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   3006篇
  免费   566篇
  国内免费   718篇
化学   1230篇
晶体学   64篇
力学   161篇
综合类   138篇
数学   232篇
物理学   791篇
无线电   1674篇
  2024年   13篇
  2023年   45篇
  2022年   118篇
  2021年   96篇
  2020年   84篇
  2019年   75篇
  2018年   79篇
  2017年   141篇
  2016年   102篇
  2015年   155篇
  2014年   214篇
  2013年   204篇
  2012年   219篇
  2011年   227篇
  2010年   205篇
  2009年   254篇
  2008年   299篇
  2007年   277篇
  2006年   226篇
  2005年   231篇
  2004年   168篇
  2003年   132篇
  2002年   124篇
  2001年   117篇
  2000年   130篇
  1999年   58篇
  1998年   47篇
  1997年   36篇
  1996年   23篇
  1995年   29篇
  1994年   16篇
  1993年   19篇
  1992年   23篇
  1991年   19篇
  1990年   13篇
  1989年   12篇
  1988年   12篇
  1987年   10篇
  1986年   7篇
  1985年   9篇
  1984年   5篇
  1982年   5篇
  1981年   1篇
  1978年   1篇
  1976年   1篇
  1971年   1篇
  1968年   2篇
  1964年   1篇
  1961年   3篇
  1959年   1篇
排序方式: 共有4290条查询结果,搜索用时 8 毫秒
991.
微波场中A型分子筛及分子筛膜合成的研究   总被引:8,自引:0,他引:8  
Effects of aging time and alkalinity as well as cation concentration(Na+) on the synthesis of A type zeolite and zeolite membrane were investigated in this paper. The results showed that aging time for synthesis of A type zeolite and zeolite membrane had remarkable influence and alkalinity could accelerate the formation of zeolite but was disadvantage to zeolite membrane synthesis. Cation concentration(Na+) also couldn′t accelerate the synthesis of zeolite and zeolite membrane but it was probable an important factor to accelerate crystal growth .Moreover, more compact zeolite membranes could be synthesized by this method while the thickness of membrane was thinner than that of convention. Zeolites and zeolite membranes were characterized by XRD, TEM as well as IR characterization techniques.  相似文献   
992.
提出了一种简明的理论分析方法,求出了三分子反应A+2X 3X在一维介质上的动力学行为的解析解,并成功地解决了该反应动力学行为对初始条件的依赖性问题。还通过数值模拟证实了提出的理论方法的正确性。  相似文献   
993.
The intelligent controlled drug delivery systems (DDS) are a series of the preparations including microcapsules or nanocapsules composed of intelligent polymers and medication. The properties of preparations can change with the external stimuli, such as pH value, temperature,chemical substance, light, electricity and magnetism etc. According to this properties, the DDS can be intelligently controlled. This paper has reviemed research on syntheses and applications of intelligent controlled DDS of polymer carriers.Drug delivery system with pH stimuliThe volume of polymer hydrogel can change with the pH value of external environment. The sensitive polymer hydrogels to pH are often as carriers. The polymer hydrogel carrying medicine is especially suitable for taking orally. In order to protect medicine from losing activation, we enwrapped medicine into polymer hydrogel with acidic group. In the acidic environment of stomach,the volume of polymer hydrogel contracts because of the hydrogen bond. The medicine in the polymer hydrogel cannot disperse out. When it goes to the intestine of basic environment, the hydrogen bond will be broken, and the medicine can release.Drug delivery system with temperatureTemperature sensitive polymer hydrogel can change its volume with changing of environmental temperature. This kind of polymer hydrogel can be also used as a carrier of medicine. At a low temperature, the polymer chains form hydrogen bond with water to swell to let medicine disperse out from the hydrogel. On the other hand, the hydrogen bond will be broken and polymer chain will lose water to contract with temperature's increasing. And the medicine will not disperse out. For example,the poly(N-isopropylacrylamide)(PNIPAAm) is the hydrogel that is swelled at lower temperature and contracted at higher temperature. PNIPAAm has the lower critical solution temperature(LCST).We can adjust its LCST to control PNIPAAm hydrogel's swelling or contraction to let medicine release or not.Drug delivery system with other stimuliThe polymer carrier drug delivery system can be intelligently controlled with the stimuli of pH value and temperature. In addition, there are still some other stimuli for DDS. For example, DDS with light; DDS with electricity(or electric field); DDS with magnetism(magnetic field); DDS with chemical substance; etc. The characteristic of intelligent polymer carrier is based on P.J.Flory's gel-swelling theory. Intelligent polymer carrier DDS will be widely used in biological and medical fields.  相似文献   
994.
