全文获取类型
收费全文 | 19277篇 |
免费 | 620篇 |
国内免费 | 18篇 |
专业分类
电工技术 | 269篇 |
综合类 | 10篇 |
化学工业 | 3817篇 |
金属工艺 | 303篇 |
机械仪表 | 345篇 |
建筑科学 | 1127篇 |
矿业工程 | 111篇 |
能源动力 | 536篇 |
轻工业 | 1533篇 |
水利工程 | 166篇 |
石油天然气 | 195篇 |
武器工业 | 1篇 |
无线电 | 1209篇 |
一般工业技术 | 3099篇 |
冶金工业 | 4447篇 |
原子能技术 | 184篇 |
自动化技术 | 2563篇 |
出版年
2022年 | 163篇 |
2021年 | 243篇 |
2020年 | 193篇 |
2019年 | 254篇 |
2018年 | 296篇 |
2017年 | 303篇 |
2016年 | 353篇 |
2015年 | 279篇 |
2014年 | 425篇 |
2013年 | 1179篇 |
2012年 | 721篇 |
2011年 | 936篇 |
2010年 | 712篇 |
2009年 | 711篇 |
2008年 | 891篇 |
2007年 | 907篇 |
2006年 | 724篇 |
2005年 | 751篇 |
2004年 | 588篇 |
2003年 | 600篇 |
2002年 | 540篇 |
2001年 | 350篇 |
2000年 | 323篇 |
1999年 | 320篇 |
1998年 | 398篇 |
1997年 | 355篇 |
1996年 | 347篇 |
1995年 | 366篇 |
1994年 | 292篇 |
1993年 | 329篇 |
1992年 | 289篇 |
1991年 | 181篇 |
1990年 | 253篇 |
1989年 | 293篇 |
1988年 | 217篇 |
1987年 | 223篇 |
1986年 | 222篇 |
1985年 | 276篇 |
1984年 | 271篇 |
1983年 | 234篇 |
1982年 | 225篇 |
1981年 | 217篇 |
1980年 | 174篇 |
1979年 | 191篇 |
1978年 | 178篇 |
1977年 | 182篇 |
1976年 | 160篇 |
1975年 | 184篇 |
1974年 | 166篇 |
1973年 | 137篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
41.
Lewis V. McCarty Richard Donelson Robert f. Hehemann 《Metallurgical and Materials Transactions A》1991,22(1):969-974
A mathematical model describes the carburization kinetics of tungsten powders mixed with carbon and heated in hydrogen. It
is based on diffusion of carbon through a shell of WC growing into particles which are modeled as spheres. The activation
energy is 58 kcal/mole in the temperature range 1056 to 1833 °C. Hydrogen gas is important to transport carbon as methane
or acetylene, but increased hydrogen pressure increases the rate of carburization so little that an adsorbed species such
as CH probably controls the carbon concentration at the particle surface.
Formerly with the Lamp Metals Laboratory, deceased, was Professor of Metallurgy at Case Western Reserve University Cleveland
OH. 相似文献
42.
We study the mechanical and conformational properties of networks of helical polymers with a combination of Monte Carlo simulations based on the Wang-Landau algorithm and the three-chain model. We find that the stress-strain behavior of these networks has novel features not observed in typical networks made of synthetic polymers. In particular, we find that as these networks are stretched they first strengthen, then soften and finally strengthen again. This non-monotonic behavior of the stress correlates with the one of the helical content and is rationalized by the elongation-induced formation and melting of the helical structure of the polymer. We complement these results with a study of other conformational properties of the polymer strands that clarify the molecular mechanisms behind the mechanical behavior of these networks. Finally, we present a qualitative comparison of some of our results with the theoretical ones recently reported by Kutter and Terentjev. 相似文献
43.
44.
