全文获取类型
收费全文 | 103649篇 |
免费 | 1468篇 |
国内免费 | 1229篇 |
专业分类
电工技术 | 1954篇 |
综合类 | 138篇 |
化学工业 | 10739篇 |
金属工艺 | 5469篇 |
机械仪表 | 3207篇 |
建筑科学 | 2056篇 |
矿业工程 | 159篇 |
能源动力 | 2976篇 |
轻工业 | 6113篇 |
水利工程 | 707篇 |
石油天然气 | 668篇 |
武器工业 | 15篇 |
无线电 | 16033篇 |
一般工业技术 | 21918篇 |
冶金工业 | 25393篇 |
原子能技术 | 1401篇 |
自动化技术 | 7400篇 |
出版年
2022年 | 464篇 |
2021年 | 704篇 |
2020年 | 530篇 |
2019年 | 679篇 |
2018年 | 1142篇 |
2017年 | 1100篇 |
2016年 | 1188篇 |
2015年 | 943篇 |
2014年 | 1502篇 |
2013年 | 4702篇 |
2012年 | 2593篇 |
2011年 | 3868篇 |
2010年 | 3129篇 |
2009年 | 3759篇 |
2008年 | 3937篇 |
2007年 | 4118篇 |
2006年 | 3713篇 |
2005年 | 3357篇 |
2004年 | 3202篇 |
2003年 | 3053篇 |
2002年 | 2700篇 |
2001年 | 2998篇 |
2000年 | 2746篇 |
1999年 | 3094篇 |
1998年 | 9485篇 |
1997年 | 6184篇 |
1996年 | 4778篇 |
1995年 | 3179篇 |
1994年 | 2807篇 |
1993年 | 2745篇 |
1992年 | 1647篇 |
1991年 | 1606篇 |
1990年 | 1529篇 |
1989年 | 1340篇 |
1988年 | 1183篇 |
1987年 | 870篇 |
1986年 | 892篇 |
1985年 | 931篇 |
1984年 | 815篇 |
1983年 | 708篇 |
1982年 | 705篇 |
1981年 | 691篇 |
1980年 | 577篇 |
1979年 | 479篇 |
1978年 | 418篇 |
1977年 | 539篇 |
1976年 | 964篇 |
1975年 | 299篇 |
1974年 | 279篇 |
1973年 | 258篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
End-to-End QoS for Video Delivery Over Wireless Internet 总被引:6,自引:0,他引:6
Zhang Q. Zhu W. Zhang Y. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》2005,93(1):123-134
Providing end-to-end quality of service (QoS) support is essential for video delivery over the next-generation wireless Internet. We address several key elements in the end-to-end QoS support, including scalable video representation, network-aware end system, and network QoS provisioning. There are generally two approaches in QoS support: the network-centric and the end-system centric solutions. The fundamental problem in a network-centric solution is how to map QoS criterion at different layers respectively, and optimize total quality across these layers. We first present the general framework of a cross-layer network-centric solution, and then describe the recent advances in network modeling, QoS mapping, and QoS adaptation. The key targets in end-system centric approach are network adaptation and media adaptation. We present a general framework of the end-system centric solution and investigate the recent developments. Specifically, for network adaptation, we review the available bandwidth estimation and efficient video transport protocol; for media adaptation , we describe the advances in error control, power control, and corresponding bit allocation. Finally, we highlight several advanced research directions. 相似文献
172.
A method for the rapid and quantitative determination of aflatoxin B1 from small quantities of liver, around 1-2 g, is described. The extraction procedure involves acidification to pH 2 of the aqueous liver homogenates, extraction with chloroform: acetone and HPLC-fluorimetric detection after derivatization with trifluoroacetic acid. Quantitative recovery of aflatoxin B1 from chick liver was achieved and detection at levels of 0.2-1 ppb was proved feasible. The aflatoxin B1 concentration in chick liver after oral administration is also shown. 相似文献
173.
