全文获取类型
收费全文 | 27616篇 |
免费 | 6284篇 |
国内免费 | 6955篇 |
专业分类
测绘学 | 2761篇 |
大气科学 | 4280篇 |
地球物理 | 7509篇 |
地质学 | 15520篇 |
海洋学 | 3572篇 |
天文学 | 1287篇 |
综合类 | 2615篇 |
自然地理 | 3311篇 |
出版年
2024年 | 240篇 |
2023年 | 669篇 |
2022年 | 1483篇 |
2021年 | 1692篇 |
2020年 | 1430篇 |
2019年 | 1653篇 |
2018年 | 1854篇 |
2017年 | 1708篇 |
2016年 | 1919篇 |
2015年 | 1941篇 |
2014年 | 2085篇 |
2013年 | 2143篇 |
2012年 | 2028篇 |
2011年 | 2071篇 |
2010年 | 2018篇 |
2009年 | 1800篇 |
2008年 | 1705篇 |
2007年 | 1490篇 |
2006年 | 1190篇 |
2005年 | 1101篇 |
2004年 | 848篇 |
2003年 | 821篇 |
2002年 | 943篇 |
2001年 | 854篇 |
2000年 | 780篇 |
1999年 | 761篇 |
1998年 | 552篇 |
1997年 | 463篇 |
1996年 | 455篇 |
1995年 | 354篇 |
1994年 | 384篇 |
1993年 | 319篇 |
1992年 | 265篇 |
1991年 | 161篇 |
1990年 | 137篇 |
1989年 | 106篇 |
1988年 | 101篇 |
1987年 | 64篇 |
1986年 | 54篇 |
1985年 | 46篇 |
1984年 | 23篇 |
1983年 | 20篇 |
1982年 | 26篇 |
1981年 | 21篇 |
1980年 | 16篇 |
1979年 | 16篇 |
1976年 | 3篇 |
1958年 | 17篇 |
1957年 | 8篇 |
1954年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
In many areas of engineering practice, applied loads are not uniformly distributed but often concentrated towards the centre of a foundation. Thus, loads are more realistically depicted as distributed as linearly varying or as parabola of revolution. Solutions for stresses in a transversely isotropic half‐space caused by concave and convex parabolic loads that act on a rectangle have not been derived. This work proposes analytical solutions for stresses in a transversely isotropic half‐space, induced by three‐dimensional, buried, linearly varying/uniform/parabolic rectangular loads. Load types include an upwardly and a downwardly linearly varying load, a uniform load, a concave and a convex parabolic load, all distributed over a rectangular area. These solutions are obtained by integrating the point load solutions in a Cartesian co‐ordinate system for a transversely isotropic half‐space. The buried depth, the dimensions of the loaded area, the type and degree of material anisotropy and the loading type for transversely isotropic half‐spaces influence the proposed solutions. An illustrative example is presented to elucidate the effect of the dimensions of the loaded area, the type and degree of rock anisotropy, and the type of loading on the vertical stress in the isotropic/transversely isotropic rocks subjected to a linearly varying/uniform/parabolic rectangular load. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
42.
采用正交试验等方法对淮山提取液(DB-Ext)代替羧甲基纤维素钠(CMC)作为调配型酸奶稳定剂的效果进行了初步探讨。结果表明以原料的6.0倍水量在110℃的温度条件下,提取40min,所得的淮山提取液的粘度为3500mPa 相似文献
43.
IntroductionThe area of eastern Liaoning is an importantmetal and nonmetal metallogenetic district in China,and the Liaohe group is one of the most importantstrata that hosts Pb, Zn, Au, B and Mg etcstratabound deposits. Up to now many geo1ogistssuch as Z… 相似文献
44.
This paper presents a method that incorporates a non‐associated flow rule into the limit analysis to investigate the influence of the dilatancy angle on the factor of safety for the slope stability analysis. The proposed method retain's the advantage of the upper bound method, which is simple and has no stress involvement in the calculation of the energy dissipation and the factor of safety. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
45.
徐江 《地球信息科学学报》1996,(1):21-25
本文阐述了地理信息系统在环境地球化学中的应用,提出建立区域环境地球化学空间信息系统的方法、功能及组成,并以红枫湖区域环境地球化学空间信息系统为例,作了分析。 相似文献
46.
