全文获取类型
收费全文 | 72407篇 |
免费 | 1187篇 |
国内免费 | 519篇 |
专业分类
测绘学 | 1912篇 |
大气科学 | 5486篇 |
地球物理 | 13934篇 |
地质学 | 25092篇 |
海洋学 | 6212篇 |
天文学 | 16710篇 |
综合类 | 162篇 |
自然地理 | 4605篇 |
出版年
2021年 | 535篇 |
2020年 | 596篇 |
2019年 | 600篇 |
2018年 | 1433篇 |
2017年 | 1415篇 |
2016年 | 1736篇 |
2015年 | 1059篇 |
2014年 | 1819篇 |
2013年 | 3669篇 |
2012年 | 1868篇 |
2011年 | 2517篇 |
2010年 | 2304篇 |
2009年 | 3002篇 |
2008年 | 2718篇 |
2007年 | 2690篇 |
2006年 | 2544篇 |
2005年 | 2185篇 |
2004年 | 2143篇 |
2003年 | 1996篇 |
2002年 | 1916篇 |
2001年 | 1778篇 |
2000年 | 1671篇 |
1999年 | 1521篇 |
1998年 | 1442篇 |
1997年 | 1494篇 |
1996年 | 1202篇 |
1995年 | 1221篇 |
1994年 | 1164篇 |
1993年 | 1021篇 |
1992年 | 1005篇 |
1991年 | 947篇 |
1990年 | 1057篇 |
1989年 | 944篇 |
1988年 | 918篇 |
1987年 | 1020篇 |
1986年 | 869篇 |
1985年 | 1160篇 |
1984年 | 1264篇 |
1983年 | 1239篇 |
1982年 | 1184篇 |
1981年 | 1096篇 |
1980年 | 1037篇 |
1979年 | 949篇 |
1978年 | 932篇 |
1977年 | 858篇 |
1976年 | 806篇 |
1975年 | 730篇 |
1974年 | 798篇 |
1973年 | 787篇 |
1972年 | 495篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
The Dergaon (H5) chondrite: Fall,classification, petrological and chemical characteristics,cosmogenic effects,and noble gas records 总被引:1,自引:0,他引:1
P. N. SHUKLA A. D. SHUKLA V. K. RAI S. V. S. MURTY N. BHANDARI J. N. GOSWAMI A. C. MAZUMDAR P. PHUKON K. DUORAH R. E. GREENWOOD I. A. FRANCHI 《Meteoritics & planetary science》2005,40(4):627-637
Abstract— A multiple fall of a stony meteorite occurred near the town of Dergaon in Assam, India, on March 2, 2001. Several fragments weighing <2 kg and a single large fragment weighing ~10 kg were recovered from the strewn field, which extended over several tens of square kilometers. Chemical, petrographic, and oxygen isotopic studies indicate it to be, in most aspects, a typical H5 chondrite, except the unusually low K content of ~340 ppm. A cosmic ray exposure of 9.7 Ma is inferred from the cosmogenic noble gas records. Activities of eleven cosmogenic radionuclides were measured. 26Al and 22Na activities as well as the 22Na/26Al activity ratio are close to the values expected on the basis of solar modulation of galactic cosmic rays. The low 60Co activity (<1 dpm/kg) is indicative of a small preatmospheric size of the meteorite. Cosmic ray heavy nuclei track densities in olivine grains range from ~106 cm?2 in samples from the largest fragment to approximately (4–9) × 105 cm?2 in one of the smaller fragments. The combined track, radionuclide, and noble gas data suggest a preatmospheric radius of ~20 cm for the Dergaon meteorite. 相似文献
92.
After briefly recalling basic facts about the metallicity gradients inspiral galaxies, we summarize two recent N-body gas-dynamical modelsthat implement a non-instantaneous gas recycling and point our their potential for an improved treatment of the chemical evolution ingalactic disks. 相似文献
93.
94.
95.
96.
Jack B. Zirker 《Solar physics》1985,100(1-2):281-287
This paper reviews research highlights of the past five years. Considerable progress has been made in observing and interpreting coronal mass ejections. The stability of coronal loops is much better understood and new observations of the onset of wind streams in coronal holes have been made. Observations from the Solar Maximum Mission should helpt to clarify the physics of the active corona.The mechanisms that heat the corona and accelerate the high-speed wind streams remain to be identified, however.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. 相似文献
97.
Ralph B. Baldwin 《Icarus》1985,61(1):63-91
This paper contains a reasonably successful attempt to determine relative ages and then absolute ages of individual craters younger than Imbrium, and the rate of infalls onto the Moon as a function of time. After the tail of the massive premare bombardment became depleted before 3 aeons (1 aeon = 109 years) ago, there was a period of minimal numbers of infalls. The rate of infalls increased rather steadily from this minimum to the present. The rate in the geologically recent past (0.3 aeon) was about two times that found for the period immediately after the last of the major lave outpourings (3.2 aeons). Absolute ages were determined for large craters (?8 km) from crater counts on the surfaces within and on the rims of the large craters. Key dates were 0 and 0.3 aeon for terrestrial meteoritic craters, 3.2, 3.5, 3.8, and 3.82 aeons for the various mare surfaces according to the determinations of D.E. Wilhelms (1980, Geologic history of the Moon, U.S. Geol. Surv. Prof. Pap.) and 3.85 aeons from the formation of Imbrium. 相似文献
98.
99.
100.
R. L. Macklin 《Astrophysics and Space Science》1985,115(1):71-83
Neutron capture measurements made on a sample of fission-product zirconium containing 20%93Zr(t
1/2=1.5×106a) at the Oak Ridge Electron Linear Accelerator time-of-flight facility resulted in the identification of 138 resonance peaks for the93Zr isotope at neutron energies up to 21.5 keV. Average capture cross sections from 20 to 300 keV were derived by subtracting neutron capture yields of the stable zirconium isotopes90, 91, 92, 94, 96Zr and additional backgrounds. The average cross sections found were significantly less than those of JENDL-1. While generally 30% higher than those of ENDF/B —V below 60 keV, the binned data overlapped the smooth ENDF/B —V curve. The average for a Maxwellian neutron spectrum withkT=30 keV is (95±10) mb and the resonance contribution to the capture resonance integral is (15.0±0.5)b.Research sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences, under contract DE-AC05-840R21400 with the Martin Marietta Energy Systems, Inc. 相似文献