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   255篇
  免费   7篇
  国内免费   4篇
系统科学   3篇
丛书文集   2篇
教育与普及   1篇
理论与方法论   9篇
现状及发展   60篇
研究方法   35篇
综合类   154篇
自然研究   2篇
  2023年   1篇
  2022年   1篇
  2019年   1篇
  2018年   5篇
  2017年   5篇
  2016年   7篇
  2015年   5篇
  2014年   7篇
  2013年   7篇
  2012年   17篇
  2011年   32篇
  2010年   19篇
  2009年   10篇
  2008年   21篇
  2007年   13篇
  2006年   14篇
  2005年   9篇
  2004年   8篇
  2003年   19篇
  2002年   12篇
  2001年   6篇
  2000年   11篇
  1999年   4篇
  1998年   1篇
  1996年   1篇
  1994年   1篇
  1992年   3篇
  1991年   4篇
  1989年   1篇
  1985年   1篇
  1984年   2篇
  1980年   2篇
  1978年   1篇
  1977年   2篇
  1976年   1篇
  1975年   1篇
  1974年   2篇
  1973年   2篇
  1972年   1篇
  1971年   1篇
  1970年   2篇
  1969年   1篇
  1966年   2篇
排序方式: 共有266条查询结果,搜索用时 15 毫秒
111.
This study investigated the extraction of 5'-phosphodiesterase from malt root and the degradation of nucleic acids by this enzyme. The extraction used grade precipitation with ammonium sulfate and enzymatic hydrolysis. Samples were assayed using the modified Bradford method and high performance liquid chromatography. The results show that 5'-phosphodiesterase is isolated by grade precipitation with 30% and 80% saturation of ammonium sulfate and can be utilized to degrade deoxyribonucleic acid. The hydrolysate has four kinds of deoxynucleotides: 5'-dCMP, 5'-dTMP, 5'-dAMP, and 5'-dGMP. The optimum reaction temperature is 70℃, and the optimum pH is 5.5-6.0 for the reaction. The percentage of deoxynucleotides indicated by the China Pharmacopoeia (2000 edition) in the product is over 70%. The extraction of 5'-phosphodiesterase from malt root is shown to be possible and economical. Products from the enzYmatic hydrolysate of DNA meet the pharmacopoeia.  相似文献   
112.
113.
Indole-3-carbinol (I3C) has been found to act against several types of cancer, while ultraviolet B (UVB) is known to induce the apoptosis of human melanoma cells. Here, we investigated whether I3C can sensitize G361 human melanoma cells to UVB-induced apoptosis. We examined the effects of combined I3C and UVB (I3C/UVB) at various dosages. I3C (200 μM)/UVB (50 mJ/cm2) synergistically reduced melanoma cell viability, whereas I3C (200 μM) or UVB (50 mJ/cm2), separately, had little effect on cell viability. DNA fragmentation assays indicated that I3C/UVB induced apoptosis. Further results show that I3C/UVB activates caspase-8, −3, and Bid and causes the cleavage of poly(ADP-ribose) polymerase. Moreover, I3C decreased the expression of the anti-apoptotic protein, Bcl-2, whereas UVB increased the translocation of Bax to mitochondria. Thus, an increased Bax/Bcl-2 ratio by I3C/UVB may result in melanoma apoptosis. In conclusion, our study demonstrated that I3C sensitizes human melanoma cells by down-regulating Bcl-2. Received 5 July 2006; received after revision 25 August 2006; accepted 11 September 2006  相似文献   
114.
