全文获取类型
收费全文 | 567030篇 |
免费 | 19377篇 |
国内免费 | 9950篇 |
专业分类
电工技术 | 17612篇 |
技术理论 | 14篇 |
综合类 | 11827篇 |
化学工业 | 84213篇 |
金属工艺 | 25656篇 |
机械仪表 | 22912篇 |
建筑科学 | 24413篇 |
矿业工程 | 6661篇 |
能源动力 | 15116篇 |
轻工业 | 40033篇 |
水利工程 | 6782篇 |
石油天然气 | 17346篇 |
武器工业 | 1402篇 |
无线电 | 71273篇 |
一般工业技术 | 99712篇 |
冶金工业 | 84579篇 |
原子能技术 | 10714篇 |
自动化技术 | 56092篇 |
出版年
2023年 | 3925篇 |
2022年 | 7524篇 |
2021年 | 10613篇 |
2020年 | 7813篇 |
2019年 | 7579篇 |
2018年 | 9816篇 |
2017年 | 10705篇 |
2016年 | 10427篇 |
2015年 | 10454篇 |
2014年 | 14949篇 |
2013年 | 27811篇 |
2012年 | 21112篇 |
2011年 | 26338篇 |
2010年 | 21400篇 |
2009年 | 22915篇 |
2008年 | 22788篇 |
2007年 | 22582篇 |
2006年 | 21438篇 |
2005年 | 19438篇 |
2004年 | 16203篇 |
2003年 | 15112篇 |
2002年 | 14067篇 |
2001年 | 13922篇 |
2000年 | 13477篇 |
1999年 | 14920篇 |
1998年 | 29940篇 |
1997年 | 21347篇 |
1996年 | 16980篇 |
1995年 | 12897篇 |
1994年 | 11155篇 |
1993年 | 10594篇 |
1992年 | 7437篇 |
1991年 | 6805篇 |
1990年 | 6356篇 |
1989年 | 6029篇 |
1988年 | 5421篇 |
1987年 | 4617篇 |
1986年 | 4562篇 |
1985年 | 4866篇 |
1984年 | 4382篇 |
1983年 | 4025篇 |
1982年 | 3793篇 |
1981年 | 3795篇 |
1980年 | 3656篇 |
1979年 | 3334篇 |
1978年 | 3295篇 |
1977年 | 3693篇 |
1976年 | 5071篇 |
1975年 | 2720篇 |
1974年 | 2659篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
径向水平井弯曲转向机构影响因素仿真研究 总被引:3,自引:1,他引:2
针对径向水平井钻进中钻杆弯曲转向前进运动困难的问题,通过建立弯曲转向机构仿真有限元模型,对其主要影响因素:滑道轨迹曲率、间隙、工作压力、滚轮形状与位置和钻杆壁厚等用ANSYS软件进行仿真研究。研究表明:影响截面变形的主要因素是滑道曲率的改变和滑道摩擦因数,钻井失败时钻杆所受阻力的增大不是因为升高工作压力导致截面变形过大与滑道干涉所致,钻杆经校直段,钻头中轴线与校直中心线存在一定的角度,即钻杆的前进轨迹就会偏离理论值,或与地层干涉,或可能再次发生弯曲变形;另一主要原因是转向器滑道工作恶化。增加钻杆壁厚,降低工作压力和滑道摩擦因数,设计合理弯曲转向机构是解决问题的关键。 相似文献
62.
Tsung-Hui Huang Han-Jan Chen Chin-Sheng Chang Lih-Shan Chen Yeong-Her Wang Mau-Phon Houng 《Microwave and Wireless Components Letters, IEEE》2006,16(6):360-362
In this letter, a novel compact ring dual-mode with adjustable second-passband for dual-band applications are presented. A ring resonator with two different geometric dimensions are derived and designed to have identical fundamental and the first higher-order resonant frequencies, and to establish appropriate couplings in the structure. Moreover, the proposed filter has smaller size as compared with the basic topology of stopband filters and stepped-impedance-resonator (SIR) filters. The measured filter performance is in good agreement with the simulated response. 相似文献
63.
Pallem C. Chowdhuri P. Demko J.A. Gouge M.J. 《Applied Superconductivity, IEEE Transactions on》2006,16(1):31-37
The objective of this work was to determine the significant parameters of a 3-GW 200-kV dc superconducting cable system which influence the transient voltage distribution in the various parts of the cable. The cable system consists of four coaxial metallic cylinders. It was found that the dielectric constant and the electrical resistivity of the soil significantly affect the severity of the transient voltages; lower dielectric constant and higher resistivity of the soil will increase the magnitudes of the transient voltages by increasing the earth-return impedance. It was also found that the effect of the conductor internal impedances of the cable is insignificant. Shorting the coaxial cylinders of the cryogen flow and the cryostat will lessen the severity of the transient voltages. Grounding the second, third, and fourth cylinders at regular intervals with low-impedance grounding impedance will also improve the transient performance of the cable. More research is needed to evaluate these procedures. 相似文献
64.