One polythiophene derivative PT3T and two low band gap copolymers,PBTT-T3T and PBTT,with different ratios of 5,6-dini-trobenzothiadiazole as the acceptor unit in the polymer backbone have been synthesized by Pd-catalyzed Stille-coupling polymerizations.Thermal stability,X-ray diffraction analyses,UV-vis absorption spectra,photoluminescence spectra and electrochemical properties of the copolymers were investigated.The band gap estimated from UV-vis-NIR spectra of the copolymers films varied from 1.39 to 1.94...  相似文献   
995.
The local maxima of water density in the space near the single-helical of Konjac glucomannan as well as the dominant conformation of the system was studied by molecular dyna-mics under the water environment. The results indicate that the hydration shell,potential energy,and its hydrogen bond network with water bridges of the left-handed single-helix conformation was favored compared to those of right-handed single-helix conformation. This work suggests that the left-handed single-helix conformation was the dominant conformation of KGM in the water environment.  相似文献   
996.
特殊结构的α-氨基酸对于生命过程研究和生物医药开发有重要意义. 不对称合成α-氨基酸的方法学中, 手性辅基方法是较多研究和应用的领域. 介绍了该方法近二十年来的发展, 对近十年的甘氨酸等当体合成策略做了详尽述评.  相似文献   
997.
Some excellent catalysts for ODE reaction,such as K-Sr-Bi-O-Cl[1], LiCl/sulfated-zirconia[2], which have high conversion of ethane and selectivity to ethylene, have been reported recently. But the relatively high reaction temperatures up to 620℃ and/or very low GHSV have been used over above catalyst systems. So the research aimed at searching for new catalysts with good performances at low temperatures appears to be desirable.  相似文献   
998.
超微NaY分子筛的合成(Ⅰ)──添加轻稀土离子的影响   总被引:11,自引:0,他引:11  
导向剂的陈化时间和反应胶的晶化温度可以影响NaY分子筛的晶化行为.随导向剂陈化时间的增加,NaY分子筛的晶化速度加快,晶粒尺寸减小;随晶化温度的升高,NaY分子筛的晶化速度加快,但晶粒尺寸增加.而向合成体系中添加轻稀土离子(Ln3+)对NaY分子筛的晶化行为影响更为明显.与未添加稀土离子时相比较,在一定的稀土离子添加量范围内[n(Ln3+)/n(Al3+)<0.2],NaY分子筛的晶化速度明显加快而晶粒尺寸显著减小,同时所合成出的NaY分子筛的n(SiO2)/n(Al2O3)比也有较大的提高.添加稀土离子使合成体系中形成异晶晶种的Ln(OH)3微晶,从而进一步引发NaY分子筛的成核.  相似文献   
999.
提出了一种基于COM-Express Nano工控板现IP调度台的方法。给出了系统硬件和软件结构,描述了软硬件各个模块的功能。该设计方案以COM-Express Nano工控板为软件和硬件平台,在此基础上实现IP视频和音频调度设备,具有容易实现可扩展性强等特点。  相似文献   
1000.
基于观测器的复杂网络辨识新方法研究   总被引:1,自引:1,他引:0       下载免费PDF全文
万佑红  王锁萍  蒋国平 《电子学报》2010,38(5):1064-1068
充分考虑复杂网络中只有部分状态变量可测量的特点,提出了辨识复杂网络拓扑结构的新方法.针对复杂网络中存在部分不可测状态变量的问题,在系统具有多输出的情况下,提出了一种利用降维观测器辨识网络拓扑的方法;进一步实现了仅利用输出变量就能辨识网络拓扑.根据Lyapunov稳定性理论,推导出了设计观测器的条件和拓扑辨识方法.与现有基于观测器的复杂网络辨识方法相比,该方法更加简单实用.最后利用该方法对复杂网络的拓扑结构进行辨识仿真,效果良好.  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