首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1541篇
  免费   329篇
  国内免费   412篇
测绘学   116篇
大气科学   310篇
地球物理   351篇
地质学   812篇
海洋学   275篇
天文学   83篇
综合类   150篇
自然地理   185篇
  2024年   4篇
  2023年   20篇
  2022年   48篇
  2021年   77篇
  2020年   72篇
  2019年   80篇
  2018年   81篇
  2017年   68篇
  2016年   93篇
  2015年   86篇
  2014年   93篇
  2013年   95篇
  2012年   92篇
  2011年   118篇
  2010年   86篇
  2009年   96篇
  2008年   126篇
  2007年   114篇
  2006年   90篇
  2005年   72篇
  2004年   84篇
  2003年   58篇
  2002年   56篇
  2001年   75篇
  2000年   59篇
  1999年   75篇
  1998年   37篇
  1997年   50篇
  1996年   37篇
  1995年   16篇
  1994年   27篇
  1993年   24篇
  1992年   12篇
  1991年   8篇
  1990年   10篇
  1989年   9篇
  1988年   7篇
  1987年   8篇
  1986年   3篇
  1985年   1篇
  1984年   2篇
  1983年   3篇
  1982年   2篇
  1981年   2篇
  1980年   1篇
  1979年   5篇
排序方式: 共有2282条查询结果,搜索用时 31 毫秒
981.
Traditional farming practices conform to sustainable rural livelihoods, while agricultural modernisation tends to undermine these practices through various perturbations. A case study in Tengchong County (western Yunnan, China) shows that transformation of traditional alder (Alnus nepalensis) and dry rice (upland rice) rotational farming to introduced Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) plantations leads to localised water scarcity and soil fertility decline. While farmers are aware of ecological sustainability of traditional farming, they prefer Chinese fir forestry because it is less labour-intensive, has a high market value, and releases time for profitable off-farm work. Farmers adapt to economic liberalization by planting high value crops and trees. However, alternatives to make local agricultural production more profitable through competitive business strategies, cooperative approaches, innovations in integrating high value crops and ‘downstream’ processing for sustainable rural livelihoods have been overlooked largely due to poor information availability and lack of organisational framework.  相似文献   
982.
对差分编码、HDB3码、码间串扰、眼图等数字基带传输系统中较为抽象难懂的内容进行了介绍.叙述了如何用Flash软件来设计制作相应的知识难点课件.在实际教学中,教师可以一边讲解一边演示这种动画课件,有助于学生的理解.  相似文献   
983.
984.
本文重点讨论了滩间山金矿床主要矿石矿物的标型特征及其地质意义,可用于矿床类比及该类型金矿床的找矿评价和预测。  相似文献   
985.
The mosaic structure of landscape of the central area of Shanghai Metropolis is studied by quantitative methods of landscape ecology based on Remote Sensing (RS) and Geographic Information System (GIS) in this pa-per. Firstly, landscapes are classified into eight categories: residential quarter, industrial quarter, road, other urban landscape, farmland, village and small town, on-building area, river and other water bodies (such as lake, etc.). Sec-ondly, a GIS is designed and set up based on the remote sensing data and field investigation, and a digital map of landscape mosaic is made. Then the indexes of diversity, dominance, fragmentation and isolation, and fractal dimen-sion of each type of landscape in different periods are calculated by using spatial analysis method of GIS. With refer-ence to the calculated results, a series of relative issues are discussed.  相似文献   
986.
Affecting Geospatial Technologies: Toward a Feminist Politics of Emotion*   总被引:1,自引:0,他引:1  
Building on earlier contributions to feminist understanding of geospatial technologies (GT), I seek to further develop feminist perspectives on GT along new directions. I argue that an attention to the importance of affect (feelings and emotions) and the performative nature of GT practices offers a distinctive critical edge to feminist work on GT. I emphasize the need for GT practitioners to contest the dominant meanings and uses of GT, and to participate in struggles against the oppressive or violent effects of these technologies. I argue that only when emotions, feelings, values, and ethics become an integral part of our geospatial practices can we hope that the use of GT will lead to a less violent and more just world.  相似文献   
987.
It had been demonstrated that individual or protoplasts isolated from Porphyra thallus by enzyme could develop into normal leafy thalli in the same way as monosproes,and that isolated cells develop in different way in liquid and on semi-solid media,The authors observed the ultrastructure of isolated vegetative cells cultured on semi-solid media and compared them with those of monospores and isolated cells cultured in liquid media,The results showed that subcellular struce-tures were quite different among cells in different conditions.In their development,isolated cells on semi-solid media did not show the characteristic subcellular eature of monospore formation ,such as production of fibrous vesicles,Callus-like cells formed on semi-solid media underwent a distinctive modification in cellular organization.They developed characteristic cell inclusions and a special 2-layer cell covering,Golgi bodies,ER ,starch grains,mitochondria,Vaculoes were not commonly found in them.  相似文献   
988.
Eucheuma gelatinae (Esper) J. Agardh has vegetative and reproductive features distinguishing it from other species ofEucheuma. The occurrence of reproductive nemathecia containing carpogonial branches, auxiliary cells and post-fertilization stages including gonimoblast and pericarp initiation, fusion cell formation stages and carposporophyte development are described and ilustrated for the first time for this species. Contribution No. 2380 of the Institute of Oceanology, Chinese Academy of Sciences.  相似文献   
989.
使用澳大利亚致密阵(ATCA)对一个红外光度高达10^11.8-10^12.8L⊙的IRAS星系样本进行了8.6,4.8,2.4和1.4GHz的射电观测,发现其中有4个源在高频为陡谱,在较低频率上流星降低,呈峰状,峰值在几GHz。讨论了造成频谱倒转的两种可能机制:同步自吸收和free-free吸收。通过对00199-7426,02587-6336,07020-5423和21547-5823的频谱分析,我们认为free-free吸收可能导致了低频倒转谱的形成。通过对观测数据的模拟,获得了这几个源的有关物理参量。  相似文献   
990.
空间太阳望远镜1m主镜支撑结构的研究   总被引:8,自引:0,他引:8  
本文对正在预研中的空间太阳望远镜的关键部件之一1m主镜,提出一种合理的支撑结构系统,此系统可以兼顾空间太阳望远镜光学系统的地面装校调试的有效性和火箭发射力学环境中的安全性以及入轨后正常观测时的高精度性。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号