全文获取类型
收费全文 | 344篇 |
免费 | 12篇 |
国内免费 | 1篇 |
专业分类
化学 | 257篇 |
晶体学 | 1篇 |
力学 | 3篇 |
数学 | 35篇 |
物理学 | 61篇 |
出版年
2023年 | 1篇 |
2022年 | 16篇 |
2021年 | 9篇 |
2020年 | 11篇 |
2019年 | 11篇 |
2018年 | 4篇 |
2017年 | 7篇 |
2016年 | 7篇 |
2015年 | 14篇 |
2014年 | 11篇 |
2013年 | 28篇 |
2012年 | 23篇 |
2011年 | 28篇 |
2010年 | 22篇 |
2009年 | 12篇 |
2008年 | 17篇 |
2007年 | 20篇 |
2006年 | 16篇 |
2005年 | 16篇 |
2004年 | 12篇 |
2003年 | 13篇 |
2002年 | 8篇 |
2001年 | 5篇 |
2000年 | 1篇 |
1999年 | 2篇 |
1998年 | 2篇 |
1997年 | 3篇 |
1995年 | 4篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1990年 | 4篇 |
1989年 | 5篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1973年 | 2篇 |
1972年 | 1篇 |
1944年 | 1篇 |
排序方式: 共有357条查询结果,搜索用时 31 毫秒
71.
An optimal control approach based on multiple parameter genetic algorithms is applied to the design of plasmonic nanoconstructs with predetermined optical properties and functionalities. We first develop nanoscale metallic lenses that focus an incident plane wave onto a prespecified, spatially confined spot. Our results illustrate the mechanism of energy flow through wires and cavities. Next we design a periodic array of silver particles to modify the polarization of an incident, linearly polarized plane wave in a desired fashion while localizing the light in space. The results provide insight into the structural features that determine the birefringence properties of metal nanoparticles and their arrays. Of the variety of potential applications that may be envisioned, we note the design of nanoscale light sources with controllable coherence and polarization properties that could serve for coherent control of molecular, electronic, or electromechanical dynamics in the nanoscale. 相似文献
72.
73.
To investigate the molecular interaction study between secondary amine with aryl and alkyl ether, we report densities, speeds of sound, viscosities and refractive indices of N-ethylaniline (NEA) with aryl (anisole, phenetole) and alkyl (tert-butyl methyl ether) ether over the entire composition regime and in the temperature range of 293.15–323.15 K at 5 K intervals. Various excess and transport properties were derived from the experimental data and the excess parameters were fitted to the Redlich–Kister polynomial equation using multiparametric non-linear regression analysis to derive the binary coefficients and to estimate the standard deviation. Molecular interactions between the unlike molecules were analysed through dipole–dipole interactions, cross-association between the components of the mixtures and H-bond formation. Viscosity and refractive index of the mixtures were predicted through several empirical equations and compared with the experimental results. Prigogine–Flory–Patterson statistical theory was used to predict the excess molar volume results. 相似文献
74.
Manjunath D. Meti Sharanappa T. Nandibewoor Shrinivas D. Joshi Uttam A. More Shivamurti A. Chimatadar 《Journal of the Iranian Chemical Society》2016,13(7):1325-1338
This study was designed to examine the interaction of histamine H2-receptor antagonist drug ranitidine (RTN) with human serum albumin by multi-spectroscopic methods. The experimental results showed the involvement of dynamic quenching mechanism which was further confirmed by lifetime spectral studies. The binding constants (K a) at three temperatures (288, 298, and 308 K) were 2.058 ± 0.020, 4.160 ± 0.010 and 6.801 ± 0.011 × 104 dm3 mol?1, respectively, and the number of binding sites (m) were 1.169, respectively; thermodynamic parameters ΔH 0 (44.152 ± 0.047 kJ mol?1), ΔG 0 (?26.214 ± 0.040 kJ mol?1), and ΔS 0 (236.130 ± 0.025 J K?1 mol?1) were calculated. The distance r between donor and acceptor was obtained (r = 3.40 nm) according to the Förster theory of non-radiative energy transfer. Synchronous fluorescence, CD, AFM and 3D fluorescence spectral results revealed the changes in secondary structure of the protein upon interaction with RTN. A molecular modeling study further confirmed the binding mode obtained by the experimental studies. 相似文献
75.
