全文获取类型
收费全文 | 52779篇 |
免费 | 466篇 |
国内免费 | 376篇 |
专业分类
测绘学 | 1044篇 |
大气科学 | 2772篇 |
地球物理 | 10083篇 |
地质学 | 20226篇 |
海洋学 | 4981篇 |
天文学 | 12442篇 |
综合类 | 193篇 |
自然地理 | 1880篇 |
出版年
2022年 | 609篇 |
2021年 | 901篇 |
2020年 | 941篇 |
2019年 | 1042篇 |
2018年 | 2196篇 |
2017年 | 2009篇 |
2016年 | 2163篇 |
2015年 | 910篇 |
2014年 | 1943篇 |
2013年 | 2919篇 |
2012年 | 2214篇 |
2011年 | 2558篇 |
2010年 | 2445篇 |
2009年 | 2782篇 |
2008年 | 2434篇 |
2007年 | 2708篇 |
2006年 | 2351篇 |
2005年 | 1364篇 |
2004年 | 1242篇 |
2003年 | 1215篇 |
2002年 | 1172篇 |
2001年 | 1143篇 |
2000年 | 945篇 |
1999年 | 702篇 |
1998年 | 713篇 |
1997年 | 752篇 |
1996年 | 558篇 |
1995年 | 576篇 |
1994年 | 537篇 |
1993年 | 434篇 |
1992年 | 453篇 |
1991年 | 431篇 |
1990年 | 478篇 |
1989年 | 422篇 |
1988年 | 390篇 |
1987年 | 412篇 |
1986年 | 339篇 |
1985年 | 465篇 |
1984年 | 474篇 |
1983年 | 454篇 |
1982年 | 433篇 |
1981年 | 388篇 |
1980年 | 398篇 |
1979年 | 349篇 |
1978年 | 378篇 |
1977年 | 320篇 |
1976年 | 289篇 |
1975年 | 293篇 |
1974年 | 275篇 |
1973年 | 344篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Using the basic Boussinesq's equation, the expression for the vertical stress distribution (σz) underneath any point on the ground surface due to a general triangular loaded region in a preferred orientation with a linearly varied loading has been successfully derived. When the triangle is not in a preferred orientation, a simple axis transformation is required and the expression will be equally applicable. Based on this expression, σz due to an arbitrarily shaped loaded foundation can simply be determined by first triangulating the loaded area and summing up the contributions from each generated triangular region. The procedures for triangulating and calculating the stress distribution can be simply automated through computer programs. 相似文献
22.
Summary
Silica-undersaturated phlogopite schists from the Cackleberry Metamorphics, Arunta Inlier, central Australia, preserve relatively
low-temperature sapphirine-bearing parageneses that developed during low-pressure upper amphibolite facies metamorphism. Peak
metamorphic phlogopite–cordierite–sapphirine assemblages are interpreted to have formed during the same event recorded in
nearby metapelites, at c.3 kbar and 650–700 °C. Initial cooling of the terrain resulted in the breakdown of sapphirine to corundum–chlorite–phlogopite
and corundum–spinel–chlorite assemblages. Further retrogression at greenschist facies conditions resulted in the replacement
of sapphirine by diaspore–chlorite intergrowths. The reaction textures are consistent with a near-isobaric heating-cooling
path at low-pressure, and provide evidence for the stability of sapphirine at c.700 °C at low pressures in rocks of an appropriate Mg- and Fe3+-rich bulk composition.
Received August 15, 2001 accepted December 27, 2001 相似文献
23.
24.
A. Bhatnagar R. M. Jain J. T. Burkepile I. M. Chertok A. Magun H. Urbarz P. Zlobec 《Astrophysics and Space Science》1996,243(1):209-213
The powerful cosmic ray flare of Sept. 29, 1989 occurred behind the limb and was observed over a wide spectral range. The analysis of optical, radio, and other relevant data suggest two phases of energy release. After an impulsive phase a prolonged post eruption energy release occurred in an extended region of the corona following the eruption of a large coronal mass ejection (CME). This phase is responsible for numerous coronal and interplanetary phenomena including the ground-level increase of cosmic rays. 相似文献
25.
