全文获取类型
收费全文 | 2301篇 |
免费 | 113篇 |
国内免费 | 12篇 |
专业分类
化学 | 1990篇 |
晶体学 | 6篇 |
力学 | 16篇 |
数学 | 242篇 |
物理学 | 172篇 |
出版年
2023年 | 15篇 |
2022年 | 17篇 |
2021年 | 61篇 |
2020年 | 62篇 |
2019年 | 55篇 |
2018年 | 29篇 |
2017年 | 43篇 |
2016年 | 89篇 |
2015年 | 75篇 |
2014年 | 84篇 |
2013年 | 125篇 |
2012年 | 161篇 |
2011年 | 178篇 |
2010年 | 100篇 |
2009年 | 101篇 |
2008年 | 143篇 |
2007年 | 136篇 |
2006年 | 149篇 |
2005年 | 120篇 |
2004年 | 103篇 |
2003年 | 95篇 |
2002年 | 84篇 |
2001年 | 33篇 |
2000年 | 22篇 |
1999年 | 21篇 |
1998年 | 18篇 |
1997年 | 11篇 |
1996年 | 21篇 |
1995年 | 18篇 |
1994年 | 12篇 |
1993年 | 9篇 |
1992年 | 10篇 |
1991年 | 11篇 |
1990年 | 9篇 |
1989年 | 8篇 |
1988年 | 8篇 |
1987年 | 3篇 |
1985年 | 4篇 |
1984年 | 6篇 |
1983年 | 13篇 |
1982年 | 7篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1979年 | 9篇 |
1978年 | 3篇 |
1976年 | 5篇 |
1973年 | 3篇 |
1958年 | 55篇 |
1957年 | 20篇 |
1934年 | 4篇 |
排序方式: 共有2426条查询结果,搜索用时 15 毫秒
41.
Harth E Van Horn B Lee VY Germack DS Gonzales CP Miller RD Hawker CJ 《Journal of the American Chemical Society》2002,124(29):8653-8660
A novel approach is presented for the controlled intramolecular collapse of linear polymer chains to give well-defined single-molecule nanoparticles whose structure is directly related to the original linear polymer. By employing a combination of living free radical polymerization and benzocyclobutene (BCB) chemistry, nanoparticles can be routinely prepared in multigram quantities with the size being accurately controlled by either the initial degree of polymerization of the linear chain or the level of incorporation of the BCB coupling groups. The latter also allows the cross-link density of the final nanoparticles to be manipulated. In analogy with dendritic macromolecules, a significant reduction of up to 75% in the hydrodynamic volume is observed on going from the starting random coil linear chains to the corresponding nanoparticles. The facile nature of the living free radical process also permits wide variation in monomer selection and functional group incorporation and allows novel macromolecular architectures to be prepared. Furthermore, the use of block copolymers functionalized with benzocyclobutene groups in only one of the blocks gives, after intramolecular collapse, a hybrid architecture in which a single linear polymer chain is attached to the globular nanoparticle. 相似文献
42.
Reitznerová E Amarasiriwardena D Kopcáková M Barnes RM 《Fresenius' Journal of Analytical Chemistry》2000,367(8):748-754
Determination of seven elements (Cu, Fe, Mg, Mn, Pb, Sr and Zn) in whole enamel and surface layers of extracted non-carious human teeth by FAAS, ETA AAS, ICP-AES and ICP-MS (Pb) is demonstrated. Techniques are described for obtaining whole enamel and its acid dissolution. Fifty microm width enamel layers from outer enamel surface to a 200 microm depth were successively etched in 1 mL of 3 M HClO4 for 3 min dissolution periods. Enamel samples were analyzed for populations under and over 20 years of age and enamel from Bronze Age teeth. Concentrations of microelements in the whole enamel and in the first surface layer (50 microm depth) were compared. With exception of Sr and Mg, all elements show significantly higher concentrations in the first layer than in whole enamel and higher concentration in teeth of individuals over 20 years, which demonstrate the cumulative effect of these elements. The Cu, Fe, Mn, Pb and Zn concentrations in four layers of erupted and non-erupted teeth decreased while Mg and Sr concentrations increased toward enamel-dentine junction. The concentrations of most elements were almost constant as they approached the 150 microm layer. This concentration gradient may result from interaction between saliva and teeth and supports the hypothesis that the surface de- and re-mineralization process is effective at most to 150 microm from the enamel surface. 相似文献
43.
