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   403897篇
  免费   3980篇
  国内免费   747篇
化学   199711篇
晶体学   5670篇
力学   19349篇
综合类   12篇
数学   62206篇
物理学   121676篇
  2020年   2843篇
  2019年   3021篇
  2018年   12243篇
  2017年   12808篇
  2016年   9884篇
  2015年   4838篇
  2014年   5837篇
  2013年   13936篇
  2012年   14883篇
  2011年   23833篇
  2010年   15379篇
  2009年   15001篇
  2008年   21224篇
  2007年   24261篇
  2006年   11151篇
  2005年   15959篇
  2004年   12177篇
  2003年   11317篇
  2002年   9217篇
  2001年   8375篇
  2000年   6937篇
  1999年   5105篇
  1998年   4446篇
  1997年   4343篇
  1996年   4394篇
  1995年   3868篇
  1994年   4112篇
  1993年   3745篇
  1992年   4224篇
  1991年   4153篇
  1990年   4121篇
  1989年   3927篇
  1988年   4023篇
  1987年   3874篇
  1986年   3692篇
  1985年   4896篇
  1984年   5137篇
  1983年   4289篇
  1982年   4437篇
  1981年   4360篇
  1980年   4287篇
  1979年   4253篇
  1978年   4578篇
  1977年   4404篇
  1976年   4496篇
  1975年   4133篇
  1974年   4175篇
  1973年   4380篇
  1972年   3028篇
  1971年   2514篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Two new compounds of fluorine: (C2H5)4N[I2F] and (C2H5)4N[Br2F], have been easily synthesized in a nearly quantitative by a direct reaction of (C2H5)4NF, I2 and Br2. The products were isolated and characterized by elemental analysis and spectroscopic methods such as: Fourier transform infrared spectroscopy (FTIR) and ultraviolet-visible spectroscopy (UV-Vis). These compounds have been studied computationally with the Scalar ZORA relativistic level of theory using the ADF program package. The molecular parameters, and vibrational spectra were calculated. The excitation energies were found by timedependent perturbation density functional theory (TD-DFT). Molecule optimization, frequencies and excitation energies were calculated with standard Slatertype-orbital (STO) basis sets with triple-zeta quality double plus polarization functions (TZ2P) for all atoms. The FTIR, UV-Vis spectra and assignment of principal transitions and total density of state (TDOS) were extracted using the GaussSum 2.2 program. The comparison between experimental and calculated values showes that the experimental results correlate well with the predicted data.  相似文献   
72.
73.
Development of biocompatible porous supports is a promising strategy in the field of tissue engineering for the repair and regeneration of bone tissues with severe damage. Graphene oxide aerogels (GOAs) are excellent candidates for the manufacture of these systems due to their porosity, ability to imitate bone structure, and mechanical resistance, and according to their surface chemical reactivity, they can facilitate osseointegration, osteogenesis, osteoinduction and osteoconduction. In this review, synthesis of GOAs from the most primary source is described, and recent studies on the use of these functionalized carbonaceous foams as scaffolding for bone tissue regeneration are presented.  相似文献   
74.
75.
76.
Applied Magnetic Resonance - The suggested approach allowed us to derive analytical expressions for modeling the shape of solid-echo signal and its time evolution for a system of three-spin groups...  相似文献   
77.
The synthesis and characterisation of a family of block codendrimers consisting of highly versatile mesogenic and carbazole‐containing 2,2‐bis(hydroxymethyl)propionic acid (bis‐MPA) dendrons are reported. The liquid‐crystal behaviour was investigated by means of differential scanning calorimetry, polarised‐light optical microscopy and X‐ray diffraction. Depending on the chemical structure of the constituent dendrons, the codendrimers show lamellar or columnar mesophases. On the basis of the experimental results, models both at the molecular level and in the mesophase are proposed. The physical properties of the block codendrimers derived from the presence of the carbazole moiety in their structure were investigated: photoluminescence in solution and in the mesophase, electrochemical behaviour and hole transport. Electrodeposition of carbazole dendrons afforded a globular supramolecular conformation in which the mesogenic molecular side plays a key role.  相似文献   
78.
79.
80.
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