全文获取类型
收费全文 | 235972篇 |
免费 | 2125篇 |
国内免费 | 766篇 |
专业分类
化学 | 126310篇 |
晶体学 | 3390篇 |
力学 | 8650篇 |
综合类 | 6篇 |
数学 | 25188篇 |
物理学 | 67088篇 |
综合类 | 8231篇 |
出版年
2020年 | 2133篇 |
2019年 | 2422篇 |
2018年 | 3331篇 |
2017年 | 3256篇 |
2016年 | 4508篇 |
2015年 | 2629篇 |
2014年 | 4142篇 |
2013年 | 9873篇 |
2012年 | 7793篇 |
2011年 | 9576篇 |
2010年 | 6597篇 |
2009年 | 6300篇 |
2008年 | 9164篇 |
2007年 | 9231篇 |
2006年 | 8671篇 |
2005年 | 8032篇 |
2004年 | 7126篇 |
2003年 | 6464篇 |
2002年 | 6281篇 |
2001年 | 6442篇 |
2000年 | 5173篇 |
1999年 | 3805篇 |
1998年 | 3188篇 |
1997年 | 3219篇 |
1996年 | 3054篇 |
1995年 | 2486篇 |
1994年 | 2581篇 |
1993年 | 2619篇 |
1992年 | 2882篇 |
1991年 | 2932篇 |
1990年 | 2819篇 |
1989年 | 2760篇 |
1988年 | 2700篇 |
1987年 | 2646篇 |
1986年 | 2540篇 |
1985年 | 3318篇 |
1984年 | 3397篇 |
1983年 | 2719篇 |
1982年 | 2969篇 |
1981年 | 2868篇 |
1980年 | 2662篇 |
1979年 | 2978篇 |
1978年 | 3049篇 |
1977年 | 3060篇 |
1976年 | 3015篇 |
1975年 | 2866篇 |
1974年 | 2911篇 |
1973年 | 2985篇 |
1972年 | 1983篇 |
1967年 | 2072篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
A new class of Hamiltonian dynamical systems with two degrees of freedom and kinetic energy of the form T = c1|p1| + c2|p2| (called “pseudo-billiards”) is studied. For any kind of interaction, the canonical equations can always be integrated on sequential time intervals; i.e. in principle all the trajectories can be found explicitly.
Depending on the potential, a dynamical system of this class can either be completely integrable or behave just as a usual non-integrable Hamiltonian system with two degrees of freedom: in its phase space there exist invariant tori, stochastic layers, domains of global chaos, etc. Pseudo-billiard models of both the types are considered.
If a potential of a pseudo-billiard system has critical points (equilibria), then trajectories close to these points (“loops”) can exist; they can be treated as images of self-localized objects with finite duration. Such a model (with quartic potential) is also studied. 相似文献
942.
V. M. Baev J. Eschner E. Paeth R. Schüler P. E. Toschek 《Applied physics. B, Lasers and optics》1992,55(6):463-477
A multi-mode diode laser with an external cavity is studied experimentally and theoretically for its application to intra-cavity spectroscopy. One facet of a typical Ga0.89Al0.11As laser diode was antireflection-coated by deposition of HfO2 such that 10–3 residual reflectivity was left over. This diode was placed in an external optical cavity. The emission spectrum of this diode laser is highly sensitive to any frequency-dependent loss in the cavity, and the detectivity of such a loss grows with the pump rate. Even close to threshold, the absorption at 780 nm of Rb atoms with a density of 5×1010 cm–3 has been detected. An adequate model for diode lasers based on rate equations and including frequency-dependent gain saturation is developed and applied to the calculations of output spectra. The sensitivity of these spectra to intra-cavity absorption is determined by the overall cavity loss — which is rather high — and the fraction of spontaneous emission in the total emission, in contrast with dye lasers where it is limited by nonlinear mode coupling. Various criteria for the sensitivity are suggested. The smallest detectable absorption with a perfectly antireflection-coated laser is 10–6 cm–1. Improvement of the characteristics of the laser diode would increase the sensitivity. 相似文献
943.
