全文获取类型
收费全文 | 10230篇 |
免费 | 2523篇 |
国内免费 | 3751篇 |
专业分类
测绘学 | 1585篇 |
大气科学 | 1999篇 |
地球物理 | 2191篇 |
地质学 | 5861篇 |
海洋学 | 1999篇 |
天文学 | 234篇 |
综合类 | 959篇 |
自然地理 | 1676篇 |
出版年
2024年 | 121篇 |
2023年 | 257篇 |
2022年 | 707篇 |
2021年 | 826篇 |
2020年 | 669篇 |
2019年 | 763篇 |
2018年 | 739篇 |
2017年 | 686篇 |
2016年 | 646篇 |
2015年 | 789篇 |
2014年 | 755篇 |
2013年 | 844篇 |
2012年 | 1005篇 |
2011年 | 966篇 |
2010年 | 891篇 |
2009年 | 844篇 |
2008年 | 839篇 |
2007年 | 733篇 |
2006年 | 677篇 |
2005年 | 575篇 |
2004年 | 375篇 |
2003年 | 316篇 |
2002年 | 337篇 |
2001年 | 273篇 |
2000年 | 210篇 |
1999年 | 145篇 |
1998年 | 96篇 |
1997年 | 62篇 |
1996年 | 49篇 |
1995年 | 48篇 |
1994年 | 39篇 |
1993年 | 34篇 |
1992年 | 30篇 |
1991年 | 29篇 |
1990年 | 23篇 |
1989年 | 11篇 |
1988年 | 10篇 |
1987年 | 13篇 |
1986年 | 8篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1980年 | 3篇 |
1979年 | 11篇 |
1958年 | 3篇 |
1957年 | 3篇 |
1954年 | 7篇 |
1928年 | 3篇 |
1925年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
511.
512.
513.
514.
基于数字高程模型不规则三角网的土方计算方法的实现 总被引:1,自引:0,他引:1
本文介绍一种基于数字高程模型(DEM)不规则三角网的土方量计算方法,其利用地形碎部点快速、方便的构建三维实体模型,计算土方精度高. 相似文献
515.
Spatial variations in dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) were surveyed in the surface microlayer and in the subsurface waters of the low productivity South China Sea in May 2005. Overall, average subsurface water concentrations of DMS and DMSP of dissolved (DMSPd) and particulate (DMSPp) fractions were 1.74 (1.00-2.50), 3.92 (2.21-6.54) and 6.06 (3.40-8.68) nM, respectively. No enrichment in DMS and DMSPp was observed in the microlayer. In contrast, the microlayer showed a DMSPd enrichment, with an average enrichment factor (EF, defined as the ratio of the microlayer concentration to subsurface water concentration) of 1.40. In the study area, none of the sulfur components were correlated with chlorophyll a. An important finding in this study was that DMS, DMSP and chlorophyll a concentrations in the surface microlayer were respectively correlated with those in the subsurface water, suggesting a close linkage between these two water bodies. The ratios of DMS:Chl-a and DMSPp:Chl-a showed a gradually increasing trend from North to South. This might be due to changes in the proportion of DMSP producers in the phytoplankton community with the increased surface seawater temperature. A clear diurnal variation in the DMS and DMSP concentrations was observed at an anchor station with the highest concentrations appearing during the day and the lowest concentrations during the night. The higher DMS and DMSP concentrations during daytime might be attributed to the light-induced increase in both algal synthesis and exudation of DMSP and biological production of DMS. The mean flux of DMS from the investigated area to the atmosphere was estimated to be 2.06 micromo lm(-2)d(-1). This low DMS emission flux, together with the low DMS surface concentrations was attributed to the low productivity in this sea. 相似文献
516.
缀锦蛤亚科(Tapetinae)贝类线粒体DNA序列的系统学分析 总被引:3,自引:0,他引:3
采用线粒体16S rRNA和COI序列扩增和测序方法,对隶属于缀锦蛤亚科5属7种动物进行了系统学分析.经Clustal x多重比对和PAuP 4.10 软件分析,得到种间序列的遗传距离并构建了邻接(NJ)系统树.实验数据显示,缀锦蛤亚科为遗传连续的同源类群,其中的浅蛤属(Gomphina)、缀锦蛤属(Tapes)和蛤仔属(Ruditapes)亲缘关系较近,结果与Fischer-Piette等(1971)的缀锦蛤亚科的分类方案基本一致.另外,菲律宾蛤仔R.philippinarum 和杂色蛤仔 R.variegata是蛤仔属在印度洋和太平洋海区的一个组群,尽管两种贝类有明显的重叠分布区和相近的贝壳形态,但二者间的16S rRNA 和COI序列遗传距离均达到了物种间差别.结果支持庄启谦(2001)有关菲律宾蛤仔与杂色蛤仔的形态分类及分布区划分的观点. 相似文献
517.
The neutralizing activities of eight monoclonal antibodies (MAbs) against white spot syndrome virus (WSSV) (2D2, 2B2,1D2, 1D5, 1C2, 4A1, 6A4 and 6B4) were analyzed by in vivo experiments. Gills from WSSV-infected shrimp were homogenized and ten-fold serially diluted by PBS, and then incubated with MAbs (hybridoma culture supernatant), respectively. The mixture of WSSV and MAbs were injected into crayfish (Procambarus clarkii). After challenge, the death rates of crayfish were counted to determine the neutralizing activities of MAbs. At the same time, the mixture of myeloma culture supernatant and WSSV or PBS was served as positive or negative control, respectively. The results showed that, at each virus dilution, the mean time to death of the crayfish injected with MAb-treated virus was significantly longer than that in the positive control, though they all showed 100% mortality within 25 d, and meanwhile, few crayfish died in the negative control. Among the eight MAbs, 2D2, 2B2, 1D2 and 1D5, especially the former two, delayed the mortality significantly, and 1 C2, 4A1 and 6A4 delayed the mortality as well but not so efficiently, while MAb 6IM was efficient only when the virus concentration increased. The results indicated that the anti-WSSV MAbs can neutralize WSSV in different virus dilutions. 相似文献
518.
519.
Wind impact on pollutant transport in a shallow estuary 总被引:4,自引:0,他引:4
A three-dimensional numerical model, EFDC ( environmental fluid dynamics code) is applied to the Pamlico River Estuary (PRE) in eastern North Carolina of the United States to examine the wind impact on pollutant age distributions and residence time. A series of model experiments representing base case, remote-wind-induced water level set-up and local winds cases are conducted. Model results indicate that the pollutant mean age and the system residence time are functions of gravitational circulation in the PRE. The system responses to remote-wind-induced water level set-up are different in different portions of the PRE. Under such condition, dissolved substances in the upstream portion of the PRE have a younger age and shorter residence time (compared with the base case) , by contrast, they have a older age and longer residence time in the downstream portion of the PRE. Upriver and downriver local winds appear to have opposite impacts on pollutant age distributions. The substances are retained much longer within the PRE under upriver wind than those under downriver wind. The model results also suggest that across - river winds may lead to longer residence time through enhanced turbulence mixing, which slows down the gravitational circulation in the PRE. 相似文献
520.