全文获取类型
收费全文 | 339篇 |
免费 | 73篇 |
国内免费 | 39篇 |
专业分类
化学 | 78篇 |
力学 | 22篇 |
综合类 | 6篇 |
数学 | 43篇 |
物理学 | 101篇 |
无线电 | 201篇 |
出版年
2023年 | 4篇 |
2022年 | 4篇 |
2021年 | 4篇 |
2020年 | 8篇 |
2019年 | 5篇 |
2018年 | 4篇 |
2017年 | 6篇 |
2016年 | 6篇 |
2014年 | 9篇 |
2013年 | 16篇 |
2012年 | 8篇 |
2011年 | 7篇 |
2010年 | 22篇 |
2009年 | 13篇 |
2008年 | 12篇 |
2007年 | 17篇 |
2006年 | 22篇 |
2005年 | 16篇 |
2004年 | 21篇 |
2003年 | 12篇 |
2002年 | 3篇 |
2001年 | 11篇 |
2000年 | 3篇 |
1999年 | 15篇 |
1998年 | 16篇 |
1997年 | 20篇 |
1996年 | 11篇 |
1995年 | 15篇 |
1994年 | 13篇 |
1993年 | 11篇 |
1992年 | 15篇 |
1991年 | 13篇 |
1990年 | 14篇 |
1989年 | 9篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1983年 | 2篇 |
1982年 | 7篇 |
1981年 | 8篇 |
1980年 | 8篇 |
1979年 | 2篇 |
1966年 | 3篇 |
1965年 | 4篇 |
1951年 | 1篇 |
1950年 | 1篇 |
1935年 | 3篇 |
1933年 | 2篇 |
排序方式: 共有451条查询结果,搜索用时 562 毫秒
71.
本文介绍了几何光学的最新发展,系统论述了经典光线力学和量子光线力学的基本概念,讨论了光线力学对传统几何光学和波动光学的影响及其发展前景. 相似文献
72.
73.
74.
A thorough investigation by means of X-rays has been carried out with the purpose to determine the nature of the ternary phase τ in Al-Cu-Ni alloys. In contrast with the conventional concept of alloy phase which is characterized by a definite type of crystal structure, systematic structure changes are found in the single phase field of τ which occupies quite an extensive area in the isothermal section of the phase diagram at room temperature. There are eight types of structures altogether, all derived from a basic rhombohedron with corners occupied by Al atoms and centres either occupied by the heavy atoms or remaining vacant. The basic rhombohedron is the building stone in the crystal architecture. By transforming the basic rhombohedron into a hexagonal prism in the usual way, all structures may be considered to be built up by stacking together a number of these hexagonal prisms along the triad. The transformation of one structure into another is quite systematic in the way that the number of the stacking stories in the unit cell increases according to the order 10, 11, 12, 13, 14, 15, 16, 17. The atomic arrangements in the different structures are closely related too, in the respect that they are all superstructures due to the presence of ordered vacancies in the rhombohedral centres.The principal factor controlling the formation of these structures has been fully considered. In view of the fact that the change of structure types follows closely with the content of Ni or Cu for alloys of constant Al content, the atomic size factor appears to be unimportant in the formation of these alloys. It has been shown that for alloy phases of the defect lattice type as the r-phase, the most fundamental factor is the average number of valency electrons per structural unit which is the basic rhombohedron in the present case. By assuming Hume-Rothery's valencies, the average number of valency electrons remains remarkably constant throughout the entire phase field, while the electron concentration varies with compositions. It has also been pointed out that for alloy phases where there is no unit cell change, the average number of electrons per structural unit is equivalent to the number of electrons per unit cell, and for alloy phase where there is no defect, this is in effect equivalent to the electron concentration. 相似文献
75.
76.
77.
本文在时间扩展ALOHA多址协议的基础上,描述了卫星无线电测定业务(RDSS)中正在研究的一种新的多址协议-扩频ALOHA多址协议,给出了该协议信道的延迟捕获特性,获得了信道的吞吐容量对延时捕获时间的关系曲线。 相似文献
78.
采用高温固相法制备了红色荧光粉Ca4LaNb(W1-x Mo x)4O20∶Eu3+并研究了样品的发光性质。Ca4LaNbW4O20∶Eu3+的激发光谱中包含一个宽的激发带,峰值位于275 nm,归属于WO2-4基团的电荷迁移跃迁。随着Mo6+离子的掺入,Ca4LaNbW4O20∶Eu3+位于275 nm处的吸收带变宽,其原因是O2--Eu3+的电荷迁移跃迁增强。在Ca4LaNb(W1-x Mo x)4O20∶Eu3+的发射光谱中,400~500 nm间较宽的发射带属于WO2-4基团的发射带,而位于591 nm和616 nm的尖锐的发射峰分别属于Eu3+的5D0→7F1磁偶极跃迁和5D0→7F2电偶极跃迁发射。随着Mo6+离子浓度的增加,WO2-4基团的发射带强度下降,从而提高了色纯度。 相似文献
80.
Considering the one-gluon exchange interaction and phenomenological quark confinement potential, an improved light-cone effective Hamiltonian for mesons and the corresponding radial mass eigen equations in angular momentum representation is obtained. Solving the J = 0 eigen equations numerically and using a set of adjustable parameters, the obtained solutions for ground states and radial excited states can simultaneously describe both pseudoscalar and scalar flavour-off-diagonal mesons. Some radial excited states are also predicted and wait for experimental test. More results for the vector and axial vector mesons are expected. 相似文献