全文获取类型
收费全文 | 21704篇 |
免费 | 1304篇 |
国内免费 | 16篇 |
专业分类
电工技术 | 431篇 |
综合类 | 70篇 |
化学工业 | 4756篇 |
金属工艺 | 670篇 |
机械仪表 | 500篇 |
建筑科学 | 1013篇 |
矿业工程 | 157篇 |
能源动力 | 372篇 |
轻工业 | 3435篇 |
水利工程 | 157篇 |
石油天然气 | 27篇 |
无线电 | 1313篇 |
一般工业技术 | 5584篇 |
冶金工业 | 791篇 |
原子能技术 | 199篇 |
自动化技术 | 3549篇 |
出版年
2023年 | 313篇 |
2022年 | 230篇 |
2021年 | 521篇 |
2020年 | 445篇 |
2019年 | 429篇 |
2018年 | 784篇 |
2017年 | 794篇 |
2016年 | 867篇 |
2015年 | 726篇 |
2014年 | 895篇 |
2013年 | 1669篇 |
2012年 | 1070篇 |
2011年 | 1257篇 |
2010年 | 1071篇 |
2009年 | 987篇 |
2008年 | 1075篇 |
2007年 | 1066篇 |
2006年 | 737篇 |
2005年 | 573篇 |
2004年 | 472篇 |
2003年 | 463篇 |
2002年 | 477篇 |
2001年 | 404篇 |
2000年 | 342篇 |
1999年 | 273篇 |
1998年 | 304篇 |
1997年 | 281篇 |
1996年 | 228篇 |
1995年 | 208篇 |
1994年 | 174篇 |
1993年 | 153篇 |
1992年 | 136篇 |
1991年 | 118篇 |
1990年 | 106篇 |
1989年 | 85篇 |
1988年 | 98篇 |
1987年 | 89篇 |
1985年 | 163篇 |
1984年 | 176篇 |
1983年 | 120篇 |
1982年 | 115篇 |
1981年 | 140篇 |
1980年 | 111篇 |
1979年 | 89篇 |
1978年 | 99篇 |
1977年 | 85篇 |
1976年 | 96篇 |
1975年 | 97篇 |
1974年 | 75篇 |
1973年 | 76篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Martin Gneiser Julia Heidemann Mathias Klier Andrea Landherr Florian Probst 《Information Systems and E-Business Management》2012,10(1):61-84
Online social networks have been gaining increasing economic importance in light of the rising number of their users. Numerous
recent acquisitions priced at enormous amounts have illustrated this development and revealed the need for adequate business
valuation models. The value of an online social network is largely determined by the value of its users, the relationships
between these users, and the resulting network effects. Therefore, the interconnectedness of a user within the network has
to be considered explicitly to get a reasonable estimate for its economic value. Established standard business valuation models,
however, do not account for these aspects sufficiently. Thus, we propose an economic model for the valuation of online social
networks, which takes into account the users’ interconnectedness within the network. Furthermore, we analyze different centrality
measures, which can be used to quantify users’ interconnectedness in online social networks and propose a measure which is
based on the PageRank-algorithm. Finally, the practical application of the model is illustrated by an example of the European
online social network XING.com. 相似文献
82.
Alexandre N. Tuch Eva E. Presslaber Markus Stöcklin Klaus Opwis Javier A. Bargas-Avila 《International journal of human-computer studies》2012,70(11):794-811
This paper experimentally investigates the role of visual complexity (VC) and prototypicality (PT) as design factors of websites, shaping users' first impressions by means of two studies. In the first study, 119 screenshots of real websites varying in VC (low vs. medium vs. high) and PT (low vs. high) were rated on perceived aesthetics. Screenshot presentation time was varied as a between-subject factor (50 ms vs. 500 ms vs. 1000 ms). Results reveal that VC and PT affect participants' aesthetics ratings within the first 50 ms of exposure. In the second study presentation times were shortened to 17, 33 and 50 ms. Results suggest that VC and PT affect aesthetic perception even within 17 ms, though the effect of PT is less pronounced than the one of VC. With increasing presentation time the effect of PT becomes as influential as the VC effect. This supports the reasoning of the information-processing stage model of aesthetic processing (Leder et al., 2004), where VC is processed at an earlier stage than PT. Overall, websites with low VC and high PT were perceived as highly appealing. 相似文献
83.
Numerical modeling of multiphase flows in microfluidics and micro process engineering: a review of methods and applications 总被引:2,自引:1,他引:1
Martin Wörner 《Microfluidics and nanofluidics》2012,12(6):841-886
This article presents a comprehensive review of numerical methods and models for interface resolving simulations of multiphase
flows in microfluidics and micro process engineering. The focus of the paper is on continuum methods where it covers the three
common approaches in the sharp interface limit, namely the volume-of-fluid method with interface reconstruction, the level
set method and the front tracking method, as well as methods with finite interface thickness such as color-function based
methods and the phase-field method. Variants of the mesoscopic lattice Boltzmann method for two-fluid flows are also discussed,
as well as various hybrid approaches. The mathematical foundation of each method is given and its specific advantages and
limitations are highlighted. For continuum methods, the coupling of the interface evolution equation with the single-field
Navier–Stokes equations and related issues are discussed. Methods and models for surface tension forces, contact lines, heat
and mass transfer and phase change are presented. In the second part of this article applications of the methods in microfluidics
and micro process engineering are reviewed, including flow hydrodynamics (separated and segmented flow, bubble and drop formation,
breakup and coalescence), heat and mass transfer (with and without chemical reactions), mixing and dispersion, Marangoni flows
and surfactants, and boiling. 相似文献
84.
