全文获取类型
收费全文 | 6244篇 |
免费 | 334篇 |
国内免费 | 59篇 |
专业分类
化学 | 4503篇 |
晶体学 | 23篇 |
力学 | 110篇 |
综合类 | 1篇 |
数学 | 1027篇 |
物理学 | 973篇 |
出版年
2023年 | 57篇 |
2022年 | 66篇 |
2021年 | 120篇 |
2020年 | 168篇 |
2019年 | 133篇 |
2018年 | 132篇 |
2017年 | 102篇 |
2016年 | 214篇 |
2015年 | 183篇 |
2014年 | 228篇 |
2013年 | 329篇 |
2012年 | 498篇 |
2011年 | 523篇 |
2010年 | 259篇 |
2009年 | 207篇 |
2008年 | 443篇 |
2007年 | 410篇 |
2006年 | 400篇 |
2005年 | 367篇 |
2004年 | 334篇 |
2003年 | 247篇 |
2002年 | 215篇 |
2001年 | 86篇 |
2000年 | 47篇 |
1999年 | 50篇 |
1998年 | 50篇 |
1997年 | 52篇 |
1996年 | 55篇 |
1995年 | 39篇 |
1994年 | 46篇 |
1993年 | 37篇 |
1992年 | 42篇 |
1991年 | 40篇 |
1990年 | 28篇 |
1989年 | 29篇 |
1988年 | 31篇 |
1987年 | 30篇 |
1986年 | 30篇 |
1985年 | 30篇 |
1984年 | 22篇 |
1983年 | 25篇 |
1982年 | 21篇 |
1981年 | 32篇 |
1980年 | 26篇 |
1979年 | 20篇 |
1978年 | 17篇 |
1976年 | 14篇 |
1975年 | 15篇 |
1974年 | 15篇 |
1973年 | 13篇 |
排序方式: 共有6637条查询结果,搜索用时 125 毫秒
31.
Jonathan S. Dordick K. C. Backman R. Balakrishnan R. Brent M. S. Ptashne L. P. Casson S. A. Goff A. L. Goldberg P. A. Cornelius R. M. Hochstrasser N. R. Kallenbach H. Rubin G. J. Todaro H. A. De Boer J. C. Delgoffe M. Lobmann N. ZyGraich L. Gehrke T. Kunkel A. Paau S. G. Platt L. Sequeira M. A. Palladino H. G. Roman D. Hultmark T. T. Rasmusan H. Steiner 《Applied biochemistry and biotechnology》1990,26(1):107-113
Protein engineering and site-directed mutagenesis is becoming immensely important in both fundamental studies and commercial applications involving proteins and enzymes in biocatalysis. Protein engineering has become a powerful tool to help biochemists and molecular enzymologists elucidate structure-function relationships in enzymic active sites, to understand the intricacies of protein folding and denaturation, and to alter the selectivity of enzymatic catalysis. Commercial applications of engineered enzymes are being developed to increase protein stability, widen or narrow substrate specificity, and to develop novel approaches for use of enzymes in organic synthesis, drug design, and clinical applications. In addition to protein engineering, novel expression systems have been designed to prepare large quantities of genetically engineered proteins. Recent US patents and scientific literature on protein engineering, site-directed mutagenesis, and protein expression systems related to protein engineering are surveyed. Patent abstracts are summarized individually and a list of literature references are given. 相似文献
32.
Applied Biochemistry and Biotechnology - 相似文献
33.
Alcohol fuels produced from biomass can improve air quality, enhance energy security, create employment opportunities, and
reduce waste disposal problems. Opportunities in California exist to produce alcohols from waste streams from various sectors
of the economy. Government agencies have promoted waste-to-alcohol activities, but efforts have been inconsistent and intermittent.
Often these efforts have been hindered by contradictory but mandate-driven policies.
A prudent approach to coordinate statewide efforts includes the development of an integrated statewide policy to examine barriers
that impede private sector business efforts to produce alcohols from biomass. A multi-agency task force to promote research,
development, commercialization, and marketing efforts for biomass-produced alcohols is desirable.
The views and opinions contained in this document do not necessarily reflect those of the California Energy Commission, its
staff, management, or the State of California. 相似文献
34.
H. Jara K. Boyer U. Johann T. S. Luk I. A. McIntyre A. McPherson C. K. Rhodes 《Applied physics. B, Lasers and optics》1987,42(1):11-15
The results of transient loss measurements performed in a self-sustained discharge KrF* amplifier are reported. Analysis of these results gives a minimum value of 20 for the effective gain to loss ratiog
0/eff, indicating that efficient extraction of energy in subpicosecond KrF* amplifiers in the 1 J range should be achievable. 相似文献
35.
