全文获取类型
收费全文 | 1206篇 |
免费 | 376篇 |
国内免费 | 396篇 |
专业分类
测绘学 | 171篇 |
大气科学 | 156篇 |
地球物理 | 347篇 |
地质学 | 780篇 |
海洋学 | 173篇 |
天文学 | 10篇 |
综合类 | 105篇 |
自然地理 | 236篇 |
出版年
2024年 | 7篇 |
2023年 | 31篇 |
2022年 | 90篇 |
2021年 | 110篇 |
2020年 | 66篇 |
2019年 | 89篇 |
2018年 | 78篇 |
2017年 | 85篇 |
2016年 | 67篇 |
2015年 | 78篇 |
2014年 | 84篇 |
2013年 | 89篇 |
2012年 | 115篇 |
2011年 | 111篇 |
2010年 | 100篇 |
2009年 | 104篇 |
2008年 | 87篇 |
2007年 | 79篇 |
2006年 | 79篇 |
2005年 | 67篇 |
2004年 | 57篇 |
2003年 | 38篇 |
2002年 | 35篇 |
2001年 | 34篇 |
2000年 | 35篇 |
1999年 | 23篇 |
1998年 | 17篇 |
1997年 | 17篇 |
1996年 | 12篇 |
1995年 | 12篇 |
1994年 | 13篇 |
1993年 | 10篇 |
1992年 | 10篇 |
1991年 | 7篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 4篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1969年 | 1篇 |
1966年 | 1篇 |
1954年 | 1篇 |
1925年 | 1篇 |
排序方式: 共有1978条查询结果,搜索用时 78 毫秒
1.
Polypeptide fromChlamys farreri (PCF), a topical polypeptide isolated fromChlamys farreri, was used in this experiment aimed to investigate the photoprotective effect of PCF against chronic skin damage induced by
ultraviolet A (UVA) and ultraviolet B (UVB) radiation. The chronic ultraviolet-irradiated guinea pig model was established,
and visible changes in the skin including wrinkling, sagging and erythema were observed. Malondialdehyde (MDA) and antioxidant
enzymes including superoxide dismutase (SOD) and glutathione peroxidase (GSH-px) in the dorsal skin were determined using
biochemical methods. The results showed: (1) PCF (5 % and 20%) could greatly protect the dorsal skin of guinea pig against
wrinkling, sagging and erythema induced by UV radiation in a concentration-dependent manner. (2) PCF could reduce MDA formation
in the dorsal skin caused by UV irradiation, while increasing the activities of SOD and GSH-px. (3) The differences among
the PCF groups and UV model group were significant (P<0.05,P<0.01). These results indicated that topical application, of PCF provided broad solar UV spectrum photoprotection; and that
the antioxidant property of PCF might play a role in photoprotection.
Project No. 39970638 supported by the NSFC, and the Science and Technology Bureau of Qingdao (No. 2001-28-50). 相似文献
2.
3.
通过精细岩芯描述、单井沉积微相分析与地震相、测井相综合研究,认为在塔里木盆地满加尔凹陷西缘奥陶系柯坪塔格组下段顶部含油气砂体中,发育3种类型的准层序:①粒级由底部粗—中下部细—上部粗的、反映整体向上变浅的海侵滞留沉积—海泛泥岩—滨岸沉积准层序,羊屋2井5460~5468 m含油气段位于该准层序内;②粒级向上变粗的滨外泥岩—临滨-前滨沉积,顶部受风暴影响的准层序,跃南2井6503~6512 m含油气段位于该准层序内;③粒级向上变细的潮下带—潮间带沉积的准层序,吉南1井5465~5486 m含油气段属于此准层序。发育于羊屋2井中的第一种准层序内含油气砂体在时间上属于较早的准层序3,发育于跃南2井、哈得13井中的第二种准层序与吉南1井中的第三种准层序内的含油气砂体属于稍晚的准层序4。早期沉积的、发育于羊屋2井顶部的第一种准层序是有利储层发育段,该井区以北靠近柯坪塔格组下段顶面尖灭线附近的准层序3时期沉积的临滨相储层是油气聚集成藏的有利部位。 相似文献
4.
