全文获取类型
收费全文 | 2631篇 |
免费 | 718篇 |
国内免费 | 490篇 |
专业分类
测绘学 | 324篇 |
大气科学 | 128篇 |
地球物理 | 318篇 |
地质学 | 1542篇 |
海洋学 | 336篇 |
天文学 | 722篇 |
综合类 | 175篇 |
自然地理 | 294篇 |
出版年
2024年 | 9篇 |
2023年 | 25篇 |
2022年 | 78篇 |
2021年 | 88篇 |
2020年 | 112篇 |
2019年 | 117篇 |
2018年 | 96篇 |
2017年 | 119篇 |
2016年 | 131篇 |
2015年 | 150篇 |
2014年 | 161篇 |
2013年 | 174篇 |
2012年 | 165篇 |
2011年 | 183篇 |
2010年 | 168篇 |
2009年 | 240篇 |
2008年 | 174篇 |
2007年 | 212篇 |
2006年 | 217篇 |
2005年 | 197篇 |
2004年 | 159篇 |
2003年 | 143篇 |
2002年 | 123篇 |
2001年 | 102篇 |
2000年 | 97篇 |
1999年 | 90篇 |
1998年 | 58篇 |
1997年 | 51篇 |
1996年 | 40篇 |
1995年 | 26篇 |
1994年 | 29篇 |
1993年 | 24篇 |
1992年 | 18篇 |
1991年 | 8篇 |
1990年 | 13篇 |
1989年 | 5篇 |
1988年 | 8篇 |
1987年 | 7篇 |
1986年 | 6篇 |
1985年 | 6篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1974年 | 1篇 |
1963年 | 2篇 |
排序方式: 共有3839条查询结果,搜索用时 36 毫秒
21.
22.
R. K. Manchanda 《Journal of Astrophysics and Astronomy》2001,22(2-3):145-154
Spectral measurement of Mkn 421 were made in the hard X-ray energy band of 20–200 keV using a high sensitivity, large area
scintillation counter telescope on November 21, 2000 and these coincided with the onset of an active X-ray phase as seen in
the ASM counting rates on board RXTE. The observed spectrum can not be fitted to a single power law similar to the PDS data
of BeppoSAX. The data can be fitted both by a two component power-law function or a combination of an exponential function
with a power law component at the high energies above 80 keV. We identify these components with those arising from the synchrotron
self compton and the high energy power-law tail arising from the upgrading of the thermal photons due to multiple Compton
scattering a la Cyg X-1. A comparison with the earlier data clearly suggests a spectral variability in the hard X-ray spectrum
of the source. We propose a continuously flaring geometry for the source as the underlying mechanism for energy release. 相似文献
23.
24.
The trouble with nature: Ambivalence in the lives of urban Australian environmentalists 总被引:1,自引:0,他引:1
The number and range of contests over the social production of nature is growing. Much environmentalist discourse, however, continues to appeal for unequivocal scientific support on clearly demarcated issues of pure, a-social nature. This paper explores the question of how participants in environmental movements view this demarcation in the context of their own lives. After introducing scholarly critique of dualistic ideas of nature and the ambivalence characteristic of environmentalist conceptions of nature, the paper draws upon a qualitative study with environmentalists in the Australian cities of Melbourne, Perth and Hobart. Inquiring into the interplay of discourse of nature and everyday worlds, interviews revealed a complex interdependence of dualistic and non-dualistic understandings of nature. This ecology of ideas was especially evident in spatial imaginaries of nature. Scientific terminology, social disaffection and pessimism were associated with abstract and mutually exclusive conceptions of society and nature. Reflections on personal experience, however, indicated more flexible and hopeful negotiations in everyday liminal spaces offering ‘the best of both worlds’. It is concluded that environmentalist resistance to dismantling of conceptual boundaries between society and nature may often stem not from failure to appreciate socionatural complexity, but from a strongly felt sense that the self can only truly be found in nature. 相似文献
25.
26.
介绍了广州地区基坑支护近年来的主要类型及其适用范围,并在分析选型各影响因素的基础上,总结出设计选型的要点。 相似文献
27.
28.
基于AGA的SVM需水预测模型研究 总被引:1,自引:0,他引:1
需水预测是一个由城市人口、工业水平、社会经济水平共同作用的多因素、多层次的复杂非线性系统.其结果将直接影响受区域水资源承载力约束的产业结构、布局形态等决策.作为一种集中参数预报方法,支持向量机方法具有对未来样本的较好的泛化性能,对于这类资料缺乏、系统结构尚欠清晰的问题可以取得较好的模拟和预测结果.基于此,本文将支持向量机方法引入需水预测领域,建立了需水预测支持向量机模型.同时,本文将加速遗传算法和支持向量机方法耦合起来,构造了支持向量机模型参数的自适应优化算法.模型在珠海市的应用实例表明:与简单遗传算法比较,AGA的模型参数寻优效率更高;与BP神经网络模型相比,SVM模型较好地解决了小样本、经验性等问题,并取得了较高的预测精度. 相似文献
29.
Rony Huys 《Marine Ecology》1991,12(1):1-34
Abstract. Two new genera of Tantulocarida are described from the Ligurian deep sea (Western Mediterranean) off Corsica. Xenalytus scotophilia is referred to the Microdajidae and differs from Microdajus GREVE in thoracopodal segmentation as well as in the presence of coupling spines on thoracopod 6 and longitudinal lamellae on the cephalic shield. Aphotocentor styx is placed in the Deoterthridae and holds an intermediate position between Deoterthron BRADFORD & HEWITT and Boreotantulus HI/YS & BOXSHALL. Some aspects of cephalon internal structure are described for the first time and give a possible explanation for the mechanism of stylet protrusion. The external structures of the oral disc are described using SEM. The discovery of well-developed muscles in the trunk and thoracopods is discussed in the light of the benthic phase the tantulus passes through. The boundary between the thorax and abdomen in the tantulus is reinterpreted, corroborating the 5–7-5 bodyplan of the hypothetical urmaxillopodan as found in the Upper Cambrian Skaracarida and possibly also Dala peilertae MOLLER. Some aspects related to dispersal, infection, and feeding are reinterpreted or approached from a different perspective. SEM of Microdajus langi GREVE gives evidence that the tantuli hatch via a conspicuous slit located at the posterior end of the female trunk sac. A worldwide key to the tantulocaridan families and genera is given, and distributional records are compiled. It is suggested that Tantulocarida might be common representatives of the temporary meiobenthos and that their present species number represents only the tip of the iceberg. 相似文献
30.
1 .IntroductionTheappraisalofagooddesignforaverylargefloatingstructure(VLFS)thatwillserveinaspeci fiedoceanarearequiresthatthestructuralsystembeeconomicallydesigned ,therequirementsforitsfunctionbesatisfiedandthestructurekeepsstableinitswholeserviceperio… 相似文献