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   8530篇
  免费   378篇
  8908篇
  2020年   119篇
  2019年   173篇
  2018年   488篇
  2017年   388篇
  2016年   489篇
  2015年   186篇
  2014年   166篇
  2013年   1959篇
  2012年   244篇
  2011年   394篇
  2010年   397篇
  2009年   259篇
  2008年   261篇
  2007年   326篇
  2006年   171篇
  2005年   139篇
  2004年   105篇
  2003年   103篇
  2002年   113篇
  2001年   143篇
  2000年   105篇
  1999年   113篇
  1998年   89篇
  1997年   80篇
  1996年   84篇
  1995年   69篇
  1994年   59篇
  1993年   72篇
  1992年   71篇
  1991年   60篇
  1990年   60篇
  1989年   56篇
  1988年   52篇
  1987年   60篇
  1986年   60篇
  1985年   69篇
  1984年   79篇
  1983年   84篇
  1982年   68篇
  1981年   67篇
  1980年   66篇
  1979年   61篇
  1978年   66篇
  1977年   67篇
  1976年   61篇
  1975年   71篇
  1974年   42篇
  1973年   38篇
  1972年   41篇
  1971年   37篇
排序方式: 共有8908条查询结果,搜索用时 15 毫秒
991.
992.
993.
A small iron gun recovered from the wreck of the Caroline warship Swan has been identified as a drake, a lightweight tapered-chamber design dating from the 1620s. It is the only known cast-iron example of such a piece made by John Browne, gunfounder to Charles I, who was the leading figure in its development and manufacture in England. The gun was found together with an associated carriage, port-lid, and case shot appropriate to its calibre.
© 2004 The Nautical Archaeology Society  相似文献   
994.
M. A. Simms  G. Garven 《Geofluids》2004,4(2):109-130
Thermal convection has the potential to be a significant and widespread mechanism of fluid flow, mass transport, and heat transport in rift and other extensional basins. Based on numerical simulation results, large‐scale convection can occur on the scale of the basin thickness, depending on the Rayleigh number for the basin. Our analysis indicates that for syn‐rift and early post‐rift settings with a basin thickness of 5 km, thermal convection can occur for basal heat flows ranging from 80 to 150 mW m?2, when the vertical hydraulic conductivity is on the order of 1.5 m year?1 and lower. The convection cells have characteristic wavelengths and flow patterns depending on the thermal and hydraulic boundary conditions. Steeply dipping extensional faults can provide pathways for vertical fluid flow across large thicknesses of basin sediments and can modify the dynamics of thermal convection. The presence of faults perturbs the thermal convective flow pattern and can constrain the size and locations of convection cells. Depending on the spacing of the faults and the hydraulic properties of the faults and basin sediments, the convection cells can be spatially organized to align with adjacent faults. A fault‐bounded cell occurs when one convection cell is constrained to occupy a fault block so that the up‐flow zone converges into one fault zone and the down‐flow zone is centred on the adjacent fault. A fault‐bounded cell pair occurs when two convection cells occupy a fault block with the up‐flow zone located between the faults and the down‐flow zones centred on the adjacent faults or with the reverse pattern of flow. Fault‐bounded cells and cell pairs can be referred to collectively as fault‐bounded convective flow. The flow paths in fault‐bounded convective flow can be lengthened significantly with respect to those of convection cells unperturbed by the presence of faults. The cell pattern and sense of circulation depend on the fault spacing, sediment and fault permeabilities, lithologic heterogeneity, and the basal heat flow. The presence of fault zones also extends the range of conditions for which thermal convection can occur to basin settings with Rayleigh numbers below the critical value for large‐scale convection to occur in a basin without faults. The widespread potential for the occurrence of thermal convection suggests that it may play a role in controlling geological processes in rift basins including the acquisition and deposition of metals by basin fluids, the distribution of diagenetic processes, the temperature field and heat flow, petroleum generation and migration, and the geochemical evolution of basin fluids. Fault‐bounded cells and cell pairs can focus mass and heat transport from longer flow paths into fault zones, and their discharge zones are a particularly favourable setting for the formation of sediment‐hosted ore deposits near the sea floor.  相似文献   
995.
996.
997.
This article examines how gender ideologies differentially inform men and women's work‐life courses in the fishing economy of southern India. Drawing on interviews conducted with fish traders in Trivandrum, India in 1993–94 and 1999, we construct economic life histories for men and women traders. These are then used to illustrate how gender creates different patterns of paid work for men and women throughout their life courses and to analyze how men and women formulate livelihood strategies differently as a result.  相似文献   
998.
999.
托忒文历史文献对清朝官方史籍编纂的影响   总被引:2,自引:0,他引:2  
M·乌兰 《清史研究》2004,(3):95-101
二十世纪三十至四十年代 ,陈垣先生曾大力倡导史源学研究。史源学的要义 ,是追寻史学论著的史料来源 ,检验立论的依据 ,考证其中的讹误。半个世纪以来 ,史源学不但在历史学界享有盛誉 ,而且扬起学术规范化的风帆 ,驶向邻近学科的学术港湾 ,产生广泛的影响。① 当然这主要讲的是对汉文文献研究的影响。在蒙文文献的研究方面 ,1 978年蒙古国学者沙·比拉在他的专著《蒙古史学史》中也关注到了这一问题。他在研究“帝国时代的蒙古史学史 (十三———十四世纪 )”的有关著述《圣武亲征录》、《元史》时就断言 ,它们的资料来源中有不少蒙古文资料…  相似文献   
1000.
Friedman and Smith's (1960) article introducing an exciting, potentially precise and inexpensive method of dating obsidian artefacts has thus far failed to reach its potential. Numerous efforts to refine, improve and even redevelop the method since that time have similarly failed to achieve the original promise. Only within the last eight years have significant improvements been made, due to both improved analytical techniques and a better understanding of the hydration process. However, most of our mechanistic understanding of the interaction of water with rhyolitic glass is based on experiments performed on melts and glasses at temperatures above their glass transitions, conditions inappropriate for investigation of near‐surface environmental conditions. Unfortunately, studies detailing the temporal evolution of the diffusion profile at low temperatures are rare, and few useful data are available on the low‐temperature diffusive hydration of silicate glasses. This paper presents data on the experimental hydration of obsidian from the Pachuca source (a.k.a. Sierra de las Navajas, Basin of Mexico) at 75°C for times ranging from 3 to 562 days, and compares these results with data for samples obtained from a stratigraphic excavation of the Chalco site in the Basin of Mexico. Samples have been analysed using secondary ion mass spectrometry (SIMS) to provide concentration/depth data. While 75°C is still significantly above the temperatures at which archaeological obsidians hydrate, it is well below the glass transition temperature (approx. 400°C) and thus processes are likely to be similar to those that occur in nature, but fast enough to be observed over a laboratory timescale. The results demonstrate that a simple square‐root‐of‐time model of the evolution of the diffusion profile is not adequate to describe the diffusion process, as measured diffusion profiles exhibit the effects of concentration‐ and time‐dependent, non‐Fickian diffusion. With progressive hydration, characteristic diffusion coefficients first decrease, then increase with time. Surface concentration increases with time, but an intermediate plateau is observed in its time evolution that is consistent with results obtained from the suite of Chalco samples. Both of these effects have been observed during diffusion in glassy polymer systems and are associated with the build‐up and relaxation of self‐stress caused by the influx of diffusing material.  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