全文获取类型
收费全文 | 149010篇 |
免费 | 20998篇 |
国内免费 | 16723篇 |
专业分类
化学 | 78861篇 |
晶体学 | 1251篇 |
力学 | 6651篇 |
综合类 | 754篇 |
数学 | 13924篇 |
物理学 | 43167篇 |
无线电 | 42123篇 |
出版年
2024年 | 572篇 |
2023年 | 3345篇 |
2022年 | 4421篇 |
2021年 | 5644篇 |
2020年 | 5535篇 |
2019年 | 4985篇 |
2018年 | 4678篇 |
2017年 | 4429篇 |
2016年 | 6279篇 |
2015年 | 6656篇 |
2014年 | 8198篇 |
2013年 | 10512篇 |
2012年 | 12241篇 |
2011年 | 12495篇 |
2010年 | 9228篇 |
2009年 | 9230篇 |
2008年 | 9576篇 |
2007年 | 8710篇 |
2006年 | 8244篇 |
2005年 | 7296篇 |
2004年 | 5332篇 |
2003年 | 4479篇 |
2002年 | 4033篇 |
2001年 | 3464篇 |
2000年 | 3129篇 |
1999年 | 3286篇 |
1998年 | 2887篇 |
1997年 | 2480篇 |
1996年 | 2471篇 |
1995年 | 2188篇 |
1994年 | 1897篇 |
1993年 | 1529篇 |
1992年 | 1363篇 |
1991年 | 1149篇 |
1990年 | 994篇 |
1989年 | 678篇 |
1988年 | 550篇 |
1987年 | 470篇 |
1986年 | 472篇 |
1985年 | 370篇 |
1984年 | 271篇 |
1983年 | 226篇 |
1982年 | 175篇 |
1981年 | 114篇 |
1980年 | 86篇 |
1979年 | 43篇 |
1978年 | 44篇 |
1977年 | 33篇 |
1976年 | 33篇 |
1975年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
由于器件的快速退化,101.5小时似乎成了Znse基蓝绿色半导体激光器难于逾越的寿命极限。分析退化机制,发现在强电流注入的半导体激光器中,热退化具有重要影响。研究表明,用作载流子限制层的宽带Ⅱ-Ⅵ族四元合金(如ZnMgSSe)只能对ZnSe中的电子有效地限制,无法对空穴很好地限制;而对BeTe,却只能对空穴进行有效的限制,无法对电子很好地限制。这导致ZnSe(或BeTe)活性层空穴(或电子)漏电发热,引起退化。本文提出以ZnSe/BeTe超晶格为蓝绿发光层,并用包络函数理论具体计算了阱宽、垒宽对载流子能级的不同影响,考察了ZnSe、BeTe厚度比和超晶格周期对带隙、载流子限制能力的调节。为研制新型长寿命蓝绿色半导体激光器提供了一条新的途径。 相似文献
102.
大型复杂高精度波导钎焊设备的研制:简介ZHS—132型真空钎焊炉 总被引:1,自引:0,他引:1
应军事电子装备,特别是军用雷达预研和生产的急需,电子工业部第二研究所近期研制的成功大型、复杂、高精度波导钎焊设备ZHS-132型真空钎焊炉。文中主要介绍了该设备的开发背景和对波导钎焊工艺的影响,该设备的设计构思、结构和技术特征,使用功能和效果,以及推广应用前景。 相似文献
103.
Airborne scatterometers: investigating ocean backscatter under low and high-wind conditions 总被引:1,自引:0,他引:1
Carswell J.R. Carson S.C. McIntosh R.E. Li F.K. Neumann G. McLaughlin D.J. Wilkerson J.C. Black P.G. Nghiem S.V. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》1994,82(12):1835-1860
Attempting to understand and predict weather on a local and global basis has challenged both the scientific and engineering communities. One key parameter in understanding the weather is the ocean surface wind vector because of its role in the energy exchange at the air-sea surface. scatterometers, radars that measure the reflectivity of a target offer a tool with which to remotely monitor these winds from tower-, aircraft-, and satellite-based platforms. This paper introduces three current airborne scatterometer systems, and presents data collected by these instruments under low-, moderate-, and high-wind conditions. The paper focuses on airborne scatterometers because of their ability to resolve submesoscale variations in wind fields. Discrepancies between existing theory and the observations are noted and the concerns in measuring low-wind speeds discussed. Finally, the application of using this technology for estimating the surface-wind vector during a hurricane is demonstrated 相似文献
104.
2-D dopant profiling in VLSI devices using dopant-selectiveetching: an atomic force microscopy study
We report a detailed mapping of a 2-D dopant profile on a fully processed industrial sample with large dynamic range and high spatial resolution by utilizing a dopant-selective etching process and Atomic Force Microscopy. The experimental results show excellent agreement with those obtained from SRP and SIMS as corroborative methods. We also discuss the most critical factors which influence the applicability, reproducibility, and reliability of this method 相似文献
105.
Chun Hu Ji Zhao Li G.P. Liu P. Worley E. White J. Kjar R. 《Electron Device Letters, IEEE》1995,16(2):61-63
The effects of the plasma etching process induced gate oxide damages on device's low frequency noise behavior are investigated on MOSFET's fabricated with different field plate perimeter to gate area ratio antennas. Abnormal 1/f noise spectrum with a shoulder centered in the frequency range of 100 and to 1 kHz was frequently observed in small geometry devices, and it is attributable to a nonuniform distribution of oxide traps induced by plasma etching process 相似文献
106.
107.
108.
Li W. Aiken M. 《IEEE transactions on systems, man and cybernetics. Part C, Applications and reviews》1998,28(2):288-294
Many real-world decision-making problems fall into the general category of classification. Algorithms for constructing knowledge by inductive inference from example have been widely used for some decades. Although these learning algorithms frequently address the same problem of learning from preclassified examples and much previous work in inductive learning has focused on the algorithms' predictive accuracy, little attention has been paid to the effect of data factors on the performance of a learning system. An experiment was conducted using five learning algorithms on two data sets to investigate how the change in labeling the class attribute can alter the behavior of learning algorithms. The results show that different preclassification rules applied on the training examples can affect either the classification accuracy or classification structure 相似文献
109.
受中国电信委托、原邮电部经济技术发展研究中心在全国进行一次农村电信市场调查,以了解当前农村通信市场发展潜力,本文公布了此次调查的结果,从经济能力、资费和潜在市场等方面提出了发展农话的建议 。 相似文献
110.