全文获取类型
收费全文 | 39192篇 |
免费 | 18071篇 |
国内免费 | 42190篇 |
专业分类
测绘学 | 2654篇 |
大气科学 | 21707篇 |
地球物理 | 13926篇 |
地质学 | 36238篇 |
海洋学 | 16133篇 |
天文学 | 2441篇 |
综合类 | 3982篇 |
自然地理 | 2372篇 |
出版年
2024年 | 139篇 |
2023年 | 247篇 |
2022年 | 333篇 |
2021年 | 688篇 |
2020年 | 1967篇 |
2019年 | 5400篇 |
2018年 | 5816篇 |
2017年 | 5566篇 |
2016年 | 5711篇 |
2015年 | 4964篇 |
2014年 | 4477篇 |
2013年 | 5238篇 |
2012年 | 4804篇 |
2011年 | 4608篇 |
2010年 | 4519篇 |
2009年 | 3950篇 |
2008年 | 3104篇 |
2007年 | 3065篇 |
2006年 | 2685篇 |
2005年 | 2548篇 |
2004年 | 2938篇 |
2003年 | 2620篇 |
2002年 | 2321篇 |
2001年 | 2068篇 |
2000年 | 1680篇 |
1999年 | 1669篇 |
1998年 | 1785篇 |
1997年 | 1878篇 |
1996年 | 1440篇 |
1995年 | 1405篇 |
1994年 | 1257篇 |
1993年 | 1206篇 |
1992年 | 1009篇 |
1991年 | 796篇 |
1990年 | 772篇 |
1989年 | 624篇 |
1988年 | 539篇 |
1987年 | 468篇 |
1986年 | 360篇 |
1985年 | 310篇 |
1984年 | 340篇 |
1983年 | 215篇 |
1982年 | 267篇 |
1981年 | 217篇 |
1980年 | 161篇 |
1979年 | 155篇 |
1978年 | 87篇 |
1977年 | 91篇 |
1972年 | 67篇 |
1971年 | 83篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
Martin M. Roth Thomas Fechner Dieter Wolter Andreas Kelz Thomas Becker 《Experimental Astronomy》2002,14(2):99-105
PMAS, the Potsdam Multi-Aperture Spectrophotometer, is a new integral field spectrograph in the optical, which is optimized
for good transmission and high image quality from 350 nm to 1 μm. We present our plan to implement a CCD charge-shuffle mode
to allow for beam switching with a very high degree of sky subtraction accuracy for faint object 3-D spectroscopy.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
102.
103.
The purpose of this work was to reinvestigate the existing hydrogeological conceptual model of the basin of Madrid, Spain. A cumulative chemical isotopic diagram which enabled the distinction between different groups of water as well as calculation of the mode of their blending was used for this investigation. It was found that the groups of discharge were lighter in their isotopic composition than that of recharge. The previous explanation of this fact, backed by carbon-14 dating, was the long residence time due to flow lines going down to depths of more than 1000 m. This flow model assumes homogenous conditions to these depths. This assumption can not be supported by evidence from deep wells. Thus a modified model is suggested which maintains homogenous conditions only to about 300 m and a deep confined aquifer below containing paleowater. The higher degree of depletion of this water has been explained by a colder climate on top of an altitude effect. Another interesting observation was the correlation between the isotopic composition of the rains, the month of the rain event and the composition of the recharge group groundwater. It could be seen that the winter rains resemble the groundwater composition, which shows that practically all the spring and summer rains were evapotranspirated. 相似文献
104.
介绍一种自行设计和加工的直接雾化石墨炉进样装置的结构、工作原理和操作方法。该装置由微机控制与石墨炉加热程序同步工作,可以实现自动控制和自动测量。对雾化进样石墨炉分析的仪器参数和实验条件进行了研究和选择。结果表明,该装置与石墨炉结合具有自动化程度高、重现性好、样品利用率高和分析速度快等特点。 相似文献
105.
106.
107.
D. H. Morgan Q. A. Parker Martin Cohen 《Monthly notices of the Royal Astronomical Society》2003,346(3):719-730
We report the discovery of the first probable Galactic [WN] central star of a planetary nebula (CSPN). The planetary nebula candidate was found during our systematic scans of the AAO/UKST Hα Survey of the Milky Way. Subsequent confirmatory spectroscopy of the nebula and central star reveals the remarkable nature of this object. The nebular spectrum shows emission lines with large expansion velocities exceeding 150 km s−1 , suggesting that perhaps the object is not a conventional planetary nebula. The central star itself is very red and is identified as being of the [WN] class, which makes it unique in the Galaxy. A large body of supplementary observational data supports the hypothesis that this object is indeed a planetary nebula and not a Population I Wolf–Rayet star with a ring nebula. 相似文献
108.
109.
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
110.
The trans-Neptunian belt has been subject to a strong depletion that has reduced its primordial population by a factor of one hundred over the solar system's age. One by-product of such a depletion process is the existence of a scattered disk population in transit from the belt to other places, such as the Jupiter zone, the Oort cloud or interstellar space. We have integrated the orbits of the scattered disk objects (SDOs) so far discovered by 2500 Myr to study their dynamical time scales and the probability of falling in each of the end states mentioned above, paying special attention to their contribution to the Oort cloud. We found that their dynamical half-time is close to 2.5 Gyr and that about one third of the SDOs end up in the Oort cloud. 相似文献