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   546篇
  免费   109篇
  国内免费   96篇
化学   714篇
晶体学   3篇
力学   16篇
综合类   3篇
物理学   15篇
  2023年   12篇
  2022年   9篇
  2021年   16篇
  2020年   35篇
  2019年   30篇
  2018年   16篇
  2017年   27篇
  2016年   39篇
  2015年   39篇
  2014年   40篇
  2013年   60篇
  2012年   69篇
  2011年   40篇
  2010年   22篇
  2009年   43篇
  2008年   50篇
  2007年   33篇
  2006年   34篇
  2005年   27篇
  2004年   35篇
  2003年   28篇
  2002年   7篇
  2001年   4篇
  2000年   5篇
  1999年   5篇
  1998年   5篇
  1997年   8篇
  1995年   4篇
  1994年   2篇
  1992年   1篇
  1991年   3篇
  1989年   1篇
  1986年   1篇
  1984年   1篇
排序方式: 共有751条查询结果,搜索用时 15 毫秒
1.
通过三点弯动态冲击实验和数值模拟方法,研究了分支交错层状仿生复合材料的动态断裂韧性。首先设计并制备了分支交错层状仿贝壳复合材料试样,即将一种脆性刚性材料和一种橡胶类材料分别作为复合材料的硬质层和软胶层;随后采用改进的分离式Hopkinson压杆装置进行了三点弯冲击实验;接着讨论了初始冲击速度、硬质材料长宽比、软质材料层厚度对复合材料试样动态断裂行为的影响;最后采用ABAQUS有限元数值模拟,研究了不同宽度和不同冲击方向对复合材料试样动态断裂韧性和裂纹扩展的影响。结果表明:随着冲击速度和硬质材料长宽比增加、软胶层厚度减小,裂纹越倾向于沿直线扩展,反之,裂纹越倾向于绕过硬质材料沿着软胶层呈折线扩展;试样的峰值动载荷和起裂时间也随之增大。有限元模拟结果表明:随着结构总宽度的增大,试样断裂韧性增加,裂纹倾向于绕过硬质材料沿着软胶层扩展;采用实验设计的冲击方向时,试样的断裂韧性高于其他方向。  相似文献   
2.
An efficient synthetic route to the resveratrol oligomers quadrangularin A and pallidol is reported. It features a scalable biomimetic oxidative dimerization that proceeds in excellent yield and with complete regioselectivity. A systematic evaluation of the natural products and their synthetic precursors as radical‐trapping antioxidants has revealed that, contrary to popular belief, this mode of action is unlikely to account for their observed biological activity.  相似文献   
3.
4.
以海藻酸钠-壳聚糖为复合囊材采用锐孔法制备桑椹红微囊,探讨了海藻酸钠浓度、壳聚糖浓度、Ca Cl2浓度、桑椹红浓度、针头孔径、下滴高度、温度、转速等因素对微囊包封率的影响。确定了最佳制备工艺条件为海藻酸钠浓度4.0%、壳聚糖浓度2.5%、氯化钙浓度2.0%、桑椹红浓度0.50%、针头孔径0.390mm、下滴高度4cm、温度为20℃、转速为300r·min-1。制得的微囊药物含量为11.28%,包封率为88.93%。  相似文献   
5.
