全文获取类型
收费全文 | 12556篇 |
免费 | 2611篇 |
国内免费 | 3439篇 |
专业分类
测绘学 | 1251篇 |
大气科学 | 2490篇 |
地球物理 | 3093篇 |
地质学 | 6087篇 |
海洋学 | 2204篇 |
天文学 | 441篇 |
综合类 | 1260篇 |
自然地理 | 1780篇 |
出版年
2024年 | 98篇 |
2023年 | 333篇 |
2022年 | 744篇 |
2021年 | 913篇 |
2020年 | 692篇 |
2019年 | 737篇 |
2018年 | 820篇 |
2017年 | 688篇 |
2016年 | 791篇 |
2015年 | 767篇 |
2014年 | 804篇 |
2013年 | 918篇 |
2012年 | 860篇 |
2011年 | 868篇 |
2010年 | 882篇 |
2009年 | 773篇 |
2008年 | 716篇 |
2007年 | 684篇 |
2006年 | 584篇 |
2005年 | 465篇 |
2004年 | 399篇 |
2003年 | 293篇 |
2002年 | 314篇 |
2001年 | 337篇 |
2000年 | 286篇 |
1999年 | 397篇 |
1998年 | 368篇 |
1997年 | 338篇 |
1996年 | 317篇 |
1995年 | 246篇 |
1994年 | 213篇 |
1993年 | 184篇 |
1992年 | 136篇 |
1991年 | 119篇 |
1990年 | 112篇 |
1989年 | 92篇 |
1988年 | 73篇 |
1987年 | 61篇 |
1986年 | 49篇 |
1985年 | 28篇 |
1984年 | 20篇 |
1983年 | 10篇 |
1982年 | 17篇 |
1981年 | 13篇 |
1980年 | 12篇 |
1979年 | 9篇 |
1978年 | 4篇 |
1976年 | 4篇 |
1958年 | 4篇 |
1957年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
X线侧位片与高分辨率CT对鼻骨骨折的对比研究 总被引:17,自引:0,他引:17
目的 通过86例鼻骨骨折的鼻骨X线侧位片与高分辨率CT(HRCT)对比研究,确定HRCT检查对鼻骨骨折的诊断价值。材料与方祛 86例鼻外伤或/和He、面外伤的患者均进行了鼻骨X线侧位摄影、HRCT扫描检查,经集体讨论,由有经验的放射医师对全部影像资料进行分析并进行统计学处理。结果HRCT显示所有的86例鼻骨骨折,但在此86例骨折中X线平片只显示了35例阳性,包括15例鼻骨汇合部骨折,7例双侧鼻骨骨折,3例复合骨折(鼻骨汇合部及双侧鼻骨骨折),l0例鼻区骨折。X线平片显示5l例为阴性。结论 对鼻骨骨折的诊断,HRCT明显优于鼻骨X线侧位片,HRCT扫描检查可提高X线隐性鼻骨骨折的诊断,并可明确鼻骨骨折的类型、范围和程度,为临床诊断和治疗提供可靠的依据。 相似文献
82.
83.
Nonlinear earthquake response analysis and energy calculation for seismic slit shear wall structures
Based on the concept of structural passive control, a new type of slit shear wall, with improved seismic performance when
compared to an ordinary solid shear wall, was proposed by the authors in 1996. The idea has been verified by a series of pseudo-static
and dynamic tests. In this paper a macro numerical model is developed for the wall element and the energy dissipation device.
Then, nonlinear time history analysis is carried out for a 10-story slit shear wall model tested on a shaking table. Furthermore,
the seismic input energy and the individual energy dissipated by the components are calculated by a method based on Newmark-β
assumptions for this shear wall model, and the advantages of this shear wall are further demonstrated by the calculation results
from the viewpoint of energy. Finally, according to the seismic damage criterion on the basis of plastic accumulative energy
and maximum response, the optimal analysis is carried out to select design parameters for the energy dissipation device. 相似文献
84.
