全文获取类型
收费全文 | 2567篇 |
免费 | 251篇 |
国内免费 | 24篇 |
专业分类
化学 | 1535篇 |
晶体学 | 23篇 |
力学 | 41篇 |
数学 | 144篇 |
物理学 | 351篇 |
无线电 | 748篇 |
出版年
2024年 | 6篇 |
2023年 | 35篇 |
2022年 | 36篇 |
2021年 | 66篇 |
2020年 | 62篇 |
2019年 | 78篇 |
2018年 | 50篇 |
2017年 | 69篇 |
2016年 | 107篇 |
2015年 | 88篇 |
2014年 | 131篇 |
2013年 | 166篇 |
2012年 | 230篇 |
2011年 | 205篇 |
2010年 | 127篇 |
2009年 | 119篇 |
2008年 | 152篇 |
2007年 | 148篇 |
2006年 | 129篇 |
2005年 | 123篇 |
2004年 | 109篇 |
2003年 | 91篇 |
2002年 | 91篇 |
2001年 | 60篇 |
2000年 | 70篇 |
1999年 | 34篇 |
1998年 | 30篇 |
1997年 | 27篇 |
1996年 | 21篇 |
1995年 | 19篇 |
1994年 | 19篇 |
1993年 | 15篇 |
1992年 | 17篇 |
1991年 | 16篇 |
1990年 | 7篇 |
1989年 | 9篇 |
1988年 | 6篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1983年 | 7篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1974年 | 7篇 |
1973年 | 6篇 |
1933年 | 2篇 |
排序方式: 共有2842条查询结果,搜索用时 15 毫秒
161.
Miae Woo Sang-Tae Kim Jong-Ho Paik Hyoung-Kyu Song Young-Hwan You 《Broadcasting, IEEE Transactions on》2007,53(3):678-684
A combination of orthogonal frequency division multiplexing (OFDM) with frequency-division multiple access (FDMA) called orthogonal frequency-division multiple access (OFDMA) is regarded as a promising solution for improving the performance of interactive wireless broadcasting systems. This paper presents our investigation into reducing the multiple access interference (MAI) introduced by symbol timing misalignment in an OFDMA uplink system. To combat the MAI, we provide a new OFDMA symbol frame employing a simple symbol repetition coupled with a cyclic time shift for typical OFDMA uplink scenarios. Theoretical analysis and computer simulations are used to assess the performance of the proposed OFDMA uplink system. It is found that this scheme can decrease the probability of destroying the orthogonality among the users and provide the MAI-free reception. 相似文献
162.
Recently electron cyclotron resonance (ECR) plasma have been explored for wafer cleaning applications, since it is known to do less damage to silicon surface than conventional plasma. Organic contaminants removal efficiency and plasma radiation damage of the ECR plasma cleaning have been investigated. In oxygen ECR plasma cleaning, the plasma exposure time needed to remove the organic contaminants on the silicon surface down to the detection limit is 40 s, but the one to reach the lowest surface roughness is 10 s. The leakage current level of the MOS capacitor made using the Si substrate exposed to oxygen ECR plasma for 40 s is 8 × 10−9 A. The optimum exposure time determined by considering the contaminants removal efficiency and the plasma radiation damage (or the leakage current level) is 40 s. Organic contaminants seem to be removed through both sputter-off mechanism by oxygen ion bombardment and evaporation mechanism by chemical reactions with excited oxygen atoms. 相似文献
163.
Yttrium aluminum garnet (YAG) particles doped with Tb3+ or double doped with Tb3+ and Ce3+ were prepared by spray pyrolysis and characterized by photo- and cathode-luminescence. It was tried to incorporate a broad band of Ce3+ activator into the line peaks of Tb3+ in YAG host without the reduction of emission intensity. Ce-codoped YAG:Tb particles showed a broad band emission due to the d-f transition of Ce3+ and a reduction in the intensity of emission peaks due to 5D3-7Fj (j=3, 4, 5, 6) transition of Tb3+ when they were excited by the ultraviolet light of 270 nm. These results supported that an effective energy transfer occurs from Tb3+ to Ce3+ in YAG host. Codoping Ce3+ ions greatly intensified the excitation peak at 270 nm for the emission at 540 nm of Tb3+, which means that more lattice defects, involving in the energy absorption and transfer to Tb3+, are formed by the Ce3+ codoping. The finding gives a promising approach for enhancing the luminescence efficiency. 相似文献
164.
