全文获取类型
收费全文 | 968篇 |
免费 | 91篇 |
国内免费 | 53篇 |
专业分类
化学 | 118篇 |
晶体学 | 1篇 |
力学 | 82篇 |
综合类 | 11篇 |
数学 | 216篇 |
物理学 | 684篇 |
出版年
2024年 | 5篇 |
2023年 | 11篇 |
2022年 | 14篇 |
2021年 | 21篇 |
2020年 | 22篇 |
2019年 | 22篇 |
2018年 | 18篇 |
2017年 | 25篇 |
2016年 | 37篇 |
2015年 | 37篇 |
2014年 | 77篇 |
2013年 | 87篇 |
2012年 | 34篇 |
2011年 | 72篇 |
2010年 | 46篇 |
2009年 | 54篇 |
2008年 | 69篇 |
2007年 | 91篇 |
2006年 | 62篇 |
2005年 | 40篇 |
2004年 | 37篇 |
2003年 | 36篇 |
2002年 | 25篇 |
2001年 | 29篇 |
2000年 | 15篇 |
1999年 | 19篇 |
1998年 | 21篇 |
1997年 | 14篇 |
1996年 | 15篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1993年 | 5篇 |
1992年 | 5篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 5篇 |
1988年 | 4篇 |
1987年 | 6篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1973年 | 1篇 |
排序方式: 共有1112条查询结果,搜索用时 15 毫秒
921.
922.
In this paper, a wind tunnel analysis of two degrees-of-freedom system represented by sectional girders is carried out. Besides an evaluation of the aeroelastic coefficients, the analysis is focused on the influence of the natural frequency ratio on the initiation of unstable vibration, which can be of practical interest. On the phenomenological level, the paper also discusses experimentally ascertained response regimes, with an emphasis on their stability character. The attention is paid to the memory effect in the response described by the hysteresis loop together with the separation curves determining the stability boundaries. The influence of initial disturbance on the stability is examined. Two types of cross-sections were investigated: (i) rectangular one with the aspect ratio 1:5, and (ii) bridge-like cross-section with comparable principal dimensions. For both types of cross-sections, the limits of the stability are significantly affected by an intentionally introduced initial disturbance. This holds especially with regard to the rectangular profile where the separation curves create very narrow sub-domains between a stable and an unstable response, while the bridge-like cross-section demonstrates much stable behaviour. 相似文献
923.
《Analytical letters》2012,45(6):1019-1032
To cope with the problem of frequency aliasing in Mallat algorithm, which makes traditional discrete wavelet transform (DWT) inappropriate for feature extraction in some cases, an improved algorithm composed of sub-band reconstruction and Fourier transform is suggested through which the original signal could be split into a series of sub-bands of different frequencies with little distortion both in time and frequency domains. This strategy is developed to extract local features from analytical signals accurately as well as straightforwardly. Some NIR spectra have been selected as examples to demonstrate the availability and application of the proposed method. 相似文献
924.
An approach for time-delay identification is pro-posed in multiple-degree-of-freedom (MDOF) linear sys-tems with multiple feedback. The applicability of the approach is discussed in detail. Based on the characteris-tics of frequency domain in feedback controlled system with multiple time-delays, this paper proposes a time-delay iden-tification approach, which is based on the pseudo impedance function of reference point. Treating feedback time-delays as the“frequencies”of the oscillation curve, the time-delays can be obtained from the“frequencies”of the curve. Numerical simulation is conducted to validate the proposed approach. The application scope of the approach is discussed with regard to different forms of feedback. 相似文献
925.
A novel algorithm for enhancing the details of the blurred infrared images based on frequency extrapolation has been raised in this paper. Unlike other researchers’ work, this algorithm mainly focuses on how to predict the higher frequency information based on the Laplacian pyramid separation of the blurred image. This algorithm uses the first level of the high frequency component of the pyramid of the blurred image to reverse-generate a higher, non-existing frequency component, and adds back to the histogram equalized input blurred image. A simple nonlinear operator is used to analyze the extracted first level high frequency component of the pyramid. Two critical parameters are participated in the calculation known as the clipping parameter C and the scaling parameter S. The detailed analysis of how these two parameters work during the procedure is figure demonstrated in this paper. The blurred image will become clear, and the detail will be enhanced due to the added higher frequency information. This algorithm has the advantages of computational simplicity and great performance, and it can definitely be deployed in the real-time industrial applications. We have done lots of experiments and gave illustrations of the algorithm’s performance in this paper to convince its effectiveness. 相似文献
926.
