全文获取类型
收费全文 | 19661篇 |
免费 | 630篇 |
国内免费 | 113篇 |
专业分类
化学 | 14106篇 |
晶体学 | 189篇 |
力学 | 378篇 |
综合类 | 1篇 |
数学 | 2956篇 |
物理学 | 2774篇 |
出版年
2022年 | 164篇 |
2021年 | 199篇 |
2020年 | 294篇 |
2019年 | 290篇 |
2018年 | 186篇 |
2017年 | 181篇 |
2016年 | 407篇 |
2015年 | 357篇 |
2014年 | 451篇 |
2013年 | 929篇 |
2012年 | 978篇 |
2011年 | 1268篇 |
2010年 | 619篇 |
2009年 | 524篇 |
2008年 | 1014篇 |
2007年 | 1092篇 |
2006年 | 1073篇 |
2005年 | 1039篇 |
2004年 | 881篇 |
2003年 | 745篇 |
2002年 | 732篇 |
2001年 | 250篇 |
2000年 | 254篇 |
1999年 | 231篇 |
1998年 | 223篇 |
1997年 | 259篇 |
1996年 | 284篇 |
1995年 | 227篇 |
1994年 | 212篇 |
1993年 | 244篇 |
1992年 | 203篇 |
1991年 | 185篇 |
1990年 | 163篇 |
1989年 | 141篇 |
1988年 | 140篇 |
1987年 | 158篇 |
1986年 | 135篇 |
1985年 | 202篇 |
1984年 | 249篇 |
1983年 | 178篇 |
1982年 | 220篇 |
1981年 | 241篇 |
1980年 | 180篇 |
1979年 | 181篇 |
1978年 | 214篇 |
1977年 | 159篇 |
1976年 | 157篇 |
1975年 | 150篇 |
1974年 | 165篇 |
1973年 | 140篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
J. Jon Paul Selvam M. Srinivasulu N. Suryakiran V. Suresh S. Malla Reddy 《合成通讯》2013,43(11):1760-1767
A mild and efficient synthesis of bis(indolyl) methanes by the reaction of indoles with various aldehydes at room temperature in the presence of a catalytic amount of a La(NO3)3 · 6H2O afforded the corresponding bis(indolyl) methanes in excellent yields under solvent‐free conditions. 相似文献
992.
Vincent J. Kalish Robert L. Shone Steven W. Kramer Paul W. Collins Kevin A. Babiak Kathleen T. McLaughlin 《合成通讯》2013,43(11):1641-1645
Hydrozirconation-iodination of a terminal acetylene followed by lithium-iodide exchange and finally dilithio cyanocuprate mediated conjugate addition to an appropriate cyclopentenone is reported as an efficient method for the synthesis of prostaglandin analogues. 相似文献
993.
A wide range of 2-Benzylindoles 2 are conveniently and efficiently prepared by heating N-benzylindoles 1 in polyphosphoric acid. Mechanistic studies suggest an intramolecular rearrangement via the corresponding 3-benzyl intermediates. 相似文献
994.
Treatment of fluoroanthracene-9,10-diones, fluorobenz[g]isoquinoline-5,10-diones and 1- or 4-fluoronitrobenzenes with sodium trimethylsilanoate in THF followed by acidification readily yields the corresponding hydroxyl analogues. 相似文献
995.
The asymmetric syntheses of novel dihydroxyhomoprolines have been achieved using the doubly diastereoselective conjugate additions of the antipodes of lithium N-benzyl-N-(α-methylbenzyl)amide to a set of four chiral α,β-unsaturated esters (derived from d-pentoses) as one of the key steps. A full account of the diastereoselectivity observed in these conjugate additions is presented and the stereochemical outcomes of these reactions have been established unambiguously via a combination of hydrogenolytic chemical correlation and single crystal X-ray diffraction analyses. A tandem hydrogenolysis/intramolecular reductive amination reaction was then used to create the corresponding enantiopure pyrrolidines, providing access to (2′S,3′S,4′R)-dihydroxyhomoproline and (2′S,3′R,4′S)-dihydroxyhomoproline after deprotection. 相似文献
996.
