全文获取类型
收费全文 | 1128篇 |
免费 | 71篇 |
国内免费 | 30篇 |
专业分类
化学 | 785篇 |
晶体学 | 7篇 |
力学 | 73篇 |
数学 | 79篇 |
物理学 | 285篇 |
出版年
2023年 | 3篇 |
2022年 | 3篇 |
2021年 | 22篇 |
2020年 | 20篇 |
2019年 | 19篇 |
2018年 | 21篇 |
2017年 | 14篇 |
2016年 | 28篇 |
2015年 | 41篇 |
2014年 | 44篇 |
2013年 | 60篇 |
2012年 | 62篇 |
2011年 | 80篇 |
2010年 | 63篇 |
2009年 | 46篇 |
2008年 | 74篇 |
2007年 | 65篇 |
2006年 | 65篇 |
2005年 | 56篇 |
2004年 | 79篇 |
2003年 | 47篇 |
2002年 | 38篇 |
2001年 | 27篇 |
2000年 | 29篇 |
1999年 | 19篇 |
1998年 | 9篇 |
1997年 | 13篇 |
1996年 | 18篇 |
1995年 | 14篇 |
1994年 | 9篇 |
1993年 | 10篇 |
1992年 | 19篇 |
1991年 | 11篇 |
1990年 | 8篇 |
1989年 | 8篇 |
1988年 | 8篇 |
1987年 | 11篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 10篇 |
1983年 | 10篇 |
1982年 | 9篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 5篇 |
1975年 | 2篇 |
1974年 | 4篇 |
排序方式: 共有1229条查询结果,搜索用时 234 毫秒
991.
992.
993.
Aaron Chin Yit Tay Benjamin J. Frogley David C. Ware Jeanet Conradie Abhik Ghosh Penelope J. Brothers 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(10):3089-3093
The first examples of diboron complexes of the tetrapyrroles octaethylporphyrazine (OEPz) and 2,9,16,23‐tetra‐t‐butyl‐phthalocyanine (Pc) are reported, counterpoints to the better known monoboron tripyrroles, subporphyrazine and subphthalocyanine. Two stereochemical possibilities are observed, with cisoid‐B2OF2(OEPz), both cisoid‐B2OPh2(OEPz) and transoid‐B2OPh2(OEPz), transoid‐B2OF2(Pc) and cisoid‐B2OPh2(Pc) having been isolated and characterised, including structure determinations for the OEPz complexes. This variation in stereochemistry, which can be extended to include the previously reported transoid‐B2OF2(porphyrin), cisoid‐[B2OF2(corrole)]?, and both transoid‐ and cisoid‐B2OF2(calixphyrin), prompted a wider DFT study to elucidate the factors influencing the stereochemical preferences. This shows that the cisoid/transoid preference is correlated to the ease with which the macrocycle accommodates a rectangularly distorted N4 cavity. 相似文献
994.
PISEMO, a separated local field experiment that can be performed with either direct (15)N (or (13)C) detection or indirect (1)H detection, is demonstrated on a single crystal of a model peptide. The (1)H signals modulated by (1)H-(15)N heteronuclear dipole-dipole couplings are observed stroboscopically in the windows of the multiple-pulse sequence used to attenuate (1)H-(1)H homonuclear dipole-dipole couplings. (1)H-detection yields spectra with about 2.5 times the signal to noise ratio observed with (15)N-detection under equivalent conditions. Resolution in both the (15)N chemical shift and (1)H-(15)N heteronuclear dipole-dipole coupling dimensions is similar to that observed with PISEMA, however, since only on-resonance pulses are utilized, the bandwidth is better. 相似文献
995.
