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   8220篇
  免费   936篇
  国内免费   853篇
化学   7075篇
晶体学   135篇
力学   103篇
综合类   63篇
数学   679篇
物理学   1954篇
  2024年   20篇
  2023年   111篇
  2022年   458篇
  2021年   386篇
  2020年   373篇
  2019年   411篇
  2018年   426篇
  2017年   326篇
  2016年   419篇
  2015年   377篇
  2014年   466篇
  2013年   639篇
  2012年   507篇
  2011年   552篇
  2010年   428篇
  2009年   569篇
  2008年   454篇
  2007年   407篇
  2006年   370篇
  2005年   329篇
  2004年   293篇
  2003年   200篇
  2002年   264篇
  2001年   150篇
  2000年   122篇
  1999年   83篇
  1998年   89篇
  1997年   76篇
  1996年   86篇
  1995年   70篇
  1994年   84篇
  1993年   78篇
  1992年   58篇
  1991年   42篇
  1990年   35篇
  1989年   25篇
  1988年   28篇
  1987年   23篇
  1986年   22篇
  1985年   14篇
  1984年   22篇
  1983年   9篇
  1982年   26篇
  1981年   15篇
  1980年   11篇
  1979年   17篇
  1978年   12篇
  1977年   8篇
  1976年   8篇
  1975年   4篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
7-氮杂-3,6-二氧杂-二环[3.3.0]辛-2-酮类化合物合成的新方法   总被引:1,自引:0,他引:1  
5-甲氧基-2-(5H)-呋喃酮与氮酸硅烷基酯通过1,3-偶极环加成反应可以为7-氮杂-4-甲氧基-3,6-二氧杂-二环[3.3.0]辛-2-酮类化合物提供新的合成途径。通过元素分析、IR、^1HNMR、^13CNMR、MS等波谱分析进行了结构表征。  相似文献   
992.
The first enantioselective total synthesis of 5-acetoxygoniothalamin 1 and 5-acetoxyisogoniothalamin oxide 2 was achieved through the Sharpless kinetic resolution of recemic secondary 2-furylmethanol 5 and the Mitsunobu reaction. At the same time we developed a short synthetic route for 6R-( )-goniothalamin 3 and (6R,7R,8R)-( )-goniothalamin oxide 4. And according to this route the configuration of 5-acetoxygoniothalamin 1 was confirmed as (5S,6S).  相似文献   
993.
IntroductionSchiffbases ,amides ,imidazolo[2 ,1 b] 1,3,4 thiadiazoles,and 7H s triazolo[3,4 b] 1,3,4 thiadi azinescontainingheterocycleshavebeenattractingmuchattentionaspotentialantifungalagents .1 51,2 ,3 tria zole ,6 7mercapto 1,2 ,4 triazole ,81,3,4 thiadiazole9 10andtheirrelatedco…  相似文献   
994.
The equilibrium solubility of CsCl-CeCl3-HCl(11%)-H2O qua-ternary system at 25℃ has been determined by the physicchemical analysis method ,and the phase diagram was plotted, Two new double salts 3CsCl.CeCl3.3H2O and CsCl.CeCl3.4H2O obtained from the complicated system were identified and characterized by XRD,TG-DTA ,DSC,UV and fluorescence spectroscopy, Studies on the fluorescence excitation and emission show that 3CsCl.CeCl3.3H2O and CsCl.CeCl3.4H2O have upconversion luminescence of infrared-visible range,and the upconversion emission intensity increases with the increase of ratio of CeCl3 in CsCl.  相似文献   
995.
通过Stille反应合成了一系列含有均苯四甲酸二酰亚胺受体单元的共轭聚合物P1~P7.该系列聚合物在常见有机溶剂中溶解性良好,在370~600 nm范围内有较强吸收.通过循环伏安法测量其LUMO能级范围在-3.66~-3.90 eV之间,HOMO能级在-5.25~-6.17 eV之间,在同类分子中接近最低值.通过改变主链中噻吩单元的数量和给电子单元,可以调节分子的能隙,使其电化学能隙在2.45~1.55 eV范围内变化.将含均苯四甲酸二酰亚胺受体单元的P1~P7应用于有机太阳能电池中,作为给体材料与PC61BM共混制成本体异质结聚合物电池,器件开路电压普遍较高.其中基于均苯四甲酸二酰亚胺与二噻吩并噻咯的聚合物P7的器件,在AM 1.5 G,86 mW/cm2光照条件下,开路电压为0.72 V,短路电流为1.22 mA/cm2,能量转换效率为0.27%.  相似文献   
996.
