全文获取类型
收费全文 | 676670篇 |
免费 | 7057篇 |
国内免费 | 2189篇 |
专业分类
化学 | 307117篇 |
晶体学 | 8841篇 |
力学 | 26233篇 |
综合类 | 14篇 |
数学 | 71492篇 |
物理学 | 180974篇 |
无线电 | 91245篇 |
出版年
2021年 | 5084篇 |
2020年 | 5563篇 |
2019年 | 6045篇 |
2018年 | 7484篇 |
2017年 | 7491篇 |
2016年 | 11468篇 |
2015年 | 7198篇 |
2014年 | 11455篇 |
2013年 | 29133篇 |
2012年 | 21963篇 |
2011年 | 26693篇 |
2010年 | 19026篇 |
2009年 | 19923篇 |
2008年 | 26137篇 |
2007年 | 27258篇 |
2006年 | 25719篇 |
2005年 | 23195篇 |
2004年 | 21498篇 |
2003年 | 19276篇 |
2002年 | 19053篇 |
2001年 | 21161篇 |
2000年 | 16827篇 |
1999年 | 13781篇 |
1998年 | 11868篇 |
1997年 | 11564篇 |
1996年 | 11117篇 |
1995年 | 10144篇 |
1994年 | 9822篇 |
1993年 | 9572篇 |
1992年 | 10261篇 |
1991年 | 10293篇 |
1990年 | 9706篇 |
1989年 | 9203篇 |
1988年 | 9079篇 |
1987年 | 8084篇 |
1986年 | 7697篇 |
1985年 | 10200篇 |
1984年 | 10310篇 |
1983年 | 8540篇 |
1982年 | 8847篇 |
1981年 | 8575篇 |
1980年 | 8266篇 |
1979年 | 8529篇 |
1978年 | 8592篇 |
1977年 | 8502篇 |
1976年 | 8425篇 |
1975年 | 8146篇 |
1974年 | 7880篇 |
1973年 | 8069篇 |
1972年 | 5276篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Kawai S. Iwatsuki K. Suzuki K. Nishi S. Saruwatari M. Sato K. Wakita K. 《Electronics letters》1994,30(3):251-252
The authors demonstrate 10 Gbit/s optical soliton transmission in a recirculating loop through the use of a monolithically integrated MQW-DFB-LD/MQW-EA modulator light source. The timing jitter due to the Gordon-Haus effect is successfully reduced by the optical bandpass filters conventionally used in the cascaded EDFAs to avoid the accumulation of amplified spontaneous emission. The transmission distance of 7200 km is achieved at a bit error rate of 10-9 相似文献
42.
43.
We consider the growth of a spherical crystal in a supersaturatedsolution. In the first part, existence and uniqueness resultsfor radially symmetric growth are obtained, provided that thesupersaturation is not too large; conversely, when the far-fieldsupersaturation exceeds a critical value, it is shown that theradially symmetric solution ceases to exist in finite time.In the second part, we examine the linear stability of a radiallysymmetric similarity solution (in which the radius grows ast?) to shape perturbations. The results are compared with previousquasi-static analyses, and, in particular, the critical radiusat which the crystal becomes unstable is found to be largerfor small supersaturations, but smaller for large supersaturations,than those predicted by the quasi-static analysis 相似文献
44.
45.
A narrow linewidth has been obtained in a new 1.5 ?m distributed feedback laser integrated monolithically with a tunable external cavity. The linewidth of 18 MHz has been achieved by tuning the current in the external cavity, and the FM response was flat from 100 kHz to 500 MHz. 相似文献
46.
R. J. Hinde 《Few-Body Systems》2006,38(2-4):187-191
We compute the vibrational coupling between two H2 molecules from ab initio quantum chemical calculations of the H2-H2 potential carried out at the full configuration interaction level of theory using the atom-centered aug-cc-pVTZ basis set
for hydrogen. We compare the full configuration interaction results with those obtained using two variants of coupled cluster
theory and find that a fully iterative treatment of connected triples may be required to estimate the H2-H2 vibrational coupling accurately using coupled cluster theory. 相似文献
47.
