全文获取类型
收费全文 | 144720篇 |
免费 | 21518篇 |
国内免费 | 16444篇 |
专业分类
化学 | 77111篇 |
晶体学 | 1241篇 |
力学 | 6312篇 |
综合类 | 714篇 |
数学 | 13744篇 |
物理学 | 41927篇 |
无线电 | 41633篇 |
出版年
2024年 | 576篇 |
2023年 | 3322篇 |
2022年 | 4449篇 |
2021年 | 5488篇 |
2020年 | 5463篇 |
2019年 | 4961篇 |
2018年 | 4666篇 |
2017年 | 4416篇 |
2016年 | 6260篇 |
2015年 | 6645篇 |
2014年 | 8191篇 |
2013年 | 10387篇 |
2012年 | 12207篇 |
2011年 | 12410篇 |
2010年 | 9142篇 |
2009年 | 9039篇 |
2008年 | 9403篇 |
2007年 | 8453篇 |
2006年 | 8084篇 |
2005年 | 7065篇 |
2004年 | 5217篇 |
2003年 | 4347篇 |
2002年 | 3943篇 |
2001年 | 3338篇 |
2000年 | 2985篇 |
1999年 | 3171篇 |
1998年 | 2678篇 |
1997年 | 2296篇 |
1996年 | 2281篇 |
1995年 | 2055篇 |
1994年 | 1742篇 |
1993年 | 1423篇 |
1992年 | 1255篇 |
1991年 | 1063篇 |
1990年 | 905篇 |
1989年 | 648篇 |
1988年 | 495篇 |
1987年 | 404篇 |
1986年 | 416篇 |
1985年 | 309篇 |
1984年 | 244篇 |
1983年 | 210篇 |
1982年 | 154篇 |
1981年 | 99篇 |
1980年 | 71篇 |
1979年 | 37篇 |
1978年 | 32篇 |
1976年 | 30篇 |
1975年 | 29篇 |
1957年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
Jihua Zhang Xi WangWeidong Yu Tao FengFumin Zhang Zhihong ZhengQiong Li Xianghuai Liu 《Solid State Communications》2003,127(4):289-293
Effects of ion impinging on the microstructure and field electron emission properties of screen-printed carbon nanotube films were investigated. We observed that the plasma treatment modified the microstructure of CNTs along with the remarkable increase of emission site density. With the prolongation of ion impinging time, the emission current falls down first, and then rises up to higher than that of the untreated films. It is proposed that the change of emission characteristics is due to the different emission mechanisms. After the treatment, electrons are emitted predominantly from the nano-nodes on the tube wall instead from the nanotube tips. 相似文献
162.
A new family of photonucleases, naphthalimide-thiazoles was synthesized and evaluated. These compounds intercalated into DNA efficiently and damaged DNA photochemically at concentrations as low as 5 μM. Mechanistic experiment suggests that a novel naphthalimide-thiazole radical produced via an excited triple state might be involved in the DNA photodamage. Different activity may arise from the impact of substituents at 2-phenyl ring of thiazole on the electron population of excited triple state according to AM1 semi-empirical calculation. 相似文献
163.
164.
Li Yingle Huang Jiying Gong Shuohong 《International Journal of Infrared and Millimeter Waves》2003,24(6):953-961
The dielectric constant for rain medium is investigated by utilizing the system identification method. The rain rate model and frequency model of permittivity in millimeter waves band for rain medium are presented. The results obtained with models are in very good agreement with references in calculating the attenuation of electromagnetic waves induced by rain, which Shows that the obtained models are valid and practicable. The cross-polar discrimination gotten with rain rate model is in agreement with references. 相似文献
165.
学生学习知识,一般来说,是从生动的具体的直观开始的,生动的直观能使学生获得关于事物生动而具体的感性认识。但是,感性认识并没有揭露事物的本质,人们对事物本质的认识要通过一系列的抽象来完成。 相似文献
166.
Resistivity-temperature characteristics of sol gel YBa2Cu3Oy samples synthesized in flowing oxygen atmosphere 下载免费PDF全文
The relationship of resistivity versus synthesizing temperature of sol gel YBa_2Cu_3O_y samples was studied when prepared under flowing oxygen conditions. A set of high-temperature ρ-T curves was obtained for the whole process. After the sample finished the test measuring, its resistivity was ρ_{300}=9.83×10^{-3 }Ω·cm at room temperature. The ρ-T curve also showed that the orthorhombic-tetragonal phase transformation of sol-gel YBa_2Cu_3O_y sample occurred at 581℃ for the sample in the rising temperature process, but at 613℃ in the cooling process, lower than that of the samples made by using the conventional powder metallurgy methods. 相似文献
167.
说明了在异步串行通信(RS-232)的工作方式下,对所接收的数据进行接收、处理,丢弃乱码,并进行保存的可靠方法,并给出了相关的VC 语言程序. 相似文献
168.
169.
A Low-Power CMOS Linear-in-Decibel Variable Gain Amplifier With Programmable Bandwidth and Stable Group Delay 总被引:1,自引:0,他引:1
Tsou S.-C. Li C.-F. Huang P.-C. 《Circuits and Systems II: Express Briefs, IEEE Transactions on》2006,53(12):1436-1440
This brief presents a new circuit architecture for linear-in-decibel, constant-bandwidth variable gain amplifier (VGA). To obtain high linearity under low-voltage operation, this VGA is a closed-loop structure. In loop amplifier design, two techniques are applied: first, the loop amplifier is given finite input impedance. This arrangement keeps the VGA bandwidth constant under different gain setting. Second, a current-buffered compensation is applied for loop stability. Compared to the Miller compensation, this method achieves wider bandwidth. The prototype chip using 0.18-mum CMOS technology demonstrates that -10- to 20-dB gain and 0.5- to 30-MHz bandwidth can be programmed independently. The group delay difference within 30-dB gain control range is smaller than 1%. The total circuit dissipates 1.35 mA from a 1.8-V supply 相似文献
170.
Charalambous C.D. Bultitude R.J.C. Li X. Zhan J. 《Wireless Communications, IEEE Transactions on》2008,7(2):434-439
This paper is concerned with modeling and identification of wireless channels using noisy measurements. The models employed are governed by stochastic differential equations (SDEs) in state space form, while the identification method is based on the expectation-maximization (EM) algorithm and Kalman filtering. The algorithm is tested against real channel measurements. The results presented include state space models for the channels, estimates of inphase and quadrature components, and estimates of the corresponding Doppler power spectral densities (DPSDs), from sample noisy measurements. Based on the available measurements, it is concluded that state space models of order two are sufficient for wireless flat fading channel characterization. 相似文献