全文获取类型
收费全文 | 1972篇 |
免费 | 85篇 |
国内免费 | 17篇 |
专业分类
化学 | 1383篇 |
晶体学 | 17篇 |
力学 | 44篇 |
数学 | 356篇 |
物理学 | 274篇 |
出版年
2023年 | 16篇 |
2022年 | 9篇 |
2021年 | 18篇 |
2020年 | 34篇 |
2019年 | 33篇 |
2018年 | 32篇 |
2017年 | 31篇 |
2016年 | 62篇 |
2015年 | 58篇 |
2014年 | 52篇 |
2013年 | 120篇 |
2012年 | 135篇 |
2011年 | 128篇 |
2010年 | 67篇 |
2009年 | 79篇 |
2008年 | 128篇 |
2007年 | 148篇 |
2006年 | 143篇 |
2005年 | 104篇 |
2004年 | 94篇 |
2003年 | 72篇 |
2002年 | 85篇 |
2001年 | 46篇 |
2000年 | 49篇 |
1999年 | 28篇 |
1998年 | 23篇 |
1997年 | 24篇 |
1996年 | 33篇 |
1995年 | 19篇 |
1994年 | 25篇 |
1993年 | 17篇 |
1992年 | 16篇 |
1991年 | 16篇 |
1990年 | 13篇 |
1989年 | 11篇 |
1988年 | 10篇 |
1987年 | 8篇 |
1986年 | 3篇 |
1985年 | 17篇 |
1984年 | 9篇 |
1982年 | 5篇 |
1980年 | 5篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1974年 | 5篇 |
1973年 | 4篇 |
1971年 | 5篇 |
1962年 | 3篇 |
排序方式: 共有2074条查询结果,搜索用时 296 毫秒
11.
Abdul Majid Jim Tunney Steve Argue Mike Post 《Journal of Sol-Gel Science and Technology》2004,32(1-3):323-326
We have investigated the synthesis of perovskite-type SrFeOx (2.5 x 3.0) using three preparative methods: sol-gel, mechanochemical processing and solid state reactions at high temperature of the corresponding oxides. The sample obtained after calcination of the gel from sol-gel method, contained the least amount of strontium carbonate impurity. The amount of strontium carbonate impurity decreased with the increase in calcination temperature. Perovskites obtained have been characterized by X-ray diffraction (XRD) and nitrogen adsorption isotherms. Samples obtained from three methods have been compared with respect to calcination temperature, crystallite size and specific surface area.Issued as NRCC No. 46479. 相似文献
12.
Shi Nian-Qing Prahl Kristine Hendrick Jim Cruz Jose Lu Ping Cho Jae-Yong Jones Sharon Jeffries Thomas 《Applied biochemistry and biotechnology》2000,84(1-9):201-216
Pichia stipitis CBS 6054 will grow on d-xylose, d-arabinose, and l-arabinose. d-Xylose and l-arabinose are abundant in seed hulls of maize, and their utilization is important in processing grain residues. To elucidate
the degradation pathway for l-arabinose, we obtained a mutant, FPL-MY30, that was unable to grow on d-xylose and l-arabinose but that could grow on d-arabinitol. Activity assays of oxidoreductase and pentulokinase enzymes involved in d-xylose, d-arabinose, and l-arabinose pathways indicated that FPL-MY30 is deficient in d-xylitol dehydrogenase (D-XDH), d- and l-arabinitol dehydrogenases, and d-ribitol dehydrogenase. Transforming FPL-MY30 with a gene for xylitol dehydrogenase (PsXYL2), which was cloned from CBS 6054 (Gen Bank AF127801), restored the D-XDH activity and the capacity for FPL-MY30 to grow on
l-arabinose. This suggested that FPL-MY30 is critically deficient in XYL2 and that the d-xylose and l-arabinose metabolic pathways have xylitolas a common intermediate. The capacity for FPL-MY30 to grow on d-arabinitol could proceed through d-ribulose. 相似文献
13.
Jim Bashkin Clive E. Briant D. Michael P. Mingos Robert W. M. Wardle 《Transition Metal Chemistry》1985,10(3):113-114
Summary [(Ph3P)AuCo(CO)3(PPh3)] has been synthesised from [(Ph3P)AuCo(CO)4], PPh3v and Me3NO in acetonitrile. Its molecular structure, determined by single-crystal x-ray crystallography, consists of an almost linear
P-Au-Co-P arrangement in which the Co atom is in a slightly distorted trigonalbipyramidal geometry, with the Au and P atoms
occupying the apical sites. The Au-Co bond length of 2.450(1) ? is shorter than that reported for [(Ph3P)AuCo(CO)4]. The carbonyl ligands are bent towards the Au atom and the mean Au-Co-C angle is 81(1)°. 相似文献
14.
15.
The reaction of the cluster complex HCC[Co2(CO)6]CCo3(CO)9 with (or without) BrCCo3(CO)9 under Cadiot-Chodkiewicz coupling conditions gave the unusual alkynyl carboxamide complex C2H5N(H)C(O)CC[Co2(CO)6]CCo3(CO)9 rather than a coupled product containing two tricobalt cluster units. Steric demands imposed by the Co3 cluster allow attack at the least hindered alkyne carbon and stabilise the formed ynamine, so allowing subsequent CO insertion. The product has been characterised by X-ray crystallography. 相似文献
16.
