全文获取类型
收费全文 | 228307篇 |
免费 | 15748篇 |
国内免费 | 8423篇 |
专业分类
电工技术 | 11658篇 |
技术理论 | 14篇 |
综合类 | 11262篇 |
化学工业 | 38151篇 |
金属工艺 | 12566篇 |
机械仪表 | 12877篇 |
建筑科学 | 16860篇 |
矿业工程 | 4986篇 |
能源动力 | 6662篇 |
轻工业 | 13830篇 |
水利工程 | 3343篇 |
石油天然气 | 11408篇 |
武器工业 | 1350篇 |
无线电 | 28501篇 |
一般工业技术 | 32813篇 |
冶金工业 | 14535篇 |
原子能技术 | 2416篇 |
自动化技术 | 29246篇 |
出版年
2024年 | 886篇 |
2023年 | 3308篇 |
2022年 | 5942篇 |
2021年 | 8087篇 |
2020年 | 5918篇 |
2019年 | 5229篇 |
2018年 | 5776篇 |
2017年 | 6550篇 |
2016年 | 6084篇 |
2015年 | 7604篇 |
2014年 | 10029篇 |
2013年 | 13726篇 |
2012年 | 13035篇 |
2011年 | 15149篇 |
2010年 | 12636篇 |
2009年 | 12648篇 |
2008年 | 12037篇 |
2007年 | 11686篇 |
2006年 | 11955篇 |
2005年 | 10620篇 |
2004年 | 7377篇 |
2003年 | 6549篇 |
2002年 | 5986篇 |
2001年 | 5417篇 |
2000年 | 5551篇 |
1999年 | 6343篇 |
1998年 | 6679篇 |
1997年 | 5343篇 |
1996年 | 4831篇 |
1995年 | 3867篇 |
1994年 | 3214篇 |
1993年 | 2591篇 |
1992年 | 1987篇 |
1991年 | 1559篇 |
1990年 | 1198篇 |
1989年 | 1074篇 |
1988年 | 826篇 |
1987年 | 606篇 |
1986年 | 463篇 |
1985年 | 401篇 |
1984年 | 262篇 |
1983年 | 237篇 |
1982年 | 198篇 |
1981年 | 183篇 |
1980年 | 153篇 |
1979年 | 111篇 |
1978年 | 78篇 |
1977年 | 77篇 |
1976年 | 102篇 |
1973年 | 52篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
以TD—SCDMA既有进展及面临的挑战与NGBWM的发展走向为基础,重点论述自主创新导向下的TD—SCDMA与NGBWM务实发展战略思考。 相似文献
82.
Chen J.J. Gao G.-B. Chyi J.-I. Morkoc H. 《Electron Devices, IEEE Transactions on》1989,36(10):2165-2172
Avalanche breakdown behavior at the collector junction of the GaAs/AlGaAs HBT (heterojunction bipolar transistor) has been studied. Junction breakdown characteristics displaying hard breakdown, soft breakdown, and negative resistance breakdown behavior were observed and are interpreted by analysis of localized microplasma effects, uniform microplasma-free behavior, and associated current gain measurements. Light emission from the collector-base junction of the GaAs/AlGaAs HBT was observed and used to investigate breakdown uniformity. Using a simple punchthrough breakdown model, the theoretical breakdown curves at different collector doping concentrations and thicknesses were computed and found to be in agreement with maximum breakdown voltages measured from devices displaying the most uniform junction breakdown. The serious current gain degradation of GaAs/AlGaAs HBTs at low current densities was analyzed in connection with the measurement of a large collector-emitter breakdown voltage. The unexpected functional relationship between the collector-emitter breakdown voltage and collector-base breakdown voltage is explained by the absence of a hole-feedback effect for devices not exhibiting transistor action 相似文献
83.
Chen Guodong 《International Journal of Pressure Vessels and Piping》1985,19(1):19-30
In this paper, according to the theory of thin shells, a basic equation in complex form is derived for conical thin shells with linearly varying wall thickness under symmetric loads, and the uniformly valid asymptotic solutions are given. 相似文献
84.
Scattering from bandlimited fractal fibers 总被引:2,自引:0,他引:2
The scattering of electromagnetic or optical waves from tenuous dielectric fibers characterized by bandlimited fractal surfaces is considered. The role of several fractal dimensions is noted, and one suitable for this problem is identified. It is found that in the fractal regime, the Rayleigh-Gans differential scattering cross section as a function of the normalized size parameter exhibits a power-law relation reminiscent of that associated with light scattering from fractal clusters and small-angle X-ray and neutron scattering by porous material. This power-law relation is monotonically related to an appropriate fractal dimension and variance of the fiber surface 相似文献
85.
