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   288篇
  免费   44篇
  国内免费   63篇
测绘学   22篇
大气科学   64篇
地球物理   64篇
地质学   148篇
海洋学   29篇
天文学   16篇
综合类   22篇
自然地理   30篇
  2024年   2篇
  2023年   5篇
  2022年   13篇
  2021年   11篇
  2020年   12篇
  2019年   8篇
  2018年   13篇
  2017年   12篇
  2016年   15篇
  2015年   18篇
  2014年   11篇
  2013年   11篇
  2012年   7篇
  2011年   9篇
  2010年   13篇
  2009年   15篇
  2008年   17篇
  2007年   11篇
  2006年   11篇
  2005年   14篇
  2004年   11篇
  2003年   23篇
  2002年   29篇
  2001年   27篇
  2000年   20篇
  1999年   16篇
  1998年   13篇
  1997年   4篇
  1996年   5篇
  1995年   1篇
  1994年   2篇
  1993年   3篇
  1992年   2篇
  1991年   1篇
  1990年   2篇
  1989年   3篇
  1988年   4篇
  1981年   1篇
排序方式: 共有395条查询结果,搜索用时 515 毫秒
251.
方钢管混凝土柱与钢梁半刚性连接节点的恢复力本构模型   总被引:1,自引:0,他引:1  
本文在试验研究和有限元模型修正的基础上,对两类方钢管混凝土柱与钢梁半刚性连接节点——缀板连接节点和穿芯螺栓-端板连接节点进行反复荷载作用下的非线性有限元参数分析,结果表明:影响缀板连接节点转动性能的主要因素是缀板的厚度,影响穿芯螺栓-端板连接节点转动性能的主要困索是端板厚度和螺栓直径等。进而通过回归分析建立了上述两类节点弯矩-转角恢复力模型的简化计算公式,可供工程设计与数值简化计算参考。  相似文献   
252.
Coronal mass ejection (CME) velocities have been studied over recent decades. We present a statistical analysis of the relationship between CME velocities and X-ray fluxes of the associated flares. We study two types of CMEs. One is the FL type associ- ated only with flares, while the other is the intermediate type associated with both filament eruptions and flares. It is found that the velocities of the FL type CMEs are strongly cor- related with both the peak and the time-integrated X-ray fluxes of the associated flares. However, the correlations between the intermediate type CME velocities and the corre- sponding two parameters are poor. It is also found that the correlation between the CME velocities and the peak X-ray fluxes is stronger than that between the CME velocities and the time-integrated X-ray fluxes of the associated flares.  相似文献   
253.
京津冀地区产业结构和竞争力空间分异研究   总被引:1,自引:0,他引:1  
对Boarnet公式的经济权重进行修正,以经济增长变化对区域增长的贡献率代替绝对量值的变化,作为区域内经济空间增长的联系强度,并将其作为偏离-份额分析空间模型的空间权重。根据偏离-份额分析空间模型计算三次产业在2002—2005年和2006—2009年2个时段的要素量值变化,用ArcGIS 9.3分析产业结构、产业竞争力的空间变化特征。结构上看,京津冀地区第一产业结构偏强,第二、三产业处于偏弱的态势;竞争力上看,京津冀地区第二产业竞争力呈现整体水平提升,第一、三产业竞争力呈现出多样化的态势。今后应继续保持对第一产业的结构调整力度,快速实现二、三产业结构效率的提升。加大投资和科技投入,保持第二产业竞争实力,逐步实现第一、三产业竞争力的整体提升。  相似文献   
254.
