy was also tested using certified reference soils with a bias not exceeding 10% relative. 相似文献