全文获取类型
收费全文 | 11291篇 |
免费 | 1873篇 |
国内免费 | 1169篇 |
专业分类
化学 | 5863篇 |
晶体学 | 86篇 |
力学 | 722篇 |
综合类 | 66篇 |
数学 | 1128篇 |
物理学 | 3424篇 |
无线电 | 3044篇 |
出版年
2024年 | 30篇 |
2023年 | 263篇 |
2022年 | 295篇 |
2021年 | 457篇 |
2020年 | 436篇 |
2019年 | 392篇 |
2018年 | 331篇 |
2017年 | 341篇 |
2016年 | 488篇 |
2015年 | 519篇 |
2014年 | 569篇 |
2013年 | 807篇 |
2012年 | 897篇 |
2011年 | 962篇 |
2010年 | 718篇 |
2009年 | 687篇 |
2008年 | 779篇 |
2007年 | 613篇 |
2006年 | 677篇 |
2005年 | 510篇 |
2004年 | 393篇 |
2003年 | 337篇 |
2002年 | 318篇 |
2001年 | 266篇 |
2000年 | 282篇 |
1999年 | 247篇 |
1998年 | 236篇 |
1997年 | 219篇 |
1996年 | 228篇 |
1995年 | 177篇 |
1994年 | 150篇 |
1993年 | 136篇 |
1992年 | 116篇 |
1991年 | 88篇 |
1990年 | 85篇 |
1989年 | 62篇 |
1988年 | 48篇 |
1987年 | 37篇 |
1986年 | 40篇 |
1985年 | 26篇 |
1984年 | 20篇 |
1983年 | 18篇 |
1982年 | 17篇 |
1981年 | 3篇 |
1980年 | 7篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1975年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Guoli Ding 《Graphs and Combinatorics》1994,10(2-4):323-328
The P4-structure of a graphG =(V,E) is a hypergraphH = (V,F) such that, for every hyperedgeA inF, the cardinality ofA is four and the subgraph of G induced byA is a path. It is proved in this paper that the P4-structure of a tree can be recognized in polynomial time. 相似文献
92.
Low-threshold, high-efficiency, high-repetition-rate optical parametric generator based on periodically poled LiNbO3
下载免费PDF全文
![点击此处可从《中国物理》网站下载免费的PDF全文](/ch/ext_images/free.gif)
We report a high-repetition-rate optical parametric generator (OPG) with a periodically poled lithium niobate (PPLN) crystal pumped by an acousto-optically Q-switched CW-diode-end-pumped Nd:YVO_4 laser. For the maximum 1064nm pump power of 970mW, the maximum conversion efficiency is 32.9% under the conditions of 250℃, 1064nm pulse repetition rate of 22.6kHz and pulse width of 12ns, and the PPLN OPG threshold in the collinear case is less than 23.7μJ. The output power increases with the increase of the crystal temperature. The 1485-1553nm signal wave and 3383-3754nm idler wave are obtained by changing the temperature and the angle of the PPLN crystal. 相似文献
93.
94.
介绍了采用不同材料类型的粘结膜、半固化片、所需设备和粘结材料,用于生产低到高层数的PTFE多层板的情况。 相似文献
95.
The propagation properties of decentered twisted Gaussian Schell-model (DTGSM) beams passing through a misaligned first-order optical system are studied. The explicit expressions for the cross-spectral density function and Wigner distribution function of the output beam are derived, which retain their form unchanged. It is shown that the DTGSM beams preserve their closed property. The second-order moments matrix and the Wigner distribution function evolve with the usual laws, whereas the first-order moments matrix varies, as if a ray passes through such system. The propagation of DTGSM beams through an aligned first-order optical system is treated as the limiting case that corresponds to the vanishing misalignment parameters. 相似文献
96.
等级化信息安全保障体系设计思想 总被引:1,自引:0,他引:1
一、前言等级化信息安全是我国信息安全的一项基本策略,实施等级化信息安全保护是实现“重点资产重点保护”,同时又“适度安全”的有效手段,其最终目的是实现安全和成本的综合平衡,达到提高信息安全建设质量,提高安全的综合性价比这一根本目的。正如任何一种新方法的提出必然面临如何有效实施的问题一样,自从2003年中办发27号文件以来,尽管国家相关部门已经陆续出台了等级化信息安全保障的相关文件、意见和实施指南,等级化在信息安全界的实施却比较困难。这是由于在实践中发现,目前出台的相关指南对于复杂的应用系统和网络系统在其操作性上… 相似文献
97.
98.
A high-efficiency CMOS +22-dBm linear power amplifier 总被引:2,自引:0,他引:2
Modern wireless communication systems require power amplifiers with high efficiency and high linearity. CMOS is the technology of choice for complete systems on a chip due to its lower costs and high integration levels. However, it has always been difficult to integrate high-efficiency power amplifiers in CMOS. In this paper, we present a new class of operation (parallel A&B) for power amplifiers that improves both their dynamic range and power efficiency. A prototype design of the new amplifier was fabricated in a 0.18-/spl mu/m CMOS technology. Measurement results show a PAE that is over 44% and the measured output power is +22 dBm. In comparison to a normal class A amplifier, this new design increases the 1-dB compression point (P1dB) by over 3 dB and reduces dc power consumption by over 50% within the linear operating range. 相似文献
99.
W. Shi Y.J. Ding C. Fang Q. Pan Q. Gu 《Applied Physics A: Materials Science & Processing》2003,77(3-4):567-570
The optimal corona-poling temperature of polymer films was accurately determined by measuring the temperature dependence of the in situ second-harmonic (SH) intensity profile under the applied poling electric field. The in situ SH intensity profile was first measured by probing both the surface voltages and the poling currents induced by surface/space charges for the corona-poled polymer films. Moreover, charge effects on the stability of the chromophore orientation were first studied by using the thermally stimulated discharge-current technique. PACS 72.20.Jv; 42.65.Ky; 73.61.Ph; 81.40.Tv 相似文献
100.
This paper is a contionuation of [1] and is concerned with multiplication of weak functions. Here the weak functions are treated as generalized expansions in Hermite functions in [2]. 相似文献