全文获取类型
收费全文 | 3570篇 |
免费 | 59篇 |
国内免费 | 5篇 |
专业分类
电工技术 | 192篇 |
综合类 | 4篇 |
化学工业 | 701篇 |
金属工艺 | 69篇 |
机械仪表 | 109篇 |
建筑科学 | 67篇 |
矿业工程 | 4篇 |
能源动力 | 115篇 |
轻工业 | 312篇 |
水利工程 | 7篇 |
石油天然气 | 9篇 |
无线电 | 433篇 |
一般工业技术 | 568篇 |
冶金工业 | 751篇 |
原子能技术 | 126篇 |
自动化技术 | 167篇 |
出版年
2023年 | 17篇 |
2022年 | 29篇 |
2021年 | 52篇 |
2020年 | 22篇 |
2019年 | 39篇 |
2018年 | 45篇 |
2017年 | 52篇 |
2016年 | 51篇 |
2015年 | 35篇 |
2014年 | 62篇 |
2013年 | 150篇 |
2012年 | 105篇 |
2011年 | 136篇 |
2010年 | 110篇 |
2009年 | 95篇 |
2008年 | 128篇 |
2007年 | 109篇 |
2006年 | 105篇 |
2005年 | 114篇 |
2004年 | 99篇 |
2003年 | 120篇 |
2002年 | 108篇 |
2001年 | 78篇 |
2000年 | 62篇 |
1999年 | 94篇 |
1998年 | 332篇 |
1997年 | 200篇 |
1996年 | 144篇 |
1995年 | 122篇 |
1994年 | 92篇 |
1993年 | 85篇 |
1992年 | 58篇 |
1991年 | 43篇 |
1990年 | 44篇 |
1989年 | 39篇 |
1988年 | 45篇 |
1987年 | 39篇 |
1986年 | 42篇 |
1985年 | 33篇 |
1984年 | 28篇 |
1983年 | 36篇 |
1982年 | 28篇 |
1981年 | 29篇 |
1980年 | 26篇 |
1979年 | 23篇 |
1978年 | 16篇 |
1977年 | 31篇 |
1976年 | 38篇 |
1975年 | 11篇 |
1974年 | 6篇 |
排序方式: 共有3634条查询结果,搜索用时 15 毫秒
61.
Nakahara S. Moriyama S. Kuroda T. Sasaki M. Yamazaki S. Yamada O. 《Broadcasting, IEEE Transactions on》1996,42(3):173-178
Requirements for a terrestrial ISDB system and the schemes for efficiently using frequencies to fulfil the requirements are discussed. One requirement of a terrestrial ISDB system is the efficient allocation of frequencies using a SFN (single frequency network). The SFN incorporates the great advantages of OFDM, which has excellent performance under multi-path conditions. The transmission characteristics of 64QAM-OFDM and the SFN capability using 64QAM-OFDM in the Kanto area, including the Tokyo metropolitan area, are analyzed. The results show that more than a 97% coverage ratio can be obtained even by using a single channel while the same area is covered currently by more than 30 analog channels. The other requirement is a scheme for efficiently using the available frequency band for various services. A transmission scheme concept called BST-OFDM (band segmented transmission-orthogonal frequency division multiplexing) is proposed as a transmission scheme for meeting the requirements of a terrestrial ISDB system. In the BST-OFDM scheme, data are transmitted in a number of OFDM blocks called a BST-segment, for which channel coding and modulation schemes can be independently defined to provide robustness in the transmission environment according to stationary, portable and mobile reception 相似文献
62.
Makoto Kasu Rangaiya Rao Susumu Noda Akio Sasaki 《Journal of Electronic Materials》1991,20(9):691-693
Properties of theDX centers in Al0.5Ga0.5As bulk alloy (b-AL), (AlAs)2 (GaSa)2 ordered superlattice (o-SL) and (AlAs)
m
(GaAs)
n
disordered superlattice (d-SL) (m = 1, 2, 3,n = 1, 2, 3) with the same macroscopic composition were measured and compared. By deconvolution of deep level transient spectroscopy
(DLTS) spectrum due to theDX center, we have found a decrease in the number of separate peaks in DLTS spectrum in an intentionally atomic ordered arrangement.