Sessler JL Roznyatovskiy V Pantos GD Borisova NE Reshetova MD Lynch VM Khrustalev VN Ustynyuk YA 《Organic letters》2005,7(23):5277-5280
[reaction: see text] Two easy-to-synthesize polypyrrolic 2,5-diamidothiophene Schiff base macrocycles are reported, along with their anion binding properties as determined via UV-vis spectroscopic titrations carried out in dichloroethane. There is a striking difference between the interactions with anions of the two macrocycles, a finding ascribed to differences in their rigidity. For example, the more flexible dipyrromethane-derived macrocycle displays a 1.2:1 hydrogen sulfate versus nitrate selectivity, while its more rigid bipyrrole-derived congener shows a 7.4:1 selectivity in favor to hydrogen sulfate. 相似文献
36.
Yu JQ Wu HC Ramarao C Spencer JB Ley SV 《Chemical communications (Cambridge, England)》2003,(6):678-679
A robust and recyclable palladium catalyst [Pd0EnCat] has been prepared by ligand exchange of polyurea-encapsulated palladium(II) acetate with formic acid, resulting in deposition of Pd(0) in the support material; Pd0EnCat is shown to be a highly efficient transfer hydrogenation catalyst for chemoselective reduction of a wide range of aryl ketones to benzyl alcohols. 相似文献
37.
A general method is described for asymmetric synthesis of both syn- and anti-1,3-amino alcohols. The first application of metalloenamines derived from N-sulfinyl imines is reported for the highly diastereoselective addition to aldehydes. The reduction of the product beta-hydroxy N-sulfinyl imines 2 with catecholborane and LiBHEt(3) provides syn- and anti-1,3-amino alcohols with very high diastereomeric ratios. This method was found to be effective for a variety of substrates incorporating either aromatic or various aliphatic substituents. The convergent and efficient asymmetric syntheses of the two natural products, (-)-8-epihalosaline and (-)-halosaline, were also accomplished. 相似文献
38.
Ana A. C. Pacheco Arnaldo F. da Silva Filho Kristoffer Kortsen Magnus W. D. Hanson-Heine Vincenzo Taresco Jonathan D. Hirst Muriel Lansalot Franck D'Agosto Steven M. Howdle 《Chemical science》2021,12(3):1016
Reversible addition–fragmentation chain transfer (RAFT) dispersion polymerisation of methyl methacrylate (MMA) is performed in supercritical carbon dioxide (scCO2) with 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) present as chain transfer agent (CTA) and surprisingly shows good control over PMMA molecular weight. Kinetic studies of the polymerisation in scCO2 also confirm these data. By contrast, only poor control of MMA polymerisation is obtained in toluene solution, as would be expected for this CTA which is better suited for acrylates. In this regard, we select a range of CTAs and use them to determine the parameters that must be considered for good control in dispersion polymerisation in scCO2. A thorough investigation of the nucleation stage during the dispersion polymerisation reveals an unexpected “in situ two-stage” mechanism that strongly determines how the CTA works. Finally, using a novel computational solvation model, we identify a correlation between polymerisation control and degree of solubility of the CTAs. All of this ultimately gives rise to a simple, elegant and counterintuitive guideline to select the best CTA for RAFT dispersion polymerisation in scCO2.RAFT dispersion polymerisation of methyl methacrylate is performed in scCO2 with 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) present as chain transfer agent (CTA) and surprisingly shows good control over PMMA molecular weight. 相似文献
39.
Oxyallyl cations generated in situ from dihaloketones have been found to undergo the Favorskii rearrangement in preference to an intramolecular Type-II 4 + 3 cycloaddition reaction in trifluoroethanol and hexafluoropropan-2-ol solvents, generating acrylate esters with high cis selectivity. A synthetic alternative to the intramolecular Type-II 4 + 3 cycloaddition has been developed on the basis of intramolecular enolate alkylation to close a 7-membered ring with an exocyclic double bond. 相似文献
40.
A simple and accurate spectrophotometric method is proposed for the determination of tannins in tea and beer samples based on the reduction of iron(III) to iron(II) by tannins at 80 degrees C for 20 min. The iron(II) was then reacted with 1,10-phenanthroline at pH 4.4 to form a coloured complex. Background correction could be effected by precipitating the tannins in the sample solution twice with gelatin and kaolin. Absorbance measurements were made at 540 nm and the calibration graph was linear from 0 to 5.5 micrograms ml-1 of tannic acid with a slope of 0.213 A p.p.m.-1. The precision for the determination of tannins in a tea sample containing 9.45% of tannins was 1.8%. Most of the ingredients commonly found in tea and beer samples do not interfere with the determination. Several tea and beer samples were analysed for their tannin content using the proposed method. 相似文献