全文获取类型
收费全文 | 10278篇 |
免费 | 2739篇 |
国内免费 | 3793篇 |
专业分类
测绘学 | 1637篇 |
大气科学 | 1400篇 |
地球物理 | 1990篇 |
地质学 | 6419篇 |
海洋学 | 2372篇 |
天文学 | 227篇 |
综合类 | 1060篇 |
自然地理 | 1705篇 |
出版年
2024年 | 66篇 |
2023年 | 318篇 |
2022年 | 779篇 |
2021年 | 880篇 |
2020年 | 731篇 |
2019年 | 806篇 |
2018年 | 757篇 |
2017年 | 704篇 |
2016年 | 706篇 |
2015年 | 788篇 |
2014年 | 724篇 |
2013年 | 930篇 |
2012年 | 961篇 |
2011年 | 982篇 |
2010年 | 917篇 |
2009年 | 830篇 |
2008年 | 819篇 |
2007年 | 694篇 |
2006年 | 618篇 |
2005年 | 544篇 |
2004年 | 438篇 |
2003年 | 311篇 |
2002年 | 375篇 |
2001年 | 327篇 |
2000年 | 269篇 |
1999年 | 157篇 |
1998年 | 63篇 |
1997年 | 66篇 |
1996年 | 36篇 |
1995年 | 20篇 |
1994年 | 30篇 |
1993年 | 28篇 |
1992年 | 27篇 |
1991年 | 10篇 |
1990年 | 22篇 |
1989年 | 10篇 |
1988年 | 11篇 |
1987年 | 11篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1979年 | 3篇 |
1977年 | 1篇 |
1974年 | 1篇 |
1958年 | 4篇 |
1957年 | 4篇 |
1954年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
The effects of different lateral confinement stress on the fatigue behavior of and cumulative damage to plain concrete are investigated experimentally. Eighty 100 mm x 100 mm x 100 mm specimens of ordinary strength concrete are tested under constant- or variable-amplitude fatigue loading and lateral confinement pressure in two orthogonal directions. A fatigue equation is developed by modifying the classical Aas-Jakobsen S-N equation for taking into account the effect of the confined stress on fatigue strength of plain concrete. The results of variable-amplitude fatigue tests indicate that the linear damage theory proposed by Palmgren and Miner is unreasonable in the biaxial stress state. A nonlinear cumulative damage model that could model the effects of the magnitude and sequence of variable-amplitude fatigue loading and lateral confinement pressure is proposed on the basis of the evolution laws of the residual strains in the longitudinal direction during fatigue tests. The residual fatigue life predic 相似文献
82.
The effect of the cone spacing of a conieal structure on the ice force is studied by model experiments. The ice force reduction coefficient presented in this paper expresses the relationship between the ice force and the arrangement of cones. The experiments prove that the mode of the ice failure before the boundary of upward-downward bending cone (UDBC) is crushing. A conclusion can also be drawn from the experiments that the ice force on the boundary of UDBC is by far less than that on a vertical pile with the same diameter. Moreover, the ice force frequencies on upright and inve-rted cones are obtained under the field condition of the platform JZ20-2, respectively. The results show that the alterna-tion of the ice force on UDBC can hardly induce resonance of platform JZ20-2. 相似文献
83.
To study ice-induced vibration of a compliant conical structure,a series of model tests were performed from 2004 to 2005.In the tests,the ice sheet before the compliant conical structure was found to fail in two-time breaking.From 2005 to 2006,this type of ice failure was studied through more groups of tests.The tests show that two-time breaking is the typical failure of ice before steep conical structures,and is controlled by other factors at the same time,such as ice speed and the angle of the cone. 相似文献
84.
The source and significance of two nutrients, nitrogen and phosphorous, were investigated by a modified dilution method performed on seawater samples from the Jiaozhou Bay, in autumn 2004. This modified dilution method accounted for the phytoplankton growth rate, microzooplankton grazing mortality rate, the internal and external nutrient pools, as well as nutrient supplied through remineralization by microzooplankton. The results indicated that the phytoplankton net growth rate increased in turn from inside the bay, to outside the bay, to in the Xiaogang Harbor. The phytoplankton maximum growth rates and microzooplankton grazing mortality rates were 1.14 and 0.92 d-1 outside the bay, 0.42 and 0.32 d-1 inside the bay and 0.98 and 0.62 d-1 in the harbor respectively. Outside the bay, the remineralized nitrogen (Kr=24.49) had heavy influence on the growth of the phytoplankton. Inside the bay, the remineralized phosphorus(Kr=3.49) strongly affected the phytoplankton growth. In the harbor, the remineralized phosphorus (Kr=3.73) was in larger demand by phytoplankton growth. The results demonstrated that the different nutrients pools supplied for phytoplankton growth were greatly in accordance with the phytoplankton community structure, microzooplankton grazing mortality rates and environmental conditions. It is revealed that nutrient remineralization is much more important for the phytoplankton growth in the Jiaozhou Bay than previously believed. 相似文献
85.
