全文获取类型
收费全文 | 20859篇 |
免费 | 4486篇 |
国内免费 | 1972篇 |
专业分类
化学 | 10938篇 |
晶体学 | 136篇 |
力学 | 719篇 |
综合类 | 98篇 |
数学 | 1254篇 |
物理学 | 7686篇 |
无线电 | 6486篇 |
出版年
2024年 | 70篇 |
2023年 | 365篇 |
2022年 | 465篇 |
2021年 | 658篇 |
2020年 | 817篇 |
2019年 | 725篇 |
2018年 | 624篇 |
2017年 | 748篇 |
2016年 | 1079篇 |
2015年 | 1163篇 |
2014年 | 1457篇 |
2013年 | 1818篇 |
2012年 | 1591篇 |
2011年 | 1614篇 |
2010年 | 1301篇 |
2009年 | 1367篇 |
2008年 | 1505篇 |
2007年 | 1503篇 |
2006年 | 1351篇 |
2005年 | 1215篇 |
2004年 | 954篇 |
2003年 | 817篇 |
2002年 | 590篇 |
2001年 | 518篇 |
2000年 | 490篇 |
1999年 | 378篇 |
1998年 | 332篇 |
1997年 | 295篇 |
1996年 | 221篇 |
1995年 | 206篇 |
1994年 | 161篇 |
1993年 | 148篇 |
1992年 | 116篇 |
1991年 | 101篇 |
1990年 | 80篇 |
1989年 | 69篇 |
1988年 | 71篇 |
1987年 | 44篇 |
1986年 | 37篇 |
1985年 | 46篇 |
1984年 | 38篇 |
1983年 | 14篇 |
1982年 | 30篇 |
1981年 | 28篇 |
1980年 | 16篇 |
1979年 | 15篇 |
1978年 | 11篇 |
1977年 | 15篇 |
1976年 | 7篇 |
1974年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 906 毫秒
141.
R. S. Kaler 《Fiber and Integrated Optics》2006,25(1):41-57
In this article, the comparison of large signal theory and small signal theory has been done with dispersive propagation of optical signal with IMDD (Intensity Modulation Direct Detection) systems for semiconductor lasers with higher-order dispersion terms. The expressions for an exact large signal theory and small signal theory including higher-order dispersion terms for propagation of an optical wave with sinusoidal amplitude and frequency modulation in a dispersive fiber have been derived. It is observed that small signal theory is more sensitive compared to large signal theory in terms of intensity modulation/direct detection systems. Also, it is reported that for large signal analysis the higher-order effects of dispersion can be ignored, whereas for small signal theory, the higher-order effects can be ignored for lower modulation frequencies only. The variation in the transfer function for various values of modulation indices are greater for small signal analysis than for large signal analysis. Also, as the intensity modulation index is increased, there is a decrease in the value of transfer function. The large signal model approximates the small signal model for lower values of the intensity modulation index. 相似文献
142.
143.
144.
Despite its capability of high spatial resolution, simulation of turbulent flows with traditional Lagrangian (front tracking) scheme is often discouraged by numerical instability caused by clustering of marker nodes and topological changes of fronts. Contour advection surgery (CAS), being a robust front tracking scheme, can limit the growth of front complexity during simulation without jeopardizing accuracy or efficiency. This allows it to open up an advantage over traditional front-tracking schemes. It has already been demonstrated that CAS, with incorporation of the reaction sheet model, can accurately simulate the propagation and advection of a turbulent premixed V-shaped flame. In this study, it is further tested with 10 values of vortex circulation. A range of upstream turbulence levels of 1.8–19.8% was obtained. Results indicate that upstream turbulence increase the average flame length, flame zone area and the overall burning rate. Flame surface density Σ was also estimated. Maximum values of Σ obtained lie in the range 0.1–1.4 mm−1. Skewness towards the burnt region was observed in all profiles of Σ. Similar to results from laboratory experiments, it was found that Σ values decreases with upstream turbulence. From this study, the ability of CAS to cope with intense turbulence is demonstrated and a better quantitative understanding on the scheme has also been acquired. 相似文献
145.
