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   18208篇
  免费   476篇
  国内免费   7篇
财政金融   2968篇
工业经济   1065篇
计划管理   2255篇
经济学   3016篇
综合类   152篇
运输经济   100篇
旅游经济   222篇
贸易经济   2251篇
农业经济   619篇
经济概况   1762篇
水利工程   4267篇
信息产业经济   2篇
邮电经济   12篇
  2021年   146篇
  2020年   228篇
  2019年   293篇
  2018年   461篇
  2017年   442篇
  2016年   435篇
  2015年   280篇
  2014年   414篇
  2013年   1754篇
  2012年   599篇
  2011年   688篇
  2010年   497篇
  2009年   512篇
  2008年   564篇
  2007年   619篇
  2006年   608篇
  2005年   507篇
  2004年   530篇
  2003年   330篇
  2002年   322篇
  2001年   335篇
  2000年   332篇
  1999年   337篇
  1998年   329篇
  1997年   293篇
  1996年   262篇
  1995年   268篇
  1994年   267篇
  1993年   267篇
  1992年   313篇
  1991年   287篇
  1990年   246篇
  1989年   232篇
  1988年   208篇
  1987年   200篇
  1986年   197篇
  1985年   270篇
  1984年   291篇
  1983年   277篇
  1982年   246篇
  1981年   238篇
  1980年   224篇
  1979年   216篇
  1978年   208篇
  1977年   174篇
  1976年   170篇
  1975年   193篇
  1974年   146篇
  1973年   154篇
  1972年   119篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
Wastewater treatment and reuse is being emphasized due to the shortage of water sources and the continuous deterioration of the aquatic environment. In this study, a novel sponge bioreactor was studied as a low cost, high efficiency alternative for an attached growth biological system. This was designed by combining of number of sponge trays. This emerging technology has many beneficial properties in wastewater treatment and reuse. The approaches towards the conditions for system design were: (i) selection of sponge types; (ii) selection of sponge shapes; and (iii) selection of designated slope of sponge tray. They were determined through a series of experiments using a laboratory-scale unit with synthetic wastewater. It was then tested with a pilot-scale unit at the predetermined optimum conditions. The results indicate that the highest biomass growth was found at the sponge type with a cell count of 70-90 cells/in(2) (6.45 cm(2)) The relationship between biomass growth and biological oxygen consumption was well established. The prism-shaped sponge (triangular polyurethane sponge of 70-90 cells/in(2) with designated slope of sponge tray at 10 degrees) led to the best performance in terms of both organic and nutrient removal efficiency.  相似文献   
932.
Quantitative analyses of flow and stage data, remote sensing and geographic information systems analysis, and field studies were used to assess the impact of dams and diversions on the point bar habitat of the Sacramento Valley Tiger Beetle (Cicindela hirticollis abrupta). The reaches of interest include sites of known historic populations of C. h. abrupta along the Sacramento River from approximately 8 km north of Colusa southward to the confluence with the Feather River and along the Feather River between Yuba City and its confluence with the Sacramento River. The results from this study show that construction of two major dams has altered flows such that prolonged and increased flows during summer, fall, and early winter have most likely disrupted life cycles, flooded larvae, drowned overwintering adults and led to high mortality. Additionally, habitat availability has decreased over time because point bars have decreased in number and area causing increased distances between populations and isolation of populations. Moreover, point bar armouring, channel scouring, altered flows, redirected flows through weirs, and lithologic controls have produced a bimodal distribution of mean grain sizes in the Sacramento River in which the more northern bars contain gravel deposits and more southern bars possess fine sands. These conditions negatively alter moisture retention and sediment compaction and, consequently, burrowing conditions needed by this tiger beetle. Additionally, more stabilized flows (reduced variability) and increased fine‐grained deposition have enabled development and encroachment of vegetation onto the sand bars. Finally, human stresses, such as foot traffic and vehicular traffic may have interfered with burrowing, ovipositing, and foraging. The combination of these stress factors has most likely led to a reduction in source populations and, ultimately, the apparent extirpation of the entire metapopulation. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   
933.
Processes affecting fish populations range in scale from local to global. Fish response is also scale‐dependent, with some activities varying locally while others depend on large‐scale connectivity within or between watersheds. These issues are still only partly recognized, with large‐scale research often affected by non‐independent sampling, weak inference, poor model testing or model over‐extrapolation. Available multi‐scale studies can reach different conclusions about factors affecting fishes from local studies, but results vary between examples. Potential explanations are (i) effects on fishes are context‐dependent; (ii) different species or life‐stages are limited in different ways; (iii) multi‐scale studies are too few for generalization. We advocate improved use of geostatistical tools to guide sampling or interpret the spatial extent of management problems, and we illustrate this using brown trout in Welsh streams. Our strongest recommendation is that fish ecologists recognize the importance of interactions across scales in quantifying effects on fishes so that management decisions can be better based on evidence rather than judgement or extrapolation. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   
934.
