全文获取类型
收费全文 | 6497篇 |
免费 | 240篇 |
专业分类
公路运输 | 1474篇 |
综合类 | 1932篇 |
水路运输 | 1945篇 |
铁路运输 | 848篇 |
综合运输 | 538篇 |
出版年
2024年 | 10篇 |
2023年 | 37篇 |
2022年 | 141篇 |
2021年 | 255篇 |
2020年 | 147篇 |
2019年 | 87篇 |
2018年 | 403篇 |
2017年 | 380篇 |
2016年 | 346篇 |
2015年 | 151篇 |
2014年 | 245篇 |
2013年 | 303篇 |
2012年 | 405篇 |
2011年 | 630篇 |
2010年 | 561篇 |
2009年 | 395篇 |
2008年 | 498篇 |
2007年 | 512篇 |
2006年 | 349篇 |
2005年 | 303篇 |
2004年 | 127篇 |
2003年 | 132篇 |
2002年 | 63篇 |
2001年 | 71篇 |
2000年 | 43篇 |
1999年 | 29篇 |
1998年 | 25篇 |
1997年 | 23篇 |
1996年 | 14篇 |
1995年 | 8篇 |
1994年 | 13篇 |
1993年 | 9篇 |
1992年 | 8篇 |
1991年 | 4篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1984年 | 1篇 |
排序方式: 共有6737条查询结果,搜索用时 125 毫秒
941.
贺斯行急性超容性血液稀释时对机体血液动力学及凝血功能的影响 总被引:5,自引:0,他引:5
目的 观察羟乙基淀粉 (6 %贺斯 2 0 0 0 .5 ,HES)行急性超容性血液稀释 (AHHD)时血液动力学、血气及凝血功能的变化。方法 选择ASAⅠ -Ⅱ级在硬外复合浅全麻下行食道癌根治及全髋置换手术患者 5 0例 ,麻醉前 30~4 0min内输入HES(15mL·kg- 1 ) ,使血容量增加 2 0 %。当Hb低于 90 g·L- 1 时输入同型异体血。术中连续监测HR、BP、SpO2 ,于HD前 (H0 )、HD后 15min(H1 )、输入同型异体血前 (H2 )以及术毕 (H3)各时点抽取动、静脉血行血气分析、血常规及凝血功能检测。结果 HR、BP在整个手术过程中无明显变化 (P >0 .0 5 ) ;Hb、Hct在HD后各时点均降低 (P <0 .0 5或P <0 .0 1)。PT在HD后H1 、H2 ,FIB在H2 时均明显降低 (P <0 .0 5或P <0 .0 1)。结论 采用HES行AHHD对心、肺、肝、肾及凝血功能正常患者行血液保护是安全有效的 ,可避免异体血不必要的输入 相似文献
942.
三种路面破损分类算法的仿真比较 总被引:2,自引:0,他引:2
路面破损分类是制约路面破损自动检测的发展和普及的重要因素之一。本文在前文提出的破损密度方法的基础上,提炼出两种基于密度因子方法的算法——基本密度因子算法和方向密度因子算法,并通过仿真,验证了其对常见的5种路面破损类型进行分类的可行性。文中作为对照的方法是由美国Lee,Byoung Jik博士提供的PROXIMITY算法。仿真结果表明:1)三种路面破损分类算法中,方向密度因子算法对5种常见路面破损类型的整体识别效果最好;2)对于常规算法最难区分的块状裂缝和龟状裂缝,基本密度因子的算法的识别效果最好。这充分验证了前文中我们提出的基于破损密度因子的路面破损分类方法的可行性和优越性。 相似文献
943.
944.
目的设计构建趋化因子受体4(CXCR4)的RNA干扰逆转录病毒载体,探讨干扰CXCR4表达对前列腺癌细胞侵袭能力的影响。方法用PCR扩增CXCR4特异性小干扰RNA(smallinterferingRNA,siRNA),转入带有增强型绿色荧光蛋白(EGFP)和启动子U6的Pgensil-1质粒中,再将重组基因片段导入到逆转录病毒真核表达载体PLX-SN中,行限制性酶切及测序鉴定。重组逆转录病毒载体转染包装细胞PA317,获取病毒上清转染前列腺癌细胞,RT-PCR检测CXCR4mRNA表达。Transwell小室侵袭实验检测前列腺癌细胞体外侵袭能力的变化。结果通过酶切及测序鉴定,成功构建了PLXSN/EGFP-U6-siCXCR4。干扰质粒抑制CXCR4mRNA的表达,与空载体组比较,PC-3m、LNCaPsiRNA对CXCR4mRNA的抑制率分别为(84.26±10.20)%和(88.17±11.23)%。Transwell侵袭实验结果显示,干扰组微膜下孔细胞数PC-3m、LNCaP分别为(14.7±3.1)和(18.9±4.2)个,空载体组分别为(46.9±5.3)和(66.7±6.0)个,两者差异显著(P<0.05)。结论PLXSN/EGFP-U6-siCXCR4可以有效地抑制前列腺癌细胞CXCR4mRNA的表达,干扰CXCR4可以显著抑制前列腺癌细胞体外侵袭转移能力。 相似文献
945.
