Hollow γ-MnO2 sphere was obtained by annealing the precursor at 400 °C with different holding time. The influences of different holding time on the morphology and crystalline structure of final products have been discussed in detail, and the microwave absorption properties of the as-products were also investigated. The results exhibited that finer crystalline feature of the γ-MnO2 and larger pore size in the hollow γ-MnO2 sphere could be obtained with the extended holding time. The MnO2/paraffin composites (50 wt% loading) present extraordinary microwave absorption performance, and the minimum reflection loss (RL) values is −51.3 dB at 4.9 GHz with the thickness of 3.5 mm. The excellent electromagnetic absorption properties can be ascribed to the hollow structure, perfect impedance matching behavior and the multiple interface polarization effect. 相似文献
With the development of computer network technology, the means of foreign language teaching have changed. Computer aided spoken English test is a
new test method, and there is a great advantage compared with the traditional oral test. In order to further understand the superiority of the computer aided
spoken English test, this study took the sophomores of Foreign Language Department in Henan University of Chinese Medicine, China, as the subjects and
carried out the traditional interview-type spoken English test and computer-assisted spoken English test. The score system based on Hidden Markov Model
(HMM) was used, and then the two tests were carried out. The performance in the two tests was compared, and the attitudes of the participants to the
computer assisted Spoken English test were analyzed in the form of questionnaires. The results showed that the computer aided spoken English test could
better reflect the true level of the students, and the teachers and students clearly stated that the computer aided spoken English test could relieve tension
and reduce the burden of teachers. The research verified the feasibility of the computer-assisted spoken English test, which provides a reference for the
promotion of the computer-assisted spoken English test. 相似文献
It is essential to develop an accurate model of proton exchange membrane fuel cell (PEMFC) for a reliable operation and analysis, in which unknown parameters usually need to be determined. The inherent nonlinear, strong coupling, and diversification of PEMFC model seriously hinder traditional methods to identify the parameters. For the sake of overcoming these thorny obstacles, Levenberg-Marquardt backpropagation (LMBP) algorithm based on artificial neural networks (ANNs) is proposed for PEMFC parameter identification. Furthermore, the performance of LMBP is thoroughly evaluated and compared with four typical meta-heuristic algorithms under three cases. Simulation results indicate that LMBP performs a higher accuracy and faster speed for parameter identification. In particular, accuracy and convergence speed can achieve as much as 99.8% and 95.9% growth via LMBP, respectively. 相似文献
Cr2O3 is a well-known corrosion resistant oxide used in refractory applications. However, it can oxidize into toxic and water-soluble Cr(VI) compounds upon reaction with calcium aluminate cement phases in the presence of oxygen, which subsequently causes disposal problems after use. This study describes the extent to which chromium in the spinel Mg(Al,Cr)2O4 phase can be oxidized to Cr(VI) when it reacts with the calcium aluminate cement phases C12A7, CA, CA2 and free CaO at 1300 °C in air, using XRD, XPS and leaching tests (TRGS 613 standard) as analytical tools. On reaction with CaO, the Mg(Al,CrIII)2O4 spinel mainly transformed into hauyne (Ca4Al6CrVIO16) and Ca5Cr3O12 which contains both Cr(IV) and Cr(VI). The reaction of C12A7 and CA with the spinel phase also resulted in the formation of Ca4Al6CrO16. Conversely, the reaction of Mg(Al,CrIII)2O4 spinel with CA2 resulted in the formation of only a trace amount of Cr(VI). Water-soluble Cr(VI) leached in large quantities (>100 mg/L) from samples where the Mg(Al,CrIII)2O4 reacted with either C12A7 or CA. Almost no Cr(VI) leached from the sample when Mg(Al,CrIII)2O4 reacted with CaO, using the standard TRGS 613 leach test, but a significant amount of Cr(VI) was released into solution when leached with a HCl solution for 12 h. Both Cr(IV) and Cr(VI) present in the Ca5Cr3O12 dissolved into acidic solution. Only a small amount of Cr(VI) leached from the sample that resulted when spinel was reacted with CA2, even after a prolonged HCl leach. Cr(III) in spinel Mg(Al,Cr)2O4 is very stable and does not leach in either distilled water or acidic solution. 相似文献
As a promising graphene analogue, two-dimensional (2D) polymer nanosheets with unique 2D features, diversified topological structures and as well as tunable electronic properties, have received extensive attention in recent years. Here in this review, we summarized the recent research progress in the preparation methods of 2D polymer nanosheets, mainly including interfacial polymerization and solution polymerization. We also discussed the recent research advancements of 2D polymer nanosheets in the fields of energy storage and conversion applications, such as batteries, supercapacitors, electrocatalysis and photocatalysis. Finally, on the basis of their current development, we put forward the existing challenges and some personal perspectives.