全文获取类型
收费全文 | 326344篇 |
免费 | 2855篇 |
国内免费 | 863篇 |
专业分类
化学 | 153547篇 |
晶体学 | 4122篇 |
力学 | 12124篇 |
综合类 | 10篇 |
数学 | 28125篇 |
物理学 | 87772篇 |
无线电 | 44362篇 |
出版年
2020年 | 2397篇 |
2019年 | 2493篇 |
2018年 | 3198篇 |
2017年 | 3185篇 |
2016年 | 5064篇 |
2015年 | 3114篇 |
2014年 | 4893篇 |
2013年 | 12473篇 |
2012年 | 9644篇 |
2011年 | 11976篇 |
2010年 | 8642篇 |
2009年 | 9214篇 |
2008年 | 12234篇 |
2007年 | 12546篇 |
2006年 | 11984篇 |
2005年 | 11202篇 |
2004年 | 10194篇 |
2003年 | 9262篇 |
2002年 | 9103篇 |
2001年 | 11011篇 |
2000年 | 8581篇 |
1999年 | 6871篇 |
1998年 | 5667篇 |
1997年 | 5609篇 |
1996年 | 5212篇 |
1995年 | 4987篇 |
1994年 | 4921篇 |
1993年 | 4547篇 |
1992年 | 5261篇 |
1991年 | 5427篇 |
1990年 | 5005篇 |
1989年 | 4881篇 |
1988年 | 4660篇 |
1987年 | 4099篇 |
1986年 | 3863篇 |
1985年 | 4851篇 |
1984年 | 4907篇 |
1983年 | 4051篇 |
1982年 | 4143篇 |
1981年 | 3915篇 |
1980年 | 3834篇 |
1979年 | 4141篇 |
1978年 | 4355篇 |
1977年 | 4145篇 |
1976年 | 4144篇 |
1975年 | 3882篇 |
1974年 | 3950篇 |
1973年 | 3933篇 |
1972年 | 2660篇 |
1971年 | 2255篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Sugawara F. Aoki K. Yamaguchi H. Sasaki K. Sasaki T. Fujisaki H. 《Electron Device Letters, IEEE》1997,18(10):483-485
A new lateral MOS-gated thyristor, called the Base-Current-Controlled Thyristor, is described. This device is designed so that most holes at the on-stage reach the P base through the floating P+ region adjacent to the P base and the on-state MOSFET. At the turn-off stage, the interruption of the hole current to the P base due to switching off the above MOSFET occurs simultaneously with the conventional turn-off operation. The concept of this device is verified experimentally by using the fabricated lateral device with the external MOSFET. This device exhibits a better trade-off relation between the on-state voltage and the turn-off time compared uith the conventional MOS-gated thyristor 相似文献
72.
Sadka A.H. Eryurthlu F. Kondoz A.M. 《Vision, Image and Signal Processing, IEE Proceedings -》1997,144(6):369-376
An algorithm is presented to enhance the resilience of block-based video-coding algorithms against channel errors. The error-resilience algorithm described suggests the use of fixed length coding to alleviate the effect of Huffman coding on the synchronisation of the decode under erroneous conditions. The proposed mechanism modifies the order of transmission of the fixed-length video parameters to increase the chance of their arrival. Synchronisation words are sent at fixed-length intervals within the bit stream to reduce the possibility of the decoder falling on similar bit patterns within the bit stream. To limit the effect of error accumulation, the motion prediction process is halted and the differential coding of motion vectors is not applied. FEC techniques are applied on some error-sensitive segments of the reordered video bitstream. The effectiveness of the proposed error-resilience algorithm is evidenced by both subjective and objective results 相似文献
73.
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
74.
Ya. V. Fattakhov M. F. Galyautdinov T. N. L’vova I. B. Khaibullin 《Technical Physics》1997,42(12):1457-1459
The first results regarding the formation of a two-dimensional periodic structure of local melting regions on a silicon surface
upon pulsed light irradiation are presented. The conditions are established, and the mechanism of the formation of such structures
is discussed.
Zh. Tekh. Fiz. 67, 97–99 (December 1997) 相似文献
75.
Cresswell M.W. Allen R.A. Guthrie W.F. Sniegowski J.J. Ghoshtagore R.N. Linholm L.W. 《Semiconductor Manufacturing, IEEE Transactions on》1998,11(2):182-193
The physical widths of reference features incorporated into electrical linewidth test structures patterned in films of monocrystalline silicon have been determined from Kelvin voltage measurements. The films in which the test structures are patterned are electrically insulated from the bulk-silicon substrate by a layer of silicon dioxide provided by SIMOX (Separation by the IMplantation of OXygen) processing. The motivation is to facilitate the development of linewidth reference materials for critical-dimension (CD) metrology-instrument calibration. The selection of the (110) orientation of the starting silicon and the orientation of the structures' features relative to the crystal lattice enable a lattice-plane-selective etch to generate reference-feature properties of rectangular cross section and atomically planar sidewalls. These properties are highly desirable for CD applications in which feature widths are certified with nanometer-level uncertainty for use by a diverse range of CD instruments. End applications include the development and calibration of new generations of CD instruments directed at controlling processes for manufacturing devices having sub-quarter-micrometer features 相似文献
76.
Analysis of low bit rate image transform coding 总被引:8,自引:0,他引:8
Calculations based on high-resolution quantizations prove that the distortion rate D(R¯) of an image transform coding is proportional to 2-2R when R¯ is large enough. In wavelet and block cosine bases, we show that if R¯<1 bit/pixel, then D(R¯) varies like R¯1-2γ, where γ remains of the order of 1 for most natural images. The improved performance of embedded codings in wavelet bases is analyzed. At low bit rates, we show that the compression performance of an orthonormal basis depends mostly on its ability to approximate images with a few nonzero vectors 相似文献
77.
F. G. Vagizov 《Hyperfine Interactions》1995,95(1):85-95
We present the results of an experimental investigation of a RF splitting of57Fe hyperfine lines in the regime of NMR and Mössbauer double resonance. The experiments have been performed as a function of RF field intensity and static magnetic field magnitude. The intensity of the RF components and the separation between them are extremely sensitive to the frequency and amplitude of the RF magnetic field. The RF splitting of hyperfine lines is inversely proportional to the strength of the static magnetic field. 相似文献
78.
Two new versions of component relevancy for multistate structure functions are introduced. They are compared with some existing component-relevance conditions and their general properties are investigated. Based on the two relevance conditions, two component-importance measures for multistate systems are defined; they are most appropriate for comparing components when a certain type of system improvement is considered. Series and parallel structures are characterized within the L -superadditive and L -subadditive structure functions by imposing the two new relevance conditions 相似文献
79.
Tajiri A. Minakuchi K. Komeda K. Bessho Y. Inoue Y. Yodoshi K. Yamaguchi T. 《Electronics letters》1993,29(4):369-370
A highly reliable 200 mW AlGaAs laser diode with a fundamental transverse mode has been developed, by optimising its structure with a 0.8 mu m thick p-cladding layer, a 1200 mu m long cavity length, and a front facet coating with a low reflectivity of 2%. The maximum output power was 500 mW, and stable fundamental transverse mode operation was obtained up to 350 mW. Stable operation under 200 mW and 50 degrees C was confirmed for more than 1200 h. Optical feedback noise was below 3*10/sup -14/ Hz/sup -1/.<> 相似文献
80.