全文获取类型
收费全文 | 112797篇 |
免费 | 6286篇 |
国内免费 | 6695篇 |
专业分类
测绘学 | 3324篇 |
大气科学 | 12495篇 |
地球物理 | 24170篇 |
地质学 | 44494篇 |
海洋学 | 9994篇 |
天文学 | 19749篇 |
综合类 | 3274篇 |
自然地理 | 8278篇 |
出版年
2022年 | 1217篇 |
2021年 | 1510篇 |
2020年 | 1478篇 |
2019年 | 1595篇 |
2018年 | 5258篇 |
2017年 | 4954篇 |
2016年 | 4395篇 |
2015年 | 2325篇 |
2014年 | 2958篇 |
2013年 | 4671篇 |
2012年 | 3880篇 |
2011年 | 6196篇 |
2010年 | 5918篇 |
2009年 | 6828篇 |
2008年 | 5834篇 |
2007年 | 6175篇 |
2006年 | 3479篇 |
2005年 | 3480篇 |
2004年 | 3328篇 |
2003年 | 3254篇 |
2002年 | 2878篇 |
2001年 | 2471篇 |
2000年 | 2676篇 |
1999年 | 2873篇 |
1998年 | 2589篇 |
1997年 | 2574篇 |
1996年 | 2258篇 |
1995年 | 2070篇 |
1994年 | 1900篇 |
1993年 | 1680篇 |
1992年 | 1473篇 |
1991年 | 1212篇 |
1990年 | 1299篇 |
1989年 | 1108篇 |
1988年 | 1002篇 |
1987年 | 1067篇 |
1986年 | 930篇 |
1985年 | 1103篇 |
1984年 | 1230篇 |
1983年 | 1162篇 |
1982年 | 1065篇 |
1981年 | 1025篇 |
1980年 | 867篇 |
1979年 | 826篇 |
1978年 | 866篇 |
1977年 | 779篇 |
1976年 | 744篇 |
1975年 | 684篇 |
1974年 | 694篇 |
1973年 | 705篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
121.
Histological study on the ontogeny of the lymphoid organs, kidney, thymus and spleen of Japanese flounder, Paralichthys olivaceus, from hatching to 40 d was carried out. The pronephric kidney duct appeared early in hatching although the primordial haemopoietic
stem cells were observed within a week after hatching. The spleen was first seen after 8d of hatching. The thymus appeared
after 15d, situated near the pronephric kidney. Small lymphoid cells appeared during the later phase of the post-larval stage
in the sequence of thymus, kidney and spleen. During the 40d of observations, there were no distinct inner or outer zones
in thymus and no red or white pulp in spleen. These results suggest that the nonspecific defense immune system plays a very
important role in the early larval stage of Japanese flounder. 相似文献
122.
123.
本文论述了珠穆朗玛峯地区土壤中稀土元素的含量,以及它们在土壤中的分布模式,并发现土壤中稀土元素的含量水平在很大程度上受土壤母质的制约。 相似文献
124.
125.
126.
V. Parro J.A. Rodríguez-Manfredi C. Compostizo E. Vez M. Moreno-Paz P. Fernández-Calvo J. Pérez-Mercader J. Gómez-Elvira 《Planetary and Space Science》2005,53(7):729-737
Recent findings by the MER rover opportunity confirming the presence of iron minerals that can only be formed in the presence of water emphasize the study of analogous environments to Mars on Earth. The study of chemolithoautotrophic communities living in acidic iron-rich habitats is highly relevant in order to identify Mars analog environment-specific biomarkers. Iron oxidizing bacteria like Leptospirillum ferrooxidans and Acidithiobacillus ferrooxidans have ways of life for which it is feasible to identify a past or present hypothetical niche on Mars. We have developed a strategy for biomarker identification based on: (i) search for biosignatures on acid and metal-rich environments; (ii) development of an immunosensor microarray; and (iii) integration into an instrument for autonomous and remote operation. The instrument that we have built, called Signs Of LIfe Detector (SOLID), is capable of processing a variety of samples for the detection of specific biomarkers. Antibodies against several bacterial strains have been developed and tested in a microarray biosensor on SOLID. Tests with field samples have been successfully performed, allowing the detection of L. ferrooxidans, A. ferrooxidans present in sediment samples. 相似文献
127.
We consider the particular solutions of the evolutionary system of equations in elements that correspond to planar and spatial circular orbits of the singly averaged Hill problem. We analyze the stability of planar and spatial circular orbits to inclination and eccentricity, respectively. We construct the instability regions of both particular solutions in the plane of parameters of the problem. 相似文献
128.
Abstract The transition from impact to post‐impact rocks in the Yaxcopoil‐1 (Yax‐1) core is marked by a 2 cm‐thick clay layer characterized by dissolution features. The clay overlies a 9 cm‐thick hardground, overlying a 66 cm‐thick crossbedded unit, consisting of dolomite sandstone alternating with thin micro‐conglomerate layers with litho‐ and bioclasts and the altered remains of impact glass, now smectite. The micro‐conglomerates mark erosion surfaces. Microprobe and backscatter SEM analysis of the dolomite rhombs show an early diagenetic, complex‐zoned, idiomorphic overgrowth, with Mn‐rich zones, possibly formed by hot fluids related to cooling melt sheet in the crater. The pore spaces are filled with several generations of coelestite, barite, K‐feldpar, and sparry calcite. XRF core scanning analysis detected high Mn values in the crossbedded sediments but no anomalous enrichment of the siderophile elements Cr, Co, Fe, and Ni in the clay layer. Shocked quartz occurs in the crossbedded unit but is absent in the clay layer. The basal Paleocene marls are strongly dissolved and do not contain a basal Paleocene fauna. The presence of a hardground, the lack of siderophile elements, shocked quartz, or Ni‐rich spinels in the clay layer, and the absence of basal Paleocene biozones P0 and Pa all suggest that the top of the ejecta sequence and a significant part of the lower Paleocene is missing. Due to the high energy sedimentation infill, a hiatus at the top of the impactite is not unexpected, but there is nothing in the biostratigraphy, geochemistry, and petrology of the Yax‐1 core that can be used to argue against the synchroneity of the end‐Cretaceous mass‐extinctions and the Chicxulub crater. 相似文献
129.
130.
W. H. Jefferys G. F. Benedict P. D. Hemenway P. J. Shelus R. L. Duncombe 《Celestial Mechanics and Dynamical Astronomy》1985,37(3):299-305
The Hubble Space Telescope (HST), a large optical telescope having an aperture of 2.4 meters and a length of 8.8 meters, is being developed by the National Aeronautics and Space Administration. This telescope will be placed into earth orbit by the space shuttle. Astrometric observations with the HST are made using a Fine Guidance Sensor which is capable of measuring the position of one object relative to another with an accuracy of ±0.002 arcseconds. The astrometric user of HST will be provided with an Astrometric Data Reduction Software package (ADRS). The variety of astrometric problems to be investigated with HST is discussed. 相似文献