全文获取类型
收费全文 | 228994篇 |
免费 | 2362篇 |
国内免费 | 723篇 |
专业分类
化学 | 110100篇 |
晶体学 | 3300篇 |
力学 | 7869篇 |
综合类 | 4篇 |
数学 | 18858篇 |
物理学 | 60746篇 |
无线电 | 31202篇 |
出版年
2016年 | 2597篇 |
2015年 | 1913篇 |
2014年 | 2755篇 |
2013年 | 8476篇 |
2012年 | 5803篇 |
2011年 | 7194篇 |
2010年 | 5088篇 |
2009年 | 5305篇 |
2008年 | 7206篇 |
2007年 | 7704篇 |
2006年 | 7512篇 |
2005年 | 6911篇 |
2004年 | 6357篇 |
2003年 | 5705篇 |
2002年 | 5603篇 |
2001年 | 7097篇 |
2000年 | 5612篇 |
1999年 | 4626篇 |
1998年 | 3850篇 |
1997年 | 3880篇 |
1996年 | 3761篇 |
1995年 | 3505篇 |
1994年 | 3379篇 |
1993年 | 3241篇 |
1992年 | 3744篇 |
1991年 | 3630篇 |
1990年 | 3433篇 |
1989年 | 3469篇 |
1988年 | 3303篇 |
1987年 | 2947篇 |
1986年 | 2762篇 |
1985年 | 3646篇 |
1984年 | 3665篇 |
1983年 | 3082篇 |
1982年 | 3227篇 |
1981年 | 3165篇 |
1980年 | 3022篇 |
1979年 | 3138篇 |
1978年 | 3375篇 |
1977年 | 3154篇 |
1976年 | 3118篇 |
1975年 | 2934篇 |
1974年 | 2878篇 |
1973年 | 2915篇 |
1972年 | 1900篇 |
1971年 | 1613篇 |
1968年 | 2038篇 |
1967年 | 2215篇 |
1966年 | 2017篇 |
1965年 | 1580篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
91.
It is often assumed that blind separation of dynamically mixed sources cannot be done with second-order statistics. It is shown that separation of dynamically mixed sources indeed can be performed using second-order statistics only. A criterion based on second-order statistics for the purpose of separating crosswise mixtures is stated. The criterion is used in order to derive a gradient-based separation algorithm, as well as a Newton-type separation algorithm. The uniqueness of the solution representing the separation is also investigated. This reveals that (1) the channel system is parameter identifiable under weak conditions, and (2) if the sources have the same color, there exists at most two solutions. The local convergence behavior of the proposed algorithm is studied and reveals a sufficient condition for local convergence. Furthermore, the estimates of the channel system are shown to be consistent or to locally minimize the criterion 相似文献
92.
P. F. Miller H. A. Gebbie 《Journal of Infrared, Millimeter and Terahertz Waves》1996,17(10):1573-1591
Measurements made in an untuned resonator by Fourier Transform Spectroscopy of atmospheric aerosols are explained by a pumped phonon model due to Fröhlich. 相似文献
93.
H. Grübmeier A. Naoumidis G. Stochniol A. Tsoga 《Fresenius' Journal of Analytical Chemistry》1995,353(3-4):393-398
Chemical interactions at the phase boundaries of materials applied for the solid oxide fuel cell (SOFC) have been studied by EPMA. The chemical reactivity at the interface of Lay-xSrxMnO3/ZrO2-Y2O3 is dependent on the stoichiometry (y) and the Sr content (x) of the perovskite. Typical reaction products (zirconates) and a diffusion zone in the ZrO2–Y2O3 have been observed. The extension of cation release (Mn) is related to the increasing chemical activity of Mn oxide in the perovskite by the Sr substitution for La. The wettability of the metal/oxide interface in the anode cermet (Ni/ZrO2–Y2O3) has been found to be influenced by chemical reactions resulting from the applied reducing atmosphere with high carbon activity. The disintegration of ZrO2–Y2O3 in contact with molten Ni or Ni-Ti and Ni-Cr alloys leads to the redeposition of Y2O3-enriched oxides and also to Zr-rich intermetallic compounds and eutectics. 相似文献
94.
The authors have implemented chirped distributed feedback (DFB) grating lasers with phase shifts [λ/8, 2λ/8 and 3λ/8] distributed continuously over various axial distances, using their recently proposed method for producing bent optical waveguides. The singlemode stability of the lasers with distributed phase shifts was found to be considerably higher than for lasers with abrupt phase shifts 相似文献
95.
Tsuji M. Shigesawa H. Frezza F. Lampariello P. Oliner A.A. 《Antennas and Propagation, IEEE Transactions on》1998,46(7):1047-1055
For pt.II ibid., vol.46, no.7, p.1042-46. The article describes the measurements that verify the validity of the theoretical expressions for this new type of antenna. Measurements were made both at millimeter wavelengths (40 to 60 GHz) and in the X-band (8 to 12 GHz) frequency range and of two different quantities: the propagation wavenumbers β and α and radiation patterns. Comparisons were made with numerical values obtained from the theoretical expressions derived in parts I and II and very good agreement was found in all cases 相似文献
96.