Transplantation of islet cells into diabetic patients is a promising therapy, provided that the islet cells are able to evade host immune rejection. With improved islet viability, this strategy may effectively reverse diabetes. We applied 2% calcium alginate to generate small and large capsules to encapsulate porcine neonatal pancreatic cell clusters (NPCCs) using an air-driven encapsulator. After encapsulation, the viability was assessed at 1, 4, 7, 14 and 28 days and secretion of functional insulin in response to glucose stimulation were tested at days 14 and 28. Selective permeability of the small alginate capsules was confirmed using various sizes of isothiocyanate-labeled dextran (FITC-dextran). Encapsulation of NPCCs was performed without islet protrusion in the small and large capsules. The viability of NPCCs in all experimental groups was greater than 90% at day 1 and then gradually decreased after day 7. The NPCCs encapsulated in large capsules showed significantly lower viability (79.50 ± 2.88%) than that of naïve NPCCs and NPCCs in small capsule (86.83 ± 2.32%, 87.67 ± 2.07%, respectively) at day 7. The viability of naïve NPCCs decreased rapidly at day 14 (75.67 ± 1.75%), whereas the NPCCs encapsulated in small capsules maintained (82.0 ± 2.19%). After 14 and 28 days NPCCs' function in small capsules (2.67 ± 0.09 and 2.13 ± 0.09) was conserved better compared to that of naïve NPCCs (2.04 ± 0.25 and 1.53 ± 0.32, respectively) and NPCCs in large capsules (2.04 ± 0.34 and 1.13 ± 0.10, respectively), as assessed by a stimulation index. The small capsules also demonstrated selective permeability. With this encapsulation technique, small capsules improved the viability and insulin secretion of NPCCs without islet protrusion.  相似文献   
997.
The synthesis and spectroscopic characterization of ruthenium complexes(R-l to R-8) of the type[Ru(A)2(B)],(where A = 1,10-phenanthroline/2,2′-bipyridine and B = 3.4.5-tri-OCH3DPC,4-CH3-DPC,4-N-(CH32-DPC,4-NO2-DPC arc described. These ligands form bidentate octahedral ruthenium complexes.The title complexes were subjected to in vitro cytotoxic activity measurements against the human cancer T-lymphocyte cell lines MTT assay.In vitro evaluation of these ruthenium complexes revealed cytotoxic activity from 0.24 to 1.4μg/mL against CEM,0.44 to 1.9μg/mL against Molt4/C8.0.28 to 1.5μg/mL against L1210,0.24 to 0.98μg/mL against HL60,and 0.25 to 1.2μg/mL against BEL7402,depending the nature of the compound.  相似文献   
998.
Bromination is used as a strategy to improve biological activity in medicinal chemistry.In order to study on the structure-activity relationships of the novel acetylcholinesterase inhibitors with 7H-thiazolo[3,2-b]-1,2,4-triazin-7-one scaffold,based on our previous work and molecular modeling,a series of novel 3-aryl-6-(bromoarylrnethyl)-7H-thiazolo[3,2-b]-1,2,4-triazin-7-one derivatives were designed by molecular docking,synthesized and characterized by mass spectra,infrared spectra,proton NMR and elemental analyses.The study of AChE inhibitory activity was carried out using the Ellman colorimetric assay with huperzine-A as the positive control.Most of all target compounds exhibited more than 45%inhibition at 10μmol/L.The preliminary structureactivity relationship was the bromine atoms and the hydroxyl group at the phenyl ring at the C6 position of the parent nucleus played significant roles in the AChE inhibitory activity of the target compounds.  相似文献   
999.
The electron density distribution in the crystal of the Meisenheimer complex of potassium 3-methyl-5′, 7′-dinitro-5′, 8′-dihydrospiro(1,3-oxazolidine-2,8′- quinolinide) (1) was studied based on the data of precision X-ray diffraction study. The experimental data were compared with the results of quantum-mechanical calculations. The electron density distributions in the regions of the nitro groups are substantially different in spite of the similarity in the geometric parameters of these groups. In addition, there is a discrepancy between the exprimental and theoretical data on this distribution. The effect of the cation on the electronic strucure of the anion in the crystalline phase is the most probable cause of the differences observed. To reveal this effect.ab initio calculations of different anion1-K+ systems were performed and a topological analysis of the electron density distributions was carried out. Depending on the mode of coordination of the cation to the anion, the former determines the contribution of alternative resonance forms to the structure of the anion. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 451–458, March, 2000.  相似文献   
1000.
Thick photosensitive inorganic-organic hybrid gel films are fabricated using a silica–PEO(poly(ethylene oxide)) polymeric network and several chelated metal alkoxides: Ti(OEt)4, Al(OBusec)3, Zr(OPr n )4. The -glycidoxypropyltrimethoxysilane (GPTS) and the metal alkoxides stabilized by -ketoester or -diketone are used as precursors. The chelated metal complex in the gel films are photodecomposed and forms the oxide network by UV exposure. The photodecomposition of the chelate ring and the photobleaching of the UV absorption bands are investigated as a function of UV exposure time. The photobleaching rates with respect to chelating agents, metal alkoxides and photon energy are compared.  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