全文获取类型
收费全文 | 109075篇 |
免费 | 16809篇 |
国内免费 | 12455篇 |
专业分类
化学 | 59186篇 |
晶体学 | 948篇 |
力学 | 5193篇 |
综合类 | 539篇 |
数学 | 9494篇 |
物理学 | 31554篇 |
无线电 | 31425篇 |
出版年
2024年 | 285篇 |
2023年 | 2588篇 |
2022年 | 3027篇 |
2021年 | 4116篇 |
2020年 | 4270篇 |
2019年 | 3959篇 |
2018年 | 3428篇 |
2017年 | 3376篇 |
2016年 | 4661篇 |
2015年 | 5003篇 |
2014年 | 5969篇 |
2013年 | 7571篇 |
2012年 | 9403篇 |
2011年 | 9508篇 |
2010年 | 7035篇 |
2009年 | 6900篇 |
2008年 | 7314篇 |
2007年 | 6587篇 |
2006年 | 6074篇 |
2005年 | 5125篇 |
2004年 | 3864篇 |
2003年 | 3257篇 |
2002年 | 3002篇 |
2001年 | 2536篇 |
2000年 | 2362篇 |
1999年 | 2416篇 |
1998年 | 2045篇 |
1997年 | 1829篇 |
1996年 | 1839篇 |
1995年 | 1534篇 |
1994年 | 1330篇 |
1993年 | 1064篇 |
1992年 | 949篇 |
1991年 | 818篇 |
1990年 | 650篇 |
1989年 | 469篇 |
1988年 | 400篇 |
1987年 | 324篇 |
1986年 | 286篇 |
1985年 | 251篇 |
1984年 | 181篇 |
1983年 | 133篇 |
1982年 | 102篇 |
1981年 | 78篇 |
1980年 | 80篇 |
1979年 | 45篇 |
1978年 | 40篇 |
1977年 | 48篇 |
1976年 | 44篇 |
1973年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 531 毫秒
871.
Retractable structures which alter their geometries according to practical requirements are widely used in roof structures of stadiums for their versatility. A retractable structure based on threefold-symmetric Bricard linkages and rotating rings of tetrahedra is proposed and developed in this paper. By replacing each link of a threefold-symmetric Bricard linkage with a tetrahedron, a retractable structure is obtained which can repeatedly open and close by rotating the tetrahedra. The geometric relationship between the retractable structure and the threefold-symmetric Bricard linkage is derived and ranges of main geometric parameters are determined to ensure a continuous and smooth movement in the deployment of the structure. Then the relative displacement of the support and the open rate of the structure, two main parameters concerned in practical applications, are investigated. Preferred ranges of main geometric parameters are proposed and a physical model is manufactured to verify them. A discussion on some possible improvements and modifications of the structure is also presented. 相似文献
872.
873.
Tie-Jun Liu Yue-Sheng Wang Chuanzeng Zhang 《Archive of Applied Mechanics (Ingenieur Archiv)》2008,78(4):267-282
The main interest of this study is a new method to solve the axisymmetric frictionless contact problem of functionally graded
materials (FGMs). Based on the fact that an arbitrary curve can be approached by a series of continuous but piecewise linear
curves, the FGM is divided into a series of sub-layers with shear modulus varying linearly in each sub-layer and continuous
at the sub-interfaces. With this model, the axisymmetric frictionless contact problem of a functionally graded coated half-space
is investigated. By using the transfer matrix method and Hankel integral transform technique, the problem is reduced to a
Cauchy singular integral equation. The contact pressure, contact region and indentation are calculated for various indenters
by solving the equations numerically.
An erratum to this article can be found at 相似文献
874.
Zhaohui Liu Dickson K. McCanny J.V. 《IEEE transactions on circuits and systems. I, Regular papers》2005,52(4):755-765
A novel application-specific instruction set processor (ASIP) for use in the construction of modern signal processing systems is presented. This is a flexible device that can be used in the construction of array processor systems for the real-time implementation of functions such as singular-value decomposition (SVD) and QR decomposition (QRD), as well as other important matrix computations. It uses a coordinate rotation digital computer (CORDIC) module to perform arithmetic operations and several approaches are adopted to achieve high performance including pipelining of the micro-rotations, the use of parallel instructions and a dual-bus architecture. In addition, a novel method for scale factor correction is presented which only needs to be applied once at the end of the computation. This also reduces computation time and enhances performance. Methods are described which allow this processor to be used in reduced dimension (i.e., folded) array processor structures that allow tradeoffs between hardware and performance. The net result is a flexible matrix computational processing element (PE) whose functionality can be changed under program control for use in a wider range of scenarios than previous work. Details are presented of the results of a design study, which considers the application of this decomposition PE architecture in a combined SVD/QRD system and demonstrates that a combination of high performance and efficient silicon implementation are achievable. 相似文献
875.
一种PN码自适应捕获门限的改进算法 总被引:1,自引:0,他引:1
文献[1]提出了一种用于直扩系统的PN码自适应门限算法。但该文献也指出,此算法对门限总数十分敏感。当门限总数设置不当时,系统的平均捕获时间将显著增加。这限制了该算法在实际中的应用。本文就此提出了改进,给出了算法及电路框图。仿真结果表明,与原方案相比,改进算法改善了对门限总数的敏感性,降低了PN码平均捕获时间。 相似文献
876.
877.
稀土及杂质元素对ZA27合金晶间腐蚀的影响 总被引:10,自引:0,他引:10
为了从本质上了解杂质与稀土元素对锌铝合金晶间腐蚀的影响 ,探索抑制合金晶间腐蚀的有效途径 ,依据晶界的大角度重位点阵理论编制出相应计算机软件 ,建立含稀土、杂质及 η相颗粒的α相大角度晶界原子集团模型 ,采用递归法计算了α相晶界间的电荷转移 ,由此讨论了杂质 (Pb、Sn、Cd)及稀土元素 (La、Y)对Zn、Al电极电位的影响 .结果表明 ,杂质Pb、Sn、Cd增大原子间的电荷转移量 ,提高Zn、Al电极电位差 ,加速合金的腐蚀 ,稀土元素减小原子间的电荷转移量 ,降低Zn、Al电极电位差 ,具有抑制晶间腐蚀的作用 . 相似文献
878.
879.
利用2~8 MeV的Naq+、Clq+(q=2,3,4,5)轰击氦原子,对碰撞的直接多重电离过程进行研究.实验采用反冲离子-散射离子飞行时间符合技术,通过反冲离子飞行时间谱区分不同价态反冲离子;利用静电偏转和位置灵敏探测技术区分不同电荷态散射离子;结合CAMAC-PC多参数获取系统得到一定价态散射离子所对应的反冲离子电荷态分布谱;经分析该谱得到直接多重电离截面与直接单电离截面之比R21.讨论了R21随入射离子速度和电荷态的变化关系. 相似文献
880.