Jacopo Canton Ramis Örlü Cheng Chin Nicholas Hutchins Jason Monty Philipp Schlatter 《Flow, Turbulence and Combustion》2016,97(3):811-827
The present study reconsiders the control scheme proposed by Schoppa & Hussain (Phys. Fluids 10, 1049–1051 1998), using a new set of numerical simulations. The computations are performed in a turbulent channel at friction Reynolds numbers of 104 (the value employed in the original study) and 180. In particular, the aim is to better characterise the physics of the control as well as to investigate the optimal parameters. The former purpose lead to a re-design of the control strategy: moving from a numerical imposition of the mean flow to the application of a volume force. A comparison between the two is presented. Results show that the original method only gave rise to transient drag reduction. The forcing method, on the other hand, leads to sustained drag reduction, and thus shows the superiority of the forcing approach for all wavelengths investigated. A clear maximum efficiency in drag reduction is reached for the case with a viscous-scaled spanwise wavelength of the vortices of 1200, which yields a drag reduction of 18 %, as compared to the smaller wavelength of 400 suggested as the most efficient vortex in Schoppa & Hussain. Various turbulence statistics are considered, in an effort to elucidate the causes of the drag-reducing effect. For instance, a region of negative production was found, which is quite unusual for developed turbulent channel flow. 相似文献
996.
One new furylhydroquinone derivative (1) and seven known compounds (2–8) were isolated from the roots of Lithospermum erythrorhizon Sieb. et Zucc (Boraginaceae). The structure of 1 was elucidated by extensive spectroscopic methods using NMR and MS. The absolute configuration of shikonofuran J (1) was unambiguously determined by aid of comparison experimental ECD with predicted ECD spectra. All the isolates were tested for their inhibitory activities against IL-6 production in HaCaT cells stimulated by tumor necrosis factor (TNF)-α. It was found that gracicleistanthoside (5) and uridine (7) remarkably down-regulated the TNF-α-induced synthesis of interleukin-6 (IL-6), a pro-inflammatory cytokine associated with cutaneous inflammation, in HaCaT cells. 相似文献
997.
Tay Meng Guan Chia Ying Ying Kuan Suzie Hui Chin Phan Tze Pei 《Transition Metal Chemistry》2019,44(3):293-301
Transition Metal Chemistry - Ru(II) complexes with diazabutadiene (R-DAB) ligands have been prepared. The reaction of RuCl3·nH2O with P(p-tolyl)3 gave a [RuCl2{P(p-tolyl)3}] precursor, whose... 相似文献
998.
999.
Dr. Ziqi Hu Aman Ullah Dr. Helena Prima-Garcia Sang-Hyun Chin Dr. Yuanyuan Wang Dr. Juan Aragó Prof. Zujin Shi Dr. Alejandro Gaita-Ariño Prof. Eugenio Coronado 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(52):13242-13248
To design molecular spin qubits with enhanced quantum coherence, a control of the coupling between the local vibrations and the spin states is crucial, which could be realized in principle by engineering molecular structures via coordination chemistry. To this end, understanding the underlying structural factors that govern the spin relaxation is a central topic. Here, we report the investigation of the spin dynamics in a series of chemically designed europium(II)-based endohedral metallofullerenes (EMFs). By introducing a unique structural difference, i. e. metal-cage binding site, while keeping other molecular parameters constant between different complexes, these manifest the key role of the three low-energy metal-displacing vibrations in mediating the spin-lattice relaxation times (T1). The temperature dependence of T1 can thus be normalized by the frequencies of these low energy vibrations to show an unprecedentedly universal behavior for EMFs in frozen CS2 solution. Our theoretical analysis indicates that this structural difference determines not only the vibrational rigidity but also spin-vibration coupling in these EMF-based qubit candidates. 相似文献
1000.
Vignesh Sekar Se Yong Eh Noum Azma Putra Sivakumar Sivanesan Kok Chun Chin Yi San Wong Dg Hafizah Kassim 《声与振动》2021,55(4):343-352
This paper presents the development and performance of micro-perforated panels (MPP) from natural fiber reinforced composites. The MPP is made of Polylactic Acid (PLA) reinforced with Oil Palm Empty Fruit Bunch Fiber (OPEFBF). The investigation was made by varying the fiber density, air gap, and perforation ratio to observe the effect on the Sound Absorption Coefficient (SAC) through the experiment in an impedance tube. It is found that the peak level of SAC is not affected, but the peak frequency shifts to lower frequency when the fiber density is increased. This phenomenon might be due to the presence of porosity in the inner wall of the holes. Increasing or decreasing the air gap and perforation ratio shifts the peaks of acoustic absorption either way. 相似文献