全文获取类型
收费全文 | 9609篇 |
免费 | 2031篇 |
国内免费 | 2586篇 |
专业分类
测绘学 | 954篇 |
大气科学 | 1514篇 |
地球物理 | 2344篇 |
地质学 | 5217篇 |
海洋学 | 1447篇 |
天文学 | 515篇 |
综合类 | 1033篇 |
自然地理 | 1202篇 |
出版年
2024年 | 69篇 |
2023年 | 192篇 |
2022年 | 642篇 |
2021年 | 682篇 |
2020年 | 571篇 |
2019年 | 648篇 |
2018年 | 659篇 |
2017年 | 616篇 |
2016年 | 699篇 |
2015年 | 632篇 |
2014年 | 683篇 |
2013年 | 689篇 |
2012年 | 693篇 |
2011年 | 650篇 |
2010年 | 625篇 |
2009年 | 558篇 |
2008年 | 511篇 |
2007年 | 452篇 |
2006年 | 449篇 |
2005年 | 323篇 |
2004年 | 254篇 |
2003年 | 244篇 |
2002年 | 229篇 |
2001年 | 233篇 |
2000年 | 244篇 |
1999年 | 291篇 |
1998年 | 226篇 |
1997年 | 224篇 |
1996年 | 187篇 |
1995年 | 187篇 |
1994年 | 167篇 |
1993年 | 139篇 |
1992年 | 122篇 |
1991年 | 78篇 |
1990年 | 64篇 |
1989年 | 69篇 |
1988年 | 36篇 |
1987年 | 30篇 |
1986年 | 33篇 |
1985年 | 31篇 |
1984年 | 19篇 |
1983年 | 15篇 |
1982年 | 11篇 |
1981年 | 6篇 |
1980年 | 9篇 |
1979年 | 7篇 |
1978年 | 3篇 |
1958年 | 11篇 |
1957年 | 2篇 |
1954年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
581.
Zhan Li Qingyu Peng Bir Bhanu Qingfeng Zhang Haifeng He 《Astrophysics and Space Science》2018,363(5):92
In order to obtain detailed information from multiple telescope observations a general blind super-resolution (SR) reconstruction approach for astronomical images is proposed in this paper. A pixel-reliability-based SR reconstruction algorithm is described and implemented, where the developed process incorporates flat field correction, automatic star searching and centering, iterative star matching, and sub-pixel image registration. Images captured by the 1-m telescope at Yunnan Observatory are used to test the proposed technique. The results of these experiments indicate that, following SR reconstruction, faint stars are more distinct, bright stars have sharper profiles, and the backgrounds have higher details; thus these results benefit from the high-precision star centering and image registration provided by the developed method. Application of the proposed approach not only provides more opportunities for new discoveries from astronomical image sequences, but will also contribute to enhancing the capabilities of most spatial or ground-based telescopes. 相似文献
582.
583.
584.
585.
本实验选用ZLI-3654型铁电液晶(FLC)以及SE-3140型取向剂制备FLC器件样品,通过改变FLC相变过程中的降温速率以及相变前的热力学平衡态来研究FLC相变前的热力学平衡态对FLC排列的影响,共进行了十组实验.实验后,将FLC器件静态对比度进行比较,得出手性向列相到近晶A相(N*-SmA)相变前的热力学平衡态对FLC实现均匀排列起着极其重要的作用的结论.实验结果表明:器件的静态对比度可高达620 ∶1, N*-SmA相变前的热力学平衡态有利于形成高对比 相似文献
586.
587.
588.
