全文获取类型
收费全文 | 494245篇 |
免费 | 7743篇 |
国内免费 | 1791篇 |
专业分类
电工技术 | 8447篇 |
综合类 | 2669篇 |
化学工业 | 73329篇 |
金属工艺 | 18810篇 |
机械仪表 | 14868篇 |
建筑科学 | 13999篇 |
矿业工程 | 1430篇 |
能源动力 | 12269篇 |
轻工业 | 47419篇 |
水利工程 | 4535篇 |
石油天然气 | 2176篇 |
武器工业 | 13篇 |
无线电 | 64525篇 |
一般工业技术 | 92439篇 |
冶金工业 | 77512篇 |
原子能技术 | 5831篇 |
自动化技术 | 63508篇 |
出版年
2021年 | 2796篇 |
2019年 | 2523篇 |
2018年 | 17798篇 |
2017年 | 16828篇 |
2016年 | 13823篇 |
2015年 | 4130篇 |
2014年 | 6085篇 |
2013年 | 19666篇 |
2012年 | 13337篇 |
2011年 | 23806篇 |
2010年 | 19305篇 |
2009年 | 18262篇 |
2008年 | 20423篇 |
2007年 | 21749篇 |
2006年 | 13018篇 |
2005年 | 13187篇 |
2004年 | 12671篇 |
2003年 | 12433篇 |
2002年 | 11479篇 |
2001年 | 11262篇 |
2000年 | 10477篇 |
1999年 | 10861篇 |
1998年 | 24095篇 |
1997年 | 17635篇 |
1996年 | 13984篇 |
1995年 | 11017篇 |
1994年 | 9800篇 |
1993年 | 9382篇 |
1992年 | 7142篇 |
1991年 | 6822篇 |
1990年 | 6406篇 |
1989年 | 6119篇 |
1988年 | 5873篇 |
1987年 | 4887篇 |
1986年 | 4841篇 |
1985年 | 5985篇 |
1984年 | 5601篇 |
1983年 | 4833篇 |
1982年 | 4501篇 |
1981年 | 4450篇 |
1980年 | 4209篇 |
1979年 | 4089篇 |
1978年 | 3771篇 |
1977年 | 4550篇 |
1976年 | 6104篇 |
1975年 | 3118篇 |
1974年 | 3014篇 |
1973年 | 2895篇 |
1972年 | 2319篇 |
1971年 | 2006篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Weaver P.M. Pechrach K. McBride J.W. 《Components and Packaging Technologies, IEEE Transactions on》2005,28(4):734-740
A novel contact opening mechanism has been developed using a piezoelectric actuator to open the contacts in a low contact opening velocity circuit breaker. The arc control on the contacts is critical for successful current interruption (10/sup 3/-10/sup 4/ A) in low voltage (<250V) devices. Previous work has shown how arc root commutation from the contact region into the arc chamber is affected by arc chamber materials, contact materials and the gap behind the moving contact for contact velocities between 1ms/sup -1/ and 10ms/sup -1/. This work is extended using a commercially available piezoelectric actuator to open the contacts. Contact opening speeds are assessed and the arc root mobility is characterized under this operating regime. A flexible test apparatus and solid-state high-speed arc imaging system are used to gather data on the arc root during the opening of the contacts. New experimental results are presented on the anode and cathode root velocity and arc root motion in an arc chamber with piezoelectrically actuated contact opening. These results can be used to improve the design of high current low voltage circuit breakers suitable for piezoelectric actuation. 相似文献
992.
