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   207篇
  免费   33篇
  国内免费   22篇
测绘学   9篇
大气科学   12篇
地球物理   43篇
地质学   85篇
海洋学   59篇
综合类   21篇
自然地理   33篇
  2024年   1篇
  2023年   3篇
  2022年   7篇
  2021年   5篇
  2020年   5篇
  2019年   6篇
  2018年   3篇
  2017年   5篇
  2016年   4篇
  2015年   8篇
  2014年   24篇
  2013年   12篇
  2012年   13篇
  2011年   12篇
  2010年   17篇
  2009年   13篇
  2008年   14篇
  2007年   6篇
  2006年   9篇
  2005年   9篇
  2004年   6篇
  2003年   4篇
  2002年   10篇
  2001年   15篇
  2000年   4篇
  1999年   5篇
  1998年   10篇
  1997年   4篇
  1996年   7篇
  1995年   4篇
  1994年   4篇
  1993年   2篇
  1992年   3篇
  1991年   3篇
  1990年   1篇
  1988年   3篇
  1987年   1篇
排序方式: 共有262条查询结果,搜索用时 15 毫秒
51.
52.
早前寒武纪花岗岩—绿岩地体中金的成矿作用   总被引:1,自引:0,他引:1       下载免费PDF全文
早前寒武纪花岗岩—绿岩地体中赋存着丰富的金矿床。我国绿岩带主要分布在华北陆台,也是重要的含金岩系。产在花岗岩—绿岩地体中的金矿可分为绿岩带中的原生和再生金矿床两大类。金矿的形成是多阶段成矿作用的产物。  相似文献   
53.
通过研究西方工业废弃地的景观再生的发展规律与开发模式,从生态、艺术以及社会三个方面分析“798”艺术中心的所面临的问题,并提出解决问题、即“798”艺术中心可持续发展的战略方法。  相似文献   
54.
Damming is a common anthropogenic intervention along the course of rivers, which is defined as "artificial-lake effect", both in China and across the world. Today as many as 48000 dams and/or reservoirs are in operation in the Changjiang River (Yangtze River) drainage area, and more are being constructed. While damming is well known to affect riverbome nutrient loads, and thus the riverine ecosystems owing to removal of carbon fixation, and removal of particles in reservoir sediments, there is limited information on the detailed early diagenesis of sediments in reservoirs including the regeneration processes of nutrients deposited in sediments and exchange flux across the sediment-water interface, which is important for mass balance of riverbome nutrients. In the present study, two large-size reservoirs, Wujiangdu Reservoir (WJDR) and Dongfeng Reservoir (DFR), located on the main steam of the Wujiang River and with uniformity hydrography and discrepancy biogeochemical activity (e. g. primary production), were selected for a comparative study on the detailed processes of nutrient regeneration. Water, pore water and sediment were sampled from these two reservoirs and dissolved organic carbon (DOC), NH4^+, NO3^-, PO4^3-, dissolved silica (DSi) in the overlying water and pore water, and total organic matter in sediments were determined. The results of correlation analysis suggested that in these two reservoirs, processes of nutrient regeneration near the sediment-water interface were significantly different. As a result of rapid decomposition of algae-derived "labile" organic matter in upper sediments, nutrient regeneration processes and upward fluxes in WJDR are dramatically stronger than those of DFR. NH4^+ upward flux from sediment in WJDR was about 17 times higher than that in DFR. PO4^3- flux in WJDR is about 13 times above that in DFR. DOC flux in WJDR is larger that in DFR by 5 times, and DSi by 1 time.  相似文献   
55.
In recent years, reclaimed wastewater as a water source for urban artificial landscape rivers is being rapidly developed in some coastal cities with limited water resource available in China. However, eutrophication occurs more often in the landscape waterbody with reclaimed wastewater than in natural rivers and lakes. The study of restoration and reconstruction of macrophytes, especially the submerged aquatic vegetation, is essential to inhibit eutrophication of reclaimed wastewater. This study investigated the seasonal variation of biomass of Ruppia maritima in the artificial landscape river using reclaimed wastewater as the only water source in Tianjin, China. It was found that there were two seasonal growth peaks of Ruppia maritima each year in the coastal reclaimedwastewater. The contents of nitrogen and phosphorus of Ruppia maritima varied with the season.  相似文献   
56.
