全文获取类型
收费全文 | 41905篇 |
免费 | 7010篇 |
国内免费 | 8720篇 |
专业分类
测绘学 | 4018篇 |
大气科学 | 5793篇 |
地球物理 | 10762篇 |
地质学 | 20185篇 |
海洋学 | 5969篇 |
天文学 | 1153篇 |
综合类 | 2980篇 |
自然地理 | 6775篇 |
出版年
2024年 | 197篇 |
2023年 | 503篇 |
2022年 | 1186篇 |
2021年 | 1657篇 |
2020年 | 1763篇 |
2019年 | 2162篇 |
2018年 | 1586篇 |
2017年 | 1907篇 |
2016年 | 1886篇 |
2015年 | 2056篇 |
2014年 | 2616篇 |
2013年 | 3121篇 |
2012年 | 2628篇 |
2011年 | 2837篇 |
2010年 | 2353篇 |
2009年 | 2772篇 |
2008年 | 2792篇 |
2007年 | 2926篇 |
2006年 | 2870篇 |
2005年 | 2301篇 |
2004年 | 2193篇 |
2003年 | 1880篇 |
2002年 | 1544篇 |
2001年 | 1353篇 |
2000年 | 1211篇 |
1999年 | 1083篇 |
1998年 | 1049篇 |
1997年 | 901篇 |
1996年 | 749篇 |
1995年 | 680篇 |
1994年 | 584篇 |
1993年 | 518篇 |
1992年 | 368篇 |
1991年 | 284篇 |
1990年 | 220篇 |
1989年 | 207篇 |
1988年 | 151篇 |
1987年 | 97篇 |
1986年 | 77篇 |
1985年 | 81篇 |
1984年 | 40篇 |
1983年 | 24篇 |
1982年 | 25篇 |
1981年 | 27篇 |
1980年 | 28篇 |
1978年 | 23篇 |
1977年 | 19篇 |
1976年 | 26篇 |
1973年 | 18篇 |
1971年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 734 毫秒
41.
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
42.
43.
The dunes in estuary of Tumen River in China lie to the area between the Jiushaping and Fangchuan in the left bank of Tumen River (Fig.1). The dis-tance is about 15~20 km between the dunes and the coast. It ranges from China to D. P. R. Korea and Russia. The range of the height of dunes is about 15~20 m and the width is 100~200 m. By the re-connaissance, we protracted 9 section planes of the dunes and collected 40 sand samples of the dunes, and identified the sedimentary environment … 相似文献
44.
YANGYong-liang SHITong-guang JLANGLU-guang LIYue 《中国地理科学(英文版)》2003,13(2):130-133
In recent years, many coal-producing countries have paid great attention to the land subsidence causedby coal cutting. In China, because of the dense population in coalfield areas, the land subsidence hazard is more seri-ous. After a brief analysis on the mechanism of land subsidence, this paper gives a comprehensive and systematical ac-count on all kinds of hazards caused by the land subsidence in China. The study shows that land subsidence has endan-gered land, buildings, traffic and communication lines, dykes and dams. It also causes damage to ecological and socialenvironment. In order to lessen the hazard of land subsidence, preventive measures should be taken to reduce the col-lapse amount, such as extraction with stowing, banded mining system, succession and coordination mining system, orhigh-pressure mudflow between rock strata. Measures of reinforcing or moving certain buildings should also be taken toreduce the destructive degree. In order to harness the subsidence land and bring them under control for fanning, mea-sures should be taken such as filling with spoil or fine breeze, excavating the deeper and covering the shallower land. 相似文献
45.
DEVELOPMENTSTRATEGIESOFWATERANDLANDRESOURCESINTHEHEXIREGION,CHINA肖洪浪,高前兆,李福兴DEVELOPMENTSTRATEGIESOFWATERANDLANDRESOURCESINTHE... 相似文献
46.
It has been found that the near-infrared flux variations of Seyfert galaxies satisfy relations of the form Fi ≈α i j +β i j Fj , where Fi , Fj are the fluxes in filters i and j ; and α i , j , β i , j are constants. These relations have been used to estimate the constant contributions of the non-variable underlying galaxies. The paper attempts a formal treatment of the estimation procedure, allowing for the possible presence of a third component, namely non-variable hot dust. In an analysis of a sample of 38 Seyfert galaxies, inclusion of the hot dust component improves the model fit in approximately half the cases. All derived dust temperatures are below 300 K, in the range 540–860 K or above 1300 K. A noteworthy feature is the estimation of confidence intervals for the component contributions: this is achieved by bootstrapping. It is also pointed out that the model implies that such data could be fruitfully analysed in terms of principal components. 相似文献
47.
1INTRODUCTIONGrain production has always been top-priority issue con-cerned by government and people (MA etal., 2002). Af-ter China put household contract responsibility system in force, its domestic issue of grain was basically settled in the mid 1980s (MA and XU, 1999). However, in the days when idea of sustainable development is broadly accepted, efficiency and sustainability of grain produc-tion are now in question, which have prompted interest in reviewing grain production patterns… 相似文献
48.
张敏先 《成都信息工程学院学报》2001,16(3):209-212
文献[1][2]认为M-PN空间是局部凸的。通过定理说明这个结论对t-模T(a,b)≥min(a,b)时成立。又通过反例表明,当t-模型T(a,b)=max(a b-1,0)时无论邻域Nθ(ε,λ)概率有界还是概率半有界结论均不成立。还讨论了Takahashi凸性并对S-凸和K-凸作了比较。 相似文献
49.
定水头注水引起的含水层水平运动和应变 总被引:2,自引:2,他引:0
基于含水层固体颗粒与孔隙水不可压缩的假设 ,本文导出了单井注水情况下泰斯承压含水层水平运动速度与水头之间的基本关系式。然后利用注水井壁处的应力、应变边界条件 ,进一步导出了单井定水头注水引起的泰斯承压含水层水平运动速度、位移和应变解析表达式。该水平位移与应变由两部分组成 :一部分为由注水压力本身引起的经典弹性力学解项 ,它仅随半径而变化 ,与注水时间无关 ;另一部分为由地下水头变化引起的水动力学位移和应变解项。其中 ,含水层水动力学水平位移随时间加长呈指数增长特征 ,水动力学径向应变则表现为近井处拉张、远井处挤压的分区特征 ,且近井拉张区随时间加长逐渐向外扩展。单井注水含水层水动力学水平位移、应变解的导出 ,完善和发展了单孔内压经典弹性平面力学问题解 相似文献
50.
周发贵 《地球科学与环境学报》2001,23(1):63-65
路基施工中保证路基填筑密实度,通常沙土路基采用水夯;普通路基采用振动碾、羊角碾等重型机具振密夯实。但在缺电、缺水,高气温、大风沙的沙漠气候等恶劣的施工条件下,风沙路基的施工如何保证路基填筑的密实度和施工机械在风沙路基上作业而不陷车的问题,就目前没有合适的经验可借鉴,也没有规范可参考。因此把在包西线神木北至延安北段的铁路工程建设中风沙路基施工所取得的经验和施工技术作一介绍。 相似文献