P. J. Wagstaffe A. Boenke E. Schnug A. S. Lindsey 《Fresenius' Journal of Analytical Chemistry》1992,344(1-2):1-7
Summary The sulphur content of three rapeseed reference materials has been certified on the basis of measurements made by inductively
coupled plasma-optical emission spectroscopy, isotope-dilution mass spectrometry and ion chromatography. Their suitability
for the calibration of x-ray fluorescence spectrometers used for measuring sulphur in rapeseed samples was established. 相似文献
944.
945.
946.
We present for the first time a self-consistent calculation of the electronic energy band structure of NiMn in the antiferromagnetic tetragonal phase. The electronic energy spectrum is calculated, along with the full and partial electronic density of states, the Fermi surface, the electronic specific heat coefficient, and the magnetic moments on each type of atom. We have studied the influence of structural distortions and antiferromagnetic ordering on the electronic structure of NiMn. Satisfactory agreement is found with the existing experimental data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 18–23, August, 1991.The authors express their appreciation to N. I. Kulikov and A. F. Tatarchenko for assistance in the computer calculations and for useful discussion of the work. 相似文献
947.
S. G. Barsov A. L. Getalov S. L. Ginsburg V. P. Koptev S. P. Kruglov L. A. Kuzmin S. V. Maleyev E. I. Maltsev S. M. Mikirtychyants N. A. Tarasov G. V. Shcherbakov V. G. Grebinnik V. N. Duginov A. B. Lazarev V. G. Olshevski S. N. Shilov V. A. Zhukov I. I. Gurevich B. F. Kirillov A. I. Klimov B. A. Nikolski A. V. Pirogov A. N. Ponomarev V. A. Suetin 《Hyperfine Interactions》1991,64(1-4):415-419
The zero field μSR-method has been used to study the magnetism in the disordered magnetic alloy Fe82−xNixCr18 near the three-critical, pointx=25. The dynamic and static local field distributions are analyzed. The difference between spin-glass states obtained either
from the paramagnetic or after the double transition is discussed. 相似文献
948.
There is a high correlation between molecular surface area (TSA) of triorganotin and triorganolead compounds and their toxicity towards a bacterium (Escherichia coli) and an alga (Selenastrum capricornutum). Parallel attempts to correlate other Group IVA organometals incorporating silicon or germanium were unsuccessful. It was further demonstrated, however, that a high correlation was obtainable between certain series of compounds with the same organic substituent but different metal centers involving all Group IVA elements. In both instances, the inability to obtain a quantitative structure-activity relationship (QSAR) for all systems studied appears to be a function of the solubility of the compounds. While organotin TSA values have been found to correlate well with their toxicities toward various organisms, this study clearly suggests that this type of QSAR can be readily extended to include other organometal systems, provided that there is no solubility problem and the toxicity is a function of the hydrophobicity of the organometal compounds. 相似文献
949.
1-(3-Alkyl-2,3-dideoxy-alpha,beta-D-erythro-pentofuranosyl)uracils and 1-(3-alkyl-2,3-dideoxy-alpha,beta-D-threo-pentofuranosyl)uracils have been prepared from (E)-4,5-di-O-acetyl-2,3-dideoxy-aldehydo-D-glycero-pent-2-enose by a Michael addition reaction of the appropriate organocopper reagent followed in subsequent order by glycosidation of the resulting 3-alkyl-4,5-diacetoxypentanal with methanolic hydrogen chloride, protection with p-methoxybenzoyl chloride, and trimethylsilyl triflate catalyzed coupling with 2,4-di-O-(trimethylsilyl)uracil. The nucleosides were deprotected by treatment with 33% methylamine in absolute ethanol and separated by reversed-phase HPLC. 相似文献
950.
The temperature dependence of the principal values of the refractive index in Sr1−x
CaxTiO3 (x=0.014) has been measured in the 17–275 K range under various conditions of sample illumination with 1.96 eV photons. The
spontaneous photorefractive contribution δn
ph to the temperature-induced variation of the refractive index of Sr1−x
CaxTiO3, which appears after illumination of the sample in the ferrophase (transition temperature T
c=32 K) and persists in the paraphase under heating up to 150 K, has been separated. The photoinduced polarization has been
estimated.
Fiz. Tverd. Tela (St. Petersburg) 39, 711–713 (April 1997) 相似文献