全文获取类型
收费全文 | 89340篇 |
免费 | 545篇 |
国内免费 | 400篇 |
专业分类
化学 | 30868篇 |
晶体学 | 856篇 |
力学 | 6976篇 |
综合类 | 1篇 |
数学 | 33314篇 |
物理学 | 18270篇 |
出版年
2021年 | 93篇 |
2020年 | 127篇 |
2019年 | 111篇 |
2018年 | 10497篇 |
2017年 | 10300篇 |
2016年 | 6229篇 |
2015年 | 1019篇 |
2014年 | 472篇 |
2013年 | 649篇 |
2012年 | 4159篇 |
2011年 | 10936篇 |
2010年 | 5923篇 |
2009年 | 6257篇 |
2008年 | 6999篇 |
2007年 | 9163篇 |
2006年 | 687篇 |
2005年 | 1716篇 |
2004年 | 1912篇 |
2003年 | 2290篇 |
2002年 | 1319篇 |
2001年 | 521篇 |
2000年 | 514篇 |
1999年 | 329篇 |
1998年 | 357篇 |
1997年 | 296篇 |
1996年 | 381篇 |
1995年 | 318篇 |
1994年 | 282篇 |
1993年 | 303篇 |
1992年 | 284篇 |
1991年 | 244篇 |
1990年 | 210篇 |
1989年 | 183篇 |
1988年 | 212篇 |
1987年 | 198篇 |
1986年 | 196篇 |
1985年 | 198篇 |
1984年 | 191篇 |
1983年 | 169篇 |
1982年 | 188篇 |
1981年 | 168篇 |
1980年 | 168篇 |
1979年 | 168篇 |
1978年 | 157篇 |
1977年 | 145篇 |
1976年 | 141篇 |
1975年 | 119篇 |
1974年 | 120篇 |
1973年 | 148篇 |
1972年 | 87篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Codée JD Stubba B Schiattarella M Overkleeft HS van Boeckel CA van Boom JH van der Marel GA 《Journal of the American Chemical Society》2005,127(11):3767-3773
A novel flexible assembly strategy is described for the modular synthesis of heparin and heparan sulfates. The reported strategy uses monomeric building blocks to construct the oligosaccharide chain to attain a maximum degree of flexibility. In the assembly, 1-hydroxyl glucosazido- and 1-thio uronic acid donors are combined in a sequential glycosylation protocol using sulfonium triflate activator systems. The key 1-thio uronic acids were obtained in an efficient manner from diacetone glucose employing a chemo- and regioselective oxidation of partially protected glucose and idose thioglycosides. 相似文献
992.
L. F. Brum Malta Marta Eloisa Medeiros 《Journal of Thermal Analysis and Calorimetry》2005,81(1):149-152
Summary This work reports the room-temperature stabilization of the Bi4V2-xFexIIO11-1.5x γ ‘ phase, a promising ionic conductive material that finds application in solid oxide fuel cell and oxygen sensor devices. The Fe(II) cation proved to be a better stabilizer than Fe(III), which was previously used, since a lower substitution degree of V5+ is needed for the former. Powder X-ray diffraction, Fourier-transform infrared spectroscopy and differential scanning calorimetry were used in these experiments. 相似文献
993.
H. R. Veenendaal P. M. van Berkel G. de Jong P. K. Baggelaar 《Accreditation and quality assurance》2007,12(7):365-368
Due to the relatively large spread in the results of microbiological proficiency tests, the z-scores are often not able to detect zero or low results as being ‘bad’ results. This paper describes an adapted z-score based on the average or the standard deviation of the 50% ‘highest’ results. The combination of the adapted z-scores of four samples enables a better judgement of the performance of each laboratory.
Papers published in this section do not necessarily reflect the opinion of the Editors, the Editorial Board or the Publisher. 相似文献
994.
Lars S. von Chrzanowski Martin Lutz Anthony L. Spek 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(7):m283-m288
The crystal structure of the title compound, [Co(C5H7O2)3], has been investigated by a multi‐temperature measurement. In contrast to the isomorphous Al compound, the title compound exists in the studied temperature range as its monoclinic α polymorph (space group P21/c) and does not undergo a phase transition. Rigid‐body TLS analyses have been performed and the anisotropic thermal expansion tensor αij has been determined. The cell axes show a linear expansion behavior with respect to the temperature, but the slope is significantly different. A possible explanation are the different strengths of different intermolecular C—H...O contacts, which run in different crystallographic directions. 相似文献
995.
Hans König 《Analytical and bioanalytical chemistry》1973,266(2):119-124
The separation of a mixture of 22 bactericides has been achieved by gas chromatography on columns with silicone rubber W-982 as stationary phase with temperatures between 100° and 300°C. The unchanged compounds as well as their silylation products have been used. The latter are more conveniently used especially for the quantitative determination. To be able to calculate the retention indices after Kovats gas chromatography has been performed isothermally at 180°C for the more volatile compounds and at 250°C for all other bactericides.The retention indices obtained under these conditions are tabulated together with the limits of detection. 相似文献
996.
