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   41616篇
  免费   1625篇
  2020年   528篇
  2019年   816篇
  2018年   913篇
  2017年   1122篇
  2016年   1217篇
  2015年   936篇
  2014年   905篇
  2013年   14071篇
  2012年   1048篇
  2011年   1079篇
  2010年   1092篇
  2009年   1083篇
  2008年   1020篇
  2007年   919篇
  2006年   829篇
  2005年   581篇
  2004年   576篇
  2003年   537篇
  2002年   506篇
  2001年   501篇
  2000年   454篇
  1999年   525篇
  1998年   464篇
  1997年   418篇
  1996年   401篇
  1995年   417篇
  1994年   360篇
  1993年   413篇
  1992年   391篇
  1991年   293篇
  1990年   291篇
  1989年   300篇
  1988年   302篇
  1987年   319篇
  1986年   297篇
  1985年   417篇
  1984年   446篇
  1983年   451篇
  1982年   388篇
  1981年   386篇
  1980年   426篇
  1979年   418篇
  1978年   397篇
  1977年   427篇
  1976年   377篇
  1975年   382篇
  1974年   294篇
  1973年   267篇
  1972年   245篇
  1970年   224篇
排序方式: 共有10000条查询结果,搜索用时 437 毫秒
971.
972.
973.
D. A. SCOTT 《Archaeometry》1996,38(2):305-311
Examination of a Sican ceremonial tumi, dated to the Middle Sican, c. AD 850–1050, showed that it was made in a silver-copper alloy of approximate composition 93% silver, 6% copper. Many separate pieces are used in the construction, joined together by both mechanical methods and by solder. The principal figure has been made from 18 separate silver sheets, with 169 separate parts identified in the head-dress, totalling 187 design elements. Metallographic examination of a fragment of repoussé decorated sheet from the figure surmounting the tumi surprisingly showed that the silver-copper alloy has a cast morphology; this demonstrates that Sican metalsmiths were pre-casting silver-copper sheet into the dimensions required for making the tumi, rather than carrying out the expected process of making the idol from hammered silver-copper sheet, shaped by extensive cycles of working and annealing, thus contradicting the usual assumptions about the manufacture of sheet-metalwork made in such cases.  相似文献   
974.
Non-destructive X-ray fluorescence analyses induced by white synchrotron radiation (SYXRF) have been performed on a page of the Gutenberg Bible and 21 other different single leaves printed in the years 1470 to 1500. Absolute elemental areal densities for the papers and the inks are reported. The known high Cu and Pb content in Gutenberg's ink is confirmed. The elemental compositions of the other inks, if measurable by XRF at all, are in general very low, governed only by a few additional elements and found to be not distinctive for a known printer. This will render an identification of the printer or printing offices by ink composition analysis in the period after 1470 difficult.  相似文献   
975.
976.
ABSTRACT. This paper uses new estimates of metropolitan factor demand and output supply functions to study how federal, state, and local fiscal policies affect metropolitan economies. We illustrate our work with findings for ten metropolitan areas in five states for changes in state corporate income taxes, local property taxes, the federal corporate income tax, an investment tax credit, interest rates, public capital stocks, output prices, and tax and regulatory policies affecting gross wages. It is clear from these simulations that a policy that is nominally the same everywhere will have repercussions that vary widely across regions and cities.  相似文献   
977.
ABSTRACT. This paper examines the socially optimal locations of branch facilities (or small stores) and main facilities (or large stores) on a finite linear market that is uniformly populated from position 0 to position 1. Each consumer has a probability w of finding the desired service (or product) at a branch facility, and a probability 1 of finding the desired service (or product) at a main facility. Two types of consumer search are considered: phone search and visit search. Different assumptions are made about the numbers of branch facilities and main facilities (each involving one or two facilities of each type). Under visit search, the socially optimal locations of branch facilities tend to be closer to main facilities than under phone search, and this tendency is more pronounced for smaller values of w.  相似文献   
978.
This study examined landcover change during 1940–1988 in two contiguous landscapes of approximately 12,400 hectares in central Ohio, USA: an inertial till plain area and a more dynamic moraine area. Agriculture dominated both landscapes throughout, however, the cover of natural vegetation and urban-suburban development increased over time on the moraine while natural vegetation decreased and agriculture increased on the till plain. Markov process models for landcover change were constructed for three 14–17-year time intervals and for the entire forty-eight-year study period. Probabilities of self-replacement for agricultural lands (0.91–0.97) were higher and those for forests (0.54–0.65) lower than those reported for other landscapes. Predictions of landcover percentages for fifty years in the future were made using Markov process models derived from actual cover changes in 1940–1957, 1971–1988, and 1940–1988. All three models produced similar predictions for the inertial till plain area. In contrast, the three models' predictions differed considerably for the moraine area. Only the 1971–1988 transition matrix was sensitive to increases in urbanization and reforestation which began on the moraine in the 1970s. These results indicate that reliance on lengthy study intervals and net rates of change can obscure spatial and temporal patterns of landcover change that are caused by natural and socioeconomic factors operating on shorter time scales.  相似文献   
979.
980.
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