全文获取类型
收费全文 | 9355篇 |
免费 | 444篇 |
国内免费 | 128篇 |
专业分类
化学 | 6209篇 |
晶体学 | 59篇 |
力学 | 418篇 |
综合类 | 1篇 |
数学 | 770篇 |
物理学 | 1278篇 |
无线电 | 1192篇 |
出版年
2024年 | 23篇 |
2023年 | 93篇 |
2022年 | 353篇 |
2021年 | 466篇 |
2020年 | 471篇 |
2019年 | 491篇 |
2018年 | 478篇 |
2017年 | 435篇 |
2016年 | 604篇 |
2015年 | 415篇 |
2014年 | 592篇 |
2013年 | 1013篇 |
2012年 | 763篇 |
2011年 | 668篇 |
2010年 | 435篇 |
2009年 | 424篇 |
2008年 | 413篇 |
2007年 | 354篇 |
2006年 | 250篇 |
2005年 | 220篇 |
2004年 | 163篇 |
2003年 | 148篇 |
2002年 | 125篇 |
2001年 | 46篇 |
2000年 | 29篇 |
1999年 | 37篇 |
1998年 | 34篇 |
1997年 | 29篇 |
1996年 | 30篇 |
1995年 | 18篇 |
1994年 | 24篇 |
1993年 | 18篇 |
1992年 | 26篇 |
1991年 | 15篇 |
1990年 | 17篇 |
1989年 | 13篇 |
1988年 | 10篇 |
1987年 | 19篇 |
1986年 | 8篇 |
1985年 | 19篇 |
1984年 | 20篇 |
1983年 | 11篇 |
1982年 | 17篇 |
1981年 | 13篇 |
1980年 | 17篇 |
1979年 | 10篇 |
1978年 | 12篇 |
1976年 | 9篇 |
1973年 | 5篇 |
1971年 | 4篇 |
排序方式: 共有9927条查询结果,搜索用时 15 毫秒
141.
Mohammad Reza Parsaei Hamid Reza Boveiri Reza Javidan Raouf Khayami 《International Journal of Communication Systems》2020,33(11)
Telemedicine is a new area based on the information and communication technology for collecting, storing, organizing, retrieving and exchanging medical information. One of the most important applications of telemedicine is indeed telesurgery in which an efficient telecommunication infrastructure between the surgery room and remote surgeons need to be established. One of the most important issues to be tackled in telesurgery is to find favorable links for routing as well as providing high Quality of Service (QoS). In this paper, an efficient model based on the hybridization of Type‐2 Fuzzy System (T2FS) and Cuckoo Optimization Algorithm (COA) over the Software Defined Networks (SDN) is proposed in order to achieve optimal and reliable routes for telesurgery application. Using T2FS, the fitness of the links is determined; then, a COA is conducted over the Constraint Shortest Path (CSP) problem to find the best routes. Delay is considered as a CSP problem which is satisfied by trying to find the paths with minimum cost. Due to the NP‐completeness of the CSP problem, an Enhanced COA (so‐called E‐COA) is proposed and utilized as a metaheuristic solver. To the best of our knowledge, this paper is the first SDN‐based communication model that applies both T2FS and E‐COA for assigning proper costs to the network's links, and solves the consequence CSP problem according to the QoS requirement for telesurgery. The model also recognizes and preserves the second‐best routes in order to keep the reliability for such a critical application. In addition to the simulations, the performance evaluation is also conducted on a real experimental scenario. Many comparisons are carried out between the proposed model and other conventional methods, and the evaluation study shows the superiority of the proposed model on all the three QoS‐related metrics, i.e. average end‐to‐end delay, packet loss ratio and PSNR. 相似文献
142.
