全文获取类型
收费全文 | 625篇 |
免费 | 34篇 |
国内免费 | 7篇 |
专业分类
化学 | 395篇 |
晶体学 | 3篇 |
力学 | 71篇 |
数学 | 69篇 |
物理学 | 128篇 |
出版年
2024年 | 1篇 |
2023年 | 5篇 |
2022年 | 29篇 |
2021年 | 32篇 |
2020年 | 36篇 |
2019年 | 27篇 |
2018年 | 40篇 |
2017年 | 27篇 |
2016年 | 41篇 |
2015年 | 26篇 |
2014年 | 42篇 |
2013年 | 78篇 |
2012年 | 57篇 |
2011年 | 58篇 |
2010年 | 25篇 |
2009年 | 21篇 |
2008年 | 13篇 |
2007年 | 13篇 |
2006年 | 10篇 |
2005年 | 2篇 |
2004年 | 3篇 |
2003年 | 6篇 |
2002年 | 6篇 |
2001年 | 3篇 |
2000年 | 7篇 |
1999年 | 2篇 |
1998年 | 4篇 |
1997年 | 4篇 |
1996年 | 5篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 9篇 |
1992年 | 1篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1977年 | 1篇 |
1953年 | 1篇 |
排序方式: 共有666条查询结果,搜索用时 15 毫秒
51.
Alberto Plaza Ciro Balestrieri Giuseppe Bifulco Arafa Hamed Sonia Piacente 《Tetrahedron》2005,61(31):7470-7480
Eight new 14,15-secopregnane glycosides, namely argelosides C-J, possessing two ketal functions involved in three five-membered rings, have been isolated from the hairy seeds of Solenostemma argel. Their structures have been established by MS and NMR experiments, combined with quantum mechanical calculations of the 13C chemical shifts for the interpretation of the experimental data. On the basis of the obtained results, the structures of argelosides A and B have been revised. Additionally, the effect of these compounds on the VEGF-induced in Kaposi's sarcoma cell proliferation was evaluated. Results indicated that all the compounds reduced the cell proliferation in a dose-dependent manner. 相似文献
52.
R. H. Abu-Eittah M. M. Hamed M. M. Abdou 《International journal of quantum chemistry》1987,31(6):855-869
The electronic absorption spectra of some phenylethylamine drugs, namely, d-pseudoepherine, l-pseudoephedrine, l-ephedrine, dl-ephedrine dl-norephedrine, phenylethylamine, methoxyphenamine, and l-noradrenaline were investigated in polar and nonpolar solvents. The observed transitions were interpreted, and the role of σ-π interaction was explored. Molecular orbital calculations were performed on representatives of the above group of compounds, namely, d-pseudoephedrine, l-ephedrine, and l-noradrenaline using the INDO procedures and adopting the best conformer of the molecule. The transition energy, band intensity, and dipole moments were calculated and corresponded satisfactorily with the experimental values. 相似文献
53.
Abbas Afkhami Farzaneh Soltani-Felehgari Tayyebeh Madrakian Hamed Ghaedi Majid Rezaeivala 《Analytica chimica acta》2013
A new chemically modified carbon paste electrode was constructed and used for rapid, simple, accurate, selective and highly sensitive simultaneous determination of cadmium, copper and mercury using square wave anodic stripping voltammetry (SWASV). The carbon paste electrode was modified by N,N′-bis(3-(2-thenylidenimino)propyl)piperazine coated silica nanoparticles. Compared with carbon paste electrode, the stripping peak currents had a significant increase at the modified electrode. Under the optimized conditions (deposition potential, −1.100 V vs. Ag/AgCl; deposition time, 60 s; resting time, 10 s; SW frequency, 25 Hz; pulse amplitude, 0.15 V; dc voltage step height, 4.4 mV), the detection limit was 0.3, 0.1 and 0.05 ng mL−1 for the determination of Cd2+, Cu2+ and Hg2+, respectively. The complexation reaction of the ligand with several metal cations in methanol was studied and the stability constants of the complexes were obtained. The effects of different cations and anions on the simultaneous determination of metal ions were studied and it was found that the electrode is highly selective for the simultaneous determination of Cd2+, Cu2+ and Hg2+. Furthermore, the present method was applied to the determination of Cd2+, Cu2+ and Hg2+ in water and some foodstuff samples. 相似文献
54.
S. Abboudy A. E. Hamed M. E. Kassem L. I. Abou-El-Nasr 《Journal of Thermal Analysis and Calorimetry》1993,39(3):301-308
The influence of stoichiometric ratio on the phase transition of sodium potassium sulphate (NaxK1–x)2SO4 crystals is investigated in the temperature range 300–500 K. The stoichiometric ratiox is chosen to bex=0.2, 0.3, 0.4, 0.8 and 0.9. The study has been carried out by using differential scanning calorimetry, DSC technique. The value of the specific heat,C
p, at the transition temperature,T
c
, increases asx is increased up tox=0.4 and then decreases for higher values ofx. The change in the specific heat, C
p, is following a relation of the form C
p C
p (T–T
c
/T
c
)– with = 0.12 which is in a fair agreement with the 3d Ising model. Insertion of Cu2+ ions into the lattice of (NaxK1–x)2SO4 crystals leads to a multiple peak in the temperature dependence ofC
p. The results are discussed from thermodynamical point of view.We would like to thank Prof. E. F. El-Wahidy, Prof. of Solid State Physics and Head of Physics Department, Faculty of Science, Alexandria University, for his interest and support of this work. 相似文献
55.
