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   15348篇
  免费   2871篇
  国内免费   2041篇
化学   11334篇
晶体学   204篇
力学   952篇
综合类   130篇
数学   1590篇
物理学   6050篇
  2024年   45篇
  2023年   341篇
  2022年   508篇
  2021年   595篇
  2020年   733篇
  2019年   752篇
  2018年   629篇
  2017年   600篇
  2016年   813篇
  2015年   854篇
  2014年   960篇
  2013年   1208篇
  2012年   1491篇
  2011年   1462篇
  2010年   979篇
  2009年   1061篇
  2008年   1118篇
  2007年   937篇
  2006年   874篇
  2005年   726篇
  2004年   541篇
  2003年   399篇
  2002年   364篇
  2001年   313篇
  2000年   246篇
  1999年   255篇
  1998年   195篇
  1997年   193篇
  1996年   158篇
  1995年   153篇
  1994年   115篇
  1993年   120篇
  1992年   97篇
  1991年   84篇
  1990年   64篇
  1989年   42篇
  1988年   43篇
  1987年   34篇
  1986年   30篇
  1985年   30篇
  1984年   16篇
  1983年   20篇
  1982年   6篇
  1981年   9篇
  1980年   6篇
  1979年   8篇
  1977年   4篇
  1975年   5篇
  1965年   4篇
  1957年   3篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A CuCl2·H2O-catalyzed tandem reaction of 2-iodophenol with isothiocyanate in water is described, which provides an environmentally benign, efficient, and practical route for the generation of 2-iminobenzo-1,3-oxathiole. It was noteworthy that most of the products were easily separated from the reaction system by simple filtration.  相似文献   
992.
993.
994.
995.
We develop a microscopic pedestrian-simulation model in which pedestrian positions are updated at discrete time steps. At each time step, each pedestrian probabilistically selects a direction of movement from a predetermined set according to a logit-type function that considers the dynamics of other pedestrians around, and then selects a step size that satisfies a certain distribution. We perform a number of field experiments on real intersecting pedestrian flows with four different angles. We then validate and calibrate the model using sample data on the deviation angles, step velocities, and velocity–density relations obtained from the experiments.  相似文献   
996.
Multiscale entropy (MSE) is an effective algorithm for measuring the complexity of a time series that has been applied in many fields successfully. However, MSE may yield an inaccurate estimation of entropy or induce undefined entropy because the coarse-graining procedure reduces the length of a time series considerably at large scales. Composite multiscale entropy (CMSE) was recently proposed to improve the accuracy of MSE, but it does not resolve undefined entropy. Here we propose a refined composite multiscale entropy (RCMSE) to improve CMSE. For short time series analyses, we demonstrate that RCMSE increases the accuracy of entropy estimation and reduces the probability of inducing undefined entropy.  相似文献   
997.
1INTRODUCTIONInrecentyears,ourresearchgrouphasstudiedsystematicalycycloadditionreac tionsoftheC=Ndoublebondsinthebenzoheteroa...  相似文献   
998.
梁琳  余岳辉  彭亚斌 《中国物理 B》2008,17(7):2627-2632
The power dissipation characteristics of pulsed power switch reversely switched dynistors (RSDs) are investigated in this paper. According to the expressions of voltage on RSD, derived from the plasma bipolar drift model and the RLC circuit equations of RSD main loop, the simulation waveforms of current and voltage on RSD are acquired through iterative calculation by using the fourth order Runge-Kutta method, then the curve of transient power on RSD versus time is obtained. The result shows that the total dissipation on RSD is trivial compared with the pulse discharge energy and the commutation dissipation can be nearly ignored compared with the quasi-static dissipation. These characteristics can make the repetitive frequency of RSD increase largely. The experimental results prove the validity of simulation calculations. The influence factors on power dissipation are discussed. The power dissipation increases with the increase of the peak current and the n-base width and with the decrease of n-base doping concentration. In order to keep a low power dissipation, it is suggested that the n-base width should be smaller than 320μm when doping concentration is 1.0×10^14cm^-3 while the doping concentration should be higher than 5.8×10^13cm^-3 when n-base width is 270μm.  相似文献   
999.
於黄忠  彭俊彪 《中国物理 B》2008,17(8):3143-3148
This paper studies the self-organization of the polymer in solar cells based on poly(3-hexylthiophene): [6, 6]-phenyl C61-butyric acid methyl ester by controlling the growth rate of active layer. These blend films are characterized by UV-vis absorption spectroscopy, charge-transport dark J - V curve, x-ray diffraction pattern curve, and atomic force microscopy. The results indicate that slowing down the drying process of the wet films leads to an enhanced selforganization, which causes an increased hole transport. Increased incident light absorption, higher carrier mobility, and balanced carrier transport in the active layer explain the enhancement in the device performance, the power conversion efficiency of 3.43% and fill factor up to 64.6% are achieved under Air Mass 1.5, 100 mW/cm^2.  相似文献   
1000.
The optical limiting properties of the mixed liquid of carbon black suspensions (CBS) and green tea solution were studied by using an 8 ns laser pulse at 532\,nm. The optical limiting effects of the CBS and mixed liquid have been compared between 5 and 10\,Hz repetition frequencies with nanosecond laser pulse. The experimental results indicate that the optical limiting threshold of the sample with the incidence laser at 10\,Hz repetition frequency is lower than at 5\,Hz repetition frequency. The possible reasons for the influence of the repetition frequency on the samples are discussed. And by observing the optical radiant distributions when the laser pulse passing through different samples, a possible mechanism for the observed effects is suggested. At the same time, the result shows that the optical limiting of CBS is the dominant factor to optical limiting of the mixed liquid.  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