Facile synthesis of a carbon-rich SiAlCN precursor and investigation of its structural evolution during the polymer-ceramic conversion process

A liquid carbon-rich SiAlCN precursor is facilely synthetized by hydrosilylation between liquid polyaluminocarbosilane (LPACS) and 1,3,5,7-tetravinyl- 1,3,5,7-tetramethylcyclotetrasilazane {[CH₃(CH₂CH₂)SiNH]₄} (TeVSZ). The structural evolution during the polymer-ceramic conversion process is investigated by various methods. The results show that the main cured mechanism is β-addition on hydrosilylation, although α-addition on hydrosilylation, polymerization of vinyl groups and dehydrocoupling reaction between N–H bonds also occur during the cured process. During the pyrolysis process, dehydrogenation and dehydrocarbonation condensation reactions, transamination reactions occur, leading to formation of a three-dimensional network inorganic structure at 400–800 °C, where part of Al–O bonds convert to Al–N bonds. Then the network inorganic structure undergoes demixing and separation into amorphous SiAlCN(O) phase, where the amorphous turbostratic free carbon phase also form at 800–1200 °C. With demixing and decomposition of the amorphous carbon-rich SiAlCN(O) phase, the crystalline β-SiC and graphitic carbon start to form at about 1400 °C, the crystalline sizes of them both enlarge with increasing temperature. However, the crystal growth of β-SiC is distinctly inhibited due to existence of the rich carbon phase, tiny amounts of Al₂O₃ and AlN. In addition, a small amount of AlN can promote the formation of α-SiC at 1800 °C.     

The magnetocaloric effect of Gd-Tb-Dy-Al-M (M = Fe,Co and Ni) high-entropy bulk metallic glasses

In this article, we report the formation of the high-entropy Gd₂₀Tb₂₀Dy₂₀Al₂₀M₂₀ (M = Fe, Co and Ni) bulk metallic glasses with good magnetocaloric properties. Compared with most of the rare earth based metallic glasses, these alloys are found to have the comparably large maximum magnetic entropy changes (ΔS_M), but much broader widths of the ΔS_M peaks, and hence larger refrigerant capacity (RC). This can be attributed to the combination of the spin glass behaviors and the complicated compositions in these alloys. Our work show that the high entropy bulk metallic glasses is a promising candidate material as the magnetic refrigerant.     

Screen-printing fabrication of electrowetting displays based on poly(imide siloxane) and polyimide

We report a screen-printing fabrication process for large area electrowetting display (EWD) devices using polyimide-based materials. The poly(imide siloxane) was selected as hydrophobic insulator layer, and relatively hydrophilic polyimide as grids material. EWD devices that use poly(imide siloxane) as hydrophobic insulator fabricated with conventional methods showed good and reversible electrowetting performance on both single droplet level and device level, which showed its potential application in EWDs. The compatibility of polyimide-based materials (hydrophobic poly(imide siloxane) and hydrophilic polyimide) guarantee the good adhesion between two layers and the capability of printable fabrication. To this end, the hydrophilic grids have been successfully built on hydrophobic layer by screen-printing directly. The resulting EWD devices showed good switch performance and relatively high yield. Compared to conventional method, the polyimide-based materials and method offer the advantages of simple, cheap and fast fabrication, and are especially suitable for large area display fabrication.     

Investigation of mechanical and tribological behaviors of multilayer graphene reinforced Ni3Al matrix composites

Multilayer graphene (MLG) shows an attractive prospect for the demanding engineering applications. This paper reports the mechanical and tribological properties of MLG reinforced Ni₃Al matrix composites (NMCs) under dry sliding at varying sliding speed. The hardness and elastic modulus of the NMCs are significantly influenced with MLG content. It is found that the hardness and elastic modulus of the NMCs are found to be increased by increasing MLG content up to 1.0 wt.%, while decreased when MLG content is above 1.0 wt.%. Tribological experiments suggest that MLG can dramatically improve the wear resistance and decrease the friction coefficient of the NMCs. Such marked improvement of wear resistance is attributed to the reinforcing mechanisms of MLG, such as crack deflection and pull-out, and reduction of friction coefficient is related to the formation of a tribofilm on the sliding contact surface.     

