首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   330802篇
  免费   3074篇
  国内免费   937篇
化学   165324篇
晶体学   5377篇
力学   15552篇
综合类   7篇
数学   38563篇
物理学   109990篇
  2021年   3155篇
  2020年   3357篇
  2019年   3789篇
  2018年   5225篇
  2017年   5472篇
  2016年   7409篇
  2015年   4163篇
  2014年   7057篇
  2013年   15969篇
  2012年   12427篇
  2011年   14875篇
  2010年   11052篇
  2009年   10937篇
  2008年   13277篇
  2007年   13234篇
  2006年   12174篇
  2005年   10547篇
  2004年   9856篇
  2003年   8697篇
  2002年   8686篇
  2001年   10400篇
  2000年   7668篇
  1999年   5971篇
  1998年   4977篇
  1997年   4816篇
  1996年   4483篇
  1995年   3870篇
  1994年   3827篇
  1993年   3708篇
  1992年   4100篇
  1991年   4312篇
  1990年   4100篇
  1989年   4011篇
  1988年   3787篇
  1987年   3922篇
  1986年   3683篇
  1985年   4636篇
  1984年   4688篇
  1983年   3895篇
  1982年   3981篇
  1981年   3703篇
  1980年   3650篇
  1979年   3916篇
  1978年   3906篇
  1977年   3915篇
  1976年   3882篇
  1975年   3681篇
  1974年   3619篇
  1973年   3660篇
  1972年   2606篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
201.
We report viscometric data collected in a Couette rheometry on dilute, single‐solvent polystyrene (PS)/dioctyl phthalate (DOP) solutions over a variety of polymer molecular weights (5.5 × 105Mw ≤ 3.0 × 106 Da) and system temperatures (288 K ≤ T ≤ 318 K). In view of the essential viscometric features, the current data may be classified into three categories: The first concerns all the investigated solutions at low shear rates, where the solution properties are found to agree excellently with the Zimm model predictions. The second includes all sample solutions, except for high‐molecular‐weight PS samples (Mw ≥ 2.0 × 106 Da), where excellent time–temperature superposition is observed for the steady‐state polymer viscosity at constant polymer molecular weights. No similar superposition applies at a constant temperature but varied polymer molecular weights, however. The third appears to be characteristic of dilute high‐molecular‐weight polymer solutions, for which the effects of temperature on the viscosity curve are further complicated at high shear rates. The implications concerning the relative importance of hydrodynamic interactions, segmental interactions, and chain extensibility with increasing polymer molecular weight, system temperature, and shear rate are discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 787–794, 2006  相似文献   
202.
In this paper, we demonstrate that laser vaporization of metals in the presence of a small concentration of butadiene vapor leads to the polymerization of butadiene and incorporation of the metal nanoparticles within the polymer matrix. The metal nanocomposites are characterized by electron microscopy, X-ray diffraction and EDX. The results from high pressure mass spectrometry indicate that multiple additions of butadiene molecules on the metal cations Fe+, Ni+ and Pt+, generated by laser vaporization, take place at room temperature thus providing an efficient means of initiating further polymerization reactions. The Pt+ reactions show extensive fragmentations and elimination steps generating hydrocarbon ions. The laser vaporization/polymerization method provides the ability to encapsulate several different metals or metal oxides which undoubtedly will play a significant role in tuning the various properties of the polymer composites.  相似文献   
203.
204.
205.
206.
207.
The solution of the Poisson-Boltzmann equation for a cloud of charges surrounding two charged dust particles treated as Debye atoms forming a Debye molecule is investigated numerically using Cassini coordinates. The electric force exerted on a dust particle by the other dust particle was determined by integrating the electrostatic pressure on the surface of the dust particle. It is shown that attractive forces appear when the following two conditions are satisfied. First, the Debye radius (corresponding to the electron density at half the mean distance between the dust particles) must be approximately equal to half the mean distance between the dust particles. Attraction between the dust particles emerges at a distance equal approximately to half the mean distance between the dust particles. Second, attraction takes place when like charges are concentrated predominantly on the dust particles. If the particles carry a small fraction of charge of the same polarity, repulsion between the particles takes place at all distances.  相似文献   
208.
The effects of stimulus frequency and intensity on response patterns (PST histograms) to tone burst stimulation were examined in differently tuned saccular fibers of the goldfish. In addition, the sensitivity of these fibers to amplitude-modulated (AM) signals of different carrier frequencies was measured. The response patterns evoked by unmodulated signals were a complex function of tuning, spontaneous activity and sensitivity of the fiber, and the frequency and intensity of the signal. Frequency-dependent response patterns were found in low-frequency fibers with best frequencies (BF) below 200 Hz. Responses in these fibers ranged from tonic to phasic in nonspontaneous fibers and included more complex patterns in spontaneously active fibers, such as suppression of evoked activity below spontaneous levels. Midfrequency fibers (BF = 500-600 Hz) showed responses similar to those in low-frequency fibers, but with less dependence on frequency. In contrast, both high-frequency (BF = 800-1000 Hz) and wideband, untuned fibers showed frequency-invariant patterns of adaptation. High-frequency fibers were equally sensitive to AM signals at all frequencies tested. The sensitivity of low-frequency fibers to AM, however, increased as a function of carrier frequency and corresponded to the degree of adaptation in response to unmodulated tones. In general, the AM sensitivity of a fiber could be predicted more by its pattern of response to unmodulated signals than by its tuning characteristics.  相似文献   
209.
It is shown that the small-scale Rossby waves in a shallow rotating fluid placed in a gravitational field can generate large-scale zonal flows by means of a two-stream-like mechanism. This mechanism is revealed under the conditions when the Lighthill instability criterion is not satisfied. The text was submitted by the authors in English.  相似文献   
210.
The interaction of water-soluble cationic 5,10,15,20-tetrakis(4-N-methyl pyridyl) porphyrin (H2TMPyP4) with some mono-and polynucleotides is studied by time-resolved and steady-state fluorescence spectroscopy, as well as by steady-state absorption spectroscopy. The fluorescence decay kinetics are analyzed by reconstructing the decay time distributions, which made it possible to describe in more detail than previously the complexes formed due to the interaction. The main effect of binding of H2TMPyP4 adenosine 5′-monophosphate and to poly(dA-dT)2 is shown to be an increase in the fluorescence lifetime from 4.6 ns in the solution to 8.3 and 12.3 ns, respectively. This effect is explained by a less polar (in comparison with water) environment of porphyrin in complexes, which leads to a decrease in the quenching action of the intramolecular charge transfer state between the porphyrin macrocycle and methyl pyridyl groups. In the case of complex formation with guanine-containing nucleotides (guanosine 5′-monophosphate and poly(dG-dC)2), the effect of decrease in the quenching action of the intramolecular charge transfer state caused by a decrease in the medium polarity is superimposed by a stronger effect of decrease in the fluorescence lifetime of porphyrin as a result of intermolecular electron transfer from guanine to excited porphyrin. A high sensitivity of this intermolecular quenching to the mutual arrangement of the electron donor and the electron acceptor makes it possible to reveal four types of complexes between H2TMPyP4 and guanosine 5′-monophosphate, which differ in the positions of four broad peaks in the porphyrin fluorescence decay time distribution (0.1, 0.7, 2.4, and 6.1 ns). For the complex with poly(dG-dC)2, a narrow peak at 2.8 ns prevails in the fluorescence decay time distribution, with the contributions from two additional narrow peaks at 1.0 and 6.2 ns being small.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号