全文获取类型
收费全文 | 25529篇 |
免费 | 4831篇 |
国内免费 | 5722篇 |
专业分类
测绘学 | 2183篇 |
大气科学 | 3189篇 |
地球物理 | 10445篇 |
地质学 | 9365篇 |
海洋学 | 4713篇 |
天文学 | 1402篇 |
综合类 | 1902篇 |
自然地理 | 2883篇 |
出版年
2024年 | 82篇 |
2023年 | 265篇 |
2022年 | 793篇 |
2021年 | 897篇 |
2020年 | 1149篇 |
2019年 | 1364篇 |
2018年 | 1038篇 |
2017年 | 1117篇 |
2016年 | 1071篇 |
2015年 | 1311篇 |
2014年 | 1412篇 |
2013年 | 1626篇 |
2012年 | 1590篇 |
2011年 | 1580篇 |
2010年 | 1294篇 |
2009年 | 1503篇 |
2008年 | 1624篇 |
2007年 | 1940篇 |
2006年 | 1966篇 |
2005年 | 1685篇 |
2004年 | 1507篇 |
2003年 | 1284篇 |
2002年 | 1113篇 |
2001年 | 846篇 |
2000年 | 1005篇 |
1999年 | 911篇 |
1998年 | 760篇 |
1997年 | 576篇 |
1996年 | 550篇 |
1995年 | 437篇 |
1994年 | 393篇 |
1993年 | 362篇 |
1992年 | 268篇 |
1991年 | 175篇 |
1990年 | 116篇 |
1989年 | 138篇 |
1988年 | 92篇 |
1987年 | 69篇 |
1986年 | 61篇 |
1985年 | 27篇 |
1984年 | 20篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 8篇 |
1980年 | 5篇 |
1979年 | 19篇 |
1978年 | 2篇 |
1977年 | 4篇 |
1954年 | 12篇 |
1877年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
241.
242.
The atomic force microscopy (AFM) is a recently developed, bench-top instrument that can image the surface structures of biological
specimens at high resolution with simultaneous measurement of their size. This paper describes the application of AFM to marine
bacteria. Both natural and cultured bacteria were retained on a filter or placed on glass, washed, air-dried and observed
by AFM. The instrumental condition, the choice of suitable filter, effect of fixation and filtration, comparison with epifluorescent
microscopic (EFM) count, and the size and shape of bacterial cells were investigated. An Isopore filter was best for concentration
and subsequent observation because of its surface flatness. Cross section images showed that both rod and coccoid cells were
flattened, the former usually having a two-humped shape. Bacterial cells were differentiated from non-living particles based
on their cross section shape and size. Bacterial counts by AFM and EFM showed good agreement. Although size measurement is
easily done by the instrument, AFM tends to overestimate the size of microspheres. More work is thus needed on the size measurement
of living organisms. Because AFM easily provides images of natural bacterial cells at high magnification, it can be used as
a new tool to study the fine structures of marine bacteria.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
243.
244.
245.
246.
247.
Are invasive species most successful in habitats of low native species richness across European brackish water seas? 总被引:5,自引:0,他引:5
Marjo Paavola Sergej Olenin Erkki Leppkoski 《Estuarine, Coastal and Shelf Science》2005,64(4):738-750
European brackish water seas (Baltic Sea, Black Sea and Sea of Azov, Caspian Sea) are subject to intense invasion of non-indigenous species (NIS). In these seas, salinity is the most important range limiting factor and native species seem to reach a minimum species richness at intermediate salinities. This trend, revealed by Remane in 1934 and later on confirmed by many other scientists, was compared to the salinity range of already established NIS in the European brackish water seas. It turned out that most NIS are well adapted to the salinities holding lowest native species richness, already in their native area, and that NIS richness maximum in brackish water seas occurs in the salinity intervals of native species richness minimum. A predictable pattern in the salinity range of NIS can be used as a tool in initial risk assessment of future invasions in brackish water seas, especially when mapping highly potential donor and recipient areas. A product of empty niches, suitable environmental conditions, and availability of proper vectors might be the most effective predictor for the invasibility of brackish water areas. 相似文献
248.
Jonathan M. Bull Martin Gutowski Justin K. Dix Timothy J. Henstock Peter Hogarth Timothy G. Leighton Paul R. White 《Marine Geophysical Researches》2005,26(2-4):157-169
Chirp sub-bottom profilers are marine acoustic devices that use a known and repeatable source signature (1–24 kHz) to produce
decimetre vertical resolution cross-sections of the sub-seabed. Here the design and development of the first true 3D Chirp
system is described. When developing the design, critical factors that had to be considered included spatial aliasing, and
precise positioning of sources and receivers. Full 3D numerical modelling of the combined source and receiver directivity
was completed to determine optimal source and receiver geometries. The design incorporates four source transducers (1.5–13 kHz)
that can be arranged into different configurations, including Maltese Cross, a square and two separated pairs. The receive
array comprises 240 hydrophones in 60 groups whose group-centres are separated by 25 cm in both horizontal directions, with
each hydrophone group containing four individual elements and a pre-amplifier.
After careful consideration, it was concluded that the only way to determine with sufficient accuracy the source–receiver
geometry, was to fix the sources and receivers within a rigid array. Positional information for the array is given by a Real
Time Kinematic GPS and attitude system incorporating four antennas to give position, heading, pitch and roll. It is shown
that this system offers vertical positioning accuracy with a root-mean-square (rms) error less than 2.6 cm, while the horizontal
positioning rms error was less than 2.0 cm. The system is configured so that the Chirp source signature can be chosen by software
aboard the acquisition vessel.
The complete system is described and initial navigational and seismic data results are presented. These data demonstrate that
the approach of using fixed source-receiver geometry combined with RTK navigation can provide complete 3D imaging of the sub-surface. 相似文献
249.
250.