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   89898篇
  免费   2494篇
  国内免费   2893篇
化学   34385篇
晶体学   1035篇
力学   7302篇
综合类   153篇
数学   33076篇
物理学   19334篇
  2023年   277篇
  2022年   544篇
  2021年   539篇
  2020年   593篇
  2019年   660篇
  2018年   10860篇
  2017年   10703篇
  2016年   6633篇
  2015年   1498篇
  2014年   1080篇
  2013年   1294篇
  2012年   4825篇
  2011年   11518篇
  2010年   6485篇
  2009年   6812篇
  2008年   7408篇
  2007年   9421篇
  2006年   909篇
  2005年   1966篇
  2004年   2079篇
  2003年   2438篇
  2002年   1536篇
  2001年   745篇
  2000年   608篇
  1999年   408篇
  1998年   353篇
  1997年   245篇
  1996年   321篇
  1995年   204篇
  1994年   173篇
  1993年   180篇
  1992年   113篇
  1991年   132篇
  1990年   90篇
  1989年   90篇
  1988年   82篇
  1987年   76篇
  1986年   78篇
  1985年   63篇
  1984年   51篇
  1983年   47篇
  1982年   42篇
  1981年   40篇
  1980年   48篇
  1979年   46篇
  1914年   45篇
  1913年   40篇
  1912年   40篇
  1909年   41篇
  1908年   40篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
标题化合物晶体属三斜晶系;空间群为P1;晶胞参数:a=8.182(2)Å,b=10.389(2)Å,c=16.261(5)Å,α=99.38(2)°,β=89.97(2)°,γ=113.18(2)°;Z=2,两个phen上的四个N原子和一个配位水中的O原子围绕中心体Cu原子形成一个畸变的三角双锥构型,配位水与阴离子ClO4-中的O原子形成氢键。  相似文献   
992.
Three unsymmetrical tetradentate Schiff base ligands, H2salipn, H2salipn-Br4 and H2salipn-Cl2, have been synthesized from the typical condensation reactions of treating 1,2-diaminopropane with salicylaldehyde, 3,5-dibromosalicylaldehyde and 5-chlorosalicylaldehyde, respectively. Treatment of [RuCl2(PPh3)3] with one equivalent of H2salipn or H2salipn-Br4 in the presence of triethylamine in tetrahydrofuran (THF) afforded the corresponding ruthenium(III) complexes [RuIIICl(PPh3)(salipn)] (1) and [RuIIICl(PPh3)(salipn-Br4)] (2). Interaction of [RuHCl(CO)(PPh3)3] with one equivalent of H2salipn-Cl2 or H2salipn-Br4 under the same conditions led to isolation of ruthenium(II) complexes [RuII(CO)(PPh3)(salalipn-Cl2)] (3) and [RuII(CO)(PPh3)(salalipn-Br4)] (4), respectively, in which one of the imine bonds was nucleophilically attacked by hydride to result in the formation of a mixed imine-amine ligand. The molecular structures of 1?1.5CH2Cl2, 2, 3?0.5CH2Cl2 and 4 have been determined by single-crystal X-ray crystallography. The electrochemical properties of 14 were also investigated. Their cyclic voltammograms displayed quasi-reversible Ru(IV)/Ru(III) and Ru(III)/Ru(II) couples with Eo ranging from 0.67 to 1.05 V and 0.74 to 0.80 V vs. Ag/AgCl (0.1 M), respectively.  相似文献   
993.
The perfect matching vector and forcing and the Kekulé-vector of cata-benzenoids are defined. Two theorems are given which set the sufficient and necessary conditions for HKZ-vector (Harary et al. J Math Chem 6:295, 1991) and Kekulé-vector in cata-benzenoids. Additional two theorems are obtained which give sharp bounds for the modules of HKZ- and Kekulé vectors. Dedicated to Professor Tadeusz Marek Krygowski on the happy occasion of his 70th birthday.  相似文献   
994.
