全文获取类型
收费全文 | 233976篇 |
免费 | 13542篇 |
国内免费 | 7361篇 |
专业分类
电工技术 | 9412篇 |
技术理论 | 3篇 |
综合类 | 10574篇 |
化学工业 | 35902篇 |
金属工艺 | 13032篇 |
机械仪表 | 12895篇 |
建筑科学 | 11549篇 |
矿业工程 | 3852篇 |
能源动力 | 6457篇 |
轻工业 | 14665篇 |
水利工程 | 4096篇 |
石油天然气 | 5154篇 |
武器工业 | 1286篇 |
无线电 | 29693篇 |
一般工业技术 | 37501篇 |
冶金工业 | 10902篇 |
原子能技术 | 2470篇 |
自动化技术 | 45436篇 |
出版年
2024年 | 487篇 |
2023年 | 1889篇 |
2022年 | 3608篇 |
2021年 | 5459篇 |
2020年 | 4016篇 |
2019年 | 3663篇 |
2018年 | 17940篇 |
2017年 | 17305篇 |
2016年 | 14219篇 |
2015年 | 6178篇 |
2014年 | 7855篇 |
2013年 | 9789篇 |
2012年 | 13826篇 |
2011年 | 21229篇 |
2010年 | 18524篇 |
2009年 | 15848篇 |
2008年 | 16907篇 |
2007年 | 17017篇 |
2006年 | 9107篇 |
2005年 | 8428篇 |
2004年 | 6790篇 |
2003年 | 5989篇 |
2002年 | 5129篇 |
2001年 | 4041篇 |
2000年 | 3385篇 |
1999年 | 2632篇 |
1998年 | 2766篇 |
1997年 | 1969篇 |
1996年 | 1614篇 |
1995年 | 1183篇 |
1994年 | 967篇 |
1993年 | 786篇 |
1992年 | 554篇 |
1991年 | 508篇 |
1990年 | 433篇 |
1989年 | 403篇 |
1988年 | 308篇 |
1987年 | 263篇 |
1986年 | 220篇 |
1985年 | 166篇 |
1984年 | 133篇 |
1983年 | 117篇 |
1982年 | 64篇 |
1981年 | 78篇 |
1980年 | 63篇 |
1977年 | 64篇 |
1976年 | 89篇 |
1959年 | 52篇 |
1955年 | 63篇 |
1954年 | 68篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
In applying pattern recognition methods in remote sensing problems, an inherent limitation is that there is almost always only a small number of training samples with which to design the classifier. A hybrid decision tree classifier design procedure that produces efficient and accurate classifiers for this situation is proposed. In doing so, several key questions are addressed, among them the question of the feature extraction techniques to be used and the mathematical relationship between sample size, dimensionality, and risk value. Empirical tests comparing the hybrid design classifier with a conventional single layered one are presented. They suggest that the hybrid design produces higher accuracy with fewer features. The need for fewer features is an important advantage, because it reflects favorably on both the size of the training set needed and the amount of computation time that will be needed in analysis 相似文献
92.
Ho A.N. Pantell R.H. Feinstein J. Huang Y.C. 《Quantum Electronics, IEEE Journal of》1991,27(12):2650-2655
A novel wiggler design for use in free-electron lasers (FELs) is proposed, consisting of a staggered array of magnetic poles situated inside the bore of a solenoid. The resultant field pattern consists of a periodic transverse magnetic field on axis, as well as a longitudinal guide field. Such a wiggler has several advantages: the longitudinal field acts to confine the electrons near the FEL axis, high fields can be attained at short wiggler periods, the field strength is easily varied, and fabrication and testing of the wiggler are relatively easy. It is planned to use this wiggler design in a far infrared FEL to be built at Stanford University 相似文献
93.
Recently, Timmermann and Nowak developed algorithms for estimating the means of independent Poisson random variables. The algorithms are based on a multiscale model where certain random variables are assumed to obey a beta-mixture density function. Timmermann and Nowak simplify the density estimation problem by assuming the beta parameters are known and only one mixture parameter is unknown. They use the observed data and the method of moments to estimate the unknown mixture parameter. Taking a different approach, we generate training data from the observed data and compute maximum likelihood estimates of all of the beta-mixture parameters. To assess the improved performance obtained by the proposed modification, we consider a denoising application using Poisson data. 相似文献
94.
