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   9307篇
  免费   2120篇
  国内免费   3102篇
测绘学   1182篇
大气科学   1425篇
地球物理   2154篇
地质学   5461篇
海洋学   1762篇
天文学   303篇
综合类   1002篇
自然地理   1240篇
  2024年   74篇
  2023年   193篇
  2022年   539篇
  2021年   632篇
  2020年   516篇
  2019年   597篇
  2018年   639篇
  2017年   586篇
  2016年   556篇
  2015年   575篇
  2014年   617篇
  2013年   656篇
  2012年   808篇
  2011年   788篇
  2010年   775篇
  2009年   708篇
  2008年   601篇
  2007年   631篇
  2006年   598篇
  2005年   514篇
  2004年   359篇
  2003年   301篇
  2002年   288篇
  2001年   260篇
  2000年   315篇
  1999年   254篇
  1998年   158篇
  1997年   182篇
  1996年   128篇
  1995年   114篇
  1994年   106篇
  1993年   103篇
  1992年   78篇
  1991年   56篇
  1990年   51篇
  1989年   31篇
  1988年   23篇
  1987年   22篇
  1986年   12篇
  1985年   9篇
  1984年   8篇
  1983年   12篇
  1982年   7篇
  1981年   6篇
  1979年   7篇
  1977年   3篇
  1976年   3篇
  1974年   5篇
  1957年   5篇
  1954年   9篇
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
471.
现有铬盐生产技术阻碍了铬盐行业的清洁化发展,研发新型铬盐生产技术实属必要。中国科学院青海盐湖研究所自行研发出一套以电化学法为基础的铬盐清洁生产工艺,涵盖三条大工艺和四种母系铬盐产品。包括氢氧化钠溶液中电化学氧化铬铁制备铬酸钠联产三氧化二铬和三氧化二铁工艺,离子膜电解法从铬酸钠合成重铬酸钠工艺,以及离子膜电解法从重铬酸钠合成铬酸酐工艺。相比现有铬盐生产技术,该技术的优势主要体现在以下五点:温和化、去渣化、高值化、自动化和高效化。目前相关研发工作业已走出实验室,走过产业化中间试验,正向产业化示范线迈进。  相似文献   
472.
Yuan  Lihua  Chen  Xiaoqiang  Wang  Xiangyu  Xiong  Zhe  Song  Changqing 《地理学报(英文版)》2019,29(9):1548-1564
Journal of Geographical Sciences - The Heihe River Basin is located in the arid and semi-arid regions of Northwest China. Here, the terrestrial ecosystem is vulnerable, making it necessary to...  相似文献   
473.
474.
An origin-destination (OD) flow can be defined as the movement of objects between two locations. These movements must be determined for a range of purposes, and strong interactions can be visually represented via clustering of OD flows. Identification of such clusters may be useful in urban planning, traffic planning and logistics management research. However, few methods can identify arbitrarily shaped flow clusters. Here, we present a spatial scan statistical approach based on ant colony optimization (ACO) for detecting arbitrarily shaped clusters of OD flows (AntScan_flow). In this study, an OD flow cluster is defined as a regional pair with significant log likelihood ratio (LLR), and the ACO is employed to detect the clusters with maximum LLRs in the search space. Simulation experiments based on AntScan_flow and SaTScan_flow show that AntScan_flow yields better performance based on accuracy but requires a large computational demand. Finally, a case study of the morning commuting flows of Beijing residents was conducted. The AntScan_flow results show that the regions associated with moderate- and long-distance commuting OD flow clusters are highly consistent with subway lines and highways in the city. Additionally, the regions of short-distance commuting OD flow clusters are more likely to exhibit ‘residential-area to work-area’ patterns.  相似文献   
475.