Taking inspiration from yeast alcohol dehydrogenase (yADH), a benzimidazolium (BI+) organic hydride‐acceptor domain has been coupled with a 1,10‐phenanthroline (phen) metal‐binding domain to afford a novel multifunctional ligand ( L BI+) with hydride‐carrier capacity ( L BI++H?? L BIH). Complexes of the type [Cp*M( L BI)Cl][PF6]2 (M=Rh, Ir) have been made and fully characterised by cyclic voltammetry, UV/Vis spectroelectrochemistry, and, for the IrIII congener, X‐ray crystallography. [Cp*Rh( L BI)Cl][PF6]2 catalyses the transfer hydrogenation of imines by formate ion in very goods yield under conditions where the corresponding [Cp*Ir( L BI)Cl][PF6] and [Cp*M(phen)Cl][PF6] (M=Rh, Ir) complexes are almost inert as catalysts. Possible alternatives for the catalysis pathway are canvassed, and the free energies of intermediates and transition states determined by DFT calculations. The DFT study supports a mechanism involving formate‐driven Rh?H formation (90 kJ mol?1 free‐energy barrier), transfer of hydride between the Rh and BI+ centres to generate a tethered benzimidazoline (BIH) hydride donor, binding of imine substrate at Rh, back‐transfer of hydride from the BIH organic hydride donor to the Rh‐activated imine substrate (89 kJ mol?1 barrier), and exergonic protonation of the metal‐bound amide by formic acid with release of amine product to close the catalytic cycle. Parallels with the mechanism of biological hydride transfer in yADH are discussed.  相似文献   
6.
The synthesis of a novel pH‐sensitive hetero[4]rotaxane molecular machine through a self‐sorting strategy is reported. The original tetra‐interlocked molecular architecture combines a [c2]daisy chain scaffold linked to two [2]rotaxane units. Actuation of the system through pH variation is possible thanks to the specific interactions of the dibenzo‐24‐crown‐8 (DB24C8) macrocycles for ammonium, anilinium, and triazolium molecular stations. Selective deprotonation of the anilinium moieties triggers shuttling of the unsubstituted DB24C8 along the [2]rotaxane units.  相似文献   
7.
Bulk hierarchical anatase‐titania/cellulose composite sheets were fabricated by subjecting an ultrathin titania gel film pre‐deposited filter paper to a solvo‐co‐hydrothermal treatment by using titanium butoxide as the precursor to grow anatase‐titania nanocrystallites on the cellulose nanofiber surfaces. The titanium butoxide specie is firstly absorbed onto the nanofibers of the cellulose substance through a solvothermal process, which was thereafter hydrolyzed and crystallized upon the subsequent hydrothermal treatment, leading to the formation of fine anatase‐titania nanoparticles with sizes of 2–5 nm uniformly anchored on the cellulose nanofibers. The resulting anatase‐titania/cellulose composite sheet shows a significant photocatalytic performance towards degradation of a methylene blue dye, and introduction of silver nanoparticles into the composite sheet yields an Ag‐NP/anatase‐titania/cellulose composite material possessing excellent antibacterial activity against both Gram‐positive and Gram‐negative bacteria.  相似文献   
8.
The chemistry of [Fe]‐hydrogenase has attracted significant interest due to its ability to activate molecular hydrogen. The intriguing properties of this enzyme have prompted the synthesis of numerous small molecule mimics aimed at activating H2. Despite considerable effort, a majority of these compounds remain nonfunctional for hydrogenation reactions. By using a recently synthesized model as an entry point, seven biomimetic complexes have been examined through DFT computations to probe the influence of ligand environment on the ability of a mimic to bind and split H2. One mimic, featuring a bidentate diphosphine group incorporating an internal nitrogen base, was found to have particularly attractive energetics, prompting a study of the role played by the proton/hydride acceptor necessary to complete the catalytic cycle. Computations revealed an experimentally accessible energetic pathway involving a benzaldehyde proton/hydride acceptor and the most promising catalyst.  相似文献   
9.
刺激响应型微胶囊由于具有独有的高稳定性、多功能性、膜结构的可调性、以及对不同芯材的运送能力,在药物封装和释放、人造细胞、催化、化学传感器等领域具有广阔的应用前景。本文综述了近年来不同刺激响应型复合微胶囊的可控释放的研究进展,包括温敏型、pH响应型、磁响应型、生物响应型、电响应型,以及光响应型微胶囊,根据释放机理的不同着重对光响应型微胶囊的释放过程进行了总结,并对微胶囊可控释放在未来的发展趋势进行了展望。  相似文献   
10.
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