Effects of supports and combined process on hydrogen purification over nickel supported catalysts

Hydrogen purification must be done to meet the different purposes of hydrogen utilization.In the present work,it is confirmed that the catalyst Ni/CeO_2 has the highest activity for total methanation(Total MET) of CO and CO_2,and is thus most suitable for hydrogen purification for ammonia synthesis.While,the catalyst Ni/ZrO_2 appears the best one for selective methanation of CO(CO-SMET) in the H_2-rich gas to produce clean fuel for proton exchange membrane fuel cell(PEMFC).In spite of this,the catalyst Ni/ZrO_2 without adding chlorine ions as promoter is not yet capable of removing the CO in the reformate gas to below 10 ppm in a wide reaction temperature range by the way of CO-SMET.Adding chlorine ions as promoter is indeed not favorable for practical application due to its gradual loss in the catalytic reaction as proved in our previous work.Therefore,a step to decrease CO_2 concentration(called as de-CO_2 step) is suggested to be set prior to the CO-SMET step in this work.It is proved that such combination of de-CO_2 step and CO-SMET step is efficient to achieve a deep removal of CO to below 10 ppm with a high selectivity more than 50% in a wide reaction temperature range of 220—280℃over the catalyst Ni/ZrO_2 without adding chlorine ions as promoter.The combined process has potential for practical application,at least in the large-scale power plant of PEMFC.     

Rare earth nanostructures based on PrO_x/CNT composites as potential electrodes for an asymmetric pseudocapacitor cell

Rare earth metal oxides have been widely used as pseudocapacitor electrodes owing to their unique physical and electronic properties.The present paper reports the synthesis and pseudocapacitor applications of praseodymium oxides,owing to their unique physical properties.PrO_x/unzipped carbonnanotubes were synthesized following a hydrothermal approach.Detailed morphological as well as electrochemical analyses were performed to elucidate how the unique properties of PrO_x affect the charge storage ability.Special emphasis was given on the effect of anion intercalation due to the surface oxygen vacancies in PrO_x which would contribute towards the pseudocapacitive energy storage.Oxygen intercalation was exploited for the first time in fluorite crystals for fast energy storage and a specific capacitance value as high as 1099 F/g was obtained with the electrodes.An asymmetric supercapacitor prototype was also fabricated with a V_2 O_5/graphene counter electrode and the energy and power density values obtained are as high as 52.08 Wh/kg and 2.9 kW/kg,respectively.     

Ambiguous temperature difference in aerodynamic levitation process: Modelling,solving and application

The aerodynamic levitation provides an efficient technique for the research on thermophysical properties and solidification behavior of refractory materials. However, there is a nonnegligible temperature differences across sample, causing unexpected uncertainty of measurement, such as, thermal expansivity and undercooling limit. We establish thermal filed model with properly simplified boundary condition, and derive quantitative expressions of this ambiguous temperature difference. Here we show that the temperature difference not only related to the average temperature, relative size and thermal conductivity of sample, but significantly influenced by the rotation pattern of sample. A huge temperature differences is almost inevitable when the sample with low thermal conductivity and high melting point is smelted in stationary suspension pattern, however, a drastically reduction of temperature difference can be fulfilled by simply making the sample rotation in up to down pattern. The thermal filed simulation was used to confirm the validity of these theoretical expressions. This work shed light on temperature difference in aerodynamic levitation. Based on this work, one can simply estimate the extent of temperature difference across the sample, and regulated that conveniently if needed, which benefit for novel material preparation and solidification mechanism study based on this technique.     

Silicon lithium-ion battery anode with enhanced performance: Multiple effects of silver nanoparticles

Silicon has been regarded as one of the most promising next generation lithium-ion battery anode. However, the poor cyclic stability of the Si based anode has severely limited its practical applications, which is even worse with high mass loading density (>1?mg?cm^?2). A new concept has been developed to enhance the electrochemical performance of the Si nanoparticle anode. Silver nanoparticles are composited with the silicon nanoparticles in a facile way for the first time. It is found that the mechanical properties of the Si electrode have been significantly improved by the incorporation of the silver nanoparticles, leading to enhanced cyclic performance. With the Si/Ag mass ratio of 4:1, the reversible specific discharge capacity is retained as 1156?mA?h?g^?1 after 100 cycles at 200?mA?g^?1, which is more than three times higher than that of the bare silicon (318?mA?h?g^?1). The rate performance has been effectively improved as well due to excellent electron conductivity of the silver nanoparticles.     

Wear resistance of Fe-based amorphous coatings prepared by AC-HVAF and HVOF

Fe₆₃Cr₈Mo_3.5Ni₅P₁₀B₄C₄Si_2.5 amorphous coatings have been prepared by the activated combustion high velocity air fuel (AC-HVAF) and high velocity oxygen fuel (HVOF) processes. The microstructure and wear resistance of the amorphous coatings are comparatively studied. The wear volume loss of the AC-HVAF coating is approximately seven times less than that of the HVOF coating, indicating that the AC-HVAF coating exhibits better wear resistance. Detailed analysis on the worn surface indicates that the enhanced wear resistance of the AC-HVAF coating is mainly attributed to the formation of a more stable oxide tribolayer and smoother worn surface, which result from the dense and complete amorphous microstructure of the AC-HVAF coating. The wear mechanism of the amorphous coatings is dominated by oxidation wear.