Rapidly solidified metal matrix composites have been produced on a laboratory scale either by (1) melt spinning a composite after introduction of the ceramic phase and extrusion of the flakes obtained, or (2) blending melt-spun powder (basic alloy) with the ceramic phase and subsequent extrusion. AlMg(Si) alloys were used as matrix material while SiC particles with diameters of 3 or 20 m were used as the ceramic phase. For the composites prepared by route 1 it was found that the basic alloy was reinforced by the addition of 3 m particles whereas for the 20m particles reinforcement was observed only for very ductile matrices. The bond between SiC particles and matrix was good. A diffusion and wetting bond was formed. For the composites prepared by route 2 it was found that reinforcement did not occur and that the bond between particles and matrix was weak. Debonding of the particles took place in the case of tensile fracture. The advantage of a rapidly solidified matrix for these composites is that relatively high ductilities are combined with good reinforcement effects. Prior contact of the ceramic phase and the aluminium melt is needed for a good bond between SiC and the matrix material. It is concluded that route 1 should be preferred for the production of rapidly solidified aluminium matrix composites. 相似文献
85.
R. G. Zoeller N. G. Ugras K. D. Möller 《Journal of Infrared, Millimeter and Terahertz Waves》1988,9(5):497-504
A Fourier Transform Spectrometer using a 250 Watt Mercury lamp, a lamellar grating and a pumped He-cooled detector is applied to the millimeter wave spectral region. The pure rotation spectrum of CH2F, the transmission of graphite particles in a cloud chamber, the interference on round copper dots and the absorption of iron whiskers have been observed. 相似文献
86.
Kähkönen E Nykyri E Ilmarinen R Ketola R Lusa S Nygård CH Suurnäkki T 《Applied ergonomics》1992,23(3):186-190
A study was made of the appraisers' effect on the estimation of metabolic rate with the Edholm scale and a table of the ISO 7243 heat stress standard. The appraisers, five experienced and five inexperienced persons, estimated the metabolic rate of three different work tasks from videotapes. Analysis of variance indicated significant ( [Formula: see text] ) differences in the appraisers' recordings of the activities. The appraisers were grouped according to the similarity of the estimated values they gave. The groups thus contained both experienced and inexperienced appraisers, and it was not possible to classify the appraisers into experienced and inexperienced groups according to their earlier experience. The metabolic rates according to the Edholm scale were higher than according to the ISO 7243 table. The differences in metabolic rates given by the individual observers varied from 38 to 118 W/m(2). The variations in the estimation of metabolic rates were greater when the Edholm scale was used. This variation caused considerable variation also in the predicted mean vote, PMV index. It is recommended that the appraisers be selected carefully, because it is not possible to know whether a randomly selected appraiser is an 'average' or an 'extreme' appraiser without a test. Before conducting extensive field surveys where several appraisers estimate the metabolic rates, it would be useful to arrange training in order to calibrate the levels of the Edholm scale as well as ISO method among the appraisers because training clearly unified the estimation. 相似文献
87.
88.
Single crystals of nearly stoichiometric Fe3Si were creep-deformed at temperatureT = 450 to 850° C and applied stress=40 to 250 MPa. While the temperature dependence of the steady-state creep rate of crystals with less than 25 at% Si can be described by an exponential function exp (–H
exp/kT), the Fe-26 at% Si samples show an exponential dependence only below 500° C and above 600° C. At intermediate temperatures the dependence is weak. It is suggested that in this intermediate range two phases exist. The experimental results are consistent with the assumption that the phase boundaries do not hinder dislocation movement, and that the disocation velocity in the two phases is different. 相似文献
89.
The meat quality of M. longissimus muscle was evaluated in 851 pigs by using the Fibre Optic Probe (FOP) at three sites in the muscle and in both halves of the carcass. A systematic difference between sites was found, with the lowest light scattering (indicating the best meat quality) in the mid-part of the muscle and higher light scattering in the anterior and posterior parts. A non-systematic variation was also observed, with the opposite pattern in some animals, even though it was not frequent. A negative influence of the shackling was found, yielding higher FOP values in the shackled half.
Drip loss measurements in the Longissimus muscle, taken from another 100 pig carcasses, were evaluated using three methods. Drip loss, too, showed a considerable variation along the Longissimus muscle, with minimum losses around the last rib. Repeatability estimates, calculated from two non-consecutive pieces of the Longissimus dorsi muscle from each carcass, varied from 0·4 when keeping samples vacuum packed for 2 days, to 0·5 when the samples were either kept in plain plastic bags for 2 days or in a meat container with a squared inset for 1 or 2 days. 相似文献
90.
W. Minor B. Schönfeld B. Lebech B. Buras W. Dmowski 《Journal of Materials Science》1987,22(11):4144-4152
We have studied the crystallization kinetics of Fe90-x
Si
x
B10 amorphous alloys withx ranging from 7 to 21, by synchrotron X-ray radiation. Using energy- dispersive X-ray diffraction, the kinetics of the different crystalline phases evolving during isothermal annealing were followed. These crystalline phases were identified as precipitation of-Fe(Si) and/or Fe3Si in the amorphous matrix. At a later time or at a higher temperature, Fe2B starts to crystallize (x < 21 ). Only at low iron concentration (x = 21) was the second phase different, namely Fe5SiB2 The hypo- and hyper-eutectic Fe-Si-B glasses were found to crystallize differently. The crystallization processes are discussed in some detail. 相似文献