用溶剂热方法合成了[Me4N]2HgGe4S10, 通过单晶X射线衍射, IR, DSC-TG手段对其进行了表征. 结果表明, 标题化合物属四方晶系, I-4空间群, 晶胞参数a=0.92687(8) nm, c=1.43739(12) nm, V=1.23484(18) nm3, Z=2, Mo Kα λ=0.071073 nm, R1=0.0570, wR2=0.1374, 空旷骨架结构由超四面体Ge4S10与HgS4四面体共用顶点连接而成, 有机模板离子在一维孔道中. 该化合物具有一定的热稳定性, 在320 ℃发生分解形成GeS2.  相似文献   
995.
以Sio2和PMo12为表面材料,用可选频脉冲TEA-CO2激光器作光源,考察了激发频率、激光强度、脉冲次数和脉冲间隔等激光参数对甲醇激光分解反应性能的影响规律。结果表明:甲醇的激光表面反应具有频率选择性,激发频率不同,甲醇分解率也不同;不同表面上激发次数影响规律不同,随着激发次数增加,甲醇转化率增加;脉冲间隔的改变只影响转化率,脉冲间隔越小,能量利用率越高,甲醇分解率越高;激光强度的提高可增加甲  相似文献   
996.
The different aspects of Vitamin D3 determination and the data reported in the literature are discussed. The spectrophotometric determination of Vitamin D3 in cod-liver oil is carried out after the alkaline saponification, extraction of unsaponified parts, precipitation of accompanying sterols and the column- and thin-layer chromatographic purification and separation of vitamin D3 on 40 cm plates from other vitamins. The dyestuff α-naphtholbenzein is suited well as standard substance for the better location and identification of vitamin D3 zone on the thin-layer plate. The results obtained from the chemical method were checked through the simultaneous biological determination.  相似文献   
997.
胺催化下的Baylis-Hillman反应   总被引:1,自引:0,他引:1  
Baylis-Hillman (B-H)反应是在催化剂作用下由活泼烯烃和亲电试剂发生的偶合反应, 其产物是一个具有多官能团的化合物. 胺类催化剂是反应中应用最广泛的一类催化剂. 综述了各种胺催化下的Baylis-Hillman反应的研究进展.  相似文献   
998.
Mono(6-deoxy-dimethylpyridinium)-β-cyclodextrins have been synthesized in reaction of mono (p-toluenesulfonyl) derivative of β cyclodextrin with the appropriate lutidine under microwave irradiation and conventional conditions. The results indicate that the mechanism consists of inclusion complex formation.  相似文献   
999.
Summary As part of an air pollution monitoring study, airborne particulate matter (PM2/PM10-2) samples were collected from 2000 to 2003 at two sampling sites in an urban region, Daejeon, the middle of Korea. Mass concentrations of both fine and coarse fractions and that of the black carbon in the fine particles were measured using the Gent stacked filter unit sampler and the smoke stain reflectometer, respectively. In the collected samples the concentrations of 24 elements were analyzed using instrumental neutron activation analysis. Monitored data were investigated for their temporal trends under different environmental conditions and their seasonal correlation patterns. Crustal enrichment factors were also estimated to establish the contribution between anthropogenic and crustal origin. Patterns for airborne particle matter (APM) and elemental concentrations, seasonal variation of some marker elements were investigated. The results can be applied for the investigation of further air pollution sources and for the evaluation of air quality.  相似文献   
1000.
From the de Broglie matter wave hypothesis and Planck’s energy quantization law, and assuming conservation of energy in the absorption of a photon and its consequent conversion to kinetic energy of motion by a material particle initially at rest, one can deduce a simple mathematical relationship between the wavelength λ (or frequency ν), of the photon absorbed by the particle at rest, and the resulting de Broglie matter wave length, λD, of the particle with kinetic energy of motion of mv2/2. The relationship so deduced, λD∝√λ, suggests that visible wavelengths of light, from about 4000 ?, in the violet, to beyond about 7000 ?, in the red, on absorption by an electron at rest, lead to material electron wavelengths, λD, of the order of the size of the electron transfer proteins seen in the photosynthetic reaction centers of photosynthesizing organisms, at about a size of 50–100 ?. In addition to understanding the mechanism of photosynthesis as a material wave mediated phenomenon, further areas of importance of the relations pointed out in this paper are in the design of experiments to gain a deeper understanding of the basic tenets of wave mechanics, and in the use of tunable lasers to probe various properties of material waves, and to precisely control their properties for applications including lithography.  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