Visiting Scholar of the Japan Society for the Promotion of Science. On leave from Department of Electrical Engineering, San
Jose State University, San Jose, California 95192-0084, USA. 相似文献
63.
This paper reports the measurement for the oscillation spectra in He-Zn II lasers at the 492.4 nm line and He-Cd II lasers at the 533.7 and 537.8 nm lines of the hollow cathode (HC) type with natural zinc or cadmium. The oscillation width of these lasers, which amounts to more than 2000 MHz, is unexpectedly broad in comparison with the theoretical gain profile; and the isotope shifts of the spectra which are caused by any two isotopes whose mass numbers differ by unity (DeltaM = 1 ) can be estimated to be as much as 300-500 MHz to explain such a broadened oscillation width. The dominant reason for the large isotope shifts is not so much the mass effects as the volume effects. The behavior of the spectra at the axial mode competition is also measured. The extraordinary appearances of the mode competition can be attributed to the influence of the isotope shifts upon the oscillation spectra. Moreover, the axial mode separation at the beginning of the mode competition is nearly coincident with the decay rate of the upper laser level. 相似文献
64.
Mode coupling effects caused by waveguide imperfections and ambient fluctuations in the stress-applied single polarization fibers have been investigated. Several origins of mode coupling, such as: 1) core deformations, 2) deformation of stress-applying parts, 3) micro-bending, and 4) temperature fluctuations, have been treated. The mode coupling parameters between the orthogonally polarized HE11 modes are compared for various kinds of waveguide imperfections and ambient fluctuations. It was clarified that the deformation of the stress-applying parts is the dominant factor in the mode coupling effects in stress-applied single polarization fibers. 相似文献
65.
The generation of phase-matched two-wave sum-frequency light has been observed in optical fibers. A Nd:YAG laser by simultaneous mode-locking andQ -switching or onlyQ -switching operation is used as a pump laser. The input power dependence and fiber length dependence of the sum-frequency light are investigated in the two pumping methods. The observed coherence length for the sum-frequency light generation is about 200 m. 相似文献
66.
Electron concentration and undoped AlGaAs spacer thickness dependencies of the mobility of a two-dimensional electron gas in a GaAs/AlGaAs single heterostructure are calculated at 4.2 K. The results predict extremely high electron mobility in this structure and agree quite well with very recent experimental data. 相似文献
67.
Summary The binding of fibrinogen with heparin and dextran sulfates, and of antithrombin III with heparin and dextran sulfate were investigated by sedimentation velocity method. From the measurement of fibrinogen-acidic polysaccharide systems, it was confirmed that heparin and low molecular weight dextran sulfate(DSC) forms soluble complexes with fibrinogen, though the amount bound of the latter was rather small compared with high molecular weight dextran sulfate(DSD). Antithrombin III was also found to be bound by heparin and DSC. The numbers of heparin molecules bound to one molecule of fibrinogen and antithrombin III were estimated to be 4.2 and 0.3, respectively. 相似文献
68.
Naoto Kimbara Akio Nishijima Toshio Sato Yuji Yoshimura Hiromichi Shimada Nobuyuki Matsubayashi Akinori Yukimura 《Catalysis Letters》1990,6(3-6):409-415
TiN supported molybdenum sulfide catalysts showed much higher activity for cleavage of C-C bonds than oxide supported molybdenum sulfide catalysts, indicating the possibility of a new generation of supports for hydroprocessing catalysts. 相似文献
69.
Ueno Y. Takahashi M. Nakamura S. Suzuki K. Shimizu T. Furukawa A. Tamanuki T. Mori K. Ae S. Sasaki T. Tajima K. 《Photonics Technology Letters, IEEE》2002,14(12):1692-1694
A control scheme for accurately optimizing (and also automatically stabilizing) the interferometer phase bias of symmetric-Mach-Zehnder (SMZ)-type ultrafast all-optical switches is proposed. In this control scheme, weak continuous-wave light is used as supervisory input light and its spectral power ratio at the switch output is used as a bipolar error signal. Our experimental results for 168-Gb/s 16:1 demultiplexing with a hybrid-integrated SMZ switch indicate the feasibility and the sensitivity of this control scheme. 相似文献
70.