全文获取类型
收费全文 | 1152篇 |
免费 | 164篇 |
国内免费 | 239篇 |
专业分类
测绘学 | 94篇 |
大气科学 | 281篇 |
地球物理 | 279篇 |
地质学 | 479篇 |
海洋学 | 138篇 |
天文学 | 52篇 |
综合类 | 113篇 |
自然地理 | 119篇 |
出版年
2024年 | 5篇 |
2023年 | 23篇 |
2022年 | 48篇 |
2021年 | 59篇 |
2020年 | 39篇 |
2019年 | 48篇 |
2018年 | 52篇 |
2017年 | 54篇 |
2016年 | 60篇 |
2015年 | 47篇 |
2014年 | 58篇 |
2013年 | 67篇 |
2012年 | 42篇 |
2011年 | 64篇 |
2010年 | 61篇 |
2009年 | 63篇 |
2008年 | 61篇 |
2007年 | 62篇 |
2006年 | 30篇 |
2005年 | 36篇 |
2004年 | 26篇 |
2003年 | 29篇 |
2002年 | 46篇 |
2001年 | 41篇 |
2000年 | 29篇 |
1999年 | 63篇 |
1998年 | 53篇 |
1997年 | 44篇 |
1996年 | 32篇 |
1995年 | 32篇 |
1994年 | 28篇 |
1993年 | 26篇 |
1992年 | 24篇 |
1991年 | 15篇 |
1990年 | 19篇 |
1989年 | 17篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 5篇 |
1985年 | 7篇 |
1984年 | 4篇 |
1983年 | 7篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1975年 | 1篇 |
1970年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有1555条查询结果,搜索用时 615 毫秒
251.
252.
253.
A method of levulinic acid production from bagasse and paddy straw is presented. Biomass waste was mixed with hydrochloride acid and water, and then liquefied at about 200°C in a pressurized reactor. Both the mass of the biomass and the amount of hydrochloride acid were kept as constants, and the effects of temperature, reaction time, and acid concentration on the levulinic acid yield were studied. It was found that the yield of levulinic acid was maximum when the reaction temperature was carried out for 45 min at 220°C using 4.45% concentration of hydrochloride acid, and the maximum yields of levulinic acid were 22.8 and 23.7% for bagasse and paddy straw, respectively. The products were analyzed using NMR, HPLC, and GC/MS. 相似文献
254.
无人机快速获取地震灾情的应用探索 总被引:2,自引:0,他引:2
快速获取地震灾情对于应急救援决策和指挥是非常重要的。大地震造成通讯和交通设施破坏,使应急救援指挥部不能及时全面了解灾情,汶川地震已经给我们惨痛的教训。为此,我们开展了用无人机快速获取地震灾情的探索。 相似文献
255.
YANG ChuPeng GENG AnSong LIAO ZeWen SUN YongGe & ZHANG LüHui State Key Laboratory of Organic Geochemistry Guangzhou Institute of Geochemistry Chinese Academy of Sciences Guangzhou China Graduate University of Chinese Academy of Sciences Beijing 《中国科学D辑(英文版)》2009,52(1):12-21
Gas washing has been known in the Tazhong area of the Tarim Basin, but its quantitative assessment has not yet been reported.
Here the influence of gas washing fractionation in the area was discussed based on the gas chromatogram data of 68 oils and
the results of the mixing experiments of a black oil and a condensate. The results show that the intensity of gas washing
fractionation decreased generally from northern to southern part and vertically from deep reservoirs to shallow reservoirs.
The gas washing fractionation was mainly controlled by fault systems in this area, with the increase of n-alkane mass depletion positively correlated to the number and scale of faults. Gas washing fractionation appears to have
affected the hydrocarbon property, and as a result the diversity of the crude oils is markedly controlled by gas washing.
In addition, the occurrence of waxy oil in this area may be resulted from multiple factors including gas washing, mixed filling
and migration fractionation.
Supported by National Basic Research Program of China (Grant No. 2006CB202303) and the National Natural Science Foundation
of China (Grant No. 40672091) 相似文献
256.
W.Phillip Yen 《地震工程与工程振动(英文版)》2009,8(1):127-135
Conventional seismic evaluation of existing bridges explores the ability of a bridge to survive under significant earthquake excitations. This approach has several major drawbacks, such as only a single structural performance of near collapse is considered, and the simplified approach of adopting strength-based concept to indirectly estimate the nonlinear behavior of a structure lacks accuracy. As a result, performance-based concepts that include a wider variety of structural performance states of a given bridge excited by different levels of earthquake intensity is needed by the engineering community. This paper introduces an improved process for the seismic evaluation of existing bridges. The relationship between the overall structural performance and earthquakes with varying levels of peak ground acceleration (PGA) can successfully be linked. A universal perspective on the seismic evaluation of bridges over their entire life-cycle can be easily obtained to investigate multiple performance objectives. The accuracy of the proposed method, based on pushover analysis, is proven in a case study that compares the results from the proposed procedure with additional nonlinear time history analyses. 相似文献
257.
258.
库姆塔格沙漠的“羽毛状沙丘”形态的观测 总被引:3,自引:0,他引:3
参照卫星照片和地形图,实地观测了库姆塔格沙漠“羽毛状沙丘”分布区域的地貌。新月型“沙垄”是该区域的主导性地貌类型,垄间地形波状起伏。如果将“沙垄”喻为“羽轴”,垄间的波状起伏地形为“羽枝”,则可构成卫星照片所显示的“羽毛状沙丘”。新月型“沙垄”的走向为NE-SW,由新月形沙丘前一沙丘的迎风坡与后一沙丘的东翼相连而成,高3~19m,宽约50m,长15~22km。“垄”间起伏地形的波长为100~300m,振幅为40~100cm,与“沙垄”成60~105°夹角。在波状起伏地形的波峰与波谷过渡区堆积颜色较浅的细沙,厚2~7cm,构成了“大沙波”。实地观测的“羽毛状沙丘”的形态与卫星照片显示的形态有一定差异。 相似文献
259.
利用气象卫星遥感图来监测大降水以及山区水流的分布、洪水形成的大小及流向 ,是利用气象现代化技术监测灾害性天气的一个有效方法。 相似文献
260.
胜利油田火山岩辉石中岩浆包裹体成分及有关成因问题 总被引:5,自引:1,他引:4
对胜利油田火山岩中辉石及其中岩浆包裹体成分的研究表明:CO2气藏区和非CO2气藏区新生代火山岩辉石及其中岩浆包裹体成分有明显区别,前者中辉石为普通辉石,成分相对富SiO2,而贫Al2O3、TiO2、MgO和挥发成分;而后者中辉石为透辉石,成分相对贫SiO2和挥发份,而富Al2O3、TiO2和MgO。前者岩浆包裹体玻相中富含CO2,包裹体中的金属子矿物多为黄铁矿;而后者岩浆包裹体玻相中贫CO2;包裹 相似文献