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   388篇
  免费   48篇
  2021年   4篇
  2020年   15篇
  2019年   18篇
  2018年   25篇
  2017年   15篇
  2016年   28篇
  2015年   17篇
  2014年   10篇
  2013年   54篇
  2012年   16篇
  2011年   21篇
  2010年   22篇
  2009年   22篇
  2008年   11篇
  2007年   14篇
  2006年   20篇
  2005年   8篇
  2004年   9篇
  2003年   3篇
  2002年   7篇
  2001年   3篇
  2000年   5篇
  1999年   9篇
  1998年   4篇
  1996年   3篇
  1995年   2篇
  1994年   5篇
  1993年   7篇
  1992年   9篇
  1991年   4篇
  1989年   4篇
  1988年   3篇
  1987年   2篇
  1986年   2篇
  1985年   3篇
  1984年   2篇
  1983年   3篇
  1982年   4篇
  1981年   3篇
  1980年   3篇
  1978年   2篇
  1976年   4篇
  1974年   2篇
  1972年   1篇
  1970年   2篇
  1968年   1篇
  1967年   1篇
  1966年   1篇
  1963年   1篇
  1950年   1篇
排序方式: 共有436条查询结果,搜索用时 46 毫秒
61.
This paper seeks to examine how federal financial support of academic scientists shapes their collaborative behavior. More specifically, it investigates two questions: (i) are there threshold sizes of federal grants that have effects on collaboration? and (ii) do the threshold sizes differ by discipline? To test and model these questions, this paper uses data collected in a 2004–2005 survey by the Research Valuing Mapping Project (a project based at Georgia Tech and led by Barry Bozeman) of 2,086 academically based research scientists and engineers. The analysis in this paper reaffirms the positive relationship between federal funding and collaboration seen in previous works, but the findings suggest that there is in fact a threshold effect—as the positive relationship has a diminishing influence. The findings of this paper are more limited with respect to how academic disciplines’ threshold levels differ. The results indicate a difference in the threshold levels between disciplines for three of the eight models.  相似文献   
62.
The transformation of yellow goethite into red hematite by heating has long been assumed for Palaeolithic red artefacts excavated close to fireplaces. However, this transformation is extremely rare. Using SEM–FEG–EDX, PIXE–PIGE, TEM and μXRD, we characterized the mineralogical and chemical compositions of four microsamples of colouring materials from the Mousterian layer B in the es‐Skhul rock‐shelter, from about 100 kya ago. For some colouring materials, the Mousterian people of es‐Skhul chose to gather remote yellow lumps for heating. Their significant transport distance provides evidence of the possible high cultural value of these colouring materials for transformation into red pigments.  相似文献   
63.
博物馆策展人的素质与能力是博物馆陈列展览特别是专题展览成功的关键因素之一.美国大都会艺术博物馆亚洲艺术部主任、国际著名博物馆策展人屈志仁先生以"忽必烈的时代"展为例指出:博物馆策展人应同时具备学者、艺术鉴赏家与展览组织者的素质与能力,通晓与展览有关的历史学、艺术史、文化史及文物与博物馆学等学科知识,组织、协调从研究策划...  相似文献   
64.
Y. LIU  G. CHI  K. M. BETHUNE  B. DUBÉ 《Geofluids》2011,11(3):260-279
The Red Lake mine trend, a deformation zone in the Archean Red Lake greenstone belt that hosts the world‐class Campbell‐Red Lake gold deposit, is characterized by abundant foliation‐parallel iron‐carbonate ± quartz veins with banded colloform‐crustiform structures and cockade breccias overprinted by silicification and gold mineralization. There is an apparent incompatibility between the cavity‐fill structures of the veins and breccias (typically developed at shallow crustal depths) and the upper greenschist to lower amphibole facies metamorphic conditions recorded in the host rocks (indicating relatively deep environments). This, together with the development of veins along the foliation plane, represents an enigmatic problem that may be related to the interplay between fluid dynamics and stress field. We approach this problem through systematic study of fluid inclusion planes (FIPs) in the vein minerals, including the orientations of the FIPs and the pressure–temperature conditions inferred from fluid inclusion microthermometry. We find that fluid inclusions in the main stage vein minerals (pregold mineralization ankerite and quartz and syn‐ore quartz) are predominantly carbonic without a visible aqueous phase, whereas many inclusions in the postore stage contain an aqueous phase. Most FIPs are subvertical, and many are subparallel to the foliation. High fluid pressure coupled with the high wetting angles of the water‐poor, carbonic fluids may have been responsible for the abundance of brittle deformation features. The development of subvertical FIPs is interpreted to indicate episodic switching of the maximum principal compressive stress (σ1) from subhorizontal (perpendicular to the foliation) to subvertical (parallel to the foliation) orientation. The subvertical σ1 is favorable for the formation of foliation‐parallel veins, as fractures are preferentially opened along the foliation in such a stress regime, the origin of which may be linked to the fluid source.  相似文献   
65.
