全文获取类型
收费全文 | 56854篇 |
免费 | 13067篇 |
国内免费 | 20271篇 |
专业分类
测绘学 | 10139篇 |
大气科学 | 7543篇 |
地球物理 | 10716篇 |
地质学 | 37798篇 |
海洋学 | 10446篇 |
天文学 | 903篇 |
综合类 | 4456篇 |
自然地理 | 8191篇 |
出版年
2024年 | 688篇 |
2023年 | 1769篇 |
2022年 | 3609篇 |
2021年 | 4395篇 |
2020年 | 3523篇 |
2019年 | 4142篇 |
2018年 | 3503篇 |
2017年 | 3343篇 |
2016年 | 3537篇 |
2015年 | 3989篇 |
2014年 | 3842篇 |
2013年 | 4554篇 |
2012年 | 5217篇 |
2011年 | 5092篇 |
2010年 | 5088篇 |
2009年 | 4784篇 |
2008年 | 4614篇 |
2007年 | 4306篇 |
2006年 | 4150篇 |
2005年 | 3423篇 |
2004年 | 2541篇 |
2003年 | 1834篇 |
2002年 | 1933篇 |
2001年 | 1837篇 |
2000年 | 1443篇 |
1999年 | 733篇 |
1998年 | 324篇 |
1997年 | 269篇 |
1996年 | 210篇 |
1995年 | 175篇 |
1994年 | 141篇 |
1993年 | 140篇 |
1992年 | 161篇 |
1991年 | 67篇 |
1990年 | 94篇 |
1989年 | 65篇 |
1988年 | 76篇 |
1987年 | 55篇 |
1986年 | 80篇 |
1985年 | 50篇 |
1984年 | 65篇 |
1983年 | 40篇 |
1982年 | 42篇 |
1981年 | 20篇 |
1979年 | 43篇 |
1978年 | 22篇 |
1977年 | 15篇 |
1976年 | 16篇 |
1957年 | 21篇 |
1954年 | 29篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
The ordinary kriging method, a geostatistical interpolation technique, was applied for developing contour maps of design storm depth in northern Taiwan using intensity–duration–frequency (IDF) data. Results of variogram modelling on design storm depths indicate that the design storms can be categorized into two distinct storm types: (i) storms of short duration and high spatial variation and (ii) storms of long duration and less spatial variation. For storms of the first category, the influence range of rainfall depth decreases when the recurrence interval increases, owing to the increasing degree of their spatial independence. However, for storms of the second category, the influence range of rainfall depth does not change significantly and has an average of approximately 72 km. For very extreme events, such as events of short duration and long recurrence interval, we do not recommend usage of the established design storm contours, because most of the interstation distances exceed the influence ranges. Our study concludes that the influence range of the design storm depth is dependent on the design duration and recurrence interval and is a key factor in developing design storm contours. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
2.
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
3.
介绍一种自行设计和加工的直接雾化石墨炉进样装置的结构、工作原理和操作方法。该装置由微机控制与石墨炉加热程序同步工作,可以实现自动控制和自动测量。对雾化进样石墨炉分析的仪器参数和实验条件进行了研究和选择。结果表明,该装置与石墨炉结合具有自动化程度高、重现性好、样品利用率高和分析速度快等特点。 相似文献
4.
STUDY ON BEIJING''''S EMERGING MOBILE COMMUNICATION INDUSTRIAL CLUSTER AND ITS POLICY IMPLICATIONS 总被引:1,自引:0,他引:1
1INTRODUCTIONClusters,definedasgeographicallyproximategroupsofinterconnectedcompaniesandassociatedinstitutionsinparticularfields,linkedbycommonalitiesandcomple-mentarities(PORTER,2000),havearousedanintenseinterestofurbanandregionalplanningresearchersandpolicymakersoverthepastdecades.Itiswidelyrecog-nizedthatclusterscanpromoteproductivityandinno-vation,developlocalcompetitiveadvantages.Popular-izedbyMichaelPORTERinhisbook“TheCompetitiveAdvantageofNationsin1990,theclusterisnotanewcon… 相似文献
5.
A new set of equations of motion for wave propagation in water with varying depth is derived in this study. The equations expressed by the velocity potentials and the wave surface elevations include first-order non-linearity of waves and have the same dispersion characteristic to the extended Boussinesq equations. Compared to the extended Boussinesq equations, the equations have only two unknown scalars and do not contain spatial derivatives with an order higher than 2. The wave equations are solved by a finite element method. Fourth-order predictor–corrector method is applied in the time integration and a damping layer is applied at the open boundary for absorbing the outgoing waves. The model is applied to several examples of wave propagation in variable water depth. The computational results are compared with experimental data and other numerical results available in literature. The comparison demonstrates that the new form of the equations is capable of calculating wave transformation from relative deep water to shallow water. 相似文献
6.
7.
Monthly and seasonal panel experiments were carried out in the Yongxing Island from September 1979 to August 1981. Altogether 119 species of fouling organisms are collected, of which, Hydroides elegans, Serpula vermicular is, Scrupocellaria longispinosa, Amathia distant, Enteromorpha spp. and Ostrea spp. are dominant species. Fouling of organisms may occur all the year round with evident seasonal variations, Species succession in the process of community formation is very evident and the climax has arrived after two years. Great variation exists between fouling organisms in different years and at different stations. 相似文献
8.
In this paper, the principle and steps for differentiating water masses by fuzzy cluster method are introduced, and a scalar formula based on Euclidean distance and a method for determining objectively the number of water masses by F-test are proposed. Consequently, a method and specific steps for differentiating modified water masses in shallow sea according to fuzzy elastic classification are given. Computation of the membership degree in which each sample belongs to every water mass determines conveniently and quantitatively the cores, boundaries of water masses and mixed zones. An example for the Huanghai Sea and East China Sea is shown and compared with previous results. 相似文献
9.
10.
雪宝顶碱性花岗岩岩石地球化学与成矿控制 总被引:7,自引:0,他引:7
对雪宝顶碱性花岗岩具有高丰度的W、Sn、Be含量和W、Sn矿化花岗岩的微量元素组合特征进行了分析,结果显示:稀土元素具有∑REE含量与LREE/HREE比值偏低和负Eu异常的特点;元素比值K/Rb,U/Th,Nb/Ta的规律性变化以及矿脉产状特征等表明W-(Sn)-Be脉状矿床形成于岩浆演化晚期,与岩浆期后的热液活动有关。 相似文献