全文获取类型
收费全文 | 85317篇 |
免费 | 983篇 |
国内免费 | 580篇 |
专业分类
测绘学 | 2299篇 |
大气科学 | 5532篇 |
地球物理 | 16390篇 |
地质学 | 32424篇 |
海洋学 | 7112篇 |
天文学 | 19278篇 |
综合类 | 321篇 |
自然地理 | 3524篇 |
出版年
2022年 | 573篇 |
2021年 | 886篇 |
2020年 | 976篇 |
2019年 | 1096篇 |
2018年 | 4274篇 |
2017年 | 3814篇 |
2016年 | 3469篇 |
2015年 | 1259篇 |
2014年 | 2265篇 |
2013年 | 3666篇 |
2012年 | 3146篇 |
2011年 | 4864篇 |
2010年 | 4306篇 |
2009年 | 5175篇 |
2008年 | 4417篇 |
2007年 | 4929篇 |
2006年 | 3057篇 |
2005年 | 2230篇 |
2004年 | 2172篇 |
2003年 | 2085篇 |
2002年 | 2015篇 |
2001年 | 1706篇 |
2000年 | 1547篇 |
1999年 | 1211篇 |
1998年 | 1238篇 |
1997年 | 1263篇 |
1996年 | 985篇 |
1995年 | 954篇 |
1994年 | 888篇 |
1993年 | 770篇 |
1992年 | 699篇 |
1991年 | 714篇 |
1990年 | 757篇 |
1989年 | 714篇 |
1988年 | 652篇 |
1987年 | 718篇 |
1986年 | 617篇 |
1985年 | 793篇 |
1984年 | 909篇 |
1983年 | 806篇 |
1982年 | 789篇 |
1981年 | 720篇 |
1980年 | 672篇 |
1979年 | 596篇 |
1978年 | 646篇 |
1977年 | 558篇 |
1976年 | 499篇 |
1975年 | 506篇 |
1974年 | 467篇 |
1973年 | 542篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
162.
The statistical results presented by Achong and Stahl (1984) may alternatively be interpreted as demonstrating a strong dependence of SID flare production on Mt. Wilson magnetic class of the parent sunspot group. 相似文献
163.
The Apollo orbital geochemistry, photogeologic, and other remote sensing data sets were used to identify and characterize geochemical anomalies on the eastern limb and farside of the Moon and to investigate the processes responsible for their formation. The anomalies are located in the following regions: (1) Balmer basin, (2) terrain northeast of Mare Smythii, (3) near Langemak crater, (4) Pasteur crater, (5) terrain northwest of Milne basin, (6) northeast of Mendeleev basin, (7) north and northeast of Korolev basin, (8) terrain north of Taruntius crater, and (9) terrain north of Orientale basin. The anomalies are commonly associated with Imbrian- or Nectarian-aged light plains units which exhibit dark-haloed impact craters. The results of recent spectral reflectance studies of dark-haloed impact craters plus consideration of the surface chemistry of the anomalies strongly indicate that those geochemical anomalies associated with light plains deposits which display dark-haloed impact craters result from the presence of basaltic units that are either covered by varying thickness of highland debris or have a surface contaminated with significant amounts of highlands material. The burial or contamination of ancient volcanic surfaces by varying amounts of highland material appears to have been an important (though not the dominant) process in the formation of lunar light plains. Basaltic volcanism on the eastern limb and farside of the Moon was more extensive in both space and time than has been accepted. 相似文献
164.
M. A. Iatì C. Cecchi-Pestellini D. A. Williams F. Borghese P. Denti R. Saija S. Aiello 《Monthly notices of the Royal Astronomical Society》2001,322(4):749-756
Recent observations of stellar composition suggest that elements in the Sun are significantly more abundant than in other stars. The reduction in the available element budget implies a drastic revision in current models of interstellar dust. Theoretical models are therefore exploring fluffy, porous physical structure for the grain material. Since a detailed exact treatment of extinction cross-sections is mandatory for a correct understanding of the nature of interstellar dust, we present a technique based on the multipole expansions of the electromagnetic field, which has proven to be general, flexible and powerful in treating scattering of light by porous, composite, arbitrarily shaped particles. The results of this study speak in favour of core–mantle structures characterized by the presence of porosities. 相似文献
165.
