全文获取类型
收费全文 | 1398篇 |
免费 | 38篇 |
国内免费 | 1篇 |
专业分类
化学 | 909篇 |
晶体学 | 16篇 |
力学 | 61篇 |
数学 | 150篇 |
物理学 | 301篇 |
出版年
2023年 | 10篇 |
2022年 | 16篇 |
2021年 | 27篇 |
2020年 | 28篇 |
2019年 | 12篇 |
2018年 | 14篇 |
2017年 | 15篇 |
2016年 | 42篇 |
2015年 | 26篇 |
2014年 | 26篇 |
2013年 | 65篇 |
2012年 | 73篇 |
2011年 | 94篇 |
2010年 | 22篇 |
2009年 | 37篇 |
2008年 | 56篇 |
2007年 | 59篇 |
2006年 | 65篇 |
2005年 | 49篇 |
2004年 | 49篇 |
2003年 | 32篇 |
2002年 | 38篇 |
2001年 | 25篇 |
2000年 | 26篇 |
1999年 | 18篇 |
1998年 | 16篇 |
1997年 | 13篇 |
1996年 | 18篇 |
1995年 | 21篇 |
1994年 | 17篇 |
1993年 | 30篇 |
1992年 | 17篇 |
1991年 | 21篇 |
1990年 | 20篇 |
1989年 | 18篇 |
1988年 | 18篇 |
1987年 | 14篇 |
1986年 | 11篇 |
1985年 | 21篇 |
1984年 | 16篇 |
1982年 | 18篇 |
1981年 | 12篇 |
1980年 | 9篇 |
1976年 | 16篇 |
1975年 | 9篇 |
1974年 | 15篇 |
1973年 | 18篇 |
1972年 | 9篇 |
1969年 | 10篇 |
1862年 | 11篇 |
排序方式: 共有1437条查询结果,搜索用时 46 毫秒
111.
Glow discharge spectroscopy (GDOS) will be shown to be a quick, informative and simple method for quantitative depth profile analysis of elements of nitrided layers well suited for their quality control. By systematic variation of all glow discharge determining parameters it is possible to get an excellent depth resolution in the order of sub-m corresponding to a comparatively large analytical activated area (50 mm2). In this paper the behaviour of a number of important parameters related to sputtering of the activated area will be discussed. Some quantitative GDOS depth profiles of carbon and nitrogen of pure iron samples nitrided by different procedures will be shown as examples for application. 相似文献
112.
Banci L Bertini I Felli IC Sarrou J 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2005,172(2):191-200
CH(alpha) residual dipolar couplings (Deltardc's) were measured for the oxidized cytochrome b562 from Escherichia coli as a result of its partial self-orientation in high magnetic fields due to the anisotropy of the overall magnetic susceptibility tensor. Both the low spin iron (III) heme and the four-helix bundle fold contribute to the magnetic anisotropy tensor. CH(alpha) Deltardc's, which span a larger range than the analogous NH values (already available in the literature) sample large space variations at variance with NH Deltardc's, which are largely isooriented within alpha helices. The whole structure is now significantly refined with the chemical shift index and CH(alpha) Deltardc's. The latter are particularly useful also in defining the molecular magnetic anisotropy parameters. It is shown here that the backbone folding can be conveniently and accurately determined using backbone restraints only, which include NOEs, hydrogen bonds, residual dipolar couplings, pseudocontact shifts, and chemical shift index. All these restraints are easily and quickly determined from the backbone assignment. The calculated backbone structure is comparable to that obtained by using also side chain restraint. Furthermore, the structure obtained with backbone only restraints is, in its whole, very similar to that obtained with the complete set of restraints. The paramagnetism based restraints are shown to be absolutely relevant, especially for Deltardc's. 相似文献
113.
This paper deals with the problem of reproducing two signals at two points in space by using two acoustic sources. While much is now known about the techniques available for the design of matrices of inverse filters that enable this objective to be achieved in practice, it is still the basic physics of the sound field produced that controls the effectiveness of such systems and which ultimately dictates their design. The basic physical processes involved in producing the cross-talk cancellation that enables the reproduction of the desired signals is revisited here by using a simple two source/two field point free field model. The singular value decomposition is used to identify those frequencies where the inversion problem becomes ill-conditioned and to explain physically the origin of the ill-conditioning. As observed previously, it is found that cross-talk cancellation becomes problematic when the path length difference between the two sources and one of the field points becomes equal to one half the acoustic wavelength. The ill-conditioned frequencies are also found to be associated with a limited spatial region of cross-talk cancellation and with large source outputs manifested in the time domain by responses of long duration. 相似文献
114.
