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   340篇
  免费   90篇
  国内免费   174篇
大气科学   64篇
地球物理   44篇
地质学   314篇
海洋学   80篇
天文学   1篇
综合类   21篇
自然地理   80篇
  2024年   2篇
  2023年   7篇
  2022年   12篇
  2021年   16篇
  2020年   21篇
  2019年   22篇
  2018年   17篇
  2017年   10篇
  2016年   16篇
  2015年   20篇
  2014年   53篇
  2013年   26篇
  2012年   14篇
  2011年   29篇
  2010年   23篇
  2009年   30篇
  2008年   26篇
  2007年   21篇
  2006年   28篇
  2005年   24篇
  2004年   20篇
  2003年   18篇
  2002年   13篇
  2001年   17篇
  2000年   22篇
  1999年   19篇
  1998年   12篇
  1997年   12篇
  1996年   7篇
  1995年   7篇
  1994年   6篇
  1993年   3篇
  1992年   9篇
  1991年   5篇
  1990年   4篇
  1989年   3篇
  1988年   3篇
  1987年   2篇
  1986年   1篇
  1983年   2篇
  1982年   1篇
  1980年   1篇
排序方式: 共有604条查询结果,搜索用时 15 毫秒
51.
利用电子探针背散射电子成像分析和微区成分分析功能,对贵州瓮福高镁磷尾矿的矿石结构和矿物组成进行了研究,以探索尾矿与其原矿之间的差异,为综合开发利用磷尾矿提供必要的依据。结果表明该高磷尾矿的矿石结构可分为单矿物结构和多矿物碎屑结构两类。前者矿物组成包括白云石单矿物、磷灰石单矿物、褐铁矿单矿物、石英单矿物;后者包括白云石-磷灰石多矿物、磷灰石-白云石-石英多矿物,属高镁低品位磷矿。矿物组成类型按其含量从高到低依次为白云石质尾矿、白云石磷灰石质尾矿、磷灰石白云石石英质尾矿、石英质尾矿、磷灰石质尾矿、褐铁矿质尾矿,白云石质尾矿是尾矿中最丰富的矿物,白云石颗粒是含量最高的碎屑类型。  相似文献   
52.
天然红辉沸石经过氯化铝及氯化镁共同改性后,制备出改性红辉沸石。研究了改性红辉沸石和天然红辉沸石对生活污水中CODcr的吸附效果,表明改性红辉沸石的吸附效果明显优于红辉沸石,并通过X射线粉末衍射、热重分析、红外光谱对改性红辉沸石表面结构进行了表征。  相似文献   
53.
地下水中钙和镁的离子色谱法同时测定   总被引:2,自引:0,他引:2  
改进了离子色谱分析地下水中钙和镁的方法。以IonPac CS12A为分离柱,稀盐酸为淋洗液,电导检测器检测,对地下水中的锂、钾、钠、钙和镁进行同时测定。方法具有较宽的线性范围和较高的灵敏度,钙、镁的浓度分别在0~500mg/L和0~250mg/L内呈良好的线性关系;钙的检出限为1.50μg/L,镁的检出限为0.89μg/L。对不同浓度钙、镁水质标准样品进行分析测定,同时对实际样品进行不同稀释倍数分析验证,方法精密度(RSD,n=8)为0.19%~1.89%,无显著的基体效应影响。方法可满足全国地下水调查评价规范要求,适于地下水样品中锂、钾、钠、钙和镁离子的同时测定。  相似文献   
54.
本文构建以掺硼金刚石(Boron-doped diamond,BDD)为阳极、不锈钢为阴极、硫酸盐为电解质的电化学体系,考察了电流密度、pH值、硫酸盐浓度以及初始四环素浓度等四个因素对电化学氧化降解废水中四环素的影响,运用响应曲面法对运行参数进行优化;通过电子自旋共振检测技术分析电化学反应中产生的自由基,探究了间歇通电模式下电化学体系持续氧化机理。结果表明,四个因素对TOC去除率的影响大小次序为:电流密度>初始四环素浓度>初始pH值>硫酸盐浓度,其中初始pH值和硫酸盐浓度与电流密度和初始四环素浓度的交互作用对TOC去除率的影响较为显著;最佳运行参数为pH值为5,电流密度为100 mA·cm-2,硫酸盐浓度为0.25 mol·L-1,初始四环素浓度为1000 mg·L-1;间歇通电模式下,BDD电极表面产生的SO4·-等高活性物质间相互转化提供了体系的可持续氧化能力。该研究结果为电化学氧化技术的实际应用提供了节省能耗的有效途径。  相似文献   
55.
纳滤膜技术以其环境友好、装置集成度高和智能化控制等优势,在卤水锂资源绿色开发领域具有良好的应用前景。实验通过添加聚乙烯醇对纳滤膜功能层进行改性,考察了聚乙烯醇相对含量对纳滤膜性能的影响。结果表明,聚乙烯醇参与界面聚合反应,纳滤膜功能层O/N比增加,聚乙烯醇分子链间氢键作用降低纳滤膜表面粗糙度。羟基官能团的引入有效改善纳滤膜材料亲水效果,膜面羧基密度随聚乙烯醇相对添加量的增加逐渐增加。聚乙烯醇改性纳滤膜截留效果优异,纳滤膜对镁离子的截留率高于95%,锂离子截留率低于-44%,不可逆通量衰减指数低于15%。聚乙烯醇改性纳滤膜材料具有较好的镁锂分离选择效果及抗污染能力,为用于盐湖卤水镁锂分离国产化纳滤膜材料的开发提供新思路。  相似文献   
56.
