全文获取类型
收费全文 | 502篇 |
免费 | 73篇 |
国内免费 | 101篇 |
专业分类
测绘学 | 44篇 |
大气科学 | 121篇 |
地球物理 | 135篇 |
地质学 | 179篇 |
海洋学 | 77篇 |
天文学 | 7篇 |
综合类 | 70篇 |
自然地理 | 43篇 |
出版年
2024年 | 2篇 |
2023年 | 6篇 |
2022年 | 28篇 |
2021年 | 24篇 |
2020年 | 17篇 |
2019年 | 14篇 |
2018年 | 25篇 |
2017年 | 21篇 |
2016年 | 21篇 |
2015年 | 26篇 |
2014年 | 28篇 |
2013年 | 32篇 |
2012年 | 25篇 |
2011年 | 36篇 |
2010年 | 21篇 |
2009年 | 28篇 |
2008年 | 30篇 |
2007年 | 27篇 |
2006年 | 30篇 |
2005年 | 21篇 |
2004年 | 14篇 |
2003年 | 15篇 |
2002年 | 17篇 |
2001年 | 15篇 |
2000年 | 12篇 |
1999年 | 24篇 |
1998年 | 14篇 |
1997年 | 12篇 |
1996年 | 24篇 |
1995年 | 14篇 |
1994年 | 4篇 |
1993年 | 10篇 |
1992年 | 4篇 |
1991年 | 11篇 |
1990年 | 5篇 |
1989年 | 7篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1984年 | 2篇 |
1982年 | 2篇 |
排序方式: 共有676条查询结果,搜索用时 187 毫秒
661.
662.
高层建筑结构的抗震可靠度分析与优化设计 总被引:12,自引:4,他引:12
本文根据我们在文献2中给出的等效随机地震静力作用模型,紧密结合规范和利用我们在文献6中提出的结构体系可靠度分析的最弱失效模式法,提出了结构构件和体系“小震不坏”和“大震不倒”及结构体系在设计基准期内的抗震可靠度分析方法;重新校准了结构构件的目标可靠度指标;综合考虑结构造价和损失期望,提出了结构体系抗震目标可靠度的优化决策方法;分别给出了满足构件抗震目标可靠指标与同时满足构件和体系抗震目标可靠指标的 相似文献
663.
664.
在郯庐断裂多期构造活动作用下,辽河坳陷东部凹陷发生了多期火山活动,致使中基性火山岩广泛发育,以玄武岩和粗面岩为主,可分为5相14亚相。以常规测井曲线特征为基础,结合电成像的分析,识别出了爆发相(火山碎屑流和热基浪亚相)、溢流相(玻质碎屑岩、板状熔岩流和复合熔岩流亚相)、侵出相(内带、中带和外带亚相)和火山沉积相(含外碎屑和再搬运火山碎屑沉积亚相)10种岩相/亚相:火山碎屑流亚相具"焊接"特征,热基浪亚相具层理特征;玻质碎屑岩亚相具高CNL的特征,板状熔岩流亚相DEN、CNL和AC测井曲线显示微齿平滑的特征,而复合熔岩流亚相测井曲线呈指状叠加的特征;侵出相内带→中带→外带电阻率逐渐减小;含外碎屑和再搬运火山碎屑沉积亚相GR值范围不同,并且再搬运火山碎屑沉积亚相具水平层理特征。火山岩相控制原生储集空间的类型,并作用于后期的次生改造,从而影响储层的物性、储集性和有效性。复合熔岩流亚相储层孔隙发育,物性好,但由于内部纵向上结构不一致,非均质性强,因而储层含油性差。火山碎屑流亚相内部在纵向上岩性及结构相对一致,因而储层物性相对均一、分布集中,为有利的火山岩储层,可以作为东部凹陷进一步勘探开发的有利相带。 相似文献
665.
L.?J.?Hou M.?LiuEmail author H.?Y.?Jiang S.?Y.?Xu D.?N.?Ou Q.?M.?Liu B.?L.?Zhang 《Environmental Geology》2003,45(1):72-78
Ammonium adsorption by sediments plays an important role in the cycle of nitrogen in the estuarine tidal flat ecosystem. Ammonium adsorption on tidal flat sediments from the Yangtze estuary was studied using the simulation experiment. The findings indicated that there was linear adsorption by the tidal flat sediments for the lower ammonium concentration (<10 mM) in the overlying waters. The adsorption coefficient of ammonium in the study area ranged from 3.81 to 9.00, and had a significant positive correlation with total organic carbon (r = 0.779, = 0.02284 , n=8), reflecting that organic matter might be one of the main factors controlling the behavior of ammonium adsorption. It also showed that salinity had a pronounced effect on ammonium adsorption, and that salinity affected the distribution of ammonium between the sediments and waters. The amount of ammonium adsorption on sediments gradually decreased with the increasing salinity. Especially within the range of lower salinity, just a little change of salinity had a dramatic influence on ammonium adsorption by sediments. 相似文献
666.
