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   2976篇
  免费   66篇
  2020年   19篇
  2019年   22篇
  2018年   324篇
  2017年   235篇
  2016年   280篇
  2015年   51篇
  2014年   31篇
  2013年   597篇
  2012年   83篇
  2011年   208篇
  2010年   246篇
  2009年   97篇
  2008年   109篇
  2007年   178篇
  2006年   70篇
  2005年   51篇
  2004年   39篇
  2003年   25篇
  2002年   19篇
  2001年   17篇
  2000年   10篇
  1999年   15篇
  1998年   15篇
  1997年   6篇
  1996年   8篇
  1995年   18篇
  1994年   11篇
  1993年   9篇
  1992年   12篇
  1991年   9篇
  1990年   8篇
  1989年   12篇
  1988年   13篇
  1987年   16篇
  1986年   11篇
  1985年   14篇
  1984年   11篇
  1983年   9篇
  1982年   10篇
  1981年   12篇
  1980年   14篇
  1979年   7篇
  1978年   8篇
  1977年   6篇
  1976年   8篇
  1975年   8篇
  1972年   10篇
  1971年   5篇
  1970年   7篇
  1969年   5篇
排序方式: 共有3042条查询结果,搜索用时 55 毫秒
951.
952.
This article offers a discussion of nuclear doctrines and their significance for war, peace and stability between nuclear‐armed states. The cases of India and Pakistan are analysed to show the challenges these states have faced in articulating and implementing a proper nuclear doctrine, and the implications of this for nuclear stability in the region. We argue that both the Indian and Pakistani doctrines and postures are problematic from a regional security perspective because they are either ambiguous about how to address crucial deterrence related issues, and/or demonstrate a severe mismatch between the security problems and goals they are designed to deal with, and the doctrines that conceptualize and operationalize the role of nuclear weapons in grand strategy. Consequently, as both India's and Pakistan's nuclear doctrines and postures evolve, the risks of a spiralling nuclear arms race in the subcontinent are likely to increase without a reassessment of doctrinal issues in New Delhi and Islamabad. A case is made for more clarity and less ambition from both sides in reconceptualizing their nuclear doctrines. We conclude, however, that owing to the contrasting barriers to doctrinal reorientation in each country, the likelihood of such changes being made—and the ease with which they can be made—is greater in India than in Pakistan.  相似文献   
953.
This article evaluates the current state of paleoethnobotany since Hastorf’s 1999 review published in this journal. We discuss advances in methods, ancient subsistence reconstructions, the origins and intensification of agriculture, and how plants inform on issues of political economy and identity. Significant methodological developments in the extraction, identification, and analysis of starch grains and phytoliths have led to advancements in our knowledge of early plant domestication and the transition to food production. Paleoethnobotanists increasingly are using more complex quantitative techniques to characterize their data, which have resulted in more nuanced interpretations of plants that fall within the purview of social archaeology and allow us to address issues related to gender, identity, and ritual practice.  相似文献   
954.
Understanding the effect of changing stress conditions on multiphase flow in porous media is of fundamental importance for many subsurface activities including enhanced oil recovery, water drawdown from aquifers, soil confinement, and geologic carbon storage. Geomechanical properties of complex porous systems are dynamically linked to flow conditions, but their feedback relationship is often oversimplified due to the difficulty of representing pore‐scale stress deformation and multiphase flow characteristics in high fidelity. In this work, we performed pore‐scale experiments of single‐ and multiphase flow through bead packs at different confining pressure conditions to elucidate compaction‐dependent characteristics of granular packs and their impact on fluid flow. A series of drainage and imbibition cycles were conducted on a water‐wet, soda‐lime glass bead pack under varying confining stress conditions. Simultaneously, X‐ray micro‐CT was used to visualize and quantify the degree of deformation and fluid distribution corresponding with each stress condition and injection cycle. Micro‐CT images were segmented using a gradient‐based method to identify fluids (e.g., oil and water), and solid phase redistribution throughout the different experimental stages. Changes in porosity, tortuosity, and specific surface area were quantified as a function of applied confining pressure. Results demonstrate varying degrees of sensitivity of these properties to confining pressure, which suggests that caution must be taken when considering scalability of these properties for practical modeling purposes. Changes in capillary number with confining pressure are attributed to the increase in pore velocity as a result of pore contraction. However, this increase in pore velocity was found to have a marginal impact on average phase trapping at different confining pressures.  相似文献   
955.
This article analyzes the indoor climate that creates risk of damages in naturally ventilated churches in the cold climate of Estonia. Indoor temperature and humidity were measured over a one-year period and the results were analyzed on the basis of damage functions: mold growth, risk of cracking and fracturing of wooden objects, and delamination of the gesso layer of panel paintings.

In unheated churches, one of the most dominant problems was very high relative humidity throughout the year, creating a high risk for mold and algae growth. Churches may need background heating to avoid freezing during a long cold winter that causes low surface temperatures of massive walls during the spring–summer period. It was found that mold risk was significantly lower in heated churches than in unheated or intermittently heated churches. The risk of mold growth was not decreased by the use of intermittent heating.

