全文获取类型
收费全文 | 76851篇 |
免费 | 2869篇 |
国内免费 | 4143篇 |
专业分类
测绘学 | 3031篇 |
大气科学 | 6852篇 |
地球物理 | 14996篇 |
地质学 | 32441篇 |
海洋学 | 6659篇 |
天文学 | 11820篇 |
综合类 | 3087篇 |
自然地理 | 4977篇 |
出版年
2023年 | 380篇 |
2022年 | 962篇 |
2021年 | 1334篇 |
2020年 | 1288篇 |
2019年 | 1389篇 |
2018年 | 6467篇 |
2017年 | 5699篇 |
2016年 | 4418篇 |
2015年 | 1527篇 |
2014年 | 2027篇 |
2013年 | 2811篇 |
2012年 | 3022篇 |
2011年 | 5187篇 |
2010年 | 4216篇 |
2009年 | 4810篇 |
2008年 | 4186篇 |
2007年 | 4710篇 |
2006年 | 2317篇 |
2005年 | 1650篇 |
2004年 | 1803篇 |
2003年 | 1899篇 |
2002年 | 1699篇 |
2001年 | 1346篇 |
2000年 | 1175篇 |
1999年 | 1095篇 |
1998年 | 1014篇 |
1997年 | 1019篇 |
1996年 | 842篇 |
1995年 | 819篇 |
1994年 | 727篇 |
1993年 | 630篇 |
1992年 | 570篇 |
1991年 | 529篇 |
1990年 | 526篇 |
1989年 | 500篇 |
1988年 | 443篇 |
1987年 | 462篇 |
1986年 | 471篇 |
1985年 | 551篇 |
1984年 | 587篇 |
1983年 | 578篇 |
1982年 | 533篇 |
1981年 | 496篇 |
1980年 | 470篇 |
1979年 | 433篇 |
1978年 | 391篇 |
1977年 | 400篇 |
1976年 | 363篇 |
1975年 | 371篇 |
1973年 | 384篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
Grid method is employed for sampling covering soil at the test field,which is reclamation area filled by coal mining wastes for cropland in the Fushun coal mine,Liaoning Province,the Northeast China.The soil samples are taken at different locations,including three kinds of covering soil,three different depths of soil layers and four different covering ages of covering soil.The spatial-temporal variation of heavy metal element content in reclamation soil is stud-ied.The results indicate that the content of heavy metal elements is decreasing year after year,the determinant reason why the content of heavy metal elements at 60cm depth layer is higher than that at 30cm depth layer and surface is fertiliz-er and manure application;the metal elements mainly come from external environment;there is no metal pollution coming from mother material (coal mining wastes)in plough layer of covering soil. 相似文献
12.
Andrew J. Westphal Christopher Snead Janet Borg Eric Quirico Pierre‐Ivan Raynal Michael E. Zolensky Gianluca Ferrini Luigi Colangeli Pasquale Palumbo 《Meteoritics & planetary science》2002,37(6):855-865
Abstract— It has now been about a decade since the first demonstrations that hypervelocity particles could be captured, partially intact, in aerogel collectors. But the initial promise of a bonanza of partially‐intact extraterrestrial particles, collected in space, has yet to materialize. One of the difficulties that investigators have encountered is that the location, extraction, handling and analysis of very small (10 μm and less) grains, which constitute the vast majority of the captured particles, is challenging and burdensome. Furthermore, current extraction techniques tend to be destructive over large areas of the collectors. Here we describe our efforts to alleviate some of these difficulties. We have learned how to rapidly and efficiently locate captured particles in aerogel collectors, using an automated microscopic scanning system originally developed for experimental nuclear astrophysics. We have learned how to precisely excavate small access tunnels and trenches using an automated micromanipulator and glass microneedles as tools. These excavations are only destructive to the collector in a very small area—this feature may be particularly important for excavations in the precious Stardust collectors. Using actuatable silicon microtweezers, we have learned how to extract and store “naked” particles—essentially free of aerogel—as small as 3 μm in size. We have also developed a technique for extracting particles, along with their terminal tracks, still embedded in small cubical aerogel blocks. We have developed a novel method for storing very small particles in etched nuclear tracks. We have applied these techniques to the extraction and storage of grains captured in aerogel collectors (Particle Impact Experiment, Orbital Debris Collector Experiment, Comet‐99) in low Earth orbit. 相似文献
13.