Biodegradation and oil mixing in Silurian sandstone reservoirs of the Tarim Basin, one of the largest composite basins in China, were investigated by analyzing the molecular characteristics and stable carbon isotopic signatures of low-molecular-weight (LMW) saturated hydrocarbons and high-molecular-weight (HMW) asphaltenes. Detection of 25-norhopanes and 17-nortricyclic terpanes in most Silurian tar sands from the Tabei Uplift in the Tarim Basin suggests a much greater degree of biodegradation here than in the Tazhong Uplift. This explains the relatively more abundant tricyclic terpanes, gammacerane, pregnane and diasteranes in tar sands from the Tabei Uplift than in those from the Tazhong Uplift. Hence, care must be taken when assigning oil source correlations using biomarkers in tar sands because of the biodegradation and mixing of oils derived from multiple sources in such an old composite basin. Asphaltenes in the tar sands seem to be part of the oil charge before biodegradation, depending on the relative anti-biodegradation characteristics of asphaltenes, the similarity in carbon isotopic signatures for asphaltenes and their pyrolysates, and the consistent product distribution for flash pyrolysis and for regular steranes in asphaltene pyrolysates, regardless of whether the tar sands were charged with fresh oil. According to the relative distributions of regular steranes and the relatively abundant 1,2,3,4-tetramethylbenzene significantly enriched in 13C, the oil sources for asphaltenes in the tar sands might be related to lower Paleozoic marine source rocks formed in euxinic conditions. Nevertheless, the relatively low abundance of gammacerane and C28 regular steranes observed in asphaltene pyrolysates and residual hydrocarbons, within limited samples investigated in this work, made a direct correlation of oils originally charged into Silurian tar sands with those Cambrian source rocks, reported so far, seem not to be possible. Comparison of carbon isotopic signatures of n-alkanes in asphaltene pyrolysates with those of LMW saturated hydrocarbons is helpful in determining if the abundant n-alkanes in tar sands are derived from fresh oil charges after biodegradation. The limited carbon isotopic data for n-alkanes in LMW saturated hydrocarbons from the tar sands can be used to classify oils charged after biodegradation in the composite basin into four distinct groups. 相似文献
589.
We conducted studies of phytoplankton and hydrological variables in a semi-enclosed bay in northern China to understand the spatial–temporal variability and relationship between these variables. Samples were collected during seven cruises in Jiaozhou Bay from November 2003 to October 2004, and were analyzed for temperature, nutrients and phytoplankton pigments. Pigments from eight possible phytoplankton classes (Diatoms, Dinoflagellates, Chlorophyceae, Prasinophyceae, Chrysophyceae, Haptophyceae, Cryptophyceae and Caynophyceae) were detected in surface water by high performance liquid chromatography (HPLC). Phytoplankton pigment and nutrient concentrations in Jiaozhou Bay were spatially and temporally variable, and most of them were highest in the northern and eastern parts of the sampling regions in spring (May) and summer (August), close to areas of shellfish culturing, river estuaries, dense population and high industrialization, reflecting human activities. Chlorophyll a was recorded in all samples, with an annual mean concentration of 1.892 μg L−1, and fucoxanthin was the most abundant accessory pigment, with a mean concentration of 0.791 μg L−1. The highest concentrations of chlorophyll a (15.299 μg L−1) and fucoxanthin (9.417 μg L−1) were observed in May 2004 at the station close to the Qingdao Xiaogang Ferry, indicating a spring bloom of Diatoms in this area. Although chlorophyll a and other biomarker pigments showed significant correlations, none of them showed strong correlations with temperature and nutrients, suggesting an apparent de-coupling between the pigments and these hydrological variables. The nutrient composition and phytoplankton community composition of Jiaozhou Bay have changed significantly in the past several decades, reflecting the increasing nutrient concentrations and decline of phytoplankton cell abundance. The unchanged total chlorophyll a levels indicated that smaller species have filled the niche vacated by the larger species in Jiaozhou Bay, as revealed by our biomarker pigment analysis. 相似文献
590.
分子鉴定方法研究大亚湾水体弧菌种类变化 总被引:2,自引:0,他引:2
借助分子鉴定方法监测大亚湾水体中弧菌Vibrio种类季节性动态的变化规律.通过增菌培养、菌株分离,在224份海水中共分离出弧菌368株,并用分子生物学辅助生化鉴定方法鉴定弧菌菌株.结果表明,在大亚湾海域水体中鉴定的弧菌种类有溶藻弧菌Vibrio alginolyticus、副溶血弧菌V.parahaemolyticus、哈氏弧菌V.harveyi和创伤弧菌V.vuinificus,没有检测到霍乱弧菌V.cholerae、拟态弧菌V.mimicus、河流弧菌V.fluvialis和霍利斯弧菌V.hollisae.溶藻弧菌和副溶血弧菌为优势菌群,分别占弧菌总数的27.99%和21.74%. 相似文献