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991.
Theoretical Analysis of the CW Doppler Ultrasonic Flowmeter 总被引:5,自引:0,他引:5
The widespread application of ultrasonic techniques for the measurement of pulsatile blood flow has been hampered by the lack of a detailed theoretical understanding of the Doppler ultrasonic flowmeter. A general model for the Doppler flowmeter based upon stochastic considerations of the scattering of ultrasound by blood is introduced in this paper. The model characterizes the back- scattered ultrasound as a Gaussian random process and the expression for the autocovariance function is derived. For the CW Doppler flowmeter, the power spectral density function is computed, and its significance is emphasized: measurement of blood flow velocity corresponds to estimation of the average frequency of the Doppler power spectrum. 相似文献
992.
Background
This research was conducted to explore the relationships between the levels of omega-3 fatty acids in serum phospholipid and key fatty acid ratios including potential cut-offs for risk factor assessment with respect to coronary heart disease and fatal ischemic heart disease. 相似文献993.
Bacher Leigh F.; Robertson Steven S.; Smotherman William P. 《Canadian Metallurgical Quarterly》2000,114(3):594
The present experiment investigated the relationship between motor activity and oral grasping of an artificial nipple in newborn rats. Pups orally grasped the artificial nipple, and they performed more and longer oral grasps in the latter portion of the nipple presentation. Motor activity was cyclical, and this cyclicity was evident before and during presentation of the artificial nipple. The onset of an oral grasp response was preceded by a period of relatively low motor activity, and the termination of a grasp was followed by relatively high motor activity. The newborn rat pup's intrinsic oscillations in motor activity may regulate the expression of discrete responses to cues important for the initiation of suckling. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
994.
995.
拿起镜头,发现你眼中的时尚,《surface))杂志一年一度的“先锋奖(Avant Guardian Project)”正在进行,本次特别为大家介绍三对新晋的双人组合与时装设计师马太·艾姆(Matthew Ames)的新作。 相似文献
996.
Robert L. Wolff Frédérique Pédrono Anne M. Marpeau William W. Christie Frank D. Gunstone 《Journal of the American Oil Chemists' Society》1998,75(11):1637-1641
The fatty acid compositions of the seeds from three Taxus (yew) species and one Torreya species belonging to the Taxaceae family [Taxus cuspidata (Japanese yew), T. chinensis (Chinese yew), T. baccata (English yew), and Torreya grandis (Chinese nutmeg yew)] have been established. These compositions were compared with those previously published for T. canadensis (Canadian yew) and Torreya nucifera. In Taxus species, as well as in Torreya species, Δ5-olefinic acids are present in the seed lipids from all species analyzed. In Taxus, 5,9-18:2 (taxoleic) acid is the prominent Δ5-olefinic acid. It represents between 9.5 and 16.2% of total fatty acids. Other
Δ5-olefinic acids that occur in low amounts are 5,9,12-18:3 (<3.5%), 5,11-20:2 (<0.3%), 5,11,14-20:3 (<2.2%), and 5,11,14,17-20:4
(<0.3%) acids. In Torreya species, the major Δ5-olefinic acid is 5,11,14-20:3 (sciadonic) acid (between 6.7 and 11.2%). In contrast to Taxus species, the 5,9-18:2 and 5,9,12-18:3 acids are scarce in Torreya species: less than 0.1%. Also, the 9,12,15-18:3 acid content is significantly lower in Torreya than in Taxus. The prominence of taxoleic acid among Δ5-olefinic acids in the seed lipids is a unique characteristic of the genus Taxus that isolates it from all other Coniferophytes analyzed so far. However, this feature is not shared by other Taxaceae species,
such as Torreya, and with regard to their seed fatty acid compositions, the family Taxaceae appears particularly heterogeneous. Our observations
favor the hypothesis that in Gymnosperm seeds, there might exist two Δ5-desaturases, one specific for unsaturated acids with
a Δ9-ethylenic bond (active in Taxus but not in Torreya), and the other specific for unsaturated acids with a Δ11-ethylenic bond (active in Torreya but not in Taxus). Our data also highlight the importance of the elongase(s) in the metabolism of fatty acids in Gymnosperm seeds. 14-Methylhexadecanoic
acid, a habitual component of Pinaceae and Ginkgo biloba seed lipids, could not be detected in the Taxaceae studied here. 13C nuclear magnetic resonance spectroscopy of the oils from both genera has confirmed that Δ5-olefinic acids are apparently
excluded from the internal position of triacylglycerols, which is a characteristic common to all Gymnosperm species analyzed
so far, and consequently of great antiquity in their life history. 相似文献
997.
