全文获取类型
收费全文 | 63358篇 |
免费 | 1272篇 |
国内免费 | 1861篇 |
专业分类
测绘学 | 2347篇 |
大气科学 | 4970篇 |
地球物理 | 12452篇 |
地质学 | 26093篇 |
海洋学 | 4545篇 |
天文学 | 10002篇 |
综合类 | 2410篇 |
自然地理 | 3672篇 |
出版年
2022年 | 317篇 |
2021年 | 495篇 |
2020年 | 499篇 |
2019年 | 561篇 |
2018年 | 5668篇 |
2017年 | 4916篇 |
2016年 | 3707篇 |
2015年 | 1026篇 |
2014年 | 1296篇 |
2013年 | 2132篇 |
2012年 | 2517篇 |
2011年 | 4597篇 |
2010年 | 3730篇 |
2009年 | 4385篇 |
2008年 | 3674篇 |
2007年 | 4178篇 |
2006年 | 1910篇 |
2005年 | 1489篇 |
2004年 | 1561篇 |
2003年 | 1444篇 |
2002年 | 1258篇 |
2001年 | 916篇 |
2000年 | 895篇 |
1999年 | 692篇 |
1998年 | 739篇 |
1997年 | 703篇 |
1996年 | 576篇 |
1995年 | 563篇 |
1994年 | 481篇 |
1993年 | 419篇 |
1992年 | 419篇 |
1991年 | 388篇 |
1990年 | 456篇 |
1989年 | 373篇 |
1988年 | 356篇 |
1987年 | 440篇 |
1986年 | 346篇 |
1985年 | 432篇 |
1984年 | 536篇 |
1983年 | 451篇 |
1982年 | 454篇 |
1981年 | 420篇 |
1980年 | 439篇 |
1979年 | 359篇 |
1978年 | 344篇 |
1977年 | 340篇 |
1976年 | 316篇 |
1975年 | 296篇 |
1974年 | 312篇 |
1973年 | 339篇 |
排序方式: 共有10000条查询结果,搜索用时 117 毫秒
181.
Dr M. J. Wooller D. L. Swain K. J. Ficken A. D. Q. Agnew F. A. Street-Perrott G. Eglinton 《第四纪科学杂志》2003,18(1):3-15
Woody, subalpine shrubs and grasses currently surround Lake Rutundu, Mount Kenya. Multiple proxies, including carbon isotopes, pollen and grass cuticles, from a 755‐cm‐long core were used to reconstruct the vegetation over the past 38 300 calendar years. Stable carbon‐isotope ratios of total organic carbon and terrestrial biomarkers from the lake sediments imply that the proportion of terrestrial plants using the C4 photosynthetic pathway was greater during the Late Pleistocene than in the Holocene. Pollen data show that grasses were a major constituent of the vegetation throughout the Late Pleistocene and Holocene. The proportion of grass pollen relative to the pollen from other plants was greatest at the last glacial maximum (LGM). Grass cuticles confirm evidence that C4 grass taxa were present at the LGM and that the majority followed the cold‐tolerant NADP‐MEC4 subpathway. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
182.
183.
We present observations of a sample of Herbig AeBe stars, as well as the FU Orionis object V1057 Cygni. Our K-band (2.2μm)
observations from the Palomar Testbed Interferometer (PTI) used baselines of 110 m and 85 m, resulting in fringe spacings
of ∼4 mas and 5 mas, respectively. Fringes were obtained for the first time on V1057 Cygnias well as V594 Cas. Additional
measurements were made of MWC147, while upper limits to visibility-squared are obtained for MWC297, HD190073, and MWC614.
These measurements are sensitive to the distribution of warm, circumstellar dust in these sources. If the circumstellar infrared
emission comes from warm dust in a disk, the inclination of the disk to the line of sight implies that the observed interferometric
visibilities should depend upon hour angle. Surprisingly, the observations of Millan-Gabet, Schloerb and Traub (2001)(hereafter
MST) did not show significant variation with hour angle. However, limited sampling of angular frequencies on the sky was possible
with the IOTA interferometer, motivating us to study a subset of their objects to further constrain these systems.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
184.
