全文获取类型
收费全文 | 39937篇 |
免费 | 19014篇 |
国内免费 | 43751篇 |
专业分类
测绘学 | 2989篇 |
大气科学 | 22329篇 |
地球物理 | 13897篇 |
地质学 | 37680篇 |
海洋学 | 16563篇 |
天文学 | 1071篇 |
综合类 | 5125篇 |
自然地理 | 3048篇 |
出版年
2024年 | 168篇 |
2023年 | 412篇 |
2022年 | 835篇 |
2021年 | 1152篇 |
2020年 | 2273篇 |
2019年 | 5662篇 |
2018年 | 6003篇 |
2017年 | 5685篇 |
2016年 | 5777篇 |
2015年 | 5004篇 |
2014年 | 4588篇 |
2013年 | 5097篇 |
2012年 | 4820篇 |
2011年 | 4644篇 |
2010年 | 4583篇 |
2009年 | 3877篇 |
2008年 | 3084篇 |
2007年 | 3146篇 |
2006年 | 2782篇 |
2005年 | 2613篇 |
2004年 | 2882篇 |
2003年 | 2616篇 |
2002年 | 2360篇 |
2001年 | 2130篇 |
2000年 | 1814篇 |
1999年 | 1993篇 |
1998年 | 2004篇 |
1997年 | 2054篇 |
1996年 | 1623篇 |
1995年 | 1600篇 |
1994年 | 1412篇 |
1993年 | 1370篇 |
1992年 | 1130篇 |
1991年 | 843篇 |
1990年 | 791篇 |
1989年 | 656篇 |
1988年 | 550篇 |
1987年 | 428篇 |
1986年 | 340篇 |
1985年 | 273篇 |
1984年 | 297篇 |
1983年 | 177篇 |
1982年 | 220篇 |
1981年 | 151篇 |
1980年 | 120篇 |
1979年 | 115篇 |
1978年 | 49篇 |
1977年 | 43篇 |
1971年 | 54篇 |
1970年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
241.
生物基可降解聚乳酸(PLA)沙障对沙丘表层沉积物粒度特征的影响 总被引:4,自引:0,他引:4
以吉兰泰盐湖北部流沙区铺设8 a的不同规格聚乳酸(PLA)沙障内沉积物为研究对象,利用激光衍射技术分析了沙丘表层0~3 cm沉积物粒度组成,计算并分析平均粒径、标准偏差等粒度参数,探讨了影响障内沉积物相对粗细的关键组分,分析PLA沙障的防沙效益。结果表明:PLA沙障内沉积物以细沙、中沙、极细沙为主。中沙、极细沙、细沙是障内沉积物的关键组分。PLA沙障的阻滞作用使得中沙、粗沙含量百分比升高,细颗粒含量百分比降低,障内沙粒分选性逐渐变差,峰态偏离正态分布,向负偏方向发展,频率分布曲线分布范围变宽,累积频率曲线变缓,表层颗粒组成趋于粗化。1 m×1 m规格PLA沙障内均以积沙为主;沙丘迎风坡和背风坡的2 m×2 m、3 m×3 m沙障内均为积沙状态,丘顶则为风蚀状态。综合考虑沙障铺设成本和防护效果,得出研究区域内2 m×2 m规格PLA沙障整体防沙效益最佳。 相似文献
242.
归来庄金矿是目前鲁西南地区唯一的大型新类型碲金型金矿床,近几年深部找矿有了进展。在充分研究归来庄金矿成矿地质特征及金矿化类型的基础上,根据归来庄金矿深部的地质勘查工程资料,以Surpac系统软件为主要工具,首次建立了归来庄深部金矿床的地质数据库, 构建了矿体的三维可视化地质模型。基于归来庄金矿深部矿体三维模型,应用距离幂次反比法对归来庄金矿深部矿体估算了资源量。将估算结果和传统地质块断法进行对比研究,结果表明用此软件进行资源量估算简便且准确,可以为矿产开发提供依据。 相似文献
243.
