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   124篇
  免费   19篇
  国内免费   19篇
化学   91篇
晶体学   1篇
力学   5篇
数学   5篇
物理学   60篇
  2024年   1篇
  2023年   2篇
  2022年   11篇
  2021年   5篇
  2020年   7篇
  2019年   9篇
  2018年   4篇
  2017年   6篇
  2016年   7篇
  2015年   7篇
  2014年   1篇
  2013年   4篇
  2012年   7篇
  2011年   11篇
  2010年   3篇
  2008年   4篇
  2007年   5篇
  2006年   7篇
  2004年   3篇
  2003年   5篇
  2002年   2篇
  2001年   4篇
  2000年   6篇
  1999年   3篇
  1998年   4篇
  1997年   4篇
  1996年   7篇
  1995年   5篇
  1994年   5篇
  1993年   5篇
  1992年   2篇
  1991年   1篇
  1990年   2篇
  1987年   1篇
  1986年   1篇
  1984年   1篇
排序方式: 共有162条查询结果,搜索用时 31 毫秒
101.
利用汉明距离优选神经网络学习样本   总被引:1,自引:0,他引:1  
鉴于学习样本对神经网络模型的模式识别性能有很大的影响 ,提出学习样本的选择应与识别模型所利用的特性相结合 ,并利用汉明 (Hamming)距离对用于旋转不变识别的级联模型的学习样本进行优选 ,计算机对三个很相似的飞机模型进行识别 ,识别结果表明对学习样本进行有效的选择不仅可以减少系统的学习训练时间而且可以提高模型的识别能力。  相似文献   
102.
The development of RNA imaging strategies in live cells is essential to improve our understanding of their role in various cellular functions. We report an efficient RNA imaging method based on the CRISPR-dPspCas13b system with fluorescent RNA aptamers in sgRNA (CasFAS) in live cells. Using modified sgRNA attached to fluorescent RNA aptamers that showed reduced background fluorescence, this approach provides a simple, sensitive way to image and track endogenous RNA with high accuracy and efficiency. In addition, color switching can be easily achieved by changing the fluorogenic dye analogues in living cells through user-friendly washing and restaining operations. CasFAS is compatible with orthogonal fluorescent aptamers, such as Broccoli and Pepper, enabling multiple colors RNA labeling or intracellular RNA–RNA interaction imaging. Finally, the visualization of severe fever with thrombocytopenia syndrome virus (SFTSV) was achieved by CasFAS, which may facilitate further studies on this virus.

The development of RNA imaging strategies in live cells is essential to improve our understanding of their role in various cellular functions.  相似文献   
103.
As the key properties of perovskite solar cells (PSCs), the hole extraction and transport capabilities of the hole transport material (HTM) affect the photovoltaic performance of PSCs to a considerable extent, while both capabilities can be adjusted by molecular planarity. Therefore, in this work, the molecular planarity of the HTM is systematically optimized to regulate the hole extraction and transport capabilities. Along with the improvement in planarity, the HTM′s HOMO level is increased, leading to the enhancement of hole extraction capability. Meanwhile, the hole transport capability can also be improved due to the intensification of molecular stacking during the film formation. As a result, the planar HTM achieves a relatively high efficiency of 18.48 %, which is higher than that of spiro-OMeTAD. Accordingly, the molecular planarity presents an important impact on the photovoltaic performance of PSCs, providing us with a promising strategy for further optimization of efficient HTMs.  相似文献   
104.
张峰  张恒  朱万诚  李言信  赵斌 《应用化学》2012,29(6):668-673
在微波作用下,由钙基膨润土和铁溶胶快速制备铁柱撑膨润土,分别用XRD、BET、SEM-EDS和TC-DSC测试技术表征了催化剂的组成和结构,结果表明,羟基铁阳离子进入膨润土层间取代Ca2+形成柱撑,膨润土孔结构基本不变,比表面积增加.考察了铁柱撑膨润土对光催化降解甲基橙反应的催化活性,确定了最佳降解反应条件为:体系pH=3,微波500W、80℃辐射10min制备的铁柱撑膨润土为催化剂,用量1.5g/L,H2O2浓度为7.345×10-3mol/L,甲基橙浓度100mg/L,紫外光照射60min,甲基橙的降解率可达98.1%.  相似文献   
105.
Yang X  Liu Y  Tian F  Yuan L  Liu Z  Luo S  Zhao E 《Optics letters》2012,37(11):2115-2117
A fiber optic integrated modulation-depth-tunable modulator based on a type of hollow optical fiber with suspended core is proposed and investigated. We synthesized magnetic fluid containing superparamagnetic Fe(3)O(4) nanoparticles and encapsulated it in the hollow optical fiber as the cladding layer of the suspended core by fusing the hollow optical fiber with the multimode optical fibers. The light with a wavelength of 632.8 nm is coupled in and out of the modulating element by a tapering technique. Experimental results show that the light attenuation in the system can be greatly influenced by only 2.0×10(-2) μL of the magnetic fluid under different magnetic field strengths. The saturated modulation depth is 43% when the magnetic field strength is 489 Oe. The response time of the system is <120 ms. Significantly, this work presents information for the development of all-fiber modulators, including other integrated electro-optic devices, such as optical switch, optical fiber filter, and magnetic sensors utilizing the special structure of this hollow optical fiber with suspended core and superparamagnetic magnetic fluid.  相似文献   
106.
The urchin-like Bi2S3 nanostructures have been grown by a facile environmentally friendly hydrothermal method. X-ray diffraction (XRD) and Raman spectrum demonstrate that the obtained samples are composed of pure orthorhombic phase Bi2S3. Scanning electron microscopy (SEM) images and transmission electron microscopy (TEM) images reveal that it is produced as uniform urchin-like pattern with spherical symmetry. High-resolution (HR) TEM and selected-area electron diffraction (SAED) demonstrate that the nanowires which grow radially from the center of the urchin-like nanostructures toward all directions are single-crystalline and grow along the [001]. It is found that the reaction time, reaction temperature and thiourea (Tu) play key roles for the formation of urchin-like Bi2S3 nanostructures. The formation mechanism is ascribed to self-assembly and the intrinsic splitting character of the Bi2S3 structure. The urchin-like Bi2S3 composed of porous nanorods, solid nanorods and nanowires could be found potential application in optical, catalysts and sensor devices.  相似文献   
107.
Xu  Lechang  Wang  Yanxin  L&#;  Junwen  Lu  Xueshi  Liu  Yaochi  Liu  Xiaoyang 《中国科学:化学(英文版)》2002,45(1):11-19

