全文获取类型
收费全文 | 79526篇 |
免费 | 6690篇 |
国内免费 | 4981篇 |
专业分类
化学 | 23461篇 |
晶体学 | 1824篇 |
力学 | 3613篇 |
综合类 | 368篇 |
数学 | 18087篇 |
物理学 | 43844篇 |
出版年
2022年 | 189篇 |
2021年 | 330篇 |
2020年 | 651篇 |
2019年 | 950篇 |
2018年 | 902篇 |
2017年 | 639篇 |
2016年 | 491篇 |
2015年 | 440篇 |
2014年 | 1131篇 |
2013年 | 1649篇 |
2012年 | 1175篇 |
2011年 | 1738篇 |
2010年 | 2328篇 |
2009年 | 6858篇 |
2008年 | 7925篇 |
2007年 | 6391篇 |
2006年 | 5824篇 |
2005年 | 3986篇 |
2004年 | 3780篇 |
2003年 | 3991篇 |
2002年 | 4722篇 |
2001年 | 3692篇 |
2000年 | 3496篇 |
1999年 | 3318篇 |
1998年 | 2744篇 |
1997年 | 1908篇 |
1996年 | 1727篇 |
1995年 | 2204篇 |
1994年 | 2135篇 |
1993年 | 1598篇 |
1992年 | 1105篇 |
1991年 | 835篇 |
1990年 | 680篇 |
1989年 | 601篇 |
1988年 | 569篇 |
1987年 | 407篇 |
1986年 | 192篇 |
1985年 | 943篇 |
1984年 | 624篇 |
1983年 | 488篇 |
1982年 | 640篇 |
1981年 | 795篇 |
1980年 | 719篇 |
1979年 | 558篇 |
1978年 | 581篇 |
1977年 | 538篇 |
1976年 | 540篇 |
1975年 | 317篇 |
1974年 | 354篇 |
1973年 | 462篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
本文设计了一种梯形的周期极化掺镁铌酸锂(PPMgLN)波导,并通过在传播方向上引入温度梯度来拓宽其倍频(SHG)过程的泵浦光源可接收带宽。通过有限差分的光束传输法,计算波导的有效折射率,并进行波导尺寸的设计。结果表明,通过改变梯形波导不同位置的温度,使其形成一个温度梯度,可拓宽泵浦光源的波长可接收带宽。本文所设计的PPMgLN波导最大泵浦光源可接收带宽为C波段,即1 530~1 565 nm,该波导可倍频C波段,得到输出波段带宽为765~782.5 nm,温度调谐范围为30~150 ℃。 相似文献
2.
A general framework for describing photofission observables of actinides at an average excitation energy below 30 MeV 下载免费PDF全文
Guan-Lin Wang Hao-Yang Lan Xiao-Ming Shi Zhi-Chao Zhu Wen Luo 《中国物理C(英文版)》2022,46(8):084102-084102-14
A reasonable prediction of photofission observables plays a paramount role in understanding the photofission process and guiding various photofission-induced applications, such as short-lived isotope production, nuclear waste disposal, and nuclear safeguards. However, the available experimental data for photofission observables are limited, and the existing models and programs have mainly been developed for neutron-induced fission processes. In this study, a general framework is proposed for characterizing the photofission observables of actinides, including the mass yield distributions (MYD) and isobaric charge distributions (ICD) of fission fragments and the multiplicity and energy distributions of prompt neutrons (np) and prompt γ rays (γp). The framework encompasses various systematic neutron models and empirical models considering the Bohr hypothesis and does not rely on the experimental data as input. These models are then validated individually against experimental data at an average excitation energy below 30 MeV, which shows the reliability and robustness of the general framework. Finally, we employ this framework to predict the characteristics of photofission fragments and the emissions of prompt particles for typical actinides including 232Th, 235, 238U and 240Pu. It is found that the 238U(γ, f) reaction is more suitable for producing neutron-rich nuclei compared to the 232Th(γ, f) reaction. In addition, the average multiplicity number of both np and γp increases with the average excitation energy. 相似文献
3.
Based on the primal mixed variational formulation, a stabilized nonconforming mixed finite element method is proposed for the linear elasticity on rectangular and cubic meshes. Two kinds of penalty terms are introduced in the stabilized mixed formulation, which are the jump penalty term for the displacement and the divergence penalty term for the stress. We use the classical nonconforming rectangular and cubic elements for the displacement and the discontinuous piecewise polynomial space for the stress, where the discrete space for stress are carefully chosen to guarantee the well-posedness of discrete formulation. The stabilized mixed method is locking-free. The optimal convergence order is derived in the $L^2$-norm for stress and in the broken $H^1$-norm and $L^2$-norm for displacement. A numerical test is carried out to verify the optimal convergence of the stabilized method. 相似文献
4.
计算机视觉的飞速发展, 使得采用视觉技术辅助无人船航行成为可能. 在无人船巡航过程中, 获取船体航向是航行控制的必备基础. 特征匹配是无人船相关视觉技术中的重要部分, 是目标识别和定位等功能的关键步骤. 获取无人船运动姿态的基本步骤是对图像前后帧进行有效的特征提取和匹配. 针对水域环境中的图像静态特征提取速度慢、精度低的问题, 本文提出一种图像匹配方法以求取无人船的航行姿态角. 首先对图像预处理, 并对有效区域进行特征提取. 其次, 设计一种基于描述子相似度的初始特征匹配策略. 再其次, 筛选特征匹配对, 优化模型参数. 最后, 通过前后帧旋转矩阵计算航行姿态角. 实验表明, 该方法能有效提取无人船的航行姿态角. 相似文献
5.
6.
7.
In this paper, estimates for a trilinear operator associated with the Hartree type nonlinearity are proved. Moreover, as application of these estimates, it is proved that after a linear transformation, the Cauchy problem for the Hartree-type equation becomes locally well posed in the Bessel potential and homogeneous Besov spaces under certain regularity assumptions on the initial data. This notion of well-posedness and the functional framework to solve the equation were firstly proposed by Y. Zhou. 相似文献
8.
9.
10.