The minimum energy required to transmit one bit of information through a network characterizes the most economical way to communicate in a network. In this paper, we show that, under a layered model of wireless networks, the minimum energy-per-bit for multicasting in a mobile ad hoc network can be found by a linear program; the minimum energy-per-bit can be attained by performing network coding. Compared with conventional routing solutions, network coding not only allows a potentially lower energy-per-bit to be achieved, but also enables the optimal solution to be found in polynomial time, in sharp contrast with the NP-hardness of constructing the minimum-energy multicast tree as the optimal routing solution. We further show that the minimum energy multicast formulation is equivalent to a cost minimization with linear edge-based pricing, where the edge prices are the energy-per-bits of the corresponding physical broadcast links. This paper also investigates minimum energy multicasting with routing. Due to the linearity of the pricing scheme, the minimum energy-per-bit for routing is achievable by using a single distribution tree. A characterization of the admissible rate region for routing with a single tree is presented. The minimum energy-per-bit for multicasting with routing is found by an integer linear program. We show that the relaxation of this integer linear program, studied earlier in the Steiner tree literature, can now be interpreted as the optimization for minimum energy multicasting with network coding. In short, this paper presents a unifying study of minimum energy multicasting with network coding and routing. 相似文献
174.
X. S. Ning K. Suganuma T. Okamoto A. Koreeda Y. Miyamoto 《Journal of Materials Science》1989,24(8):2879-2883
Two kinds of additive-free silicon nitride ceramics were brazed with aluminium; one was with as-ground faying surfaces and
the other was with faying surfaces heat-treated at 1073K for 1.8 ksec in air. The heat-treatment of the silicon nitride ceramics
formed a silicon oxynitride layer on the faying surfaces and increased the brazing strength of the joints. A silica-alumina
non-crystalline layer and a β′-sialon layer were formed successively from the aluminium side at the interface of the joints.
The heat-treatment which made the former layer thicker is a necessary process in making reliable, strong brazed joints. 相似文献
175.
Kitakami O. Ogawa Y. Fujiwara H. Kugiya F. Suzuki M. 《IEEE transactions on magnetics》1989,25(3):2607-2611
A Co-Cr film deposited directly on a substrate has an initial growth layer with low coercivity. However, the existence of a Ti underlayer prevents the formation of such a layer. As a result, Co-Cr film deposited on a Ti underlayer has high perpendicular anisotropy and coercivity even in cases of extremely thin film thickness (200 Å). As for the read-write characteristics of Co-Cr thin-film media, the existence of such an initial growth layer greatly improves the reproduced output level. The cause for this is considered to be that the free charges which appear on the back surface of the perpendicular recording layer are reduced and the demagnetization field acting on the recorded magnetization subsequently decreases due to the existence of the initial growth layer 相似文献
176.
177.
Y. Nath P. C. Dumir R. S. Bhatiaf 《International journal for numerical methods in engineering》1985,21(3):565-578
The present work investigates the efficacy and applicability of interior global orthogonal point collocation method to the axisymmetric nonlinear analysis of elastic circular plates and shallow spherical shells subjected to uniformly distributed transverse load. Spacewise discretisation has been carried out using a polynomial expansion with the zeros of a Chebyshev polynomial as collocation points. Timewise integration has been carried out with Newmark k-β scheme corresponding to average acceleration method. The static response and snap-through buckling results, as well as, the dynamic response and dynamic buckling results under a uniformly distributed step load have been obtained and found to agree closely with the available results. 相似文献
178.
C. Y. Liao S. N. Atluri 《International journal for numerical methods in engineering》1991,32(6):1339-1361
An efficient method, based on the Schwarz–Neumann alternating technique, is presented for computing weight functions of a general solid (3-D as well as 2-D), with embedded or surface-flaw configurations. The total rate of change of the crack-opening displacements, due to simple perturbations of crack-dimension characteristics, is conveniently decomposed into the infinite-domain and boundary-correction parts. The former is determined from available analytical solutions of ideal-shaped cracks, whereas the latter is computed numerically by imposing nil boundary-traction requirements for the displacement field corresponding to the weight functions. Numerical examples, with solutions for 3-D weighted-average and local stress intensity factors, indicate that the proposed method is very accurate and efficient. 相似文献
179.
Flux pinning characteristics have been investigated for the Nb x Ti100-x /Nb(x = 65, 50 and 28) and Nb 28 Ti 72/Nb 65 Ti 35 superconductor(S)/superconductor(S′) multilayers. The maximum of the pinning force F p⊥ max perpendicular to the layer plane as a function of the structure modulation length λ has a peak in the quasi-two-dimensional region (λ~20 nm). The maximum. values of the F p⊥ max versus λ curve are proportional to the difference of the GL coherence length (ξ GL ) between the superconductive sublayers S and S′. The results suggest that the large F p⊥ max in the S/S′ multilayer is caused by the repulsive pinning force due to Nb layers with larger ξ GL . 相似文献
180.