全文获取类型
收费全文 | 6875篇 |
免费 | 109篇 |
专业分类
公路运输 | 2005篇 |
综合类 | 1307篇 |
水路运输 | 1965篇 |
铁路运输 | 1025篇 |
综合运输 | 682篇 |
出版年
2023年 | 31篇 |
2022年 | 116篇 |
2021年 | 144篇 |
2020年 | 128篇 |
2019年 | 64篇 |
2018年 | 108篇 |
2017年 | 101篇 |
2016年 | 130篇 |
2015年 | 122篇 |
2014年 | 235篇 |
2013年 | 537篇 |
2012年 | 419篇 |
2011年 | 509篇 |
2010年 | 476篇 |
2009年 | 490篇 |
2008年 | 479篇 |
2007年 | 604篇 |
2006年 | 569篇 |
2005年 | 372篇 |
2004年 | 129篇 |
2003年 | 107篇 |
2002年 | 84篇 |
2001年 | 105篇 |
2000年 | 122篇 |
1999年 | 79篇 |
1998年 | 64篇 |
1997年 | 61篇 |
1996年 | 56篇 |
1995年 | 69篇 |
1994年 | 44篇 |
1993年 | 45篇 |
1992年 | 26篇 |
1991年 | 30篇 |
1990年 | 34篇 |
1989年 | 20篇 |
1988年 | 20篇 |
1987年 | 16篇 |
1986年 | 16篇 |
1985年 | 20篇 |
1984年 | 21篇 |
1983年 | 14篇 |
1982年 | 22篇 |
1981年 | 27篇 |
1980年 | 18篇 |
1979年 | 33篇 |
1978年 | 12篇 |
1977年 | 18篇 |
1976年 | 8篇 |
1975年 | 10篇 |
1974年 | 8篇 |
排序方式: 共有6984条查询结果,搜索用时 15 毫秒
991.
992.
由于货物导致的船损事故主要有火灾、船体破损、船舶倾覆、船体腐蚀等,这类事故的共同特点是起因的隐蔽性、损坏的永久性、事故的可预防性等。之所以会产生由于货物导致的船损事故,主要是由于货物的积载、管理不当,以及外界环境,船舶条件的影响和限制,但人的因素为主要原因。 相似文献
993.
Hydrographic cruises off northwest Africa: the Canary Current and the Cape Ghir region 总被引:3,自引:0,他引:3
J.L. Pelegrí A. Marrero-Díaz A. Ratsimandresy A. Antoranz J. Cisneros-Aguirre C. Gordo D. Grisolía A. Hernndez-Guerra I. Liz A. Martínez G. Parrilla P. Prez-Rodríguez A. Rodríguez-Santana P. Sangr 《Journal of Marine Systems》2005,54(1-4):39
We present hydrographic data for several sections located along the African coastline and off Cape Ghir, carried out at times of weak surface winds (October 1995 and September 1997). The main sections are near the continental slope, at mean distances between 100 and 150 km from the coastline. North of Cape Ghir (31°N) the geostrophic transport (referenced to 650 m) of North Atlantic Central Water through these sections is 3.7 and 2.0 Sv for 1995 and 1997, respectively. This confirms that a major fraction of the water transport by the Canary Current flows east, into the continental slope off northwest Africa, at latitudes above Cape Ghir. Most of this flow continues south past Cape Ghir, along the coast and probably through the eastern passages of the Canary Archipelago. A significant fraction, however, may escape through surface Ekman transport (0.3–0.5 Sv during the early fall season) and by offshore flow at Cape Ghir (1.1 Sv in September 1997, referenced to 650 m). Despite the weak winds the Cape Ghir filament was clearly visible, characterized by localized coastal upwelling associated to a cyclonic shallow structure and cold (and fresh) waters stretching offshore as a very shallow feature (50–100 m deep). The satellite images show that the surface temperature field is highly variable, in rapid response to the surface winds, always with a core region of relatively cold water and commonly with one or two associated eddies. Our results support the existence of two recirculation cells in the area: a horizontal one that connects the interior eastern boundary currents with the coastal region and a vertical one related to both wind-induced and filament upwelling. The data also show a salinity subsurface maximum at the root of the filament, linked to water inflow from northern latitudes, and a subsurface anticyclonic eddy over the Agadir canyon, likely related to the poleward slope undercurrent. 相似文献
994.
Management of coastal eutrophication: Integration of field data, ecosystem-scale simulations and screening models 总被引:5,自引:0,他引:5
A.M. Nobre J.G. Ferreira A. Newton T. Simas J.D. Icely R. Neves 《Journal of Marine Systems》2005,56(3-4):375-390
A hybrid approach for eutrophication assessment in estuarine and coastal ecosystems is presented. The ASSETS screening model (http://www.eutro.org) classifies eutrophication status into five classes: High (better), Good, Moderate, Poor and Bad (worse). This model was applied to a dataset from a shallow coastal barrier island system in southwest Europe (Ria Formosa), with a resulting score of Good. A detailed dynamic model was developed for this ecosystem, and the outputs were used to drive the screening model. Four scenarios were run on the research model: pristine, standard (simulates present loading), half and double the current nutrient loading. The Ria Formosa has a short water residence time and eutrophication symptoms are not apparent in the water column. However, benthic symptoms are expressed as excessive macroalgal growth and strong dissolved oxygen fluctuations in the tide pools. The standard simulation results showed an ASSETS grade identical to the field data application. The application of the screening model to the other scenario outputs showed the responsiveness of ASSETS to changes in pressure, state and response, scoring a grade of High under pristine conditions, Good for half the standard scenario and Moderate for double the present loadings. The use of this hybrid approach allows managers to test the outcome of measures against a set of well-defined metrics for the evaluation of state. It additionally provides a way of testing and improving the pressure component of ASSETS. Sensitivity analysis revealed that sub-sampling the output of the research model at a monthly scale, typical for the acquisition of field data, may significantly affect the outcome of the screening model, by overlooking extreme events such as occasional night-time anoxia in tide pools. 相似文献
995.
996.
997.
998.
999.
LiuZhengwen(刘正稳);ZhangChengwen(张成文);LiuQingzhen(刘清珍);HanSuxia(韩苏夏);WuSuge(吴素歌)STUDIESONDYNAMICCHANGESANDCOMPARISONSOFSERUMSPE... 相似文献
1000.
内燃机连杆设计计算的若干理论研究 总被引:2,自引:0,他引:2
吴昌华 《大连铁道学院学报》1994,15(1):46-50
在对多种连杆大量计算的基础上,总结了连杆设计的经验,指出连杆大端对称于中心线的两侧刚度分布均匀,是保证其应力分布均匀的前提,是连杆合理设计的一个重要原则.文中提出,为了保证精度,连杆计算应采用多体接触模型,并对影响计算精度的各种因素做了分析。讨论了螺栓位置和预紧力应如何选取以及齿形制造误差对强度的影响。 相似文献