Disturbance of the grounding of the shaft of a turboset or its absence for technical reasons can result in failure, misoperation, limitation of the operating zone, errors or limitation of the sensitivity of devices for control and protection of exciting circuits from single-point leakage, and electroerosion of elements of the turboset structure. Measures for increasing the reliability of the grounding of the shaft are discussed. __________ Translated from élektricheskie Stantsii, No. 3, March 2006, pp. 61–66.  相似文献   
935.
Operation of the dam in a transitional mode is described. __________ Translated from Gidrotekhnicheskoe Stroitel’stvo, No. 1, January 2006, pp. 8–15.  相似文献   
936.
Problems arising with connecting the second exhaust-heat boiler to the first exhaust-heat boiler under load in the case of flow circuits of combined-cycle plants of type PGU-450 are considered. Similar problems exist for two-boiler single-turbine units of type PGU-325 and for two-boiler single-turbine units of traditional steampower plants with an output of 200–300 MW upon connection of the second furnace of a two-furnace boiler to the operating first furnace. Analysis of the temperature-induced stressed state and of the cyclic strength of thick-walled elements of the steam-water conduit of PGU-450 and of two-furnace boilers of two-boiler single-turbine units with an output of 200–300 MW (the check valve, the inlet header of the water economizer) is used for determining the steam temperature differences in the high-pressure conduits to the steam turbine and the order of connection of the second exhaust-heat boiler to the operating fist one. Modes of enabling a two-boiler single-turbine unit of a combined-cycle plant, which are most “rigid” from the standpoint of thermal strength, are determined. __________ Translated from élektricheskie Stantsii, No. 2, February 2006, pp. 11–17.  相似文献   
937.
Problems, methods, and results of instrumental seismic risk microzoning (SRM) of areas of hydrotechnical construction are considered. New approaches are suggested for processing and analyzing the data, which are also applicable for practical SRM of inhabited localities and civil and industrial construction sites. __________ Translated from Gidrotekhnicheskoe Stroitel’stvo, No. 11, November 2005, pp. 26–33.  相似文献   
938.
Results of a study of tube specimens cut from hot-water boilers show that film-forming octadecyl amine (ODA) used in the process of preservation removes deposition from the surface of the metal. An ODA film is preserved on the surface after repeated washing of the latter with hot water. The concentration of chlorides at the surface of the metal after treatment with ODA is lower than before the treatment. __________ Translated from élektricheskie Stantsii, No. 11, November 2005, pp. 15–18.  相似文献   
939.
In recent months, strong global growth, rebounding commodity prices, and relatively accommodative financial conditions have benefited the Indonesian economy. The first quarter of 2017 in Indonesia saw resilient GDP growth, moderate inflation, stable exchange rates, an increase in the growth of non-oil exports, and an investment upgrade from ratings agency Standard & Poor's. Investment growth, however, did not pick up enough to drive overall growth to a higher rate. The poor quality of banking-sector assets and the gaps in tax revenue—despite the fulfilment of the government's tax-amnesty program—are two of the most immediate economic concerns. President Joko Widodo (Jokowi), who is well into the second half of his term, is under pressure to deliver on his development platform, which includes making progress in sustainable development and climate change mitigation. The effective management of forests is key to this platform. There has been longstanding tension over Indonesia's forests between the protection of environmental values, including carbon storage, and the production of valuable commodities, including timber, palm oil, and pulpwood, which generate revenue and employment. We survey recent developments in four storylines related to forestry and climate change: first, Indonesia's commitment to reducing emissions to 29%–41% below projected business-as-usual levels by 2030, as well as the international climate agreements and finance that can help achieve this commitment; second, land-use rights and regulations, including a moratorium on clearing, draining, or setting fires on peatland; third, measures to prevent catastrophic forest fires like those during the 2015 El Niño, including the establishment of the Peatland Restoration Agency; and, fourth, the actions of non-state actors, especially large agribusinesses, in managing forests and peatland. We conclude by discussing differences in the approaches of Jokowi's administration and those of former president Susilo Bambang Yudhoyono's administration and by questioning whether Indonesia's budgeted resources, actions, and results to date are commensurate with its climate commitments.  相似文献   
940.
Using 2013–2015 National Health Interview Survey data, we reproduce a well‐documented finding that self‐identified lesbians earn significantly more than comparable heterosexual women. These data also show — for the first time in the literature — that self‐identified gay men also earn significantly more than comparable heterosexual men, a difference on the order of 10% of annual earnings. We discuss several possible explanations for the new finding of a gay male earnings premium and suggest that reduced discrimination and changing patterns of household specialization are unlikely to be the primary mechanisms.  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