Earth fissures in Jiangsu Province, China have caused serious damages to properties, farmlands, and infrastructures and adversely affected the local or regional economic development. Under the geological and environmental background in Jiangsu Province, this paper presents the earth fissures caused by excessive groundwater withdrawal and coupled by distinctive geological structures such as Ancient Yellow River Fault in Xuzhou karst area, and Ancient Yangtze River Course and bedrock hills in Suzhou, Wuxi, and Changzhou area. Although all the earth fissures are triggered by groundwater exploitation, the characteristics are strongly affected by the specific geological and hydrogeological settings. In particular, in the water-thirsty Xuzhou city, the cone of depression caused by groundwater extraction enlarged nearly 20 times and the piezometric head of groundwater declined 17 m over a decade. As groundwater is extracted from the shallowly buried karst strata in the Ancient Yellow River Fault zone, the development of earth fissures is highly associated with the development of karstic cavities and sinkholes and their distribution is controlled by the Ancient Yellow River Fault with all the 17 sinkholes on the fault. On the other hand, in the rapidly developing Southern Jiangsu Province, groundwater is mainly pumped from the second confined aquifer in the Quaternary, which is distributed neither homogeneously nor isotropically. The second confined aquifer comprises more than 50 m thick sand over the Ancient Yangtze River Course, but this layer may completely miss on the riverbank and bedrock hills. With a typical drawdown rate of 4–6 m per annum, the piezometric head of groundwater in the second confined aquifer has declined 76 m at Maocunyuan since 1970s and 40 m at Changjing since mid-1980s, and a large land subsidence, e.g., 1,100 mm at Maocunyuan, is triggered. Coupled with the dramatic change of the bedrock topography that was revealed through traditional geological drilling and modern seismic reflection methods, the geological-structure-controlled differential settlement and earth fissures are phenomenal in this area.  相似文献   
255.
淅川柑橘汁多个大、酸甜适度、无籽、耐储藏,是淅川县重要经济作物.2008年1月份出现的连续15天低温雨雪冰冻气,极端最低气温达-8℃,造成了淅川柑橘种植以来最严重的冻害.为防治柑橘冻害,首要考虑选用抗寒品种和砧木,应以枳砧、早中熟蜜柑为主;在柑橘种植上要选择环库区背风向阳的坡地,避免在风口和地势低洼处栽种.冻害发生后,要适时修剪受害的枝叶.  相似文献   
256.
Carboniferous deep‐water marine strata have been insufficiently studied in western Junggar, NW China where the deep‐water facies successions have long been disputed in terms of age constraints, sequence and palaeoenvironmental reconstruction. This paper introduces some views in the light of new materials obtained from this region in recent years. The presence of the Visean plant fossils from the upper Ta'erbahatai Formation in the Tarbgatay Mountains indicates that the formation can be extended to the Early Carboniferous epoch in age. This unit also displays obvious diachroneity, which is of Late Devonian to Early Tournaisian age in the Saur Mountains and Late Devonian to Visean age in the Tarbgatay Mountains. The Xibeikulasi, Baogutu and Tailegula formations are widely distributed in northwestern Karamay areas. The scouring structures and graded bedding near the boundaries between the three formations confirm the stratal sequence that they were originally assigned, namely the Xibeikulasi, Baogutu and Tailegula formations in ascending order. The ‘fossil chaos’ of the three formations is due to mistaking fossils of other stratigraphic units for fossils of these three formations. After revision, only the Early Carboniferous fossils are considered reliable, and combined with the newly found plant fossils, the Xibeikulasi, Baogutu and Tailegula formations are re‐assigned to the early Visean, late Visean, and latest Visean to Serpukhovian ages, respectively. An extension of the lower Hala'alate Formation was recognized in the southwestern Hala'alate Mountains. The presence of the latest Early Carboniferous brachiopods constrains the Hala'alate Formation as late Serpukhovian to Bashkirian in age, bearing the mid‐Carboniferous boundary. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   
257.
Ultracompact dark matter minihalos(UCMHs) would be formed during the early universe if there were large density perturbations.If dark matter can decay into particles described by the standard model,such as neutrinos,these objects would become potential astrophysical sources of emission which could be detected by instruments such as IceCube.In this paper,we investigate neutrino signals from nearby UCMHs due to gravitino dark matter decay and compare these signals with the background neutrino flux which is mainly from the atmosphere to obtain constraints on the abundance of UCMHs.  相似文献   
258.