Security issues in hybrid networks with a satellite component 总被引:3,自引:0,他引:3
Roy-Chowdhury A. Baras J.S. Hadjitheodosiou M. Papademetriou S. 《Wireless Communications, IEEE》2005,12(6):50-61
Satellites are expected to play an increasingly important role in providing broadband Internet services over long distances in an efficient manner. Most future networks will be hybrid in nature - having terrestrial nodes interconnected by satellite links. Security is an important concern in such networks, since the satellite segment is susceptible to a host of attacks, including eavesdropping, session hijacking and data corruption. In this article we address the issue of securing communication in satellite networks. We discuss various security attacks that are possible in hybrid satellite networks, and survey the different solutions proposed to secure data communications in these networks. We look at the performance problems arising in hybrid networks due to security additions like Internet security protocol (IPSec) or secure socket layer (SSL), and suggest solutions to performance-related problems. We also point out important drawbacks in the proposed solutions, and suggest a hierarchical key-management approach for adding data security to group communication in hybrid networks. 相似文献
993.
Mutual information-based analysis of JPEG2000 contexts. 总被引:4,自引:0,他引:4
Context-based arithmetic coding has been widely adopted in image and video compression and is a key component of the new JPEG2000 image compression standard. In this paper, the contexts used in JPEG2000 are analyzed using the mutual information, which is closely related to the compression performance. We first show that, when combining the contexts, the mutual information between the contexts and the encoded data will decrease unless the conditional probability distributions of the combined contexts are the same. Given I, the initial number of contexts, and F, the final desired number of contexts, there are S(I, F) possible context classification schemes where S(I, F) is called the Stirling number of the second kind. The optimal classification scheme is the one that gives the maximum mutual information. Instead of using an exhaustive search, the optimal classification scheme can be obtained through a modified generalized Lloyd algorithm with the relative entropy as the distortion metric. For binary arithmetic coding, the search complexity can be reduced by using dynamic programming. Our experimental results show that the JPEG2000 contexts capture the correlations among the wavelet coefficients very well. At the same time, the number of contexts used as part of the standard can be reduced without loss in the coding performance. 相似文献
994.
Stable scheduling policies for fading wireless channels 总被引:1,自引:0,他引:1
We study the problem of stable scheduling for a class of wireless networks. The goal is to stabilize the queues holding information to be transmitted over a fading channel. Few assumptions are made on the arrival process statistics other than the assumption that their mean values lie within the capacity region and that they satisfy a version of the law of large numbers. We prove that, for any mean arrival rate that lies in the capacity region, the queues will be stable under our policy. Moreover, we show that it is easy to incorporate imperfect queue length information and other approximations that can simplify the implementation of our policy. 相似文献
995.
Wastewater discharge from coal refining plants contains a number of biologically toxic compounds; 2000-2500 mg/l of COD of which 40% is composed of phenol, 100-400 mg/l of thiocyanate, 10-40 mg/l of cyanide, 100-250 mg/l of NH4+-N and 150-300 mg/l of total nitrogen. In order to treat this kind of high strength wastewater, we have developed a high performance biofilm process using fluidizing bio-carriers of the tube chip type. The fluidizing biofilm carriers are made of a composite of polyethylene and several inorganic materials, whose density is controlled at 0.97-0.98 g/ml. The fluidizing biofilm carriers show sound fluidization characteristics inside bioreactors. The wastewater is treated using three consecutive series reactors in oxic-anoxic-oxic arrangement. Each reactor is charged with the fluidizing biofilm carriers of 50 vol%. Furthermore, newly cultured active microorganisms for the thiocyanate biodegradation are added in the biofilm process. At total hydraulic retention time of 2.2 days, this process can achieve steady state removal efficiencies: COD, 99%; thiocyanate, 99%; NH4+-N, 99% and total nitrogen, 90%. 相似文献
996.
