全文获取类型
收费全文 | 138564篇 |
免费 | 11822篇 |
国内免费 | 11095篇 |
专业分类
化学 | 65696篇 |
晶体学 | 1896篇 |
力学 | 6751篇 |
综合类 | 1075篇 |
数学 | 14110篇 |
物理学 | 40721篇 |
无线电 | 31232篇 |
出版年
2023年 | 1597篇 |
2022年 | 2851篇 |
2021年 | 3514篇 |
2020年 | 3521篇 |
2019年 | 3710篇 |
2018年 | 4044篇 |
2017年 | 4344篇 |
2016年 | 4743篇 |
2015年 | 4198篇 |
2014年 | 5660篇 |
2013年 | 7551篇 |
2012年 | 7897篇 |
2011年 | 8253篇 |
2010年 | 7173篇 |
2009年 | 7416篇 |
2008年 | 7683篇 |
2007年 | 7274篇 |
2006年 | 6669篇 |
2005年 | 6079篇 |
2004年 | 4972篇 |
2003年 | 4277篇 |
2002年 | 4258篇 |
2001年 | 3875篇 |
2000年 | 3406篇 |
1999年 | 2467篇 |
1998年 | 2053篇 |
1997年 | 1767篇 |
1996年 | 1653篇 |
1995年 | 1387篇 |
1994年 | 1402篇 |
1993年 | 1360篇 |
1992年 | 1309篇 |
1991年 | 1301篇 |
1990年 | 1256篇 |
1989年 | 1158篇 |
1988年 | 1024篇 |
1987年 | 1023篇 |
1986年 | 948篇 |
1985年 | 966篇 |
1984年 | 983篇 |
1983年 | 889篇 |
1982年 | 845篇 |
1979年 | 828篇 |
1978年 | 821篇 |
1977年 | 822篇 |
1976年 | 921篇 |
1975年 | 818篇 |
1974年 | 849篇 |
1973年 | 848篇 |
1972年 | 755篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Code-division multiple-access (CDMA) implemented with direct-sequence spread spectrum (DS/SS) signaling is a promising multiplexing technique for cellular telecommunications services. The efficiency of a direct-sequence spread-spectrum code-division multiple-access (DS-CDMA) system depends heavily on the shape of the spectrum of the spread signal. Maximum efficiency is obtained with an ideal brick-wall bandpass spectrum. There are two approaches toward achieving such a spectrum. One is to use a simple spreader that produces a broad spectrum and then follow it with a precise, high order filter to band limit the spectrum. A second approach, which is the approach taken in this paper, is to use a spreader that produces a spectrum close to the ideal spectrum and then employ a simple filter to control the out-of-band power. The proposed spreader/despreader is based on a simple hybrid function and can be easily implemented. An analysis provides a compact expression for the signal-to-noise ratio (SNR) of a RAKE receiver. The expression includes the effects of baseband, intermediate frequency (IF) and RF filtering as well as the effects of the spectral densities of the spreading/despreading functions. The analysis shows that the proposed spreader/despreader yields superior performance over a conventional pseudo noise (PN) spreading/despreading mechanism 相似文献
22.
23.
S. A. Gurevich T. A. Zaraiskaya S. G. Konnikov V. M. Mikushkin S. Yu. Nikonov A. A. Sitnikova S. E. Sysoev V. V. Khorenko V. V. Shnitov Yu. S. Gordeev 《Physics of the Solid State》1997,39(10):1691-1695
The concentration and chemical state of copper in the subsurface region of Cu/SiO2 composite films obtained by simultaneous magnetron sputtering from two sources (Cu and SiO2) are determined by x-ray photoelectron spectroscopy (XPS). It is established that copper in the as-grown film is primarily in the form of unoxidized atoms dispersed in a SiO2 matrix. Annealing of the film results in practically no oxidation, but about 70% of the copper atoms condense into metallic clusters with sizes below 10 Å in the subsurface region and about 50 Å in the bulk of the film. The changes in the binding energy of core electrons, and especially in the energies of Auger electrons, are so large in this situation that photoelectron and Auger spectroscopy are efficient methods for monitoring the chemical state of this composite material. 相似文献
24.
In this paper, symbolic code matrix ,constant matrix and count matrix are defined .The first twomatrices are used to describe the elemental expression of augmented matrix and the nede admittance equa-tion is thus obtained. The third matrix is used to obtain the incoming degree matrix, and according to thematrix all the 1- factors of the Coates graph are given. By using the data code, the determinant is expandedand the same items in the expansion are merged. Thus the symbolic network function in which no term can-cellation occurs is generated. 相似文献
25.
A. L. Apanasenko A. V. Kuznichenko Yu. B. Govyadovskii V. G. Yakunin 《Journal of Applied Spectroscopy》1991,54(3):271-276
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 3, pp. 438–444, March, 1991. 相似文献
26.
Yu. K. Voron'ko A. B. Kudryavtsev V. V. Osiko E. V. Sorokin 《Journal of Applied Spectroscopy》1991,55(4):953-957
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 55, No. 4, pp. 535–540, October, 1991. 相似文献
27.
The excitation of eigen surface waves by tubular electron beams in cylindrical discharge devices is studied. The influence of the wave‐field azimuthal structure on the excitation efficiency and nonlinear stage of the plasmabeam instability is investigated both numerically and analytically. Analytical expressions for the saturation amplitude and excitation efficiency of the wave under study are derived. They are found to agree well with results obtained by numerical modelling of the plasma‐beam interaction presented in this paper. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
28.
Organic conductor is a kind of organic compound which has special electronic and magnetic properties. The research of the organic compounds has received considerable attention because of their potential applications in many areas. The molecular conductive units are theoretically investigated as well as their energy gap and charge distribution. The relationship of conductivity and micro-mechanism is discussed. 相似文献
29.
A. R. Volkov B. V. Shul'gin T. I. Polupanova V. N. Lebedev A. A. Nagornyi V. L. Petrov Yu. F. Kargin 《Journal of Applied Spectroscopy》1991,54(6):585-590
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 6, pp. 970–975, June, 1991. 相似文献
30.
禹思敏 《电气电子教学学报》2002,24(2):29-32
对数字通信中MPSK系统的调制与解调问题进行了较为的分析,并以8PSK和循环码16PSK系统为例,阐述其调制器电路的逻辑设计方法。在此基础上给出了8PSK系统调制与解调的一种方案,讨论了循环码和自然二进制码16PSK系统的调制器的具体设计过程,该设计方法可应用于一般MPSK系统的逻辑设计。 相似文献