结合工程实例,阐述了在输水水质腐蚀性不大的情况下,输水钢管的内壁防腐采用水泥砂浆衬里的必要性和优点,并介绍了设计要点和安装施工注意事项。 相似文献
65.
Yu. V. Baldokhin V. V. Vavilova P. Ya. Kolotyrkin Yu. K. Kovneristyi N. A. Palii A. S. Solomatin 《Inorganic Materials》2003,39(6):562-567
Mössbauer effect measurements and physicochemical analysis demonstrate that annealing of amorphous Fe–P–Mn alloys leads to the formation of a nanocrystalline structure. 相似文献
66.
Mantooth B.A. Weiss P.S. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》2003,91(11):1785-1802
One of the ultimate miniaturizations in nanotechnology is molecular electronics, where devices will consist of individual molecules. There are many complications associated with the use of molecules in electronic devices, such as the electronic perturbations in the molecule associated with being bonded to an electrode, how electrons traverse the metal-molecule junction, and the difficulty of macroscopically addressing single to very few molecules. Whether fabricating a test structure or a usable device, the use of self-assembly is fundamental to the fabrication of molecular electronic devices. We will discuss how to fabricate self-assembled monolayers into test assemblies and how to use intermolecular interactions to direct molecules into desired positions to create nanostructures and to connect functional molecules to the outside world. These assemblies serve as test structures for measurements on single or bundled molecules. The development of several experimental techniques, including various scanning probes, mercury drop junctions, break junctions, nanopores, crossed wires, and other techniques using nanoparticles have enabled the ability to test these structures and make reproducible measurements on single molecules. Many of these methods have been developed to test molecules with potential for integration into devices such as oligo (phenylene-ethynylene) molecules and other /spl pi/-conjugated molecules, in ensemble or single-molecule measurements. 相似文献
67.
H. L. Du S. R. Rose Z. D. Xiang P. K. Datta X. Y. Li 《Materialwissenschaft und Werkstofftechnik》2003,34(4):421-426
The oxidation/sulphidation behaviour of a Ti‐46.7Al‐1.9W‐0.5Si alloy with a TiAl3 diffusion coating was studied in an environment of H2/H2S/H2O at 850oC. The kinetic results demonstrate that the TiAl3 coating significantly increased the high temperature corrosion resistance of Ti‐46.7Al‐1.9W‐0.5Si. The SEM, EDX, XRD and TEM analysis reveals that the formation of an Al2O3 scale on the surface of the TiAl3‐coated sample was responsible for the enhancement of the corroison resistance. The Ti‐46.7Al‐1.9W‐0.5Si alloy was also modified by Nb ion implantation. The Nb ion implanted and as received sampels were subjected to cyclic oxidation in an open air at 800oC. The Nb ion implantation not only increased the oxidation resistance but also substantially improved the adhesion of scale to the substrate. 相似文献
68.
Biao Chen Hao Wang 《Signal Processing, IEEE Transactions on》2004,52(7):2047-2057
Blind deterministic estimation of the orthogonal frequency division multiplexing (OFDM) frequency offset via oversampling is proposed in this paper. This method utilizes the intrinsic phase shift of neighboring sample points incurred by the frequency offset that is common among all subcarriers. The proposed method is data efficient - it requires only a single OFDM symbol to achieve reliable estimation, hence making it more suitable to systems with stringent delay requirement and mobility-induced channel variation. The proposed scheme is devised to perfectly retrieve frequency offset in the absence of noise. Quite remarkably, we show that in the presence of channel noise, this intuitive scheme is indeed the maximum likelihood estimate of the carrier frequency offset. The possible presence of virtual carriers are also accommodated in the system model, and some interesting observations are obtained. The Cramer-Rao lower bound is derived for the oversampling-based signal model, and we show through numerical simulation that the proposed algorithm is efficient. Practical issues such as identifiability, the front-end filter bandwidth, and the possible presence of correlated noises are also carefully addressed. 相似文献
69.
70.
Field Static Load Test on Kao-Ping-Hsi Cable-Stayed Bridge 总被引:1,自引:0,他引:1
Field load testing is an effective method for understanding the behavior and fundamental characteristics of a cable-stayed bridge. This paper presents the results of field static load tests on the Kao-Ping-Hsi cable-stayed bridge, the longest cable-stayed bridge in Taiwan, before it was open to traffic. A total of 40 loading cases, including the unit and distributed bending and torsion loading effects, were conducted to investigate the bridge behavior. The atmospheric temperature effect on the variations of the main girder deflections was also monitored. The results of static load testing include the main girder deflections, the flexural strains of the prestressed concrete girder, and the variations of the cable forces. A three-dimensional finite-element model was developed. The results show that the bridge under the planned load test conditions has linear superposition characteristics and the analytical model shows a very good agreement with the bridge responses. Further discussion of deflection and cable forces of the design specifications for a cable-stayed bridge is also presented. 相似文献