Mohammad Mahdi Tavakoli Giovanni Azzellino Marek Hempel Ang‐Yu Lu Francisco J. Martin‐Martinez Jiayuan Zhao Jingjie Yeo Tomas Palacios Markus J. Buehler Jing Kong 《Advanced functional materials》2020,30(31)
A roll‐to‐roll (R2R) transfer technique is employed to improve the electrical properties of transferred graphene on flexible substrates using parylene as an interfacial layer. A layer of parylene is deposited on graphene/copper (Cu) foils grown by chemical vapor deposition and are laminated onto ethylene vinyl acetate (EVA)/poly(ethylene terephthalate). Then, the samples are delaminated from the Cu using an electrochemical transfer process, resulting in flexible and conductive substrates with sheet resistances of below 300 Ω sq?1, which is significantly better (fourfold) than the sample transferred by R2R without parylene (1200 Ω sq?1). The characterization results indicate that parylene C and D dope graphene due to the presence of chlorine atoms in their structure, resulting in higher carrier density and thus lower sheet resistance. Density functional theory calculations reveal that the binding energy between parylene and graphene is stronger than that of EVA and graphene, which may lead to less tear in graphene during the R2R transfer. Finally, organic solar cells are fabricated on the ultrathin and flexible parylene/graphene substrates and an ultra‐lightweight device is achieved with a power conversion efficiency of 5.86%. Additionally, the device shows a high power per weight of 6.46 W g?1 with superior air stability. 相似文献
143.
Wireless Networks - Wireless sensor networks include a large number of sensor nodes which are developed in an environment. These networks have military, commercial, industrial, and medical... 相似文献
144.
A Novel UWB Pulse Shape Modulation System 总被引:4,自引:1,他引:4
Ghavami Mohammad Michael Lachlan B. Haruyama Shinichiro Kohno Ryuji 《Wireless Personal Communications》2002,23(1):105-120
In this paper novel modified Hermite polynomial functions for use in impulse radio (ultra-wideband) communications are proposed. With these functions pulse shapes which are orthogonal and have nearly constant pulse width regardless of the pulse order are generated. These properties hold under the effects of differentiation. An M-ary communication system is constructed using these pulse shapes. A Matlab model for generating the pulses is designed and the effect of timing jitter on the performance of the system is investigated by computer simulation. 相似文献
145.
Mohammad?Shukri?AhmadEmail author Osman?Kukrer Aykut?Hocanin 《Circuits, Systems, and Signal Processing》2012,31(2):703-710
The recently proposed Recursive Inverse (RI) algorithm has shown a significant performance improvement compared to that of
the Recursive Least Squares (RLS) algorithm, in various noise environments. However, both algorithms fail to converge in certain
impulsive noise environments, especially if the Signal-to-Noise Ratio (SNR) is low. In this paper, a Robust RI algorithm is
proposed. Analytical results show that robustness against impulsive noise is achieved by choosing the weights on the basis
of the L
1 norms of the autocorrelation matrix and the cross-correlation vector. Simulation results confirm that the proposed algorithm
provides an improved performance, with a reduction in computational complexity, compared to those of the RLS and the Robust
RLS in white and correlated impulsive noise. 相似文献
146.
Mohammad Tariqul Islam Ahmed Toaha Mobashsher Norbahiah Misran 《Journal of Infrared, Millimeter and Terahertz Waves》2010,31(12):1427-1437
A novel and simple coplanar waveguide fed compact antenna is introduced in this paper. The antenna structure combines the
advantages of CPW with those of the broadband antenna and simplifies the structure of the antenna by reducing the number of
metallization level to construct uni-planar antenna. Prototype of the proposed antenna have been constructed and studied experimentally.
The measured results agrees well with the simulated prediction and shows a broad bandwidth of 6 GHz ranging from 3.5 GHz to
9.5 GHz with VSWR ≤2 (return loss ≤−10 dB), which is equivalent to 92.3% impedance bandwidth centered at 6.5 GHz. The proposed
antenna shows stable radiation characteristics, gain and axial ratio of less than 1 dB over the whole operating bandwidth.
Furthermore, an extensive parametric study was performed to realize the relationship between the resonance frequencies of
the broadband antennas and different parameters which is helpful for advancement of the antenna design. 相似文献
147.
