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   31345篇
  免费   889篇
  国内免费   5篇
财政金融   4484篇
工业经济   1749篇
计划管理   4497篇
经济学   6325篇
综合类   269篇
运输经济   229篇
旅游经济   381篇
贸易经济   4632篇
农业经济   1239篇
经济概况   2889篇
水利工程   5461篇
信息产业经济   3篇
邮电经济   81篇
  2023年   219篇
  2021年   302篇
  2020年   459篇
  2019年   650篇
  2018年   884篇
  2017年   947篇
  2016年   925篇
  2015年   577篇
  2014年   850篇
  2013年   3277篇
  2012年   1315篇
  2011年   1228篇
  2010年   986篇
  2009年   1037篇
  2008年   1060篇
  2007年   1057篇
  2006年   1020篇
  2005年   900篇
  2004年   810篇
  2003年   599篇
  2002年   526篇
  2001年   602篇
  2000年   518篇
  1999年   485篇
  1998年   532篇
  1997年   488篇
  1996年   482篇
  1995年   418篇
  1994年   416篇
  1993年   406篇
  1992年   420篇
  1991年   436篇
  1990年   389篇
  1989年   316篇
  1988年   322篇
  1987年   319篇
  1986年   288篇
  1985年   412篇
  1984年   409篇
  1983年   376篇
  1982年   339篇
  1981年   336篇
  1980年   357篇
  1979年   333篇
  1978年   286篇
  1977年   232篇
  1976年   253篇
  1975年   209篇
  1974年   209篇
  1973年   203篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
伊朗卡尔黑多目标水利枢纽,首台机组于2002年9月投运。介绍了工程历史,并从导流系统遂洪道、渗漏控制和惟幕灌浆等方面介绍了工程的技术要求。  相似文献   
12.
An Imhoff tank was reconstructed into a 250 m3 UASB reactor in order to treat a malting plant wastewater. The UASB was inoculated with sludge from an anaerobic lagoon used for slaughterhouse wastewater treatment. After two months of operation the reactor achieved full load with an HRT of 17 h, a COD removal higher than 80% and a biogas production of 300 m3/day (77% average methane content), with an organic loading rate of 3.6 kgCOD/m3.d (0.24 kgCOD/kgVSS.d). A yield coefficient of 0.09 gVSS/gCODrem was found from a mass balance. The fat present in the inoculated sludge (48 mg/gSSV) did not affect the start up performance. Sludge from the inoculum with high content of fat (270 mg/gSSV), was separated by flotation in the first week of operation. The COD removal efficiency was scarcely influenced by the reactor operation temperature (17-25 degrees C).  相似文献   
13.
Anaerobic co-digestion of organic wastes from households, slaughterhouses and meat processing industries was optimised in a half technical scale plant. The plant was operated for 130 days using two different substrates under organic loading rates of 10 and 12 kgCOD.m(-3).d(-1). Since the substrates were rich in fat and protein components (TKN: 12 g.kg(-1) the treatment was challenging. The process was monitored on-line and in the laboratory. It was demonstrated that an intensive and stable co-digestion of partly hydrolysed organic waste and protein rich slaughterhouse waste can be achieved in the balance of inconsistent pH and buffering NH4-N. In the first experimental period the reduction of the substrate COD was almost complete in an overall stable process (COD reduction >82%). In the second period methane productivity increased, but certain intermediate products accumulated constantly. Process design options for a second digestion phase for advanced degradation were investigated. Potential causes for slow and reduced propionic and valeric acid degradation were assessed. Recommendations for full-scale process implementation can be made from the experimental results reported. The highly loaded and stable codigestion of these substrates may be a good technical and economic treatment alternative.  相似文献   
14.
15.
The performance of service industries in Canada has been lower than that of good industries over the last four decades, with noticeable exceptions such as for railways and telecommunication carriers. Service industries were less economically (and technically) efficient in that they generated less output value (quantity) per hour worked (level and growth) or per combined unit of labour and capital (multifactor productivity growth) than good industries. The relative output price of services declined slightly over time compared with goods. At the disaggregated level, changing relative output prices were substantial and proved to be an important factor explaining the relative satisfactory economic performance of many service industries despite their low technical performance. Nevertheless, the output share of service industries increased over that period, sustained, mainly, by the growing recourse of all firms to outsourcing of services.  相似文献   
16.