我国盐湖资源丰富,对盐湖中的锂镁资源进行深度开发已经成为开发盐湖资源的研究热点之一。基于ISI WoK Thomson Innovation专利数据库(TI),利用TDA分析工具对盐湖锂镁分离提取技术领域专利文献进行分析,系统揭示盐湖锂镁分离提取技术的研发现状、热点以及技术分布与格局,最后根据分析结果对盐湖锂镁分离提取技术发展提出对策建议。  相似文献   
57.
从硫酸锂粗矿中分离富集锂的实验研究   总被引:2,自引:2,他引:0  
针对从西藏查波措盐湖卤水中获得的硫酸锂粗矿,开展从该矿物中分离提取锂的研究。根据该矿物的组成,设计了去除与锂离子共存的镁、硫酸根等主要离子的工艺路线,通过除镁、硫酸根、钙等过程和蒸发浓缩、结晶分离等步骤,得到了杂质离子含量较低的富锂溶液。实验对除镁步骤的加水量、加料方式、反应时间、Ca O加入量等反应关键控制因素进行了考察,确定了较优的分离条件。  相似文献   
58.
Sulfate chitosan derivatives have good solubility and therapeutic effect on the cell model of NAFLD. The aim of this study was to examine the therapeutic effect of sulfate chitosan derivatives on NAFLD. The male Wistar rats were orally fed high fat emulsion and received sulfate chitosan derivatives for 5 weeks to determine the pre-treatment effect of sulfate chitosan derivatives on NAFLD. To evaluate the therapeutic effect of sulfate chitosan derivatives on NAFLD, the rats were orally fed with high concentration emulsion for 5 weeks, followed by sulfate chitosan derivatives for 3 weeks. Histological analysis and biomedical assays showed that sulfate chitosan derivatives can dramatically prevent the development of hepatic steatosis in hepatocyte cells. In animal studies, pre-treatment and treatment with sulfate chitosan derivatives significantly protected against hepatic steatohepatitis induced by high fat diet according to histological analysis. Furthermore, increased TC, ALT, MDA, and LEP in NAFLD were significantly ameliorated by pre-treatment and treatment with sulfate chitosan derivatives. Furthermore, increased TG, AST, and TNF-α in NAFLD were significantly ameliorated by treatment with sulfate chitosan derivatives. Sulfate chitosan derivatives have good pre-treatment and therapeutic effect on NAFLD.  相似文献   
59.
Intense thermochemical sulfate reduction (TSR) and up to 18% H2S are found in the Upper Permian Changxing Formation (P3ch) in the northeast (NE) Sichuan Basin, China, despite that rare gypsum or anhydrite was found in this formation. Here, we present new concentration data of carbonate-associated sulfate (CAS) from carbonate host rocks, C, O, and Sr isotope data for TSR-related calcites, and S isotope data for sulfur compounds obtained during this study. These data along with spatial-temporal changes in palaeogeopressure conditions, hydraulic conductivity and the physical capacity indicate that the H2S was generated locally from TSR within the P3ch reservoirs. We propose that the reactive sulfates were derived from CAS released during dolomitization and recrystallization of earlier dolomite within the P3ch Fm. and from the cross-formational migration of evaporative brines from the Lower Triassic Feixianguan Formation (T1f) to P3ch Fm. Our calculation shows that the two sources could provide enough SO42− for the generation of H2S within the P3ch reservoirs. Early downward migration of sulfate-rich evaporative brines from the T1f formation occurred in near-surface and shallow burial diagenetic settings (mainly <1000 m). The evaporative brines seeped into porous grainstones and displaced preexisting seawater, causing pervasive dolomitization within the P3ch Fm. Subsequently, TSR calcites precipitated from the pore water have high Sr concentrations (up to 7767 ppm), close to the T1f TSR calcites, and 87Sr/86Sr ratios mainly from 0.7074 to 0.7078, which are significantly higher than those of Late Permian seawater but within the range of early Triassic seawater.  相似文献   
60.
Gas occurrences consisting of carbon dioxide (CO2), hydrogen sulfide (H2S), and hydrocarbon (HC) gases and oil within the Dodan Field in southeastern Turkey are located in Cretaceous carbonate reservoir rocks in the Garzan and Mardin Formations. The aim of this study was to determine gas composition and to define the origin of gases in Dodan Field. For this purpose, gas samples were analyzed for their molecular and isotopic composition. The isotopic composition of CO2, with values of −1.5‰ and −2.8‰, suggested abiogenic origin from limestone. δ34S values of H2S ranged from +11.9 to +13.4‰. H2S is most likely formed from thermochemical sulfate reduction (TSR) and bacterial sulfate reduction (BSR) within the Bakuk Formation. The Bakuk Formation is composed of a dolomite dominated carbonate succession also containing anhydrite. TSR may occur within an evaporitic environment at temperatures of approximately 120–145 °C. Basin modeling revealed that these temperatures were reached within the Bakuk Formation at 10 Ma. Furthermore, sulfate reducing bacteria were found in oil–water phase samples from Dodan Field. As a result, the H2S in Dodan Field can be considered to have formed by BSR and TSR.As indicated by their isotopic composition, HC gases are of thermogenic origin and were generated within the Upper Permian Kas and Gomaniibrik Formations. As indicated by the heavier isotopic composition of methane and ethane, HC gases were later altered by TSR. Based on our results, the Dodan gas field may have formed as a result of the interaction of the following processes during the last 7–8 Ma: 1) thermogenic gas generation in Permian source rocks, 2) the formation of thrust faults, 3) the lateral-up dip migration of HC-gases due to thrust faults from the Kas Formation into the Bakuk Formation, 4) the formation of H2S and CO2 by TSR within the Bakuk Formation, 5) the vertical migration of gases into reservoirs through the thrust fault, and 6) lateral-up dip migration within reservoir rocks toward the Dodan structure.  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