全文获取类型
收费全文 | 24754篇 |
免费 | 181篇 |
国内免费 | 921篇 |
专业分类
测绘学 | 1412篇 |
大气科学 | 2001篇 |
地球物理 | 4586篇 |
地质学 | 11662篇 |
海洋学 | 1031篇 |
天文学 | 1755篇 |
综合类 | 2162篇 |
自然地理 | 1247篇 |
出版年
2020年 | 4篇 |
2019年 | 3篇 |
2018年 | 4765篇 |
2017年 | 4040篇 |
2016年 | 2578篇 |
2015年 | 240篇 |
2014年 | 90篇 |
2013年 | 40篇 |
2012年 | 1009篇 |
2011年 | 2743篇 |
2010年 | 2027篇 |
2009年 | 2332篇 |
2008年 | 1895篇 |
2007年 | 2380篇 |
2006年 | 73篇 |
2005年 | 208篇 |
2004年 | 414篇 |
2003年 | 422篇 |
2002年 | 268篇 |
2001年 | 52篇 |
2000年 | 54篇 |
1999年 | 25篇 |
1998年 | 30篇 |
1997年 | 3篇 |
1996年 | 3篇 |
1995年 | 4篇 |
1994年 | 2篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 11篇 |
1981年 | 27篇 |
1980年 | 24篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 16篇 |
1975年 | 5篇 |
1974年 | 6篇 |
1973年 | 4篇 |
1972年 | 3篇 |
1971年 | 4篇 |
1970年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
The effects of natural fish oil,DHA oil and soybean lecithin in microparticulate diets on stress tolerance of larval gilthead seabream(Sparus aurata)were investigated after 15 days feeding trials.The tolerance of larval gilthead seabream to various stress factors such as exposure to air(lack of dissolved oxygen),changes in water temperature(low)and salinity(high) were determined.This study showed that microparticulate diet with natural fish oil and soybean lecithin was the most effective for in-creasing the tolerance of larval gilthead seabream to various stresses,and that microparticulate diet with natural fish oil and palmitic acid(16:0)was more effective than microparticulate diet with DHA oil and soybean lecithin. 相似文献
2.
Transport by southeastern rivers has insignificant influence on the lead concentration of southeastern shelf waters. If the rate of mobilization and transport of lead by these rivers is representative of uncontaminated fluvial transport during the Pleistocene, only about 5% of the prehistoric output of lead to the North Atlantic can be accounted for by river input.Lead concentrations in southeastern shelf waters are similar to those of North Atlantic Water from the upper 1000 m, which is probably similar to the intrusion source water for the shelf. Atmospheric inputs to the shelf of the same magnitude as observed for the Western North Atlantic are difficult to reconcile given the residence time of shelf waters and their lead concentration unless the rate of loss of lead to shelf sediments is about the same as the atmospheric flux. 相似文献
3.
4.
JINFeng-jun 《中国地理科学(英文版)》2003,13(2):97-103
It is very important to establish cooperative mechanism to guarantee all members to develop their e-conomies in the Yellow Sea Rim. In this paper, the development strategies ofshipplng centers and transportation networkare discussed based on economic giobalization tendency. The results argue that a united transportation network should bebuilt in order to promote the economic competition of Northeast Asia in the world. As a key component of the economiccooperation, a hierarchical shipping centers network should be established with Hong Kong, Shanghai, Pusan, Koho,and Tokyo as cores. The authorities of China, Japan, R. O. Korea and D. P. B. Korea should make more efforts tobuild a set of cooperation institutions based on raising the transportation efficiency. 相似文献
5.
Ralph D. Lorenz 《Meteoritics & planetary science》2004,39(4):617-623
Abstract— A model for an impact ejecta landform peculiar to Saturn's moon Titan is presented. Expansion of the ejecta plume from moderate‐sized craters is constrained by Titan's thick atmosphere. Much of the plume is collimated along the incoming bolide's trajectory, as was observed for plumes from impacts on Jupiter of P/Shoemaker‐Levy‐9, but is retained as a linear, diagonal ejecta cloud, unlike on Venus where the plume “blows out.” On Titan, the blowout is suppressed because the vertically‐extended atmosphere requires a long wake to reach the vacuum of space, and the modest impact velocities mean plume expansion along the wake is slow enough to allow the wake to close off. Beyond the immediate ejecta blanket around the crater, distal ejecta is released into the atmosphere from an oblique line source: this material is winnowed by the zonal wind field to form streaks, with coarse radar‐bright particles transported less far than fine radar‐dark material. Thus, the ejecta form two distinct streaks faintly reminiscent of dual comet tails, a sharply W‐E radar‐dark one, and a less swept and sometimes comma‐shaped radar‐bright one. 相似文献
6.
