全文获取类型
收费全文 | 9677篇 |
免费 | 1202篇 |
国内免费 | 783篇 |
专业分类
化学 | 5265篇 |
晶体学 | 37篇 |
力学 | 367篇 |
综合类 | 16篇 |
数学 | 808篇 |
物理学 | 2348篇 |
无线电 | 2821篇 |
出版年
2024年 | 14篇 |
2023年 | 223篇 |
2022年 | 195篇 |
2021年 | 371篇 |
2020年 | 354篇 |
2019年 | 310篇 |
2018年 | 270篇 |
2017年 | 231篇 |
2016年 | 358篇 |
2015年 | 365篇 |
2014年 | 479篇 |
2013年 | 608篇 |
2012年 | 783篇 |
2011年 | 778篇 |
2010年 | 550篇 |
2009年 | 499篇 |
2008年 | 610篇 |
2007年 | 544篇 |
2006年 | 535篇 |
2005年 | 448篇 |
2004年 | 304篇 |
2003年 | 306篇 |
2002年 | 226篇 |
2001年 | 229篇 |
2000年 | 225篇 |
1999年 | 236篇 |
1998年 | 173篇 |
1997年 | 150篇 |
1996年 | 185篇 |
1995年 | 173篇 |
1994年 | 116篇 |
1993年 | 99篇 |
1992年 | 93篇 |
1991年 | 77篇 |
1990年 | 100篇 |
1989年 | 72篇 |
1988年 | 49篇 |
1987年 | 42篇 |
1986年 | 30篇 |
1985年 | 25篇 |
1984年 | 17篇 |
1983年 | 24篇 |
1982年 | 21篇 |
1981年 | 14篇 |
1979年 | 28篇 |
1978年 | 25篇 |
1977年 | 16篇 |
1975年 | 16篇 |
1974年 | 10篇 |
1973年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
11.
Chong D. Y. R. Lim B. K. Rebibis K. J. Pan S. J. Sivalingam K. Kapoor R. Sun A. Y. S. Tan H. B. 《Advanced Packaging, IEEE Transactions on》2006,29(4):674-682
The recent advancement in high- performance semiconductor packages has been driven by the need for higher pin count and superior heat dissipation. A one-piece cavity lid flip chip ball grid array (BGA) package with high pin count and targeted reliability has emerged as a popular choice. The flip chip technology can accommodate an I/O count of more than five hundreds500, and the die junction temperature can be reduced to a minimum level by a metal heat spreader attachment. None the less, greater expectations on these high-performance packages arose such as better substrate real estate utilization for multiple chips, ease in handling for thinner core substrates, and improved board- level solder joint reliability. A new design of the flip chip BGA package has been looked into for meeting such requirements. By encapsulating the flip chip with molding compound leaving the die top exposed, a planar top surface can be formed. A, and a flat lid can then be mounted on the planar mold/die top surface. In this manner the direct interaction of the metal lid with the substrate can be removed. The new package is thus less rigid under thermal loading and solder joint reliability enhancement is expected. This paper discusses the process development of the new package and its advantages for improved solder joint fatigue life, and being a multichip package and thin core substrate options. Finite-element simulations have been employed for the study of its structural integrity, thermal, and electrical performances. Detailed package and board-level reliability test results will also be reported 相似文献
12.
Boon Tiong Tan Siou Teck Chew Mook Seng Leong Ban Leong Ooi 《Microwave Theory and Techniques》2003,51(8):1906-1910
A dual-mode ring bandpass filter with two pairs of capacitors has been designed. The capacitors are used to control the location of the even- and odd-mode frequencies independently, allowing weak coupling for narrow-band filter design with realizable capacitance values. Theoretical expressions have been derived for these frequencies. A 4% bandwidth bandpass filter centered at 1.9 GHz was designed and tested with good agreement between theoretical and measured results. 相似文献
13.
14.
Ying Tan 《The Journal of VLSI Signal Processing》2002,32(1-2):45-54
In this paper, we propose a new approach for signal detection in wireless digital communications based on the neural network with transient chaos and time-varying gain (NNTCTG), and give a concrete model of the signal detector after appropriate transformations and mappings. It is well known that the problem of the maximum likelihood signal detection can be described as a complex optimization problem that has so many local optima that conventional Hopfield-type neural networks fail to solve. By refraining from the serious local optima problem of Hopfield-type neural networks, the NNTCTG makes use of the time-varying parameters of the recurrent neural network to control the evolving behavior of the network so that the network undergoes the transition from chaotic behavior to gradient convergence. It has richer and more flexible dynamics rather than conventional neural networks only with point attractors, so that it can be expected to have much ability to search for globally optimal or near-optimal solutions. After going through a transiently inverse-bifurcation process, the NNTCTG can approach the global optimum or the neighborhood of global optimum of our problem. Simulation experiments have been performed to show the effectiveness and validation of the proposed neural network based method for the signal detection in digital communications. 相似文献
15.
本文对采用区内C/I平衡的多区蜂窝CDMA系统下行链路的性能进行分析,给出两种区内C/I平衡算法并比较它们的性能,考虑呈对数正态分布的阴影和R^-4规律的路径损失的综合影响,采用与传统不同的小区平均中断概率来评价系统的性能。 相似文献
16.
本文用密度泛函理论(DFT)的总能计算研究了一氧化碳和氢原子在Ni(111)表面上p(2×2)共吸附系统的原子结构和电子态,结果表明CO和H原子分别被吸附于两个对角p(1×1)元胞的hcp和fcc位置.以氢分子和CO分子作为能量参考点,总吸附能为2.81 eV,相应的共吸附表面功函数φ为6.28 eV.计算得到的C—O,C—Ni和H—Ni的键长分别是1.19?, 1.96?和 1.71?,并且CO分子以C原子处于hcp的谷位与金属衬底原子结合.衬底Ni(111)的最外两层的晶面间距在吸附后的相对变化分别是
关键词:
Fisher-Tropsch反应
催化作用
Ni(111) p(2×2)/(CO+H)
共吸附 相似文献
17.
18.
A design of diode-pumped high-efficiency Nd:YVO4/LBO red laser is reported. Using critical phase-matching (CPM) LBO, 671 nm red laser was obtained from 1342 nm light by intracavity frequency doubling. With an incident pump laser of 800 mW, using type-I and type-II CPM LBO, 97 and 52 mWTEM00 mode red laser outputs were obtained, with optical-to-optical conversion efficiencies of up to 12.1% and 6.5%, respectively. 相似文献
19.
We study the structure of invertible substitutions on three-letter alphabet. We show that there exists a finite set
of invertible substitutions such that any invertible substitution can be written as Iwσ1σ2σk, where Iw is the inner automorphism associated with w, and
for 1jk. As a consequence, M is the matrix of an invertible substitution if and only if it is a finite product of non-negative elementary matrices. 相似文献
20.