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   14698篇
  免费   3670篇
  国内免费   910篇
化学   5315篇
晶体学   562篇
力学   208篇
综合类   80篇
数学   130篇
物理学   12983篇
  2024年   38篇
  2023年   131篇
  2022年   319篇
  2021年   339篇
  2020年   359篇
  2019年   322篇
  2018年   313篇
  2017年   518篇
  2016年   617篇
  2015年   598篇
  2014年   889篇
  2013年   1360篇
  2012年   1086篇
  2011年   1013篇
  2010年   777篇
  2009年   903篇
  2008年   1120篇
  2007年   1006篇
  2006年   971篇
  2005年   848篇
  2004年   738篇
  2003年   658篇
  2002年   589篇
  2001年   547篇
  2000年   501篇
  1999年   451篇
  1998年   384篇
  1997年   354篇
  1996年   271篇
  1995年   277篇
  1994年   232篇
  1993年   170篇
  1992年   142篇
  1991年   91篇
  1990年   69篇
  1989年   58篇
  1988年   48篇
  1987年   43篇
  1986年   27篇
  1985年   27篇
  1984年   23篇
  1983年   7篇
  1982年   16篇
  1981年   7篇
  1980年   6篇
  1979年   4篇
  1976年   1篇
  1975年   3篇
  1973年   3篇
  1970年   3篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
991.
992.
旋转滑动弧氩等离子体裂解甲烷制氢   总被引:3,自引:0,他引:3  
采用切向气流和磁场协同驱动的旋转滑动弧氩等离子体,先通过光谱分析法计算了其电子温度和电子密度,了解其物理特性,将其应用于甲烷裂解制氢,研究了进气流量和CH_4/Ar比对反应效果的影响。结果表明,该滑动弧系统电子温度为1.0-2.0 e V,电子密度高达1015cm~(-3),是介于热与低温等离子体之间的一种等离子体形式,具有独特的物理特性,可以在达到较高反应效率的同时,保持较大的处理量;在CH_4裂解制氢实验中,CH_4转化率可达22.1%-70.2%,并随进气流量和CH_4/Ar比的增大均逐渐降低;H_2选择性为21.2%-61.2%,并随进气流量的增大先基本不变后有所增大,随CH_4/Ar比的增大逐渐降低;与应用于甲烷裂解的不同形式的低温等离子体对比(如微波、射频、介质阻挡放电等)可以发现,旋转滑动弧在获得较高甲烷转化率、较高H_2选择性和较低制氢能耗的同时,还可以保持较大的处理量,即进气流量可达6-20 L/min。  相似文献   
993.
While extensive theoretical work has been devoted to analyzing scattering behavior for nonspherical particles, few experimental studies of the light‐scattering properties of such particles are available, largely because of the difficulty of synthesizing such particles with uniform geometries. Here we report the synthesis of highly uniform, volume‐equivalent rod‐shaped colloidal particles prepared from their commercial spherical counterparts, on which we performed light scattering experiments as a function of scattering angle for micro rods with varying aspect ratio and volume. These results were compared to values calculated using the T‐Matrix method. Good agreement with theoretical predictions was found for the experimentally measured scattering cross sections and the angular dependence of the scattering intensity. An increase in the forward scattering intensity is observed and predicted for particles with larger aspect ratios relative to their volume equivalent spheres, with only minor differences observed at both mid‐range and backscattering angles. Furthermore, the light scattering results for the rod‐shaped particles did not show the scattering fringes seen in scattering by the spheres, indicating that as three‐dimensional symmetry is broken, the associated Lorenz–Mie resonances are strongly attenuated. This observation also was predicted by theory. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1889–1895  相似文献   
994.
Tetrakis(4‐(1‐bromoethyl)phenyl)silane is synthesized and utilized to initiate the atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) to generate bromo‐terminated four‐armed PMMA macroinitiators, which further initiate the ATRP of methylacryloyloxyl‐2‐hydroxypropyl perfluorooctanoate (FGOA) to create fluorinated star‐shaped block copolymers PMMA‐b‐poly(FGOA)s with fluorine content ranging from 0 to 31.7 wt %. The polymerizations are well controlled with the polydispersity indices <1.30. The polymers readily dissolve in common organic solvents and show good film‐formation. Compared with the nonfluorinated sample, the fluorinated films exhibit significantly increased water contact angles owing to the enrichment of fluorine on the surface. The enhanced hydrophobicity is advantageous for the optical stability when the devices work under a moist environment. Moreover, the films possess high thermo‐optic coefficients, tunable refractive indices, and extremely low birefringence coefficients because of the presence of bulky and rigid tetraphenylsilane core and star‐shaped topological structure, showing potential application in optical waveguide devices. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1969–1977  相似文献   
995.
Herein we report the synthesis and detailed studies of the anion‐binding properties of two 20‐membered macrocyclic tetramide receptors: one symmetrical, containing two identical azulene‐based bisamide units, the other a hybrid, containing a dipicolinic bisamide unit and an azulene‐based bisamide unit. Analysis of the crystal structures of the macrocyclic receptors revealed their preference for adopting similar well‐preorganized bent‐sheet conformations, both as free receptors and in their complexes with anions. Studies of the optical properties of both receptors revealed abilities to selectively sense phosphate anions (H2PO4?, HP2O73?), allowing for naked‐eye detection of the presence of these guests in DMSO. Binding studies in solution confirmed that the receptors bind strongly to a series of anions even in highly demanding media, such as mixtures of DMSO with water or with methanol. Comparison of the anion affinity of linear analogues with that of the macrocyclic receptors evidenced the importance of macrocyclic topology. Quantitative analysis revealed that the macrocyclic receptors are selective for H2PO4? over other anions. The affinity to H2PO4? seen for the symmetrical receptor, containing two azulene‐based subunits, is much higher than for the hybrid macrocycle containing both the azulene‐based and pyridine‐derived subunits. This highlights that the azulene‐based building block serves efficiently as both a binding site and a structure‐preorganizing motif.  相似文献   
996.
997.
In the crystallographic literature, there is an ever-increasing number of publications on crystals that are referred to as ‘new non-linear optical materials’, many of them reporting salts of amino acids. However, the term NLO material is used improperly in most cases. In fact, the establishment of any crystal species as such a material requires several experimental and computational procedures, which are seldom satisfied when characterizing a supposedly new species. Here, some frequent hazards and problems are addressed and observations of papers reporting amino acid compounds as supposedly new NLO materials are made.  相似文献   
998.
An investigation of the optical properties of a hydrogenic donor in spherical parabolic quantum dots has been performed by using the matrix diagonalization method. The optical absorption coefficient between the ground (L = 0) and the first excited state (L = 1) have been examined based on the computed energies and wave functions. The results are presented as a function of the incident photon energy for the different values of the confinement strength. These results show the effects of the quantum size and the impurity on the optical absorption coefficient of a donor impurity quantum dot.  相似文献   
999.
Based on the generalized diffraction integral, we derive an analytical formula for generalized Mathieu–Gauss beams (gMGBs) passing through an apertured misaligned optical system. Furthermore, we use the fact that a hard aperture function can be expanded into a finite sum of complex Gaussian functions to establish an approximate propagation equation of gMGBs through paraxial circularly apertured optical system. As an example, the propagation of ordinary and modified zeroth order MGBs through a misaligned thin lens is studied numerically.  相似文献   
1000.
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