全文获取类型
收费全文 | 1564篇 |
免费 | 51篇 |
国内免费 | 6篇 |
专业分类
化学 | 1125篇 |
晶体学 | 10篇 |
力学 | 42篇 |
数学 | 135篇 |
物理学 | 309篇 |
出版年
2022年 | 19篇 |
2021年 | 35篇 |
2020年 | 35篇 |
2019年 | 20篇 |
2018年 | 23篇 |
2017年 | 20篇 |
2016年 | 37篇 |
2015年 | 33篇 |
2014年 | 37篇 |
2013年 | 72篇 |
2012年 | 87篇 |
2011年 | 123篇 |
2010年 | 54篇 |
2009年 | 45篇 |
2008年 | 94篇 |
2007年 | 79篇 |
2006年 | 90篇 |
2005年 | 103篇 |
2004年 | 80篇 |
2003年 | 59篇 |
2002年 | 59篇 |
2001年 | 25篇 |
2000年 | 21篇 |
1999年 | 18篇 |
1998年 | 22篇 |
1997年 | 32篇 |
1996年 | 25篇 |
1995年 | 13篇 |
1994年 | 11篇 |
1993年 | 14篇 |
1992年 | 20篇 |
1991年 | 13篇 |
1989年 | 8篇 |
1988年 | 12篇 |
1987年 | 12篇 |
1986年 | 7篇 |
1985年 | 17篇 |
1984年 | 14篇 |
1983年 | 12篇 |
1982年 | 8篇 |
1981年 | 5篇 |
1980年 | 10篇 |
1979年 | 5篇 |
1978年 | 17篇 |
1977年 | 10篇 |
1976年 | 12篇 |
1975年 | 6篇 |
1974年 | 7篇 |
1972年 | 5篇 |
1971年 | 7篇 |
排序方式: 共有1621条查询结果,搜索用时 109 毫秒
101.
Langone Marta A. P. De Abreu Melissa E. Rezende Michelle J. C. Sant’Anna Geraldo L. 《Applied biochemistry and biotechnology》2002,98(1-9):987-996
The synthesis of monocaprin, monolaurin, and monomyristin in a solvent-free system was conducted by mixing a commercial immobilized
lipase with the organic reactants (glycerol and fatty acids) in a 20-mL batch reactor with constant stirring. The effects
of temperature, fatty acid/glycerol molar ratio, and enzyme concentration on the reaction conversion were determined. The
addition of molecular sieves in the assays of monomyristin synthesis was also evaluated. The reactions were carried out for
5 to 6 h and the nonpolar phase was analyzed by gas chromatography. The best results in terms of selectivity and conversion
(defined as the percentage of fatty acid consumed) were achieved when the stoichiometric amount of reagents (molar ratio=1)
and 9% (w/w) commercial enzyme were used and the reaction was performed at 60°C. The addition of molecular sieves did not
improve the synthesis of monomyristin. Conversions as high as 80%, with monoglycerides being the major products, were attained.
After 5 h of reaction, the concentration of monoglyceride was about twice that of diglyceride, and only trace amounts of triglyceride
were found. The results illustrate the technical possibility of producing medium chain monoglycerides in a solvent-free medium
using a simple batch reactor. 相似文献
102.
Bobette D. Nourse Robert L. Hettich Michelle V. Buchanan 《Journal of the American Society for Mass Spectrometry》1993,4(4):296-305
Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA Differentiation of the seven isomers of methyl guanine has been accomplished by monitoring gas-phase hydrogen/deuterium (H/D) exchange reactions of the protonated molecular ions with deuterium oxide (D2O) in a Fourier transform mass spectrometer. In each case a distinctive reaction rate for the first H/D exchange was observed, and exchanges of up to three deuterium atoms occurred with characteristic ion abundances that could be used to differentiate the isomers. O6-Methyl guanine, for example, showed only one slow H/D exchange with D2O, whereas l-methyl guanine exchanged two hydrogen atoms at a significantly faster rate. On comparison of the possible resonance structures of each protonated isomer with the experimental information about the number and rate of H/D exchanges observed, a reaction mechanism involving a concerted proton abstraction-deuterium cation donation was proposed. 相似文献
103.
104.
Carmen L. Huffman Melissa Lee Williams D. Michelle Benoist Richard E. Overstreet Emily E. Jellen‐McCullough 《Rapid communications in mass spectrometry : RCM》2011,25(16):2299-2306
In collision‐induced dissociation mass spectrometry experiments, the collision energy required for dissociation linearly depends on the degrees of freedom in the precursor ion. The magnitude of the slope of this relationship previously has been shown to qualitatively correlate to the relative binding strength of a noncovalently bound, monovalent complex. The goal of the work presented here is to determine if a similar methodology can be applied for assessing relative binding strengths in multivalent species. We have tested the method on complexes formed from 18‐crown‐6 and a variety of protonated, primary alkylamines, [CnH2n+1NH3]+ (n = 9, 12, 14, 16 and 18) and alkyldiamines, [H3NCnH2nNH3]2+ (n = 3, 5, 6, 9 and 12), and compared our results with dissociation energies calculated using density functional theory at the B3LYP/6‐31G* level. We found that the method correctly assessed the stronger crown ether/headgroup interaction in the two divalent species (1:1 and 2:1 complexes formed from the diaminoalkanes) compared with the weaker interaction in the monovalent species (1:1 complexes formed from mono‐aminoalkanes). However, the experimental method could not distinguish between the binding strengths of the two divalent complexes, perhaps because their calculated dissociation energies were quite similar. Our preliminary results suggest that this method could potentially be used for a quick and simple analysis of binding strengths in multivalent species if the binding strengths of the species are significantly different from one another. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
105.
