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   314篇
  免费   98篇
  国内免费   139篇
大气科学   65篇
地球物理   40篇
地质学   250篇
海洋学   71篇
天文学   3篇
综合类   23篇
自然地理   99篇
  2024年   3篇
  2023年   1篇
  2022年   12篇
  2021年   13篇
  2020年   13篇
  2019年   20篇
  2018年   14篇
  2017年   9篇
  2016年   15篇
  2015年   21篇
  2014年   33篇
  2013年   17篇
  2012年   13篇
  2011年   22篇
  2010年   24篇
  2009年   29篇
  2008年   25篇
  2007年   25篇
  2006年   28篇
  2005年   26篇
  2004年   27篇
  2003年   18篇
  2002年   14篇
  2001年   15篇
  2000年   16篇
  1999年   16篇
  1998年   13篇
  1997年   10篇
  1996年   8篇
  1995年   5篇
  1994年   7篇
  1993年   5篇
  1992年   9篇
  1991年   5篇
  1990年   7篇
  1989年   4篇
  1988年   3篇
  1987年   4篇
  1986年   1篇
  1983年   1篇
排序方式: 共有551条查询结果,搜索用时 62 毫秒
141.
Water samples from both hot and artesian springs in Kuantzeling in west-central Taiwan have been collected on a regular basis from July 15, 1999 to the end of August 2001 to measure cation and anion concentrations as a tool to detect major earthquake precursors. The data identify chloride and sulfate ion anomalies few days prior to major quakes and lasting a few days afterward. These anomalies are characterized by increases in Cl- concentrations from 34.9% to 41.2% and 71.5% to 138.1% as well as increases in SO42- concentrations from 232.7% to 276.8% and 100.0% to 155.1% above the means in both hot and artesian springs. The occurrence of these anomalies is probably explained first as stress/strain-induced pressure changes in the subsurface water systems which then generate precursory limited geochemical discharges at the levels of subsurface reservoirs. Therefore, finally leading to the mixing of previously separated subsurface water bodies occurs. This suggests that the hot and artesian springs in the Kuantzeling area are possible ideal sites for recording strain changes serving well as earthquake precursors.  相似文献   
142.
《中国地球化学学报》2006,25(B08):270-271
Rock weathering by carbonic acid is one of the important atmosphere CO2 sequestration. Actually, it depends on whether carbonic acid or other acids as weathering agents, which is important to understand the model of global carbon cycle. For example, sulfuric acid derived from oxidation of pyrite takes part in the rock weathering, which might counteract one part of CO2 drawdown by silicate weathering. In this study, chemicals and carbon isotopic composition of waters were determined in the Beipan River, Guizhou. The δ^13C values of dissolved inorganic carbon in the province, Southwest China. The values of the samples range from -13.1‰ to -2.4‰, which show a good negative correlation with the equivalent ratio of [HCO3]/([Ca^2++Mg^2+]) and a good positive correlation with the equivalent ratio of [SO4^2-]/([Ca^2++Mg^2+]) and [SO4^2-]/[HCO3^-]. The relationships suggest that sulfuric acid took part in carbonate weathering. Acid rain is thus a significant source of sulfuric acid to the karstic rivers of Guizhou Province.  相似文献   
143.
Diosgenin is produced from Dioscrea L, mainly using D.zingiberensis as the raw material. During the process, particularly in the process of acidic hydrolysis, large quantities of high-strength wastewaters are discharged. The waste liquids are characteristic of strong acidity, usually with pH value less than 1, and high concentrations of organic matter with average CODcr (chemical organic demand) in the range of 20000-3000 mg/L. Without proper treatment, their free disposal would pose serious threat to natural ecosystem and cause considerable environmental problems. Since diosgenin wastewaters contain high load organics and acidity, appropriate pretreatment is needed before they are biologically degraded. Coagulation is a common method for eliminating colloid particles and/or high molecule organic chemicals from wastewaters. Poly-ferric sulfate is found to be most effective in removing organic matter. The objective of this study is to investigate the effectiveness and feasibility of poly-ferric sulfate to reduce CODcr and turbidity from diosgenin wastewaters. Due to its high acidic nature, the pH value of diosgenin wastewater was neutralized with Ca(OH)2 solution before being coagulated. Various factors that influence the coagulation process, such as pH value, the dosage of poly-ferric sulfate, polyelectrolyte aids and mixing conditions were examined in the experiments.  相似文献   
144.
加氢和TSR反应对天然气同位素组成的影响   总被引:2,自引:1,他引:2  
天然气形成过程中的加氢作用和 TSR 反应是有机-无机相互作用的重要方式。相邻水体和深部来源的氢,是天然气形成的重要氢源,塔里木盆地天然气的甲烷氢同位素组成明显表现出不同沉积水体对甲烷氢同位素的控制作用,大宛105~25井和阿克1井具有深部流体加氢的特征;TSR 反应中硫同位素在不同反应阶段和反应过程具有不同的分馏特征,这种特征在四川盆地高舍硫天然气中具有很好的表现,TSR 反应硫同位素分馏一般小于20‰,而单体硫、黄铁矿和硫酸盐矿物等其它反应过程的产物硫同位素分馏不明显。  相似文献   
145.
