全文获取类型
收费全文 | 378727篇 |
免费 | 4800篇 |
国内免费 | 1122篇 |
专业分类
电工技术 | 6590篇 |
综合类 | 280篇 |
化学工业 | 56659篇 |
金属工艺 | 17048篇 |
机械仪表 | 12106篇 |
建筑科学 | 8362篇 |
矿业工程 | 3157篇 |
能源动力 | 9262篇 |
轻工业 | 28145篇 |
水利工程 | 4807篇 |
石油天然气 | 10592篇 |
武器工业 | 28篇 |
无线电 | 41697篇 |
一般工业技术 | 78804篇 |
冶金工业 | 68089篇 |
原子能技术 | 10354篇 |
自动化技术 | 28669篇 |
出版年
2021年 | 3396篇 |
2019年 | 3309篇 |
2018年 | 5755篇 |
2017年 | 5870篇 |
2016年 | 6250篇 |
2015年 | 3747篇 |
2014年 | 6289篇 |
2013年 | 16601篇 |
2012年 | 9932篇 |
2011年 | 13197篇 |
2010年 | 10630篇 |
2009年 | 11980篇 |
2008年 | 12181篇 |
2007年 | 12177篇 |
2006年 | 10408篇 |
2005年 | 9664篇 |
2004年 | 8977篇 |
2003年 | 8829篇 |
2002年 | 8374篇 |
2001年 | 8517篇 |
2000年 | 8186篇 |
1999年 | 8362篇 |
1998年 | 20579篇 |
1997年 | 14520篇 |
1996年 | 11339篇 |
1995年 | 8371篇 |
1994年 | 7483篇 |
1993年 | 7493篇 |
1992年 | 5815篇 |
1991年 | 5679篇 |
1990年 | 5371篇 |
1989年 | 5392篇 |
1988年 | 5260篇 |
1987年 | 4575篇 |
1986年 | 4544篇 |
1985年 | 5200篇 |
1984年 | 4853篇 |
1983年 | 4552篇 |
1982年 | 4091篇 |
1981年 | 4166篇 |
1980年 | 4079篇 |
1979年 | 4155篇 |
1978年 | 4142篇 |
1977年 | 4529篇 |
1976年 | 5691篇 |
1975年 | 3698篇 |
1974年 | 3527篇 |
1973年 | 3635篇 |
1972年 | 3113篇 |
1971年 | 2876篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
911.
Aqueous solutions of five selected non-ionic surfactants: Triton (i-octylphenolethoxylates), Tergitol (2,6,8-trimethyl-4-nonanoloxethylates), Symperonic (n-nonylphenol-oxethylates) and Brij (fatty alcohol ethoxylates) were investigated in this study. Using the bioluminescent bacteria Vibrio fischeri the toxicity of the surfactants solutions were determined. An attempt was made to relate rather low biodegradability of nonionic surfactant solutions measured by the BOD5/COD ratio (ranging from 1 to 17%) to their toxicity. The ozonation process was carried out in a 1.5 dm3 stirred cell reactor equipped with two ozone detectors. The following parameters were analyzed: pH, COD, BOD5, DOC, TOC, polarography as well as UV spectrum. The positive effect of ozonation, represented by decay of UV absorption, was visible in almost complete destruction of the surfactants, with exception of Triton X-705 (only 65% degradation after absorption of 2000 mgO3/dm3). The most striking results were obtained in toxicity tests for ozonated solutions of the non-ionic surfactants – an increase of the bacterial growth inhibition (1.5 to 4 times increase in toxicity due to ozonation). The obtained results were discussed taking into account the literature and our own experimental data on mechanisms of ozonation and biodegradation of non-ionic surfactants of the Triton-type and similar chemical structures. 相似文献
912.
A combination of concept (the product portfolio and the product life cycle) and empirical research (the PIMS study) can aid in clarifying the key issues involved in the selection of a firm's core strategies. Strategy control is multistage: periodic re-evaluation of strategy alternatives by use of the marketing audit; an analytical framework for control of chosen strategies by breaking down components and reporting their performance variances; and 'marketing segment' analysis for control of the detailed marketing plan.? 相似文献
913.
914.
915.
916.
917.
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
918.
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
919.
Alinur Büyükaksoy 《电信纪事》1985,40(7-8):402-410
An asymptotic solution to the problem of the diffraction of high-frequency waves by the edge of a cylindrically curved surface whose convex and concave faces present soft and hard boundary conditions is obtained. The results permit us to define new diffraction coefficients related to this kind of mixed boundary conditions. When the radius of the curvature becomes infinitely large, the edgediffraction coefficient reduces to this known for the softhard half-plane. 相似文献
920.
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