Mostafa A. Hamed 《Experimental Mechanics》1987,27(3):250-254
A basic theory of acoustical speckle and cross-correlation is described which utilizes the ubiquitous wave characteristics
in ultrasonics. A computer-based pulse-echo system has been developed. It is interfaced to a VAX11/780 computer which operates
the system and performs the data analysis. Data from three tests using specimens of steel, aluminum and Plexiglas are discussed.
On each specimen, a pulse-echo scan of a scattering surface is made over some area which represents reference configuration.
After the surface is displaced, a pulse-echo scan is made over the same area. A small region of the displaced configuration
is correlated with the reference as a measure of object motion. 相似文献
56.
Mohamed Rafat Mahmoud Maher Mohamed Ahmed Hamed Ali Mohamed Shaker 《Journal of solution chemistry》1986,15(9):765-776
Kinetics of base hydrolysis of new heterocyclic azomethines derived from active methyl quaternary salts and aromatic nitroso compounds were investigated in the presence of 70% (wt/wt) water-methanol. The base hydrolysis of these compounds is strictly first-order with respect to OH– and azomethine. The rate determining step is suggested to be the attack of the hydroxide ion on the free base. Effects of water content and nature of organic hydroxylic solvent have been studied. It is concluded that specific solute-solvent interactions through dispersion forces play a major role in the base hydrolysis rate of the azomethines investigated. The effect of pH (2.98 – 12.24) on hydrolysis rates of compounds having a diethylamino substituent in the presence of 30% methanol has been studied. In acidic media, the rate determining step is probably the water attack on the protonated substrate. 相似文献
57.
58.
Ali Sarafraz-Yazdi Soroosh Sepehr Zarrin Es’haghi Hamed Piri Moghadam 《Chromatographia》2010,72(9-10):923-931
In this research, the sol–gel technology was applied for the preparation of solid-phase microextraction fibers for extracting of methyl tert-butyl ether (MTBE) from environmental water samples. For this purpose, two different polymers such as poly(ethylene glycol) (PEG) and combination of PEG and multiwall carbon nanotubes (MWCNTs) were prepared using sol–gel technology as coating procedure for the fibers. The pre-concentration process followed by GC–FID determination was used and the results evidenced that pre-concentration factor for PEG/CNTs fiber was approximately five times higher than PEG. Parameters affecting the extraction efficiency such as temperature, extraction time, stirring speed and salt effect for each fiber were investigated and optimized. On the optimal conditions, the linear range for MTBE with PEG and PEG/CNT fibers were 10–3,000 and 1–1,000 ng mL?1 and the detection limits (S/N = 3) were 1.0 and 0.3 ng mL?1, respectively. The sol–gel PEG/CNTs fiber has good performance and therefore relatively better figures of merit and experimental results such as thermal stability (up to 320 °C), average of life time (over 150 times) and repeatability (RSD < 4) in comparison to conventional PDMS/Carboxen fiber, which was already reported for determination of MTBE. 相似文献
59.
Seyyed Hossein Asadpour Mostafa Sahrai Rasoul Sadighi-Bonabi Amir Soltani Hamed Mahrami 《Physica E: Low-dimensional Systems and Nanostructures》2011,43(10):1759-1762
The giant Kerr nonlinearity with reduced linear and nonlinear absorption in a four-level quantum dot by employing the tunnel coupling is investigated. It is shown that by enhancement of tunnel coupling value the Kerr nonlinearity increases and at the same time linear and nonlinear absorption reduces at the long wavelength which is very important for communicational applications. Enhanced of Kerr nonlinearity in a double quantum dots is investigated. It is found that the electron tunneling has an essential role to reducing the linear absorption and increasing the Kerr nonlinearity at long wavelength. 相似文献
60.
One of the most important concerns for managing public health is the prevention of infectious diseases. Although vaccines provide the most effective means for preventing infectious diseases, there are two main reasons why it is often difficult to reach a socially optimal level of vaccine coverage: (i) the emergence of operational issues (such as yield uncertainty) on the supply side, and (ii) the existence of negative network effects on the consumption side. In particular, uncertainties about production yield and vaccine imperfections often make manufacturing some vaccines a risky process and may lead the manufacturer to produce below the socially optimal level. At the same time, negative network effects provide incentives to potential consumers to free ride off the immunity of the vaccinated population. In this research, we consider how a central policy-maker can induce a socially optimal vaccine coverage through the use of incentives to both consumers and the vaccine manufacturer. We consider a monopoly market for an imperfect vaccine; we show that a fixed two-part subsidy is unable to coordinate the market, but derive a two-part menu of subsidies that leads to a socially efficient level of coverage. 相似文献