In recent years it has been observed that resorcin[4]arenes and pyrogallol[4]arenes form hydrogen-bonded hexameric capsules in nonpolar solvents. In the present study we have used NMR spectroscopy, with an emphasis on diffusion NMR, to investigate the self-assembly and the aggregation mode of solutions of octahydroxypyridine[4]arene (1 b) in chloroform. In spectroscopic studies, the hexameric capsule of C-undecylresorcin[4]arene (2 b) was used as a reference compound and in some cases also as an internal reference. The current diffusion NMR spectroscopy study shows, in contrast to a previous report, that compound 1 b self-assembles spontaneously into hexameric and dimeric aggregates in solutions in chloroform. The (1)H NMR and diffusion NMR spectroscopic studies on a solution of 1 b in CHCl(3) show the presence of new upfield-shifted peaks, which diffuse with the same diffusion coefficient as the hexameric peaks in the spectrum. Therefore, these new upfield-shifted peaks were attributed to encapsulated CHCl(3) molecules. Interestingly, diffusion NMR measurements showed that the addition of trifluoroacetic acid (6.7 equiv), which had no effect on the hexameric capsules of 2 b, led to the disassembly of the hexamer and the dimer of 1 b into its monomers. Therefore, we conclude that compound 1 b self-assembles spontaneously into hexameric capsules in nonpolar organic solvents, as do resorcin[4]arenes 2 b and 2 c and pyrogallol[4]arenes 3 a and 3 b. 相似文献
76.
Evan-Salem T Frish L van Leeuwen FW Reinhoudt DN Verboom W Kaucher MS Davis JT Cohen Y 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(7):1969-1977
Cation-templated self-assembly of the lipophilic isoguanosine (isoG 1) with different monovalent cations (M(+)=Li(+), Na(+), K(+), NH(4) (+), and Cs(+)) was studied in solvents of different polarity by using diffusion NMR spectroscopy. Previous studies that did not use diffusion NMR techniques concluded that isoG 1 forms both pentamers (isoG 1)(5)M(+) and decamers (isoG 1)(10)M(+) in the presence of alkali-metal cations. The present diffusion NMR studies demonstrate, however, that isoG 1 does not form (isoG 1)(5)M(+) pentamers. In fact, the diffusion NMR data indicates that both doubly charged decamers of formula (isoG 1)(10)2 M(+) and singly charged decamers, (isoG 1)(10)M(+), are formed with lithium, sodium, potassium, and ammonium tetraphenylborate salts (LiB(Ph)(4), KB(Ph)(4), NaB(Ph)(4) and NH(4)B(Ph)(4)), depending on the isoG 1:salt stoichiometry of the solution. In the presence of CsB(Ph)(4), isoG 1 affords only the singly charged decamers (isoG 1)(10)Cs(+). By monitoring the diffusion coefficient of the B(Ph)(4) (-) ion in the different mixtures of solvents, we also concluded that the anion is more strongly associated to the doubly charged decamers (isoG 1)(10)2 M(+) than to the singly charged decamers (isoG 1)(10)M(+). The (isoG 1)(10)2 M(+) species can, however, exist in solution without the mediation of the anion. This last conclusion was supported by the finding that the doubly charged decamers (isoG 1)(10)2 M(+) also prevail in 1:1 CD(3)CN:CDCl(3), a solvent mixture in which the B(Ph)(4) (-) ion does not interact significantly with the self-assembled complex. These diffusion measurements, which have provided new and improved structural information about these decameric isoG 1 assemblies, demonstrate the utility of combining diffusion NMR techniques with conventional NMR methods in seeking to characterize labile, multicomponent, supramolecular systems in solution, especially those with high symmetry. 相似文献
77.
