全文获取类型
收费全文 | 71212篇 |
免费 | 10375篇 |
国内免费 | 7444篇 |
专业分类
化学 | 37422篇 |
晶体学 | 641篇 |
力学 | 3078篇 |
综合类 | 328篇 |
数学 | 5959篇 |
物理学 | 20050篇 |
无线电 | 21553篇 |
出版年
2024年 | 307篇 |
2023年 | 1840篇 |
2022年 | 2454篇 |
2021年 | 3117篇 |
2020年 | 2945篇 |
2019年 | 2661篇 |
2018年 | 2274篇 |
2017年 | 2336篇 |
2016年 | 3128篇 |
2015年 | 3453篇 |
2014年 | 4062篇 |
2013年 | 5061篇 |
2012年 | 6053篇 |
2011年 | 5955篇 |
2010年 | 4414篇 |
2009年 | 4295篇 |
2008年 | 4552篇 |
2007年 | 3994篇 |
2006年 | 3694篇 |
2005年 | 3201篇 |
2004年 | 2373篇 |
2003年 | 1915篇 |
2002年 | 1766篇 |
2001年 | 1441篇 |
2000年 | 1289篇 |
1999年 | 1519篇 |
1998年 | 1221篇 |
1997年 | 1116篇 |
1996年 | 1083篇 |
1995年 | 947篇 |
1994年 | 813篇 |
1993年 | 696篇 |
1992年 | 578篇 |
1991年 | 500篇 |
1990年 | 397篇 |
1989年 | 294篇 |
1988年 | 216篇 |
1987年 | 181篇 |
1986年 | 181篇 |
1985年 | 156篇 |
1984年 | 103篇 |
1983年 | 98篇 |
1982年 | 62篇 |
1981年 | 45篇 |
1980年 | 30篇 |
1979年 | 19篇 |
1977年 | 21篇 |
1976年 | 21篇 |
1975年 | 23篇 |
1972年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
In this paper, we investigate the problem of determining the optimal bandwidth allocation for a Dynamic Bandwidth Allocation Scheme (DBAS). The objective is to minimize the total amount of bandwidth required to satisfy the Quality of Service (QoS) requirements of all traffic streams. It is shown that when the performance functions satisfy a certain number of conditions, there exists a unique optimal bandwidth allocation such that, for each traffic stream, either its QoS is just satisfied or its QoS is over-satisfied and it is allocated zero bandwidth. Such an allocation is said to be efficient. It is also shown that there exists a unique efficient allocation in the entire feasible region. An iterative algorithm is developed to compute the efficient allocation employing its special properties. Numerical examples are presented to demonstrate how the algorithm works. Future extensions of this work are also discussed.Partially supported by NSERC of Canada through grants OGP14020 and STRIN-200. 相似文献
42.
43.
44.
本文提出了数字遥测信号与模拟图象信与混合的无线电测控系统中用于捕获和跟踪带钟复合码的跟踪双环方案,分析了双环系统的捕获性能,理论分析和实验结果表明,双环系统不仅捕获时间短,而且具有强的抗干扰能力。 相似文献
45.
A technique for SiO2 formation by liquid-phase deposition (LPD) at nearly room temperature for low-temperature processed (LTP) polysilicon thin-film transistor (poly-Si TFT) was developed. LPD SiO2 film with a lower P-etch rate shows a dense structure. LPD SiO2 also exhibits good electrical characteristics. LTP poly-Si thin-film transistors (TFTs) with LPD SiO 2 as the gate insulator have been fabricated and investigated. Their characteristics indicate performance adequate for their use as pixel transistors in liquid crystal displays (LCDs) 相似文献
46.
低压铝箔交流腐蚀研究 总被引:4,自引:2,他引:2
在30Hz频率下,通过铝箔在HCl+H2SO4+HNO3+H3PO4体系中的交流腐蚀,研究腐蚀液组成中腐蚀主体及缓蚀剂对铝箔腐蚀的作用,探讨腐蚀过程中电源频率、腐蚀液温度、电流密度及腐蚀时间对铝箔腐蚀的影响。腐蚀液组成的配比恰当,有利于比容的提高。在特定的频率下采用合适的腐蚀液温度、适宜的电流密度和腐蚀时间可以提高铝箔的静电容量。 相似文献
47.
在线性近似条件下,量子化了非线性薛定谔方程,用后向传播法数值求解了光孤子相互作用对压缩比的影响。数值结果表明,在碰撞距离附近,压缩比显著增加。 相似文献
48.
Da-Zhi Li Li Gan Zi-Qiang Yang Zheng Liang Jia-Yin Li 《Journal of Infrared, Millimeter and Terahertz Waves》1997,18(10):1885-1896
The dispersion characteristic of the plasma-loaded relativistic backward wave oscillator has been analyzed. The theoretical model has been established and the numerical calculations accord with the experimental results, which provides some useful suggestions on the designing of slow-wave structure of BWO. 相似文献
49.
In theory, both polarity and steric hindrance are basic factors which affect molecular interactions. To investigate the optical properties and steric structures of chiral compounds having different chiral moieties which affect the wavelength of light reflection in liquid crystal (LC) cells, a series of novel chiral compounds and azobenzene derivatives were synthesized. The liquid crystalline phases of the compounds were identified using small angle X-ray diffraction, differential scanning calorimetry and polarizing optical microscopy. Cholesteric LC cells with various synthesized chiral dopants which selectively reflect visible light were first prepared, the photochemical switching behaviour of colours was then investigated, with special reference to the change in transmittance in cholesteric LC cells containing an azobenzene derivative as a photoisomerizable guest molecule. Reversible isomerization of azobenzene molecules occurred in the cholesteric systems, resulting in a depression of TChI and a shift of the selectively reflected wavelength. We discuss the photochemically driven change in the helical pitch of the cholesteric LCs with respect to structural effects involving the chiral moieties. Molecular interactions caused by the added dopants, reliability and stability of the photoisomerization, and UV irradiation effects on the cholesteric LC cells were also investigated. A real image was recorded through a mask on a cholesteric LC cell fabricated in this investigation. 相似文献
50.
Dual-slope converters use time to perform analog-to-digital conversion but require 2/sup N+1/ clock cycles to achieve N bits of precision. We describe a novel current-mode algorithm that also uses time to perform analog-to-digital conversion but requires 5N clock cycles to achieve N bits of precision via a successive subranging technique. The algorithm requires one asynchronous comparator, two capacitors, one current source, and a state machine. Amplification of two is achieved without the use of an explicit amplifier by simply doing things twice in time. The use of alternating voltage-to-time and time-to-voltage conversions provides natural error cancellation of comparator offset and delay, 1/f noise, and switching charge-injection. The use of few components and an efficient mechanism for amplification and error cancellation allow for energy-efficient operation: in a 0.35-/spl mu/m implementation, we were able to achieve 12 bit of DNL limited precision or 11 bit of thermal noise-limited precision at a sampling frequency of 31.25 kHz with 75 /spl mu/W of total analog and digital power consumption. These numbers yield a thermal noise-limited energy efficiency of 1.17 pJ per quantization level, making it one of the most energy-efficient converters to date in the 10-12 bit precision range. 相似文献