全文获取类型
收费全文 | 243272篇 |
免费 | 3151篇 |
国内免费 | 1241篇 |
专业分类
电工技术 | 4829篇 |
综合类 | 264篇 |
化学工业 | 35412篇 |
金属工艺 | 10861篇 |
机械仪表 | 7066篇 |
建筑科学 | 6238篇 |
矿业工程 | 481篇 |
能源动力 | 6273篇 |
轻工业 | 24147篇 |
水利工程 | 1836篇 |
石油天然气 | 1335篇 |
武器工业 | 3篇 |
无线电 | 31035篇 |
一般工业技术 | 44878篇 |
冶金工业 | 50106篇 |
原子能技术 | 3860篇 |
自动化技术 | 19040篇 |
出版年
2021年 | 1564篇 |
2019年 | 1424篇 |
2018年 | 2321篇 |
2017年 | 2223篇 |
2016年 | 2444篇 |
2015年 | 1907篇 |
2014年 | 3160篇 |
2013年 | 10793篇 |
2012年 | 5449篇 |
2011年 | 7786篇 |
2010年 | 6284篇 |
2009年 | 6962篇 |
2008年 | 7558篇 |
2007年 | 7658篇 |
2006年 | 7031篇 |
2005年 | 6519篇 |
2004年 | 6244篇 |
2003年 | 6261篇 |
2002年 | 6239篇 |
2001年 | 6371篇 |
2000年 | 5801篇 |
1999年 | 6417篇 |
1998年 | 16558篇 |
1997年 | 11597篇 |
1996年 | 8948篇 |
1995年 | 6689篇 |
1994年 | 5907篇 |
1993年 | 5727篇 |
1992年 | 4051篇 |
1991年 | 3994篇 |
1990年 | 3582篇 |
1989年 | 3504篇 |
1988年 | 3440篇 |
1987年 | 2907篇 |
1986年 | 2842篇 |
1985年 | 3413篇 |
1984年 | 3050篇 |
1983年 | 2743篇 |
1982年 | 2547篇 |
1981年 | 2568篇 |
1980年 | 2430篇 |
1979年 | 2271篇 |
1978年 | 2235篇 |
1977年 | 2808篇 |
1976年 | 4171篇 |
1975年 | 1899篇 |
1974年 | 1795篇 |
1973年 | 1783篇 |
1972年 | 1460篇 |
1971年 | 1317篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
951.
This paper tests the assertion that experienced contractors are more competitive than inexperienced contractors by measuring the effect of experience on bidding competitiveness for building contracts procured by a regular client. Contractors are grouped into experienced and inexperienced contractors, and two levels of contractors’ experience are identified: (1) bidding experience only and (2) bidding plus construction experience. Results of two-sample t tests show that experienced contractors are more competitive than inexperienced contractors. Contractors’ competitiveness in respect of school contracts and other contracts are further compared in evaluating the relationship between the project type and experience effect. Experienced contractors are, on average, more competitive in competing for school contracts. It emerges that the factor of project type familiarity amplifies the experience effect. A comparison of experienced contractors’ competitiveness in bidding for (1) new building works and (2) alteration and extension works shows that the effect of experience would be more significant when contract works packages are highly standardized. 相似文献
952.
K. Lefmann B. Lake G. Aeppli S.-W. Cheong N. B. Christensen K. N. Clausen S. Hayden T. E. Mason D.F. McMorrow H. A. Mook H. M. Rønnow H. Takagi 《Journal of Low Temperature Physics》2004,135(5-6):621-664
One important area of modern condensed matter research is the investigation of the nature of the superconducting cuprates. Much progress in this field has been obtained with the technique of neutron scattering. We here present a review of neutron scattering studies of the high-temperature superconductor La2?xSrxCuO4, performed at Risø National Laboratory. We review the work on the mapping of the incommensurate spin fluctuations, the investigation of the gap in the fluctuation spectrum, the magnetic properties of the vortices appearing in an applied field, and the quantum critical behaviour of the system. We discuss our findings in the light of results of neutron scattering from other groups, on other cuprate systems, and results from other experimental methods, e.g. NMR, μSR, STM, X-ray diffraction, and ARPES. We end with a discussion on the implications of the experimental results for the progress in the general understanding of high-temperature superconductivity. PACS numbers: 61.12.Ex, 74.20.Mn, 74.72.Dn, 75.25.+z, 78.70.Nx. 相似文献
953.
