全文获取类型
收费全文 | 60120篇 |
免费 | 8402篇 |
国内免费 | 10954篇 |
专业分类
测绘学 | 9538篇 |
大气科学 | 7022篇 |
地球物理 | 9288篇 |
地质学 | 26568篇 |
海洋学 | 6078篇 |
天文学 | 9010篇 |
综合类 | 4243篇 |
自然地理 | 7729篇 |
出版年
2024年 | 229篇 |
2023年 | 621篇 |
2022年 | 1742篇 |
2021年 | 2122篇 |
2020年 | 2086篇 |
2019年 | 2591篇 |
2018年 | 1804篇 |
2017年 | 2332篇 |
2016年 | 2272篇 |
2015年 | 2521篇 |
2014年 | 3280篇 |
2013年 | 3577篇 |
2012年 | 3588篇 |
2011年 | 3863篇 |
2010年 | 3349篇 |
2009年 | 4164篇 |
2008年 | 4154篇 |
2007年 | 4333篇 |
2006年 | 4174篇 |
2005年 | 3728篇 |
2004年 | 3305篇 |
2003年 | 2966篇 |
2002年 | 2500篇 |
2001年 | 2212篇 |
2000年 | 2121篇 |
1999年 | 1862篇 |
1998年 | 1568篇 |
1997年 | 1092篇 |
1996年 | 926篇 |
1995年 | 816篇 |
1994年 | 775篇 |
1993年 | 692篇 |
1992年 | 475篇 |
1991年 | 402篇 |
1990年 | 283篇 |
1989年 | 234篇 |
1988年 | 190篇 |
1987年 | 103篇 |
1986年 | 76篇 |
1985年 | 68篇 |
1984年 | 41篇 |
1983年 | 29篇 |
1982年 | 35篇 |
1981年 | 19篇 |
1980年 | 27篇 |
1979年 | 11篇 |
1978年 | 25篇 |
1977年 | 30篇 |
1976年 | 8篇 |
1954年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
James Etherington Witold Maciejewski 《Monthly notices of the Royal Astronomical Society》2006,367(3):1003-1010
If supermassive black holes in centres of galaxies form by merging of black hole remnants of massive Population III stars, then there should be a few black holes of mass one or two orders of magnitude smaller than that of the central ones, orbiting around the centre of a typical galaxy. These black holes constitute a weak perturbation in the gravitational potential, which can generate wave phenomena in gas within a disc close to the centre of the galaxy. Here, we show that a single orbiting black hole generates a three-arm spiral pattern in the central gaseous disc. The density excess in the spiral arms in the disc reaches values of 3–12 per cent when the orbiting black hole is about 10 times less massive than the central black hole. Therefore, the observed density pattern in gas can be used as a signature in detecting the most massive orbiting black holes. 相似文献
82.
The characteristics of Asian dust events in Northeast Asia during the springtime from 1993 to 2004 总被引:3,自引:1,他引:2
The characteristics of Asian dust events that occurred in Northeast Asia during the springtime from 1993 to 2004 are investigated using 3-hourly SYNOP reports (World Meteorological Organization). Occurrences of blowing sand and dust storm are low in 1997 and 1999, but have increased rapidly since 2000. The maximum occurrence was recorded in 2002. Wind velocity of 6.5 m s− 1 as a threshold wind velocity is not so exactly consistent with the occurrence of blowing sand. However, wind velocity of 14 m s− 1 as a strong wind causing dust storm had similar tendency to those of dust storm and Dust Storm Index.Source regions of Asian dust are divided into three regions (A: dry arid, B: semi-arid, and C: cultivated), based upon the occurrence of blowing sand and dust storm. Eight meteorological stations are selected in three regions, which have frequent occurrences of blowing sand. Source regions of Asian dust that affect the Korean peninsula are gradually extending eastward. Positive anomalies of NDVI occurred in 1994, 1995, and 1998 when temperature was high and precipitation was heavy. However, the frequent occurrence of the dust phenomena is not always consistent with lots of vegetation, high temperature, and much precipitation in this study. 相似文献
83.
Virtual Huanghe River System: Framework and Technology 总被引:2,自引:0,他引:2
LU Heli LIU Guifang SUN Jiulin 《中国地理科学(英文版)》2006,16(3):255-259
1 Introduction Huanghe (Yellow) River basin is located in 32°–42°N, 96°–119°E. The area of the catchment is more than 752,000km2. The river is 5464km long with a drop in elevation of 4830m. Among the whole area, the moun- tainous and stone area accounts for 29%, loess and hills area 46%, sandy area 11% and plain area 14%, respec- tively. Different natural landscapes exist in this area. The Huanghe River flows through the Loess Plateau, where the soil is eroded seriously (Wang, 2002;… 相似文献
84.
J. C. A. Miller-Jones R. P. Fender E. Nakar 《Monthly notices of the Royal Astronomical Society》2006,367(4):1432-1440
We present a collation of the available data on the opening angles of jets in X-ray binaries, which in most cases are small (≲10°). Under the assumption of no confinement, we calculate the Lorentz factors required to produce such small opening angles via the transverse relativistic Doppler effect. The derived Lorentz factors, which are in most cases lower limits, are found to be large, with a mean >10, comparable to those estimated for active galactic nuclei (AGN) and much higher than the commonly assumed values for X-ray binaries of 2–5. Jet power constraints do not, in most cases, rule out such high Lorentz factors. The upper limits on the opening angles show no evidence for smaller Lorentz factors in the steady jets of Cygnus X-1 and GRS 1915+105. In those sources in which deceleration has been observed (notably XTE J1550−564 and Cygnus X-3), some confinement of the jets must be occurring, and we briefly discuss possible confinement mechanisms. It is however possible that all the jets could be confined, in which case the requirement for high bulk Lorentz factors can be relaxed. 相似文献
85.
86.
87.
88.
89.
PatMoore 《成都信息工程学院学报》2002,17(4):289-299
尽管语法在交际教学法鼎盛时期一度遭到冷遇,但现在又重新被英语语言教学所接纳,作为一种新的语法教学方式,发现原则综合了传统归纳发和演绎法的长处,概述了发现原则的理论依据,指出发现原则旨在鼓励学习者积极参与而非被动接受,符合当前认知主义对二语学习的认识。 相似文献
90.
The results of a photometric monitoring of the quasar 4C 38.41, performed at the optical R and B bands in 2002 February–March, are presented. With a 60/90 cm Schmidt telescope at the Xinglong station of the National Astronomical Observatories of China, we observed the source exhibiting amplitude variations of up to 0.78 mag in both bands during the whole campaign. Intraday and even intranight variations are detected as well. A typical variability time-scale of about 36 d is derived from our 2-month observations at the optical bands, which is identical to that found at a radio wavelength of 92 cm, suggesting a common origin for the variations in 4C 38.41 from optical to radio bands. 相似文献