全文获取类型
收费全文 | 72249篇 |
免费 | 1509篇 |
国内免费 | 658篇 |
专业分类
测绘学 | 1790篇 |
大气科学 | 5179篇 |
地球物理 | 14758篇 |
地质学 | 25621篇 |
海洋学 | 6304篇 |
天文学 | 16281篇 |
综合类 | 215篇 |
自然地理 | 4268篇 |
出版年
2022年 | 408篇 |
2021年 | 706篇 |
2020年 | 821篇 |
2019年 | 848篇 |
2018年 | 1949篇 |
2017年 | 1802篇 |
2016年 | 2313篇 |
2015年 | 1383篇 |
2014年 | 2238篇 |
2013年 | 3876篇 |
2012年 | 2352篇 |
2011年 | 3229篇 |
2010年 | 2693篇 |
2009年 | 3614篇 |
2008年 | 3350篇 |
2007年 | 3112篇 |
2006年 | 2937篇 |
2005年 | 2396篇 |
2004年 | 2296篇 |
2003年 | 2157篇 |
2002年 | 1959篇 |
2001年 | 1851篇 |
2000年 | 1743篇 |
1999年 | 1409篇 |
1998年 | 1495篇 |
1997年 | 1401篇 |
1996年 | 1018篇 |
1995年 | 1139篇 |
1994年 | 965篇 |
1993年 | 867篇 |
1992年 | 845篇 |
1991年 | 763篇 |
1990年 | 847篇 |
1989年 | 708篇 |
1988年 | 651篇 |
1987年 | 801篇 |
1986年 | 667篇 |
1985年 | 862篇 |
1984年 | 923篇 |
1983年 | 872篇 |
1982年 | 830篇 |
1981年 | 710篇 |
1980年 | 665篇 |
1979年 | 615篇 |
1978年 | 611篇 |
1977年 | 561篇 |
1976年 | 540篇 |
1975年 | 513篇 |
1974年 | 506篇 |
1973年 | 478篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
31.
A study of presolar material in hydrated lithic clasts from metal-rich carbonaceous chondrites 下载免费PDF全文
We report on the investigation of presolar grain inventories of hydrated lithic clasts in three metal-rich carbonaceous chondrites from the CR clan, Acfer 182 (CH3), Isheyevo (CH3/CBb3), and Lewis Cliff (LEW) 85332 (C3-un), as well as the carbon- and nitrogen-isotopic compositions of the fine-grained clast material. Eleven presolar silicate grains as well as nine presolar silicon carbide (SiC) grains were identified in the clasts. Presolar silicate abundances range from 4 to 22 parts per million (ppm), significantly lower than in pristine meteorites and interplanetary dust particles (IDP), and comparable to recent findings for CM2s and CR2 interchondrule matrix. SiC concentrations lie between 9 and 23 ppm, and are comparable to the values for CI, CM, and CR chondrites. The results of our investigation suggest similar alteration pathways for the clast material, the interchondrule matrix of the CR2 chondrites, and the fine-grained fraction of CM2 chondrites. Fine-grained matter of all three meteorites contains moderate to high 15N-enrichments (~50‰ ≤ δ15N ≤ ~1600‰) compared to the terrestrial value, indicating the presence of primitive organic material. We observed no correlation between 15N-enrichments and presolar dust concentrations in the clasts. This is in contrast to the findings from a suite of primitive IDPs, which display in several cases enhanced bulk 15N/14N ratios and high presolar grain abundances of several hundred or even thousand ppm. The bulk 15N/14N ratios of the clasts are comparable to the range for primitive IDPs, suggesting a nitrogen carrier less susceptible to destruction by aqueous alteration than silicate stardust. 相似文献
32.
33.
Izvestiya, Atmospheric and Oceanic Physics - The problem of atmospheric correction for shortwave channels of a multispectral low-resolution scanning radiometer onboard the Meteor-M No. 2 satellite... 相似文献
34.
Korzhenkov A. M. Gadzhiev M. S. Ovsyuchenko A. N. Lar’kov A. S. Rogozhin E. A. 《Izvestiya Atmospheric and Oceanic Physics》2020,56(7):633-656
Izvestiya, Atmospheric and Oceanic Physics - As a result of studies conducted in the city of Derbent, it is established that its monumental fortification structures erected at the end of the 560s... 相似文献
35.
Yu Xinran Ahmadinia Masoud Shariatipour Seyed M. Lawton Don Osadetz Kirk Saeedfar Amin 《Natural Resources Research》2020,29(4):2735-2752
Natural Resources Research - Carbon capture and storage is part of Canada’s climate change action plan to reduce greenhouse gas emissions. The Containment and Monitoring Institute Field... 相似文献
36.
Abstract— Active capture is a new process for the incorporation of large quantities of heavy noble gases into growing surfaces. Adsorption in the conventional sense involves surface bonding by polarization (Van der Waals forces). What is referred to as “anomalous adsorption” of heavy noble gases involves chemical bonds and can occur when other (more chemically active) species are not available to preempt sites with unfilled bonds. Anomalous adsorption has been observed under conditions of fracture, vacuum deposition and ionizing radiation. Active capture depends upon anomalous adsorption to retain noble gases on a surface long enough to be captured in a growing surface film as it is deposited. The fundamental principle may be the impingement onto the growing film with sufficient energy to liberate surface electrons (work function energy of a few electronvolts) so that they are retained by anomalous adsorption long enough to be entrapped in the growing surface. Trapping efficiencies of ?1% have been observed for Kr and Xe in laboratory experiments, implying a fundamentally new mechanism for the incorporation of heavy noble gases onto surfaces. It may play a role in explaining the large concentrations of planetary noble gases contained in phase‐Q. 相似文献
37.
Abstract— The possibility of volcanism on Mercury has been a topic of discussion since Mariner 10 returned images of half the planet's surface showing widespread plains material. These plains could be volcanic or lobate crater ejecta. An assessment of the mechanics of the ascent and eruption of magma shows that it is possible to have widespread volcanism, no volcanism on the surface whatsoever, or some range in between. It is difficult to distinguish between a lava flow and lobate crater ejecta based on morphology and morphometry. No definite volcanic features have been identified on Mercury. However, known lunar volcanic features cannot be identified in images with similar resolutions and viewing geometries as the Mariner 10 dataset. Examination of high‐resolution, low Sun angle Mariner 10 images reveals several features which are interpreted to be flow fronts; it is unclear if these are volcanic flows or ejecta flows. This analysis implies that a clear assessment of volcanism on Mercury must wait for better data. MESSENGER (MErcury: Surface, Space ENvironment, GEochemistry, Ranging) will take images with viewing geometries and resolutions appropriate for the identification of such features. 相似文献
38.
39.
We present our spectroscopic observations of the novae V1425, V1493, and V1494 Aql carried out with the 125-cm telescope at the Crimean Station of the Sternberg Astronomical Institute in the wavelength range 4000–11000 Å. We measured the emission-line intensities, determined the nova shell expansion velocities from the line profile FWHMs and components, and estimated the interstellar reddening from the first members of the Balmer series. The chemical composition of the nova shells is analyzed. Nitrogen and oxygen were found to be overabundant in V1425 and V1494 Aql; the helium abundance turned out to be normal in the two stars. 相似文献
40.