全文获取类型
收费全文 | 2953篇 |
免费 | 6篇 |
国内免费 | 250篇 |
专业分类
化学 | 2778篇 |
力学 | 7篇 |
数学 | 13篇 |
物理学 | 411篇 |
出版年
2022年 | 2篇 |
2021年 | 1篇 |
2018年 | 1篇 |
2017年 | 2篇 |
2016年 | 5篇 |
2015年 | 1篇 |
2014年 | 6篇 |
2013年 | 1篇 |
2012年 | 258篇 |
2011年 | 325篇 |
2010年 | 42篇 |
2009年 | 129篇 |
2008年 | 429篇 |
2007年 | 305篇 |
2006年 | 352篇 |
2005年 | 351篇 |
2004年 | 246篇 |
2003年 | 192篇 |
2002年 | 144篇 |
2001年 | 87篇 |
2000年 | 108篇 |
1999年 | 36篇 |
1998年 | 14篇 |
1997年 | 8篇 |
1996年 | 38篇 |
1995年 | 20篇 |
1994年 | 21篇 |
1993年 | 47篇 |
1992年 | 24篇 |
1991年 | 4篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1986年 | 3篇 |
1979年 | 1篇 |
排序方式: 共有3209条查询结果,搜索用时 93 毫秒
991.
HUA Guo-Pinga ZHANG Xiao-Jingb TU Shu-Jiangb② ZHU Song-Leib LI Tuan-Jieb ZHU Xiao-Tongb ZHANG Jin-Pengb a 《结构化学》2005,24(4):399-402
1 INTRODUCTION Since the discovery of pharmacological effectsof 1,4-dihydropyridine (1,4-DHPs) as calcium mo-dulators[1], a great deal of work has been directedtowards the synthesis of 1,4-DHPs acting as cal-cium antagonists[2]. The chemical modifications onthe DHP ring, such as the introduction of differentsubstituents on heteroatoms[3], have allowed theexpansion of researches on structure-activity rela-tionship to have new insight into the molecularinteractions at the receptor l… 相似文献
992.
YANG Jian-Guoa PAN Fu-Youb② a 《结构化学》2005,24(12):1403-1407
1 INTRODUCTION 2. 1 Physical measurements Azole derivatives, such as pyrazole, imidazole, All solvent and chemicals were commercial rea- triazole(including benzotriazole), tetrazole and indole, gents and used without further purification. Ele- have extensive biological activities. They have be- mental analyses were performed on a PE 1700 CHN come the central focus of studies for agricultural che- auto elemental analyzer. IR spectra were recorded on micals, medicines, plant growth regul… 相似文献
993.
HUANG Lia ZHOU Ya-Jua YE De-Yonga① CHEN Min-Qinb a 《结构化学》2005,24(8):945-951
1 INTRODUCTION The success of strategies for total synthesis con-tinues to depend on high levels of stereoselectivityobserved in substrate-directed diastereoselective re-actions[1]. In this context, one of the most widelyused processes is diastereoselective epoxidation ofcyclic alkenes with allylic O- and N-directing group,which is very useful in building stereodefined syn-thetic blocks[2, 3] .However, in some circumstances,the assignment of resulting epoxides is not straight-forward[4~8… 相似文献
994.
1 INTRODUCTIONResearchonSHGeffecthasbeenonaremarkablepositioninorganicnonlinearoptical(NLO)materialsfield.Theconjugateddelocalizedπelectronspossiblyresultinthesecond-orderNLOpropertyoforganiccompounds,soanumberofnitroanilinederivatives,suchas2-methyl… 相似文献
995.
María Natividad Pérez-Camacho Jehad Abu-Dahrieh Alexre Goguet Kening Sun David Rooney 《催化学报》2014,(8)
Gas-to-liquid processes are generally used to convert natural gas or other gaseous hydrocarbons into liquid fuels via an intermediate syngas stream. This includes the production of liquid fuels from biomass‐derived sources such as biogas. For example, the dry reforming of methane is done by reacting CH4 and CO2, the two main components of natural biogas, into more valuable products, i.e.,CO and H2. Nickel containing perovskite type catalysts can promote this reaction, yielding good conversions and selectivities; however, they are prone to coke laydown under certain operating conditions. We investigated the addition of high oxygen mobility dopants such as CeO2, ZrO2, or YSZ to reduce carbon laydown, particularly using reaction conditions that normally result in rapid cok‐ing. While doping with YSZ, YDC, GDC, and SDC did not result in any improvement, we show that a Ni perovskite catalyst (Na0.5La0.5Ni0.3Al0.7O2.5) doped with 80.9 ZrO2 15.2 CeO2 gave the lowest amount of carbon formation at 800 °C and activity was maintained over the operating time. 相似文献
996.