Ralph B. Baldwin 《Icarus》1985,61(1):63-91
This paper contains a reasonably successful attempt to determine relative ages and then absolute ages of individual craters younger than Imbrium, and the rate of infalls onto the Moon as a function of time. After the tail of the massive premare bombardment became depleted before 3 aeons (1 aeon = 109 years) ago, there was a period of minimal numbers of infalls. The rate of infalls increased rather steadily from this minimum to the present. The rate in the geologically recent past (0.3 aeon) was about two times that found for the period immediately after the last of the major lave outpourings (3.2 aeons). Absolute ages were determined for large craters (?8 km) from crater counts on the surfaces within and on the rims of the large craters. Key dates were 0 and 0.3 aeon for terrestrial meteoritic craters, 3.2, 3.5, 3.8, and 3.82 aeons for the various mare surfaces according to the determinations of D.E. Wilhelms (1980, Geologic history of the Moon, U.S. Geol. Surv. Prof. Pap.) and 3.85 aeons from the formation of Imbrium. 相似文献
7.
CATASTROPHIC ECO-ENVIRONMENTAL CHANGE IN THE SONGNEN PLAIN, NORTHEASTERN CHINA SINCE 1900S 总被引:1,自引:1,他引:0
WANGHai-xia WANZhong-juan YUShao-peng LUOXin-zheng SUNGuang-you 《中国地理科学(英文版)》2004,14(2):179-185
Although the Songnen Plain in the northeastern China was developed relatively late in the temperate zone of the world, its eco-environment has changed greatly. This paper analyzes the changes of land cover and the rates and trends ofdesertification during the past 100 years in the Songnen Plain. According to the macroscopic analysis, we find that the eco-environment in the plain has reached to the threshold of catastrophic change since the 1950s. The Thorn Needle Catastrophic Model was used to determine and validate this conclusion. Human activities, including large-scale construction projects, such as huge dams and dikes, and excessive grazing were the primary factors contributing to regional eco-environmental catastrophe. And irrational reclamation of the wilderness also affected the eco-environmental change. The results reveal the complex human-land interactions. 相似文献
8.
Ken Ebisawa Aya Kubota Tsunefumi Mizuno Piotr Życki 《Astrophysics and Space Science》2001,276(1):11-14
The Ultra-luminous Compact X-ray Sources (ULXs)in nearby spiral galaxies and the Galactic super-luminaljet sources sharethe common spectral characteristic that they haveextremely high disk temperatures which cannot be explainedin the framework of the standard accretion disk modelin the Schwarzschild metric. We have calculated an extreme Kerr disk model to examine if the Kerr disk model can instead explain the observed `too hot' accretion disk spectra.We found that the Kerr disk spectrum becomes significantly hardercompared to the Schwarzschild disk only when the disk is highlyinclined.For super-luminal jet sources, which are known to beinclined systems, the Kerr disk model may thuswork if we choose proper values for the black hole angular momentum. For the ULXs, however, the Kerr disk interpretation will be problematic,as is is highly unlikely that their accretion disks are preferentiallyinclined. 相似文献
9.
Ralph D. Lorenz A. J. Timothy Jull Timothy D. Swindle Jonathan I. Lunine 《Meteoritics & planetary science》2002,37(6):867-874
Abstract— We explore the likely production and fate of 14C in the thick nitrogen atmosphere of Saturn's moon Titan and investigate the constraints that measurements of 14C might place on Titan's photochemical, atmospheric transport and surface‐atmosphere interaction processes. Titan's atmosphere is thick enough that cosmic‐ray flux limits the production of 14C: absence of a strong magnetic field and the increased distance from the Sun suggest production rates of ?9 atom/cm2/s, ?4x higher than Earth. The fate and detectability of 14C depends on the chemical species into which it is incorporated: as methane it would be hopelessly diluted even in only the atmosphere. However, in the more likely case that the 14C attaches to the haze that rains out onto the surface (as tholin, HCN or acetylene and their polymers), haze in the atmosphere or recently deposited on the surface would be quite radioactive. Such radioactivity may lead to a significant enhancement in the electrical conductivity of the atmosphere which will be measured by the Huygens probe. Measurements with simple detectors on future missions could place useful constraints on the mass deposition rates of photochemical material on the surface and identify locations where surface deposits of such material are “freshest”. 相似文献
10.
Technique for Controlling Spread of Limnotic Oncomelania 总被引:1,自引:0,他引:1
Schistosomiasis is a parasitic disease mostly found in areas along the Changjiang River of China. The disease is spread solely through an intermediary named oncomelania, so its spread of schistosomiasis can be controlled by properly designing water intakes which prevent oncomelania from entering farming land or residential areas. This paper reports a successful design process and a new oncomelania-free intake device. The design of the new intake is based on a sound research program in which extensive experimental studies were carried out to gain knowledge of oncomelania eco-hydraulic behavior and detailed flow field information through CFD simulation. 相似文献