全文获取类型
收费全文 | 2173篇 |
免费 | 583篇 |
国内免费 | 145篇 |
专业分类
化学 | 107篇 |
晶体学 | 26篇 |
力学 | 247篇 |
综合类 | 13篇 |
数学 | 116篇 |
物理学 | 1172篇 |
综合类 | 1220篇 |
出版年
2024年 | 16篇 |
2023年 | 44篇 |
2022年 | 100篇 |
2021年 | 79篇 |
2020年 | 78篇 |
2019年 | 107篇 |
2018年 | 69篇 |
2017年 | 87篇 |
2016年 | 125篇 |
2015年 | 110篇 |
2014年 | 153篇 |
2013年 | 151篇 |
2012年 | 151篇 |
2011年 | 151篇 |
2010年 | 114篇 |
2009年 | 128篇 |
2008年 | 128篇 |
2007年 | 144篇 |
2006年 | 109篇 |
2005年 | 85篇 |
2004年 | 90篇 |
2003年 | 91篇 |
2002年 | 79篇 |
2001年 | 75篇 |
2000年 | 61篇 |
1999年 | 65篇 |
1998年 | 49篇 |
1997年 | 28篇 |
1996年 | 31篇 |
1995年 | 34篇 |
1994年 | 30篇 |
1993年 | 21篇 |
1992年 | 9篇 |
1991年 | 21篇 |
1990年 | 15篇 |
1989年 | 13篇 |
1988年 | 11篇 |
1987年 | 8篇 |
1986年 | 5篇 |
1985年 | 9篇 |
1984年 | 4篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1976年 | 1篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1971年 | 5篇 |
1955年 | 1篇 |
排序方式: 共有2901条查询结果,搜索用时 218 毫秒
991.
992.
993.
The characteristics of typical AE signals initiated by mechanical component damages are analyzed. Based on the extracting principle of acoustic emission(AE) signals from damaged components,the paper introduces Wigner high-order spectra to the field of feature extraction and fault diagnosis of AE signals. Some main performances of Wigner binary spectra,Wigner triple spectra and Wigner-Ville distribution (WVD) are discussed,including of time-frequency resolution,energy accumulation,reduction of crossing items and noise elimination. Wigner triple spectra is employed to the fault diagnosis of rolling bearings with AE techniques. The fault features reading from experimental data analysis are clear,accurate and intuitionistic. The validity and accuracy of Wigner high-order spectra methods proposed agree quite well with simulation results. Simulation and research results indicate that wigner high-order spectra is quite useful for condition monitoring and fault diagnosis in conjunction with AE technique,and has very important research and application values in feature extraction and faults diagnosis based on AE signals due to mechanical component damages. 相似文献
994.
通过试验研究声发射信号在金属板中传播的特性,对试验中的模拟声发射源进行定位分析.通过对一钢板进行模拟声发射试验,获取了40组声发射试验数据,对采集信号的波形进行互相关分析,对声发射信号源进行定位计算.通过对各组试验数据进行分析,比较实际测得的声发射源位置和通过计算所得到的声发射源的位置,结果表明采用相关分析法对声发射源进行定位是可行的. 相似文献
995.
Simulation experiment of AE-based localization damage and deformation characteristic on covering rock in mined-out area
下载免费PDF全文
![点击此处可从《矿物冶金与材料学报》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The localization damage and deformation characteristics of multiple rock in mined-out area under thick soil layer were investigated by acoustic emission (AE)-based physical similar simulation experiment and damage test combined with the AE theory on rock damage. During different mining courses, the degree of wall rock suffered from mining disturbed process in mined-out area was different, so did the AE density of initial damage and fracture. Some characteristic parameters, including large events, total events, and energy rate, presented fluctuations with temporal and spatial change and take on a certain extent statistic relation. Subsequently, the percent of damage degree was defined and divided into 4 stages, and the localization damage and deformation characteristics of surrounding rock in mined-out area and coal pillars were analyzed quantitatively. 相似文献
996.