Dolmatova L. S. Eliseykina M. G. Timchenko N. F. Kovaleva A. L. Shitkova O. A. 《中国海洋湖沼学报》2003,21(4):293-304
Pure fraction (92%–95%) of phagocytes (FP) and a mixture of amoebocytes (62%) and morula cells (38%)-FPMC- of the holothurianEupentacta fraudatrix' (Holothuroidea, Dendrochirota) were obtained by using ficoll-verographine step gradient. Basal production of reactive oxygen species (ROS) in FP quantified
by using reduction of nitroblue tetrazolium (NBT) was more than twice that in FPMC. Thermostable toxin ofYersinia pseudotuberculosis (TST) at different concentrations (0.2; 0.5; 2.5 μg/ml, but not 0.1 μg/ml) stimulated NBT reduction in FPMC after 24 h incubation.
In FP, TST at concentrations of 0.1 and 0.2 μg/ml inhibited and at concentrations of 0.5 and 2.5 μg/ml stimulated NBT reduction
after 24 h incubation. Maximal effect was observed in FP and FPMC at TST concentrations of 0.5 and 0.2 μg/ml, respectively.
Addition of catalase (0.7 μg/ml) to the cells treated with TST (2.5 μg/ml) was followed by a decrease in NBT reduction compared
to that under toxin treatment alone. TST stimulated superoxide dismutase activity in concentration-dependent manner (maximum
at 0.5 μg/ml concentration in FP) after 24 h treatment, and this stimulation was prevented by a commercial catalase. Plant
lectin concanavalin A stimulated NBT reduction more than 5-fold in FPMC compared to the control. With addition of TST, lectin
stimulated ROS to lesser extent than that with lectin alone. When catalase, TST, and lectin were added into the FPMC simultaneously,
ROS increase was similar to that under lectin treatment alone. On the whole, data obtained indicated that ROS generation in
holothurian coelomocytes especially occurs in both stimulated and not stimulated phagocytes, and that changes in ROS production
by these cells may be one of the mechanisms of antibacterial protection of holothurians.
This work was financially supported by the Russian Foundation for Fundamental Research Grant (No. 00-04-48949). 相似文献
185.
The present paper reports 9 species of pontoniine shrimps collected from Hainan IS-land ,South China Sea ,by the Joint Chinese-German Marine Biology Expeditions,in which Peri-climenaeus arabicus (Calman,1939) and Periclimenaeus hecate (Nobili,1904) are recorded for the first time from Hainan Island. 相似文献
186.
Lightning discharge generated in the protoplanetary nebula is viewed as a temporally isolated surge in the flow of electrically charged particles, similar to that of terrestrial lightning. If the current is intense enough, a powerful circular impulse magnetic field is generated around the instantaneous virtual electric conductor. Such magnetic field is capable of magnetizing dust grains containing ferromagnetic components present in its vicinity to their saturation levels. As a result, dust grains attract one another, forming the aggregates. This magnetically driven attraction suggests an important process possibly operational at an early stage of the planetary accretion. Based on both a classical model for electric conductor, and the theory of Lienard–Wiechert electromagnetic potentials, our calculations show that the magnetic impulse due to a discharge channel of a few cm in diameter transferring a charge of about 104 electrons reaches as high as 10 T. At these magnetic fields, the ferromagnetic dust grains, and possibly the already-formed larger aggregates as well, are easily magnetized to the saturation levels, producing compact clusters exhibiting permanent magnetic moments. 相似文献
187.
188.
189.
APPLICATION OF GEOGRAPHICAL PARAMETER DATABASE TO ESTABLISHMENT OF UNIT POPULATION DATABASE 总被引:1,自引:1,他引:0
Now GIS is turning into a good tool in handling geographical, economical, and population data, so we can obtain more and more information from these data. On the other hand, in some cases, for a calamity, such as hurricane, earthquake, flood, drought etc., or a decision-making, such as setting up a broadcasting transmitter, building a chemical plant etc., we have to evaluate the total population in the region influenced by a calamity or a project. In this paper, a method is put forward to evaluate the population in such special region. Through exploring the correlation of geographical parameters and the distribution of people in the same region by means of quantitative analysis and qualitative analysis, unit population database (1km× 1km) is established. In this way, estimating the number of people in a special region is capable by adding up the population in every grid involved in this region boundary. The geographical parameters are obtained from topographic database and DEM database on the scale of 相似文献
190.