全文获取类型
收费全文 | 602篇 |
免费 | 16篇 |
专业分类
化学 | 403篇 |
晶体学 | 19篇 |
力学 | 5篇 |
数学 | 32篇 |
物理学 | 159篇 |
出版年
2023年 | 7篇 |
2022年 | 13篇 |
2021年 | 17篇 |
2020年 | 20篇 |
2019年 | 21篇 |
2018年 | 16篇 |
2017年 | 10篇 |
2016年 | 20篇 |
2015年 | 14篇 |
2014年 | 26篇 |
2013年 | 31篇 |
2012年 | 37篇 |
2011年 | 40篇 |
2010年 | 29篇 |
2009年 | 46篇 |
2008年 | 44篇 |
2007年 | 30篇 |
2006年 | 23篇 |
2005年 | 31篇 |
2004年 | 15篇 |
2003年 | 14篇 |
2002年 | 26篇 |
2001年 | 6篇 |
2000年 | 8篇 |
1999年 | 3篇 |
1998年 | 1篇 |
1997年 | 10篇 |
1996年 | 4篇 |
1995年 | 3篇 |
1994年 | 3篇 |
1993年 | 9篇 |
1992年 | 7篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 3篇 |
1979年 | 1篇 |
1976年 | 2篇 |
1974年 | 1篇 |
排序方式: 共有618条查询结果,搜索用时 15 毫秒
161.
Concentrations of K, Ca, Cr, Fe, Ni, Cu, Zn, Se, Rb, Sr, Cd, Hg and Pb were measured in the muscle, digestive glands and gonads of Octopus vulgaris, Lolligo vulgaris and Sepia officinalis captured in the west and south coast of Portugal. Essential elements (K, Ca, Fe, Cu, Zn, Se, Rb and Sr) were obtained by energy dispersive x‐ray fluorescence spectrometry, while toxic elements (Cr, Ni, Cd, Hg and Pb) were obtained by atomic absorption spectrometry. The latter technique was used owing to its higher sensitivity, since these elements were not detected by the first technique. The tissues of the three species present a similar pattern for essential elements. Potassium and Ca are present at the highest concentration in all the studied samples. The highest levels for toxic elements were found in the digestive glands, exceeding the maximum allowed values in the case of Se and Cd. However, in the muscle tissue these values were never reached. Comparing both coasts, we can say that the west coast presents higher levels of Pb while the south coast presents enhanced levels of Hg. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
162.
Nunes FM Barros-Filho BA de Oliveira MC Mafezoli J Andrade-Neto M de Mattos MC Silveira ER Pirani JR 《Magnetic resonance in chemistry : MRC》2005,43(2):180-183
One- and two-dimensional NMR experiments were used for the unambiguous assignment of the 1H and 13C NMR chemical shifts of 3,3-diisopentenyl-N-methyl-2,4-quinoldione and five novel reaction derivatives. 相似文献
163.
D.F. de Sousa L.A.O. Nunes J.H. Rohling M.L. Baesso 《Applied physics. B, Lasers and optics》2003,77(1):59-63
Laser emission at 1077 nm (4F3/24I11/2) was demonstrated in Nd3+-doped low silica content calcium aluminosilicate (LSCA) glass under 810-nm pumping by a Ti:sapphire laser. The slope efficiency of the laser emission was found to be 34% with a 20-mW threshold power. The excited-state absorption (ESA) and stimulated emission were measured between 850 and 1520 nm. The results showed two ESA bands from 931 to 1070 nm (4F3/22D3/2,2G9/2,4G11/2,2K15/2) and from 1160 to 1414 nm (4F3/2,4G9/24G7/2,2K13/2). The stimulated emission for the laser transition was found to be 45-nm broad with a peak value of 1.8×10-20 cm2. Thermal and mechanical properties of the LSCA glass were compared to the ED-2 and LHG-8 glasses. While the mechanical properties of the LSCA glass are superior to those of the commercial glasses, its temperature coefficient of optical path length is the highest among the three hosts. PACS 42.60.Pk; 42.70.Ce; 78.66.Jg 相似文献
164.