A series of Pd/Co3O4 catalysts were prepared by Self-Propagating High-Temperature Synthesis (SHS) method in this study, and electric field was applied for catalytic combustion of lean methane over Pd/Co3O4 catalysts at low temperature. When electric field was applied, the catalytic combustion performance of Pd/Co3O4 catalysts was greatly improved, and the application of electric field could reduce the load of active element Pd to some extent while maintaining the same efficiency. Based on experimental tests and the analysis results of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H2-temperature-programmed reduction (H2-TPR) and in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS), the mechanism of catalytic oxidation of CH4 over Pd/Co3O4 catalysts in electric field was proposed. The catalytic combustion of CH4 occurs only when the temperature is higher than 250 °C normally, but when electric field was applied, the whole process of CH4 oxidation was promoted significantly and the reaction temperature was reduced. Electric field could promote the reduction of the support Co3O4 to release the lattice oxygen, resulting in the increase of PdOx and the surface chemisorbed oxygen, which could provide more active sites for the low-temperature oxidation of CH4. Furthermore, electric field could accelerate the dehydroxylation of CoOOH to further enhance the activity of the catalysts. 相似文献
946.
Sepsisischaracterizedbyreleaseofexcessiveamountofproinflammatorycytokinesinthecircu lation ,leadingtoa generalizedanduncontrolledhostresponse .Aconsiderablenumberofexperimen talandclinicalinvestigatorshavereportedemploy inghemofiltration (HF)toeliminatecytokinesandchemokines .Thisexcessiveinflammatoryresponsecouldbedownregulatedwithremovalofcytokinesandothermediators .Thesedatashowedthatthemechanismofmediatorseliminationwaslinkedtoconvectionandadsorptionbyhemofiltration .Ad sorptionappearstob… 相似文献
947.
本文提出了一种分析已有公交专用道运行状态的方法.通过计算公交车道和社会车道上的饱和度,来确定公交车道和社会车道的服务水平,并结合公交专用道上各车型所占比例,分析公交专用道的运行情况及其对社会车道的影响,指出存在的问题和可能的改进措施. 相似文献
948.
提出了在用Arena软件对供应链进行仿真建模研究中的一些思路,通过计算机对供应链系统的仿真分析,不仅可以使得管理人员对由于从销售端到供应端的需求变化引起的“牛鞭效应”进行较为深入和直观的分析,同时也可以在模型中快速对不同物流管理策略和方案进行测评.整个供应链和供应链中各环节的平均库存水平和服务水平是所介绍的模型中的主要供应链绩效指标.文中采用了由供应商、制造商、分销商、零售商和客户所组成的传统供应链结构进行仿真建模,同时介绍了完成的第一个Arena仿真模型. 相似文献
949.
An asynchronous wrapper with novel handshake circuits for data communication in globally asynchronous locally synchronous (GALS) systems is proposed. The handshake circuits include two communication ports and a local clock generator. Two approaches for the implementation of communication ports are presented, one with pure standard cells and the others with Mttller-C elements. The detailed design methodology for GALS systems is given and the circuits are validated with VHDL and circuits simulation in standard CMOS technology. 相似文献
950.
王英林 《上海交通大学学报(英文版)》2016,21(6):692-701
Nowadays, software requirements are still mainly analyzed manually, which has many drawbacks (such as a large amount of labor consumption, inefficiency, and even inaccuracy of the results). The problems are even worse in domain analysis scenarios because a large number of requirements from many users need to be analyzed. In this sense, automatic analysis of software requirements can bring benefits to software companies. For this purpose, we proposed an approach to automatically analyze software requirement specifications (SRSs) and extract the semantic information. In this approach, a machine learning and ontology based semantic role labeling (SRL) method was used. First of all, some common verbs were calculated from SRS documents in the E-commerce domain, and then semantic frames were designed for those verbs. Based on the frames, sentences from SRSs were selected and labeled manually, and the labeled sentences were used as training examples in the machine learning stage. Besides the training examples labeled with semantic roles, external ontology knowledge was used to relieve the data sparsity problem and obtain reliable results. Based on the SemCor and WordNet corpus, the senses of nouns and verbs were identified in a sequential manner through the K-nearest neighbor approach. Then the senses of the verbs were used to identify the frame types. After that, we trained the SRL labeling classifier with the maximum entropy method, in which we added some new features based on word sense, such as the hypernyms and hyponyms of the word senses in the ontology. Experimental results show that this new approach for automatic functional requirements analysis is effective. 相似文献