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   94676篇
  免费   2883篇
  国内免费   2532篇
化学   36977篇
晶体学   1043篇
力学   7611篇
综合类   187篇
数学   33507篇
物理学   20766篇
  2023年   267篇
  2022年   524篇
  2021年   575篇
  2020年   616篇
  2019年   625篇
  2018年   10811篇
  2017年   10637篇
  2016年   6780篇
  2015年   1617篇
  2014年   1220篇
  2013年   1579篇
  2012年   5287篇
  2011年   11951篇
  2010年   6704篇
  2009年   6952篇
  2008年   7663篇
  2007年   9653篇
  2006年   1165篇
  2005年   2195篇
  2004年   2233篇
  2003年   2593篇
  2002年   1711篇
  2001年   718篇
  2000年   718篇
  1999年   535篇
  1998年   474篇
  1997年   405篇
  1996年   447篇
  1995年   352篇
  1994年   262篇
  1993年   264篇
  1992年   213篇
  1991年   199篇
  1990年   172篇
  1989年   157篇
  1988年   144篇
  1987年   114篇
  1986年   118篇
  1985年   109篇
  1984年   75篇
  1983年   65篇
  1982年   62篇
  1981年   58篇
  1980年   55篇
  1979年   51篇
  1914年   45篇
  1913年   40篇
  1912年   40篇
  1909年   41篇
  1908年   40篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
使用Monte Carlo模拟的方法得出了随机粗糙表面,在Kirchhoff近似的基础上,利用数值分析的方法分析了一维随机粗糙表面的散射特征,得出了一维随机粗糙表面散射分布曲线,我们还考虑了遮蔽效应的影响,并且讨论了Kirchhoff近似的有效性。  相似文献   
4.
5.
6.
7.
Structural hierarchy of rock masses and the mechanisms of its formation   总被引:1,自引:0,他引:1  
The formation mechanisms for the structural hierarchy in geological media are discussed. It is shown that the formation of such a hierarchy is caused by certain external and internal circumstances. The first ones consist in the fact that, because of external actions, the Earth’s poles continuously execute the translational and rotational motions responsible for a regular structure of rock fracture and for the appearance of the scale factor √2. The second ones consist in the fact that, under the action of many random factors and because of external actions, during the formation of geological media there appear dissipative structures and, hence, some self-similar fractal structures are formed.  相似文献   
8.
Nicholas Nathan tries to resist the current version of the causal argument for sense-data in two ways. First he suggests that, on what he considers to be the correct re-construction of the argument, it equivocates on the sense of proximate cause. Second he defends a form of disjunctivism, by claiming that there might be an extra mechanism involved in producing veridical hallucination, that is not present in perception. I argue that Nathan’s reconstruction of the argument is not the appropriate one, and that, properly interpreted, the argument does not equivocate on proximate cause. Furthermore, I claim that his postulation of a modified mechanism for hallucinations is implausibly ad hoc.  相似文献   
9.
A poly(p‐phenylenevinylene) (PPV) derivative containing a bulky (2,2‐diphenylvinyl)phenyl group in the side chain, EHDVP‐PPV, was synthesized by Gilch route. The reduced tolane‐bisbenzyl (TBB) defects, as well as the structure of the polymer, was confirmed by various spectroscopic methods. The intramolecular energy transfer from the (2,2‐diphenylvinyl)phenyl side group to the PPV backbone was studied by UV‐vis and photoluminescence (PL) of the obtained polymer and model compound. The polymer film showed maximum absorption and emission peaks at 454 and 546 nm, respectively, and high PL efficiency of 57%. A yellow electroluminescence (λmax = 548 nm) was obtained with intensities of 6479 cd/m2 when the light‐emitting diodes of ITO/PEDOT/EHDVP‐PPV/LiF/Al were fabricated. The maximum power efficiency of the devices was 0.729 lm/W with a turn‐on voltage of 3.6 V. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5636–5646, 2004  相似文献   
10.
A series of polyimides were synthesized from 2,2‐Bis(3,4‐dicarboxyphenyl)hexafluoropropane, 2,2‐bis(3‐amino‐4‐hydroxyphenyl)‐hexafluoropropane, and 4,4′‐oxydianiline by chemical imidization. The effects of the diamine ratios on the properties of the films were evaluated through the study of their thermal, electrical, and morphological properties. All the polymers exhibited better solubility in most of the organic solvents and hence were easily processable. Polyimides with more 2,2‐bis(3‐amino‐4‐hydroxyphenyl)‐hexafluoropropane exhibited better solubility and a low refractive index, which is highly desired for microelectronic applications. The dielectric constant and birefringence were strongly dependent on the fluorine content. With an increase in the fluorine substitution, both the dielectric constant and birefringence decreased. All the polymers exhibited high thermal stability (>400 °C). The absence of crystalline melting in differential scanning calorimetry and broad wide‐angle X‐ray diffraction patterns revealed the amorphous nature of the polymers, which was due to the presence of bulky CF3 groups and hinged ether linkages of the diamine component. The residual stress values decreased with an increase in the 4,4′‐oxydianiline content, and the results were in agreement with the dielectric constant. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4303–4312, 2004  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