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871.
Marco Lorenzini Gian Luca Morini Torsten Henning Jürgen Brandner 《International Journal of Thermal Sciences》2009,48(2):282-289
The promising performance of microchannels has given rise to intensive research on pressure drop and heat transfer characteristics of flows at the small scale. To check the classical models and to validate new ones, experiments need to be conducted, which are particularly difficult given the characteristic dimensions involved and the magnitude of the fluxes to be measured. Although more care has been devoted lately to the design of experiments in terms of control of the geometry and of the boundary conditions, the uncertainties which inevitably affect each measurement do not seem to have been given the proper consideration. Correctly calculating uncertainties not only allows a correct assessment of the experimental data obtained, but can also be used to decide which measurements need to have the highest precision to achieve a certain accuracy, thus saving money on the others. In this paper, a quantitative criterion is given to assess the accuracy achievable in the determination of the friction factor in the laminar regime for the flow of a fluid in a circular microtube. The influence of the six quantities (pressure drop, outlet pressure, temperature, length, diameter and volume flow rate) measured to determine f in the laminar regime are studied separately and when combined. It is found that at low Reynolds numbers flow rate and pressure drop measurements are determinant for the final value of the uncertainty, while at larger Reynolds numbers the influence of the accuracy in measuring the hydraulic diameter prevails and also limits the minimum value that the total uncertainty can take. 相似文献
872.
Orth Ulrich; Robins Richard W.; Trzesniewski Kali H.; Maes Jürgen; Schmitt Manfred 《Canadian Metallurgical Quarterly》2009,118(3):472
Data from two large longitudinal studies were used to analyze reciprocal relations between self-esteem and depressive symptoms across the adult life span. Study 1 included 1,685 participants aged 18 to 96 years assessed 4 times over a 9-year period. Study 2 included 2,479 participants aged 18 to 88 years assessed 3 times over a 4-year period. In both studies, cross-lagged regression analyses indicated that low self-esteem predicted subsequent depressive symptoms, but depressive symptoms did not predict subsequent levels of self-esteem. This pattern of results replicated across all age groups, for both affective–cognitive and somatic symptoms of depression, and after controlling for content overlap between the self-esteem and depression scales. The results suggest that low self-esteem operates as a risk factor for depressive symptoms at all phases of the adult life span. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
873.
874.
Vicente Braz Trindade Ulrich Krupp Hans-Jürgen Christ 《Journal of Materials Engineering and Performance》2008,17(6):915-919
The oxidation behavior of steels containing low-Cr concentrations (0.5-2.25 wt.%) has been studied in laboratory air in the
temperature range of 400-550 °C. The oxidation rate of the steels was lower than that of pure iron, but higher than that of
pure iron when a small amount of rare earth element cerium (0.03 wt.%) is added to the 2.25Cr1Mo steel. The mass change follows
a nearly parabolic law for the case of pure iron and the steel without Ce addition, while linear behavior describes the oxygen
uptake for the case of the 2.25Cr1Mo+0.03Ce steel. SEM cross-section observations and thermodynamic calculations confirm that
there is no wustite (FeO) formation during oxidation of pure iron and low-Cr steels at 550 °C, whereas FeO might be formed
in the oxide scale of 2.25Cr1Mo+0.03Ce at the same oxidation conditions (temperature, atmosphere, and exposure time). By investigating
the temperature for FeO stability, this study reveals that the temperature for FeO formation on pure iron is 568 °C, for the
2.25Cr1Mo steel 589 °C, and 471 °C for the 2.25Cr1Mo+0.03Ce. This low value for the FeO stability temperature found for the
steel 2.25Cr1Mo+0.03Ce steel explains why this steel oxidizes very fast at 550 °C. 相似文献
875.
George Makrides Bastian Zinsser Matthew Norton George E. Georghiou Markus Schubert Jürgen H. Werner 《Renewable & Sustainable Energy Reviews》2010,14(2):754-762
Renewable energy sources derived principally from solar energy have been gaining ground over the last few years and are now beginning to contribute to the global energy mix. Solar energy in the form of direct electricity conversion (photovoltaics) is already very popular in countries such as the United States, Germany and Japan. The enormous potential of photovoltaic (PV) technology is also obvious and favourable in countries with high irradiation such as the Mediterranean region. The objective of this paper is to review the different up and coming PV technologies, to explore the potential of different PV systems in countries with high solar irradiation and to compare their performance through the assessment of thirteen different types of PV systems that have been installed side by side in Nicosia, Cyprus and Stuttgart, Germany. Finally useful insight into the performance of the PV systems as a function of the meteorological conditions and location will be highlighted. 相似文献
876.
Erik Svanes Mie Vold Hanne Møller Marit Kvalvåg Pettersen Hanne Larsen Ole Jørgen Hanssen 《Packaging Technology and Science》2010,23(3):161-175
This study describes a holistic methodology for sustainable packaging design. This methodology studies the combined systems of packaging and the packaged products across the whole distribution chain from manufacturer to end consumer and the life cycle from raw material extraction to the waste phase. It contains a number of indicators that are grouped into the following main categories: environmental sustainability, distribution costs, product protection, market acceptance and user friendliness. The methodology integrates a number of different analytical methods. It is intended to be used in packaging design and optimisation, for idea generation, decision support and as documentation of properties of existing packaging systems. The study describes experiences with the methodology from one case study in the Norwegian Food Industry. The experiences show that the methodology is very comprehensive, and gives a good overview of the properties of a packaging solution. It enables quantitative comparisons between different packaging solutions throughout the design process. The methodology reduces the risk of implementing sub‐optimal packaging solutions. An additional benefit of the methodology is gained by working in cross‐functional teams. One potential drawback is that the methodology can be resource and data intensive. The methodology can be used as a tool box in packaging design, i.e. it is not necessary to use all methods and quantify all indicators to gain benefit. However, all indicators and requirements should be evaluated and considered. In all cases, it should be considered to include additional indicators if important sustainability issues have not been addressed. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
877.
