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   342747篇
  免费   2377篇
  国内免费   994篇
化学   165007篇
晶体学   4915篇
力学   18475篇
综合类   9篇
数学   60430篇
物理学   97282篇
  2019年   2298篇
  2018年   13248篇
  2017年   13090篇
  2016年   10361篇
  2015年   3462篇
  2014年   4555篇
  2013年   10956篇
  2012年   11657篇
  2011年   19907篇
  2010年   12497篇
  2009年   12696篇
  2008年   15636篇
  2007年   17877篇
  2006年   9086篇
  2005年   9273篇
  2004年   8877篇
  2003年   8617篇
  2002年   7599篇
  2001年   7545篇
  2000年   5936篇
  1999年   4495篇
  1998年   4037篇
  1997年   3974篇
  1996年   3772篇
  1995年   3441篇
  1994年   3569篇
  1993年   3283篇
  1992年   3723篇
  1991年   3730篇
  1990年   3654篇
  1989年   3522篇
  1988年   3634篇
  1987年   3522篇
  1986年   3347篇
  1985年   4413篇
  1984年   4677篇
  1983年   3912篇
  1982年   4047篇
  1981年   3971篇
  1980年   3956篇
  1979年   3927篇
  1978年   4229篇
  1977年   4125篇
  1976年   4193篇
  1975年   3870篇
  1974年   3908篇
  1973年   4078篇
  1972年   2869篇
  1971年   2387篇
  1970年   2185篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
(Na, K)NbO3 crystals with a perovskite structure and a KNbO3 content up to 40 mol % were grown from flux with the use of the solvent NaBO2. The dielectric measurements of the crystals grown revealed phase transitions that had never been observed before in ceramic samples.  相似文献   
92.
 Results are presented of an experimental investigation of vortex ring formation by a fluid drop contacting a free surface with negligible velocity. The pool fluid is mixed with fluorescein dye, and a laser sheet is used to illuminate a plane of the flow. A series of representative images is recorded by a CCD camera and speculation is made regarding specific sources of vorticity flux through the free surface. Two scaling analyses previously presented by other investigators are demonstrated to be equivalent under the assumptions of this experiment, and they provide the motivation for a series of test runs in which the duration of the coalescence process, τ*, is related to variations in drop diameter L and fluid surface tension σ. Experimental results are in agreement with the analyses, showing τ*∼σ-1/2 and τ*L 3/2. Received: 22 December 1995 / Accepted: 15 October 1996  相似文献   
93.
Summary For accurate speciation analyses it is important to know the stability of the respective species, especially in the case of metal complexes. Factors affecting the chromatographic stability of such metal species are investigated. By using thermodynamic models for complex formation and chromatographic retention equilibria the influence of species concentration, stoichiometry and excess of ligand is calculated and compared with experimental results for iron complexes (lactate, gluconate and citrate species). Iron citrate is the only species, that is chromatographed as 1:2 complex (metal: ligand), while iron lactate and gluconate are transformed to 1:1 species. Problems resulting from the coelution of different species are discussed.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday  相似文献   
94.
Summary Observation of fluorescence reactions on TLC plates is a valuable additional tool within the scope of screening procedures for many toxicologically relevant substances commonly encountered in clinical- and forensic-toxicological analysis. The reactions are based on native fluorescence without any treatment and on reactions obtained with an approved derivatization procedure. Due to the enormous sensitivity of the fluorescence detection, the method is also applicable to very low concentrations and small amounts of biological materials. The procedures described in this article have proven their high pragmatic usefulness in many practical cases.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday  相似文献   
95.
Summary X-ray reflectivity may be used to determine the internal structure of thin polymer films. An electron density difference of 10% for polystyrene and polyisoprene is sufficient to distinguish between a random distribution of lamellae, complete orientation parallel to the substrate surface and a surface induced formation of lamellae. The disappearance of the lamellar Bragg-peaks, with heating of the film, shows the transition into the disordered state.  相似文献   
96.
Electric fields of the anions, cations and neutral forms of 2-aminopurine and 6-thioguanine have been mapped. Certain important features of the maps are similar to those found earlier in the neutral and ionic forms of adenine and guanine. The computed electric field patterns satisfactorily explain reactive sites and biological activity of the molecules.  相似文献   
97.
Investigations to the causes and effects of contaminants at the rf diode deposition of CrSi films in a non-heated high vacuum apparatus were carried out comparing an oxygen-free fusion target with an oxygen-containing cermet target. The films of the fusion target contained considerable amounts of oxygen; analogously the oxygen concentration in the films was increased in the case of the cermet target. Moreover, hydrogen was found. The concentration of the contaminants decreased with the sputtering power, however the rate of incorporation increased. For high sputtering rates the contamination process was reproducible and independent of the plant conditioning; the deposited films were depth-homogeneous. The reasons for this behaviour lie in water sources which are activated first of all by the discharge itself. The electrical properties found are explained by the contaminants and the deposition conditions.  相似文献   
98.
99.
In 1980 Chern and Terng defined a Bäcklund transformation for affine minimal surfaces. In this paper we show that this Bäcklund transformation can be simply represented by an involution and translation of the affine conormal.  相似文献   
100.
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