Antoniades N. Boskovic A. Tomkos I. Madamopoulos N. Lee M. Roudas I. Pastel D. Sharma M. Yadlowsky M.J. 《Selected Areas in Communications, IEEE Journal on》2002,20(1):149-165
This paper demonstrates the use of computer simulation for topological design and performance engineering of transparent wavelength-division multiplexing metropolitan-area networks. Engineering of these networks involves the study of various transport-layer impairments such as amplifier noise, component ripple, chirp/dispersion, optical crosstalk, waveform distortion due to filter concatenation, fiber nonlinearities, and polarization effects. A computer simulation methodology composed of three main simulation steps is derived and implemented. This methodology obtains performance estimations by applying efficient wavelength-domain simulations on the entire network topology, followed by time-/frequency-domain simulations on selected paths of the network and finally Q-budgeting on an identified worst case path. The above technique provides an efficient tool for topological design and network performance engineering. Accurate simulation models are presented for each of the performance impairments, and the computer simulation methodology is used for the design and engineering of a number of actual metro network architectures 相似文献
48.
Buttari D. Chini A. Meneghesso G. Zanoni E. Moran B. Heikman S. Zhang N.Q. Shen L. Coffie R. DenBaars S.P. Mishra U.K. 《Electron Device Letters, IEEE》2002,23(2):76-78
Pre-metal-deposition reactive ion etching (RIE) was performed on an Al0.3Ga0.7N/AlN/GaN heterostructure in order to improve the metal-to-semiconductor contact resistance. An optimum AlGaN thickness for minimizing contact resistance was determined. An initial decrease in contact resistance with etching time was explained in terms of removal of an oxide surface layer and/or by an increase in tunnelling current with the decrease of the AlGaN thickness. The presence of a dissimilar surface layer was confirmed by an initial nonuniform etch depth rate. An increase in contact resistance for deeper etches was experienced. The increase was related to depletion of the two-dimensional (2-D) electron gas (2-DEG) under the ohmics. Etch depths were measured by atomic force microscopy (AFM). The contact resistance decreased from about 0.45 Ωmm for unetched ohmics to a minimum of 0.27 Ωmm for 70 Å etched ohmics. The initial thickness of the AlGaN layer was 250 Å. The decrease in contact resistance, without excessive complications on device processing, supports RIE etching as a viable solution to improve ohmic contact resistance in AlGaN/GaN HEMTs 相似文献
49.
In this work, a quantitative analysis is applied to resolve the newly reported polarity-dependent charge-to-breakdown (Q/sub BD/) data from thick oxides of 6.8 nm down to ultrathin oxides of 1.9 nm. Three independent sets of Q/sub BD/ data, i.e., n/sup +/poly/NFET stressed under inversion and accumulation, and p/sup +/ poly/PFET under accumulation are carefully investigated. The Q/sub BD/ degradation observed for p-type anodes, either poly-Si or Si-substrate, can be nicely understood with the framework of maximum energy released by injected electrons. Thus, this work provides a universal and quantitative account for a variety of experimental observations in the time-to-breakdown (T/sub BD/) and Q/sub BD/ polarity-dependence of oxide breakdown. 相似文献
50.
Losada M.A. Garces I. Mateo J. Salinas I. Lou J. Zubia J. 《Lightwave Technology, Journal of》2002,20(7):1160-1164
We have studied the optical power losses due to multiple curvatures in polymethylmetacrylate (PMMA) plastic optical fibers (POFs) of different numerical apertures (NAs) and attenuation. The fibers were tested for several configurations in order to assess the influence of different types of curved-to-straight fiber transitions in the amount of power radiation. We found that losses are below the standards for all tested fiber types, and thus, they are a suitable choice for local area network (LAN) applications. In addition, our results revealed the presence of modal interactions as confirmed using an experimental procedure to estimate the mode coupling strength for the same fibers. 相似文献