Early Yanshanian post-orogenic granitoids in the Nanling region
Petrological constraints and geodynamic settings 总被引:1,自引:0,他引:1
Petrological constraints and geodynamic settings 总被引:1,自引:0,他引:1
Early Yanshanian magmatic suites predominate absolutely in the Nanling granite belt. They consist mainly of monzogranite and K-feldspar granite. There occur associations of early Yanshanian A-type granitoids (176 Ma-178 Ma) and bimodal volcanic rocks (158 Ma-179 Ma) in southern Jiangxi and southwestern Fujian in the eastern sector of the granite belt and early Yanshanian basalts (177 Ma-178 Ma) in southern Hunan in the central sector of the belt. Both the acid end-member rhyolite in the bimodal volcanic rock association and A-type granitoids in southern Jiangxi have the geochemical characteristics of intraplate granitic rocks and the basic end-member basalt of the association is intraplate tholeiite, while the basaltic rocks in southern Hunan include not only intraplate tholeiite but also intraplate alkali basalt. Therefore the early Yanshanian magmatic suites in the Nanling region are undoubtedly typical post-orogenic rock associations. Post-orogenic suites mark the end of a post-collision or late orogenic event and the initiation of Pangaea break-up, indicating that a new orogenic Wilson cycle is about to start. Therefore it may be considered that the early Yanshanian geodynamic settings in the Nanling region should be related to post-orogenic continental break-up after the Indosinian orogeny and the break-up did not begin in the Cretaceous. 相似文献
85.
1 Introduction Soil is not only the major natural resource on which human being depends for the production of food, feed, fiber, renewable energy and raw materials, but also plays a key role in maintaining the complex terrestrial ecosystems and climate systems of this planet. Recent rapid increase in the human population is placing a great strain on the worlds soil resources. Only about 11% of the global land surface covered by the soils are being used to raise crops and livestock, in other … 相似文献
86.
云南省防洪空间信息系统构建 总被引:4,自引:1,他引:4
通过讨论应用地理信息系统(GIS)技术,立足现实对相关空间数据进行集成和整合处理,实现防洪空间信息的数字化、标准化,为优化防汛指挥工作手段提供基础。系统数据库饭知不同比例尺精度的空间基础数据以满足不同目的的需求,对相关水文、防洪设施、全省洪涝风险和重点区风险图、土地利用等图件进行系统整合。系统有较强的管理功能和分析功能,可提高云南省防洪空间信息的共享度和系统有效管理,为政府统一组织和指挥抗洪救灾提供重要的辅助决策支持手段。 相似文献
87.
88.
岫岩-海城5.4级地震前小震震源机制解与记录特征分析 总被引:4,自引:2,他引:4
采用Pn、Pg初始波初动符号,利用乌尔夫网上半球投影,用作图方法求解了岫岩-海城震区(1999年1月-1999年11月29日)主震前辽宁数字地震台网记录(ML≥2.5)的41个小震的震源机制参数。结合前震记录的某些特征,对主震前应力方向的时空变化,震源错动性质进行分析和讨论。 相似文献
89.
论述了GPS导航定位技术在伊拉克战争中的作用,着重分析了GPS在精确制导武器中的应用形式和效果,并预测了美国在GPS应用方面未来将要进行的努力。 相似文献
90.
W. Lu W. Hustrulid 《Fragblast: International Journal for Blasting and Fragmentation》2003,7(4):231-255
Airdecking is used in mining for two quite different applications. One is to enhance the fragmentation by amplifying the induced fracturing and the second is for pre-split blasting in which the borehole fracturing is reduced. This paper deals with the first of these effects. A forth coming paper will describe pre-splitting by airdecking. The use of air decks to enhance rock fragmentation and so to reduce explosive costs has been the practice for quite long time. Although a number of studies has been conducted to verify the advantages of blasting with air decks and to investigate the mechanisms involved, the proposed mechanisms still cannot explain clearly the phenomena observed in practice and the design approach adopted for this kind of blasting is still primary based on rules-of-thumb. In this paper, the theory of shock tubes is adopted to (a) investigate the processes of the expanding detonation products, (b) study the interactions between the explosion products and the stemming or bottom of blasthole, and (c) to decide the distribution of the changing pressure of explosion products along blasthole. Numerical simulation and theoretical analyses are then performed to study the physical process of blasting with air decks. Finally, a reasonable value for the airdecking ratio is decided theoretically. It is shown that the pressure-unloading process caused by the propagation of the rarefaction wave and the reflected rarefaction waves in the detonation products plays an important role in the enhanced fragmentation of rock when blasting with air decks. The unloading process can induce tensile stresses of rather high magnitude in the rock mass surrounding blasthole. This favors fracturing of the rock. The reflected shock wave with a magnitude of gas pressure higher than that of the average detonation pressure in a fully charged blasthole acts as the main energy source to break the rock in the air deck and stemming portions. The second and succeeding strain waves induced by the unloading or reloading of the pressurewithin the blasthole also contribute to form the initial fracture network in the rock around the blasthole. It is also revealed that there exists a reasonable range of values for the airdecking ratio. For ANFO, this value varies from 0.13-0.40. 相似文献