全文获取类型
收费全文 | 1789篇 |
免费 | 249篇 |
专业分类
公路运输 | 376篇 |
综合类 | 366篇 |
水路运输 | 1094篇 |
铁路运输 | 173篇 |
综合运输 | 29篇 |
出版年
2024年 | 21篇 |
2023年 | 19篇 |
2022年 | 35篇 |
2021年 | 42篇 |
2020年 | 76篇 |
2019年 | 57篇 |
2018年 | 55篇 |
2017年 | 74篇 |
2016年 | 50篇 |
2015年 | 64篇 |
2014年 | 137篇 |
2013年 | 139篇 |
2012年 | 201篇 |
2011年 | 162篇 |
2010年 | 129篇 |
2009年 | 120篇 |
2008年 | 108篇 |
2007年 | 141篇 |
2006年 | 128篇 |
2005年 | 79篇 |
2004年 | 64篇 |
2003年 | 28篇 |
2002年 | 21篇 |
2001年 | 27篇 |
2000年 | 17篇 |
1999年 | 15篇 |
1998年 | 6篇 |
1997年 | 2篇 |
1996年 | 7篇 |
1995年 | 4篇 |
1994年 | 2篇 |
1992年 | 3篇 |
1991年 | 1篇 |
1990年 | 3篇 |
1989年 | 1篇 |
排序方式: 共有2038条查询结果,搜索用时 546 毫秒
1.
本文以陕西省引汉济渭工程岭北TBM施工段为依托,为解决TBM在狭小空间内较大部件更换维修困难难题,开展大直径TBM刀盘边块在狭小空间内更换技术研究。通过大量研究,创新性提出新刀盘边块更换方法——基坑法,即通过专用设备配合TBM固有性能,在无扩大洞室工况下完成刀盘边块更换。经岭北TBM刀盘边块更换实例证明,此方法操作简单、适用性高,尤其在狭小空间内更具有优越性,且能节省大量维修时间,有效提高施工工效,可为同类TBM施工提供有益借鉴。 相似文献
2.
ABSTRACT This paper explores the online channel strategies adopted by Chinese manufacturers with regard to the Belt and Road. It also investigates the effect of offline channel power structures and maritime transportation costs on online channel mode selection and pricing strategies. The offline channel power structures are classified into three types, namely, Chinese manufacturer Stackelberg (MS) structure, vertical Nash (VN) structure, and foreign retailer Stackelberg (RS) structure. Furthermore, a game model is developed among the Chinese manufacturer, foreign retailer, and cross-border e-tailer to investigate the interactions among offline channel power structures, maritime transportation costs, and Chinese manufacturers’ online channel mode selection. The results show that Chinese manufacturers’ preferences for the online agency selling mode increases correspondingly with the decrease in their market position and bargaining power. Moreover, foreign retailers can obtain higher profits in the online agency selling mode under the VN structure. However, the level of profit for foreign retailers depends on the intensity of their competition with cross-border e-tailers under the MS and RS structures. Finally, consumers in countries along the Belt and Road can obtain a higher surplus in the online agency selling mode under all of the structures. 相似文献
3.
4.
温州乐清湾港区试挖槽监测分析 总被引:1,自引:0,他引:1
乐清湾避风条件好,潮流动力强,含沙量小,具有较好的建港条件。介绍了乐清湾试挖槽情况,对试挖槽进行了监测分析,指出数据分析表明试挖槽基本处于冲淤阶段,乐清湾港池开挖实行浅水深用的开发方案是可行的。 相似文献
5.
带中间渠道船闸运转方式的试验研究 总被引:1,自引:0,他引:1
带中间渠道的分散梯级船闸是高坝通航建筑物的布置型式之一,它具有调度灵活、通过能力大等特点。但其上下级船闸采用不同运转方式时,所产生的非恒定流水流条件十分复杂,严重时将影响船舶(队)停泊和航行条件,还会对船闸运转及人字闸门产生不利影响。通过1:40的水工概化模型试验,对断面为规则的矩形中间渠道进行了上下级船闸单独、同时与错时灌泄水3种运转方式渠道内的水力特性研究,否定了船闸同时运转方式;对渠长2 000 m和1 400 m在特定条件下的船闸错时运转方式,找出了最佳错开时间;对3种运转方式的水力要素进行综合对比后,最终推荐采用先泄后灌运转方式。 相似文献
6.
本文在分析川江上游运输船舶现状的基础上,提出船型比选的原则,并阐述了对现有船型比选的思路和方法,最后介绍了采用VB开发的计算机比选程序。 相似文献
7.
Organic carbon budget for the Gulf of Bothnia 总被引:1,自引:0,他引:1
Grete Algesten Lars Brydsten Per Jonsson Pirkko Kortelainen Stefan Lfgren Lars Rahm Antti Rike Sebastian Sobek Lars Tranvik Johan Wikner Mats Jansson 《Journal of Marine Systems》2006,63(3-4):155-161
We calculated input of organic carbon to the unproductive, brackish water basin of the Gulf of Bothnia from rivers, point sources and the atmosphere. We also calculated the net exchange of organic carbon between the Gulf of Bothnia and the adjacent marine system, the Baltic Proper. We compared the input with sinks for organic carbon; permanent incorporation in sediments and mineralization and subsequent evasion of CO2 to the atmosphere. The major fluxes were riverine input (1500 Gg C year− 1), exchange with the Baltic Proper (depending on which of several possible DOC concentration differences between the basins that was used in the calculation, the flux varied between an outflow of 466 and an input of 950 Gg C year− 1), sediment burial (1100 Gg C year− 1) and evasion to the atmosphere (3610 Gg C year− 1). The largest single net flux was the emission of CO2 to the atmosphere, mainly caused by bacterial mineralization of organic carbon. Input and output did not match in our budget which we ascribe uncertainties in the calculation of the exchange of organic carbon between the Gulf of Bothnia and the Baltic Proper, and the fact that CO2 emission, which in our calculation represented 1 year (2002) may have been overestimated in comparison with long-term means. We conclude that net heterotrophy of the Gulf of Bothnia was due to input of organic carbon from both the catchment and from the Baltic Proper and that the future degree of net heterotrophy will be sensible to both catchment export of organic carbon and to the ongoing eutrophication of the Baltic Proper. 相似文献
8.
9.
10.
首先论述了导弹旅电子化指挥系统中无线通信的重要性以及无线通信仿真的必要性,给出了几种典型的无线通信的模型,提出了一种信道仿真实现方法。 相似文献