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   79006篇
  免费   393篇
  国内免费   381篇
化学   25072篇
晶体学   795篇
力学   6730篇
数学   31994篇
物理学   15189篇
  2020年   34篇
  2019年   47篇
  2018年   10461篇
  2017年   10267篇
  2016年   6084篇
  2015年   890篇
  2014年   334篇
  2013年   381篇
  2012年   3847篇
  2011年   10553篇
  2010年   5678篇
  2009年   6077篇
  2008年   6673篇
  2007年   8807篇
  2006年   306篇
  2005年   1385篇
  2004年   1600篇
  2003年   2017篇
  2002年   1046篇
  2001年   253篇
  2000年   297篇
  1999年   159篇
  1998年   198篇
  1997年   151篇
  1996年   203篇
  1995年   120篇
  1994年   80篇
  1993年   99篇
  1992年   59篇
  1991年   68篇
  1990年   53篇
  1989年   58篇
  1988年   61篇
  1987年   60篇
  1986年   62篇
  1985年   52篇
  1984年   43篇
  1983年   37篇
  1982年   46篇
  1981年   47篇
  1980年   51篇
  1979年   47篇
  1978年   37篇
  1914年   45篇
  1913年   40篇
  1912年   40篇
  1909年   41篇
  1908年   40篇
  1907年   32篇
  1904年   28篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
993.
994.
995.
Wei Yuan  Ji Yan  Zhiyuan Tang  Li Ma 《Ionics》2012,18(3):329-335
A novel ultrasonic-assisted sol–gel method is proposed to prepare Li3V2(PO4)3/C cathode material. X-ray diffraction analyses show that both Li3V2(PO4)3/C(A) synthesized by the ultrasonic-assisted sol–gel method and Li3V2(PO4)3/C(B) synthesized by a traditional sol–gel method have monoclinic structure. Scanning electron microscopy images indicate that the Li3V2(PO4)3/C(A) composite has a more uniform morphology than that of the Li3V2(PO4)3/C(B) composite. In the voltage range of 3.0–4.3 V (vs. Li/Li+), the initial specific discharge capacities of the Li3V2(PO4)3/C(A) and Li3V2(PO4)3/C(B) samples are 129.8 and 125.9 mAh g−1 at 1C rate (1C = 133 mA g−1), respectively. Furthermore, at 2-C charge/10-C discharge rate, the specific discharge capacity of the Li3V2(PO4)3/C(A) composite retains 113.2 mAh g−1 after 50 cycles, but the Li3V2(PO4)3/C(B) composite only presents a capacity of 94.8 mAh g−1.  相似文献   
996.
Charge distribution characteristics were investigated for nanoparticles synthesized in a diffusion flame aerosol reactor. The nanoparticles considered were pristine TiO2 and Cu–TiO2, with Cu dopant concentrations ranging from 1 to 5 wt% with particle size from 25 to 60 nm. In situ measurements were conducted by integrating a tandem differential mobility analyzer (TDMA) experimental setup with the flame aerosol reactor. A charging model was used to identify the important parameters that govern the two charging mechanisms (diffusion and thermo-ionization) in the flame and their relative importance at different operating parameters. The results indicate that TiO2 and Cu–TiO2 nanoparticles carry single as well as double unit charges. The charged fraction depends on particle size as well as on dopant concentration. The charged fraction increased with increasing particle size and decreased with copper dopant concentration. Measured charged fractions were similar for both the polarities at different mobility diameters. Based on the flame operating parameters, the calculations indicate that diffusion charging is dominant in the flame, which is consistent with the experimental results.  相似文献   
997.
In order to solve the hierarchy problem, several extra-dimensional models have received considerable attention. We have considered a process where a Higgs boson is produced in association with a KK-graviton (G KK) at the LHC. At the leading order, this process occurs through the gluon fusion mechanism gghG KK via a quark loop. We compute the cross section and examine some features of this process in the ADD model. We find that the quark in the loop does not decouple in the large quark-mass limit just as in the case of ggh process. We compute the cross section of this process for the case of the RS model also. We examine the feasibility of this process being observed at the LHC.  相似文献   
998.
In this paper, we propose a highly nonlinear dispersion flattened hexagonal photonic crystal fiber (HNDF-HPCF) with nonlinear coefficients as large as 57.5W−1 km−1 at 1.31 μm wavelength for dental optical coherence tomography (OCT) applications. This HNDF-HPCF offers not only large nonlinear coefficient but also very flat dispersion slope and very low confinement losses. Using these characteristics of our proposed PCF, it is shown through simulations by using finite difference method with an anisotropic perfectly matched boundary layer that this PCF offers the efficient supercontinuum (SC) generation for dental OCT applications at 1.31 μm wavelength using a picosecond pulse easily produced by commercially available less expensive laser sources. Coherent length of light source using SC is found 10 μm and the spatial resolutions in the depth direction for dental applications of OCT are found about 6.1 μm for enamel and 6.5 μm for dentin.  相似文献   
999.
A five dimensional cosmological model with FLRW type Kaluza-Klien metric has been investigated with static extra dimensions and varying cosmological constant. The field equations with static extra dimension are solved by considering the cosmological constant as a function of time for different cases. The effective pressure is considered as the difference of pressure corresponding to the extra dimension and the usual four dimensions. The conditions for acceleration of the universe are then discussed.  相似文献   
1000.
The periodic precipitation pattern formation in gelatinous media is interpreted as a moving boundary problem. The time law, spacing law and width law are revisited on the basis of the new scenario. The explicit dependence of the geometric structure on the initial concentrations of the reactants is derived. Matalon—Packter law, which relates the spacing coefficient with the initial concentrations is reformulated removing many ambiguities and impractical parameters. Experimental results are discussed to establish the significance of moving boundary concept in the diffusion controlled pattern forming systems  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