在乔治王岛中部的阿德默勒尔蒂湾 ,凯勒半岛、于尔曼山嘴和埃内坎角是第三纪火山岩集中分布区。野外地质调查和同位素年代学研究表明 ,凯勒半岛发生了三期火山喷发 (三个火山喷发旋回 ) ,同时伴随着火山活动中心的不断北移 ;在第二期火山喷发之后 ,火口塌陷 ,形成破火山 ,之后在半岛北部又发生火山喷发 ,并向东迁移到于尔曼山嘴。因此凯勒半岛是一复活破火山 ,火山活动具有随时间迁移的特性。埃内坎角火山活动虽与上述两地区大致同时 ,但化学成分上不存在明显演化关系 ,应属另一相对独立的火山活动中心 相似文献
244.
245.
246.
247.
The characteristics of rill development and their effects on runoff and sediment yield under different slope gradients 总被引:8,自引:0,他引:8
Rill formation is the predominant erosion process in slope land in the Loess Plateau, China. This study was conducted to investigate rill erosion characteristics and their effects on runoff and sediment yielding processes under different slope gradients at a rate of 10°, 15°, 20° and 25° with rainfall intensity of 1.5 mm min-1 in a laboratory setting. Results revealed that mean rill depth and rill density has a positive interrelation to the slope gradient. To the contrary, width-depth ratio and distance of the longest rill to the top of the slope negatively related to slope gradient. All these suggested that increasing slope steepness could enhance rill headward erosion, vertical erosion and the fragmentation of the slope surface. Furthermore,total erosion tended to approach a stable maximum value with increasing slope, which implied that there is probably a threshold slope gradient where soil erosion begins to weaken. At the same time, the correlation analysis showed that there was a close connection between slope gradient and the variousindices of soil erosion: the correlation coefficients of slope gradient with maximal rill depth, number of rills and the distance of the longest rill from the top of the slope were 0.98, 0.97 and-0.98, respectively,indicating that slope gradient is the major factor of affecting the development of rills. Furthermore,runoff was not sensitive to slope gradient and rill formation in this study. Sediment concentration,however, is positively related to slope gradient and rill formation, the sediment concentrations increased rapidly after rill initiation, especially. These results may be essential for soil loss prediction. 相似文献
248.
Other Index
List of Forthcoming Papers 相似文献249.
YuLong Duan FaSi Wu DongPeng He RuiHong Xu HuYuan Feng Tuo Chen GuangXiu Liu WanFu Wang 《寒旱区科学》2021,13(6):522-532
In this study, a systematic survey of cultural airborne fungi was carried out in the occurrence environments of wall paintings that are preserved in the Tiantishan Grottoes and the Western Xia Museum, China. A bio-aerosol sampler was used for sampling in four seasons in 2016. Culture-dependent and -independent methods were taken to acquire airborne fungal concentration and purified strains; by the extraction of genomic DNA, amplification of fungal ITS rRNA gene region, sequencing, and phylogenetic analysis, thereafter the fungal community composition and distribution characteristics of different study sites were clarified. We disclosure the main environmental factors which may be responsible for dynamic changes of airborne fungi at the sampling sites. The concentration of cultural airborne fungi was in a range from 13 to 1,576 CFU/m3, no significant difference between the two sites at the Tiantishan Grottoes, with obvious characteristics of seasonal variation, in winter and spring were higher than in summer and autumn. Also, there was a significant difference in fungal concentration between the inside and outside of the Western Xia Museum, the outside of the museum was far more than the inside of the museum in the four seasons, particularly in the winter. Eight fungal genera were detected, including Cladosporium, Penicillium, Alternaria, and Filobasidium as the dominant groups. The airborne fungal community structures of the Tiantishan Grottoes show a distinct characteristic of seasonal variation and spatial distribution. Relative humidity, temperature and seasonal rainfall influence airborne fungal distribution. Some of the isolated strains have the potential to cause biodeterioration of ancient wall paintings. This study provides supporting information for the pre-warning conservation of cultural relics that are preserved at local sites and inside museums. 相似文献
250.
Hong SUN Xihong ZHAO 《东北亚地学研究》2006,9(2):156-160
The deformation, of embankment has serious influences on neighboring structure and infrastructure. A trial embankment is reanalyzed by elastoplastic damage model coupling Blot' s consolidation theory. With the increase in time of loading, the damage accumulation becomes larger. Under the centre and toe of embankment, damage becomes serious. Under the centre of embankment, vertical damage values are bigger than horizontal ones. Under the toe of embankment, horizontal damage values are bigger than vertical ones. 相似文献