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   219939篇
  免费   3000篇
  国内免费   2070篇
化学   115057篇
晶体学   3484篇
力学   9636篇
综合类   56篇
数学   24702篇
物理学   72074篇
  2021年   2062篇
  2020年   2064篇
  2019年   2307篇
  2018年   2977篇
  2017年   2867篇
  2016年   4352篇
  2015年   2782篇
  2014年   4158篇
  2013年   10289篇
  2012年   8520篇
  2011年   10105篇
  2010年   6821篇
  2009年   6938篇
  2008年   8875篇
  2007年   8996篇
  2006年   8519篇
  2005年   7737篇
  2004年   7577篇
  2003年   6567篇
  2002年   6319篇
  2001年   6887篇
  2000年   5255篇
  1999年   4046篇
  1998年   3247篇
  1997年   3227篇
  1996年   2972篇
  1995年   2762篇
  1994年   2569篇
  1993年   2475篇
  1992年   2804篇
  1991年   2743篇
  1990年   2673篇
  1989年   2461篇
  1988年   2508篇
  1987年   2501篇
  1986年   2343篇
  1985年   3048篇
  1984年   3299篇
  1983年   2695篇
  1982年   2859篇
  1981年   2828篇
  1980年   2689篇
  1979年   2760篇
  1978年   2849篇
  1977年   2855篇
  1976年   2912篇
  1975年   2694篇
  1974年   2669篇
  1973年   2788篇
  1972年   1776篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
The properties of a polymer-concrete composed of polyester matrix and locally available rock aggregate are investigated. The formula of the concrete is found by an experimental-calculation approach in such a way as to attain a closer packing of the aggregate particles on the one hand, and to ensure the needed processing characteristics (placeability) of the mix on the other. It is shown experimentally that the material obtained has a rather high compression strength. Under prolonged compression loads, the polymer-concrete exhibits a noticeable creep behavior with a linear relation between the creep strains and stresses. After the action of half the ultimate load over 3000 h, the total strains exceed the instantaneous ones by 2.0 to 2.2 times. The accumulation of irreversible strains is also observed; however, their contribution to the total strain is small. It is found that the stress-strain relation can be represented by the equation of linear hereditary creep theory.Institute of Polymer Mechanics, University of Latvia, Riga, LV-1006, Latvia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 2, pp. 147–162, March–April, 1999.  相似文献   
152.
153.
Selective photoconductivity at frequency ω=155 cm was discovered in PbTe(Ga) narrow-gap semiconductors at liquid-nitrogen temperatures. The corresponding energy is much lower than all characteristic energies of the electronic spectrum of the semiconductor. The effect is attributed to optical excitation of a local vibrational mode of an impurity center leading to delocalization of the electrons. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 5, 342–346 (10 March 1996)  相似文献   
154.
Phase Relations and Sodium Ion Conductivity within the Quasi-binary System Na2SiF6/Na2AIF6 . The phase diagram of the Na2SiF6/Na3AlF6 system has been determined by means of x-ray powder diffraction, thermal analysis and conductivity measurements in the sub-solidus region. Na3AlF6 accomodates up to 73 mol.-% Na2SiF6 maintaining the crystal structure type. The sodium ion conductivity increases by about five orders of magnitude upon doping Na3AlF6 with Na2SiF6.  相似文献   
155.
Oxidation of the title compounds yields, besides the reported isoimides 3 and/or the amides 4 , also the imides 5 . The observed product dichotomy is considered as the result of an intramolecular nucleophilic attack on the aroyl group, of the pressumed zwitterionic intermediate 2 , by O or N present in the ambident N-aroylimine site of 2 . The results of AM1 calculations agree with the product studies and both permit the formulation of a set of rules correlating structure and selectivity.  相似文献   
156.
Surface relief formation at holographic recording on amorphous selenium films was demonstrated and investigated. The presence of this optical phase modulation component is essential for ensuring significant, stable and erasable optical recording in a-Se films at 290–320 K temperatures, where conventional photodarkening was known as insignificant and unstable. Photocrystallization can only be observed in super-exposed a-Se films at the given experimental conditions of hologram recording. Erasing behavior of surface relief gratings under heat treatment was also investigated in order to reveal further details of the mechanism. Photoinduced structural transformations within the amorphous phase, connected to local ordering under the condition of light-induced fluidity, are proposed as an explanation for the relief formation and erasing. The observed reversible optical recording process may be useful for the various optoelectronic applications of photoconductive a-Se layers. Received: 12 June 2000 / Accepted: 6 June 2001 / Published online: 30 August 2001  相似文献   
157.
This paper reports on the first experimental observation of quantum-well states and sp-type resonances in thin single-crystal gold, silver, and copper layers formed on single-crystal W(110) surfaces, which result from spatial localization of Bloch-type electronic wave functions in a quantum well with potential barriers at the vacuum/metal and metal/W(110) interfaces. The quantization of the valence-band electronic structure in Au/W(110), Ag/W(110), and Cu/W(110) systems was studied experimentally using angle-resolved photoelectron spectroscopy.  相似文献   
158.
We present a stratification by “normal flatness” associated to an analytic mapping, analogous to Hironaka's classical result for analytic spaces. Our construction is based on a generic normal flatness theorem for mappings, proved using techniques concerning the variation of modules of meromorphically parametrized formal power series [1]. The existence of such a stratification was announced by Hironaka [13], but the other claims made in [13] are false. Counterexamples are also presented here.  相似文献   
159.
We consider the growth of a spherical crystal in a supersaturatedsolution. In the first part, existence and uniqueness resultsfor radially symmetric growth are obtained, provided that thesupersaturation is not too large; conversely, when the far-fieldsupersaturation exceeds a critical value, it is shown that theradially symmetric solution ceases to exist in finite time.In the second part, we examine the linear stability of a radiallysymmetric similarity solution (in which the radius grows ast?) to shape perturbations. The results are compared with previousquasi-static analyses, and, in particular, the critical radiusat which the crystal becomes unstable is found to be largerfor small supersaturations, but smaller for large supersaturations,than those predicted by the quasi-static analysis  相似文献   
160.
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