全文获取类型
收费全文 | 7540篇 |
免费 | 160篇 |
国内免费 | 47篇 |
专业分类
化学 | 4330篇 |
晶体学 | 79篇 |
力学 | 264篇 |
数学 | 1341篇 |
物理学 | 1733篇 |
出版年
2021年 | 45篇 |
2020年 | 68篇 |
2019年 | 61篇 |
2018年 | 48篇 |
2017年 | 46篇 |
2016年 | 119篇 |
2015年 | 108篇 |
2014年 | 121篇 |
2013年 | 323篇 |
2012年 | 332篇 |
2011年 | 431篇 |
2010年 | 203篇 |
2009年 | 175篇 |
2008年 | 350篇 |
2007年 | 340篇 |
2006年 | 333篇 |
2005年 | 330篇 |
2004年 | 264篇 |
2003年 | 242篇 |
2002年 | 239篇 |
2001年 | 195篇 |
2000年 | 154篇 |
1999年 | 105篇 |
1998年 | 78篇 |
1997年 | 79篇 |
1996年 | 117篇 |
1995年 | 105篇 |
1994年 | 129篇 |
1993年 | 128篇 |
1992年 | 121篇 |
1991年 | 122篇 |
1990年 | 104篇 |
1989年 | 121篇 |
1988年 | 80篇 |
1987年 | 88篇 |
1986年 | 82篇 |
1985年 | 130篇 |
1984年 | 118篇 |
1983年 | 79篇 |
1982年 | 115篇 |
1981年 | 103篇 |
1980年 | 90篇 |
1979年 | 86篇 |
1978年 | 93篇 |
1977年 | 77篇 |
1976年 | 90篇 |
1975年 | 93篇 |
1974年 | 84篇 |
1973年 | 123篇 |
1972年 | 44篇 |
排序方式: 共有7747条查询结果,搜索用时 515 毫秒
91.
Bergen HR Abraham RS Johnson KL Bradwell AR Naylor S 《Biomedical chromatography : BMC》2004,18(3):191-201
Primary systemic amyloidosis (AL) is characterized by the overproduction of immunoglobulin light chain proteins by a monoclonal, terminally differentiated B-lymphocyte or plasma cell clone. The free immunoglobulin light chains are deposited in an abnormal conformation as amyloid in a variety of organs in the body. The mechanism of amyloid formation is not well understood, but appears to be associated with some form of cleavage of the immunoglobulin light chain with subsequent aggregate formation. In an attempt to characterize the structure of amyloid-forming light chain proteins we developed an on-line immunoaffinity purification and subsequent characterization of free kappa and free lambda immunoglobulin light chains by electrospray ionization mass spectrometry. The methodology is totally automated and requires 20 micro L of serum. Mass spectral analysis of Bence Jones proteins under non-denaturing conditions was also utilized to examine the tertiary and quaternary structure of light chain proteins and clearly shows covalent dimer formation of lambda type light chain. This type of on-line assay may prove helpful in elucidating distinguishing features capable of discriminating AL from benign monoclonal gammopathies of undetermined significance as well as diagnosing AL. 相似文献
92.
Dario Braga Fabrizia Grepioni Paul J. Dyson Brian F. G. Johnson 《Journal of Cluster Science》1992,3(3):297-311
The molecular and crystal structures of a number of ruthenium and osmium clusters of nuclearity between three and six containing arene fragments such as C6H6, C6H3Me3, C6H4Me2 and C6H5Me have been investigated. Attention has been focused on the relationship between the terminal (
6-coordination) and face-capping (
3:
2:
2:
2-coordination) bonding modes. Empirical packing potential energy calculations have been employed to investigate the intermolecular organization in the crystal. It has been shown that the arene fragments in mono-arene clusters form ribbons, while in bis-arene clusters graphitic-like interactions throughout the crystal are established. The factors controlling the ease of arene reorientational motion in the solid state has also been investigated in relation to the shape, size and geometry of the molecules and of their interlocking modes. 相似文献
93.
J. R. Vogt C. C. Graham M. D. Glascock R. H. Cobean 《Journal of Radioanalytical and Nuclear Chemistry》1982,69(1-2):271-289
An extensive study of Mesoamerican obsidian source samples from 37 sites has been begun to better differentiate and define
the trace element content at known Mesoamerican obsidian quarries, with particular emphasis on those in Mexico. This paper
reports on the first results in that study as well as studies of sample contamination by grinding and results for the new
National Bureau of Standards No. 278 Obsidian SRM. 相似文献
94.
