全文获取类型
收费全文 | 1455465篇 |
免费 | 24155篇 |
国内免费 | 6961篇 |
专业分类
电工技术 | 34065篇 |
综合类 | 6337篇 |
化学工业 | 268258篇 |
金属工艺 | 63503篇 |
机械仪表 | 40618篇 |
建筑科学 | 44980篇 |
矿业工程 | 11311篇 |
能源动力 | 50141篇 |
轻工业 | 105598篇 |
水利工程 | 14495篇 |
石油天然气 | 37380篇 |
武器工业 | 128篇 |
无线电 | 194535篇 |
一般工业技术 | 274599篇 |
冶金工业 | 140337篇 |
原子能技术 | 33845篇 |
自动化技术 | 166451篇 |
出版年
2021年 | 15470篇 |
2020年 | 11746篇 |
2019年 | 14498篇 |
2018年 | 14523篇 |
2017年 | 13753篇 |
2016年 | 20648篇 |
2015年 | 17174篇 |
2014年 | 28529篇 |
2013年 | 87532篇 |
2012年 | 33833篇 |
2011年 | 45315篇 |
2010年 | 41162篇 |
2009年 | 49984篇 |
2008年 | 42684篇 |
2007年 | 39623篇 |
2006年 | 43352篇 |
2005年 | 37866篇 |
2004年 | 40306篇 |
2003年 | 40245篇 |
2002年 | 39549篇 |
2001年 | 35995篇 |
2000年 | 34716篇 |
1999年 | 33364篇 |
1998年 | 37551篇 |
1997年 | 34755篇 |
1996年 | 32467篇 |
1995年 | 29033篇 |
1994年 | 27433篇 |
1993年 | 27318篇 |
1992年 | 25658篇 |
1991年 | 22693篇 |
1990年 | 23098篇 |
1989年 | 22164篇 |
1988年 | 20645篇 |
1987年 | 19030篇 |
1986年 | 18292篇 |
1985年 | 21664篇 |
1984年 | 22052篇 |
1983年 | 20025篇 |
1982年 | 19066篇 |
1981年 | 19177篇 |
1980年 | 17781篇 |
1979年 | 18363篇 |
1978年 | 17609篇 |
1977年 | 17099篇 |
1976年 | 17510篇 |
1975年 | 15903篇 |
1974年 | 15426篇 |
1973年 | 15517篇 |
1972年 | 12994篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
The ray-tracing (RT) algorithm has been used for accurately predicting the site-specific radio propagation characteristics, in spite of its computational intensity. Statistical models, on the other hand, offers computational simplicity but low accuracy. In this paper, a new model is proposed for predicting the indoor radio propagation to achieve computational simplicity over the RT method and better accuracy than the statistical models. The new model is based on the statistical derivation of the ray-tracing operation, whose results are a number of paths between the transmitter and receiver, each path comprises a number of rays. The pattern and length of the rays in these paths are related to statistical parameters of the site-specific features of indoor environment, such as the floor plan geometry. A key equation is derived to relate the average path power to the site-specific parameters, which are: 1) mean free distance; 2) transmission coefficient; and 3) reflection coefficient. The equation of the average path power is then used to predict the received power in a typical indoor environment. To evaluate the accuracy of the new model in predicting the received power in a typical indoor environment, a comparison with RT results and with measurement data shows an error bound of less than 5 dB 相似文献
992.
干涉条纹图估测基线的方法研究 总被引:1,自引:1,他引:0
根据SAR干涉测量原理,提出了利用干涉相干条纹估测基线的方法,并以欧洲空间局资源卫星1号和2号携带的SAR系统在中国新疆伽师试验区获取的串接式(Tan-dem)干涉测量数据为数据源,对提出的方法进行了验证,结果表明,该方法能有效地估测基线,并弥补不能有效获取卫星轨道参数时,对基线进行估测的缺陷,该方法尤其适用于差分干涉测量的研究。 相似文献
993.
Nicholas Biery Marc De Graef Jack Beuth Rafel Raban Andrew Elliott Tresa M. Pollock Curt Austin 《Metallurgical and Materials Transactions A》2002,33(10):3127-3136
Tensile experiments have been performed on specimens of four different investment-cast TiAl-based alloys with variations in
casting conditions. The average ductilities obtained in these experiments vary between approximately 0.8 to 2.0 pct plastic
strain to failure in tension. By using the three-parameter form of the Weibull relation, with the 0.2 pct offset yield strength
as the minimum failure strength, the resulting variability in the data can be quantified and is found to be similar for those
alloys with similar microstructural scale (grain size). Large variations in lamellar volume fraction, segregation, and phase
distribution have a minor influence on property variability, compared to changes in the scale of the grain structure caused
by either variations in cooling rate during casting or the addition of grain refiners. 相似文献
994.