Using the basic Boussinesq's equation, the expression for the vertical stress distribution (σz) underneath any point on the ground surface due to a general triangular loaded region in a preferred orientation with a linearly varied loading has been successfully derived. When the triangle is not in a preferred orientation, a simple axis transformation is required and the expression will be equally applicable. Based on this expression, σz due to an arbitrarily shaped loaded foundation can simply be determined by first triangulating the loaded area and summing up the contributions from each generated triangular region. The procedures for triangulating and calculating the stress distribution can be simply automated through computer programs. 相似文献
166.
High nitrate contents (up to 200 mgl) have been observed in wells drilled into fractured aquifers lying beneath layers of weathered and decayed rock in the humid tropics where annual rainfall is over 1000 mm and where there is no notable pollution. The source of the nitrates is linked mainly with localized deforestation by man. 相似文献
167.
This study examines the depletion of ferromagnesian silicate minerals from a sequence of thin, distal, mainly rhyolitic tephra layers of Holocene age preserved in an acid peat bog (Kopouatai), North Island, New Zealand. The rate of such depletion has been fast, as indicated by the complete loss of biotite from one tephra layer (Kaharoa Tephra), in which it is normally dominant, in only ca. 770 yr. Chemical dissolution is advocated as the likely cause for the depletion, with amphiboles and other mineral grains commonly showing etch pits, microcaves, and other characteristic surface solution features. Theoretical thermodynamic and kinetic models show a marked increase in the rate of dissolution of all ferromagnesian minerals under conditions of low pH (< 4), but that where silica concentrations in solution are high the relative proportions of minerals remaining are unaffected. However, where concentrations of dissolved silica are low, as in most bog environments, the relative proportions of ferromagnesian minerals are affected as well as absolute amounts being decreased. Amphiboles are depleted relative to pyroxenes, consistent with kinetic studies. The results show that the identification and correlation of tephras on the basis of relative abundances of ferromagnesian minerals alone may be unreliable, and emphasise the need to use multiple criteria in such studies. 相似文献
168.
V.V. Orlov A.V. Petrova V.G. Tarantaev 《Monthly notices of the Royal Astronomical Society》2001,325(1):133-137
The intrinsic, three-dimensional shapes of small galaxy groups, containing between three and eight members, are evaluated using three different statistics: (i) the mean sum of square sines of angles in all possible triangles formed by members of the group; (ii) the variance of square paired separations in the group; (iii) the axial ratio of a rectangle containing the group. The mean values of these parameters and their rms deviations are calculated for observed galaxy groups and simulated groups with members that are distributed randomly within prolate or oblate spheroids. Comparison of observational data and simulations shows that the observed galaxy groups have shapes consistent with the projected shapes of prolate or oblate spheroids with axial ratios of 3:1, regardless of their multiplicity, but inconsistent with the projected shapes of spherical objects. 相似文献
169.
The axisymmetric elastic response of circular footings and anchor plates in a linearly non-homogeneous elastic soil is analysed. It is assumed that footings/anchors are flexible and subjected to axisymmetric vertical loads. The response of the footing/anchor is modelled by using the classical Poisson–Kirchhoff thin plate theory. A variational technique is used to analyse the interaction problem. A representation for the contact stress is established by using a fundamental solution corresponding to a unit vertical pressure acting over an annular region in the interior of the non-homogeneous soil. The fundamental solution can be derived by using rigorous analytical procedures. The influence of the footing flexibility and the degree of soil non-homogeneity on the displacements, bending moments and contact stresses of a surface footing is examined over a wide range of governing parameters. In the case of anchor plates the influence of depth of embedment, degree of soil non-homogeneity and anchor flexibility on the anchor displacement is investigated. 相似文献
170.