全文获取类型
收费全文 | 1170篇 |
免费 | 248篇 |
国内免费 | 228篇 |
专业分类
测绘学 | 84篇 |
大气科学 | 234篇 |
地球物理 | 436篇 |
地质学 | 525篇 |
海洋学 | 103篇 |
天文学 | 44篇 |
综合类 | 87篇 |
自然地理 | 133篇 |
出版年
2024年 | 3篇 |
2023年 | 17篇 |
2022年 | 39篇 |
2021年 | 71篇 |
2020年 | 48篇 |
2019年 | 47篇 |
2018年 | 70篇 |
2017年 | 60篇 |
2016年 | 50篇 |
2015年 | 73篇 |
2014年 | 77篇 |
2013年 | 81篇 |
2012年 | 68篇 |
2011年 | 81篇 |
2010年 | 64篇 |
2009年 | 61篇 |
2008年 | 68篇 |
2007年 | 53篇 |
2006年 | 42篇 |
2005年 | 36篇 |
2004年 | 46篇 |
2003年 | 37篇 |
2002年 | 59篇 |
2001年 | 53篇 |
2000年 | 47篇 |
1999年 | 42篇 |
1998年 | 35篇 |
1997年 | 25篇 |
1996年 | 27篇 |
1995年 | 11篇 |
1994年 | 34篇 |
1993年 | 27篇 |
1992年 | 18篇 |
1991年 | 12篇 |
1990年 | 10篇 |
1989年 | 9篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1983年 | 1篇 |
1982年 | 7篇 |
1981年 | 1篇 |
1980年 | 4篇 |
1977年 | 1篇 |
1974年 | 1篇 |
1964年 | 1篇 |
1958年 | 1篇 |
1949年 | 1篇 |
排序方式: 共有1646条查询结果,搜索用时 15 毫秒
991.
Hybrid experimental performance of a full‐scale two‐story buckling‐restrained braced RC frame 下载免费PDF全文
An‐Chien Wu Keh‐Chyuan Tsai Hsun‐Horng Yang Jie‐Luen Huang Chao‐Hsien Li Kung‐Juin Wang Hsen‐Han Khoo 《地震工程与结构动力学》2017,46(8):1223-1244
This paper proposes a novel implementation of buckling‐restrained braces (BRB) in new reinforced concrete (RC) frame construction. Seismic design and analysis methods for using a proposed steel cast‐in anchor bracket (CAB) to transfer normal and shear forces between the BRB and RC members are investigated. A full‐scale two‐story RC frame with BRBs (BRB‐RCF) is tested using hybrid and cyclic loading test procedures. The BRBs were arranged in a zigzag configuration and designed to resist 70% of the story shear. The gusset design incorporates the BRB axial and RCF actions, while the beam and column members comply with ACI 318‐14 seismic design provisions. Test results confirm that the BRBs enhanced the RCF stiffness, strength, and ductility. The hysteresis energy dissipation ratios in the four hybrid tests range from 60% to 94% in the two stories, indicating that BRBs can effectively dissipate seismic input energy. When the inter‐story drift ratio for both stories reached 3.5% in the cyclic loading test, the overall lateral force versus deformation response was still very stable. No failure of the proposed steel CABs and RC discontinuity regions was observed. This study demonstrates that the proposed design and construction methods for the CABs are effective and practical for real applications. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
992.
Cesária Gomes Ana Nobre Silva Rui Taborda Filipe Santos Luís Rebelo Aníbal Medina 《地球表面变化过程与地形》2019,44(14):2810-2822
Beach erosion poses significant threat to small island economies which are generally highly dependent on coastal tourism. This work investigates the evolution of the low-lying sandy coast of Boa Vista through an integrated characterization of coastline and shoreline indicators (over the past four decades) based on aerial imagery. It was found that tandem use of the two indicators was important to obtain a reliable perspective of the Boa Vista low-lying coastal evolution across a wide range of coastal environments. Results indicate that between 1968 and 2010 the coast was relatively stable, although some spatial variability was recognized. The largest changes were observed at the tips of embayed beaches and a clear coastal progradation was found at the southern (downwind) coastal sectors. Coastal evolution has been dominated by sediment budget and the results put in evidence the sedimentary connections between the beaches across the island, either through bypass and overpass processes. Findings show that understanding coastal evolution at low-lying islands should be supported on island-scale observations, being the only scale capable to capture the sedimentary connections between beach systems, that often control coastal evolution. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd. 相似文献
993.
994.
菰(Zizania latifolia)是长江中下游流域常见的挺水植物.为了解菰在野外的扩张和退化机制,本研究通过模拟实验分析了不同时期菰幼苗对短期淹没的形态学响应.实验共选择株高36和70 cm两种幼苗,设计不淹没、50%淹没和100%淹没3个处理.研究结果表明:两组菰幼苗地上部分的形态学响应有一定差异.在100%淹没处理下,36 cm幼苗组株高最低,茎粗和最大叶宽均显著低于其他两个处理;而70 cm幼苗组在不淹没处理下株高最低,茎粗和最大叶宽在50%和100%淹没处理下均无显著差异.两组菰幼苗根系的形态学响应也不一致.36 cm幼苗组,100%淹没处理组的所有6个根系指标均与其他两种处理有显著差异;70 cm幼苗组,不淹没处理组的6个根系指标均与其他两种处理有显著差异.随着淹没深度的增加,两组菰幼苗总生物量和根茎比均逐渐下降;但36 cm幼苗组在100%淹没处理下总生物量和根茎比均显著低于其他两个处理,而70 cm幼苗组在50%和100%淹没处理下却无显著差异.综合以上分析,本研究结果表明随着菰幼苗的生长,其对淹没的形态学响应能力增强.本研究结果对长江中下游湖泊菰的生态调控具有重要意义. 相似文献
995.