J. Fang 《Optics & Laser Technology》1991,23(6):349-352
A rotational mismatch of gratings is used to generate a fringe carrier of an in-plane moiré pattern so that specimen deformation shows itself in the form of modulation of the carrier frequency. As both the unmodulated and modulated carriers are the patterns without fringe-loop or fringe-connection, they are automatically numbered with monotonical increasing orders by a digital image processing system without any ambiguity. The orders at every pixel of the image are determined by interpolation of the orders of the tracked fringes to establish two grey image files, from the difference of which, the orders of the in-plane displacement moiré are displayed with grey-level variation. 相似文献
146.
We have generalized the coherent potential approximation (CPA) of Tripathi and Behera to the case of ann-component alloy. It is seen that then-component CPA density of states reproduces the binary, ternary quartenary alloys etc when the appropriate limits are adopted. 相似文献
147.
用密度泛函B3LYP/6-311++G(d,p)方法和相对论有效实势(Lanl2dz基组)对VOn±(n=0,1,2)分子离子的势能函数及光谱常数进行了分析. 结果表明它们都能稳定存在, 其基态电子状态分别是:4Σ(VO2-), 3Σ(VO-), 4Σ(VO), 3Σ(VO+)和2Σ(VO2+). 其中VO2-和VO2+的势能函数曲线呈“火山口”型, 属于亚稳态分子离子. 用七参数Murell-Sorbie势拟合VO2-和VO2+分子亚稳态双原子分子离子势能函数, 发现其拟合曲线与势能函数曲线符合得很好. 同时,讨论了电荷对势能函数和能级的影响.
关键词:
分子离子
密度泛函理论
势能函数
能级 相似文献
148.
M. Kleiber M. Bode R. Ravli N. Tezuka R. Wiesendanger 《Journal of magnetism and magnetic materials》2002,240(1-3):64-69
The magnetic properties of the Cr(0 0 1) surface have been studied by spin-polarized scanning tunneling spectroscopy (SP-STS). Spatially resolved mapping of the spectroscopic dI/dU signal at an energy close to the spin-polarized Cr(0 0 1) surface state allows the confirmation of the topological antiferromagnetic order of the Cr(0 0 1) surface. It is shown that the presence of screw dislocations leads to the formation of domain walls which exhibit a width of 120–170 nm. A long-period modulation of the SP-STS signal was not observed indicating that the bulk spin-density wave is modified at the surface due to symmetry breaking. 相似文献
149.
对形如Nt×Nl型准二维无序系统,只考虑格点之间的最近邻跳跃积分,采用特殊的格点编号方案,在单电子近似下,系统的哈密顿量可表示为简明对称矩阵,借助豪斯荷尔德变换将其约化为对称三对角矩阵,再利用负本征值理论及传输矩阵等方法,对系统态密度、局域长度及电导等电子结构特性进行数值计算. 重点研究了准一维四平行链和五平行链无序系统, 将结果与一维单链、准一维双链及三链系统进行对比,发现随维度的增加,系统的能带有所展宽,能态密度分布发生很大的变化,其峰值数量呈偶数规律增加. 并且在能带中心处存在有局域长度大于系统大小的扩展态,处于这些态下的系统具有较大电导. 从单链到多链,相当于扩大了系统的关联范围,使系统出现了类似非对角长程关联的行为.
关键词:
准二维无序系统
态密度
局域长度
电导 相似文献
150.
本文介绍的调制脉冲声测量系统是TDS和数字测量技术的进一步发展,其特点是简化了TEF系统的实现条件,解决了全频带的测量动态问题。本系统曾在上海音像公司录音棚中进行过测量,取得了一定的效果。 相似文献