全文获取类型
收费全文 | 4207篇 |
免费 | 770篇 |
国内免费 | 815篇 |
专业分类
测绘学 | 115篇 |
大气科学 | 133篇 |
地球物理 | 1099篇 |
地质学 | 2302篇 |
海洋学 | 740篇 |
天文学 | 43篇 |
综合类 | 228篇 |
自然地理 | 1132篇 |
出版年
2024年 | 10篇 |
2023年 | 71篇 |
2022年 | 184篇 |
2021年 | 211篇 |
2020年 | 186篇 |
2019年 | 209篇 |
2018年 | 195篇 |
2017年 | 171篇 |
2016年 | 204篇 |
2015年 | 202篇 |
2014年 | 233篇 |
2013年 | 270篇 |
2012年 | 235篇 |
2011年 | 255篇 |
2010年 | 213篇 |
2009年 | 257篇 |
2008年 | 251篇 |
2007年 | 295篇 |
2006年 | 268篇 |
2005年 | 197篇 |
2004年 | 251篇 |
2003年 | 180篇 |
2002年 | 157篇 |
2001年 | 139篇 |
2000年 | 145篇 |
1999年 | 102篇 |
1998年 | 112篇 |
1997年 | 93篇 |
1996年 | 86篇 |
1995年 | 97篇 |
1994年 | 71篇 |
1993年 | 52篇 |
1992年 | 46篇 |
1991年 | 30篇 |
1990年 | 11篇 |
1989年 | 26篇 |
1988年 | 20篇 |
1987年 | 11篇 |
1986年 | 7篇 |
1985年 | 12篇 |
1984年 | 6篇 |
1983年 | 5篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1979年 | 5篇 |
1976年 | 1篇 |
1973年 | 1篇 |
1971年 | 2篇 |
1954年 | 4篇 |
排序方式: 共有5792条查询结果,搜索用时 203 毫秒
1.
This paper investigates an approach to limit the fullness of ‘tuning’ provided by wave-by-wave impedance matching control of wave energy devices in irregular waves. A single analytical formulation based on the Lagrange multiplier approach of Evans [1] is used to limit the velocity amplitude while also limiting the closeness of the phase match between velocity and exciting force. The paper studies the effect of the present technique in concurrently limiting the device velocity and the required control/actuation force. Time domain application requires wave-profile prediction, which here is based on a deterministic propagation model. Also examined in the time domain is the effect of possible violation of the displacement constraint, which for many designs implies impacts at hard stops within the power take-off mechanism. Time domain simulations are carried out for a 2-body axisymmetric converter (with physical end-stops) in sea states reported for a site off the US east coast. It is found that the approach leads to effective power conversion in the less energetic sea states, while as desired, considerable muting of the optimal response is found in the larger sea states. Under the assumptions of this work, the end-stop collisions are found to have a minor effect on the power conversion. The present approach could be used to guide the design of power take-off systems so that their displacement stroke, maximum force, and resistive and reactive power limits are well-matched to the achievable performance of a given controlled primary energy converter. 相似文献
2.
本文回顾了自然和自然贡献情景模型发展的背景、历史和内容,概括总结了自然和自然贡献情景模型的发展进程以及联合国生物多样性与生态系统服务政府间科学—政策平台(IPBES)情景模型的概念框架,讨论了自然和自然贡献情景模型存在的问题和发展方向。为了在全球层面解决现有综合集成模型存在的问题,根据地球表层建模基本定理和生态环境曲面建模基本定理,提出了具有中国原创特点的自然与自然贡献情景模型概念框架。 相似文献
3.
Mobile Agent技术是一门新兴的技术,具有移动性、智能性和异步计算的特点,已成为计算机网络和分布式系统最具活力的发展方向.现对Mobile Agent的概念、系统结构及其关键技术进行了分析和探讨. 相似文献
4.
1INTRODUCTIONTheIGBP-IHDP(InternationalGeosphere-BiosphereProgramme,andInternationalHumanDimensionPro-gramme)jointproject—LUCC(landuse/coverchange)isaninterdisciplinaryprogramdesignedtoimprovetheunderstandingofthedynamicsofland-useandland-coverchangeasinputstoandconsequencesofglobalenvironmentalchangeandsustainabledevelopment(CHARLOTTE,2002).Inparticularthedifferentbio-geochemicalmodeling(e.g.theglobalcarboncycle)ac-tivitiesandcomparativestudies,suchasthoseconduct-edbyIGBP-G… 相似文献
5.
ZHANGXue-qin GEQuan-sheng ZHENGJing-yun 《中国地理科学(英文版)》2003,13(3):207-215
How land-cover has been changed by human use over the last 300 years is one of the five overarching questions guiding the Land-use/Cover Change (LUCC) Science/Research Plan. China has variety of historical docu-ments providing unique data superiority. So the characteristics of farmland area in Shandong Province during the Qing Dynasty (1644-1911) are summarized firstly: 1) the rising trend of farmland area was striking; 2) farmland area had re-markable fluctuation; 3) farmland area per capita decreased dramatically; 4) wasteland reclamation index increased rapidly. Then, the driving forces of farmland area change are analyzed. It is concluded that natural and human factors are jointly influential. Among the driving forces, human dimensions are the main factors of farmland area change,which direct the general tendency of the changes mentioned above. And the natural factors influence the stability of farmland area as well. Variation of the natural factors would act as the major contributory factor to farmland area change during years or periods of abrupt climatic changes, or during the intensive occurring periods of natural hazards.Besides, the passive aspects of human factors, such as war chaos also influenced the fluctuation of the farmland area.This research indicates that it is feasible to study the land-use/cover change by Chinese historical literatures, which has huge potential to provide a comprehensive picture of the growing dominance of human land-use and land-cover pat-terns that can be used in many global change research oroiects. 相似文献
6.
刘满全 《地球科学与环境学报》1994,(2)
本文介绍紫阳骨坡治理工程中有关抗滑桩的设计与施工。文中详细地论述了设计中设计人员应根据工程的具体情况去灵活处理的一些技术问题,如确定有关参数的选择原则,计算上的条件简化和一些具体技术细节的处理方法等. 相似文献
7.
G.H.L. Read 《Estuarine, Coastal and Shelf Science》1985,21(3):313-324
A dry (1979–1980) and a wet (1980–1981) season had a marked effect on the freshwater inflow into the Keiskamma estuary. Under low inflow conditions, which results in elevated salinities in the upper reaches, an upstream migration of adult Macrobrachium petersi (Hilgendorf) to freshwater takes place. During periods of increased river inflow adult M. petersi move downstream to the more saline reaches of the estuary. These two migratory responses have been interpreted as (a) a breeding migration under high inflow conditions which ensures that larvae are in close proximity to salinities that favour growth and development, and (b) an adult upstream migration back to freshwater to escape elevated estuarine salinities as a result of the low freshwater inflow. 相似文献
8.
9.
10.