全文获取类型
收费全文 | 559993篇 |
免费 | 4109篇 |
国内免费 | 1172篇 |
专业分类
化学 | 264110篇 |
晶体学 | 7713篇 |
力学 | 30663篇 |
综合类 | 5篇 |
数学 | 84377篇 |
物理学 | 178406篇 |
出版年
2020年 | 4724篇 |
2019年 | 5697篇 |
2018年 | 12816篇 |
2017年 | 13203篇 |
2016年 | 13223篇 |
2015年 | 6059篇 |
2014年 | 9948篇 |
2013年 | 21713篇 |
2012年 | 19367篇 |
2011年 | 27600篇 |
2010年 | 19416篇 |
2009年 | 19586篇 |
2008年 | 25178篇 |
2007年 | 27273篇 |
2006年 | 17113篇 |
2005年 | 18516篇 |
2004年 | 15891篇 |
2003年 | 15033篇 |
2002年 | 14058篇 |
2001年 | 13828篇 |
2000年 | 10943篇 |
1999年 | 8151篇 |
1998年 | 7330篇 |
1997年 | 7218篇 |
1996年 | 6752篇 |
1995年 | 6125篇 |
1994年 | 6061篇 |
1993年 | 6080篇 |
1992年 | 6362篇 |
1991年 | 6738篇 |
1990年 | 6582篇 |
1989年 | 6547篇 |
1988年 | 6376篇 |
1987年 | 6336篇 |
1986年 | 5927篇 |
1985年 | 7493篇 |
1984年 | 7860篇 |
1983年 | 6604篇 |
1982年 | 7023篇 |
1981年 | 6590篇 |
1980年 | 6272篇 |
1979年 | 6814篇 |
1978年 | 7125篇 |
1977年 | 7034篇 |
1976年 | 7368篇 |
1975年 | 6664篇 |
1974年 | 6687篇 |
1973年 | 7150篇 |
1972年 | 5241篇 |
1971年 | 4425篇 |
排序方式: 共有10000条查询结果,搜索用时 609 毫秒
991.
James W. Pavlik Theppawut Israsena Na Ayudhaya Chennagiri R. Pandit Supawan Tantayanon 《Journal of heterocyclic chemistry》2004,41(1):61-67
Trifluromethyl substitution on the pyrazole ring was found to enhance photoreactivity via the P4 pathway which involves interchange of the N2‐C3 ring atoms. Thus, 1‐methyl‐3‐(trifluromethyl)pyrazole (1) and 1‐methyl‐5‐(trifluromethyl)pyrazole (3) transposed with a P4/P6 or P4/P6+P7 ratio of 2.1:1. 1‐methyl‐4‐(trifluromethyl)pyrazole (2) transposed regiospecifically by the P4 transposition pathway. Compounds 2 and 3 were also observed to undergo photocleavage to yield enaminonitrile and enaminoisocyanide products. 相似文献
992.
In different regimes of exposure of metals to pulsed laser radiation, we show the role played by the liquid-drop phase formed from the target material in a laser-induced erosion plume due to bulk vaporization, in the dynamics of this plume. For some metals, the ranges of power densities of the acting laser radiation, at which the condensed-phase particles influence the passage of laser radiation to the target surface, have been determined experimentally. The mechanism of realization of a low-threshold breakdown has been revealed. 相似文献
993.
Modification of Paving Asphalts with Sulfur 总被引:2,自引:0,他引:2
Syroezhko A. M. Begak O. Yu. Fedorov V. V. Gusarova E. N. 《Russian Journal of Applied Chemistry》2003,76(3):491-496
Sulfur dissolved in oil components of asphalt is a binder, filler, and chemical co-reagent at once. The structure of sulfur-extended asphalt depends on its formula and heating temperature. Road pavement based on sulfur-extended asphalt demonstrates higher durability as compared to that with the conventional binders. 相似文献
994.
