全文获取类型
收费全文 | 179192篇 |
免费 | 14344篇 |
国内免费 | 18950篇 |
专业分类
测绘学 | 12513篇 |
大气科学 | 15352篇 |
地球物理 | 34691篇 |
地质学 | 79071篇 |
海洋学 | 21152篇 |
天文学 | 30067篇 |
综合类 | 4413篇 |
自然地理 | 15227篇 |
出版年
2023年 | 1267篇 |
2022年 | 3801篇 |
2021年 | 5148篇 |
2020年 | 4453篇 |
2019年 | 5359篇 |
2018年 | 6557篇 |
2017年 | 6129篇 |
2016年 | 6928篇 |
2015年 | 5678篇 |
2014年 | 7245篇 |
2013年 | 11120篇 |
2012年 | 8986篇 |
2011年 | 10301篇 |
2010年 | 9595篇 |
2009年 | 10729篇 |
2008年 | 9944篇 |
2007年 | 9609篇 |
2006年 | 9140篇 |
2005年 | 7174篇 |
2004年 | 6203篇 |
2003年 | 5273篇 |
2002年 | 5153篇 |
2001年 | 4743篇 |
2000年 | 4282篇 |
1999年 | 2971篇 |
1998年 | 2720篇 |
1997年 | 2469篇 |
1996年 | 2103篇 |
1995年 | 2072篇 |
1994年 | 1765篇 |
1993年 | 1661篇 |
1992年 | 1592篇 |
1991年 | 1497篇 |
1990年 | 1588篇 |
1989年 | 1360篇 |
1988年 | 1243篇 |
1987年 | 1502篇 |
1986年 | 1307篇 |
1985年 | 1589篇 |
1984年 | 1790篇 |
1983年 | 1702篇 |
1982年 | 1593篇 |
1981年 | 1426篇 |
1980年 | 1307篇 |
1979年 | 1271篇 |
1978年 | 1209篇 |
1977年 | 1055篇 |
1976年 | 1039篇 |
1975年 | 995篇 |
1973年 | 1036篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
81.
M. A. Mansour 《Astrophysics and Space Science》1990,166(2):269-275
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
82.
Faith Vilas David J. Tholen Larry A. Lebofsky Humberto Campins Glenn J. Veeder Richard P. Binzel Alan T. Tokunaga 《Icarus》1985,63(2):201-205
Visual and infrared observations were made of Amor asteroid 1982 DV during its discovery apparition. Broadband visual and near-infrared photometry shows that it is an S-class asteroid. Narrowband spectrophotometry shows an absorption feature due to olivine or pyroxene or both centered at 0.93 μm. Applying a nonrotating thermal model to 10-μm photometry, the geometric albedo is calculated to be approximately 0.27. The geometric albedo for a slowly rotating, rocky surface was calculated for 1 night to be 0.15, consistent with S-class asteroid albedos. Thus, 1982 DV is either one of the most reflective S-class asteroids known, or a significant amount of bare rock is exposed on the asteroid's surface. For the nonrotating model, ellipsoidal dimensions for 1982 DV are 3.5 × 1.4 × 1.4 km. 相似文献
83.
84.
A numerical scheme is developed in order to simulate fluid flow in three dimensional (3‐D) microstructures. The governing equations for steady incompressible flow are solved using the semi‐implicit method for pressure‐linked equations (SIMPLE) finite difference scheme within a non‐staggered grid system that represents the 3‐D microstructure. This system allows solving the governing equations using only one computational cell. The numerical scheme is verified through simulating fluid flow in idealized 3‐D microstructures with known closed form solutions for permeability. The numerical factors affecting the solution in terms of convergence and accuracy are also discussed. These factors include the resolution of the analysed microstructure and the truncation criterion. Fluid flow in 2‐D X‐ray computed tomography (CT) images of real porous media microstructure is also simulated using this numerical model. These real microstructures include field cores of asphalt mixes, laboratory linear kneading compactor (LKC) specimens, and laboratory Superpave gyratory compactor (SGC) specimens. The numerical results for the permeability of the real microstructures are compared with the results from closed form solutions. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
85.
隧道工程地质评价的内容和方法 总被引:2,自引:0,他引:2
结合工程隧道实际,首先阐述了隧道工程的基本地质环境和工程地质条件,然后针对隧道工程可能出现的不良地质现象和可采取的工程措施,从大气降水、围岩稳定、围岩压力、洞口稳定、隧道比选等角度探讨隧道工程地质评价的主要方法和一般内容,进行隧道工程地质评价,为隧道施工、支护提供了依据. 相似文献
86.
87.
88.
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
89.
M.S. Hanner E. Tedesco A.T. Tokunaga G.J. Veeder D.F. Lester F.C. Witteborn J.D. Bregman J. Gradie L. Lebofsky 《Icarus》1985,64(1):11-19
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
90.
黄土高原古植被与再造山川秀美 总被引:7,自引:4,他引:3
李秉成 《地球科学与环境学报》2004,26(1):85-88
要想恢复秀美的山川,主要在于恢复被人类严重破坏的原始植被.黄土高原的原始植被到底是什么样子,学者们说法不一.孢粉分析是最重要的方法.目前,研究结果已揭示出黄土高原古植被在第四纪以来显示着时间上、空间上的变化.古土壤代表间冰期夏季风盛行的湿热气候,植被比较繁盛;黄土代表冰期冬季风盛行的干冷气候,植被凋凌.全新世中期5000~6000年前,现代间冰期气候最佳时期,半坡人生存之时,森林茂密并有亚热带植物入侵,但兰州一带1万年来一直保持着荒漠草原的面貌.因此,黄土高原植被的恢复应参照各地的原始植被并考虑现代气候条件,因地制宜地实施.在东南部可以恢复森林,中西部可以恢复森林草原,西北部只能种草恢复草原. 相似文献