全文获取类型
收费全文 | 276篇 |
免费 | 7篇 |
国内免费 | 2篇 |
专业分类
化学 | 163篇 |
晶体学 | 6篇 |
力学 | 19篇 |
数学 | 12篇 |
物理学 | 85篇 |
出版年
2023年 | 3篇 |
2022年 | 17篇 |
2021年 | 17篇 |
2020年 | 5篇 |
2019年 | 6篇 |
2018年 | 10篇 |
2017年 | 6篇 |
2016年 | 8篇 |
2015年 | 4篇 |
2014年 | 7篇 |
2013年 | 12篇 |
2012年 | 12篇 |
2011年 | 27篇 |
2010年 | 11篇 |
2009年 | 15篇 |
2008年 | 7篇 |
2007年 | 8篇 |
2006年 | 7篇 |
2005年 | 15篇 |
2004年 | 7篇 |
2003年 | 12篇 |
2002年 | 9篇 |
2001年 | 8篇 |
2000年 | 2篇 |
1999年 | 1篇 |
1998年 | 1篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1987年 | 4篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1980年 | 4篇 |
1979年 | 1篇 |
1978年 | 5篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有285条查询结果,搜索用时 31 毫秒
91.
K. Prabhakaran K. V. P. M. Shafi Y. Yamauchi K. Tsubaki A. Ulman Y. Homma T. Ogino 《Applied Surface Science》2002,190(1-4):161-165
Here, we demonstrate a “plug and play” approach to achieve multi-functionalization of Si. In this approach, externally synthesized functional nanoparticles are introduced onto device quality Si wafers and the surface chemical bonds are manipulated. Sonochemically synthesized Fe2O3 nanoparticles are introduced onto Si from an alcohol suspension. On annealing this sample in ultra-high vacuum, the oxygen atoms change the bonding partner from Fe to Si and desorb as SiO at 750 °C. This results in the formation of nanoparticles of Fe on the surface and exhibits ferromagnetic behavior. Deposition of a thin layer (2 nm) of Si onto the sample containing the metallic Fe nanoparticles followed by annealing at 560 °C leads to optically active Si. Photoluminescence measurements show that this sample emits light at three different wavelengths, namely 1.57, 1.61 and 1.63 μm, when excited by He–Ne or Ar lasers. Oxidation of this material results in the formation of a selective capping layer of SiO2. Thus we obtain multi-functional Si in an “all in one” form and we believe that this approach is universal. 相似文献
92.
We study the long-distance behaviour of pure unified SU(5) gauge theory in the limit when the electroweak subgroup is unbroken. We show that the symmetry breaking pattern SU(5)→SU(3)c×SU(2)×U(1)Y, with SU(3)c and SU(2) ×U(1)Y realized, respectively, in confining and coulombic phases, is a possible dynamical phase of the SU(5) theory. The proof relies on showing that the duality equation of 't Hooft, relating the electric and magnetic flux, is exactly satisfied for the above symmetry breaking pattern. The infrared structure of SU(5), broken down to SU(3)c×SU(2)×U(1)Y, is not self-dual. 相似文献
93.
N. G. Kozlenko V. V. Abaev V. S. Bekrenev S. P. Kruglov A. A. Koulbardis I. V. Lopatin A. B. Starostin B. Draper S. Hayden J. Huddleston D. Isenhower C. Robinson M. Sadler C. Allgower R. Cadman H. Spinka J. Comfort K. Craig A. Ramirez T. Kycia M. Clajus A. Marusic S. McDonald B. M. K. Nefkens N. Phaisangittisakul S. Prakhov J. W. Price W. B. Tippens J. Peterson W. J. Briscoe A. Shafi I. I. Strakovsky H. Staudenmaier D. M. Manley J. Olmsted D. Peaslee N. Knecht G. Lolos Z. Papandreou I. Supek I. Slaus A. Gibson D. Grosnic D. Koetke R. Manweiler S. Stanislaus 《Physics of Atomic Nuclei》2003,66(1):110-113
The first results obtained in 1998 by the Crystal Ball collaboration from a measurement of the total and differential cross sections for the reaction π ? p → ηn are presented. These new experimental results for the total cross sections are compared with the predictions of the K-matrix model for pion-nucleon scattering. The angular distribution at momenta near the reaction threshold (685 MeV/c) is determined by the S-wave contribution. The P-wave contribution begins to manifest itself from a momentum of 720 MeV/c. 相似文献
94.
