全文获取类型
收费全文 | 588175篇 |
免费 | 4724篇 |
国内免费 | 1465篇 |
专业分类
化学 | 269077篇 |
晶体学 | 7645篇 |
力学 | 25990篇 |
综合类 | 17篇 |
数学 | 67779篇 |
物理学 | 163577篇 |
无线电 | 60279篇 |
出版年
2021年 | 5477篇 |
2020年 | 6048篇 |
2019年 | 6903篇 |
2018年 | 9128篇 |
2017年 | 9230篇 |
2016年 | 12512篇 |
2015年 | 6527篇 |
2014年 | 11296篇 |
2013年 | 24940篇 |
2012年 | 19340篇 |
2011年 | 22961篇 |
2010年 | 17650篇 |
2009年 | 18190篇 |
2008年 | 22974篇 |
2007年 | 23244篇 |
2006年 | 21294篇 |
2005年 | 19241篇 |
2004年 | 18020篇 |
2003年 | 16328篇 |
2002年 | 16194篇 |
2001年 | 16900篇 |
2000年 | 13586篇 |
1999年 | 10773篇 |
1998年 | 9581篇 |
1997年 | 9330篇 |
1996年 | 8551篇 |
1995年 | 7805篇 |
1994年 | 7878篇 |
1993年 | 7577篇 |
1992年 | 7822篇 |
1991年 | 8278篇 |
1990年 | 7907篇 |
1989年 | 7679篇 |
1988年 | 7389篇 |
1987年 | 6770篇 |
1986年 | 6460篇 |
1985年 | 8012篇 |
1984年 | 8306篇 |
1983年 | 7021篇 |
1982年 | 7186篇 |
1981年 | 6718篇 |
1980年 | 6390篇 |
1979年 | 6823篇 |
1978年 | 7064篇 |
1977年 | 6938篇 |
1976年 | 6856篇 |
1975年 | 6560篇 |
1974年 | 6398篇 |
1973年 | 6705篇 |
1972年 | 4891篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
The effect of secondary scattering that is produced by the induced current in an infinitely long, finitely conducting wire over a plane, finitely conducting earth from an incident electromagnetic pulse is investigated. This is accomplished by finding the field of a Hertzian dipole over the earth and identifying the part that is due to secondary scattering. Frequency-domain results for the overall induced current require that certain infinite integrals be numerically evaluated. Fourier inversion to the time domain in turn requires the numerical evaluation of another infinite integral. When the results are compared to those obtained when secondary scattering is ignored, it is found that secondary scattering slightly increases the current for the case of broadside incidence and reduces it for the case of grazing incidence by as much as two orders of magnitude. Peak currents are much smaller than previously thought. These general results hold for all of the earth parameters that were used in the study 相似文献
72.
Alsing P. Peterson P. Cardimona D. Gavrielides A. 《Quantum Electronics, IEEE Journal of》1987,23(5):557-568
We investigate the saturation effects of power broadening, Stark shifting, and population transfer on Stokes conversion in stimulated Raman scattering. We do not make the usual rotating wave approximation because the detuning from the next electronic state is assumed to be in the optical regime. Retaining the counter-rotating terms allows an exact determination of the pump and Stokes indexes of refraction. Steady-state solutions for the Stokes intensity and phase are obtained and the effects of making the rotating wave approximation (RWA) are discussed. Finally, we examine the behavior of these solutions for Stokes conversion in hydrogen gas when geometric propagation is appropriate. 相似文献
73.
74.
A. D. Smirnov 《Journal of Applied Spectroscopy》1991,54(1):33-38
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 1, pp. 46–50, January, 1991. 相似文献
75.
76.
A systematic study on the electrochemical behavior of diosmin in Britton-Robinson buffer (pH 2.0-10.0) at a glassy carbon electrode (GCE) was made. The oxidation process of the drug was found to be quasi-reversible with an adsorption-controlled step. The adsorption stripping response was evaluated with respect to various experimental conditions, such as the pH of the supporting electrolyte, the accumulation potential and the accumulation time. The observed anodic peak current at +0.73 V vs. Ag/AgCl reference electrode increased linearly over two orders of magnitude from 5.0x10(-8) M to 9.0x10(-6) M. A limit of detection down to 3.5x10(-8) M of diosmin at the GCE was achieved with a mean recovery of 97+/-2.1%. Based on the electrochemical data, an open-circuit accumulation step in a stirred sample solution of BR at pH 3.0 was developed. The proposed method was successfully applied to the determination of the drug in pharmaceutical formulations. The results compared favorably with the data obtained via spectrophotometric and HPLC methods. 相似文献
77.
A. Bozkurt 《Journal of Physics and Chemistry of Solids》2002,63(4):685-690
Ionic conductivity of dry quaternary ammonium polymer, polydiallyldimethylammonium with Cl− and H2PO4− counter ions was studied in a wide range of frequencies (10−2-107 Hz) and temperatures (300-470 K). The data were analysed within the formalism of permittivity. WAXS, and thermal properties of this polymer were investigated prior to conductivity measurements. 相似文献
78.
Small-signal and temperature noise model for MOSFETs 总被引:1,自引:0,他引:1
79.
80.
Kuh S.S. Strozier T.A. Ryan C.R. 《Selected Areas in Communications, IEEE Journal on》1989,7(9):1462-1469
A continuous phase quadrature phase shift keyed (CPQPSK) modulation technique is presented. This method utilizes a conventional QPSK modulator and a phase trajectory converter to approximate M =4, h =1/4 continuous phase signal and allows low cost, low complexity, and high rate (>1 Gbit/s) CPM modem implementation for bandwidth efficient transmission through nonlinear satellite channels. Using a communications analysis computer program it has been found that CPQPSK has 99 percent out-of-band power of 0.8R (MSK has 99 percent out-of-band power of 1.2 R where R is defined as bit rate), continuous phase trajectories, and nearly constant envelope amplitude. Simulation of realistic hardware designs indicate that the CPQPSK will require an Eb /No of 14 dB to achieve a bit error rate (BER) of 10-6. Forward error correcting techniques using block codes with an overhead of 10 percent indicate that the Eb /No requirements can be reduced to 11.2 dB for 10-6 BER 相似文献