Alex von Bohlen Henriette Brink-Kloke Christiane Althoff 《Analytica chimica acta》2003,480(2):327-335
An aqua regia extraction procedure for heavy metals in soils optimised for total reflection X-ray fluorescence analysis is presented. The procedure is applied to 92 soil samples of medieval layers from the city area of Dortmund. Sixteen elements (P, S, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Rb, Sr, Ag, Sn, and Pb) were used to characterise 17 sample sites. The results are projected onto the medieval urban structure of Dortmund. Two sites loaded with non-ferrous heavy metal could be detected and correlated with archaeological data. The efficiency and repeatability of the proposed extraction procedure is discussed. 相似文献
997.
Valérie Chevalier-Billosta Jean-Paul Joseleau Alain Cochaux Katia Ruel 《Cellulose (London, England)》2007,14(2):141-152
The composition and ultrastructural arrangement of cell wall polymers in wood fibres have determining influence on the properties
of wood derived materials. It is therefore important to improve our understanding of the relationship between fibres organisation,
the modifications induced by pulping treatments, and the resulting paper sheet mechanical properties. The different treatments
to which fibres are subjected during the manufacturing of pulps and papers induce morphological and micro-structural alterations
due to the removal of wall constituents and of microfibrillar elements. The impact of pulping processes on fibres was investigated
at the ultrastructural scale of transmission electron microscopy. Particular attention was given to the effects of beating
in refiners at various intensities on the ultrastructure of fibres. The most characteristic effects consisted of delaminations,
microfibril disorganisation, and even fractures, of varying importance depending on the intensity of the mechanical refining.
The consequences of internal alterations and surface modifications of the fibres were examined in relation to the paper sheet
mechanical properties. Correlations between the type of alteration observed in the fibres and its possible impact on a given
paper mechanical property are suggested. With similar approaches, the effects of drying and recycling were studied. 相似文献
998.
Stereoselectivity of reductive amination of (R)-1-hydroxy-1-phenylpropan-2-one by methylamine was studied. From the four isomers possible, only two are produced by this
reaction. These are marked as (−)-(1R,2S)-ephedrine (desired product) and (+)-(1S,2R)-ephedrine. The reaction stereoselectivity depends both on the type of the catalyst and reaction conditions. The most suitable
type is the supported platinum. However, this catalyst rapidly deactivates. With a decreasing activity of Pt catalyst, the
stereoselectivity decreases. It is also decreased during the production of the second liquid phase (water) in the reaction
mixture. 相似文献
999.
Gabriela Flores-Ramírez Manuel Rivera Alfredo Morales-Pablos Joel Osuna Xavier Soberón Paul Gaytán 《BMC chemical biology》2007,7(1):1
Background
The effect of single and multiple amino acid substitutions in the green fluorescent protein (GFP) from Aequorea victoria has been extensively explored, yielding several proteins of diverse spectral properties. However, the role of amino acid deletions in this protein -as with most proteins- is still unknown, due to the technical difficulties involved in generating combinatorial in-phase amino acid deletions on a target region. 相似文献1000.
Wang Q Barclay JE Blake AJ Davies ES Evans DJ Marr AC McInnes EJ McMaster J Wilson C Schröder M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(14):3384-3396
A novel [NiS4Fe2(CO)6]cluster (1: 'S(4)'=(CH(3)C(6)H(3)S(2))(2)(CH(2))(3)) has been synthesised, structurally characterised and has been shown to undergo a chemically reversible reduction process at -1.31 V versus Fc(+)/Fc to generate the EPR-active monoanion 1(-). Multifrequency Q-, X- and S-band EPR spectra of (61)Ni-enriched 1(-) show a well-resolved quartet hyperfine splitting in the low-field region due to the interaction with a single (61)Ni (I=3/2) nucleus. Simulations of the EPR spectra require the introduction of a single angle of non-coincidence between g(1) and A(1), and g(3) and A(3) to reproduce all of the features in the S- and X-band spectra. This behaviour provides a rare example of the detection and measurement of non-coincidence effects from frozen-solution EPR spectra without the need for single-crystal measurements, and in which the S-band experiment is sensitive to the non-coincidence. An analysis of the EPR spectra of 1(-) reveals a 24 % Ni contribution to the SOMO in 1(-), supporting a delocalisation of the spin-density across the NiFe(2) cluster. This observation is supported by IR spectroscopic results which show that the CO stretching frequencies, nu(CO), shift to lower frequency by about 70 cm(-1) when 1 is reduced to 1(-). Density functional calculations provide a framework for the interpretation of the spectroscopic properties of 1(-) and suggest that the SOMO is delocalised over the whole cluster, but with little S-centre participation. This electronic structure contrasts with that of the Ni-A, -B, -C and -L forms of [NiFe] hydrogenase in which there is considerable S participation in the SOMO. 相似文献