全文获取类型
收费全文 | 112897篇 |
免费 | 7884篇 |
国内免费 | 13422篇 |
专业分类
化学 | 73264篇 |
晶体学 | 1304篇 |
力学 | 5395篇 |
综合类 | 1318篇 |
数学 | 27814篇 |
物理学 | 25108篇 |
出版年
2024年 | 126篇 |
2023年 | 848篇 |
2022年 | 1884篇 |
2021年 | 2198篇 |
2020年 | 2778篇 |
2019年 | 2723篇 |
2018年 | 2365篇 |
2017年 | 3371篇 |
2016年 | 3621篇 |
2015年 | 2887篇 |
2014年 | 4330篇 |
2013年 | 8395篇 |
2012年 | 7258篇 |
2011年 | 6226篇 |
2010年 | 5347篇 |
2009年 | 7099篇 |
2008年 | 7380篇 |
2007年 | 7666篇 |
2006年 | 6970篇 |
2005年 | 6058篇 |
2004年 | 5484篇 |
2003年 | 4770篇 |
2002年 | 5904篇 |
2001年 | 3663篇 |
2000年 | 3412篇 |
1999年 | 3168篇 |
1998年 | 2738篇 |
1997年 | 2212篇 |
1996年 | 1820篇 |
1995年 | 1699篇 |
1994年 | 1453篇 |
1993年 | 1246篇 |
1992年 | 1150篇 |
1991年 | 818篇 |
1990年 | 668篇 |
1989年 | 610篇 |
1988年 | 464篇 |
1987年 | 375篇 |
1986年 | 361篇 |
1985年 | 324篇 |
1984年 | 323篇 |
1983年 | 182篇 |
1982年 | 267篇 |
1981年 | 244篇 |
1980年 | 244篇 |
1979年 | 237篇 |
1978年 | 230篇 |
1977年 | 170篇 |
1976年 | 139篇 |
1973年 | 76篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
2.
《Mendeleev Communications》2022,32(6):777-779
The reactions of aryllithium reagents o-LiC6H4CH2NR2 with (MeO)2CO afford two new tris(aryl)carbinols bearing pendant-NR2 donor groups in the side chain [o-R NCH C H ] COH [R = Me, R + R = (CH) ]. These alcohols feature helical chirality due to differently inclined aromatic fragments and are presented in a crystalline cell as two M and P enantiomers. Carbinol (R = Me) readily reacts with (Me3SiCH2)3Sc(THF)2 to give a scandium bis(alkyl) complex [(o-C6H4CH2NMe2)3CO]Sc(CH2SiMe3)2 featuring rigid binding of the alkoxy anion through a κ1-O, κ2-N chelating coordination mode 相似文献
3.
4.
Jiraya Kiriratnikom Xun Zhang Xiaohan Cao Bo Chu Chengjian Zhang Xinghong Zhang 《Journal of polymer science. Part A, Polymer chemistry》2022,60(15):2262-2268
Facile construction of sulfur-rich polymers using readily available raw chemicals is an area aggressively pursued but challenging. Herein we use common feedstocks of ethylene oxide (EO), propylene oxide (PO), and carbonyl sulfide (COS) to synthesize copoly(thioether)s which are traditionally produced from unpleasant and difficult to store episulfides. In this protocol, the EO/COS coupling selectively generates a pure poly(ethylene sulfide) (PES) with melting temperature (Tm) values up to 172°C and high yields up to 98%. The EO/PO/COS terpolymerization leads to the incorporation of soft poly(propylene sulfide) (PPS) and hard PES segments together, affording a random PES-co-PPS copoly(thioether) with the complete consumption of EO and PO. Additionally, by simply varying the EO/PO feeding ratio, the obtained copoly(thioether)s possess tunable thermal properties, Tm values in the range of 76–144°C, and excellent solubility. These copolymerizations are conducted in one-pot/one-step at industrially favored reaction temperatures of 100–120°C using catalysts of common organic bases, suggesting a facile and practical manner. Especially, the copoly(thioether) exhibits high refractive indices up to 1.68 owing to its high sulfur content, suggesting a broad application prospect in optical materials. 相似文献
5.
《Mendeleev Communications》2022,32(4):507-509
We report on the synthesis of new Ru(bpy)2(phen) catalyst for the oscillatory Belousov–Zhabotinsky chemical reaction and on the preparation of novel Ru(bpy)2(phen)-based self-oscillating gels. The synthesized gels exhibit high-amplitude autonomous mechanical oscillations when the Belousov–Zhabotinsky reaction proceeds inside these gels 相似文献
6.
Chemical bonding in representative astrophysically relevant neutral,cation, and anion HCnH chains
下载免费PDF全文
![点击此处可从《中国物理 B》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Ioan Baldea 《中国物理 B》2022,31(12):123101-123101
Most existing studies assign a polyynic and cumulenic character of chemical bonding in carbon-based chains relying on values of the bond lengths. Building on our recent work, in this paper we add further evidence on the limitations of such an analysis and demonstrate the significant insight gained via natural bond analysis. Presently reported results include atomic charges, natural bond order and valence indices obtained from ab initio computations for representative members of the astrophysically relevant neutral and charged HC2k/2k+1H chain family. They unravel a series of counter-intuitive aspects and/or help naive intuition in properly understanding microscopic processes, e.g., electron removal from or electron attachment to a neutral chain. Demonstrating that the Wiberg indices adequately quantify the chemical bonding structure of the HC2k/2k+1H chains—while the often heavily advertised Mayer indices do not—represents an important message conveyed by the present study. 相似文献
7.
Marcus Carlsson 《Expositiones Mathematicae》2021,39(1):149-157
von Neumann’s inequality in matrix theory refers to the fact that the Frobenius scalar product of two matrices is less than or equal to the scalar product of the respective singular values. Moreover, equality can only happen if the two matrices share a joint set of singular vectors, and this latter part is hard to find in the literature. We extend these facts to the separable Hilbert space setting, and provide a self-contained proof of the “latter part”. 相似文献
8.
In this article, we construct and analyze a residual-based a posteriori error estimator for a quadratic finite volume method (FVM) for solving nonlinear elliptic partial differential equations with homogeneous Dirichlet boundary conditions. We shall prove that the a posteriori error estimator yields the global upper and local lower bounds for the norm error of the FVM. So that the a posteriori error estimator is equivalent to the true error in a certain sense. Numerical experiments are performed to illustrate the theoretical results. 相似文献
9.
10.
In this work, a vanillin complex is immobilized onto MCM-41 and characterized by FT-IR, X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, thermogravimetric analysis, and BET techniques. This supported Schiff base complex was found to be an efficient and recoverable catalyst for the chemoselective oxidation of sulfides into sulfoxides and thiols into their corresponding disulfides (using hydrogen peroxide as a green oxidant) and also a suitable catalyst for the preparation of 2,3-dihydroquinazolin-4(1H)-one derivatives in water at 90°C. Using this protocol, we show that a variety of disulfides, sulfoxides, and 2,3-dihydroquinazolin-4(1H)-one derivatives can be synthesized in green conditions. The catalyst can be recovered and recycled for further reactions without appreciable loss of catalytic performance. 相似文献