The Florida Department of Transportation (FDOT) is in the process of implementing Pontis, a Bridge Management System, to provide decision support to engineers in the headquarters and district offices as they make routine policy, programming, and budgeting decisions regarding the preservation and improvement of the state’s bridges. As part of this effort, an ongoing research program is underway to adapt the system to FDOT needs as well as to advance the state of the art in several areas important to the Department. Most of the research results are organized around a new project-level decision support framework that complements and builds on Pontis’ existing network-level analysis. Specific new models include accident risk and user cost due to roadway width and alignment deficiencies; user cost of load capacity and vertical clearance restrictions, and moveable bridge openings; project-level prediction models for bridge element condition and costs; and prediction of economics of scale and scoping possibilities. The new models are built into a highly graphical spreadsheet model for decision support use. 相似文献
45.
Osemwengie Iyoha Bret Howard Bryan Morreale Richard Killmeyer Robert Enick 《Topics in Catalysis》2008,49(1-2):97-107
The hydrogen permeance of 1 mm-thick Pd80wt%Cu foils was measured in the presence of equimolar mixtures of H2 with CO, CO2 or H2O over the temperature and total pressure ranges of 623–1,173 K and 0.62–2.86 MPa, respectively. In all cases, permeance losses at 623 and 738 K were very modest. At higher temperatures, more significant decreases in membrane permeance were observed with the highest reduction of about 50% occurring when macroscopic carbon deposition occurred on the membrane surface during H2–CO exposure at 908 K. The more worrisome effects of exposure to these gases, however, were the micron-scale surface defects observed at 908 and 1,038 K. Although the 1 mm thick disk membranes retained their mechanical integrity, such defects could cause catastrophic failure of ultra-thin, Pd–Cu membranes (1–5 μm thick) deposited on porous substrates. 相似文献
46.
J. Giacomin M. S. Shayaa E. Dormegnie L. Richard 《International Journal of Industrial Ergonomics》2004,33(6):56-541
The human perception of rotational hand–arm vibration has been investigated by means of a test rig consisting of a rigid frame, an electrodynamic shaker unit, a rigid steering wheel, a shaft assembly, bearings and an automobile seat. Fifteen subjects were tested while seated in a driving posture. Four equal sensation tests and one annoyance threshold test were performed using sinusoidal excitation at 18 frequencies in the range from 3 to 315 Hz. In order to guarantee the generality of the equal sensation data, the four tests were defined to permit checks of the possible influence of three factors: reference signal amplitude, psychophysical test procedure and temporary threshold shift caused by the test exposure. All equal sensation tests used a reference sinusoid of 63 Hz at either 1.0 or 1.5 m/s2 r.m.s. in amplitude. The four equal sensation curves were similar in shape and suggested a decrease in human sensitivity to hand–arm rotational vibration with increasing frequency. The slopes of the equal sensation curves changed at transition points of approximately 6.3 and 63 Hz. A frequency weighting, called Ws, was developed for the purpose of evaluating steering wheel rotational vibration. The proposed Ws has a slope of 0 dB per octave over the frequency range from 3 to 6.3 Hz, a slope of −6 dB per octave from 6.3 to 50 Hz, a slope of 0 dB per octave from 50 to 160 Hz and a slope of −10 dB per octave from 160 to 315 Hz. Ws provides a possible alternative to the existing Wh frequency weighting defined in International Standards Organisation 5349-1 (2001) and British Standards Institution BS 6842 (1987).
Relevance to industry
For the manufacturers of tyres, steering systems and other vehicular components the proposed Ws frequency weighting provides a more accurate representation of human perception of steering wheel rotational vibration than the Wh weighting of ISO 5349-1 and BS6842. 相似文献
47.