The Ecological-living-productive land(ELPL) classification system was proposed in an effort to steer China's land pattern to an ecological-centered path, with the development model shifting from a single function into more integrated multifunction land use. The focus is coordinating the man-land contradictions and developing an intensive, efficient and sustainable land use policy in an increasingly tense relationship between humans and nature. Driven by socioeconomic change and rapid population growth, many cities are undergoing urban sprawl, which involves the consumption of cropland and ecological land and threatens the ecological balance. This paper aims to quantitatively analyze the critical effects of ELPL changes on eco-environmental quality according to land use classification based on leading function of ecology, living and production from 1990 to 2015 with a case study of Xining City. Also, four future land use scenarios were simulated for2030 using the Future Land Use Simulation(FLUS) model that couples human and natural effects. Our results show a decrease in productive land(PL) and an increase in ecological land(EL) and living land(LL) in Xining City. Forestry ecological land(FEL) covered the top largest proportion; agriculture productive land(APL) showed the greatest reduction and urban and rural living land(U-RLL) presented a dramatic increase. The eco-environmental quality improved in 1990-2010, mainly affected by the conversion of APL to FEL and GEL. However, the encroachment of U-RLL into APL, other ecological land(OEL) and FEL was the main contributor to the decline in eco-environmental quality in 2010-2015 as well as the primary reason for the increase area of lower-quality. The Harmonious Development(HD)-Scenario, characterized by a rational allocation of LL and PL and a better eco-environment, would have implications for planning and monitoring future management of ELPL, and may represent a valuable reference for local policy-makers.  相似文献   
476.
Beijing, Tianjin and Hebei each contributed to the comprehensive governance of bulk coal to treat bulk coal pollution in a mutually beneficial way in 2017. The cooperative game theory is used in this paper to study the environmental benefits and cost effectiveness brought about by this comprehensive governance strategy, primarily focusing on the issue of how to maximize the environmental benefits by choosing an appropriate strategy since the benefits to Beijing, Tianjin and Hebei are closely related. Therefore, the linear optimization, game theory and Shapley value method in the cooperative game model are used to find the ways to minimize the total governance cost of bulk coal in the three areas. In addition, the issues of how to carry out rational distribution and transfer of governance capital among the three places are explored according to the actual amounts of consumption of bulk coal, the influence of the coal burning on the PM2.5 and the actual cost of coal governance in Beijing, Tianjin and Hebei in 2017. The results show that the governance task in Hebei Province is the most onerous, and requires more investment than the other two cities. Thus, it requires the support from other two cities, with the amount of increased capital required of about 600 million Yuan. At the same time, the cost saved after optimization in Tianjin is calculated to be the largest, which thus can be adjusted appropriately and allocated to Hebei for the governance of bulk coal. The model constructed in this paper can not only be used to solve the issues related to bulk coal consumption in Beijing, Tianjin and Hebei, but also to carry out the effective distribution of capital, by which a win-win scenario among the three places can be achieved.  相似文献   
477.
China’s tourism industry has witnessed rapid progress in recent years, and is now an important part of global tourism in dealing with climate change. Within a framework of Pressure-State-Response (PSR), this paper focuses on the emission reduction pressure, carbon emission status, and responses of stakeholders in China’s tourism industry. Findings include: 1) The central government’s strategy and rapid growth of the industry scale exert rising pressure on China’s tourism to reduce carbon emissions. 2) Carbon emissions of China's tourism account for 13%-14.6% of global tourism, and about 3% of China’s emissions overall. Chinese tourists’ per capita carbon emission is lower than half of the global level. 3) The Chinese government attaches great importance to energy-saving and carbon emission reduction. In the tourism industry, documents, standards and other regulative measures have been issued to ensure that business practitioners set up green operational and managerial systems. In the field of tourism transportation, China's high-speed rail, new energy vehicles, and urban shared bicycles, have developed very rapidly in recent years, and they have effectively reduced the carbon emissions in traveling. Furthermore, this paper finds that Chinese tourists already have awareness and willingness for low-carbon tourism.  相似文献   
478.
利用新三维地震资料对松南-宝岛凹陷反转构造带发育特征、形成期次进行研究,并从构造背景及力学机制两个方面探讨反转构造的成因机制。研究结果表明,松南-宝岛凹陷反转构造带主要发育一系列"上凸下凹"的大型褶皱背斜,伴生NWW向弱走滑断裂构造和NNW向张剪构造。反转构造及其伴生构造符合NEE右旋剪切应力场特征,形成时间与东沙运动一致,表明该反转构造带可能受晚中新世南海东北部东沙运动产生的右旋走滑应力场作用控制。反转构造有利于研究区圈闭的重建和改造,对琼东南盆地东部新区油气运移和重新优选分配的认识,具有重要的油气地质意义。  相似文献   
479.