In heated churches, overheating (room temperature >+10ºC) causes a RH below 50% during cold periods, and the favorable period for irreversible response of panel paintings was significantly longer, so there is a higher risk that the gesso may crack or delaminate. From the point of view of the cracking and fracturing of wood, indoor climate conditions are in the safe range for most of the year.  相似文献   

956.
People create narratives of their maritime past through the remembering and forgetting of seafaring experiences, and through the retention and disposal of maritime artefacts that function mnemonically to evoke or suppress those experiences. The sustenance and reproduction of the resulting narratives depends further on effective media of intergenerational transmission; otherwise, they are lost. Rapid socio-economic transformation across Saudi Arabia in the age of oil has disrupted longstanding seafaring economies in the Red Sea archipelago of the Farasan Islands, and the nearby mainland port of Jizan. Vestiges of wooden boatbuilding activity are few; long-distance dhow trade with South Asia, the Arabian-Persian Gulf and East Africa has ceased; and a once substantial pearling and nacre (mother of pearl) collection industry has dwindled to a tiny group of hobbyists: no youth dive today. This widespread withdrawal from seafaring activity among many people in these formerly maritime-oriented communities has diminished the salience of such activity in cultural memory, and has set in motion narrative creation processes, through which memories are filtered and selected, and objects preserved, discarded, or lost. This paper is a product of the encounter of the authors with keepers of maritime memories and objects in the Farasan Islands and Jizan. An older generation of men recall memories of their experiences as boat builders, captains, seafarers, pearl divers and fishermen. Their recounted memories are inscribed, and Arabic seafaring terms recorded. The extent of the retention of maritime material cultural items as memorials is also assessed, and the rôle of individual, communal and state actors in that retention is considered. Through this reflection, it becomes clear that the extra-biological memory and archive of the region’s maritime past is sparse; that intergenerational transmission is failing; that the participation of state agencies in maritime heritage creation is highly limited; and that, as a result, memories current among the older generation have limited prospect of survival. These memories, recorded and interpreted here, identify the Farasan Islands as a former centre of the pearling industry in the Red Sea, and identify them and Jizan as open to far-reaching maritime-mediated cultural influences in an era before the imposition of the attributes of the modern nation-state.  相似文献   
957.
In the first century B.C., a stone carrier sank off the Aegean coast of Turkey at K?z?lburun transporting all the elements of a monumental marble column, including a single Doric capital and eight drums. The 60-tonne cargo lay at a depth of 45–48 m and was excavated in its entirety by the Institute of Nautical Archaeology at Texas A&M University between 2005 and 2011. Ongoing research has shown that the Doric column pieces in the K?z?lburun cargo originated in the marble quarries on Proconnesus Island in the Sea of Marmara and were very likely heading for the Temple of Apollo at Claros when the ship sank just 50 km short of its destination. The complete recovery of the ship’s cargo posed a unique set of methodological challenges involving lifting the multi-tonne marble drums without disturbing the delicate waterlogged wooden hull remains preserved beneath. This report summarizes the solutions developed over five seasons in order to rig, hoist, move, and ultimately raise to the surface eight large ancient marble column drums (as well as all the other marble artifacts in the cargo) from a depth of almost 50 m under water.  相似文献   
958.
Maritime archaeology has a tremendous capacity to engage with climate change science. The field is uniquely positioned to support climate change research and the understanding of past human adaptations to climate change. Maritime archaeological data can inform on environmental shifts and submerged sites can serve as an important avenue for public outreach by mobilizing public interest and action towards understanding the impacts of climate change. Despite these opportunities, maritime archaeologists have not fully developed a role within climate change science and policy. Moreover, submerged site vulnerabilities stemming from climate change impacts are not yet well understood. This article discusses potential climate change threats to maritime archaeological resources, the challenges confronting cultural resource managers, and the contributions maritime archaeology can offer to climate change science. Maritime archaeology’s ability to both support and benefit from climate change science argues its relevant and valuable place in the global climate change dialogue, but also reveals the necessity for our heightened engagement.  相似文献   
959.
Investigation of the inter-tidal heritage of the Orkney Islands is used to interpret a previously perplexing complex at Weelie’s Taing on Papa Westray. The study revealed a previously unknown type of harbour since identified in several locations around Orkney. Situated in exposed environmental situations, shelter is formed by an ‘ayre’, a type of spit that encloses a loch, and which has been used historically as a landing place or crossing of the inter-tidal zone. A complex landing area, pier, tower and ship-blockage suggest Weelie’s Taing was used as a harbour. Important fishing grounds exploited since the Neolithic are nearby, and Papa Westray was the site of water-focussed religious communities. It is suggested that Weelie’s Taing was in use in the medieval period when Papa Westray was less isolated than today with the presence of ecclesiastical communities and situation on the Orkney-Shetland route.  相似文献   
960.
Contact with nature is vital for the development of children and teenagers. In the past, informal urban green spaces (IGS) such as vacant lots appear to have been used for such purposes. We need to better understand how previous generations used IGS to make sure young people today can also enjoy its social, mental, emotional and physical health benefits. This study quantitatively compared adult residents' memories of IGS use in their childhood and teenage years in two geographically and culturally distinct cities: Brisbane, Australia, and Sapporo, Japan. The results showed most respondents (>70%) remembered using IGS in the past, and preferred it over other green space because it was easily accessible. Most (>70%) recalled experiencing no problems (e.g. danger of injury) when using IGS, a contrast to recently increasing parental concern for children's safety. Such factors may limit present IGS use and prevent it from fulfilling the important role it played for previous generations' recreation.  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