M. Siddique Rao Tahir Ali Khan M. Shafi 《Journal of Radioanalytical and Nuclear Chemistry》2008,277(3):531-537
Mössbauer effect technique has been used for the comparative study of Cu1?x Zn x Fe2O4 and Cu1?x Cd x Fe2O4 ( x = 0.0?1.0) ferrites. Both Zn2+ and Cd2+ cations are divalent, non-magnetic ions with different ionic radii. With the substitution of these non-magnetic cations the average internal magnetic field decreases and paramagnetic behavior is dominated at x = 0.7 in both series. It is observed that the occupancy of Cu2+ ions for tetrahedral site is not constant for all compositions but fluctuate between 8–15%. It is also found that Cu2+ ions have more preference for tetrahedral site in Cu-Zn system as compared to the Cu-Cd system. Zn2+ and Cd2+ both ions occupy tetrahedral site completely and form normal spinels for x = 1.0. 相似文献
95.
This review covers the biosynthesis of aminocyclitol-aminoglycoside antibiotics and related compounds, particularly from the molecular genetic perspectives. 195 references are cited. 相似文献
96.
Shahid P. Shafi Rylan J. Lundgren Lachlan M.D. Cranswick Mario Bieringer 《Journal of solid state chemistry》2007,180(12):3333-3340
We report the preparation and stability of ScVO3.5+x and the novel phase InVO3.5+x. AVO3.5+x (A=Sc, In) defect fluorite structures are formed as metastable intermediates during the topotactic oxidation of AVO3 bixbyites. The oxidation pathway has been studied in detail by means of thermogravimetric/differential thermal analysis and in-situ powder X-ray diffraction. The oxidation of the bixbyite phase follows a topotactic pathway at temperatures between 300 and 400 °C in air/carbon dioxide. The range of accessible oxygen stoichiometries for the AVO3.5+x structures following this pathway are 0.00x0.22. Rietveld refinements against powder X-ray and neutron data revealed that InVO3.54 and ScVO3.70 crystallize in the defect fluorite structure in space group Fm-3 m (227) with a=4.9863(5) and 4.9697(3)Å, respectively with A3+/V4+ disorder on the (4a) cation site. Powder neutron diffraction experiments indicate clustering of oxide defects in all samples. Bulk magnetic measurements showed the presence of V4+ and the absence of magnetic ordering at low temperatures. Powder neutron diffraction experiments confirmed the absence of a long range ordered magnetic ground state. 相似文献
97.
Shakeel Ahmad Khan Shafi Ullah Khan Fozia Najeeb Ullah Mohibullah Shah Riaz Ullah Ijaz Ahmad Amal Alotaibi 《Molecules (Basel, Switzerland)》2021,26(7)
Admittedly, the disastrous emergence of drug resistance in prokaryotic and eukaryotic human pathogens has created an urgent need to develop novel chemotherapeutic agents. Onosma chitralicum is a source of traditional medicine with cooling, laxative, and anthelmintic effects. The objective of the current research was to analyze the biological potential of Onosma chitralicum, and to isolate and characterize the chemical constituents of the plant. The crude extracts of the plant prepared with different solvents, such as aqueous, hexane, chloroform, ethyl acetate, and butanol, were subjected to antimicrobial activities. Results corroborate that crude (methanol), EtoAc, and n-C6H14 fractions were more active against bacterial strains. Among these fractions, the EtoAc fraction was found more potent. The EtoAc fraction was the most active against the selected microbes, which was subjected to successive column chromatography, and the resultant compounds 1 to 7 were isolated. Different techniques, such as UV, IR, and NMR, were used to characterize the structures of the isolated compounds 1–7. All the isolated pure compounds (1–7) were tested for their antimicrobial potential. Compounds 1 (4′,8-dimethoxy-7-hydroxyisoflavone), 6 (5,3′,3-trihydroxy-7,4′-dimethoxyflavanone), and 7 (5′,7,8-trihydroxy-6,3′,4′-trimethoxyflavanone) were found to be more active against Staphylococcus aureus and Salmonella Typhi. Compound 1 inhibited S. typhi and S. aureus to 10 ± 0.21 mm and 10 ± 0.45 mm, whereas compound 6 showed inhibition to 10 ± 0.77 mm and 9 ± 0.20 mm, respectively. Compound 7 inhibited S. aureus to 6 ± 0.36 mm. Compounds 6 and 7 showed significant antibacterial potential, and the structure–activity relationship also justifies their binding to the bacterial enzymes, i.e., beta-hydroxyacyl dehydratase (HadAB complex) and tyrosyl-tRNA synthetase. Both bacterial enzymes are potential drug targets. Further, the isolated compounds were found to be active against the tested fungal strains. Whereas docking identified compound 7, the best binder to the lanosterol 14α-demethylase (an essential fungal cell membrane synthesizing enzyme), reported as an antifungal fluconazole binding enzyme. Based on our isolation-linked preliminary structure-activity relationship (SAR) data, we conclude that O. chitralicum can be a good source of natural compounds for drug development against some potential enzyme targets. 相似文献
98.
