全文获取类型
收费全文 | 148278篇 |
免费 | 1338篇 |
国内免费 | 401篇 |
专业分类
化学 | 80997篇 |
晶体学 | 2392篇 |
力学 | 6101篇 |
综合类 | 3篇 |
数学 | 13767篇 |
物理学 | 46757篇 |
出版年
2021年 | 1080篇 |
2020年 | 1211篇 |
2019年 | 1281篇 |
2018年 | 1685篇 |
2017年 | 1726篇 |
2016年 | 2593篇 |
2015年 | 1601篇 |
2014年 | 2410篇 |
2013年 | 6123篇 |
2012年 | 4924篇 |
2011年 | 6018篇 |
2010年 | 4255篇 |
2009年 | 4282篇 |
2008年 | 5803篇 |
2007年 | 5581篇 |
2006年 | 5448篇 |
2005年 | 4982篇 |
2004年 | 4495篇 |
2003年 | 4141篇 |
2002年 | 4029篇 |
2001年 | 5566篇 |
2000年 | 3979篇 |
1999年 | 3095篇 |
1998年 | 2120篇 |
1997年 | 2222篇 |
1996年 | 2077篇 |
1995年 | 1927篇 |
1994年 | 1921篇 |
1993年 | 1734篇 |
1992年 | 2217篇 |
1991年 | 2301篇 |
1990年 | 2142篇 |
1989年 | 2106篇 |
1988年 | 2078篇 |
1987年 | 2088篇 |
1986年 | 1912篇 |
1985年 | 2374篇 |
1984年 | 2395篇 |
1983年 | 1911篇 |
1982年 | 1917篇 |
1981年 | 1753篇 |
1980年 | 1738篇 |
1979年 | 2026篇 |
1978年 | 2101篇 |
1977年 | 2046篇 |
1976年 | 1975篇 |
1975年 | 1861篇 |
1974年 | 1842篇 |
1973年 | 1874篇 |
1972年 | 1235篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Field emission projector studies of fullerene coatings deposited on tungsten tip field emitters reveal specific ordered patterns in the form of doublets, quadruplets, rings, disks, and other forms in the emitter images. The ways in which these types of ordered emission patterns arise and their relation to C60 microformations have been established. Possible causes of the emergence of the ordered emission images are analyzed on the basis of published data and experimental results obtained. A modification of the model of field emission from the surface of microformations taking into account internal reflection of the electronic waves from the formation boundaries has been proposed. 相似文献
92.
The most promising next generation Image Velocimtry (IV) is the high-speed Dynamic PIV. It requires the development of innovative
high-speed video camera sensors. We started by specifying the required performance of these new sensors, for measurements
in air and water flows. These criteria founded on the most recent developments in PIV algorithms and incorporate results from
a large questionnaire survey of users of high-speed video cameras in Japan. The results suggest that the followings are required:
(1) frame rate of 1,000,000 fps, (2) pixel count of 1,000,000 pixels, (3) frame storage capacity of 100–200 frames for tracing
a single event and 10,000 frames for turbulent measurements, (4) gray levels of 4–8 bits for PTV; 12 bits for observation.
Finally, we reviewed the state of the art of high-speed video-image sensors. Currently the standard parallel-readout sensors
can operate at 1 Kfps with a pixel count of approximately 1 Kpixels. The In-situ Storage Image Sensor (ISIS) developed by
the authors has recently achieved frame rates of 1 Mfps. 相似文献
93.
