全文获取类型
收费全文 | 3489篇 |
免费 | 73篇 |
专业分类
公路运输 | 658篇 |
综合类 | 1062篇 |
水路运输 | 1022篇 |
铁路运输 | 406篇 |
综合运输 | 414篇 |
出版年
2025年 | 1篇 |
2024年 | 21篇 |
2023年 | 25篇 |
2022年 | 54篇 |
2021年 | 62篇 |
2020年 | 55篇 |
2019年 | 33篇 |
2018年 | 367篇 |
2017年 | 315篇 |
2016年 | 262篇 |
2015年 | 57篇 |
2014年 | 84篇 |
2013年 | 85篇 |
2012年 | 155篇 |
2011年 | 341篇 |
2010年 | 366篇 |
2009年 | 202篇 |
2008年 | 270篇 |
2007年 | 251篇 |
2006年 | 127篇 |
2005年 | 138篇 |
2004年 | 70篇 |
2003年 | 88篇 |
2002年 | 43篇 |
2001年 | 28篇 |
2000年 | 17篇 |
1999年 | 4篇 |
1998年 | 12篇 |
1997年 | 5篇 |
1996年 | 7篇 |
1995年 | 3篇 |
1994年 | 4篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1984年 | 1篇 |
排序方式: 共有3562条查询结果,搜索用时 0 毫秒
951.
应用3H-TdR及14C-UR参入淋巴细胞的双标记方法,观察了淋巴细胞转化试验中细胞DNA及RNA的合成功能。结果表明,肿瘤患者在多抗甲素治疗前淋巴细胞的参入计数显著低于正常人,治疗后显著高于治疗前,但仍低于正常人。 相似文献
952.
利用差迷离心法分离提取大鼠心、肝组织富含溶酶体部分,并以溶酶体之标志酶酸性磷酸酶(ACP)及组织蛋白酶D(CD)的游离酶和总酶活性比值为观察溶酶体膜稳定性变化的指标。分别观察了低硒的克山病病区粮及病区粮加亚硒酸钠或蛋氨酸喂养的大鼠之心、肝溶酶体酶(ACD和CD)的活性变化。发现用低的病区粮喂养的大鼠心肝组织溶酶体膜的稳定性下降,而加硒或蛋氨酸后,其稳定性增加。考虑这种变化与大鼠心肝组织中的GSH-P_x活性变化有关。 相似文献
953.
目的探讨糖链抗原50(CA50)在大肠癌及癌旁组织中表达的意义。方法采用免疫组化检测正常大肠黏膜(10例)、大肠癌远旁(40例)、近旁(40例)和癌(40例)组织中CA50的表达。结果CA50阳性表达率按正常大肠黏膜、大肠癌远旁、近旁、癌组织的顺序依次增高,且阳性表达率在4组间有显著性差异(P<0.05)。CA50表达与大肠癌的分化程度、Duke s分期和淋巴结转移有关(P<0.05),与组织学类型无关(P>0.05)。结论CA50的表达对评价大肠癌的生物学特性、判断预后具有重要的临床意义。 相似文献
954.
普通适应值共享的小生境遗传算法是基于群体内个体适应度的共享,但这种小生境划分一般基于个体适应度、基因空间距离等属性,忽视了群体基因特征之间的关系.因此将生物学中的群体间共享机制引入到小生境遗传算法中,有效地利用了群体中的优良因素,并且利用了精英选择机制,增强了算法的全局和局部搜索能力,实验表明,普通适应值共享的小生境遗传算法在搜索能力和收敛性能上更有效. 相似文献
955.
根据船舶减摇鳍控制系统在不同海情下的期望减摇效果,设计了不同的性能指标,运用遗传算法离线优化设计模糊控制器,组建控制系统知识库.控制程序根据船舶实时的运行情况,调用系统知识库中相应的控制器,并在线修正GA-FC控制器的比例因子,进一步增强系统的自适应能力.仿真结果表明,基于遗传模糊算法的船舶减摇鳍在线控制系统,具有很好的减摇控制效果,并减缓了鳍角饱和概率和转鳍角速度,从而降低了鳍伺服系统的功耗. 相似文献
956.
