全文获取类型
收费全文 | 9456篇 |
免费 | 1916篇 |
国内免费 | 697篇 |
专业分类
化学 | 1248篇 |
晶体学 | 33篇 |
力学 | 599篇 |
综合类 | 54篇 |
数学 | 1133篇 |
物理学 | 2740篇 |
无线电 | 6262篇 |
出版年
2024年 | 34篇 |
2023年 | 147篇 |
2022年 | 213篇 |
2021年 | 234篇 |
2020年 | 283篇 |
2019年 | 243篇 |
2018年 | 294篇 |
2017年 | 330篇 |
2016年 | 355篇 |
2015年 | 434篇 |
2014年 | 649篇 |
2013年 | 625篇 |
2012年 | 686篇 |
2011年 | 743篇 |
2010年 | 592篇 |
2009年 | 579篇 |
2008年 | 644篇 |
2007年 | 682篇 |
2006年 | 605篇 |
2005年 | 576篇 |
2004年 | 435篇 |
2003年 | 438篇 |
2002年 | 319篇 |
2001年 | 306篇 |
2000年 | 268篇 |
1999年 | 246篇 |
1998年 | 205篇 |
1997年 | 174篇 |
1996年 | 140篇 |
1995年 | 123篇 |
1994年 | 107篇 |
1993年 | 85篇 |
1992年 | 67篇 |
1991年 | 59篇 |
1990年 | 30篇 |
1989年 | 24篇 |
1988年 | 15篇 |
1987年 | 19篇 |
1986年 | 11篇 |
1985年 | 12篇 |
1984年 | 10篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 9篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1957年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
随着兰州重离子加速器冷却储存环(HIRFL-CSR)外靶实验终端中子墙的建立, 为实验测量高能中子提供了机遇. 为确定CSR外靶实验终端对中子流测量的可行性, 基于BUU理论模型分别对对称系统(Ni+Ni, Pb+Pb)和非对称系统(Pb+Ni)进行了模拟计算, 发现当系统能量达到几百MeV/u时, 中子流信号相当明显, 并与碰撞参数有明显的依赖关系. 模拟结果表明, 在前角20°的覆盖范围内, 可以较好地实现中子流测量所需要的反应平面确定及碰撞参数选择. 对双击事件及其对中子流的影响进行了简单的讨论. 相似文献
42.
自由空间二维榕树网实现方法 总被引:1,自引:1,他引:0
鉴于榕树网在自由空间光子交换网络中具有重要的应用价值,分析了榕树网的特点和4×4二维榕树网的空间拓扑结构,通过偏振光分光棱镜、微闪耀光栅阵列、平面反射镜、半反半透镜和液晶空间光调制器的集成,构建二维的榕树交换网实验模块,利用微闪耀光栅的衍射特性,控制每块微闪耀光栅的周期,以实现入射光信号不同方向的闪耀输出,最终完成二维榕树网自由空间水平和竖直方向上的交叉互连,直通则由平面镜反射实现。对二维榕树网实验模块的功能分析表明,该实验模块理论上可以完成4×4二维面阵内光信号(或数据)的排序、交换、组播、广播、矩阵变换等操作,具有交换透明、速度快、空间带宽高等特点,在全光交换和光通信中具有一定的应用。 相似文献
43.
44.
The photonic quantum ring (PQR) laser is a three dimensional whispering gallery (WG) mode laser and has anomalous quantum wire properties, such as microampere to nanoampere range threshold currents and √T‐dependent thermal red shifts. We observed uniform bottom emissions from a 1‐kb smart pixel chip of a 32×32 InGaAs PQR laser array flip‐chip bonded to a 0.35 µm CMOS‐based PQR laser driver. The PQR‐CMOS smart pixel array, now operating at 30 MHz, will be improved to the GHz frequency range through device and circuit optimization. 相似文献
45.
R.A. Gunasekaran M. Agarwal A. Singh P. Dubasi P. Coane K. Varahramyan 《Optics and Lasers in Engineering》2005,43(6):1086
A novel small fluid controlled optical lens system that is capable of displaying dynamic variation of its focal length and field-of-view (FOV) is designed and fabricated. In this active lens system, appropriate volume of the optical fluid can be pumped into or out of the lens chamber to provide double-convex (DCX) or double-concave (DCV) lens effect. Simple optical imaging experiments were performed using different sets of glass lenses with fixed focal lengths to determine the optimum lens configuration required for designing a dynamic optical lens system. The experimental results obtained from the glass lenses demonstrate that a combination of a single DCX lens with three DCV lenses provides a wider FOV. The flexible membranes for fluid controlled lenses were fabricated using polydimethylsiloxane polymer material, which has good optical transparency and elasticity. A simple fluid injection system is used to vary the radius of curvature of the lenses, and thereby to change the focal length. A dynamic optical lens system with a combination of one DCX and multiple variable focal length DCV lenses as designed here can image an object with a wide range of focal length and FOV. With this fluid controlled optical system, the FOV and focal length could be continuously varied and a maximum FOV of 118.3° could be achieved. The smallest f-number (f/#) for this fluid controlled single lens system was found to be 1.3, which corresponds to the numerical aperture value of 0.35. 相似文献
46.
47.
利用自成像效应进行半导体列阵激光器锁相是一种重要的外腔锁相技术,所构成的共振腔可分为Talbot腔和带相位补偿板的Lau腔。我们用Fox-Li方法计算了共振模式。本文给出了Talbot腔的计算结果和分析,表明同相和反相模的损耗随腔长变化存在临界点。 相似文献
48.
The charge storage characteristics of P-channel Ge/Si hetero-nanocrystal based MOSFET memory has been investigated and a logical array has been constructed using this memory cell. In the case of the thickness of tunneling oxide T_ox=2nm and the dimensions of Si- and Ge-nanocrystal D_Si=D_Ge=5nm, the retention time of this device can reach ten years(~1×10~8s) while the programming and erasing time achieve the orders of microsecond and millisecond at the control gate voltage |V_g|=3V with respect to N-wells, respectively. Therefore, this novel device, as an excellent nonvolatile memory operating at room temperature, is desired to obtain application in future VLSI. 相似文献
49.
Victor Pambuccian 《Mathematical Logic Quarterly》1992,38(1):345-348
We proved in the first part [1] that plane geometry over Pythagorean fields is axiomatizable by quantifier-free axioms in a language with three individual constants, one binary and three ternary operation symbols. In this paper we prove that two of these operation symbols are superfluous. 相似文献
50.
Marlies Gerber 《Geometriae Dedicata》2003,98(1):123-160
We establish some criteria for the existence or nonexistence of focal points near closed geodesics on surfaces. These criteria are in terms of the curvature of the manifold along the closed geodesic and the average values of the partial derivatives of the curvature in the direction perpendicular to the geodesic. Our criteria lead to a new family of examples of surfaces with no focal points. We also show that if S is a compact surface with no focal points and an inequality relating the curvature of the surface to the curvature of the horocycles holds, then the horocycles (considered as curves in S) are uniformly C
2+Lipschitz. 相似文献