全文获取类型
收费全文 | 192083篇 |
免费 | 18645篇 |
国内免费 | 10456篇 |
专业分类
电工技术 | 14413篇 |
技术理论 | 8篇 |
综合类 | 19245篇 |
化学工业 | 21085篇 |
金属工艺 | 8036篇 |
机械仪表 | 18099篇 |
建筑科学 | 21081篇 |
矿业工程 | 7084篇 |
能源动力 | 8855篇 |
轻工业 | 13810篇 |
水利工程 | 7849篇 |
石油天然气 | 8746篇 |
武器工业 | 2550篇 |
无线电 | 10762篇 |
一般工业技术 | 20058篇 |
冶金工业 | 7466篇 |
原子能技术 | 2811篇 |
自动化技术 | 29226篇 |
出版年
2024年 | 755篇 |
2023年 | 2449篇 |
2022年 | 5032篇 |
2021年 | 5961篇 |
2020年 | 6080篇 |
2019年 | 5127篇 |
2018年 | 4972篇 |
2017年 | 6139篇 |
2016年 | 7293篇 |
2015年 | 7632篇 |
2014年 | 12307篇 |
2013年 | 12544篇 |
2012年 | 14061篇 |
2011年 | 15704篇 |
2010年 | 11157篇 |
2009年 | 11519篇 |
2008年 | 10714篇 |
2007年 | 12875篇 |
2006年 | 11184篇 |
2005年 | 9328篇 |
2004年 | 7929篇 |
2003年 | 6748篇 |
2002年 | 5452篇 |
2001年 | 4516篇 |
2000年 | 3847篇 |
1999年 | 3264篇 |
1998年 | 2795篇 |
1997年 | 2338篇 |
1996年 | 1937篇 |
1995年 | 1603篇 |
1994年 | 1403篇 |
1993年 | 1067篇 |
1992年 | 969篇 |
1991年 | 726篇 |
1990年 | 592篇 |
1989年 | 540篇 |
1988年 | 432篇 |
1987年 | 285篇 |
1986年 | 248篇 |
1985年 | 232篇 |
1984年 | 272篇 |
1983年 | 253篇 |
1982年 | 210篇 |
1981年 | 99篇 |
1980年 | 95篇 |
1979年 | 68篇 |
1978年 | 54篇 |
1977年 | 46篇 |
1976年 | 43篇 |
1959年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 656 毫秒
51.
《International Journal of Hydrogen Energy》2020,45(38):19720-19732
The need to reduce PEMFC systems cost as well as to increase their durability is crucial for their integration in various applications and especially for transport applications. A new simplified architecture of the anode circuit called Alternating Fuel Feeding (AFF) offers to reduce the development costs. Requiring a new stack concept, it combines the simplicity of Dead-End Anode (DEA) with the operation advantages of the hydrogen recirculation. The three architectures (DEA, recirculation and AFF) are compared in terms of performance on a 5-kW test bench in automotive conditions, through a sensitivity analysis. A gain of 17% on the system efficiency is observed when switching from DEA to AFF. Moreover, similar performances are obtained both for AFF and for recirculation after an accurate optimization of the AFF tuning parameters. Based on DoE data, a gain of 25% on the weight of the anodic line has been identified compared to pulsed ejector architecture and 43% with the classic recirculation architecture with blower only (Miraï). 相似文献
52.
An integrated approach to measure the cost efficiency of the postal network of Universal Service Provider is proposed. An integrated approach enables the measurement of cost efficiency for delivery and non-delivery postal network units. The proposed approach is verified and tested on the postal network of the selected provider and the results were derived by using Data Envelopment Analysis (DEA). The results show that the main sources of inefficiency are inadequate allocation of resources relative to the network units. In addition, the study indicates that economies of scale have a positive impact on the efficiency of postal network units. 相似文献
53.
54.
This paper deals with the application of wavelet transforms for the detection, classification and location of faults on transmission lines. A Global Positioning System clock is used to synchronize sampling of voltage and current signals at both the ends of the transmission line. The detail coefficients of current signals of both the ends are utilized to calculate fault indices. These fault indices are compared with threshold values to detect and classify the faults. Artificial Neural Networks are employed to locate the fault, which make use of approximate decompositions of the voltages and currents of local end. The proposed algorithm is tested successfully for different locations and types of faults. 相似文献
55.
