全文获取类型
收费全文 | 20367篇 |
免费 | 983篇 |
国内免费 | 99篇 |
专业分类
化学 | 13037篇 |
晶体学 | 120篇 |
力学 | 420篇 |
数学 | 2894篇 |
物理学 | 3399篇 |
无线电 | 1579篇 |
出版年
2023年 | 132篇 |
2022年 | 156篇 |
2021年 | 310篇 |
2020年 | 424篇 |
2019年 | 431篇 |
2018年 | 281篇 |
2017年 | 269篇 |
2016年 | 646篇 |
2015年 | 593篇 |
2014年 | 701篇 |
2013年 | 1084篇 |
2012年 | 1189篇 |
2011年 | 1318篇 |
2010年 | 787篇 |
2009年 | 735篇 |
2008年 | 1096篇 |
2007年 | 1081篇 |
2006年 | 1008篇 |
2005年 | 924篇 |
2004年 | 831篇 |
2003年 | 694篇 |
2002年 | 651篇 |
2001年 | 349篇 |
2000年 | 323篇 |
1999年 | 309篇 |
1998年 | 270篇 |
1997年 | 259篇 |
1996年 | 291篇 |
1995年 | 276篇 |
1994年 | 243篇 |
1993年 | 232篇 |
1992年 | 215篇 |
1991年 | 186篇 |
1990年 | 189篇 |
1989年 | 184篇 |
1988年 | 162篇 |
1987年 | 135篇 |
1986年 | 120篇 |
1985年 | 175篇 |
1984年 | 173篇 |
1983年 | 134篇 |
1982年 | 159篇 |
1981年 | 165篇 |
1980年 | 141篇 |
1979年 | 145篇 |
1978年 | 166篇 |
1977年 | 127篇 |
1976年 | 108篇 |
1975年 | 121篇 |
1974年 | 96篇 |
排序方式: 共有10000条查询结果,搜索用时 753 毫秒
91.
V. Katchkanov J.F.W. Mosselmans S. Dalmasso K.P. ODonnell S. Hernandez K. Wang R.W. Martin O. Briot N. Rousseau G. Halambalakis K. Lorenz E. Alves 《Superlattices and Microstructures》2004,36(4-6):729
The local structure of Tm3+ ions incorporated into GaN epilayers was studied by means of Extended X-ray Absorption Fine Structure. The samples were doped either in situ during growth by Molecular Beam Epitaxy or by ion implantation of layers grown by Metal Organic Chemical Vapour Deposition. The implantation was done at ion energy of 300 keV and different nominal fluences of 3×1015, 4×1015 cm−2 and 5×1015 cm−2. The concentration of Tm in the samples studied was measured by Wavelength Dispersive X-ray analysis. For the in situ doped sample with concentration of 0.5%, and for all of the implanted samples, Tm was found on the Ga site in GaN. The ion implanted sample and an in situ doped sample with a similar concentration of Tm showed the same local structure, which suggests that the lattice site occupied by Tm does not depend on the doping method. When the average Tm concentration for in situ doped samples is increased to 1.2% and 2.0%, Tm is found to occupy the Ga substitutional site and the presence of a substantial number of Tm ions in the second coordination sphere indicates dopant clustering in the films. The formation of pure TmN clusters was found in an in situ doped sample with a dopant concentration of 3.4%. 相似文献
92.
Tan W.S. Uren M.J. Houston P.A. Green R.T. Balmer R.S. Martin T. 《Electron Device Letters, IEEE》2006,27(1):1-3
A novel guarded surface leakage test structure is used to isolate the surface and bulk leakage contributions to gate current in AlGaN/GaN HFETs. Passivation with various recipes of SiN/sub x/ always resulted in the commonly observed increase in gate leakage, which was found to be dominated by bulk leakage through the AlGaN. However, high temperature deposited SiN/sub x/ recipes gave a 1-2 orders reduction in surface leakage, whereas low temperature deposition gave an increase. Gate lag measurements were found to correlate closely with the surface leakage component, giving direct evidence that the key device problem of current slump is associated with current flow at the AlGaN surface. 相似文献
93.
Martin Weese 《Mathematical Logic Quarterly》1992,38(1):325-326
94.
