全文获取类型
收费全文 | 246554篇 |
免费 | 7313篇 |
国内免费 | 4147篇 |
专业分类
电工技术 | 7579篇 |
技术理论 | 10篇 |
综合类 | 4725篇 |
化学工业 | 31443篇 |
金属工艺 | 15742篇 |
机械仪表 | 12943篇 |
建筑科学 | 8708篇 |
矿业工程 | 2758篇 |
能源动力 | 5506篇 |
轻工业 | 12366篇 |
水利工程 | 3120篇 |
石油天然气 | 5767篇 |
武器工业 | 541篇 |
无线电 | 31096篇 |
一般工业技术 | 50073篇 |
冶金工业 | 34399篇 |
原子能技术 | 3209篇 |
自动化技术 | 28029篇 |
出版年
2023年 | 1321篇 |
2022年 | 2239篇 |
2021年 | 3465篇 |
2020年 | 2699篇 |
2019年 | 2401篇 |
2018年 | 12073篇 |
2017年 | 12762篇 |
2016年 | 8148篇 |
2015年 | 4089篇 |
2014年 | 4848篇 |
2013年 | 8525篇 |
2012年 | 9588篇 |
2011年 | 17230篇 |
2010年 | 15364篇 |
2009年 | 14534篇 |
2008年 | 14373篇 |
2007年 | 16487篇 |
2006年 | 6964篇 |
2005年 | 9987篇 |
2004年 | 7354篇 |
2003年 | 6561篇 |
2002年 | 5026篇 |
2001年 | 4549篇 |
2000年 | 4496篇 |
1999年 | 5039篇 |
1998年 | 11077篇 |
1997年 | 7483篇 |
1996年 | 6104篇 |
1995年 | 4277篇 |
1994年 | 3777篇 |
1993年 | 3419篇 |
1992年 | 2172篇 |
1991年 | 1988篇 |
1990年 | 1841篇 |
1989年 | 1607篇 |
1988年 | 1390篇 |
1987年 | 999篇 |
1986年 | 987篇 |
1985年 | 985篇 |
1984年 | 856篇 |
1983年 | 737篇 |
1982年 | 748篇 |
1981年 | 713篇 |
1980年 | 598篇 |
1979年 | 488篇 |
1978年 | 420篇 |
1977年 | 541篇 |
1976年 | 970篇 |
1975年 | 301篇 |
1974年 | 279篇 |
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
81.
82.
83.
84.
85.
86.
87.
The design of a microstrip‐fed annular‐ring slot antenna (ARSA) with circular polarization (CP) radiation is initially studied. To obtain CP radiation with broad 3‐dB axial ratio (AR) bandwidth that can cover the WiMAX 2.3 GHz (2305–2320 MHz, 2345–2360 MHz) and WLAN 2.4 GHz (2400–2480 MHz) bands, a novel technique of extending an inverted L‐shaped slot from the bottom section of the annular‐ring is proposed. To suppress the harmonic modes induced by the CP ARSA, the technique of integrating a defected ground structure into the annular‐ring slot is further introduced. From the measured results, 10‐dB impedance bandwidth and 3‐dB AR bandwidth of 44.86 and 9.68% were achieved by the proposed harmonic suppressed CP ARSA. Furthermore, average gain and radiation efficiency of ~4.7 dBic and 71%, respectively, were also exhibited across the bands of interest. © 2014 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:337–345, 2015. 相似文献
88.
Haitang Yang Zilong Lu Bixiong Bie Zeyu Fu Jianling Yue Xiaozhong Huang 《Ceramics International》2019,45(9):11395-11402
SiCf/PyC/SiC and SiCf/BN/SiC mini-composites comprising single tow SiC fibre-reinforced SiC with chemical vapor deposited PyC or BN interface layers are fabricated. The microstructure evolutions of the mini-composite samples as the oxidation temperature increases (oxidation at 1000, 1200, 1400, and 1600?°C in air for 2?h) are observed by scanning electron microscopy, energy dispersive spectrometry, and X-ray diffraction characterization methods. The damage evolution for each component of the as-fabricated SiCf/SiC composites (SiC fibre, PyC/BN interface, SiC matrix, and mesophase) is mapped as a three-dimensional (3D) image and quantified with X-ray computed tomography. The mechanical performance of the composites is investigated via tensile tests.The results reveal that tensile failure occurs after the delamination and fibre pull-out in the SiCf/PyC/SiC composites due to the volatilization of the PyC interface at high temperatures in the air environment. Meanwhile, the gaps between the fibres and matrix lead to rapid oxidation and crack propagation from the SiC matrix to SiC fibre, resulting in the failure of the SiCf/PyC/SiC composites as the oxidation temperature increases to 1600?°C. On the other hand, the oxidation products of B2O3 molten compounds (reacted from the BN interface) fill up the fracture, cracks, and voids in the SiC matrix, providing excellent strength retention at elevated oxidation temperatures. Moreover, under the protection of B2O3, the SiCf/BN/SiC mini-composites show a nearly intact microstructure of the SiC fibre, a low void growth rate from the matrix to fibre, and inhibition of new void formation and the SiO2 grain growth from room to high temperatures. This work provides guidance for predicting the service life of SiCf/PyC/SiC and SiCf/BN/SiC composite materials, and is fundamental for establishing multiscale damage models on a local scale. 相似文献
89.
According to the definition of spectral integral,a new spectral characteristic parameter,with the name Reversed Spectral Absorption Integral(RSAI),is proposed and used to retrieve the chromium content based on the Partial Least Squares Regression(PLSR) model.The contrastive study with other traditional spectral characteristic parameters,including differential transformation,inverse transformation,absorption area,etc.indicates that(1) the first derivation of square root transformed model can predict the chromium content quantitatively in terms of spectral transformations.(2) the stability of the absorption area model is slightly poor,and the chromium content of samples can only be estimated roughly.(3) However,as to the inversed spectral absorption integral model,the adjustment determination coefficient(Ad-R2) of the modeling and verification is 0.73 and 0.77,while the Root Mean Squared Error(RMSE) is 2.63 mg/kg and 2.36 mg/kg respectively with Relative Percent Deviation(RPD) being 3.21,which shows that the RSAI model has excellent prediction ability.So,the inversed spectral absorption integral new model can improve the accuracy and stability used to retrieve the chromium content,which provides a new idea for monitoring the chromium contamination in soil. 相似文献
90.