95.
Yu-Li Hsueh Jaedon Kim Ching-Fong Su Rabbat R. Hamada T. Tian C. Kazovsky L.G. 《Lightwave Technology, Journal of》2006,24(1):44-53
A sublambda traffic-grooming scheme on wavelength-division-multiplexed (WDM) rings, which is called optical burst transport (OBT), is proposed. The network protocol and architecture allow increased flexibility to tailor the transport network behavior for efficient delivery of bursty data traffic. Using different network parameters, its performance is analyzed via simulation, and the implementation issues including the media-access-control (MAC) protocol, tunable-filter controller, and burst-mode receiver are addressed. Finally, the feasibility of the OBT with an experimental testbed built by the authors is demonstrated and a streaming-video application is used to present its overall functionality. 相似文献
96.
The stability of five major phenolics, namely (−)-epicatechin (EC), procyanidin B2 (PC-B2), chlorogenic acid (ChA), hyperoside (HP) and isoquercitrin (IQ), in hawthorn fruits and a canned hawthorn drink were evaluated during 6 months of storage in the dark at three different temperatures (4, 23 and 40 °C). HPLC with a diode-array detector was used to determine the contents of the individual compounds. The results showed that the studied phenolics in the hawthorn fruits and the drink were both stable at 4 °C and relatively unstable at 23 and 40 °C with varied extents of degradation. At room temperature (23 °C), marked degradations of EC and PC-B2 were observed in both the fruits and the drink with around 50% and 30% decrease after a 6-month storage, respectively. A more significant decrease of the phenolics was observed at 40 °C, especially for EC and PC-B2, which were almost completely degraded after a 6-month storage. HP, IQ and ChA were relatively stable at 23 °C, but unstable at 40 °C. Therefore, low-temperature storage is recommended for maintaining the quality and efficacy of hawthorn fruits and its preparations. 相似文献
97.
The relatively fast diffusion of Au atoms in eutectic PbSn matrix is considered one of the contributing factors to the Au
embrittlement problem. In this study, we further investigated the Au embrittlement problem in high-Sn solders. Experimentally,
Sn3.5Ag (wt.%) spheres with 500-μm diameter were soldered over the Au/Ni soldering pads. It was found that some of the AuSn4 needles that formed after reflow inside the solder migrated back to the solder/pad interface during thermal aging. However,
the migration kinetics in high-Sn solders was slower compared to that in eutectic PbSn. The difference in migration kinetics
of AuSn4 in eutectic PbSn and SnAg was ascribed to the difference in the magnitudes of the Au flux and the Ni flux. In eutectic PbSn,
the Au flux was much greater than that of the Ni flux, and the Au and Ni flux were in the same order of magnitude in eutectic
SnAg. The relative magnitude of the Au and Ni flux changed in eutectic PbSn and SnAg because the homologous temperatures of
PbSn and SnAg were different. 相似文献
98.
99.
100.
Coal is potentially a very inexpensive source of clean hydrogen fuel for use in fuel cells, turbines, and various process applications. To realize its potential however, efficient low-cost gas separation systems are needed to provide high purity oxygen that will enhance the coal gasification reaction and to extract hydrogen from the resulting gas product stream. Several types of inorganic membranes are being developed for hydrogen or oxygen separation, including porous alumina, transition metal oxide perovskites, and zirconia. Because they form the heart of the working device, numerous advances have been made in the fabrication and performance of these membrane materials. However, less emphasis has been placed on the materials that will be used in the balance of the device; in particular, the seals that bond the functional ceramic to the metallic structural component. In an effort to begin addressing this issue, we have examined ceramic-to-metal brazing as a method of sealing a model set of gas separation component materials: yttria-stabilized zirconia and stainless steel. In comparative high-temperature exposure testing of joints prepared using commercial brazes and a newly conceived braze alloy, the commercial material proved to be unsuitable due to excessive oxidation. On the other hand, the new material not only displayed superior oxidation resistance, but also excellent hermeticity in prototypic membrane testing. 相似文献