全文获取类型
收费全文 | 49656篇 |
免费 | 2488篇 |
国内免费 | 156篇 |
专业分类
电工技术 | 708篇 |
综合类 | 65篇 |
化学工业 | 10598篇 |
金属工艺 | 2136篇 |
机械仪表 | 3242篇 |
建筑科学 | 1104篇 |
矿业工程 | 25篇 |
能源动力 | 2113篇 |
轻工业 | 3887篇 |
水利工程 | 271篇 |
石油天然气 | 88篇 |
武器工业 | 2篇 |
无线电 | 7834篇 |
一般工业技术 | 10663篇 |
冶金工业 | 3881篇 |
原子能技术 | 658篇 |
自动化技术 | 5025篇 |
出版年
2024年 | 50篇 |
2023年 | 570篇 |
2022年 | 872篇 |
2021年 | 1485篇 |
2020年 | 1069篇 |
2019年 | 1176篇 |
2018年 | 1429篇 |
2017年 | 1413篇 |
2016年 | 1751篇 |
2015年 | 1294篇 |
2014年 | 2088篇 |
2013年 | 3007篇 |
2012年 | 3266篇 |
2011年 | 3889篇 |
2010年 | 2815篇 |
2009年 | 2922篇 |
2008年 | 2812篇 |
2007年 | 2185篇 |
2006年 | 2033篇 |
2005年 | 1723篇 |
2004年 | 1571篇 |
2003年 | 1513篇 |
2002年 | 1329篇 |
2001年 | 1134篇 |
2000年 | 996篇 |
1999年 | 928篇 |
1998年 | 1557篇 |
1997年 | 991篇 |
1996年 | 803篇 |
1995年 | 557篇 |
1994年 | 461篇 |
1993年 | 406篇 |
1992年 | 290篇 |
1991年 | 274篇 |
1990年 | 258篇 |
1989年 | 241篇 |
1988年 | 205篇 |
1987年 | 168篇 |
1986年 | 119篇 |
1985年 | 115篇 |
1984年 | 92篇 |
1983年 | 63篇 |
1982年 | 38篇 |
1981年 | 39篇 |
1980年 | 30篇 |
1979年 | 31篇 |
1978年 | 30篇 |
1977年 | 38篇 |
1976年 | 61篇 |
1973年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
181.
182.
183.
184.
Sang‐Heung Lee Seung‐Yun Lee Hyun‐Cheol Bae Ja‐Yol Lee Sang‐Hoon Kim Bo Woo Kim Jin‐Yeong Kang 《ETRI Journal》2005,27(5):569-578
The purpose of this paper is to describe the implementation of monolithically matching circuits, interface circuits, and RF core circuits to the same substrate. We designed and fabricated on‐chip 1 to 6 GHz up‐conversion and 1 to 8 GHz down‐conversion mixers using a 0.8 µm SiGe hetero‐junction bipolar transistor (HBT) process technology. To fabricate a SiGe HBT, we used a reduced pressure chemical vapor deposition (RPCVD) system to grow a base epitaxial layer, and we adopted local oxidation of silicon (LOCOS) isolation to separate the device terminals. An up‐conversion mixer was implemented on‐chip using an intermediate frequency (IF) matching circuit, local oscillator (LO)/radio frequency (RF) wideband matching circuits, LO/IF input balun circuits, and an RF output balun circuit. The measured results of the fabricated up‐conversion mixer show a positive power conversion gain from 1 to 6 GHz and a bandwidth of about 4.5 GHz. Also, the down‐conversion mixer was implemented on‐chip using LO/RF wideband matching circuits, LO/RF input balun circuits, and an IF output balun circuit. The measured results of the fabricated down‐conversion mixer show a positive power conversion gain from 1 to 8 GHz and a bandwidth of about 4.5 GHz. 相似文献
185.
K- and Q-bands CMOS frequency sources with X-band quadrature VCO 总被引:1,自引:0,他引:1
Sangsoo Ko Jeong-Geun Kim Taeksang Song Euisik Yoon Songcheol Hong 《Microwave Theory and Techniques》2005,53(9):2789-2800
Fully integrated 10-, 20-, and 40-GHz frequency sources are presented, which are implemented with a 0.18-/spl mu/m CMOS process. A 10-GHz quadrature voltage-controlled oscillator (QVCO) is designed to have output with a low dc level, which can be effectively followed by a frequency multiplier. The proposed multipliers generate signals of 20 and 40 GHz using the harmonics of the QVCO. To have more harmonic power, a frequency doubler with pinchoff clipping is used without any buffers or dc-level shifters. The QVCO has a low phase noise of -118.67 dBc/Hz at a 1-MHz offset frequency with a 1.8-V power supply. The transistor size effect on phase noise is investigated. The frequency doubler has a low phase noise of -111.67 dBc/Hz at a 1-MHz offset frequency is measured, which is 7 dB higher than a phase noise of the QVCO. The doubler can be tuned between 19.8-22 GHz and the output is -6.83 dBm. A fourth-order frequency multiplier, which is used to obtain 40-GHz outputs, shows a phase noise of -102.0 dBc/Hz at 1-MHz offset frequency with the output power of -18.0 dBm. A large tuning range of 39.3-43.67 GHz (10%) is observed. 相似文献
186.
We propose an asymmetric integral imaging method to adjust the resolution and depth of a three‐dimensional image. Our method is obtained by use of two lenticular sheets with different pitches fabricated under the same F/#. The asymmetric integral imaging is the generalized version of integral imaging, including both conventional integral imaging and one‐dimensional integral imaging. We present experimental results to test and verify the performance of our method computationally. 相似文献
187.
The solar photocatalytic degradation of methyl parathion was investigated using a circulating TiO2/solar light reactor. Under solar photocatalysis condition, parathion was more effectively degraded than solar photolysis and TiO2-only conditions. With solar photocatalysis, 20 mg/L of parathion was completely degraded within 60 min with a TOC decrease of 63% after 150 min. The main ionic byproducts during photocatalysis recovered from parathion degradation were mainly as NO3-, NO2- and NH4+, 80% of the sulphur as SO4(2-), and 5% of phosphorus as PO4(3-). The organic intermediates 4-nitrophenol and methyl paraoxon were also identified, and these were further degraded in solar photocatalytic condition. Two different bioassays (Vibrio fischeri and Daphnia magna) were used to test the acute toxicity of solutions treated by solar photocatalysis and photolysis. The Microtox test using V. fischeri showed that the toxicity expressed as EC50 (%) value increased from 5.5% to >82% in solar photocatalysis, indicating that the treated solution is non-toxic, but only increased from 4.9 to 20.5% after 150 min in solar photolysis. The acute toxicity test using D. magna showed that EC50 (%) increased from 0.05 to 1.08% under solar photocatalysis, but only increased to 0.12% after 150 min with solar photolysis, indicating the solution is still toxic. The pattern of toxicity reduction parallels the decrease in TOC and the parathion concentrations. 相似文献
188.
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
189.
190.
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