全文获取类型
收费全文 | 503506篇 |
免费 | 6959篇 |
国内免费 | 1889篇 |
专业分类
化学 | 233201篇 |
晶体学 | 6071篇 |
力学 | 18388篇 |
综合类 | 14篇 |
数学 | 50410篇 |
物理学 | 127634篇 |
无线电 | 76636篇 |
出版年
2020年 | 3091篇 |
2019年 | 3038篇 |
2018年 | 3147篇 |
2017年 | 3035篇 |
2016年 | 6125篇 |
2015年 | 4970篇 |
2014年 | 6853篇 |
2013年 | 20687篇 |
2012年 | 15830篇 |
2011年 | 19732篇 |
2010年 | 12844篇 |
2009年 | 13792篇 |
2008年 | 19875篇 |
2007年 | 20956篇 |
2006年 | 20259篇 |
2005年 | 18835篇 |
2004年 | 17098篇 |
2003年 | 15230篇 |
2002年 | 14892篇 |
2001年 | 16660篇 |
2000年 | 13362篇 |
1999年 | 10856篇 |
1998年 | 9205篇 |
1997年 | 8936篇 |
1996年 | 8821篇 |
1995年 | 8237篇 |
1994年 | 7838篇 |
1993年 | 7636篇 |
1992年 | 8138篇 |
1991年 | 8013篇 |
1990年 | 7346篇 |
1989年 | 6981篇 |
1988年 | 7121篇 |
1987年 | 5913篇 |
1986年 | 5725篇 |
1985年 | 7980篇 |
1984年 | 7998篇 |
1983年 | 6601篇 |
1982年 | 7023篇 |
1981年 | 6914篇 |
1980年 | 6689篇 |
1979年 | 6643篇 |
1978年 | 6705篇 |
1977年 | 6628篇 |
1976年 | 6508篇 |
1975年 | 6351篇 |
1974年 | 6144篇 |
1973年 | 6367篇 |
1972年 | 3825篇 |
1971年 | 2824篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
71.
72.
73.
74.
A model is developed for the formation and propagation of cracks in a material sample that is heated at its top surface, pyrolyses, and then thermally degrades to form char. In this work the sample is heated uniformly over its entire top surface by a hypothetical flame (a heat source). The pyrolysis mechanism is described by a one-step overall reaction that is dependent nonlinearly on the temperature (Arrhenius form). Stresses develop in response to the thermal degradation of the material by means of a shrinkage strain caused by local mass loss during pyrolysis. When the principal stress exceeds a prescribed threshold value, the material forms a local crack. Cracks are found to generally originate at the surface in response to heating, but occasionally they form in the bulk, away from ever-changing material boundaries. The resulting cracks evolve and form patterns whose characteristics are described. Quantities examined in detail are: the crack spacing in the pyrolysis zone; the crack length evolution; the formation and nature of crack loops which are defined as individual cracks that have joined to form loops that are disconnected from the remaining material; the formation of enhanced pyrolysis area; and the impact of all of the former quantities on mass flux. It is determined that the mass flux from the sample can be greatly enhanced over its nominal (non-cracking) counterpart. The mass efflux profile qualitatively resembles those observed in Cone Calorimeter tests. 相似文献
75.
76.
Immobilized Catalysts for Iridium‐Catalyzed Allylic Amination: Rate Enhancement by Immobilization 下载免费PDF全文
Dr. Chandi C. Malakar Prof. Dr. Günter Helmchen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(19):7127-7134
The first immobilized catalyst for Ir‐catalyzed asymmetric allylic aminations is described. The catalyst is a cationic (π‐allyl)Ir complex bound by cation exchange to an anionic silica gel support. Preparation of the catalyst is facile, and the supported catalyst displayed considerably enhanced activity compared with the parent homogeneous catalyst. Up to 43 consecutive amination runs were possible in recycling experiments. 相似文献
77.
Physical aging in amorphous poly(ethylene furanoate): Enthalpic recovery,density, and oxygen transport considerations 下载免费PDF全文
Steven K. Burgess Christopher R. Mubarak Robert M. Kriegel William J. Koros 《Journal of Polymer Science.Polymer Physics》2015,53(6):389-399
The current work utilizes three separate techniques to study the physical aging process in amorphous poly(ethylene furanoate) (PEF), which is a recently introduced engineering thermoplastic with enhanced properties compared to petroleum‐sourced poly(ethylene terephthalate). Differential scanning calorimetry aging experiments were conducted at multiple aging temperatures and times, and the resultant enthalpic recovery values compared to the theoretical maximum enthalpy loss evaluated from calculations involving extrapolation of the equilibrium liquid line. Density measurements reveal densification of the matrix for the aged versus unaged samples, and provide an estimate for the reduction in free volume for the aged samples. Complementary oxygen permeation and pressure‐decay sorption experiments provide independent verification of the free volume reduction mechanism for physical aging in glassy polymers. The current work provides the first detailed aging study for PEF. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 389–399 相似文献
78.
79.
80.
Susana Estopiñá-Durán Liam J. Donnelly Euan B. Mclean Bryony M. Hockin Prof. Alexandra M. Z. Slawin Dr. James E. Taylor 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(15):3950-3956
A combination of pentafluorophenylboronic acid and oxalic acid catalyses the dehydrative substitution of benzylic alcohols with a second alcohol to form new C−O bonds. This method has been applied to the intermolecular substitution of benzylic alcohols to form symmetrical ethers, intramolecular cyclisations of diols to form aryl-substituted tetrahydrofuran and tetrahydropyran derivatives, and intermolecular crossed-etherification reactions between two different alcohols. Mechanistic control experiments have identified a potential catalytic intermediate formed between the aryl boronic acid and oxalic acid. 相似文献