Reliable research synthesis is of high significance for a transport policy which tries to base its decisions on available
empirical evidence. There is growing doubt whether the frequently used narrative approach provides a scientifically defensible
way of synthesising a body of quantitative research results. The present paper demonstrates meta-analysis as a more objective
and powerful alternative for this task. For this purpose evaluation data synthesised in two earlier narrative reviews on the
effectiveness of work travel plans [Cairns et al. Making travel plans work: research report (former), Department for Transport,
Local Government and the Regions (DTLR), London, 2002; Smarter choices—changing the way we travel. (Final report of the research project: The influence of soft factor interventions
on travel demand. Research report for the Department for Transport.) London. Retrieved 1 December 2005, from , 2004] are meta-analytically re-analysed and compared with the conclusions drawn from the narrative synthesis of these data. Our
meta-analyis provides only limited support for the conclusion that addressing parking is the hallmark of successful work travel
plans. Our meta-analysis indicates that site and organisational factors as well as characteristics of the monitoring process
are significant predictors of effective work travel plans, whereas Cairns et al. view little evidence for the impact of these
factors. 相似文献
957.
上市公司信息披露制度是一项复杂的系统工程,由于信息披露法律关系的特殊性,并涉及多方主体的利益,实践中出现的层层问题还是不能得到解决,杭萧钢构事件充分说明了这一点。从理顺信息披露法律制度的基本原理,认识信息披露制度缺陷的角度,探讨信息披露制度的进一步完善。 相似文献
958.
This article concerns thermal radiation hazards associated with unconfined liquefied natural gas (LNG) spills on water. Consequence assessment methods were compared to clarify their model characteristics in large-scale LNG spills from an LNG carrier (LNGC). The consequences of LNG release, pool spread, and pool fire hazards were estimated using the following practical methods: the Federal Energy Regulatory Commission’s (FERC) method, the Sandia National Laboratories’ method, and the Fay method. The sensitivity of consequence analysis results to the breach size of a tank was examined under the assumption that LNG is released from a common type LNGC of 125000 m3 cargo capacity. Consequently, it was found that the FERC method is useful from the practical viewpoint of being applicable to any breach size. Finally, thermal radiation hazards from pool fires involving spills from one of the latest and largest LNGCs (250 000 m3 cargo capacity), which are currently considered for construction, were investigated using the recommended FERC method, and the results are discussed in comparison with those for common type LNGCs. As a result, it was found that the maximum thermal hazard distance is longer by only about 24% compared with the common type LNGC, whereas the spill volume is twice as much. 相似文献
959.
Appropriate modeling of roll damping is one of the key issues in accurately predicting ship roll motion. The difficulties in modeling roll damping arise from the nonlinear nature of the phenomena. In this study, we report a new effort in modeling the bilge keel roll damping effect based on the blocking mechanisms of an object in the potential flow. This effect can be implemented as a component of appropriate ship motion models. We used our digital, self-consistent, ship experimental laboratory (DiSSEL) ship motion model to test its effectiveness in predicting ship roll motion. Our numerical experiment demonstrated clearly that the implementation of this roll damping component improves significantly the accuracy of numerical model results (the results were compared with ship experiment data from the Naval Surface Warfare Center, Carderock Division, Maneuvering and Seakeeping Facility). 相似文献
960.
Philip J. Zwart Philippe G. Godin Justin Penrose Shin Hyung Rhee 《Journal of Marine Science and Technology》2008,13(4):346-355
An accurate, efficient algorithm for solving free surface flows around ship hulls is described. Accuracy is achieved using a compressive advection discretization which maintains a sharp free surface interface representation without relying on a small time step. Efficiency is obtained using a solution algorithm which implicitly couples velocity, pressure, and volume fractions. The algorithm has been implemented in a computational fluid dynamics code called CFX and is validated by means of a comparisonwith experimental data of benchmark cases in both steady and transient conditions. 相似文献