全文获取类型
收费全文 | 115244篇 |
免费 | 18554篇 |
国内免费 | 42660篇 |
专业分类
测绘学 | 4418篇 |
大气科学 | 26597篇 |
地球物理 | 27919篇 |
地质学 | 62374篇 |
海洋学 | 23394篇 |
天文学 | 21154篇 |
综合类 | 4119篇 |
自然地理 | 6483篇 |
出版年
2022年 | 839篇 |
2021年 | 1438篇 |
2020年 | 2765篇 |
2019年 | 6271篇 |
2018年 | 7737篇 |
2017年 | 7329篇 |
2016年 | 7722篇 |
2015年 | 5902篇 |
2014年 | 6472篇 |
2013年 | 9217篇 |
2012年 | 6909篇 |
2011年 | 7211篇 |
2010年 | 7151篇 |
2009年 | 7278篇 |
2008年 | 6124篇 |
2007年 | 6144篇 |
2006年 | 5512篇 |
2005年 | 4689篇 |
2004年 | 4973篇 |
2003年 | 4618篇 |
2002年 | 4302篇 |
2001年 | 3826篇 |
2000年 | 3342篇 |
1999年 | 3133篇 |
1998年 | 3208篇 |
1997年 | 3284篇 |
1996年 | 2644篇 |
1995年 | 2524篇 |
1994年 | 2380篇 |
1993年 | 2172篇 |
1992年 | 1987篇 |
1991年 | 1707篇 |
1990年 | 1796篇 |
1989年 | 1516篇 |
1988年 | 1387篇 |
1987年 | 1444篇 |
1986年 | 1217篇 |
1985年 | 1453篇 |
1984年 | 1541篇 |
1983年 | 1406篇 |
1982年 | 1408篇 |
1981年 | 1240篇 |
1980年 | 1117篇 |
1979年 | 1091篇 |
1978年 | 1019篇 |
1977年 | 900篇 |
1976年 | 851篇 |
1975年 | 769篇 |
1974年 | 839篇 |
1973年 | 854篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
511.
Infiltration experiments have been performed at three sites along a well-known catena under virgin tropical rain forest using a portable sprinkling infiltrometer. Experimentally determined infiltration curves are presented. Infiltration curves are also simulated on the basis of the Mein-Larson equation. The parameters for this model have been obtained from the infiltration curves (saturated conductivity) and simple soil moisture determinations (fillable porosity). The agreement between experimentally determined and modelled infiltration is reasonable, provided (a) saturated conductivity as derived from the experimental data is corrected, (b) a storage parameter, also derived from the experimental data, is added to the Mein-Larson model, and (c) the decline in soil porosity with depth is either small or occurs abruptly at shallow depth. Comparison of observed infiltration rates with rainfall intensity shows that Horton Overland Flow has to occur naturally at least on the middle and lower section of the catena. Despite the fact that most parameters can be estimated in principle from basic soil data, it remains advisable to obtain sprinkling infiltrometer field measurements, because of soil variability due to dynamic surface conditions, macroporosity, air entrapment, and irregularity of the wetting front. 相似文献
512.
513.
R. K. Manchanda 《Journal of Astrophysics and Astronomy》2001,22(2-3):145-154
Spectral measurement of Mkn 421 were made in the hard X-ray energy band of 20–200 keV using a high sensitivity, large area
scintillation counter telescope on November 21, 2000 and these coincided with the onset of an active X-ray phase as seen in
the ASM counting rates on board RXTE. The observed spectrum can not be fitted to a single power law similar to the PDS data
of BeppoSAX. The data can be fitted both by a two component power-law function or a combination of an exponential function
with a power law component at the high energies above 80 keV. We identify these components with those arising from the synchrotron
self compton and the high energy power-law tail arising from the upgrading of the thermal photons due to multiple Compton
scattering a la Cyg X-1. A comparison with the earlier data clearly suggests a spectral variability in the hard X-ray spectrum
of the source. We propose a continuously flaring geometry for the source as the underlying mechanism for energy release. 相似文献
514.
515.
516.
517.
利用三维有限元模拟两层地球模型(包括弹性层和粘弹性层)介质的横向不均匀和断层错动的复杂性对震后地表位移场的影响,其数值结果包括弹性解和粘弹性解。结果表明,震后位移场的分布特征不仅与粘弹性层的流变特性有关,还与弹性层介质的剪切模量以及断层的同震错动有关。 相似文献
518.
B. R. Johnson M. E. Abroe P. Ade J. Bock J. Borrill J. S. Collins P. Ferreira S. Hanany A. H. Jaffe T. Jones A. T. Lee L. Levinson T. Matsumura B. Rabii T. Renbarger P. L. Richards G. F. Smoot R. Stompor H. T. Tran C. D. Winant 《New Astronomy Reviews》2003,47(11-12):1067
We discuss MAXIPOL, a bolometric balloon-borne experiment designed to measure the E-mode polarization anisotropy of the cosmic microwave background radiation (CMB) on angular scales of 10′ to 2°. MAXIPOL is the first CMB experiment to collect data with a polarimeter that utilizes a rotating half-wave plate and fixed wire-grid polarizer. We present the instrument design, elaborate on the polarimeter strategy and show the instrument performance during flight with some time domain data. Our primary dataset was collected during a 26 h turnaround flight that was launched from the National Scientific Ballooning Facility in Ft. Sumner, New Mexico in May 2003. During this flight five regions of the sky were mapped. Data analysis is in progress. 相似文献
519.
520.
A high-velocity ionized outflow and XUV photosphere in the narrow emission line quasar PG1211+143 总被引:1,自引:0,他引:1
K. A. Pounds J. N. Reeves A. R. King K. L. Page P. T. O'Brien M. J. L. Turner 《Monthly notices of the Royal Astronomical Society》2003,345(3):705-713
We report on the analysis of a ∼60-ks XMM–Newton observation of the bright, narrow emission line quasar PG1211+143. Absorption lines are seen in both European Photon Imaging Camera and Reflection Grating Spectrometer spectra corresponding to H- and He-like ions of Fe, S, Mg, Ne, O, N and C. The observed line energies indicate an ionized outflow velocity of ∼24 000 km s−1 . The highest energy lines require a column density of N H ∼ 5 × 1023 cm−2 , at an ionization parameter of log ξ∼ 3.4 . If the origin of this high-velocity outflow lies in matter being driven from the inner disc, then the flow is likely to be optically thick within a radius of ∼130 Schwarzschild radii, providing a natural explanation for the big blue bump (and strong soft X-ray) emission in PG1211+143. 相似文献