首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   112973篇
  免费   1530篇
  国内免费   775篇
测绘学   2574篇
大气科学   7466篇
地球物理   22093篇
地质学   40290篇
海洋学   10109篇
天文学   26152篇
综合类   296篇
自然地理   6298篇
  2022年   718篇
  2021年   1227篇
  2020年   1325篇
  2019年   1448篇
  2018年   3073篇
  2017年   2830篇
  2016年   3344篇
  2015年   1760篇
  2014年   3274篇
  2013年   5870篇
  2012年   3560篇
  2011年   4688篇
  2010年   4214篇
  2009年   5447篇
  2008年   4719篇
  2007年   4835篇
  2006年   4477篇
  2005年   3284篇
  2004年   3221篇
  2003年   3023篇
  2002年   2990篇
  2001年   2587篇
  2000年   2526篇
  1999年   2048篇
  1998年   2140篇
  1997年   1980篇
  1996年   1720篇
  1995年   1722篇
  1994年   1475篇
  1993年   1379篇
  1992年   1293篇
  1991年   1328篇
  1990年   1338篇
  1989年   1185篇
  1988年   1087篇
  1987年   1293篇
  1986年   1118篇
  1985年   1396篇
  1984年   1589篇
  1983年   1515篇
  1982年   1377篇
  1981年   1335篇
  1980年   1176篇
  1979年   1102篇
  1978年   1083篇
  1977年   953篇
  1976年   951篇
  1975年   926篇
  1974年   896篇
  1973年   990篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
942.
A simple and general technique to obtain a six-component earthquake input motion of a rigid foundation using three-linear-components earthquake record is presented. Using the multicomponent input, the maximum structural response to an arbitrarily oriented earthquake is derived. The method is applied to determine the response spectrum for a two-d.o.f. laterally and torsionally coupled linear system. The effects of foundation geometry, structural properties and apparent wave velocity on the structural response are investigated. It is found that increasing the foundation size reduces the peak values of the input motion; this reduction is more pronounced for a square foundation than for a circular one.  相似文献   
943.
Libya occupies a large part of the Sahara Desert, and is a predominantly arid country with extremely scarce freshwaters. Despite aridity, a distinct higher rainfall zone exists along its mediterranean coastal belt. A lotic environment (perennial stream) in Ain-Scersciara, Tarhunãh in northwest Libya was studied for the species composition of diatom communities and their ecological characteristics. The waters are generally alkaline (pH = 7.5) and of hard-water (conductivity = 2008 μS/cm) type. The epiphytic diatom growth on benthic filamentous algae was high (32 taxa) with Amphora veneta as a very common taxon. Gomphonema parvulum var. micropus was an important taxon found in scrapings from the leaves of macrophytes and stony substratum. Nitzschia and Navicula showed high species richness (9 taxa in each) followed by Gomphonema (6 taxa). The ecological attributes of the majority of taxa show an oligohalobe status and alkaliphilous indicator.  相似文献   
944.
945.
Examples show that the sampling operation–i.e., the change from the continuous time domain to the discrete time domain–does not necessarily preserve the minimum-phase property. Further examples can be constructed to show that the resampling operation on the discrete time domain does not necessarily preserve the minimum-phase property. Finally it can be shown that the minimum-phase property can be either created or destroyed by sampling or resampling.  相似文献   
946.
In a companion paper1 the authors show that the parameters of an absorber which will minimize the resonant response of a simple elastic body can be determined from known results by treating the body as an equivalent single degree-of-freedom system. In this paper cylindrical shells are considered as examples of dynamically complex structures, for which the ratio of the natural frequencies of adjacent modes tends towards unity. It is shown that as dynamic complexity increases optimum absorber parameters for the reduction of resonant response deviate increasingly from those for an equivalent single degree-of-freedom system. Absorbers can be used also to reduce the random response of structures. Simple expressions for optimum parameters are given for an undamped main system, which has one degree of freedom and is subjected to white noise excitation. Optimum absorber parameters for beams, plates and cylindrical shells show similar qualitative behaviour for random and harmonic response with the concept of an equivalent single degree-of-freedom system being applicable only for the simpler structures.  相似文献   
947.
It is shown that the so-called Kirchhoff-summation operator is of a very wide-band nature and even contains an evanescent part. As a consequence, discretization may cause serious aliasing errors, particularly for small extrapolation steps. It is proposed to use in all practical cases band-limited versions of the summation operator, the spatial cut-off frequency being determined by the spatial Fourier spectrum of the coherent noise.  相似文献   
948.
A statistical analysis of volcanic aerosol based on 1,083 variables including concentration, grain-size and site variabilities establishes correlations between sites and among chemical elements.Variation curves of element concentration show parallel behaviour. High coefficients of correlation between anions and cations confirm associations between elements.  相似文献   
949.
Scattering of antiplane shear waves (SH) in two dimensions by surface and near-surface defects in a homogeneous, isotropic elastic semi-infinite medium has been studied. Attention has been focused here in the range of medium to long wavelengths. A combined finite element and analytical technique has been used to study the problems of scattering by semi-circular and triangular canyons. The results for the former case are compared with the known exact solution and those for the latter case are compared with some available approximate solutions. Finally a problem of multiple scattering by a triangular canyon and a nearby circular tunnel is studied. Numerical results are presented showing the effects of multiple scattering and different angles of incidence. These results are of interest in earthquake engineering.  相似文献   
950.
A reservoir of water is contained by a concrete valley block, a ferrocement wall and a steel plate. Both wall and plate contain an array of pressure transducer sockets (Figures 1 and 2). Using the M.A.M.A.1 equipment pure modes of vibration are excited. Frequency and mode shape are measured with the reservoir empty. When the reservoir is full hydrodynamic pressure is also measured. These hydrodynamic pressures are compared with Chopra's2 two-dimensional, series solution, which includes compressibility of water, and with two- and three-dimensional finite element solutions of Laplace's equation, which do not include compressibility. Chopra's solution is unsatisfactory for modes which contain a vertical node line. The best agreement between experimental and theoretical hydrodynamic pressure is obtained when the latter is obtained from three-dimensional solutions of Laplace's equations, indicating that compressibility does not play a significant rǒle. This conclusion is supported by agreement between experimental frequencies (reservoir full) and those calculated using added mass obtained from the Laplace solution. Similar conclusions were reached from tests on a floating steel plate, suspended in the surface of the reservoir by a soft spring. Here, dynamic pressure measurements were not made, reliance being placed on agreement between calculated and measured frequencies and mode shapes.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号