M. V. Sazhin V. E. Zharov T. A. Kalinina 《Monthly notices of the Royal Astronomical Society》2001,323(4):952-964
Parallax measurements allow distances to celestial objects to be determined. Together with measurements of their position on the celestial sphere, they give a full three-dimensional picture of the location of the objects relative to the observer. The distortion of the parallax value of a distant source affected by weak microlensing is considered. This means that the weak microlensing leads to distortion of the distance scale. The gravitational deflection causes a retrograde apparent motion of the image, which is revealed as a negative parallax. It is shown that the distortions may significantly change the parallax values when they amount to several microseconds of arc. In particular, at this level many measured values of parallaxes should be negative. 相似文献
26.
M. A. Vashkov’yak 《Astronomy Letters》2001,27(7):464-469
We consider the structural peculiarities of Uranus’s satellite system associated with its separation into two groups: inner equatorial satellites moving in nearly circular orbits and distant irregular satellites with retrograde motion in highly elliptical orbits. The intermediate region is free from satellites in a wide range of semimajor axes. By analyzing the evolution of satellite orbits under the combined effect of solar attraction and Uranus’s oblateness, we offer a celestial-mechanical explanation for the absence of equatorial satellites in this region. M.L. Lidov’s studies during 1961–1963 have served as a basis for our analysis. 相似文献
27.
V. Garnier D. Ohnenstetter G. Giuliani P. Blanc D. Schwarz 《Mineralogy and Petrology》2002,76(3-4):179-193
Summary ?Mong Hsu rubies of the “trapiche” type are sporadically seen in the gem market. However, they have never been described in
the field. The study of the nature of solid inclusions, the variation of trace-element contents, as well as the cathodoluminescence
behaviour of six “trapiche” rubies permit the conclusion that these rubies crystallised in the same geological environment
(marble-type deposit) as the normal rubies from Mong Hsu: (1) Cr and V are the main chromophorous elements in both ruby types;
they act, together with Ti, as activators or quenchers for cathodoluminescence; (2) calcite, dolomite, rutile, mica, diaspore,
apatite, chlorite, and feldspar are solid inclusions found in both ruby types; (3) the presence of bastn?site in trapiche
ruby and fluorite in non-trapiche ruby indicates the circulation of F-bearing fluids during ruby deposition; (4) the distribution
of trace-element contents in the crystal is similar for both ruby types. In the Cr2O3 vs. Fe2O3 and Cr2O3 vs. Fe2O3/TiO2 diagrams, the population fields of Mong Hsu “trapiche” and non-“trapiche” rubies overlap. They are distinct from those of
rubies and sapphires hosted in basalts from South-east Asia.
Received October 30, 2001; revised version accepted March 25, 2002 相似文献
28.
29.
30.
Surface waters of Alsea Bay, an unpolluted estuary on the Oregon coast, were analysed for nitrous oxide, nitrate and nitrite on a weekly or biweekly basis during the summer of 1979. The estuary was found to be a variable source of N2O to the atmosphere. Large and rapid increases in the concentrations of N2O, NO3?, and NO2? occurred at the beginning of the sampling period and are attributed to the influx of nutrient-rich upwelling water into the estuary with the tide. The subsequent decline in concentrations of nitrate, nitrite and nitrous oxide over the remainder of the summer is attributed to a decrease in upwelling intensity, a decline in nitrification rates and to assimilatory nitrate reduction. Measurements of nitrous oxide at six stations along the Alsea River were also made in September and October before and after the onset of the rainy season. Samples taken after flood conditions were established were systematically 50% higher than pre-flood samples. The data suggest that soil runoff results in elevated concentrations of N2O in rivers. 相似文献