全文获取类型
收费全文 | 508篇 |
免费 | 26篇 |
国内免费 | 8篇 |
专业分类
化学 | 205篇 |
晶体学 | 1篇 |
力学 | 26篇 |
数学 | 146篇 |
物理学 | 73篇 |
无线电 | 91篇 |
出版年
2024年 | 1篇 |
2023年 | 4篇 |
2022年 | 9篇 |
2021年 | 18篇 |
2020年 | 19篇 |
2019年 | 20篇 |
2018年 | 17篇 |
2017年 | 13篇 |
2016年 | 25篇 |
2015年 | 11篇 |
2014年 | 25篇 |
2013年 | 52篇 |
2012年 | 34篇 |
2011年 | 27篇 |
2010年 | 22篇 |
2009年 | 21篇 |
2008年 | 30篇 |
2007年 | 19篇 |
2006年 | 24篇 |
2005年 | 22篇 |
2004年 | 19篇 |
2003年 | 14篇 |
2002年 | 17篇 |
2001年 | 7篇 |
2000年 | 6篇 |
1999年 | 3篇 |
1998年 | 5篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1995年 | 4篇 |
1993年 | 3篇 |
1992年 | 5篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 4篇 |
1885年 | 1篇 |
排序方式: 共有542条查询结果,搜索用时 203 毫秒
71.
The condensation of benzyne in aprotic medium with the enolates of the 2,2,6-trimethyl, 3,3,5,5-tetramethyl, 4-4-dimethyl, 4-methyl, 4-t-butyl cyclohexanones as well as with those of α-tetralone and α-benzosuberone leads to the corresponding α-phenyl ketones and several tri and tetracyclic benzocyclobutenols. 相似文献
72.
Butadienal 1, propadienol 2 and propadienamine 3 are synthesized by flash vacuum thermolysis of their anthracenic Diels-Alder adducts and characterized at low temperature. 相似文献
73.
74.
Ferrioxamine B analogues: targeting the FoxA uptake system in the pathogenic Yersinia enterocolitica
Kornreich-Leshem H Ziv C Gumienna-Kontecka E Arad-Yellin R Chen Y Elhabiri M Albrecht-Gary AM Hadar Y Shanzer A 《Journal of the American Chemical Society》2005,127(4):1137-1145
A series of ferrioxamine B analogues that target the bacterium Yersinia enterocolitica were prepared. These iron carriers are composed of three hydroxamate-containing monomeric units. Two identical monomers consist of N-hydroxy-3-aminopropionic acid coupled with beta-alanine, and a third unit at the amino terminal is composed of N-hydroxy-3-aminopropionic acid and one of the following amino acids: beta-alanine (1a), phenylalanine (1b), cyclohexylalanine (1c), or glycine (1d). Thermodynamic results for representatives of the analogues have shown a strong destabilization (3-4 orders of magnitude) of the ferric complexes with respect to ferrioxamine B, probably due to shorter spacers and a more strained structure around the metal center. No significant effect of the variations at the N-terminal has been observed on the stability of the ferric complexes. By contrast, using in vivo radioactive uptake experiments, we have found that these modifications have a substantial effect on the mechanism of iron(III) uptake in the pathogenic bacteria Yersinia enterocolitica. Analogues 1a and 1d were utilized by the ferrioxamine B uptake system (FoxA), while 1b and 1c either used different uptake systems or were transported to the microbial cell nonspecifically by diffusion via the cell membrane. Transport via the FoxA system was also confirmed by uptake experiments with the FoxA deficient strain of Yersinia enterocolitica. A fluorescent marker, attached to 1a in a way that did not interfere with its biological activity, provided additional means to monitor the uptake mechanism by fluorescence techniques. Of particular interest is the observation that 1a was utilized by the uptake system of ferrioxamine B in Yersinia enterocolitica (FoxA) but failed to use the ferrioxamine uptake route in Pseudomonas putida. Here, we present a case in which biomimetic siderophore analogues deliberately designed for a particular bacterium can distinguish between related uptake systems of different microorganisms. 相似文献
75.
Mourad?E.?H.?IsmailEmail author Inna?Nikolova Plamen?Simeonov 《The Ramanujan Journal》2005,8(4):475-502
We prove that any set of polynomials orthogonal with respect to a discrete measure supported on equidistant points contained in a half line satisfy a second order difference equation. We also give a discrete analogue of the discriminant and give a general formula for the discrete discriminant of a discrete orthogonal polynomial. As an application we give explicit evaluations of the discrete discriminants of the Meixner and the Hahn polynomials. A difference analogue of the Bethe Ansatz equations is also mentioned.Research partially supported by NSF grant DMS 99-70865 相似文献
76.
S.A. Mourad 《Journal of Quantitative Spectroscopy & Radiative Transfer》1974,14(8):661-677
An accurate numerical method has been devised to solve half-range problems for the scattering of azimuth-independent polarized radiation in stellar atmospheres. The use of the method is illustrated by solving the Milne problem for the radiation intensity at any optical depth, for the extrapolation distance, and for the law of darkening. 相似文献
77.
Facile syntheses of pyrazole, pyridine, pyrimidine and pyrazolo[3,4-d]pyrimidine derivatives have been achieved by the reaction of β-enamino nitrile 2a with hydrazines, phenyl isothiocyanate, thiourea and active methylene reagents. The β-enamino ketone 2e with the same reagents affords 3,5-dimethyl heterocyclic ring systems. 相似文献
78.
D. Mourad G. Czycholl 《The European Physical Journal B - Condensed Matter and Complex Systems》2010,78(4):497-507
Zero-dimensional nanocrystals, as obtained by chemical synthesis, offer a broad range of applications, as their spectrum and
thus their excitation gap can be tailored by variation of their size. Additionally, nanocrystals of the type A
x
B1-
x
C can be realized by alloying of two pure compound semiconductor materials AC and BC, which allows for a continuous tuning
of their absorption and emission spectrum with the concentration x. We use the single-particle energies and wave functions calculated
from a multiband sp
3 empirical tight-binding model in combination with the configuration
interaction scheme to calculate the optical properties of
Cd
x
Zn1-
x
Se nanocrystals with a spherical shape. In contrast to common mean-field approaches like the virtual crystal approximation
(VCA), we treat the disorder on a microscopic level by taking into account a finite number of realizations for each size and
concentration.
We then compare the results for the optical properties with recent experimental data and calculate the optical bowing coefficient
for further sizes. 相似文献
79.
The photodarkening (PD) mechanisms of ytterbium-doped silica optical fibers have still not been elucidated, although hardening routes have been proposed. Most basic questions are still under debate about the assignment of the darkening excitation bands into the UV range, the nature of absorbing centers (photoionized centers or trapped carriers?), or of traps accepting photo-released carriers (electron or hole traps?). We used thermoluminescence measurements to characterize traps populated by different radiation types. It is notably demonstrated that photodarkening involves silica hole traps. The popular idea that color centers are formed upon carrier trapping is not consistent with our observations. 相似文献
80.
Kerboua Mourad 《随机分析与应用》2018,36(2):209-223
In this paper, the approximate controllability for Sobolev-type fractional neutral stochastic evolution equations with fractional stochastic nonlocal conditions and fractional Brownian motion in a Hilbert space are studied. The results are obtained by using semigroup theory, fractional calculus, stochastic integrals for fractional Brownian motion, Banach's fixed point theorem, and methods adopted directly from deterministic control problems for the main results. Finally, an example is given to illustrate the application of our result. 相似文献