Susann Paul W. Bernd Schweizer Graham Rugg Hans Martin Senn Ryan Gilmour 《Tetrahedron》2013,69(27-28):5647-5659
Herein, we report the synthesis and X-ray structural analysis of a collection of fluorinated metal N-heterocyclic carbenes (Ag, Au, Pd, Rh, Ir) and their precursor salts. The common structural feature of these species is a flanking fluoroethyl group, which is either freely rotating or embedded within a bicyclic framework. Solid state analysis confirmed a gauche conformational preference in all cases with the fluorine adopting a syn clinal arrangement (?[NCCF]~60°) with respect to the triazolium nitrogen at the vicinal position of the NHC. A density functional theory analysis was employed to quantify these effects and evaluate the influence of electronic modulation of the carbenic carbon [(CN+); neutral carbene (C:); metal-bound carbene (CM)], on the relative gauche/anti preference, thus highlighting the potential of this conformational phenomenon as a useful molecular design strategy for controlling the topology of organometallic complexes. 相似文献
997.
Firas Awaja Michael Gilbert Minoo Naebe Georgina Kelly Bronwyn Fox Russell Brynolf Paul J. Pigram 《Polymer Degradation and Stability》2013
Thermoplastic polyolefin panels were treated with a flame, flame & water, and accelerated thermo molecular adhesion process (ATmaP) treatment. XPS, contact angle and adhesion test (pull off) results were acquired over a one year period to determine the changes in the elemental composition, surface energy and adhesion strength respectively over time. All surface-treated thermoplastic polyolefin samples showed a sharp decline in adhesion strength up to an ageing period totalling 6 months. The decline in adhesion strength was correlated with a decline in the nitrogen-containing constituents and C–O functional groups at the surface and a decline in surface energy for the flame & water-treated sample. There was no significant change in adhesion strength for all samples for ageing periods greater than 6 months. ATmaP-treated thermoplastic polyolefin outperformed the other two surface treatments in adhesion strength tests due to ATmaP retaining nitrogen-based functional groups (mainly nitrogen oxides) over the year long study. This retention of functionality allowed for a slower ageing process for ATmaP-treated surfaces in comparison to the other surface treatments. 相似文献
998.
Paul D. Sylvester Helen E. Ryan Craig D. Smith Aaron S. Micallef Carl H. Schiesser Uta Wille 《Polymer Degradation and Stability》2013
A profluorescent nitroxide possessing an isoindoline nitroxide moiety linked to a perylene fluorophore was developed to monitor radical mediated degradation of melamine-formaldehyde crosslinked polyester coil coatings in an industry standard accelerated weathering tester. Trapping of polyester-derived radicals (most likely C-radicals) that are generated during polymer degradation leads to fluorescent closed-shell alkoxy amines, which was used to obtain time-dependent degradation profiles to assess the relative stability of different polyesters towards weathering. The nitroxide probe couples excellent thermal stability and satisfactory photostability with high sensitivity and enables detection of free radical damage in polyesters under conditions that mimic exposure to the environment on a time scale of hours rather than months or years required by other testing methods. There are indications that the profluorescent nitroxide undergoes partial photo-degradation in the absence of polymer-derived radicals. Unexpectedly, it was also found that UV-induced fragmentation of the NO–C bond in closed-shell alkoxy amines leads to regeneration of the profluorescent nitroxide and the respective C-radical. The maximum fluorescence intensity that could be achieved with a given probe concentration is therefore not only determined by the amount of polyester radicals formed during accelerated weathering, but also by the light-driven side reactions of the profluorescent nitroxide and the corresponding alkoxy amine radical trapping products. Studies to determine the optimum probe concentration in the polymer matrix revealed that aggregation and re-absorption effects lowered the fluorescence intensity at higher concentrations of the profluorescent nitroxide, but too low probe concentrations, where these effects would be avoided, were not sufficient to trap the amount of polyester radicals formed upon weathering. The optimized experimental conditions were used to assess the impact of temperature and UV irradiance on polymer degradation during accelerated weathering. 相似文献
999.
Brecht Van Hooreweder David Moens Rene Boonen Jean-Pierre Kruth Paul Sas 《Polymer Testing》2013,32(5):972-981
This paper describes the influence of dynamic tension/compression loading on notched and unnotched nylon specimens fabricated by Injection Molding (IM) and Selective Laser Sintering (SLS). The main objective of this work is to analyze and describe the differences in material structure and fatigue properties of as-built nylon parts produced by IM and SLM from the same polyamide 12 powder. The differences in dimensional quality, density, surface roughness, crystal structure and crystallinity are systematically measured and linked to the mechanical fatigue properties. The fatigue properties of the unnotched SLS specimens are found to be equal to those of the unnotched IM specimens. The presence of pores in the sintered samples does not lead to rapid failure, and the microvoid coalescence failure mechanism is delayed. The notched specimens show more brittle failure and increased fatigue resistance which is caused by local notch-strengthening. The results enable improved understanding of the difference in material structure and fatigue behavior of selective laser sintered and injection molded polyamide. 相似文献
1000.