Placement and characterization of pairs of luminescent molecules in spatially separated regions of nanostructured thin films 总被引:1,自引:0,他引:1
Minoofar PN Hernandez R Chia S Dunn B Zink JI Franville AC 《Journal of the American Chemical Society》2002,124(48):14388-14396
Methods of making mesostructured sol-gel silicate thin films containing two different molecules deliberately placed in two different spatially separated regions in a one-step, one-pot preparation are developed and demonstrated. When the structure-directing agent is the surfactant cetyltrimethylammonium bromide, the structure is 2-D hexagonal with lattice spacings between 31.6 and 42.1 angstroms depending on the dopant molecules and their concentrations. The three general strategies that are used to place the molecules are philicity (like dissolves like), bonding, and bifunctionality. These strategies take advantage of the different chemical and physical properties of the regions of the films. These regions are the inorganic silicate framework, the hydrophobic organic interior of the micelles, and the ionic interface between them. Luminescent molecules that possess the physical and chemical properties appropriate for the desired strategies are chosen. Lanthanide and ruthenium complexes with condensable trialkoxysilane groups are incorporated into the silicate framework. 1,4-Naphthoquinone, pyrene, rhodamine 6G and coumarin 540A, and lanthanides with no condensable trialkoxysilanes occupy the hydrophobic core of micelles by virtue of their hydrophobicity. The locations of the molecules are determined by luminescence spectroscopy and by luminescence lifetime measurements. In all cases, the long-range order templated into the thin film is verified by X-ray diffraction. The simultaneous placement of two molecules in the structured film and the maintenance of long-range order require a delicate balance among film preparation methodology, design of the molecules to be incorporated in specific regions, and concentrations of all of the species. 相似文献
106.
We demonstrate strong lasing on the Ni-like 4d(1)S(0)?4p(1)P(1) transition at 18.9, 20.3, 22.0, and 24.0 nm for Mo, Nb, Zr, and Y ions, respectively, using the transient collisional excitation scheme. Approximately 5 J of laser energy in a combination of a 600-ps pulse and a 1-ps pulse from the Compact Multipulse Terawatt (COMET) tabletop laser system is used to irradiate slab targets of these materials. Small-signal gains of 17-26cm (-1) are determined on the 4d?4p transition, with overall gain-length products gL of 11-12. Lasing is observed and gain is measured on the 4f(1)P(1)?4d(1)P(1) transition, which is pumped by collisional excitation combined with self-photopumping, for what is to our knowledge the first time. 相似文献
107.
Continuous-wave optical parametric oscillator based on periodically poled KTiOPO(4) and its application to spectroscopy 总被引:1,自引:0,他引:1
We report a continuous-wave, doubly resonant optical parametric oscillator (OPO) based on the nonlinear material periodically poled KTiOPO(4) and its application to spectroscopy. The OPO, which is pumped by a diode-pumped frequency-doubled Nd:YLF laser at 523 nm, has a low pump-power threshold of 25 mW and can deliver 10 mW of single-frequency output at 1.65 mum for a pump power of 200 mW. The idler wavelength can be temperature tuned at a rate of 0.73 nm/( degrees )C , and smooth tuning of the output frequency over ~3 GHz is achieved by smooth tuning of the pump laser. We demonstrate the practicality of the OPO by recording the absorption spectrum of methane near 1649 nm. 相似文献
108.
Experiments were conducted to assess the ability of a particle-counter-sizer velocimeter (PCSV) to measure droplet speeds in the developing region of an electrohydrodynamic spray. The working fluid, ethanol, was pumped through a stainless steel capillary tube at a constant mass flow rate. The capillary tube was held at various positive potential voltages with respect to an externally grounded funnel located below the tube. Data were acquired at two axial stations, directly under the capillary tip, in the developing region of the subject spray. The results indicated that the PCSV, with an experimental modification to the speed reduction process, performed well in this region. Speeds were measured at the near station within an average uncertainty of 2.2% and a corresponding maximum uncertainty of 3.0%; speeds at the far station were measured within an average uncertainty of 4.6% and a corresponding maximum uncertainty of 6.0%. 相似文献
109.
Kevin M. Dunn Péter Pulay Christian Van Alsenoy James E. Boggs 《Journal of Molecular Spectroscopy》1984,103(2):268-280
The geometry, complete harmonic force field, and dipole moment derivatives of cubane, C8H8, have been calculated at the Hartree-Fock level using a 4–21 Gaussian basis set. The infrared and Raman spectra of cubane and four deuterated derivatives were calculated and compared with previously observed spectra. A set of five scale factors for the calculated force constants was then derived by least-squares fitting of the fundamental vibrational frequencies calculated from the scaled force field to the frequencies obtained by direct experimental measurement. The resulting scaled quantum-mechanical (SQM) force field, containing 73 unique elements, is believed to give an accurate representation of the harmonic vibrational potential of cubane. In most cases, the spectral assignments previously made from purely empirical considerations were confirmed, but a few corrections are proposed. The only major alteration is for an A2u mode revised to appear at 1030 cm?1 in the undeuterated molecule. Coriolis constants and approximate infrared intensities are also calculated. 相似文献
110.