目的: 系统评价益气养血中药联合雄激素、环孢素治疗再生障碍性贫血(AA)的临床疗效。方法:检索中国知网(CNKI)、万方数据(WANFANG DATA)、维普中文科技期刊数据库(VIP)、中国生物医学文献服务系统(SinoMed)、PubMed、EMBase、 The Cochrane Library从建库至2021年11月28日发表的益气养血中药联合雄激素、环孢素治疗AA的临床对照研究,对照组采用环孢素、雄激素治疗,由2位研究员根据统一标准独立筛选文献和提取信息,并进行质量及偏倚评价,采用RevMan 5.3软件进行Meta分析。结果:共纳入16篇文献,1118 例患者,其中治疗组593 例,对照组525 例。Meta分析结果显示:治疗组总有效率[RR=1.34,95%CI(1.24,1.45),P<0.00001]、完全缓解率[RR=1.66,95%CI(1.32,2.09),P<0.0001]优于对照组。在亚组分析中,黄芪注射液联合西药与单用西药治疗相比,总有效率[RR=1.39,95%CI(1.26,1.54),P<0.00001]、完全缓解率[RR=1.64,95%CI(1.27,2.12),P=0.0002]差异均有统计学意义;参麦注射液亚组总有效率亦优于对照组[RR=1.30,95%CI(1.06,1.60),P=0.01]。治疗组外周血白细胞水平[MD=0.62,95%CI(0.42,0.81),P<0.00001]、血红蛋白水平[MD=10.10,95%CI(7.16,13.04),P<0.00001]、血小板水平[MD=11.41,95%CI(7.15,15.67),P<0.00001]较对照组上升更明显。结论:益气养血中药联合雄激素、环孢素治疗AA疗效优于单用西药,且能更好地促进患者血常规指标恢复。  相似文献   
57.
Based on six-degree-of-freedom three-dimensional shaking table tests, the seismic response of a recycled aggregate concrete (RAC) frame was obtained. The analysis results indicate that the maximum story shear force and overturning moment reduce proportionally along the height of the model under the same earthquake wave. The story shear force, base shear coefficient and overturning moment of the structure increase progressively as the acceleration amplitude increases. The base shear coefficient is primarily controlled by the peak ground acceleration (PGA). The relationships between the PGA and the shear coefficient as well as between the PGA and the dynamic amplification factor are obtained by mathematical fitting. The dynamic amplification factor decreases rapidly at the elastic-plastic stage, but decreases slowly with the development of the elastic-plasticity stage. The results show that the RAC frame structure has reasonable deformability when compared with natural aggregate concrete frame structures. The maximum inter-story drift ratios of the RAC frame model under frequent and rare intensity 8 test phases are 1/266 and 1/29, respectively, which are larger than the allowable value of 1/500 and 1/50 according to Chinese seismic design requirements. Nevertheless, the RAC frame structure does not collapse under base excitations with PGAs from 0.066 g up to 1.170 g.  相似文献   
58.
2009年4月利用植株枚订移植法在山东荣成天鹅湖海域开展大叶藻植株移植,于5~10月监测移植大叶藻植株的存活率,并逐月采集自然和移植大叶藻植株,比较了植株生长、叶绿素含量和类胡萝卜素含量的变化,分析了移植操作胁迫对大叶藻存活、生长、叶绿素含量和类胡萝卜素含量的影响。结果显示,移植后1~2个月,移植植株的平均成活率较高,至移植后4个月,平均成活率降至57.8%,之后保持稳定。移植后6个月内,叶长移植植株均极显著低于自然植株;根长和茎节长在移植后1个月时移植植株显著低于自然植株,移植2个月后与自然植株无明显差异,甚至显著高于自然植株;叶绿素含量和类胡萝卜素含量在移植后1个月时移植植株显著低于自然植株,移植2个月后则与天然植株无明显不同,甚至显著高于自然植株,但10月份移植植株的叶绿素含量明显下降。结果表明,移植操作对大叶藻植株的地上部分产生了严重的胁迫作用和较长的胁迫时间,而移植植株的地下部分及叶片叶绿素和类胡萝卜素在经过短期胁迫后,能够通过自身的补偿机制分别实现快速生长和显著增加,从而利于植株的扎根、固着和提高植株的光合作用能力。  相似文献   
59.
在饲养条件下,对青岛辛岛湾潮间带的三崎柱头虫(Balanoglossus misakiensis)的栖息和再生进行研究。6条柱头虫沿生殖翼中后部自切为2段,后段失去摄食、钻洞穴等行为能力和再生前段器官的能力,生存约1个月后解体;前段在1、2天内恢复了摄食、钻洞穴等生活习性,2个月内完成了后段缺失器官的再生,最长存活时间达156d,体长能达到母体体长的一半。113个个体通过手术横切为2段、3段或4段,发现42%的含有吻、领及鳃区的前段虫体,也能够再生全部失去的其他器官;没有吻、领和鳃区的其它片段失去了再生能力。  相似文献   
60.
方娟 《西部资源》2014,(3):67-67
<正>废旧电池、电子废弃物、塑料包装物等废弃物资,是放错位置的"资源"。"资源有限、循环无限"。因此,废弃资源再生利用被人形象地喻为挖掘"城市矿产"。作为全国首批26个再生资源体系建设试点城市之一,合肥按照"政府积极引导,社区作为载体,市场机制运作"原则,引导再生资源行业健康可持续发展,深耕"城市矿产"。从废旧物资中寻找"富矿"以废旧电视机为例,废旧零件通过深加工可以获取各种"矿  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