对均匀来流下靠近壁面处在垂直流向做强迫振荡运动的光滑圆柱的水动力特性进行了试验研究. 试验在拖曳水池中进行, 雷诺数为2× 105, 通过采集顺流向和垂直流向的力, 得到了阻力系数、升力系数、相位角等与间隙比、振荡频率和振幅之间的关系. 通过研究得到如下结论: 1)振荡圆柱的平均阻力系数在近壁面处随间隙比的减小而骤降; 2)振荡圆柱泄涡受到完全抑制的临界间隙比要小于静止圆柱; 3)近壁面的存在对振荡圆柱的能量传递有着重要的影响, 自由边界圆柱强迫振荡所得到的水动力系数不能用来预报海底管道的涡激振动; 4)对于振荡圆柱, 附加质量系数只有在一定的频率范围内才是定值, 且在低频率区域其绝对值随间隙比减小而增大; 5)圆柱在进行强迫振荡时, 其平均阻力系数、振荡阻力系数和振荡升力系数均随无因次振幅的增加而增大.
关键词:
海底管道
强迫振荡
水动力特性
涡激振动 相似文献
927.
基于小波能量和光斑尺寸的干扰图像尺度分析 总被引:2,自引:2,他引:0
介绍了激光干扰对CCD成像系统的影响,从影响光电成像系统目标检测性能的角度阐述了激光干扰图像与背景杂波的相似性.针对激光干扰图像的频率特性,利用小波分析在图像处理中的优势,结合机器视觉中背景杂波的量化尺度,提出了基于小波能量和光斑尺寸的综合图像尺度.对典型激光干扰图像的小波分析显示,随着激光干扰功率的增大,干扰图像中的低频分量和高频分量会同时增加,图像中的光斑可被看作是低频分量,光斑周围“斑驳状”的散斑是高频分量,类似于盐噪音.激光视场内干扰实验及数值计算表明:激光干扰会降低Otsu算法分割准确度,增加相关检测虚警概率,该尺度将干扰图像的高频分量和低频分量结合起来,同时克服了单一光斑尺度和小波能量尺度的局限性,能够较好地评估视场内激光干扰CCD成像系统的效果. 相似文献
928.
Spectra excited in a hollow cathode discharge operating at high frequency and direct currents have been studied. Samples (La2O3 and Y2O3) were placed in the copper hollow cathodes and argon was a carrier gas. The excitation temperatures of Ar I, Ar II, Cu I, Y I, Y II and La II in high frequency and direct current hollow cathode discharges have been determined and compared. 相似文献
929.
Ana M. Reis Marcos V. D. Vermelho Dário L. Nicácio Evandro A. Gouveia Artur S. Gouveia-neto 《Fiber and Integrated Optics》2013,32(2):179-193
Abstract The generation of near-infrared and intense visible light through stimulated multi-wave mixing processes in single-mode silica-based optical fibers pumped by a Q-switched and mode-locked Nd:YAG laser operated at 1.319 μm is described. The experimental results show that intense infrared light around 1.2 μm is produced via selp-hase-matched four-photon mixing at the minimum group velocity dispersion region of pure SiO2?core and P2O5?doped silica fibers. In the visible spectral region, from 580 nm to 600 nm, 20 W peak power 100-ps pulses were generated by pumping single spans of single-mode P2O5?doped and undoped SiO2?core fibers with 1.319-μm laser pulses. The signal light generated in such fibers propagated in the LP02 fiber mode and exhibited a threshold power that depended upon the fiber length and a critical length that was power dependent. Also, it exhibited an asymmetrical spectrum of a few nanometers bandwidth, with a long tail toward high frequencies. For GeO2?doped silica-based fibers, a multiple-wavelength visible signal propagating in several high-order fiber modes was generated. 相似文献
930.