全文获取类型
收费全文 | 15604篇 |
免费 | 3029篇 |
国内免费 | 3920篇 |
专业分类
测绘学 | 1240篇 |
大气科学 | 3338篇 |
地球物理 | 4146篇 |
地质学 | 7765篇 |
海洋学 | 2127篇 |
天文学 | 684篇 |
综合类 | 1575篇 |
自然地理 | 1678篇 |
出版年
2024年 | 54篇 |
2023年 | 250篇 |
2022年 | 683篇 |
2021年 | 768篇 |
2020年 | 669篇 |
2019年 | 696篇 |
2018年 | 789篇 |
2017年 | 759篇 |
2016年 | 919篇 |
2015年 | 719篇 |
2014年 | 922篇 |
2013年 | 901篇 |
2012年 | 814篇 |
2011年 | 903篇 |
2010年 | 878篇 |
2009年 | 853篇 |
2008年 | 792篇 |
2007年 | 798篇 |
2006年 | 663篇 |
2005年 | 633篇 |
2004年 | 476篇 |
2003年 | 477篇 |
2002年 | 522篇 |
2001年 | 491篇 |
2000年 | 535篇 |
1999年 | 736篇 |
1998年 | 639篇 |
1997年 | 629篇 |
1996年 | 568篇 |
1995年 | 487篇 |
1994年 | 455篇 |
1993年 | 404篇 |
1992年 | 338篇 |
1991年 | 285篇 |
1990年 | 217篇 |
1989年 | 190篇 |
1988年 | 158篇 |
1987年 | 82篇 |
1986年 | 100篇 |
1985年 | 58篇 |
1984年 | 44篇 |
1983年 | 28篇 |
1982年 | 49篇 |
1981年 | 28篇 |
1980年 | 24篇 |
1979年 | 21篇 |
1978年 | 10篇 |
1977年 | 6篇 |
1962年 | 6篇 |
1958年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
Parametric transduction offers valuable advantages for underwater acoustic communications. Perhaps the most significant benefit is the fact that high directivity is achieved by means of a physically small transmit transducer. This feature may, ultimately, be employed to permit long-range, low-frequency communication using a compact source. The high directivity is desirable to combat multipath propagation and to achieve data communications in water which is shallow by comparison with range. A real-time, high data-rate “model” differential phase shift keying (DPSK) communication system has been constructed and demonstrated. This system uses parametric transduction, with a 300-kHz primary frequency and a 50-kHz secondary frequency. Experimental results show that the system can be employed to combat multipath propagation in shallow water and can achieve high data-rate text and color image transmission at 10 and 20 kb s-1 for 2-DPSK and 4-DPSK, respectively, through a transmission bandwidth of 10 kHz. The “model” system was developed to confirm performance predictions for a future, operational long-range link employing a 50-kHz primary frequency and a 5-kHz secondary frequency 相似文献
12.
Maarten A. Prins Hongbo Zheng Kay Beets Simon Troelstra Patrick Bacon Ilse Kamerling Wouter Wester Martin Konert Xiangtong Huang Wang Ke Jef Vandenberghe 《第四纪科学杂志》2009,24(1):75-84
The Mangshan Plateau is located on the south bank of the Huang He (Yellow River) just west of the city of Zhengzhou, well outside the Loess Plateau in central China. Mixing models of the grain‐size data indicate that the loess deposits are mixtures of three loess components. Comparison of the mixing model with existing models established for a series of loess–palaeosol sequences from the Loess Plateau indicates that the Mangshan loess has been supplied from a proximal dust source, the Huang He floodplain, during major dust outbreaks. The high accumulation rates, the composition of the loess components, and especially the high proportions of a sandy loess component support this. Owing to the exceptionally high accumulation rates, the Mangshan grain size, magnetic susceptibility and carbonate records provide a high‐resolution archive of environmental and climate change. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
13.
地壳对海洋潮汐的响应 总被引:2,自引:2,他引:0
应用三维动态有限元方法研究了中国北部地区的地壳对邻近的渤海与黄海海平面变化的响应。虽然此应力场过于微弱不足以引发地震,但发现应力集中的位置及应力场变化较大的位置恰好与某些现代地震的震中一致。这一结果表明研究地壳对广泛分布的载荷的响应对研究区域地震构造是有帮助的。 相似文献
14.
We use the following numerical model for the collapse stage of a Type II supernova of 15 M⊙. Our electron capture rate includes the effects of the inverse reaction and the neutron-proton mass difference. This decreases the electron density at the collapse stage and led to rather large values of the maximum inward velocity and of the corresponding mass (Umax = 3.06 × 109cm/s, Mmax=0.76 M⊙). These larger values are more favourable for the propagation of shock after the rebounce and the triggering-off of a Type-II supernova explosion. For neutrino transport, we use a leakage model and an equilibrium diffusion model, respectively, for the thin and thick stages and a grey atmosphere model to assess the effect of neutrino precipitation on the collapse. We found this effect to be small, the energy precipitation to be not more than 10?5 the neutrino energy loss and the momentum precipitation not more than 10?6 the gravitational acceleration. 相似文献
15.
16.
Xie Rui-xiang Wang Min Shi Shuo-biao Xu Chun Li Wei-hua Yan Yi-hua 《Chinese Astronomy and Astrophysics》2003,27(4):426-434
Using the decimetric (700–1500 MHz) radio spectrometer and the synchronous observational system with high temporal resolution at four frequencies (1420, 2130, 2840 and 4260 MHz) of Yunnan Observatory, two rare events were observed on 2001 June 24 and 1990 July 30. The former was a small radio burst exhibiting pulsations with short periods (about 29, 40 and 100 ms) in the impulsive phase. The latter was a large radio burst, which at 2840 MHz produced radio pulsations with period of about 30 ms. This paper focuses on pulsations with very short periods in the range of 29–40 ms. The mechanism of generation of such pulsations may be modulation of radio radiation by the periodic trains of whistler packets originating in unstable regions of the corona. Alternatively, these pulsations can be attributed to wave-wave non-linear interactions of electrostatic upper hybrid waves driven by beams of precipitating electrons in flaring loops. 相似文献
17.
18.
同时采用4个台站的国际超导重力仪长期连续重力观测资料和国际地球自转服务中心提供的同步地球自转参数,研究了极移引起的地球重力场变化特征。利用自回归模型估计了各序列的功率谱密度和积谱密度,结果表明极移导致的重力效应的主要能量集中在Chandler摆动和周年项附近,叠积后实际重力观测与极移重力信号理论值之间的差异分别为0.4%和3.9%,说明超导重力仪可有效监测极移导致的重力变化。 相似文献
19.
本文从Zernike多项式出发 ,推导了激光导引星自适应光学倾斜校正的非等晕性方差 ,以其方差 1rad2 为判据 ,导出了倾斜等晕角的表达式 ,得出了与Parenti和Sasiela推导的相同的结果 相似文献
20.