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   31042篇
  免费   3524篇
  国内免费   2818篇
化学   13127篇
晶体学   1110篇
力学   2224篇
综合类   212篇
数学   11741篇
物理学   8970篇
  2024年   40篇
  2023年   265篇
  2022年   667篇
  2021年   566篇
  2020年   733篇
  2019年   873篇
  2018年   905篇
  2017年   863篇
  2016年   1054篇
  2015年   949篇
  2014年   1479篇
  2013年   2332篇
  2012年   1542篇
  2011年   2099篇
  2010年   1987篇
  2009年   2169篇
  2008年   2206篇
  2007年   2260篇
  2006年   1883篇
  2005年   1537篇
  2004年   1445篇
  2003年   1228篇
  2002年   992篇
  2001年   826篇
  2000年   764篇
  1999年   653篇
  1998年   619篇
  1997年   498篇
  1996年   572篇
  1995年   481篇
  1994年   491篇
  1993年   400篇
  1992年   331篇
  1991年   212篇
  1990年   174篇
  1989年   178篇
  1988年   133篇
  1987年   126篇
  1986年   132篇
  1985年   156篇
  1984年   101篇
  1983年   59篇
  1982年   92篇
  1981年   68篇
  1980年   50篇
  1979年   31篇
  1978年   38篇
  1977年   43篇
  1976年   32篇
  1974年   16篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
1 INTRODUCTION The interfaces between metals and oxide play a vital role in many industrial applications: hetero- geneous catalysis, microelectronics, thermal barriers, corrosion protection, metal processing and so on[1]. In catalysis, the choice of metal and oxide support is critical in order to obtain a desired reactivity and selectivity[2]. This is due in part to the inherent reac- tivity of the two components. Also the size and shape of the metal particle, which depend on the choice…  相似文献   
952.
The theoretical equations for friction coefficients of spheres having sticky or hairy surfaces are presented. In the present treatment, the sticky surface is represented by a higher viscosity of fluid in the surface layer and the hairy surface is assumed to have a porous structure. The friction coefficients are given as functions of the thickness of the surface layer, the fluid viscosity in the surface layer and the segmental density of hairy chains.  相似文献   
953.
Colloidal silica and titanium dioxide were surface-modified by chemisorption of octadecyl dimethylmethoxy silane. The surface density of these alkyl silane groups was adjusted to less than 7% of the available surface hydroxyls, leaving the adsorbents hydrophilic and electrically charged in aqueous solution.Ionic surfactants (tetradecylpyridinium chloride and sodium lauryl sulfate) are adsorbed onto the surface-modified silica and titanium dioxide from aqueous solution, even in the case where the surface of the adsorbents exhibits the same sign of electrical charge as the surfactant ionic head groups. According to the adsorption model of Gu the chemiadsorbed alkyl chains are supposed to serve as anchors for small surface aggregates of the ionic surfactants.  相似文献   
954.
Preparation, 11B NMR, Vibrational Spectra, and Crystal Structure of [(C5H5N)2CH2][1-(O2N)B10H9] By reaction of [B10H10]2? in aqueous acetonitrile with a saturated solution of NO2 in dichloromethane [1-(O2N) · B10H9]2? and [B10H9(NO)B10H9]3? are formed which can be separated by ion exchange chromatography on diethylaminoethyl(DEAE) cellulose from the starting compound. The X-ray structure determination of [(C5H5N)2CH2][1-(O2N)B10H9] (triclinic, space group P1 , a = 7.1530(9), b = 8.3753(8), c = 15.198(2) Å, α = 96.00(1), β = 95.48(1), γ = 95.60(1)°, Z = 2) reveals the coordination of the NO2 group via N with a B1? N distance of 1.535(5) Å and an O2? N? O1 angle of 119.3(3)°. The 11B NMR spectrum exhibits the characteristic feature (1 : 1 : 4 : 4) of an apical monosubstituted B10 cluster with a strong downfield shift of the ipso-B atom at +13.4 ppm. The IR and Raman spectra show strong NO stretching vibrations at 1381 und 1420 cm?1.  相似文献   
955.
