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   132784篇
  免费   2672篇
  国内免费   1161篇
测绘学   3343篇
大气科学   9125篇
地球物理   27326篇
地质学   47520篇
海洋学   11451篇
天文学   29772篇
综合类   531篇
自然地理   7549篇
  2022年   771篇
  2021年   1311篇
  2020年   1466篇
  2019年   1551篇
  2018年   3776篇
  2017年   3528篇
  2016年   4110篇
  2015年   2238篇
  2014年   3839篇
  2013年   6873篇
  2012年   4435篇
  2011年   5578篇
  2010年   4747篇
  2009年   6047篇
  2008年   5539篇
  2007年   5559篇
  2006年   5140篇
  2005年   4540篇
  2004年   4447篇
  2003年   4023篇
  2002年   3422篇
  2001年   3146篇
  2000年   2868篇
  1999年   2377篇
  1998年   2419篇
  1997年   2403篇
  1996年   1951篇
  1995年   1950篇
  1994年   1733篇
  1993年   1582篇
  1992年   1541篇
  1991年   1495篇
  1990年   1667篇
  1989年   1465篇
  1988年   1359篇
  1987年   1544篇
  1986年   1295篇
  1985年   1691篇
  1984年   1891篇
  1983年   1776篇
  1982年   1695篇
  1981年   1544篇
  1980年   1422篇
  1979年   1310篇
  1978年   1344篇
  1977年   1191篇
  1976年   1146篇
  1975年   1065篇
  1974年   1114篇
  1973年   1109篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
71.
The interaction of free convection with thermal radiation of the oscillatory flow past a vertical plate is studied. The Rosseland approximation is used to describe the radiative heat flux in the energy equation.  相似文献   
72.
A transformation of the differential equations of motion of the two-body problem in the spherical coordinates to oscillator form is derived. It is shown that the independent variable transformation dt/ds=r2 is a transformation which makes the oscillator form possible.  相似文献   
73.
74.
75.
The results of a general theoretical investigation of three commonly used types of inductive conductivity sensors, i.e., the single transformer, the double transformer, and the double transformer with an additional loop, are presented. The resulting formulas describe the dependence of the sensor output signal not only on the conductivity of the seawater but also on the parameters of the electrical circuit, among them the permeability of the transformer core(s), which-unlike the other parameters-shifts considerably during oceanographic in situ measurements. A mathematical discussion of these formulas shows that for certain circuit configurations, the sensor output is independent of changes in permeability. Most of these configurations form the basis of existing oceanographical conductivity sensors, among them the "classical" sensors developed by H. Hinkelmann [3], [4], and by N. L. Brown [14], while some others make evident further possibilities for eliminating the unwanted effects of shifting permeability. In the era of microelectronics, the latter might lead to a reassessment, especially of the single transformer-type sensor.  相似文献   
76.
Transverse secondary circulations involving surface convergence, observed in a well-mixed estuary in North Wales, are made visible by the collection of surface material along an axial line which extends continuously for many kilometres through the estuary. The circulation and axial convergence, however, are seen only during the flood phase of the tide and no similar behaviour has been observed during the ebb phase.Convergent circulations in the estuary are associated with small but steady transverse density gradients in the cross-section, produced by non-uniform advection of the longitudinal gradient through the channel. A diagnostic model, using measured mean distributions of cross-sectional density, indicates surface transverse velocities (~0.1 ms?1) similar to those observed in the estuary. The model further predicts appreciable transverse divergent currents at a fractional depth of 0.75: a prediction which has been tested in the estuary using a vertical array of accurately resolving current direction indicators.  相似文献   
77.
78.
The dust coma of Comet P/Churyumov-Gerasimenko was monitored in the infrared (1–20 μm) from September 1982 to March 1983. Maximum dust production rate of ~2 × 105 g/sec occured in December, 1 month postperihelion. The ratio of dust/gas production was higher than that in other short-period comets. No silicate feature was visible in the 8- to 13-μm spectrum on 23 October. The mean geometric albedo of the grains was ~0.04 at 1.25 μm and ~0.05 at 2.2 μm.  相似文献   
79.
A coupled continuum‐discrete hydromechanical model was employed to analyse the liquefaction of a saturated loose deposit of cohesionless particles when subjected to a dynamic base excitation. The pore fluid flow was idealized using averaged Navier–Stokes equations and the discrete element method was employed to model the solid phase particles. A well established semi‐empirical relationship was utilized to quantify the fluid–particle interactions. The conducted simulations revealed a number of salient micro‐mechanical mechanisms and response patterns associated with the deposit liquefaction. Space and time variation of porosity was a major factor which affected the coupled response of the solid and fluid phases. Pore fluid flow was within Darcy's regime. The predicted response exhibited macroscopic patterns consistent with experimental results and case histories of the liquefaction of granular soil deposits. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   
80.
Two distinct age estimates for eclogite-facies metamorphism in the Sanbagawa belt have been proposed: (i) c.  120–110 Ma based on a zircon SHRIMP age for the Western Iratsu unit and (ii) c.  88–89 Ma based on a garnet–omphacite Lu–Hf isochron age from the Seba and Kotsu eclogite units. Despite the contrasting estimates of formation ages, petrological studies suggest the formation conditions of the Western Iratsu unit are indistinguishable from those of the other two units—all ∼20 kbar and 600–650 °C. Studies of the associated geological structures suggest the Seba and Western Iratsu units are parts of a larger semi-continuous eclogite unit. A combination of geochronological and petrological studies for the Western Iratsu eclogite offers a resolution to this discrepancy in age estimates. New Lu–Hf dating for the Western Iratsu eclogite yields an age of 115.9 ± 0.5 Ma that is compatible with the zircon SHRIMP age. However, petrological studies show that there was significant garnet growth in the Western Iratsu eclogite before eclogite facies metamorphism, and the early core growth is associated with a strong concentration of Lu. Pre-eclogite facies garnet (Grt1) includes epidote–amphibolite facies parageneses equilibrated at 550–650 °C and ∼10 kbar, and this is overgrown by prograde eclogite facies garnet (Grt2). The Lu–Hf age of c.  116 Ma is strongly skewed to the isotopic composition of Grt1 and is interpreted to reflect the age of the pre-eclogite phase. The considerable time gap ( c.  27 Myr) between the two Lu–Hf ages suggests they may be related to separate tectonic events or distinct phases in the evolution of the Sanbagawa subduction zone.  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