全文获取类型
收费全文 | 2309篇 |
免费 | 121篇 |
国内免费 | 13篇 |
专业分类
化学 | 1621篇 |
晶体学 | 32篇 |
力学 | 50篇 |
数学 | 207篇 |
物理学 | 533篇 |
出版年
2023年 | 12篇 |
2022年 | 23篇 |
2021年 | 44篇 |
2020年 | 34篇 |
2019年 | 46篇 |
2018年 | 22篇 |
2017年 | 31篇 |
2016年 | 67篇 |
2015年 | 72篇 |
2014年 | 81篇 |
2013年 | 145篇 |
2012年 | 129篇 |
2011年 | 158篇 |
2010年 | 92篇 |
2009年 | 93篇 |
2008年 | 132篇 |
2007年 | 125篇 |
2006年 | 93篇 |
2005年 | 85篇 |
2004年 | 85篇 |
2003年 | 60篇 |
2002年 | 84篇 |
2001年 | 48篇 |
2000年 | 75篇 |
1999年 | 33篇 |
1998年 | 41篇 |
1997年 | 28篇 |
1996年 | 28篇 |
1995年 | 37篇 |
1994年 | 33篇 |
1993年 | 32篇 |
1992年 | 29篇 |
1991年 | 37篇 |
1990年 | 33篇 |
1989年 | 24篇 |
1988年 | 26篇 |
1987年 | 20篇 |
1986年 | 21篇 |
1985年 | 26篇 |
1984年 | 29篇 |
1983年 | 17篇 |
1982年 | 19篇 |
1981年 | 17篇 |
1980年 | 10篇 |
1979年 | 6篇 |
1978年 | 8篇 |
1977年 | 12篇 |
1976年 | 7篇 |
1975年 | 7篇 |
1974年 | 6篇 |
排序方式: 共有2443条查询结果,搜索用时 62 毫秒
31.
Garcia AL Ista LK Petsev DN O'Brien MJ Bisong P Mammoli AA Brueck SR López GP 《Lab on a chip》2005,5(11):1271-1276
This report presents a study of electrokinetic transport in a series of integrated macro- to nano-fluidic chips that allow for controlled injection of molecular mixtures into high-density arrays of nanochannels. The high-aspect-ratio nanochannels were fabricated on a Si wafer using interferometric lithography and standard semiconductor industry processes, and are capped with a transparent Pyrex cover slip to allow for experimental observations. Confocal laser scanning microscopy was used to examine the electrokinetic transport of a negatively charged dye (Alexa 488) and a neutral dye (rhodamine B) within nanochannels that varied in width from 35 to 200 nm with electric field strengths equal to or below 2000 V m-1. In the negatively charged channels, nanoconfinement and interactions between the respective solutes and channel walls give rise to higher electroosmotic velocities for the negatively charged dye than for the neutral dye, towards the negative electrode, resulting in an anomalous separation that occurs over a relatively short distance (<1 mm). Increasing the channel widths leads to a switch in the electroosmotic transport behavior observed in microscale channels, where neutral molecules move faster because the negatively charged molecules are slowed by the electrophoretic drag. Thus a clear distinction between "nano-" and "microfluidic" regimes is established. We present an analytical model that accounts for the electrokinetic transport and adsorption (of the neutral dye) at the channel walls, and is in good agreement with the experimental data. The observed effects have potential for use in new nano-separation technologies. 相似文献
32.
J.M. Pingarron Carrazon A. Gordon Vergara A.J. Reviejo Garcia L.M. Polo Diez 《Analytica chimica acta》1989
An electroanalytical study of the oxidation processes of umbelliferone and hymecromone at a glassy carbon electrode in micellar solution and emulsified medium by different voltammetric techniques is described. The non-ionic surfactant Triton X-405 in acetate-buffered medium at pH 4.8 was found to be the most suitable. Different ranges of linearity were obtained in the micellar solutions, depending on the technique used; the limits of determination for differential pulse voltammetry (DPV) at a stationary electrode were 2.9×10?6 mol l?1 and 3.3×10?6 mol l?1 for umbelliferone and hymecromone, respectively. In the emulsified medium formed with a mixture of toluene and ethyl acetate (3:2), the oxidation processes yielded similar results. With DPV, linear calibration plots were obtained in the ranges 1.0×10?5–9.0×10?7 mol l?1 umbelliferone and 1.0×10?5–2.0×10?6 mol l?1 hymecromone. The media used are predominantly aqueous so that special reference electrodes and solvent purification are not needed. 相似文献
33.
Picquart M. López T. Gómez R. Torres E. Moreno A. Garcia J. 《Journal of Thermal Analysis and Calorimetry》2004,76(3):755-761
Sol-gel zirconia was characterized using high-resolution thermogravimetry (Hi-Res TG) and differential scanning calorimetry
(DSC) and compared with X-ray diffraction, Raman scattering, and UV-Vis spectroscopy. ZrO2-x(OH)2x·yH2O annealed below 400°C show typical behavior of amorphous material. As the annealing temperature is increased, the tetragonal
and monoclinic phases crystallize. Typical Hi-Res TG curve shows that the samples are continuously dehydrated in a long temperature
range, between room temperature and 600°C. The total mass loss relative to the initial mass is of about 29%. The DSC analysis
coupled with TG and structural information, indicate that the exothermic processes about 355 and 447°C can be related to the
nucleation process of the formation of tetragonal zirconia, with bulk crystallization at 447°C.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
34.