AbstractRecently, a literature has emerged using empirical techniques to study the evolution of international cities over many centuries; however, few studies examine long-run change within cities. Conventional models and concepts are not always appropriate and data issues make long-run neighbourhood analysis particularly problematic. This paper addresses some of these points. First, it discusses why the analysis of long-run urban change is important for modern urban policy and considers the most important concepts. Second, it constructs a novel data set at the micro level, which allows consistent comparisons of London neighbourhoods in 1881 and 2001. Third, the paper models some of the key factors that affected long-run change, including the role of housing. There is evidence that the relative social positions of local urban areas persist over time but, nevertheless, at fine spatial scales, local areas still exhibit change, arising from aggregate population dynamics, from advances in technology, and also from the effects of shocks, such as wars. In general, where small areas are considered, long-run changes are likely to be greater, because individuals are more mobile over short than long distances. Finally, the paper considers the implications for policy. 相似文献
56.
Controllability and observability of multi-agent systems with heterogeneous and switching topologies
ABSTRACTThis paper focuses on controllability and observability of multi-agent systems with heterogeneous and switching topologies, where the first- and the second-order information interaction topologies are different and switching. First, based on the controllable state set, a controllability criterion is obtained in terms of the controllability matrix corresponding to the switching sequence. Next, by virtue of the subspace sequence, two necessary and sufficient algebraic conditions are established for controllability in terms of the system matrices corresponding to all the possible topologies. Furthermore, controllability is considered from the graphic perspective. It is proved that the system is controllable if the union graph of all the possible topologies is controllable. With respect to observability, two sufficient and necessary conditions are derived by taking advantage of the system matrices and the corresponding invariant subspace, respectively. Finally, some simulation examples are worked out to illustrate the theoretical results. 相似文献
57.
An intertwined supply network (ISN) is an entirety of interconnected supply chains (SC) which, in their integrity secure the provision of society and markets with goods and services. The ISNs are open systems with structural dynamics since the firms may exhibit multiple behaviours by changing the buyer-supplier roles in interconnected or even competing SCs. From the positions of resilience, the ISNs as a whole provide services to society (e.g. food service, mobility service or communication service) which are required to ensure a long-term survival. The analysis of survivability at the level of ISN requires a consideration at a large scale as resilience of individual SCs. The recent example of coronavirus COVID-19 outbreak clearly shows the necessity of this new perspective. Our study introduces a new angle in SC resilience research when a resistance to extraordinary disruptions needs to be considered at the scale of viability. We elaborate on the integrity of the ISN and viability. The contribution of our position study lies in a conceptualisation of a novel decision-making environment of ISN viability. We illustrate the viability formation through a dynamic game-theoretic modelling of a biological system that resembles the ISN. We discuss some future research areas. 相似文献
58.
59.
The lack of consensus on the meaning of energy security and how it should be measured hinders the evaluation of alternatives for improving the Cape Verdean energy sector. The objective of the paper is to develop an energy security index adjusted to the country's context. The methods used are the multi-criteria analysis and the Delphi survey. Different alternatives to improve energy security are assessed: electrically interconnect Cape Verde's islands, increase the share of renewable energy, and a combination of the previous alternatives. The results of our analysis indicate that increasing renewable energy is the optimal alternative for Cape Verde. 相似文献
60.
为研究喷射混凝土套拱加固前后二次衬砌与混凝土套拱的受力状况,依托陕西汉中至留坝段八里关隧道,运用有限元软件建立隧道混凝土结构套拱加固分析模型,并通过现场监测获取二次衬砌与套拱间的接触压力、套拱格栅拱架钢筋应力、套拱混凝土应力,将数值模拟结果与现场测试结果相结合,得出套拱结构的一般受力变化规律。结果表明:套拱加固前衬砌结构的最不利荷载位置位于施工缝附近的拱顶、拱肩与拱脚处; 由于衬砌局部变形与温度应力的影响,套拱混凝土应力变化呈现反复“上升-下降-上升”最后趋于稳定的特点; 套拱的作用是控制二次衬砌的进一步变形,套拱施作后所承受荷载较小,套拱反作用力远小于围岩作用于衬砌的应力,在应力计算中不应将衬砌与套拱作为整体计算; 套拱结构数值模拟所得的结果与现场测试套拱结构内力(轴力、弯矩)的大小及分布相似度高,但在衬砌裂损严重部位,数值模拟所得结果误差较大,应以现场测试结果为准。 相似文献