全文获取类型
收费全文 | 25589篇 |
免费 | 557篇 |
国内免费 | 105篇 |
专业分类
化学 | 14459篇 |
晶体学 | 157篇 |
力学 | 794篇 |
综合类 | 9篇 |
数学 | 3525篇 |
物理学 | 7307篇 |
出版年
2023年 | 116篇 |
2022年 | 326篇 |
2021年 | 406篇 |
2020年 | 365篇 |
2019年 | 328篇 |
2018年 | 413篇 |
2017年 | 371篇 |
2016年 | 656篇 |
2015年 | 528篇 |
2014年 | 615篇 |
2013年 | 1013篇 |
2012年 | 1300篇 |
2011年 | 1486篇 |
2010年 | 819篇 |
2009年 | 797篇 |
2008年 | 1251篇 |
2007年 | 1225篇 |
2006年 | 1094篇 |
2005年 | 1531篇 |
2004年 | 1554篇 |
2003年 | 1112篇 |
2002年 | 750篇 |
2001年 | 614篇 |
2000年 | 572篇 |
1999年 | 352篇 |
1998年 | 298篇 |
1997年 | 300篇 |
1996年 | 372篇 |
1995年 | 316篇 |
1994年 | 313篇 |
1993年 | 332篇 |
1992年 | 323篇 |
1991年 | 263篇 |
1990年 | 217篇 |
1989年 | 205篇 |
1988年 | 179篇 |
1987年 | 177篇 |
1986年 | 177篇 |
1985年 | 231篇 |
1984年 | 215篇 |
1983年 | 183篇 |
1982年 | 186篇 |
1981年 | 187篇 |
1980年 | 172篇 |
1979年 | 150篇 |
1978年 | 179篇 |
1977年 | 139篇 |
1976年 | 135篇 |
1974年 | 134篇 |
1973年 | 123篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
A tentative assignment of the vibrational modes of sulphadiazine is made, based on the infrared and Raman data. In addition, dirhodium tetracarboxylates and their adducts with sulphadiazine are investigated by Raman spectroscopy, with emphasis in the identify the metal-metal and metalligand stretching modes. 相似文献
992.
993.
Residual‐based stabilization of the finite element approximation to the acoustic perturbation equations for low Mach number aeroacoustics 下载免费PDF全文
Oriol Guasch Patricia Sánchez‐Martín Arnau Pont Joan Baiges Ramon Codina 《国际流体数值方法杂志》2016,82(12):839-857
The acoustic perturbation equations (APE) are suitable to predict aerodynamic noise in the presence of a non‐uniform mean flow. As for any hybrid computational aeroacoustics approach, a first computational fluid dynamics simulation is carried out from which the mean flow characteristics and acoustic sources are obtained. In a second step, the APE are solved to get the acoustic pressure and particle velocity fields. However, resorting to the finite element method (FEM) for that purpose is not straightforward. Whereas mixed finite elements satisfying an appropriate inf–sup compatibility condition can be built in the case of no mean flow, that is, for the standard wave equation in mixed form, these are difficult to implement and their good performance is yet to be checked for more complex wave operators. As a consequence, strong simplifying assumptions are usually considered when solving the APE with FEM. It is possible to avoid them by resorting to stabilized formulations. In this work, a residual‐based stabilized FEM is presented for the APE at low Mach numbers, which allows one to deal with the APE convective and reaction terms in its full extent. The key of the approach resides in the design of the matrix of stabilization parameters. The performance of the formulation and the contributions of the different terms in the equations are tested for an acoustic pulse propagating in sheared‐solenoidal mean flow, and for the aeolian tone generated by flow past a two‐dimensional cylinder. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
994.
Felipe Wodtke Fernando R. Xavier Samuel R. Mendes Alfredo R.M. de Oliveira Rogério A. Gariani 《Tetrahedron letters》2017,58(3):231-234
A novel application of the highly stable niobium oxide phosphate (NbOPO4) as an efficient catalyst for the synthesis of β-enamino ketones under solvent-free conditions is described. This protocol, exhibits attractive yields, short reaction periods, lower loading of catalyst and high chemoselectivity. 相似文献
995.
996.
997.
