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   460356篇
  免费   5301篇
  国内免费   1540篇
化学   236983篇
晶体学   7167篇
力学   21620篇
综合类   13篇
数学   54071篇
物理学   147343篇
  2021年   3866篇
  2020年   4189篇
  2019年   4656篇
  2018年   6061篇
  2017年   6302篇
  2016年   9138篇
  2015年   5578篇
  2014年   8899篇
  2013年   21839篇
  2012年   16949篇
  2011年   20701篇
  2010年   14636篇
  2009年   14557篇
  2008年   18660篇
  2007年   18680篇
  2006年   17297篇
  2005年   15399篇
  2004年   14219篇
  2003年   12452篇
  2002年   12401篇
  2001年   14534篇
  2000年   10976篇
  1999年   8524篇
  1998年   6896篇
  1997年   6834篇
  1996年   6481篇
  1995年   5662篇
  1994年   5492篇
  1993年   5307篇
  1992年   5933篇
  1991年   6106篇
  1990年   5686篇
  1989年   5589篇
  1988年   5406篇
  1987年   5440篇
  1986年   5140篇
  1985年   6895篇
  1984年   6903篇
  1983年   5686篇
  1982年   5958篇
  1981年   5620篇
  1980年   5492篇
  1979年   5701篇
  1978年   5776篇
  1977年   5764篇
  1976年   5658篇
  1975年   5447篇
  1974年   5308篇
  1973年   5399篇
  1972年   3640篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
191.
192.
Exposure of cimetidine (CIM) to dry heat (160–180 °C) afforded, upon cooling, a glassy solid containing new and hitherto unknown products. The kinetics of this process was studied by a second order chemometrics-assisted multi-spectroscopic approach. Proton and carbon-13 nuclear magnetic resonance (NMR), as well as ultraviolet and infrared spectroscopic data were jointly used, whereas multivariate curve resolution with alternating least squares (MCR-ALS) was employed as the chemometrics method to extract process information. It was established that drug degradation follows a first order kinetics.  相似文献   
193.
194.
195.
Kim  F. H.  Moylan  S. P.  Phan  T. Q.  Garboczi  E. J. 《Experimental Mechanics》2020,60(7):987-1004
Experimental Mechanics - Insufficient data are available to fully understand the effects of metal additive manufacturing (AM) defects for widespread adoption of the emerging technology....  相似文献   
196.
A model is developed for the formation and propagation of cracks in a material sample that is heated at its top surface, pyrolyses, and then thermally degrades to form char. In this work the sample is heated uniformly over its entire top surface by a hypothetical flame (a heat source). The pyrolysis mechanism is described by a one-step overall reaction that is dependent nonlinearly on the temperature (Arrhenius form). Stresses develop in response to the thermal degradation of the material by means of a shrinkage strain caused by local mass loss during pyrolysis. When the principal stress exceeds a prescribed threshold value, the material forms a local crack. Cracks are found to generally originate at the surface in response to heating, but occasionally they form in the bulk, away from ever-changing material boundaries. The resulting cracks evolve and form patterns whose characteristics are described. Quantities examined in detail are: the crack spacing in the pyrolysis zone; the crack length evolution; the formation and nature of crack loops which are defined as individual cracks that have joined to form loops that are disconnected from the remaining material; the formation of enhanced pyrolysis area; and the impact of all of the former quantities on mass flux. It is determined that the mass flux from the sample can be greatly enhanced over its nominal (non-cracking) counterpart. The mass efflux profile qualitatively resembles those observed in Cone Calorimeter tests.  相似文献   
197.
Russian Physics Journal - Using the potentials, generated by a tight binding approximation, the equilibrium atomic structure and the phonon spectra of Cr and Ni thin films (one monolayer) deposited...  相似文献   
198.
199.
200.
π-Extended molecules are key components for the development of materials science. In fact, polyaromatic structures are fundamental for the scientific and technological progress of fields such as organic electronics and bio-applications. Beneficial properties of π-extended structures are absorption in the visible region, often luminescence, high electron mobilities and stability. Common approaches to adjust the properties of polyaromatic structures to functional setups involve changes in shape and size at the molecular level. Recently, incorporating hetero-elements emerged as successful approach. In this regard, organophosphorus conjugated molecules are new materials holding great promise for potential applications. In this review, we comprehensively discuss the design/development of polyaromatic phosphorus materials and their applicability. We establish structure/property/applicability relationships to provide key guidelines for the engineering of newer, future applications. This article thus provides a source of information for the further development of this rapidly evolving field of research.  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