[reaction: see text] A study of new nucleophilic addition/ring-closure (NARC) sequences has resulted in the development of a stereoselective synthetic route to 3-deoxy-8-oxatropanes. The new sequences consisted of either a syn or anti aldol addition, employing an omega-alkenoyl sultam, followed by two-step bicyclic ring construction involving, consecutively, ring-closing metathesis and intramolecular oxymercuration. 相似文献
115.
The radical reactions of a series of bromides 9a-c and selenides 12-15 have been investigated to determine the factors that are important for a successful radical-mediated 1,2-imino migration. 相似文献
116.
Yu LH DiMauro L Doyuran A Graves WS Johnson ED Heese R Krinsky S Loos H Murphy JB Rakowsky G Rose J Shaftan T Sheehy B Skaritka J Wang XJ Wu Z 《Physical review letters》2003,91(7):074801
We report the first experimental results on a high-gain harmonic-generation (HGHG) free-electron laser (FEL) operating in the ultraviolet. An 800 nm seed from a Ti:sapphire laser has been used to produce saturated amplified radiation at the 266 nm third harmonic. The results confirm the predictions for HGHG FEL operation: stable central wavelength, narrow bandwidth, and small pulse-energy fluctuation. 相似文献
117.
The reaction of alpha-angelica lactone with alkylamines under aqueous conditions afforded 5-hydroxy-5-methylpyrrolidinones in high yield. When the reaction was carried out under anhydrous conditions, the only products obtained were the corresponding 4-oxopentanoic acid amides. Treatment of either class of compound with triflic anhydride (Tf(2)O) in pyridine resulted in the formation of various substituted sulfonamidofurans. The suggested mechanism involves initial formation of an iminium ion which is subsequently transformed into a transient imino triflate. Cyclization of the highly electrophilic imine onto the oxygen atom of the adjacent carbonyl group generates an imino dihydrofuran intermediate. This species reacts further with another equivalent of Tf(2)O to give the observed product. The nature of the Lewis acid used was found to affect the outcome of the cyclization reaction. In certain cases, the sulfonamide furan was utilized as a cycloaddition substrate for the synthesis of indolines and related heterocyclic systems. 相似文献
118.
Darbeau RW Perez EV Sobieski JI Rose WA Yates MC Boese BJ Darbeau NR 《The Journal of organic chemistry》2001,66(17):5679-5686
N-Alkyl-N-nitrosoamides undergo competitive reactions whose rates are dependent upon the interplay of a number of factors. There already exists a significant body of work delineating the effects of pH on the partitioning of the nitrosoamides along their deaminative (-N(2)) and denitrosative (-"NO(+)") pathways. In this paper, the issue of pH dependence is discussed with particular attention to nitrosoamide decompositions in nonaqueous media. The role of the acidity of the medium in the partitioning of the nitrosoamide between deamination and denitrosation and in the choice of deaminative pathways is revisited. In nonaqueous media under near-neutral conditions, the partitioning's pH dependence is evidently accompanied by a sensitivity to structural features in the nitrosoamide. Thus, diminution of steric crowding around the N-nitroso moiety as well as the presence of strongly electron-withdrawing acyl units (i.e., those derived from strong acids, e.g., tosyl and trifyl) increase the relative yield of amides by encouraging the denitrosative pathway. A mechanism for thermal denitrosation of nitrosoamides under near-neutral conditions is proposed in which rapid protonation at the acyl O rather than slow protonation at the amidic N is the first step in the reaction profile. A rate-limiting, bimolecular reaction between the O-conjugate acid and adventitious nucleophiles at the nitrosyl group then occurs followed by rapid tautomerization to amide. 相似文献
119.
The transmitted angular distribution of a 527 nm nearly diffraction-limited laser is measured after it propagates through a plasma with supersonic transverse flow. The laser beam is deflected by as much as 10 degrees and exhibits bowlike features in the flow direction, which is attributed to flow-induced beam steering. The finite interaction volume allows for direct comparison with a 3D hydrodynamic simulation, which is in good agreement with details of the experiment. 相似文献
120.
P. Rose T. Richter B. Terhalle J. Imbrock F. Kaiser C. Denz 《Applied physics. B, Lasers and optics》2007,89(4):521-526
We investigate the formation of fundamental discrete solitons and dipole-mode gap solitons in triangular photonic lattices
imprinted in photorefractive nonlinear media. These lattices are strongly affected by the photorefractive anisotropy, resulting
in orientation-dependent refractive index structures with reduced symmetry. It is demonstrated that two different orientations
of the lattice wave enable the formation of fundamental discrete solitons in the total internal reflection gap. Furthermore,
it is shown that one lattice orientation additionally supports dipole-mode solitons in the Bragg reflection gap. The experimental
results are corroborated by numerical simulations using the full anisotropic model.
PACS 42.65.Tg; 42.65.Wi; 42.70.Qs 相似文献