全文获取类型
收费全文 | 171篇 |
免费 | 103篇 |
国内免费 | 42篇 |
专业分类
化学 | 86篇 |
晶体学 | 8篇 |
力学 | 9篇 |
综合类 | 5篇 |
数学 | 10篇 |
物理学 | 198篇 |
出版年
2024年 | 1篇 |
2023年 | 9篇 |
2022年 | 10篇 |
2021年 | 6篇 |
2020年 | 8篇 |
2019年 | 9篇 |
2018年 | 12篇 |
2017年 | 10篇 |
2016年 | 9篇 |
2015年 | 16篇 |
2014年 | 27篇 |
2013年 | 16篇 |
2012年 | 11篇 |
2011年 | 27篇 |
2010年 | 8篇 |
2009年 | 26篇 |
2008年 | 19篇 |
2007年 | 17篇 |
2006年 | 15篇 |
2005年 | 10篇 |
2004年 | 8篇 |
2003年 | 3篇 |
2002年 | 5篇 |
2001年 | 8篇 |
2000年 | 7篇 |
1999年 | 1篇 |
1998年 | 1篇 |
1997年 | 1篇 |
1996年 | 1篇 |
1995年 | 5篇 |
1994年 | 1篇 |
1992年 | 1篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1985年 | 1篇 |
1982年 | 2篇 |
1955年 | 1篇 |
排序方式: 共有316条查询结果,搜索用时 140 毫秒
41.
42.
Numerical Calculation of SAW Propagation Properties at the x-Cut of Ferroelectric PMN-33%PT Single Crystals
下载免费PDF全文
![点击此处可从《中国物理快报》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Surface acoustic wave (SAW) properties at the x-cut of relaxor-based 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 (PMN- 33%PT) ferroelectric single crystals are analyzed theoretically when poled along the [001]c cubic direction. It can be found that PMN-33%PT single crystal is a kind of material with a low phase velocity and high electromechanical coupling coefficient, and the single crystal possesses some cuts with zero power flow angle. The results are based on the material parameters at room temperature. The conclusions provide device designers with a few ideal cuts of PMN-33%PT single crystals. Moreover, choosing an optimal cut will dramatically improve the performance of SAW devices, and corresponding results for crystal systems working at other temperatures could also be figured out by employing the method. 相似文献
43.
44.
45.
A method of solving the stiffness problem in Biot's poroelastic equations using a staggered high-order finite-difference
下载免费PDF全文
![点击此处可从《中国物理》网站下载免费的PDF全文](/ch/ext_images/free.gif)
In modelling elastic wave propagation in a porous medium, when the ratio between the fluid viscosity and the medium permeability is comparatively large, the stiffness problem of Blot's poroelastic equations will be encountered. In the paper, a partition method is developed to solve the stiffness problem with a staggered high-order finite-difference. The method splits the Biot equations into two systems. One is stiff, and solved analytically, the other is nonstiff, and solved numerically by using a high-order staggered-grid finite-difference scheme. The time step is determined by the staggered finite-difference algorithm in solving the nonstiff equations, thus a coarse time step may be employed. Therefore, the computation efficiency and computational stability are improved greatly. Also a perfect by matched layer technology is used in the split method as absorbing boundary conditions. The numerical results are compared with the analytical results and those obtained from the conventional staggered-grid finite-difference method in a homogeneous model, respectively. They are in good agreement with each other. Finally, a slightly more complex model is investigated and compared with related equivalent model to illustrate the good performance of the staggered-grid finite-difference scheme in the partition method. 相似文献
46.
基于叶绿素荧光光谱分析的稻瘟病害预测模型 总被引:3,自引:0,他引:3
为了实现稻瘟病的快速、准确和无损检测,力求构建稻瘟病害预测模型。根据水稻叶片相对病害面积将稻瘟病划分为3个等级,通过激光诱导法采集不同病害等级的活体水稻叶片叶绿素荧光光谱。选取502~830 nm波段激光诱导叶绿素荧光光谱(LICF)作为研究对象,利用Savitzky-Golay平滑法(SG)和一阶导数变换(FDT)对光谱信息进行预处理,通过主成分分析(PCA)方法获取经SG-FDT预处理后光谱的特征向量,根据累积贡献率和方差选取前3个主成分进行分析。将试验样本分为建模样本和检验样本,以稻瘟病害等级为预测指标,利用建模样本的133片叶片的光谱和病害信息分别结合判别分析(DA)、多类逻辑回归分析(MLRA)和多层感知器(MLP)建立稻瘟病的预测模型,利用检验样本的89片叶片的光谱和病害信息对所建模型进行预测检验,完成对PCA-DA、PCA-MLRA和PCA-MLP的对比寻优。结果表明,PCA-DA,PCA-MLRA和PCA-MLP模型均能完成对稻瘟病害的预测,但PCA-MLP模型的平均预测准确率能够达到91.7%,相比PCA-DA和PCA-MLRA模型,在稻瘟病害3个等级上均具有较好的分类和预测能力。 相似文献
47.
微地震监测技术是利用岩石破裂位置的时空分布对地下裂缝进行成像和分析,因此,震源定位是微地震资料处理中最基础和最重要的内容。该文首先介绍了微地震定位方法的研究现状,特别是近年来相对定位法在微地震定位应用中的研究进展。然后,详细阐述了两种波形叠加类相对定位方法的原理,并结合实际矿山微地震数据分析了两种方法的成像分辨率特征、计算效率和定位结果,验证了该文研究方法的可行性。最后,对波形叠加类定位方法及微地震监测技术的发展方向进行了讨论和展望。 相似文献
48.
49.
This paper focuses on the simulation analysis of stripe formation and dynamic features of intersecting pedestrian flows.The intersecting flows consist of two streams of pedestrians and each pedestrian stream has a desired walking direction.The model adopted in the simulations is the social force model, which can reproduce the self-organization phenomena successfully. Three scenarios of different cross angles are established. The simulations confirm the empirical observations that there is a stripe formation when two streams of pedestrians intersect and the direction of the stripes is perpendicular to the sum of the directional vectors of the two streams. It can be concluded from the numerical simulation results that smaller cross angle results in higher mean speed and lower level of speed fluctuation. Moreover, the detailed pictures of pedestrians' moving behavior at intersections are given as well. 相似文献
50.
为了实现更为简单的三维测量同时解决传统立体视觉方法中基线的限制问题,提出一种单摄像机微小角度旋转结合神经网络进行三维测量的方法.方法通过单摄像机小角度旋转采集二维图像数据;利用电动平台移动标定板构建三维空间坐标系;基于摄像机的线性成像模型思想,将代表图像坐标和三维坐标映射关系的投影矩阵替换为BP神经网络,得到二维坐标到... 相似文献