Dimethylsulfide (DMS) in surface seawater and the air, methanesulfonic acid (MSA) and non-sea-salt sulfate (nss-SO4 2–) in aerosol, and radon-222 (Rn-222) were measured in the northern North Pacific, including the Bering Sea, during summer (13 July – 6 September 1997). The mean atmospheric DMS concentrations in the eastern region (21.0 ± 5.8 nmole/m3 (mean ± S.D.), n=30) and Bering Sea (19.9 ± 9.8 nmole/m3, n=10) were higher than that in the western region (11.1 ± 6.4 nmole/m3, n=31) (p<0.05), although these regions did not significantly differ in the mean DMS concentration in surface seawater. Mean sea-to-air DMS flux in the eastern region (21.0 ± 10.4 mole/m2/day, n=19) was larger than those in the western region (11.3 ± 16.9 mole /m2/day, n=22) and Bering Sea (11.2 ± 7.8 mole/m2/day, n=7) (p<0.05). This suggests that the longitudinal difference in atmospheric DMS was produced by that in DMS flux owing to wind speed, while the possible causes of the higher DMS concentrations in the Bering Sea include (1) later DMS oxidation rates, (2) lower heights of the marine boundary layer, and (3) more inactive convection. The mean MSA concentrations in the eastern region (1.18 ± 0.84 nmole/m3, n=35) and Bering Sea (1.17 ± 0.87 nmole/m3, n=13) were higher than that in the western region (0.49 ± 0.25 nmole/m3, n=28) (p < 0.05). Thus the distribution of MSA was similar to that of DMS, while the nss-SO4 2– concentrations were higher near the continent. This suggests that nss-SO4 2– concentrations were regionally influenced by anthropogenic sulfur input, because the distribution of nss-SO4 2– was similar to that of Rn-222 used as a tracer of continental air masses.  相似文献   
146.
等离子体发射光谱法测定重晶石中钡铁和硅   总被引:3,自引:0,他引:3  
杨德君  陈利平 《岩矿测试》1998,17(4):317-318
重晶石经Na2CO3熔融,水浸取,过滤。沉淀经HCl溶解后,用电感耦合等离子体发射光谱法同时测定主成分BaSO4和次量成分Fe2O3以及滤液中的SiO2,结果与部颁标准方法符合。方法精密度好,主量组分BaSO4测定的RSD(n=6)为0.24%,次量组分Fe2O3和SiO2的RSD(n=6)分别是3.69%和1.45%。  相似文献   
147.
描述了用工业硫酸和氯化钾为原料,萃取法制取硫酸钾新工艺及其工程化的新进展。研究了该工艺三相萃取过程的基本原理,列出了试验数据及经济技术评价。  相似文献   
148.
新疆阿尔泰钾长石矿物学特征及开发利用   总被引:3,自引:0,他引:3  
王元龙  邢慧 《矿产与地质》1997,11(2):119-124
阿尔泰钾长石是新疆优势非金属矿产资源之一,主要赋存在花岗伟晶岩矿脉、钾长花岗岩和碱性花岗岩中,钾长石资源储量大、品位高,易采易选,但尚未很好开发利用。通过阿尔泰钾长石矿物的物理特征、光学性质、化学成分以及工艺矿物学特征的研究,阐述了钾长石资源在工农业中的应用状况和应用范围,经与国内同类型矿种对比,指出新疆阿尔泰钾长石开发利用有着广阔的前景。  相似文献   
149.
The concentrations of H+, nitrate (NO3 -), and sulfate (SO4 2-) in rainwater and their temporal changes were analyzed on the basis of continuous observation from 1 July 1991 to 30 June 1992 at a suburb of Nagoya, Japan. The yearly average for pH was 4.4. In general, an increasing pH with increase in precipitation amount was observed for rain events. Relatively high pH rainwater was sometimes observed at the beginning of rainfall, even though high concentrations of NO3 - and SO4 2- were involved. The high pH values were considered to be caused by the neutralization process with particulate matter containing cations. The yearly averaged ratio of equivalent concentration of nitrate to sulfate (N/S) in rainwater was 0.58. In the early stage of rain, the N/S value was usually more than 1.0 due to the difference of scavenging process between NO3 - and SO4 2-. High values of N/S ranging from 5 to 10 were found under the atmospheric conditions of calm winds and low humidity, during which it is possible that atmospheric particles float for a long time in the air before a rain event. The adsorption of NO3 - in the early stage of rainfall by particulate matter was suggested from the difference in scavenging processes of NO3 - and SO4 2-. A possible scavenging process, called limb cloud scavenging, is presented to explain the interaction of particles and nitrate ions at the early stage of rain. In limb cloud scavenging, the repeated migration of cloud particles or raindrops between the inside and outside of clouds increases the absorption of ions to a highly condensed level, thus increasing the N/S value of rainwater. The influence of global scale seasonal phenomena with large amounts of particulates, such as typhoons or Asian dust storms, was also studied.  相似文献   
150.
本文用含铝矿石生产硫酸铝。在反复试验的基础上所拟定的最佳工艺流程,经建厂投产质量达标。并对工艺条件和影响质量因素进行了有效的探索。  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