Recharge source and hydrogeochemical evolution of shallow groundwater in a complex alluvial fan system,southwest of North China Plain 总被引:4,自引:0,他引:4
Fadong Li Guoying Pan Changyuan Tang Qiuying Zhang Jingjie Yu 《Environmental Geology》2008,55(5):1109-1122
Many cities around the world are developed at alluvial fans. With economic and industrial development and increase in population,
quality and quantity of groundwater are often damaged by over-exploitation in these areas. In order to realistically assess
these groundwater resources and their sustainability, it is vital to understand the recharge sources and hydrogeochemical
evolution of groundwater in alluvial fans. In March 2006, groundwater and surface water were sampled for major element analysis
and stable isotope (oxygen-18 and deuterium) compositions in Xinxiang, which is located at a complex alluvial fan system composed
of a mountainous area, Taihang Mt. alluvial fan and Yellow River alluvial fan. In the Taihang mountainous area, the groundwater
was recharged by precipitation and was characterized by Ca–HCO3 type water with depleted δ18O and δD (mean value of −8.8‰ δ18O). Along the flow path from the mountainous area to Taihang Mt. alluvial fan, the groundwater became geochemically complex
(Ca–Na–Mg–HCO3–Cl–SO4 type), and heavier δ18O and δD were observed (around −8‰ δ18O). Before the surface water with mean δ18O of −8.7‰ recharged to groundwater, it underwent isotopic enrichment in Taihang Mt. alluvial fan. Chemical mixture and ion
exchange are expected to be responsible for the chemical evolution of groundwater in Yellow River alluvial fan. Transferred
water from the Yellow River is the main source of the groundwater in the Yellow River alluvial fan in the south of the study
area, and stable isotopic compositions of the groundwater (mean value of −8.8‰ δ18O) were similar to those of transferred water (−8.9‰), increasing from the southern boundary of the study area to the distal
end of the fan. The groundwater underwent chemical evolution from Ca–HCO3, Na–HCO3, to Na–SO4. A conceptual model, integrating stiff diagrams, is used to describe the spatial variation of recharge sources, chemical
evolution, and groundwater flow paths in the complex alluvial fan aquifer system. 相似文献
5.
甲酸和乙酸稳定碳同位素组成(δ^13C)的分析对环境、食品、制药和自然产品等的研究具有重要的应用价值。但目前尚缺乏有效的测定方法。本研究利用最近出现的针式固相微萃取技术(NeedlEX),以吹扫.捕集方式对水溶液中的有机酸进行了萃取,然后利用气相色谱.同位素比值质谱联用仪(GC—IRMS)对所萃取的有机酸分子进行了占δ^13C的测定。结果显示,质谱计的信号强度与水溶液中有机酸的浓度存在显著的线性相关关系(R^2〉0.99,P〈0.05),表明Needl EX对水溶液中有机酸具有稳定的萃取能力。在甲酸与乙酸含量分别不低于300μg/mL与200μg/mL的水溶液中,1000mL的吹扫体积可以使两者δ^13C多次分析结果的相对误差分别保持在3%和1%左右,且整个实验流程没有造成可检测的碳同位素分馏作用。低于这两个浓度界线,则分析误差随浓度的降低迅速增加。本研究虽然是针对水溶液中有机酸δ^13C的测定,其萃取方法对其他水溶性挥发和半挥发有机物δ^13C的分析也同样具有应用价值。 相似文献
6.
7.
滑坡体三维地质建模与可视化分析 总被引:1,自引:0,他引:1
针对滑坡地质研究的自身特点,提出了面向滑坡地质体三维建模的NURBS-BRep混合数据结构和地质结构单元实体构造技术.通过对滑坡基础地质数据的预处理、滑坡数字地形和滑动面的三维建模、地质结构面的系统构造、地质结构体的生成和显示,形成了一套完整的滑坡三维地质模型的构建方法.将该方法应用于某水库滑坡,建立了相应的三维滑坡地质模型,并基于此模型进行了三维剖切分析、数字钻孔、等值线生成、滑块自动剖分、滑坡失稳可视化动态模拟和滑坡体方量精确计算等一系列实用的可视化分析,为滑坡稳定性的准确计算和客观评价提供了有力的支持. 相似文献
8.
9.
将模糊化(拓扑)导算子进行了公理化,并且给出了若干模糊化(拓扑)导算子的刻画;同时建立了模糊化(拓扑)导算子和模糊化(拓扑)闭包算子之间的关系。 相似文献
10.
Tellurium is known to be a poisonous element to human health. In recent years,determination of trace tellurium has been attached great attention in connection with thewide application of its compounds and/or with environmental pollution. Literature citedreported the determination of ppb level tellurium in waste water and artificial watersamples by atomic-absorption spectrophotometry. As the tellurium content in naturalwater is extremely low, there is not yet any sensitive analytical method for determination ofits background value in various natural water. In chemical oceanography, data abouttellurium content in seawater are rather scarce. This paper will summarize and evaluate a series of our investigations on the analytical 相似文献