S. Klages M. Schöck C. Sürgers H. v. Löhneysen 《Journal of Low Temperature Physics》2004,137(3-4):509-522
The morphology and electronic transport of ultrathin Au films with thicknesses d = 1 ? 5 monolayers (ML) deposited on Si(111)7 × 7 surfaces is investigated by in situ scanning tunneling microscopy and electrical resistance measurements for temperatures T = 2 ? 300 K. With decreasing film thickness, i.e. decreasing sheet conductance Gs, a transition from a weakly conducting regime described by a logarithmic temperature dependence to an insulating regime occurs. In the insulating regime, the temperature dependence is described by Gs ∝ exp[?(T 0 /T) n] with an exponent n which gradually changes from 0.69 to 1 with decreasing film thickness. In contrast, for the Si(111)6 × 6-Au reconstruction obtained after annealing, an exponent n = 1/2 is found suggesting the formation of a soft Coulomb gap due to electron-electron interaction. PACS numbers: 68.37.-d, 68.55.-a, 73.50.-h, 73.25.+i, 81.15.-z 相似文献
954.
955.
N. Karasawa H. Kakehata K. Mishina J. Yamamoto S. Kobayashi 《Photonics Technology Letters, IEEE》2005,17(1):31-33
Using a cross-correlation frequency-resolved optical gating apparatus, the phases and amplitudes of ultrabroad-band optical pulses from a taper fiber have been measured for various input pulse peak powers. The measured waveforms have been compared with calculated waveforms using a finite-difference in the frequency domain method, in which no envelope approximations were used and the variation of the taper shape was taken into account. Excellent agreement between the measured and the calculated waveforms was obtained when the additional dispersion of a normal fiber and an objective was considered. 相似文献
956.
S. Danto P. Houizot C. Boussard‐Pledel X.‐H. Zhang F. Smektala J. Lucas 《Advanced functional materials》2006,16(14):1847-1852
Conditions of formation of bulk tellurium chalcogenide glasses, containing 70–80 at % Te associated to Ga and Ge are described. The characteristic temperatures are measured: the glass‐transition temperature, Tg, is situated in the range 140–185 °C and the difference Tx – Tg, where Tx is the onset crystallization temperature, is in the range 76–113 °C. Moreover, the optical transmission window is exceptional, ranging from 1.99 μm in the bandgap up to 28 μm in the phonon region. Developed within the framework of requirements for the Darwin mission (a search for and study of extrasolar planets), feasibility of infrared optical fibers formed from these glasses is studied. Drawing experiments are conducted with the glass Ga2‐Ge3Te15, which possesses the maximum Tx – Tg, and a first optical fiber is presented. 相似文献
957.
958.
H. Maurer O. Univ.-Prof. Dr. phil. 《e & i Elektrotechnik und Informationstechnik》2003,120(11):411-413
In this article it is argued that in ten years’ time personal computers as we know them now won’t exist anymore — all their functions will by then be integrated in mobiles which will be our steady companions. As such, mobiles will change our lives to an enormous extent. Technology as well as consequences will be discussed in this paper. 相似文献
959.
Calvente J. Martinez-Salamero L. Valderrama H. Vidal-Idiarte E. 《Power Electronics Letters, IEEE》2004,2(2):58-62
A dynamic analysis of the boost converter with an output filter reveals that magnetic coupling between inductors allows transfer of the zeros to the left half-plane of the control-to-output transfer function. Similar results requiring smaller magnetic components are obtained by combining magnetic coupling with damping of the output filter. The analysis is based on the application of the Routh-Hurtwitz's criterion to the numerator of the transfer function in order to derive the design conditions for the converter parameters. A design example illustrates the procedure, and experimental results verify the theoretical predictions. The application of these techniques will allow the design of high efficiency voltage boost-based regulators with dynamic behavior similar to buck-derived structures. 相似文献
960.
Ideally selective diisocyanate building blocks: New perspectives for dendrimers and coating binders 总被引:3,自引:0,他引:3
R. A. T. M. van Benthem A. Hofland H. W. I. Peerlings E. W. Meijer 《Progress in Organic Coatings》2003,48(2-4):164-176
Using zirconium (IV) salts as catalyst, the reaction of a diisocyanate carrying one tertiary and one primary isocyanate group (IMCI, DIMP) with alcohols can be conducted with complete regioselectivity. This unique selectivity enables the use of a diisocyanate building block in unprecedented ways. Incomplete regioselectivity of diisocyanates, as commonly encountered in currently commercial ones like IPDI and TDI, inevitably leads to polydisperse products in reaction with polyols. The ultimate monodisperse polymer architecture, dendrimers, can now be made in a facile, straightforward manner using IMCI as the ideal diisocyanate building block. Coating applications of this unique building block are not restricted to these perfect branching polymers, but also include isocyanate-functional coating resins. Without any increase in polydispersity, a hydroxyl-functional polyester can be end-capped with the IMCI diisocyanate and subsequently serve as a crosslinker in powder coatings. 相似文献