全文获取类型
收费全文 | 19181篇 |
免费 | 2531篇 |
国内免费 | 4486篇 |
专业分类
测绘学 | 2094篇 |
大气科学 | 2949篇 |
地球物理 | 3635篇 |
地质学 | 6173篇 |
海洋学 | 2940篇 |
天文学 | 5064篇 |
综合类 | 1426篇 |
自然地理 | 1917篇 |
出版年
2024年 | 66篇 |
2023年 | 178篇 |
2022年 | 525篇 |
2021年 | 573篇 |
2020年 | 788篇 |
2019年 | 895篇 |
2018年 | 656篇 |
2017年 | 779篇 |
2016年 | 757篇 |
2015年 | 935篇 |
2014年 | 922篇 |
2013年 | 1072篇 |
2012年 | 1045篇 |
2011年 | 992篇 |
2010年 | 840篇 |
2009年 | 1217篇 |
2008年 | 1253篇 |
2007年 | 1507篇 |
2006年 | 1499篇 |
2005年 | 1375篇 |
2004年 | 1197篇 |
2003年 | 1071篇 |
2002年 | 985篇 |
2001年 | 768篇 |
2000年 | 873篇 |
1999年 | 829篇 |
1998年 | 757篇 |
1997年 | 356篇 |
1996年 | 271篇 |
1995年 | 230篇 |
1994年 | 195篇 |
1993年 | 176篇 |
1992年 | 151篇 |
1991年 | 98篇 |
1990年 | 70篇 |
1989年 | 86篇 |
1988年 | 47篇 |
1987年 | 36篇 |
1986年 | 39篇 |
1985年 | 24篇 |
1984年 | 14篇 |
1983年 | 9篇 |
1982年 | 9篇 |
1981年 | 6篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1977年 | 2篇 |
1954年 | 4篇 |
1880年 | 2篇 |
1877年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
121.
A. G. Lyne R. S. Pritchard F. Graham-Smith 《Monthly notices of the Royal Astronomical Society》2001,321(1):67-70
Between 1997 August and October, the radio pulses from the Crab pulsar were followed by discrete moving echoes, which appear to be reflections from part of an ionized shell in the outer part of the Crab Nebula, crossing the line of sight to pulsar. Similar events have now been recognized in recordings from the past 30 yr, and it seems that the Nebula must contain a large number of ionized shell-like surfaces on a much finer scale than recognized hitherto. 相似文献
122.
Da-Ming Chen 《中国天文和天体物理学报》2001,1(4):287-290
1 INTRODUCTIONObservations of the most famous starburst galaxy M82 with the High-Resolution Cameraon board the Chandra X-Ray Observatory, showed that there are nine sources in the celltral11 x 11 region, but no son-roe was detected at the galactic center (Matsumoto et al. 2001).Comparing the observations on 1999 October 28 and those on 2000 January 20, the authorsfound an extremely large time variability of the source CXO M82 J095550.2 694047, which islocated gll away from the galact… 相似文献
123.
Randall D. Campbell 《Experimental Astronomy》2002,14(1):57-60
Keck's Long Wavelength Spectrometer (LWS), is the facility instrument used for imaging and spectroscopy in the wavelength
range of 3–28 μm at the Keck Observatory. LWS uses an 128 × 128 Si:As blocked impurity band (BIB) array manufactured by the
Boeing Corporation. This paper discusses the method used for optimizing the detector's operating parameters at a temperature
of 8.5 K and bias voltage of 1.2 V. A process for characterizing detective quantum efficiency of BIB detectors is also presented.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
124.
根据我国测绘生产单位的实际情况 ,测绘项目管理的具体实施大致包括对项目的识别、选定、启动与筹集资金、计划、成本估算、执行计划、控制、收尾等主要步骤。当项目确定后 ,需要启动和筹集资金。而计划是项目管理的基本组成部分 ,包括明确项目目标和工作范围。项目成本估算有因素估算法和利用WBS方法的详细估算。项目质量管理规划的依据有质量目标、范围说明、标准和规范等。 相似文献
125.
WANG Yachun 《东北亚地学研究》2002,(1)
The paper deals with the methods of formation pressure evaluation for a single well by using the very common ac-cepted parameters, such as drilling exponent , and flowline temperature , etc. which is part of compiling the end well report. 相似文献
126.
R. J. Protheroe 《Monthly notices of the Royal Astronomical Society》2003,341(1):230-238
I re-examine the brightness temperature problem in PKS 0405-385, which is an extreme intra-day variable radio quasar with an inferred brightness temperature of ∼5 × 1014 K at 5 GHz, well above the Compton catastrophe limit of ∼1011 K that is reached when the synchrotron photon energy density exceeds the energy density of the magnetic field. If one takes into account the uncertainty in the distance to the ionized clouds responsible for interstellar scintillation causing rapid intra-day variability in PKS 0405-385, it is possible that the brightness temperature could be as low as ∼1013 K at 5 GHz, or even lower. The radio spectrum can be fitted by optically thin emission from mono-energetic electrons, or an electron spectrum with a low-energy cut-off such that the critical frequency of the lowest energy electrons is above the radio frequencies of interest. If one observes optically thin emission along a long narrow emission region, the average energy density in the emission region can be many orders of magnitude lower than calculated from the observed intensity if one assumed a spherical emission region. I discuss the physical conditions in the emission region and find that the Compton catastrophe can then be avoided using a reasonable Doppler factor. I also show that MeV to 100-GeV gamma-ray emission at observable flux levels should be expected from extreme intra-day variable sources such as PKS 0405-385. 相似文献
127.
128.
K. S. Stankevich A. M. Aslanyan V. P. Ivanov R. M. Martirosyan Ye. Terzian 《Astrophysics》2003,46(4):429-433
Radio emission of the historical supernovae remnants Tycho (SNR1572) and Kepler (SNR1604) and evolution of their luminosity are considered. Measurement data of secular luminosity decrease rate, obtained earlier by the authors, were corrected with account of variation in time of the flux density of the reference sources. As a result, it is found that the SNR1604 luminosity at 1667 MHz is weakening with an annual mean rate equal to (0.2 ± 0.07)%. The corresponding rate for SNR1572 is (0.47 ± 0.05)%. Since the radio luminosity evolution, as well as energy densities of magnetic field and relativistic electrons inside SNR1604 and SNR1572 are essentially different, these remnants should be considered as different types of supernovae. Bandiera classified SN1604 as type SNIb or SNII. 相似文献
129.
D. H. Morgan Q. A. Parker Martin Cohen 《Monthly notices of the Royal Astronomical Society》2003,346(3):719-730
We report the discovery of the first probable Galactic [WN] central star of a planetary nebula (CSPN). The planetary nebula candidate was found during our systematic scans of the AAO/UKST Hα Survey of the Milky Way. Subsequent confirmatory spectroscopy of the nebula and central star reveals the remarkable nature of this object. The nebular spectrum shows emission lines with large expansion velocities exceeding 150 km s−1 , suggesting that perhaps the object is not a conventional planetary nebula. The central star itself is very red and is identified as being of the [WN] class, which makes it unique in the Galaxy. A large body of supplementary observational data supports the hypothesis that this object is indeed a planetary nebula and not a Population I Wolf–Rayet star with a ring nebula. 相似文献
130.