全文获取类型
收费全文 | 2496篇 |
免费 | 61篇 |
国内免费 | 6篇 |
专业分类
化学 | 1556篇 |
晶体学 | 17篇 |
力学 | 29篇 |
数学 | 272篇 |
物理学 | 528篇 |
无线电 | 161篇 |
出版年
2023年 | 21篇 |
2022年 | 31篇 |
2021年 | 23篇 |
2020年 | 16篇 |
2019年 | 25篇 |
2017年 | 19篇 |
2016年 | 43篇 |
2015年 | 42篇 |
2014年 | 47篇 |
2013年 | 100篇 |
2012年 | 105篇 |
2011年 | 147篇 |
2010年 | 77篇 |
2009年 | 69篇 |
2008年 | 134篇 |
2007年 | 143篇 |
2006年 | 132篇 |
2005年 | 134篇 |
2004年 | 101篇 |
2003年 | 108篇 |
2002年 | 76篇 |
2001年 | 39篇 |
2000年 | 38篇 |
1999年 | 40篇 |
1998年 | 29篇 |
1997年 | 45篇 |
1996年 | 54篇 |
1995年 | 37篇 |
1994年 | 32篇 |
1993年 | 35篇 |
1992年 | 34篇 |
1991年 | 34篇 |
1990年 | 35篇 |
1989年 | 28篇 |
1988年 | 26篇 |
1987年 | 21篇 |
1986年 | 24篇 |
1985年 | 47篇 |
1984年 | 21篇 |
1983年 | 25篇 |
1982年 | 23篇 |
1981年 | 22篇 |
1980年 | 20篇 |
1979年 | 22篇 |
1978年 | 25篇 |
1977年 | 20篇 |
1976年 | 21篇 |
1975年 | 26篇 |
1974年 | 25篇 |
1973年 | 20篇 |
排序方式: 共有2563条查询结果,搜索用时 15 毫秒
991.
Mobile Networks and Applications - The advent of cloud computing has been so enormous that, cloud has been seen as the most enduring technology of today’s technology scenario. It is known to... 相似文献
992.
993.
Abachi S Derrick M Kooijman P Musgrave B Price L Repond J Sugano K Blockus D Brabson BB Brom J Jung C Ogren H Rust DR Snyder A Cork B Akerlof C Chapman J Errede D Ken MT Kesten P Meyer DI Neal H Nitz D Thun R Tschirhart R Baringer P Bylsma BG Debonte R Low EH McIlwain RL Miller DH Ng CR Rangan K Shibata E 《Physical review D: Particles and fields》1990,41(7):2045-2056
994.
Kyhn LA Tougaard J Thomas L Duve LR Stenback J Amundin M Desportes G Teilmann J 《The Journal of the Acoustical Society of America》2012,131(1):550-560
Monitoring abundance and population trends of small odontocetes is notoriously difficult and labor intensive. There is a need to develop alternative methods to the traditional visual line transect surveys, especially for low density areas. Here, the prospect of obtaining robust density estimates for porpoises by passive acoustic monitoring (PAM) is demonstrated by combining rigorous application of methods adapted from distance sampling to PAM. Acoustic dataloggers (T-PODs) were deployed in an area where harbor porpoises concurrently were tracked visually. Probability of detection was estimated in a mark-recapture approach, where a visual sighting constituted a "mark" and a simultaneous acoustic detection a "recapture." As a distance could be assigned to each visual observation, a detection function was estimated. Effective detection radius of T-PODs ranged from 22 to 104 m depending on T-POD type, T-POD sensitivity, train classification settings, and snapshot duration. The T-POD density estimates corresponded to the visual densities derived concurrently for the same period. With more dataloggers, located according to a systematic design, density estimates would be obtainable for a larger area. This provides a method suitable for monitoring in areas with densities too low for visual surveys to be practically feasible, e.g., the endangered harbor porpoise population in the Baltic. 相似文献
995.
996.