Amplitude equations, as well as the effective dispersion and nonlinearity parameters, which define the dynamics of a wave
packet formed by two strongly coupled modes, are derived with allowance for the frequency dependence of the linear mode coupling
coefficient. These equations are used to study the onset of the modulation instability of the two-mode wave packet, soliton-like
pulses, and compression modes. Unlike single-mode systems, the last two effects in optical waveguides may arise for both a
negative and positive disper-sion of the waveguide material. 相似文献
995.
V. Ballarini G. Barucca E. Bennici C. F. Pirri C. Ricciardi E. Tresso F. Giorgis 《Physica E: Low-dimensional Systems and Nanostructures》2003,16(3-4):591
All amorphous silicon-nitride planar optical microcavities operating in the visible range have been grown by plasma enhanced chemical vapor deposition. The luminescence intensity of the N-rich silicon-nitride layer from a microcavity with 6 period distributed Bragg reflectors (DBRs) is two order of magnitude higher than that of the luminescent layer without the cavity. Moreover, a strong directionality of the microcavities emission can be observed. Such results can be ascribed to the anisotropic optical density of states induced in the Fabry–Perot structure. The quality factors of the resonators are strictly correlated to the number of periods of the DBRs. 相似文献
996.
G. Lujanienë 《Czechoslovak Journal of Physics》2003,53(1):A57-A65
In order to understand better the way in which radioactive contamination may be transferred from the atmosphere to other ecological compartments it is necessary to obtain information of the deposition mechanisms of radionuclides. Concentration and speciation of 137Cs and 7Be in the atmospheric aerosol and deposition were studied. The mixed deposition velocities of water-soluble and insoluble 137Cs and 7Be were determined. The relation between activity concentrations of insoluble 7Be in the atmosphere and its amount in deposition was observed. It is supposed that in the deposition process of radiocesium the precipitation plays an important role in scavenging of water-soluble radiocesium by falling raindrops, in contrast to insoluble 137Cs where the dry deposition becomes predominant. 相似文献
997.
The method of nonlinear moments, when used to solve the Boltzmann equation, necessitates the calculation of collision integral
matrix elements. The matrix elements are hard to calculate numerically, especially at large indices. The asymptotics of the
matrix elements are constructed. In terms of the model of pseudopower particle interaction, a formula free of summation is
derived. This makes it possible to find the asymptotic behavior of linear and nonlinear elements when two indices are large.
For an arbitrary interaction cross section, asymptotic expansions of linear and nonlinear matrix elements in one index are
obtained. For Maxwellian molecules, asymptotic formulas are derived for three large indices. 相似文献
998.
O. V. Sizova E. P. Sokolova V. I. Baranovskii D. A. Rozmanov O. A. Tomashenko 《Journal of Structural Chemistry》2004,45(5):762-770
Ab initio MP2 calculations with several basis sets proved the existence of a stable benzonitrile dimer with a planar structure and a short contact between the H atom of one molecule and the N atom of another. The structure is greatly stabilized by attraction between the neighboring oppositely directed dipoles and donor-acceptor interaction between the orbital of the lone electron pair of the N′ atom and the vacant antibonding orbital localized on the C-H bond.Original Russian Text Copyright © 2004 by O. V. Sizova, E. P. Sokolova, V. I. Baranovskii, D. A. Rozmanov, and O. A. Tomashenko__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 5, pp. 807–815, September–October, 2004. 相似文献
999.
Relationships between the electrodynamic (wave propagation constants) and quantum-mechanical parameters (probability of induced transitions) of interaction between energy quanta and particles of the medium are derived for dipole and paramagnetic interaction types. It is demonstrated that quantum-mechanical parameters are generally complex. 相似文献
1000.
Aromatic nitro compounds were reduced to the corresponding symmetrically substituted azo compounds using lead as catalyst and triethylammonium formate as hydrogen donor. Various azo compounds containing additional reducible substituents including halogens, nitrile, acid, phenol, ester, methoxy functions, etc, have been synthesized in a single step by the use of this reagent. The conversion is reasonably fast, clean, high yielding and occurs at room temperature in methanol. 相似文献