Transition metal complexes that reversibly bind to DNA have been studied for almost 30 years. In the last few years a variety of new systems have been developed, employing a range of metal ions and ligand architectures. In many cases, high affinity binding and specific selectivities have been observed. These complexes display properties that make them attractive as probes of DNA structure and function, suggesting that they may find a r?le as prototypical tools for a spectrum of applications, from basic molecular biology to medicine. This review presents an overview of some of the structures and properties of such complexes. 相似文献
17.
Pyrolytic process has a promising potential for the environmentally friendly upgrading of lignocellulosic and plastic waste.
Thermogravimetry and pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS) were used to get information about the reactive
decomposition of PCL in binary mixtures with microcrystalline cellulose (MC) or sisal fibres (SF). Preliminary thermogravimetric
investigation showed that biomass is thermally degraded at lower temperatures than PCL and this process has a predominant
influence on the thermal behaviour of the mixtures. Discrepancies between the experimental and predicted TG/DTG profiles were
considered as a measurement of the extent of interactions occurring on co-pyrolysis. It was found that reactivity of PCL was
slightly increased in PCL-SF binary mixtures. Evolution of acidic products from cellulose and hemicelluloses decomposition
may promote PCL degradation in binary mixtures with SF. It seems that the co-pyrolysis process could have potential for the
environmentally friendly transformation of biocomposites. 相似文献
18.
Peri F Jiménez-Barbero J García-Aparicio V Tvaroska I Nicotra F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(6):1433-1444
N(OMe)-linked disaccharide analogues, isosteric to the corresponding natural disaccharides, have been synthesized by chemoselective assembly of unprotected natural monosaccharides with methyl 6-deoxy-6-methoxyamino-alpha-D-glucopyranoside in an aqueous environment. The coupling reactions were found to be chemo- and stereoselective affording beta-(1-->6) disaccharide mimics when using Glc and GlcNAc; in the case of Gal, the beta-anomer was prevalent (beta:alpha=7:1). An iterative method for the synthesis of linear N(OMe) oligosaccharide analogues was demonstrated, based on the use of an unprotected monosaccharide building block in which an oxime functionality at C-6 is converted during the synthesis into the corresponding methoxyamino group. The conformational analysis of these compounds was carried out by using NMR spectroscopy, ab initio, molecular mechanics, and molecular dynamics methods. Optimized geometries and energies of fourteen conformers for each compound have been calculated at the B3LYP/6-31G* level. Predicted conformational equilibria were compared with the results based on NMR experiments and good agreement was found. It appears that N(OMe)-linked disaccharide analogues exhibit a slightly different conformational behavior to their parent natural disaccharides. 相似文献
19.
Bernardi A Potenza D Capelli AM García-Herrero A Cañada FJ Jiménez-Barbero J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(20):4598-4612
As a step to delineate a strategy of ligand design for cholera toxin (CT), NMR studies were performed on several mimics of the GM1 ganglioside oligosaccharide. The conformation of these analogues was investigated first in solution and then upon binding to cholera toxin by transferred nuclear Overhauser effect (TR-NOE) measurements. It was demonstrated that CT selects a conformation similar to the global minima of the free saccharides from the ensemble of presented conformations. No evidence of major conformational distortions was obtained, but one or two of the available conformers of the hydroxyacid side chain appear to be selected in the bound state. The NMR data were interpreted with the aid of computer models, generated and analyzed by using a combination of different approaches (MacroModels' MC/EM and MC/SD, Autodock, and GRID). Analysis of the NMR data supported by computational studies allowed us to interpret the experimental observations and to derive workable models of the ligand:toxin complexes. These models suggest that the higher affinity of the (R)-lactic acid derivative 3 may stem from lipophilic interactions with a hydrophobic area in the toxin binding site located in the vicinity of the sialic acid side chain binding region of the CT:GM1 complex, and formed by the side chain of Ile-58 and Lys-34. Thus, the models obtained have allowed us to make useful design suggestions for the improvement of ligand affinity. 相似文献
20.
Rapid screening method for the determination of diethylstilbestrol in edible animal tissue by column liquid chromatography with electrochemical detection 总被引:10,自引:0,他引:10
A rapid and sensitive screening method for the determination of residues of diethylstilbestrol in edible animal tissue is described. The analyte was extracted from the tissue with tert.-butyl methyl ether, reextracted with 1 M sodium hydroxide and further cleaned up by solid-phase extraction with C18 cartridges. Analysis was performed by isocratic elution with a phosphate-buffered mobile phase, methanol-0.05 M phosphate buffer pH 3.5 (67:33), on a Nucleosil 5-microns C18 column with electrochemical detection at +0.90 V. The average recovery of trans-diethylstilbestrol in spiked samples is 66%, with a standard deviation of 14% (n = 22) in the range 0.5-2.0 microgram/kg. The detection limit is 0.1-0.2 microgram/kg, although at this level other compounds may interfere and give rise to false positive results. 相似文献