We prepared surface-grafted polystyrene (PS) beads with comb-like poly(ethylene glycol) (PEG) chains. To accomplish this, conventional gel-type PS beads (35-75 microm) were treated with ozone gas to introduce hydroperoxide groups onto the surface. Using these hydroperoxide groups, poly(methyl methacrylate) (PMMA, Mn= 22,000-25,000) was grafted onto the surface of the PS beads. The ester groups of the grafted PMMA were reduced to hydroxyl groups with lithium aluminum hydride (LAH). After adding ethylene oxide (EO) to the hydroxyl groups, we obtained the PS-sg-PEG beads, which had a rugged surface and a diameter of 80-150 microm. We could obtain several kinds of the PS-sg-PEG beads by controlling the chain lengths of the grafted PMMA and the molecular weights of the PEG chains. The grafted PEG layer was about 30-50 microm thick, which was verified from the cross-sectioned views of the fluorescamine-labeled beads. These fluorescence images proved that the beads possessed a pellicular structure. Furthermore, we found that the surface-grafted PEG chains had the characteristic property of reducing non-specific protein adsorption on the beads.  相似文献   
956.
DNA分子在气液界面的组装相变特性及其LB膜结构研究   总被引:5,自引:0,他引:5  
戴树玺  张兴堂  杜祖亮  党鸿辛 《化学学报》2003,61(12):2013-2015
对十八胺与DNA在气液界面上组装及其相变过程进行了研究,利用AFM观察了不 同压力下转移的DNA复合LB膜结构。发现在低表面压时,DNA复合单分子膜表现为技 术发散的分形结构;随着压力的升高,DNA复合膜逐渐由紧密的网状排布结构变为 团聚的块状和团簇结构。表明通过调节膜压,可使膜内DNA分子的构象发生大的变 化,从而生成具有特定形态的二维纳米图案。这种具有特殊形态和结构的DNA LB膜 可望为合成新型生物纳米结构有序功能体系提供模板。  相似文献   
957.
Sm(II)-modified periodic mesoporous silica (PMS), Sm[N(SiHMe2)2]2(THF)x@MCM-41, was used for the synthesis of Sm(II) alkyl, alkoxide, and indenyl surface species via secondary ligand exchange. The performance of the novel Sm(II)-based organometallic–inorganic hybrid materials as initiators for the graft polymerization of methyl methacrylate (MMA) is reported. All of the Sm(II) hybrid materials including the new PMMA–PMS composites were characterized via N2 physisorption, elemental analysis, FTIR spectroscopy, and scanning electron microscopy (SEM). The organic–inorganic composites revealed complete pore blockage as well as enrichment and strong adhesion of the polymer at the exterior of the porous silica material.  相似文献   
958.
Two compounds janoxepin (1) and brevicompanine B (2) were isolated from the fungus Aspergillus janus and the structures elucidated by one- and two-dimensional NMR spectroscopic methods and mass spectrometry. Janoxepin is a novel oxepin derivative with a rare d-leucine incorporated. Brevicompanine B has previously only been isolated from Penicillium brevicompactum. Both compounds were tested in antimicrobial assays and found to be active against the malaria parasite Plasmodium falciparum 3D7 (IC50-values of 28 and 35 mg/ml, respectively). However, no activity was observed in antifungal or antibacterial assays.  相似文献   
959.
Fe3+-TiO2/SiO2光催化降解罗丹明B的研究   总被引:25,自引:0,他引:25  
以硅胶为载体,采用溶胶-凝胶法制备了不同掺杂量的Fe3+-TiO2/SiO2光催化剂,并采用SEM,Raman和DRS等手段对其进行了分析和表征.以氙灯为光源,通过对可溶性染料罗丹明B的降解反应,考察了Fe3+-TiO2/SiO2催化剂的光催化活性,探讨了光催化反应中溶液pH值和起始浓度对催化反应的影响.  相似文献   
960.
吸附质在两相界面上的吸附行为对多相催化、界面化学等有重要意义[1,2]。本文采用程序升温法分别测定了13种醇在α Al2O3和硅胶G表面上的脱附温度td,发现td与醇的沸点及烷基化效应指数[3](PEI)有定量关系。1 实验红外干燥箱100℃干燥30分钟后的吸附剂在所研究的吸附质的饱和蒸汽压、室温下平衡24-48h,以达到吸附平衡;取20-25mg吸附平衡后的吸附剂,用PCT 1A型差热天平进行TG/DTA分析,升温速率为2 5℃/min,升温范围从室温到250℃,由计算机进行数据采集(RSZ 200热分析系统软件),从DTA曲线的最高峰值确定脱附温度,每个样品重复3-5…  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