A Subcarriers Allocation Scheme for Cognitive Radio Systems Based on Multi-Carrier Modulation 总被引:1,自引:0,他引:1
Tao Luo Tao Jiang Weidong Xiang Hsiao-Hwa Chen 《Wireless Communications, IEEE Transactions on》2008,7(9):3335-3340
Cognitive radio (CR) is a dynamic spectrum access technology as a solution to spectrum under-utilization problem in some licensed bands. Operating over an exceedingly wide spectrum, CR systems usually adopt multi-carrier modulation (MCM) to implement flexible channelization. Consequently, efficient channel allocation scheme becomes extremely important to an MCM based CR (MCM-CR) system. In this paper, a maximum likelihood detection model is developed to detect the presence and locations of licensed users (LUs) signals in the frequency domain. Performance of the detection model, including the optimal detection region, detection probability and false alarm probability, is analyzed. A one-order two-state Markovian chain model is proposed to predict channel status information. In particular, a novel subcarrier allocation scheme for MCM-CR systems is proposed, taking into account the confidence of channel estimation, quality of services (QoS) of rental users (RUs) and throughput. To validate the analytical results, simulations have been conducted to show effectiveness of the proposed scheme. 相似文献
86.
La doping effects on intergrowth Bi2WO6–Bi3TiNbO9 ferroelectric ceramics were studied by X-ray diffraction, electron probe microanalysis and dielectric spectroscopy. It was found that the La3+ distribution, ferroelectric phase transition and dielectric relaxation behavior are apparently affected by La doping. With increasing La3+ content, the site of dopant ion varies, the grain growth of Bi5TiNbWO15 is restrained, the Curie temperature is reduced and broadened. Furthermore, two dielectric relaxation loss peaks were observed both in temperature and frequency spectra. The calculated relaxation parameters revealed the oxygen vacancy related to the relaxation process. 相似文献
87.
M.-F. Foulon T. Crepin X. Melique D. Lippens 《Microwaves, Antennas & Propagation, IET》2007,1(1):23-28
Co-planar strip (CPS) transmission lines, aimed at operating up to sub-millimetre wavelengths, were periodically loaded by lumped capacitances and inductances in series and shunt configuration, respectively. Under this condition, propagation on the line is backward with group and phase velocities of opposite signs. The transmission lines were fabricated onto a quartz substrate by e-beam patterning and thin film deposition, silicon nitride and gold layers, on a deep sub-micron scale (~200 nm). A direct evidence of left-handedness between 160 and 380 GHz was achieved via the tracking of phase advance in a differential phase experiment using electro-optic sampling. The comparison between modelling results by electromagnetic full wave circuit analysis and experiment was performed in the time and frequency domains. It shows a good agreement in the dispersion characteristics and ultra-fast variations in the transmitted signals. At last, we took benefit of a lumped element fitting approach in order to discuss of the frequency capability of this non-resonant approach 相似文献
88.
89.
It is well established that the indentation hardness of metallic alloys shows a reasonable correlation with their yield strength or ultimate strength. Experiments illustrate that such a unique correlation is nonexistent for discontinuously reinforced metal matrix composites, even when the indentation size is much greater than the reinforcement size. For aluminum alloys reinforced with silicon carbide particles, the same composite yield strength and tensile strength with different reinforcement fractions do not lead to similar hardness, or vice versa. Finite element analyses are carried out to rationalize the experimental findings. The modeling utilizes a two-dimensional plane-strain formulation. Discrete particles are included in the material model, and the overall stress-strain response and the indentation response are numerically simulated. The results confirm the lack of unique correspondence between the composite hardness and strength. The alteration of local heterogeneity in the composite is found to affect the indentation response. Effects of the geometrical arrangement of particles and thermal residual stresses on the indentation response are also investigated numerically. 相似文献
90.
Polyester‐based polyurethane/nano‐silica composites were obtained via in situ polymerization and investigated by Fourier‐transform infrared spectroscopy (FTIR), or FTIR coupled with attenuated total reflectance (FTIR‐ATR), Transmission electron microscopy (TEM), atomic force microscopy (AFM), an Instron testing machine, dynamic mechanical analysis (DMA) and ultraviolet‐visible spectrophotometry (UV‐vis). FTIR analysis showed that in situ polymerization provoked some chemical reactions between polyester molecules and nano‐silica particles. FTIR‐ATR, TEM and AFM analyses showed that both surface and interface contained nano‐silica particles. Instron testing and DMA data showed that introducing nano‐silica particles into polyurethane enhanced the hardness, glass temperature and adhesion strength of polyurethane to the substrate, but also increased the resin viscosity. UV‐vis spectrophotometry showed that nano‐silica obtained by the fumed method did not shield UV radiation in polyurethane films. Copyright © 2003 Society of Chemical Industry 相似文献