全文获取类型
收费全文 | 48237篇 |
免费 | 901篇 |
国内免费 | 458篇 |
专业分类
测绘学 | 1105篇 |
大气科学 | 3570篇 |
地球物理 | 10094篇 |
地质学 | 17020篇 |
海洋学 | 3939篇 |
天文学 | 10567篇 |
综合类 | 79篇 |
自然地理 | 3222篇 |
出版年
2020年 | 321篇 |
2019年 | 322篇 |
2018年 | 764篇 |
2017年 | 701篇 |
2016年 | 934篇 |
2015年 | 703篇 |
2014年 | 975篇 |
2013年 | 2277篇 |
2012年 | 1173篇 |
2011年 | 1695篇 |
2010年 | 1447篇 |
2009年 | 2018篇 |
2008年 | 1811篇 |
2007年 | 1745篇 |
2006年 | 1684篇 |
2005年 | 1439篇 |
2004年 | 1489篇 |
2003年 | 1425篇 |
2002年 | 1364篇 |
2001年 | 1201篇 |
2000年 | 1151篇 |
1999年 | 1076篇 |
1998年 | 1052篇 |
1997年 | 1051篇 |
1996年 | 842篇 |
1995年 | 818篇 |
1994年 | 778篇 |
1993年 | 716篇 |
1992年 | 656篇 |
1991年 | 573篇 |
1990年 | 651篇 |
1989年 | 547篇 |
1988年 | 543篇 |
1987年 | 639篇 |
1986年 | 556篇 |
1985年 | 778篇 |
1984年 | 854篇 |
1983年 | 868篇 |
1982年 | 718篇 |
1981年 | 694篇 |
1980年 | 705篇 |
1979年 | 637篇 |
1978年 | 661篇 |
1977年 | 573篇 |
1976年 | 611篇 |
1975年 | 568篇 |
1974年 | 574篇 |
1973年 | 572篇 |
1972年 | 378篇 |
1971年 | 316篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
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
13.
14.
Mars Global Surveyor accelerometer observations of the martian upper atmosphere revealed large variations in density with longitude during northern hemisphere spring at altitudes of 130-160 km, all latitudes, and mid-afternoon local solar times (LSTs). This zonal structure is due to tides from the surface. The zonal structure is stable on timescales of weeks, decays with increasing altitude above 130 km, and is dominated by wave-3 (average amplitude 22% of mean density) and wave-2 (18%) harmonics. The phases of these harmonics are constant with both altitude and latitude, though their amplitudes change significantly with latitude. Near the South Pole, the phase of the wave-2 harmonic changes by 90° with a change of half a martian solar day while the wave-3 phase stays constant, suggesting diurnal and semidiurnal behaviour, respectively. We use a simple application of classical tidal theory to identify the dominant tidal modes and obtain results consistent with those of General Circulation Models. Our method is less rigorous, but simpler, than the General Circulation Models and hence complements them. Topography has a strong influence on the zonal structure. 相似文献
15.
16.
Abstract— The Vredefort Granophyre represents impact melt that was injected downward into fractures in the floor of the Vredefort impact structure, South Africa. This unit contains inclusions of country rock that were derived from different locations within the impact structure and are predominantly composed of quartzite, feldspathic quartzite, arkose, and granitic material with minor proportions of shale and epidiorite. Two of the least recrystallized inclusions contain quartz with single or multiple sets of planar deformation features. Quartz grains in other inclusions display a vermicular texture, which is reminiscent of checkerboard feldspar. Feldspars range from large, twinned crystals in some inclusions to fine‐grained aggregates that apparently are the product of decomposition of larger primary crystals. In rare inclusions, a mafic mineral, probably biotite or amphibole, has been transformed to very fine‐grained aggregates of secondary phases that include small euhedral crystals of Fe‐rich spinel. These data indicate that inclusions within the Vredefort Granophyre were exposed to shock pressures ranging from <5 to 8–30 GPa. Many of these inclusions contain small, rounded melt pockets composed of a groundmass of devitrified or metamorphosed glass containing microlites of a variety of minerals, including K‐feldspar, quartz, augite, low‐Ca pyroxene, and magnetite. The composition of this devitrified glass varies from inclusion to inclusion, but is generally consistent with a mixture of quartz and feldspar with minor proportions of mafic minerals. In the case of granitoid inclusions, melt pockets commonly occur at the boundaries between feldspar and quartz grains. In metasedimentary inclusions, some of these melt pockets contain remnants of partially melted feldspar grains. These melt pockets may have formed by eutectic melting caused by inclusion of these fragments in the hot (650 to 1610 °C) impact melt that crystallized to form the Vredefort Granophyre. 相似文献
17.
Roberto F. Viotti Lucio Angelo Antonelli Sonja Rebecchi Corinne Rossi 《Journal of Astrophysics and Astronomy》2002,23(1-2):19-22
We have investigated with BeppoSAX the long term behaviour of the harder X-ray component of the supposed supermassive binary
system η Car along its 5.52 year cycle. We have found that in March 1998 during egress from the last December 1997 eclipse,
this component was the same as outside eclipse, but for a large (×3.5) increase of NH
h
, that can be attributed to the presence or formation of opaque matter in front of the source near periastron. Unexpectedly,
at that time the iron 6.7 keV emission line was 40% stronger. BeppoSAX has for the first time found ahard X-ray tail extending to at least 50 keV, that cannot be adequately fitted with an additional hotter thermal component. The 2–100 keV
spectrum of η Car is instead well fitted with an absorbed powerlaw spectrum with photon index 2.53, suggesting non-thermal
emission as an alternative model for the core source. 相似文献
18.
19.
20.
IwrRODUcrlONStainlessstalhasbeenusedwidelyasbuildingrnateria1,especiallyasdecoratingrnate-rialinbuildings.Ordinarystainlesssteehasonlyonemonot0noussilverywhitCcolourwhichsomeimesdoesnotrnatchwithitSsurmundingenvironment.lnordert0improveitSdereratingperfonnaneeandexpanditsuse,muchresearchhasbeenconductedsincethe4O'st0trytoformabright,uniform,highcorrosi0nandwearresistantco1ourfilmonastainlesssteesurface.Duringtheeariystage,anirnmersionedhodwasused,i.e.inimrs-ingthestainlesssteelinahots0luti… 相似文献