Tamar Ziegler 《Journal of the American Mathematical Society》2007,20(1):53-97
Let be an ergodic probability measure-preserving system. For a natural number we consider the averages where , and are integers. A factor of is characteristic for averaging schemes of length (or -characteristic) if for any nonzero distinct integers , the limiting behavior of the averages in (*) is unaltered if we first project the functions onto the factor. A factor of is a -universal characteristic factor (-u.c.f.) if it is a -characteristic factor, and a factor of any -characteristic factor. We show that there exists a unique -u.c.f., and it has the structure of a -step nilsystem, more specifically an inverse limit of -step nilflows. Using this we show that the averages in (*) converge in . This provides an alternative proof to the one given by Host and Kra.
78.
Lipases in organic solvents catalyse lactonisation or polymerisation of ω-hydroxyesters. Under these conditions the enzymes exhibit both enantioselectivity and prochiral selectivity. This approach was used to develop the synthesis of chiral γ-lactones and A-B type polyesters from racemic or prochiral hydroxyester substrate. 相似文献
79.
Titanium dioxide (TiO2) films were fabricated on fluorine doped tin oxide (FTO) coated glass substrate using successive ionic layer adsorption and reaction (SILAR) method. The X-ray diffraction, scanning electron microscopy, transmission electron microscopy, optical absorption and contact angle measurement were applied to study the structural, surface morphological, optical and surface wettability properties of the as-deposited and annealed TiO2 films. The X-ray diffraction studies revealed both as-deposited and annealed TiO2 films are amorphous. Irregular shaped spherical grains of random size and well covered to the fluorine doped tin oxide coated glass substrates were observed from SEM studies with some cracks after annealing. The optical band gap values of virgin TiO2, annealed, methyl violet and rose bengal sensitized TiO2 were found to be 3.6, 3.5, 2.87 and 2.95 eV, respectively. Surface wettability studied in contact with liquid interface, showed hydrophobic nature as water contact angles were greater than 90°. The adsorption of dyes, as confirmed by the photographs, is one of the prime requirements for dye sensitized solar cells (DSSC). 相似文献
80.
Enhanced field emission from pulsed laser deposited nanocrystalline ZnO thin films on Re and W 总被引:1,自引:0,他引:1
Dattatray J. Late Pankaj Misra B. N. Singh Lalit M. Kukreja Dilip S. Joag Mahendra A. More 《Applied Physics A: Materials Science & Processing》2009,95(2):613-620
Nanocrystalline ZnO thin films have been deposited on rhenium and tungsten pointed and flat substrates by pulsed laser deposition
method. An emission current of 1 nA with an onset voltage of 120 V was observed repeatedly and maximum current density ∼1.3 A/cm2 and 9.3 mA/cm2 has been drawn from ZnO/Re and ZnO/W pointed emitters at an applied voltage of 12.8 and 14 kV, respectively. In case of planar
emitters (ZnO deposited on flat substrates), the onset field required to draw 1 nA emission current is observed to be 0.87
and 1.2 V/μm for ZnO/Re and ZnO/W planar emitters, respectively. The Fowler–Nordheim plots of both the emitters show nonlinear
behaviour, typical for a semiconducting field emitter. The field enhancement factor β is estimated to be ∼2.15×105 cm−1 and 2.16×105 cm−1 for pointed and 3.2×104 and 1.74×104 for planar ZnO/Re and ZnO/W emitters, respectively. The high value of β factor suggests that the emission is from the nanometric features of the emitter surface. The emission current–time plots
exhibit good stability of emission current over a period of more than three hours. The post field emission surface morphology
studies show no significant deterioration of the emitter surface indicating that the ZnO thin film has a very strong adherence
to both the substrates and exhibits a remarkable structural stability against high-field-induced mechanical stresses and ion
bombardment. The results reveal that PLD offers unprecedented advantages in fabricating the ZnO field emitters for practical
applications in field-emission-based electron sources. 相似文献