全文获取类型
收费全文 | 15758篇 |
免费 | 3837篇 |
国内免费 | 5913篇 |
专业分类
测绘学 | 2975篇 |
大气科学 | 2327篇 |
地球物理 | 2825篇 |
地质学 | 10629篇 |
海洋学 | 2948篇 |
天文学 | 217篇 |
综合类 | 1391篇 |
自然地理 | 2196篇 |
出版年
2024年 | 105篇 |
2023年 | 372篇 |
2022年 | 942篇 |
2021年 | 1182篇 |
2020年 | 879篇 |
2019年 | 1106篇 |
2018年 | 989篇 |
2017年 | 932篇 |
2016年 | 927篇 |
2015年 | 1159篇 |
2014年 | 1115篇 |
2013年 | 1380篇 |
2012年 | 1482篇 |
2011年 | 1451篇 |
2010年 | 1367篇 |
2009年 | 1334篇 |
2008年 | 1407篇 |
2007年 | 1329篇 |
2006年 | 1287篇 |
2005年 | 973篇 |
2004年 | 843篇 |
2003年 | 607篇 |
2002年 | 534篇 |
2001年 | 496篇 |
2000年 | 446篇 |
1999年 | 227篇 |
1998年 | 113篇 |
1997年 | 100篇 |
1996年 | 70篇 |
1995年 | 59篇 |
1994年 | 34篇 |
1993年 | 31篇 |
1992年 | 37篇 |
1991年 | 30篇 |
1990年 | 25篇 |
1989年 | 15篇 |
1988年 | 10篇 |
1987年 | 20篇 |
1986年 | 8篇 |
1985年 | 7篇 |
1984年 | 13篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 5篇 |
1979年 | 6篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1957年 | 9篇 |
1954年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
冲绳海槽北部表层沉积物中的钙质超微化石及其环境特征 总被引:4,自引:2,他引:4
为探讨冲绳海槽表层沉积物中钙质超微化石的分布规律及其影响因素 ,对海槽北部槽坡区 ( 2 9°5 8.33′— 31°31 .2 3′N ,1 2 7°39.5 5′— 1 2 8°5 9.61′E) 34个表层样品进行了分析和研究。共发现 35种钙质超微化石 ,其中优势种为Gephyrocapsaoceanica、Emilianiahuxleyi和Florisphaeraprofun da ;G .oceanica、E .huxleyi等的百分含量变化和特征属种U .tenuis的出现 ,表明该区钙质超微化石组合面貌与东海外陆架相似 ,和东海及南海都属于北太平洋中央水团组合。G .oceanica的含量与海水营养盐存在一定的关系 ,但两者之间并不存在明显的正相关性 ;暖水分子U .irregularis、U .sibogae和O .fragilis的较高含量和冷水分子G .caribbeanica和Calcidiscusleptoporus的较低含量 ,进一步反映该区钙质超微化石的分布受到黑潮及对马暖流的影响 ,其中温度、盐度和表层海水结构性质 ,如海水透明度等因素可能为主要控制因素。 相似文献
102.
运用扫描电镜(SEM)和X-射线衍射(XRD)技术研究了电化学脱盐处理对钢样-砂浆界面微观结构的影响。试验结果表明,经电化学脱盐处理后,钢样-砂浆界面的微观形貌特征与组成发生了明显的变化。 相似文献
103.
The incompressible viscous uniform and shear flow past a circular cylinder is studied. The two-dimensional Navier-Stokes equations are solved by a finite element method. The governing equations are discretized by a weighted residual method in space. The stable three-step scheme is applied to the momentum equations in the time integration. The numerical model is firstly applied to the computation of the lid-driven cavity flow for its validation. The computed results agree well with the measured data and other numerical results. Then, it is used to simulate the viscous uniform and shear flow over a circular cylinder for Reynolds numbers from lO0 to lO00. The transient time interval before the vortex shedding occurs is shortened considerably by introduction of artificial perturbation. The computed Strouhal number, drag and lift coefficients agree well with the experimental data. The computation shows that the finite element model can be successfully applied to the viscous flow problem. 相似文献
104.
105.
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
106.
107.
AbstractBased on hydrological data obtained during the 7th to 9th Chinese National Arctic Research Expeditions in the summers of 2016–2018, the main water structure on the shelf of the northern Bering Sea and the volume and heat fluxes of the Bering Strait throughflow were analyzed. Distinct variability was identified in the three Pacific water masses feeding the strait - Anadyr Water (AW), Bering Shelf Water (BSW) and Alaskan coastal water (ACW). Overall, the temperature and salinity of the entire section increased each year, with 2018 showing significant anomalies, i.e., a temperature anomaly of up to 1?°C and a maximum salinity anomaly of 2. From 2016 to 2018, the extent of the ACW gradually narrowed in the eastern part of this section, while the AW expanded eastward each year. The net volume transport through each of the three sections increased poleward from 1.65?Sv to 2.76?Sv, with the AW increasing from 0?Sv to 1.03?Sv, the BSW varying between 0.52–1.65?Sv, and the ACW gradually decreasing from 1.04?Sv to disappearing completely. The net heat fluxes were also poleward, varying between 25.77 TW and 61.50 TW, and showing a significant increase. Significant variations in magnitude and extent were observed in each water mass of the Bering Strait throughflow, which could produce widespread effects in the Arctic Ocean and the global ocean beyond. 相似文献
108.
109.
110.
2004年10月至2005年8月,对不同季节千岛湖蚤状潘的垂直分布情况以及昼夜迁移进行了研究。结果表明,蚤状潘在千岛湖分布广泛,春季和夏季蚤状潘主要分布在15-25m水层,而在秋冬季分布相对均匀,从表层到60m水深都有分布;比较了蚤状漫在不同季节的迁移现象,春季和秋季蚤状潘为夜间迁移模式,而在夏季和冬季虽然都存在迁移现象,但不同于常见的三种迁移模式。 相似文献