Leatherman MD Svejda SA Johnson LK Brookhart M 《Journal of the American Chemical Society》2003,125(10):3068-3081
The synthesis of a series of (alpha-diimine)NiR(2) (R = Et, (n)Pr) complexes via Grignard alkylation of the corresponding (alpha-diimine)NiBr(2) precursors is presented. Protonation of these species by the oxonium acid [H(OEt(2))(2)](+)[BAr'(4)](-) at low temperatures yields cationic Ni(II) beta-agostic alkyl complexes which model relevant intermediates present in nickel-catalyzed olefin polymerization reactions. The highly dynamic nature of these agostic alkyl cations is quantitatively addressed using NMR line broadening techniques. Trapping of these complexes with ethylene provides cationic Ni alkyl ethylene species, which are used to determine rates of ethylene insertion into primary and secondary carbon centers. The Ni agostic alkyl cations are also trapped by CH(3)CN and Me(2)S to yield Ni(R)(L)(+) (L = CH(3)CN, Me(2)S) complexes, and the dynamic behavior of these species in the presence of varied [L] is discussed. The kinetic data obtained from these experiments are used to present an overall picture of the ethylene polymerization mechanism for (alpha-diimine)Ni catalysts, including effects of reaction temperature and ethylene pressure on catalyst activity, polyethylene branching, and polymer architecture. Detailed comparisons of these systems to the previously presented analogous palladium catalysts are made. 相似文献
95.
Dendritic nanowire ultraviolet laser array 总被引:16,自引:0,他引:16
Yan H He R Johnson J Law M Saykally RJ Yang P 《Journal of the American Chemical Society》2003,125(16):4728-4729
Self-organized dendritic crystal growth is explored to assemble uniform semiconductor nanowires into highly ordered one-dimensional microscale arrays that resemble comb structures. The individual ZnO nanowires have uniform diameters ranging from 10 to 300 nm. They are evenly spaced on a stem with a regular periodicity of 0.1-2 micrometer. Under optical excitation, each individual ZnO nanowire serves as a Fabry-Perot optical cavity, and together they form a highly ordered nanowire ultraviolet laser array. 相似文献
96.
97.
Kate J. Graham Chris P. Schaller Brian J. Johnson John B. Klassen 《The Chemical Educator》2002,7(6):376-378
The incorporation of research projects into undergraduate chemistry courses provides a perspective that is fundamentally unavailable in most laboratory experiences. While independent, multistep synthesis projects in organic chemistry have been reported previously, most efforts have been directed at relatively restricted, closely guided research plans with modest student participation in the experimental design. We have implemented a more open-ended synthesis project, limited principally by cost, safety and availability of materials. In the second semester of the sophomore organic sequence, students develop multiple drafts of a plan for a three-to-four-step synthesis. Subsequently, students obtain their own literature protocols for the individual steps. The synthesis is performed over three four-hour laboratory periods. The students conclude this project with a poster presentation of the results at the end of the semester. Evaluation of the students work focuses not only on the successful synthesis of the target but also on planning, troubleshooting, purification, and spectral analysis. 相似文献
98.
Several aromatic compounds containing one or two C6F5S groups have been prepared by nucleophilic displacement reactions using CuSC6F5 in DMF solution. Aromatic iodine or bromine, rather than chlorine of fluorine is replaced by the SC6F5 group using CuSC6F5. A mechanism is postulated. New compounds prepared include -(C6F6F5S)2C6H4, - and -(C6F5S)2C6F4 and XC6H4SC6F5(X=C1, NO2, I, CH3, CO2C2H5). 相似文献
99.
100.
Direct-current cyclic voltammetry is used to investigate the suitability of a ferrocene derivative as a mediator with galactose, glycolate and l-amino acid oxidases. The three enzymes coupled catalytically to ferrocene monocarboxylic acid exhibiting homogeneous second-order rate constants in the range 0.4 × 105 to 8.5 × 105 l mol?1 s?1. Enzyme electrodes which responded to d-galactose, glycolate or l-amino acids were constructed. The appropriate oxidase was retained behind a dialysis membrane at a carbon paste electrode containing the poorly soluble derivative 1,1′-dimethylferrocene. All the electrodes responded rapidly to millimolar concentrations of their respective substrates producing 95% of the steady-state current response in <2 min. This general method of biosensor construction should be widely applicable to oxidases and other oxidoreductase enzymes. 相似文献