全文获取类型
收费全文 | 3335篇 |
免费 | 418篇 |
国内免费 | 271篇 |
专业分类
化学 | 991篇 |
晶体学 | 4篇 |
力学 | 20篇 |
综合类 | 89篇 |
数学 | 47篇 |
物理学 | 694篇 |
无线电 | 2179篇 |
出版年
2024年 | 42篇 |
2023年 | 130篇 |
2022年 | 144篇 |
2021年 | 146篇 |
2020年 | 110篇 |
2019年 | 179篇 |
2018年 | 57篇 |
2017年 | 132篇 |
2016年 | 145篇 |
2015年 | 133篇 |
2014年 | 223篇 |
2013年 | 205篇 |
2012年 | 238篇 |
2011年 | 218篇 |
2010年 | 184篇 |
2009年 | 202篇 |
2008年 | 236篇 |
2007年 | 195篇 |
2006年 | 184篇 |
2005年 | 154篇 |
2004年 | 121篇 |
2003年 | 156篇 |
2002年 | 118篇 |
2001年 | 86篇 |
2000年 | 50篇 |
1999年 | 30篇 |
1998年 | 35篇 |
1997年 | 40篇 |
1996年 | 36篇 |
1995年 | 19篇 |
1994年 | 18篇 |
1993年 | 7篇 |
1992年 | 13篇 |
1991年 | 8篇 |
1990年 | 11篇 |
1989年 | 12篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1982年 | 1篇 |
1951年 | 1篇 |
排序方式: 共有4024条查询结果,搜索用时 15 毫秒
991.
Pornographic image/video recognition plays a vital role in network information surveillance and management. In this paper, its key techniques, such as skin detection, key frame extraction, and classifier design, etc, are studied in compressed domain. A skin detection method based on data-mining in compressed domain is proposed firstly and achieves the higher detection accuracy as well as higher speed. Then, a cascade scheme of pornographic image recognition based on selective decision tree ensemble is proposed in order to improve both the speed and accuracy of recognition. A pornographic video oriented key frame extraction solution in compressed domain and an approach of pornographic video recognition are discussed respectively in the end. 相似文献
992.
993.
本文基于AIBO机器狗通用平台,研究了一种自主识别不同材质地表的方法。通过提取z轴加速度传感信号的震动功率模式,作为反馈信号,并设计数字PI控制器,调节运动控制系统的性能,实现对各种地表的识别,产生自适应运动。实验结果表明这一方法是可行的。 相似文献
994.
摘要:采用SMIC 0.18um CMOS工艺设计并实现了一个5-Gb/s在片集成时钟提取功能的2:1复接器,且该时钟提取子电路具有自动相位对准功能.芯片面积为670um*780um.在1.8V电压下,总功耗为112 mW, 输入灵敏度在50 mV以下, 输出单端摆幅大于300 mV. 测试结果表明,该复接器能够在不需要任何外接元件、参考时钟或外部相位调整下可靠地工作在1.8 Gb/s至2.6 Gb/s之间的任何输入数据速率. 该芯片可被用在并行光互连系统中. 相似文献
995.
High frequency intrinsic small-signal model parameter extraction for microwave SiGe heterojunction bipolar transistors is studied, with a focus on the main feedback elements including the emitter series resistor, internal and external base-collector capacitors as well as the base series resistor, all of which are important in determining the behavior of the device equivalent circuit. In accordance with the respective features of definition of the Y- and Z-parameters, a novel combined use of them succeeds in reasonably simplifying the device equivalent circuit and thus decoupling the extraction of base-collector capacitances from other model parameters. As a result, a very simple direct extraction method is proposed. The proposed method is applied for determining the SiGe HBT small-signal model parameters by taking numerically simulated Y- and Z-parameters as nominal "measurement data" with the help of a Taurus-device simulator. The validity of the method is preliminarily confirmed by the observation of certain linear relations of device frequency behavior as predicted by the corresponding theoretical analysis. Furthermore, the extraction results can be used to reasonably account for the dependence of the extracted model parameters on device geometry and process parameters, reflecting the explicit physical meanings of parameters, and especially revealing the distributed nature of the base series resistor and its complex interactions with base-collector capacitors. Finally, the accuracy of our model parameter extraction method is further validated by comparing the modeled and simulated S-parameters as a function of frequency. 相似文献
996.
Gallium nitride (GaN) based light emitting diodes (LEDs) with a thick and high quality ZnO film as a current spreading layer grown by metal-source vapor phase epitaxy (MVPE) are fabricated successfully. Compared with GaN-based LEDs employing a Ni/Au or an indium tin oxide transparent current spreading layer, these LEDs show an enhancement of the external quantum efficiency of 93% and 35% at a forward current of 20 mA, respectively. The full width at half maximum of the ZnO (002) ω-scan rocking curve is 93 arcsec, which corresponds to a high crystal quality of the ZnO film. Optical microscopy and atomic force microscopy are used to observe the surface morphology of the ZnO film, and many regular hexagonal features are found. A spectrophotometer is used to study the different absorption properties between the ZnO film and the indium tin oxide film of the GaN LED. The mechanisms of the extraction quantum efficiency increase and the series resistance change of the GaN-based LEDs with ZnO transparent current spreading layers are analyzed. 相似文献
997.
To enhance the light extraction efficiency of traditional light-emitting diodes(LEDs) by reducing the total internal reflection,an omni-directional reflector(ODR) and photonic crystal are adopted in the paper.The structures of photonic crystal and the ODR are designed by diffraction theory and finite difference time domain(FDTD) method.The photonic crystal is employed in the p-GaN layer and the ODR composed of TiO2/SiO2 is designed between the active region and substrate.The simulation results indicate that the light extraction of LEDs can be enhanced by 11.6 times,and the external quantum efficiency of LEDs will be effectively improved. 相似文献
998.
CMOS标准单元后仿真及其时序信息的建立 总被引:1,自引:0,他引:1
时序信息是标准单元特征化参数的重要组成部分.针对全定制标准单元的版图,在进行LVS验证之后,采用寄生参数提取工具对其进行晶体管级的寄生参数提取,得到单元内部的详细寄生电容和电阻值.提出了一种建立标准单元时序信息的方法,并以一个具体标准单元为例对其进行了版图后仿真,结果表明该法行之有效. 相似文献
999.
提出了一种有效的星像坐标提取方法,该方法根据星像拖尾的特点,通过对拖尾星像进行预处理使之接近理想的星像特征,然后采用重心算法提取其星像坐标.仿真结果表明该方法提取的星像坐标误差比直接采用重心算法提取的星像坐标误差明显减少,从而提高了星敏感器的精度和机动性. 相似文献
1000.