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71.
72.
The role of conductive pathways in the conductivity and rheological behavior of poly(methyl methacrylate)–graphite composites 下载免费PDF全文
Up to now, research on the dynamic process of conductive network formation has tended to focus on composite particles with one‐dimensional geometry, such as carbon black and carbon nanotubes. However, studies on this subject based on fillers with two‐dimensional structure, such as graphite, are rare in the literature. In this work, the dynamic percolation and rheological properties of poly(methyl methacrylate) (PMMA)–graphite composites under an electric field were investigated. The activation energies of conductive network formation and polymer matrix mobility were calculated from the temperature dependence of the percolation time and the zero‐shear viscosity. It was found that the activation energy calculated from the zero‐shear viscosity was not influenced by the electric field in the concentration range investigated, but the electric field had an effect on the activation energy calculated from the percolation time. This finding emphasizes that the electrical and rheological properties have different physical origins. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43810. 相似文献
73.
Temperature Dependence of Efficiency in GaInN/GaN Light‐Emitting Diodes with a GaInN Underlayer 下载免费PDF全文
Kyurin Kim David S. Meyaard Guan‐Bo Lin E. Fred Schubert Jong Kyu Kim Jaehee Cho 《International Journal of Applied Ceramic Technology》2016,13(2):234-238
The temperature dependence of the efficiency (i.e., temperature‐mediated efficiency droop) in blue light‐emitting diodes (LEDs) is investigated. A GaInN/GaN LED with a GaInN underlayer having an indium mole fraction of 8% shows less temperature dependence of efficiency, compared to the LED without an underlayer. Better carrier confinement in the active region of the LED with a GaInN underlayer is proposed to reduce carrier leakage from the active region at high temperature. The results indicate that the insertion of an underlayer leads to an improvement of the LED's radiative efficiency and its high‐temperature‐tolerant performance. 相似文献
74.
An original simulation program to explore the effects of μ-law companding in digital communication systems is presented. The program allows the user to explore companding for arbitrary input signals. Reconstructed analog signals and quantization noise for both companded and uncompanded systems are graphically displayed. Quantization signal-to-noise ratio is calculated and compares favorably with theory. The program was written using the mathematical simulation package MathCAD 相似文献
75.
R. Tschentscher M. Schubert A. Bieberle T.A. Nijhuis J. van der Schaaf U. Hampel J.C. Schouten 《Chemical engineering science》2011,(14):14
Rotating solid foam reactors have already proven to show high mass transfer rates and to be a potential alternative to slurry reactors. The rotation of a foam block stirrer results in a high mass transfer and in the development of different reactor sections showing specific hydrodynamics and gas holdup distributions. In order to optimize the reactor system the hydrodynamics in a lab scale reactor are studied using γ-ray tomography, a powerful method to measure the gas holdup in three-phase reactors. The influence of liquid properties, such as viscosity and surface tension, and the rotational speed on the gas/liquid distribution in the different reactor sections is investigated. Especially the viscosity has a strong effect on the entrapment of gas bubbles in the foam block structure, while the surface tension is the dominant parameter in the outer reactor section. The influence of these parameters on the inset of foaming and the collapse of the gas/liquid dispersion is investigated. Conclusions on the mass transfer performance are drawn and recommendations for further optimizations of the reactor design and the operational conditions depending on the liquid properties are developed. 相似文献
76.
The long-term high-temperature cyclic oxidation (100 cycles, 104 h, 1500°C) of a Si3 N4 material and a Si3 N4 /MoSi2 composite, both fabricated with Y2 O3 as a sintering additive, was studied. Both materials exhibited similar oxidation rates because of surface SiO2 formation described by an almost parabolic law and a total weight gain of 3–4 mg/cm2 after 104 h. As a consequence of oxidation processes in the bulk, microstructural damage was found in the Si3 N4 material. These effects were not observed in the composite. The remarkable microstructural stability observed offers the high potential of Si3 N4 /MoSi2 composites for long-term structural applications at elevated temperatures up to 1500°C. 相似文献
77.
