全文获取类型
收费全文 | 216935篇 |
免费 | 1812篇 |
国内免费 | 638篇 |
专业分类
化学 | 119053篇 |
晶体学 | 3280篇 |
力学 | 8390篇 |
综合类 | 6篇 |
数学 | 23639篇 |
物理学 | 65017篇 |
出版年
2020年 | 1999篇 |
2019年 | 2295篇 |
2018年 | 3227篇 |
2017年 | 3131篇 |
2016年 | 4294篇 |
2015年 | 2395篇 |
2014年 | 3860篇 |
2013年 | 9342篇 |
2012年 | 6986篇 |
2011年 | 8561篇 |
2010年 | 6126篇 |
2009年 | 5985篇 |
2008年 | 8229篇 |
2007年 | 8324篇 |
2006年 | 7728篇 |
2005年 | 7013篇 |
2004年 | 6247篇 |
2003年 | 5710篇 |
2002年 | 5560篇 |
2001年 | 5963篇 |
2000年 | 4718篇 |
1999年 | 3491篇 |
1998年 | 3024篇 |
1997年 | 3053篇 |
1996年 | 2844篇 |
1995年 | 2356篇 |
1994年 | 2446篇 |
1993年 | 2463篇 |
1992年 | 2621篇 |
1991年 | 2670篇 |
1990年 | 2571篇 |
1989年 | 2546篇 |
1988年 | 2475篇 |
1987年 | 2445篇 |
1986年 | 2374篇 |
1985年 | 3052篇 |
1984年 | 3211篇 |
1983年 | 2572篇 |
1982年 | 2842篇 |
1981年 | 2725篇 |
1980年 | 2512篇 |
1979年 | 2726篇 |
1978年 | 2842篇 |
1977年 | 2866篇 |
1976年 | 2841篇 |
1975年 | 2699篇 |
1974年 | 2676篇 |
1973年 | 2813篇 |
1972年 | 1876篇 |
1967年 | 1831篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Michiel Janssen Prof. Dirk E. De Vos 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(45):10724-10734
The practical application of Shilov-type Pt catalysis to the selective hydroxylation of terminal aliphatic C−H bonds remains a formidable challenge, due to difficulties in replacing PtIV with a more economically viable oxidant, particularly O2. We report the potential of employing FeCl2 as a suitable redox mediator to overcome the kinetic hurdles related to the direct use of O2 in the Pt reoxidation. For the selective conversion of butyric acid to γ-hydroxybutyric acid (GHB), a significantly enhanced catalyst activity and stability (turnover numbers (TON)>30) were achieved under 20 bar O2 in comparison to current state-of-the-art systems (TON<10). In this regard, essential reaction parameters affecting the overall activity were identified, along with specific additives to attain catalyst stability at longer reaction times. Notably, deactivation by reduction to Pt0 was prevented by the addition of monodentate pyridine derivatives, such as 2-fluoropyridine, but also by introducing varying partial pressures of N2 in the gaseous atmosphere. Finally, stability tests revealed the involvement of PtII and FeCl2 in catalyzing the non-selective overoxidation of GHB. Accordingly, in situ esterification with boric acid proved to be a suitable strategy to maintain enhanced selectivities at much higher conversions (TON>60). Altogether, a useful catalytic system for the selective hydroxylation of primary aliphatic C−H bonds with O2 is presented. 相似文献
82.
Doklady Physics - In this paper, we study the statistical characteristics of the Riemann wave and the integral of it. Such problems arise in the physics of fronts’ propagation, in particular,... 相似文献
83.
Norbornene derivatives from a metal‐free,strain‐promoted cycloaddition reaction: New building blocks for ring‐opening metathesis polymerization reactions 下载免费PDF全文
Christopher E. Hobbs Binhong Lin Thomas Malinski 《Journal of polymer science. Part A, Polymer chemistry》2015,53(20):2357-2362
The preparation of new ring opening metathesis polymerization (ROMP) monomers using a 1,3‐dipolar cycloaddition between aryl azides and norbornadiene is described. Various norbornenetriazolines, obtained through a solvent‐and catalyst‐free reaction, can subsequently be incorporated into polymer backbones through ROMP reactions. Furthermore, thermal decomposition of the triazoline moiety can allow for further polymer functionalization. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2357–2362 相似文献
84.
85.
G.E. Farr 《Discrete Mathematics》2019,342(5):1510-1519
This paper continues the study of combinatorial properties of binary functions — that is, functions such that , where is a finite set. Binary functions have previously been shown to admit families of transforms that generalise duality, including a trinity transform, and families of associated minor operations that generalise deletion and contraction, with both these families parameterised by the complex numbers. Binary function representations exist for graphs (via the indicator functions of their cutset spaces) and indeed arbitrary matroids (as shown by the author previously). In this paper, we characterise degenerate elements – analogues of loops and coloops – in binary functions, with respect to any set of minor operations from our complex-parameterised family. We then apply this to study the relationship between binary functions and Tutte’s alternating dimaps, which also support a trinity transform and three associated minor operations. It is shown that only the simplest alternating dimaps have binary representations of the form we consider, which seems to be the most direct type of representation. The question of whether there exist other, more sophisticated types of binary function representations for alternating dimaps is left open. 相似文献
86.
Differential inequalities for polynomials generalizing the well-known Smirnov, Rahman, Schmeisser, and Bernstein inequalities are obtained.
相似文献87.
Tereshchenko E. D. Cherniakov S. M. Yurik R. Yu. Rietveld M. T. Häggström I. 《Radiophysics and Quantum Electronics》2020,62(10):667-676
Radiophysics and Quantum Electronics - We present the results of comparing the total electron content measurements based on GLONASS satellite signals and the EISCAT UHF incoherent scatter radar... 相似文献
88.
Journal of Structural Chemistry - An analysis is presented for the results obtained in the studies initiated by A. E. Shvelashivili, Corresponding Member, Academy of Sciences of Georgia, and... 相似文献
89.
90.
Pan Huang Dawen Xu Robert M. Reich Felix Kaiser Boping Liu Fritz E. Kühn 《Tetrahedron letters》2019,60(24):1574-1577
A first example of an Et2Zn mediated silylation of 1-aklynes is reported. A series of functional groups are tolerated in this reaction. Mechanistic studies support Zn alkynilides as intermediates in the reaction. This reaction protocol provides a practical method for the preparation of alkynylsilanes and expands the application of organometallic zinc in organic synthesis. 相似文献