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21.
The general objective of this study is to contribute to a better understanding of heat transfer in a nucleate boiling regime. The aim is to determine the heat transfer characteristics under controlled operating conditions (thermodynamics of the fluid, noncondensable gas, surface state). Experimental investigations have been carried out in natural convection and nucleate boiling regimes. An experimental device was realized to perform boiling experiments using a boiling-meter, allowing investigations for different orientations of the wall. The boiling-meter is designed to investigate boiling for single and multiple nucleation sites. The purpose of this paper is to detail the experimental setup as well as the boiling-meter. This device allows the determination of the temporal heat transfer characteristics evolutions. In particular, this new device allows bringing to light the phenomenon of nucleation, growth, and detachment of generated vapor bubbles on a single artificial nucleate site, as well as for multiple natural nucleation sites. First results of the influence of the orientation of the heating wall for multiple and single nucleation sites on heat transfer are presented and analyzed.  相似文献   
22.
A miniature co‐extrusion technique, to produce a concentric multilayered glass fiber‐optic preform of ~3 mm diameter, is modeled and experimentally demonstrated. A three‐dimensional, incompressible, noncavitating, and nonisothermal Computational Fluid Dynamics (CFD) model, similar to one developed in our previous work, is used to predict the dimensions of an alternating four‐layer glass stack feed required to produce the desired layer dimensions in a multilayered‐glass preform extrudate, using a miniaturized and thus more economical co‐extrusion. Strong agreement in the cross‐sectional geometrical proportions of the simulated and experimentally obtained preform supports the prowess of the predictive modeling. Nevertheless, some small deviations between the simulated and experimentally obtained dimensions indicate topics for future rheological study. Performing the co‐extrusion process under vacuum helps to minimize the inter‐layer defects in the multi‐layered fiber‐optic preform. The miniature co‐extrusion potentially removes the need for a postextrusion draw‐down prior to fiber drawing, avoiding devitrification issues possible in non‐oxide novel glass compositions.  相似文献   
23.
The influences of pressure, shear stress and temperature on the structure of zinc orthophosphate and zinc metaphosphate was investigated. Tribological tests were performed to study the combined effect of pressure and shear stress at two temperatures. Friction tests were carried out in the boundary lubrication regime from dispersions of zinc polyphosphates in base oil. The effects of pressure alone were investigated using a diamond anvil cell in order to separate it from those of shearing. Raman spectroscopy was used to follow in situ or ex situ the structural changes of the zinc polyphosphate powders and the tribo-stressed areas. Tribofilms obtained with both polyphosphates display a partial and full depolymerization of the zinc metaphosphate at ambient and high (120 °C) temperature, respectively. The large stress and strain conditions of the tribological tests are necessary to induce a tribochemical reaction between zinc metaphosphate and iron oxide leading to a depolymerization of the phosphate in the tribofilm. The tribochemical reaction and antiwear tribofilm formation are significantly enhanced by the modest temperature increase from ambient to 120 °C. Pressure alone induces only disordering in the structure of zinc polyphosphates, with only minor changes of the chain length in phosphates and does not contribute significantly to the observed structural changes in tribofilms.  相似文献   
24.
A three‐dimensional, incompressible and noncavitating model of a glass‐stack coextrusion process, under isothermal and non‐isothermal conditions is numerically simulated by means of computational fluid dynamics. A dynamic mesh approach is taken in a domain‐subdomain type setup to simulate the transient steps in the steady‐velocity phase of the experimental co‐extrusion. The multiphase setup consists of a glass‐stack which is composed of different glass compositions. Experimentally measured glass properties, such as the temperature coefficient of the viscosity of the supercooled glass melts are used to define the flow behavior of the glasses in the starting stack when extruded. The modeled extrudate is numerically verified for transient and spatial errors, leading to the choice of a suitable mesh. Excellent agreement is found between modeling and experiment when plotting the core/cladding dimensions of a step‐index extruded fiber‐optic preform along the length of the preform. This approach can identify the stable part of the preform, in terms of constant core/cladding layer geometry, obviating costly and time‐consuming experimental iteration. Also, the modeling allows prediction of the starting glass‐stack dimensions for a specified fiber design.  相似文献   
25.