Schubert U.-A. Anderle F. Spengler J. Zühlke J. Eberle H.-J. Grasselli R.K. Knözinger H. 《Topics in Catalysis》2001,15(2-4):195-200
Titania-supported vanadia catalysts were modified by addition of antimony oxide for application in o-xylene selective oxidation to phthalic anhydride. It was shown that active and selective catalysts can be prepared by ball-milling mixtures of powders of TiO2, V2O5and Sb2O3followed by calcination. X-ray photoelectron spectroscopy proves the formation of highly dispersed overlayers of vanadium oxide and antimony oxide, in which V5+is partially reduced to lower oxidation states and Sb3+is partially oxidized to Sb5+. Antimony oxide segregated into the outermost surface layers. It is therefore inferred that the presence of the antimony oxide modifier spatially separates V–O species and leads to site isolation which may be responsible for the positive effect of the modifier for the catalyst's selectivity. 相似文献
78.
Remendable polymers via reversible Diels–Alder cycloaddition of anthracene‐containing copolymers with fullerenes 下载免费PDF全文
Julia Kötteritzsch Robert Geitner Johannes Ahner Marcus Abend Stefan Zechel Jürgen Vitz Stephanie Hoeppener Benjamin Dietzek Michael Schmitt Jürgen Popp Ulrich S. Schubert Martin D. Hager 《应用聚合物科学杂志》2018,135(10)
Poly(lauryl methacrylate)s with anthracene moieties in the side chain were converted with C60‐fullerene and phenyl‐C61‐butyric acid methyl ester (PCBM), resulting in new remendable (self‐healing) polymeric materials. The utilization of differently substituted anthracene monomers enabled the tuning of the reactivity and the resulting mechanical properties. Copolymers with different contents of the anthracene moieties were synthesized and characterized using size exclusion chromatography, 1H nuclear magnetic resonance (NMR) spectroscopy as well as differential scanning calorimetry (DSC). 1H NMR spectroscopic studies were utilized in order to investigate the reversibility of the Diels–Alder reaction between copolymers with C60‐fullerene and PCBM, respectively, in solution. In order to investigate the conversion of the polymers with C60‐fullerene and PCBM in bulk, additionally, DSC, nanoindentation, rheology, atomic force microscopy (AFM), 3D microscopy, simultaneous thermal analysis (STA) and FT‐Raman investigations were performed. The fullerene‐containing copolymers could be healed in a temperature range of 40–80 °C. Consequently, a new generation of low temperature remendable polymers could be established. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45916. 相似文献
79.
Oil-in-water(o/w) emulsions were produced with a membrane emulsification system. The effect of the continuous phase viscosity on the emulsification was studied. The theoretical analyses show that the continuous phase viscosity influences not only the flow field of the continuous phase but also the interfacial tension. The droplet size distribution and disperse phase flux for different continuous phase viscosity were investigated experimentally at constant wall shear stress and constant volume flow rate of the continuous phase respectively. 相似文献
80.
Markus Schubert Günther Hessel Cornelius Zippe Rüdiger Lange Uwe Hampel 《Chemical engineering journal (Lausanne, Switzerland : 1996)》2008,140(1-3):332-340
Trickle bed reactor performance and safety may suffer from radial and axial liquid maldistribution and thus from non-uniform utilization of the catalyst packing. Therefore, experimental analysis and fluid dynamic simulation of liquid–gas flow in trickle bed reactors is an important topic in chemical engineering. In the present study for the first time a truly high-resolution gamma ray tomography technique was applied to the quantitative analysis of the liquid flow texture in a laboratory cold flow trickle bed reactor of 90 mm diameter. The objective of this study was to present the comparative analysis of the liquid flow dynamics for two different initial liquid distributions and two different types of reactor configurations. Thus, the hydrodynamic behavior of a glass bead packing was compared to a porous Al2O3 catalyst particle packing using inlet flow from a commercial spray nozzle (uniform initial liquid distribution) and inlet flow from a central point source (strongly non-uniform initial liquid distribution), respectively. The column was operated in downflow mode at a gas flow rate of 180 L h−1 and at liquid flow rates of 15 and 25 L h−1. 相似文献