全文获取类型
收费全文 | 4503篇 |
免费 | 842篇 |
国内免费 | 1197篇 |
专业分类
测绘学 | 101篇 |
大气科学 | 986篇 |
地球物理 | 1378篇 |
地质学 | 1969篇 |
海洋学 | 859篇 |
天文学 | 491篇 |
综合类 | 199篇 |
自然地理 | 559篇 |
出版年
2024年 | 19篇 |
2023年 | 71篇 |
2022年 | 150篇 |
2021年 | 167篇 |
2020年 | 209篇 |
2019年 | 219篇 |
2018年 | 173篇 |
2017年 | 199篇 |
2016年 | 229篇 |
2015年 | 265篇 |
2014年 | 332篇 |
2013年 | 316篇 |
2012年 | 262篇 |
2011年 | 325篇 |
2010年 | 274篇 |
2009年 | 314篇 |
2008年 | 285篇 |
2007年 | 328篇 |
2006年 | 301篇 |
2005年 | 275篇 |
2004年 | 221篇 |
2003年 | 183篇 |
2002年 | 193篇 |
2001年 | 145篇 |
2000年 | 146篇 |
1999年 | 129篇 |
1998年 | 111篇 |
1997年 | 97篇 |
1996年 | 100篇 |
1995年 | 86篇 |
1994年 | 78篇 |
1993年 | 57篇 |
1992年 | 57篇 |
1991年 | 53篇 |
1990年 | 31篇 |
1989年 | 32篇 |
1988年 | 21篇 |
1987年 | 10篇 |
1986年 | 15篇 |
1985年 | 12篇 |
1984年 | 10篇 |
1983年 | 11篇 |
1982年 | 9篇 |
1981年 | 8篇 |
1980年 | 1篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1973年 | 1篇 |
1954年 | 3篇 |
排序方式: 共有6542条查询结果,搜索用时 15 毫秒
21.
活动星系核(AGN)是宇宙中最奇特的天体之一。它是真正意义上的全波天体,其中X射线波段的发射功率占到全波段功率的50%左右。AGN的X射线辐射研究涉及天体物理中的最基本问题,例如能量产生、辐射机制和宇宙论等,而Chandra X射线卫星的高分辨率图像和光谱对这一研究有着重要作用。以Chandra卫星的部分观测结果为例,简要介绍了几类不同类型AGN的X射线辐射研究进展:(1)宽吸收线类星体APM08279+5255(z=3.91)的X射线谱分析,以及高红移类星体的观测概况;(2)Seyfert星系NGC 4151延展X射线发射问题的解决,及NGC 1068X射线辐射与光学波段的高激发态发射线([OⅢ]λ5007)有很强相关性的发现;(3)6个BLLac天体样本的X射线环境分析;(4)射电星系X射线喷流的观测等。 相似文献
22.
The energy budget ofBellamya earuginosa in a shallow algal lake, Houhu Lake (Wuhan, China) was investigated by the measurement of flesh production (32.8 kJ/(m2·a)), egestion (337.7 kJ/(m2·a)), metabolism (246.7 kJ/(m2·a)), and estimation of excretion (21.4kJ/(m2·a)). The net growth efficiency of the species is about 10.9%, which accords with the generally reported value for gastropods.
In addition, the relationships between starvation respiration (R, mgO2/(Ind·d)), body weight (Wd, mg in dry wt) and temperature (T, °C) were also determined. The regression equationR=0.044Wd
0.537
e
0.061T
was obtained by the least square method, The measured SDA of the species is 26.51% of its gross metabolism.
Project No. 3960019 and 39430101 supported by NSFC and also a granted for the East Lake Ecological Experimental Station, CAS,
Wuhan. 相似文献
23.
D. Prandle 《Estuarine, Coastal and Shelf Science》1985,20(5):615-635
An examination is made of the circulation in narrow estuaries subject to a predominant tidal forcing. Velocity structures are derived separately for residual flow components associated with (a) river flow, (b) wind stress, (c) a well-mixed longitudinal density gradient and (d) a fully stratified saline wedge. Dimensionless parameters are introduced to indicate the magnitude of each component and these parameters are evaluated for 9 major estuaries, thereby revealing their sensitivity to each component.For a channel of constant breadth and depth, formulae are deduced for the length of saline intrusion, L. Comparisons with observed data show that such formulae may be used with confidence to predict changes in L arising from variations in river flow, tidal range or channel depths.The level of stratification is shown to be related to a product of two parameters, one associated with velocity structure and a second involving the square of the ‘flow ratio’ (i.e. residual velocity/amplitude of the tidal velocity). This relationship provides a simple classification system for estuarine stratification which can be used to indicate the sensitivity of any particular estuary to changing conditions. 相似文献
24.
Wang Yanhui 《水文研究》1992,6(2):241-251
Black locust (Robina pseudoacacia) has become one of the most important shelter species in the loess area of northwest China. This paper summarizes recent research concerning its hydrological influence, including canopy interception, litter absorption capacity, its effect on rainfall kinetic energy, infiltration rates, surface runoff, soil moisture, and evapotranspiration, and its role in soil conservation. Several predictive models are listed. on the basis of existing results, optimum characteristics for an effective plantation are defined, and problems requiring further research are identified. 相似文献
25.
G. Palladino A. Basili G. Di Cocco T. Franceschini G. Landini S. Silvestri A. Barbini M. Galimberti L. A. Gizzi 《Experimental Astronomy》2006,21(3):169-187
This paper describes the design of a star sensor based upon a high dynamic range CCD in order to reach an arcsec-level attitude
determination in balloon-borne missions. A custom star identification software was developed and laboratory-tested on a prototype
assembled using commercial components. A set of numerical simulations have been carried out to study the dependence on the
pointing precision of the centroid position accuracy, the number of detected stars and the effect of the image focusing. Moreover,
the role of the electronic noise and the discrete pixel structure on the light signals is identified by the analysis of numerical
simulations. Laboratory tests confirm that the arcsec pointing accuracy with a 1 Hz update rate can be achieved with our combination
of custom-developed software and selected hardware components. 相似文献
26.
27.
M. G. Revnivtsev 《Astronomy Letters》2003,29(6):383-386
Analysis of the RXTE slew data in October 1996 revealed a weak X-ray burst from the millisecond pulsar SAX J 1808.4-3658. The 3–20-keV energy spectrum of the source can be described by a power law with an index of 2.0 and a(3-to 20 keV) luminosity of ~1.4×1035 erg s?1 (the distance to the source was taken to be 2.5 kpc). Because of the short exposure time, we failed to detect weak pulsations at a frequency of 401 Hz in the source. The (2σ) upper limit of the pulse fraction is ~13%. 相似文献
28.
29.
30.