H. Y. Woo D. Vak D. Korystov A. Mikhailovsky G. C. Bazan D.‐Y. Kim 《Advanced functional materials》2007,17(2):290-295
Two water‐soluble conjugated polyelectrolytes, poly(9,9′‐bis(6‐N,N,N‐trimethylammoniumhexyl)fluorene‐alt‐1,4‐(2,5‐bis(6‐N,N,N‐trimethylammoniumhexyloxy))phenylene) tetrabromide ( P1i ) and poly((10,10′‐bis(6‐N,N,N‐trimethylammoniumhexyl)‐10H‐spiro(anthracene‐9,9′‐fluorene))‐alt‐1,4‐(2,5‐bis(6‐N,N,N‐trimethylammoniumhexyloxy))phenylene) tetrabromide ( P2i ) are synthesized, characterized, and used in fluorescence resonance energy transfer (FRET) experiments with fluorescein‐labeled single‐stranded DNA (ssDNA‐Fl). P1i and P2i have nearly identical π‐conjugated backbones, as determined by cyclic voltammetry and UV‐vis spectroscopy. The main structural difference is the presence of an anthracenyl substituent, orthogonal to the main chain in each of the P2i repeat units, which increases the average interchain separation in aggregated phases. It is possible to observe emission from ssDNA‐Fl via FRET upon excitation of P2i . Fluorescein is not emissive within the ssDNA‐Fl/ P1i electrostatic complex, suggesting Fl emission quenching through photoinduced charge transfer (PCT). We propose that the presence of the anthracenyl “molecular bumper” in P2i increases the distance between optical partners, which decreases PCT more acutely relative to FRET. 相似文献
165.
Jung-Kyen Moon Jung-Won Park Woo Song Lee Young-Jin Kang Hyun-A Chung Mi-Seon Shin Yong-Jin Yoon Ki Hun Park 《Journal of heterocyclic chemistry》1999,36(3):793-798
This paper presents the synthesis of 2-amino-, 2-acetamido- and 2-benzamidothioxanthones and their 10,10-dioxides. 相似文献
166.
Various phenomenon-mimicking algorithms, such as genetic algorithm, simulated annealing, tabu search, ant colony optimization, and particle swarm optimization, have their own algorithm parameters. These parameters need to be skillfully assigned in order to obtain good results. It is burdensome, especially to novice users, to assign these parameters. The same is true for the harmony search algorithm which was inspired by music performance. Thus, this study proposes a novel technique to eliminate tedious and experience-requiring parameter assigning efforts. The new parameter-setting-free (PSF) technique which this study suggests contains one additional matrix which contains an operation type (random selection, memory consideration, or pitch adjustment) for every variable in harmony memory. Three examples illustrate that the PSF technique can find good solutions robustly. 相似文献
167.
The interposition of surfactants between polymer and liquid crystal (LC) droplets was theoretically predicted by the positive spreading coefficient (0 < λ31) and utilized to interpret the morphology, grating formation kinetics, diffraction efficiency, and switching of the holographic polymer dispersed liquid crystal (HPDLC), prepared from various types (octanoic acid, poly oxyethylene octyl phenyl ether, and perfluoro‐1‐butanesulfonyl fluoride) and amounts (0–9 wt%) of surfactant and molecular weights of polyol (PPG). Regardless of the surfactant type, diffraction efficiency increased with the addition and increasing amount of surfactant, a tendency consistent with increasing value of spreading coefficient, which is determined by the formulations of grating formation. In contrast, diffraction efficiency showed a maximum with the polypropylene glycol (PPG) molecular weight. Surfactant effectively reduced the anchoring energy and electrically drove the film which otherwise was not driven. Overall, surfactant with greater λ31 gave smaller droplet, greater diffraction efficiency, driving voltage, contrast ratio, and smaller response time. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
168.
Young Woo Kim Seung Joon Lee Jeong Ho Kim Young Hoon Kwon Dan Keun Sung 《Wireless Personal Communications》2000,14(1):29-47
This paper is concerned with capacity estimation in multiple-chip-rate (MCR)DS/CDMA systems supporting multimedia services with different informationrates and quality requirements. Considering both power spectral density (PSD)over a radio frequency (RF) band and the effect of RF input filtering onthe receiver, capacity that satisfies the requirementof the bit energy-to-interference PSD ratio is derived.The optimum value of the received power which causes the leastinterference for other users while maintaining an acceptablequality-of-service(QoS) requirement is also derived. The results show that system performanceisstrongly affected by a selected channel assignment strategy. Therefore, it is critical to efficiently assign radio resources in MCR-DS/CDMA systems that support high capacity and a low blocking rate. 相似文献
169.
Low-power 3D graphics processors for mobile terminals 总被引:1,自引:0,他引:1
Ju-Ho Sohn Yong-Ha Park Chi-Weon Yoon Woo R. Se-Jeong Park Hoi-Jun Yoo 《Communications Magazine, IEEE》2005,43(12):90-99
A full 3D graphics pipeline is investigated, and optimizations of graphics architecture are assessed for satisfying the performance requirements and overcoming the limited system resources found in mobile terminals. Two mobile 3D graphics processor architectures, RAMP and DigiAcc, are proposed based on the analysis, and a prototype development platform (REMY) is implemented. REMY includes a software graphics library and simulation environment developed for more flexible realization of mobile 3D graphics. The experimental results demonstrate the feasibility of mobile 3D graphics with 3.6 Mpolygons/s at 155 mW power consumption for full 3D operation. 相似文献
170.
The effects of preamorphization implantation (PAI) on the interface properties between hafnium-silicate (HfSiO) gate dielectrics and silicon substrates were examined. In the case of an NH/sub 3/ nitrided interface, it was found that the PAI can improve the interface trap density (D/sub IT/) compared with the no PAI case. However, for the PAI samples, it was also found that samples with sacrificial screening oxide (Sac Ox) had worse interface properties compared with the samples without Sac Ox. It is attributed to the recoiled oxygen from Sac Ox during PAI. 相似文献