本文提出了一种基于散射计风场数据的台风定强方法。该方法定义了台风所在海区的一个圆形区域上的风速平均值来对台风进行定强,并使用海洋二号卫星散射计历史数据对该方法进行了验证。验证结果表明,该台风定强方法可以克服散射计反演台风风速过低的问题。与台风强度监测的历史记录比较的结果显示,在台风强度低于"强台风"这一强度等级时,该方法能够有效估算台风强度。  相似文献   
480.
中国物理海洋学研究70年:发展历程、学术成就概览   总被引:2,自引:2,他引:0  
本文概略评述新中国成立70年来物理海洋学各分支研究领域的发展历程和若干学术成就。中国物理海洋学研究起步于海浪、潮汐、近海环流与水团,以及以风暴潮为主的海洋气象灾害的研究。随着国力的增强,研究领域不断拓展,涌现了大量具有广泛影响力的研究成果,其中包括:提出了被国际广泛采用的“普遍风浪谱”和“涌浪谱”,发展了第三代海浪数值模式;提出了“准调和分析方法”和“潮汐潮流永久预报”等潮汐潮流的分析和预报方法;发现并命名了“棉兰老潜流”,揭示了东海黑潮的多核结构及其多尺度变异机理等,系统描述了太平洋西边界流系;提出了印度尼西亚贯穿流的南海分支(或称南海贯穿流);不断完善了中国近海陆架环流系统,在南海环流、黑潮及其分支、台湾暖流、闽浙沿岸流、黄海冷水团环流、黄海暖流、渤海环流,以及陆架波方面均取得了深刻的认识;从大气桥和海洋桥两个方面对太平洋–印度洋–大西洋洋际相互作用进行了系统的总结;发展了浅海水团的研究方法,基本摸清了中国近海水团的分布和消长特征与机制,在大洋和极地水团分布及运动研究方面也做出了重要贡献;阐明了南海中尺度涡的宏观特征和生成机制,揭示了中尺度涡的三维结构,定量评估了其全球物质与能量输运能力;基本摸清了中国近海海洋锋的空间分布和季节变化特征,提出了地形、正压不稳定和斜压不稳定等锋面动力学机制;构建了“南海内波潜标观测网”,实现了对内波生成–演变–消亡全过程机理的系统认识;发展了湍流的剪切不稳定理论,提出了海流“边缘不稳定”的概念,开发了海洋湍流模式,提出了湍流混合参数化的新方法等;在海洋内部混合机制和能量来源方面取得了新的认识,并阐述了混合对海洋深层环流、营养物质输运等过程的影响;研发了全球浪–潮–流耦合模式,推出一系列海洋与气候模式;发展了可同化主要海洋观测数据的海洋数据同化系统和用于ENSO预报的耦合同化系统;建立了达到国际水准的非地转(水槽/水池)和地转(旋转平台)物理模 型实验平台;发展了ENSO预报的误差分析方法,建立了海洋和气候系统年代际变化的理论体系,揭示了中深层海洋对全球气候变化的响应;初步建成了中国近海海洋观测网;持续开展南北极调查研究;建立了台风、风暴潮、巨浪和海啸的业务化预报系统,为中国气象减灾提供保障;突破了国外的海洋技术封锁,研发了万米水深的深水水听器和海洋光学特性系列测量仪器;建立了溢油、危险化学品漂移扩散等预测模型,为伴随海洋资源开发所带来的风险事故的应急处理和预警预报提供科学支撑。文中引用的大量学术成果文献(每位第一作者优选不超过3篇)显示,经过70年的发展,中国物理海洋学研究培养了一支实力雄厚的科研队伍,这是最宝贵的成果。这支队伍必将成为中国物理海洋学研究攀登新高峰的主力军。  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