Robert H. Hunt J.E. Lolck Alan G. Robiette Linda R. Brown Robert A. Toth 《Journal of Molecular Spectroscopy》1982,92(1):246-256
Approximately 500 infrared absorption lines with room-temperature strengths between 3 × 10?5 and 1 × 10?2 atm?1 were assigned to the 2ν2 band of 12CH4 in the region from 2930 to 3250 cm?1. These determine 207 of the 212 upper-state energy levels through J′ = 12 as well as a number of levels with J′ = 13 and 14. All but 17 of the levels with J′ ≤ 12 are calculated to 0.03 cm?1 or better on the basis of a Hamiltonion that contains Coriolis and Fermi interaction terms coupling the upper states of the five bands, 2ν4, ν2 + ν4, ν1, ν3, and 2ν2. 相似文献
997.
998.
Richard J. Morris Ashley J. Mustafa Christopher R. McCrea Linda P. Fowler Christopher Aspaas 《Journal of voice》2007,21(5):568-575
Acoustic differences were evaluated among three choral arrangements and two choral textures recorded at three microphone locations. A choir was recorded when singing two musical selections of different choral texture, one homophonic and one polyphonic. Both musical selections were sung in three choral arrangements: block sectional, sectional-in-columns, and mixed. Microphones were placed at the level of the choristers, the conductor, and the audience. The recordings at each location were analyzed using long-term average spectrum (LTAS). The LTAS from the mixed arrangement exhibited more signal amplitude than the other arrangements in the range of 1000-3500Hz. When considering the musical selections, the chorus produced more signal amplitude in the region of 1800-2200Hz for the homophonic selection. In addition, the LTAS produced by the choir for the homophonic selection varied across the microphone locations. As for the microphone location, the LTAS of the signal detected directly in front of the chorus had a greater slope than the other two locations. Thus, the acoustic signal near the choristers differed from the signals near the conductor and in the audience. Conductors may be using acoustic information from the region of the second and third formants when they decide how to arrange a choir for a particular musical selection. 相似文献
999.
Adriana Predoi-Cross Linda R. Brown M. Brawley-Tremblay 《Journal of Molecular Spectroscopy》2005,232(2):231-246
The Lorentz self-broadening (halfwidths) and self-induced pressure-shift coefficients were measured for the first time in the octad region of methane. All spectra were recorded at 0.011 cm−1 resolution using the McMath-Pierce Fourier transform spectrometer located at the National Solar Observatory on Kitt Peak, Arizona. 12C-enriched CH4 gas samples at room temperature were used, and high signal-to-noise ratios of 2000:1 were achieved. The multispectrum nonlinear least squares fitting technique enabled us to fit simultaneously a total of 10 high-resolution laboratory absorption spectra consisting of six self-broadened and four air-broadened spectra. In this paper, we report the self-broadened widths and self-induced pressure shift coefficients for 1423 transitions belonging to five bands with a maximum J of 16. The numbers of measurements by bands are: 71 for ν2 + 2ν4, 202 for ν1 + ν4, 824 for ν3 + ν4, 58 for 2ν2 + ν4, and 268 for ν2 + ν3. The observed widths varied from 0.045 to about 0.090 cm−1 atm−1 at 296 K. The measured pressure-shift coefficients had values extending from about −0.020 to −0.005 cm−1 atm−1 at 298.3 ± 1.2 K. The results obtained for the broadening coefficients in the various bands were compared with each other and with measurements reported in the literature for other methane bands. 相似文献
1000.
The pure rotational spectrum of the complex Kr-AgCl has been measured between 8-15 GHz using a cavity pulsed-jet Fourier transform microwave spectrometer. The complex was found to be linear and relatively rigid, with a Kr-Ag bond length of approximately 2.641 ?. The Kr-Ag stretching frequency was estimated to be 117 cm(-1). Ab initio calculations performed at the MP2 level of theory gave the geometry, vibration frequencies, Kr-Ag bond dissociation energy, and orbital populations. The Kr-Ag bond dissociation energy was estimated to be approximately 28 kJ mol(-1). The Kr-Ag force constant and dissociation energy are greater than those of Ar-Ag in Ar-AgCl. The chlorine nuclear quadrupole coupling constants show slight changes on complex formation. Ab initio orbital population analysis shows a small shift in sigma-electron density from Kr to Ag on complex formation. The combined experimental and ab initio results are consistent with the presence of a weak Kr-Ag covalent bond. Copyright 2001 Academic Press. 相似文献