Hartung J. Jacquin A. Pawlyk J. Rosenberg J. Okada H. Crouch P.E. 《Selected Areas in Communications, IEEE Journal on》1998,16(1):42-55
This paper describes a prototype video coding platform meant for the conception and testing of multimedia products such as next-generation videophones. The platform is largely based on ITU-T Recommendation H.263, with a number of additional object-oriented quality enhancement features which make it especially well suited for very low bit-rate coding of “head-and-shoulders” video material typical of real-time multimedia applications, video teleconferencing, and video telephony. These features consist of: (1) segmentation into objects of interest, (2) segmentation-based prefiltering, (3) model-assisted rate control, (4) adaptive vector quantization, and finally (5) segmentation-based postfiltering. In the spirit of Recommendation H.263, these enhancements are modular and can be selectively turned on or off, thereby enabling a wide variety of coding modes 相似文献
97.
M. S. Han S. R. Hahn H. C. Kwon Y. Bin T. W. Kang J. H. Leem Y. B. Hou H. C. Jeon J. K. Hyun Y. T. Jeoung H. K. Kim J. M. Kim T. W. Kim 《Journal of Electronic Materials》1998,27(6):680-683
Double-crystal x-ray rocking curve (DCRC) and secondary-ion mass-spectroscopy (SIMS) measurements have been performed to investigate the effect of rapid thermal annealing on the interdiffusion behavior of Hg in HgTe/CdTe superlattices grown on Cd0.96Zn0.04Te (211)B substrates by molecular beam epitaxy. The sharp satellite peaks of the DCRC measurements on a 100-period HgTe/CdTe (100Å/100Å) superlattice show a periodic arrangement of the superlattice with high-quality interfaces. The negative direction of the entropy change obtained from the diffusion coefficients as a function of the reciprocal of the temperature after RTA indicates that the Hg diffusion for the annealed HgTe/CdTe superlattice is caused by an interstitial mechanism. The Cd and the Hg concentration profiles near the annealed HgTe/CdTe superlattice interfaces, as measured by SIMS, show a nonlinear behavior for Hg, originating from the interstitial diffusion mechanism of the Hg composition. These results indicate that a nonlinear interdiffusion behavior is dominant for HgTe/CdTe superlattices annealed at 190°C and that the rectangular shape of HgTe/CdTe superlattices may change to a parabolic shape because of the intermixing of Hg and Cd due to the thermal treatment. 相似文献
98.
Buffer analysis has traditionally relied heavily on the use of traffic models with short range (exponentially decaying) autocorrelation, e.g. Poisson and Markov modulated Poisson processes. Recent literature has suggested the presence of traffic with slowly (hyperbolically) decaying autocorrelation functions; these sources collectively and individually seem to be exhibiting the phenomenon of long range dependence (LRD). Furthermore, it has been claimed that this sort of traffic may exhibit behaviour, when buffered, that is advantageous in that a smaller buffer may be required than would be the case with non-LRD traffic. This phenomenon has been named the crossover effect, and the authors quantify its relevance to ATM networking, using simulation studies of a homogenous mix of chaotic sources. These show that there is a crossover effect, but the key finding is that this effect is much nearer to a buffer length of zero than was predicted by previous theoretical studies, actually occurring at a value so low that it has no practical significance 相似文献
99.
A. J. Kalkman H. P. M. Pellemans T. O. Klaassen W. Th. Wenckebach 《Journal of Infrared, Millimeter and Terahertz Waves》1996,17(3):569-577
The low temperature lifetime of electrons excited in the 2p?1 donor level of n-GaAs has been studied in a far-infrared pump-probe experiment. The measurement has been carried out using a pulsed far-infrared molecular gas laser working at a wavelength of 292µm, with the sample in a magnetic field of 5.1 T, resonant with the 1so?2p?1 transition. Two FIR pulses are sliced from one FIR-laser pulse by means of optical switching techniques using two Q-switched Nd:YAG lasers. The first pulse is used to saturate the transition, while the second pulse probes the return of the population in the excited state towards thermal equilibrium as a function of the time delay after the excitation pulse. The value of 350±50 ns found for the lifetime falls in line with CW saturation results on materials with other doping concentrations. 相似文献
100.
H.-J. Sue E. I. Garcia-Meitin N. A. Orchard 《Journal of Polymer Science.Polymer Physics》1993,31(5):595-608
The fracture behavior of a core-shell rubber (CSR) modified epoxy is investigated using both fracture mechanics and microscopy tools. The CSR-modified epoxy is found to be toughened via numerous line-array cavitations of the CSR particles, followed by plastic flow of the epoxy matrix. The toughening effect via the above craze-like damage process is found to be as effective as that of the well-known widespread rubber cavitation/matrix shear yielding mechanisms. The conditions for triggering the craze-like damage appear to be both stress state and rubber concentration dependent. The type of rubber tougheners utilized also plays a critical role in triggering this rather unusual craze-like damage in epoxy systems. © 1993 John Wiley & Sons, Inc. 相似文献