Investigations on the efficiency of cardiac-gated methods for the acquisition of diffusion-weighted images 总被引:5,自引:0,他引:5
Nunes RG Jezzard P Clare S 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2005,177(1):102-110
Diffusion-weighted images are inherently very sensitive to motion. Pulsatile motion of the brain can give rise to artifactual signal attenuation leading to over-estimation of the apparent diffusion coefficients, even with snapshot echo planar imaging. Such miscalculations can result in erroneous estimates of the principal diffusion directions. Cardiac gating can be performed to confine acquisition to the quiet portion of the cycle. Although effective, this approach leads to significantly longer acquisition times. On the other hand, it has been demonstrated that pulsatile motion is not significant in regions above the corpus callosum. To reduce acquisition times and improve the efficiency of whole brain cardiac-gated acquisitions, the upper slices of the brain can be imaged during systole, reserving diastole for those slices most affected by pulsatile motion. The merits and disadvantages of this optimized approach are investigated here, in comparison to a more standard gating method and to the non-gated approach. 相似文献
165.
166.
Regina C. Reis Nunes Mar Lpez‐Gonzlez Evaristo Riande 《Journal of Polymer Science.Polymer Physics》2005,43(16):2131-2140
This work reports sorption processes of oxygen, carbon dioxide, methane, ethylene, and propylene in films of both vulcanized natural rubber and vulcanized rubber–regenerated cellulose composites. The curves representing the pressure dependence of the concentration of carbon dioxide in the composites clearly exhibit a slight concavity with respect to the abscissa axis as a result of adsorption processes taking place in Langmuir sites located in the glassy cellulose component. Adsorption processes are also detected in the sorption curves of ethylene at low pressures. The concavity with respect to the ordinate axis of the curve concentration of propylene versus pressure at high pressure is pretty well described by the Flory‐Huggins formalism. The solubilities of the other gases mainly obey Henry's behavior, adsorption processes in the glassy component being in most cases negligible. Values of the interaction χ parameter for gas–natural rubber and gas–natural rubber composites are obtained from the comparison of the experimental solubility coefficients with those predicted by the Flory‐Huggins theory. The theory suggests that Henry's constant is a linear function of the boiling temperature of the gases. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2131–2140, 2005 相似文献
167.
O.A.C. Nunes 《Solid State Communications》1982,44(2):177-180
The magnon damping in an electron-magnon system in the presence of a laser beam and a d.c. magnetic field is discussed. It is shown that near the laser-cyclotron resonance, and for the laser beam propagating perpendicularly to the magnetic field the magnon population in a relatively narrow range of k may grow with time. 相似文献
168.
We formulate a general model for the growth of scale-free networks under filtering information conditions-that is, when the nodes can process information about only a subset of the existing nodes in the network. We find that the distribution of the number of incoming links to a node follows a universal scaling form, i.e., that it decays as a power law with an exponential truncation controlled not only by the system size but also by a feature not previously considered, the subset of the network "accessible" to the node. We test our model with empirical data for the World Wide Web and find agreement. 相似文献
169.