Mihaela Nedelcu Mohammad S. M. Saifullah David G. Hasko Arang Jang David Anderson Wilhelm T. S. Huck Geraint A. C. Jones Mark E. Welland Dae Joon Kang Ullrich Steiner 《Advanced functional materials》2010,20(14):2317-2323
The fabrication of very narrow metal lines by the lift‐off technique, especially below sub‐10 nm, is challenging due to thinner resist requirements in order to achieve the lithographic resolution. At such small length scales, when the grain size becomes comparable with the line‐width, the built‐in stress in the metal film can cause a break to occur at a grain boundary. Moreover, the line‐width roughness (LWR) from the patterned resist can result in deposited metal lines with a very high LWR, leading to an adverse change in device characteristics. Here a new approach that is not based on the lift‐off technique but rather on low temperature hydrogen reduction of electron‐beam patterned metal naphthenates is demonstrated. This not only enables the fabrication of sub‐10 nm metal lines of good integrity, but also of low LWR, below the limit of 3.2 nm discussed in the International Technology Roadmap for Semiconductors. Using this method, sub‐10 nm nickel wires are obtained by reducing patterned nickel naphthenate lines in a hydrogen‐rich atmosphere at 500 °C for 1 h. The LWR (i.e., 3 σLWR) of these nickel nanolines was found to be 2.9 nm. The technique is general and is likely to be suitable for fabrication of nanostructures of most commonly used metals (and their alloys), such as iron, cobalt, nickel, copper, tungsten, molybdenum, and so on, from their respective metal–organic compounds. 相似文献
148.
Reza Talebzadeh Mohammad Soroosh Yousef S. Kavian Farhad Mehdizadeh 《Photonic Network Communications》2017,34(2):248-257
In this paper, we employed multilayer ring resonators in a silicon rod base structure to realize 6-channel and 8-channel demultiplexers based on two-dimensional photonic crystals. Both the main rings and basic structures are composed of silicon rods, and the interior rings of the multilayer rings are composed of carbon. Employing silicon and carbon rods of different radii in multilayer ring resonators enhanced the coupling efficiency between the rings and waveguides. The average quality factor and power transmission efficiency were 4320 and 93%, respectively. Crosstalk values from \(-11\) to ?46 dB in conjunction with the mentioned characteristics suggest the use of the device for optical communication applications. The compact size of the proposed structure and the materials used make the proposed demultiplexer suitable for optical integrated circuits. 相似文献
149.
The advent of interactive digital platforms has led people to progressively interact on such platforms, urging organizations to create online communities to engage customers with them and with each other to enhance brand loyalty. This study attempts to investigate what motivates customers to engage in these brand communities. Through a questionnaire survey of 430 Facebook users, this study investigates whether and how the unique characteristics (information quality, system quality, virtual interactivity, and rewards) of online brand communities affect customer engagement. The consequent effect of customer engagement on brand loyalty is also examined. This study frames and empirically validates a model for engaging customers with online brand communities on Facebook, considering the moderating role of gender. The Stimulus-Organism-Response paradigm is solicited to justify the theoretical background of this study. The data were analyzed using structure equation modelling. Results reveal that each of the characteristics positively influences customer engagement, with information quality and virtual interactivity bearing the strongest influence. Customer engagement also exhibits a strong positive impact on brand loyalty. This results further reveal that gender gap in the online environment is declining as the impact of all the four characteristics of online brand communities on customer engagement was invariable across male and female members. 相似文献
150.
This paper presents a simple distance estimation for implicit polynomial fitting. It is computed as the height of a simplex built between the point and the surface (i.e., a triangle in 2-D or a tetrahedron in 3-D), which is used as a coarse but reliable estimation of the orthogonal distance. The proposed distance can be described as a function of the coefficients of the implicit polynomial. Moreover, it is differentiable and has a smooth behavior . Hence, it can be used in any gradient-based optimization. In this paper, its use in a Levenberg-Marquardt framework is shown, which is particularly devoted for nonlinear least squares problems. The proposed estimation is a generalization of the gradient-based distance estimation, which is widely used in the literature. Experimental results, both in 2-D and 3-D data sets, are provided. Comparisons with state-of-the-art techniques are presented, showing the advantages of the proposed approach. 相似文献