Josefina Garcia Robert Greenhouse Joseph M Muchowski Jose Antonio Ruiz 《Tetrahedron letters》1985,26(15):1827-1830
2-Alkyl-3-cyanoindoles are obtained when 1-alkylmethyl-2-chloro-(or 2-phenylsulfonyl)-3-phenylsulfonylindoles are reacted with excess azide ion (90°/DMF). The reaction is considered to occur by a fragmentation recombination process in which the Schiff's base is of central importance. This proposal is supported by the formation of 2-substituted indole-3-carboxylates from aldehydes and the α-phenylsulfonyl--aminophenylacetic acid ester derivative . 相似文献
35.
F. M. Bautista J. M. Campelo A. Garcia D. Luna J. M. Marinas A. A. Romero M. R. Urbano 《Reaction Kinetics and Catalysis Letters》1997,62(1):47-54
The vapor-phase catalytic alkylation of phenol with methanol and dimethyl carbonate on a series of differently prepared CrPO4 (Cr/P=1) and CrPO4-AlPO4 (CrAIP) catalysts, has been studied at different temperatures (473–673 K). The reaction is first order in phenol, giving
a mixture of O- and C-alkylated products (C-alkylation taking place preferentially at theortho-position). Moreover, dimethyl carbonate is a better methylating agent than methanol. 相似文献
36.
The intergrowth of 1,10-decanedicarboxylic acid and urea give infinite hydrogen-bonded chains of the guest included in the hexagonal urea host. A deuterium high-resolution solid-state NMR study of the selectively deuterated intergrowth compound 1,10-decanedicarboxylic acid/hydrogenated urea at variable temperature in the range 90 < or = T < or = 300 K was performed on a single crystal. The analysis of the second moment as a function of temperature is shown to be compatible with the known phase transition occurring near T(c) = 203 K. Moreover, the spectra indicate that the orientational disorder is strong, and is compared to an axial uniform disorder. For this purpose, the general equation for the second moment of a system with uniform two-dimensional axial orientational disorder is given, and a method to take into account the non-uniform excitation of the pulse sequence is proposed. 相似文献
37.
Cimpeanu V Parvulescu VI Amorós P Beltrán D Thompson JM Hardacre C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(18):4640-4646
Heterogeneous catalytic oxidation of a series of thioethers (2-thiomethylpyrimidine, 2-thiomethyl-4,6-dimethyl-pyrimidine, 2-thiobenzylpyrimidine, 2-thiobenzyl-4,6-dimethylpyrimidine, thioanisole, and n-heptyl methyl sulfide) was performed in ionic liquids by using MCM-41 and UVM-type mesoporous catalysts containing Ti, or Ti and Ge. A range of triflate, tetrafluoroborate, trifluoroacetate, lactate and bis(trifluoromethanesulfonyl)imide-based ionic liquids were used. The oxidations were carried out by using anhydrous hydrogen peroxide or the urea-hydrogen peroxide adduct and showed that ionic liquids are very effective solvents, achieving greater reactivity and selectivity than reactions performed in dioxane. The effects of halide and acid impurities on the reactions were also investigated. Recycling experiments on catalysts were carried out in order to evaluate Ti leaching and its effect on activity and selectivity. 相似文献
38.
M. E. Garcia G. M. Pastor K. H. Bennemann 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1993,26(1):293-295
The electronic properties of neutral and ionized divalent-metal clusters have been studied using a microscopic theory, which takes into account the interplay between van der Waals (vdW) and covalent bonding in the neutral clusters, and the competition between hole delocalization and polarization energy in the ionized clusters. By calculating the ground-state energies of neutral and ionized Hg n clusters, we determine the size dependence of the bond character and the ionization potentialI p (n). For neutral Hg n clusters we obtain a transition from van der Waals to covalent behaviour at the critical sizen c ~10–20 atoms. Results forI p (Hg n ) withn≤20 are in good agreement with experiments, and suggest that small Hg n + clusters can be viewed as consisting of a positive trimer core Hg 3 + surrounded byn?3 polarized neutral atoms. 相似文献
39.
The reaction of several α-amino acids and peptides (containing Gly, L-Ala, L-Leu, L- or DL-Phe, and/or L- or D-Val) with air-diluted nitrogen oxides has been studied to roughly mimic the N-nitrosation of peptide bonds that the contaminated urban air might produce in pulmonary tissues. Most N-protected α-amino acids give practically quantitative yields of N-nitroso derivatives. N-Protected dipeptides afford either dinitrosated peptides, mixtures of di- and mononitrosated compounds, selectively mononitrosated products, or no reaction at all, depending mainly on steric effects. The same trends are observed for some higher peptides. The (po1y)nitrosated Peptides, which retain the chirality of the starting materials, have been characterized by 1H and 13C NMR spectroscopy and are cleaved by pyrrolidine and amino esters under mild conditions to give (new) amides or peptides plus diazo derivatives. 相似文献
40.
Mohamed Mongi Ftini Mohamed Krifa Haddad Amor 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(8):i106-i108
The title compounds, potassium pentamolybdenum oxide, KMo5O13, and potassium niobate antimonate or potassium niobium antimony oxide (1/1.76/3.24), KNb1.76Sb3.24O13, were synthesized by solid‐state reactions and are isomorphous in space group Cmcm. The structure of the Mo complex has three unique Mo atoms and consists of MoO6 octahedra sharing edges to form Mo2O6 pairs and Mo3O9 triplets, which, in turn, form layers by sharing corners. These layers are linked together in the [100] direction, yielding a three‐dimensional network similar to that of KSb5O13. This framework delimits interconnected tunnels, running approximately along the [110] and [10] directions, in which K+ ions are located. In the isomorphous KNb1.76Sb3.24O13 structure, one of the Mo sites in KMo5O13 is replaced by Sb and the other two Mo sites have been replaced by Nb/Sb. 相似文献