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   8631篇
  免费   1486篇
  国内免费   1182篇
化学   7250篇
晶体学   68篇
力学   299篇
综合类   48篇
数学   337篇
物理学   3297篇
  2024年   27篇
  2023年   99篇
  2022年   241篇
  2021年   281篇
  2020年   389篇
  2019年   311篇
  2018年   262篇
  2017年   276篇
  2016年   409篇
  2015年   390篇
  2014年   502篇
  2013年   786篇
  2012年   594篇
  2011年   607篇
  2010年   489篇
  2009年   567篇
  2008年   570篇
  2007年   495篇
  2006年   499篇
  2005年   443篇
  2004年   445篇
  2003年   383篇
  2002年   310篇
  2001年   225篇
  2000年   219篇
  1999年   198篇
  1998年   193篇
  1997年   153篇
  1996年   151篇
  1995年   114篇
  1994年   113篇
  1993年   87篇
  1992年   88篇
  1991年   60篇
  1990年   47篇
  1989年   46篇
  1988年   42篇
  1987年   26篇
  1986年   37篇
  1985年   12篇
  1984年   21篇
  1983年   7篇
  1982年   19篇
  1981年   10篇
  1980年   10篇
  1979年   15篇
  1978年   6篇
  1977年   7篇
  1976年   5篇
  1973年   4篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
62.
Titanium dioxide (TiO(2)) photoelectrodes with micro/nano hierarchical branched inner channels have been prepared by an electrohydrodynamic (EHD) technique and assembled to form dye-sensitized solar cells (DSSCs). Excellent penetration of ionic-liquid electrolytes and enhanced light harvesting in the longer wavelength region are realized within the composite-structure electrode, thus a better fill factor (ff) of 75.3 % and higher conversion efficiency (eta) of 7.1 % are obtained for viscous ionic-liquid electrolytes compared to pure nanostructured films. Hierarchical branched channels in the photoanodes can efficiently improve the transport properties of redox-active species in viscous electrolytes, which is demonstrated by electrical impedance spectroscopy (EIS). The incident monochromatic photon-to-electron conversion efficiency (IPCE) shows that enhanced light scattering in the composite film is of benefit for light harvesting and thus for solar energy conversion efficiency.  相似文献   
63.
64.
研究了4-位苯基取代基旋转受阻和旋转自由的2,4,6-三苯基氧盐的光物理性质。实验结果表明,当4-位苯基取代基旋转受阻时,氧盐化合物在激发态时引起的分子内极化程度比4-位取代基旋转自由的氧盐化合物大,即在激发态时旋转受阻氧盐化合物发生的分子内电荷转移能力较强;4-位取代基旋转自由的化合物的荧光量子产率随溶剂粘度的增大而有所增大,但旋转受阻化合物在相同的条件下则出现相反的结果。实验结果还表明,4-位取代苯基旋转受阻对化合物的荧光发射不利。  相似文献   
65.
The crystal and molecular structures of 2,2-bis(4-hydroxyphenyl)propane (Bisphenol A, BPA) (1), benzyl 4-hydroxybenzoate (2), 1,7-bis(4-hydroxyphenylthio)-3,5-dioxaheptane (3) and 4-hydroxyphenyl 4-isopropoxyphenyl sulfone (4) have been determined by X-ray crystal structure analysis. Theoretical calculations of the steric hindrance and semiempirical quantum chemical calculations to determine the color characteristics have been carried out. It is clear that the energy barriers for the variation of the orientation of phenol group in 1 to 4 are due to steric hindrance caused by the other moiety and the peak profiles are due to repulsive interactions of the other moiety. Net atomic charges on the hydrogen of the OH group are larger than those on the other atoms in the molecules. This high electron charge of the para orientation will cause the different thermosensitivity and stabilization.  相似文献   
66.
根据CTP在离子交换树脂上的吸附容量和分离因数的大小,确定Duolite A-30树脂适合CTP与CDP,CMP之间的分离.对CTP在Duolite A-30树脂上的吸附动力学和热力学研究表明,在283.15K~303.15K之间,CTP的质量浓度在7.5g/L以上时,Duolite A-30树脂对CTP的吸附主要受颗粒扩散的控制,其有效扩散系数为D=3.47×10-7cm2/s,溶液的质量浓度≤1.0g/L时,CTP与Duolite A-30树脂之间的交换速率主要受液膜控制,其液膜扩散系数为Kf =4.112×10-4/s.同时测定了不同条件下三磷酸胞苷溶液在Duolite A-30树脂固定床离子交换柱中的穿透曲线,研究了进口浓度、进口流速、原料液温度、原料液的pH值及柱高对穿透曲线的影响.用二阶动力学推动力模型描述固定床动态过程,考察了轴向返混对穿透曲线的影响,并从穿透曲线回归得到总传质系数,模型计算值与实验数据符合良好.  相似文献   
67.
In this paper,p-tricyanovinyl-N,N-dialkylanilines were synthesized as model compounds and through their spectroscopic behavior,the photo-induced electron transfer,charge separation were discussed and the aggregation in DMSO-H_2O system was investigated.  相似文献   
68.
Composite membranes were prepared by (a) infiltrating NAFION with SiO2-P2O5-ZrO2 sol, and (b) recasting a film using NAFION solution containing SiO2-P2O5-ZrO2 sol. The membranes were characterized by Differential Thermal Analysis and ac-impedance spectroscopy as a function of relative humidity. The influences of the heat treatment (80°C–150°C) and cleaning on the electrical properties were investigated. The incorporation of SiO2-P2O5-ZrO2 gels into NAFION lead to improvements in its thermal stability and proton conductivity.  相似文献   
69.
A novel asymmetric, edge-sharing bioctahedral complex with formamidinato ligands ( 2-DAniF)Mo(-DAniF)2(-O,Cl)MoCl2, 1, (DAniF=N,N-di-p-anisylformamidinate) has been isolated as a byproduct from the preparation of the dimolybdenum(II,III) compound Mo2(DAniF)3Cl2. An X-ray crystallographic study shows that the structure consists of edge-sharing bioctahedra, with the shared edge defined by the vector joining the: -O and -Cl ligands. Compound 1 is best formulated as a mixed-valent MoIIIMoV compound, the Mo(V) ion being that with the two terminal Cl ligands. The cyclic voltammogram of 1 shows a reversible reduction at –0.472 V and a reversible oxidation at 1.028 V vs. the Ag/AgCl reference electrode, while the absorption spectrum of 1 reveals an intense low energy absorption at 523 nm ( max=12 500) which is attributed to an intervalence charge transfer transition. Crystallographic data for 1 are as follows: a=13.387(1) Å, b=15.500(2) Å, c=21.855(2) Å, =98.786(2)°, V=4481.8(8) Å3, P21/c, R1 (wR2)=0.082 (0.177).  相似文献   
70.
A new layered cobalt-zinc phosphite, Co(H2O)4Zn4(HPO3)6·C2N2H10 has been synthesized in the presence of ethylenediamine as the structure-directing agent. The compound crystallizes in the monoclinic system, space group Cc (No. 9), a=18.2090(8), b=9.9264(7), c=15.4080(7) Å, β=114.098(4)°, V=2542.3(2) Å3, Z=4, R=0.0323, wR=0.0846. The structure consists of ZnO4 tetrahedra, CoO6 octahedra and HPO3 pseudopyramids through their vertices forming bimetallic phosphite layers parallel to the ab plane. Organic cations, which reside between the inorganic layers, are mobile and can be exchanged by NH4+ cations without the collapse of the framework.  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