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   1336596篇
  免费   29818篇
  国内免费   7737篇
化学   665456篇
晶体学   20347篇
力学   74719篇
综合类   117篇
数学   243327篇
物理学   370185篇
  2021年   13641篇
  2020年   16095篇
  2019年   16208篇
  2018年   12819篇
  2016年   28192篇
  2015年   21024篇
  2014年   30718篇
  2013年   74886篇
  2012年   37913篇
  2011年   34710篇
  2010年   37259篇
  2009年   39655篇
  2008年   34087篇
  2007年   29744篇
  2006年   35667篇
  2005年   28405篇
  2004年   29494篇
  2003年   27883篇
  2002年   29026篇
  2001年   27491篇
  2000年   24403篇
  1999年   22301篇
  1998年   21002篇
  1997年   21052篇
  1996年   21251篇
  1995年   19319篇
  1994年   18794篇
  1993年   18379篇
  1992年   18162篇
  1991年   18417篇
  1990年   17522篇
  1989年   17615篇
  1988年   17171篇
  1987年   17189篇
  1986年   16071篇
  1985年   22523篇
  1984年   23804篇
  1983年   19986篇
  1982年   21697篇
  1981年   20928篇
  1980年   20299篇
  1979年   20397篇
  1978年   21735篇
  1977年   21309篇
  1976年   21038篇
  1975年   19701篇
  1974年   19374篇
  1973年   19807篇
  1972年   14306篇
  1967年   12553篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
871.
Methods for phase stabilization of ammonium nitrate were sought for in order to considerably expand the application area of this oxidizing agent in various-purpose self-combustible formulations, including that in a new generation of gas-generating formulations for automobile air bags. New methods for stabilization of ammonium nitrate were studied and, in particular, a search was made for organic compounds that can stabilize ammonium nitrate even at their low content. The mechanism of phase state stabilization of ammonium nitrate by compounds of this kind was examined.  相似文献   
872.
873.
In this work, the variation of the magnetic moments of the Ni/Pt multilayers are studied using the linearized augmented plane waves (LAPW) method in the framework of the density functional theory (DFT) implemented in the version of WIEN2K program. The systems have been modeled by seven layers slab separated in z direction by a vacuum region of four substrate layers. We present the results of the dependence of the magnetic properties with respect to the thickness variation of the different multilayers. The modeling of these systems finds an important empirical support. Experiment and theory show the same trends for the magnetic moments: hybridization effects between Ni and Pt are mostly localized at the interface.  相似文献   
874.
In this study, stoichiometric protonation constants of L-tyrosine, L-cysteine, L-tryptophane, L-lysine, and L-histidine, and their methyl and ethyl esters in water and ethanol–water mixtures of 30, 50, and 70% ethanol (v/v), were determined potentiometrically using a combined pH electrode system calibrated as the concentration of hydrogen ion. Titrations were performed at 25C and the ionic strength of the medium was maintained at 0.10 mol⋅L−1 using sodium chloride. Protonation constants were calculated by using the BEST computer program. The effect of solvent composition on the protonation constants is discussed. The log10 K2 values of esters generally decreased with increasing ethanol content. However, the log10 K1 values of the esters of L-tyrosine, L-cysteine, and L-tryptophane were found to increase with increasing ethanol content in contrast those of L-lysine and L-histidine esters.  相似文献   
875.
Results from 5D induced-matter and membrane theory with null paths are extended to show that a particle obeys the 4D Klein-Gordon equation but with a variable mass. The Dirac equation also follows, but raises concerns about 4D quantization in the two natural 5D gauges, and reopens the question of a Regge-like trajectory for the spin angular momenta and squared masses of gravitationally-dominated systems.  相似文献   
876.
In this work the results of the statistical topometric analysis of fracture surfaces of soda-lime-silica glass with and without ionic exchange treatment are reported. In this case, the mechanism of substitution is K+-Na+. atomic force microscopy (AFM) was employed to record the topometric data from the fracture surface. The roughness exponent (ζ) and the correlation length (ξ) were calculated by the variable bandwidth method. The analysis for both glasses (subjected and non-subjected to ionic exchange) for ζ shows a value ∼0.8, this value agrees well with that reported in the literature for rapid crack propagation in a variety of materials. The correlation length shows different values for each condition. These results, along with those of microhardness indentations suggest that the self-affine correlation length is influenced by the complex interactions of the stress field of microcracks with that resulting from the collective behavior of the point defects introduced by the strengthening mechanism of ionic exchange.  相似文献   
877.
Holographic sensors for monitoring ionic strength have been fabricated from charged sulphonate and quaternary ammonium monomers, incorporated into thin, polymeric hydrogel films which were transformed into volume holograms. The diffraction wavelength or reflected colour of the holograms was used to characterise their swelling or de-swelling behaviour as a function of ionic strength in various media. The effects of co-monomer structure, buffer composition, ion composition, pH and temperature were evaluated, whilst the reversibility and reproducibility of the sensor was also assessed. An acrylamide-based hologram containing equal molar amounts of negatively and positively charged monomers was shown to be able to quantify ionic strength independent of the identity of the ionic species present in the test solution. The sensor was fully reversible, free of hysteresis and exhibited little response to pH between 3 and 9 and temperature within the range 20-45 °C. The system was successfully used to quantify the ionic strength of milk solutions, which contain a complex mixture of ions and biological components.  相似文献   
878.
Using the simplest (sinusoidal) corrugation profile in the approximation of absence of losses in the material as an example, we study the regimes of complete transformation of an incident wave to the diffraction lobe by a corrugated metal surface or a corrugated interface of two dielectric media. A numerical-intuitive pattern of appearance and evolution of such regimes with increasing corrugation amplitude is revealed. It is shown that the regimes of complete transformation of an incident wave to the diffraction lobe are possible in both the case of autocollimation and the case of a considerable deviation from it. The examination is based on a numerical method of solving the integral equation by means of a specially created interactive processing system in Visual Fortran.  相似文献   
879.
880.
The possibility of realizing SASE Free Election Laser in x-ray waveband with EM-wave wiggler is studied in this paper. SASE effect including saturation length, saturation power and nonlinear process is explored through a 3-D simulation code. A utilization of seed light from laser plasma x-ray is also analyzed, which demonstrates the feature of shortening the interaction length for saturation. The results show that sizeable output power of x-ray laser would be generated with a middle energy electron beam.  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