1 前言 CONTOP~R工艺最初是由德国科隆的KHDHumboldt Wedag AG公司针对有色金属工业而开发的。两台迄今为止最大的CONTOP~R熔炼旋风炉建于美国德克萨斯的ASARCO公司,每台能力为32t/h铜精矿混和料。 1998年,奥钢联工程技术公司(VAI)获得了 相似文献
995.
Mingjun Li Kosuke Nagashio Ph.D. Kazuhiko Kuribayashi 《Metallurgical and Materials Transactions A》2002,33(8):2677-2683
A mullite (3Al2O3·2SiO2) sample has been levitated and undercooled in an aero-acoustic levitator, so as to investigate the solidification behavior
in a containerless condition. Crystal-growth velocities are measured as a function of melt undercoolings, which increase slowly
with melt undercoolings up to 380 K and then increase quickly when undercoolings exceed 400 K. In order to elucidate the crystal
growth and solidification behavior, the relationship of melt viscosities as a function of melt undercoolings is established
on the basis of the fact that molten mullite melts are fragile, from which the atomic diffusivity is calculated via the Einstein-Stokes equation. The interface kinetics is analyzed when considering atomic diffusivities. The crystal-growth
velocity vs melt undercooling is calculated based on the classical rate theory. Interestingly, two different microstructures are observed;
one exhibits a straight, faceted rod without any branching with melt undercoolings up to 400 K, and the other is a feathery
faceted dendrite when undercoolings exceed 400 K. The formation of these morphologies is discussed, taking into account the
contributions of constitutional and kinetic undercoolings at different bulk undercoolings. 相似文献
996.
J. C. Williams R. G. Baggerly N. E. Paton 《Metallurgical and Materials Transactions A》2002,33(3):837-850
Single crystals of Ti-Al alloys containing 1.4, 2.9, 5, and 6.6 pct Al (by weight) were oriented for 〈a〉 slip on either basal or prism planes or loaded parallel along the c-axis to enforce a nonbasal deformation mode. Most of the tests were conducted in compression and at temperatures between
77 and 1000 K. Trace analysis of prepolished surfaces enabled identification of the twin or slip systems primarily responsible
for deformation. Increasing the deformation temperature, Al content, or both, acted to inhibit secondary twin and slip systems,
thereby increasing the tendency toward strain accommodation by a single slip system having the highest resolved stress. In
the crystals oriented for basal slip, transitions from twinning to multiple slip and, finally, to basal slip occurred with
increasing temperature in the lower-Al-content alloys, whereas for Ti-6.6 pct Al, only basal slip was observed at all temperatures
tested. A comparison of the critically resolved shear stress (CRSS) values for basal and prism slip as a function of Al content
shows that prism slip is favored at room temperature in pure Ti, but the stress to activate these two systems becomes essentially
equal in the Ti-6.6 pct Al crystals over a wide range of temperatures.
Compression tests on crystals oriented so that the load was applied parallel to the c-axis showed extensive twinning in lower Al concentrations and 〈c+a〉 slip at higher Al concentrations, with a mixture of 〈c+a〉 slip and twinning at intermediate compositions. A few tests also were conducted in tension, with the load applied parallel
to the c-axis. In these cases, twinning was observed, and the resolved shear for plastic deformation by twinning was much lower that
that for 〈c+a〉 slip observed in compression loading.
This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals
and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following
ASM committees: Materials Science and Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic
Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee. 相似文献
997.
998.
A theory related to the statistics of the structure and properties of inhomogeneous materials has been proposed, dealing with the distribution of properties from sampling of inhomogeneous structures by line transects. It is demonstrated that the empirical cumulative distribution functions of the intercepts from the microstructural constituents and their variances are important fingerprints of the inhomogeneous structures.Simulated sampling of the images of the microstructure by line transects has been proposed as a powerful method for characterization the distribution of properties of inhomogeneous structures. One of the advantages of the method based on the expected values of the order statistics of the intercepts is that the probability bounds of the property are determined without prior knowledge of the functional relationship between the property and the intercepts from the structural constituents.The concept intercept variance has an important application in determining the minimum transect length that stabilizes the variation of the intercept at a low value. Additionally, the concept ‘intercept variance’ can be used for topological optimization of the microstructure regarding the risk of intercepting a large amount of the weaker constituent. Equations regarding the variance of the intercepted fraction characterizing transect lengths with a specified distribution have also been derived. 相似文献
999.
1000.