全文获取类型
收费全文 | 758篇 |
免费 | 125篇 |
国内免费 | 197篇 |
专业分类
测绘学 | 30篇 |
大气科学 | 169篇 |
地球物理 | 165篇 |
地质学 | 342篇 |
海洋学 | 94篇 |
天文学 | 79篇 |
综合类 | 111篇 |
自然地理 | 90篇 |
出版年
2024年 | 2篇 |
2023年 | 16篇 |
2022年 | 50篇 |
2021年 | 36篇 |
2020年 | 25篇 |
2019年 | 37篇 |
2018年 | 47篇 |
2017年 | 56篇 |
2016年 | 47篇 |
2015年 | 23篇 |
2014年 | 39篇 |
2013年 | 38篇 |
2012年 | 44篇 |
2011年 | 52篇 |
2010年 | 46篇 |
2009年 | 52篇 |
2008年 | 35篇 |
2007年 | 44篇 |
2006年 | 24篇 |
2005年 | 26篇 |
2004年 | 22篇 |
2003年 | 20篇 |
2002年 | 30篇 |
2001年 | 18篇 |
2000年 | 20篇 |
1999年 | 34篇 |
1998年 | 32篇 |
1997年 | 25篇 |
1996年 | 19篇 |
1995年 | 7篇 |
1994年 | 21篇 |
1993年 | 19篇 |
1992年 | 12篇 |
1991年 | 14篇 |
1990年 | 9篇 |
1989年 | 5篇 |
1988年 | 5篇 |
1987年 | 8篇 |
1986年 | 2篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1980年 | 1篇 |
1977年 | 2篇 |
排序方式: 共有1080条查询结果,搜索用时 375 毫秒
211.
Amplified fragment length polymorphism (AFLP) technique was used to analyze the fingerprinting of four successive generations
ofFenneropenaeus chinensis to reveal their disease-resistance traits. Some loci showed quite different genetic frequencies due to artificial selection,
which implied that these fragments were putative markers related to the disease-resistance trait. We developed a simple and
effective method to further characterize these AFLP fragments. Specific AFLP bands were cut directly from polyacrylamide gels,
re-amplified, cloned and sequenced. Eight putative genetic markers were sequenced and their sizes ranged from 63 to 209 bp.
The sequences were submitted to dbGSS (database of Genome Sequence Survey); and the BLAST analysis showed low similarity to
the function genes, indicating these markers were tightly linked to a disease-resistance trait but were not functional genes.
This research was supported by special funds from the National Key Basic Research Program (G1999012007) and the National High-Tech
Research and Development Program of China (863 Program. 2001AA620105). 相似文献
212.
Systematic study on sorption behavior of o-nitrophenol on marine sediments was conducted.Isotherms of sorption of o-nitrophenol on marine sediments could be described by Freundlich model; and the isotherm of sorption of o-nitrophenol on HCl-treated sediment could be described by Langmuir model. The sorption behavior was affected by various factors including organic carbon content, aqueous solution salinity,temperature, and acidity. The sorption amount of o-nitrophenol increased when salinity and acidity of the aqueous solution increase, but decreased with increasing temperature. Organic carbon content in sediments had apparent effect on the behavior except for HCl-treated sediments. 相似文献
213.
Jingyi Cong Gang Hu Tara N. Jonell Zhongpeng Yuan Xianghuai Kong Yong Zhang Yonghong Wang 《中国海洋大学学报(英文版)》2020,19(4):790-800
Large amounts of sediments originating from the Changjiang(Yangtze) River are deposited in the subaqueous delta and in the adjacent muddy area off the mouth and on the inner shelf of the East China Sea. The terrestrial sediments deposited in these areas not only contain information about the composition and environment of the source area, but they also record changes in anthropogenic activities. A sediment piston core(CJ0702) was extracted from the Changjiang subaqueous depocenter(31.00°N, 122.67°E) in a water depth of 22.0 m. The core was subsampled at 1 – 2 cm intervals and analyzed for grain size, clay mineralogy, and major element geochemistry. Results indicate a relatively high sediment accumulation rate of approximately 3.11 cm yr~(-1). These parameters exhibited only minor cyclical fluctuations in the core, which resulted from many factors. During the past 120 years, the Changjiang River-derived sediment is the primary source of sediment in the offshore mud area without evidence for the Yellow River-derived sediment increasing. After the trunk stream shifted from the North Branch to South Branch, the variations of proxies are controlled by the periodic fluctuation possibly linked to El Ni?o-Southern Oscillation(ENSO) and the Pacific Decadal Oscillation(PDO). In addition, anthropogenic heavy metal concentrations can be divided into three stages, which coincide well with economic development and environmental protection policies. 相似文献
214.