W. A. Rodrigues Jr. Q. A. G. de Souza J. Vaz Jr. P. Lounesto 《International Journal of Theoretical Physics》1996,35(9):1849-1900
In this paper we study Dirac-Hestenes spinor fields (DHSF) on a four-dimensional Riemann-Cartan spacetime (RCST). We prove that these fields must be defined as certain equivalence classes of even sections of the Clifford bundle (over the RCST), thereby being certain particular sections of a new bundle named the spin-Clifford bundle (SCB). The conditions for the existence of the SCB are studied and are shown to be equivalent to Geroch's theorem concerning the existence of spinor structures in a Lorentzian spacetime. We introduce also the covariant and algebraic Dirac spinor fields and compare these with DHSF, showing that all three kinds of spinor fields contain the same mathematical and physical information. We clarify also the notion of (Crumeyrolle's) amorphous spinors (Dirac-Kähler spinor fields are of this type), showing that they cannot be used to describe fermionic fields. We develop a rigorous theory for the covariant derivatives of Clifford fields (sections of the Clifford bundle, CB) and of Dirac-Hestenes spinor fields. We show how to generalize the original Dirac-Hestenes equation in Minkowski spacetime for the case of RCST. Our results are obtained from a variational principle formulated through the multiform derivative approach to Lagrangian field theory in the Clifford bundle. 相似文献
94.
Chemical kinetics of benzonitrile nitration with mixed acid is investigated in the temperature range 283–299 K. Pseudo-first-order rate constants are evaluated by means of rate experiments on homogeneous reacting mixtures having large stoichiometric excesses of nitric acid. The second-order kinetic constants for nitronium ion attack to the aromatic substrate are derived on the basis of the assessed nitration mechanism. An activation energy of 604 ± 37 kJ mol?1 is calculated for this reaction step. © 1993 John Wiley & Sons, Inc. 相似文献
95.
One of the shortcomings of R&D evaluation is a lack of emphasis on analytical assessment of the value of an on-going R&D project. This paper addresses the problem concerning the distributive aspect of access to superior knowledge. Decision tree analysis and probability models appear to be appropriate tools for assessing the values of an intermediate result and patent reward of a firm's R&D decision in an environment of perfect information and oligopolistic competition. The assessed values are used as minimum prices acceptable to the firm when the knowledge is disseminated to the public. This paper will attempt to resolve this problem through the determination of the appropriate values of the reservation price of the first-stage invention in terms of the final reward and of a patent reward for which the inventor is willing to apply. 相似文献
96.
Comparative measurements were conducted for the backscattered intensities of light from uniform random and fractal aggregated media. Different features are found for the backscattered intensity peak shapes. A crossover between the θ1-D and θ-2 dependences of the backscattered intensity occurs in the case of fractal aggregated medium, where D indicates the fractal dimension. 相似文献
97.
T. B. Anders W. Jachmann A. O. Barut 《Zeitschrift fur Physik C Particles and Fields》1992,56(4):561-563
The polarization asymmetries related to usual polarizations combine at least 4 different helicity amplitudes. The introduction of ±45° inclined transverse polarizations allows us to obtain the complete set of new polarization asymmetries combining only up to 3 helicity amplitudes. There are no further polarization asymmetries depending on a smaller number of helicity amplitudes. These expressions are most economical to determine the helicity amplitudes from observed data. Some of them are suitable to study especially the spin-flip effects. We give a complete Table of all such polarization asymmetries. 相似文献
98.
Although the most important use of nuclear magnetic resonance imaging (MRI) continues to be for diagnostic medicine, recognition is being gained for many nonmedical applications. Examples include the following areas: petrogeology, food, agriculture, polymers and polymer-composites, and pharmaceuticals. These areas all involve studies of species that have short spin-spin relaxation times, and consequently need far fast gradient switching. These technical details are discussed and typical applications given. 相似文献
99.
Semi-empirical molecular orbital calculations were carried out for the compounds (C2H5)3As, (C2H5)3Ga and RAsH2 (R = C2H5, i-C3H7, i-C4H9, and t-C4H9) by using the CNDO/2-U program, and their capability of β-elimination reaction is compared on the basis of the torsion energy to the transition state, electrostatic interactions and orbital overlapping between the central atom and the β-hydrogen, and bond order of the metal-carbon, and carbon-hydrogen bond. In the comparison of (C2H5)3As with (C2H5)3Ga, we found that the β-elimination of (C2H5)3As could hardly be expected to take place in the thermal decomposition. The capability of β-elimination would be smaller in C2H5AsH2 than that in (C2H5)3As. Moreover when the ethyl group is replaced by a t-butyl group in RAsH2, the β-elimination reaction appears to become more difficult and a large possibility for a radical process is suggested. 相似文献
100.