岳佳霏  潘柳榕  宗会明 《热带地理》2022,42(7):1096-1106
选取南海周边国家和地区37个集装箱港口为研究对象,选用全球前八名船公司航线数据,就其现有航线分布构建二向网络,采用度数中心度、接近中心度、介数中心度及权利指数等复杂网络中心性指标和网络结构模型对其港口体系特征和网络结构进行研究,得出的结论主要有:1)近年来,南海沿线国家和地区间贸易愈加频繁,相关港口吞吐量不断增加,在世界集装箱港口体系内地位总体呈上升态势;各港吞吐量排名总体相对稳定,南海周边7国集装箱港口运力分布不均衡,主要集中在中国与新加坡沿线。2)深圳港、新加坡港、香港港、高雄港、青岛港与上海港等港航线较多,可与较多区域直接进行贸易往来,受其他港口联系影响较小;上海港、深圳港、宁波舟山港、广州港等在国际集装箱航线中起重要的串联中介作用,对航运网络控制能力较强。3)基于港口吞吐规模与中心程度分异情况将港口网络地位分为4层。第一层为国际航线母港,包括上海港、新加坡港、深圳港、宁波舟山港、广州港、青岛港与香港港;第二层为国际航线枢纽港,包括高雄港、丹戎帕拉帕斯港、厦门港、天津港、林查班港、巴生港与胡志明港,中心性发育程度较好;第三层为国际航线挂靠港,包括海防港、钦州港、大连港、丹绒布鲁港、马尼拉港、丹戎佩拉港、连云港、盖梅港、烟台港、营口港、日照港与福州港,港口相对中心度偏低,吞吐量相对受限;第四层为区域性喂给港,包括锦州港、太仓港、台中港、东莞港、唐山港、南京港、泉州港、南通港、珠海港、嘉兴港与海口港,因其地理区位特点成为较热门转运港口。4)整体港口网络可划分为4个港口社群。其中,社群一中心属性明显,社群二与其联通关系紧密,社群三是其喂给港群,社群四小世界属性明显,群内港口航线相对独立。  相似文献   
259.
宗雯  康丛轩  杨献忠 《江苏地质》2022,46(4):366-374
岩石的变形机制和力学性质对认识活动断裂带的孕震机制和发震过程都具有重要意义。圣·安德列斯(San Andreas)转换断层是太平洋板块与北美板块的构造边界,也是重要的地震活动带,总结了美国南加州地震中心对圣·安德列斯断层钻孔断层岩的研究进展。对断层泥的高速摩擦实验发现:动力弱化现象只在高速滑移时才发生,伴随着摩擦生热,岩石的摩擦系数显著降低。断层泥中富镁黏土矿物的富集可以显著降低断层的摩擦系数,促进蠕滑,黏土矿物的不均一分布可导致断裂带变形行为和地震分布的突变。与断层泥和碎裂岩不同,粉碎岩形成于快速传播地震的断裂端部,具高孔隙率和高渗透率,流体活动性强。此外,断层岩的有机质成熟度可用来追溯古地震造成的热异常。在川滇中国地震科学实验场开展科学钻探项目,研究断层岩样品的显微结构、物理性质和变形历史,以期为理解大陆汇聚与转换挤压过程中复杂的地震活动提供重要信息。  相似文献   
260.
陶兰初  朱星强  张七道  马一奇  魏总  庞龙  涂春霖  和成忠  刘红豪 《地质论评》2022,68(6):2022112006-2022112006
为支撑滇东富源县古敢水族乡热水塘温泉旅游的开发和可持续利用,通过采集区内水样进行水化学及环境同位素分析,对温泉的形成机理及其理疗价值进行了研究。结果表明:热水塘温泉温度为37. 8~42. 2 ℃,pH值为7. 24~7. 75,TDS(溶解固态物质总量)的质量浓度为1245~1840 mg/L,属于中低温弱碱性热矿水。温泉水中偏硅酸的质量浓度为48. 20~60. 57 mg/L,F为2. 86~2. 94mg/L,Sr为8. 60~14. 40mg/L,均达到了硅水、氟水、锶水的理疗水质标准,此外温泉水中222Rn浓度达到了129. 4 Bq/L,接近于氡水的理疗水质标准,具有较大的开发利用价值。热水塘温泉水中阳离子以Ca2+、Mg2+为主,阴离子以SO2-4、HCO-3为主,水化学类型为SO2-4—Ca2+·Mg2+型。石膏和碳酸盐岩矿物的溶解是控制热水塘温泉水化学特征的主要因素,其中石膏的溶解占据了主导地位。参与水岩反应的CO2的δ13C CO2(V- PDB)值为-11. 64‰~-14. 24‰,具有明显的沉积有机质来源特征。14C测年结果表明热水塘温泉年龄为 3291a BP,氢氧同位素特征表明热水塘温泉由大气降水补给,补给高程为2101. 9~2138. 3 m,补给区温度为5. 9~6. 2 ℃。利用硅—焓方程法计算得出热水塘温泉的冷水混合比例为0. 84~0. 89,综合硅—焓方程计算的热储温度和校正后的SiO2地热温标计算的热储温度,得出热水塘温泉的热储温度为173. 1~220. 3 ℃,循环深度为2059. 7~2661. 1 m。  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