在产量预测模式研究中常用的回归参数估计方法是最小二乘法,但该方法以某些统计假设为前提,计算也比较复杂,特别是在应用短序列资料建模时会受到很大的限制。而累积法是对原始数据序列按一定的规律进行相应叠加,用之建立线性模型,其估计量具有无偏、线性、有效、唯一等特点,估计效果与最小二乘法相同且计算过程更简便。因此,应用累积法,建立了安徽省桐城市一季稻产量预测模式,并介绍了累积法的建模过程和误差分析方法。应用累积法建立的桐城一季稻产量预测模式的历史回代误差率平均为3.90%,2011、2012两年试报准确率分别为95.7%和97.0%,与最小二乘法建立的预测模式误差率相近且略小,其估计精度完全符合业务要求。投入业务使用后,2013、2014年实际预报准确率分别为92.9%、98.5%。累积法的不足之处是随着自变量个数和样本容量的增大,正规方程组系数矩阵的病态性也会随之加剧,从而影响参数估计的精度。 相似文献
215.
地下气体动态特征与包头6.4级地震 总被引:2,自引:0,他引:2
初步总结了1996年5月3日内蒙古治区包头西Ms6.4地震前后,首都圈北京市昌平县太平庄台自流井溶解气体CO2、逸出气体He,河北省怀来县后郝窑台断层土壤气体CO2,北京市昌平县白浮台断层土壤气体Hg的动态特征,并对有关的问题进行了初步讨论。 相似文献
216.
217.
Four successive mass selection lines of the Pacific oyster, Crassostrea gigas, selected for faster growth in breeding programs in China were examined at ten polymorphic microsatellite loci to assess the level of allelic diversity and estimate the effective population size. These data were compared with those of their base population. The results showed that the genetic variation of the four generations were maintained at high levels with an average allelic richness of 18.8–20.6, and a mean expected heterozygosity of 0.902–0.921. They were not reduced compared with those of their base population. Estimated effective population sizes based on temporal variances in microsatellite frequencies were smaller to that of sex ratio-corrected broodstock count estimates. Using a relatively large number of broodstock and keeping an equal sex ratio in the broodstock each generation may have contributed to retaining the original genetic diversity and maintaining relatively large effective population size. The results obtained in this study showed that the genetic variation was not affected greatly by mass selection progress and high genetic variation still existed in the mass selection lines, suggesting that there is still potential for increasing the gains in future generations of C. gigas. The present study provided important information for future genetic improvement by selective breeding, and for the design of suitable management guidelines for genetic breeding of C. gigas. 相似文献
218.
219.
基于日本海洋信息中心提供的东海黑潮PN断面CTD资料,本文采用动力高度法计算了1991-2011年间90个航次的断面流速,并对流场结构、最大流速、流幅和流量进行了统计分析。结果表明:东海黑潮PN断面流场存在单核、双核、多核3种结构;其中单核结构出现的概率为50%,双核结构为39%,多核结构为11%。东海黑潮的流结构存在显著的季节变化:秋季多核结构所占的比重为4个季节最大,平均流核数最多;冬季主要为单核结构,平均流核数最少;夏季和春季则没有明显的倾向性,单核、双核、多核3种结构出现概率相近,平均流核数介于秋季和冬季之间。其次东海黑潮的流量也存在显著的季节变化:冬季与夏季最强,秋季最小,春季居中。最后东海黑潮的最大流速和流幅也存在季节变化:夏季最大,秋季最小,春季和冬季居中。 相似文献
220.
Hydropower development in Xizang (Tibet) Autonomous Region plays a vital role in co-control of local air pollutants and greenhouse gas (GHG) in China. According to emission factors of local air pollutants and GHG of coal-fired power industry in different hydropower service regions, we estimate the effect and synergy of local air pollutants and GHG reduction achieved by hydropower development in Tibet, examine the main factors constraining the effect and synergy, using correlation analysis and multiple regression analysis. The results show that: 1) During the period from 2006 to 2012, the effect of local air pollutants and GHG reduction achieved by hydropower development in Tibet decreased as a whole, while the synergy increased first and decreased afterwards. 2) The effect and synergy of local air pollutants and GHG reduction achieved by hydropower development in Tibet vary significantly across different hydropower service regions. The effect based on emission levels of Central China power grid (CCPG) and Northwest China power grid (NCPG) was more significant than that based on emission level of national power grid (NPG) from 2006 to 2012, and the synergy based on emission levels of CCPG and NCPG was also more significant than that based on emission level of NPG from 2010 to 2012. 3) The main factors constraining the effect and synergy based on emission levels of NCPG and CCPG included SO2 removal rate and NO x removal rate, the effect and synergy based on emission level of NPG was mainly influenced by net coal consumption rate. 4) Transferring hydropower from Tibet to NCPG and CCPG, and substituting local coal-fired power with hydropower can greatly help to co-control local air pollutants and GHG, transform the emission reduction pattern of the power industry and optimize energy structure. 相似文献