Humeres E Nunes RJ Machado VG Gasques MD Machado C 《The Journal of organic chemistry》2001,66(4):1163-1170
The rate constants of the S(N)2 reaction of sodium 4-nitrophenoxide (1) and iodomethane were determined by UV-visible spectrophotometry in acetone-water mixtures at 25, 30, and 35 degrees C. The rate-Xwater (mole fraction of water) profile shows that the reaction depends strongly on the medium. The fastest rate constant was obtained in pure acetone, and a minimum occurred at Xwater= 0.4, whereas the observed second-order rate constants increases again in the water-rich region. In pure acetone, in the presence of dicyclohexano-[18]-crown-6, increases linearly with the concentration of the crown ether as a result of the complexation of the sodium ion (KS = 104.8 M) of the ion-pair and the increase in the effective concentration of free 4-nitrophenoxide ion, which was assumed to be the only reactive species. Ion-pairing was also detected at Xwater= 0.65 with a dissociation constant Kd = 7.82 x 10(-4) M(-1). The solvatochromic behaviors of 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-1-phenoxide (2), 4-[(1-methyl-4(1H)-pyridinylidene)ethylidene]-2,5-cyclohexadien-1-one (3), and 1-methyl-8-oxy-quinolinium betaine (4) were investigated in the entire range of acetone-water mixtures. The dyes presented an increasing order of hydrophilicity compatible with their chemical structure, i.e., 2 < 3 < 4. Kinetic parameters for the methylation of 1 and the ET values of the dyes show a linear correlation of the polarity in the region of Xwater = 1.0-0.40 for 3 and 4, and it was observed that the more hydrophilic the dye the better the correlation coefficient, because of the structural similarity with 1. The activation parameter-Xwater profile shows extrema at Xwater < 0.4, reflecting an important change in the structure of the solvent that is responsible for the changes in the solvation of the reactive species including ion-pairs. These results suggest that the addition of water to acetone reduces abruptly the rate of substitution due to the preferential solvation (PS) of the phenoxide ion by the hydrogen-bonding donor (HBD) solvent. Nevertheless, the real second-order rate constant is "masked" by the association involving Na+ and 4-nitrophenoxide that extends even to water-rich mixtures. A model, based on the assumption that the free-energy terms involved in the second-order rate constant and the dissociation constant of the ion-pair have two components, is invoked to explain the kinetic data. One of the components depends on electrostatic interactions for which the main variable is the dielectric constant of the solvent mixture, and the other depends on the specific solute-solvent interactions, expressed by the activity coefficients of transfer of the species involved. The model indicates that in the range of Xwater = 1.0-0.40 the interactions are exclusively electrostatic, while for the rest of the acetone-rich region they are specific with a large contribution of the 4-nitrophenoxide ion. 相似文献
170.
Ana Luiza Ribeiro de Souza Tatiana Andreani Fernando M. Nunes Douglas Lopes Cassimiro Adélia Emília de Almeida Clóvis Augusto Ribeiro Victor Hugo Vitorino Sarmento Maria Palmira Daflon Gremião Amélia M. Silva Eliana B. Souto 《Journal of Thermal Analysis and Calorimetry》2012,108(1):353-360
Praziquantel (PZQ) is the drug of choice for oral treatment of schistosomiasis and other fluke infections that affect humans.
Its low oral bioavailability demands the development of innovative strategies to overcome the first pass metabolism. In this
article, solid lipid nanoparticles loaded with PZQ (PZQ-SLN) were prepared by a modified oil-in-water microemulsion method
selecting stearic acid as lipid phase after solubility screening studies. The mean particle size (Z-Ave) and zeta potential
(ZP) were 500 nm and −34.0 mV, respectively. Morphology and shape of PZQ-SLN were analysed by scanning electron microscopy
revealing the presence of spherical particles with smooth surface. Differential scanning calorimetry suggested that SLN comprised
a less ordered arrangement of crystals and the drug was molecularly dispersed in the lipid matrix. No supercooled melts were
detected. The entrapment efficiency (EE) and loading capacity of PZQ, determined by high performance liquid chromatography,
were 99.06 ± 0.3 and 17.48 ± 0.05, respectively. Effective incorporation of PZQ into the particles was confirmed by small
angle X-ray scattering revealing the presence of a lipid lamellar structure. Stability parameters of PZQ-SLN stored at room
temperature (25 °C) and at 4 °C were checked by analysing Z-Ave, ZP and the EE for a period of 60 days. Results showed a relatively
long-term physical stability after storage at 4 °C, without drug expulsion. 相似文献