全文获取类型
收费全文 | 18406篇 |
免费 | 4562篇 |
国内免费 | 4714篇 |
专业分类
测绘学 | 2065篇 |
大气科学 | 2822篇 |
地球物理 | 5321篇 |
地质学 | 10313篇 |
海洋学 | 2355篇 |
天文学 | 1044篇 |
综合类 | 1621篇 |
自然地理 | 2141篇 |
出版年
2024年 | 90篇 |
2023年 | 308篇 |
2022年 | 889篇 |
2021年 | 1041篇 |
2020年 | 882篇 |
2019年 | 1191篇 |
2018年 | 1250篇 |
2017年 | 1232篇 |
2016年 | 1358篇 |
2015年 | 1288篇 |
2014年 | 1416篇 |
2013年 | 1547篇 |
2012年 | 1436篇 |
2011年 | 1418篇 |
2010年 | 1310篇 |
2009年 | 1201篇 |
2008年 | 1202篇 |
2007年 | 1035篇 |
2006年 | 898篇 |
2005年 | 691篇 |
2004年 | 578篇 |
2003年 | 561篇 |
2002年 | 619篇 |
2001年 | 612篇 |
2000年 | 490篇 |
1999年 | 466篇 |
1998年 | 381篇 |
1997年 | 353篇 |
1996年 | 346篇 |
1995年 | 277篇 |
1994年 | 238篇 |
1993年 | 189篇 |
1992年 | 155篇 |
1991年 | 131篇 |
1990年 | 118篇 |
1989年 | 92篇 |
1988年 | 75篇 |
1987年 | 71篇 |
1986年 | 58篇 |
1985年 | 34篇 |
1984年 | 29篇 |
1983年 | 12篇 |
1982年 | 21篇 |
1981年 | 20篇 |
1980年 | 21篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1976年 | 7篇 |
1954年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
21.
基于ARP协议的网络主机地址的识别技术研究 总被引:3,自引:0,他引:3
从移动IPv6工作原理入手,IP地址管理器是针对越来越严重的IP地址盗用现象而设计的一套有效的监控和管理IP地址的方案。分析与研究IP地址管理器中的信息收集模块后,以Winpcap为开发工具,利用ARP,附等协议完成对网络中活动主机的扫描以及信息(主机名、MAC、操作系统)的收集和识别。模块实现了对网络的自动实时监控,有效地提高了管理效率。 相似文献
22.
Wei-Hao Bian Quan-Ling Cui Li-Hua Chao Department of Physics Institute of Theoretical Physics Nanjing Normal University Nanjing Key Laboratory for Particle Astrophysics Institute of High Energy Physics Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2006,6(3):281-286
We used multi-component profiles to model the Hβ and [OⅢ]λλ4959,5007 lines of SDSS J143030.22-001115.1, a narrow-line Seyfert 1 galaxy (NLS1) in a sample of 150 NLS1 candidates selected from the Sloan Digital Sky Survey (SDSS), Early Data Release (EDR). After subtracting the Hβ contribution from narrow line regions (NLRs), we found that its full width half maximum (FWHM) of broad Hβ line is nearly 2900km s~(-1) , significantly larger than the customarily adopted criterion of 2000 km s~(-1) . With its weak Fe Ⅱ multiples, we believe that SDSS J143030.22-001115.1 should not be classified as a genuine NLS1. When we calculate the virial black hole masses of NLS1s, we should use the Hβ linewidth after subtracting the NLR component. 相似文献
23.
对CPLD(复杂可编程逻辑器件)技术的基本特征和发展趋势作了简要介绍,揭示了该技术在现代数字系统中的重要地位及作用.利用CPLD对时统设备IRIG-B码产生器进行集成,其实验结果表明,集成了的B码产生器不但简单、可靠,而且便于调试,克服了以往硬件电路复杂的缺点. 相似文献
24.
在常规照相天体测量工作中 ,对暗天体的定位是采用逐级定标星过渡的方法实现的。由于星表系统差和局部差的存在 ,最终定位结果可能包含难以把握的误差积累。在CCD小视场观测情况下时常难以找到足够数量的定标星。鉴于此 ,推导了CCD观测联合平差方法的严格矢量表达式 ,以期通过共同星的过渡 ,实现大天区统一平差。原理上此方法可实现相对于河外天体的定位 ,以削弱星表系统差和局部差对定位结果的影响。讨论了CCD联合平差方法实现中的问题 ,包括底片常数模型选择、切点位置改正、共同星的较差改正和法方程解算方法等 ,并分析了具体的处理措施。用实例初步检验了CCD联合平差方法的效果 ,表明通过对多张CCD观测的联合平差 ,可以削弱单张CCD观测参考星数目过少、单张CCD观测参考星分布不均、个别参考星存在位置偏差和局部区域参考星存在系统性位置偏差等不利因素对归算结果的影响 ,进而达到扩大视场和提高归算精度的目的 相似文献
25.