14.
15.
AN ILLUSTRATIVE STUDY ON LOCAL LANDSCAPEAND ITS LONG-TERM CHANGES BASED ON IKONOS AND HISTORICAL AERIAL PHOTO 总被引:2,自引:0,他引:2
RUANRen-zong ErieCELLISt 《中国地理科学(英文版)》2004,14(2):162-169
Nowadays, the research works on landscape at fine scales using high-resolution images are uncommon.This research is based on the analysis of the combination of remote sensing data (1KONOS imagery acquired in 2002 and historical aerial photo taken in 1942). In the paper, the ecotopes in Qiujiadou and Xishao villages in Yixing City of Jiangsu Province in 1942 and 2002 were compared and landscape changes as well as the causes of the considerable changes were analyzed. It was found that the ecotope changes were at greater level in some aspects such as water surface and perennial vegetation coverage etc. This study at fine scale is globally significant for the rural areas, especially for the subsistence agricultural land, which occupies larger percentage in the earth. And it analyzes the structure of landscape based on a new landscape classification system--stratifications method. 相似文献
16.
Mallory S. E. Roberts Crystal L. Brogan Bryan M. Gaensler Jason W. T. Hessels C.-Y. Ng Roger W. Romani 《Astrophysics and Space Science》2005,297(1-4):93-100
A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET γ-ray sources. X-ray and radio imaging studies of unidentified EGRET sources have resulted in the discovery of at least six new pulsar wind nebulae (PWN). Stationary PWN (SPWN) appear to be
associated with steady EGRET sources with hard spectra, typical for γ-ray pulsars. Their toroidal morphologies can help determine the geometry of the
pulsar which is useful for constraining models of pulsed γ-ray emission. Rapidly moving PWN (RPWN) with more cometary morphologies
seem to be associated with variable EGRET sources in regions where the ambient medium is dense compared to what is typical for the ISM. 相似文献
17.
E. C. Hopewell M. J. Barlow J. E. Drew Y. C. Unruh Q. A. Parker M. J. Pierce P. A. Crowther C. Knigge S. Phillipps A. A. Zijlstra 《Monthly notices of the Royal Astronomical Society》2005,363(3):857-866
We report the discovery of five massive Wolf–Rayet (WR) stars resulting from a programme of follow-up spectroscopy of candidate emission-line stars in the Anglo-Australian Observatory United Kingdom Schmidt Telescope (AAO/UKST) Southern Galactic Plane Hα survey. The 6195–6775 Å spectra of the stars are presented and discussed. A WC9 class is assigned to all five stars through comparison of their spectra with those of known late-type WC stars, bringing the known total number of Galactic WC9 stars to 44. Whilst three of the five WC9 stars exhibit near-infrared (NIR) excesses characteristic of hot dust emission (as seen in the great majority of known WC9 stars), we find that two of the stars show no discernible evidence of such excesses. This increases the number of known WC9 stars without NIR excesses to seven. Reddenings and distances for all five stars are estimated. 相似文献
18.
19.
ENVIRONMENTAL SUSTAINABILITY AND SCENARIOS OF URBANIZATION IN ARID AREA——A Case Study in Wuwei City of Gansu Province 总被引:1,自引:0,他引:1
YOUFei LIYu DONGSuo-cheng 《中国地理科学(英文版)》2005,15(2):120-130
1IN T R O D U C T IO N The western China isfacinggrowing problems of eco- logicaland economic development. The disparitiebse- tween the socio-economy of the western and eastern China were enlarging, and the major ecologicalprob- lems stilelvolve as past(H… 相似文献
20.
The main properties of the first- and second-order moments of polarized hydrogen lines, forming in the presence of stationary electric and magnetic fields, are reviewed. The analytical results presented here apply directly to the case of optically-thin emission lines in the LTE regime. Some applications of such results to electric- and magnetic-field diagnostics in (solar) plasmas are then briefly considered.On leave from the Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, Largo E. Fermi 5, I-50125 Firenze, Italy 相似文献