86.
87.
济阳坳陷博兴洼陷西部沙三段层序地层 总被引:1,自引:0,他引:1
选取以基准面为参照面的高分辨率层序地层学的理论与分析技术,对博兴洼陷西部沙三段开展层序地层分析工作。在博兴洼陷沙三段识别出5个层序界面和4个较大规模的洪泛面,由此将研究层段划分为4个长期基准面旋回(相当于3级层序),并通过长期旋回内部次级转换面的识别,细分出8个中期旋回(大致相当于4级层序)。通过对比建立了研究区的高分辨率层序地层格架,并分析了各层序的地层发育特征。以层序格架为基础,探讨了研究区各层序的沉积演化特征,建立了辫状三角洲—浊积扇层序发育模式,认为研究区辫状三角洲和浊积扇均具有加积作用特点;斜坡区为辫状三角洲发育区,而洼陷区为浊积扇发育区;中期基准面旋回下降期辫状三角洲发育,上升期浊积扇发育;浊积扇体的发育规模与湖泛规模相关。综合分析认为,浊积扇是形成岩性圈闭的主要储集砂体类型,其发育的有利层位是MSC8、MSC7、MSC6、MSC5旋回的上升半旋回,岩性圈闭发育的有利区是博兴南部斜坡坡折带之下的洼陷区。 相似文献
88.
南海深海盆表层沉积物氮的地球化学特征与生态学功能 总被引:4,自引:1,他引:4
研究了南海深海盆区域(南沙海槽西南部)表层沉积物中氮的形态、分布及其在生物地球化学循环中的功能.研究表明,表层沉积物中不同形态氮的含量不同.其中,氧化还原转化态的氮(SOEF-N)含量最高,平均为68.3μg/g,占总氮(TN)的7.08%;弱酸转化态氮(WAEF-N)含量最小,仅占总氮(TN)的1.09%.离子交换态(IEF-N),WAEF-N,S OEF-N及TN的地球化学分布特征存在一定相似性:均由海槽东西两侧向中央递增,并在槽底呈高含量分布;SAEF-N(强碱转化态)分布则与该趋势相反.IEF-N,SAEF-N和SOEF-N的分布主要受沉积物中有机碳含量(OC)控制;而WAEF-N则与碳酸盐(CaCO3)存在显著的负相关关系;TN与OC不具有显著意义的相关,间接说明二者来源的不同.同时,各形态氮的分布还与沉积物粒度类型密切联系.此外,研究区域内由沉积物提供的氮源很大程度上补偿了浮游植物对水体中营养盐的消耗,对维持该海域的初级生产力水平起到一定作用.其中,IEF-N和SOEF-N的释放对浮游植物生长及初级生产力的贡献较为显著. 相似文献
89.
Analysis of Dynamic Characteristics of Submarine Free Spanning Pipelines by Complex Damping Method 总被引:2,自引:0,他引:2
Considering the effect of the internal flowing fluid and the external marine environmental condition, the differential equation for the vortex-induced vibration (V1V) of the free spanning pipeline is derived and is discretized by the Hermit interpolation function. The free vibration equation with the damping term is solved by the complex damping method for the natural frequency, and then the effect of fluid damping on the natural frequency of the free spanning pipeline is analyzed.The results show that fluid damping has a significant influence on the damped natural frequency of the free spanning pipeline in the lock-in state, while it has little influence when the pipeline is out of the lock-in state. In the meantime,the change of the free span length has the same effect on the damped natural frequency and the undamped natural frequency. 相似文献
90.
简要阐述地震小区划在现代城市建设中的重要性以及在实施地震小区划的研究中浅层地震探测的重要作用及其方法原理,并给出应用浅层地震探测在城市地震小区划中的实例。 相似文献