全文获取类型
收费全文 | 390697篇 |
免费 | 4102篇 |
国内免费 | 779篇 |
专业分类
电工技术 | 6600篇 |
综合类 | 216篇 |
化学工业 | 60704篇 |
金属工艺 | 21058篇 |
机械仪表 | 14226篇 |
建筑科学 | 7348篇 |
矿业工程 | 4752篇 |
能源动力 | 7878篇 |
轻工业 | 20932篇 |
水利工程 | 5905篇 |
石油天然气 | 16122篇 |
武器工业 | 41篇 |
无线电 | 36818篇 |
一般工业技术 | 91876篇 |
冶金工业 | 59039篇 |
原子能技术 | 13973篇 |
自动化技术 | 28090篇 |
出版年
2021年 | 4028篇 |
2019年 | 3933篇 |
2018年 | 7426篇 |
2017年 | 7677篇 |
2016年 | 8152篇 |
2015年 | 4490篇 |
2014年 | 7645篇 |
2013年 | 16490篇 |
2012年 | 11029篇 |
2011年 | 13950篇 |
2010年 | 11313篇 |
2009年 | 12610篇 |
2008年 | 12847篇 |
2007年 | 12396篇 |
2006年 | 10351篇 |
2005年 | 9324篇 |
2004年 | 8859篇 |
2003年 | 8611篇 |
2002年 | 8341篇 |
2001年 | 8247篇 |
2000年 | 8005篇 |
1999年 | 7524篇 |
1998年 | 16274篇 |
1997年 | 12016篇 |
1996年 | 9131篇 |
1995年 | 7078篇 |
1994年 | 6395篇 |
1993年 | 6685篇 |
1992年 | 5376篇 |
1991年 | 5452篇 |
1990年 | 5383篇 |
1989年 | 5202篇 |
1988年 | 5109篇 |
1987年 | 4796篇 |
1986年 | 4755篇 |
1985年 | 5191篇 |
1984年 | 4964篇 |
1983年 | 4762篇 |
1982年 | 4430篇 |
1981年 | 4462篇 |
1980年 | 4518篇 |
1979年 | 4706篇 |
1978年 | 4827篇 |
1977年 | 5027篇 |
1976年 | 5767篇 |
1975年 | 4445篇 |
1974年 | 4464篇 |
1973年 | 4555篇 |
1972年 | 4054篇 |
1971年 | 3701篇 |
排序方式: 共有10000条查询结果,搜索用时 937 毫秒
991.
A 10.6 μm scan laser has been constructed and operated with an off-axis cathode ray tube, high reflectance multilayer thin-film structures, and a tapered plasma discharge tube. Equations are given for the switching time of a high-reflectance spot on the VO2 and for the relation of scan laser output power to cavity geometry, cavity losses, and the gain of the active CO2 medium. A scan capability of2.1 times 10^{3} easily resolvable directions was demonstrated, and sequential and randomly addressed spot rates of 105/s were achieved. The equations relating output power and cavity mode size were experimentally verified using a nonscanned beam. 相似文献
992.
V. I. Gorynin 《Strength of Materials》1991,23(5):524-527
The influence of the level of defectiveness of design and service origin (grooves and cracks) of a fastener and an impact specimen on the accuracy of determination of the allowable values of the ductile-to-brittle transition temperature is investigated. An evaluation of the allowable ductile-to-brittle transition temperatures making it possible to impose more soundly based requirements for effectiveness of the material in the stages of design, production, and service in relation to the production condition of the steel, the level of defectiveness of the fastener, and the specified operating life of the threaded joint is proposed.Translated from Problemy Prochnosti, No. 5, pp. 35–37, May, 1991. 相似文献
993.
R.A. Cunningham N.E. Sanderson W.N.J. Snodgrass D.W. Banes S.D. Hoath J.N. Mo 《Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment》1985,234(1):67-80
The design, construction and performance of a modular, two pressure focal plane detector, 550 mm long, 1000 mm deep and with an active height of 60 mm are described. The detector has been operated with ions ranging from 50 MeV 1H to 200 MeV 36S. Typical results achieved are 0.5 mm and 0.3° for the spatial and angular resolutions, and 0.7% and 3% for the total energy and energy loss measurements. A height signal is also provided. 相似文献
994.
Computer simulation is employed to assess jitter performance of a clock recovery circuit as a function of the characteristics of the rectifier being used. Several types of rectifiers are compared, some operating at baseband, others at intermediate frequency (IF). It is shown that the best choice between them depends both on the modulation format and on the excess bandwidth factor of the pulse spectrum. In QPSK systems, fourth-law rectifiers outperform the others for rolloff factors up to 0.2 while, for higher values, baseband absolutevalue rectifiers are preferable. In the case of 9QPRS, baseband absolutevalue rectifiers provide jitter reductions of one order of magnitude at high signal-to-noise ratios. 相似文献
995.
996.
Cross-layer design: a survey and the road ahead 总被引:25,自引:0,他引:25
Of late, there has been an avalanche of cross-layer design proposals for wireless networks. A number of researchers have looked at specific aspects of network performance and, approaching cross-layer design via their interpretation of what it implies, have presented several cross-layer design proposals. These proposals involve different layers of the protocol stack, and address both cellular and ad hoc networks. There has also been work relating to the implementation of cross-layer interactions. It is high time that these various individual efforts be put into perspective and a more holistic view be taken. In this article, we take a step in that direction by presenting a survey of the literature in the area of cross-layer design, and by taking stock of the ongoing work. We suggest a definition for cross-layer design, discuss the basic types of cross-layer design with examples drawn from the literature, and categorize the initial proposals on how cross-layer interactions may be implemented. We then highlight some open challenges and new opportunities for cross-layer design. Designers presenting cross-layer design proposals can start addressing these as they move ahead. 相似文献
997.