Sean E. Landwehr Gregory E. Hilmas William G. Fahrenholtz Inna G. Talmy 《Journal of the American Ceramic Society》2007,90(7):1998-2002
The chemical compatibility of ZrC and Mo was investigated in carburizing and carbon-free environments at temperatures from 1700° to 2200°C. Heating in the carburizing atmosphere resulted in the complete reaction of Mo with C, while the carbon-free atmosphere resulted in retained metallic phase with a maximum of 13.8 mol% Mo2 C formed. The presence of Mo2 C was not detected at 2100°C in the carbon-free atmosphere, confirming the existing phase equilibria in the Zr–Mo–C system. Heat treatments in the carbon-free atmosphere also showed liquid formation at 2200°C, as evident from microstructure analysis. Liquid formation was consistent with the interaction between Mo and Mo2 C. The liquid was found to comprise at least 7 vol% of the total component, based on a phase diagram for the Mo–C system. The formation of a liquid should allow for the processing of ZrC–Mo cermets by liquid-phase pressureless sintering. 相似文献
998.
A New Model for Tensile Creep of Silicon Nitride 总被引:2,自引:0,他引:2
William E. Luecke Sheldon M. Wiederhorn 《Journal of the American Ceramic Society》1999,82(10):2769-2778
The tensile creep rate of most commercial grades of Si3 N4 increases strongly with stress. Although the usual power-law functions can represent the creep data, the data often show curvature and systematic variations of slope with temperature and stress. In this article, we present a new approach to understanding the creep of ceramics, such as Si3 N4 , where a deformable second phase bonds a deformation-resistant major phase. A review of experimental data suggests that the rate of formation and growth of cavities in the second phase controls creep in these materials. The critical step for deformation is the redistribution of the second phase away from the cavitation site to the surrounding volume. The effective viscosity of the second phase and the density of active cavities determine the creep rate. Assuming that the hydrostatic stresses in pockets of the second phase are normally distributed leads to a model that accurately describes the tensile creep rate of grades of Si3 N4 . In this model, the creep rate increases exponentially with the applied stress, is independent of Si3 N4 grain size, is inversely proportional to the effective viscosity of the deformable phase, and is proportional to the cube of the volume fraction of the deformable phase. 相似文献
999.
Geraldine S. Cheok William C. Stone Robert R. Lipman Christoph Witzgall 《Automation in Construction》2000,9(5-6):463-477
Work at the National Institute of Standards and Technology (NIST) on laser radar imaging of a construction site is described. The objective of the NIST research is to make measurements required in a construction project quicker and cheaper than current practice and to do so without impacting existing operations. This can be done by developing techniques for real-time assessment and documentation in terms of 3-D as-built models of the construction process. Once developed, this technology may be used for other applications such as condition assessment of a hazardous environment where human intervention would be impossible. 相似文献
1000.
Erik Delarue Kris Voorspools William D’haeseleer 《Canadian Metallurgical Quarterly》2008,134(2):40-46
The European Union has implemented the European Union emission trading scheme (EU ETS) as an instrument to facilitate greenhouse gas (GHG) emission abatement stipulated in the Kyoto protocol. Empirical data show that in the early stages of the EU ETS, the value of a ton of CO2 has already led to emission abatement through switching from coal to gas in the European electric power sector. In the second part of this paper, an electricity generation simulation model is used to perform simulations on the switching behavior in both the first and the second trading periods of the EU ETS. In 2005, the reduction in GHG emissions in the electric power sector due to EU ETS is estimated close to 88?Mton. For the second trading period, a European Union allowance (EUA) price dependent GHG reduction curve has been determined. The obtained switching potential turns out to be significant, up to 300?Mton/year, at sufficiently high EUA prices. 相似文献