铁(氢)氧化物介导的溶解性有机质(DOM)和无机磷(DIP)的固定在水生态系统中普遍存在,对碳、磷元素的生物地球化学循环有重要的影响. 铁(氢)氧化物主要通过吸附和共沉淀两种过程固定DOM和DIP,且铁(氢)氧化物、DOM和DIP三者存在复杂的相互作用. 本文主要从铁(氢)氧化物对DOM和DIP的固定,铁(氢)氧化物、DOM和DIP之间的相互影响等方面综述了相关研究进展,梳理了铁(氢)氧化物、DOM和DIP在吸附和共沉淀过程中的相互作用机制与影响因素. DOM的存在会通过占据铁(氢)氧化物表面吸附点位、络合、抑制铁水解沉淀等途径影响铁(氢)氧化物对DIP的固定; 且不同机制与DOM的不同性质如分子大小、芳香组分、羧基官能团含量等有关. 而DIP的存在会改变铁(氢)氧化物对DOM的固定分馏过程,改变溶液中DOM的组分和性质. 在明晰三者相互作用基础上,探讨了铁(氢)氧化物介导的DOM和DIP的固定过程对湖泊内源磷释放和碳埋藏的可能影响,并对未来的研究方向进行了展望. 相似文献
996.
以山西省左权、王曲电厂等为期一年的铁塔气象观测资料和各邻近地面气象站同期观测资料为例,说明如何选取典型年以及相关性较好的对比气象站,通过对电厂空冷梯度的主要气象要素分析,结合选取的对比气象站长时间序列的逐时气象资料,采用相关统计分析并进行回归检验,重建厂址区域风、温场资料,并针对风资料转换中存在的问题进行了探讨.结果表明:在两地风资料相关较差时.利用条件概率结合线性回归以及风矢量相关等方法补充订正厂址区域风资料效果较好.其结果对风、温场历史资料的重建有一定的指导意义. 相似文献
997.
998.
Li Feng Huang Shuiying Lu Xiaoxia Wang Jun Lin Mingwei An Yu Wu Shaoting Cai Minggang 《中国海洋湖沼学报》2018,36(5):1851-1859
Journal of Oceanology and Limnology - An algal astaxanthin feeding trial was carried out to investigate the effects of natural astaxanthin from Haematococcus pluvialis as feed additives on growth,... 相似文献
999.
Garnet, as a major constitutive mineral of eclogite, is important for Sm–Nd dating of eclogite due to its high Sm/Nd ratio and its stability during retrogression. However, a comprehensive study of the petrography, mineral chemistry, garnet water content, and Sm–Nd isotopic composition of eclogites from the Bixiling massif, Central Dabie Zone (CDZ), reveals significant modification of the Sm–Nd isotopic system in garnet as a result of retrogression. This problem constitutes a challenge for Sm–Nd dating of the Bixiling eclogites, with the Sm–Nd isochron ages of 218 ± 4 to 210 ± 9 Ma reported in the literature being younger than 226 ± 3 Ma, which is the generally accepted peak metamorphic age of the CDZ. Petrographic analysis reveals heterogeneity in colour within single fractured garnet grains. There are light‐pink garnet (Grt‐P) and red garnet (Grt‐R) types that possess distinct chemical compositions. Compared to Grt‐P, Grt‐R has higher Fe and andradrite contents but lower Al and grossular contents. Grt‐P also has lower water contents (15–35 ppm) than Grt‐R (34–65 ppm), which, together with the spatial association between Grt‐R and fractures, suggests that the colour change is related to fluid alteration. Grt‐P is an ultra‐high‐pressure (UHP) mineral, and Grt‐R is the product of the interaction between Grt‐P and a fluid during retrogression. Moreover, Grt‐R features lower Sm and Nd contents but higher Sm/Nd ratios than Grt‐P. The Sm–Nd isochrons defined by UHP minerals (Grt‐P+Omp+Rt or Grt‐P+Cpx+WR) from three eclogite samples yield consistent ages of 226.0 ± 3.8 Ma, 225.0 ± 3.9 Ma and 226.2 ± 6.9 Ma, which are identical to the peak metamorphic age of 226 ± 3 Ma for the CDZ. The retrogressed garnet (i.e., Grt‐R), omphacite and rutile, together define a pseudoisochron with younger ages of 218.9 ± 5.9 to 202.8 ± 4.8 Ma, which are geologically meaningless. The increase in the Sm/Nd ratio with constant or lower 143Nd/144Nd ratios during the transformation of Grt‐P to Grt‐R was probably the cause of these younger ages. 相似文献
1000.
雷达波段对多极化SAR海面溢油检测极化特征参数的影响 总被引:1,自引:1,他引:0
多极化SAR数据海面溢油检测研究日益受到重视。本文研究不同波段极化SAR数据的海面溢油检测能力,为最大程度减小观测条件、环境因素等的影响,选取准同步获取的SIR-C/X多极化SAR数据。针对海面油膜、生物油膜和低风区疑似溢油现象,研究L波段和C波段的共极化相位差、一致性系数、极化熵、各向异性和平均散射角等极化特征对海面油膜以及不同海面暗斑现象的检测能力。研究结果表明:在海面溢油检测以及探测不同暗斑现象间差异方面,C波段总体优于L波段;L波段,极化分解特征各向异性参数优于共极化相位差和一致性系数;C波段,共极化相位差、一致性系数特征优于极化分解特征各向异性和极化熵,结合平均散射角特征有助于滤除生物油膜和低风区。 相似文献