The optical character of waste stabilization ponds (WSPs) is of concern for several reasons. Algal photosynthesis, which produces oxygen for waste oxidation in WSPs, is influenced by attenuation of sunlight in ponds. Disinfection in WSPs is influenced by optical characteristics because solar UV exposure usually dominates inactivation. The optical nature of WSPs effluent also affects assimilation by receiving waters. Despite the importance of light behaviour in WSPs, few studies have been made of their optical characteristics. We discuss simple optical measures suitable for routine monitoring of WSPs (including at sites remote from laboratories): optical density of filtrates - an index of dissolved coloured organic (humic) matter, visual clarity - to provide an estimate of the beam attenuation coefficient (a fundamental quantity needed for optical modelling) colour (hue) - as an indicator of general WSP 'condition' and irradiance attenuation quantifying depth of light penetration. The value of optical characterisation of WSPs is illustrated with reference to optical data for WSPs in NZ (including high-rate algal ponds) treating dairy cattle wastewater versus domestic sewage. We encourage increased research on optical characteristics of WSPs and the incorporation of optical measures in monitoring and modelling of WSP performance. 相似文献
997.
This work investigates the impact of nine new product development (NPD) acceleration approaches (supplier involvement, lead user involvement, speeding up activities and tasks, reduction of parts and components, training and rewarding of employees, implementation of support systems and techniques, stimulating interfunctional cooperation, emphasis on the customer, and simplification of organizational structure) on development speed and new product profitability. Our findings from 233 manufacturing firms show that lead user involvement and training/rewarding of employees increase both development speed and profitability. Supplier involvement, speeding up activities and tasks, and a simplification of the organizational structure also enhance development speed, while an emphasis on the customer has an additional positive impact on new product profitability. Both new product speed and profitability increase firm financial performance. Our results further show that pioneers and fast followers should not select the same NPD acceleration approaches as the speed and profitability impact of the majority of the acceleration approaches depends on the new product strategy of the firm. These results are important as they provide guidance for pioneers and fast followers regarding which NPD acceleration approaches to select in order to enhance speed and profitability and, hence, firm financial performance. 相似文献
998.
Hadjem A. Lautru D. Dale C. Man Fai Wong Hanna V.F. Wiart J. 《Microwave Theory and Techniques》2005,53(1):4-11
This paper gives a first comparison of specific absorption rate (SAR) induced in a child-sized (CS) head and an adult head using a dual-band mobile phone. In the second study, the visible human head is considered and comparison of SAR induced in a CS or child-like (CL) head and an adult head using a dual-band mobile phone is given. All the peaks of average SAR over a mass of 10 and 1 g in the head and the power budget are determined in the two comparisons using the finite-difference time-domain method. The differences between the results for adult and CS or CL heads are given at 900 and 1800 MHz. No important differences are noted for the peak SAR averaged over 10 g (SAR10 g), between the two adult head models, as well as between the two child head models. The peak SAR10 g in the brain of the CS or CL head is slightly more significant than that for the adult one. 相似文献
999.
J.-P. Han S.M. Koo E.M. Vogel E.P. Gusev C. DEmic C.A. Richter J.S. Suehle 《Microelectronics Reliability》2005,45(5-6):783
Anomalous threshold voltage roll-up behavior, commonly referred as reverse short channel effect (RSCE), has been observed in high-k (HfO2 on SiON buffer, Al2O3 on SiON buffer) gated submicron nMOSFETs, while the SiO2 or SiON control samples show normal short channel effect (SCE) behavior. The possible causes such as inhomogeneous channel doping profile and gate oxide thickness variation near S/D ends have been ruled out. The results indicate that interface trap density that dependents on channel length is the main cause of the RSCE observed here. In addition, oxide charge also plays a role. 相似文献
1000.
Dielectric reliability in Al2O3(2–3.1nm)–HfO2(3nm) stack capacitor with Metal–Insulator–Si(MIS) structure is investigated in this paper. We propose an optimized capacitor process through the Time–Dependent Dielectric Breakdown (TDDB) data under various process conditions. Furthermore, due to asymmetric current at both negative and positive voltage stress polarities, we show different lifetime extrapolation by a fluence–driven model. As a result, the maximum allowed operating voltage is projected to be 1.7V (failure rate 10ppm during 10year @ 85°C) for Data “0” retention lifetime. 相似文献