全文获取类型
收费全文 | 426740篇 |
免费 | 5011篇 |
国内免费 | 1186篇 |
专业分类
化学 | 214878篇 |
晶体学 | 6013篇 |
力学 | 19770篇 |
综合类 | 7篇 |
数学 | 62134篇 |
物理学 | 130135篇 |
出版年
2020年 | 2311篇 |
2019年 | 2305篇 |
2018年 | 11164篇 |
2017年 | 12046篇 |
2016年 | 8418篇 |
2015年 | 4592篇 |
2014年 | 5096篇 |
2013年 | 14984篇 |
2012年 | 15934篇 |
2011年 | 26593篇 |
2010年 | 16112篇 |
2009年 | 15988篇 |
2008年 | 23569篇 |
2007年 | 27079篇 |
2006年 | 13158篇 |
2005年 | 18973篇 |
2004年 | 14182篇 |
2003年 | 12671篇 |
2002年 | 10505篇 |
2001年 | 10505篇 |
2000年 | 8216篇 |
1999年 | 6378篇 |
1998年 | 5130篇 |
1997年 | 4945篇 |
1996年 | 5145篇 |
1995年 | 4665篇 |
1994年 | 4383篇 |
1993年 | 4201篇 |
1992年 | 4670篇 |
1991年 | 4494篇 |
1990年 | 4219篇 |
1989年 | 4045篇 |
1988年 | 4324篇 |
1987年 | 3960篇 |
1986年 | 3829篇 |
1985年 | 5524篇 |
1984年 | 5611篇 |
1983年 | 4553篇 |
1982年 | 4953篇 |
1981年 | 4998篇 |
1980年 | 4729篇 |
1979年 | 4854篇 |
1978年 | 4896篇 |
1977年 | 4760篇 |
1976年 | 4730篇 |
1975年 | 4638篇 |
1974年 | 4469篇 |
1973年 | 4654篇 |
1972年 | 2638篇 |
1971年 | 1939篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
41.
42.
43.
A (di)graph is supereulerian if it contains a spanning eulerian sub(di)graph. This property is a relaxation of hamiltonicity. Inspired by this analogy with hamiltonian cycles and by similar results in supereulerian graph theory, we analyze a number of sufficient Ore type conditions for a digraph to be supereulerian. Furthermore, we study the following conjecture due to Thomassé and the first author: if the arc‐connectivity of a digraph is not smaller than its independence number, then the digraph is supereulerian. As a support for this conjecture we prove it for digraphs that are semicomplete multipartite or quasitransitive and verify the analogous statement for undirected graphs. 相似文献
44.
45.
A. Lashgari S. Ghammamy R. Ramirez-Tagle G. Salgado-Moran 《Journal of Structural Chemistry》2015,56(8):1505-1513
Two new compounds of fluorine: (C2H5)4N[I2F] and (C2H5)4N[Br2F], have been easily synthesized in a nearly quantitative by a direct reaction of (C2H5)4NF, I2 and Br2. The products were isolated and characterized by elemental analysis and spectroscopic methods such as: Fourier transform infrared spectroscopy (FTIR) and ultraviolet-visible spectroscopy (UV-Vis). These compounds have been studied computationally with the Scalar ZORA relativistic level of theory using the ADF program package. The molecular parameters, and vibrational spectra were calculated. The excitation energies were found by timedependent perturbation density functional theory (TD-DFT). Molecule optimization, frequencies and excitation energies were calculated with standard Slatertype-orbital (STO) basis sets with triple-zeta quality double plus polarization functions (TZ2P) for all atoms. The FTIR, UV-Vis spectra and assignment of principal transitions and total density of state (TDOS) were extracted using the GaussSum 2.2 program. The comparison between experimental and calculated values showes that the experimental results correlate well with the predicted data. 相似文献
46.
Martin Kellert Jan-Simon Jeshua Friedrichs Nadine Anke Ullrich Alexander Feinhals Jonas Tepper Peter Lnnecke Evamarie Hey-Hawkins 《Molecules (Basel, Switzerland)》2021,26(7)
The development of novel, tumor-selective and boron-rich compounds as potential agents for use in boron neutron capture therapy (BNCT) represents a very important field in cancer treatment by radiation therapy. Here, we report the design and synthesis of two promising compounds that combine meta-carborane, a water-soluble monosaccharide and a linking unit, namely glycine or ethylenediamine, for facile coupling with various tumor-selective biomolecules bearing a free amino or carboxylic acid group. In this work, coupling experiments with two selected biomolecules, a coumarin derivative and folic acid, were included. The task of every component in this approach was carefully chosen: the carborane moiety supplies ten boron atoms, which is a tenfold increase in boron content compared to the l-boronophenylalanine (l-BPA) presently used in BNCT; the sugar moiety compensates for the hydrophobic character of the carborane; the linking unit, depending on the chosen biomolecule, acts as the connection between the tumor-selective component and the boron-rich moiety; and the respective tumor-selective biomolecule provides the necessary selectivity. This approach makes it possible to develop a modular and feasible strategy for the synthesis of readily obtainable boron-rich agents with optimized properties for potential applications in BNCT. 相似文献
47.
Farsi R. Mohammadi M. K. Saghanezhad S. J. 《Russian Journal of Organic Chemistry》2021,57(12):2002-2009
Russian Journal of Organic Chemistry - Herein, we report the sulfonamide-functionalized covalent organic framework (COF-SO3H), prepared by condensation of melamine and terephthalaldehyde and... 相似文献
48.
49.
Timm Bergholz Dr. Benjamin Oelkers Dr. Benedikt Huber Prof. Bernhard Roling Prof. Dr. Jörg Sundermeyer 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(6):2613-2620
New salts based on imidazolium, pyrrolidinium, phosphonium, guanidinium, and ammonium cations together with the 5‐cyanotetrazolide anion [C2N5]? are reported. Depending on the nature of cation–anion interactions, characterized by XRD, the ionic liquids (ILs) have a low viscosity and are liquid at room temperature or have higher melting temperatures. Thermogravimetric analysis, cyclic voltammetry, viscosimetry, and impedance spectroscopy display a thermal stability up to 230 °C, an electrochemical window of 4.5 V, a viscosity of 25 mPa s at 20 °C, and an ionic conductivity of 5.4 mS cm?1 at 20 °C for the IL 1‐butyl‐1‐methylpyrrolidinium 5‐cyanotetrazolide [BMPyr][C2N5]. On the basis of these results, the synthesized compounds are promising electrolytes for lithium‐ion batteries. 相似文献
50.