A three-dimensional coupled-mode model for the acoustic field in a two-dimensional waveguide with perfectly reflecting boundaries
下载免费PDF全文
![点击此处可从《中国物理 B》网站下载免费的PDF全文](/ch/ext_images/free.gif)
This paper presents a three-dimensional(3D) coupled-mode model using the direct-global-matrix technique as well as Fourier synthesis. This model is a full wave, two-way three-dimensional model, and is therefore capable of providing accurate acoustic field solutions. Because the problem of sound propagation excited by a point source in an ideal wedge with perfectly reflecting boundaries is one of a few three-dimensional problems with analytical solutions, the ideal wedge problem is chosen in this work to validate the presented three-dimensional model. Numerical results show that the field results by analytical solutions and those by the presented model are in excellent agreement, indicating that the presented model can serve as a benchmark model for three-dimensional sound propagation problems involving a planar two-dimensional geometry as well as a point source. 相似文献
997.
Demodulation of acoustic telemetry binary phase shift keying signal based on high-order Duffing system
下载免费PDF全文
![点击此处可从《中国物理 B》网站下载免费的PDF全文](/ch/ext_images/free.gif)
In order to grasp the downhole situation immediately, logging while drilling(LWD) technology is adopted. One of the LWD technologies, called acoustic telemetry, can be successfully applied to modern drilling. It is critical for acoustic telemetry technology that the signal is successfully transmitted to the ground. In this paper, binary phase shift keying(BPSK) is used to modulate carrier waves for the transmission and a new BPSK demodulation scheme based on Duffing chaos is investigated. Firstly, a high-order system is given in order to enhance the signal detection capability and it is realized through building a virtual circuit using an electronic workbench(EWB). Secondly, a new BPSK demodulation scheme is proposed based on the intermittent chaos phenomena of the new Duffing system. Finally, a system variable crossing zero-point equidistance method is proposed to obtain the phase difference between the system and the BPSK signal. Then it is determined that the digital signal transmitted from the bottom of the well is ‘0' or ‘1'. The simulation results show that the demodulation method is feasible. 相似文献
998.
999.
本文研究了S0兰姆波在渐变折射率声子晶体中的聚焦现象.通过调整正方晶格声子晶体的主轴方向与波传播的相对方向,设计了ΓX和ΓM方向两组不同的渐变折射率声透镜.采用有限单元法模拟S0波在不同声透镜内的传播,并与S0波在声透镜内的传播迹线做了对比,数值结果和理论结果基本一致.数值计算结果显示:ΓX方向的声透镜的焦距fl略大于其理论值,而ΓM方向的声透镜的fl则略小于其理论值.ΓX方向的声透镜的横向分辨率FWHM小于ΓM方向的声透镜,而聚焦强度MFI则大于ΓM方向的声透镜.结果表明,通过调整声子晶体的主轴方向与波传播的相对方向,可以显著地改变声透镜的聚焦特征.这为S0兰姆波声透镜的设计提供了重要依据. 相似文献
1000.
高亚声速湍流喷流气动噪声数值分析 总被引:3,自引:1,他引:2
为适应航空噪声管制规定要求,发动机喷流噪声控制成为目前气动声学研究中的重要课题,预测分析喷流噪声辐射并揭示其产生机理将为噪声控制奠定基础.采用高精度并行LES(large eddy simulation)方法计算分析马赫数0.9高亚声速喷流的湍流演化和气动噪声现象.首先,仔细验证喷流LES湍流场计算保真性,并分析流场中不同尺度涡结构的演化形态.其次,利用可穿透面FW-H(Ffowcs Williams and Hawkings)方法外推喷流近场声源数据获得精确声辐射远场,进而分析声场主导声模态特性.最后,通过分析声源机制、分离声模态等方法研究势流核末端大尺度拟序涡运动演化形成的低波数波包在噪声主导声模态产生中的重要作用.数值结果表明LES结合可穿透面FW-H方法可精确预测高亚声速喷流的流场及声场特征,且数值分析揭示涡环对并形成的大尺度拟序结构在喷流中心线上沿径向融合,产生了在远场低方位角占优的主导声模态,并构成强指向性声场,噪声峰值方位角约为30°. 相似文献