A.K.M. Akther Hossain S.T. Mahmud M. Seki T. Kawai H. Tabata 《Journal of magnetism and magnetic materials》2007
Structural, electrical, and magnetic properties of Ni1−xZnxFe2O4 (x=0.2, 0.4) samples sintered at various temperatures have been investigated thoroughly. The bulk density of the Ni0.8Zn0.2Fe2O4 samples increases as the sintering temperature (Ts) increases from 1200 to 1300 °C and above 1300 °C the bulk density decreases slightly. The Ni0.6Zn0.4Fe2O4 samples show similar behavior of changes to that of Ni0.8Zn0.2Fe2O4 samples, except that the bulk density is found to be the highest at 1350 °C. The DC electrical resistivity, ρ(T), decreases as the temperature increases indicating that the samples have semiconductor-like behavior. As the Zn content increases, the Curie temperature (Tc), resistivity, and the activation energy decrease while the magnetization, initial permeability, and the relative quality factor (Q) increases. A Hopkinson peak is obtained near Tc in the real part of the initial permeability vs. temperature curves. The ferrite with higher permeability has a relatively lower resonance frequency. The initial permeability and magnetization of the samples has been found to correlate with density, average grain sizes. Possible explanation for the observed structural, magnetic, and changes of resistivity behavior with various Zn content are discussed. 相似文献
99.
Krohn K Cludius-Brandt S Schulz B Sreelekha M Shafi PM 《Natural product communications》2011,6(11):1595-1596
Several biologically active alkaloids (1-4, 6), including a new quinazoline-6-carboxylic acid (1), were isolated from the medicinal plant Zanthoxylum rhetsa, an evergreen tree, native to subtropical areas. Whereas the pharmacological properties of the plant extract and single constituents have been widely tested, we now show that all of the metabolites have antialgal activities, all but 6 are antibacterial, and 6 and the reduction product 5 (derived from 4) are also antifungal. 相似文献
100.
Khan IA Khanam AJ Sheikh MS Kabir-ud-Din 《Journal of colloid and interface science》2011,359(2):467-473
Micellization of binary systems of a cationic gemini surfactant butanediyl-1,4-bis(dimethylcetylammonium bromide) (16-4-16) and cationic/nonionic hydrotropes (aniline-hydrochloride, 2-methylanilinehydrochloride, 4-methylanilinehydrochloride, hydroxybenzene, 1,3-benzenediol, benzene-1,2,3-triol) have been studied using a conductometric technique. The critical micelle concentrations (cmc) for different mixing mole fractions at different temperatures have been calculated. To explain and compare the results, theoretical models of Clint, Rubingh and Motomura have been used to obtain the ideal cmc, mixed micelle composition, interaction parameters (β(m)), free energies of micellization, and activity coefficients. The mixtures show nonideal behavior and the interactions between the surfactants and the hydrotropes are synergistic in nature which is confirmed by high negative β(m) values and low values of the activity coefficients. Thermodynamic parameters were also obtained from the temperature dependence of the cmc values. 相似文献