全文获取类型
收费全文 | 1831篇 |
免费 | 71篇 |
国内免费 | 10篇 |
专业分类
测绘学 | 26篇 |
大气科学 | 202篇 |
地球物理 | 410篇 |
地质学 | 642篇 |
海洋学 | 128篇 |
天文学 | 303篇 |
综合类 | 3篇 |
自然地理 | 198篇 |
出版年
2021年 | 15篇 |
2020年 | 26篇 |
2019年 | 28篇 |
2018年 | 27篇 |
2017年 | 50篇 |
2016年 | 65篇 |
2015年 | 49篇 |
2014年 | 53篇 |
2013年 | 133篇 |
2012年 | 45篇 |
2011年 | 76篇 |
2010年 | 64篇 |
2009年 | 85篇 |
2008年 | 61篇 |
2007年 | 50篇 |
2006年 | 58篇 |
2005年 | 41篇 |
2004年 | 57篇 |
2003年 | 58篇 |
2002年 | 57篇 |
2001年 | 41篇 |
2000年 | 36篇 |
1999年 | 43篇 |
1998年 | 40篇 |
1997年 | 31篇 |
1996年 | 25篇 |
1995年 | 32篇 |
1994年 | 24篇 |
1993年 | 22篇 |
1992年 | 26篇 |
1991年 | 19篇 |
1990年 | 21篇 |
1989年 | 18篇 |
1988年 | 17篇 |
1987年 | 20篇 |
1986年 | 12篇 |
1985年 | 20篇 |
1984年 | 36篇 |
1983年 | 25篇 |
1982年 | 26篇 |
1981年 | 30篇 |
1980年 | 30篇 |
1979年 | 25篇 |
1978年 | 30篇 |
1977年 | 24篇 |
1976年 | 19篇 |
1975年 | 26篇 |
1974年 | 14篇 |
1973年 | 17篇 |
1972年 | 13篇 |
排序方式: 共有1912条查询结果,搜索用时 15 毫秒
1.
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
2.
The HCN emission features near 3 μm recently detected by Geballe et al. (2003, Astrophys. J. 583, L39) are analyzed with a model for fluorescence of sunlight in the ν3 band of HCN. The emission spectrum is consistent with current knowledge of the atmospheric temperature profile and the HCN distribution inferred from millimeter-wave observations. The spectrum is insensitive to the abundance of HCN in the thermosphere and the thousand-fold enhancement relative to photochemical models suggested by Geballe et al. (2003, Astrophys. J. 583, L39) is not required to explain the observations. We find that the spectrum can be matched with temperatures from 130 to 200 K, with slightly better fits at high temperature, contrary to the temperature determination of 130±10 K of Geballe et al. (2003, Astrophys. J. 583, L39). The HCN emission spectrum is sensitive to the collisional de-excitation probability, P10, for the ν3 state and we determine a value of 10−5 with an accuracy of about a factor of two. Analysis of absorption lines in the C2H2ν3 band near 3 μm, detected in the same spectrum, indicate a C2H2 mole fraction near 0.01 μbar of 10−5 for P10=10−4. The derived mole fraction, however, is dependent upon the value adopted for P10 and lower values are required if P10 at Titan temperatures is less than its room temperature value. 相似文献
3.
4.
5.
Ezio Caroli Natalia Auricchio Lorenzo Amati Yuriy Bezsmolnyy Carl Budtz-JøRgensen Rui M. Curado da Silva Filippo Frontera Alessandro Pisa Stefano Del Sordo John B. Stephen Giulio Ventura 《Experimental Astronomy》2005,20(1-3):341-351
The energy range above 60 keV is important for the study of many open problems in high energy astrophysics such as the role of Inverse Compton with respect to synchrotron or thermal processes in GRBs, non thermal mechanisms in SNR, the study of the high energy cut-offs in AGN spectra, and the detection of nuclear and annihilation lines. Recently the development of high energy Laue lenses with broad energy bandpasses from 60 to 600keV have been proposed for a Hard X ray focusing Telescope (HAXTEL) in order to study the X-ray continuum of celestial sources. The required focal plane detector should have high detection efficiency over the entire operative range, a spatial resolution of about 1mm, an energy resolution of a few keV at 500keV and a sensitivity to linear polarization. We describe a possible configuration of the focal plane detector based on several CdTe/CZT pixelated layers stacked together to achieve the required detection efficiency at high energy.
Each layer can operate both as a separate position sensitive detector and polarimeter or work with other layers to increase the overall photopeak efficiency. Each layer has a hexagonal shape in order to minimize the detector surface required to cover the lens field of view. The pixels would have the same geometry so as to provide the best coupling with the lens point spread function and to increase the symmetry for polarimetric studies. 相似文献
6.
7.
8.
Long-range sidescan sonar can be used to map sediment distributions over wide expanses of deep ocean floor. Seven acoustic facies that arise from differing sediment or rock types have been mapped over the low-relief Saharan continental rise and Madeira abyssal plain. These have been calibrated with sampling, profiling and camera studies and the facies can be traced confidently on a regional scale using the sidescan data. The mapping of the sediment distribution shows that a complex interplay of turbidity current and debris flow processes can occur at a continental rise/abysaal plain transition over 1000 km from the nearest continental slope. 相似文献
9.
In this paper the second order characteristic (discontinuous bifurcation) condition is derived for the granular flow (fully plastic) equations. This second order bifurcation equation is shown to be formally identical to the first order localization requirement during steady elastoplastic deformation provided the elastic compliance tensor is substituted for the product of the plastic multiplier with the flow Hessian. For isotropic yield and flow functions the invariant form of the characteristic condition is given in detail, as well as an alternative expression in adapted co‐ordinates. The characteristic condition can be regarded as defining a hardening function which is maximized to identify the critical angles. When the method is applied to 3D Coulomb flow, Mohr's 3D fracture plane conditions are obtained uniquely. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
10.
S. Maaløe 《Mineralogy and Petrology》2003,77(1-2):1-24
Summary ?Partial melting of the mantle is polybaric which implies that the phase relations change during partial melting. In addition
to the pressure the composition of the melt depends on the melting mode. Various melting models have been suggested. Here
the basic phase relations of polybaric batch, percolative, and critical melting are considered, using a simple ternary system.
The percolative melts are in equilibrium with their residua, but differ somewhat in composition from those of batch melting.
Critical melting is a fractional type of melting where the residuum contain interstitial melt. The critical melts differ in
composition from batch melts. The linear trends of peridotites from ophiolites show that the extracted melts had nearly constant
compositions, and therefore were extracted within a small pressure interval. A comparison between the trends of mantle peridotite
and experimental batch melts suggests strongly that the melt extracted from the peridotites are in equilibrium with their
residua. This could suggest that either batch or percolative melting are relevant melting modes for the mantle. However, isotopic
disequilibria favor instead a critical mode of melting. This inconsistency can be avoided if the ascending melts are accumulated
within a source region and equilibrate with the residuum before the melt is extracted from the source region. The evidence
for equilibrium suggests that multisaturation of tholeiitic compositions in PT-diagrams is relevant for estimating pressure
and temperature of generation of primary tholeiitic magmas.
Received September 2, 2001; revised version accepted March 20, 2002 相似文献