全文获取类型
收费全文 | 5336篇 |
免费 | 1056篇 |
国内免费 | 1387篇 |
专业分类
测绘学 | 555篇 |
大气科学 | 904篇 |
地球物理 | 1167篇 |
地质学 | 2848篇 |
海洋学 | 774篇 |
天文学 | 222篇 |
综合类 | 604篇 |
自然地理 | 705篇 |
出版年
2024年 | 22篇 |
2023年 | 81篇 |
2022年 | 193篇 |
2021年 | 265篇 |
2020年 | 225篇 |
2019年 | 255篇 |
2018年 | 314篇 |
2017年 | 279篇 |
2016年 | 301篇 |
2015年 | 253篇 |
2014年 | 377篇 |
2013年 | 393篇 |
2012年 | 324篇 |
2011年 | 368篇 |
2010年 | 355篇 |
2009年 | 349篇 |
2008年 | 316篇 |
2007年 | 312篇 |
2006年 | 288篇 |
2005年 | 241篇 |
2004年 | 179篇 |
2003年 | 169篇 |
2002年 | 161篇 |
2001年 | 173篇 |
2000年 | 205篇 |
1999年 | 233篇 |
1998年 | 152篇 |
1997年 | 155篇 |
1996年 | 140篇 |
1995年 | 121篇 |
1994年 | 132篇 |
1993年 | 89篇 |
1992年 | 77篇 |
1991年 | 62篇 |
1990年 | 32篇 |
1989年 | 27篇 |
1988年 | 33篇 |
1987年 | 32篇 |
1986年 | 23篇 |
1985年 | 14篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1982年 | 10篇 |
1981年 | 7篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1976年 | 4篇 |
1974年 | 3篇 |
1958年 | 3篇 |
1954年 | 2篇 |
排序方式: 共有7779条查询结果,搜索用时 265 毫秒
1.
Revisiting the existence of an effective stress for wet granular soils with micromechanics
下载免费PDF全文
![点击此处可从《国际地质力学数值与分析法杂志》网站下载免费的PDF全文](/ch/ext_images/free.gif)
A possible effective stress variable for wet granular materials is numerically investigated based on an adapted discrete element method (DEM) model for an ideal three‐phase system. The DEM simulations consider granular materials made of nearly monodisperse spherical particles, in the pendular regime with the pore fluid mixture consisting of distinct water menisci bridging particle pairs. The contact force‐related stress contribution to the total stresses is isolated and tested as the effective stress candidate for dense or loose systems. It is first recalled that this contact stress tensor is indeed an adequate effective stress that describes stress limit states of wet samples with the same Mohr‐Coulomb criterion associated with their dry counterparts. As for constitutive relationships, it is demonstrated that the contact stress tensor used in conjunction with dry constitutive relations does describe the strains of wet samples during an initial strain regime but not beyond. Outside this so‐called quasi‐static strain regime, whose extent is much greater for dense than loose materials, dramatic changes in the contact network prevent macroscale contact stress‐strain relationships to apply in the same manner to dry and unsaturated conditions. The presented numerical results also reveal unexpected constitutive bifurcations for the loose material, related to stick‐slip macrobehavior. 相似文献
2.
Zhi Xu Cheng Fang Wei-Qun Can Astronomy Department of Nanjing University Nanjing Purple Mountain Observatory Chinese Academy of Sciences Nanjing 《中国天文和天体物理学报》2005,5(5):519-530
Chromospheric lines, including Ha, Lyα, Lyβand CaⅡK, CaⅡ8542, are systemically and quantitatively investigated with respect to the non-thermal excitation and ionization due to particle beam bombardment for a series of solar semi-empirical atmospheric models. As a result we propose to use the contrast in the integrated intensity of hydrogen lines to estimate the total energy flux of the bombarding beam during the solar flare impulsive phase. Partial frequency redistribution is considered in the Lyαline calculation and a smaller intensity enhancement in the Ha line-centers is found than in the previous results of Fang et al. 相似文献
3.
4.
为建立社会主义市场经济的地矿工作新秩序,本文对国有地矿企业的经营者(厂长、经理)这一特殊群体进行了分析。着重研究了新形势下企业经营者实行年薪制的问题。指出年薪制就是以企业一个生产经营周期为单位确定经营者的报酬,这一周期通常为一年。进而阐述了为什么要实行企业经营者年薪制;企业经营者的收入应与职工工资完全脱钩;采用基础报酬加效益报酬的方法确定收入等等。并构想了实行经营者年薪制所需要的外部环境:建立经营者人才市场、经营者要能进能出、实行任职资格制度、建立风险机制、按劳动力市场经济规律办事。 相似文献
5.
CATASTROPHIC ECO-ENVIRONMENTAL CHANGE IN THE SONGNEN PLAIN, NORTHEASTERN CHINA SINCE 1900S 总被引:1,自引:1,他引:0
WANGHai-xia WANZhong-juan YUShao-peng LUOXin-zheng SUNGuang-you 《中国地理科学(英文版)》2004,14(2):179-185
Although the Songnen Plain in the northeastern China was developed relatively late in the temperate zone of the world, its eco-environment has changed greatly. This paper analyzes the changes of land cover and the rates and trends ofdesertification during the past 100 years in the Songnen Plain. According to the macroscopic analysis, we find that the eco-environment in the plain has reached to the threshold of catastrophic change since the 1950s. The Thorn Needle Catastrophic Model was used to determine and validate this conclusion. Human activities, including large-scale construction projects, such as huge dams and dikes, and excessive grazing were the primary factors contributing to regional eco-environmental catastrophe. And irrational reclamation of the wilderness also affected the eco-environmental change. The results reveal the complex human-land interactions. 相似文献
6.
探讨用遗传算法对数学模型进行优化。考虑到控制系统稳健性的要求 ,用遗传算法寻找出控制系统最佳稳定域 ,实现控制系统数学模型的寻优。阐述了用遗传算法求解问题的步骤和参数的取值 ,并用仿真实验对优化结果进行了检验。结果表明 :在控制系统数学模型的优化中 ,遗传算法具有其他算法无可比拟的优越性。 相似文献
7.
从ROSAT巡天和定点观测结果的对比出发,发现了几个低银纬的巡天源表中的X射线源在定点观测中没有出现.本文计算了在这些位置的定点观测中出现点源的流强上限,发现比巡天源表中这些源的流强值低很多,这表明它们的流强变化很大,可能是X射线双星系统中的一类———X射线暂现源. 相似文献
8.
9.
STUDY ON GIS FOR YIELD ESTIMATION BY REMOTE SENSING IN JILIN MAIZE BELTSTUDYONGISFORYIELDESTIMATIONBYREMOTESENSINGINJILINMAIZ... 相似文献
10.