Organic‐acid mediated bulk polymerization of ε‐caprolactam and its copolymerization with ε‐caprolactone 下载免费PDF全文
Ana Sanchez‐Sanchez Andere Basterretxea Daniele Mantione Agustin Etxeberria Cristina Elizetxea Amaia de la Calle Sonia García‐Arrieta Haritz Sardon David Mecerreyes 《Journal of polymer science. Part A, Polymer chemistry》2016,54(15):2394-2402
Polyamides (PA) constitute one of the most important classes of polymeric materials and have gained strong position in different areas, such as textiles, fibers, and construction materials. Whereas most PA are synthesized by step‐growth polycondensation, PA 6 is synthesized by ring opening polymerization (ROP) of ε‐caprolactam (ε‐CLa). The most popular ROP methods involve the use of alkaline metal catalyst difficult to handle at large scale. In this article, we propose the use of organic acids for the ROP of ε‐CLa in bulk at 180 °C (below the polymer's melting point). Among evaluated organic acids, sulfonic acids were found to be the most effective for the polymerization of ε‐CLa , being the Brønsted acid ionic liquid: 1‐(4‐sulfobutyl)?3‐methylimidazolium hydrogen sulfate the most suitable due to its higher thermal stability. End‐group analysis by 1H nuclear magnetic resonance and model reactions provided mechanistic insights and suggested that the catalytic activity of sulfonic acids was a function of not only the acid strength, but of the nucleophilic character of conjugate base as well. Finally, the ability of sulfonic acid to promote the copolymerization of ε‐CLa and ε‐caprolactone is demonstrated. As a result, poly(ε‐caprolactam‐co‐ε‐caprolactone) copolymers with considerably randomness are obtained. This benign route allows the synthesis of poly(ester amide)s with different thermal and mechanical properties. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2394–2402 相似文献
998.
Cristiane S. Schwalm Ilton B.D. de Castro Jailton Ferrari Fábio L. de Oliveira Ricardo Aparicio Carlos Roque D. Correia 《Tetrahedron letters》2012,53(13):1660-1663
Pentrabromopseudilin and other 2 and 3-arylpyrrole derivatives were synthesized through the Heck–Matsuda reaction involving endocyclic enecarbamates and N-protected 3-pyrrolines, respectively. The overall processes permitted an easy and efficient access to these structural motifs present in several bioactive compounds. Attempts to synthesize the compound isopentabromopseudilin led to a tribromo aryl maleimide. We hypothesize that this latter compound is the putative product arising from the unusual thermal instability of isopentabromopseudilin. 相似文献
999.
N. Buarque de Gusmáo M. Kaouadji F. Seigle-Murandi R. Steiman F. Thomasson 《光谱学快报》2013,46(8):1373-1380
Acrodontiolamide, a new fungal metabolite has been isolated from the cell-free culture medium of Acrodontium salmoneum. Its purification was achieved by centrifugal TLC on silica gel and the structural analysis was based on spectroscopic data relative to both the natural product and its diacetyl derivative. This secondary metabolite, identified with 3-(p-nitrophenyl)-3-hydroxy-4-dichloromethoxy-isobutanamide exhibited antifungal properties. 相似文献
1000.
Ren Y Acuña UM Jiménez F García R Mejía M Chai H Gallucci JC Farnsworth NR Soejarto DD Carcache de Blanco EJ Kinghorn AD 《Tetrahedron》2012,68(12):2671-2678
Six new (1-6) and eight known germacranolide-type sesquiterpene lactones, along with several known phenylpropanol coumarates and methylated flavonoids, were isolated from the leaves of Piptocoma rufescens, collected in the Dominican Republic. The new compounds were identified by analysis of their spectroscopic data, with the molecular structure of 3 being established by single-crystal X-ray diffraction. The absolute configurations of the sesquiterpene lactones isolated were determined from their CD and NOESY NMR spectra, together with the analysis of Mosher ester reactions. Bioassay screening results showed the majority of the sesquiterpene lactones isolated (1-13) to be highly cytotoxic toward the HT-29 human colon cancer cell line, with the most potent compound being 15-deoxygoyazensolide (10, IC(50), 0.26 μM). In addition, several of the sesquiterpene lactones exhibited NF-κB (p65) inhibitory activity. 相似文献