全文获取类型
收费全文 | 63190篇 |
免费 | 782篇 |
国内免费 | 1201篇 |
专业分类
测绘学 | 2287篇 |
大气科学 | 4795篇 |
地球物理 | 12100篇 |
地质学 | 25868篇 |
海洋学 | 4558篇 |
天文学 | 9968篇 |
综合类 | 2256篇 |
自然地理 | 3341篇 |
出版年
2022年 | 274篇 |
2021年 | 439篇 |
2020年 | 467篇 |
2019年 | 503篇 |
2018年 | 5764篇 |
2017年 | 4955篇 |
2016年 | 3727篇 |
2015年 | 856篇 |
2014年 | 1233篇 |
2013年 | 2114篇 |
2012年 | 2244篇 |
2011年 | 4286篇 |
2010年 | 3455篇 |
2009年 | 4111篇 |
2008年 | 3481篇 |
2007年 | 3919篇 |
2006年 | 1636篇 |
2005年 | 1311篇 |
2004年 | 1508篇 |
2003年 | 1412篇 |
2002年 | 1247篇 |
2001年 | 973篇 |
2000年 | 970篇 |
1999年 | 702篇 |
1998年 | 702篇 |
1997年 | 746篇 |
1996年 | 567篇 |
1995年 | 590篇 |
1994年 | 574篇 |
1993年 | 472篇 |
1992年 | 448篇 |
1991年 | 452篇 |
1990年 | 458篇 |
1989年 | 406篇 |
1988年 | 417篇 |
1987年 | 459篇 |
1986年 | 399篇 |
1985年 | 523篇 |
1984年 | 501篇 |
1983年 | 547篇 |
1982年 | 494篇 |
1981年 | 460篇 |
1980年 | 503篇 |
1979年 | 408篇 |
1978年 | 385篇 |
1977年 | 369篇 |
1976年 | 328篇 |
1975年 | 325篇 |
1974年 | 332篇 |
1973年 | 318篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
221.
222.
MAO Duan-qian ZHANG Jie BAO Hao-sheng 《中国地理科学(英文版)》2005,15(3):275-280
1IN T R O D U C T IO NAn image isa setofbelief,s ideasand impressionsthatapersonholdsaboutan object(KOTLER,1991).Similar-lydefinitionistheimage of a destinationas thesum ofbelief,sideasand impressionsthatpeoplehave ofa placeor destination(CROMPTON,1979;KO… 相似文献
223.
224.
Bjerrum-Niese C. Bjorno L. Pinto M.A. Quellec B. 《Oceanic Engineering, IEEE Journal of》1996,21(2):143-149
The paper discusses the development of a simulation tool to model high data-rate acoustic communication in shallow water. The simulation tool is able to generate synthetic time series of signals received at a transducer array after transmission across a shallow-water communication channel. The simulation tool is suitable for testing advanced signal processing techniques for message recovery. A channel model has been developed based on the physical aspects of the acoustic channel. Special emphasis has been given to fluctuations of the signal transmission caused by time-varying multipath effects. At shorter ranges, the temporal variations are dominated by acoustic scattering from the moving sea surface. Therefore, the channel model produces a coherence function which may be interpreted as a time-varying reflection coefficient for the surface scattered acoustical path. A static, range-independent ray model identifies the significant multipaths, and the surface path is modulated with the time-varying reflection coefficient. The advantages and limitations of the channel model are discussed and assumptions necessary to overcome the limitations are emphasised. Based on the assumptions, an algorithm has been developed and implemented to model how a binary message will be modulated when transmitted by a transducer, is distorted in the channel and finally is received by a transducer array 相似文献
225.
This study on the distribution features of petroleum hydrocarbon in water and sedi-ment off the Fujian shore using data obtained from the baseline research on oceanic contamination in Fujian showed that:during the research preiod,petroleum hydrocarbon concentrations in water varied from 5.77μg/L to 37.28μg/L,averaged 14.48 μg/L;was lower in the wet seasson than in the dry season;and was highest in the Minjiang Estuary and Jiulong Estuary in both seasons ,The petorleum hydrocarbon concentrations in shore sediment varied from 14.48 mg/kg to 784.36 mg/kg,averaged 133.3 mg/kg ,and was closely related to sediment types (granularity). 相似文献
226.
Is Precipitation the Dominant Controlling Factor of High Inorganic Nitrogen Content in the Changjiang River and Its Mouth? 总被引:1,自引:1,他引:0
沈志良 《中国海洋湖沼学报》2003,21(4):368-376
The main reasons for the high content of inorganic N and its increase by several times in the Changjiang River and its mouth during the last 40 years were analysed in this work. The inorganic N in precipitation in the Changjiang River catchment mainly comes from gaseous loss of fertilizer N, N resulting from the increases of population and livestock, and from high temperature combustions of fossil fuels. N from precipitation is the first N source in the Changjiang River water and the only direct cause of high content of inorganic N in the Changjiang River and its mouth. The lost N in gaseous form and from agriculture non-point sources fertilizer comprised about 60% of annual consumption of fertilizer N in the Changjiang River catchment and were key factors controlling the high content of inorganic N in the Changjiang River mouth. The fate of the N in precipitation and other N sources in the Changjiang River catchment are also discussed in this paper. 相似文献
227.
228.
田斌 《紫金山天文台台刊》2003,(2)
在本文中我们对两种不同的 βCephei变星的理论脉动不稳定带作了比较。它们具有明显的不同之处。为了确定这种变星的脉动不稳定带的轮廓 ,我们根据 4 9颗 βCephei变星的观测资料作了统计研究。在从样本星的色指数 (B -V) ,(U -B)和视差得到它们的有效温度和光度之后 ,我们发现在赫罗图上它们大多数是位于主序带内的。而且这些样本星的质量都位于 7M⊙ 到 30M⊙ 之间。和理论模型的比较表明我们提出的具有光度上边界和红蓝边界的 βCephei变星的理论脉动不稳定带跟观测吻合得更好 相似文献
229.
O. J. Bennett A. S. Dickinson T. Leininger F. X. Gadéa 《Monthly notices of the Royal Astronomical Society》2003,341(1):361-368
The role in radiative association of narrow resonances arising from quasi-bound levels is examined. For the rate coefficient the contribution from such levels is readily calculated in terms of the Einstein A -coefficient from bound rovibrational levels generated using a suitable alternative boundary condition. By locating systematically all the quasi-bound levels, an enhancement by more than an order of magnitude of the rate coefficient for Li(2s) + H(1s) association above about 600 K is obtained. The effect on the LiH abundance in the early Universe is no more than an increase of a factor of 3 for limited ranges of values of the redshift. For the Li(2p) state the effect of narrow resonances appears to have been taken into account in previous calculations. 相似文献
230.
DharmalingamVENUGOPAL 《山地科学学报》2004,1(1):74-80
The Nilgiri Mountains of south India is considered unique by anthropologists, geologists, climatologists, botanists as well as tourists. It has remained a subject of constant study and research over the last two centuries. Man-nature balance had continued undisturbed in the Nilgiris for thousands of years until the early 19th century when it became a British colony attracting, in due course, various developmental activities. Subsequently, the Nilgiris and its popular hill stations emerged as favourite places for the British population in India for rest and recuperation, game and for raising commercial plantations. In the process, the traditional indigenous crops were replaced by “English“ vegetables and the natural forests gave way to commercial plantations of coffee, tea and other exotic species of trees. 相似文献