全文获取类型
收费全文 | 14346篇 |
免费 | 2775篇 |
国内免费 | 4641篇 |
专业分类
测绘学 | 1071篇 |
大气科学 | 2500篇 |
地球物理 | 3646篇 |
地质学 | 9177篇 |
海洋学 | 1696篇 |
天文学 | 636篇 |
综合类 | 1637篇 |
自然地理 | 1399篇 |
出版年
2024年 | 50篇 |
2023年 | 218篇 |
2022年 | 501篇 |
2021年 | 593篇 |
2020年 | 459篇 |
2019年 | 501篇 |
2018年 | 586篇 |
2017年 | 485篇 |
2016年 | 596篇 |
2015年 | 542篇 |
2014年 | 599篇 |
2013年 | 647篇 |
2012年 | 580篇 |
2011年 | 1038篇 |
2010年 | 855篇 |
2009年 | 947篇 |
2008年 | 623篇 |
2007年 | 739篇 |
2006年 | 870篇 |
2005年 | 882篇 |
2004年 | 1400篇 |
2003年 | 953篇 |
2002年 | 809篇 |
2001年 | 663篇 |
2000年 | 511篇 |
1999年 | 614篇 |
1998年 | 658篇 |
1997年 | 578篇 |
1996年 | 500篇 |
1995年 | 392篇 |
1994年 | 339篇 |
1993年 | 479篇 |
1992年 | 420篇 |
1991年 | 251篇 |
1990年 | 152篇 |
1989年 | 146篇 |
1988年 | 93篇 |
1987年 | 117篇 |
1986年 | 77篇 |
1985年 | 37篇 |
1984年 | 25篇 |
1983年 | 19篇 |
1982年 | 22篇 |
1981年 | 18篇 |
1980年 | 15篇 |
1979年 | 11篇 |
1977年 | 16篇 |
1976年 | 14篇 |
1937年 | 16篇 |
1933年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
A formula for the thickness of a shear band formed in saturated soils under a simple shear or a combined stress state has been proposed. It is shown that the shear band thickness is dependent on the pore pressure properties of the material and the dilatancy rate, but is independent of the details of the combined stress state. This is in accordance with some separate experimental observations. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
2.
Lu Sun 《国际地质力学数值与分析法杂志》2003,27(1):69-84
A closed‐form deflection response of a beam rest is presented in this paper using the integral transform method. The theory of linear partial differential equations is used to represent the deflection of beam subjected to a moving harmonic line load in integration form. The solution is finally carried out using the inverse Fourier transform. To evaluate the integration analytically, poles of the integrand are identified with the help of algebraic equation theory. Residue theorem is then utilized to represent the integration as a contour integral in the complex plane. Closed‐form deflections and numerical results are provided for different combinations of load frequency and velocity. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
3.
4.
A new compilation of N‐isotope and abundance data for metasedimentary rocks, and hyrdothermal micas that proxy for bulk crust, show systematic patterns. (1) δ15N values of kerogen in Precambrian cherts are more negative relative to siliciclastic counterparts, probably due to a mantle hydrothermal component. (2) There is a secular trend from average δ15N 15.3 ± 1.8‰ in Archean shales, through intermediate values in the Proterozoic, to Phanerozoic counterparts where δ15N averages +3.5‰. (3) Hydrothermal micas in metamorphic hydrothermal systems of Palaeozoic and Mesozoic age that proxy for crust have δ15N within the range of contemporaneous sedimentary rocks. (4) Hydrothermal micas track the secular trend of δ15N for kerogen from 2.7 Ga to the Phanerozoic. (5) Within Precambrian datasets δ15N does not increase with decreasing N content; accordingly, high δ15N values cannot stem either from metamorphism or form Rayleigh fractionation. (6) Previous studies show isotopic shifts during metamorphism are only +1 to +3‰ up to amphibolite facies. Values of 10–24‰ are attributed to a high δ15N Archean atmosphere, a residual signature of CI carbonaceous chondrites where δ15N is +30‰ to + 42‰. 相似文献
5.
6.
I INTRODUCTIONWetland has very important effect on retainingwater and against drought, regUlating climate, controlling soil erosion and so on ac. Today men often attachimportance to economic effect of wetland reclamation,but neglect and destroy other functions and effectswhich result in blind wetland development, a lot ofwetland degeneration. The area of freshwater wetland inthe 1950s was 134 x 10'ha, but in the 1970s 70% hadchanged into farmland (MITSCH, 1993). The area ofwetland in d… 相似文献
7.
Northern Hemispheric“polar lows”,i.e.maritime,small,but fairly intense cyclonic systems have been studied formore than two decades.Recently researchers have turned their attention towards the occurrence of polar lows and othersmall-scale cyclonic disturbances on the Southern Hemisphere.In this paper a short account of our present knowledgeof Northern Hemispheric polar lows will be given,followed by some preliminary results of an investigation of the natureand occurrence of polar lows on the Southern Hemisphere. 相似文献
8.
Shi Zhi-dong Li Zhong-yuanUniversity of Science Technology of China Hefei PRC Advanced Centre for Earth Science Astronomy Third World Academy of Science 《紫金山天文台台刊》1996,(2)
Areview about K-H instability related to comet tail is presented.The effect of dust graing on KHI is theoretically analysed and relations of critical shear to excit KHI with the properties of dust grains both for sheared ion flow and for sheared dust flow re put out.Their possible application in comet tail is suggested. 相似文献
9.
PROPAGATION OF 30—60 DAY LOW FREQUENCY OSCILLATIONS AND THEIR INFLUENCE UPON THE SUBTROPICAL WESTERLIES JET STREAM DURING NORTHERN HEMISPHERE WINTER 下载免费PDF全文
Based on daily ECMWF gridpoint data of two winters during 1981—1983 including an ENSOyear,propagation of low frequency oscillations(LFO)during Northern Hemisphere winters andtheir influences upon 30—60 day oscillations of the subtropical jet stream are studied with the sta-tistical methods as complex empirical orthogonal function(CEOF)and so on.Results show that inthe winter of a normal year(1981—1982),30—60 day oscillations in the subtropical zone aremainly in the northern and southern flanks of exit region of jet stream.In the ENSO year(1982—1983),they are mainly in the vicinity of entrance and exit regions of jet stream.Intraseasonalchanges of subtropical jet stream manifested themselves as latitudinal fluctuation or longitudinalprogression or regression of about 40 day period.There are marked differences between propagat-ing passages of low frequency modes responsible for changes of subtropical jet stream in the normalyear(1981—1982)and in the ENSO year(1982—1983).Changes of oscillation amplitude showobvious phases.In general,the one in late winter is stronger than that in early winter,strongestone occurs in February. 相似文献
10.
Yi-Zhong Fan Zi-Gao Dai Yong-Feng Huang Tan Lu Department of Astronomy Nanjing University Nanjing 《中国天文和天体物理学报》2002,2(5):449-453
The optical flash accompanying GRB 990123 is believed to be powered by the reverse shock of a thin shell. With the best-fit physical parameters for GRB 990123 and the assumption that the parameters in the optical flash are the same as in the afterglow, we show that: 1) the shell is thick rather than thin, and we have provided the light curve for the thick shell case which coincides with the observation; 2) the theoretical peak flux of the optical flash accounts for only 3×10~-4 of the observed. In order to remove this discrepancy, the physical parameters, the electron energy and magnetic ratios, εe and εB, should be 0.61 and 0.39, which are very different from their values for the late afterglow. 相似文献