全文获取类型
收费全文 | 86734篇 |
免费 | 1354篇 |
国内免费 | 806篇 |
专业分类
测绘学 | 2298篇 |
大气科学 | 5966篇 |
地球物理 | 16820篇 |
地质学 | 30702篇 |
海洋学 | 7708篇 |
天文学 | 20787篇 |
综合类 | 272篇 |
自然地理 | 4341篇 |
出版年
2022年 | 514篇 |
2021年 | 890篇 |
2020年 | 967篇 |
2019年 | 1006篇 |
2018年 | 2287篇 |
2017年 | 2142篇 |
2016年 | 2667篇 |
2015年 | 1452篇 |
2014年 | 2543篇 |
2013年 | 4504篇 |
2012年 | 2753篇 |
2011年 | 3598篇 |
2010年 | 3180篇 |
2009年 | 4208篇 |
2008年 | 3551篇 |
2007年 | 3610篇 |
2006年 | 3376篇 |
2005年 | 2540篇 |
2004年 | 2585篇 |
2003年 | 2416篇 |
2002年 | 2339篇 |
2001年 | 2065篇 |
2000年 | 1937篇 |
1999年 | 1628篇 |
1998年 | 1697篇 |
1997年 | 1571篇 |
1996年 | 1369篇 |
1995年 | 1331篇 |
1994年 | 1177篇 |
1993年 | 1043篇 |
1992年 | 1000篇 |
1991年 | 1019篇 |
1990年 | 1102篇 |
1989年 | 908篇 |
1988年 | 884篇 |
1987年 | 1020篇 |
1986年 | 891篇 |
1985年 | 1130篇 |
1984年 | 1253篇 |
1983年 | 1145篇 |
1982年 | 1064篇 |
1981年 | 1027篇 |
1980年 | 914篇 |
1979年 | 877篇 |
1978年 | 860篇 |
1977年 | 737篇 |
1976年 | 702篇 |
1975年 | 709篇 |
1974年 | 663篇 |
1973年 | 715篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
Mallory S. E. Roberts Crystal L. Brogan Bryan M. Gaensler Jason W. T. Hessels C.-Y. Ng Roger W. Romani 《Astrophysics and Space Science》2005,297(1-4):93-100
A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET γ-ray sources. X-ray and radio imaging studies of unidentified EGRET sources have resulted in the discovery of at least six new pulsar wind nebulae (PWN). Stationary PWN (SPWN) appear to be
associated with steady EGRET sources with hard spectra, typical for γ-ray pulsars. Their toroidal morphologies can help determine the geometry of the
pulsar which is useful for constraining models of pulsed γ-ray emission. Rapidly moving PWN (RPWN) with more cometary morphologies
seem to be associated with variable EGRET sources in regions where the ambient medium is dense compared to what is typical for the ISM. 相似文献
13.
14.
15.
16.
In recent times it has been emphasized that the present kinematical structures of asteroid families should be evolved with respect to the original post-impact situations, according to numerical simulations performed taking into account also the previously neglected Yarkovsky effect. In this paper we show that also a “classical” approach based on an analysis of the current kinematical properties of families leads to conclude that the distributions of proper eccentricities and semimajor axes of family members exhibit evidence of an evolution. The importance of this approach is that it yields a fully independent and quantitative estimate of an evolutionary spreading of the proper elements. In particular, we find that the original post-impact families had to be on the average about twice more compact than the families we observe now, when considering family members down to about 5 km in size. This result can be used in future analyses to derive estimates of the ages of different families, and to better constrain the typical values of the ejection velocities of the fragments in family-forming events. 相似文献
17.
The stability and evolution of cold, shock-bounded slabs is studied using numerical hydrodynamic simulations. We confirm the analysis of Vishniac (1994) [ApJ, 428, 186], who showed that such slabs are unstable if they are perturbed by a displacement larger than their width. The growth rate of this nonlinear thin shell instability (NTSI) is found to increase with decreasing wavelength, in qualitative agreement with Vishniac's analysis. The NTSI saturates when the bending angle becomes large and the growth in the width of the slab pinches off the perturbation. After saturation, the slab remains greatly extended with an average density much less than the original slab density, supported primarily by supersonic turbulence within the slab. Linear perturbations are also found to be unstable in that they can lead to turbulent flow within the slab, although this response to linear perturbations is distinct from, and much less violent than the NTSI.Richard McCray 相似文献
18.
In astronomical photometry, the sensitivity of observations is limited by the dark counts of the photomultiplier tube. In the present work, the effect of dark count noise in photon counting systems is investigated by theory and experimental measurements. Dark counts are considered to be originating from two sources, namely: dc fluctuations and random pulses.Experimental measurements were carried out to determine noise effects in different operating regions of noise dominance. The results provide strong evidence that: in normal operating mode, where the effect of random pulses is dominant, dark counts do not follow Poisson statistics. The observed noise shows strong (1/f) power spectrum, where the observed noise power is found to increase with time of observation.The results are important in photon counting systems operating under dark count limited mode. The conclusions drawn can be useful in obtaining more accurate error estimates and in assessing astronomical photometric observations and data reduction techniques. 相似文献
19.
20.
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