Kristina Klinkhammer Julia Bockelmann Chariklia Simitzis Gary A. Brook Dirk Grafahrend Jürgen Groll Martin Möller Jörg Mey Doris Klee 《Journal of materials science. Materials in medicine》2010,21(9):2637-2651
Microfibers produced with electrospinning have recently been used in tissue engineering. In the development of artificial
implants for nerve regeneration they are of particular interest as guidance structures for cell migration and axonal growth.
Using electrospinning we produced parallel-orientated biocompatible fibers in the submicron range consisting of poly(ε-caprolactone)
(PCL) and star shaped NCO-poly(ethylene glycol)-stat-poly(propylene glycol) (sPEG). Addition of the bioactive peptide sequence glycine-arginine-glycine-aspartate-serine (GRGDS)
or the extracellular matrix protein fibronectin to the electrospinning solution resulted in functionalized fibers. Surface
characteristics and biological properties of functionalized and non-functionalised fibers were investigated. Polymer solutions
and electrospinning process parameters were varied to obtain high quality orientated fibers. A polymer mixture containing
high molecular weight PCL, PCL-diol, and sPEG permitted a chemical reaction between hydroxyl groups of the diol and isocyanante
groups of the sPEG. Surface analysis demonstrated that sPEG at the fiber surface minimized protein adhesion. In vitro experiments
using dorsal root ganglia explants showed that the cell repellent property of pure PCL/sPEG fibers was overcome by functionalization
either with GRGDS peptide or fibronectin. In this way cell migration and axonal outgrowth along fibers were significantly
increased. Thus, functionalized electrospun PCL/sPEG fibers, while preventing non-specific protein adsorption, are a suitable
substrate for biological and medical applications. 相似文献
878.
A low‐density sintered ferro‐molybdenum is presented as a new alloying agent. The paper describes laboratory studies about the dissolution in liquid iron of briquettes of this alloy and of classical high density ferro‐molybdenum pieces. It presents further a mathematical model of the melting and dissolution process. During dissolution, an approximately 1mm thick layer infiltrated by melt forms on the particle surface. The infiltrated melt solidifies in the plane where the temperature has reached the eutectic temperature in the system iron‐molybdenum. Internal dissolution of alloy material in the layer is weak, which means that the dissolution proceeds almost exclusively from the outer surface of the briquette. Dissolution rate increases with sinking briquette density. The lower molybdenum content per volume in the briquettes which is proportional to the density has the effect that the liquidus concentration and the liquidus temperature at the solid‐liquid interface decrease in comparison with compact material. This reduces the mass transfer rate and increases the heat transfer rate. The effect is a faster movement of the interface. Below a critical density of approximately 5200 kg/m3 for the alloy considered, the molybdenum concentration on both sides of the interface becomes equal. From this moment, the alloy is liquefied solely by melting of the moving interface. Mass transfer from the interface gets negligible and distribution of the molten alloy into the bulk melt takes place only by the outer mixing process. From the described behaviour it follows, that below the critical density the melting rate is solely determined by heat transfer. Since heat transfer is faster than mass transfer, melting of alloys with reduced density is correspondingly accelerated. The extremely slow dissolution rates of high melting alloys can thus, be overcome by giving the alloy a certain porosity. The mathematical process model describes the phenomena quantitatively. 相似文献
879.
Waxes are important as building material and for the chemical communication of the honeybee Apis mellifera carnica. In this study chemometric tools were established for classifying the different waxes inside the hive. By using gas chromatography in combination with mass spectrometry, components of different types of waxes were analyzed. By considering different substance classes of waxes, discriminant function analyses revealed distinct subtypes of comb waxes and of cuticular waxes. It is shown that the aging of comb wax is in part a spontaneous physicochemical process due to differential volatilities of compound classes with different chain length ranges. On the other hand it is directly influenced by the bees by adding lipolytic enzymes to the comb wax. The data suggest that the varying cuticular wax and comb wax compositions could serve as cues for bees to recognize castes, sexes, or comb age. 相似文献
880.
The local conformation and the defect Gibbs energy of stereoregularity defects in isotactic poly(propylene), i-PP, is calculated by means of the thermodynamic-integration approach. For i-PP with high isotacticity, where the atactic inclusions do not interfere with each other, we found changes in the torsion angles next to the defect, which bring the methyl unit to the position it would have in the conformation of the purely isotactic chain. The Gibbs energy of this inclusion is low and enables cocrystallization corresponding to the uniform inclusion model. For sequences of adjacent defects in the polymer chain, the local chain deformation was found to be much more complex, and a considerable defect Gibbs energy remains that makes the inclusion of atactic sequences less likely. 相似文献