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971.
This research proposes a model that predicts the effect of the anode diffusion layer and membrane properties on the electrochemical performance and methanol crossover of a direct methanol fuel cell (DMFC) membrane electrode assembly (MEA). It is an easily extensible, lumped DMFC model. Parameters used in this design model are experimentally obtainable, and some of the parameters are indicative of material characteristics. The quantification of these material parameters builds up a material database. Model parameters for various membranes and diffusion layers are determined by using various techniques such as polarization, mass balance, electrochemical impedance spectroscopy (EIS), and interpretation of the response of the cell to step changes in current. Since the investigation techniques cover different response times of the DMFC, processes in the cell such as transport, reaction and charge processes can be investigated separately. Properties of single layers of the MEA are systematically varied, and subsequent analysis enables identification of the influence of the layer's properties on the electrochemical performance and methanol crossover. Finally, a case study indicates that the use of a membrane with lower methanol diffusivity and a thicker anode micro-porous layer (MPL) yields MEAs with lower methanol crossover but similar power density. 相似文献
972.
Ce0.9Gd0.1O1.95 with various Mg doping contents was synthesized by citric acid-nitrate low temperature combustion process and sintered under different conditions. The crystal structures, microstructures and electrical properties were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and ac impedance spectroscopy. Low solubility of Mg2+ in Ce0.9Gd0.1O1.95 lattice was evidenced by XRD and FESEM micrographs. The samples sintered at 1300 °C exhibited the higher total conductivity than those sintered at 1100 and 1500 °C, with the maximum value of 1.48 × 10−2 S cm−1 (measured at 600 °C) at the Mg doping content of 6 mol%, corresponding to the minimum total activation energy (Etol) of 0.84 eV (150–400 °C). The effect of Mg doping on the electrical conductivity was significant particularly at higher sintering temperatures. At the sintering temperature of 1500 °C, the addition of Mg (10 mol%) enhanced the grain boundary conductivity by over 102 times comparing with that of undoped Ce0.9Gd0.1O1.95, which may be explained by the optimization of space charge layer due to the segregation of Mg2+ to the grain boundaries. 相似文献
973.
974.
In this article we discuss a formal framework for casting the inverse problem of detecting the location and shape of an insonified
scatterer embedded within a two-dimensional homogeneous acoustic host, in terms of a partial-differential-equation-constrained
optimization approach. We seek to satisfy the ensuing Karush–Kuhn–Tucker first-order optimality conditions using boundary
integral equations. The treatment of evolving boundary shapes, which arise naturally during the search for the true shape,
resides on the use of total derivatives, borrowing from recent work by Bonnet and Guzina [1–4] in elastodynamics. We consider
incomplete information collected at stations sparsely spaced at the assumed obstacle’s backscattered region. To improve on
the ability of the optimizer to arrive at the global optimum we: (a) favor an amplitude-based misfit functional; and (b) iterate
over both the frequency- and wave-direction spaces through a sequence of problems. We report numerical results for sound-hard
objects with shapes ranging from circles, to penny- and kite-shaped, including obstacles with arbitrarily shaped non-convex
boundaries.
Partial support for this work was provided by the US National Science Foundation under grant award CMS-0348484. 相似文献
975.
Emmanuel Fern ndez-Gaucherand Aristotle Arapostathis Steven I. Marcus 《Systems & Control Letters》1990,15(5):425-432
Necessary conditions are given for the existence of a bounded solution to the optimality equation arising in Markov decision processes, under a long-run, expected average cost criterion. The relationships of some of our results to known sufficient conditions are also shown. 相似文献
976.
In this paper we will discuss the first order multiplicative intuitionistic fragment of linear logic, MILL1, and its applications to linguistics. We give an embedding translation from formulas in the Lambek Calculus to formulas in
MILL1 and show this translation is sound and complete. We then exploit the extra power of the first order fragment to give an account
of a number of linguistic phenomena which have no satisfactory treatment in the Lambek Calculus.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
977.
Joachim Windel 《Quality and Reliability Engineering International》1990,6(2):93-105
Low-power rectifier diodes were stressed by different forward currents IF, reverse voltages UR, and ambient temperatures a for a time interval (0, t*) in order to analyse the resulting distributions. Owing to the limitations in the test time, t*, the gathering of comprehensive information about their performance requires high stress levels without exceeding either physical limits or the limits for initiating thermal runaway. First, the results of lifetime experiments related to the middle stress region were described by a Weibull distribution with constant shape parameter and stress-dependent scale parameter. This model, however, did not fit sufficiently the typical features of the real-life distribution. Therefore a non-parametric measure called the average initial reliability in (O,t*) was defined using the experimental results. Its stress dependence was estimated. The results are guiding figures of stress conditions both for accelerated tests and for circuits with high demands on reliability. 相似文献
978.
Ioannis S. Arvanitoyannis Athanassios Mavromatis Anestis Rodiatis & Christas Goulas 《International Journal of Food Science & Technology》2007,42(7):819-826
Five landraces of Greek common dry bean (Phaseolus vulgaris L.), were analysed for thirteen physico‐chemical and eighteen sensory variables. Brightness, acceptance, odour and hardness were the most important sensory variables for the overall acceptability of the cooked product. Application of principal component analysis revealed a rather weak relationship between the sensory attributes and the physico‐chemical variables. However, a certain grouping of cultivars with corresponding properties [(PC1 vs. PC2, Kastoria with saltiness, hardness, grassiness, intensity, protein and metal; Byzitsa bushy with fat, odour and sourness; Ksanthi and Belestino with tenaciousness, chroma, pH and overall impression), (PC1 vs. PC3 Byzitsa bushy with odour and acridness; Ksanthi with metal, hardness and pH; Kastoria with colour, protein and intensity)] was made. On the contrary, cluster analysis showed a much more satisfactory grouping of dry bean varieties based on their geographical origin. The multiple regression equation explained 72.3% of the variation in total acceptance of dry beans. The relative importance of brightness, acceptance, smell and hardness in predicting the overall impression of dry beans was 13%, 13.7%, 26% and 10%, respectively. These results may indicate that smell, brightness and acceptance are the most important factors determining dry bean total acceptability, so effort should be directed at improving these attributes in an attempt to satisfy consumer experience. 相似文献
979.
980.