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   1811篇
  免费   47篇
  国内免费   8篇
化学   1309篇
晶体学   7篇
力学   24篇
数学   149篇
物理学   377篇
  2021年   17篇
  2020年   32篇
  2019年   24篇
  2016年   38篇
  2015年   31篇
  2014年   26篇
  2013年   75篇
  2012年   64篇
  2011年   80篇
  2010年   46篇
  2009年   39篇
  2008年   73篇
  2007年   61篇
  2006年   70篇
  2005年   80篇
  2004年   51篇
  2003年   46篇
  2002年   61篇
  2001年   55篇
  2000年   41篇
  1999年   36篇
  1998年   21篇
  1997年   21篇
  1996年   30篇
  1995年   21篇
  1994年   28篇
  1993年   23篇
  1992年   35篇
  1991年   21篇
  1989年   22篇
  1988年   21篇
  1987年   14篇
  1986年   31篇
  1985年   19篇
  1984年   17篇
  1983年   17篇
  1981年   18篇
  1980年   20篇
  1979年   17篇
  1978年   20篇
  1977年   15篇
  1976年   21篇
  1975年   21篇
  1974年   16篇
  1973年   18篇
  1972年   13篇
  1970年   11篇
  1969年   12篇
  1968年   10篇
  1967年   12篇
排序方式: 共有1866条查询结果,搜索用时 27 毫秒
101.
The splitting of water into hydrogen and oxygen molecules using sunlight is an attractive method for solar energy storage. Until now, photoelectrochemical hydrogen evolution is mostly studied in acidic solutions, in which the hydrogen evolution is more facile than in alkaline solutions. Herein, we report photoelectrochemical hydrogen production in alkaline solutions, which are more favorable than acidic solutions for the complementary oxygen evolution half‐reaction. We show for the first time that amorphous molybdenum sulfide is a highly active hydrogen evolution catalyst in basic medium. The amorphous molybdenum sulfide catalyst and a Ni–Mo catalyst are then deposited on surface‐protected cuprous oxide photocathodes to catalyze sunlight‐driven hydrogen production in 1 M KOH. The photocathodes give photocurrents of ?6.3 mA cm?2 at the reversible hydrogen evolution potential, the highest yet reported for a metal oxide photocathode using an earth‐abundant hydrogen evolution reaction catalyst.  相似文献   
102.
The synthesis of titanium–carboxylate metal–organic frameworks (MOFs) is hampered by the high reactivity of the commonly employed alkoxide precursors. Herein, we present an innovative approach to titanium‐based MOFs by the use of titanocene dichloride to synthesize COK‐69, the first breathing Ti MOF, which is built up from trans‐1,4‐cyclohexanedicarboxylate linkers and an unprecedented [TiIV33‐O)(O)2(COO)6] cluster. The photoactive properties of COK‐69 were investigated in depth by proton‐coupled electron‐transfer experiments, which revealed that up to one TiIV center per cluster can be photoreduced to TiIII while preserving the structural integrity of the framework. The electronic structure of COK‐69 was determined by molecular modeling, and a band gap of 3.77 eV was found.  相似文献   
103.
104.
In this work, fresh and CO2-exposed specimens of Ba0.5Sr0.5Co0.8Fe0.2O3–δ (BSCF) are examined by atomic force microscopy (AFM) using amplitude-modulated Kelvin probe force microscopy (KPFM) and also electrostatic force microscopy (EFM) to characterize the early stages of the formation of reaction products due to reaction with gaseous CO2. A comparison is made with results from electron microscopy on the same samples. BSCF specimens exposed for 24 and 240 h to an atmosphere of 99.9 % CO2 at 900 °C, respectively, were analyzed and compared with non-exposed specimens. The observation of interconnected carbonate islands on BSCF forming a continuous carbonate layer after some exposure to CO2 indicates a Stranski–Krastanov or Volmer–Weber growth mechanism of the carbonate layer. Our results demonstrate that the measurement of surface potential variations by means of KPFM and EFM constitutes a very sensitive technique to detect the formation of reaction layers on gas permeation membranes such as BSCF. In contrast to electron microscopy techniques, scanning probe techniques permit the investigation of the topography and of electrochemical characteristics of the sample surface as received and without further preparation.  相似文献   
105.
A model of heterogeneous medium taking into account the friction between the particles and liquid, as well as the relaxation of the small-size particles to the equilibrium on the stress, has been proposed to describe the propagation of the elastic waves in a suspension. A system of wave equations describing the propagation of a plane longitudinal wave has been formulated for the components of the medium. Analytical expressions for the sound velocity in a suspension has been obtained in the approximation in which the particles are completely carried away by liquid in the limiting cases in which the particles are in equilibrium under stress with the liquid or equilibrium is absent. The dependence of the sound velocity in the medium on the volumetric portion and the size of the inclusions has been studied. The obtained results agree with the experimental data and obtained analytical expressions for the sound velocity. The dynamics of the components of the medium at the propagation of the plane longitudinal monochromatic wave has been studied.  相似文献   
106.
107.
The treatment of α‐chiral secondary alkyl iodides with tBuLi at ?100 °C leads to the corresponding secondary alkyllithiums with high retention of configuration. Subsequent quenching with various electrophiles such as Bu2S2, DMF, MeOB(OR)2, or Et2CO provides the desired products with retention of configuration. Furthermore, a transmetalation with CuBr?P(OEt)3 also allows retentive trapping with acid chlorides and ethylene oxide. The quenching of the resulting alkyllithiums with ClCO2Et furnishes stereoselectively syn‐ and anti‐ethyl‐2,3‐dimethyl ester carboxylates (d.r.>94 %). Related esters bearing three adjacent stereo‐controlled centers (stereotriads) have also been prepared. This method has been applied to the synthesis of the ant pheromone (±)‐lasiol in 26 % overall yield (four steps) with d.r.=97:3 starting from commercially available cis‐2,3‐epoxybutane.  相似文献   
108.
We present five new variance reduction techniques applicable to Monte Carlo simulations of radiative transfer in the atmosphere: detector directional importance sampling, n-tuple local estimate, prediction-based splitting and Russian roulette, and circum-solar virtual importance sampling. With this set of methods it is possible to simulate remote sensing instruments accurately and quickly. In contrast to all other known techniques used to accelerate Monte Carlo simulations in cloudy atmospheres - except for two methods limited to narrow angle lidars - the presented methods do not make any approximations, and hence do not bias the result. Nevertheless, these methods converge as quickly as any of the biasing acceleration techniques, and the probability distribution of the simulation results is almost perfectly normal. The presented variance reduction techniques have been implemented into the Monte Carlo code MYSTIC (“Monte Carlo code for the physically correct tracing of photons in cloudy atmospheres”) in order to validate the techniques.  相似文献   
109.
110.
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