Martin Celli 《Comptes Rendus Mathematique》2005,340(7):513-518
The Note deals with rigid solutions of the N-Body Problem, i.e. solutions with constant mutual distances between the bodies. It is shown that for these motions, the configuration is balanced in the sense of Albouy and Chenciner [Invent. Math. 131 (1998) 151–184] even when the masses are of different signs. This fact was proved only for positive masses, using the scalar product they define. A consequence of the result is the constancy of the rotation velocity. It is also shown that any configuration can generate non-planar rigid motions for certain masses. Such motions do not exist with positive masses. All the results can be generalized to systems with N charged particles. To cite this article: M. Celli, C. R. Acad. Sci. Paris, Ser. I 340 (2005). 相似文献
95.
An adaptive analog continuous-time biquadratic filter is realized in a 2-μm digital CMOS process for operation at 300 kHz. The biquad implements the notch, bandpass and low-pass transfer functions. The only parameter adapted is the resonant frequency of the biquad, which is identical to the notch frequency and the bandpass center frequency. The update method is based on a least-means-square algorithm which adapts the notch frequency to minimize the power at the notch filter output. The actual update is modified to reduce the circuit complexity to one biquad and one correlator. When the filter is tracking a sinusoid, this update generates a ripple-free gradient that decreases tracking error. Applications include phase-frequency detectors, FM demodulators (linear and frequency shift keying), clock extractors, and frequency acquisition aids for phase-locked loops and Costas loops. Measured results from experimental prototypes are presented. Nonidealities of an all-analog implementation are discussed, along with suggestions to improve performance 相似文献
96.
Chang C.-R. Steer M.B. Martin S. Reese E. Jr. 《Microwave Theory and Techniques》1991,39(10):1735-1745
The authors present a newly developed free-running steady-state oscillator analysis algorithm suited to large-signal oscillator analysis. Kurokawa's oscillation condition is coupled with the modified nodal admittance form of the circuit equations to avoid degenerate solutions. The algorithm was implemented by using both harmonic balance and frequency-domain spectral balance techniques. It was used in the simulation of monolithically integrated varactor-tuned MESFET oscillator. Good agreement between simulated power and oscillation frequency results was obtained 相似文献
97.
H.W. Choi C.W. Jeon M.D. Dawson P.R. Edwards R.W. Martin 《Photonics Technology Letters, IEEE》2003,15(4):510-512
High-performance, two-dimensional arrays of parallel-addressed InGaN blue micro-light-emitting diodes (LEDs) with individual element diameters of 8, 12, and 20 /spl mu/m, respectively, and overall dimensions 490 /spl times/490 /spl mu/m, have been fabricated. In order to overcome the difficulty of interconnecting multiple device elements with sufficient step-height coverage for contact metallization, a novel scheme involving the etching of sloped-sidewalls has been developed. The devices have current-voltage (I-V) characteristics approaching those of broad-area reference LEDs fabricated from the same wafer, and give comparable (3-mW) light output in the forward direction to the reference LEDs, despite much lower active area. The external efficiencies of the micro-LED arrays improve as the dimensions of the individual elements are scaled down. This is attributed to scattering at the etched sidewalls of in-plane propagating photons into the forward direction. 相似文献
98.
99.
Models are developed for decision making about monitoring andmaintenance of systems whose performance through time is describedby a general stochastic process. The system is monitored andpreventive and corrective maintenance actions are carried outin response to the observed system state. The decision processis simplified by using the maximum process as a decision variable.The models developed generalize age replacement models and othersimple maintenance strategies. The approach can deal with failuresthat prevent the system functioning further, and also failuresthat are defined by regulation or economic considerations. Attentionis restricted to perfect repair and inspection, but the structureprovides the framework for further developments. 相似文献
100.
Martin Breza 《Czechoslovak Journal of Physics》1992,42(2):201-207
The electronic structure of YBa2Cu3Ox (x=6 and 7) is investigated using the CNDO molecular orbital method. Electronic structures of model clusters [Cu3O10]–15, [Ba8Cu3O10]+1, [Y8Cu3O10]+9 of the non-superconducting (x=6) and [Cu3O12]–17, [Ba8Cu3O12]–1, [Y8Cu3O12]+7 of the superconducting (x=7) phases are compared. Y and Ba layers cause a considerable electron density transfer from the central Cu(1) region. 相似文献