Layered low permeability rock units, like shales, represent seals or ‘cap‐rocks’ in a variety of geological settings. A continuous increase in the fluid pressure gradients across a virtually impermeable rock layer will ultimately lead to hydro‐fracturing. Depending on the boundary conditions, such fracturing may lead to the formation of a set of sub‐parallel cracks oriented more or less perpendicular to the cap‐rock layer. In this article, we propose a new numerical model that describes interactions between multiple cross‐cutting fractures in an elastic low permeability rock layer. The width of each fracture and the spacing between them are modeled as a force balance between the fluid pressure and the elastic forces in the cap‐rock and between each fracture. The model indicates that the system of fractures evolves toward a spatially periodic steady‐state distribution with a fixed fracture spacing and aperture. The results are similar for incompressible and compressible fluids. The steady‐state conditions depend on only two dimensionless parameters, and the fracture spacing is only weakly dependent on the cap‐rock thickness. This is in contrast to fracturing produced by simple extension of an elastic rock layer beyond the fracture strength, in which case fracture spacing is proportional to layer thickness.  相似文献   
66.
Lead isotope compositions provide a direct means of assessing provenance. The lead contents in bronzes cannot be used to argue against lead isotopes. Any claim for the origin of the source material for the origin of Yin‐Shang bronzes must satisfy the measured lead isotope compositions. Thus far, only southern African sources are found to meet this criteria.  相似文献   
67.

This article deals with the early colonization of the Russian Barents Sea Coast (the so-called Murman Coast) in the middle of the 19th and early 20th century. It focuses mainly on the household economy and economic adaptation in the area by groups of Finnish, Russian, Karelian, Norwegian and Sami colonists. The opening up of the new territory by the colonists resulted partly in the employment of traditional methods of using natural resources and partly in the culture of these groups being transformed. Based on this, two different patterns of using natural resources were singled out within the ethnic groups that settled on the Murman Coast (western and eastern). These patterns were more suited to the new living conditions in each of the respective areas, and became the basis for different trends in the economic development of the Murman Coast.  相似文献   
68.
69.
Notwithstanding the relative abundance of bioarchaeological evidence of pre‐20th century gunshot wounds discovered at European and American archaeological sites, similar findings in East Asian countries have been very rare. In this study, we report evidence of possible gunshot wounds sustained by two individuals buried in Joseon tombs of Korea. The shapes of the trauma seemed to indicate low‐velocity projectiles, most likely musket balls. To our knowledge, this is the first bioarchaeological report specifically concerning possible gunshot‐wound evidence in East Asian skeletons recovered from archaeological fields. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   
70.
In this paper, the damage potential of an earthquake ground motion is evaluated in terms of the total power of the acceleration of the ground motion. By assuming an appropriate spectral shape for the input energy spectrum, and using the well-known Parseval theorem for evaluating the total power of a random signal, the peak amplification factor for the equivalent input energy velocity spectrum can be determined. It is shown that the peak amplification factor for the input energy spectrum depends on the peak-ground-acceleration to peak-ground-velocity ratio and duration of the strong motion phase of the ground motion. Values for the equivalent input energy velocity amplification factor vary from about 2 to 10 for most of the recorded ground motions used in this study. Although a considerable scatter of data is observed in this study, the peak amplification factor predicted by the Fourier amplitude spectrum of the ground acceleration provides a fairly good estimate of the mean value of the peak input energy compared to that determined from inelastic dynamic time history analyses, particularly for systems with high damping and low lateral strength. The peak amplification factor derived in this paper provides a more consistent approach for estimation of seismic demand when compared to an earlier empirical expression used for the formulation of duration-dependent inelastic seismic design spectra, even though only a slight difference in the required lateral strength results from the use of the new formula.  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