Herein, we outline a method that is able to generate truly minimal basis sets that accurately describe either a group of bands, a band, or even just the occupied part of a band. These basis sets are the so-called NMTOs, muffin-tin orbitals of order N. For an isolated set of bands, symmetrical orthonormalization of the NMTOs yields a set of Wannier functions that are atom-centered and localized by construction. They are not necessarily maximally localized, but may be transformed into those Wannier functions. For bands that overlap others, Wannier-like functions can be generated. It is shown that NMTOs give a chemical understanding of an extended system. In particular, orbitals for the pi and sigma bands in an insulator, boron nitride, and a semimetal, graphite, will be considered. In addition, we illustrate that it is possible to obtain Wannier-like functions for only the occupied states in a metallic system by generating NMTOs for cesium. Finally, we visualize the pressure-induced s-->d transition. 相似文献
44.
Mats Johansson Eva Malmstrm Anders Hult 《Journal of polymer science. Part A, Polymer chemistry》1993,31(3):619-624
A model two-step synthesis of a saturated hyperbranched hydroxyl-terminated ester has been developed to show a synthesis route. Three different series of hyperbranched esters with different terminations have been synthesized to relate some of their properties to their structures. This route has then been used to synthesize three different allyl ethermaleate functional hyperbranched ester resins in a two-step procedure. The resins have been characterized with respect to rheology, structure, and properties, and the differences are discussed. The allyl ether-maleate functional resins have also been studied with respect to curing performance and final film properties. © 1993 John Wiley & Sons, Inc. 相似文献
45.
PHOTOSENSITIZING PROPERTIES OF BACTERIOCHLOROPHYLLIN a and BACTERIOCHLORIN a, TWO DERIVATIVES OF BACTERIOCHLOROPHYLL a 总被引:2,自引:0,他引:2
Eva M. Beems Tom M. A. R. Dubbelman Johan Lugtenburg Jaap A. van Best Monique F. M. A. Smeets J. Pierre J. Boegheim 《Photochemistry and photobiology》1987,46(5):639-643
Abstract The photosensitizing properties of two water soluble derivatives of bacteriochlorophyll a , bacteriochlorophyllin a and bacteriochlorin a (lacking the central Mg-ion) were investigated and compared to those of hematoporphyrin derivatives. At physiological pH the oxygen consumption rate of histidine, tryptophan, dithiothreitol and guanosine upon illumination was 3 to 4 times higher when bacteriochlorin a was used as photosensitizer than when hematoporphyrin derivatives were used. Especially bacteriochlorin a proved to be an effective sensitizer for the killing of L929 cells. Because bacteriochlorin a has an absorption maximum at 765 nm in phosphate buffered saline (allowing a light penetration in tissue about ten times larger than at 630 nm) and a high molar absorption coefficient (32 000 M cm−1 ) it has promising possibilities for the application in photodynamic therapy. 相似文献
46.
Mixed-mode hydrophilic interaction/cation-exchange chromatography (HILIC/CEX) is a novel high-performance technique which has excellent potential for peptide separations. Separations by HILIX/CEX are carried out by subjecting peptides to linear increasing salt gradients in the presence of high levels of acetonitrile, which promotes hydrophilic interactions overlaid on ionic interactions with the cation-exchange matrix. In the present study, HILIC/CEX has been compared to reversed-phase liquid chromatography (RP-HPLC) for separation of mixtures of diastereomeric amphipathic alpha-helical peptide analogues, where L- and D-amino acid substitutions were made in the centre of the hydrophilic face of the amphipathic alpha-helix. Unlike RP-HPLC, temperature had a substantial effect on HILIC/CEX of the peptides, with a rise in temperature from 25 to 65 degrees C increasing the retention times of the peptides as well as improving resolution. Our results again highlight the potential of HILIC/CEX as a peptide separation mode in its own right as well as an excellent complement to RP-HPLC. 相似文献
47.
