全文获取类型
收费全文 | 2106篇 |
免费 | 164篇 |
国内免费 | 294篇 |
专业分类
测绘学 | 190篇 |
大气科学 | 164篇 |
地球物理 | 159篇 |
地质学 | 812篇 |
海洋学 | 125篇 |
天文学 | 920篇 |
综合类 | 75篇 |
自然地理 | 119篇 |
出版年
2024年 | 5篇 |
2023年 | 14篇 |
2022年 | 28篇 |
2021年 | 13篇 |
2020年 | 12篇 |
2019年 | 28篇 |
2018年 | 14篇 |
2017年 | 21篇 |
2016年 | 35篇 |
2015年 | 41篇 |
2014年 | 108篇 |
2013年 | 80篇 |
2012年 | 146篇 |
2011年 | 50篇 |
2010年 | 88篇 |
2009年 | 133篇 |
2008年 | 159篇 |
2007年 | 166篇 |
2006年 | 201篇 |
2005年 | 206篇 |
2004年 | 126篇 |
2003年 | 148篇 |
2002年 | 140篇 |
2001年 | 117篇 |
2000年 | 65篇 |
1999年 | 102篇 |
1998年 | 39篇 |
1997年 | 35篇 |
1996年 | 45篇 |
1995年 | 39篇 |
1994年 | 28篇 |
1993年 | 45篇 |
1992年 | 30篇 |
1991年 | 20篇 |
1990年 | 12篇 |
1989年 | 8篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 8篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1977年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有2564条查询结果,搜索用时 31 毫秒
461.
462.
463.
A bistatic space debris observation system using a radio telescope as the receiving part is introduced. The detection capability of the system at different working frequencies is analyzed based on real instruments. The detection range of targets with a fixed radar cross section and the detection ability of small space debris at a fixed range are discussed. The simulations of this particular observation system at different transmitting powers are also implemented and the detection capability is discussed. The simulated results approximately match the actual experiments. The analysis in this paper provides a theoretical basis for developing a space debris observation system that can be built in China. 相似文献
464.
本文基于多种资料,梳理了1912—1949年中国地理学者姓名、生卒年、籍贯、领域、教育与工作经历等数据,从来源、增长、结构3个层面讨论当时中国地理学者发展进程。研究发现1912—1949年间:① 地理学者在来源层面可分舆地学者、外聘地理学者、现代学制培养地理学者(含有交集关系的国内培养学者、留学培养学者、跨学科学者)3种类型,不同类型群体对地理学者发展的影响差异明显;② 地理学者数量发展可分1912—1927年波动增长,1928—1939年持续增长,1940—1941年急速增长和1942—1949年放缓增长4个时期;③ 中国地理学者学科结构、领域结构、学缘结构、地域结构、年龄结构、性别结构等方面均表现为失衡但失衡程度趋于降低的态势;④ 地理学者发展与地理教学、研究机构发展关系密切,特别是高等地理教学机构对地理学者发展有多重影响。1912—1949年中国地理学者的发展为中国科学地理学的持续进步奠定了人才基础,其过程同时是中国近现代地理学转型的一个侧面。 相似文献
465.
Benthic macrophytes as a tool for delineating, monitoring and assessing ecological status: the case of Slovenian coastal waters 总被引:1,自引:0,他引:1
Benthic macrophytes from 51 sites in the upper infralittoral zone along the Slovenian rocky coast were sampled (randomly stratified) in order to (1) confirm water body (WB) boundaries, (2) select sampling points for a surveillance monitoring programme, and (3) assess preliminary Ecological Status Classes (ESC) within the European Water Framework Directive (WFD). Coverage data of macrophytes were analyzed by using cluster analysis, Simper analysis and the Ecological Evaluation Index (EEI). The vegetation was rather homogenously classified into two Cystoseiretum crinitae subassociations: Halopithetosum incurvae, and Cystoseiretosum compressae, and into Cystoseiretum barbatae association. Seven monitoring sites at a kilometer scale were selected in two significant sized WBs following the mixing zone principle and the EEI successional model. The preliminary assessment of the ESC with the EEI was in agreement with existing human pressures in the area. In view of the present results, benthic macrophytes and EEI could be valuable tools for the implementation of the WFD within the Mediterranean eco-region. 相似文献
466.
One of the founders of structural control theory and its application in civil engineering,Professor Emeritus Tsu T.Soong,envisioned the development of the integral design of structures protected by active control devices.Most of his disciples and colleagues continuously attempted to develop procedures to achieve such integral control.In his recent papers published jointly with some of the authors of this paper,Professor Soong developed design procedures for the entire structure using a design-redesign proce... 相似文献
467.
非线性波浪波面追踪的一种新模式 总被引:1,自引:0,他引:1
基于Laplace方程的Green积分表达式和波面BemouUi方程所建立的非线性波动数学模型,是一个时域上具有初始值的边值问题,而精确地追踪自由表面的波动位置,给出波面运动瞬时的波面高度和波面势函数,是建立时域内非线性波浪数值模式的基础。本文采用0-1混合型边界元剖分计算域边界并离散Laplace方程的Green积分表达式,采用有限元剖分自由水面并推导满足自由表面非线性边界条件的波面有限元方程,联立计算域内以节点波势函数和波面位置高度的时间增量为未知量的线性方程组,通过时步内的循环迭代,给出每个时步上的波面位置和波面势函数,从而建立了一种新的非线性波浪波面追踪模式。数值造波水槽内的波浪试验表明,其数值模拟结果具有良好的计算精度。 相似文献
468.
A number of cold seeps have been discovered in the northern South China Sea (SCS) including the Haiyang 4 cold-seep area where Core 973-5 was collected. Intact polar lipids (IPLs) and core lipids (CLs) were analyzed separately in sediments from Core 973-5. The most abundant lipid biomarkers were isoprenoidal GDGTs (isoGDGTs), with Crenarchaeol and GDGT-0 predominating. IPL-isoGDGTs and CL-isoGDGTs were mainly derived from Thaumarchaeota. IPL-isoGDGTs were mainly produced and retained in situ thus containing most of the in situ microbiological information. Branched GDGTs were predominantly derived from generated in marine production, and mixed with some terrestrial inputs. All IPLs groups presented a high value in the sulfate-methane transition zone (SMTZ). Furthermore, IPL and CL-MI, IPL-R0/4 showed the highest values within the SMTZ, while IPL and CL-R4/i had the lowest values at the SMTZ, suggesting that the contribution of Methanophila and methanogenic to GDGTs increased, while the contribution of ammonia-oxidizing Archaea to GDGTs decreased at the SMTZ. 相似文献
469.
470.