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   40877篇
  免费   1623篇
  2020年   524篇
  2019年   816篇
  2018年   897篇
  2017年   1113篇
  2016年   1198篇
  2015年   924篇
  2014年   900篇
  2013年   14038篇
  2012年   1037篇
  2011年   1064篇
  2010年   1065篇
  2009年   1066篇
  2008年   999篇
  2007年   901篇
  2006年   817篇
  2005年   565篇
  2004年   563篇
  2003年   521篇
  2002年   494篇
  2001年   480篇
  2000年   414篇
  1999年   478篇
  1998年   445篇
  1997年   415篇
  1996年   391篇
  1995年   405篇
  1994年   349篇
  1993年   404篇
  1992年   380篇
  1991年   284篇
  1990年   283篇
  1989年   288篇
  1988年   296篇
  1987年   308篇
  1986年   281篇
  1985年   397篇
  1984年   436篇
  1983年   431篇
  1982年   380篇
  1981年   374篇
  1980年   412篇
  1979年   406篇
  1978年   396篇
  1977年   419篇
  1976年   371篇
  1975年   376篇
  1974年   282篇
  1973年   257篇
  1972年   240篇
  1970年   217篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Ceramic vessels from Later Stone Age sites in the Seacow river valley, South Africa, were heavily tempered with C4 grasses. However, most δ13C values of the temper fall between the known δ13C ranges for C4 and C3 grasses. Among possible C3 additives that could skew the signal, ground bone, animal dung, alpine grass, Karoo scrub ash, and humic acid contamination are eliminated. The sherd surfaces yield fatty chars with C3-like values. Although these are not enough to account for all the skewing, they represent only the final cooking event. They were preceded by many such events in which fat was absorbed into the C4 grass then charred to soot by later periods in the fire. Thus organic carbon with the isotope composition of the fat built up while the vessel remained in use.  相似文献   
992.
K. B. PERSSON 《Archaeometry》1997,39(2):441-443
A new method for soil phosphate analysis has been developed for field measurement which will increase the applicability of phosphate mapping in archaeological prospection.  相似文献   
993.
The characterization of various samples of white marble from the archaeological site of Tharros (Sardinia) is the starting point for the reexamination of the capabilities of electron spin resonance (ESR) spectroscopy as a tool for identifying the quarries of origin of ancient marble artefacts. Existing methods and data bases are employed for the assignment of our samples, but also reviewed to see if a more extensive use of the technique is feasible. It turns out that ESR spectroscopy, coupled with other available methods, may readily provide valuable information, often yielding the final answer to the problem of marble provenance. However, it is also clear that much work has still to be done to standardize the data collection and analysis procedures and to expand the available data base.  相似文献   
994.
A combination of analytical techniques, including thin-section observations, scanning electron microscopy, X-ray diffraction and X-ray fluorescence analysis, was applied to study the nature and origin of an adhesive used for mounting flint sickle blades to a wooden handle, recovered from early Eighteenth Dynasty culture layers of the multi-layer settlement at Tell el Dab'a, Lower Eygpt. The results obtained from these investigations led to the conclusion that the adhesive is a pure lime plaster, thus providing the oldest evidence for the use of this material as a binding agent so far known in ancient Egypt.  相似文献   
995.
Some 630 Roman silver coins excavated at Augusta Raurica (Switzerland) have been analysed by non-destructive X-ray fluorescence and X-ray diffraction. Furthermore, the density and the weight have been determined. The measured average density of plated and massive depletion-silvered coins is lower than the density calculated from the chemical composition, whereas massive coins display equal values for both of the densities. Based on experimental X-ray investigations of modern silver (tempering, acid treatment, production of corrosion layers), the nature of corrosion products on silver, their impact on surface analysis, and aspects of wearing-off by circulation are briefly discussed.  相似文献   
996.
Although assessments by conventional macro measures show considerable improvement in food security in India, the levels of cereal consumption and calorie intake of even the general population are still below the normative threshold limits. This article brings out the dynamics of changes in institutional and production conditions and their implications for consumption patterns and food security at the micro level. The article uses empirical evidence to raise a number of issues, and calls for emphasis on investment in human capital by way of improvements in food and calorie intake for efficiency and economic growth.  相似文献   
997.
SPATIAL DEPENDENCY OF SEGREGATION INDICES   总被引:3,自引:0,他引:3  
A few researchers have mentioned the scale sensitivity of segregation index, D. In this paper, I discuss analytically and empirically why using large enumeration areal units usually results in low segregation measures, and using small areal units produces relatively high segregation measures. The discussion is also applicable to the multi-group variant of D. A major finding is that if people of the same ethnic groups are positively spatially auto-correlated, increasing the size of areal units of analysis may not lower D initially, because only people of the same group are added. But enlarging the areal units subsequently may include population of other ethnic groups, and therefore could lower D. However, if the boundaries of the larger enumeration units are drawn to include only population of the same group, then D will not change significantly. Both the spatial autocorrelation of ethnic group population and zonal pattern are critical factors in determining the scale sensitivity of D.  相似文献   
998.
999.
1000.
The empirical analysis in this paper explores the interurban variation in family income distribution. The results point to increasing urban development, rising female-headship, a widening educational distribution, and changes in the industrial and occupational mix as major contributing factors to rising inequality. However, the increase in the relative number of multiple worker families was a significant mitigating force to rising inequality. A decomposition of 1979 and 1989 cross-sectional models revealed that while changes in urban family and industrial characteristics have been sources of rising inequality, there has been significant structural change in the urban models acting to decrease inequality.  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