Pco2 of air and seawater samples from the East China Sea(ECS) were measured in situ in autumn, 1994,Ocean currents,terrestrial fluviation,biological activities,etc.,Pco2 char-acters in air and seawater were investigated,CO2 flux and its character in the East China Sea are discussed on the basis of the Pco2 profiles of air and seawater,It was clear that the nearshore was the source of CO2;and tht the oulter sea area was the sink of CO2; and that the shelf area of the EXS is a net sink for atmospheric CO2 in autumn. 相似文献
26.
27.
The ordinary kriging method, a geostatistical interpolation technique, was applied for developing contour maps of design storm depth in northern Taiwan using intensity–duration–frequency (IDF) data. Results of variogram modelling on design storm depths indicate that the design storms can be categorized into two distinct storm types: (i) storms of short duration and high spatial variation and (ii) storms of long duration and less spatial variation. For storms of the first category, the influence range of rainfall depth decreases when the recurrence interval increases, owing to the increasing degree of their spatial independence. However, for storms of the second category, the influence range of rainfall depth does not change significantly and has an average of approximately 72 km. For very extreme events, such as events of short duration and long recurrence interval, we do not recommend usage of the established design storm contours, because most of the interstation distances exceed the influence ranges. Our study concludes that the influence range of the design storm depth is dependent on the design duration and recurrence interval and is a key factor in developing design storm contours. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
28.
Kunde Yang Yuanliang Ma Chao Sun Miller J.H. Potty G.R. 《Oceanic Engineering, IEEE Journal of》2004,29(4):964-972
This paper discusses the results of geoacoustic inversion carried out using explosive charge data from the Asian Seas International Acoustic Experiment (ASIAEX) East China Sea (ECS) Experiment. A multifrequency incoherent matched-field inversion processor and a genetic algorithm (GA) are used for the inversion. A multistep matched field inversion approach is presented, which makes use of the varying sensitivities of wave fields at various frequencies to reduce the inversion problem into a sequence of smaller inversions with fewer unknowns to estimate at each stage. Different parameters are estimated using data at different frequencies according to their sensitivities. Inversion results for different areas in the ECS region are summarized and compared with core data. 相似文献
29.
γ射线暴的研究进展(Ⅳ:特强γ射线暴和软γ重复暴 总被引:2,自引:0,他引:2
最近CGRO上的仪器观测到了两个迄今为止发现的最强的λ暴GB930131和GB940217,其光子最高能量均达GeV量级;GB930131的峰值计数率达2×10~6个光子s~(-1);而GB940217的持续时间竟长达90min。另外沉寂了多年的软重复暴源SGR1900 14和SGR1806-20的再次爆发也由BATSE发现。一系列的观测还发现这些软重复暴均与X射线源对应且与超新星遗迹成协。文中对特强暴和软重暴(SGR)的性质及研究进展作了较为详细的评述。 相似文献
30.
In many areas of engineering practice, applied loads are not uniformly distributed but often concentrated towards the centre of a foundation. Thus, loads are more realistically depicted as distributed as linearly varying or as parabola of revolution. Solutions for stresses in a transversely isotropic half‐space caused by concave and convex parabolic loads that act on a rectangle have not been derived. This work proposes analytical solutions for stresses in a transversely isotropic half‐space, induced by three‐dimensional, buried, linearly varying/uniform/parabolic rectangular loads. Load types include an upwardly and a downwardly linearly varying load, a uniform load, a concave and a convex parabolic load, all distributed over a rectangular area. These solutions are obtained by integrating the point load solutions in a Cartesian co‐ordinate system for a transversely isotropic half‐space. The buried depth, the dimensions of the loaded area, the type and degree of material anisotropy and the loading type for transversely isotropic half‐spaces influence the proposed solutions. An illustrative example is presented to elucidate the effect of the dimensions of the loaded area, the type and degree of rock anisotropy, and the type of loading on the vertical stress in the isotropic/transversely isotropic rocks subjected to a linearly varying/uniform/parabolic rectangular load. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献