The Woodman Point Wastewater Treatment Plant (WWTP) in Western Australia has experienced two separate problems causing avoidable maintenance costs: the build-up of massive struvite (MgNH4PO4. 6H2O) scaling downstream of the anaerobic digester and the formation of hydrogen sulfide (H2S) levels in the digester gas to levels that compromised gas engine operation and caused high operating costs on the gas scrubber. As both problems hang together with a chemical imbalance in the anaerobic digester, we decided to investigate whether both problems could be (feasibly and economically) addressed by a common solution (such as dosing of iron solutions to precipitate both sulfide and phosphate), or by using separate approaches. Laboratory results showed that, the hydrogen sulfide emission in digesters could be effectively and economically controlled by the addition of iron dosing. Slightly higher than the theoretical value of 1.5 mol of FeCl3 was required to precipitate 1 mol of dissolved sulfide inside the digester. Due to the high concentration of PO4(3-) in the digested sludge liquor, significantly higher iron is required for struvite precipitation. Iron dosing did not appear an economic solution for struvite control via iron phosphate formation. By taking advantage of the natural tendency of struvite formation in the digester liquid, it is possible to reduce the risk of struvite precipitation in and around the sludge-dewatering centrifuge by increasing the pH to precipitate struvite out before passing through the centrifuge. However, as the Mg2+/PO4(3-) molar ratio in digested sludge was low, by increasing the pH alone (using NaOH) the precipitation of PO4(3-) was limited by the amount of cations (Ca2+ and Mg2+) available in the sludge. Although this would reduce struvite precipitation in the centrifuge, it could not significantly reduce PO4(3-) recycling back to the plant. For long-term operation, maximum PO4(3-) reduction should be the ultimate aim to minimise PO4(3-) accumulation in the plant. Magnesium hydroxide liquid (MHL) was found to be the most cost-effective chemical to achieve this goal. It enhanced struvite precipitation from both, digested sludge and centrate to the point where more than 95% PO4(3-) reduction in the digested sludge was achieved.  相似文献   
17.
Managing sewerage systems is a highly complex task due to the dynamic nature of the facilities. Their performance strongly depends on the know-how applied by the operators. In order to define optimal operational settings, two decision support tools based on mathematical models have been developed. Moreover, easy-to-use interfaces have been created as well, aiding operators who presumably do not have the necessary skills to use modelling software. The two developed programs simulate the behaviour of both wastewater treatment plants (WWTP) and sewer network systems, respectively. They have essentially the same structure, including raw data management and statistical analysis, a simulation layer using the application programming interface of the applied software and a layer responsible for the representation of the obtained results. Four user modes are provided in the two software including the simulation of historical data using the applied and novel operational settings, as well as modes concerning prediction of possible operation periods and updates. Concerning the WWTP software, it was successfully installed in Nantes (France) in June 2004. Moreover, the one managing sewer networks has been deployed in Saint-Malo (France) in January 2005. This paper presents the structure of the developed software and the first results obtained during the commissioning phase.  相似文献   
18.
This article surveys recent research of the Spanish colonial era in the Philippines since the late eighteenth century. While highlighting imperfections in our understanding, the article establishes the parameters with which the Philippine economy entered the twentieth century. It outlines the intensification of Spanish colonial rule through changes in the taxation system, particularly the expansion of forced tobacco cultivation until its abolition in 1882. Since then, the Spanish set out to further change and intensify colonial rule but contradictions in the system of colonial rule caused the effort to come to an abrupt end in 1898.  相似文献   
19.
The number of firms using alliances as part of their corporate venturing or market entry strategies has surged over the past decade. Three common reasons cited for pursuing alliances are technology convergence, market access and alliance partners' complementary resources. This paper contrasts the alliance strategies of HP and IBM, two major competitors in electronic services (i.e. Internet-based 'e- service') businesses, using the Familiarity Matrix as a display tool to portray the strategies. Whereas the HP strategy is to attempt to establish its technology infrastructure as the standard e-services infrastructure on the Internet, IBM aims to position its IBM Global Services, rather than its technology, at the center of this ecosystem.  相似文献   
20.
This paper examines monthly wage data collected by the Agro Economic Survey from seven villages in the Cimanuk River Basin of West Java, where data on agricultural and non-agricultural wages and prices of nine essential commodities were collected twice monthly between 1977 and 1983. Movements in money wages deflated by both the rice price and a weighted index of basic commodities are analysed in detail and differences between lowland and upland villages are discussed.  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