全文获取类型
收费全文 | 36730篇 |
免费 | 6390篇 |
国内免费 | 9888篇 |
专业分类
测绘学 | 3120篇 |
大气科学 | 11323篇 |
地球物理 | 6411篇 |
地质学 | 14633篇 |
海洋学 | 3937篇 |
天文学 | 6789篇 |
综合类 | 2116篇 |
自然地理 | 4679篇 |
出版年
2024年 | 175篇 |
2023年 | 512篇 |
2022年 | 1194篇 |
2021年 | 1454篇 |
2020年 | 1489篇 |
2019年 | 1822篇 |
2018年 | 1406篇 |
2017年 | 1585篇 |
2016年 | 1598篇 |
2015年 | 1752篇 |
2014年 | 2377篇 |
2013年 | 2461篇 |
2012年 | 2626篇 |
2011年 | 2611篇 |
2010年 | 2377篇 |
2009年 | 2929篇 |
2008年 | 2700篇 |
2007年 | 2964篇 |
2006年 | 2679篇 |
2005年 | 2245篇 |
2004年 | 2062篇 |
2003年 | 1735篇 |
2002年 | 1396篇 |
2001年 | 1250篇 |
2000年 | 1178篇 |
1999年 | 1049篇 |
1998年 | 983篇 |
1997年 | 701篇 |
1996年 | 665篇 |
1995年 | 548篇 |
1994年 | 548篇 |
1993年 | 459篇 |
1992年 | 334篇 |
1991年 | 253篇 |
1990年 | 192篇 |
1989年 | 181篇 |
1988年 | 145篇 |
1987年 | 63篇 |
1986年 | 55篇 |
1985年 | 48篇 |
1984年 | 37篇 |
1983年 | 27篇 |
1982年 | 34篇 |
1981年 | 18篇 |
1980年 | 28篇 |
1979年 | 9篇 |
1978年 | 18篇 |
1977年 | 19篇 |
1954年 | 11篇 |
1875年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 343 毫秒
51.
Polypeptide fromChlamys farreri (PCF), a topical polypeptide isolated fromChlamys farreri, was used in this experiment aimed to investigate the photoprotective effect of PCF against chronic skin damage induced by
ultraviolet A (UVA) and ultraviolet B (UVB) radiation. The chronic ultraviolet-irradiated guinea pig model was established,
and visible changes in the skin including wrinkling, sagging and erythema were observed. Malondialdehyde (MDA) and antioxidant
enzymes including superoxide dismutase (SOD) and glutathione peroxidase (GSH-px) in the dorsal skin were determined using
biochemical methods. The results showed: (1) PCF (5 % and 20%) could greatly protect the dorsal skin of guinea pig against
wrinkling, sagging and erythema induced by UV radiation in a concentration-dependent manner. (2) PCF could reduce MDA formation
in the dorsal skin caused by UV irradiation, while increasing the activities of SOD and GSH-px. (3) The differences among
the PCF groups and UV model group were significant (P<0.05,P<0.01). These results indicated that topical application, of PCF provided broad solar UV spectrum photoprotection; and that
the antioxidant property of PCF might play a role in photoprotection.
Project No. 39970638 supported by the NSFC, and the Science and Technology Bureau of Qingdao (No. 2001-28-50). 相似文献
52.
CARBON DYNAMICS OF WETLAND IN THE SANJIANG PLAIN 总被引:2,自引:0,他引:2
SONG Chang-chun WANG Yi-yong WANG Yue-si YAN Bai-xing WANG De-xuan ZHAO Zhi-chun LOU Yan-jing 《中国地理科学(英文版)》2003,13(3):228-231
1INTRODUCTIONWetlandsplayanimportant roleintheprocessofcar-bonstorage.Thetotalcarbonstoredindifferentkindsofwetlandsisabout15%-35%ofthetotalcarboninthegloballandsoils(POSTetal.,1982;GORHAM,1991).Inaddition,wetlandsaresignificantnaturalsources fortheatmospheric CH4 (MOORE,1994).It isestimatedthatabout110×1012gCH4 originates fromanaerobicdecompositioninthenaturalwetlands,CH4 emission fromthenaturalwetlandsis15%-30%oftheglobalCH4 emission andtheCH4 emission from thepeat land at hi… 相似文献
53.
54.
J. C. A. Miller-Jones R. P. Fender E. Nakar 《Monthly notices of the Royal Astronomical Society》2006,367(4):1432-1440
We present a collation of the available data on the opening angles of jets in X-ray binaries, which in most cases are small (≲10°). Under the assumption of no confinement, we calculate the Lorentz factors required to produce such small opening angles via the transverse relativistic Doppler effect. The derived Lorentz factors, which are in most cases lower limits, are found to be large, with a mean >10, comparable to those estimated for active galactic nuclei (AGN) and much higher than the commonly assumed values for X-ray binaries of 2–5. Jet power constraints do not, in most cases, rule out such high Lorentz factors. The upper limits on the opening angles show no evidence for smaller Lorentz factors in the steady jets of Cygnus X-1 and GRS 1915+105. In those sources in which deceleration has been observed (notably XTE J1550−564 and Cygnus X-3), some confinement of the jets must be occurring, and we briefly discuss possible confinement mechanisms. It is however possible that all the jets could be confined, in which case the requirement for high bulk Lorentz factors can be relaxed. 相似文献
55.
56.
57.
Radiation exposure and Mission Strategies
for Interplanetary Manned Missions (REMSIM) 总被引:1,自引:0,他引:1
C. Cougnet N. B. Crosby S. Eckersley C. Foullon V. Guarnieri S. Guatelli D. Heynderickx A. Holmes-Siedle C. Lobascio S. Masiello P. Nieminen G. Parisi P. Parodi M. A. Perino M. G. Pia R. Rampini P. Spillantini V. Tamburini E. Tracino 《Earth, Moon, and Planets》2004,94(3-4):279-285
Cosmic radiation is an important problem for human interplanetary missions. The “Radiation Exposure and Mission Strategies for Interplanetary Manned Missions–REMSIM” study is summarised here. They are related to current strategies and countermeasures to ensure the protection of astronauts from radiation during interplanetary missions, with specific reference to: radiation environment and its variability; radiation effects on the crew; transfer trajectories and associated fluences; vehicle and surface habitat concepts; passive and active shielding concepts; space weather monitoring and warning systems. 相似文献
58.
59.
The results of a photometric monitoring of the quasar 4C 38.41, performed at the optical R and B bands in 2002 February–March, are presented. With a 60/90 cm Schmidt telescope at the Xinglong station of the National Astronomical Observatories of China, we observed the source exhibiting amplitude variations of up to 0.78 mag in both bands during the whole campaign. Intraday and even intranight variations are detected as well. A typical variability time-scale of about 36 d is derived from our 2-month observations at the optical bands, which is identical to that found at a radio wavelength of 92 cm, suggesting a common origin for the variations in 4C 38.41 from optical to radio bands. 相似文献
60.