Ye Feng XueShan Zhao LiJun Gou YuFeng Li James FSteiner Javier AGarc´ıa Yuan Wang Nan Jia ZhenXuan Liao HuiXian Li 《中国科学:物理学 力学 天文学(英文版)》2022,(1)
AT2019 wey is a new galactic X-ray binary that was first discovered as an optical transient by the Asteroid Terrestrial-impact Last Alert System(ATLAS)on December 7,2019.AT2019 wey consists of a black hole candidate as well as a low-mass companion star(Mstar≤1.0 M■)and is likely to have a short orbital period(Porb≤16h).Although AT2019 wey began activation in the X-ray band on March 8,2020,it did not enter the soft state during almost the entire outburst.In this study,we present a detailed spectral analysis of AT2019 wey in the low/hard state during its X-ray outburst on the basis of Nuclear Spectroscopic Telescope Array(Nu S T AR)observations.We obtain tight constraints on several of its important physical parameters by applying the state of the art relxill relativistic reflection model family.In particular,we determine that the measured inner radius of the accretion disk is most likely to have extended to the innermost stable circular orbit(ISCO)radius,i.e.,Rin=1.38+0.23-0.16RISCO.Hence,assuming Rin=RISCO,we find the spin of AT2019 wey to be a*~0.97,which is close to the extreme and an inner disk inclination angle of i~22°.Additionally,according to our adopted models,AT2019 wey tends to have a relatively high iron abundance of AFe~5AFe,■and a high disk ionization state of logξ~3.4. 相似文献
997.
In this paper, we characterize the odd positive integers n satisfying the congruence∑n -1 j=1 j n-1/2 ≡ 0 (mod n). We show that the set of such positive integers has an asymptotic density which turns out to be slightly larger than 3/8. 相似文献
998.
In this paper, a recently introduced cellular automata (CA) model is used for a statistical analysis of the inner micro-scopic structure of synchronized traffic flow. The analysis focuses on the formation and dissolution of clusters or platoons of vehicles, as the mechanism that causes the presence of this synchronized traffic state with a high flow. This platoon formation is one of the most interesting phenomena observed in traffic flows and plays an important role both in manual and automated highway systems (AHS). Simulation results, obtained from a single-lane system under periodic boundary conditions indicate that in the density region where the synchronized state is observed, most vehicles travel together in pla- toons with approximately the same speed and small spatial distances. The examination of velocity variations and individual vehicle gaps shows that the flow corresponding to the synchronized state is stable, safe and highly correlated. Moreover, results indicate that the observed platoon formation in real traffic is reproduced in simulations by the relation between vehicle headway and velocity that is embedded in the dynamics definition of the CA model. 相似文献
999.
We present fine-structure-resolved collisional rate coefficients for the NH(X(3)Σ(-))-He van der Waals complex. The calculations are based on the state-of-the-art potential energy surface [Cybulski et al., J. Chem. Phys. 122, 094307 (2005)]. Close-coupling calculations of the collisional excitation cross sections of the fine-structure levels of NH by He are calculated for total energies up to 3500 cm(-1), which yield, after thermal average, rate coefficients up to 350 K. The fine-structure splitting of rotational levels is taken into account rigorously. The propensity rules between fine-structure levels are reported, and it is found that F-conserving cross sections are much larger than F-changing cross sections, as expected from theoretical considerations. The calculated rate coefficients are compared with available experimental measurements at room temperature and a fairly good agreement is found between experimental and theoretical data. The agreement confirms the relatively good quality of the scattering calculations and also the accuracy of the potential energy surface used in this work. The new set of thermal rate coefficients for this system may be used for improvements in astrophysical and atmospherical modeling. 相似文献
1000.
Cerveny S Arrese-Igor S Dolado JS Gaitero JJ Alegría A Colmenero J 《The Journal of chemical physics》2011,134(3):034509
The behavior of water dynamics confined in hydrated calcium silicate hydrate (C-S-H) gel has been investigated using broadband dielectric spectroscopy (BDS; 10(-2)-10(6) Hz) in the low-temperature range (110-250 K). Different water contents in C-S-H gel were explored (from 6 to 15 wt%) where water remains amorphous for all the studied temperatures. Three relaxation processes were found by BDS (labeled 1 to 3 from the fastest to the slowest), two of them reported here for the first time. We show that a strong change in the dielectric relaxation of C-S-H gel occurs with increasing hydration, especially at a hydration level in which a monolayer of water around the basic units of cement materials is predicted by different structural models. Below this hydration level both processes 2 and 3 have an Arrhenius temperature dependence. However, at higher hydration level, a non-Arrhenius behavior temperature dependence for process 3 over the whole accessible temperature range and, a crossover from low-temperature Arrhenius to high-temperature non-Arrhenius behavior for process 2 are observed. Characteristics of these processes will be discussed in this work. 相似文献