Use of rare earth element patterns to trace the provenance of the atmospheric dust near Beijing, China 总被引:2,自引:1,他引:2
Atmospheric dust is one of the important indicators of urban air quality. In this study, atmospheric dust and topsoil samples were collected monthly from four sites in Beijing and surrounding areas, and analyzed for rare earth elements (REEs). Total REE concentrations (∑REE) in the dusts ranged from 43.10 to 164.43 mg kg?1; concentrations in Beijing and Miyun were higher than those in Zhangbei and Fengning. Most of sampling sites showed the same seasonal depletion in dust ∑REE concentrations in summer, probably caused by the “dilution effect” of added organic matter in summertime samples. Most of the dusts have consistent Ce-anomaly (Ce/Ce*) values (0.91–1.00) and Eu anomaly (Eu/Eu*) values (0.93–1.36); some dusts show slight positive Eu anomalies. The REE patterns of both the dusts and topsoils (normalized to post-Archean shales from Australia, PASS) show different signatures at different sites, whereas the temporal patterns are consistent at each site. These results suggest that local topsoils are likely to be the main provenance of the dusts in the study area. 相似文献
667.
Yun Chen Qiming FengYanhai Shao Guofan ZhangLeming Ou Yiping Lu 《International Journal of Mineral Processing》2006
Extraction of molybdenum and vanadium from ammonia leaching residue (main chemical composition: 2.05% Mo, 0.42% V, 65.6% Al2O3 and 10.7% SiO2) of spent catalyst was investigated by roasting the residue with soda carbonate, followed by hydrometallurgical treatment of the roasted products. In the roasting process, over 91.3% of molybdenum and 90.1% of vanadium could be extracted when a charge containing a sodium carbonate to spent catalyst ratio of 0.15 was roasted at 750 °C for 45 min and the roasted mass was leached with water (liquid to solid ratio of 2) at 80–90 °C for 15 min. After the purification of leach liquor, an extraction solvent consisting of 20 vol.% trialkylamine (N235, commercialized in China) and 10 vol.% secondary octyl alcohol (phase modifier) dissolved in sulfonated kerosene was used to extract molybdenum and vanadium in leach liquor. 10 wt.% ammonia water was used as stripping agent. Adding 30 g/l NH4NO3 to the stripping solution and adjusting the pH to 7–8.5, over 99% of vanadium can be crystallized as ammonium metavanadate. Over 98% of molybdenum can be crystallized as ammonium polymolybdate when pH is between 1.5 and 2.5 (pH is adjusted by HNO3). Ammonium metavanadate and ammonium polymolybdate were calcinated at 500–550 °C, the purity of MoO3 and V2O5 was 99.08% and 98.06% respectively. In the whole process, 88.2% of molybdenum and 87.1% of vanadium could be achieved. The proposed roasting, leaching and separation steps give a feasible alternative for the processing of ammonia leaching residue of spent catalyst and can be applied in the comprehensive utilization of low grade molybdenum ores. 相似文献
668.
本文以一座三跨总长60 m的整体桥为案例桥,分别试设计了同跨径的半整体桥、延伸桥面板桥和常规连续梁桥。通过Midas/Civil软件建立四种桥型的有限元模型,并对其进行了E1和E2反应谱分析和时程分析,对比了四种桥型的结构反应峰值(墩顶位移、桥墩及桩基剪力与弯矩、台底位移、桥台桩基剪力与弯矩)。计算结果表明:当桥梁存在15°的斜交角,整体桥、半整体桥在地震动沿平行于桥台长边方向及其垂直方向输入时更不利,而延伸桥面板桥和常规连续梁桥在地震动沿顺桥向和横桥向输入时更不利。四种桥型在地震作用下:整体桥抗震性能最优异,但其台底位移、桥台桩基的剪力和弯矩最大;半整体桥台底位移、桥台桩基的剪力和弯矩最小,其墩顶位移、桥墩及桩基的剪力和弯矩仅比整体桥大;延伸桥面板桥和常规连续梁桥的墩-梁相对位移远大于整体桥和半整体桥,不适用于地震基本烈度高的区域。 相似文献
669.
670.