YH Kim  JS Heo  TH Kim  S Park  MH Yoon  J Kim  MS Oh  GR Yi  YY Noh  SK Park 《Nature》2012,489(7414):128-132
Amorphous metal-oxide semiconductors have emerged as potential replacements for organic and silicon materials in thin-film electronics. The high carrier mobility in the amorphous state, and excellent large-area uniformity, have extended their applications to active-matrix electronics, including displays, sensor arrays and X-ray detectors. Moreover, their solution processability and optical transparency have opened new horizons for low-cost printable and transparent electronics on plastic substrates. But metal-oxide formation by the sol-gel route requires an annealing step at relatively high temperature, which has prevented the incorporation of these materials with the polymer substrates used in high-performance flexible electronics. Here we report a general method for forming high-performance and operationally stable metal-oxide semiconductors at room temperature, by deep-ultraviolet photochemical activation of sol-gel films. Deep-ultraviolet irradiation induces efficient condensation and densification of oxide semiconducting films by photochemical activation at low temperature. This photochemical activation is applicable to numerous metal-oxide semiconductors, and the performance (in terms of transistor mobility and operational stability) of thin-film transistors fabricated by this route compares favourably with that of thin-film transistors based on thermally annealed materials. The field-effect mobilities of the photo-activated metal-oxide semiconductors are as high as 14 and 7?cm(2)?V(-1)?s(-1) (with an Al(2)O(3) gate insulator) on glass and polymer substrates, respectively; and seven-stage ring oscillators fabricated on polymer substrates operate with an oscillation frequency of more than 340?kHz, corresponding to a propagation delay of less than 210?nanoseconds per stage.  相似文献   
115.
116.
117.
In this paper,to study the power loss of converter for elevator,The analysis is to establish relationship between parameters(current,voltages and losses in the inverter and converter) relevant for sizing of major components of the drive was done.) For high PWM(pulse width modulation) frequency like in elevator applications of fpwm =10kHz,switching losses are dominant and are about 2/3 of the total losses on IGBT switch.Transition from continuous 3 phase to discontinuous 2 phase PWM results in 50% reduction of switching loses on IGBT devices providing that PWM is not done over 60deg angle in a particular phase when current has maximum value.Total losses on IGBT(conduction + switching) are reduced approximately by ~1/3 what is still a significant reduction.Two phase PWM with reduced losses can be used for applications when acoustic noise due to increased current ripple is not significant and fall back solution to regular 3 phase PWM when drive operates under rare extreme conditions resulting in increased heat sink temperature.The analysis will be examined by further laboratory testing simulating 60% duty cycle on a dynamometer.  相似文献   
118.
This paper presents a system to alert of dangerous a child situation of a child by applying context information collected from a home network to ontology that is capable of inference. Radio frequency Identification (RFID) and sensors were used for the configuration of a home network, to obtain the raw data to convert into context information. To express the ontology, web ontology language (OWL) was used to provide the inference of context information. Then, simple object access protocol (SOAP) messages were used to notify of the dangerous situations that a child may be involved in via mobile devices. The proposed system consists of Context Manager, Service Manager, and Notification Manager. The child's safety management system can proactively detect the context data of a child on the basis of context awareness. In the experiment, the Jena 2.0 by ontology reasoner and the OSGi(Open Service Gateway initiative) Gateway developed using open source software Knopflerfish 1.3.3 were used to implement the service frame work.  相似文献   
119.
1 Results Polymeric light-emitting diodes (PLEDs) have attracted much scientific and technological research interest due to a number of advantages over inorganic or organic small molecules for use in LEDs: better processability,lower operating voltages,faster response times,lower production costs,and high flexibility.Polyfluorene has been selected for the polymer backbone,because of its large band gap,facile substitution at the C9 position of fluorene,good chemical and thermal stability,and high photolu...  相似文献   
120.
Gated Single Assignment (GSA) form is used to transform an imperative program into a form suitable for dataflow interpretation. We describe a GSA-formed Control Flow Graph (CFG) that contains gating functions and the information of switches. We also present an algorithm to transform an imperative program into a GSA-formed CFG. Transformation of an imperative program into a GSA-formed CFG provides the basis for generating dataflow graphs(DFG). By using GSA-formed CFG, we can transform an imperative program into a DFG more simply comparing with previous methods, and show an expectation of use with demand- and control-driven model. As we create an intermediate form which has essential information for translation, it will be used to do transformations for various kinds of dataflow models.  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