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   386篇
  免费   56篇
  国内免费   70篇
测绘学   4篇
大气科学   11篇
地球物理   77篇
地质学   270篇
海洋学   82篇
天文学   12篇
综合类   12篇
自然地理   44篇
  2024年   1篇
  2023年   4篇
  2022年   11篇
  2021年   20篇
  2020年   12篇
  2019年   13篇
  2018年   7篇
  2017年   6篇
  2016年   14篇
  2015年   28篇
  2014年   25篇
  2013年   14篇
  2012年   30篇
  2011年   25篇
  2010年   27篇
  2009年   17篇
  2008年   41篇
  2007年   44篇
  2006年   42篇
  2005年   26篇
  2004年   11篇
  2003年   13篇
  2002年   12篇
  2001年   9篇
  2000年   9篇
  1999年   4篇
  1998年   7篇
  1997年   11篇
  1996年   4篇
  1995年   6篇
  1994年   2篇
  1993年   4篇
  1992年   2篇
  1990年   2篇
  1989年   2篇
  1988年   3篇
  1986年   2篇
  1985年   1篇
  1984年   1篇
排序方式: 共有512条查询结果,搜索用时 234 毫秒
101.
13X沸石分子筛的比表面积和孔分布   总被引:2,自引:0,他引:2  
白峰  马鸿文 《现代地质》2008,22(5):838-844
对利用钾长石粉水热合成的13X沸石分子筛分别进行了比表面积和孔分布测定。结果表明:13X沸石分子筛比表面积约为621.50~747.40 m2/g,总孔体积约为0.344 70~0.393 70 cm3/g;具有高比例且孔径小于2.50 nm的微孔,中-小型实验样品(SXZ-10g和SXZ-1kg)的微孔体积比例相近,为81.33%~84.70%,而扩大实验样品(SXZ-10kg)的微孔体积比例为67.90%~67.98%。13X沸石分子筛微孔的含量是决定13X沸石分子筛具有高比表面积和高效吸附性能的前提。  相似文献   
102.
本文应用单颗粒锆石SHRIMP U—Pb法和角闪石^40Ar/^39Ar法,对出露在冈底斯构造带上的门巴花岗岩体进行了同位素测年,时代分别为207Ma和215Ma,表明它们形成于晚印支期。地质与地球化学研究显示该时期形成的岩体主要为黑云角闪花岗闪长岩和黑云母二长花岗岩,它们形成于岛弧环境,与其南侧墨竹工卡一带出露的晚三叠世钙碱性火山岩(叶巴组)共同构成与板块俯冲有关的特征性岩石组合,揭示新特提斯洋的俯冲时代应早于晚三叠世。  相似文献   
103.
荒漠植物叶片碳同位素组成及其水分利用效率   总被引:29,自引:17,他引:29  
水分通常是影响荒漠植物生长的主要限制因子,然而当前很少有关于荒漠群落中植物水分利用效率的报道。作为指示水分利用效率的可靠指标,叶片稳定碳同位素组成(δ13 C值)可以用来探讨植物适应干旱环境的强弱程度。对阜康和金塔同种或同属的植物叶片δ13 C的测量结果表明,干旱可使植物叶片δ13 C升高:年降水量每增加1mm,叶片δ13 C则降低001‰~ 0015‰。阜康荒漠灌木叶片δ13 C值明显高于草本,这样的趋势也存在于甘肃金塔主要荒漠植物中,与前人的报道也基本一致。说明灌木可能更适应干旱胁迫,并且这种现象可能是全球荒漠生态系统的一种共性。对阜康四种荒漠代表植物红砂、梭梭、补血草和骆驼刺的SOD活性的测定结果间接地支持了这一结果。进一步的分析表明,藜科、豆科和某些禾本科植物适应干旱环境的能力相对较强。  相似文献   
104.
The lateral transport of bicarbonate as dissolved inorganic carbon (DIC) to the oceans is an integral component of the global carbon budget and can represent the sequestration of CO2 from the atmosphere. Recently studies have implicated land use change, in particular agricultural development, as an accelerator of bicarbonate export. However, due to the co-variation of land use, bedrock and surficial geologies, and the relationship between bicarbonate export and climate, the impact of anthropogenic activities on DIC export remains an important research question. In order to examine the land use controls on DIC export from small temperate watersheds we sampled 19 streams draining catchments of varying land uses with similar bedrock and surficial geologies. In addition to an agricultural effect, there was a strong correlation between the percent of watershed in urban development and DIC concentrations and DIC yields. Urban watersheds exported 7.8 times more DIC than their nearby forested counterparts and 2.0 times more DIC than nearby agricultural catchments. Isotopic data suggest that excess DIC export from altered systems results from increased chemical weathering, enhanced CO2 production within urban green spaces, and as a result of organic matter loading from septic systems and leaky sewer lines. Furthermore, we found that nitrogen additions (e.g. fertilizers and manure) are aiding in the dissolution of lime, increasing the total export of DIC from agricultural watersheds. Calculated anthropogenic loading rates ranged from 0.43 to 0.86 mol C m− 2 yr− 1. These loading rates suggest that a significant portion of global DIC export might be attributable to human activities, although the impacts on CO2 sequestration are difficult to determine.  相似文献   
105.
