全文获取类型
收费全文 | 89847篇 |
免费 | 28875篇 |
国内免费 | 54633篇 |
专业分类
测绘学 | 13694篇 |
大气科学 | 26618篇 |
地球物理 | 21837篇 |
地质学 | 64811篇 |
海洋学 | 24287篇 |
天文学 | 1669篇 |
综合类 | 8815篇 |
自然地理 | 11624篇 |
出版年
2024年 | 625篇 |
2023年 | 1647篇 |
2022年 | 4273篇 |
2021年 | 5269篇 |
2020年 | 5043篇 |
2019年 | 8182篇 |
2018年 | 7771篇 |
2017年 | 7337篇 |
2016年 | 7437篇 |
2015年 | 7805篇 |
2014年 | 7214篇 |
2013年 | 8721篇 |
2012年 | 9137篇 |
2011年 | 9051篇 |
2010年 | 9121篇 |
2009年 | 8113篇 |
2008年 | 7601篇 |
2007年 | 7443篇 |
2006年 | 6806篇 |
2005年 | 5894篇 |
2004年 | 5242篇 |
2003年 | 4241篇 |
2002年 | 4202篇 |
2001年 | 3756篇 |
2000年 | 3154篇 |
1999年 | 2320篇 |
1998年 | 1967篇 |
1997年 | 1995篇 |
1996年 | 1535篇 |
1995年 | 1444篇 |
1994年 | 1293篇 |
1993年 | 1230篇 |
1992年 | 1064篇 |
1991年 | 773篇 |
1990年 | 756篇 |
1989年 | 616篇 |
1988年 | 527篇 |
1987年 | 419篇 |
1986年 | 338篇 |
1985年 | 264篇 |
1984年 | 299篇 |
1983年 | 168篇 |
1982年 | 220篇 |
1981年 | 156篇 |
1980年 | 112篇 |
1979年 | 138篇 |
1978年 | 49篇 |
1977年 | 49篇 |
1971年 | 56篇 |
1954年 | 47篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
Ren Tinawi Marc Sarrazin Andr Filiatrault 《Soil Dynamics and Earthquake Engineering》1993,12(8):469-477
The 1990 edition of the National Building Code of Canada (Associate Committee of the National Building Code, National Research Council, Ottawa, 1990) makes a clear distinction between eastern and western Canada in terms of seismic acceleration and velocity zones. While it is well established that ground motions can be amplified significantly through loose clay deposits, no results are available that take into consideration the typical high frequency content of ground motions in eastern Canada. This paper develops ground amplification curves for clays having depths between 10 and 70 m excited by typical eastern Canadian ground motions scaled to two different values of peak horizontal accelerations. Simplified free-field spectral design curves, which could be used by structural designers, are proposed. The curves show that maximum spectral accelerations occur for structural periods between 0.2 and 0.5 s. In addition, soil depth does not appear to be an important parameter controlling the response of typical clay deposits in eastern Canada. 相似文献
82.
83.
在20℃、4℃、-10℃、-20℃四种温度下贮藏马面卵巢,对其中磷脂含量的变化进行了测定,并利用薄层层析分析测定了不同温度下贮存3d后马面卵巢中磷脂各组分的相对百分比。结果表明马面卵巢中磷脂的损失率随贮藏温度的上升而增加;在不同温度下贮藏3d,磷脂各组分的相对百分比都发生了变化;随贮藏温度的升高磷脂酰胆碱的降解速度大大加快,所占的百分比快速下降,而溶血磷脂酰胆碱的比例有不同程度的增加。相对而言,-20℃下贮藏数天,磷脂的含量及组分的变化较小。 相似文献
84.
Hong Lin Zhang Douglas H. Sampson 《Monthly notices of the Royal Astronomical Society》2001,322(3):433-437
Calculations are made of the resonance contribution to electron-impact excitation of H-like 13 C and Li-like 23 Na, 25 Mg, 27 Al and 29 Si to the upper hyperfine levels that produce millimetre (mm) lines of likely astrophysical interest. The resonance contribution is found to be very important for these Li-like ions, considerably more important than for Li-like 57 Fe considered previously. However, resonances are found to be rather unimportant for H-like 13 C. The effect of radiative decay on the resonance contribution is found to be insignificant in all of the present calculations. 相似文献
85.
86.
BIAN Jian-min LIN Nian-feng TANG Jie College of Environment Resources Jilin University Changchun Jilin China 《东北亚地学研究》2005,(Z1)
Desertification is an environmental issue in the world. The salt-alkalization desertification land area formed by both primary and secondary salt-alkalization has extended in a large scale, which has become a significant eco-environmental problem. Based on the characteristics of eco-environment and the situation of desertification in western Songnen plain, this paper reports the analyzes of its formation in background and cause. An early warning system on the salt-alkalization desertification is established and the GIS technology is used to abstract the information of desertification evaluation index. Supported by the integrated technology of the GIS and ANN, the orientation and quantitative result of desertification are gained, which is helpful to the eco-environment protection and resource development in western Songnen Plain. 相似文献
87.
月日潮汐摩擦和地球惯量矩变化是日长长期变化的主要原因.在本文中,利用最新的地球物理和古生物钟数据,对过去15亿年以来的月日潮汐摩擦、地球惯量矩变化和日长长期变化等作了数值对比研究.由此得到二个重要结论:一是仅利用地球的自转形变不能解释J2的变化,这说明地球的重力分异现象至今仍存在着;其二是在几亿年前的潮汐摩擦比现在大得多,若取潮汐耗散与距离的立方成反比时,理论结果与由古生物钟得到的回归年日数和朔望月日数数据较为符合。 相似文献
88.
秦岭凤太矿田层控铅锌(铜)矿床的金属物质、硫和成矿溶液主要来自深部基底的岩石,属海底喷流—沉积成因矿床。从这一理论认识出发可进一步分析控矿地质条件及今后在该区有效地寻找同类矿床。 相似文献
89.
断层相关褶皱理论在准噶尔盆地南缘山前带构造研究的应用 总被引:8,自引:4,他引:4
利用断层相关褶皱的构造几何分析方法,对准噶尔盆地南缘山前复杂构造带内基于地震剖面进行了构造解析,搭建了中、东段的构造轮廓和构造组合样式,认为东段阜康断裂带主要表现为至地表的推覆逆掩。由于位移量大部分转移至地表,阜康断裂带的前陆部分无喜山期构造带;西段造山带内的挤压往前陆方向传递过程中以前列式不断释放其位移量,造成在纵向上呈现三排主要的断层相关褶皱带。根据正演平衡地质剖面制作技术对山前复杂构造区地震剖面反射波的构造识别进行了模拟与探讨。 相似文献
90.
The energy flow ofBranchiura sowerbyi was studied for the first time in China in a shallow macrophytic lake, Biandantang Lake, Hubei Province. The energy flow
was calculated from the measurement of flesh production (12.5241kJ/m2a), egestion (517.7302kJ/m2a), metabolism (38.3273 kJ/m2a), and excretion (4.3798kJ/m2a). The net growth efficiency of the species is about 22.7%, which accords well with the generally reported value for oligochaetes.
In addition, the relationship between starvation respiration (R, mgO2/ind·d), wet weight (Ww, mg) and temperature (T, °C) were also measured, with the regression function beingR=0.008Ww0.736 e0.050T.
Project supported by NSFC (30270278, 3960019), the foundation of the government of Hubei Province (No. 2000J109), and the
foundation of Ecological Station, CAS in the Institute of Hydrobiology. 相似文献