The distribution of radioactive pollutants, such as222Rn, U, Th and226Ra in the air, surface waters, soils and crops around the Lincang uranium mine, Yunnan Province, China, is studied. The mechanical, geochemical and biogeochemical processes responsible for the transport and fate of the radioactive elements are discussed based on the monitoring data. The pollutants concentrations of effluents from the mine tunnels were dependent on pH and which were controlled by biochemical oxidation of sulfide in the ore/host rocks. Radon anomalies in air reached 4 km from the tailings pile depending on radon release from the site, topography and climate.238U and226Ra abnormities in stream sediments and soil were 40—90 cm deep and 790—800 m away downstream. Anomalies of radioactive contaminants of surface watercourses extended 7.5—13 km from the discharge of effluents of the site mainly depending on mechanical and chemical processes. There were about 2.86 ha rice fields and 1.59 km stream sediments contaminated. Erosion of tailings and mining debris with little or no containment or control accelerated the contamination processes.

  相似文献   
108.
We present a new theoretical strategy, ab initio rate constants plus integration of rate equations, that is used to characterize the role of entropy in driving high-temperature/low-pressure hydrocarbon chemical kinetics typical of filament-assisted diamond growth environments. Twelve elementary processes were analyzed that produce a viable pathway for converting methane in a feed gas to acetylene. These calculations clearly relate the kinetics of this conversion to the properties of individual species, demonstrating that (1) loss of translational entropy restricts addition of hydrogen (and other radical species) to unsaturated carbon-carbon bonds, (2) rotational entropy determines the direction of the rate-limiting abstraction reactions, and (3) the overall pathway is enhanced by high beta-scission reaction rates driven by translational entropy. These results suggest that the proposed strategy is likely applicable to understand gas-phase chemistry occurring in the systems of combustion and other chemical vapor depositions.  相似文献   
109.
Using a new, parameter-free first principles strategy for modeling sublimation growth, we show that while Al and N2 dominate gas concentrations in AlN sublimation growth chambers under typical growth conditions, N2 is undersaturated with respect to the crystal and therefore cannot be a growth precursor. Instead, our calculations predict that the nitrogen-containing precursors are Al(n)N (n=2,3,4), in stark contrast to assumptions used in all previous modeling studies of this system.  相似文献   
110.
本文分析了影响液晶显示屏对比度的种种因素,根据偏振光在液晶中传输的性质,提出了一种有效改进液晶电视空间光调制器对比度的新方法。通过实验得到了液晶电视最佳对比度的条件,从而使原来4∶1的最佳对比度提高到19∶1。  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