以吉林省长白山地区的天然浮石为原料,成功地合成了13X型分子筛,对其合成路线,反应条件进行了研究,确定了合成13X型分子筛的最佳工艺条件,并通过X射线衍射法及差热分析等手段证明了产物的结构  相似文献   
106.
Examination of a series of coalified gymnospermous woods ranging in rank from brown coal to subbituminous coal by solid-state 13C NMR and analytical pyrolysis has provided sufficient information to construct structural models depicting the changes that occur to lignin, the primary precursor of vitrinite, during coalification. Progressive changes in the chemistry of coalified wood suggest the following series of reactions: (1) demethylation to form catechol-like structures that are dominant components of brown coal and lignite A; (2) cleavage of aryl ether linkages to form phenols and reactive carbocations that alkylate the catechol rings; (3) dehydration of the catechol rings; (3) dehydration of the catechol-like structures to form the structures of subbituminous coal dominated by alkylphenols; and (4) reduction of the 3-carbon alkyl side chain derived from lignin to form propyl substituents. The models developed for each stage of coalification are derived from chemical modifications of the structure of lignin.  相似文献   
107.
The standard E – model generates aplanetary boundary layerthat appears to be much too deep. The cause of theproblem is traced to the equation for the dissipationrate () of turbulent kinetic energy (E), specifically theparameterization of dissipation production anddestruction. In the context of atmosphericboundary-layer modelling, we argue that a part of thedissipation production should be modelled as the inputto the spectral cascade from the energy-containingpart of the spectrum, with a characteristic length , while the equilibrium imbalancebetween local production and destruction ofdissipation is modelled as proportional toE2/E, as in the standard model. Wepropose an E – – turbulence closurescheme, in which both the mixing length, m, and are prescribed. The importance ofthe equation is diminished, though itstill determines the dissipation rate in the Eequation.  相似文献   
108.
A 911-m-long sediment core from Lake Biwa, Japan, provides a record of organic matter delivery and accumulation in this large lake during a succession of tectonic and climatic changes dating back to the latest Pliocene. Sediments deposited since 430 ky are profundal; older sediments vary in setting between shallow-water and fluviodeltaic conditions, with occasional deep-water intervals. C/N ratios identify algal production as the dominant source of organic matter throughout the core, although the proportion of land-derived contributions episodically increases in the fluviodeltaic and shallow-water sediments. Rates of organic matter delivery and burial in lake sediments change in response to glacial-interglacial climate changes over the past 430 ky. Sediments deposited during interglacial intervals have organic carbon mass accumulation rates up to 9 times greater than those from glacial intervals, reflecting interglacial climates that were wetter than glacial climates. Algal production of organic matter increased during interglacial times because of greater wash-in of soil nutrients, and organic matter preservation was enhanced because of faster sedimentation rates.  相似文献   
109.
成烃理论的发展——(Ⅱ)煤成油及其初次运移模式   总被引:9,自引:0,他引:9  
近几年来,我国针对侏罗系煤系地层的油气勘探,取得了重大突破。在我国西北区的一些含煤盆地中都发现了丰富油气矿藏。煤成油的勘探开发和研究,是对陆相生油理论的重大发展,本文介绍了秦匡宗提出的一种利用^13C NMR谱确定油潜力碳、气潜力碳和芳构碳评价生油气潜力的新方法,以及一种煤成烃的阶段性初次运移模式。  相似文献   
110.
全新世早期是太阳辐射加强、全球温度上升,并伴随着冰盖消融的重要时期,而其间发生的冷事件以及亚洲季风区的弱夏季风事件的成因一直是全新世早期研究的重点。对亚洲季风-海洋-极地联系研究有着重要的意义。通过分析湖南莲花洞LHD5石笋28个U/Th年龄和535个氧同位素数据重建了全新世亚洲季风演化特征,其中全新世早期分辨率达8年。LHD5石笋记录到YD结束时间为11748±30 a B.P.,全新世开始于11684±39 a B.P.,转换时间约为64年,与格陵兰gicc05记录在误差范围内一致。LHD5石笋记录到全新世早期6次弱夏季风事件,事件年龄中心点分别为11461±34 a B.P.、10354±36 a B.P.、9957±25 a B.P.、9062±36 a B.P.、8744±23 a B.P.、8144±24 a B.P.,其δ18O值的波动幅度分别为1.08‰、0.94‰、0.66‰、0.90‰、0.55‰、1.02‰,这些弱季风事件在亚洲季风区具有普遍的区域意义。除8.2 ka事件之外,10 ka B.P.之前的弱季风事件除了受到太阳活动的影响,还受到北大西洋IRD事件的影响,而之后更多地受到太阳活动和ITCZ南移的影响。  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