J Byrne S Kienzle D Johnson G Duke V Gannon B Selinger J Thomas 《Water science and technology》2006,53(10):327-334
Long-term trends in alpine and prairie snow pack accumulation and melt are affecting streamflow within the Oldman River Basin in southern Alberta, Canada. Unchecked rural and urban development also has contributed to changes in water quality, including enhanced microbial populations and increased waterborne pathogen occurrence. In this study we look at changing environment within the Oldman River Basin and its impact on water quality and quantity. The cumulative effects include a decline in net water supplies, and declining quality resulting in increased risk of disease. Our data indicates that decreases in the rate of flow of water can result in sedimentation of bacterial contaminants within the water column. Water for ecosystems, urban consumption, recreation and distribution through irrigation is often drawn from waterholding facilities such as dams and weirs, and concern must be expressed over the potential for contaminate build-up and disproportionate potential of these structures to pose a risk to human and animal health. With disruption of natural flow rates for water resulting from environmental change such as global warming and/or human intervention, increased attention needs to be paid to use of best management practices to protect source water supplies. 相似文献
998.
M.G. Pujar N. Parvathavarthini Sidhartha S. Jena B.V.R. Tata R.K. Dayal H.S. Khatak 《Journal of Materials Engineering and Performance》2008,17(6):793-801
In the present work 316LN as well as 316 stainless steel (SS) coupons each of dimensions (0.025 × 0.018 × 0.006 m3) in well-polished condition were used as two nominal electrodes which were immersed in the aerated solution of 0.5 M NaCl.
Correlated current and potential electrochemical noise (EN) signals were collected at 1 Hz sampling frequency for 1 h daily
over a period of 30 days. The detrended EN data were used to calculate the noise resistance (R
N) as well as the spectral noise resistance at zero frequency (R
SNo) values and other statistical parameters. To study the nature of pits and distribution of their diameters as well as depths,
extensive observations of the pitted and the blank specimens were carried out using Confocal Laser Scanning Microscopy (CLSM).
The current and the potential records of the two alloys showed distinct differences in their corrosion behavior. It was observed
that within less than 4 h of immersion, 316SS showed signals indicative of unstable pitting and onset of stable pitting was
noticed after 48 h of exposure. However, until about 24 h, 316LN showed just the random signals and unstable pitting was observed
after 28 h. The signals clearly indicated continuous growth of the stable pits in 316SS as against the repassivation of the
unstable pits in 316LN after 7 days exposure. It was observed that R
N values showed a continuous decrease in the case of 316SS, but were quite stable in the case of 316LN over the exposure period.
Concurrent to these observations it was observed that 316SS specimen was extensively pitted. The frequency distributions of
pit diameters as well pit depths were observed to be highest at 10-20 μm and 5-10 μm, respectively. However, pits with as
large as 70-80 μm diameter and as deep as 20-25 μm too were observed. No pits were observed in case of 316LN even after 30 days
of exposure, an observation that corroborates well with the stable R
N values. Thus, in the present investigation, the long-term tests using EN technique coupled with CLSM studies conclusively
prove that many unstable pits initiated in 316SS turn into stable ones resulting in insidious localized corrosion attack whereas
the unstable pits initiated in 316LN get passivated in the simulated coastal environment. 相似文献
999.
The switchable optical properties of Pd-protected RCo2-type Ho0.6Mm0.4Co2 alloy thin films have been investigated in a KOH electrolyte. The reversible optical switching has been carried out simultaneously by measuring transmitted light through the thin film during electrochemical charging–discharging of hydrogen. The dependence of switching speed and cyclic durability of the film on the charging and discharging current density as well as concentration of KOH electrolyte has been studied. In addition, cyclic voltammetric measurements have been performed to examine the hydride formation and decomposition reactions. 相似文献
1000.
Stress analysis of spontaneous Sn whisker growth 总被引:5,自引:0,他引:5
K. N. Tu Chih Chen Albert T. Wu 《Journal of Materials Science: Materials in Electronics》2007,18(1-3):269-281
Spontaneous Sn whisker growth is a surface relief phenomenon of creep, driven by a compressive stress gradient. No externally
applied stress is required for the growth, and the compressive stress is generated within, from the chemical reaction between
Sn and Cu to form the intermetallic compound Cu6Sn5 at room temperature. To obtain the compressive stress gradient, a break of the protective oxide on the Sn surface is required
because the free surface of the break is stress-free. Thus, spontaneous Sn whisker growth is unique that stress relaxation
accompanies stress generation. One of the whisker challenging issues in understanding and in finding effective methods to
prevent spontaneous Sn whisker growth is to develop accelerated tests of whisker growth. Use of electromigration on short
Sn stripes can facilitate this. The stress distribution around the vicinity and the root of a whisker can be obtained by using
the micro-beam X-ray diffraction utilizing synchrotron radiation. A discussion of how to prevent spontaneous Sn whisker growth
by blocking both stress generation and stress relaxation is given. 相似文献