全文获取类型
收费全文 | 561899篇 |
免费 | 50631篇 |
国内免费 | 29404篇 |
专业分类
电工技术 | 46250篇 |
技术理论 | 80篇 |
综合类 | 66129篇 |
化学工业 | 61034篇 |
金属工艺 | 21224篇 |
机械仪表 | 35620篇 |
建筑科学 | 73054篇 |
矿业工程 | 26114篇 |
能源动力 | 30010篇 |
轻工业 | 35499篇 |
水利工程 | 24835篇 |
石油天然气 | 25534篇 |
武器工业 | 5485篇 |
无线电 | 38827篇 |
一般工业技术 | 46554篇 |
冶金工业 | 28141篇 |
原子能技术 | 7239篇 |
自动化技术 | 70305篇 |
出版年
2024年 | 2150篇 |
2023年 | 6827篇 |
2022年 | 13070篇 |
2021年 | 15280篇 |
2020年 | 16101篇 |
2019年 | 13457篇 |
2018年 | 12704篇 |
2017年 | 15521篇 |
2016年 | 18394篇 |
2015年 | 19863篇 |
2014年 | 35010篇 |
2013年 | 32712篇 |
2012年 | 39435篇 |
2011年 | 43042篇 |
2010年 | 32879篇 |
2009年 | 33937篇 |
2008年 | 31432篇 |
2007年 | 38854篇 |
2006年 | 35312篇 |
2005年 | 29892篇 |
2004年 | 25145篇 |
2003年 | 22155篇 |
2002年 | 18283篇 |
2001年 | 15374篇 |
2000年 | 13000篇 |
1999年 | 10657篇 |
1998年 | 8304篇 |
1997年 | 7075篇 |
1996年 | 6302篇 |
1995年 | 5256篇 |
1994年 | 4653篇 |
1993年 | 3437篇 |
1992年 | 3035篇 |
1991年 | 2314篇 |
1990年 | 2015篇 |
1989年 | 1717篇 |
1988年 | 1416篇 |
1987年 | 948篇 |
1986年 | 752篇 |
1985年 | 659篇 |
1984年 | 647篇 |
1983年 | 492篇 |
1982年 | 439篇 |
1981年 | 317篇 |
1980年 | 277篇 |
1979年 | 201篇 |
1978年 | 124篇 |
1977年 | 125篇 |
1976年 | 96篇 |
1975年 | 85篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
Geraint Johnes Swati Virmani 《International Transactions in Operational Research》2020,27(4):1869-1885
Data from the Young Lives study are used to evaluate the efficiency of education systems in four low and middle income countries: Ethiopia, India, Peru and Vietnam. A meta‐frontier variant of data envelopment analysis is used to assess the relative performance of each country's system, and, within each country, to evaluate the impact of public and private schooling, and of urban and rural location. Comparisons are drawn between the four countries; the results indicate that in no country does the educational system perform uniformly badly or well. Conditioning on the inputs available, rural areas are often indicative of higher levels of efficiency, thus suggesting a number of implications for policy. 相似文献
83.
84.
Output feedback controller for polytopic systems exploiting the direct searching of the design space
This paper deals with the problem of designing a robust static output feedback controller for polytopic systems. The current research that tackled this problem is mainly based on LMI method, which is conservative by nature. In this paper, a novel approach is proposed, which considers the design space of the controller parameters and iteratively partitions the space to small simplexes. Then, by assessing the stability in each simplex, the solution space for design parameters is directly determined. It has been theoretically proved that, if there exists a feasible solution in the design space, the algorithm can find it. To validate the result of the proposed approach, comparative simulation examples are given to illustrate the performance of the design methodology as compared to those of previous approaches. 相似文献
85.