Heavy metal sorption by calcium alginate beads from Laminaria digitata   总被引:1,自引:0,他引:1  
Alginate with a high M/G ratio, extracted from Laminaria digitata, was evaluated for Cu(2+), Cd(2+) and Pb(2+) sorption in acidic solutions, in the form of calcium cross-linked beads. The high M/G ratio of alginate extracted from this algal species is most likely the determining factor for the increased adsorption capacity of the investigated metals, indicating that the mannuronic acid is responsible for the ion exchange mechanism. The data obtained from the batch experiments have been interpreted with Langmuir, Freundlich and Sips models. The Sips equation provided the best fit with the experimental results, indicating sorption sites heterogeneity for the material. The pH was found to have a significant effect on the process, with sorption capacity reaching a maximum at pH 4.5, indicating a competition mechanism between H(+) and metal ions. Kinetic experiments were performed at the optimum pH. For the interpretation of the kinetic experiments the Linear Adsorption Model was employed and diffusion coefficients were determined. The model fits the experimental data at higher concentrations, where the adsorbed quantity remains almost constant. Finally, a simplified expression of the batch kinetic adsorption model was employed. The model, predicts adequately, not only the diffusivity values, but also the concentration profiles inside the spherical particles.  相似文献   
26.
向市场推出新手机的时坷要求正在变得越来越紧追。在面市时间日益缩短的趋势之下.产品要想取得成功.就需要具备更多的功能;伴随着手机市场的发展.手机支持的功能也在不断增多:除了基本的话音通信功能之外.照相机,因特网浏览、流视频、MP3,游戏以及PDA功能等都在被越来越多的手机所拥有。  相似文献   
27.
We have undertaken systematic calculations of transition metal sulfides bulk crystal structures, electronic and energetic properties at the first principles level (DFT, GGA, PW-USPP, PBC, implemented in the Vienna Ab initio Simulation Package, VASP). Relaxed cell parameters and ionic positions showed an excellent agreement with the experimental values. Computed and experimental cohesive energies agreed within 3%. We re-defined the metal–sulfur (M–S) bond strength as the cohesive energy per metal–sulfur bond: we show that all experimental HDS activities (Pecoraro and Chianelli, 1981) fit nicely on a single volcano master curve when plotted against this simple energetic parameter. Metallic (i.e. zero gap) ionic sulfides consistently exhibit the weakest M–S bonds and semi-conductor iono-covalent sulfides the strongest. However, the Sabatier principle suggests a simple kinetic interpretation of this master curve. This new interpretation also accounts for the well known synergetic effects in mixed sulfides and therefore opens new prospects for exploratory applied research.  相似文献   
28.
29.
The catalytic performance of the LaCoO3 thin films toward CO and ethanol oxidation was studied in a reactor designed for the investigation of model planar catalysts. The effect of the thickness on the catalytic efficiency and the surface oxygen vacancies offers a straightforward method to identify or discriminate the oxygen species involved in the catalytic reaction. The weight-normalized reaction rate is demonstrated to be an intrinsic parameter and thus confirms the participation of the bulk lattice-oxygen in the studied catalytic reactions. Compared to other perovskite oxide powder catalysts, thin film catalysts show a remarkably higher activity toward ethanol and CO oxidation.  相似文献   
30.
UASB工艺常温处理木薯加工废水   总被引:6,自引:0,他引:6  
郑平  胡宝兰 《太阳能学报》2002,23(6):774-777
为了控制喀麦隆木薯加工废水的污染并回收沼气 ,选用上流式厌氧污泥床反应器 (UASB) ,进行了常温厌氧处理木薯加工废水的试验。结果表明 :采用UASB工艺处理COD浓度为 14190mg/L的木薯加工废水 ,COD去除率可达90 %以上 ,容积COD负荷达 6.81g/L·d ,容积沼气产率 3 .2 0L/L·d。在实用中 ,进水COD浓度宜控制在 140 0 0mg/L左右 ;容积COD负荷宜控制在 7.0 0 g/L·d左右 ;水力停留时间宜控制在 2d。  相似文献   
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