Abstract— Abundances and isotopic compositions of noble gases in metal and graphite of the Bohumilitz IAB iron meteorite were measured. The abundance ratios of spallogenic components in metal reveal a 3He deficiency which is due to the diffusive loss of parent isotopes, that is, tritium (Tilles, 1963; Schultz, 1967). The diffusive loss likely has been induced by thermal heating by the Sun during cosmic‐ray exposure (~160 Ma; Lavielle et al, 1999). Thermal process such as impact‐induced partial loss may have affected the isotopic composition of spallogenic Ne. The 129Xe/131Xe ratio of cosmogenic components in the metal indicates an enhanced production of epi‐thermal neutrons. The abundance ratios of spallogenic components in the graphite reveal that it contained small amounts of metal and silicates. The isotopic composition of heavy noble gases in graphite itself was obtained from graphite treated with HF/HCl. The isotopic composition of the etched graphite shows that it contains two types of primordial Xe (i.e., Q‐Xe and El Taco Xe). The isotopic heterogeneity preserved in the Bohumilitz graphite indicates that the Bohumilitz graphite did not experience any high‐temperature event and, consequently, must have been emplaced into the metal at subsolidus temperatures. This situation is incompatible with an igneous model as well as the impact melting models for the IAB‐IIICD iron meteorites as proposed by Choi et al. (1995) and Wasson et al (1980). 相似文献
47.
In this paper, we reconstruct the finite energy force-free magnetic field of the active region NOAA 8100 on 4 November 1997
above the photosphere. In particular, the 3-D magnetic field structures before and after a 2B/X2 flare at 05:58 UT in this
region are analyzed. The magnetic field lines were extrapolated in close coincidence with the Yohkoh soft X-ray (SXR) loops accordingly. It is found that the active region is composed of an emerging flux loop, a complex loop
system with differential magnetic field shear, and large-scale, or open field lines. Similar magnetic connectivity has been
obtained for both instants but apparent changes of the twisting situations of the calculated magnetic field lines can be observed
that properly align with the corresponding SXR coronal loops. We conclude that this flare was triggered by the interaction
of an emerging flux loop and a large loop system with differential magnetic field shear, as well as large-scale, or open field
lines. The onset of the flare was at the common footpoints of several interacting magnetic loops and confined near the footpoints
of the emerging flux loop. The sheared configuration remained even after the energetic flare, as demonstrated by calculated
values of the twist for the loop system, which means that the active region was relaxed to a lower energy state but not completely
to the minimum energy state (two days later another X-class flare occurred in this region). 相似文献
48.
Steven J. Ostro Lance A. M. Benner Christopher Magri Jon D. Giorgini Randy Rose Raymond F. Jurgens Donald K. Yeomans Alice A. Hine Michael C. Nolan Daniel J. Scheeres Stephen B. Broschart Mikko Kaasalainen Jean‐Luc Margot 《Meteoritics & planetary science》2005,40(11):1563-1574
Abstract— We present June 2004 radar images of asteroid 25143 Itokawa (1998 SF36) that improve upon the longitude‐latitude coverage of images obtained in 2001 by Ostro et al. (2004) and use the 2001–2004 data to refine that paper's constraints on Itokawa's shape. The 2004 images, the first of the asteroid's southern side, look distinctly different from the 2001 images, revealing leading edges that are much more curved and rugged than the nearly convex leading edges seen at northern latitudes in 2001. Itokawa is shaped like a slightly asymmetrical, bent, lumpy ellipsoid with dimensions along the principal axes within 10% of 594 times 320 times 288 m. To illustrate the uncertainty space associated with shape reconstruction from images with suboptimal orientational coverage, we present two alternative three‐dimensional models of the object. 相似文献
49.
50.
To seek nonlinear solutions of force-free magnetic fields, some symmetries or approximations are usually invoked. We consider magnetic fields lying on coordinate surfaces of an orthogonal curvilinear coordinate system. We conclude that only fields on parallel planes or spherical shells can be expressed in the form provided by Low in 1980s. These force-free fields are stable against small perturbations with rigid boundaries. Fields on cylindrical shells are also considered. 相似文献