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   7482篇
  免费   255篇
  2020年   94篇
  2019年   110篇
  2018年   159篇
  2017年   175篇
  2016年   196篇
  2015年   153篇
  2014年   145篇
  2013年   2014篇
  2012年   212篇
  2011年   228篇
  2010年   200篇
  2009年   213篇
  2008年   245篇
  2007年   173篇
  2006年   138篇
  2005年   108篇
  2004年   106篇
  2003年   81篇
  2002年   89篇
  2001年   141篇
  2000年   102篇
  1999年   104篇
  1998年   74篇
  1997年   67篇
  1996年   81篇
  1995年   71篇
  1994年   70篇
  1993年   82篇
  1992年   72篇
  1991年   67篇
  1990年   56篇
  1989年   71篇
  1988年   69篇
  1987年   83篇
  1986年   64篇
  1985年   83篇
  1984年   83篇
  1983年   119篇
  1982年   85篇
  1981年   88篇
  1980年   96篇
  1979年   72篇
  1978年   85篇
  1977年   95篇
  1976年   88篇
  1975年   69篇
  1974年   57篇
  1973年   55篇
  1972年   54篇
  1971年   61篇
排序方式: 共有7737条查询结果,搜索用时 31 毫秒
61.
62.
An attempt has been made to relate steatite artifacts to the source of rock from which they were manufactured, using the distribution patterns of rare earth elements. Radiochemical- and instrumental-neutron activation analyses of Shetland steatites suggest that the use of rare earth elements in artifact provenancing may be unreliable. Concentrations of rare earth elements in many Shetland steatites are too low to be measured reliably by a cost effective method. The relative importance of primary igneous and secondary metasomatic processes in controlling steatite rare earth element geochemistry is discussed and geological arguments against the existence of unique rare earth ‘fingerprints’ for steatite are presented.  相似文献   
63.
A simulation model of a multiregional labor migration process is developed. Information about employment conditions in alternative regions is transmitted in a combination of three channels. The adopted emission policies directly control the amount of information available to potential migrants which, in turn, determines the potential migrant's perception of the level of welfare in the other regions. The effect of several different advertising policies on migration streams is examined. Employers can pursue a number of policies designed to maximize their return. Other agencies adopt policies that facilitate the matching process between unemployed workers and vacant jobs. Results are presented for a number of simulation experiments.  相似文献   
64.
65.
66.
67.
Digital analysis of Landsat data is applied to up-dating land cover maps for the Goondiwindi district of southern, inland Queensland. The imagery was paired with climax cover maps obtained from land systems reports with analysis conducted separately for each of three climax cover types (open forest, woodland and open woodland). Computer classification using this approach produced readily interpreted and reliable results. This work demonstrates that Landsat can be used in conjunction with existing, land systems derived maps to produce data that combine the advantages of two approaches to environmental survey, the former being based on current land cover/land use and the latter on the original environment and its land use potential.  相似文献   
68.
A Roman wreck, named Plemmirio B[1]i, lies on the rocky talus below the southern cliffs of Capo Murro di Porco, near Costa Bianca del Plemmirio (Penisola della Maddalena), prov. Siracusa, Sicily. The cargo of amphoras and iron bars, relatively well-preserved, is situated between 22 m and 47 m depth. Following brief examinations in 1974–1982, a University of Bristol expedition spent four weeks surveying the site in July-August 1983[2] This paper presents an interim summary of these campaigns; a comprehensive report will be published after completion of excavations at the site[3]. The archaeological deposit at Plemmirio B is characterized by concentrations of fragmented amphoras. There is no direct evidence for the structure of the ship, much of which may have disintegrated during the initial wreck process, but study of the artefact distribution suggests a vessel of fairly large capacity. The amphora consignment (which may have numbered no more than 200) comprised cylindrical African containers, 80% of which were form Africana 2A and 20% Africana 1[4] w. The Africana 2A amphoras had internal resinous linings, and so may have contained a fish product rather than olive oil. Other amphoras exposed on the site were at least one Mauretanian Dressel 30 (Keay type 1) and two possibly intrusive amphoras of unclassified types. Ferrous concretions contained voids (hollow casts) which may once have been about 39 wrought iron bars, of two distinctive shapes. These, and several other concretions of unidentified forms, may represent a consignment of iron originally weighing approximately one tonne. Other finds from the wreck are two cooking pots, a small bowl or cup, a sounding lead, three fragments of tegula rooftiles, and two small stone blocks. One of the cooking pots confirms a date for the wreck based on the amphora association of circa AD 180–250, most likely in the first decade of the 3rd century. The amphora assemblage belongs to an important phase of commerce from North Africa represented by at least 20 known wreck cargoes; however, few of these sites have been scientifically recorded or are as closely dated as Plemmirio B, and the diversity of container types and cargo consignments on this wreck is of particular interest.  相似文献   
69.
70.
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