Orte A Talavera EM Maçanita AL Orte JC Alvarez-Pez JM 《The journal of physical chemistry. A》2005,109(39):8705-8718
2',7'-Difluorofluorescein (Oregon Green 488, OG488) is a novel fluorescein dye derivative which presents important advantages for improving the fluorimetric applications in the biomedical and biochemical sciences. In aqueous solution it displays four prototropic forms, namely cation (C), neutral (N), monoanion (M), and dianion (D). In previous works, we found (J. Phys. Chem. A 2005, 109, 734-747, 2840-2846) that OG488 undergoes excited-state proton transfer reactions, which may affect the results from applications using this dye. We established that the excited-state proton transfer (ESPT) reactions between neutral, monoanionic, and dianionic forms of OG488 are promoted by acetate buffer, and we characterized the ground and excited species involved. We also solved the kinetics of the prototropic reactions using global compartmental analysis. In the present paper, we extend our study on the ESPT reactions of OG488 to acidic media, in which only the three prototropic species cation, neutral, and monoanion coexist. We have solved the kinetics of the three-state ESPT reaction by means of global three-compartmental analysis of a fluorescence decay surface in moderately acidic media (pH between 1.1 and 3.0), recovering the kinetic and spectral parameters of this three-state system. This system is one of the most complex solved to date, due to the strong overlap of the absorption and emission spectra of the neutral and monoanionic forms of OG488. We also found that the cation behaves as "super" photoacid, showing a very high deprotonation rate constant (1.04 x 10(11) s(-1)) and an enhanced acidity. Therefore, we also carried out experiments at very high perchloric acid concentrations, dealing with some other effects which become noteworthy at these [H(+)]. The presence of xanthylium cation quenching due to "free" water molecules, and the reduction in the amount of water clusters acting as proton acceptors, are processes which alter notably the time course of the excited-species in this high [H(+)] range. 相似文献
48.
A capillary electrophoresis method with UV detection was developed for the determination of 8-hydroxy-2'-deoxyguanosine (8-OHdG) in untreated urine samples. The calibration graph for 8-OHdG in urine is linear in the concentration range 10-500 mg/l. and the detection limit is 5 mg/l (17 microM). 8-OHdG was determined in urine from oncological patients treated by radiation therapy. Its concentrations relative to creatinine were found to be in the range 10-47 microg 8-OHdG/l mg creatinine (4-19 micromol 8-OHdG/mmol creatinine). The overall time of the analysis of a urine sample was less than 15 min. 相似文献
49.
Search on ruggedness of fast gas chromatography-mass spectrometry in pesticide residues analysis 总被引:1,自引:0,他引:1
Kirchner M Matisová E Otrekal R Hercegová A de Zeeuwb J 《Journal of chromatography. A》2005,1084(1-2):63-70
In this study, suitability of fast gas chromatography-mass spectrometry (GC-MS) on a narrow-bore column with a programmed temperature vaporizer for the analysis of pesticide residues in non-fatty food was evaluated. The main objectives were ruggedness and stability of chromatographic system with regards to co-extractives injected. The chromatographic matrix induced response enhancement was found to be strongly dependent on the concentration of residues and is reaching up to 700% compared to the pesticides solutions in a neat solvent. However, the responses of pesticides in matrix-matched standards at different concentration levels do not significantly change during 130 injections. Response enhancement/or decrease is influenced by the sample preparation technique. External calibration with matrix-matched calibration standards should, therefore, provide results with good precision also at the concentration level of 0.005 mg kg(-1). Special attention is given to the performance of the chromatographic column and retention gap with regards to peak widths, peak tailing and different sample preparation methods. During approximately 460 matrix sample injections, the performance of the analytical column was acceptable. GC-MS set-up with 0.15 mm i.d. column can be successfully utilized for the pesticide residues analysis. 相似文献
50.
Insertion of the glycosylidene carbenes, derived from the gluco- and the manno-diazirines 1 and 2 , into the Sn? H bond of R3SnH (R = Bu or Ph) leads to the fully substituted stannoglycosides 3 – 8 (53–77%). The 1,2-cis-configurated products are formed preferentially (α-D /β -D ranges from 2.5:1 to 5.1:1 with 1 , and 1:1.3 to 1:4.2 with 2 ). Relative to CH2Cl2, THF favors formation of the equatorial Sn-glycosides. The stannylated (benzyloxy)glucals 9 and 10 were isolated as side products. The reaction of 1 with (Bu3Sn)2 yielded 9 (17% in CH2Cl2; 36% in CCl4) together with the azines 11 and the benzyloxyglucal 12 . NMR Data of the Sn-glycosides 3 – 8 show evidence for an anomeric effect, 1J(C(1),H) being larger in the axial and 1J(Sn,C(1)) larger in the equatorial anomers. 相似文献