Most real-world vehicle nodes can be structured into an interconnected network of vehicles. Through structuring these services and vehicle device interactions into multiple types, such internet of vehicles becomes multidimensional heterogeneous overlay networks. The heterogeneousness of the overlays makes it difficult for the overlay networks to coordinate with each other to improve their performance. Therefore, it poses an interesting but critical challenge to the effective analysis of heterogeneous virtual vehicular networks. A variety of virtual vehicular networks can be easily deployed onto the native network by applying the concept of SDN (Software Defined Networking). These virtual networks reflect their heterogeneousness due to their different performance goals, and they compete for the same physical resources of the underlying network, so that a sub-optimal performance of the virtual networks may be achieved. Therefore, we propose a Deep Reinforcement Learning (DRL) approach to make the virtual networks cooperate with each other through the SDN controller. A cooperative solution based on the asymmetric Nash bargaining is proposed for co-existing virtual networks to improve their performance. Moreover, the Markov Chain model and DRL resolution are introduced to leverage the heterogeneous performance goals of virtual networks. The implementation of the approach is introduced, and simulation results confirm the performance improvement of the latency sensitive, loss-rate sensitive and throughput sensitive heterogeneous vehicular networks using our cooperative solution. 相似文献
86.
Fog computing has emerged to support the requirements of IoT applications that could not be met by today’s solutions. Different initiatives have been presented to drive the development of fog, and much work has been done to improve certain aspects. However, an in-depth analysis of the different solutions, detailing how they can be integrated and applied to meet specific requirements, is still required. In this work, we present a unified architectural model and a new taxonomy, by comparing a large number of solutions. Finally, we draw some conclusions and guidelines for the development of IoT applications based on fog. 相似文献
87.
The aim of this paper is to investigate the long and short-run relationship between spot and futures prices of the energy, precious metals, and base metals markets. We analyze daily data from January 1985 to February 2019. The empirical findings based on the cointegration test, which follows a nonlinear process, suggest that the spot prices of energy and metals assets have long-run relationships with their futures prices. Nonparametric Granger causality test results also indicate bi-directional causality among futures and spot prices. These findings indicate that the energy and metals markets are informationally efficient in the sense of Fama (1970). 相似文献
88.
A 2D computational fluid dynamics (Eulerian–Eulerian) multiphase flow model coupled with a population balance model (CFD-PBM) was implemented to investigate the fluidization structure in terms of entrance region in an industrial-scale gas phase fluidized bed reactor. The simulation results were compared with the industrial data, and good agreement was observed. Two cases including perforated distributor and complete sparger were applied to examine the flow structure through the bed. The parametric sensitivity analysis of time step, number of node, drag coefficient, and specularity coefficient was carried out. It was found that the results were more sensitive to the drag model. The results showed that the entrance configuration has significant effect on the flow structure. While the dead zones are created in both corners of the distributors, the perforated distributor generates more startup bubbles, heterogeneous flow field, and better gas–solid interaction above the entrance region due to jet formation. 相似文献
89.
The recent trend of integration among new network services such as the long-term evolution (LTE) based on internet protocol (IP) needs reputable analyses and prediction information on the internet traffic. The IP along with increased internet traffics due to expanding new service platforms such as smartphones will reflect policies such as network QoS according to new services. The establishment of monitoring methods and analysis plans is thus required for the development of internet traffics that will analyze their status and predict their future. The paper with the speed of Internet traffic model is developed for monitoring the state of the experiment and verified. The problem is that the proposed service Internet service provider (ISP) to resolve the conflict between the occurrences can be considerably Internet traffic and that the state of data may be helpful in understanding. The paper advancement policy to reflect the network traffic volume of Internet services and users irradiation with increased traffic due to the development and management of the analysis was carried out experimental measurements. 相似文献
90.
The main objective of the present work is to improve the performance of bonded joints in carbon fiber composite structures through introducing Multi-Walled Carbon Nanotubes (MWCNTs) into Epocast 50-A1/946 epoxy, which was primarily developed for joining and repairing of composite aircraft structures. Results from tension characterizations of structural adhesive joints (SAJs) with different scarf angles (5–45°) showed improvement up to 40% compared to neat epoxy (NE)–SAJs. Special attention was considered to investigate the performance of SAJs with 5° scarf angle under different environments. The tensile strength and stiffness of both NE-SAJs and MWCNT/E-SAJs were dramatically decreased at elevated temperature. Water absorption showed a marginal drop of about 2.0% in the tensile strength of the moist SAJs compared to the dry one. Cracks initiation and propagation were detected effectively using instrumented-SAJs with eight strain gauges. The experimental results agree well with the predicted using three-dimensional finite element analysis model. 相似文献