This research investigates the utility of passive microwave remote sensing instruments to accurately determine snow water equivalent (SWE) over large spatial extents. Three existing Special Sensor Microwave Imager (SSM/I) snow water equivalent algorithms produced by Chang, Tait and Goodison were evaluated for their ability to determine snow water equivalent in a snowpack containing substantial depth hoar, large faceted snow crystals. The Kuparuk River Watershed (8140 km2) test site on the North Slope of Alaska was chosen for its snowpack containing a think depth hoar layer and long history of ground truth data. A new regional snow water equivalent algorithm was developed to determine if it could produce better results than the existing algorithms in an area known to contain significant depth hoar. The four algorithms were tested to see how well they could determine snow water equivalent: (1) on a per pixel basis, (2) across swath-averaged spatial bands of approximately 850 km2, and (3) on a watershed scale. The algorithms were evaluated to see if they captured the annual spatial distribution in snow water equivalent over the watershed. Results show that the algorithms developed by Chang and from this research are generally within 3 cm of the spatially averaged snow water equivalents over the entire watershed. The algorithms produced by Chang, Tait, and in this research were able to predict the basin-wide ground measured snow water equivalent value within a percent error range from −32.4% to 24.4% in the years with a typical snowpack. None of the algorithms produce accurate results on a pixel-by-pixel scale, with errors ranging from −26% to 308%. 相似文献
48.
Yanqing Ji Hao Ying John Yen Shizhuo Zhu Daniel C. Barth‐Jones Richard E. Miller R. Michael Massanari 《国际智能系统杂志》2007,22(8):827-845
Discovering unknown adverse drug reactions (ADRs) in postmarketing surveillance as early as possible is highly desirable. Nevertheless, current postmarketing surveillance methods largely rely on spontaneous reports that suffer from serious underreporting, latency, and inconsistent reporting. Thus these methods are not ideal for rapidly identifying rare ADRs. The multiagent systems paradigm is an emerging and effective approach to tackling distributed problems, especially when data sources and knowledge are geographically located in different places and coordination and collaboration are necessary for decision making. In this article, we propose an active, multiagent framework for early detection of ADRs by utilizing electronic patient data distributed across many different sources and locations. In this framework, intelligent agents assist a team of experts based on the well‐known human decision‐making model called Recognition‐Primed Decision (RPD). We generalize the RPD model to a fuzzy RPD model and utilize fuzzy logic technology to not only represent, interpret, and compute imprecise and subjective cues that are commonly encountered in the ADR problem but also to retrieve prior experiences by evaluating the extent of matching between the current situation and a past experience. We describe our preliminary multiagent system design and illustrate its potential benefits for assisting expert teams in early detection of previously unknown ADRs. © 2007 Wiley Periodicals, Inc. Int J Int Syst 22: 827–845, 2007. 相似文献
49.
Richard A. Walton 《Israel journal of chemistry》1985,25(2):196-203
The structural and reaction chemistry of dimetal complexes containing metal—metal triple bonds of the electron-rich type (σ2π4δ2δ*2 configuration) is reviewed. Metal cores possessing this unit are Mo2+2, Re4+2, Ru6+2, and Os6+2, of which only Re4+2 has yet been subject to a detailed study. Emphasis is placed upon the more recent developments in this field. 相似文献
50.
Rate constants for the reversible deprotonation of acetylacetone were measured in carboxylate and amine buffers in water and in 50%, 90% and 95% Me2SO at 20°C. The Brønsted plot for the carboxylate ions is curved in the Me2SO—water mixtures, but straight in water. The curvature is in the direction predicted by the Reactivity—Selectivity Principle (RSP). However, the Brønsted plot for the reaction with primary amines is straight in all solvents. This suggests that the curvature observerd with the carboxylate ions is caused by loss of solvation of the base; this loss of solvation is ahead of bond formation in the transition state rather than being a manifestation of the RSP. (Note that all Brønsted plots are based on pKa values measured in the respective solvents.) The intrinsic rate constant (k0) for proton transfer increases with the addition of Me2SO, and more so with the carboxylate buffers than with the amines. This increase in k0 is attributed to delayed solvation of the developing enolate ion in the transition state; with the carboxylate buffers, an additional factor is the early loss of solvation of the base. The various solvation effects observed in this study can all be understood in the context of the Principle of Imperfect Synchronization (PIS). 相似文献